Abstract
Introduction
An equal knee joint height during flexion and extension is of critical importance in optimizing soft-tissue balancing following total knee arthroplasty (TKA). However, there is a paucity of data regarding the in-vivo knee joint height behavior. This study evaluated in-vivo heights and anterior-posterior (AP) translations of the medial and lateral femoral condyles before and after a cruciate-retaining (CR)-TKA using two flexion axes: surgical transepicondylar axis (sTEA) and geometric center axis (GCA).
Methods
Eleven patient with advanced medial knee osteoarthritis (age: 51–73 years) who scheduled for a CR TKA and 9 knees from 8 healthy subjects (age: 23–49 years) were recruited. 3D models of the tibia and femur were created from their MR images. Dual fluoroscopic images of each knee were acquired during a weight-bearing single leg lunge. The OA knee was imaged again one year after surgery using the fluoroscopy during the same weight-bearing single leg lunge. The in vivo positions of the knee along the flexion path were determined using a 2D/3D matching technique. The GCA and sTEA were determined based on existing methods. Besides the anterior-posterior translation, the femoral condyle heights were determined using the distances from the medial and lateral epicondyle centers on the sTEA and GCA to the tibial plateau surface in coronal plane (Fig. 1). The paired t-test was applied to compare the medial and lateral condyle motion within each group (Healthy, OA, and CR-TKA). Two-way ANOVA followed post hoc Newman–Keuls test was adopted to detect significant differences among the groups. p<0.05 was considered significant.
Results
The results demonstrated that following TKA, the medial and lateral femoral condyle heights were not equal at mid-flexion (15° to 45°, medial condyle lower then lateral by 2.4mm at least, p<0.01), although the knees were well-balanced at 0° and 90° (Fig. 2). While the femoral condyle heights increased from the pre-operative values (>2mm increase on average, p<0.05), they were similar to the intact knees except that the medial sTEA was lower than the intact medial condyle between 0 and 90°. At deep flexion (>90°), both condyles were significantly higher (>2mm, p <0.01) than the healthy knees. Anterior femoral translation of the TKA knee was more pronounce at mid-flexion (Fig. 3), whereas limited posterior translation was found at deep flexion.
Conclusion
Femoral condyle heights and AP translations of the CR TKA knees were significantly different from the healthy knees during the weight bearing flexion activity when measured using both the sTEA and GCA, especially at mid-flexion (15° to 45°) and deep flexion (>90°). These results suggest that a well-balanced knee intra-operatively might not necessarily result in mid-flexion and deep flexion balance during functional weight-bearing motion, implying mid-flexion instability and deep flexion tightness of the knee. The data could be useful for improvement of future prostheses designs and surgical techniques in treatment of patients with end-stage medial knee OA.
