Mechanical Axis Deviation Shift in Limb Lengthening Over the Anatomical Axis, a Retrospective Analysis

Recently, limb lengthening devices have shifted from external fixators to telescoping magnetic intramedullary lengthening nails (MILNs), which lengthen strictly along the bone's anatomic axis. Baumgart proposed the reverse planning method, overcorrecting lateral shift with a distal osteotomy and slight varus angulation. The untested assumption that antegrade lengthening along the anatomic axis causes lateral mechanical axis deviation (MAD) prompted our study, which examines MILNs' effect on lower limb alignment.

We retrospectively evaluated records for 154 femoral antegrade MILNs inserted in 122 adult patients for limb lengthening. We excluded patients who underwent concomitant corrective osteotomies or tibial lengthening, or who had malunion, nonunion, mechanical failure, or revision surgery for any reason. Long-leg standing radiographs were taken preoperatively, at the end of lengthening, around 3 months postoperatively, and at the culmination of consolidation (approximately 6 months postoperatively). MAD and anatomic mechanical angle (AMA) were assessed as primary outcomes at each radiographic time point for sequential comparison. The predicted MAD was derived from the trigonometric formula (Predicted MAD = lengthening × sin [AMA]).

Average preoperative MAD was 2.4 mm medial (SD = 10.6), diminishing to 1.9 mm medial (SD = 13.2) by the end of lengthening. On assessment at consolidation, average MAD had equilibrated back to 2.6 mm medial. Our results showed a net shift of 0.18 mm, whereas the predicted shift was 5.4 mm. The mean preoperative AMA was 5.9 mm (SD = 1.49). At the end of lengthening, the average AMA had decreased to 4.8 mm (SD = 1.4).

Our data indicated minimal to no impact on the mechanical axis or joint alignment of the lower limb after antegrade lengthening using a telescoping femoral MILN in a deformity-free femur. Study results suggested that the femur typically realigned in a way that minimized mechanical deviation while preserving joint alignment.

Level III. See Instructions for Authors for a complete description of levels of evidence.