Naoki Sasaki, DVM, PhD; Takuto Minami, DVM; Kazutaka Yamada, DVM, PhD; Haruo Yamada, DVM, PhD; Yoshinobu Inoue, PhD; Mitsutoshi Kobayashi, DVM, PhD; Yasuhiko Tabata, PhD, DMS, DP American Journal of Veterinary Research December 2008 69 12 1555-1559 Objective—To evaluate the effect of intra-articular injection of gelatin hydrogel microspheres containing basic fibroblast growth factor (bFGF) on experimentally induced defects in third metacarpal bones (MC3s) of horses, in vivo. Animals—6 healthy adult Thoroughbreds. Procedures—Horses were anesthetized, and a hole (diameter, 4.5 mm) was drilled into the medial condyle of both MC3s of each horse. One milliliter (100 μg) of a solution of gelatin hydrogel microspheres (2 mg) containing bFGF was injected into the joint capsule of the right metacarpophalangeal joint of each horse (bFGF joint). One milliliter of saline (0.9% NaCl) solution was injected into the left metacarpophalangeal joint (control joint). Radiography was performed 1 day and 4, 8, 12, and 16 weeks after surgery to evaluate bone defect refilling. Sixteen weeks after surgery, multidetector-row computed tomography (MDRCT) was performed to determine the degree of refilling at the bone defect site. Results—Radiography revealed healing of bone defects at 4 to 12 weeks after surgery in bFGF joints and at 8 to 16 weeks after surgery in control joints. In addition, MDRCT revealed a higher degree of healing in bFGF versus control joints. Mean ± SD MDRCT score for bFGF joints (411.7 ± 135.6 Hounsfield units) was significantly higher than that for control joints (240.8 ± 133.1 Hounsfield units). Conclusions and Clinical Relevance—Treatment of horses with gelatin hydrogel microspheres that contained bFGF enhanced bone regeneration and healing of experimentally induced defects. This treatment strategy may be useful for treating horses with fractures.