WO2018092054A1

WO2018092054A1 - Method for calculating leg length in hip replacement surgery

Info

Publication number: WO2018092054A1
Application number: PCT/IB2017/057164
Authority: WO
Inventors: Garth Peter GROBLER; Brendan John DOWER
Original assignee: Grobler Garth Peter; Dower Brendan John
Priority date: 2016-11-18
Filing date: 2017-11-16
Publication date: 2018-05-24
Also published as: GB201619540D0; GB2556086A

Abstract

A method, that is typically computer implemented, is provided for selecting prefabricated components of a hip prosthesis comprising a femoral stem (1), a femoral head (2) and an acetabular cup component (3) for use in hip replacement surgery. The method includes determining a planned leg length correction pre-operatively from measurements and x- rays or other images and identifying a prosthetic implant assembly aimed at achieving the planned leg length correction. A measured vertical height (b) of a resected femoral head (2) of a patient's femur is determined in the vertical direction and the diameter or radius (x) of a suitable acetabular cup component is determined following the preparation of a receiving acetabular site. The measured vertical height (b) is deducted from a predetermined prosthetic implant assembly height (a) to provide a calculated leg length correction, and the planned leg length correction is compared with the calculated leg length correction. The calculated leg length correction may be recalculated with alternative components.

Description

METHOD FOR CALCULATING LEG LENGTH IN HIP REPLACEMENT SURGERY CROSS-REFERENCE(S) TO RELATED APPLICATIONS This application claims priority from United Kingdom patent application number GB 1619540.6 filed on 18^th November 2016, which is incorporated by reference herein.

FIELD OF THE INVENTION This invention relates to a method for controlling leg length in hip replacement surgery including leg length correction with the aim of enabling a planned post-operative length of femur and attached hip prosthesis to be achieved which in turn dictates leg length. More particularly, the invention relates to a selection method that is preferably computer implemented whereby the selection of individual components of a hip prosthesis is facilitated with a view to effectively achieving a particular planned post-operative leg length that may involve a leg length correction.

BACKGROUND TO THE INVENTION

Total hip replacement is a surgical operation aimed at replacing a damaged hip joint with a prosthetic hip joint. A total hip replacement (total hip arthroplasty) consists of replacing both the acetabulum (socket) and the femoral head that is carried by a prosthetic femoral stem. Hip replacement is currently a very common orthopaedic operation with patient satisfaction high when the biomechanics are correctly restored. A patient requiring a hip replacement will generally have lost height in the vertical plane due to wear of the articulating surface. There are in addition a number of other factors that may contribute to a leg length discrepancy. This loss of height in the vertical plane needs to be measured in order that it can be corrected during the surgery. The difference in the vertical plane between the dimensions of the resected bone and the implanted hip replacement prosthesis dictates any final change in leg length.

The resected bone is difficult to measure as it is a complex three dimensional shape. The resected bone needs to be measured between specific inferior and superior reference points. The inferior reference point is generally considered to be at the position of the most medial point of the femoral component at the base of the femoral neck. Such an inferior reference point is indicated by the letter "R" in Figure 1 . The superior reference point is the highest point on the head of the natural femur and is indicated by the letter "S" in Figure 1 . A total hip replacement replaces the acetabulum (socket) and the head of a femur with a prosthetic joint. The prosthetic joint comprises a femoral stem generally formed into a neck at its upper end, a femoral head component carried by the neck and an acetabular cup component. It should be mentioned that the femoral head component typically has a socket that receives the upper end of the neck and a selection of different femoral head components that are otherwise the same, but with each having a different depth of socket, is made available. This provides an added facility for correcting leg length by selecting a femoral head component that has a socket with a depth that is most appropriate to a situation. Each assembly of a femoral stem, femoral head component having a socket of a different depth and an acetabular cup component may be provided with its own supplier's specified height that is generally in the form of a basic height for that particular implant to which a femoral neck adjustment figure is added according to the depth of the socket in the head and that facility is herein termed "effective head height adjustment". In some cases, however, such composite assembly heights need to be measured and recorded. The prosthesis may be manufactured from a variety of different materials having suitable properties.

In a natural hip joint or a prosthetic hip joint the neck portion of the femoral head component is usually at an angle of about 135° relative to the longitudinal axis of the femur. During an exemplary hip replacement a cross-sectional cut is made through the femoral neck. The resected femoral head and neck are removed from the hip joint and replaced with the appropriate prosthesis.

