GB2586871A - Tibial component of knee replacement - Google Patents

Abstract

A tibial component of a knee replacement has a contoured under-surface providing one or more convexities 4 which follow a top bearing surface having at least one concavity 1, 2 such that the thickness of the component is substantially constant across the region of the concavity. The convexity/convexities resist shear forces/stress along an interface between the component and bone. The projections may be surrounded by a flat surface forming a flange and either the flange surface or the contoured under-surface may be provided with lugs/pegs 5 which facilitate a stable interface between the component and the tibial plateau 9 of a patient.

Description

TIBIAL COMPONENT OF KNEE REPLACEMENT

Background

Knee replacements comprise a femoral component that forms a bearing with a tibial component. See Figures land 2. The tibial component may be of a monobloc-type, typically formed of high molecular weight polyethylene, or of a composite-type with the polyethylene bearing surface insert attached to a baseplate. Failure of the tibial component is a much more common occurrence than failure of the femoral component, and leads to a need for expensive and high-risk revision surgery. Knee replacement can be total (tricompartment), uni-compartment, or bi-compartment.

The tibial component typically fails by loosening its attachment to the bone of the tibial plateau. Conventionally, the baseplate is substantially flat, usually incorporating shallow pockets to improve the component-cement bond in the cemented type. In uncemented types the undersurface of the tray is also substantially flat. There is usually a central keel projection, and/or other pegs or lugs to augment fixation. Preparation of the bone surface to receive the tibial component is achieved by preparing a flat cut surface using a saw, mill, or burr. A drill and punch may then be used to form the cavity which receives the central keel.

See Figure 2. Shear forces are generated along this substantially flat interface during daily activities. In particular, strong deceleration with the knee flexed, as is encountered when descending steep stairs or slopes at pace, generates a considerable shear forces tangential to the bearing surfaces, and these shear forces resolve into corresponding shear forces (see arrows in Fig 2) along the interface between the tibial component and the bone (with a thin cement layer interposed in the case of cemented knee replacements which is subjected to this shear force). Shear forces are conventially countered by anchoring the tibial component with a substantial central keel. The shear generates a moment couple acting on the keel, due to the spatial separation between the shear force vector and the keel. This moment couple acts to toggle and loosen the tibial component from the bone. In many series, about 2-3% of tibial baseplates have loosened after an interval of 10 years after implantation. This usually leads to expensive and technically-demanding revision surgery.

A second mode of failure is breaking up of the polyethylene material block. The thickness of the polyethylene is a critical consideration: on the one hand if the minimum thickness is too thin (4mm is generally regarded as a minimum) then the polyethylene may fail; on the other hand if the polyethylene is increased to 20 or 25mm then this implies that a considerable amount of tibial plateau bone has been resected, and increases the moment couple of the bearing surface shear forces acting via the component / bone interface on to the keel. The precise polyethylene thickness is determined by the operating surgeon, balancing knee stability against movement. The tibial component may be a monobloc-type, attached with bone cement. The tibial component may a composite-type, attached either by bone cement or by uncemented fixation (bone in-growth or bone on-growth). The shear forces generated at the bearing surface that act to toggle and loosen the tibial component are the same for cemented and uncemented, monobloc and composite types.

A total knee tibial component bearing surface has two concavities, which form the knee joint by acting against the two convexities of the femoral condyles. The degree of congruency (degree of matching of the radii of surface curvatures of the femoral and tibial bearing surfaces) can vary between the medial and lateral compartments, and also widely varies between different designs. However, this general configuration of convex femoral bearing surfaces and concave tibial tibial bearing surfaces is universal. As a consequence, a minimum bearing material thickness occurs at the deepest point of the concavities, where the bearing surface material is under maximum load (see Figures land 2). In recent years it has been shown in multiple studies that the medial compartment takes greater load than the lateral compartment. The exact ratio varies according to the limb alignment, and varies between studies. However, it is reasonable to assume that in a typical knee 55-75% of the load passes through the medial compartment. In consequence, the maximum load on the insert material occurs at the deepest point of the medial concavity.

