WO2015095931A1

WO2015095931A1 - Improved surgical implant, method of surgery and method of designing a surgical implant

Info

Publication number: WO2015095931A1
Application number: PCT/AU2014/050445
Authority: WO
Inventors: Samuel John MACDESSI; Darren Beazley CHEN; Bradley Gene SEETO; Gregory Caldwell WERNECKE
Original assignee: Individual
Current assignee: Individual
Priority date: 2013-12-23
Filing date: 2014-12-23
Publication date: 2015-07-02
Anticipated expiration: 2016-06-23

Abstract

A tibial tray comprises an asymmetrical body comprising a medial section and a lateral section; the medial section comprising a medial overlay and a posteromedial shoulder; the lateral section comprising a lateral overlay and a posterolateral shoulder; both the medial section and lateral section comprising a shape and size to increase tibial coverage, reduce posterolateral overhang and minimise or prevent rotational misalignment of the tray about an axis of tibial rotation. Also provided is a method of producing or designing the tibial tray to have suitable dimensions to fit a wide distribution of the human population.

Description

I

TITLE

IMPROVED SURGICAL IMPLANT, METHOD OF SURGERY AND

METHOD OF DESIGNING A SURGICAL IMPLANT

FIELD THIS INVENTION described herein relates generally to a surgical implant, method of surgery using suc an implant and a method of designing a surgical, implant. In particular, the invention is directed to an asymmetric tibial tray with improved rotational alignment with respect to the tibial rotational axis and equal or improved tibial plateau co verage and reduced posterolateral overhang.

BACKGROUND

Total knee arthroplasty (T A) or total knee replacement (TKR) is a surgical technique in which a surgical implant is inserted to replace the knee. The surgical implant includes a tibial tray or plate. Various tibial trays are available; however the available trays all have disad vantages,

Proximal tibial anatomy has long been known to have multi-plane asymmetry and yet prosthetic design in. the axial plane has failed to suitably account for this for decades. More recentl attempts to provide a better fit for tibial prostheses brought asymmetric trays to the market. However, when roiationally aligned, these trays often demonstrate a varying amount of over or under coverage of the tibial, plateau.

Axial component rotation in total knee arthroplasty is crucial in maintaining palello femoral kinematics as well as flexion gaps. There are several inlra-opefative techniques to assess rotation of both components. The femoral component may be orientated by Whitesides line, posterior condylar axis (PC A), femoral ti'ansepicondylar axis (TEA), or b the use of compute navigation. The TEA has been shown to be the most accurate non-navigated axis of orientation for the femoral component. The tibial component can then either be aligned according to the femoral component (the so called range of motion or ROM method), toward the medial 1/3 of the tibial tubercle, the posterior tibial condyles, the ankle malleolar axis or to the medial border of the patella ligament' s insertion to the tibia (Akagi's Line). Despite these numerous methods of alignment, patellofemoral symptoms remai a major reason for TK revisions in Australia and second only to infection for reoperation rates (Newbern et a!., J Arthroplasty, 2006; Akagi et al, Clin Orthop Relat Res, 1999).

Tibial component rotation is particularly important due to implications on pate!lo femoral kinematics, polyethylene wear and flexion instability when combined with excessive posterior slope. When an asymmetric tibial tray is not. rotationaliy aligned with the tibial rotational axis it may either disrupt patellofemoral kinematics because of this misalignment or it may lead to overhang of the component once it has rotated to the correct position. For exam le, a prope medial- lateral tibial tray fit can lead to posterolateral overhang if the tray is externally rotated. Accordingly, an improved asymmetric tibial tray is required that takes into account component alignment about an axis of tibial and/or femorotibiai rotation.

SUMMARY

The present invention is broadly directed to an asymmetric tibial tray. The present inventors have designed an asymmetric tibial tray that has improved rotational alignment with the tibial axis along with equal or greater tibial plateau coverage and decreased posterolateral oversizing and/or overhang. Such tibial trays may reduce or avoid the problems associated with other tibial trays, such as patella tnaltracking, flexion gap mismatch, component overhang and subsequent soft tissue impingement, loss of knee extension arid pain,

In one aspect , there is provided a tibial tray comprising:

an asymmetrical body comprising a medial section and a lateral section; the medial section comprising a medial o verlay and a posteromedial shoulder; the lateral section comprising a lateral overlay and a posterolateral shoulder; both the medial section and lateral section comprising a shape and size to increase tibial coverage and reduce posterolateral overhang, wherein the shape and or size are selected to minimise, restrict or prevent rotational misalignment of the tray about an axis of tibial rotation.

In a second aspect the invention provides a method for making a tibial tra including;

forming an asymmetrical body comprising a medial section: and a lateral section;

the medial section comprising a medial overlay and a posteromedial shoulder; the lateral section comprising a lateral overlay and a posterolateral shoulder; both the medial section and lateral section comprising a shape and size to increase tibial coverage, reduce posterolateral overhang, wherei the shape and size are selected to minimise, restrict or prevent rotational misalignment of the tray about an axis of tibial rotation, to thereby make the tibial tray.

