WO2013117909A1

WO2013117909A1 - A device to prevent prosthetic dislocation and wear

Info

Publication number: WO2013117909A1
Application number: PCT/GB2013/050227
Authority: WO
Inventors: Glen Alan TURLEY; Damian Russel GRIFFIN
Original assignee: University of Warwick
Current assignee: University of Warwick
Priority date: 2012-02-08
Filing date: 2013-02-01
Publication date: 2013-08-15
Anticipated expiration: 2014-08-08
Also published as: GB201202126D0

Abstract

The invention concerns a trial femoral head for use with an acetabular cup wherein said head has at least one indicator representative of at least one of the following parameters a) acetabular cup inclination (α) and/or angle between the femoral neck and the transverse plane (a); and/or b) acetabular cup anteversion (β) and/or femoral neck version (b); a kit of parts including said head and a method to prevent prosthetic dislocation using said head.

Description

A device to prevent prosthetic dislocation and wear

The invention relates to a trial femoral head for use with an acetabular cup to prevent prosthetic dislocation and wear; a kit of parts comprising at least one trial femoral head in accordance with the invention and at least one acetabular cup; and a method for preventing prosthetic dislocation comprising the use of said head or kit of parts.

INTRODUCTION

Total hip arthroplasty (THA) is successful in restoring lost mobility to patients suffering from osteoarthritis and acute trauma. THA became commonplace during the 1970s. Today, THA is one of the most frequently performed reconstructive operations with excellent intermediate to long term results. However, there are still complications associated with the procedure, the most significant of which is dislocation. In the UK in 2010, 52,964 primary THA procedures were performed and 10,567 THA revisions. A little under half (44%) of these revisions were due to dislocation, representing 7.5% of all the THA procedures performed. A successful THA normalises the biomechanics of the hip joint enabling a patient to regain mobility without pain or discomfort. Normalisation of hip joint biomechanics is dependent upon achieving joint stability and also an ideal range of motion to be able to fulfil daily activities. In studies of dislocations it has been found that 90% of these dislocations show evidence of impingement. Prosthetic impingement occurs when the neck of the femoral component contacts the rim of the acetabular cup, shown in Figure 1 . Further motion beyond the impingement point leads to subluxation of the femoral head until the joint dislocates. Therefore, improved range of motion without impingement is directly correlated to improved resistance to dislocation.

In biomechanics it is important that range of motion is expressed in clinical terms to ensure meaningful comprehension by the healthcare community. To do this, motion is expressed relative to three mutually perpendicular cardinal planes - coronal, sagittal and transverse planes, Figure 2. For the hip joint, flexion and extension occur in the sagittal plane, abduction and adduction in the coronal plane and internal and external rotation in the transverse plane. These motions can be visualised more specifically using the illustrations in Figure 3. Many daily activities such as walking, stair climbing, sitting on a low chair, shoe tying, squatting and stooping could be achieved within the range of motion parameters defined in Figure 3. Consequently, if after THA a patient can achieve the required amount of joint motion in each of the clinical planes without impingement occurring between the neck of the prosthetic femoral stem and the rim of the prosthetic acetabular cup then the patient is far less likely to experience dislocation.

It has been found that the range of motion of a THA is dependent upon five factors:

(1 ) The oscillation angle (Θ)

(2) The inclination of the acetabular cup (a)

(3) The anteversion of the acetabular cup (β)

(4) The angle between the femoral neck and the transverse plane (a), and (5) The version angle of the femoral neck (b) with respect to the coronal plane.

The oscillation angle (Θ), determines the amount of movement that a THA can achieve and is dependent upon the design of the THA prosthetic components. The other four factors are determined by the orientation of the prosthetic components and are dependent upon the skill of the surgeon. These factors are inter-dependent and when combined determine the relative amounts of flexion, extension, abduction, adduction and internal/external rotation that can be achieved within the defined limits set by the oscillation angle. Using Figure 4 for illustration, if the centre of the hip joint is considered to be the origin of the anatomical coordinate system, as the hip moves the knee centre would follow the contour of a sphere. Therefore, the range of motion of a THA would form a sphere segment, whose size is dependent upon the oscillation angle (Θ) and whose position is dependent upon the factors (α), (β), (a) and (b). This effect is visualised in Figure 4, where the prosthetic THA range of motion is shown in purple and the gold area represents the required range of motion for a person to fulfil their daily activities - 120^° flexion, 30^° extension, 45^° abduction, 35^° adduction and 45^° internal and external rotation. It can be seen that the THA in Figure 4b, is better positioned due to the orientation of the prosthetic components, when compared with the example shown in Figure 4a. Therefore, this patient is more likely to fulfil their activities of daily living without risk of impingement.

Hitherto, to control prosthetic component placement during an operation, surgeons have used surgical navigation, which can track the position of the pelvis and femur in space and fit prosthetic components relative to these known positions. However, this is an expensive and time consuming option for the surgeon. Consequently, the vast majority of surgeries are performed without the use of navigation. It has been estimated that only 47% of THA patients have a cup orientation within both the ideal defined cup inclination and anteversion limits. Consequently, an aid or device is required for the surgeon in order to be able to control the relative THA component orientations to achieve the ideal range of motion parameters and also to reduce the risk of wear and tear.

