Newitt A, German AJ, Barr FJ.
Department of Veterinary Clinical Science, University of Liverpool, Small Animal Teaching Hospital, Crown Street, Liverpool, L7 7EX, UK.

Vet Radiol Ultrasound. 2008 Jan-Feb;49(1):35-41. 

Radiographs of vertebral segments from 200 cats were retrospectively assessed to determine the incidence of congenital axial skeletal abnormalities. The axial skeleton was divided into cervical, thoracic, lumbar, sacral, and caudal segments and the presence and types of congenital abnormalities present were recorded for each segment. A total of 598 axial skeletal segments were examined. Congenital abnormalities were found in 46 cats and involved 54 spinal segments. Block vertebrae were identified in three cats.

No hemivertebrae or spina bifida were noted. Transitional abnormalities were most common, and were identified in 51 spinal segments. Abnormalities at the sacrocaudal junction were the most common of these; 19 examples were found. The second most common transitional abnormality was thoracicization of L1, with 18 such cats identified.

Extra ribs on L1 were often smaller and fragmented and it may be possible for an inexperienced clinician to confuse these with rib fractures. The last rib(s) are also often used for anatomic localization of surgical procedures; the presence of a transitional lumbar vertebra with one or two extra ribs introduces the possibility of error or confusion about choice of surgical site.