The difference in the vertical plane between the dimensions of the resected bone and the size and position of the implanted prosthesis dictates any final change in leg length. The vertical height of the resected bone is difficult to measure so therefore a unique measuring device has been developed as set out in published international patent application number WO2016166587 entitled "FEMORAL HEAD MEASUREMENT DEVICE" the content of which is imported herein by reference. In order to accurately measure the resected bone in the vertical plane the angle of the cut should correspond, at least rather closely, to the angle of the base plate of a jig or inclined jaw on another measuring device.

Digital X-ray images may be used as templates for pre-operative planning in order to determine a required leg length correction. A number of techniques have been described to assess leg length correction intra-operatively.

It should be mentioned that a composite hip prosthesis is composed of a number of different components that may either be made or supplied by the same or different manufacturers or suppliers and each component needs to be selected by a surgeon in forming a composite hip prosthesis during a hip replacement operation according to individual patient characteristics including the shape, size and configuration of the natural hip joint. It is the choice of the individual components, each one from a range of different components having different height implications, that the surgeon must choose, or change an original choice, as an operation progresses.

The surgeon performing a hip replacement procedure is therefore faced with a number of different choices and decisions that need to be made during the course of a hip replacement procedure commencing with the selection of an acetabular cup once the site for receiving same has being suitably prepared, typically by reaming. The acetabular cup assembly typically comprises an outer shell that is attached or simply engaged with the prepared site with the shell receiving a complementary liner that may also be a selected one of a number of different liners compatible with the shell. The liner defines a part spherical socket having a centre of curvature.

The socket in the installed position receives a part spherical prosthetic femoral head that is typically attached to a prosthetic femoral component in a manner that adjusts the position of the centre of curvature of the prosthetic femoral head from a neck of a femoral stem component. The head typically has a socket that receives a neck of the femoral stem component and a range of heads each of which has a different depth of socket is provided so that a surgeon must then choose a head that has a suitable depth of socket that in turn affects the post operative leg length.

All this follows an original choice of femoral stem component from a whole range of different femoral stem components each of which will affect the leg length to a different extent by virtue of its design and the extent to which it will enter the femur in the installed position. The surgeon thus has a large number of different permutations and combinations of components that may be suitable. The difficulty is in choosing a particular combination that will most closely provide a target leg length or leg length correction.

The difficulties facing a surgeon in total hip replacement surgery as regards leg length discrepancy are discussed in some more detail at https://aorecon.aofoundation.ora/education/surqical-insiahts/108.html. The present practice is, in applicant's opinion, rather haphazard and unsatisfactory in that there is nothing concrete to guide a surgeon towards any particular combination of components that may achieve the desired leg length or leg length correction effectively. It should be noted that leg length is of great importance in the conduct of hip replacement surgery because if the post-operative leg length is not within acceptable limits the surgeon responsible may be exposed to serious malpractice litigation.

There is thus a need to alleviate such difficulties, at least to some extent.

The preceding discussion of the background to the invention is intended only to facilitate an understanding of the present invention. It should be appreciated that the discussion is not an acknowledgment or admission that any of the material referred to was part of the common general knowledge in the art as at the priority date of the application.

SUMMARY OF THE INVENTION

In accordance with a first aspect of this invention there is provided a method of selecting prefabricated components of a hip prosthesis comprising a femoral stem, a femoral head and an acetabular cup component for use in hip replacement surgery, the method comprising determining a planned leg length correction pre-operatively from measurements and x-rays or other images, identifying a prosthetic implant assembly having components that when assembled have an overall height aimed at achieving the planned leg length correction, wherein the method includes measuring a resected femoral head of a patient's femur in the vertical direction that corresponds to a standing patient with the femoral head orientated in its normal anatomical position to determine a measured vertical height of the resected femoral head, and selecting a suitable acetabular cup having a radius (or diameter) based on a prepared receiving acetabular site, the method being characterized in that the measured vertical height of the resected femoral head is deducted from a predetermined prosthetic implant assembly height being a manufacturer's or supplier's specified height or a previously determined height to provide a calculated leg length correction, and the planned leg length correction is compared with the calculated leg length correction, and if the difference exceeds a predetermined maximum, the calculated leg length correction is recalculated using one or more alternative components of the hip prosthesis until the difference between the planned leg length correction and the calculated leg length correction is less than said predetermined maximum. A further feature of the invention provides for the calculated leg length to be re-calculated in the event that said difference exceeds a predetermined maximum of 6 mm so as to achieve a calculated leg length correction of less than 6 mm; preferably for the calculated leg length to be re-calculated in the event that said difference exceeds a predetermined maximum of 4 mm so as to achieve a calculated leg length correction of less than 4 mm; and most preferably for the calculated leg length to be re-calculated in the event that said difference exceeds a predetermined maximum of 2 mm so as to achieve a calculated leg length correction of less than 2 mm.

