CROSS-REFERENCE(S) TO RELATED APPLICATIONS This application claims priority from United Kingdom patent application number 1619259.3 filed on 14 ^th November 2016, which is incorporated by reference herein.

FIELD OF THE INVENTION This invention relates to a femoral head measurement device that is aimed primarily at enabling a target post-operative length of femur and attached hip prosthesis to be achieved which in turn dictates leg length. In particular, it relates to a measurement device for measuring the size of a resected head of a femur during hip replacement surgery in order to select a suitably sized prosthesis whereby a correct or near correct orientation of the resected head is achieved during measurement.

Still more particularly the invention relates to a femoral head measurement device of the general nature set out in international patent application number PCT/IB2015/058658 entitled "FEMORAL HEAD MEASUREMENT DEVICE" the content of which is imported herein by reference.

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. Hip replacement is currently a very common orthopaedic operation with patient satisfaction being high when the biomechanics are correctly restored. A patient requiring a hip replacement will probably 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 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 a specific inferior and superior reference point. The inferior point is at the position of the most medial point of the femoral component at the base of the femoral neck.

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, a femoral head component and an acetabular cup component. 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 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 is removed from the hip joint and replaced with the appropriate prosthesis. In order to accurately measure the resected bone in the vertical plane the angle of the cut should correspond to the angle of the base plate of a jig or inclined jaw on the measuring device.

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 our international patent application number PCT/IB2015/058658 entitled "FEMORAL HEAD MEASUREMENT DEVICE" the content of which is imported herein by reference. Digital X-ray images are commonly used as templates for pre-operative planning in order to determine the required leg length correction. A number of techniques have been described to assess leg length correction intra-operatively.

It should be mentioned 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.

Our earlier international patent application number PCT/IB2015/058658 entitled "FEMORAL HEAD MEASUREMENT DEVICE" describes a measurement device in the form of a jig that can be used for measuring at least the vertical height of the resected bone as is more fully described therein. Nevertheless, further developmental work carried out with the aim of commercializing the jig of that application has led to additional features being developed and additional marketing strategies being envisaged. More especially, consideration has centred around the effect of sterilisation of the measuring device described in our earlier application and it has been decided that there is a need to obviate the necessity for re-sterilising a device of this nature. 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 the invention there is provided a measurement device for measuring a resected femoral head comprising a base with a post extending from the base in a direction that corresponds to a height of a resected femoral head being measured in the device; an inclined generally flat femoral head support surface carried by the base adjacent a proximal end of the post wherein the femoral head support surface is configured to contact a severed end of a femoral head at a selected incline relative to the base and post with a height of the resected femoral head extending in a general direction of the post; a height indicating arm extending transversely to the post and being movable along the length of the post towards the femoral head support surface to engage a most superior point of the resected femoral head opposite the severed end thereof with severed end is contacted by the femoral head support surface; wherein the post and height indicating arm are provided with a rack and pawl assembly extending along the length of the post with the rack and pawl being configured to permit movement of the height indicating arm only in a direction towards the femoral head support surface.

Further features of the invention provide for the base to have a bottom support surface at right angles to an axis of the post; for the femoral head support surface to be inclined relative to the bottom support surface of the base at an angle of between 35 ^e and 60 ^e , preferably between 42 ^e and 48 ^e (or relative to an axis of the post at an angle of between 55 ^e and 30 ^e preferably between 48 ^e and 42 ^e ); and for the femoral head support surface to be provided with a friction affording surface to inhibit slippage of a severed end of a resected femoral head with the femoral head support surface preferably having a series of integrally moulded transverse ridges that are preferably of saw tooth shape in cross-section with the edges of the saw tooth shapes directed towards to a distal end of the post.

Still further features of the invention provide for the base and post to be a single integral component of a suitable plastics material; for the post and base to be hollow in that they are made as a wall defining a tubular post and an inverted cup shaped base; for a rack to be moulded integral with the post preferably on the outer surface thereof and preferably toward one side or the rear thereof; for the post to have a pair of oppositely directed side faces with one of them accommodating the rack; for one of a pair of oppositely directed side faces to have moulded therein graduations denoting heights or other indicators that correspond with different sizes and configurations of prosthetic femoral heads and femoral stems; for the height indicating arm to have a passage through it for receiving the post; and for the pawl to be integral with a connecting resiliently biasing hinge portion of the height indicating arm extending into the passage for automatic co-operation with the rack, in use.

It is preferred, in the simplest implementation of the invention, to manufacture a two component measurement device consisting of the base and post with an integral rack as one integral unit and the height measurement arm with an integral pawl and hinge assembly as another self- contained integral unit.

