GB2465750A

GB2465750A - Prosthesis positioning system

Info

Publication number: GB2465750A
Application number: GB0821541A
Authority: GB
Inventors: Alec Birkbeck; Ryan Collins
Original assignee: DePuy International Ltd
Current assignee: DePuy International Ltd
Priority date: 2008-11-26
Filing date: 2008-11-26
Publication date: 2010-06-02
Also published as: GB0821541D0

Abstract

The system comprises an instrument 6 and a counterbalance. The instrument 6 comprises a handle having a first end couplable to a prosthesis 8 to be implanted into a bone cavity. The counterbalance comprises an elongate rod 2 having a first end couplable via coupling means 4 to the instrument 6 and a weight 14 coupled to the rod. The weight may be slidable along the rod such that the centre of mass of the counter balance is adjustable. In use the counterbalance extends from the instrument such that the centre of mass of the counterbalance is positioned on an opposite side of the prosthesis from the centre of mass of the instrument 6 and the prosthesis positioning system is self supporting.

Description

A Prosthesis Positioning System The present invention relates to a prosthesis positioning system including an instrument counterbalance. In particular, the counterbalance is couplable to a surgical instrument for inserting a prosthesis into a bone cavity to allow the instrument to be self-supporting such that the surgeon can temporarily let go of the instrument to assess the position of the instrument relative to the patient.

When implanting a prosthetic acetabular cup into a patient's pelvis, it is important to ensure that the cup is accurately aligned. A cup inserter may be used to manipulate the cup in a prepared socket within the pelvis. The cup inserter comprises an elongate handle having a first end arranged to couple to the acetabular cup and a second end having an impaction surface. Typically, at least part of the elongate handle is aligned with the axis of the cup.

When a patient is positioned upon an operating table lying on their side, the axis of the cup, and therefore the longitudinal axis of the cup inserter, should be aligned at approximately 45 0 inclination and 150 version angle relative to the local anatomical references on the patient's pelvis. An impaction force may then be applied to the cup inserter impaction surface in order to secure the cup in position. When the surgeon thinks that the instrument is in the correct position, it can be desirable for the surgeon to be able to let go of the cup inserter and stand back from the patient to assess the alignment from different viewpoints before applying the impaction force. This is because it can be difficult to assess whether the correct inclination and version angles have been achieved when close to the instrument due to a parallax occurring between the surgeon's viewpoint and the cup inserter.

Cup inserters are typically heavy. Currently the only option for a surgeon who wants to assess the position of the cup from further away is to ask someone else to hold the cup inserter. This is not ideal, especially if the cup inserter moves while being handed over.

It is an object of embodiments of the present invention to obviate or mitigate one or more of the problems associated with the prior art, whether identified herein or elsewhere.

According to a first aspect of the present invention there is provided a prosthesis positioning system comprising: an instrument comprising a handle having a first end couplable to a prosthesis to be implanted into a bone cavity; and a counterbalance comprising an elongate rod having a first end couplable to the instrument; wherein when the counterbalance is coupled to the instrument, the instrument is coupled to a prosthesis and the prosthesis is inserted into a bone cavity such that the instrument extends from the bone at an inclination within a predetermined anglular range, the centre of mass of the counterbalance is positioned on an opposite side of the prosthesis from the centre of mass of the instrument and the prosthesis positioning system is self supporting.

An advantage of the present invention is that when coupled to the instrument the counterbalance substantially balances the weight of the instrument about a pivot point within the prosthesis. This allows the instrument to be self-supporting such that the surgeon can temporarily let go of the instrument to assess the position of the instrument and the prosthesis relative to the patient. When the instrument extends from the bone within a predetermined range, the prosthesis positioning system is balanced upon the prosthesis. That is, the prosthesis positioning system is not supported at any other point other than where the instrument couples to the prosthesis. The surgeon may safely let go of the instrument without its inclination changing.

The prosthesis positioning system may be arranged to pivot about a prosthesis coupled to the first end of the instrument such that the moment of the counterbalance substantially balances the moment of the instrument about the pivot point.

The prosthesis positioning system may further comprise a prosthesis coupled to the first end of the instrument having a textured bone engaging surface arranged to increase the friction between the prosthesis and a bone cavity.

The counterbalance may further comprise a weight coupled to the elongate rod.