A further feature of the invention provides for the calculated leg length correction to include a deduction of any acetabular cup height correction consequent on removal of some of the natural acetabular cup bone and tissue in preparation for receiving the prosthetic acetabular cup to provide said prepared receiving acetabular site.

Further features of the invention provide for the calculated leg length correction to include an addition of any femoral stem height correction occasioned by a prosthetic femoral stem not entering the femur sufficiently to cause the inferior reference point to be in registration with the corresponding point on the prosthetic femoral stem that thereby adds to the calculated leg length correction; and for the calculated leg length correction to include any effective head height adjustment that may be occasioned by the extent to which a neck portion of the prosthetic femoral stem enters a socket in a particular size of prosthetic femoral head of which a selection of different heads of the same size is available with different depths of sockets. The calculated leg length correction is preferably determined according to the formula:-

Calculated leg length correction = predetermined prosthetic implant assembly height (a) - resected femoral head height (b) - acetabular cup adjustment height (c) + femoral stem adjustment height (d). The various heights (a), (b), (c), and (d) are shown in Figure 1 and the effective head height adjustment that forms part of (a) is indicated as one of a group (h) in Figure 2 whereof an appropriate one will be applicable to each of a series of otherwise identical femoral head prostheses that have different sockets in the head for receiving different extents of the femoral neck of a prosthetic femoral stem.

This can be stated to be the formula:- calculated leg length correction = a - b - c +d wherein

"a" is a predetermined prosthetic implant assembly height that is calculated as comprising the prosthetic acetabular cup outer radius (indicated as x in Figure 1 ) plus the femoral component leg length correction height plus (or possibly minus) the effective head height adjustment (as indicated in the rectangle "h" in Figure 2 and that are together indicated as y in Figure 1 );

"b" is the measured vertical height of the resected femoral head (as indicated in Figure 1 ); "c" is the vertical height of the vertically highest point of the prosthetic acetabular cup component above the superior reference point "S" (the height of the highest point on the natural acetabulum) and this height should be the same as the amount of acetabular tissue and bone reamed in a vertical plane, as shown in Figure 1 ; and,

"d" is the vertical height above the inferior reference point of a point on the prosthetic femoral stem that is designed to correspond to the inferior reference point and that may be considered to be the bottom end of the prosthetic femoral neck (as indicated as in Figure

1 )-

In accordance with a second aspect of the invention there is provided a computer-implemented selection method of selecting components of a hip prosthesis that comprises a femoral stem, a femoral head and an acetabular cup component in hip replacement surgery; wherein a data base is provided that is accessible to a mobile or a fixed computer device wherein the data base has data stored thereon as to dimensions of multiple hip prostheses each of which is composed of components selected to co-operate with each other, the method being carried out using a computer program on or accessible to said mobile or fixed computer device to obtain a calculated leg length correction wherein data is input into the computer device as to a planned leg length correction that is a result of a pre-operative determination using clinical and radiological techniques; the computer program being configured to receive data as to a required outer radius (or diameter) of a suitable acetabular cup component; to receive data as to a measured vertical height of a resected femoral head; to receive data as to a vertical correction height consequent on removal of acetabular tissue and bone in a vertical plane in preparation for receiving the acetabular cup component; wherein the program is configured to identify an appropriate prosthetic femoral stem and determine a calculated leg length correction associated therewith and to display the calculated leg length correction and the planned leg length correction, or any difference between them, or both; providing data input to the computer device as to the value of the vertical height above the inferior reference point of a point on the prosthetic femoral stem that is designed to correspond to the inferior reference point and selecting a prosthetic femoral head according to any difference between the planned leg length correction and the calculated leg length correction to vary the effective head height adjustment and cause the calculated leg length correction to more closely approach the planned leg length correction; and If the difference is within an acceptable range the selection may be accepted and if not, then the components may be changed appropriately.

