WO2025233637A1

WO2025233637A1 - Surgical tool

Info

Publication number: WO2025233637A1
Application number: PCT/GB2025/051008
Authority: WO
Inventors: Sarah Muirhead-Allwood
Original assignee: Individual
Current assignee: Individual
Priority date: 2024-05-10
Filing date: 2025-05-09
Publication date: 2025-11-13
Anticipated expiration: 2026-11-10

Abstract

A tool (1) for inserting a surgical implant (20) in a subject, the tool comprising: a main body portion (2) configured to transmit an impact force to the implant to insert the implant into the subject, during an impaction phase of the impact, wherein elastic potential energy is stored in the implant and surrounding tissue of the subject from the tool during the impaction phase; a decoupling mechanism (3) configured to couple the main body portion to the implant during the impaction phase and decouple the main body portion from the implant during a rebound phase of the impact, wherein a portion of the elastic potential energy stored in the implant and surrounding tissue of the subject during the impaction phase is released from the implant and surrounding tissue of the subject back into the tool during the rebound phase; wherein the decoupling mechanism is configured to be proximate to the implant, such that the decoupling mechanism is configured to reduce the mass that is coupled to the implant, thereby reducing the force on the implant acting to dislodge the implant out of the subject during the rebound phase.

Description

SURGICAL TOOL

TECHNICAL FIELD

A tool for installing a surgical implant in a subject, an implant assembly comprising an implant and the tool, and an implant configured for use with the tool. In a specific example, the implant is an acetabular cup.

BACKGROUND ART

Modern hip replacements utilize cementless implants, meaning they are fixed through a press-fit (also known as interference fit) between the implant and the bone cavity, ensuring mechanical stability and fixation. The long-term success of this implant installation relies on bio-integration (osseointegration), wherein surrounding bone remodels and grows onto the implant's surface over months and years post-surgery. To achieve optimal bone integration, minimal motion between the implant and bone is crucial, and the gap between the implant's outer surface and the cavity surface should be minimal.

To establish the press-fit, a hemispherical cavity is reamed into the pelvis that is slightly smaller than the implant's geometry. This size difference enables an interference fit between the implant and the cavity. Force is applied to drive the implant into the cavity, causing elastic deformation of the interfacing cavity. This deformation creates a reaction force between the bone and implant that increases progressively as the implant is inserted deeper into the cavity. The resulting high radial reaction force around the implant's periphery generates a frictional force capable of preventing the cup from dislodging from the cavity.

Surgeons use various methods to apply the force needed for implant insertion, including a weighted mallet, powered impaction device, or robotic surgical system. The traditional and most common method involves a weighted mallet and introducer with a handle, but several powered impaction devices are currently in development and use.

To propel the implant into the reamed cavity in the right orientation, the surgeon uses a series of mallet strikes to apply force via an introducer. The introducer is a metallic rod which at one end attaches to the implant (typically through a threaded or proprietary coupling) and a strike end with a handle for the surgeon to hold and a strike pad for impact. Usually, multiple mallet strikes are needed to position the implant correctly. During each strike, the surgeon's energy is transferred to the implant by swinging the weighted mallet and hitting the introducer at its strike pad end. The mallet's momentum is then transmitted through the introducer directly to the implant via the distal coupling.

The elastic nature of bone offers benefits by enabling the build-up of radial reacting force around the implant's periphery, which help secure the implant. However, it also poses challenges. During the impaction process, a portion of the energy transferred from the implant to the bone (and surrounding soft tissue) is stored as elastic deformation and later transferred back to the implant during a rebound phase. Furthermore, a proportion of the elastic energy from deformation of the implant will contribute to the rebound energy released during the rebound phase. If the combined rebound energy is sufficient to overcome the frictional force at the implant-bone interface, it can cause the implant to move back out of the cavity. This has several detrimental consequences including reduced fixation of the implant, increased wear at the bone-implant interface, greater void between the implant's outer wall and the bone cavity wall, requirement for more strikes to properly seat the cup, leading to increased risk of tissue fractures.

For the patient, both short-term and long-term fixation are compromised. Peak mechanical fixation is not achieved because the cup fails to reach full depth, thereby reducing the peripheral reaction force. Additionally, excess relative motion between the cavity and implant during the rebound phase can lead to wear of the cavity wall, further compromising fixation.

Furthermore, when the cup fails to fully seat into the cavity, it increases the distance that native bone must bridge during the bone remodelling and bio integration phase. A large void in this area may impede long-term fixation and compromise the overall success of the implant. These challenges highlight the importance of addressing rebound-related issues to ensure optimal patient outcomes.

The rebound issue also poses challenges for the surgeon during the implantation procedure. Firstly, it adds complexity to the surgery, as achieving proper implant seating becomes more difficult with the risk of rebound affecting the implant's positioning. The technique is not forgiving to excess strikes, meaning that if the implant is repeatedly struck beyond what is necessary due to rebound, it can lead to potential damage or complications in the surrounding tissues or bone structure.

Moreover, dealing with rebound effectively requires additional time and energy during surgery. Surgeons must take care to minimize excess strikes and carefully manage the impaction process to ensure optimal implant placement. This not only extends the duration of the surgery but also demands precise execution to mitigate the impact of rebound on the overall success of the procedure. Addressing these issues is crucial to enhancing the efficiency and effectiveness of hip replacement surgeries.

Powered impactors are able to execute strikes at a higher frequency (5-6Hz) which has the potential to reduce surgery time, however, it also increases the risk of excess strikes (and therefore degradation of fixation). In some cases, it may require 10 strikes to install the implant. At a rate of 5Hz, the surgeon would execute 10 strikes in just 2 seconds, so excess strikes is highly plausible if not inevitable when using a powered impactor.

Similar problems occur for other forms of implant.

It is an aim of the present disclosure to at least partially solve some of the above problems.