The weight may be slidable along the rod such that the centre of mass of the counterbalance is adjustable.

The prosthesis positioning system may further comprise a prosthesis coupled to the first end of the instrument, wherein the prosthesis comprises part of a replacement bone joint.

The prosthesis may comprise a socket part of a ball and socket joint. The prosthesis may comprise an acetabular cup.

The instrument may be arranged to be self supporting when the instrument extends from the bone at inclination of between 30° and 60°.

According to a second aspect of the present invention there is provided a method of positioning a prosthesis comprising: coupling a prosthesis to a first end of an instrument comprising a handle; coupling a counterbalance comprising an elongate rod to the instrument; and inserting the prosthesis into a bone cavity such that the instrument extends from the bone at an inclination within a predetermined anglular range such that the centre of mass of the counterbalance is positioned on an opposite side of the prosthesis from the centre of mass of the instrument and the prosthesis positioning system is self supporting.

The counterbalance may further comprise a weight slidably coupled to the elongate rod, and the method may further comprise sliding the weight along the rod to adjust the centre of mass of the counterbalance.

The prosthesis may comprise an acetabular cup and the method may further comprise aligning the rod with the long axis of the patient such that the prosthesis is inserted into the bone cavity at a predetermined version angle.

The present invention will now be described, by way of example only, with reference to figure 1 which illustrates a counterbalance in accordance with an embodiment of the present invention.

In accordance with the present invention there is provided a counterbalance couplable to a surgical instrument to allow the instrument to be self-supporting such that the surgeon can temporarily let go of the instrument to assess the position of the instrument relative to the patient. Figure 1 illustrates an exemplary counterbalance. The counterbalance comprises an elongate rod 2 with a first end having a coupling means 4 for coupling to a cup inserter 6. The cup inserter 6 has a first end for coupling to a prosthetic acetabular cup 8 and a second end comprising an impaction surface 10. Coupling means 4 may comprise any suitable means for temporarily connecting rod 2 to the cup inserter 6. For instance, the coupling means may simply comprises a free end of the rod arranged to be received in a socket within a connection block 12 attached to the cup inserter 6.

Attached to the rod 2 is a weight 14. The weight 14 may be adjustable in position along the rod 2 or may be fixed in position. Alternatively, several weights may be provided to adjust the mass of the counterbalance. Weight 14 is chosen to have a mass and a position such that the mass of the cup inserter 6 is balanced out about a pivot point at the centre of the cup 8.

Acetabular cup 8 is typically formed from a metal provided with a surface treatment, such as Porocoat, which is rough and allows for bone ingrowth. The cup is implanted into the reamed socket without bone cement, being held instead initially by friction between the roughened surface and ultimately by bone forming within the gaps in the roughened surface. When the cup is correctly positioned within a socket reamed in the pelvis, an impaction force is applied to impaction surface 10 to secure the cup within the socket.

However, before the impaction force is applied, friction between the cup and the reamed bone socket assists in holding the cup inserter 6 steady. That is, friction between the surface of the cup 8 and the reamed bone socket means that it is not necessary for the counterbalance weight 14 to exactly match the weight of the cup inserter. The counterbalance and the cup inserter need not be exactly balanced about the pivot point of the system at the centre of the cup. Furthermore, while it will typically be required to insert the cup at an inclination of 45°, the exact inclination may vary according to clinical need, and also the exact position of the patient upon the operating table. The friction provided by the cup ensures that the cup inserter may be self-supporting through an angular range about the typical inclination angle of +1-10-15°.

Preferably, rod 2 extends through a point vertically above the pivot point at the centre of the cup. However, due to the friction provided by the surface of the cup, the cup inserter 6 will be self supporting through a small angular range either side of this vertical position.

The counterbalance allows the surgeon to let go of the cup inserter to step back and take an alternative view of the alignment of the cup, while the cup remains stationary in the socket.

The counterbalance may be kept in place when impacting the cup into the socket, or alternatively it may be removed.

As noted above, the position of weight 14 along rod 2 may be adjustable. By adjusting the position of the weight 14 a wider angular tolerance may be provided. The rod 2 may having marks spaced apart along its length corresponding to the nominal required inclination angle (should clinical indications require an inclination other than 45°) which may be set when the weight is in each position. Furthermore, the weight may be adjustable along rod 2 in order to allow the counterbalance to be used in combination with different cup inserters, or other surgical instruments, which weigh differing amounts or have a different weight distribution.