STATEMENTS OF INVENTION

According to a first aspect of the invention there is provided a trial femoral head for use with a prosthetic acetabular cup comprising:

an attachment for releasably attaching said head to a prosthetic femoral stem and on, or in, an outer surface of said head at least one indicator representative of at least one of the following parameters

a) acetabular cup inclination (a) and/or angle between the femoral neck and the transverse plane (a); and/or

b) acetabular cup anteversion (β) and/or femoral neck version (b)

wherein said indicator(s) extends across or in said surface by an amount that is related to the amount of acetabular cup inclination (a), angle between the femoral neck and the transverse plane (a), acetabular cup anteversion (β) or femoral neck version (b), respectively;

whereby, in use, when said head is attached to said prosthetic femoral stem and positioned in said acetabular cup, and a limb to which said prosthetic femoral stem is attached is positioned in a standing posture, the point at which a rim of said acetabular cup intercepts or obscures said indicator(s) provides a measure of the amount of acetabular cup inclination (a), angle between the femoral neck and the transverse plane (a), acetabular cup anteversion (β) or femoral neck version (b), respectively.

The term angle between the femoral neck and the transverse plane (a) is also referred to herein as femoral neck axis angle (a).

In a preferred embodiment of the invention said trial femoral head comprises a single indicator for measuring one of said parameters.

In another preferred embodiment of the invention said trial femoral head comprises a plurality of indicators for measuring a plurality of said parameters. In a preferred embodiment of the invention said trial femoral head comprises two indicators representative of any two of the above four parameters.

In a preferred embodiment of the invention said trial femoral head comprises three indicators for measuring any three of the above four parameters.

Preferably, in the instance where 3 indicators are provided they represent, and measure, the following parameters acetabular cup inclination (a), acetabular cup anteversion (β) and femoral neck version (b). In the instance where 3 indicators are provided said trial femoral head ideally includes a partially visible inner sphere that can rotate with respect to said head and which has on, or in, its visible part at least one of said indicators. In a preferred embodiment of the invention said trial femoral head comprises four indicators representative of each one of the above four parameters. In a further preferred embodiment of the invention said trial femoral head comprises a single indicator for measuring either the combined acetabular cup inclination (a) and femoral neck axis angle (a) i.e. a + a; or the combined acetabular cup anteversion (β) and femoral neck version (b) i.e. β + b. In the instance where combined acetabular cup anteversion (β) and femoral neck version (b) are to be measured said trial femoral head is adapted so that it has only one rotational degree of freedom of movement and so, ideally can only move about the superior-inferior axis in the transverse plane. Preferably, this adaption is provided by the use of obstructing members or stoppers or locking means that are arranged to prevent movement other than in the afore axis. Ideally, said attachment for the prosthetic femoral stem comprises at least one, ideally two, abutment members that impinge against an inner part of the trial femoral head thus preventing movement other than in the specified way i.e. about the superior-inferior axis in the transverse plane.

In a preferred embodiment of the invention said trial femoral head comprises two indicators, a first representative of the combined acetabular cup inclination (a) and femoral neck axis angle (a) i.e. a + a, and a second representative of the combined acetabular cup anteversion (β) and femoral neck version (b) i.e. β + b.

Preferably, the trial femoral head also includes a marker, typically an unsealed line, although any other suitable marker may be used, that is used to orientate the trial femoral head with respect to the transverse plane when in use and, ideally, the other indicator(s) is/are positioned on the head in a manner compatible with this orientation whereby its/their respective parameter(s) can be measured. According to a yet further aspect of the invention there is provided a trial femoral head which is characterised by any one or more, including any combination of, the following traits:

1 . an indicator on, or in, an outer surface of said head representative of acetabular cup inclination (a); and/or

2. an indicator on, or in, an outer surface of said head representative; femoral neck axis angle (a); and/or

3. an indicator on, or in, an outer surface of said head representative; acetabular cup anteversion (β); and/or

4. an indicator on, or in, an outer surface of said head representative femoral neck version (£>).

According to a yet further aspect of the invention there is provided a trial femoral head which is characterised by any one or more of the following traits:

1 . an indicator on, or in, an outer surface of said head representative of combined acetabular cup inclination (a) and femoral neck axis angle (a);and/or 2. an indicator on, or in, an outer surface of said head representative of combined acetabular cup anteversion (β) and femoral neck version (£>). According to a yet further aspect of the invention there is provided a trial femoral head which is characterised by any one or more of the following traits:

1 . an indicator on, or in, an outer surface of said head representative of combined acetabular cup inclination (a) and femoral neck axis angle (a);and/or

2. an indicator on, or in, an outer surface of said head representative; acetabular cup anteversion (β); and/or

3. an indicator on, or in, an outer surface of said head representative femoral neck version (b).

