Mechanical Study of Various Pedicle Screw Systems including Percutaneous Pedicle Screw in Trauma Treatment.

Background and Objectives: Spine surgery using a percutaneous pedicle screw placement (PPSP) is widely implemented for spinal trauma. However, percutaneous systems have been reported to have weak screw-rod connections. In this study, conventional open and percutaneous systems were biomechanically evaluated and compared. Material and Methods: The experiments were performed in two stages: the first stage was a break test, whereas the second stage was a fatigue test. Four systems were used for the experiments. System 1 was intended for conventional open surgery (titanium rod with a 6.0 mm diameter, using a clamp connecting mechanism). System 2 was a percutaneous pedicle screw (PPS) system for trauma (titanium alloy rod with a 6.0 mm diameter, using ball ring connections). System 3 was a PPS system for trauma (cobalt-chromium alloy rod with a 6.0 mm diameter, using sagittal adjusting screw connections). System 4 was a general-purpose PPS system (titanium alloy rod with a 5.5 mm diameter, using a mechanism where the adapter in the head holds down the screw). Results: Stiffness values of 54.8 N/mm, 43.1 N/mm, 90.9 N/mm, and 39.3 N/mm were reported for systems 1, 2, 3, and 4, respectively. The average number of load cycles in the fatigue test was 134,393, 40,980, 1,550,389, and 147,724 for systems 1 to 4, respectively. At the end of the test, the displacements were 0.2 mm, 16.9 mm, 1.2 mm, and 8.6 mm, respectively. System 1, with a locking mechanism, showed the least displacement at the end of the test. Conclusion: A few PPS systems showed better results in terms on stiffness and life than the open system. The experiments showed that mechanical strength varies depending on the spinal implant. The experiments conducted are essential and significant to provide the mechanical strength required for surgical reconstruction.

Cervical Spine Osteoradionecrosis.

Osteoradionecrosis (ORN), a well-known complication of radiotherapy in the mandibular bone, is very rare in the cervical spine. The authors report the result of a 3-year follow-up of a 63-year-old female patient with ORN of the cervical spine. The patient had a history of laryngeal carcinoma and was treated with chemotherapy and radiation therapy with a total of 120 Gy. Eight years later, she developed acute, severe neck pain due to cervical spine necrosis. The authors performed vascularized fibular bone graft and posterior pedicle screw fixation to reconstruct her cervical spine. The patient was successfully treated with surgery, and cervical alignment was preserved. She had neither neurological deficits nor severe neck pain at her final follow-up 3 years later. Delaying treatment of ORN may be life threatening, so the early diagnosis of this condition is important for patients who receive radiotherapy. Otolaryngologists and spine surgeons should understand this potential complication to speed diagnosis and treatment as early as possible.

Clinical application of a pedicle nail system with polymethylmethacrylate for osteoporotic vertebral fracture.

The instrumentation of the osteoporotic spine may sometimes result in failure due to the loosening or pullout of the conventional pedicle screw. Moreover, augmentation of screws with polymethylmethacrylate (PMMA) has risks of complications. We developed a new and original pedicle nail system with PMMA for osteoporotic vertebral fractures. A clinical evaluation of this novel pedicle nail system utilized in patients with an osteoporotic vertebral collapse was performed to determine the effectiveness and safety of this technique. Thirty-four elderly patients who suffered from osteoporotic compression fractures were treated by posterolateral fusion using the pedicle nail system. The mean follow-up period was 37 months. Of the 25 patients with neurological symptoms, two patients improved two stages at the Frankel level. Fifteen patients improved one stage at the Frankel level, and eight other patients improved, however, their improvement did not exceed a Frankel level. Nine cases with neuralgia symptoms improved from 4.4 to 2.2 points on average on the Denis pain scale (p < 0.01). The fusion rate was 94% as determined by X-rays of flexion and extension, and the correction of the compression fracture site was maintained well. A pedicle nail system stabilizes the spinal column with osteoporosis and reduces the instrumentation failure. The technique for the insertion of the pedicle nail reduces complication from cement augmentation. The authors speculate that the strategy using the pedicle nail system for osteoporotic spine may be effective and safe when the surgery is performed through a posterior approach.

Implant Failure of Titanium Versus Cobalt-Chromium Growing Rods in Early-onset Scoliosis.

STUDY DESIGN: Retrospective case series of one institute database. OBJECTIVE: To investigate the differences in the metallic strength of rods used for implant failure in the dual growing rod technique and evaluate clinical outcomes. SUMMARY OF BACKGROUND DATA: The dual growing rod technique in which implanted rods extend with the growth of the spine is a useful treatment for early onset scoliosis. However, many complications, particularly those associated with rods, exist. Especially, the implant failure of growing rod focused on metallic strength is unknown. METHODS: Thirteen patients (42 lengthening surgeries) who underwent surgery by this technique at our hospital from 2007 were divided into a titanium rod plus titanium connector group (T group, n = 4, 26 lengthening surgeries) and cobalt-chromium rod plus titanium connection group (C group, n = 9, 16 lengthening surgeries). The incidence of implant failure and the site of fracture were retrospectively investigated. RESULTS: Implant failure occurred in three patients in the T group, because of rod fracture in two patients and connector fracture in one. In the C group, implant failure occurred in six patients, because of rod fracture in one patient and connector fracture in seven. Fracture occurred twice in two patients. The rod fracture rate decreased with the use of cobalt-chromium rods but the rate of connector fracture increased. We performed a stress distribution analysis using the finite element method to clarify the mechanisms underlying implant failure in both groups. Regardless of the rod type, the greater load was placed on the distal rod. However, differences in the metallic strength caused the rod to fracture when titanium rods were used and connectors (weak metallic strength) to fracture when cobalt-chromium rods were used. CONCLUSION: Rod fractures occurred more commonly with titanium rods and connector fractures with cobalt-chromium rods.

