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Bone regeneration during fracture healing has been demonstrated repeatedly, yet the regeneratioil of articular cartilage and joints has not yet been achieved. It has been recognized however that the mechanical environment during fracture healing can be correlated to the contributions of either the endochondral or intramembranous processes of bone formation, and to resultant tissue architecture. Using this information, the goal of this study was to test tlie hypothesis that induced motion can directly regulate osteogenic and chondrogenic tissue forination in a rat mid-femoral bone defect and thereby influence the anatomical result. Sixteen male Sprague Dawley rats (400 + 20 g) underwent productioii of a mid-diaphyseal, lion-critical sized 3.0 inn1 segmental femoi-a1 defect with rigid external fixation using a custom designed four pin fixator. One group of eight animals represented tlie controls and underwent surgery and constant rigid fixation. In tlie treatment group the custom external fixator was used to introduce daily interfragmentary bending strain in the eight treatment animals ( 12" angular excursion), with a hypothetical symmetrical bending load centered within tlie gap. The eight animals in tlie treatment group received motion at 1.0 Hz, for 10 inin a day, with a 3 days on, one day off loading protocol for tlie first two weeks, and 2 days on, one day off for the remaining three weeks. Data collection included histological and iniiiiiiiioliistological identification of tissue types, and mean collagen fiber angles and angular conformity between individual fibers in superficial, intermediate, and deep zones within the cartilage. These parameters were compared between tlie treatment group, rat knee articular cartilage, and tlie control group as a structural outcome assessment. After 35 days tlie control aniiiials demonstrated varying degrees of osseous union of the defect with soiiie animals showing partial union. In every individual within the niechanical treatment group the defect coinpletely failed to unite. Bony arcades developed in the experimental group, capping tlie termini of the bone seginents on both sides of the defect in four out of six animals completing tlie study. These new structures were typically covered with cartilage, as identified by specific histological staining for Type I1 collagen and proteoglycans. The distribution of collagen within analogous superficial, intermediate, and deep zones of tlie newly formed cartilage tissue demonstrated preferred fiber angles consistent with those seen in articular cartilage. Although not resulting in complete joint development, these iwuurthroses show that the induced motion selectively controlled the formation of cartilage and bone during fracture repair, and that it can be specifically directed. They further demonstrate that the spatial organization of molecular components within the newly formed tissue. at both microanatomical and gross levels, are influenced by their local mechanical environment, confirming previous theoretical models. 0...