SUMMARY OF THE INVENTION

According to an aspect of the disclosure there is provided a tool for inserting a surgical implant in a subject, the tool comprising: a main body portion configured to transmit an impact force to the implant to insert the implant into the subject, during an impaction phase of the impact, wherein elastic potential energy is stored in the implant and surrounding tissue of the subject from the tool during the impaction phase; a decoupling mechanism configured to couple the main body portion to the implant during the impaction phase and decouple the main body portion from the implant during a rebound phase of the impact, wherein a portion of the elastic potential energy stored in the implant and surrounding tissue of the subject during the impaction phase is released from the implant and surrounding tissue of the subject back into the tool during the rebound phase; wherein the decoupling mechanism is configured to be proximate to the implant, such that the decoupling mechanism is configured to reduce the mass that is coupled to the implant, thereby reduce reducing the force on the implant acting to dislodge the implant out of the subject during the rebound phase.

According to a second aspect of the disclosure there is provided a tool for inserting a surgical implant in a subject, the tool comprising: a main body portion configured to transmit an impact force to the implant to insert the implant into the subject, during an impaction phase of the impact, wherein elastic potential energy is stored in the implant and surrounding tissue of the subject from the tool during the impaction phase; a decoupling mechanism configured to couple the main body portion to the implant during the impaction phase and decouple the main body portion from the implant during a rebound phase of the impact, wherein a portion of the elastic potential energy stored in the implant and surrounding tissue of the subject during the impaction phase is released from the implant and surrounding tissue of the subject back into the tool during the rebound phase; wherein the decoupling mechanism is configured to reduce the force on the implant acting to dislodge the implant out of the subject during the rebound phase.

Optionally, the decoupling mechanism is configured to reduce friction at an interface between respective portions associated with the main body portion and the implant during the rebound phase.

Optionally, the decoupling mechanism comprises one or more bearings.

Optionally, the decoupling mechanism comprises one or more low friction materials configured to reduce friction. Optionally, the low friction material is a solid material. Optionally, the low friction material is a lubricating liquid or gel.

Optionally, the decoupling mechanism is configured to linearly decouple the main body portion from the implant in an axial direction.

Optionally, the decoupling mechanism comprises respective mating portions associated with the main body portion and the implant, configured to mate in the axial direction to couple the main body portion to the implant. Optionally, the decoupling mechanism is additionally configured to rotationally decouple the main body portion from the implant.

Optionally, at least one of the respective mating portions has a substantially cylindrical or spherical surface for facilitating rotational decoupling.

Optionally, the decoupling mechanism is configured to rotationally couple the main body portion to the implant around the axial direction, and retain the rotational coupling around the axial direction during the rebound phase.

Optionally, at least one of the respective mating portions has a substantially flat surface for providing rotational coupling.

Optionally, the decoupling mechanism is configured to selectively retain the rotational coupling or rotationally decouple the main body portion from the implant around the axial direction during the rebound phase.

Optionally, at least one of the respective mating portions has a substantially cylindrical surface for facilitating rotational decoupling and a substantially flat surface for providing rotational coupling, wherein the flat surface is configured to be selectively engaged by the other of the respective mating portions.

Optionally, one respective mating portion is configured to contact another respective mating portion at a contact interface, wherein the surface area of the contact interface is substantially smaller than the surface area of one of the mating portions facing the other mating portion.

Optionally, the tool further comprises a connecting portion provided between the main body portion and the implant, wherein the decoupling mechanism is provided at an interface between the main body portion and the attachment portion.

Optionally, the decoupling mechanism is provided at an interface between the main body portion and the implant. Optionally, the main body portion comprises a first impact surface at a proximal end configured to transmit the impact force to the implant.

Optionally, the main body portion comprises a second impact surface at a distal end configured to receive the impact force, such as from a mallet.

Optionally, the main body portion comprises an attachment mechanism at a distal end configured to attach to a powered impactor.

Optionally, the main body portion comprises an elongate shaft.

Optionally, the tool is configured to partly enter the body of the subject to position the implant.

Optionally, the tool is configured to directly couple to the implant during the impaction phase.

Optionally, a mass of the main body portion is at least 40%, optionally at least 50%, further optionally at least 70%, further optionally at least 85% of a mass of the tool

Optionally, the tool further comprises a sensor configured to detect a change in relative distance between the main body portion and the implant.

Optionally, the sensor is configured to detect a change in relative distance between the main body portion and the implant during the impaction phase and/or rebound phase.

Optionally, the sensor configured to detect a change in relative distance between the main body portion and the implant is one or more of an accelerometer or a Hall effect sensor disposed within the main body portion.

Optionally, when the decoupling mechanism is configured to linearly decouple the main body portion from the implant in an axial direction, the sensor is configured to detect a change in relative distance between the main body portion and the implant along the axial direction.

Optionally, the tool further comprises an indication system configured to emit a signal when the change in relative distance between the main body portion and the implant as sensed by the sensor reaches a predetermined threshold.

Optionally, the predetermined threshold corresponds to a value indicative of implant insertion depth within the cavity.

Optionally, the signal comprises at least one of a visual, auditory or haptic signal.

Optionally, the indication system comprises a feedback system configured to analyse the output of the sensor and trigger the signal when the change in relative distance reaches a predetermined threshold.

Optionally, the feedback system is configured to compare the detected change in relative distance to one or more reference values indicative of known relative distances associated with different degrees of implant insertion.

Optionally, the tool further comprises a computer-readable storage medium configured to store the reference values accessible to the feedback system.

Optionally, the reference values are stored on a local storage device forming part of the tool and/or on a remote device in communication with the feedback system. Optionally, if the reference values are stored on a local storage device forming part of a remove device, then the feedback system is configured to communicate with the remote device via a wired or wireless connection.

Optionally, the indication system is configured to adjust the predetermined threshold based on a user input or data received from the external device.

Optionally, the sensor is configured to transmit data to an external device configured to analyse the data and generate an output signal, such as at least one of a visual, auditory or haptic signal, when the change in relative distance reaches a predetermined threshold. Optionally, the output signal issued by the indication system or the feedback system is configured to instruct or control an automated system, such as a robotic surgical system (e.g. a robotic impaction system).

