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We sought to test the hypothesis that a pedicle screw that has two parallel threads of different heights throughout the full length of the screw could increase both bone purchase and pull-out strength compared with a standard single-threaded screw of similar dimensions. A single-threaded pedicle screw and a double-threaded pedicle screw were respectively placed into the paired pedicles of 21 vertebral bodies. The screws were then pulled out of the pedicles, and output parameters were measured. Although insertional torque was, on average, 14.5% higher (p = 0.039) for the single-threaded screw, maximum pull-out strength (p = 0.12), energy-to-failure (p = 0.39), and stiffness (p = 0.54) were not statistically different for the two screw types. It is concluded that a second, smaller inner thread on a double-threaded pedicle screw does not translate into either increased bone purchase or higher pull-out strengths.

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To improve the understanding of the functional requirements of trabecular bone substitutes, the structure-function relationships of coralline hydroxyapatite were determined and compared to those of trabecular bone from a variety of anatomic sites. Mechanical properties and permeability of cylindrical coralline hydroxyapatite specimens were measured and related to various morphological parameters that were obtained from analysis of high-resolution (20 microm) computer reconstructions of each specimen. Results indicated the average (+/-SD) Young's modulus (2900 +/- 1290 MPa, n = 20) and permeability (0.50 +/- 0.19 x 10(-9) m2, n = 21) of the coralline hydroxyapatite were within the range of values exhibited by high density trabecular bone; ultimate stress (5.87 +/- 1.92 MPa, n = 13), while in the range of mid-density trabecular bone, was low considering its high volume fraction (31.3 +/- 1.9%, n = 49); and ultimate strain (0.22 +/- 0.03%, n = 13) was much lower than that of trabecular bone from any anatomic site. The only correlation found between mechanical and morphological parameters was between Young's modulus and "fabric" (a scalar measure of architecture that combined the degree of microstructural anisotropy with orientation). These results provide insight into the in vivo performance of this implant, as well as the biomechanical requirements for successful trabecular bone substitutes in general.

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