Abstract
INTRODUCTION
Rotational alignment of the femoral and tibial component in total knee arthroplasty (TKA) are separately determined based on the anatomy of each bone. Popular references are the transepicondylar axis (TEA) for femoral component, and medial one-third of the tibial tubercle for the tibial component. It was reported that these references are not in accordance with each other in osteoarthritic (OA) knees and rotational mismatch could occur even when the components were accurately aligned. There has been, however, a paucity of data as for the rotational mismatch after TKA for OA knees. The purpose of this study was to evaluate the rotational mismatch between the femoral and tibial component after TKA for OA knees.
SUBJECTS & METHODS
Eighty-four knees which underwent primary TKA for the varus osteoarthritis of the knee were analyzed. Those knees were chosen by the retrospective confirmation of the precise rotational alignments of both femoral and tibial components by postoperative computed tomography (with ±3 degrees to the targeted reference lines described below). The femoral reference line was the surgical epicondylar axis and the tibial reference line was Akagi's line; a line connecting the midpoint of the tibial insertion of the posterior cruciate ligament and the medial border of patellar tendon. Intraoperative, dynamic evaluations of the rotational mismatch between femoral and tibial components was performed with a special device attached to the mobile-bearing trials at full extension and in neutral, passive external rotation and passive internal rotation.
RESULTS
The average rotational mismatch (in neutral position) was 4.3 ± 4.1º internal rotation of the tibial component relative to the femoral component. There was a wide range of variation from 11ºinternal rotation to 6ºexternal rotation of the tibial components. The average rotational allowance of the mobile mechanism (Between passive Internal & external rotation) was13.8 ± 5.4º (range, 6 to 30°). Neutral position was located approximately in the center of rotational allowance and 17 knees were not correctable to neutral even in the presence of mobile mechanism.
DISCUSSION AND CONCLUSION
It was demonstrated that rotational mismatch between femoral and tibial components exists in extension even when both components aligned to the anatomical reference correctly. The rotational allowance of the mobile mechanism was approximately 14º and has enough tolerance to forgive the rotational mismatch to the both direction. However, 20% of the knee were not correctable to neutral rotation in the presence of mobile mechanism and operating surgeons should be aware of this fact especially when fixed and rotational motion guided knee (e.g. medial pivot knee).
