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 such an implant and a method of designing a surgical implant. In particular, the invention is directed to an asymmetric tibial tray with equal or improved tibial plateau coverage and reduced posterolateral overhang, although the scope of the invention is not necessarily limited thereto.

BACKGROUND

Total knee arthroplasty (TKA) 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 disadvantages.

Most current tibial trays have a symmetric shape. Asymmetric trays have been designed in an attempt to improve the coverage of the native tibial bone. However, the fit of even these asymmetric trays is not ideal. An improved asymmetric tibial tray is required.

SUMMARY

The present invention is broadly directed to an asymmetric tibial tray. The present inventors have realised that an asymmetric tibial tray provides equal or greater tibial plateau coverage and decreased posterolateral oversizing and/or overhang which may reduce or avoid the problems associated with other tibial trays such as, subsidence, bone bleeding, soft tissue impingement, loss of knee extension and 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 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 and reduce posterolateral overhang. In a second aspect the invention provides a method for making a tibial tray 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 and reduce posterolateral overhang, to thereby make the tibial tray.

In a third aspect the invention provides 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 of 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 instructions for use.

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

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

According to any of the above aspects the gently curved or substantially straight section of the shoulder profiles 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 posterolateral overhang.

According to any above aspects the tibial tray may comprise a mediolateral length of between approximately or about 60 and 90mm.

In preferred aspects the mediolateral length may be approximately or about

66, 71 or 80mm. According to any above aspect the medial section may comprise a medial section anteroposterior width of between approximately or about 40 and 65 mm.

In preferred aspects the medial section anteroposterior width may be approximately or about 43, 44, 46, 50, 51 or 55mm.

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 mediolateral section may comprise a lateral section anteroposterior width of approximately or about 34, 39, 40, 42, 44 or 48mm.

According to any above aspect the medial aspect ratio may be between approximately or about 0.6 and 0.75. 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) to the mediolateral tibial width.

In preferred aspects the medial aspect ratio may be approximately or about 0.62, 0.625, 0.651, 0.688, 0.697 or 0.718.

According to any above aspect the lateral aspect ratio may be between approximately or about 0.5 and 0.65. 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) to the mediolateral tibial width.

In preferred aspects the lateral aspect ratio may be approximately or about 0.515, 0.525, 0.563, 0.591, 0.6 or 0.62.

A preferred tibial tray according to any above aspect may have a mediolateral length of 66mm, a medial section anteroposterior width of 43mm, a lateral section anteroposterior width of 34mm, a medial aspect ratio of 0.652 and a lateral aspect ratio of 0.515.

Another preferred tibial tray according to any above aspect may have a mediolateral length of 66mm, a medial section anteroposterior width of 46mm, a lateral section anteroposterior width of 39mm, a medial aspect ratio of 0.697 and a lateral aspect ratio of 0.591.

Another preferred tibial tray according to any above aspect may have a mediolateral length of 71 mm, a medial section anteroposterior width of 44mm, a lateral section anteroposterior width of 40mm, a medial aspect ratio of 0.62 and a lateral aspect ratio of 0.563.

Another preferred tibial tray according to any above aspect may have a mediolateral length of 71mm, a medial section anteroposterior width of 51mm, a lateral section anteroposterior width of 44mm, a medial aspect ratio of 0.718 and a lateral aspect ratio of 0.62.

Another preferred tibial tray according to any above aspect may have a mediolateral length of 80mm, a medial section anteroposterior width of 50mm, a lateral section anteroposterior width of 42mm, a medial aspect ratio of 0.625 and a lateral aspect ratio of 0.525.

Another preferred tibial tray according to any above aspect may have a mediolateral length of 80mm, a medial section anteroposterior width of 55mm, a lateral section anteroposterior width of 48mm, a medial aspect ratio of 0.688 and a lateral aspect ratio of 0.6.

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

interrogating a data set comprising sizing 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 imaging data set to obtain the sizing information.

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

The surgical implant may be a tibial tray.

In a seventh aspect the invention provides a method for designing a size range to cover or partially cover a normally distributed population, the method including: interrogating a data set comprising sizing information on a normally distributed population to obtain one or more dimensions;

determining an upper value and lower value from the obtained one or more dimensions;

dividing the range between the upper value and the lower value into a number of sizes to thereby design the size range.