In a third aspect the invention provide a method for knee arthroplasty or reconstruction including inserting a tibial tray according to the first aspect or a tibial tray made according to the second aspect into a knee joint in need o f arthroplasty or ^■reconstruction, to thereby perform the knee arthroplasty or knee reconstruction,

In a fourth aspect the invention provides a kit comprising the tibial tray according to the first aspect or made according to the second aspect.

The kit of the fourth aspect may also comprise instruction for use.

According to any above aspects the posteromedial and/or posterolateral shoulder profiles may comprise a gently curved or substantiall straight section.

According to any above aspects the medial and/or lateral overlays ma comprise a gently curved and/or a substantially straight section.

According t any of the above aspect the gently carved and or substantially straight section of the sh ulder pro tiles and/or the overlays may comprise a radius of curvature of, or greater than, approximately or about 4.0-12.0 cm. In some embodiments, this may be approximately or about 5, 175, 5.2, 5.3, 5.4, 5.5, 5.6, 5.7, 5.8, 5.9, 6.0, 6.5, 7,0, 7.5, 8.0, 8.5, 9.0, 9.5 or 10.0 cm.

According to any of the above aspects the radius of curvature may be- selected to increase tibial coverage and/or reduce po terolateral overhang.

According to any above aspects the tibial tray may comprise a mediolateral length of between approximately or about 50 and 95 mm

In preferred aspects the mediolateral length may be approximately or about 58, 61, 64, 67, 70, 73, 76, 79, 83 and 87 mm.

According to an above aspect the medial section may comprise a medial section anteroposterior width of between approximately or about 35 and; 65 mm.

In preferred aspects the medial section anteroposterior width may be approximately or about 40, 42, 43, 44, 46, 47, 48, 49, 50, 52, 53, 54 or 55 mm.

According to any above aspect the lateral section may comprise a lateral section anteroposterior width of between approximately or about 30 and 55 mm.

In preferred aspects the lateral section may comprise a lateral section anteroposterior width of approximately or about 36, 37, 38, 39, 40, 42, 43, 44, 46, 47, 48 or 49 mm.

According to any above aspect the medial aspect ratio may be between approximately or about 0,56 and 0.76. Suitably, this is a ratio of the anteroposterior distance of the medial tibial plateau (typically parallel to the anteroposterior axis of the tibia from the medial-middle 1/3 of patellar tendon to posterior cruciate ligament insertion) t the mediolateral tibial width.

In preferred aspects the medial aspect ratio may be approximately or about 0.575, 0.59, 0.608, 0.618, 0.629, 0.630, 0.632, 0.642, 0.651 , 0.671, 0.672, 0.684,

0.685, 0.69, 0,701, 0.705, 0.724, 0.734 or 0.754.

According to any above aspect the lateral aspect, ratio may be between approximately or about 0.5 and 0.68. Suitably, this is a ratio of the anteroposterior distance of the lateral tibial plateau (typically parallel to the anteroposterior axis of the tibia from the medial-middle 1/3 of patellar tendon to posterior cruciate ligament insertion) t the mediolateral tibial width.

In preferred aspects the lateral aspect ratio may be approximately or about

0.506, 0.518, .526, 0.532, 0.534, 0.543, 0.552, 0.563, 0.578, 0.589, 0.596, 0.599,

0.605, 0.607, 0.621 , 0.627, 0.656 or 0.672.

A preferred tibial tray according to any above aspect may ha ve a mediolateral length of approximately or about 58 mm, a medial section anteropo terior width of approximately or about 40 mm, a lateral section anteroposterior width of approximately or about 36 mm, a medial aspect ratio of approximately or about 0.690 and a lateral aspect rati of approximately or ab ut 0.621.

Another preferred tibial tray according to any above aspect may have a. mediolaleral length of approximately or about 58 .ran. a medial section anteroposterior width of approximately or about 42 mm, a lateral section anteroposterior width of approximately or about 39 mm, a medial aspect ratio of approximately or about 0,724 and a lateral aspect ratio of approximately or about 0.672.

Another preferred tibial tray according to any above aspect may have a. medio lateral length of approximately or about 61 mm, a medial section anteroposterior width of approximately or about 43 mm, a lateral section anteroposterior width of approximately or about 37 mm, a medial aspect ratio of approximately o about 0,705 and a lateral aspect ratio of approximately or about 0.607.

Another preferred tibial tray according to an above aspect may have a mediolaleral length of .approximately or about 61 mm, a medial section anteroposterior width of approximately or about 46 mm, a lateral section anteroposterior width of approximately or about 40 mm, a medial aspect ratio of approximately or about 0.754 and a lateral aspect ratio of approximately or about 0.656.

Another preferred tibial tray according to any above aspect may have a mediolaterai length of approximately or about 64 mm, a medial section anteroposterior width of approximately or about 43 mn¾ a lateral section anteroposterior width of approximately or about 37 mm, medial aspect ratio of approximately or about 0.672 and a lateral aspect ratio of approximately or about 0.578.

Another preferred tibial tra accordin to an above aspect may have a mediolaleral length of approximately or about 64 mm. a medial section anteroposterior width of approximately or about 47 mm, a lateral section anteroposterior widt of approximately or about 42 mm, a medial aspect ratio of approximately or about 0,734 and a lateral aspect ratio of approximately or about 0.656.

