Strength of Humeral and Ulnar Intramedullary Screw Fixation

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.

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.

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.

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.

This may be beneficial in ensuring fixation between arthroplasty components and bone.