Force required for bone segment transport in the treatment of large bone defects using medullary nail fixation

In distraction osteogenesis, different factors influence quality and quantity of bone formation. These factors are rigidity of fixation, frequency, and velocity of transport; and integrity of periosteum, endosteum, and intramedullary vascularization. Little data are available on forces related to distraction osteogenesis and limb lengthening, and no data on forces related to bone segment transport. This experiment investigates the correlation of the bone regeneration process with transport forces. Bone segment transport over an intramedullary nail provides an excellent model for measurement of external forces during distraction osteogenesis because soft-tissue and muscle resistance is reduced as compared with limb lengthening. In an animal experiment (sheep), tibial shaft defects of 20 mm and 45 mm were treated. External transport forces were measured daily during bilateral wire transport of a bone segment over an intramedullary nail. Forces increased during transport. Overall transport forces for large defects were slightly higher than those for small defects, reaching 350 N by the end of transport. For large defects, transport forces leveled off during bone transport and rose again at the end. Correspondingly, for large defects, stress relaxation within the tissues increased during the third and the sixth week of transport. This plateau may be interpreted as a reduction of internal forces within the repair tissues at the distraction site, implying a spreading of the healing phases during the bone regeneration process or geometrically different shapes of the regenerate.