Development of a Rat Caudal Interbody Fusion Model: A Pilot Feasibility Study

Rat caudal vertebrae are widely used in bone regeneration and spinal fusion research as convenient sites for testing bone graft materials and implants. Prior rat tail fusion models have achieved intervertebral fusion through disc removal and bone grafting, or with external fixation devices. Nonetheless, no such small-animal model has been developed that uses an interbody fusion cage analogous to that used clinically. It was hypothesized that an evaluable bone fusion environment could be established without neurological complications by performing a posterior cage insertion mimicking clinical procedures, supplemented with plate fixation, in the rat caudal vertebrae. This preliminary pilot study (n = 1) was designed to evaluate technical feasibility. A surgical approach was developed in which a miniaturized titanium cage was precisely inserted into the resected disc space between the adjacent caudal vertebrae, closely mirroring the human surgical procedure. Titanium was selected for the cage owing to its widespread clinical use and biocompatibility. The initial results demonstrated the technical feasibility of this approach. The implanted cage maintained disc height and spinal alignment. Progressive bone ingrowth through the cage was observed, culminating in confirmed continuous bone bridging across the intervertebral space by 12 weeks postoperatively. The presence of continuous trabecular structure and the absence of intervening fibrous tissue were confirmed radiologically and histologically. Overall, this cage-based rat tail fusion model provides a promising platform for evaluating novel biomaterials and investigating mechanisms of intervertebral bone fusion. However, the lack of a control group and statistical analysis, due to the single-subject design, limits the generalizability of these findings and necessitates further validation.