Graded limb lengthening by callus distraction is a widely used surgical procedure to correct tubular bone deformities and can result in dramatic functional improvements in children. We used a model of tibial lengthening in rabbits to study the postoperative pain pattern during limb lengthening and morphological changes in the dorsal root ganglia (DRG), including alteration of substance P (SP) expression. Four groups of animals (naïve; OG: osteotomised only; SDG/FDG: slow/fast distraction with 1mm/3mm lengthening a day) were used. Signs of increasing postoperative pain were detected till the10th postoperative day in all groups; then it decreased in OG, whereas remained higher in SDG/FDG until the distraction finished. This suggests that pain response is based mainly on surgical trauma until the 10th day: the lengthening extended its duration and increased its intensity. The only morphological change observed in the DRGs was the presence of large vacuoles in large neurons of all operated groups. Although osteotomy was conducted in OG/SDG/FDG groups, significant de novo SP-expression in the large DRG cells appeared only in OG and significant decrease in the number of SP-immunoreactive small DRG neurons was detected solely in the SDG/FDG groups. Faster and larger distraction resulted in more severe pain sensation and lowered further the number of SP-positive small cells. Our data suggest that down-regulation of SP in the small cells in lengthened animals is associated with the stretch nerve injury, whereas de novo expression of the peptide in the large cells in OG is likely to correspond to the undergoing regeneration.

The response of the muscle is critical in determining the functional outcome of limb lengthening. We hypothesised that muscle response would vary with age and therefore studied the response of the muscles during tibial lengthening in ten young and ten mature rabbits. A bromodeoxyuridine technique was used to identify the dividing cells.

The young rabbits demonstrated a significantly greater proliferative response to the distraction stimulus than the mature ones. This was particularly pronounced at the myotendinous junction, but was also evident within the muscle belly.

Younger muscle adapted better to lengthening, suggesting that in patients in whom a large degree of muscle lengthening is required it may be beneficial to carry out this procedure when they are young, in order to achieve the optimal functional result.