Constrained medial pivot arthroplasty

Abstract

A knee arthroplasty comprising an internal face 7 of the medial femoral condyle formed as a segment of a sphere and having a lesser radius than the medial femoral condylar articular surface 6, but wit

Description

Background Primary Total Knee Arthroplasty (TKA) aims to provide a joint replacement that moves freely through it's range of movement, with adequate stability provided by the patient's soft tissue envelope (joint capsule and ligaments) moving in concert with the surface geometry of the arthroplasty components. A conventional condylar resurfacing design accommodates flexion-extension 10; axial rotation 11; and limited translation of a few mm in the anteroposterior (A / P) and medial-lateral (M / L) directions. No constraint is provided against varus-valgus moments in a conventional primary knee arthroplasty: stability to varus and valgus forces is provided by an intact soft tissue envelope. Medially-stable, or so-called 'medial pivot' TKA is a variant arthroplasty that locates the longitudinal axis of rotation 11 in the medial compartment. See Fig 1. There is a substantial body of expert opinion that agrees that this allows a close-to-physiological movement in the replaced joint. This is achieved by forming the medial compartment in a 'ball-in-socket' configuration 6,13 that resembles a hip arthroplasty; and keeping the lateral compartment relatively unconstrained. In deep flexion, the lateral femoral condyle 8 translates posteriorly with respect to the tibial insert surface 14, while the part-spherical medial femoral condyle does not translate: it's centre of rotation 5 remains co-located with the geometrical radial centre of the part-spherical depression in the tibial component. Flexion-extension 10 and axial rotation around the long axis 11 are permitted by this arrangement; but sudden slips or translations are prevented, which increases the degree of stability experienced by the patient. Again, no constraint is provided against varus-valgus moments by the medial-pivot TKA. Figure 1 illustrates a medial pivot TKA that has been implanted into a soft-tissue envelope with a deficient, or stretched, medial collateral ligament (MCL) 3: a valgus moment is simulated, and the medial compartment is opening such that the articular surfaces are separating. This is an undesirable situation, that is likely to result in an unstable knee reconstruction. Constrained knee arthroplasties are utilised in situations where the soft tissue envelope is deficient: commonly for valgus knees with deficient MCL 3; and less commonly for severe varus knees with deficient lateral collateral ligament (LCL) 4; and rarely for cases where there has been significant soft tissue destruction, such as multi-ligament injuries, and tumour cases. Constrained knee arthroplasties are also commonly used in the revision scenario, where the soft tissue envelope is deficient, and bone loss has compromised ligament origin and insertion anchors: Hinged knee: The femoral and tibial components are connected by a hinge that allows flexion / extension. Often a 'sloppy hinge' is utilised, with sufficient laxity to allow a few degrees of axial rotation. Alternatively, axial rotation may be permitted by allowing the tibial insert to rotate on the tibial tray. The hinge provides constraint against varus and valgus moments. (Not illustrated). Central post: The femoral and tibial components interlock with a post, similar in nature to a standard 'posterior-stabilised' or 'PCL-sacrificing' design; but the post is more substantial, with vertical sides, and fits with sufficient congruence in the space between the femoral condyles to impart constraint against varus and valgus moments. Minimal axial rotation is possible at this articulation, but may be permitted by allowing the tibial insert to rotate on the tibial tray. (Not illustrated). Field of invention Total knee replacement. Description A constrained medial pivot knee arthroplasty is described, that provides resistance to varus and valgus moments, while retaining the kinematics (movement) of the conventional medial pivot arthroplasty. See Figure 2. The internal face of the medial femoral condyle 7 is formed as a segment of a sphere, of lesser radius than the medial condylar articular surface 6, but with concentric geometric centre 5. See Figure 3. This internal face 7 forms a bearing surface with the central post 15, the bearing surface being a fully congruent (closely shape matched) articulation in the form of a segment of a sphere, between the internal face 7 and the post medial surface concavity 16. It is emphasised that the differential radii of these two bearing surfaces 7 and 6 are concentric with the medial sphere geometric centre 5. In this way, flexion-extension around the axis 10 and axial rotation around the axis 11 are both permitted, and mechanical conflict with the smooth functioning of the medial-pivot femoro-tibial bearing is avoided. The concentricity of these two bearing surface pairs 7-16 and 6-13 is the core feature of this paper. The reduced radius of the medial condylar internal face 7 creates space in the intercondylar notch for the post 15. See Figures 3 and 4. The internal face of the lateral femoral condyle 9 is formed as a segment of a sphere, and forms a bearing surface with the central post 15, the articulation being in the form of the part-spherical internal face 9 moving around the curve of the post lateral surface track 17. This movement along the curve of the track 17 permits axial rotation around the longitudinal axis 11; while the part-spherical internal face 9 permits flexion-extension of the knee around the axis 10, at any point along the track 17. Compare Figure 3 to Figure 1. The present invention provides resistance to valgus and varus moments, while still allowing flexion-extension and axial rotation, by incorporating a post 15 into the established medially-stable design. The close-to-physiological kinematics of the medially-stable knee design are retained. In cases where the soft tissue envelope is deficient - very commonly the scenario is a stretched or attenuated MCL - the required stability in response to varus and valgus forces on the reconstructed limb is provided. Essential features The femoral component is provided with an internal face 7 to the medial condyle that is part-spherical, of radius less than that of the medial condyle articular surface 6, the geometric centres of these surfaces 7,6 being concentric. The femoral component is provided with an internal face 9 to the lateral condyle that is part-spherical. The geometric centre of this face 9 lies on the flexion-extension axis 10. A tibial post 15 is provided; having a medial surface concavity 16 that forms a segment of a sphere and is congruent with the internal face 7; and having a lateral surface track 17 that curves around the longitudinal axial rotation axis 11, and which articulates with the internal face 9. The tibial post 15 may be of composite construction (not shown), with central metal reinforcement sandwiched by material that is suitable for use as a bearing material, such as UHMWPE. The tibial post 15 may be modular, such that the surfaces 16,17 and a central reinforcement (not shown) may be assembled in-situ. Figure List Fig 1 Prior art. Coronal c / s view through the sphere geometric centre of a medially-stable knee arthroplasty. The medial collateral ligament is deficient: there is medial opening in response to valgus moment on the limb Fig 2 Present invention. Coronal c / s view through the medial sphere geometric centre, femoral component only, showing the concentric radii of the medial condylar articular surface and the medial condylar internal face; the respective concentric spheres that these surfaces lie on being illustrated by dotted lines. Note the lateral condylar internal face which is also comprised of a segment of a sphere, the geometric centre of which lies on the flexion-extension axis Fig 3 Present invention. Coronal c / s view through the medial sphere geometric centre, showing the articulation between the bearing surfaces Fig 4 Present invention. Axial c / s view through the sphere geometric centre, showing the articulation between the central post and the medial and lateral condylar internal faces Figure Key 1 Femur 2 Tibia 3 Medial collateral ligament (MCL) 4 Lateral collateral ligament (LCL) 5 Medial sphere geometric centre 6 Medial condylar articular surface 7 Medial condylar internal face 8 Lateral condylar articular surface 9 Lateral condylar internal face 10 Flexion-extension axis 11 Longitudinal axial rotation axis 12 Tibial insert 13 Insert medial articular surface 14 Insert lateral articular surface 15 Post 16 Post medial surface concavity 17 Post lateral surface curved track