Another preferred tibial tray according to any above aspect may have a. mediolateral length of approximately or about 67 .mm. a medial section anteroposterior width of approximately or about 43 mm, a lateral section anteroposterior width of approximately or about 37 mm, a medial aspect ratio of approximately or about 0,642 and a lateral aspect ratio of approximately or about 0.5S2.

Another preferred tibial tray according to any above aspect may have a. mediolateral length of approximately or about 67 mm, a medial section anteroposterior width of approximately or about 47 mm, a lateral section anteroposterior width of approximatel or about 42 mm, a medial aspect ratio of approximately or about 0,701 and a lateral aspect ratio of approximately or about 0.627.

Another preferred tibial tray according to an above aspect may have a mediolateral length of .approximately or about 70 mm, a medial section anteroposterior width of approximately or about 44 mm, a lateral section anteroposterior width of approximately or about 38 mm, a medial aspect ratio of approximately or about 0.629 and a lateral aspect ratio of approximately or about 0.543.

Another preferred tibial tray according to any above aspect may have a mediolateral length of approximately or about 70 mm, a medial section anteroposterior width of approximately or about 47 mn¾ a lateral section anteroposterior width of approximately or about 41 .92 mm, a medial aspect ratio of approximately or about 0.671 and a lateral aspect ratio of approximately or abou 0.599.

Another preferred tibial tra accordin to an above aspect may have a mediolaleral length of approximately or about 73 mm. a medial section anteroposterior width of approximately or about 46 mm, a lateral section anteroposterior width of approximately or about 38.99 mm, a medial aspect ratio of approximately or about 0,630 and a lateral aspect ratio of approximately or about 0.534.

Another preferred tibial tray according to any above aspect may have a. mediolaleral length of approximately or about 73 .ran. a medial section anteroposterior width of approximately or about 50 mm, a lateral section anteroposterior width of approximately or about 43 mm, a medial aspect ratio of approximately or about 0,685 and a lateral aspect ratio of approximately or about 0.589.

Another preferred tibial tray according to any above aspect may have mediolateral length of approximately or about 76 mm, a medial section anteroposterior width of approximately or about 47 mm, a lateral section anteroposterior width of approximatel or about 40 mm, a medial aspect ratio of approximately o about 0,618 and a lateral aspect ratio of approximately or about 0.526.

Another preferred tibial tray according to an above aspect may have a mediolateral length of .approximately or about 76 mm, a medial sectio anteroposterior width of approximately or about 52 mm, a lateral section anteroposterior width of approximately or about 46 mm, a medial aspect ratio of approximately or about 0.684 and a lateral aspect ratio of approximately or about 0.605.

Another preferred tibial tray according to any above aspect may have a mediolateral length of approximately or about 79 mm, a medial section anteroposterior width of approximately or about 48.05 mm. a lateral section anteroposterior width of approximately or about 42.05 mm, a medial aspect ratio of approximately or about 0.608 and a lateral aspect ratio of approximately or about 0.532.

Another preferred tibial tra accordin to an above aspect may have a mediolaleral length of approximately or about 79 mm. a medial section anteroposterior width of approximately or about 53.05 mm, a lateral section anteroposterior width of approximately or about 47.05 mm, a medial aspect ratio of approximately or about 0,67 and a lateral aspect ratio of approximately or about 0.596.

Another preferred tibial tray according to any above aspect may have a. mediolateral length of approximately or about 83 .ran. a medial section anteroposterior width of approximately or about 49 mm, a lateral section anteroposterior width of approximately or about 43 mm, a medial aspect ratio of approximately or about 0,590 and a lateral aspect ratio of approximately or about 0.518.

Another preferred tibial tray according to any above aspect may have a. mediolateral length of approximately or about 83 mm, a medial section anteroposterior width of approximately or about 54 mm, a lateral section anteroposterior width of approximately or about 48 mm, a medial aspect ratio of approximately or about 0,651 and a lateral aspect ratio of approximately or about 0.578.

Another preferred tibial tray according to an above aspect may have a mediolateral length of .approximately or about 87 mm, a medial section anteroposterior width of approximately or about 50 mm, a lateral section anteroposterior width of approximately or about 44 mm, a medial aspect ratio of approximately or about 0.575 and a lateral aspect ratio of approximately or about 0.506.

Another preferred tibial tray according to any above aspect may have a mediolateral length of approximately or about 8? mm, a medial section anteroposterior width of approximately or. about 55 mn¾ a lateral section anteroposterior width of approximately or about 49 mm, medial aspect ratio of approximately or about 0.632 and a lateral aspect ratio of approximately or about 0.563.

In a fifth aspect the invention provides a method for designing a surgical implant or prosthesis, the method including:

interrogating a data set comprisin one or more tibial and/or femoral axes of rotation information for the population into which the surgical implant is to be implanted to obtain one or more dimensions for the surgical implant;

based on the obtained one or more dimensions designing the surgical implant. The method of the fifth aspect may further include the step of interrogating an imaging data set to obtain the one or more tibial and/or femoral axes of rotation information.

In a sixth aspect the invention provides a surgical implant or prosthesis designed by the method of the fifth aspect.

The surgical implant may be a tibial tray, suc as that according to the first aspect.

As used herein, except where the context requires otherwise, the term "comprise" and variations of the term, such as ''comprising", "comprises" and "comprised", are not intended to exclude further additives, components, integers or steps,

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention, may be readily understood and put into practical effect, reference will now be made to the accompanying illustrations, wherein like reference numerals are used to refer to like elements.

