The accuracy of bony resection from patient-specific guides during total knee arthroplasty.

PURPOSE: In patient-specifically instrumented (PSI) total knee arthroplasty, the correlation between the pre-operative surgical plan, accuracy of the cutting block, and intra-operative resection size is unclear. The aim of this study was to evaluate the ability to accurately execute the PSI surgical plan and to add to the merging information with respect to this technology with the hypothesis that the PSI blocks would demonstrate good accuracy with regard to the bony thickness of the resections.

METHODS: One hundred and thirty TKAs using PSI (MRI/long-leg radiographs) were retrospectively analysed. All surgeries were conducted via similar surgical approach and technique, with resection performed after guide placement and alignment assessment. The bony cut thicknesses of the medial (MTP) and lateral tibial plateau (LTP), distal medial (DM), distal lateral (DL), posterior medial (PM) and posterior lateral (PL) femur were measured with a vernier calliper. The measured resection thickness was subtracted from the planned resection. Errors were defined as ≤1.5 mm (acceptable), 1.5-2.5 mm (borderline), and >2.5 mm (outliers).

RESULTS: Overall, 81 (62.3 %) of the knees were free of outliers. The distal femur cut had the highest proportion of acceptable cut error with 209 of 260 total cuts acceptable (80.4 %). The tibial cuts had the lowest proportion of "acceptable" cuts (68.9 %). Tibial cuts had more outliers (33 of 260 cuts, 12.7 %) than the femur (39 of 520 cuts, 7.5 %) (p = 0.01). Pre-operative varus (n = 97) and valgus (n = 33) deformities demonstrated 7.7 % (45/482) and 13.6 % (27/198) of cuts which were outliers, respectively (p = 0.01).

CONCLUSION: PSI showed only fair to moderate accuracy with 62.3 % of the knees presenting no outliers. The tibia cutting guide was less accurate than the femur. Specific attention is needed when cutting the tibia and in correction of valgus deformity. Moreover, intra-operative verifying measurements can provide feedback to the accuracy of the surgical plan.

LEVEL OF EVIDENCE: IV, case series with no comparison group.