How Precise Is Computer-navigated Gap Assessment in TKA?

Methods to improve gap balancing in total knee arthroplasty (TKA) include the development of calibrated distractors and various devices to determine the distances of the gaps. However, few studies have validated the accuracy or precision of computer navigation to determine these measurements, especially gaps created after bone cuts have been made; doing so would be important, because optimal surgical technique relies on appropriate gap spacing. We investigated the ability of a new image-free computer navigation surface registration protocol to measure gap distances in TKA. Eight embalmed cadaveric specimens of the lower extremity were used. A surface registration software protocol defined the most distal and posterior surface points of the femoral condyles and the navigation system measured the distance of the most distal femoral condyle point to the surface of the tibia after tibial resection. The tibial resection was perpendicular to the mechanical axis and was cut with a 7A degrees posterior slope. The navigation system measured gaps spaced by modular spacing blocks at 5A degrees intervals from full extension to 120A degrees of flexion. Repeatability assessed repeated measures by one surgeon. Reproducibility was assessed by performing the same measurements after complete reregistration of the computer protocol to the cadaver bones. The gaps measured by the computer were statistically the same as those assessed with the use of blocks with a maximum measurement error of 1 mm. Reregistration did introduce error into the measurement. The gaps changed with position of knee flexion, and there was gradual and significant stretching of the gaps with repeated measurements. Preliminary testing shows that computer navigation can reproduce static measurements reliably and with equal accuracy as spacer blocks. We have not demonstrated that this could be applied in a dynamic setting. This computer navigation system has sufficient precision to warrant investigation in the clinical setting for measuring gaps created during the surgical procedure.