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Assigning an appropriate density-modulus relationship is an important factor when applying inhomogeneous material properties to finite element models of bone. The purpose of this study was to develop a customized density-modulus equation for the distal ulna, using beam theory combined with experimental results. Five custom equations of the form E= ap(b) were used to apply material properties to models of eight ulnae. All equations passed through a point (1.85, Ec), where p = 1.85 g/cm3 represents the average density of cortical bone. For custom equations (1) to (3), Ec was predicted using beam theory, and the value of b was varied within the range reported in the literature. Custom equations (4) and (5) used other values of Ec from the literature, while keeping b constant. Results obtained from the custom equations were compared with those from other equations in the literature, and with experimental results. The beam theory analysis predicted Ec = 21 +/- 1.6 GPa, and the three custom equations using this value tended to have the lowest errors. The power of the equations did not affect the results as much as the value used for Ec. Overall, a customized density-modulus relationship for the ulna was generated, which provided improved results over using previously reported density-modulus equations.

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Distal ulnar arthroplasty is becoming a popular treatment option for disorders of the distal radioulnar joint; however, few studies have investigated how load transfer in the ulna is altered after insertion of an implant. The purpose of our study was to compare bone stresses before and after insertion of two commercially available cemented distal ulnar implants: an implant with a titanium stem and an implant with a cobalt chrome stem. Appropriately sized implants of both types were inserted into eight previously validated subject-specific finite element models, which were created by using information derived from computed tomography scans. The von Mises stresses were compared at eight different regions pre-and post-implantation. The bone stresses with the titanium stem were consistently closer to the pre-implantation stresses than with the cobalt chrome stem. For the loading situation and parameters investigated, results of these models show that insertion of the E-Centrix 1 ulnar Head may result in less stress shielding than the SBI uHead TM stem. Future studies are required to investigate other implant design parameters and loading conditions that may affect the predicted amount of stress shielding. ß