Single-rod versus dual-rod anterior instrumentation for idiopathic scoliosis: a biomechanical study.

STUDY DESIGN: Anterior single- and dual-rod instrumented human and ovine thoracolumbar spines, with and without structural interbody support (SIS), were biomechanically tested and compared in flexion, lateral bending, and torsion. OBJECTIVE: To determine significant differences in global stiffness of the constructs in an attempt to clarify specific indications for each in the treatment of spinal deformities. SUMMARY OF BACKGROUND DATA: Single- and dual-rod anterior systems have been used without any consensus as to indications for one versus the other. The potential added benefit of incorporating SIS and transverse connectors (dual-rod) with these constructs has also not been fully explored. METHODS: Four human cadaveric and six ovine spines were instrumented in single- and dual-rod constructs and biomechanically tested intact, postdiscectomy with and without SIS, with single- and dual-rod constructs, and with and without transverse connectors (ovine only). Biomechanical testing modes were flexion, lateral bending, and torsion. RESULTS: In the human cadaveric specimens, testing in flexion revealed that SIS was the major contributing factor for construct stiffness. In lateral bending, stiffness of single- and dual-rod constructs with and without SIS was equivalent. In torsion, both single- and dual-rod instrumentation and SIS appeared to contribute to global stiffness. In ovine specimens, dual rods were stiffer than single-rod constructs and SIS played only a minor role. Transverse connectors appeared to significantly stiffen dual-rod constructs in torsion only. CONCLUSIONS: Dual-rod constructs with SIS appear to be the best combination for providing stiffness in anterior instrumentation. The addition of cross-links to anterior constructs does not appear to increase stiffness except in torsion.

A biomechanical study of regional endplate strength and cage morphology as it relates to structural interbody support.

STUDY DESIGN: An in vitro biomechanical investigation to quantify the endplates resistance to compressive loads, in the thoracic and lumbar spine. Comparisons were made to determine the regional strength of the endplate, the optimal size and geometry of interbody support, and the effects of endplate removal on structural strength. OBJECTIVES: To biomechanically assess the regional variation of endplate strength in the thoracic and lumbar spine, the optimal geometry and cross-sectional area for structural interbody support, and endplate preparation techniques with respect to endplate failure or subsidence. SUMMARY OF BACKGROUND DATA: Anterior column interbody support plays an important role in spinal reconstruction. Subsidence of interbody structural support is a common problem and may be related to regional weakness of the endplate, the size and/or geometry of structural support, and the preparation of the endplate. Biomechanical data related to these issues should be of importance to spine surgeons and reduce the risk of subsidence and its inherent complications. METHODS: The indentation tests were performed in three subgroups, each with a different set of test variables. The first test consisted of 65 vertebrae at six different endplate test positions using a 9.53-mm diameter indenter. The second test was performed on 48 vertebrae at a central endplate test site using three hollow and two solid cylindrical indenters of varying diameter. The third test was done using 24 vertebrae with the endplate intact, partially removed, or fully removed. All tests were run using human cadaveric specimen using both the superior and inferior endplates. The maximum load to failure (MLF) was determined for each test performed. RESULTS: For all levels tested, the highest MLF occurred in the posterolateral region of the endplate. The lowest value occurred in the central and anterocentral regions for levels T7-L5 and T1-T6, respectively. Hollow indenters with a small diameter had the lowest MLF, whereas solid large-diameter indenters had the highest MLF. The ultimate compressive strength for all hollow indenters was significantly higher than all solid indenters. There was a significant reduction in the endplate strength with the complete removal of the endplate. CONCLUSIONS: The posterolateral region of the endplate provides the greatest resistance to subsidence while the central region provides the least resistance. A larger-diameter solid support has the greater MLF and the lower the risk of subsidence, suggesting a more efficient transfer of force to the endplate with the hollow indenters. Parameters such as the geometry of structural support and the position and preparation of the endplate can influence the resistance of an interbody support to subside. Partial removal of the endplate may provide both, for adequate mechanical advantage and a highly vascular site for fusion.

Anterior single rod instrumentation for thoracolumbar adolescent idiopathic scoliosis with and without the use of structural interbody support.