A femoro-tibial uni-compartment replacement can be implanted in either, or both, of the medial and lateral compartments. The general configuration is of the convex femoral component articulating with the concave tibial component. As in total knee designs, the tibial component can be monobloc or composite, cemented or uncemented.

In both total knee and uni-compartmental variants, the undersurface of the tibial component comprises a flat surface with projecting keel and/or lugs which act to improve cemented or uncemented fixation to the bone.

To summarise there is an inherent tension when considering the design of the tibial component: increased polyethylene thickness protects against material failure of the polyethylene bearing surface block, but increasing polyethylene thickness also increases the moment couple of shear forces at the bearing surface which act to loosen the tibial component.

Field of Invention

Tibial component of knee replacement.

Description

See Figures 3 and 4. A knee replacement tibial component is provided with an undersurface convex projection which is contoured to substantially follow the concavity of the bearing surface. The effect is to provide a more substantially constant thickness of material (typically high molecular weight polyethylene) across the region of the bearing surface concavity. The undersurface convex projection is substantially in the form of a segment of a sphere or ovoid (see Figures 5 and 6).

This contoured undersurface in the form of a convex projection surrounded by a flat flange maintains the polyethylene thickness in the zone of maximal load on the material, at the deepest part of the concavity of the medial compartment bearing surface. This provides flexibility to the detailed design of the component: for example the component may be formed of polyethylene of the absolute minimum thickness that the material properties allow (generally regarded as about 4mm), maintaining this thickness in the zone of maximum load, while minimising unnecessary bone resection across the remainder of the tibial plateau (Figure 3).

The convexity in the undersurface is well-situated in order to resist shear forces generated local to this area, at the bearing surface of the medial compartment. The form of the convexity acts to resist shear (Figure 4). Fixation of the convexity is achieved by pegs or lugs, utilising the well-proven methods of cemented or uncemented fixation. Fixation of the flange area may be similarly augmented by pegs or lugs (Figure 7).

Another embodiment is now described. Uni-compartment knee replacement may be of the medial, lateral or patello-femoral compartments. Bi-compartment replacement may be either of the combination of the medial and lateral compartments, or of the medial and patello-femoral compartments. Either a medial or a lateral uni-compartment replacement, or both combined as a bi-compartment replacement, can also utilise the described tibial component: the contour of the undersurface substantially following the contour of the bearing surface. This may be surrounded by a flange area -necessarily small, due to the nature of uni-compartment replacement.

Returning to total knee replacement, a further embodiment is now described. Twin convexities in the tibial component are provided, one under the medial compartment bearing surface concavity, and one under the lateral compartment bearing surface concavity. Although the load through the lateral compartment is not as high, polyethylene thickness in the lateral compartment can be maintained in a similar fashion to that previously described for the medial compartment, while still minimising bone resection in the remaining, flanged area of the tibial component undersurface.

Now turning to a technical consideration of how the bone may be prepared in order to receive a tibial component with such a contoured undersurface as has been described. Conventionally a saw is used to prepare a flat cut in the appropriate plane in the tibial plateau, followed by use of further instruments such as drill and punch to form a cavity for a central keel and further lugs, guided by a trial base-plate. To properly utilise the described invention, a cavity must be accurately shaped in the bone of the tibial plateau in order to receive the convexity in the undersurface. This cavity may be shaped using well-proven current technology in the form of a spherical reamer as is used in preparation of the acetabulum during hip replacement, the reamer being guided by a circular aperture in a trial base-plate. Alternatively, robot technology may be utilised to guide a burr or reamer to form the necessary cavity in the shape of a segment of a sphere or ovoid. This robot technology is now available: robots are used to prepare the acetabulum during hip replacement using a reamer, and are used to prepare the surfaces for uni-compartment knee replacement, using a burr.