Optionally, the predetermined threshold upon which the indication system operates corresponds to a change in relative distance between the main body portion and the implant that is indicative of an implant insertion depth within the bone cavity leaving a gap of 2 mm or less between the deepest part of the implant within the bone cavity and the base of the bone cavity (which may be the part of the bone surface within the bone cavity that is adjacent the deepest part of the implant within the bone cavity). Optionally, the predetermined threshold corresponds to a gap of 2.0 mm or less, optionally 1.9 mm or less, 1.8 mm or less, 1.7 mm or less, 1.6 mm or less, 1.5 mm or less, 1.0 mm or less, or 0.5 mm or less, between the deepest part of the implant within the bone cavity and the base of the bone cavity.

Optionally, the implant is a hemispherical acetabular cup, and the gap corresponds to a polar gap defined as the distance between the pole of the acetabular cup and the base of the bone cavity.

According to as third aspect of the disclosure there is provided an implant assembly comprising an implant and the tool of any preceding claim attached to the implant.

Optionally, after decoupling of the main body portion from the implant, a total mass of the implant and any part of the tool rigidly coupled thereto is less than 250 g, optionally less than 200 g, further optionally less than 125 g, further optionally less than 100 g.

According to as fourth aspect of the disclosure there is provided an implant configured for use with the tool of any one of claims 1 to 21 or to form the implant assembly of claim 22.

Optionally, for any preceding aspect, the implant is a cementless implant configured to press-fit into a cavity in the subject.

Optionally, for any preceding aspect, the implant is an acetabular cup. BRIEF DESCRIPTION OF THE DRAWINGS

Further features of the disclosure will be described below, by way of non-limiting examples and with reference to the accompanying drawings, in which:

Fig. 1 shows a tool forming part of the state of the art in use;

Fig. 2 shows an example tool according to the disclosure;

Fig. 3 shows the example tool of Fig. 2 in use;

Figs. 4 to 6 show an example tool according to the disclosure;

Figs. 7 to 9 show another example tool according to the disclosure;

Figs 10 and 11 show another example tool according to the disclosure;

Fig. 12 shows another example tool according to the disclosure;

Fig. 13 shows experimental data indicating the measured displacement of the main body portion of an example tool according to the disclosure following a strike, grouped by polar gap; and

Fig. 14 shows experimental data indicating polar gap and measured displacement as a function of strike number during insertion of an example tool according to the disclosure.

DETAILED DESCRIPTION

Fig. 1 shows a tool forming the state of the art, in use, during an impaction process. As described above, the tool 100 (introducer) is coupled rigidly to an implant 200 (cup) to be located in a cavity within a bone 300 (hip).

Part a) of Fig. 1 shows an impaction phase. During the impaction phase, a force is applied to the tool 100 (e.g. using a mallet or powered impaction device). This force is transmitted directly through the tool 100 to the implant 200 via a rigid coupling (e.g. either threaded or proprietary). Because they are rigidly coupled, the tool 100 and implant 200 act as a single rigid body that gains momentum from the strike. This momentum opposes the friction at the implant-cavity interface, causing the cavity to deform and increasing the reacting force, thereby enhancing fixation. Consequently, the implant moves deeper into the cavity.

Parts b) and c) of Fig. 1 show a rebound phase. As shown in part b) of Fig. 1, in the rebound phase, a portion of the energy transferred to the bone 300 and surrounding tissue is rebounded back towards the implant 200 due to elastic potential energy stored in the tissue. This rebounded energy is then transferred back to the implant-tool system 100, 200, causing it to gain momentum.

As shown in part c) of Fig. 1, when the transfer of energy to the implant-tool system 100, 200 is complete, the implant 200 begins to decelerate as it starts to dislodge back out of the cavity. Throughout this phase, momentum of the implant-tool system 100, 200 acts against the friction force generated by the interference fit. The greater the momentum implant-tool system 100, 200, the more the implant 200 will dislodge.

Fig. 2 shows a first example tool 1 according to the disclosure. As shown, the tool 1 comprises a main body portion 2. The main body portion 2 is configured to transmit an impact force to the implant 20 to insert the implant into the subject during an impaction phase of the installation. As described above, during the impaction phase of the impact, elastic potential energy is stored in the implant and surrounding tissue of the subject from the tool 1.

As shown in Fig. 2, the tool 2 comprises a decoupling mechanism 3. The decoupling mechanism 3 is configured to couple the main body portion 2 to the implant 20 during the impaction phase. This is shown in part a) of Fig. 3. The decoupling mechanism 3 is further configured to decouple the main body portion from the implant during a rebound phase of the impact. As described above, during the rebound phase, a portion of the elastic potential energy stored in the implant and surrounding tissue of the subject during the impaction phase is released from the implant and surrounding tissue of the subject back into the tool 1. The transmission of the elastic potential energy and decoupling of the main body portion from the implant are shown in part b) and c) of Fig. 3 respectively. Accordingly, the decoupling mechanism is configured to reduce the force on the implant acting to dislodge the implant out of the subject during the rebound phase.

The decoupling mechanism 3 is provided proximate to or adjacent to the implant 20. As shown in Fig. 2, the decoupling mechanism 3 may be provided at a proximal end of the main body portion 2 closest to the implant 20. A distal end of the main body portion 2 (furthest from the implant 20) may be configured to receive the impact force to be transmitted, from, for example, a surgical mallet or a power impactor. Because the decoupling mechanism 3 is provided proximate to the implant 20, and not at a distance removed from the implant 20, the decoupling significantly reduces (or minimises) the mass that is coupled to the implant 20 during the rebound phase. This therefore reduces, or minimises, the force on the implant 20 that would otherwise act to dislodge the implant out of the subject during the rebound phase. The mass (of the tool 1) coupled to the implant 20 may be reduced by at least 40%, at least 50%, at least 70%, or at least 85% in the rebound phase, relative to the impaction phase, by the decoupling mechanism 3.

