Effects of the Fusionless Instrumentation on the Disks and Facet Joints of the Unfused Segments: A Pig Model

Background: Growing rod (GR) is a state-of-the-art procedure favored when curvatures of the spine cannot be managed nonoperatively in early-onset scoliosis. Although some postulate that multiple distractions and/or relative immobilization of the unfused segments affect the health of disk and facet joint (FJ) and cause degeneration and/or spontaneous fusion, this has not thoroughly been investigated. In this study, changes in the unfused segment after a spine-based fusionless instrumentation (SBFI) are investigated and compared with the control (CG) and instrumented fusion (IF) groups. Methods: A total of 13 piglets, 10 to 14 weeks of age, were used. SBFI and IF were performed on 7 and 3 piglets, respectively, and 3 formed the CG. In SBFI, lengthening procedures of 5 mm were applied once monthly for 3 months, and, after 4 months, all piglets were euthanized. Histologic sections of the unfused disks and FJ were analyzed, and morphometric histologic analysis was performed. Results: On the basis of the Gries criteria, unfused disk median grades were 1, 2, and 4 for control, SBFI, and IF, respectively, that revealed a statistical difference (P < 0.001). Unfused FJ median grades were 1 and 2 for control and SBFI, respectively, that revealed a statistical difference (P < 0.001). The mean hypertrophic zone (HZ) heights were 69.78, 84.20, and 66.14 mu m; HZ chondrocyte cell widths were 19.03, 18.76, and 17.36 mu m; and HZ chondrocyte cell heights were 15.01, 15.04, and 12.42 mu m in the CG, SBFI, and IF groups, respectively. Statistically, for HZ heights, SBFI was different compared with CG and IF (P < 0.001), and, for HZ chondrocyte cell widths and heights, IF was different compared with CG and SBFI (P < 0.001). Conclusions: Morphometric analysis in this study supports the findings that SBFI preserves the growth potential of the spine. Furthermore, changes in the HZ heights show that distractive forces stimulate the apophyseal growth of the axial skeleton describing how the growth rate of the spine in GR might surpass the normal growth rate. Overall, although some degenerative changes are observed, SBFI and repeated distractions alone are not solely responsible for FJ arthrosis and disk degeneration, given that they are structurally preserved. Clinical Relevance: GR and regular lengthening procedures do not impair disk health and preserve the growth potential of the spine if it is applied with a meticulous technique.