The method of the invention allows for intra-operative guidance, monitoring and information communication. Stored and measured data are entered into fields provided by the computer program. This information can be processed on either a mobile or a fixed computer device. The results of the calculation and the various adjustments can be broadcast to a display panel that is visualised by the surgical team. Feature features of the second aspect of the invention provide for measured data to be received as input by the computer by way of entry into fields as shown on a screen driven by the computer device; for the screen to be either a part of a self-contained computer or to be a separate screen that can be in communication with the computer device by way of a wireless or hard wired connection; for the screen to be a screen that is part of existing operating theatre equipment and that is capable of being used for intraoperative procedure guidance, monitoring or information communication; in the alternative for the computer device to be a part of a mobile communications device such as a smart phone, a tablet or a laptop computer device; for the data base to be either an on board data base or one that is accessible to or by way of a remote computer or computer server such as that of a service provider or, alternatively, or in addition, for the computer device to be that of a smart phone, tablet or other wireless or wired communications device having a suitable computer; and for the programming of the computer device to be constituted by a so-called App installed in the computer memory and having a computer program product comprising a computer-readable medium having stored thereon computer-readable program code for performing the method of this invention.

The method of the invention includes the option of accessing authorized user financial account information to allow the user to billed for each use stored in association with authorized user identification data, or If the user is not currently authorized, the invention may include a method of requesting and receiving user information to allow temporary authorization and a payment undertaking.

A third aspect of the invention extends to a computer program product for selecting prefabricated components of a hip prosthesis that comprises a femoral stem, a compatible femoral head and a co-operating acetabular cup component for use as an implant assembly in hip replacement surgery and wherein the selection is carried out in a computer device, the computer program product comprising a computer-readable medium having stored computer- readable program code for performing the steps of receiving a planned leg length correction of a leg of a patient that is to receive a hip prosthesis; receiving data as to the radius or diameter of a prosthetic acetabular cup required for a hip replacement in that patient; receiving or determining from a data base an overall height of a suitable prosthetic femoral implant assembly that is aimed at providing that planned leg length correction wherein the identification is based on recorded heights of hip prostheses as specified by a manufacturer or supplier or as determined empirically if no such heights have been provided by a manufacturer or supplier; receiving as input a measured vertical height of a resected femoral head of a patient's femur and causing the computer device to subtract that measured vertical height from the recorded height of the prosthetic femoral implant assembly; receiving data as to the vertical height of the vertically highest point of the prosthetic acetabular cup component above the superior reference point at the vertically highest point thereof and to subtract that height from the recorded height, and causing the computer to add any measured vertical height of the femoral stem above a lowermost possible position thereof relative to the resected femoral neck that is received by the computer as input to provide a projected height difference and therefore a projected leg length correction; causing the computer to compare the planned leg length correction with the projected leg length correction; and, as may be necessary, causing the computer to select one or more of an alternative compatible femoral stem, an alternative femoral head having different dimensions with the selection being made from a data base containing appropriate manufacturers or supplier's data; causing the computer to recalculate the projected height correction once or more than once successively, and causing the computer to select a set of cooperating components that result in a projected height correction that most closely approaches, or is the same as, the planned leg length correction.

Further features provide for the computer-readable medium to be a non-transitory computer- readable medium and for the computer-readable program code to be executable by a processing circuit.

The measured vertical height of a resected femoral head of a patient's femur measured in the vertical direction corresponding to a standing patient with the femoral head orientated substantially in its normal anatomical position may be measured in any convenient way such as by using a jig as is described in WO2016166587 entitled "FEMORAL HEAD MEASUREMENT DEVICE". The jig has a femoral head support surface inclined relative to a bottom support surface of the base at an angle of between 35^e and 60^e, preferably at a presently preferred angle of about 45^e or relative to an axis of the post of the jig at an angle of between 55^e and 30^e and a presently preferred angle of about 45^e.

However, there are other ways in which the resected femoral head can be measured in the required direction and these may include callipers whereof one jaw is provided with the necessary inclination in order to achieve the orientation of the resected femoral head to result in the necessary vertical measurement.

