Evaluating the target of mechanical alignment with conventional instrumentation: establishing modern landmarks?

In the current era of evolving alignment philosophies and emerging technologies for total knee arthroplasty (TKA), established radiographic alignment parameters and anatomical landmarks can be used to evaluate the accuracy of conventional instrumentation in achieving mechanical alignment (MA).

The primary aim of the study was to evaluate the accuracy of conventional instrumentation in achieving MA in TKA. The secondary study hypothesis was that patients classified as MA outliers would have inferior clinical outcomes.

A single center prospective study was performed including 100 patients undergoing TKA. The primary outcome was the accuracy of the surgeon in achieving the target values after surgery for hip-knee-ankle angle (HKA), lateral distal femoral angle (LDFA), medial proximal tibial angle (MPTA) and joint line obliquity (JLO). Additionally, patients post-operative knee coronal deformity was evaluated. All patients underwent TKA using conventional instrumentation aiming for mechanical alignment: HKA 180 °, LDFA 90 °, MPTA 90 °, JLO 0 °. Long leg radiographs were performed before and three months after the surgery to measure the angular correction. Clinical outcomes were evaluated using the KOOS score at two years postoperatively.

The mean postoperative HKA was 178.2 ° ± 3.6 ° and the accuracy to hit the HKA target was 3.2 ° ± 2.4 °. The mean correction of HKA performed was 5.9 ° ± 4.2 °. The mean postoperative MPTA was 89.9 ° ± 1.9 ° and the accuracy to hit the MPTA target was 1.5 ° ± 1.2 °. The mean postoperative LDFA was 90.8 ° ± 2.7 ° and the accuracy to hit the LDFA target was 2.2 ° ± 1.8 °. The mean post-operative JLO was -0.8 ° ± 2.4 ° with variation to the target was 1.9 ° ± 1.6 °. After the surgery, 39 patients had neutral alignment and 61 patients were considered outliers to the ± 2 ° target of 178 ° - 182 °. Forty-two patients had modification of their pre-operative coronal deformity. KOOS Total Score improved from pre-operative to post-operative, the group where knee deformity was modified had lower mean KOOS Total Score even though not statistically significant.

Aiming for MA with conventional instrumentation, the accuracy of the surgeon on the HKA target was 3.2 ° ± 2.4 °, with 1.5 ° ± 1.2 ° for the proximal tibial cut and 2.2 ° ± 1.8 ° for the distal femoral cut. The accuracy and the risk of outliers must be known by the surgeon performing TKA with conventional instrumentation.

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