As shown in Fig. 2 the tool 1 may further comprise a connecting portion 4 provided between the main body portion 2 and the implant 20. As shown, the connecting portion 4 is configured to connect the tool 1 to the implant 20. This connection may be in the form of a rigid coupling. This may be provided by a threaded connecting means 41, for example, e.g. where a threaded proximal portion (nearest the implant) of the connecting portion 4 is screwed into a corresponding threaded opening in the implant 20. As shown in Fig. 2, the decoupling mechanism 3 may be provided at an interface between the main body portion 2 and the connecting portion 4.

As shown in Fig. 2, the main body portion may comprise an elongate shaft 21. As shown, the shaft 21 may be substantially straight, but in other examples the shaft 21 may be curved, e.g. in order to avoid impinging on soft tissue of the subject. As shown, the main body portion 2 may comprise a first impact surface 22 at a proximal end of the main body 2 configured to transmit the impact force to the implant 20, e.g. via the connecting portion 4 in this example.

The tool 1 may be configured to partly enter the body of the subject receiving the implant 20 in order to position the implant 20 within the cavity. For example, where a connecting portion 4 is provided, the connecting portion may partially or wholly enter the body of the subject. The main body portion 2 may be configured to partially enter the body, at least during the impaction phase.

As shown in Fig. 2, the main body portion 2 may comprise a second impact surface at a distal end (furthest from the implant) configured to receive the impact force, such as from a mallet, powered impactor, or surgical robot. In other examples, the main body portion may alternatively or additionally, comprise an attachment mechanism at a distal end configured to attach to a powered impactor, e.g. via a rigid coupling.

As shown in Fig. 2, the connecting portion 4 may comprise an elongate shaft or pin 42. As shown, the pin 42 may be substantially straight. As shown, the connecting portion may comprise an impact surface 43 configured to receive the impact force from the main body portion 2, e.g. via the first impact surface 22 of the main body portion 2.

As shown in Fig. 2, the tool 1 may be configured to directly couple to the implant during the impaction phase. For example, the connecting portion 4 of the tool 1 may directly couple to the implant 20 during the impaction phase.

A mass of the connection portion 4 may be significantly less than a mass of the main body portion 2. Therefore, decoupling of the main body portion 2 from the implant 20, while the connection portion 4 remains coupled to the implant 20, significantly reduces the mass that is coupled to the implant 20. For example, the mass of the main body portion 2 may be at least 40%, optionally at least 50%, further optionally at least 70%, further optionally at least 85% of the total mass of the tool (i.e. including the connection portion 4).

The total mass of the implant 20 and any part of the tool 1 (such as the connection portion 4) rigidly coupled to the implant 20, after decoupling (i.e. during the rebound phase) may be less than 250 g, optionally less than 200 g, further optionally less than 125 g, further optionally less than 100 g.

As shown in Fig. 2, the decoupling mechanism 3 may comprise respective mating portions associated with the main body portion 2 and the implant 20. As shown, the mating portions may be configured to mate in the axial direction of the tool 1 to couple the main body portion 2 to the implant 20. The axial direction may refer to a direction substantially corresponding to the direction of the impact force. The axial direction may be through the shaft 21 of the main body portions 2 and/or the pin 42 of the connecting portion 4. The decoupling mechanism 3 may be configured to linearly decouple the main body portion 2 from the implant 20 in the axial direction. As shown in Fig. 2, the mating portions may be provided by a female mating portion 24 associated with the main body portion 2 and a male mating portion 42 associated with the implant 20. In alternative examples, a male mating potion may be associated with the main body portion 2 and a female mating portion may be associated with the implant 20. In the present example, the mating portion associated with the implant 20 is part of the connecting portion 4. In other examples, the mating portion associated with the implant 20 may be part of the implant 20 itself. In the present example, the mating portion 24 associated with the main body portion 2 is part of the main body portion 2.

As shown in Fig. 2, the main body portion 2 may comprise a handle portion 25. The handle portion 25 is configured to be gripped by the user of the tool 1. As shown, the handle may be fixed relative to the rest of the main body portion, e.g. the shaft and the second impact surface 23 in particular. In alternative examples, the handle portion 25 may be moveable, e.g. configured to slide in an axial direction. This movement may increase the force transmitted through the main body for a given impact force.

Fig. 3 shows the tool 1 of Fig. 2 in use. Part a) of Fig. 1 shows the impaction phase. As shown in part a) of Fig. 3, energy is transferred to the main body portion 2, e.g. from a mallet strike. This energy is transferred from the main body portion 2 to the connecting portion 4, which is coupled to the implant 20. The kinetic energy of the implant 20 propels the implant and connecting portion 2 into the reamed bone cavity deforming the wall of the cavity.

Pars b) and c) of Fig. 3 show the rebound phase. As shown in part b) of Fig. 3, elastic potential energy stored in the bone and surrounding material during the impaction phase is transferred to the implant 20, and connecting portion 4, which form a rigidly coupled system. As shown, the connecting portion and main body portion are initially coupled, e.g. via respective impact surfaces 43 and 22. Accordingly, both the connecting portion 4 and the main body portion 2 are accelerated together by the elastic rebound of the bone and surrounding tissue. The connecting portion 4 is unable to accelerate beyond the main body portion 2 due to physical contact between the two. As a result, the velocity of the connecting portion 4 may not exceed the velocity of the main body portion 2. As shown in part c) of Fig. 3, a deceleration of the system formed by the implant 20 and connecting portion 4 occurs. With a traditional tool as shown in Fig. 1, the momentum of the implant and tool would be working to overcome the frictional forces between the bone and the implant 20. However, the main body portion 2 and the connecting portion 4 are able to decouple and move as separate bodies. This means that only a portion of the rebound energy is retained by the system formed by the implant 20 and connecting portion 4. Thus, a smaller proportion of the rebound energy is working to overcome the frictional fixation of the implant 20.

The decoupled main body portion 2 receives (and effectively dissipates) the substantial remainder of the rebound energy. The proportion of rebound energy retained by the system formed by the implant 20 and connecting portion 4 can be reduced by decreasing the mass of the system formed by the implant 20 and connecting portion 4 compared to the mass of the main body portion 2.

