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The Medial Rotation Knee™ combines the design attributes of all four basic philosophies in Total Knee Replacement namely: Fixed Condylar, Mobile Bearing, Posterior Stabilised and Bicruciate Substituting. It does this in a unique way to improve implant survivorship and gives the patient confidence to the point that they forget they have an implant!
Medial Rotation Knee™
The unique geometry of the Medial Rotation Knee™ system has evolved to combine enhanced stability with natural knee kinematics. The concept is based on the medial centred rotation and lateral patella tracking as exhibited by the normal knee.
The Medial Rotation Knee™ is the first of a new generation of total knee replacements but has been in clinical use since 1994.
Rotation in flexion occurs in the normal knee and is therefore desirable in the replaced knee. For many years the literature has described rotation about the medial condyle which is now illustrated with MRI and Fluoroscopy studies.
The Medial Rotation Knee™ is able to rotate in flexion about the prominent spherical medial condyle whilst maintaining high area contact and low surface stress in the Ultra High Molecular Weight Polyethylene (UHMWPE) insert throughout the full range of motion (ROM). The medial condyle is positioned to ensure the overall contact area is optimised during rotation.
The smaller lateral condyle is cylindrical in shape and acts as an outrigger to stabilise the knee and control rotation. A gentle anterior slope of the tibial insert biases the lateral condyle to the area of full contact posteriorly. A unique ‘closing' mechanism reduces rotation as the knee approaches full extension. Like the normal knee, rotation is excluded when weight-bearing in full extension.
High contact area throughout the ROM significantly reduces wear rates. It has been shown by a number of authors that UHMWPE contact stresses are reduced as contact area increases, significantly reducing wear rates.
In the replaced hip joint, high contact area is achieved with a close-fitting ball-in-socket design which allows movement in several axes. The ball-in-socket joint is uniquely present in the Medial Rotation Knee™ to maintain low contact stresses throughout the range of motion, including second axis rotation.
On the lateral side, the fixed radius cylindrical condyle provides area contact in the biased posterior position. Full contact is maintained up to 100 degrees flexion. Beyond 100 degrees, flexion is not limited, but contact diminishes slightly. The patella component also has a high contact area due to its conforming saddle shape.
The Medial Rotation Knee™ has the same inset patella configuration as the Freeman-Samuelson Knee which has achieved exceptional long term results. Patella problems are very rare due to enhanced stability, high contact area and retained strength in the patella bone.
The patella groove on the femoral component is positioned on the lateral side of the mid-line throughout flexion and extension in order to reflect the natural position of the patella (defined by the tendon attachment on the tibia and the lateral bias of the quadriceps muscle). The deep groove has a smooth single radius providing stability whether the patella is replaced or not.
The distinctive saddle-shaped patella component provides contact area and eliminates rocking. It is designed for cemented or cementless fixation in a counter-bored hole to have the articulating surface flush with the bone surface. This ensures maximum support from the surrounding bone and patella strength is retained. The patella component has a single peg and will self-align during final reduction.
Secure locking mechanism and micromotion-attenuation
The design of the locking mechanism between the UHMWPE tibial insert and the CoCr tibial tray of the proven FS1000 knee system was enhanced during the development of the Medial Rotation Knee™ system.
The objective of this enhancement was to practically eliminate any micromotion between the components and to maximise the pull-off strength required for dis-association. It is recognised that a reliable fixed bearing knee must address micromotion and its associated backside wear of components to offer satisfactory long-term in-vivo performance.
The evolution of the locking mechanism is part of the ethos of continual product improvement and feedback from many eminent senior surgeons.
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