Unicondylar meniscal bearing knee replacement

Abstract

A kit of parts for use in unicondylar meniscal bearing knee replacement comprises a plurality of meniscal bearings, each meniscal bearing comprising a body defining a dished first bearing surface on one side thereof and a second surface on an opposing side of the body. Each meniscal bearing has an entrapment between 3.2 mm and 3.8 mm. Meniscal bearings and methods of performing unicondylar meniscal bearing replacements are also described.

Claims

1. A system of parts for use in unicondylar meniscal bearing knee replacement, the system comprising; a plurality of meniscal bearings ranging in size including an extra small bearing having an anterior-posterior length of 29.2?0.5 mm, a small bearing, a medium bearing, a large bearing, and an extra large bearing having an anterior-posterior length of 39.5?0.5 mm, each meniscal bearing comprising a body defining a dished first bearing surface on one side thereof and a second surface on an opposing side thereof; wherein each meniscal bearing has an entrapment defined as the difference in thickness between the smaller of the maximum thicknesses between the first and second surfaces at an anterior and posterior end and the minimum thickness between the first and second surfaces, the entrapment of each meniscal bearing being between 3.2 mm at least for the extra small bearing and 3.8 mm at most for the extra large bearing.

2. The system of claim 1, wherein the entrapments of the meniscal bearings are all approximately the same.

3. The system of claim 2, wherein the entrapments of the meniscal bearings are all approximately 3.5 mm.

4. The system of claim 1, wherein the meniscal bearings have a plurality of different lengths.

5. The system of claim 1, wherein the length of at least one of the meniscal bearings is at least 39 mm.

6. The system of claim 1, wherein the length of at least one of the meniscal bearings is less than 31.6 mm.

7. The system of claim 1, wherein the meniscal bearings have a plurality of different minimum thicknesses between the first and second surfaces.

8. The system of claim 1, wherein the meniscal bearings of a given length have approximately the same entrapment.

9. The system of claim 1, wherein the meniscal bearings are trial bearings for use in fitting a prosthesis or implantable bearings for use with the prosthesis.

10. The system of claim 1, further comprising; a set of trial bearings and a set of implantable bearings, each implantable bearing corresponding to one of the trial bearings.

11. The system of claim 10, wherein the correspondence between the trial and implantable bearings is such that, after a thickest grade trial bearing insertable into a gap between tibial and femoral components of a patient's knee is identified, the implantable bearing one grade thinner than the thickest grade trial bearing is selected for implantation.

12. The system of claim 1, wherein at least one of the meniscal bearings is symmetrical about either or both of a coronal plane and a sagittal plane.

13. The system of claim 1, further comprising; at least one femoral prosthetic component having a spherical articular surface with a radius of curvature, the first surfaces of the meniscal bearings having the same radius of curvature.

14. The system of claim 1, further comprising; at least one tibial prosthetic component.

15. The system of claim 1, wherein at least one of the meniscal bearings comprises at least one protrusion at a posterior end thereof, an anterior end thereof, or both ends thereof.

16. The system of claim 15, wherein each protrusion is of even depth along the end on which it is located.

17. The system of claim 15, wherein each protrusion has a reduced depth at its upper end compared with its lower end, giving a sloped edge.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The invention will now be described by way of example only, with reference to the accompanying drawings, of which:

(2) FIG. 1 shows a medial unicondylar arthroplasty;

(3) FIGS. 2a and 2b show schematically a meniscal bearing of the prosthesis of FIG. 1;

(4) FIG. 2c shows schematically a prior art anatomically-shaped meniscal bearing;

(5) FIG. 2d shows schematically a prior art meniscal bearing;

(6) FIGS. 3a and 3b show a trial fitting of a trial meniscal bearing in a procedure to implant the unicondylar implant of FIG. 1;

(7) FIG. 4 shows a meniscal bearing and geometry for illustrating the relationship between radius of curvature R, and some geometrical features of the bearing; and

(8) FIG. 5 shows schematically a unicondylar arthroplasty;

(9) FIG. 6a shows a meniscal bearing implantation along the tibial component;

(10) FIG. 6b shows a meniscal bearing implantation along the femoral component;

(11) FIG. 7 shows schematically another embodiment of a unicondylar arthroplasty; and

(12) FIG. 8 shows another embodiment of a meniscal bearing implantation along the femoral component.

