New segmental long bone defect model in sheep: Quantitative analysis of healing with dual energy X-ray absorptiometry

An appropriate animal model is required for the study of treatments that en hance bone healing. A new segmental long bone defect model was developed fo r this purpose, and dual energy x-ray absorptiometry was used to quantify h ealing of this bone defect. In 15 sheep, a 3-cm segmental defect was create d in the left tibia and fixed with an interlocking intramedullary nail. In seven animals, the defect was left empty for the assessment of the spontane ous healing response. In eight animals serving as a positive control, autol ogous bone grafting was performed. After 12 weeks, healing was evaluated wi th radiographs. a torsional test to failure, and dual energy x-ray absorpti ometry. The mechanical test results were used for the assessment of unions and nonunions. Radiographic determination of nonunion was not reliably acco mplished in this model. By means of dual energy x-ray absorptiometry, bone mineral density and content were measured in the middle of the defect. Bone mineral density was 91 +/- 7% (mean +/- SEM) and 72 +/- 6% that of the con tralateral intact tibia in, respectively, the autologous bone-grafting and empty defect groups (p = 0.04). For bone mineral content, the values were, respectively, 117 +/- 18 and 82 +/- 9% (p = 0.07), Torsional strength and s tiffness were also higher, although not significantly, in the group with au tologous bone grafting than in that with the empty defect. Bone mineral den sity and content were closely related to the torsional properties (r(2) ran ged from 0.76 to 0.85, less than or equal to 0.0001). Because interlocking intramedullary nailing is a very common fixation method in patients, the ne wly developed segmental defect model has clinical relevance. The interlocki ng intramedullary nail provided adequate stability without implant failure. This model may be useful for the study of treatments that affect bone heal ing, and dual energy x-ray absorptiometry may be somewhat helpful in the an alysis of healing of this bone defect.