Sustained Dynamic Compression Device Adaptively Compensates to Bone Resorption Following Arthrodesis in Ovine Hindlimb Model

Abstract

Category: Midfoot/Forefoot; Basic Sciences/Biologics Introduction/Purpose: Charcot neuroarthropathy (CN) is a degenerative disease, which often impacts the midfoot and ankle, potentially leading to dislocations/fractures in the boney structures of the foot. The degenerative nature of this disease challenges the surgical intervention (beaming of the medial and lateral columns) for this condition by decelerating the fusion time, exposing the hardware to increased loading magnitude and duration, ultimately leading to increased hardware failure rate in these patients/procedures. To address this challenging patient population, a device was designed to apply sustained dynamic compression for use in midfoot fusion. While similar devices have been successfully implemented clinically in tibiotalocalcaneal fusion, this study sought to confirm mechanical durability via bench top testing and efficacy of this dynamic device in an ovine model of midfoot arthrodesis. Methods: A 7 x 100 mm dynamic device and an equivalent sized static device were used in this study. This 2x2 study divided the thirty-two mature Rambouillet Cross ewes into four groups based on bone quality (normal vs depleted) and device type (static vs dynamic) (CSU IACUC #1199). A pre-established model was utilized to reduce the bone quality of the target limb prior to arthrodesis via external fixation unloading, which has previously demonstrated a 29% decrease in bone mineral density. Multi-joint arthrodesis surgery was performed through the calcaneus-tarsus-metatarsal complex, followed by serial radiographic evaluations every four weeks. Sheep were euthanized 8-months postoperatively for biomechanical testing, micro-computed tomography analysis, and histology assessment of bone-device constructs. The study was conducted under strict ethical guidelines and approved by the Colorado State University Animal Care and Use Committee. Separate benchtop testing was conducted to confirm resorption capacity of both static and dynamic devices. Results: In vivo radiographic assessment revealed no significant differences in bone resorption/joint settling distances across bone quality at all timepoints (p≥.40), with ≥52.3% of total resorption/settling occurring within the first two months post-implantation. At the end of the 8-month study period, the normal and depleted bone quality samples exhibited 2.2 and 1.6mm of total resorption (4mm maximum device capability), respectively (p=.40). Four-point bending of treated calcaneus-tarsus-metatarsus constructs revealed no significant differences between device type or bone quality (p≥0.21). MicroCT assessments revealed significantly increased arthrodesis bone volume fraction (p=.036,Figure1) and mean density bone volume (p=.041,Figure1) in the dynamic group as compared to the static group. Benchtop testing revealed the dynamic device maintained compression through its 4mm design target; whereas the static device lost all compression within 0.5mm. Conclusion: These data illustrate that bone resorption of less than 1mm will eliminate all compressive force when using static fixation hardware, highlighting the benefit of this dynamic device to apply the requisite continued interfragmentary compression in this clinically challenging situation. This capability was confirmed by the in vivo radiographic measurements of dynamic device movement, similar biomechanical stiffness values, and increased resultant arthrodesis bone volume as measured by microCT. These benchtop and preliminary data highlight the capability of a dynamic device to compensate for bony resorption/joint settling in a multiple segment arthrodesis procedure for patients with both good and poor bone quality.