The final alignment of Patient Specific Instrumentation (PSI) TKA relies on the accuracy and the correct placement of the 3-D moulds, precision of saw cuts, soft tissue balancing and cementing technique. We aimed to compare the predicted alignment between PSI and Articulated Surface Mounted (ASM) computer navigation. Eight consecutive patients underwent knee replacement using MRI based PSI (Zimmer) with planning of 0º femoral and tibial mechanical alignment. After placing the conventional cutting blocks over the pins (placed according to PSI), the predicted alignment of cuts was verified with ASM navigation. PSI technique was used regardless of navigation values and alignment was compared.

Good correlation was found for tibial cuts (maximum variation: coronal plane – 1º, sagittal plane – 2º) and femoral cuts in the sagittal plane (maximum variation 2º). However, in two patients the coronal plane variation in femur was > 2º (3º and 4º respectively). Navigation predicted combined final alignment of 5º valgus and 4º valgus in these patients. However, long leg standing x-rays revealed neutral and 1º valgus alignment in those two patients respectively, matching closely with PSI prediction. Final alignment in long-leg standing x-rays were independently reported by a musculoskeletal radiologist. Six knees were in neutral mechanical alignment (including the 2 navigation predicted outliers). The remaining 2 knees had a maximum deviation of 2 degrees from neutral. We conclude that there was overall good correlation between PSI and navigation. Even in cases where navigation predicted more than 3º combined varus/valgus alignment, PSI prediction was more accurate on long leg views.