Abstract
Introduction
In total knee arthroplasty (TKA), tibial insert thickness is determined intraoperatively by applying forces that generate varus-valgus moments at the knee and estimating the resulting gaps. However, how the magnitude of applied moments and the surgeon's perception of gaps affect the thickness selection is unclear. We determined this relationship using an in vitro human cadaveric model.
Methods
Six pelvis-to-toe specimens (72±6 years old, four females) were implanted by an expert surgeon with a PS TKA using measured resection. Pliable sensors were wrapped around medial and lateral aspects of the foot and ankle to measure the applied forces. The forces were scaled by limb length to obtain the moments generated at the knee. Six surgeons with different experience levels independently assessed balance by applying moments in extension and 90° of flexion and choosing the insert they believed fit each knee. Peak moments and the accompanying extension and flexion gap openings as perceived by surgeons were recorded. The two measures were then related to insert choice using a generalized estimating equation.
Results
The peak applied moments varied among surgeons (mean of 14±2.5 Nm in extension and 10±3 Nm in flexion). In extension, surgeons perceived a medial gap of 1.3±0.8 mm and a lateral gap of 0.9±0.7 mm. In flexion, surgeons perceived a medial gap of 1.1±0.9 mm and a lateral gap of 1.7±1.6 mm. Despite these differences, surgeons' choices of insert thickness varied by at most 1 mm, with no association found between the selected thickness and either peak moments or surgeons' gap perception (p>0.05).
Conclusions
In a controlled setting, surgeons of varying experience levels showed remarkable convergence in insert thickness selection. This notable consistency was unrelated to either the applied moments or their estimation of extension and flexion gaps, indicating that other factors may be driving this decision.