The new technique to determine the leg length correction uses the basic principle that the leg length correction is determined by the difference in the vertical plane between what is implanted and what is removed. A desired leg length correction is determined pre-operatively using measured data and patient specific requirements. The final leg length correction is then compared to pre-operatively desired correction. The planned leg length correction and the calculated leg length correction should preferably be within about 2 mm of each other. It should be noted that the order in which the various items of data encapsulating measurements are received by the computer device can be varied according to circumstances and the order in which the relevant measurements become available and the invention is not to be interpreted as being limited to any particular order. Also, simply for the sake of perspective and completeness, the vertical height of the predetermined prosthetic implant assembly (a) is typically within the range of 40 mm to 80 mm with the effective head height adjustment that forms part of (a) that is indicated as (h) typically falling within the range of -4 mm to +16 mm; the vertical height of the resected femoral head (b) is within the same general range although may be somewhat shorter by the amount of the planned leg length correction; the vertical height of the vertically highest point of the prosthetic acetabular cup component above the superior reference point "S" (c) is generally between 0 and 4 mm; the vertical height above the inferior reference point of a point on the prosthetic femoral stem that is designed to correspond to the inferior reference point (d) is between 0 and 15 mm; and the diameter of the part spherical prosthetic femoral head is generally from about 28 mm to about 40 mm.

In order that the features of the invention defined above, as well as other features thereof may be more fully understood, one embodiment and implementation and possible variations thereof will now be described with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic illustration of a natural hip joint on the left hand side of the

Figure and the same hip joint fitted with a prosthetic implant on the right-hand side with the various heights referred to above being shown thereon;

Figure 2 is a schematic elevation of a prosthetic femoral stem and head assembly illustrating how the the effective head height adjustment works and the vertical adjustment of height that can be achieved by using different prosthetic femoral heads having different sockets for receiving the end of the prosthetic femoral neck;

Figure 3 is an elevation of a femoral head measurement jig suitable for

determining the vertical height of a resected femoral head; Figure 4 is a schematic illustration of one implementation of computer device according to the invention;

Figure 5 illustrates a first screen that can be displayed on a screen associated with the computer device;

Figure 6 illustrates a second screen that can be displayed on a screen associated with the computer device; and,

Figure 7 illustrates a third screen that can be displayed on a screen associated with a computer device.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS In one embodiment of the invention the method of selecting prefabricated components of a hip prosthesis for use in hip replacement surgery is described as follows. The hip prosthesis is illustrated schematically in the right hand side of Figure 1 and a typical femoral stem and head component of which are shown in Figure 2. The hip prosthesis comprises a femoral stem (1 ), a femoral head (2) and an acetabular cup component (3). The head has a socket (4) that receives the upper end of a neck (5) at the upper end of the femoral stem. The femoral stem has a reference point (6) that is intended to correspond to the inferior reference point of the femur (7) in the event that the stem can be fully introduced into the femur, which is not generally achieved. In order to assist a surgeon in assessing leg length correction, the edge of the femoral stem below the reference point is, in this instance, provided with a graduated scale (8) from 0 to 30 mm, although such a facility is not always provided.

The method of the invention requires the result of an initial determination of a planned leg length correction pre-operatively from measurements and x-rays, preferably digital x-rays. Whilst the invention can be implemented in other ways, it is primarily aimed at a computer implemented method of selecting prefabricated components of a hip prosthesis and for the purpose of expedience it will be described as such whilst those skilled in the art will understand how the other aspects of the invention are to be put into practice from this description.

A data base indicated by numeral (1 1 ) in Figure 4 is provided that is accessible to, or embodied in, a mobile computer device such as a smart phone (12) or laptop computer (or notebook) (13) or a fixed computer device or server (14) wherein the data base has data stored thereon by make and type, of the leg length dimensions of multiple hip prostheses each of which is composed of prefabricated components selected to co-operate with each other to form a composite hip implant in the assembled condition. The reference can be to many different sizes, configurations and head heights of prosthetic femoral heads on a prosthetic femoral stem with various different sizes of part spherical heads and sizes of acetabular cup components. The method is carried out using a computer program that is on, or accessible to, the mobile or fixed computer device to obtain a calculated leg length correction. Communication between any of the computer devices or server can be either hard wired or wireless, as may be required.