1 . an indicator on, or in, an outer surface of said head representative of combined acetabular cup anteversion (β) and femoral neck version (b) and/or 2. an indicator on, or in, an outer surface of said head representative of acetabular cup inclination(a) ; and/or

2. an indicator on, or in, an outer surface of said head representative; femoral neck axis angle (a).

In any of the afore aspects or embodiments of the invention said indicator(s) may be either a labelled scale which is used to provide a numeric measure of the parameter to be determined and/or a zonal or coded region one part of which is indicative of an acceptable measure of the parameter to be determined and another part of which is indicative of a non-acceptable measure of the parameter to be determined. The indicator(s) may travel across, on or in, the entire surface of the head or cover only a selected region thereof. In some embodiments the indicator(s) is merely intercepted by the rim of the acetabular cup to provide either a scaled numeric measure or information as to whether the positioning of the prosthetic members falls within an acceptable measure. In other embodiments, particularly where the indicator(s) only traverse a part of the head, the rim of the acetabular cup may obscure the indicator from view and this obscuring provides information that the numeric measure of the parameter to be determined is less than or greater than the associated numeric scale or is within or without the acceptable measure, according to a user's instructions.

Preferably the indicator used to measure combined version provided a measure of between 29-55° , ideally 30-50° and preferably 42° as this preferred combined version enables motions of 120^° flexion, 30^° extension, 45^° abduction, 35^° adduction and 45^° internal/external rotation.

Preferably the indicator used to measure combined inclination provided a measure preferably above 45° as this preferred combined version enables motions of 120° flexion, 30° extension, 45° abduction, 35° adduction and 45° internal/external rotation.

In one preferred embodiment of the invention the measurements shown in table 3, upper row, are used to provide guidance to a surgeon regarding acceptable orientation of prosthetic components during THA. Thus, in any of the afore aspects or embodiments of the invention, an acetabular cup inclination (a) of 45°, a femoral neck axis angle (a) of 45°, an acetabular cup anteversion (β) of 25° and a femoral neck version (b) of 15° is preferred for when working the invention.

In one other preferred embodiment of the invention the measurements shown in table 2, middle row, are used to provide guidance to a surgeon regarding acceptable orientation of prosthetic components during THA. Thus, in any of the afore aspects or embodiments of the invention, an acetabular cup inclination (a) of 42°, a femoral neck axis angle (a) of 45°, an acetabular cup anteversion (β) of 23° and a femoral neck version (b) of 15° is preferred for when working the invention.

It will be apparent from the above that an an acetabular cup inclination (a) of 40- 50°, a femoral neck axis angle (a) of 40-50°, an acetabular cup anteversion (β) of 20-30° and a femoral neck version (b) of 10-20° is preferred for when working the invention. Ideally an an acetabular cup inclination (a) or a femoral neck axis angle (a) of 41 , 42, 43, 44, 45, 46, 47, 48 or 49° is preferred. Ideally also an acetabular cup anteversion (β) of 21 , 22, 23, 24, 25, 26, 27, 28 or 29° is preferred and and a femoral neck version (b) of 1 1 , 12, 13, 14, 15, 16, 17, 18 or 19° is preferred for when working the invention. The invention also includes any combination of the aforementioned measurements.

However, those skilled in the art will appreciate, it is not possible to identify the precise measure of each of the parameters of the invention because the optimum positioning of each prosthetic member will change according to the anatomy of the patient and, indeed, the geometry of the chosen prosthetics. However, the device and kit described herein do facilitate the correct positioning and choice of prosthetic members having regard to the anatomy of a patient. Thus, when used in combination with a surgeon's skill and judgment the invention increases the success of THA and reduces the possibility of postoperative revision. The invention therefore provides a useful tool that is easy to make, inexpensive to buy and easy to use and, moreover, it helps a surgeon to make more informed decisions.

According to a further aspect of the invention there is provided a kit of parts comprising at least one trial femoral head in accordance with any aspect or embodiment of the invention, or as described herein, and at least one acetabular cup as described herein.

Preferably, said acetabular cup is a perfect hemisphere and so has no fillet or chamfer. According to a further aspect of the invention there is provided a method for preventing prosthetic dislocation comprising the following steps, but not necessarily in the following order,:

a) positioning a trial femoral head in accordance with the invention on a prosthetic femoral stem,

b) locating said head in an acetabular cup,

c) positioning a leg including said prosthetic femoral stem to which said trial femoral head has been attached in a standing posture,

d) optionally, aligning a transverse marker with the transverse plane of the patient,

e) reading the point at which a rim of the said cup intercepts or obscures at least one indicator(s) on said head to obtain information about one or more of the following parameters the amount of acetabular cup inclination (a), angle between the femoral neck and the transverse plane (a), acetabular cup anteversion (β) or femoral neck version (£>).