DESCRIPTION OF EMBODIMENTS

(13) FIG. 1 shows a right leg femur 10 having a medial condyle 12 and a lateral condyle 14, a tibia 16, and a medial unicondylar implant 18. The implant has three components: a metal tibial plate, or component, 20, a metal femoral component 22 and a plastics material (e.g. Ultra high Molecular weight polyethylene, UHMWPE) meniscal bearing 24. The materials named are those commonly used but they are not essential.

(14) The bottom, second, surface of the meniscal bearing is flat or generally flat, whereas the upper, first, surface is a dished, concave, surface which surrounds the domed surface of the femoral component 22.

(15) FIGS. 2a and 2b show schematically the meniscal bearing of FIG. 1. In this embodiment, the meniscal bearing 24 has a posterior end 40, an anterior end 42 and two generally parallel sides 44. The meniscal bearing 24 has a flat base 46 forming the second surface and a central dished upper portion 48 forming the first surface. The thickness of this bearing (reference t.sub.max in FIG. 2b) may not be the same at the anterior and posterior ends. The base surface 50 of the dished portion 48 is a distance t.sub.min above the flat base 46. The surface of the dished portion is generally part of a spherical surface. The entrapment of the bearing is t.sub.max-t.sub.min.

(16) FIG. 2c shows schematically an alternative design of meniscal bearing which has an anatomical shape. While the lateral side 52 and the medial side 54 are still generally parallel, the medial side 54 is shorter than the lateral side 52, and the medial corners 58 and 60 are of larger radii than those of lateral corners 56, which generally have equal radii. Anteromedial corner 58 may have a larger radius than posteromedial corner 60 to minimise bearing overhang in extension. The bearing also includes protrusions 62 extending from the lower portion of lateral corners 56 to the lower portion of medial corners 58 and 60. The anatomical shape reduces the likelihood of the bearing rotating on its flat base around a vertical axis, as the additional material comes into contact with the tibial wall.

(17) FIGS. 3a and 3b show the trial fitting of a trial meniscal bearing in a surgical operation. People come in different sizes, and prosthetic knee components come in different sizes, requiring femoral components of different radii & tibial components with different areas in plan view. After removing bone from a patient's damaged medial femoral condyle and fitting the femoral component 22 to it, and after removing bone from the damaged tibia and fitting the tibial component or plate 20 to it, there is a gap between the upper surface of the tibial plate and the domed surface of the femoral component. The meniscal bearing 24 is to fit in that gap.

(18) The surgeon positions a patient's leg to a desired position, e.g. with the femur and tibia at about 90? to each other. The surgeon then selects a trial bearing 30, and inserts it into the gap to test the fit. The surgeon articulates the knee joint with the trial bearing in place to see if it will move properly, without problems, under a range of movement. There should still be a gap between the trial bearing and the other joint components, and that gap should remain more or less constant through the range of movement of the knee. The surgeon may choose to try a thicker bearing if he thinks that there is too much slack in the knee, or a thinner bearing if there is not enough slack.

(19) The medial ligaments of the knee are strong and thick. It is hard to distract the joint (push the joint open) against their resistance. A surgeon often wants to ensure that the joint is not loose and so often tries to push into the gap the thickest meniscal bearing possible.

(20) This can result in accidentally overstuffing the knee joint, which can cause problems later in use of the knee. The present invention helps to avoid such problems, especially with less experienced surgeons.

(21) Up until now the entrapment of meniscal bearings has varied from about 3 mm for small patients, to about 4 mm for very large patients.

(22) Previous bearings used in unicondylar knee replacements, as shown in FIG. 4, have been constructed with a posterior lip or projecting edge having a bearing thickness of between 3 mm and 4 mm above the base of the bowl or dish of the meniscal bearing. The anterior lip or projecting edge of the bearing has had a thickness of around 5 mm above the base of the bowl. We have determined that, by reducing the entrapment to a range of about 3.2 mm for very small patients to about 3.8 mm for very large patients, or perhaps maintaining the entrapment of the meniscal bearing at around 3.5 mm for all sizes, it is more difficult to overstuff the joint in small patients, assisting in reducing post-operative pain to the patient, and it is easier to implant the correct thickness of bearing in large patients.