The computer program, that may take the form of an App (application), is configured to receive data input into the computer device as to the planned leg length correction which is a result of a pre-operative determination using clinical and radiological techniques. Figure 5 shows the first display screen, screen no 1 , with the planned leg length correction entered.

The computer program is configured to receive data as to a required outer radius or diameter of a suitable acetabular cup component which is information that becomes available intraoperative^ when the acetabular tissue and bone has been prepared, typically by reaming, for receiving a prosthetic acetabular cup component as the size of that prepared site will dictate the outer radius of the acetabular cup component that can be accommodated. As shown in Figure 6, that radius is displayed on the next screen which for present purposes can be referred to as screen no 2 of the display screen sequence.

At that stage the program is configured to identify, optionally with additional manual input as to the desired make, type and size of prosthesis, an appropriate prosthetic femoral stem and a calculated leg length correction associated therewith. This height includes a head height adjustment that may be manually selected or selected by the computer program. The possibilities of head height adjustment associated with a selected prosthesis are displayed on display screen no 2 and the selection is highlighted and can be changed manually if required, typically at a later stage.

On that basis the computer program determines a calculated leg length correction that is displayed on a subsequent display screen, namely screen no 3 that is illustrated in Figure 7. The discrepancy between the planned leg length correction and the calculated leg length correction is quite apparent as the planned leg length correction and calculated leg length correction are displayed in close proximity to each other. Screen no 3 also displays the total implant height.

The computer program is configured to receive data as to the measured vertical height of a resected femoral head that is also developed intra-operatively. The determination of a measure of the resected femoral head will be quite apparent from the foregoing and a measurement jig that is one expedient for measuring the resected femoral head height is illustrated in Figure 3. In this instance the jig has a base (21 ) with an inclined support surface (22) for the inclined severed end of a femoral head (23). A post (24) extends from the base in what would be a vertical direction with graduations (25) on it to enable the vertical height of the resected femoral head to be read off the graduations. Further details of this type of jig can be obtained as may be necessary from our earlier published patent application WO2016166587entitled "FEMORAL HEAD MEASUREMENT DEVICE".

Data as to the vertical correction height consequent on removal of acetabular tissue and bone in a vertical plane in preparation for receiving the acetabular cup component that is typically between 0 and 4 mm may then be entered.

Thereafter data is input to the computer device as to the value of the vertical height above the inferior reference point of the reference point on the prosthetic femoral stem that is designed to correspond to the inferior reference point on the natural femur.

Finally data may be input regarding the selection of a prosthetic femoral head according to any difference between the planned leg length correction and the calculated leg length correction to vary the effective head height adjustment and cause the calculated leg length correction to more closely approach the planned leg length correction

In the event that the difference is within an acceptable range of 0 to 6 mm and preferably 0 to 2 mm, the selection may be accepted. If not, then the components may be changed appropriately with what may be regarded as fine tuning being a final change of the effective head height. It should be noted that the screen number 3 (Figure 7) has a reset button to enable the selection to be changed.

As regards the display of any screen as described above and illustrated in Figures 5 to 7, it should be noted that the number of screens displayed at any one time, and indeed the number of screens in total, can be varied according to requirements and also according to the size of the display screen of the computer device. As an alternative, the display screen could be made as one continuous display screen with the facility being provided to scroll up and down to the required positions. Thus, for example, if the computer device is a smart phone, or similarly dimensioned item, then only one screen would generally be displayed at any one time and different screens could be viewed sequentially in either a forwards or a rearwards direction or by scrolling. On the other hand, if the screen is a larger screen such as that of a desktop computer or a display screen in an operating theatre, then more than one screen can be displayed at any one time and possibly all three simultaneously.

In implementing the invention, the computer program is preferably configured such that the corrections are calculated according to the formula:- calculated leg length correction = a - b - c +d as described above.

The method of the invention includes the option of accessing authorized user financial account information to allow the user to be billed for each use with the data being stored in association with authorized user identification data. On the other hand, if the user is not currently authorized, the computer program may include a method of requesting and receiving user information to allow temporary authorization and a payment undertaking as indicated above.

The system allows for intra-operative guidance, monitoring and information communication. Stored and measured data are entered into fields provided by the computer program. This information can be processed on either a mobile or a fixed computer device. The results of the calculation and the various adjustments can be broadcast to a display panel that is visualised by the surgical team. The computer program product for selecting prefabricated components of a hip prosthesis will be quite apparent to those of ordinary skill in the art in the light of the foregoing.

