Q29 - Cerclage Wire for Affordable Plate Fixation in Murine Critical Sized Defect Models

Introduction: : Bone can naturally repair itself relatively easily when affected by a small, well-stabilized defect. However, defects caused by trauma, osteomyelitis, or cancer resection that are greater than ~2.5cm or 2x the injured bone’s diameter usually cannot heal spontaneously despite adequate stabilization. These critical-sized defects (CSDs) require robust reconstruction interventions to preserve limb function and avoid amputation. Orthopedic research has established that rodents largely parallel the bone repair mechanisms of humans, making rodents valuable models to investigate bone repair and CSD treatment. Replicating CSDs in mice is technically challenging due to the exceptionally small dimensions and quite costly since precision implants, tools, and surgical guides are needed. Ideally, internal plate fixation can be used to better replicate clinical implementation and also limit secondary, transcutaneous infection. Unfortunately, commercially available microscrews for integrating bone and the limb-stabilizing plate can be cost-prohibitive as they can easily cost well over $60 per animal plus additional premium costs for the accompanying precision drill/saw guides, drill bits, and screwdrivers. The objective of this study was to create a more affordable alternative for CSD research using cerclage wire. To thoroughly challenge this method, plates printed from Tough 1500, a relatively weak 3D resin from FormLabs, were used to simulate inadequate plate fixation. We then used the current standard polyether ether ketone (PEEK) plates, a radiolucent polymer with strong mechanical properties, to simulate ideal fixation. We hypothesized that cerclage wires could be used to achieve satisfactory fixation at a much lower cost, even with high motion implants.

Materials and

Methods: : Material properties of Tough 1500 and PEEK plates were characterized in vitro and then their efficacy was evaluated in vivo. In vitro testing was achieved by exposing specimens to different conditions: 1XPBS vs air, room (25°C) vs body (40°C) temperature, 24 vs 48 hours. All plates (n=3-4/group/material, N=31-32/material) were subjected to 3-point bend testing (preload=3N, 0.1mm/s displacement to failure) and analyzed via MANOVA. IACUC-approved CSD survival surgeries (Fig1A-B) involving internal plate fixation of femoral osteotomy (3 mm) were performed. One cohort of 13 mice (8M, 5F, 26.9±7.1 weeks old) received Tough 1500 plates while a second cohort of 23 mice (13M, 10F, 16.4±0.54 weeks old) received PEEK plates. To assess plate stability and wire integration up to 20 weeks post-operation, two unbiased assessors were given a standard grading to blindly grade randomized x-ray pairs (≤10 timepoints/animal, N=73). The plates were graded on a scale of 1 to 4 with the latter being broken while the wires were graded on a scale of 1 to 3 with the latter having both lysis and overgrowth (Fig1C). Their independent scores were averaged and tested via Kruskal-Wallis. Blinded x-ray grading is ongoing with the resulting PEEK plates as the animals are 18 weeks or less post-operation (Fig1D-E). Preliminary microCT analysis of femurs with the PEEK plates was conducted to evaluate bone regeneration in the defect and wire integration (MicroCT 50, ScanCo Medical, Brüttisellen, Switzerland; 70kVp, 114μA,8W, 15μm). MicroCT images were analyzed using Dragonfly software (Version 2022.2, Object Research Systems [ORS] Inc., Montreal, Canada).

Results, Conclusions, and Discussions:: Mechanical testing data confirmed Tough 1500 plates in fluid were compromised regardless of temperature or time (Fig2AC). Tough 1500 properties were significantly different for 14/16 parameters in medium while PEEK properties were significantly different in only 4/16 parameters in medium (Fig2B). Material was significantly different in every parameter (Fig2D). X-ray grading confirmed an unstable environment was achieved in vivo (Fig3A). While plates greatly deformed after 2-weeks as evidenced by severe bone misalignment, the bones moved together when medially rotated, indicating plate intactness. Regardless, the wires were relatively undamaged and secured the plate without compromising the bone material (Fig3B).

In vitro mechanical testing of Tough 1500 and PEEK plates indicated the former is much weaker, especially when exposed to internal body conditions. In contrast, PEEK plates were either unaffected or strengthened by exposure to PBS, confirming their superiority for internal fixation. These results support that both unstable and ideal environments were achieved. X-ray grading of Tough 1500 plates showed that while plates often failed 4-weeks post-implantation, the cerclage wires were relatively undamaged and continued plate fixation without compromising the bone via lysis or new bone growth. This is promising for future use of cerclage wires in PEEK. Given the better mechanical properties of PEEK, similar or better results are expected due to better stabilization. Animals with PEEK plates are ongoing and will undergo blinded x-ray grading. Additionally, microCT imaging and histology will be conducted on the PEEK plates to evaluate bone growth and the presence of fibrous tissue, respectively. Preliminary results have shown animals were unable to achieve union, even 20 weeks post-op, indicating a true CSD model was achieved (Fig3C). Additionally, bone growth directly upon the cerclage wires was observed, indicating satisfactory fixation (Fig3D).

Cerclage wires were able to maintain fixation without causing extreme damage to the bone material despite plate instability, indicating that cerclage wires are a viable option for plate fixation in murine CSD models. Developing a fixation method that is both affordable and durable could make mouse CSD repair research more accessible to labs with limited resources and thereby expedite the improvement of CSD treatments.

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