Figure 1 A: shows an outline drawing of an embodiment of the tibial tray of size 67 A according to one aspect, of the invention.

Figure 1 B : show an outline drawing of a further embodiment of the tibial tra of size 67 B according to one aspect of the invention,

Figure 2: shows an axial MRI image at the level of the distal femur showing the transepicondylar axis (TE A) and posterior condylar axis (PCA).

Figure 3: shows an axial MRI image of the proximal tibia at a level of 1.0mm of the lateral tibial articular surface which represents an average depth of tibial resection in T.KA. Insali's line, which represents an axis of tibial component alignment, is also shown.

Figure 4: shows the TEA and PCA transposed onto an axial tibial MRI image. Figure 5 ; shows an example of the assessment of the angular differences between the femoral axes (TEA and PCA) and Insali's line.

Figure 6: show an example of the assessment of the tibial tubercle to trochlear groove (TT-TG) distance. Figure 7: shows a plot of absolute medial and lateral anteroposterior (AP) measurements versus their respective mediolateral (ML) width in all knees studied. Figure 8; shows a plot of absolute lateral and medial AP measurements versus their respective ML width in males.

Figure 9: shows a plot of absolute lateral and medial AP measurements versus their respective ML width in females.

Figure 0: shows the comparative percentage of downsizing required of the asymmetric tibial tray according to the invention

Figure 1 1: shows the comparative percentage of posterolateral AP fit for the asymmetric tibial tray according to the invention.

Figure 12: shows the comparative percentage of posteromedial AP fit for the asymmetric tibial tra according to the invention.

Figure 13: shows the graphical representation of 20 preferred tibial trays of the invention. Tibial tray 200 has the same mediolateral length, but i larger in size and has a different asymmetrical sizing aspect ratio to tibial tra 100.

DETAILED DESCRIPTION

The inventors have recognised that when tibial trays are not aligned, or are poorly aligned with the tibial rotational xis, this may have severe, negative consequences for a patient fitted with the tibial tray. A misaligned tibial component may cause patella maltracking, flexion gap mismatch and pain. Further, a misaligned tibial tray may subsequently rotate in an axial plane in response to this misalignment, leading to component overhang and subsequent soft tissue impingement, loss of knee extension and pain,

The inventors ha ve further recognised that an asymmetrical tibial tra provides equal or greater tibial plateau coverage and decreased posterokteral oversking. In this regard, an undersized tibiai tray may cause subsidence, bone bleeding and femoral component size matching problems whilst an oversized tibial tray may cause soft tissue impingement, loss of knee extension and pain,

Surprisingly, as exemplified herein, the in ventors have provided an asymmetric tray which has an increased tibial c verage compared to other tibial trays and which has a reduced posterolateral overhang well below that achieved with other tibial trays. Furthermore, the improved tibial tray provided by the inventors is of significant advantage by virtue of reducing the incidence of misalig ment ami/or malrotation relative to the tibial axis of rotation which thereby eliminates or at least, reduces patella nialtracking, flexion gap mismatching and/or component overhang.

By "rotational misalignment of the tray about an axis of tibial rotation^* is meant that the rotational axis of the tibia! tray in an axial plane differs from the rotational axis of the tibia in an axial plane, such that the angle between these two axes represents the degree of misalignment of the tray. This rotational misalignment may be in either an internal or external direction. Numerous landmarks or axes, alone or in combination, can be used for tibial rotational alignment of the tibial component, including the femoral component (the ROM method), the f ransmalleolar axis, the second metatarsal bone, the po terior tibial condylar line, the medial border of the patella ligament's insertion to the tibia (Akagi's line), the line extending from the junction of the medial and middle 1 /3 of the tibial tubercle to the centre of the posterior cruciate hgament (Insall's line), the tibial tubercle and the insertion of the posterior cruciate ligament on the posterior border of the tibia, the transcondylar tibial line, toward the media! 1/3 of the tibial, tubercle, but without limitation thereto.

While the princip les described herein are based on tibial trays for humans, this invention may also be extended to other mammals such as livestock (e.g. cattle, sheep), performance animals (e.g. racehorses) and domestic pets {e.g. dogs, cats), although without limitation thereto.

As used herein, the terms "appro imately" and "about" refer to tolerances or variances associated wit numerical values recited herein. The extent of such tolerances and variances are well understood by persons skilled in the art. Typically, such tolerances and variance do no t compromise the structure and/or fu nction of the tibial tray.

As used herein "absolute underside" means a tibial tray is greater than 3mm under size. As used herein "relative undersize" means a t ibial tray is between 1 and 3mm undersize.

As used herein "optimal fit" means a tibial tray is less than 1mm over or undersize.

As used herein "relative oversize" means a tibial tray is between 1 a id 3 mm oversize.

As used herein "absolute oversize" means a tibial tray is greater than 3mm oversize.

The tibial tray according to the invention comprises asymmetric lateral and medial sections.

FIG. 1A shows one embodiment of a tibial tray 100 according to the invention. Tibial tray 100 comprises a body 102 which comprises a medial section 110 and a lateral section .120 which are located on either side of mid-section 1.04. Midsection 104 comprises an anterior mid-section 106 and a posterior mid- ection 108.

