Progressive limb lengthening with a centromedullary nail versus an external fixator: experimental study in sheep

Purpose of the study Progressive limb lengthening with an external fixator often leads to pin-related complications. A new technique allowing progress ive lengthening with a centromedullary nail without external fixation has b een developed. This original double-locked device consists of matching male and female components fitted with a continuous thread. Lengthening is achi eved via a one-way ratchet system. Twelve back-and-forth movements produce 1.25 mm lengthening. Material and methods We tested this new device on 20 sheep and compared res ults with external fixation lengthening in 20 other sheep. The animals were divided into groups for sacrifice on days 5, 10, 20, 45 and 90. Serial x-r ay were obtained for all animals. In the 45-day and 90-day groups, histomor phometric (trichrome goldner coloration and polarized light microscopy) and densitometric studies were also performed. Bone mineral density (BMD) was determined and bone trabecular density (BTD) and trabecular bone volume (TB V) were expressed in percent of bone trabecular surface area. Results Mean lengthening in the 45-day and 90-day groups was 39 mm for the nail and 20 mm for external fixation (1 mm/day). At 90 days, 3 sheep out of 4 had consolidated radiologically with external fixation and 2 out of 4 wi th the nail. BMD was slightly better for external fixation (0.811 vs 0.695/ cm(2)). This difference could probably be attributed to the greater lengthe ning obtained with the nail. At 45 days, BMD was the same (0.6 g/cm(2)) for both devices. BTD was nearly two-fold higher for the nail compared with ex ternal fixation (59.65% vs 32.61% at 90 days), most probably due to primary bone formation. The histomorphometric study allowed an analysis of the ost eoid border. Bone quality obtained in the bone regenerate with the nail was superior to that obtained with external fixation. Primary bone formation r esulted from membrane ossification with direct transformation of fibroblast s into osteoblasts. Conclusion This work demonstrated that progressive lengthening can be achie ved with a specifically designed centromedullary nail without iterative ope ning of the operative site. Tolerance to this type of device and quality of the bone regenerate are altogether satisfactory.