Abstract
INTRODUCTION
The extension and flexion gaps are affected by different factors in total knee arthroplasty (TKA). Flexion but not extension gap measurements are influenced by posterior cruciate ligament (PCL) preservation or resection and patella reduction or eversion and thigh weight. If the flexion gap is measured with the thigh placed on the tibia, the measurement results must include the thigh weight; nevertheless, there is no detailed report regarding the thigh weight influence on the flexion gap. In this study, we investigated how thigh weight affected flexion gap measurement.
METHODS
Four knees of whole-body fresh-frozen cadavers (Mongolian race) were investigated. The femur and tibia were dissected with a standard measured resection technique. After the femoral component was set, the flexion gap was measured with a knee balancer. The distraction force of 20, 30, and 40 pounds were loaded at the joint level. For each measurement, the influences of the patella reduced or everted (PR or PE) and the PCL preserved or resected (CR or PS) were estimated. The flexion gap was measured five times in four different categories (CR/PR, CR/PE, PS/PR, PS/PE) and the thigh weight was reduced by weights (0, 0.5, 1.0, 2.0, 3.0 kg) using a string and pulley system. During measurement, the femur was just placed on the tibia, and the knee flexion angle was maintained at 90 degrees with a goniometer. After all measurements, the lower limbs were resected, and the thighs were weighed with a scale. Steel-Dwasstest (non-parametric multiple comparison test) were performed for statistical analysis, and p < 0.05 was considered significant.
RESULTS
Flexion gap measurement results show over 10 mm difference between the maximum gap (PS/PE, 40 lbs, 3 kg weight reduction) and the minimum gap (CR/PR, 20 lbs distraction, no weight reduction) in this study. When a 0.5 kg weight reduction was applied, there were no significant flexion gap increases compared to no weight reduction situation in almost all categories except for “CR/PR and 40 lbs distraction”. According to the increase of the weight reduction, the flexion gap became larger in all categories. When a 3 kg weight reduction was applied, there were significant flexion gap increases compared to no weight reduction situation in all categories (Table 1-3). The mean thigh weight was 2.3 kg (2.0–2.6 kg).
DISCUSSION
The flexion gap is usually measured with the thigh placed on the tibia in TKA, and the measurement results are considered to include the influence of the thigh weight even though this has not been discussed in the literature. From our results, the influence of the thigh weight reduction on the flexion gap was different according to heaviness of the reduction weight. When the reduction weight was over the thigh weight, flexion gap increase relative to the flexion gap without weight reduction was significant in all categories nevertheless different situations of the PCL, patella position, and joint distraction forces. To estimate adequate flexion gap and avoid post-operative flexion gap looseness, the thigh weight should be reduced when the flexion gap is measured.