Exercise of the invention also has the advantage that details as to the implants that have been selected and the basis on which they have been selected can be recorded in a data base and a hospital or other medical institution can maintain records of each hip replacement operation for future reference and possible protection of the medical fraternity. Such a data base can be available for interrogation by authorised persons at appropriate times.

Simply for completeness sake the data that is stored in the data base would at least include that normally put out by manufacturers or suppliers of prosthetic hip implants and an example of one table of data is as follows:- Size Type Leg Adjustment Length with head (mm)

-2 1.5 5 8.5 12 15.5

0 Std or High 23 25 27 30 32 34

1 Std or High 23 25 27 30 32 34

2 Std or High 24 26 28 31 33 35

3 Std or High 25 27 29 32 34 36

4 Std or High 26 28 30 33 35 37

5 Std or High 26 29 31 33 35 38

6 Std or High 28 30 32 34 37 39

7 Std or High 28 31 33 35 37 40

8 Std or High 30 32 34 36 39 41

9 Std or High 30 33 35 37 39 42

10 Std or High 32 34 36 38 41 43

11 Std or High 32 34 36 38 41 43

12 Std or High 32 34 36 38 41 43

The foregoing description of the embodiments of the invention has been presented for the purpose of illustration; it is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the above disclosure.

Throughout the specification and claims unless the contents requires otherwise the word 'comprise' or variations such as 'comprises' or 'comprising' will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Claims

CLAIMS:

A method of selecting prefabricated components of a hip prosthesis comprising a femoral stem, a femoral head and an acetabular cup component for use in hip replacement surgery, the method comprising determining a planned leg length correction pre-operatively from measurements and x-rays or other images, identifying a prosthetic implant assembly having components that when assembled have an overall height aimed at achieving the planned leg length correction, wherein the method includes measuring a resected femoral head of a patient's femur in the vertical direction that corresponds to a standing patient with the femoral head orientated in its normal anatomical position to determine a measured vertical height of the resected femoral head, and selecting a suitable acetabular cup having a radius (or diameter) based on a prepared receiving acetabular site, the method being characterized in that the measured vertical height of the resected femoral head is deducted from a predetermined prosthetic implant assembly height being a manufacturer's or supplier's specified height or a previously determined height to provide a calculated leg length correction, and the planned leg length correction is compared with the calculated leg length correction, and if the difference exceeds a predetermined maximum, the calculated leg length correction is recalculated using one or more alternative components of the hip prosthesis until the difference between the planned leg length correction and the calculated leg length correction is less than said predetermined maximum.

A method as claimed in claim 1 in which the calculated leg length is re-calculated in the event that said difference exceeds a predetermined maximum of 6 mm so as to achieve a calculated leg length correction of less than 6 mm.

A method as claimed in claim 2 in which the calculated leg length is re-calculated in the event that said difference exceeds a predetermined maximum of 4 mm so as to achieve a calculated leg length correction of less than 4 mm.

A method as claimed in claim 3 in which the calculated leg length is re-calculated in the event that said difference exceeds a predetermined maximum of 2 mm so as to achieve a calculated leg length correction of less than 2 mm.

A method as claimed in any one of the preceding claims in which the calculated leg length correction includes a deduction of any acetabular cup height correction consequent on removal of some of the natural acetabular cup bone and tissue in preparation for receiving the prosthetic acetabular cup. A method as claimed in any one of the preceding claims in which the calculated leg length correction includes an addition of any femoral stem height correction occasioned by a prosthetic femoral stem not entering the femur sufficiently to cause the inferior reference point to be in registration with the corresponding point on the prosthetic femoral stem that thereby adds to the calculated leg length correction.

A method as claimed in any one of the preceding claims in which the calculated leg length correction includes any effective head height adjustment that may be occasioned by the extent to which a neck portion of the prosthetic femoral stem enters a socket in a particular size of prosthetic femoral head of which a selection of different heads of the same size is available with different depths of sockets ij each of them.

A method as claimed in any one of the preceding claims in which the calculated leg length correction is determined according to the formula:-

Calculated leg length correction = predetermined prosthetic implant assembly height (a) - resected femoral head height (b) - cup adjustment height (c) + femoral stem adjustment height (d).