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", mean "including but not limited to" and do not exclude other moieties, additives, components, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. All references, including any patent or patent application, cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. Further, no admission is made that any of the prior art constitutes part of the common general knowledge in the art. Preferred features of each aspect of the invention may be as described in connection with any of the other aspects.

Other features of the present invention will become apparent from the following examples. Generally speaking, the invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including the accompanying claims and drawings). Thus, features, integers, characteristics, compounds or chemical moieties described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein, unless incompatible therewith.

Moreover, unless stated otherwise, any feature disclosed herein may be replaced by an alternative feature serving the same or a similar purpose. The invention will now be described by way of example only with reference to the following figures and examples wherein: Figure 1 shows an example of component-on-component impingement;

Figure 2 shows an expression of human motion

(i) The anatomical coordinate frame

(ii) Clinical rotations in the anatomical coor-dinate frame;

Figure 3 shows motion of the hip joint;

Figure 4 shows Effect of acetabular and femoral prosthetic component orientation on range of motion - (i) Poorly orientated prosthetic components, (ii) Correctly orientated components;

Figure 5 shows change in impingement point due to femoral neck-shaft angle and its effect on hip joint flexion - (i) Influence of neck-shaft angle on impingement point and (ii) Femoral head markings;

Figure 6 shows effect of acetabular cup orientation upon range of motion;

Figure 7 shows trial femoral head concept one;

Figure 8 shows acetabular cup and femoral stem orientations - (i) Acetabular cup component orientations (ii) Femoral stem component orientation;

Figure 9 shows acetabular cup and femoral stem orientations (i) Anatomical acetabular cup component orientation and (ii) Anatomical femoral stem component orientation

Figure 10 shows Anatomical combined version and the effect of femoral neck version (b) - (i) Anatomical combined version, (ii) Change in femoral neck version angle;

Figure 1 1 shows a trial femoral combined version head;

Figure 12 shows the femoral trial version head process - (i) alignment of trial head with transverse plane and (ii) Measurement of combined version;

Figure 13 shows a colour coded combined version head - (i) The trial head (ii)

Combined version achieved and (iii) Combined version not achieved;

Figure 14 shows the additive property of acetabular cup inclination (a) and femoral neck axis away from transverse plane (a) (i) The femoral neck plane

(shown in grey), (ii) Combined inclination;

Figure 15 shows the femoral trial inclination head - (i) The trial head and (ii)

Measurement of combined inclination.

Figure 16 shows the femoral trial inclination head process, (i) the combined inclination head, (ii) combined inclination achieved, and (iii) combined inclination not achieved;

Figure 17 shows a redesigned inner head - (i) measured combined inclination and (ii) zoned green/red combined inclination;

Figure 18 shows a combined concept in a single head - (i) measured combined version & inclination and (ii) zoned green/red combined version & inclination.

Figure 19 shows the single femoral trial head process; and

Figure 20 shows the effect of acetabular cup design of femoral head measurement. Example 1

In the following description of the invention we will describe how an indicator or a number of indicators on a trial femoral head can be used to provide information to a surgeon, allowing him/her to assess whether or not he/she has identified the correct prosthetic orientation to enable the patient to fulfil their daily activities without risk of dislocation. To simplify the explanation, one range of motion value and one orientation parameter will be used as an example. The range of motion value is 120° flexion, shown in Figure 3i. The orientation parameter is femoral neck axis away from the transverse plane (a), shown in Figure 5i. This last parameter is influenced by the femoral stem neck-shaft angle selected by the surgeon, An increase in neck-shaft angle increases the angle of the femoral neck axis away from the transverse plane. Significantly, it also changes the point on the femoral neck which will impinge against the rim of the acetabular cup. Therefore, separate indicators in the form of arcs of flexion can be made on the femoral head. Each arc, shown in Figure 5ii, represents 120° of hip flexion for five different femoral neck-shaft angles, including the three different neck types shown in Figure 5i in double dotted and dashed line, dotted and dashed line and dashed line. The start of the arc represents the position of the femoral neck impingement point when the subject is posed in the standing posture. The end of the arc represents 120° of flexion. Consequently, when the trial femoral head is in use the whole of this arc should be visible and not obscured by the rim of the acetabular cup after the surgeon has placed the leg in the standing posture.

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Received at the EPO on May 01 , 2013 15:37:06. Page 2 of 21 For example, and with reference to Figure 6, when the surgeon has located the femoral head into the acetabular cup and placed the leg in the standing posture, if all five of the indicators are visible and not obscured by the rim of the acetabular cup then all available neck-shaft angles are acceptable and achieve at least 120° of flexion, as is the case in Figure 6i. However, if the orientation of the acetabular cup with regard to inclination (a) or anteversion (β) differs, then not all neck-shaft angle options may be acceptable, as is the case in Figures 6ii and 6iii. Table 1 , summarises the results with regard to the prosthetic component orientations shown in Figure 6.

This example successfully shows how indicators on the femoral head can be used to provide the surgeon with feedback in relation to either prosthetic component position or range of motion. This feedback is dependent upon which markings are visible and which are obscured by the acetabular cup.