Medial section 11.0 comprises a medial overlay 11.2 and a posteromedial shoulder 1 16.

Similarly lateral section 120 comprises a lateral overlay 122 and a posterolateral shoulder 126.

Medial overlay 112 and lateral, o verlay 122 comprise a medial overlay profile 114 and lateral overlay profile 124. respectively, selected to conform to tibial anatomy. As shown in FIG. lA, profiles 114 and 124 are substantially curvilinear. Profiles 114 and 124 comprise a gently curved or substantially straight section 1 15, 125. The combination of the curved and/or straight sections results in a better fit once implanted int a joint.

Posteromedial shoulder 1 16 and posterolateral shoulder 126 comprise a posteromedial shoulder profile 118 and a posterolateral shoulder profile 128, respectively, selected to conform to tibial anatomy. Shoulder profiles 114 and 124 also comprise a gently curved or substantially straight section 1 19, 1 9. As above, inclusion of the gently curved and/or substantiall straight sections 1 19, 129 result in a better fit once implanted into a joint. As shown in FIG. 1 A the radius of curvature of gently curved or substantially straight sections 115, 125, 119,. 129 is relatively large and approaching a straight line. In other embodiments gently curved or substantially straight sections 1 15, 125, 119, 129 may comprises a radius of curvature greater than approximately or about 5 , 175, 5.2, 5.3, 5.4, 5.5. 5.6, 5.7, 5.8, 5.9, 6.0, .5, 7.0, 7.5, 8.0, 8.5, 9.0, 9.5 or 10.0 em.

The radius of curvature may be selected to increase tibial coverage and/or reduce posterolateral overhang

Medial section .1.10 and lateral section 120 are asymmetrical. As indicated on FIG. I A tibial tray 100 has a medioiateral length (ML) of 67 mm, a medial section 110 anteroposterior width (API) of 43 mm and a laterai section 120 anteroposterior width (AP2) of 37 mm. The asymmetrical sizing of tibial tray 100 can be quantified by calculating the medial aspect ratio by dividing API /ML and by calculating the lateral aspect ratio by dividing AP2/ML. Tray 100 has a medial aspect ratio of 0.642 and a. lateral aspect, ratio of 0,552.

As a re suit, o f the asymmetric al sizing of medial sec tion 1 10 and lateral section

120, the body 102 is asymmetrical.

Tibial tray 100 ha a different size and different asymmetrical sizing aspect ratio to tibial tray 200 shown, in FIG. IB.

Tibial tray 200 has the same medioiateral length as tray 100, but a different API of 47 mm and AP2 of 42 mm, which gives a medial aspect ratio of 0.701 and a lateral aspect ratio of 0,627.

The tibial tray of the invention may be sized to fit an individual. The tibial tra of the invention may comprise a medioiateral length of between approximately or about 50 and 95 mm. The medial section anteroposterior width may be between approximately or about 35 and 65 mm. The lateral section anteropo sterior width may be between approximately or about 30 and 55 mm. The medial aspect ratio may be between approximately or about 0.56 to 0.76, The lateral aspect ratio may he between approximately or about 0,50 to 0.68.

The asymmetrical tibial tray according to the invention may also be manufactured in smaller and larger sizes, as provided in Table 3 and Figure 13 and described herein.

The invention also provides a method for making a tibial tray including forming the asymmetrical body comprising the medial section and the lateral section.

Further, the invention provides a method for. knee arthroplasty including inserting a tibial tray according to the invention.

The inventors have also provided a kit comprising the tibial tray according to the invention. The kit of the invention may also comprise instructions for use.

The invention also provides a method for designing a surgical implant or prosthesis including interrogating a data set comprising one or more tibial and/or femoral axes o f rotation mformation for the population into which the surgical implant is to be implanted to obtain one or more dimensions for the surgical implant,

Once the one or more dimensions is obtained the surgical implant may be designed,

The method may further include a step of interrogating imaging data to obtain the one or more tibial and/or femoral axes of rotation information. The imaging data may be medical imaging data such as, magnetic resonance imaging, x-ray imaging, positron emission imaging or any suitable imaging method.

The method may further include a virtual testing step in which the designed surgical implant is applied to the data set. The application to the data set may comprise superimposing the designed surgical implant or prosthesis on the data set.

The following non-limiting examples illustrate the tibial tray and methods of the invention. These examples should not be construed as limiting: the examples are included for the purposes of illustration only. The tibial trays and methods discussed in the Examples will be understood to represent, an exemplification of the invention.

Examples

Example I: Stu4y of Axial Component Rotation in Total Knee Arthroplasty

A retrospective MM analysis of the rotational mismatch between the rotational axes of the femur and tibia in 101 skeletally mature patients (males outnumbered females 74 to 27} unde the age of 50 (mean age of 32) with normal .bo«y anatomy. The goal of this st udy was to make an assessment of femorotibial rotational differences and use this in the design of our improved tibia! tray .