(23) In our bearing we have realised that we may want consistent entrapment for any bearing thickness at its centre. That is to say, our bearing will sit with about the same level of slackness once it is in situ, and therefore we will avoid (or reduce the chance of) overstuffing. In the prior art the entrapment (t.sub.max?t.sub.min) train is normally about 3 mm to about 4 mm for the range of sizes of bearings. We would have consistent entrapment of about 3.5 mm, or a reduced entrapment range of 3.2 mm to 3.8 mm, increasing entrapment in the smaller sizes and decreasing it in the larger ones.

(24) Table 1 below shows the relationship between t.sub.max and t.sub.min for some of the known prior art medial unicondylar meniscal bearings. In these cases, the anterior and posterior thicknesses are equal:

(25) TABLE-US-00001 TABLE 1 (Extra Large Bearings) anterior- posterior t.sub.max t.sub.min t.sub.max ? length L Size (mm) ? 0.25 (mm) ? 0.25 t.sub.min (mm) ? 0.5 3XL 7.54 3.5 4.04 39.5 4XL 8.54 4.5 4.04 39.5 5XL 9.54 5.5 4.04 39.5 6XL 10.54 6.5 4.04 39.5 7XL 11.54 7.5 4.04 39.5 8XL 12.54 8.5 4.04 39.5 9XL 13.54 9.5 4.04 39.5

(26) In a large prior art bearing the sizes are shown in Table 2:

(27) TABLE-US-00002 TABLE 2 anterior- posterior t.sub.max t.sub.min t.sub.max ? length L Size (mm) ? 0.25 (mm) ? 0.25 t.sub.min (mm) ? 0.5 3L 7.26 3.5 3.76 36.8 4L 8.26 4.5 3.76 36.8 5L 9.26 5.5 3.76 36.8 6L 10.26 6.5 3.76 36.8 7L 11.26 7.5 3.76 36.8 8L 12.26 8.5 3.76 36.8 9L 13.26 9.5 3.76 36.8

(28) In a medium prior art bearing the sizes are as shown in Table 3:

(29) TABLE-US-00003 TABLE 3 anterior- posterior t.sub.max t.sub.min t.sub.max ? length L Size (mm) ? 0.25 (mm) ? 0.25 t.sub.min (mm) ? 0.5 3M 7.0 3.5 3.5 34.2 4M 8.0 4.5 3.5 34.2 5M 9.0 5.5 3.5 34.2 6M 10.0 6.5 3.5 34.2 7M 11.0 7.5 3.5 34.2 8M 12.0 8.5 3.5 34.2 9M 13.0 9.5 3.5 34.2

(30) In a prior art small bearing the sizes are as shown in Table 4:

(31) TABLE-US-00004 TABLE 4 anterior- posterior t.sub.max t.sub.min t.sub.max ? length L Size (mm) ? 0.25 (mm) ? 0.25 t.sub.min (mm) ? 0.5 3S 6.73 3.5 3.23 31.6 4S 7.73 4.5 3.23 31.6 5S 8.73 5.5 3.23 31.6 6S 9.73 6.5 3.23 31.6 7S 10.73 7.5 3.23 31.6 8S 11.73 8.5 3.23 31.6 9S 12.73 9.5 3.23 31.6

(32) In an extra small prior art bearing the sizes are as shown in Table 5:

(33) TABLE-US-00005 TABLE 5 anterior- posterior t.sub.max t.sub.min t.sub.max ? length L Size (mm) ? 0.25 (mm) ? 0.25 t.sub.min (mm) ? 0.5 3XS 6.48 3.5 2.98 29.2 4XS 7.48 4.5 2.98 29.2 5XS 8.48 5.5 2.98 29.2 6XS 9.48 6.5 2.98 29.2 7XS 10.48 7.5 2.98 29.2 8XS 11.48 8.5 2.98 29.2 9XS 12.48 9.5 2.98 29.2

(34) FIG. 2d shows a prior art meniscal bearing for a medial unicondylar arthroplasty. The height t.sub.p is lower than the height t.sub.a: that is to say the bearing has a thicker dimension at its anterior end than at its posterior end. As discussed in relation to Tables 1 to 5, the entrapment at its posterior end ranges from about 4 mm for the extra large bearing range to 3 mm in the extra small size range.

(35) We have discovered that there may be advantages in having the entrapment proportionally larger in smaller patients and proportionally smaller in larger patients.

