Accuracy and early outcomes in medial unicompartmental knee arthroplasty performed using patient specific instrumentation.

Unicompartmental knee arthroplasty (UKA) is a technically demanding procedure and poor implant positioning has been identified as a factor in early failure. The aim of this study was to evaluate the accuracy and clinical outcomes of the patient specific instrumentation implementation technique with a fixed bearing UKA. We carried out a prospective study of 41 patients (44 procedures) between December 2011 and April 2013. The preoperative planned sizes of implants used were accurate to within one implant size change in 96% of cases. The mean post-operative limb alignment was 3.8° varus. The Oxford Knee Scores (OKS) (0-48) improved from a mean preoperative score of 23.8 to 35.6 at six weeks and 44.5 at one year. The mean improvement in OKS from preoperative to one year was 20.7. The mean one year FJS (0-100) was 80.6. At a mean follow-up of 24 months there were no complications identified and there was a 100% survivorship. This technique may offer a particular advantage to surgeons who perform lower volumes of UKA with the potential to improve both clinical outcomes and implant survivorship in UKA to achieve greater consistency of results.

The kinematics and stability of single-radius versus multi-radius femoral components related to mid-range instability after TKA.

There continues to be some dissatisfaction with the function of total knee arthroplasties (TKA). "Mid-range instability" has been linked to multi-radius femoral components allowing transient ligament slackness and instability during knee flexion. Single-radius designs have been introduced to avoid this. We compared the kinematics and stability of eight natural knees versus multi-radius and single-radius TKAs in vitro. The loading conditions imposed across the range of active knee extension were anterior-posterior drawer forces, internal-external rotation torques, and varus-valgus moments. Significant differences were not found between the biomechanical behavior of the two TKAs. Both were significantly different from the natural knee in allowing greater anterior drawer laxity near extension, probably caused by excision of the anterior cruciate ligament, but no difference occurred beyond 30° flexion. No differences were found for any of the other degrees-of-freedom of movement. A geometric analysis suggested that the multi-radius design may tense the MCL more than the single-radius in mid-flexion, contrary to expectation. These kinematic and stability tests did not find mid-range instability of the knees, and so they could not demonstrate enhanced mid-range stability of the single-radius TKA over the older multi-radius implant. This suggests that mid-range instability may relate to unrecognized ligament laxity during surgery, rather than being inherent to a specific feature of implant design.