Introduction: We determined (1) the range of the hip-knee-ankle (HKA) angle in the native or pre-arthritic limbs of patients with a contralateral total knee arthroplasty (TKA); and when mechanical alignment is planned (2) the relationships between the HKA angle and the tibial width, and the relative narrowing between the medial and lateral compartments and (3) the effect of tibial width on the range of narrowing.
Methods: The HKA angle, distal lateral femoral angle (DLFA), and proximal medial tibial angle (PMTA) were measured on the native limb of 102 subjects (53 female) treated with contralateral TKA. The sine of the angle of the resection gap (PMTA minus 90° subtracted from the DLFA minus 90°) multiplied by the tibial width and by narrow (59 mm), average (75 mm), and wide (91 mm) tibias computed relative narrowing.
Results: The HKA angle ranged from 8° varus to - 7° valgus; 20% had constitutional varus (≥ 3°) and 11% constitutional valgus (≤ - 3°). The HKA angle strongly predicted (r 2 = 0.87) and tibial width weakly predicted (r 2 = 0.06) relative narrowing. For narrow, average, and wide tibias, the maximum medial narrowing was 9, 11, 14 mm and maximum lateral narrowing was 7, 9, and 11 mm, respectively (p < 0.0001).
Conclusion: When mechanical alignment is planned, there is greater relative narrowing between compartments when the pre-arthritic limb greatly deviates from a 0° HKA angle and the tibia is wide. These limbs may need soft-tissue releases until neutral postoperative limb alignment of 0° and negligible varus-valgus laxity are achieved.
Level of evidence: IV, therapeutic study.
Keywords: Knee arthroplasty; Mechanical alignment; Soft-tissue release.