Example 2

In the following example indicators on the trial femoral head provide the surgeon with measurement values for acetabular cup inclination (a), acetabular cup anteversion (β) and femoral version (£>). This trial femoral head is shown in Figure 7. It consists of an inner sphere that can freely rotate inside the outer trial femoral head. The outer trial femoral head locates within the acetabular cup and provides information relating to the acetabular cup inclination (a) and anteversion (β). These two orientation variables can be expressed using three different conventions - anatomical, operative and radiographic. The angles determined by these different conventions differ because they have different spatial arrangements. The design in this section uses radiographic inclination (a) and operative anteversion (β), shown in Figure 8ii. However, those skilled in the art will appreciate either of the other two conventions may be used with appropriate adjustment to the nature of the information portrayed on the trial femoral head. In use, and with reference to Figure 7, the numbers on the anterior (front) aspect of the femoral head indicate acetabular anteversion (β). The numbers on the lateral aspect indicate inclination (a). The inner sphere provides information with regard to femoral version (b), whereby the blank line indicates 0° version and each spaced black (b) line represents 5° increments of either anteversion or retroversion. Further, this trial femoral head also includes a marker, in this instance an unsealed blank line - although any other suitable marker may be used, that is used to orientate the trial femoral with respect to the transverse plane when in use.

The process of using this trial femoral head begins after the surgeon has dislocated the hip joint, prepared the trial acetabular cup, performed the femoral osteotomy and fitted the femoral stem in the medullary canal of the femur. The surgeon then locates the trial femoral head on the tapered interface of the femoral stem neck. Next the surgeon re-locates the hip joint and holds the treated leg in the standing posture ensuring the blank horizontal unsealed line is aligned with the transverse plane. At this point, the trial femoral head can be read to determine the acetabular cup orientation and femoral stem version. To determine the acetabular cup anteversion <β), the measurement scale is read at the point where the acetabular cup rim intersects the blank scaled vertical stripe, ideally the centre of the blank vertical stripe, marked β. For example in Figure 7i, it is 30^° of anteversion. To determine inclination the same approach is used but this time using the lateral measurement scale, marked a, shown in Figure 7iii. The inclination in this example is 49°. Finally, femoral version is determined by counting the number of spaced black lines on the inner sphere from the blank line on the inner sphere which indicates 0° of femoral version (b) to the point where one of the spaced black lines falls within the blank vertical stripe, marked (a) on the lateral aspect of the outer femoral sphere. Using the example of Figure 7iii, four spaced black lines each representing 5° of version, so the mean femoral anteversion is 20°.

Table 2, details the effect that different measurement readings of acetabular cup and femoral stem orientation have upon range of motion. Those motions shown

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Received at the EPO on May 01 , 2013 15:37:06. Page 3 of 21 in red are when the required minimum range of motion has not been achieved. Table 2, reveals the inter-dependent nature of the orientation parameters and the following section uses this interdependency to simplify the measurement process.

Example 3

In this example we investigated a trial femoral head which assesses the acceptability of four orientation parameters - acetabular cup inclination (a), acetabular cup anteversion (β), femoral neck angle away from the transverse plane (a) and femoral neck version angle (£>). As above, optimum component positioning or implant acceptability was dependent upon the visibility of certain colour coded zones. Although those skilled in the art will appreciate any type of different indicators may be used. The trial prosthetic femoral head presented in Example 3 provides the surgeon with feedback with regard to four orientation parameters - acetabular cup inclination (a), acetabular cup anteversion (β), femoral neck angle away from the transverse plane (a) and femoral neck version angle (£>). However this example describes how the measurement process detailed in the previous example can be simplified using, ideally, colour coded zones to assess implant acceptability. The measurements presented are generic to any implant, while, in some embodiments, the colour coded zones can be specified based on the geometry of an individual implant.

It is known that to achieve the ideal hip joint range of motion, to avoid prosthetic impingement, requires a certain orientation with regard to the acetabular cup inclination (a) and anteversion (β) as well as femoral neck version (b) and its angle to the transverse plane (a). These variables can be expressed using the anatomical orientation defined by Murray (J. Bone & joint Surgery, British Volume 75.2, pp. 228-232, 1993) and are shown in Figure 9. It should be noted that using the anatomical convention, both acetabular cup anteversion (β) and femoral neck version (b) are measured in the transverse plane, while acetabular cup inclination (a) and femoral neck angle to the transverse plane (a) are measured as an angle from the transverse plane. Therefore the variables (a) and (a) and the variables (β) and (b) have simple additive properties. The simple additive properties are exploited in the design of a trial femoral head which follows a two stage process - one to test for combined version, the other for combined inclination.

The design of the trial femoral head to assess relative acetabular cup anteversion and femoral neck version uses the parameters (β) (acetabular anteversion) and (b) (femoral neck version angle). Collectively, by adding these variables together the sum provides a combined version value which is measured in the transverse plane, as Figure 10i shows. Similar to the design presented in Example 1 , the newly defined combined version measurement can be placed on the surface of the femoral head, as Figure 1 1 shows.

