Investigations of 3D printed orthopaedic cortical bone screws to improve biomechanical pullout strength

Abstract

Bone fractures are a common hazard due to external forces acting on the bone, causing misalignment, or breakage of bones. Bone drilling is imperatively required for fractured bone fixation used during orthopaedics surgeries, joint replacement, plate implantation, and total knee arthroplasty. The fractured bone is repaired with bone screws and implants to secure and retain the accurate position of fracture parts for early mobilization and early union. Bone screws are used in almost every orthopaedic surgery, as they provide inter-fragmentary compression to support the fractured bone. The success of osteosynthesis is affected by bone-screw interface holding power, type of screw used, screw purchase strength, the number of screws used and bone mass available for fixation, the orientation of screws and bone mineral density. The poor pullout strength and holding power at the bone-screw interface may lead to intolerable pain, screw loosening and implant failure at the fracture site can cause reoperation and revision surgery. These orthopaedic bone screws are made up of metallic biomaterials such as titanium, and stainless steel that are strong, ductile, fatigue-resistant, and highly biocompatible. However, due to the high stiffness and high strength of metallic screws, they may cause stress corrosion, the stress-shielding effect, tear and wear in the surrounding tissues. Further, strong integration at the bone-screw interface by sharp threads and soft bony tissues complicates the post-operative revision surgery while removing the metallic screws. This problem can be solved with biodegradable bone screws. Three-dimensional (3D) printing technology provides the ideal tailor-made solution for different orthopaedic applications. Various researchers used this technology for the fabrication of patient-specific prosthetics and customized implants. Limited research is reported for the fabrication of orthopaedic bone screws using 3D printing technology. To overcome the aforesaid problems associated with surgical metallic bone screws, a novel 3D-printed bone screw is introduced. In the present work, fused filament fabrication-based 3D printing technology is used to fabricate orthopaedic cortical bone screws. The influence of different process parameters on the mechanical strength of 3D-printed cortical screws was monitored. The fracture and fatigue crack propagation at the bone screw interfaces were observed at the microscopic level. The process parameters of 3D printing technology are optimized with different machine learning (ML) models. A total of 100 data points were used for the training and testing of different predictive ML models. Ensemble ML models were also leveraged to improve the robustness and accuracy of base learning ML models. The bone drilling experiments were performed using rotary ultrasonic-assisted bone drilling (RUABD) and conventional bone drilling (CBD) to monitor the biomechanical pullout strength at the bone-screw interface. The influence of various drilling angles or inclination angles was observed on the pullout strength of bone screws. The micrographs at the bone-tool interface and drilled hole morphology were also observed to estimate the initial screw stability and holding strength at the bone-screw interface. The in-vitro degradation behaviour of 3D-printed cortical screws was observed in different biological solutions. Three different biological solutions were used that have similar ion concentrations to human blood plasma. The influence of different process parameters of 3D printing technology was observed on the degradation rate. The compressive strength of 3D-printed screws was tested before and after 24 weeks of preservation to understand the effect of degradation rate on the compressive strength of 3D-printed screws. So, it can be concluded that 3D-printed bone screws can be used for fracture fixation of small bones or anterior cruciate ligament fixation. Also, it can reduce the stress-shielding effect and eliminate the problem of post-operative revision surgery