Title: Shape memory bone and soft tissue fixations
Funder: Engineering and Physical Sciences Research Council

Abstract: Building on our current research, we will explore enhanced bone to bone and soft tissue to bone fixation devices made from bionert and bioresorbable shape memory polymers (SMP). These recover or partially recover an initial shape when subject to an appropriate stimulus (usually heating) above their glass transition temperature. SMPs will be structured by processing operations, to obtain controlled molecular orientation, whose recovery drives shape reversion. The research will be undertaken in close collaboration with Innovate Orthopaedics, a medical device company and Clinicians from the Fortius Clinic. The proposed research will address: (i) specific unmet clinical needs in major market segments, (including shoulder and knee ligament injuries, meniscal tears and scaphoid fractures); (ii) treatment of specific patient subgroups to achieve a reduced overall recovery time, including a faster 'return to function' time, e.g. older people - over 50's are more susceptible to traumatic and degenerative injuries; rising levels of obesity are a risk factor in arthritis and meniscal injury; high performance athletes, or military personnel/ manual workers suffer sports or job-related trauma; for most cases, such injuries bring a significant socio-economic cost; (iii) overcoming known deficiencies in existing products (e.g. loosening metal compression screws); (iv) enhanced functionality for surgeons, with the potential for shorter theatre times and simplified procedures, and significantly reduced inventories. In our research: (i) SMPs will be formulated for specific applications, by including plasticisers to control reversion onset temperature, and other additives; (ii) SMPs will be structured by (a) solid phase processing, to obtain controlled molecular orientation, whose recovery drives shape reversion, (b) by precision injection moulding, and (c) by hybrid processing using a range of techniques including: overmoulding of SMP cores; reversion moulding; reverting SMPs into moulded end features; developing novel geometries to reduce heat transfer to tissue; 3-d printing of two materials for solid phase processing.; (ii) Triggering of shape recovery by body T fluid (saline 37C); micro resistance heating probe; a hybrid route - trigger start of recovery by heating probe, with prolonged body T reversion by plasticisers; local ultrasound heating (iii) controlled reversion in clinically relevant timescales - fast or slow according to application, using controlled reversion triggering temperature, including body temperature - and degree of reversion; (iv) the balance between bioresorption rate and interference force and pull out force from bone (Leeds University facilities); (iii) the value of shape memory behaviour in accommodating variations in bone and fixing hole quality; (v) match of bone properties/ osteogenesis including incorporation of nano hydroxyapatite and antiinflammatories being developed in our labs; (v) modelling (finite element analysis) of stresses in SMP fixations to inform optimal design. Partner Roles: Clinicians - a) define Clinical challenges/ requirements for specific indications, especially scaphoid compression screws and soft tissue fixations, including ACL and rotator cuff repair; b) input to design of fixation devices and c) help remove barriers to clinical adoption; Industry - evaluate a) market size, b) process economics, c) routes to market, d) device designs Regulatory - we will engage with MHRA and BSI via our MeDe Innovation Centre.

Lay abstract: The overall aim of this collaborative project is to exploit unique capabilities in shape memory polymers for enhanced biomedical cementless fixations using controlled shape reversion, in clinically relevant timescales, and open up opportunities for new solutions in orthopaedic repairs, which will have a defined route to regulatory certification and a clear patent pathway. Specifically, building on grant EP/L020572/1, with vital industry and clinician collaboration and guidance, we aim at applications in a range of soft tissue to bone fixation devices (e.g. plugs and anchors) and bone to bone (e.g. compression screws) made from approved biocompatible bioinert or bioresorbable shape memory polymers (SMP). SMPs recover or partially recover an initial shape when subject to an appropriate stimulus above their glass transition temperature. The proposed research will address: (i) specific unmet clinical needs in major market segments, (including shoulder and knee ligament injuries, meniscal tears and scaphoid fractures); (ii) treatment of specific patient subgroups to achieve a reduced overall recovery time, including a faster 'return to function' time, e.g. older people - over 50's are more susceptible to traumatic and degenerative injuries; rising levels of obesity are a risk factor in arthritis and meniscal injury; high performance athletes, or military personnel/ manual workers suffer sports or job-related trauma; for most cases, such injuries bring a significant socio-economic cost; (iii) overcoming known deficiencies in existing products (e.g. loosening metal compression screws); (iv) enhanced functionality for surgeons, with the potential for shorter theatre times and simplified procedures, and significantly reduced inventories. We aim to : 1. further develop SMPs, using essentially amorphous bioinert and bioresorbable polymers; 2. control polymer structures, namely molecular orientation and distribution of orientation achieved by solid phase die drawing and precision micro-injection moulding, to control the degree and rate of shape recovery achievable, and to explore controlled distribution of properties in fixations; 3. explore novel hybrid processes to add features, e.g. screw threads or holding/ surgical tooling features onto shape memory cores, by (a) over-moulding, (b) reversion moulding , (c) novel initial geometry SMP fixations to reduce heat transfer to tissue, and (d)3-d printing of 2 materials for solid phase forming of 'distributed property' products; 4. control reversion using thermal probes, SMP novel geometries and body temperature reversion SMPs using plasticisers to lower the glass transition temperatures of the polymers, which lowers the temperature required for recovery of molecular orientation; 5. incorporate bioactive ingredients in the SMPs, for promotion of bone growth, or counter inflammation or infection in SMP soft tissue fixations; 6. test all fixations in vitro, (i) SMP recovery behaviour (recovery-temperature-fixation force- time) including fixation/pull out force measurements (Leeds University facilities in our MeDe consortium) SMP geometries during recovery (micro-CT & imaging); (ii) exploration of fixations in different bone qualities, and control of recovery required clinically; 7. design - we will undertake all of the research in the context of particular fixation product design requirements, including clinical procedure requirements, for fixations in full consultation with our project partners. Design principles and specific clinician operational requirements, including the potential for minimally invasive surgery where possible (e.g. rotator cuff) - will be complemented by finite element analysis of stress distributions (in device and device to tissue) to inform optimal designs. 8. incorporate surgical practice issues to guide all of the above, including tools for triggering and fixation aimed at minimally invasive surgery.

Research activity codes: 1.3 CHEMICAL AND PHYSICAL SCIENCES;5.3 MEDICAL DEVICES
Health categories: INJURIES AND ACCIDENTS;MUSCULOSKELETAL