MRI images (GE Hdx 1.5 T MRI system,. General Electric, Waukesha WI) were taken with 3-5mm thickness with patients in the supine position with the leg in full, extension. All measurements were completed using Osirix Dicom Viewer (version 3.6.1) (Osiri Foundation, Geneva). The femoral trarisepicondylar axi (TEA) was identified using both coronal and axial images as the widest point on both the media! and lateral epico.ndy.ies as these are what most surgeons will palpate intraoperative ly (Figure 1). The po terio condylar axis (PCA) was also measured as travelling along the articular surface of the femoral condyles (Figure 1 ).

The tibial tuberosity was identified on the anterior surface of the tibia in the axialplane. The tibial slice chosen estimated the normal or average resection level of a TKA as 10 mm below the lateral tibial plateau. This axial image was then used for the assessment of femoro tibial rotation.

InsaU's line, extending fromthe junction of the medial and middle one third. of the tibial tubercle to the centre of the posterior cruciate ligament (PCL) at the back of the knee, was then drawn thereon (Figure 3). TheTCA and PCA were then translated distally to the chosen tibial resect ion slice and extended to the cortical edges of the tibia (Figure 4).

Angular differences between: (a) the TEA and InsaU's line; (b) the PCA and InsalFs line; and (c) the PCA and TEA were measured using simple geometry (Figure 5) . If the TEA or PCA were externally rotated relative to InsalF s line, or the TEA was externally rotated relative to the PCA, then this rotation was afforded a positive value. Conversely, internal rotation was given a negative value.

The tibial tubercle to trochlear groove (TT-TG) distance (i.e. the distance from the midpoin of the tibial t ubercle to the deepest point of the trochlear groove) was assessed to ensure thai the study cohort, had norma! knee morphology (Figure 6). The average TT-TG distances for both males and females were within expected ranges with an absence of outliers (Tables 1. and 2).

The angular difference in degrees between the two femoral rotational axes assessed (TEA and PCA) relative to Insalf s line and each other is provided in Table 1. The TEA was the femoral axis which most closely matched; Insall's line with an average angular difference of 1.5 degrees of external rotation (Table 1), The PCA showed consistent internal rotation relative to both the TEA and Insall's line.

The gender specific values of femorotibial rotation are provided in Table 2, A gender difference wa s observed for each measure of femorotibial ro tation obtained in this study (Table 2), Of note, female patients had a closer correlation between the TEA and Insall's line than male patients (Table 2). The gender difference in TT-TG distance has been well documented in previous studies. Our rotational axes show that while lateral translation of the tubercle may contribute to an increased TT-TG distance, there is also an external rotatory component.

This study demonstrates that in the disease-free knee, Insall's line most closely matches the TE A, which is considered to be the most accurate assessment of femoral rotation. Accordingly, using Insall's line as a marker of tibial component rotation is therefore most likely to allow for matched component articulation whilst optimizing patello femoral kinematics.

Example 2: Study of Proximal Tibial Ana tomy and its Role in Tibial Tray Design An asymmetric tibial tray was designed based upon proximal tibial anatomy, The dimensions and aspect rat ios of 568 magnetic resonance images of a knee of 259 females and 309 males with, a mean age of 42.3 (female) and 38.8 (male) were studied.

AP (anteroposterior or front to back) depths of the medial and lateral plateau as well as the ML (mediolateral or side to side) width were first measured. These three values were then used to create aspect ratios to give an impression of size (Figure 7).

From Figure 7, the medial plateau is deeper than the lateral in the sagittal plane for a given coronal width. Further, it is clear from the clustering of the plot in Figure 7 that for an given tibial width (i.e. ML dimension) there can be a wide variation in AP depth.

Unsurprisingly, when separated out by gender, females represented the smaller ML and AP values (Table 4, Figure 8 and 9). Nonetheless, the aspect ratio (AP/ML) did not show marked gender differences. Furthermore, the clustering that was seen, especially in males, made the design of a tibial tray according to aspect ratios near impossible.

Instead, the evaluation of the ML values allowed segregation into groups whereby smaller increments in Size were used near the mean ML value because more patients would fell within this size. Larger increments in size were used towards the minimum & maximum ML values as fewer patients fall into these sections.

Within each ML increment there was a wide range of AP values both medially and laterally. For example, from Figure 7 a ML range of 70- 73mm demonstrates AP values from 35 to 54, Accordingly, it was considered that the ML value represents the most important dimension and that AP values should be tailored according to this width.

By way of example, an ML width of 70mm covers a range of AP values from 35mm to 54mm. If we opted to use 35mm for an AP value we would ensure that all the patients with a 70mm ML width would have an AP depth that fit. However, tibial coverage is an important factor and so if we increased the AP value to 54 then the coverage would be very good for a few patients but there would be massive posterior overhang on the remaining smaller patients, if the AP value were 45mm then our tray would fit 50% of the group moderately well, hut the other 50% would exhibit significant component overhang.

Consequently two AP values were allocated to each of the ten ML values chosen, The first of these AP values was smaller wit the goal of fitting any patient within that category of ML width (35 mm in the example above). The second value would fit the larger 50% of patients with the ML category to optimise bone coverage (45mm in the example above)).

This seemed to be the only reasonable way to addres the large variation in human proximal tibial anatomy. Having the availability of multiple AP depths for each ML width allows the surgeon to guarantee that he or she can fit a tray without overhang but also increase coverage area if bone stock allows. This led to the formulation of 20 sizes with the sizes shown in Table 3 and Figure 13.

