Prevelance of Coronal Deformity Reversal in Flexion and Its Relation to Total Knee Arthroplasty Instability

Introduction The prevalence of reversing of extension coronal deformity during flexion and how that may change the routine algorithm of soft tissue balancing in total knee arthroplasty (TKA) has not been published. We name this phenomenon, the reversing coronal deformity (RCD). We observed 12% (45 patients) of coronal deformities consistently reverse in flexion in the osteoarthritic knees before surgery. We conclude that RCD phenomena need to be addressed in every TKA and collateral ligament release need to be modified or avoided; otherwise postoperative flexion instability may be inevitable. Femoral rotation adjustment with posterior capsule release has to be attempted first in RCD patients. Method We define RCD as the reversing of a coronal extension deformity of more than 2° while the knee reaches 90°of flexion. That is to say a 2° or more varus knee in extension becomes a 2° or more valgus at 90° of flexion or vice versa. We retrospectively analyzed, in a multicenter study the alignment patterns of 387 (US = 270, UK = 117) consecutive computer navigated TKA subjects (June 2004–May 2008). 364/387 (US = 252, UK = 112) subjects were eligible for analysis (23 subjects had incomplete data: US = 18, UK = 5). The coronal deformity kinematics was observed during the range of motion and the range of medial /lateral deflections were analyzed. Result: 260/364 subjects had varus knees and 104/364 subjects had valgus knees. 18 subjects (7%) of the varus knees reversed to valgus and 27 subjects (26%) of valgus knees reversed to varus by 90°pre-operatively. Therefore, the total number of arthritic knees that reversed their coronal deformity from extension to 90° flexion was 45 (12.4%). Knee alignment in extension was 0° ± 2° in 99% of patients. 1% (4 subjects) had more than 2°of varus or valgus in extension. Collateral ligament was released in 4/45 RCD patients in which all had flexion instability of more than 10° (medial/lateral at 90°). The other 40 patients had posterior capsule release with or without femoral rotation adjustment to balance the flexion gap. None of them had flexion instability (medial /later gaping was 4° or less). The preoperative mean femoral rotation was 3.05° of external rotation (ER) in varus knees and 1.9° ER in valgus knee. While in RCD varus knees, the mean femoral rotation was 1.5 ° ER and RCD valgus knees 2.5°ER. Discussion and Conclusion Our observation has shed the light on a new concept in the kinematics of the knee, namely the reversing of the coronal deformity (RCD) during flexion which occurs in 12% of patients undergoing TKA. Basically, a varus knee in extension behaves like a valgus knee in flexion and vice versa. It is crucial to be aware of this phenomenon when attempting to do soft tissue release to balance the gaps in TKA. Otherwise, widening one gap in extension to correct a fixed deformity may result in an unacceptable overcorrection of the same gap in flexion in those knees that manifest the reverse coronal deformity phenomena. Soft tissue balance algorithm was noted to be different in such cases in which early collateral ligament release resulted in flexion instability.