UC Davis

The main objective of a total knee arthroplasty (TKA) is to restore native (i.e. healthy) function, and a crucial step is determining the correct insert thickness for each patient. Two methods used to determine the insert thickness are by manually assessing the joint laxity and by using a trial insert with goniometric markings that measures the internal-external rotation orientation of the trial with respect to a mark on the femoral component. The former is qualitative and depends on the surgeon’s experience and ‘feel’ and while the latter is quantitative, it can be used only with an insert with medial ball-in-socket insert conformity. An unexplored method is to measure the force required to push a trial insert into position., which would require a specialized tool which can measure the push force. Hence, a new push force measuring tool was designed following the necessary design criteria to measure the push force intraoperatively accurately and safely. To determine whether this method has merit, the relationship between push force and insert thickness must be addressed through a clinical study. Using the new push force measuring tool, the push force was measured in 30 patients during unrestricted kinematically aligned TKA. The peak push forces going from an insert 1 mm thinner than the correct thickness as determined by an insert goniometer and from correct thickness to 1 mm thicker were measured. The mean peak forces for the different insert thicknesses were 127 ± 104 N, 127 ± 95 N, and 144 ± 96 N for 1 mm thinner, correct, and 1 mm thicker, respectively, and did not differ (p = 0.3210). As a result, measurement of peak force during trial positioning of a tibial insert cannot be used to identify the correct insert thickness.