Evaluating strength of 3D printed screw threads for patient-specific osteosynthesis plates

Objectives: The angular stability of locking screws has made them ubiquitous in osteosynthesis plates, due to the importance of maintaining the correction during the course of healing. Bespoke and personalised implants have been made possible by advances in additive manufacture using titanium alloy, however, printed threads are a challenging feature to incorporate in additively manufactured parts due to the feature size in comparison to the main part. This study evaluated the potential to print locking screw threads within an osteosynthesis plate.
Methods: Tapered, double-start threaded Ti-6A-4V screws were custom-made to similar dimensions to the most commonly available locking screws on the market: 6.5mm maximum head diameter, 0.5mm pitch threads and 14 degree total taper angle. One hundred and nine corresponding female threaded specimens were additively manufactured (Renishaw PLC, UK) at different build orientations: 0°, 20°, 45° and 90° with different numbers of threads: 3, 4, 5 and 6. An initial power study determined that at least n=8 per group was required for a power of 80%. The main outcome measure was the ‘thread capacity’ defined as the maximum force recorded during destructive push-out testing of the screwsample threaded interlock (strain rate according to ISO6892-1:2016). Mann-Whitney statistical test was used to evaluate the differences between the groups.
Results: A steeper orientation of build direction was generally found to increase the thread capacity for any number of threads. A 90° build orientation was found to produce significantly (p=0.029) larger thread capacity than 0° for all thread numbers. The mean capacity of 5 threads was 2886±584N for 0° build orientation compared to 1435±407N for 90°. Increasing thread numbers increased the thread capacity by 202.3±86.7N per thread for 90° orientation.
Conclusion: The build orientation significantly influenced the thread capacity and we found that a vertical build orientation is superior for push-out thread resistance, however, there was a large variability in thread capacity. We examined a worst-case scenario; in reality the threads would be exposed to a combination of shear and bending.