3D Printed Locking Osteosynthesis Screw Threads Have Comparable Strength to Machined or Hand Tapped Screw Threads

Additive manufacturing, aka 3D printing, is increasingly being used for personalised orthopaedic implants. Additively manufactured components normally undergo further processing, in particular 3D printed locking osteosynthesis plates require post-printing screw thread creation. The aim of this study was to compare 3D printed threads with machined and hand tapped threads for a locking plate application. Pushout tests were performed on 115 additively manufactured specimens with tapered screw holes; additive manufacture was performed at 0°, 20°, 45° or 90° build orientations. The screw holes were either machined, hand tapped or 3D printed. The 3D printed screw holes were left as printed, or run through with a tap lubricated with water or with thread cutting oil. Printed threads run through using oil, with a build orientation of 90°, had comparable pushout force (median 6377 N 95%CI 5616 to 7739 N) to machined (median 6757 N 95%CI 6682 to 7303 N) and hand tapped (median 7805 N 95%CI 7154 to 7850 N) threads. As printed threads and those run through using water had significantly lower pushout forces. This study shows for the first time that 3D printed screw threads for a locking osteosynthesis plate application have comparable strength to traditionally produced screw threads.