Abstract An in vitro biomechanical study compared the influence of connecting plates on construct stiffness and motion of adjacent segments. Twelve porcine lumbar spines were destabilized by laminectomy and instrumented at the L3 and L4 vertebrae by one of three transpedicular screw fixation systems: Cotrel‐Dubousset, Diapason, and a newly designed implant (FPI). The current study demonstrated that connecting plates significantly increased the construct compression and flexion stiffness and added resistance to axial rotation. The upper intact disc had greater rotational displacement than the unfixed intact spine, but the lower intact disc had more anterior translational displacement. When the construct was flexed with a 1‐second period, the upper intact disc (L2/L3) flexed much more than with a 5‐second period (P<0.05). This study also showed a significant correlation between flexion stiffness and compensatory rotational displacement at the upper intact disc. Key words: spinal implantcross‐linkageloading rateadjacent segmentstiffness Notes Corresponding author. (Tel: 886–2–23631922; Fax: 886–2‐ 23631922; Email: sohon@ntu.edu.tw)
Study Design Rigid post-traumatic kyphosis after fracture of the thoracolumbar and lumbar spine represents a failure of initial management of the injury. Kyphosis moves the center of gravity anterior. The kyphosis and instability may result in pain, deformity, and increased neurologic deficits. Management for symptomatic post-traumatic kyphosis always has presented a challenge to orthopedic surgeons. Objectives To evaluate the surgical results of one-stage posterior correction for rigid symptomatic post-traumatic kyphosis of the thoracolumbar and lumbar spine. Summary of Background Data The management for post-traumatic kyphosis remains controversial. Anterior, posterior, or combined anterior and posterior procedures have been advocated by different authors and show various degrees of success. Methods One vertebra immediately above and below the level of the deformity was instrumented posteriorly by a transpedicular system (internal fixator AO). Posterior decompression was performed by excision of the spinal process and bilateral laminectomy. With the deformed vertebra through the pedicle, the vertebral body carefully is removed around the pedicle level, approximating a wedge-shape. The extent to which the deformed vertebral body should be removed is determined by the attempted correction. Correction of the deformity is achieved by manipulation of the operating table and compression of the adjacent Schanz screws above and below the lesion. Results Thirteen patients with post-traumatic kyphosis with symptoms of fatigue and pain caused by slow progression of kyphotic deformities received posterior decompression, correction, and stabilization as a definitive treatment. The precorrection kyphosis ranged from 30-60°, with a mean of 40° ± 10.8°. After correction, kyphosis was reduced to an average of 1.5° ± 3.8°, with a range from -5° to 5°. The average angle of correction was 38.8° ± 10.4°, with a range from 25° to 60°. Significant difference was found between pre-and post-operative kyphosis measures (P < 0.001). The follow-up period for all patients was 2 years, and the average kyphosis angle measured at the moment was 3.8° ± 3°, with a range from -3° to 8°. Substantial overall improvement was achieved in the 13 patients. Conclusion This method provides single-stage posterior decompression, correction, and stabilization as definitive management for post-traumatic kyphosis of the thoracolumbar and lumbar spine.
Study Design. A newly designed spinal implant was tested to evaluate multicycle stiffness and fatigue resistance. Objectives. To investigate the effect of different materials, connecting plate, and pedicle screw design on the mechanical performance of the spinal implant. Summary of the Background Data. The addition of cross-linkages did not significantly increase implant compression/flexion stiffness, but accelerated fatigue failure at the rod junctions. Both Ti-6Al-4V spinal implants and the 316L stainless-steel counterparts have been used extensively for clinical cases; however, design factors establishing the proposed superiority of the Ti-6Al-4V implant for fatigue resistance have not, as yet, been extensively studied. Methods. Twenty implants with connecting plates (two materials by two screw designs by five implants) and five implants without connecting plates were assembled to UHMWPE blocks and cyclically loaded from 60 N to 600 N at a frequency of 5 Hz. Results. Failure sites for the tested prototypes were at the cephalic screw hubs or rod–plate junctions. All Ti-6Al-4V implants demonstrated reduced stiffness compared to the structurally identical 316L analogs. The use of connecting plates raised the stiffness of the 316L prototypes without cross-links. However, elimination of the connecting plate avoided stress concentration at the rod/plate junctions and increased fatigue life. The Ti-6Al-4V new system with the minimal notch effect at the screw hubs achieved greater fatigue resistance than its 316L counterpart. By contrast, enlargement of the inner-hub diameter resulted in greater gains for fatigue resistance than for stiffness, especially for Ti-6Al-4V variants. Conclusions. Although Ti-6Al-4V was superior to 316L for endurance-limit properties, structural design of the Ti-6Al-4V implant dramatically affects fatigue resistance. This may explain the differences between existing studies and the current report, comparing fatigue life forimplantsmade from these two materials. Our results reveal that Ti-6Al-4V must be carefully treated because of sensitivity to notch, with special consideration given to screw-hub design.
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