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

Area of Science:

• Orthopedic surgery
• Biomechanical engineering

Background:

• Limb lengthening devices have evolved from external fixators to telescoping magnetic intramedullary lengthening nails (MILNs).
• Antegrade lengthening with MILNs occurs along the bone's anatomic axis, raising questions about potential mechanical axis deviation (MAD).

Purpose of the Study:

• To investigate the effect of antegrade femoral lengthening using MILNs on lower limb alignment.
• To evaluate the assumption that antegrade lengthening causes lateral MAD.

Main Methods:

• Retrospective review of 154 antegrade femoral MILNs in 122 adult patients.
• Exclusion of patients with concomitant procedures or complications.
• Sequential assessment of mechanical axis deviation (MAD) and anatomic mechanical angle (AMA) via long-leg standing radiographs.

Main Results:

• Average preoperative MAD was 2.4 mm medial, with minimal change during lengthening (1.9 mm medial) and at consolidation (2.6 mm medial).
• Net shift in MAD was 0.18 mm, significantly less than the predicted 5.4 mm.
• Average AMA decreased from 5.9 mm preoperatively to 4.8 mm at the end of lengthening.

Conclusions:

• Antegrade femoral lengthening with telescoping MILNs has minimal impact on the mechanical axis and joint alignment in deformity-free femurs.
• The femur demonstrates self-realignment during lengthening, minimizing mechanical deviation and preserving joint alignment.