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 q uantity of bone formation. These factors are rigidity of fixation, fre quency, and velocity of transport; and integrity of periosteum, endost eum, and intramedullary vascularization. Little data are available on forces related to distraction osteogenesis and limb lengthening, and n o data on forces related to bone segment transport. This experiment in vestigates the correlation of the bone regeneration process with trans port forces. Bone segment transport over an intramedullary nail provid es an excellent model for measurement of external forces during distra ction osteogenesis because soft-tissue and muscle resistance is reduce d as compared with limb lengthening. In an animal experiment (sheep), tibial shaft defects of 20 mm and 45 mm were treated. External transpo rt forces were measured daily during bilateral wire transport of a bon e segment over an intramedullary nail. Forces increased during transpo rt. Overall transport forces for large defects were slightly higher th an those for small defects, reaching 350 N by the end of transport. Fo r large defects, transport forces leveled off during bone transport an d rose again at the end. Correspondingly, for large defects, stress re laxation within the tissues increased during the third and the sixth w eek of transport. This plateau may be interpreted as a reduction of in ternal forces within the repair tissues at the distraction site, imply ing a spreading of the healing phases during the bone regeneration pro cess or geometrically different shapes of the regenerate.