Thomas S. Whitecloud

Effects of bone mineral density on pedicle screw fixation

Study Design In an attempt to evaluate the effects of bone mineral density on the quality of fixation of pedicle screws in the lumbar spine, the axial pullout force was determined and compared in normal and osteoporotic human lumbar spines. Objectives Four techniques of screw hole preparation were evaluated. Two pedicle screw/offset laminar hook constructs also were evaluated to determine whether the adjunct fixation of the laminar hooks would improve quality of fixation to a level sufficient to allow their use in the osteoporotic lumbar spine. Methods Pedicle screws were inserted by one of the listed techniques into fresh frozen cadaveric human spines. The fixation strength then was evaluated by pullout on a uniaxial testing frame. Results Bone mineral density was a strong influence on axial pullout force. In normal bone, the method of screw hole preparation did not significantly affect the quality of fixation. However, in the osteoporotic spine, either an untapped screw hole or the tapping of a screw hole with a 5.5 mm tap improved the pullout force a statistically significant amount (P < 0.003). Also, a pedicle screw with offset hooks at two adjacent levels improved the fixation significantly, increasing the pullout force to twice the expected value. Conclusion Pedicle screw pullout strength was highly correlated with bone mineral density. A 5.5 mm tap or preparation with a ganglion knife improved pullout strength. Use of pedicle screws in conjunction with laminar hooks at two levels improved pullout strength.

In vivo evaluation of recombinant human osteogenic protein (rhOP-1) implants as a bone graft substitute for spinal fusions

Study Design. Posterior, spinal fusion segments were evaluated in adult mongrel dogs at 6, 12, and 26 weeks post-implantation. Four sites on each animal received implants consisting of recombinant human osteogenic protein-1 on a bone collagen carrier, bone collagen carrier alone, autogenous iliac crest bone, or no implant material. Objective. To determine the efficacy of recombinant human osteogenic protein-1 as a bone graft substitute in achieving posterior spinal fusion and compare the results to those obtained using autogenous bone graft. Summary of Background Data. Posterior spinal fusion generally includes onlay grafting of autogenous or allogeneic bone after decortication of bony surfaces of the vertebral elements. The search for an acceptable bone graft substitute material has in recent years centered upon proteins capable of inducing bone in vivo. Recombinant human osteogenic protein-1 has demonstrated efficacy in healing large segmental osteoperiosteal defects in rabbits, dogs, and monkeys and appears ideally suited as a bone graft substitute for spinal fusions. Methods. The quality of fusion and new bone formation was evaluated using plain films, computed tomography, and magnetic resonance imaging. Results. Radiographic and histologic studies demonstrated that recombinant human osteogenic protein-1-treated fusion segments attained a stable fusion by 6 weeks post-implantation and were completely fused by 12 weeks. The autograft sites demonstrated fusion at 26 weeks post-implantation. Conclusions. The results indicated that recombinant human osteogenic protein-1 is an effective bone graft substitute for achieving stable posterior spinal fusions in a significantly more rapid fashion than can be achieved with autogenous bone graft.

Complications with the variable spinal plating system.

From January 1986 to June of 1987, 40 patients underwent transpedicle fixation and fusion using the variable spinal plate system. Nineteen patients had undergone surgery at the same level or levels, and 21 patients had undergone no previous surgery. Diagnostic categories include spondylolisthesis, thoracolumbar and lumbar fractures, Internal disc derangement, spinal stenosis, pseudarthrosis, mechanical instability, and fracture mal-union. Minimum follow-up has been 14 months, with the average being 20 months. Overall results showed 13 excellent, 12 good, seven fair, and eight poor. The overall complication rate was 45%. In those patients undergoing no previous surgery, It was 29%, but with those patients having previous surgery, It was 63%. Most of these complications were minor in nature and resolved before discharge. Implant failure occurred in seven patients, and consisted of screw breakage. Design modifications currently available should help minimize this complication. Although this method of internal fixation and fusion is technically demanding and has a high complication rate, it is considered to be indicated in lumbar fractures, revision of pseudarthrosis, spondylolisthesis with or without reduction, and failed surgery with marked instability.

Cervical spondylotic myelopathy and myeloradiculopathy : anterior decompression and stabilization with autogenous fibula strut graft

Operative treatment for cervical spondylotic myelopathy and myeloradiculopathy by anterior decompression produced functional improvement of one grade (Nuricks rating system) in 16 of 21 patients evaluated at 32 months average follow-up period. The best results occurred in patients with symptoms for less than one year and classified as grades I-III. The anterior approach for decompression is preferred because it is directed toward the degenerative structures responsible for cord and root compression. The autogeneic fibula dove-tailed strut graft is favored over an iliac crest bone graft because with multilevel decompression in the cervical spine, it provided structural stability and a high union rate. There were no neurologic complications in this series of 21 cases.

Biomechanical evaluation and preliminary clinical experience with an expansive pedicle screw design

The advantages of pedicle screw fixation depend on their ability to retain bony purchase until the fusion mass is stable. Osteoporotic bone and removal and replacement of pedicle screws in revision procedures substantially reduce screw mechanical fixation strength and can lead to clinical failure. The objective of this study was to determine if an expansive pedicle screw design could be used to improve biomechanical fixation in bone of compromised quality. Axial mechanical pullout testing was performed on paired expansive and conventional pedicle screws placed in fresh, unembalmed cadaveric vertebrae. Bone mineral density measurements (made using a dual-energy X-ray absorption meter) were used to characterize bone quality. A preliminary clinical and radiographic evaluation of 14 patients was also performed at a minimum 2-year follow-up. The mean axial pullout force in bone of all qualities was increased 30% when the expansive pedicle screw design was used. This included an appropriate 50% increase in pullout force in bone of poor quality (low bone mineral density). The preliminary clinical and radiographic results were supportive of the biomechanical design rationale and mechanical testing. The results were similar to those expected for spinal instrumentation using pedicle screws, even though compromised bone was present in two thirds of the cases in which the expansive screw was used.

