It has been shown that the bone, nerve, tendon, and muscle can generate new tissue when a leg is lengthened. In this study we have examined the muscles to see whether the proliferative response occurs uniformly along the fibres or whether it is a disproportionate occurrence, and also to see whether the muscles of animals of different age responded differently.

In five adult (more than 25 weeks) and five young (8 to 9 weeks) New Zealand White rabbits, a mid-diaphyseal tibial osteotomy was created and stabilised with an Orthofix (M-100) external fixator. After seven days, lengthening was carried out at a rate of 1.6 mm/day until a 20% increase in the tibial length had occurred. One hour prior to sacrifice, all of the animals were injected with bromodeoxyridine (BrdUrd, 40mg/kg).

Proliferative response of muscle tissue was assessed by measuring the positive staining index (PSI) of BrdUrd in a two-step indirect immunohistochemistry using the monoclonal antibody Bu20a. We accomplished this staining in transverse sections (between the proximal and middle third, and between the middle and distal third of the muscle belly) and in longitudinal sections along the proximal, middle and distal third of the myotendinous junction (MTJ) of the lengthened flexor digitorum longus muscle belly. The opposite limb was used as a control for each animal.

All of the muscles showed a proliferative response that was significantly higher on the experimental side. There was no difference between the PSI of the proximal transverse sections and the distal transverse sections. The young animals demonstrated significantly increased PSI in all sections compared with the adult animals (immature distal transverse section PSI: 4.91%; mature distal transverse section PSI: 1.67%). The PSI of the longitudinal sections of MTJ showed significantly higher values than in the muscle belly (PSI at the MTJ in adults: 5.23%; PSI at the MTJ in the young: 13.2 %). The PSI result was increased at the distal third of the MTJ in mature and immature rabbits (p0.05).

The muscles show a proliferative response to elongation forming new muscle tissue. The proliferative reaction to lengthening is far greater in the muscles of growing animals compared to adults. The myotendinous junction demonstrates much more intensive proliferative activity than the muscle belly. The distal third of the myotendinous junction shows the highest PSI results. The results of this study help to interpret the results of the animal model for clinical studies and also indicate an advantage in carrying out lengthening on young individuals.