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Session: Poster session I
Session starts: Thursday 24 January, 15:00

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Martijn Wessels (Universiteit Twente)
Edsko Hekman (Universiteit Twente)
Bart Verkerke (Universiteit Twente / Rijksuniversiteit Groningen)

Abstract:
The rigid nature of the majority of current implantable scoliosis correction systems leads to several negative side-effect. One effect is vertebral fusion, resulting in a rigid section of the spine. Another negative result is that once the implant is fixed to the spine further growth is arrested. We designed two flexible implants with the specific objective to correct idiopathic scoliosis without introducing fusion. The XS-LAT is a system made of NiTinol, which essentially provides a lateral bending force. The XS-TOR, made of a high grade Titanium alloy, essentially provides axial torsion. The correction process is force-driven: in fact, the spine is “guided” into growing straight again. Another characteristic feature of both implants is that they can slide in the anchors which connect them to the spine. Since fusion is prevented, functional bending of the spine and growth are largely unaffected. The systems have been tested by scoliosis induction in a porcine animal model. Objective was to determine the degree of deformation which the implants could induce, and to affirm that no fusion would occur. Six animals received the XS-LAT system while six other animals received the XS-TOR system. The implants were placed posteriorly and fixed to the T12, T15 and L2 vertebrae. Care was taken to avoid periosteal damage during surgery. Radiographs were taken preoperatively, peroperatively and 1, 4 and 8 weeks post-operatively. After 8 weeks the animals where sacrificed and analyses were performed investigating fusion, growth, tissue response, actual induced deformation and status of the implant. The XS-TOR induced a mean torsion of 12° between T12 and T15 and of 7.1° between T15 and L2, but only limited lateral deformation (1°). The mean Cobb-angle induced by XS-LAT was 19°, whereas induced torsion was 5°. There were no signs of obstruction of growth or decrease of kyphosis. Mean length progression of the implants in 8 weeks was 19.5%. In all pigs, some heterotopic ossification at different levels was observed, as well as cartilage degeneration in the facet joints of the instrumented vertebrae T12, T15 and L2. However, fusion of joints was not observed anywhere, and intervertebral discs showed no ossification. Tissue response to the implant was characterized as normal. We tested the implants is a scoliosis induction model, the only animal model available. A study by Meijer [Meijer, 2010] indicates that correction results may be different but not necessarily worse. We found that the system is capable of inducing significant deformities in two different directions without inhibiting spinal growth. The implants effect the intended directions, suggesting that splitting up the functionality of the implants in bending and torsion enables patient specific tuning of the corrective actions. With respect to fusion, this test can be considered as worst-case since bone formation response in young animals is always more severe than in humans [Eitel, 1981]. Long-term behaviour of the implant, including osseo-induction remains unknown. REFERENCES [1] G.J.M. Meijer: Development of a non-fusion scoliosis correction device: numerical modelling of scoliosis correction, PhD-thesis, Enschede 2011 [2] F. Eitel, F. Klapp, W. Jacobson, L. Schweiberer., “Bone regeneration in animals and in man.” Arch Orthop Trauma Surg. 1981;99(1):59-64.