RESUMO
Disabilities can emerge due to traumatic spinal fractures. In terms of sacrococcygeal spine, because of its unique anatomic structure with minimal movement, the possibility for it to have a disability is relatively low. In Korea, unlike most disability criteria, private insurance companies acknowledge angular deformities caused by vertebral fractures as disabilities according to their degree, so there were several cases where patients required compensation, arguing angular deformity caused by sacrococcygeal fracture, which in some cases led to legal conflicts. Except the Act Welfare of Persons with Disabilities which recognizes only severe angular deformity affecting internal organs as disability and the industrial accident disability evaluation which does not recognize coccygeal fracture as disability but rarely recognizes sacral vertebra deformity equivalent to compressive deformation, there is little or no case where angular deformity is recognized as disability. Given the impairment evaluation standards in social insurance, McBride system, American Medical Association (AMA) guides, and newly proposed standards by the Korean Academy of Medical Sciences (KAMS), the most contentious point in the general terms and conditions of private insurance is spinal deformity. To overcome controversy over disability evaluation, the private insurance sector is now applying criteria for axial skeleton to sacrococcygeal vertebrae through revision of standards. Under these circumstances, it is fair to recognize sacrococcygeal fracture as impairment in terms of the pelvis only when the fracture leaves serious deformity and neurological symptoms with clear relevancy. Though it may not be easy to develop accurate disability evaluation standards, improvement is necessary to remove any irrationalities and make the standards as objective as possible.
RESUMO
OBJECTIVE: To investigate the effects of posterior implant rigidity on spinal kinematics at adjacent levels by utilizing a cadaveric spine model with simulated physiological loading. METHODS: Five human lumbar spinal specimens (L3 to S1) were obtained and checked for abnormalities. The fresh specimens were stripped of muscle tissue, with care taken to preserve the spinal ligaments and facet joints. Pedicle screws were implanted in the L4 and L5 vertebrae of each specimen. Specimens were tested under 0 N and 400 N axial loading. Five different posterior rods of various elastic moduli (intact, rubber, low-density polyethylene, aluminum, and titanium) were tested. Segmental range of motion (ROM), center of rotation (COR) and intervertebral disc pressure were investigated. RESULTS: As the rigidity of the posterior rods increased, both the segmental ROM and disc pressure at L4-5 decreased, while those values increased at adjacent levels. Implant stiffness saturation was evident, as the ROM and disc pressure were only marginally increased beyond an implant stiffness of aluminum. Since the disc pressures of adjacent levels were increased by the axial loading, it was shown that the rigidity of the implants influenced the load sharing between the implant and the spinal column. The segmental CORs at the adjacent disc levels translated anteriorly and inferiorly as rigidity of the device increased. CONCLUSION: These biomechanical findings indicate that the rigidity of the dynamic stabilization implant and physiological loading play significant roles on spinal kinematics at adjacent disc levels, and will aid in further device development.