Summary

Mechanical alignment with manual or navigational instruments creating a 0° HKA can cause a 15 mm range of soft tissue tightness between the medial and lateral sides of the knee, which can be predicted by the HKA angle of the native limb.

Abstract

Background

Mechanically aligned total knee arthroplasty (MA TKA) sets the hip-knee-ankle (HKA) angle at 0° and is reported to require release of tight soft-tissues in knee extension in up to 60% of knees. Changing the native alignment of a limb with a constitutional varus or valgus HKA angle of > 3° to a mechanical alignment of 0° creates a trapezoidal gap and tightness of the medial or lateral compartment, respectively. Since the native knee has negligible varus-valgus laxity in full extension, the compartment tightness might require a soft tissue release to convert the trapezoidal gap to a rectangular gap and align the limb at a 0° HKA angle 1,2,3. The present study determines 1) the range of the HKA angle in the contralateral native limb of patients treated with a TKA, 2) the frequency of occurrence (i.e. proportion) of constitutional varus and valgus, 3) whether the native HKA angle predicts the angle of the trapezoidal gap and millimeters of compartment tightness, and 4) the frequency of occurrence of > 2 mm medial or lateral compartment tightness.

Methods

A retrospective analysis of 562 consecutive patients treated with a primary TKA between August 2014 and March 2016 was performed. A post-operative anterior-posterior CT scanogram of both limbs was available for 561 subjects. Excluded were 459 subjects because the scanogram showed an arthroplasty or degenerative arthritis in the contralateral limb (N=305), malrotation of the limb projection (N=133), a healed fracture, or an arthroplasty of the hip or ankle (N=41). The HKA angle was measured. Simulated resections of the distal femur and proximal tibia were performed perpendicular to the mechanical axes of the femur and tibia. The angle of the trapezoidal gap formed by the resections was measured. Compartment tightness was computed as the height of the opposite side of a right triangle with use of the angle of the trapezoidal gap and an adjacent side of 70 mm, which was the average width of the tibia at the level of the resection (+ tight medial compartment/ - tight lateral compartment).

Results

The native limb was analyzed on 102 subjects (63 female) with a mean BMI of 28. The HKA angle ranged from -7° valgus to 8° varus, with 10% having constitutional valgus < -3°, 70% with a HKA angle within 0 ± 3°, and 20% having constitutional varus > 3°. The native HKA angle strongly predicted the angle of the trapezoidal gap and tightness of the compartment, which ranged from 7 mm lateral to 8 mm medial (r2 = 0.82, p = < 0.0001). The proportion of native limbs with a > 2 mm tightness of the medial or lateral compartment was 27% and 24%, respectively.

Conclusions

Surgeons that perform mechanical alignment with manual, navigational, robotic, and patient-specific instrumentation should be aware that creating a 0° HKA can cause a 15 mm range of compartment tightness from 7 mm lateral to 8 mm medial, which can be predicted by the HKA angle of the native limb. Mechanical alignment creates a > 2 mm tightness in the medial or lateral compartment in over 50% of native limbs, which is comparable to the reported frequency of soft tissue releases by clinical studies of MA TKA.

References:
1. Gu, Roth, et al: JBJS 2014
2. Bellemans , William, et al: Clin Orthop Relat Res 2012
3. Eckhoff , Bach, et al: JBJS 2005