In contrast, with the traditional tool shown in Fig. 1, the rigid coupling of the tool 100 to the implant 200 means they act as a single rigid body. In this case, all of the rebound energy is retained by the implant/tool system, which works to working to overcome the frictional fixation of the implant 200.

Figs. 4 and 5 show cross-sections through the decoupling mechanism 3 of an example tool according to the disclosure, such as an example tool 1 having the features described above in relation to Figs. 2 and 3. The decoupling mechanism 3 shown in Figs 4 and 5 may be additionally configured to rotationally decouple the main body portion 2 from the implant 20 around the axial direction. As shown, at least one of the respective mating portions - in this example, the pin 42 - may have a substantially circular cross-section for facilitating rotational decoupling, e.g. defining a substantially cylindrical surface.

Figs. 7 shows a cross-section through the decoupling mechanism 3 of another example tool according to the disclosure, such as an example tool 1 having the features described above in relation to Figs. 2 and 3. The tool of Fig. 7 is also shown in Figs. 8 and 9. The decoupling mechanism 3 may be configured to rotationally couple the main body portion 2 to the implant 20 around the axial direction, and retain the rotational coupling during the rebound phase. As shown, at least one of the respective mating portions - in this example, the pin 42 - may have a substantially flat surface, e.g. polygonal cross-section, for facilitating rotational coupling. The rotational coupling allows torque to be transferred between the respective mating portions.

As shown in Figs. 7 to 9, the relevant mating portion 42 may have a hexagonal crosssection at the decoupling mechanism 3. However, in alternative examples, other shapes may be used.

Fig. 6 shows a variation of the example shown in Figs. 4 and 5 comprising blocking portion, e.g. in the form of a blocking pin 44, extending from the pin 42 in a direction orthogonal to the axial direction of the tool 1. The blocking pin 44 is configured to engage with the mating portion 24 of the main body, to rotationally couple the main body 2 to the implant 20. Accordingly, Fig. 6 shows a further example tool for which the decoupling mechanism 3 is configured to rotationally couple the main body portion 2 to the implant 20 around the axial direction, and retain the rotational coupling during the rebound phase.

In some example’s tools, the decoupling mechanism 3 may be configured to selectively retain the rotational coupling or rotationally decouple the main body portion 2 from the implant 20 around the axial direction during the rebound phase. For example, at least one of the respective mating portions may have a substantially cylindrical surface for facilitating rotational decoupling and a substantially flat surface for providing rotational coupling. In some examples, the flat surface may be configured to be selectively engaged by the other of the respective mating portions. Alternatively, or additionally, some examples tools one or both of the respective mating portions may have actuated portions configured to selectively engage, such as an actuated blocking pin 44 in the example tool shown in Fig. 6, which may be configured to extend and retract.

As show in Fig. 4, the relevant mating portion 42 may have a completely cylindrical surface at the decoupling mechanism 3. However, in alternative examples, the relevant mating portion may have a partial cylindrical surface, e.g. ³/4 of a cylinder. The remaining non-cylindrical surface may be a flat surface for example. Other shapes may be used, e.g. provided they do not interfere with rotation. Figs. 10 and 11 shows a further example tool, such as an example tool 1 having the features described above in relation to Figs. 2 and 3. The decoupling mechanism 3 may be configured to rotationally decouple the main body portion 2 from the implant 20 around an axis orthogonal to the axial direction - on other words to tilt an axis through the main body portion 2 with respect to an axis through the implant 20. As shown, at least one of the respective mating portions - in this example, the pin 42 - may have a substantially spherical surface for facilitating rotational decoupling. The spherical surface also provides rotational decoupling around the axical direction of the tool.

As shown in both Figs. 4 to 9, the other respective mating portion - in these examples, the mating portion 24 of the main body portion 2 - is configured to contact the abovedescribed mating portion at a contact interface. As shown in Figs. 4 to 9 the contact interface is provided by one or more bearings 26, through a contact surface 27 of each bearing 26.

The example of Figs. 4 to 6 has eight bearings 26. As shown, these bearings may be convex protrusions from a surface of the mating portion 24 of the main body 2 that faces the other mating portion 42. The number of bearings 26 is not limited, but in general they are arranged to fit the mating portions.

The example of Figs. 7 to 9 has three bearings 26. Ass shown, these bearings are substantially flat surfaces of the mating portion 24 of the main body 2 that faces the other mating portion 42. The number of bearings 26 is not limited, but in general they are arranged to fit the mating portions.

The example of Figs 10 and 11 have a single bearing, e.g. a spherical surface at the distal end of the pin 42. This provides another type of convex bearing surface. In this example, the bearing is provided by the mating portion 42 associated with the implant, rather than the mating portion 24 associated with the main body portion.

As shown in Figs. 4 to 10, the bearings may be slide bearings, e.g. providing with a sliding surface, e.g. one or more flat or convex surfaces. In other example tools, rolling bearings, such as ball bearings or roller bearings may be provided. The bearings 26 may be configured to reduce friction and facilitate decoupling. The bearings 26 may be formed from a low friction material in some examples.

As shown in Figs. 4 to 9, the surface area of the contact interface, e.g. provided by the contact surfaces 27 of the bearings 26, may be substantially smaller than the surface area of one of the respective mating portions at the decoupling mechanism 3. As shown, the area of contact with the mating portion 24 of the main body 2 is substantially smaller than the outer surface area of the pin 42 facing the mating portion 24 of the main body 2. Accordingly, friction between the mating portions is reduced to facilitate decoupling.

For example, a first part of a surface of one of the respective mating portions facing the other of the respective mating portions may be configured to contact the other of the respective mating portions, and a second part of a surface of one of the respective mating portions facing the other of the respective mating portions may be configured not to contact the other of the respective mating portions.

