Abstract
Introduction Unicompartmental knee arthroplasty has become increasingly popular in the USA. Minimally invasive techniques for implant placement has augmented the procedure and allowed for rapid rehabilitation and return to activities of daily living. Nevertheless, with new technologies come learning curves and rediscovery of the past. Complications do occur with placement of unicompartmental knee devices. Examples of proper and improper implantation techniques and the radiographic results are presented.
Methods I present the results of a review of the literature and personal experience.
Results Overcorrection deformity has been identified as a precursor to early failure. When sizing and placing the implants an attempt should be made to “leave alignment alone” and there should be relative pseudo-laxity of the joint with the new implants. Problems with varus or valgus tibial cuts are well known. Posterior slope is less understood. While it is reasonable to reproduce the natural inclination of the tibia, over-correction or under-correction leads to balance abnormalities. Improper slope may be a prelude to subsidence particularly if an inset design is employed. Coalescence of the pin tracts used to fix cutting blocks and sagittal tibial cuts (along the spine) have been identified as problematic. The small surface area and stress loads on the tibia predispose this area to fracture, particularly when these stress risers are present. Patellar impingement can lead to pain and disability. It may be avoided by appropriate sizing, slight recession of the femoral component, or a modest resection of the medial facet. The posterior cruciate ligament is at risk during resection of the proximal tibia. Injury to the ligamentous complex will lead to instability problems not manifest in the more conforming articulation of a TKA. One unavoidable problem is the relatively large tibial resection required in “small” knees. While we attempt to be minimal in the tibial bone resections, six to eight millimetres appears large in a diminutive knee. The most difficult positioning problem appears to be internal-external rotation positioning. Several implant systems utilize the tibial cut to position the femur. The tibial platform matches or links the femoral varus-valgus (correct or not) to the tibial cut. The rotation is more free-hand and is not well coordinated by the landmarks used in TKA. Edge loading will result from rotational malposition.
Conclusions The techniques for placement, the instrumentation, and the unicompartmental implant designs have evolved to the point where many of the problems encountered in the USA in the 1980’s have been alleviated. Attention to common positional and implantation errors will result in more satisfactory outcomes. While the less conforming articulation of these devices is forgiving, it may also penalize in the long term. Many of the problems encountered after unicompartmental knee arthroplasty could be avoided with the simple awareness that promotes improved surgical technique.
In relation to the conduct of this study, one or more of the authors has received, or is likely to receive direct material benefits.
The abstracts were prepared by Mr Jerzy Sikorski. Correspondence should be addressed to him at the Australian Orthopaedic Association, Ground Floor, William Bland Centre, 229 Macquarie Street, Sydney NSW 2000, Australia.
