Abstract
Introduction:
Total knee arthroplasty (TKA) should aim to adjust the component gap (CG) difference between extension and flexion. However, this difference cannot be measured without placement of a femoral component. The bone gap reportedly decreases in extension after component setting. In contrast, it may be possible to use the mean value of the CG difference in several patients to adjust femoral resection amount beforehand. The purpose of this study is to evaluate the technique of adjusting CG difference using the mean values with measured resection technique (MRT) in TKA.
Materials and methods:
The subjects were 222 knees (40 male knees, 182 female knees; mean age 70.4 years). To adjust the CG difference after estimation, the femoral posterior condylar pre-cut technique was used. Extension gap was created by usual bone resection; 4 mm of the femoral posterior condyle was pre-cut, and after all osteophytes and soft tissues had been treated, a pre-cut trial component (thickness of 8 mm for distal femur and 4 mm for posterior condyle without the anterior portion) was mounted, achieving the same condition as the setting of a femoral component in MRT (Fig. 1). When the posterior cruciate ligament (PCL) could be easily preserved by intraoperative gap assessments, the PCL was preserved (190 knees, 86%).
Results:
The CG measured intraoperatively were 9.4 ± 2.8 mm (mean ± S.D.) in extension and 12.2 ± 2.8 mm in flexion, and the difference (flexion - extension) was 2.8 ± 2.6 mm. The mean difference was not large but the variation was large (−3 ∼ 11 mm). When the acceptable range of CG difference (flexion - extension) is set at 0 to 3 mm, only 57% of the patients were included within this range; when 2 mm of the distal femur was cut beforehand in all patients considering the mean CG difference of 2.8 mm, 53% of the patients were within the acceptable range and 42% had 3 mm resection (Fig. 2). When the acceptable range was expanded to −2 mm to 3 mm, 64% of the patients were in this range, whereas this figure was 76% with an additional 2 mm resection of the distal femur and 72% with 3 mm resection. Even after expanding the acceptable range for CG difference to 5 mm and adjusting the distal femoral cut, one fourth of the patients were outside of the acceptable range (Fig. 3).
Discussion:
This study showed that the CG using MRT was larger in flexion by a mean of 2.8 mm; however, the variation was too large to manage by larger femoral distal cut according to the mean difference beforehand. Although the PCL was preserved in 190 knees, it is anticipated that gaps in flexion will enlarge when PCL resection is selected for all patients, which may further increase the gap requiring adjustment. To resolve such issues, we use the femoral pre-cut technique and pre-cut trial components. With this method, we can control the CG as we want by adjustment of final femoral bone resection in each patient.