STUDY DESIGN: A radiographic and clinical outcomes analysis of 41 patients treated for thoracolumbar adolescent idiopathic scoliosis utilizing a single anterior rigid rod construct. OBJECTIVES: To evaluate the necessity of structural interbody support to improve primary curve correction and preserve or augment lordosis when used in conjunction with a single anterior rigid rod construct, to identify parameters that predict horizontalization of the lowest instrumented vertebra, adjacent disc angulation, and distal uninstrumented vertebrae, and to assess patient satisfaction following surgery. BACKGROUND DATA: Instrumentation-induced kyphosis has been a concern with nonrigid anterior systems used in the past for the treatment of scoliosis. Interbody structural support has been recommended to maintain appropriate sagittal profile when anterior systems are utilized. It has also been suggested that the use of structural interbody support creates a fulcrum to increase curve correction when compression is applied to the convexity of the deformity. However, the necessity of interbody structural support when used in conjunction with a rigid anterior system has not been previously evaluated in patients with adolescent idiopathic scoliosis. MATERIALS AND METHODS: Forty-one patients mean age 15.9 years (range 12.1-18.6 years) with thoracolumbar adolescent idiopathic scoliosis underwent anterior spinal fusion using a single 6.0 to 6.5 mm solid rod construct between June 1995 and August 1999 performed by the senior author (T.G.L.). Four additional patients with thoracolumbar curves with similar anterior instrumentation over the same time period were lost to follow-up or had incomplete records and were not included in the study. Structural interbody support was used in 21 patients and packed morselized autograft alone was used in 20 patients. The patients in the group with packed morselized bone alone generally underwent surgery earlier in the series before the author began using structural interbody support on a regular basis. Each patient had a minimum follow-up of 3 years. Preoperative, initial, and most recent (>3 years) follow-up radiographs were reviewed to determine in each group Cobb angle measurements, flexibility of primary, secondary, and fractional curves, apical and end vertebral translation, lowest instrumented vertebral and caudal disc angulation, global coronal and sagittal balance, and sagittal Cobb measurements in both instrumented levels as well as lumbar lordosis (T12-S1). In addition, the SRS outcomes instrument was completed by 38 of 41 patients. RESULTS: The mean preoperative primary curve in patients with structural support was 47 degrees (Group II) and 45 degrees in patients without structural support (Group I). Mean curve correction was to 13 degrees in Groups I and II. One patient in Group II became slightly more unbalanced at final follow-up; otherwise all were improved after surgery. Sagittal measurements over instrumented segments as well as total lumbar lordosis (T12-S1) was maintained between preoperative and final postoperative values in both groups. Similarly, in both groups, when horizontalization of the distal end instrumented vertebra was achieved on the preoperative reverse side-bending radiograph, more normal relationships were achieved between instrumented and distal noninstrumented segments (adjacent disc angulation and fractional lumbar curve) at final follow-up (P

Histologic characterization of acute spinal cord injury treated with intravenous methylprednisolone.

OBJECTIVE: Many substances have been investigated for attenuation of spinal cord injury after acute trauma; however, pharmacologically only steroid administration has shown clinical benefits. This study attempts to characterize local spinal cord histologic response to human dose equivalent (HDE) intravenous methylprednisolone (MP) administration in a rodent model of acute spinal cord injury. DESIGN: Forty-eight Sprague-Dawley rats were divided equally into control and experimental groups. Each group was subdivided into eight sets of three animals each, according to postinjury intervals. Paraplegia after lower thoracic laminectomy was achieved using a standardized weight drop technique. INTERVENTION: Within one hour, experimental animals were treated with HDE MP followed by 23-hour continuous infusion of HDE MP. Spinal cords were harvested at variable intervals postinjury and prepared for histologic/immunohistochemistry examination. MAIN OUTCOME MEASUREMENTS: Edema, necrosis, and glial fibrillary acidic protein (GFAP) positivity in the specimens from treated/control groups were graded by microscopy and immunohistochemistry staining and compared in a blinded manner by a qualified neuropathologist and senior authors. RESULTS: Minimal differences were observed between control and MP-treated animals at zero and four hours. At eight hours, increased white matter and medullary edema was evident in control versus MP-treated rats. This trend continued through twelve, sixteen, twenty-four, forty-eight, and seventy-two hours. No difference was observed in the astrocytic response to injury by GFAP immunohistochemistry between the groups. CONCLUSIONS: Histologically, MP reduces the development of severe edema and preserves spinal cord architecture adjacent to the site of injury. In contrast, MP does not alter the development of spinal cord necrosis or astrocytic response at the zone of injury.

Unilateral transforaminal posterior lumbar interbody fusion (TLIF): indications, technique, and 2-year results.

A prospective analysis of consecutive cases of lumbar fusion using the unilateral transforaminal posterior lumbar interbody fusion (TLIF) technique with pedicle screw fixation. The objective of the study was to assess the clinical and radiographic outcome of TLIF and describe the technique and indications in the treatment of degenerative disease of the lumbar spine. Forty patients treated with TLIF for degenerative diseases of the lumbar spine were followed up for a minimum of 2.5 years (mean: 36 months; range: 30-42 months). Twenty-three patients had degenerative disc disease alone, 13 had associated isthmic or degenerative spondylolisthesis, and 4 had recurrent disc herniations at the L4-L5 level. Thirty-six (90%) had solid fusions radiographically at latest follow-up. Seventy-nine percent had excellent or good clinical outcomes. Our patients demonstrated high fusion rates and patient satisfaction.