Case Report: Secondary ulnar overgrowth and elbow incongruity following chronic forelimb disuse in a dog.

Chronic limb disuse during the active growth phase can induce complex secondary deformities, including paradoxical bone overgrowth. A 12-month-old neutered male Toy Poodle presented with non-weight-bearing right forelimb lameness resulting from a chronic shoulder malunion following failed stabilization of a medial luxation. Computed tomography revealed a complete nonunion of the shoulder joint with severe varus deformity, alongside a significant secondary ulnar overgrowth (6.08% elongation compared with the contralateral ulna) that created a functional 'short-radius' configuration and elbow incongruity. A combined surgical approach was employed: shoulder arthrodesis was performed utilizing patient-specific 3D-printed osteotomy and reduction guides to ensure precise alignment and fixation at a target angle of 105°, concurrent with a proximal ulnar osteotomy to restore ulnar congruity. Postoperative recovery was rapid, with functional weight-bearing observed at 1 month. At 9 months postoperatively, the dog exhibited normal limb function without lameness, and radiographs confirmed solid arthrodesis and corrected elbow alignment. These findings remained unchanged at 15 months postoperatively. This case highlights chronic disuse as a potential etiology for paradoxical ulnar overgrowth and demonstrates a successful combined surgical strategy for the management of complex, multi-joint limb deformities.