PhD in 3D Printing Mechanically Adaptive Regenerative Interfaces for Improved Bone Screw Fixation

Low back pain is the leading cause of disability worldwide, and presents a significant socioeconomic burden from direct treatment costs, and indirect costs due to losses in productivity. An ageing population, combined with increases in inactivity and obesity is expected to further increase this burden. Low back pain is initially treated conservatively, but approximately 20% of patients will require surgery. However, spinal surgery is expensive, clinical outcomes are relatively poor, and reoperation rates are relatively high (~20% within 10 years).  Spinal surgery often uses pedicle screws and rods to provide stabilisation.  

However, screw loosening and pull-out are major complications, particularly in patients with poor bone quality (e.g., osteoporosis), which can result in pain and instability, ultimately requiring expensive revision surgery.  The design of screws has been adapted to mitigate loosening, including larger or longer screws, altered thread geometry, expandable screws, and bone cement augmentation. Non-biodegradable sleeves (e.g. Ogmend) have been used to distribute screw loading to increase pull-out force. However, clinical evidence of these mitigation strategies is inconsistent, or limited, and many devices are evaluated pre-clinically using insertion torque, uniaxial pull-out tests, or simplified cyclic loading, which do not replicate the complex multi-axis loading experienced in vivo. Furthermore, they do not actively regenerate the screw–bone interface or progressively transfer load to newly formed bone.   

The aim of this project is to develop a new generation of bioresorbable, regenerative scaffolds that engineer the screw-bone interface to improve primary and secondary fixation. By controlling scaffold architecture, mechanical properties, bioactivity and degradation, the scaffold will enhance immediate fixation whilst promoting osseointegration and progressive load transfer to newly formed bone. The project will combine biomaterials and tissue engineering with advanced pre-clinical evaluation using Exeter’s six-axis bioreactor. COSA Ltd will provide expertise in medical imaging, anatomical modelling, and multi-material 3D printing to develop reproducible bone-mimicking models for in vitro mechanical testing, whilst clinical guidance will help ensure that scaffold development is aligned with surgical practice.   

The project will:  

1. Develop and evaluate functionally graded and anisotropic scaffold architectures using additive manufacturing that are designed to distribute interfacial stresses, reduce damaging micromotion, and improve resistance to screw loosening and pull-out. 
2. Engineer material compositions to promote bone formation and tissue integration. Bioactive and mechanically responsive material strategies will also be explored to determine whether physiological loading can be harnessed to stimulate regeneration at the screw–bone interface (e.g., piezoelectric biomaterials). 
3. Integrate scaffold development with physiologically relevant six-axis mechanical bioreactor testing. Candidate designs will be evaluated under complex loading that better reflects activities of daily living than conventional tests. This will reveal how scaffold architecture influences load transfer, micromotion, and biological response, directly informing iterative design and optimisation.   

The project will therefore establish a design–manufacture–test process for engineering mechanically adaptive screw–bone interfaces. Although pedicle screw fixation will provide the primary clinical application, the underlying principles could ultimately be translated to other orthopaedic fixation devices where poor bone quality and implant loosening remain major challenges.  In order to achieve these goals, the project will be supported by an interdisciplinary supervisory team consisting of academics, clinicians, and industry with expertise in biomaterials, tissue engineering, additive manufacturing, and  mechanobiology. 

Location

University of Exeter, Penryn Campus