Abstract
OBJECTIVES: Optimal coronal plane alignment for total knee arthroplasty (TKA) remains controversial. Understanding the pre-operative soft tissue status is important for optimizing the soft tissue envelope during TKA. The purpose of this study was to define the corrected, neutral (“pre-disease”) HKAA of end stage osteoarthritic knees prior to TKA and from this point measure the medial and lateral laxity of varus and valgus knees in maximum extension, 20 and 90° of flexion prior to TKA. METHODS: We conducted an observational cohort study. During surgery, the lower limb was manipulated using computer navigation, prior to surgical releases, whilst observing the joint to ensure congruence to allow the limb weight-bearing axis to pass through the knee center in maximum extension, 20° and 90° of flexion. Coronal plane laxity was measured as medial and lateral displacement from this point and compared to published values for healthy subjects. RESULTS: The corrected, neutral HKAA in 89 knees in maximum extension prior to TKA was -1.22° +/- 1.4°. The corrected HKAA in maximum extension was within +/- 3° of 0° in 91.0% of patients. 12.8% (10/78) of varus knees displayed a medial contracture. Of these 10 knees, five also displayed abnormal lateral laxity. In maximum extension, 19.2% (15/78) of knees had abnormally increased lateral laxity. 10 of these 15 knees did not have a medial contracture. The remaining 5 knees with increased lateral laxity or 6.4% (5/78) displayed a medial contracture. Lateral laxity increased significantly with increasing varus deformity. Medial laxity did not significantly decrease when comparing varus deformity of 5 -10° versus >10°. 29 Valgus knees were examined. In maximum extension and 20° of flexion 27.6% of subjects (8/29) and 6.9% (2/29) of subjects respectively had a lateral contracture. On the medial side abnormally increased laxity was seen in 40.7% (11/27) in maximum extension; 75% (21/28) in 20° and 0% in 90° of flexion. On the lateral side abnormally increased laxity was seen in 3.7% (1/27) in maximum extension; 3.6% (1/27) in 20° of flexion and 72.4% in 90° of flexion. CONCLUSION: Varus and valgus knees did not record a corrected, neutral HKAA at the opposite end of the 0 +/-3° range. Optimal TKA alignment might be better and more specifically defined by the corrected neutral axis of each knee. The majority of varus knees with deformity of up to 15° have neither a medial contracture nor abnormal lateral laxity when referenced to the neutral axis of the knee. Lateral laxity is a more consistent feature of the varus knee. In the valgus OA knee a lateral contracture is not present in most patients and typically only present in maximum extension. The pattern then reverses to an abnormal increase in lateral laxity in flexion for many subjects. The pattern on the medial side is for maximum soft tissue disturbance in 20° of flexion before normalising in 90° of flexion. These findings demonstrate potentially problematic scenarios for balancing the valgus OA knee. The patterns of contracture and laxity found are variable and correlate poorly to deformity.