The trial femoral head with this combined version measurement, consists of two parts, an outer part and an inner part. The inner part has one rotational degree of freedom. This is achieved using appropriate obstruction members or stoppers, well known to those skilled in the art. This is so that when the trial femoral head is located onto the femoral neck, the neck angle away from the transverse plane (a) does not affect the measurement of combined version. Therefore, when the head is correctly fitted to the neck, any change in the femoral neck version angle (b) will rotate the head about the superior-inferior axis in the transverse plane, as shown in Figure 10ii. This means that the position of the femoral head is only affected by one factor, femoral neck version (b), which allows combined version to be determined when the femoral head is located in the acetabular cup. It should be noted in Figure 10i that anatomical anteversion (β) is measured from the sagittal plane, while femoral neck version (Jb) is measured from the coronal plane. The angle between these planes is 90^° and is not included in the measurement of combined version. Therefore, combined version is the addition of acetabular cup anteversion (β) as measured from the sagittal plane plus femoral neck version (j ) as measured from the coronal plane. Further, this trial femoral head also includes a marker, in this instance an unsealed blank line - although any other suitable marker may be used, that is used to orientate the trial femoral head with respect to the transverse plane when in use.

The process of using the trial femoral version head begins after the surgeon has dislocated the hip joint, prepared the acetabular cup, performed the femoral osteotomy and fitted the femoral stem in the medullary canal of the femur. The surgeon then locates the trial femoral head on the tapered interface of the femoral neck and rotates it until the blue line on the trial head is aligned with the transverse plane, as shown in Figure 12. Next the surgeon re-locates the hip joint and holds the treated leg in the standing posture. At this point, the combined version measurement on the trial femoral head can be read. It is read where the blank line of the femoral head and the rim of the acetabular cup intersect. In Figure 12ii, this measurement is 40°.

Measurement of combined version in this way can be used to define an acceptable range required to be achieved for a particular implant. This can be defined in a preferred, but not exclusive, embodiment by simple colour coded zones to indicate an acceptable range (left to right descending lines) and non-acceptable range (left to right ascending lines) of motion for the patient.

A randomised control trial at the Coventry and Warwickshire NHS trust, based on the in-vivo surgical navigation measurements of acetabular cup and femoral stem orientation, assessed whether impingement would occur between the rim of the acetabular cup and femoral neck in each of the 97 patients measured. It was found that patients who did not suffer impingement whilst simulating the required motions of 120^° flexion, 30^° extension, 45^° abduction, 35^° adduction and 45^° internal/external rotation, had a combined version of on average of 42° (standard deviation 13°). Therefore, based on this information, and surgical skill and judgment, for the trial head design an ideal combined version of between 30^° and 50^° was defined. Figure 13, provides this design with the colour coded zones shown. The left to right descending line area represents the area where anatomical combined version is between 30° and 50°. If the blank line of the femoral head and the rim of the acetabular cup intersect in the left to right descending line area then the ideal combined version has been attained, Figure 13ii. If the intersection is in the left to right ascending lines area then alteration to the femoral or acetabular component is required to achieve ideal outcome, Figure 13iii. Further, this trial femoral head also includes a marker, in this instance an unsealed blank line - although any other suitable marker may be used, that is used to orientate the trial femoral head with respect to the transverse plane when in use.

Once the ideal combined version has been attained, the other two factors, acetabular cup incfination (a) and femoral neck angle away from the transverse plane (a) have to be considered. Again, these two factors are additive in a plane. However, this plane does not lie in any of the anatomical planes. This is because when considering rotations mathematically, they are non-commutative, which means they follow a strict temporal order, i.e. a sequence. Therefore, the femoral and acetabular version takes the acetabular axis and femoral neck axis away from the coronal plane. Consequently, a new plane has to be defined, the femoral neck plane, which is the plane in which the femoral neck axis lies, as shown in grey in Figure 14i. This is the plane in which the femoral neck angle away from the transverse plane (a) is measured. Therefore, if acetabular cup inclination (a) is also measured in this plane then (a) and (a) would have the same additive property as (β) and (b) in the combined version measurement. This combined measurement will be termed herein as combined inclination, and is shown in Figure 14».

Using the above design, indicators on the surface of a second trial femoral head can be used to provide measurement of combined inclination, as shown in Figure 15. This head has been termed the trial inclination head.

The process of using the femora! trial inclination head begins after the surgeon has determined the correct prosthetic combined version using the femoral trial version head. After, the joint is dislocated and the trial version head is interchanged for the trial inclination head. The surgeon rotates the trial head until the vertical blue line is in line with the femoral neck, as shown in Figure 15. Next the surgeon re-locates the hip joint and holds the treated leg in the standing posture. At this point, the trial femoral head can be read to determine the combined inclination, which in Figure 15 is 65°.

