Design And Experimental Study Of A Novel Growth-preserving Spinal Instrumentation For Scoliosis In Young Children

Background. Treatment of children with progressive scoliosis is challenging, especially the early onset scoliosis. Left untreated, progressive curves may produce significant thoracic deformity, leading to deleterious effects on the cardiopulmonary. The goal of surgery is to obtain scoliosis correction, control ongoing deformity while preserving spinal growth, and resume the normal volume of the thoracic cavity. Unfortunately, there is no ideal surgical technique now. Objectives. We develope a novel implant system for the treatment of scoliosis without arthrodesis, using pedicle screw with a sliding ring, to accommodate growth of the immature spine. The purpose of this in vivo study was to evaluate the growth preservation ability of the system which using pedicle screw with a specially designed sliding ring. With a 50 degree curve nickel-titanium shape memory alloy rod, we determine the mechanical modulation of vertebral body growth effects of the novel implant system, and thus to infer whether it is capable of correcting scoliosis in the immature spine. Concerning about the wear of the sliding ring with the nitinol rod, we evaluate the histologic response in the surrounding tissues, identify the metal debris generated by the sliding ring, quantify the amount of Ni.Methods. 1. we review all the contemporary growing rod techniques and develop a novel implant system for scoliosis surgery, which named "a growing system with sliding ring pedicle screw". 2. Animal study: i. Five fresh specimens of the vertebrae (T5, L1) of immature goat (age, 2 months) were used to determine the method of placing a 4 mm diameter pedicle screw, ii. Nine goats (age, 2-3 months) were randomized into there groups. First group (few screws group): L5 with locked pedicle screw, T10, L1 with sliding ring pedicle screw, the nitinol rod with no curve. Second group (multiple screws group): L5 with locked pedicle screw, T10, T12, L1, L3 with sliding ring pedicle screw, the nitinol rod with no curve. Third group (scoliosis group): L1, L2 with locked pedicle screw, T5, T6 with sliding ring pedicle screw, the nitincl rod with 50 degree curve. X ray was takedimmediately postoperative and 3 months later to document the sliding of the growing rod. CT 3-dimensional construction was used in the scoliosis group to observe the wedging of apical spinal segments. The height of growth plate chondrocytes on the convex, concave of the apical vertebra were studied histologically. iii. All animals were sacrificed there months postoperatively, we evaluate the histological response in the surrounding tissues, identify the metal debris generated by the sliding ring by spectrum analysis, quantify the amount of Ni by graphite furnace atomic adsorption spectrometry.Results. 1. Contemporary techniques can be classified into two categories by the mode of lengthen: one by serial surgeries and the other lengthened automatically. The growing system with sliding ring pedicle screw belonged to the latter. 2. Animal study: i. Start point of pedicle screw in T5 located at the ridge of the distal end of the transverse process, while L1 at the middle point of the origin of the lumbar transverse process. The direction parallel the endplate and in 45 degree introversion, ii. In the first group, the total lengthen of L5 to T10 averaged 4.1cm, the growth preservation of each vertebral is 0.51cm. In the second group, the total lengthen of L5 to T10 averaged 3.9cm, the growth preservation of each vertebra is 0.49cm. In the third group, the total lengthen averaged 4.8cm, the growth preservation of each vertebral is 0.48cm. In the third group, the Cobb's angle is 45 degree. CT 3-D scan indicate the wedging of apical vertebral and intervertabral disc. The height of growth plate chondrocytes of the apical vertebra on the convex side is higher than that on the concave side. iii. The tissue around the sliding ring is discoloration obviously, which around locked screw is nomal. Histologic examination showed fibrosis, foreign body reaction and inflammation associated with many metal particles, indicating metallosis around sliding ring. Ti, Al, V, Ni element was identified in the decoloration tissue. Tissue concentrations of Ni were higher in tissue around sliding ring than locked screw.(945.78±149.17 vs 5.16±2.02 mg/kg of dry tissue).Conclusions. 1. The novel implant system for scoliosis surgery, which named "a growing system with sliding ring pedicle screw", can glide smoothly to accommodate growth of the immature spine in vivo. It can preserve the spine growth effectively. 2. Loading asymmetric force to the immature spine by thisimplant can modulate the growth of vertebra, and thus to infer that it is capable of correcting scoliosis in the immature spine without fusion. 3. The sliding mechanism make it prone to wear. The tissue around the sliding ring is discoloration obviously, foreign body reaction and inflammation associated with many metal particles occur, indicating metallosis around sliding ring. Local concentrations of Ni was higher in tissue around sliding ring. The novel implant deserves further study and improvement.