According to the seventh aspect the sizing profile may be for a surgical implant or prosthesis.

In an eighth aspect the invention provides a surgical implant comprising a size designed according to the eighth 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.

FIG. 1 A: shows an outline drawing of a tibial tray of size 3 according to one aspect of the invention.

FIG. IB: shows an outline drawing of a tibial tray of size 4 according to one aspect of the invention.

FIG.2 shows comparative percent coverage of the asymmetric tibial tray according to the invention.

DETAILED DESCRIPTION

The inventors have recognised that poor fitting tibial trays may have dire consequences. An undersized tibial tray may cause subsidence, bone bleeding and femoral component size matching problems. An oversized tibial tray may cause soft tissue impingement, loss of knee extension and pain.

The inventors have further recognised that an asymmetrical tibial tray provides equal or greater tibial plateau coverage and decreased posterolateral oversizing.

Surprisingly, as exemplified herein, the inventors have provided an asymmetric tray which has an increased tibial coverage compared to other tibial trays and which has a reduced posterolateral overhang well below that achieved with other tibial trays. The improved tibial tray provided by the inventors is of significant advantage by virtue reducing the incidence of overhang of the tibial tray which thereby eliminates or at least reduces the impingement on and possible trauma to surrounding soft tissue structures. While the principles described herein are based on tibial trays for humans, this invention may also be extended to other mammals such as livestock, performance animals (e.g. racehorses) and domestic pets (e.g. dogs, cats), although without limitation thereto.

As used herein, the terms "approximately" and "about" refer to tolerances or variances associated with numerical values recited herein. The extent of such tolerances and variances are well understood by persons skilled in the art. Typically, such tolerances and variances do not compromise the structure and/or function of the tibial tray.

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

As used herein "relative undersize" means a tibial 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 and 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 104. Mid-section 104 comprises an anterior mid-section 106 and a posterior mid-section 108.

Medial section 110 comprises a medial overlay 112 and a posteromedial W

7 shoulder 116.

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

Medial overlay 112 and lateral overlay 122 comprise a medial overlay profile 5 114 and lateral overlay profile 124, respectively, selected to conform to tibial anatomy. As shown in FIGS. 1A and IB profiles 114 and 124 are substantially curvilinear. Profiles 114 and 124 comprise a gently curved or substantially straight section 115, 125. The combination of the curved and straight sections result in a better fit once implated into a joint.

10 Posteromedial shoulder 116 and posterolateral shoulder 126 comprise a posteromedial shoulder profile 118 and a posterolateral should profile 128, respectively, selected to conform to tibial anatomy. Shoulder profiles 114 and 124 also comprise a gently curved or substantially straight section 119, 129. As above, inclusion of the straight sections 119, 129 result in a better fit once implated into a 15 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 115, 125, 119, 129 may comprises a radius of curvature greater than approximately or about 5.175,

20 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.

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

Medial section 110 and lateral section 120 are asymmetrical. As indicated on FIG. 1 A tibial tray 100 has a mediolateral length (ML) of 66mm, a medial section

25 110 anteroposterior width (AP 1 ) of 43mm and a lateral section 120 anteroposterior width (AP2) of 34 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 0.651 and a lateral aspect ratio of 0.515.

30 As a result of the asymmetrical sizing of medial section 110 and lateral section 120 the 102 is asymmetrical. Tibial tray 100 is categorised as size A 1 and has a different size and different asymmetrical sizing aspect ratio to tibial tray 200 shown in FIG. IB which is categorised as size A2.

Tibial tray 200 has the same mediolateral length as tray 100, but different an API of 46 mm and an AP2 of 39 mm which gives a medial aspect ratio of 0.697 and a lateral aspect ratio of 0.591.

The tibial tray of the invention may be sized to fit an individual. The tibial tray of the invention may comprise a mediolateral length of between approximately or about 60 and 90mm. The medial section anteroposterior width may be between approximately or about 40 and 65 mm. The lateral section anteroposterior width may be between approximately or about 30 and 55 mm. The medial aspect ratio may be between approximately or about 0.6 to 0.75. The lateral aspect ratio may be between approximately or about 0.5 and 0.65.

