After five years experience with the Lubinus patello-femoral arthroplasty it was evident that a modern design of arthroplasty was required to improve tracking, stability and wear problems experienced with this prosthesis. (Ref. 4) In 1994 the design criteria for a new surface replacement patellofemoral arthroplasty were laid down. The articulation was based on the Kinemax Plus total knee replacement designed by Professor Peter Walker (Ref 1), which has excellent patellofemoral kinematics and follow-up studies have shown a very low rate of patellofemoral complications (<0.3%). (Ref. 2-3). The trochlear axis is in line with the mechanical axis of the leg not the anatomical axis of the shaft of the femur. Thus an anatomical left and right prosthesis is not required.

Design criteria : 1994

• Surface replacement (minimal bone resection)
• External rotation of femoral component
• Broad trochlea surface, unconstrained in extension
• Patella captured and stable in flexion
• Congruous articulation throughout range
• Improved patello-femoral tracking in the mechanical axis
• Dome patella with 3mm medial offset and medial odd facet
• 4 component sizes

CAD CAM  technology was used to design a femoral trochlear based on the above characteristics and several prototype models were produced in plastic. In full extension and up to 20° of flexion the patella is unconstrained in its groove allowing slight medial lateral motion.

As the knee is flexed so the patella is captured by the groove and in the mid range there is close conformity and stability of the articulation to improve resistance to wear. The dome of the patella is offset by 3mm medially to resemble the anatomical patella.

After 100° the central part of the patella completes its contact with the centre of the trochlea and the medial and lateral facets begin to bear on the articular surface of the medial and lateral femoral condyles.

As flexion continues the patella internally rotates and the odd facet comes to articulate with the lateral part of the medial femoral condyle. The medial edge of the patella replacement is rounded off to present a smooth surface similar to the odd facet to reduce the risks of wear and impingement.

Several simple instruments were designed to create the bone cuts and template the prosthesis. Dry bone workshop studies were followed by three cadaver trials and the best fit for a medium sized prosthesis was chosen. Clinical trials commenced in September 1996 and 36 cases were performed. All cases were successful with excellent tracking and patella stability. Large and small prostheses were then developed with a set of instruments to allow easy insertion.

A definitive prospective trial was commenced in September 1997 with a full range of three prostheses and a full set of instrumentation. An extra medullary alignment tower was introduced later to facilitate alignment of the anterior cutting jig.

Biomechanical studies have been performed at Imperial College, London on the stability of the natural patella compared to that of the Avon patellofemoral arthroplasty. These showed that the pattern of stability with the arthroplasty was very similar to that in the natural joint. In full extension the joint was relatively unconstrained requiring less force for a specific degree of displacement. As flexion of the joint occurred so the patella became more stable requiring a larger force for displacement, this followed almost exactly the pattern noted in the natural joint.

Further kinematic studies were performed on a group of patients with over two years follow up. Step-up tests under radiographic control allowed measurement of the angle between the patella tendon and the tibia. This is know as the patella tendon angle and follows a constant pattern of movement throughout the range of flexion. Comparison with the AGC total knee replacement and normal knees showed that the patella tendon angle in the Avon patella cases followed almost exactly the pattern that occurs in the normal knee and was significantly different to that occurring after total knee replacement with absence of the anterior cruciate ligament. (Ref. 5) . The Avon cases were However slightly eleveted compared to the normal suggesting increaded tension due to insufficient depth of the Trochlear groove.

References

1. Walker PS. Design of Kinemax total knee replacement bearing surfaces. Acta Orthop Belgica 1991; 57 suppl.II: 108-113.
2. Harwin SF. Patellofemoral complications in symmetrical total knee arthroplasty. J Arthroplasty 1998; 13: 153-162.
3. Black DL, Cannon SR, Hilton A, Bankes MJK, Briggs TWR. The Kinemax total knee arthroplasty. J Bone Joint Surg (Br) 2001; 83-B: 359-363.
4. Tauro B, Ackroyd CE, Newman JH, Shah NA. The Lubinus patellofemoral arthroplasty. (J Bone Joint Surg (Br) 2001; 83-B: 696-701.
5. Hollinghurst D, Stoney J, Ward T, Pandit H, Beard DJ, Murray DW. In Vivo sagittal plane kinematics of the Avon patellofemoral arthroplasty. J Arthroplasty 2002; 22; 117-23.