(36) It will be appreciated that smaller people need smaller prosthetic components, including smaller bearings. It is well known to have several sizes of femoral, tibial and bearing components. Each patient is adjudged by a medical practitioner (possibly the surgeon in a pre-operative review) to be extra small, small, medium, large, or extra large. At the time of surgery the surgeon is provided with a kit of trial components that are extra small, small, medium, large, or extra large. In each kit there is a range of different sizes of trial tibial components, a range of different sizes of trial femoral components and a range of different sizes of trial meniscal bearings.

(37) It will be appreciated that the surgeon is in the operating theatre with the patient and, e.g., 6 or 7 trial tibial components, 6 or 7 trial femoral components, and 6 or 7 trial sets of meniscal bearings. As he selects/tries out the trial components he selects the correct size to be used. The implantable prosthetic components corresponding to the trial size are then ordered and delivered, typically from a store outside of the operating theatre, and the implantable components are fitted into place.

(38) The surgeon may first remove a slice of bone from the medial tibial plateau, exposing a plane surface. He selects the correct size of tibial template, a plate with flat upper and lower surfaces, which best fits the exposed surface. He removes a sliver of bone from the posterior femoral condyle. He confirms that the femoral and tibial components are correctly placed so that the minimum gap between the components in extension is the same as that in flexion. The surgeon then has to select the correct thickness of bearing.

(39) After the surgeon has fitted the femoral component (having first tried selected trial components and having selected the size that best suits the patient after femoral bone has been removed), and after fitting the tibial component (having first tried selected trial components to find the size of tibial component that best suits the patient after tibial bone has been removed), the surgeon has to select the correct thickness of bearing.

(40) The surgeon has a range of, for example, large left leg medial trial meniscal bearings to choose from, each with an associated cylindrical gap gauge. He inserts a gap gauge into the gap between the tibial and femoral components and flexes and extends the knee. He progressively removes bone from the damaged distal femoral condyle until the minimum gap between the components in extension is the same as that in flexion. If the gap gauge seems too loose he takes it out and inserts a thicker gap gauge. He confirms his choice of bearing thickness by inserting the corresponding trial bearing. Traditionally, surgeons are afraid of having the joint too loose and so they tend to choose the thickest size bearing they can force in. This can result in overstuffing.

(41) We have realised that in the prior art, the lower/thinner posterior end of the bearing also means that the entrapment to the rear of the bearing is lower than the entrapment to the front of the bearing (see FIG. 2d).

(42) In our invention, the entrapment at the rear may be larger than in the prior art for small and extra small bearings, and smaller than in the prior art for large and extra large bearings. The entrapment at the rear may be around 3.5 mm for all bearing sizes in our invention.

(43) By entrapment, we may mean the difference in thickness between the lowermost portion of the dished surface of the bearing and whichever is the thinnest/shallowest of the anterior or posterior end regions of the bearing if they have different thicknesses.

(44) The anterior-posterior length of the bearing varies between sizes of bearing (e.g. between extra small, small, medium, large and extra large).

(45) For a part spherical bearing surface, FIG. 4 illustrates that the entrapment is related to the radius of curvature R of the part spherical surface and the horizontal distance x.sub.p from the posterior extreme of the curved surface to the lower extreme of the curved surface.

(46) The length of the bearing L is determined by the size of the patient's bone structureit should not be too long. The distance s.sub.a and s.sub.p from the anterior and posterior extremities of the bearing respectively to the start of the part-spherical bearing surface are shown in FIG. 4. In some embodiments s.sub.a=s.sub.p, but in others it does not.

(47) As shown in FIG. 4, by Pythagoras' Theorem,
e=R??{square root over (R.sup.2?x.sub.p.sup.2)}
or expressed another way:
x.sub.p=?{square root over (2Re?e.sup.2)}
where e=the entrapment, R=the radius of curvature, and x the length of the chord from the vertical centre line to the posterior of the curved bearing surface.

(48) If s.sub.a=s.sub.p=about 4 mm, e=3.5 mm and the anterior-posterior bearings are as in Tables 1 to 5 of the prior art, then this gives:

(49) TABLE-US-00006 Size of Bearing R x.sub.p Extra large 27.92 13.53 Large 26.08 13.05 Medium 24.25 12.55 Small 22.35 12.01 Extra small 20.7 11.5

(50) We have appreciated that for some of our embodiments, instead of, as in the prior art, having the posterior end of the bearing less thick than the anterior endmaking it easier to push in, we should have the posterior end not substantially thinner, or no thinner, than the anterior end (and in some embodiments the same thickness).