Biomechanical effect of the C2 laminar decortication on the stability of C2 intralaminar screw construct and biomechanical comparison of C2 intralaminar screw and C2 pars screw.

BACKGROUND: There have been no reports of biomechanical stability of C1-2 constructs after decortication of the C2 lamina. In addition, few studies have compared the stability of C2 laminar screw and pars screw constructs. OBJECTIVE: To compare the biomechanical stability of 3 different C1-2 construct conditions (C2 pars screw, C2 intralaminar screw, C2 intralaminar construct with C2 laminar decortication). METHODS: Fourteen fresh-frozen cadaveric cervical specimens (C1-3) were used. In 7 specimens, pure moments of 1.5 Nm were applied in flexion/extension, lateral bending, and axial rotation. Each specimen was tested in the normal state, in the destabilized state (after odontoidectomy and resection of transverse atlantal ligament), and after application of constructs. After kinematic study, these 7 specimens underwent axial pullout strength testing of pars screw and 50% decorticated C2 intralaminar screws. In another 7 specimens, insertion torque and pullout strength were measured to compare the pars screw and intact C2 intralaminar screw. RESULTS: : There were no statistically significant differences between the intact C2 intralaminar and 50% decorticated C2 intralaminar screw constructs in terms of range-of-motion limitations. The C2 pars screw construct was significantly superior to the C2 laminar screw construct in lateral bending (P < .01) and axial rotation (P < .01) and equivalent to the C2 laminar screw construct in flexion/extension (P = .42). There was no significant pullout strength difference between the 3 kinds of C2 screw. CONCLUSION: The C1 lateral mass-C2 pars screws construct was stronger than the C1 lateral mass-C2 intralaminar screw construct. Decortication of C2 laminar (up to 50%) did not affect the immediate stability of the C1-2 construct.

Posterior spinal fusion using a pedicle nail system with polymethylmethacrylate in a paraplegic patient after vertebral collapse caused by osteoporosis.

BACKGROUND CONTEXT: Because posterior decompression and fusion for vertebral collapse in an osteoporotic spine sometimes results in a pullout of the pedicle screw. Several authors reported that fixation of pedicle screws in severely osteoporotic bone could be improved by inserting polymethylmethacrylate (PMMA) into the hole before inserting the screw. However, pedicle screws were not designed to be used with PMMA. PURPOSE: To report a patient with vertebral collapse who were treated using a novel-concept, pedicle nail using with PMMA. STUDY DESIGN: Case report. METHODS: The patient was a 74-year-old female who experienced back pain after some heavy lifting 3 months before. She was laid up for 2 months because of the back pain and weakness of her lower legs. Radiographs and magnetic resonance imaging showed vertebral collapse and neural severe compression from the posterior wall of vertebra T12. The pedicle nail consists of an outer sheath with evenly spaced holes and an internal, removable, threaded component. The outer sheath and the internal component were connected before insertion. The pedicle nail attaches to and is bound by the PMMA through the holes of the outer sheath. We performed a T11-L1 posterior fusion and laminectomy using the pedicle nail, and we used a hydroxyapatite block to perform a transpedicular vertebroplasty of T12. RESULTS: Bone union was observed on a lateral radiograph without pedicle nail loosening. CONCLUSIONS: The authors speculate that posterior spinal fusion and vertebroplasty using the pedicle nail will be a reasonable choice for delayed paraplegia after osteoporotic vertebral fracture.

Comparative biomechanical analysis of an improved novel pedicle screw with sheath and bone cement.

STUDY DESIGN: A human cadaveric biomechanical study of fixation strength of an improved novel pedicle screw (NPS) with cement and a conventional screw. OBJECTIVE: To clarify whether the NPS has adequate fixation strength without leakage in vertebrae with low bone quality. SUMMARY OF BACKGROUND DATA: The fixation strength of pedicle screws decreases in frail spines of elderly osteoporotic patients. Augmentation of screw fixation with bone cement must be balanced against increased difficulty of screw removal and risk of cement leakage. We developed the NPS consisting of an internal screw and an outer sheath to mitigate the disadvantages of cement augmentation. METHODS: The T12 and L1 vertebrae obtained from 18 formalin preserved cadavers (11 males and 7 females; mean age, 82.7 y) were used. The mean bone mineral density was 0.39 +/- 0.14 g/cm2. The NPS was inserted into one pedicle of each vertebra and the control screw, a Compact CD2 screw, was inserted into the contralateral pedicle. Both screws were 6mm in diameter and 40 mm in length. Pull-out tests were performed at a crosshead speed of 10 mm/min. Cyclic loading tests were performed with a maximum 250 N load at 2 Hz until 30,000 cycles. RESULTS: Cement leakage did not occur in any of the specimens tested. The mean maximum force at pull-out was 760 +/- 344 N for the NPS and 346 +/- 172N for the control screw (P < 0.01). Loosening of 50% of the screws was observed after 17,000 cycles of the NPS and after 30 cycles of the control screw. The hazard ratio of loosening was 19.6 (95% confidence interval 19.3-19.9) (P < 0.001). CONCLUSIONS: The NPS showed a significantly higher mechanical strength than the control screw in both pull-out tests and cyclic loading tests. The NPS showed more than adequate strength without cement leakage.