A computer-implemented selection method of selecting components of a hip prosthesis that comprises a femoral stem, a femoral head and an acetabular cup component in hip replacement surgery; wherein a data base is provided that is accessible to a mobile or a fixed computer device wherein the data base has data stored thereon as to dimensions of multiple hip prostheses each of which is composed of components selected to cooperate with each other, the method being carried out using a computer program on or accessible to said mobile or fixed computer device to obtain a calculated leg length correction wherein data is input into the computer device as to a planned leg length correction that is a result of a pre-operative determination using clinical and radiological techniques; the computer program being configured to receive data as to a required outer radius (or diameter) of a suitable acetabular cup component; to receive data as to a measured vertical height of a resected femoral head; to receive data as to a vertical correction height consequent on removal of acetabular tissue and bone in a vertical plane in preparation for receiving the acetabular cup component; wherein the program is configured to identify an appropriate prosthetic femoral stem and determine a calculated leg length correction associated therewith and to display the calculated leg length correction and the planned leg length correction, or any difference between them, or both; providing data input to the computer device as to the value of the vertical height above the inferior reference point of a point on the prosthetic femoral stem that is designed to correspond to the inferior reference point and selecting a prosthetic femoral head according to any difference between the planned leg length correction and the calculated leg length correction to vary the effective head height adjustment and cause the calculated leg length correction to more closely approach the planned leg length correction; and If the difference is within an acceptable range the selection may be accepted and if not, then the components may be changed appropriately.

10. A computer-implemented selection method as claimed in claim 9 in which measured data is received as input by the computer by way of entry into fields as shown on a screen driven by the computer device.

1 1 . A computer-implemented selection method as claimed in claim 10 in which the screen is selected from a part of a self-contained computer, a separate screen that can be in communication with the computer device by way of a wireless or hard wired connection and a screen that is part of existing operating theatre equipment and that is capable of being used for intraoperative procedure guidance, monitoring or information communication.

12. A computer-implemented selection method as claimed in any one of claims 9 to 1 1 in which the computer device is a part of a mobile communications device.

13. A computer-implemented selection method as claimed in any one of claims 9 to 12 in which the calculated leg length correction is determined according to the formula:- Calculated leg length correction = predetermined prosthetic implant assembly height (a) - resected femoral head height (b) - cup adjustment height (c) + femoral stem adjustment height (d).

14. A computer-implemented selection method as claimed in any one of claims 9 to 13 in which there is included an option of accessing authorized user financial account information to allow a user to be billed for each use stored in association with authorized user identification data.

15. A computer program product for selecting prefabricated components of a hip prosthesis that comprises a femoral stem, a compatible femoral head and a co-operating acetabular cup component for use as an implant assembly in hip replacement surgery and wherein the selection is carried out in a computer device, the computer program product comprising a computer-readable medium having stored computer-readable program code for performing the steps of receiving a planned leg length correction of a leg of a patient that is to receive a hip prosthesis; receiving data as to the radius or diameter of a prosthetic acetabular cup required for a hip replacement in that patient; receiving or determining from a data base an overall height of a suitable prosthetic femoral implant assembly that is aimed at providing that planned leg length correction wherein the identification is based on recorded heights of hip prostheses as specified by a manufacturer or supplier or as determined empirically if no such heights have been provided by a manufacturer or supplier; receiving as input a measured vertical height of a resected femoral head of a patient's femur and causing the computer device to subtract that measured vertical height from the recorded height of the prosthetic femoral implant assembly; receiving data as to the vertical height of the vertically highest point of the prosthetic acetabular cup component above the superior reference point at the vertically highest point thereof and to subtract that height from the recorded height, and causing the computer to add any measured vertical height of the femoral stem above a lowermost possible position thereof relative to the resected femoral neck that is received by the computer as input to provide a projected height difference and therefore a projected leg length correction; causing the computer to compare the planned leg length correction with the projected leg length correction; and, as may be necessary, causing the computer to select one or more of an alternative compatible femoral stem, an alternative femoral head and an alternative acetabular cup component having different dimensions with the selection being made from a data base containing appropriate manufacturers or supplier's data and optionally empirically determined data; causing the computer to recalculate the projected height correction once or more than once successively, and causing the computer to select a set of co-operating components that result in a projected height correction that most closely approaches, or is the same as, the planned leg length correction.