The resultant tibial tray design was then tested on 105 knees. Medio lateral fit, any requirement for down-sizing of the tray as determined by component overhang, posterolateral fit and posteromedial fit were assessed in each patient.

As observed in Figure 10, there was no requirement to down-size the improved tibial tray design in any patient. Further, in Figures 1 1 and 12, this t ibial tray design i associated with significant improvements in posterolateral and posteromedial fit respectively. The asymmetric tray of the invention is of significant advantage at least because it has improved tibial axis alignment, which thus acts to optimize patello femoral kinematics and further limit any component malrotation and resultant overhang, compared to other tibial trays. Additionally, component overhang is reduced well below that achieved with othe tibial trays.

Throughout the specification the aim has been to describe the preferred embodiments of the invention without limiting the inventio to any one embodiment or specific collection of features. It will therefore- be appreciated by those of skill in the art that, in light of the instant disclosure, various modificat ions and changes can be made in the particular embodiments exemplified without departing from the scope of the present invention.

All computer programs, algorithms, patent and scientific literature referred to herein is incorporated herein by reference.

Table 1: Angular- difference between the femoral and tibial rotational axes

* Angular differences are measured in degrees with po itive values indicating external rotation and negative values indicating internal rotation. Table 2: Gender compariso of fem ra tibial r taion

Anguar d erences are measure n egrees wt postve values n icatng rotation and negative values indicating internal rotation.

Table 3: Sizes and Aspect Ratios of Tibial Trays

Table 4: Mean Length of Proximal Tibial Dimensions by Gender

Claims

1. A tibial tray comprising:

an asymmetrical body comprising a medial section and a lateral sect ion; the medial section comprising a medial verlay and a posteromedial shoulder; the lateral section comprising a lateral overlay and a posterolateral shoulder; both the medial section and the lateral section comprising a shape and size to increase tibial coverage and reduce posterolateral overhang, wherem the shape and size are selected to .minimise, restrict or prevent, rotational misalignment of the tray about an axis of tibial rotation,

2. A method for making a tibial tray including;

forming an asymmetrical body comprising a medial section and a lateral section;

the medial section comprisin a medial overlay and a posteromedial shoulder; the lateral sectio comprising a lateral overlay and a posterolaterai shoulder; both the medial section and lateral section comprising a shape and size to increase tibial coverage and reduce posterolateral overhang, wherein the shape and size are selected to minimise, restrict or prevent rotational misalignment of the tray about an axis of tibial rotation, to thereby make the tibial tray,

3- The tibial tray of Claim 1 or the method of Claim 2, wherein the posteromedial and/or posterolateral shoulder profiles comprise a curved and/or a substantially straight section.

4, The tibial tray of Claim 1 or Claim 3 or the method of Claim 2 or Claim 3, wherei the medial and/or lateral overlays comprise a curved and/or substantially straight section.

5, The tibial tray or method of Claim 3 or Claim 4, wherein the curved and/or substantially straight section(s) of the shoulder profiles and/or the overlays comprise a radius of curvature selected to increase tibial coverage and or reduce posterolateral overhang.

6. The tibial tray or method of Claim 5, wherein the radius of curvature is in a. range of approximately or about 4.0-12.0 cm,

7. The tibial tray or method of any preceding claim, wherein the tibial tray comprises a mediolateral length of between approximately or about 50 and 95 aim.

8. The tibial tray or method of any preceding claim, wherein the media! section comprises a medial section anteroposterior w idth of between approximately or about

35 and 65 mm.

9. The tibial tray or method of any preceding claim wherein the lateral section comprises a lateral section anteroposterio width of between approximately or about 30 and 55 mm.

10, The tibial tray or method o f any preceding claim having a medial aspect ratio between . approximately or about 0.56 and 0.76.

11. The tibial tray or method of any preceding claim having lateral aspect ratio between approximately or abou 0.50 and 0.68.