As shown in Figs. 4 to 9 one of the mating portions may have a substantially continuous outer surface around the axial direction, and the other of the mating portions may have a discontinuous outer surface around the axial direction, e.g. comprising cut-out portions. The cut-out portions may be cut-out as compared to a shape that completely covers or contacts the entirety of the surface of the opposing mating portion.

In the example of Figs. 4 to 6 the pin 42 has a continuous cylindrical surface, whereas the mating portion 24 has cut-out portions 28 between the bearings 26 and contact surfaces 27. In the example of Figs 7 to 9, the pin 42 has an outer surface around the axial direction has a continuous hexagonal prism surface, whereas the mating portion 24 has cut-out portions 28 between the bearings 26 and contact surfaces 27. In the example of Figs. 7 to 9 the bearings 26 are only provided adjacent a subset of the facets of the hexagonal prism surface. Further, the contact surface 27 of each bearing 27 is smaller than the surface of the adjacent facet of the pin 42.

As shown in Figs. 5 and 6, the cut-out portions 28 may extend partially through the mating portion 24 of the main body 2 stopping before the proximal end, such that they are closed at the distal end by the structure forming the mating portion 24. Alternatively, as shown in Figs. 8 and 9, the cut-out portions 28 may extend through the mating portion 24 of the main body 2 completely to the distal end, such that they are open at the distal end.

As shown in Fig. 6, a closed cut-out portion 28, when combined with a blocking portion, such as a blocking pin 44 may limit the relative movement of the respective mating portions 24 and 42 in the axial direction. For example, this may limit the amount of linear decoupling to provide the benefits of decoupling, while preventing complete disconnection of the respective mating portions. This may improve handling of the tool 1. In some examples, the linear decoupling may be selectively limited, e.g. with an actuatable blocking pin 44.

Fig. 12 shows a further example tool 1, in which a part 29 of the mating portion 24 of the main body 2, e.g. including a cut-out 28 configured to engage with a blocking pin 44, is actuatable to selectively limit the linear decoupling. As shown, the actuatable part 29 is a side of the mating portion 24 and comprises a closed cut-out 28. As shown, the actuatable part 29 may be actuatable by a mechanism 30 comprising levers and pivots for moving at least the distal portion of the cut-out 28 to allow the blocking pin 44 to enter the mating portion 24.

In alterative examples, the pin 42 and the mating portions 24 of the main body member may be reversed compared to the examples of Figs 4 to 9. In other words, the mating portion 24 may be the male mating portion shown in Figs. 4 and 5, and the pin 42 may be the female mating portion. In other examples, more complex mating portions may be provided, such that more than one mating portion is provided associated with each of the main body portion and the implant 20. Further, mating portions associated with the main body portion 2 may comprise all female mating portions, all male mating portions or a combination of both male and female mating portions, and correspondingly for the mating portions associated with the implant 20.

In general, the decoupling mechanism 3 may be configured to reduce friction at an interface between respective portions associated with the main body portion 2 and the implant 20 during the rebound phase, e.g. in order to facilitate decoupling. The decoupling mechanism may comprise one or more bearings, in some example tools as shown in Figs. 4 and 5. The decoupling mechanism 3 may comprises one or more low friction materials configured to reduce friction in some example tools. The low friction material may be a solid material or a lubricating liquid or gel, for example, or a combination may be used.

Bearing surfaces may formed from materials forming the respective mating parts, which may be formed from materials forming the portions of the tool with which they are associated, e.g. the connecting portion 4 and main body portion 2. This is likely to be variants of titanium. However, the bearing surface may also be formed from different materials specifically chosen for their bearing qualities. This might include coated variants of steel, other metals or low friction polymer materials such as PTFE based polymers.

As shown in Fig. 8, for example, the pin 42 may additional comprise a feature, such as a hex socket, for engaging another tool for applying torque to the connecting portion 4, e.g. to connect and disconnect the connecting portion 42 from the implant 20.

In further examples, the example tools disclosed herein may further comprise a sensor (not shown in the Figs) configured to detect displacement, preferably linear displacement, of the main body portion 2 relative to the implant 20 and/or connecting portion 4 (i.e. sense a change in relative distance between the main body portion 2 and the implant 20 and/or connecting portion 4) when they move as separate bodies as made possible by the decoupling mechanism 3, such as during the rebound phase (after a strike has occurred) and/or during a strike. The sensor may also be configured to sense the rate of displacement of the main body portion 2 relative to an implant 20 and/or connecting portion 4, such as during the rebound phase and/or during a strike.

Any suitable sensor may be used as the means for sensing the linear displacement, including (but not limited to) one or more of an accelerometer configured to detect acceleration or vibration changes within the tool, or a Hall effect sensor disposed within the main body portion 2 and configured to detect the relative position of a magnet comprised within, or detachably attached to, the connecting portion 4 and/or implant 20.

It has been found that once an implant has been inserted into a bone cavity to a certain extent, the amount of linear displacement of the main body portion 2 relative to the implant 20 and/or connecting portion 4 during a rebound can be used to indicate the degree of implant insertion, i.e. the linear displacement provides an indication of the size of gap between the base of the bone cavity (the deepest part of the cavity) and the deepest part of the implant within the cavity (e.g. the pole of an acetabular cup). Advantageously, the linear displacement can be used to indicate when the implant has reached a suitable depth and is well seated within the bone cavity.

The relationship between the extent of linear displacement of the main body portion 2 relative to the connecting portion 4 and/or the implant 20 during the rebound phase, and the size of the gap between the base of the bone cavity (e.g. the deepest part of the cavity) and the deepest part of the implant within the bone cavity (e.g. the pole of an acetabular cup) will vary depending on the dimensions of the bone cavity and the implant being used. This gap may correspond to a “polar gap” in the case of hemispherical implants (defined as the distance between the pole (i.e. the deepest central point) of the implant and the adjacent bone surface). This relationship can be determined empirically through testing, for example by way of a cadaveric study, to determine the linear displacement values indicative of specific gap sizes for given implants and cavities, such as the linear displacement observed when a desired gap size is achieved indicating that the implant is inserted in the bone cavity to the desired depth.