(51) This will, for the same anterior thickness, make the bearing harder to push in. This reduces the chance of overstuffingany given level of force to push the test bearing results in the choice of a final bearing that is a little thinner at the anterior end than was previously the case, and so for the same sized (length anterior-posterior and radius of curvature R) bearing we have a similar entrapment, and, compared to the prior art, a thinner (as measured at the depth of the spherical socket) bearing, but a thicker bearing, as measured at the posterior end, that is harder to push in.

(52) In some embodiments, our bearing is symmetric about a coronal plane and also about a sagittal central plane. This means that our bearing is no longer handedwe no longer need a left knee medial bearing and a different, handed, right knee medial bearing. Similarly, our test bearings need then not be handedthey could be symmetric. This can reduce parts inventory. They can also be inserted either way around. They do not have different anterior and posterior profiles and heights: they are the same.

(53) In another embodiment, our bearing is implanted by holding it against the anterior surface of the femoral component and sliding it round into the gap between the femoral and tibial components. This may require smaller distraction of the femoral component and a smaller force required for distraction for a given entrapment. This is true for a smaller measurement of s.sub.p from the posterior extremity of the bearing to the start of the part-spherical bearing surface.

(54) FIG. 5 shows a unicondylar knee replacement including a bearing with negligible values of s.sub.a and s.sub.p.

(55) The vertical thickness of the posterior extremity of the bearing t.sub.p is given by
t.sub.p=t.sub.min+R??{square root over (R.sup.2?x.sub.p.sup.2)}

(56) R is the external radius of the femoral component and the radius of the concavity of the upper surface of the bearing and x.sub.p is the posterior half length of the concavity of the upper surface of the bearing.

(57) t.sub.min is minimum thickness of the meniscal bearing. For a given t.sub.min+R, t.sub.p can be increased by increasing x.sub.p The maximum radial thickness of the posterior end of the bearing t.sub.r is given by
t.sub.r=?{square root over ((R+t.sub.min).sup.2+x.sub.p.sup.2)}?R

(58) For negligible values of s.sub.a, the vertical thickness of the posterior extremity t.sub.p will always be larger than the maximum radial thickness t.sub.r. Therefore less distraction and force will be required when implanting the bearing along the femoral component, as shown in FIG. 6b.

(59) The distraction d.sub.f required for implantation of the bearing along the femoral component, as shown in FIG. 6b, is as follows:
d.sub.f=t.sub.rt.sub.min=?{square root over ((R+t.sub.min).sup.2+x.sub.p.sup.2)}?R?t.sub.min

(60) From this, it can be deduced that, as the minimum thickness of the bearing t.sub.min is increased, the distraction, and therefore the force, required for implantation along the femur decreases.

(61) In some embodiments, the distraction and force required to insert the bearing horizontally along the tibia is equal to the distraction required to insert the bearing along the femur. Using the geometry in FIG. 7, it can be shown that the vertical thickness of the posterior extremity will be equal to the radial thickness when s.sub.p is calculated as follows:

(62) $\mathrm{For}{t}_p=t_r,s_p=\sqrt{{\left(t_{\min }+2R-\sqrt{R^2-x_p^2}\right)}^2-{\left(t_{\min }+R\right)}^2}-x_p$

(63) TABLE-US-00007 Size of Bearing x.sub.p R s.sub.p Extra large 15.5 32 1.9 Large 14 26.5 2.1 Medium 13 23 2.3 Small 11.5 18.5 2.5 Extra small 10.5 16 2.6

(64) In some embodiments, the distraction and force required to insert the bearing horizontally along the tibia are larger than the distraction and force required to insert the bearing along the femur. This will be the case when s.sub.p is larger than the corresponding value given above.

(65) In some embodiments, the distraction and force required to insert the bearing horizontally along the tibia is larger than the force and distraction required to insert the bearing along the femur. This will be the case when s.sub.p is smaller than the corresponding value given above.

(66) In order to increase the posterior half-length of the bearing while not significantly increasing the minimum posterior radial thickness of the bearing, material can be added to the posterior vertical surface in a semi-cylindrical shape, as shown in FIG. 8, to form a protrusion.