Strength of Humeral and Ulnar Intramedullary Screw Fixation.

PURPOSE: The goal of this study was to test the pullout strength of intramedullary (IM) screws from within the humerus to establish their ability to seat an uncemented elbow arthroplasty. METHODS: Six humerus and 6 ulna Sawbones specimens were drilled with a drill bit diameter of 5/16 inches, and the inner cortex was hand tapped for a ⅜-16 thread. A ⅜-16 custom-made titanium screw with an outer bolt diameter of 3/8 inches and 16 threads per inch was inserted by hand into the tapped holes. The specimens were then axially tensile loaded at a rate of 5 mm per minute until either the screw began to pull out from the bone or a fracture was noted. RESULTS: Intramedullary screw fixation in the humerus achieved an average pullout strength of 1,439 pound-force (6,401 N), and IM screw fixation in the ulna achieved an average pullout strength of 882 pound-force (3,923 N). A fracture was noted in 3 humeral specimens, with 3 screws pulling out. In the ulna, the IM axial load caused a fracture in 5 specimens, and in 1 specimen, the screw pulled out. CONCLUSIONS: Our findings demonstrate that IM screw fixation can create a tensile force within the screw that is greater than that required to generate the calculated level of compression between the implant and bone. CLINICAL RELEVANCE: This may be beneficial in ensuring fixation between arthroplasty components and bone.