In order that the features of the invention defined above as well as other features thereof may be more fully understood, one embodiment of the invention will now be described with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

Figure 1 is a schematic isometric view of one embodiment of measurement device according to the invention;

Figure 2 is an enlargement of the lower corner of the base thereof showing in some more detail the friction affording nature of the femoral head support surface;

Figure 3 is a side elevation of the measurement device illustrated in Figure 1 and showing in ghosted lines a resected femoral head in position thereon;

Figure 4 is a sectional view taken along line IV to IV in Figure 3; and, Figure 5 is an enlarged view of the pawl and rack assembly. DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

In the embodiment of measurement device for measuring a resected femoral head that is illustrated in the drawings, a first integral component (1 ) comprises a base (2) with an integral post (3) extending from the base in a direction that operatively corresponds to a height of a resected femoral head (4) (see Figure 3) being measured using the device.

The base and post are moulded as a single integral component of a suitable plastics material and the composite component is hollow in that it is made as a wall that defines a tubular post and an integral hollow inverted cup shaped base, as shown most clearly in Figure 4. The base has a bottom support surface (5) defined by a periphery of the hollow inverted cup shaped base that is in a plane at right angles to an axis (6) of the post. The base also has an oppositely directed flat, inclined femoral head support surface (7) that is, in this instance, inclined relative to the bottom support surface (5) of the base at an angle of about 45 ^e . It should be noted that this angle could vary depending on requirements and experience and the angle of 45 ^e was selected as accommodating most angles at which, in the experience of the inventors, the femoral head is actually resected from the femur in practice. Other practices may have different experiences. Thus it is perfectly possible that other embodiments of the invention in which the angles are different could be manufactured, such as one in which the femoral head support surface is inclined at 40 ^e to the bottom support surface, for example.

The femoral head support surface is configured to be engaged by a severed end of a femoral head at a selected incline relative to the bottom support surface of the base such that a height of the resected femoral head extends in the general direction of the post. The femoral head support surface is provided with a multitude of transverse ridges (8) that together define a friction affording surface. The ridges are of saw tooth shape in cross-section with the edges of the saw tooth shapes directed towards to a distal end (1 1 ) of the post. The arrangement is such that slippage of a severed end of a resected femoral head that is in contact with the femoral head support surface is restricted or inhibited.

A proximal end (12) of the post extends at right angles from a platform surface (13) that is parallel to the bottom support surface (5). The post is of basically oval shape in cross-section with two rounded ends and two flat sides (14, 15). In the one flat side (14) is moulded an integral rack (16) consisting of a series or teeth of saw tooth shape in cross-section with the saw tooth shape in this instance directed towards the platform surface at the proximal end of the post. This arrangement is aimed at allowing movement of the other component in the form of a height indicating arm (17) in one direction only, that is towards the base so that it can engage the upper end of a resected femoral head.

The other flat side (14) has moulded therein graduations (18) denoting heights or other indicators in the form of icons or logos that correspond with different sizes and configurations of commonly available prosthetic femoral heads and femoral stems.

The height indicating arm has an integral collar (19) with a passage through it for receiving the post and a lower (or upper) extremity of which can be used as a read off point for the height of a resected femoral head, in use. The height indicating arm extends transversely to the post and is movable along the length of the post towards the femoral head support surface so as to be capable of engaging an extremity in the form of a most superior point of a resected femoral head opposite the severed end thereof when such severed end is in contact with the femoral head support surface

A pawl (21 ) is moulded integral with a connecting resiliently biasing hinge portion (22) on the inside of the collar of the height indicating arm so as to extend into the passage through the collar for automatic co-operation with the rack. The arrangement of the pawl and rack may be akin to that of the widely used plastic cable ties in which a rack is formed along the length of the cable tie itself that passes through a collar having an integral pawl and hinge arrangement. The rack and pawl are thus configured to permit movement of the height indicating arm only in a direction towards the femoral head support surface. At the same time, engagement of the pawl with the rack creates a resistance to movement of the height indicating arm towards the femoral head support surface so that unwanted movement of the height indicating arm is obviated, at least to a substantial extent.

In this embodiment of the invention it is only necessary to manufacture two separate components of the measurement device consisting of the base and post with an integral rack as one integral component and the height measurement arm with an integral pawl and hinge assembly as another self-contained component.

The advantage of the invention is that it renders the measurement device a truly disposable device that is a single use device. This therefore does away with any necessity for sterilising the device after use.

The above description is by way of example only and it should be appreciated that numerous changes and modifications may be made to the measurement device described without departing from the scope of the invention. For example, the post need not be tubular and could be of a U-shape in cross-section.

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.