Abstract
Introduction
Several studies have described the relationship between the joint line and bony landmarks around the knee. However, high inter-patient variation makes these absolute values difficult in use.
This study was set up to validate the previously described distances and ratios on calibrated full limb standing X-rays and to investigate the accuracy and reliability of these ratios as a tool for joint line reconstruction
Methods:
One hundred calibrated full-leg standing radiographs obtained from healthy volunteers were reviewed (fig 1). Distances from the medial epicondyle, the lateral epicondyle, the adductor tubercle, the fibular head and the proximal center of the knee (CJD) to the virtual prosthetic joint line were determined (fig 3). This prosthetic joint line was created by introducing a virtual distal femoral cutting block with a valgus angle of 6° on the full-leg radiographs.
The adductor ratio was defined as the distance from adductor tubercle to the joint line divided by the femoral width.
The correlation with the femoral width, the CJD and the limb alignment was analysed using linear regression analysis. The accuracy and reliability of the use of the ratio of the distance of the adductor tubercle, the medial epicondyle and the CJD relative to the femoral width to reconstruct the joint line was calculated.
Results:
The average distance to the joint line from the medial epicondyle, the lateral epicondyle, the adductor tubercle and the fibular head was 28 mm (SD 2.97), 27 mm (SD 2.67), 44 mm (SD 4,27) and 15 mm (SD 3.69) respectively.
The distance from the adductor tubercle (R = 0,82) and the CJD (R = 0,96) to the joint line showed a strong and significant linear correlation with the femoral width. The medial epicondyle, the lateral epicondyle and the fibular head showed less strong correlations. There was no significant correlation with the limb alignment.
The adductor ratio was found to be 0.52 (SD 0.027) with only small inter-individual variation. The use of the adductor ratio reconstructed the joint line within 4 mm of its original level in 92% of the cases.
Discussion
The absolute distances and ratios for determining joint line position as previously described, were confirmed on calibrated full-limb standing radiographs. Recently, the adductor tubercle has been described as a reliable landmark for determining joint line position. As a rule of thumb, the femoral width as measured on the preoperative radiograph or intra-operative, is divided by 2. Intra-operative, the distance from the adductor tubercle to the distal cutting block that has been inserted with a 6° distal cutting angle, is adjusted to equal the calculated value. Fixation of the cutting block at this level will automatically reconstruct the joint line at its original level (fig 2). Modern instrumentation techniques will allow you to immediately select the appropriate size distal femoral augment to reconstruct this joint level.
Conclusion
The adductor ratio was found to be the most useful and accurate tool to restore the joint line to its original level in revision TKA.