12. The tibial tray or method of any preceding claim, the tibial tray having; (i) a mediolateral length of approximately or about 58 mm, a medial section anteroposterior width of approximately or about 40 mm, a lateral section anteroposterior width of -approximately or about 36 mm, a medial aspect ratio of approximately or about 0.690 and lateral aspect ratio of approximately or about 0.621: (ii) a mediolateral length of approximately or about 58 mm, a medial section anteroposterior width of 42 mm, a lateral section anteroposterior width of approximately or about 39 mm, a medial a spect ratio of appro xirnatel or about 0.724 and a lateral aspect ratio of approximately or about 0.672; (Hi) a mediolateral length of appro xiinately or about 61 mm, a medial section anteroposterior width of approximately or about 43 mm. a lateral section anteroposterior width of approximately or about 37 mm, a medial aspect ratio of approximately or about 0.705 and a lateral aspect ratio of approximately or about 0,607; (iv) a mediolateral length of approximately or about 61 mm, a medial section anteroposterior width of approximately or about 46 mm, a lateral section anteroposterior width of 40mm, a medial aspect ratio of approximately or about 0.754 and a lateral aspect ratio of approximately or about 0.656; (v) a mediolateral length of approximately or about 64 mm, a medial section anteroposterior width of approximately or about 43 mm, a lateral section anteroposterior width of 37 mm, a medial aspect ratio of approximately or about 0.672 and a lateral aspect ratio of approximately or about 0.578; vi) a. mediolateral length of approximately or about 64 mm, a medial section anteroposterior width of approximately or about 47 mm, a lateral sectio anteroposterior width of approximately or about 42 mm, a medial aspect ratio of approximately or about 0.734 and a lateral aspect ratio of approximately or about 0.656; (vii) a mediolateral length of approximately or about 67 mm, a medial section anteroposterior width of approximatel or about 43 mm, a lateral section anteroposterior width of appr ximately or about 37 mm, a medial aspect ratio of approximately or about 0.642 and a lateral aspect ratio of approximately or about 0.552; (viiij a mediolateral length of approximately or about 67 mm, a medial section anteroposterior width of approximately or about 47 mm, lateral section anteroposterior width of approximately or about 42 mm, a medial aspect ratio of approximately or about 0.701 and a lateral aspect ratio of approximately or about 0.627; (k) a mediolateral length of approximately or about 70 mm, a medial section anteroposterior width of approximately or about 44 mm, a lateral section anteroposterior width of approximately or about 38 mm, a medial aspect ratio of approximately or about 0.629 and a lateral aspect ratio of approximately or about 0.543; (x) a mediolateral length of approximately or about 70 mm, a medial section anteroposterior width of approximately or about 47 mm, a. lateral section anteroposterior width of approximately or about 41.92 ram, a medial aspect ratio of approximately or about 0.671 and a lateral aspect ratio of approximately or about 0.599; (xi) a mediolateral length of approximately or about 73 mm, a medial section anteroposterior width of approximately or about 46 mm, a lateral section anteroposterior width of approximately or about 39 mm, a medial aspect ratio of approximately or about 0,630 and a lateral aspect ratio of approximately or about 0.534; (xii) a mediolateral length of approximately or about 73 nan, a medial section anteroposterior width of approximately or about 50 mm, a lateral section anteroposterior width of approximately or about 43 mm, a medial aspect ratio of approximately -or about 0.685 and a lateral aspect ratio of approximately or about 0.589; (xiii) a mediolateral length of approximately or about 76 mm, a medial section anteroposterior width of approximately or about 47 mm, a lateral section anteroposterior width of approximately or about 40 mm, a medial aspect ratio of approximatel or about 0.618 and a lateral aspect ratio of approximately or about 0.526; (xiv) a mediolateral length of approximately or about 76 mm, a medial section anteroposterior width of approximately or about 52 mm. a lateral section anteroposterior width of approximately o about 46 mm, a medial aspect ratio of approximately or about 0.684 and lateral aspect ratio of approximately or about 0.605; (xv) a mediolateral length of approximately or about 79 mm, a medial section anteroposterior width of approximately o about 48.05 mm. a lateral section anteroposterior width of approximately or about 42.05 mm, a medial aspect ratio of approximately or about 0,608 and a lateral aspect ratio of approximately or about 0.532; (xvi) a mediolateral length of approximately or about 79 mm, a medial section anteroposterior width of approximately or about 53.05 mm, a lateral section anteroposterior width of approximately or about 47.05 mm, a medial aspect ratio of approximately or about 0.672 and lateral aspect ratio of approximately or about 0.596; (xvii) a mediolateral length of approximately or abou 83 mm, a medial section anteroposterior width of approximately or about 49 mm, a lateral section anteroposterior width of approximately or about 43 mm, a medial aspect ratio of approximately or about 0.590 and a lateral aspect ratio of approximately or about 0,518; (xviii) a mediolateral length of approximately or about 83 ram, a medial section anteroposterior width of approximately or about 54 mm, a lateral section anteroposterior width of approximately or about 48 mm, a medial aspect ratio of approximately or about 0.6 1 and lateral aspect ratio of approximately or about 0.578; (xix) a mediolateral length of approximately or about 87 mm, a medial section anteroposterior width of approximately or about 50 mm, a lateral section anteroposterior width of approximately or about 44 mm, a medial aspect ratio of approximately or about 0.575 and a lateral aspect ratio of approximately or about 0.506; or (xx) a mediolateral length of approximately or about 87 mm, a medial section anteroposterior width of approximately or about 55 ram, a lateral section anteroposterior width of approximately or about 49 mm, a medial aspect ratio of approximately or about 0.632 and a lateral aspect ratio of approximately or about 0.563.

13. A method for knee arthroplasty or reconstruction including inserting a tibial tray according to any one of Claims 1-12 or produced according to the method of any one of Claims 2-12, into a knee joint in need of arthroplasty or reconstruction, to thereby perform the knee arthroplasty or reconstruction.

.

14. A kit comprising a tibial tray according to any one of Claims 1 - 12 or produced according to the method of any one of Claims 2-12 and instructions for use.

15. A method for designing a surgical implant, or prosthesis, the method including: interrogating a data set comprisin one or more tibial and/or femoral axes of rotation information for the population into which the surgical implant is to be implanted to obtain one or more dimensions for the surgical implant; and

based on the obtained one or more dimensions designing the surgical implant.

16. The method of Claim 15 which further includes the step of interrogating an imaging data set to obtain the one or more tibial and/or femoral axe of rotation information.

17. The method of Claim 15 or. Claim 16, wherein the surgical implant or prosthesis is the tibial tray of any one of Claims 1 - 12.