In an alternative embodiment of the present invention, the counterbalance weight 14 may not comprise a separate component. Instead the necessary mass for the counterbalance may be provided by a thickened rod 2 such that the weight is distributed along the length of the rod 2. Providing the weight in a distributed form advantageously makes the counterbalance more stable and better able to accommodate a wider range of inclination angles.

In certain embodiments of the present invention the rod 2 may be used to assist in setting the correct version angle for the acetabular cup. The rod 2 may be arranged such that aligning the rod 2 with the patient's long axis ensures that the cup will be inserted at a predetermined version angle.

As described above, the counterbalance forms a separate component to the cup inserter.

The counterbalance may be arranged to couple to existing cup inserters, or may require a specially modified cup inserter provided with the appropriate coupling means.

Alternatively, the counterbalance may be integrated with and permanently attached to the cup inserter.

The present invention has primarily been described above in the context of balancing a cup inserter for inserting an acetabular cup. However, the present invention is not limited to this. The counterbalance may be readily applied to other surgical instruments used during other surgical procedures where there is a need to manipulate a prosthesis using an instrument when implanting the prosthesis into the patient. In particular, the prosthesis positioning system of the present invention is particularly suited to implantation procedures where it is important to ensure accurate alignment of the prosthesis, for instance implantation of knee or shoulder prosthetic components.

Other modifications to, and applications of, the present invention will be readily apparent to the appropriately skilled person from the teaching herein, without departing from the scope of the appended claims.

Claims

CLAIMS: 1. A prosthesis positioning system comprising: an instrument comprising a handle having a first end couplable to a prosthesis to be implanted into a bone cavity; and a counterbalance comprising an elongate rod having a first end couplable to the instrument; wherein when the counterbalance is coupled to the instrument, the instrument is coupled to a prosthesis and the prosthesis is inserted into a bone cavity such that the instrument extends from the bone at an inclination within a predetermined anglular range, the centre of mass of the counterbalance is positioned on an opposite side of the prosthesis from the centre of mass of the instrument and the prosthesis positioning system is self supporting.
2. A prosthesis positioning system according to claim 1, wherein the prosthesis positioning system is arranged to pivot about a prosthesis coupled to the first end of the instrument such that the moment of the counterbalance substantially balances the moment of the instrument about the pivot point.
3. A prosthesis positioning system according to claim 1 or claim 2, further comprising a prosthesis coupled to the first end of the instrument having a textured bone engaging surface arranged to increase the friction between the prosthesis and a bone cavity.
4. A prosthesis positioning system according to any preceding claim, wherein the counterbalance further comprises a weight coupled to the elongate rod.
5. A prosthesis positioning system according to claim 4, wherein the weight is slidable along the rod such that the centre of mass of the counterbalance is adjustable.
6. A prosthesis positioning system according to any preceding claim, further comprising a prosthesis coupled to the first end of the instrument, wherein the prosthesis comprises part of a replacement bone joint.
7. A prosthesis positioning system according to claim 6, wherein the prosthesis comprises a socket part of a ball and socket joint.
8. A prosthesis positioning system according to claim 7, wherein the prosthesis comprises an acetabular cup.
9. A prosthesis positioning system according to any preceding claim, wherein the instrument is arranged to be self supporting when the instrument extends from the bone at inclination of between 300 and 60°.
10. A method of positioning a prosthesis comprising: coupling a prosthesis to a first end of an instrument comprising a handle; coupling a counterbalance comprising an elongate rod to the instrument; and inserting the prosthesis into a bone cavity such that the instrument extends from the bone at an inclination within a predetermined anglular range such that the centre of mass of the counterbalance is positioned on an opposite side of the prosthesis from the centre of mass of the instrument and the prosthesis positioning system is self supporting.
11. A method according to claim 10, wherein the counterbalance further comprises a weight slidably coupled to the elongate rod, the method further comprising sliding the weight along the rod to adjust the centre of mass of the counterbalance.
12. A method according to claim 10 or claim 11, wherein the prosthesis comprises an acetabular cup, wherein the method further comprises: aligning the rod with the long axis of the patient such that the prosthesis is inserted into the bone cavity at a predetermined version angle.