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A radiographic evaluation of the normal as well as the progressively widened tibiofibular interval in the area of the syndesmosis was done using 12 fresh cadaver lower extremities. The width of the tibiofibular "clear space" and the amount of tibiofibular overlap was determined on accurately positioned anterior-posterior and mortise radiographs. Based on a 95% confidence interval, measurements obtained for the intact specimens would support the following criteria as consistent with a normal tibiofibular relationship: (1) a tibiofibular "clear space" on the anterior-posterior and mortise views of less than approximately 6 mm; (2) tibiofibular overlap on the anterior-posterior view of greater than approximately 6 mm or 42% of fibular width; (3) tibiofibular overlap on the mortise view of greater than approximately 1 mm. The width of the tibiofibular "clear space" on both anterior-posterior and mortise views appeared to be the most reliable parameter for detecting early syndesmotic widening.

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Three porous ceramic bone graft materials were compared with regard to their ability to heal a 2.5 cm defect created surgically in a bilateral canine radius model. The ceramic materials were analyzed at 12 and 24 weeks after surgery and included tricalcium phosphate, hydroxyapatite, and collagen hydroxyapatite, which contained a mixture of 35% tricalcium phosphate and 65% hydroxyapatite with added collagen. Each material was evaluated alone and with added bone marrow aspirate. All the implants were compared with a graft of autogenous cancellous bone in the contralateral radius. Biomechanical testing and radiographic evaluation revealed that the addition of bone marrow aspirate was essential for tricalcium phosphate and hydroxyapatite to achieve results comparable with those of cancellous bone. Collagen hydroxyapatite performed well without the addition of bone marrow, although the addition of marrow did have a positive effect. Further qualitative radiographic and histological analysis demonstrated that tricalcium phosphate was the only ceramic that showed any sign of degradation at 24 weeks. This observed degradation proved to be an important factor in evaluating radiographs because the radiodensity of collagen hydroxyapatite and hydroxyapatite interfered with the determination of radiographic union. At 24 weeks, tricalcium phosphate with bone marrow was the material that performed most like cancellous bone. In this study, the biomechanical and radiographic parameters of tricalcium phosphate with bone marrow were roughly comparable with those of cancellous bone at 12 and 24 weeks. Tricalcium phosphate was the only implant that showed significant evidence of degradation at 24 weeks by both histological and radiographic evaluations, and this degradation took place only after a degree of mechanical competence necessary for weight-bearing was achieved.

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Seventeen cadaveric human lumbar motion segments from eight spines were cyclically loaded in vitro under axial compression. Loading frequency and magnitude were chosen to simulate rigorous activity within an in vivo physiological level. The load magnitude was determined as a percentage of the ultimate compressive load, the latter estimated from the bone mineral content (BMC) of lumbar vertebrae determined by dual-photon absorptiometry. Following testing, the degree of macroscopic disc degeneration was assessed and the type of fracture in each specimen was determined from serial sagittal sections. Fractures were found in all but one specimen. Three types of fractures were formed: the node of Schmorl and Junghanns (type I), central endplate fracture (type II), and a crush or burst fracture (type III). The results suggested that type I fractures were predominantly associated with segments with normal discs, type II fractures were found primarily in segments with moderately degenerated discs, and type III fractures were associated with segments that failed on the first cycle. Segment stiffness and fatigue strength (cycles to failure) were correlated with disc degeneration, age, and segment BMC, the latter an in vivo measure of bone density. Fatigue strength also decreased in proportion to a power coefficient with increasing relative stress (cyclic stress range/ultimate stress).

The in vitro viscoelastic "creep" behavior was examined in 18 cadaveric human lumbar motion segments subjected to static axial compressive loads. Axial deformation was followed for 30 min under constant applied load. Compressive material constants (moduli and viscosity coefficients) were then determined for each intervertebral disc using a linearization method based on a Taylor series expansion of experimental data for the "three parameter" viscoelastic creep model. The degree of disc degeneration and bone mineral content (BMC) were also assessed. Good correlation between the experimentally determined and model predicted strain values were found, with the average error less than 1%. We found that motion segments from older and more degenerated lumbar discs were less stable and had lower material constants than segments from younger and less degenerated discs. Material constants and BMC correlated closely, suggesting that an interdependency of disc and vertebral body properties exists. No correlation between the creep characteristics and disc height, disc area, segment level, or sex were noted.