Complications of multilevel cervical corpectomies and reconstruction with titanium cages and anterior plating.

The ideal surgical treatment of multilevel cervical spondylosis remains unclear. This study analyzed the complications in using titanium cages and plating to reconstruct multilevel cervical corpectomies. This was a retrospective analysis of 21 consecutive patients who had multilevel cervical corpectomies and reconstruction with titanium cages and anterior plating. Sixteen had 2-level, one had 2.5-level, three had 3-level, and one had 3.5-level corpectomies. All had reconstruction with titanium cages and anterior plating. Thirty-three percent of the patients developed complications. Radiographs revealed bony consolidation in 95% of patients. Reconstructing multilevel cervical corpectomies with titanium cages and plating is associated with complications. Advantages include rigid immobilization and the avoidance of iliac crest bone graft harvesting. Major complications are largely the result of failures of the cage and plate construct, especially in patients with osteopenic bone. Supplemental posterior stabilization may be considered for cases with spasticity or greater than 2-level corpectomies with profound osteoporosis.

Cervical discogenic syndrome. Results of operative intervention in patients with positive discography.

In order to determine the validity of cervical discography in the diagnosis and treatment of patients presenting with cervical discogenic syndrome, a retrospective analysis of 34 patients who underwent cervical arthrodesis on the basis of positive cervical discography was performed. The symptomatic cervical levels were selected by reproduction of the patients symptoms at the time of injection. No patient had radicular symptoms, and other diagnostic modalities such as computed tomography (CT) scanning or myelography had been within normal limits. Seventy percent of patients who underwent surgical intervention had good or excellent results. With proper utilization, cervical discography is a valid diagnostic study.

Trabecular bone mineral density in idiopathic scoliosis

The association of idiopathic scoliosis with an osteoporotic state has been indicated previously. The present study compared the trabecular bone mineral densities of 44 adolescent idiopathic scoliotic girls with 44 age-, weight-, sex-, and race-matched controls. Their lumbar spine and femoral neck bone mineral densities were evaluated using dual-photon absorptiometry. Radiographs and scoliotic curve data were also obtained for the experimental group. The scoliotic subjects exhibited significantly lower lumbar and femoral neck bone mineral densities than the control subjects. No effect was found with respect to treatment, degree, or progression of curvature. The results of this study indicate that there is a generalized state of osteoporosis in idiopathic scoliotic girls when compared with matched controls.

Fibular Strut Graft in Reconstructive Surgery of the Cervical Spine

The procedures for operative treatment of cervical spinal disease and injury have proven to be technically simple and to have a low incidence of postoperative morbidity. There are occasional technical difficulties, however, which have been encountered during these procedures, one of the most common being graft failure for a variety of reasons. Work currently being done on the efficacy of a technique to circumvent graft failure is reported here. Segments of fibula have been used to secure stability and arthrodesis in the management of 26 patients with disease or injury of the cervical spine. Surgical objectives have been uniformly achieved in 19 patients with proper selection and adequate followup. Early technical problems are described.

Lumbosacral fixation using expandable pedicle screws. an alternative in reoperation and osteoporosis.

BACKGROUND CONTEXT Pedicle screw fixation in osteoporotic bone and in revision of previous pedicle screw fixation cases presents a significant challenge to spine surgeons. Biomechanical tests have shown that a pedicle screw that expands within the vertebrae body can substantially improve fixation in the presence of compromised bone. PURPOSE To review the clinical and radiographic results with the use of expandable pedicle screws. STUDY DESIGN One hundred forty-five patients received one or more expandable pedicle screws from the Omega21 spinal fixation system (EBI, L.P., Parsippany, NJ) to obtain thorocolumbar or lumbosacral stabilization. PATIENT SAMPLE The indications for use of the expandable screws were osteoporosis (21 cases), reoperation of previous pedicle instrumentation (27 cases), intraoperative screw relocation (17 cases), construct reinforcement (23 cases), and sacral anchoring to avoid the necessity of anterior penetration of the sacral cortex (57 cases). OUTCOME MEASURES The presence of radiographic fusion and complications arising from the instrumentation were reviewed at a mean follow-up period of 35 months (range, 24-72 months). METHODS A retrospective clinical and radiographic review was performed. Fusion was evaluated based on anterior-posterior and lateral radiographs as well as dynamic radiographs in flexion and extension. RESULTS Radiographic evidence of fusion was obtained in 125 of the 145 cases (86%). Eighty-six percent of patients with osteoporosis and 89% of reoperations fused. There were no instances of screw loosening or pullout of the expandable screws. Screw breakage occurred in four patients (2.8%), including three patients where fusion was not obtained. In six patients the expandable screws were removed without difficulty after fusion because of local discomfort. CONCLUSION The results of this study have shown that expandable pedicle screws can be efficacious in cases in which pedicle screw fixation is difficult and adds a valuable tool to the growing armamentarium of spinal instrumentation.