Reducing the gap between the implant’s outer surface and the bone cavity surface is advantageous for optimising bone growth onto the implant for long-term fixation. Providing the sensor for sensing any linear displacement of the main body 2 relative to the connecting portion 4 and/or the implant can advantageously provide an indication to a user of the tool when the gap between the implant’s outer surface and the bone cavity surface is sufficiently reduced, thereby avoiding any further unnecessary strikes which could result in damage and microfracture to the bone, deterioration of the mechanical press-fit fixation of the implant within the bone cavity, increased surgery time, and extra energy expenditure of the surgeon.

The example tools disclosed herein may further comprise an indication system configured to emit a signal to indicate the change in relative distance between the main body portion 2 and the connecting portion 4 and/or implant 20 during the rebound phase, so that a user can then use that data to determine gap size and therefore extent of implant insertion into the bone cavity. Alternatively or in addition, the indication system may be configured to emit a signal to indicate when the change in relative distance between the main body portion 2 and the connecting portion 4 and/or implant 20 as detected by the sensor reaches a predetermined threshold. It could be, for example, that the tool is configured so that a user manually assigns the threshold for linear displacement based on known gap size at such displacement. The signal emitted from the indication system may be a visual, auditory, or haptic.

The indication system may comprise a feedback controller configured to receive and analyse signals from the sensor to determine extent of insertion of the implant into the bone cavity, based on known linear displacements associated with certain degrees of insertion or gap sizes. These known displacement values may be stored on a computer- readable storage medium that is accessible by the feedback controller, for example, on a local storage device forming part of the tool or on a remote storage device in communication with the feedback controller (e.g. via a wired or wireless connection). When the detected linear displacement indicates that the gap has reached a desired size (e.g., 2 mm, 1.5 mm, 1 mm, 0.5 mm), the feedback controller may be configured to trigger an output signal to indicate completion of insertion. This output may be delivered via a visual, auditory, or haptic interface to inform the user or an automated system such as a robotic surgical system (e.g. a robotic impaction system). Alternatively, no such indication system may be provided, and the sensor may be configured to transmit sensed data to external devices (e.g. a computer or monitoring unit) configured to receive and analyse the signals in a similar manner.

EXAMPLE

Figs. 13 and 14 show experimental results from a cadaveric study and demonstrate that the magnitude of rebound phase displacement of a decoupling introducer (displacement of the main body portion 2) is inversely correlated with the size of the gap between the base of the bone cavity (the deepest part of the cavity) and the deepest part of the implant within the cavity.

For this example, a tool according to this disclosure, as shown in Fig. 3, was used to insert an acetabular cup into a prepared bone cavity. The size of the acetabular cup and the corresponding reamer was selected for this example by a consultant orthopaedic surgeon, taking into account the anatomy of the cadaveric specimen. In the tests conducted for this example, acetabular cup sizes and their corresponding ream sizes included outer diameters of 48 mm (ream 48 mm), 50 mm (ream 50 mm), 52 mm (ream 52 mm), 54 mm (ream 54 mm), 56 mm (ream 56 mm). The same test can be carried out with other implant sizes or geometries, using a corresponding manufacturer specified reamer used to prepare the bone cavity for press-fit implantation. In some cases, the reamer size could be slightly smaller than the implant size to provide an underreamed fit, depending on surgical preference or implant design. A hemispherical acetabular cup of matching nominal size was then inserted. During testing, the distance between the deepest part of the implanted acetabular cup (the "pole") and the surrounding bone at the base of the socket was defined as the polar gap-

To measure the polar gap, a passive marker was rigidly affixed to the decoupled main body portion of the instrument. An additional passive marker array was attached to the bone system via a solid plinth to which it was mounted. A stereo optical tracking system (Polaris Vega®) was employed to monitor the spatial positions of both the bone system and the decoupled main body portion of the tool throughout the experiment.

Prior to an impaction/strike, a trial acetabular cup was secured to the decoupled main body portion. This trial cup was designed to seat fully within the acetabular cavity - bottoming out without achieving a press-fit - thereby defining a reference configuration corresponding to zero millimetres of polar gap. Tracking data from subsequent impaction trials were calculated relative to this reference configuration, enabling precise determination of the polar gap.

Fig. 13 demonstrates that displacement of the main body portion 2 during the rebound phase increases as the polar gap decreases, indicating a inverse relationship useful for determining when the polar gap is suitably small.

Fig. 14, shows a plot of the polar gap and displacement of the main body portion 2 during the rebound phase as a function of strike number. The solid line (left Y-axis) corresponds to the polar gap measured between the implant and the base of the bone cavity. The dashed line (right Y-axis) corresponds to the displacement of the main body portion following each strike event (during the rebound phase). It was found that for the tested tool, implant, and bone cavity combination, as the polar gap decreases below approximately 2 mm, the relative displacement (bounce) of the main body portion 2 away from the implant and implant adapter increases.

Claims

1. A tool for inserting a surgical implant in a subject, the tool comprising: a main body portion configured to transmit an impact force to the implant to insert the implant into the subject, during an impaction phase of the impact, wherein elastic potential energy is stored in the implant and surrounding tissue of the subject from the tool during the impaction phase; a decoupling mechanism configured to couple the main body portion to the implant during the impaction phase and decouple the main body portion from the implant during a rebound phase of the impact, wherein a portion of the elastic potential energy stored in the implant and surrounding tissue of the subject during the impaction phase is released from the implant and surrounding tissue of the subject back into the tool during the rebound phase; wherein the decoupling mechanism is configured to be proximate to the implant, such that the decoupling mechanism is configured to reduce the mass that is coupled to the implant, thereby reducing the force on the implant acting to dislodge the implant out of the subject during the rebound phase.

