The use of jig instrumentation has significantly improved the reproducibility of total knee replacement (TKR). The establishment of mechanical axis, femoral rotation and joint line can be considerably enhanced by jig instrumentation. Soft tissue balance is less amenable to the jig solution. Medial compartmental osteoarthritis can be considered extension gap disease; there is no defect in the flexion gap. Although the radiographic studies demonstrate loss of joint space, the biologic defect includes laxity of the ACL and MCL with a lateral tibial thrust in the extension gap with no ligament imbalance in the flexion gap. It is critical to rebalance the ligaments to restore knee function.

Statistical studies have demonstrated 8–10 year 90% survivability with unicompartmental arthroplasty; however, statistics beyond 10 years have been disappointing. It can be assumed a significant number of uni patients will come to revision. It is therefore necessary to preserve bone for future TKR. The major difficulty in converting a uni to a TKR is loss of medial tibial buttress. Preserving a rim of medial tibial bone, the medial tibial buttress, by inlay technique simplifies conversion to TKR. It is also necessary to preserve adequate sclerotic bone to support the prosthetic system.

The major advantages of jigs when performing TKR, establishing mechanical axis, femoral rotation and joint line do not apply to uni technology, while the major requirements for a successful uni, medial tibial buttress preservation, sclerotic bone preservation and soft tissue balance are not improved by the use of jigs.

Whereas jigs may have limitations, the procedure is not without guides. The usual articular surface defect in the extension gap is 6–8 mm while there is no defect in the flexion gap. The prosthetic components are approximately 10 mm thick. The bur utilised is 5.5 mm in diameter, a useful reference point. Various sized femoral and tibial guides are utilised. Ligament tension can be palpated, while a thin shim can be used to check ligament tension in the flexion and extension gap.

The various anatomical reference points, guides, their advantages and disadvantages will be discussed. With due regard to the limitations of guide and jig instrumentation, consistent reproducibility requires a reasonable degree of surgeon experience with the procedure.