Both tray sizes Al and A2 are classified as small. The asymmetrical tibial tray according to the invention may also be manufactured in medium and large sizes.

Medium size trays are categorized as sizes Bl and B2.

A size Bl tray comprises a mediolateral length of 71 mm, a medial section anteroposterior width of 44mm, a lateral section anteroposterior width of 40mm, a medial aspect ratio of 0.62mm and a lateral aspect ratio of 0.563mm.

A size B2 tray comprises a mediolateral length of 71mm, a medial section anteroposterior width of 51mm, a lateral section anteroposterior width of 44mm, a medial aspect ratio of 0.718mm and a lateral aspect ratio of 0.62mm.

Large size trays are categorized as sizes CI and C2.

A size CI tray comprises a mediolateral length of 80mm, a medial section anteroposterior width of 50mm, a lateral section anteroposterior width of 42mm, a medial aspect ratio of 0.625mm and a lateral aspect ratio of 0.525mm.

A size C2 tray comprises a mediolateral length of 80mm, a medial section anteroposterior width of 55mm, a lateral section anteroposterior width of 48mm, a medial aspect ratio of 0.688mm and a lateral aspect ratio of 0.6mm.

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

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. 5 The invention also provides a method for designing a surgical implant or prosthesis including interrogating a data set comprising sizing information 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 10 designed.

The method may further include a step of interrogating imaging data to obtain the sizing 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.

15 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.

Another method provided by the inventors is for designing a size range to cover or partially cover a normally distributed population. This method may include

20 the steps of interrogating a data set comprising sizing information on a normally distributed population to obtain one or more dimensions and determining an upper value and a lower value from the obtained one or more dimensions.

The range between the upper value and the lower value may then be divided into a number of sizes to thereby design the size range.

25 The method for designing the sizing range may also include a step of interrogating imaging data to obtain the data set. The imaging data maybe magnetic resonance imaging, x-ray imaging, positron emission imaging or any suitable imaging method.

The sizing profile may use the Zimmer concept of odd and even sizing. 30 According to the Zimmer concept odd sizes have a small aspect ratio, i.e. a smaller anteroposterior sizing and even sizes have a larger aspect ratio. 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 1: Study of Tibial Tray Sizing and Bone Coverage

A retrospective magnetic resonance imaging (MRI) study of 101 patients with suspected soft tissue injuries was conducted. The mean age was 31.8 with a range of 17-60 and a male : female ratio of 74 : 27.

Proton density fat suppressed magnetic resonance (MR) images were previously acquired and studied using a GE Hdx 1.5T MRI system (General Electric, Waukesha, WI) and an Osirix Dicom Viewer (version 3.6.1, Osirix Foundation, Geneva). Axial images of tibia identified 8- 10mm from the lateral compartment articular surface.

Total knee arthroplasty baseplates were digitally scanned with magnification marker (HP PhotoSmart C4500 series, Hewlett Packard, Palo Alto, CA). The digital images were superimposed on axial MR cut. There was no anterior, medial or lateral tray oversizing and orientation was on or near Insall's line. Insall's line is a line originating from the junction of the medial 1/3 and lateral 2/3 of the tibial tubercle and extending to the tibial insertion of the posterior cruciate ligament.

Percent Tibial Coverage was calculated as per below:

% Tibial Coverage = (CSABASEPLATE - CSAOVERHANG)/CSA _T IBIAL SURFACE

CSA = cross sectional area.

Statistical Analysis was performed with SAS version 9.1 (Cary, NC) using chi-squared testes for comparisons of proportions and paired and unpaired t-tests for comparisons of continuous variables.

Tibial component bone coverage results were as shown in Table 1 below. There was no correlation between the number of available sites and total coverage (p=0.3).

Posterolateral tibial component position results demonstrated that 28.2 % demonstrated an optimal fit (+/- 1 mm). The NextGen implant had the least number of oversized trays in the relative (l-3mm) and absolute (>3mm) groups. The Genesis II implant had the most in both categories. 48.8% had theoretical impingement on the popliteus tendon.

Posteromedial tibial components position results demonstrated that only 7.6% had an optimal fit. Genesis II had significantly more optimal fit compared to all other prostheses used (p<0.001). 69.9% had absolute undersizing (>3mm).