Again, using the same method as presented for the trial version head, the measurement of combined inclination can be separated into acceptable left to right descending lines and unacceptable left to right ascending lines zones for the surgeon to read. The range of motion parameter that is most affected by combined inclination is that of abduction, shown Figure 3. This is defined as a lateral rotation of the hip in the coronal plane. It is generally known that a THA should deliver a minimum of 45° of abduction to avoid risk of impingement and dislocation. Therefore, if an angle of more than 45^° exists between the rim of the acetabular cup and the femoral neck in the femoral neck plane, then this amount of abduction will be achieved. The femoral trial inclination head was modified based on these parameters, as shown in Figure 16ii. The left to right ascending line area represents an angle of 45° or less in the femoral neck plane, while the left to right descending line area represents an angle of between 45° and 90^°. If, following the same surgical process outlined above, when the leg is relocated into the hip and positioned in the standing posture, the rim of the acetabular cup intersects in the left to right descending line area then combined inclination has been attained, Figure 16ii. This means the patient will have more than 45° of prosthetic abduction post-operatively. However, if the rim of the acetabular cup intersects in the left to right ascending zone then combined inclination has not been attained, Figure 16iii. Once the ideal combined version and combined inclination has been determined the surgeon can fit the final components and complete the THA procedure.

Table 3, provides the range of motion information for the two cases presented, the first where combined version and abduction have been achieved. The second when neither parameter has been achieved.

Example 4 In this example we investigated a single trial femoral head which assesses the acceptability of all the afore parameters.

In Example 3, two trial femoral heads were described. One to measure combined version and the other to measure combined inclination. As mentioned, the combined version head in Figure 4.7 comprised a part having one rotational degree of freedom, this was so that when the trial femoral head is located onto the femoral neck, the neck angle away from the transverse plane (a) would not affect the measurement of combined version. Consequently, this inner part of the head can be used to measure combined inclination as it is not affected by the measurement of combined version.

In this further Example the inner part of the trial version head was redesigned using the principle for measuring combined inclination presented in Example 3. Two embodiments of the inner head are presented in Figure 17. The first shows an indicator using a scale in degrees on the surface of the inner head and is used to provide a measurement of combined inclination. The second inner head separates this angular measurement into a separate acceptable left to right descending line zone and an unacceptable left to right ascending line zones for the surgeon to read. The left to right ascending line zone represents an angle of 45° or less in the femoral neck plane, while the left to right descending line area represents an angle of between 45° and 90°. This corresponds with an acceptable amount of abduction as defined in section Example 3.

The final assembly of both these heads are shown in Figure 18, where the outer head is the same design as the combined version trial head presented in section Example 3. Further, this trial femoral head also includes a marker, in this instance an unsealed blank line - although any other suitable marker may be used, that is used to orientate the trial femoral head with respect to the transverse plane when in use.

The process of using the new combined femoral head begins after the surgeon has dislocated the hip joint, prepared the trial acetabular cup, performed the femoral osteotomy and fitted the femoral stem in the medullary canal of the femur. The surgeon then locates the trial combined femoral head on the tapered interface of the femoral neck and rotates it until the blue line, i.e. the marker for the transverse plane, on the trial head is aligned with the transverse plane. Next the surgeon relocates the hip joint and holds the treated leg in the standing posture. At this point, both the combined version measurement and combined inclination measurement can be read using the design on the trial head presented in Figure 18 or acceptability assessed using the coloured zones as shown in Figure 19. Combined version is read where the blank line of the outer part of the femoral head and the rim of the acetabular cup intersect. Combined inclination is read where the inner part of the femoral head and the rim of the acetabular cup intersect

Example 5

The indicators on the trial femoral head and the unsealed marker used to orientate the trial femoral head with respect to the transverse plane presented in this report are used to provide a measurement of boiih acetabular and femoral prosthetic component orientation. These measurements are principally associated with reading the femoral head marking at the point where it intersects with the rim of the acetabular cup. The design of the acetabular cup can affect these readings. All designs have used an acetabular cup with no fillet or chamfer, as show in Figure 20 i.e. one that is a perfect hemisphere. The designs presented in this report are able to cope with non-hemispherical acetabular cups by off-setting the femoral head markings by an amount equal to the amount by which the acetabular cup is over or under size from a perfect hemisphere as shown in Figure 20.

Conclusions

Post-operative dislocation is a common complication for THA patients and is caused by a range of motion outside of the limit of the prosthetic joint. Typically, the neck of the prosthetic femoral component impinges against the rim of the pelvic acetabular cup causing the femoral head to 'jump out' of its socket. Prosthetic range of motion is dependent upon five parameters, four of which are related to the orientation of the prosthetic components. This invention uses indicators or markings on a trial femoral head to provide information about a prosthetic component orientation and the degree of impingement-free motion it can provide. Further the four operative parameters - acetabular cup inclination (a), acetabular cup anteversion (β), femoral neck angle away from the transverse plane (a) and femoral neck version (b) - have been simplified into two parameters, combined inclination (a + a) and combined version (β + b). This has allowed a single, but two-readings, trial femoral head to be designed which allows prosthetic component orientation to be assessed.

as e

Table 1 : Effect of acetabular cup orientation on femoral component choice.

