Principles of the 'SAIPH® Medially Stabilised' Knee
The SAIPH® Knee design is based on the principle of medial stability in the normal knee, which has been described in historical literature2 and widely in recent literature3,4,5.
In normal, healthy knees the shapes of the medial and lateral tibial condyles are different: the medial side is concave; the lateral side is convex. Stability is provided collectively by the collateral ligaments (MCL and LCL), both cruciate ligaments (ACL and PCL) and the menisci.
The shapes of the articular surfaces and the arrangement of stabilising soft tissue structures collectively provide greater stability about the medial condyle. Knee flexion is accompanied by axial rotation of the femur with respect to the tibia, which is achieved with a limited freedom for antero-posterior (AP) movement of the lateral femoral condyle relative to the tibia.
Stability throughout flexion is crucial to normal knee function: a knee with a deficient ACL or medial meniscus, for example, is likely to be unstable and may require corrective surgery. Standard TKRs require removal of the menisci, ACL and commonly the PCL, and they do not fully substitute for their functions. Those that intend to retain the PCL may not reliably do so6.
The SAIPH® Knee is different: it substitutes for all the removed structures. Inherent stability is provided throughout the full range of motion (ROM) with a medial deep-dish ball-and-socket articulation7. A semi conforming lateral articulation permits AP translation during activities that require it while limiting excessive (unnatural) movement7.
This clinical rationale describes the clinical evidence that the SAIPH® Knee provides inherent stability, a near normal tibiofemoral kinematic pattern and no restriction to the patient’s range of motion. It also describes data that links these features to a demonstrably higher rate of patient satisfaction.
Whether or not the patella is resurfaced, TKR surgery includes replacing the patellofemoral articulation. Hence, the patellofemoral joint (PFJ) design is equally important for any high-functioning TKR device. The normal trochlea is lateral to the midline8,9 and with an asymmetric patella the normal patella tracks laterally in flexion9,10. The lateralised patella also plays a role in stabilising the lateral tibiofemoral articulation.
Most standard TKR devices are restricted to a centrally located trochlea – a necessity for standard femoral condylar design11 – and the resulting patella tracking does not compare well to the normal knee12. The SAIPH® Knee, however, features a physiologically lateralised trochlea, like the MRK11,12, which exhibits a similar amount of lateral patella translation during flexion as patients without a TKR12.
With the right trochlea design, choosing not to replace the patella has not been shown to influence outcomes13. Nevertheless, the SAIPH® Knee is available with the same unique saddle-shaped patella, which can rotate to match the femur for a fully conforming interface, and has 40 years of successful clinical heritage13,14,15,16,17. The SAIPH® Knee is also available with a cemented dome-shaped patella button.
SAIPH® Clinical Rationale
SAIPH® Operative Technique
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Clinical Heritage: Success of the MRK™
The MRK™, also manufactured by MatOrtho® (previously Finsbury), is the original 1994 ‘medial ball-and-socket knee’ and remains in popular use. The clinical success of the MRK™ has strong relevance as a ‘proof of concept’ to the expected long-term outcomes for the SAIPH® Knee, which is an evolution of the original design.