This study showed that asymmetric tibial trays (Genesis Π) compared to symmetric designs are rotationally aligned and this leads to improved bone coverage and a greater incidence of oversizing.

Example 2: Asymmetric Tibial Tray Design

An asymmetric tibial tray was designed based upon proximal tibial anatomy.

The dimensions and aspect ratios of 346 magnetic resonance images of a knee of 136 females and 210 males with a mean age of 41 (female) and 37 (male) were studied.

The images were classified into three different mediolateral groups: small, medium and large. The Zimmer concept of odd and even sizing was used.

The design was then tested on the 101 knees used in the tibial tray study above.

The results are shown in Table 2 below.

Based on the mediolateral distance (ML) values three groups were identified.

In the cohort ML values ranged from 62.3 to 88.6 the difference (26.3) divided by 3 is 8.8. Beginning from 62.3 and increasing in 9mm increments gives ML values of:

1. Small: 62 to 71 ; 104 patients, mean ML = 68mm

2. Medium: 71 to 80; 140 patients, mean ML = 76.1mm

3. Large: 80 to 89; 102 patients, mean ML = 82.7mm.

The 346 knees were split into three groups to show negligible differences in the aspect ratios. The results are shown in Table 3 below.

This led to formulation of six sizes Al , A2, B 1 , B2, CI and C2 with the sizes shown in Table 4 below. Sizes Al and A2 are small. Sizes Bl and B2 are medium and sizes C 1 and C2 are large.

The fit of the tibial trays of the invention are as shown in Table 5 below. Tibial trays 100 and 200 shown in FIGS. 1 A and IB show the ML, API and AP2 lengths of tibial trays of sizes Al and A2, respectively.

The percent coverage of the asymmetric tibial tray according to the invention, labeled SKS1, is shown in FIG. 2 as compared to conventional tibial trays.

Comparative posterolateral and posteromedial fits of tibial trays according to the invention, labeled SKS1, are shown in Table 6 below.

The asymmetric tray of the invention is of significant advantage at least because it has an increased coverage compared to other tibial trays. Additionally, posterolateral overhang is reduced well below that achieved with other tibial trays.

Throughout the specification the aim has been to describe the preferred embodiments of the invention without limiting the invention 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 modifications 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 : Tibial Component Bone Coverage:

<0.0001 compared to all other designs in paired t-tests.

Table 2: 210 Male (M) and 136 Female (F) anatomical data

Min F 62.3 39.2 34.2 0.592 0.485

M 63 40.3 32.6 0.580 0.496

Max F 84.9 60.1 52.8 0.761 0.664

M 88.6 61.6 53.8 0.739 0.678

SD F 5.07 3.8 4.0 0.0316 0.0341

M 4.6 3.7 3.54 0.030 0.0312

Table 3: Differences in Aspect Ratios

SMALL MEDIUM LARGE

(ML = 62-71) (ML = 71-8 0) (ML = 80-. 39) medial lateral medial lateral medial lateral ratio ratio ratio ratio ratio ratio

MEAN 0.676 0.580 0.667 0.586 0.660 0.583

MIN 0.590 0.485 0.592 0.487 0.580 0.496

MAX 0.750 0.664 0.761 0.678 0.737 0.648

STDEV 0.02983 0.03375 0.030 0.0327 0.0316 0.03052

Table 4: Sizes and Aspect Ratios of Tibial Plate

Table 5: Fit of Tibial Tray

Table 6: Comparative posterolateral (PL) and posteromedial (PM) fit:

Absolute Relative Optimal Relative Absolute undersizing undersizing sizing oversizing oversizing

PL PM PL PM PL PM PL PM PL PM

NextGen 8 90 33 8 34 2 22 1 5 0

LCS 2 71 13 21 32 7 37 2 17 0

PFC 10 87 18 12 29 1 33 1 10 0

Scorpio 4 76 19 20 26 3 32 2 20 0

Triathlon 3 68 17 22 25 9 35 2 21 0

Genesis II 1 30 11 32 25 24 38 14 26 1

N 14 43 27 32 29 19 22 7 9 0

SKS1 28 49 37 37 31 14 4 1 1 0