Table 2: Effect of prosthetic component orientation or range of motion.

Table 3. Effect of prosthetic component orientation on range of motion,

Claims

zmnp /wn as filed 2013/117909 PCT/GB2013/050227 CLAIMS

1 . A trial femoral head for use with a prosthetic acetabular cup comprising: an attachment for releasably attaching said head to a prosthetic femoral stem and on, or in, an outer surface of said head at least one indicator representative of at least one of the following parameters

i) acetabular cup inclination (a) and/or angle between the femoral neck and the transverse plane (a); and/or

ii) acetabular cup anteversion (β) and/or femoral neck version (b)

2. A trial femoral head according to claim 1 wherein said trial femoral head comprises a single indicator for measuring one of said parameters.

3. A trial femoral head according to claim 1 wherein said trial femoral head comprises a plurality of indicators for measuring a plurality of said parameters.

4. A trial femoral head according to claim 1 wherein said trial femoral head comprises two indicators representative of any two of the four parameters.

5. A trial femoral head according to claim 1 wherein said trial femoral head comprises three indicators for measuring any three of the four parameters.

6. A trial femoral head according to claim 5 wherein said 3 indicators represent the following parameters acetabular cup inclination (a), acetabular cup anteversion (β) and femoral neck version (b).

7. A trial femoral head according to claims 5 or 6 wherein said trial femoral zmn^p /wn as filed

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head includes a partially visible inner sphere that can rotate with respect to said head and which has on, or in, its visible part at least one of said indicators.

8. A trial femoral head according to claim 7 wherein said indicator on said inner sphere, in use, is read having regard to the position of the other two indicators on said head.

9. A trial femoral head according to claim 1 wherein said trial femoral head comprises four indicators representative of each one of the four parameters.

10. A trial femoral head according to claim 1 wherein said trial femoral head comprises a single indicator for measuring either the combined acetabular cup inclination (a) and femoral neck axis angle (a) i.e. a + a; or the combined acetabular cup anteversion (β) and femoral neck version (b) i.e. β + b.

1 1 . A trial femoral head according to claim 10 wherein where combined acetabular cup anteversion (β) and femoral neck version (b) are to be measured said trial femoral head is adapted so that it has only one rotational degree of freedom of movement.

12. A trial femoral head according to claim 1 1 wherein said head is provided with at least one obstructing member or stopper or lock that is/are arranged to prevent movement other than about the superior-inferior axis in the transverse plane, with respect to the standing posture of a patient receiving said implant.

13. A trial femoral head according to claim 1 wherein said trial femoral head comprises two indicators, a first representative of the combined acetabular cup inclination (a) and femoral neck axis angle (a) i.e. a + a, and a second representative of the combined acetabular cup anteversion (β) and femoral neck version (b) i.e. β + b.

14. A trial femoral head according to any preceding claim wherein the trial femoral head also includes a marker that is used to orientate the trial femoral head, with respect to the transverse plane of a patient when in use, and said at least indicator(s) is/are positioned on the head in a manner compatible with his orientation whereby the respective parameter(s) of said indicator(s) can be measured.

15. A trial femoral head according to any preceding claim wherein said indicator(s) is either a labelled scale which is used to provide a numeric measure

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of the parameter to be determined and/or a zonal or coded region, one part of which is indicative of an acceptable measure of the parameter to be determined and another part of which is indicative of a non-acceptable measure of the parameter to be determined.

16. A trial femoral head according to any preceding claim wherein said indicator(s) travel(s) across, on or in, the entire surface of the head or cover(s) only a selected region thereof.

17. A trial femoral head according to any one of claims 10 to16 wherein the indicator for combined acetabular cup anteversion (β) and femoral neck version (b) i.e. β + b provides a measure of between 29-55°, ideally 30-50°

18. A trial femoral head according to any one of claims 10 to16 wherein the indicator for combined acetabular cup inclination (a) and femoral neck axis angle (a) i.e. a + a provides a measure of above 45°

19. A kit of parts comprising at least one trial femoral head according to claims 1 -18 and at least one acetabular.

20. A kit according to claim 19 wherein said acetabular cup is a perfect hemisphere and so has no fillet or chamfer.

21 . A method for preventing prosthetic dislocation comprising the following steps, but not necessarily in the following order,:

a) positioning a trial femoral head according to claims 1 -18 on a prosthetic femoral stem,

b) locating said head in an acetabular cup,

e) reading the point at which a rim of the said cup intercepts or obscures at least one indicator(s) on said head or to obtain information about one or more of the following parameters the amount of acetabular cup inclination (a), angle between the femoral neck and the transverse plane (a), acetabular cup anteversion (β) or femoral neck version (b).

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22. A method according to claim 21 wherein part j) thereof alternatively involves reading an indicator having regard to the position of two other indicators on said head.

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