The described inventive tibial component may be formed of a single block of material, typically high molecular-weight polyethylene, cemented into the tibial plateau using methylmethacrylate bone cement. Figures 3, 4, 5 and 6 show lugs formed into the undersurface convexity which facilitate the cement bond. For clarity, the cement layer itself is not depicted in the Figures. The inventive tibial component may be of composite construction, formed in two material layers, typically high molecular-weight polyethylene forming the bearing surface and metal forming the contoured undersurface (the baseplate), the two layers either permanently bonded or reversibly attached (see Figure 7). The baseplate may utilise cemented fixation, with projecting lugs / pegs. The baseplate may utilise uncemented fixation (bone in-growth or bone on-growth), with projecting lugs / pegs.

Figures List Figure 1. Coronal plane (view from the front) section through conventional tibial component of monobloc all-polyethylene type, with keel Figure 2. Sagittal plane (view from the side) section through the medial compartment of conventional knee replacement, showing the femoral component, all-polyethylene tibial component and parts of the distal femur and proximal tibia. Arrows indicate shear forces acting at the bearing surface Figure 3. Coronal plane section through an embodiment of the current invention: a monobloc polyethylene embodiment with single undersurface convexity Figure 4. Sagittal plane section through an embodiment of the current invention: section through the medial compartment, with a monobloc polyethylene tibial component with undersurface convexity. Arrows indicate shear forces acting at the bearing surface Figure 5. Illustration showing monobloc single convexity embodiment viewed obliquely from the front and above Figure 6. Illustration showing single convexity embodiment viewed obliquely from the side and above Figure 7. Coronal plane section through an embodiment of the current invention: a composite-type tibial component comprised of a polyethylene block with baseplate, with the concavity of the bearing surface of the medial compartment substantially followed by the convexity of the baseplate Figures Key 1 Medial bearing surface concavity 2 Lateral bearing surface concavity 3 Flat undersurface 4 Contoured undersurface: convexity and surrounding flange Undersurface pegs! lugs 6 Keel 7 Femoral component 8 Femur 9 Tibial plateau Baseplate -uncemented t-med Polyethylene thickness at base of medial bearing surface concavity t-lat Polyethylene thickness at base of lateral bearing surface concavity -> Shear forces at bearing surface

Claims (1)

1. CLAIMS1. Tibial component of knee replacement with a contoured undersurface that provides one or two convexities that substantially follow one or both of the concavities of the bearing surface, causing the thickness of the component to be substantially constant across the region of the bearing surface concavity 2. Tibial component of uni-compartment knee replacement with a contoured undersurface that provides a convexity that substantially follows the bearing surface concavity, causing the thickness of the component to be substantially constant across the region of the bearing surface concavity 3. Tibial component as claimed in Claim 1 or Claim 2 where the convex projection(s) of the undersurface are surrounded by a substantially flat surface that acts as a flange around the convex projection(s) 4. Tibial component as claimed in Claims 1-3 where the convex projection(s) form a segment of a sphere or ovoid into the contour of the component undersurface 5. Tibial component as claimed in Claim 1, where the two convexities can be of equal or unequal dimensions 6. Tibial component as claimed in Claim 1 or Claim 2 where the convex projection(s) has multiple pegs or lugs to facilitate the stable interface with bone cement 7. Tibial component as claimed in Claim 3 where the flange surface has multiple pegs or lugs to facilitate the stable interface with bone cement 8. Tibial component as claimed in Claims 1-3 where the contoured undersurface has a surface treatment that supports bone in-growth or on-growth 9. Tibial component as claimed in Claim 1 or Claim 2 where the component is formed of a single block of material, typically high molecular-weight polyethylene 10. Tibial component as claimed in Claim 1 or Claim 2 where the component is formed in two material layers, typically high molecular-weight polyethylene forming the bearing surface and metal forming the contoured undersurface, the two layers either permanently bonded to each other or reversibly attached 11. Tibial component substantially as described in the preceding Claims, the accompanying Description, and the accompanying Figures