Relationship Between Thoracic Cage and Spine Deformity in Rabbits with Experimental Scoliosis

• a - Synthes Spine, North America

Purpose: Investigate the relationship between growth of the spine and thorax under conditions that create symmetrical or asymmetrical growth disturbances of the spine or rib cage in a growing rabbit.

Background: Campbell developed the opening wedge thoracostomy in conjunction with implantation of a Vertical Expandable Prosthetic Titanium Rib (VEPTR) to expand the constricted hemithorax in children with congenital, infantile and neuromuscular scoliosis, acquired or congenital anomalies of the thorax and skeletal dysplasia. Understanding the relationship between growth of the rib cage and growth of the spine will help produce more predictable outcomes relative to the development of the thorax, lung and spine when using the VEPTR device.

Hypothesis: Growth of the thoracic spine and growth of the rib cage are directly related - a disturbance of one induces a deformity in the other.

Methods: Symmetric or asymmetric growth disturbances of the spine or rib cage were established in 15 five-week-old rabbits by means of tethering either the transverse processes of T5 - T8 unilaterally (3) or bilaterally (3) or ribs 5 - 8 unilaterally (3) or bilaterally (3). 3 rabbits were controls. Unilateral tethers were on the left. At time points during growth of the rabbits through maturity, structural changes in the spine and rib cage were assessed using serial CT scans of the entire spine and chest wall. Right, left and total lung volumes were calculated by integrating the area of lung tissue imaged on each transaxial CT summed over sequential slices T1 - T13. Measures of spine deformity, rib cage deformity and pulmonary function were related to one another across the experimental groups using ANOVA and Fisher’s probable least significant difference.

Results: The mean Cobb angle of the resultant thoracic scoliosis was significantly greater for the unilateral tethered rib group (13°) compared to: bilateral tethered rib group (4.3°, p <0 .001), unilateral tethered spine group (4.3°, p<0.001), bilateral (1.5°, p<0.001) and control (3°, p<0.001). The distortion of the thoracic cage measured by maximal chest rotation angle at apex scoliosis was significantly greater for rib (8.4°) compared to (3.5°, p="0.04)." (3.6°, There were no significant differences in total lung volume among experimental groups. However right:left ratio different (p="0.02)." Normalizing volumes weight animal, left equivalent right whereas 62% group.

Conclusions: A unilateral deformity of the spine or rib cage induces both a scoliosis and thoracic cage deformity with asymmetric lung growth. The deformity induced by unilateral tethered ribs is much greater than the deformity induced by unilateral tethered spine. The rib provides a longer moment arm than the transverse process so that tethered ribs create a larger deforming moment on the spine.

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