allowed for a high exion angle. However Freeman, working with others including Tuke and Blaha, determined that the posterior displacement occurred primarily on the lateral side. On the medial side, while there were small anterior-posterior dis­placements at the extremes of exion, in the major arc of exion, the knee acted almost as a “medial pivot.” This was consistent with the completely different shape between the lateral and medial bearing surfaces, as well as the shapes and mobility of the menisci. As a result of this work, and later studies showing a similar result, several “medial pivot” knees were designed, with limited AP laxity on the medial side. Fluoroscopic studies have shown that indeed these designs do pivot on the medial side during exion, although generally, the degrees of rotation and the AP displacement on the lateral side have been smaller than in the normal intact knee.

axial forces. The PS showed higher AP sliding than the ultra-congruent. This was due to higher femoral-tibial conformity of the latter design. Also, in the PS, the added AP constraint of the PS only acts in high exion. In another study [39], the AP motions were compared between UC and MC designs, for a sit to stand activity. The implants had the same CR type femoral components, and tibial baseplate, but the inserts were different. The mean medial AP displacements were 2–3mm, for the lateral side, both 8mm. This equality in displacements was surprising because of the asymmetric MC conformity and the symmetric UC conformity. This indicated that the motions were more affected by the activity itself rather than the insert con­formities. In a recent review of medial pivot and medially congruent designs, Hodgeson etal. [42] concluded that “Medial pivot total knee arthroplasty implant designs (as well as medially congruent) function similar to that of the native knee with a relatively xed medial center of rotation and a less conforming lateral com­partment that follows an arcuate path.” Nonetheless, other uoroscopic studies have shown less axial rotation values than in the normal knee, even though the overall displacement and rotation patterns have been similar.

The concept is that a computer model of the pre-arthritic condition of the patient’s knee is generated based on a preoperative CT scan. The pre-diseased “normal” liga­ment length patterns are generated. In the computer, osteophytes are removed, and the total knee components are inserted using the robotic surgical technique (Fig.3.13). The positions and sizes of the components are manipulated in computer software, to nd the closest match to the normal ligament lengths. The surgery is then conducted robotically using the optimized data. This innovative approach has the advantage of being “personalised” to the individual patient, as well as being “optimised.” However one limitation may be in the design of the components them­selves; using present design congurations, how closely can normal kinematics actually be produced.