2. A tool for inserting a surgical implant in a subject, the tool comprising: a main body portion configured to transmit an impact force to the implant to insert the implant into the subject, during an impaction phase of the impact, wherein elastic potential energy is stored in the implant and surrounding tissue of the subject from the tool during the impaction phase; a decoupling mechanism configured to couple the main body portion to the implant during the impaction phase and decouple the main body portion from the implant during a rebound phase of the impact, wherein a portion of the elastic potential energy stored in the implant and surrounding tissue of the subject during the impaction phase is released from the implant and surrounding tissue of the subject back into the tool during the rebound phase; wherein the decoupling mechanism is configured to reduce the force on the implant acting to dislodge the implant out of the subject during the rebound phase

3. The tool of any preceding claim, wherein the decoupling mechanism is configured to reduce friction at an interface between respective portions associated with the main body portion and the implant during the rebound phase.

4. The tool of any preceding claim, wherein the decoupling mechanism comprises one or more bearings.

5. The tool of any preceding claim, wherein the decoupling mechanism comprises one or more low friction materials configured to reduce friction.

6. The tool of claim 5, wherein the low friction material is a solid material.

7. The tool of claim 5, wherein the low friction material is a lubricating liquid or gel.

8. The tool of any preceding claim, wherein the decoupling mechanism is configured to linearly decouple the main body portion from the implant in an axial direction.

9. The tool of any preceding claim, wherein the decoupling mechanism comprises respective mating portions associated with the main body portion and the implant, configured to mate in the axial direction to couple the main body portion to the implant.

10. The tool of claim 8 or 9, wherein the decoupling mechanism is additionally configured to rotationally decouple the main body portion from the implant.

11. The tool of claim 10, when also dependent on claim 8, wherein at least one of the respective mating portions has a substantially cylindrical or spherical surface for facilitating rotational decoupling.

12. The tool of claim 9, wherein the decoupling mechanism is configured to rotationally couple the main body portion to the implant around the axial direction, and retain the rotational coupling around the axial direction during the rebound phase.

13. The tool of claim 12, wherein at least one of the respective mating portions has a substantially flat surface for providing rotational coupling.

14. The tool of claim 10, wherein the decoupling mechanism is configured to selectively retain the rotational coupling or rotationally decouple the main body portion from the implant around the axial direction during the rebound phase.

15. The tool of claim 14, when also dependent claim 8, wherein at least one of the respective mating portions has a substantially cylindrical surface for facilitating rotational decoupling and a substantially flat surface for providing rotational coupling, wherein the flat surface is configured to be selectively engaged by the other of the respective mating portions.

16. The tool of one of claims 9 to 15, wherein one respective mating portion is configured to contact another respective mating portion at a contact interface, wherein the surface area of the contact interface is substantially smaller than the surface area of one of the mating portions facing the other mating portion.

17. The tool of any preceding claim, further comprising connecting portion provided between the main body portion and the implant, wherein the decoupling mechanism is provided at an interface between the main body portion and the attachment portion.

18. The tool of any preceding claim, wherein the decoupling mechanism is provided at an interface between the main body portion and the implant.

19. The tool of any preceding claim, wherein the main body portion comprises a first impact surface at a proximal end configured to transmit the impact force to the implant.

20. The tool of any preceding claim, wherein the main body portion comprises a second impact surface at a distal end configured to receive the impact force, such as from a mallet.

21. The tool of any preceding claim, wherein the main body portion comprises an attachment mechanism at a distal end configured to attach to a powered impactor.

22. The tool of any preceding claim, wherein the main body portion comprises an elongate shaft.

23. The tool of any preceding claim, wherein the tool is configured to partly enter the body of the subject to position the implant.

24. The tool of any preceding claim, wherein the tool is configured to directly couple to the implant during the impaction phase.

25. The tool of any preceding claim, wherein a mass of the main body portion is at least 40%, optionally at least 50%, further optionally at least 70%, further optionally at least 85% of a mass of the tool.

26. The tool of any preceding claim, further comprising a sensor configured to detect a change in relative distance between the main body portion and the implant.

27. The tool of claim 26, when dependent on claim 8, wherein the sensor is configured to detect a change in relative distance between the main body portion and the implant along the axial direction.

28. The tool of claim 26 or claim 27, further comprising an indication system configured to emit a signal when the change in relative distance between the main body portion and the implant as sensed by the sensor reaches a predetermined threshold.

29. The tool of claim 28, wherein the signal comprises at least one of a visual, auditory or haptic signal.

30. The tool of claim 28 or claim 29, wherein the indication system comprises a feedback system configured to analyse the output of the sensor and trigger the signal when the change in relative distance reaches a predetermined threshold.

31. The tool of claim 26 or claim 27, wherein the sensor is configured to transmit data to an external device configured to analyse the data and generate an output signal when the change in relative distance reaches a predetermined threshold.

32. The tool of claim 31, wherein the predetermined threshold corresponds to a value indicative of implant insertion depth within the bone cavity.

33. The tool of any one of claims 28 to 32, wherein the output signal issued by the indication system or the feedback system is configured to instruct or control an automated system, such as a robotic surgical system.

34. The tool of any one of claims 28 to 33, wherein the predetermined threshold corresponds to a change in relative distance between the main body portion and the implant that is indicative of an implant insertion depth within the bone cavity leaving a gap of 2 mm or less between the deepest part of the implant within the bone cavity and the base of the bone cavity.

35. The tool of claim 34, wherein the implant is a hemispherical acetabular cup, and the gap corresponds to a polar gap defined between the pole of the acetabular cup and the base of the bone cavity.

36. An implant assembly comprising an implant and the tool of any preceding claim attached to the implant.

37. The implant assembly of claim 36, wherein, after decoupling of the main body portion from the implant, a total mass of the implant and any part of the tool rigidly coupled thereto is less than 250 g, optionally less than 200 g, further optionally less than 125 g, further optionally less than 100 g.

38. An implant configured for use with the tool of any one of claims 1 to 35 or to form the implant assembly of claim 36 or 37.

39. The tool, implant assembly or implant of any preceding claim, wherein the implant is a cementless implant configured to press-fit into a cavity in the subject.

40. The tool, implant assembly or implant of any preceding claim, wherein the implant is an acetabular cup.