Claims

Constrained medial pivot arthroplastyClaim 1 A constrained medially-stable knee arthroplasty is provided, comprising: convex internal surface of the medial femoral condyle formed as a segment of a sphere and facing substantially laterally, with radius of lesser value than the radius of the medial condylar femoro-tibial bearing surface, these two bearing surfaces having a concentric geometric centreClaim 2 A knee arthroplasty as claimed in Claim 1, where the convex internal surface of the medial femoral condyle articulates with a part-spherical concavity formed into a tibial post, the post being contiguous or connected to the tibial bearing surface componentClaim 3 A knee arthroplasty as claimed in Claim 1 and Claim 2, where a convex internal surface of the lateral femoral condyle is provided, formed as a segment of a sphere and facing substantially medially, with radial geometric centre lying on the flexion-extension axis of the femoral condylesClaim 4 A knee arthroplasty as claimed in any preceding Claim, where the convex internal surface of the lateral femoral condyle articulates with a curved track formed into the tibial post, the long axis of the curved track being co-axial with the axial long axis of rotation of the knee, these axes passing through the geometric centre of the part-spherical medial femoral condyle articular surface and the part-spherical surface of the convex internal surface of the medial femoral condyleClaim 5 A knee arthroplasty as described in the accompanying Description and Figures

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