Kevin R. Gurr

The effect of spinal implant rigidity on vertebral bone density. A canine model.

An animal model of anterior and posterior cojumn instability was developed to allow in vivo observation of bone remodeling and arthrodesis following spinal instrumentation. After an initial anterior and posterior destabilizing lesion was created at the L5-L6 vertebral levels in 63 adult beagles, various spinal reconstructive surgical procedures were performed—with or without bilateral posterolateral bone grafting, with or without bilateral oophorectomies, and with or without spinal instrumentation (Harrington distraction, Luque rectangular, Cotrel- Dubousset pedicular, or Steffee pedicular implants). Observation 6 months after surgery revealed a significantly jmproved probability of achieving a spinal fusion if spinal instrumentation had been used (X2=5.84, P=.016). Nondestructive mechanical testing after removal of all metal instrumentation in torsion, axial compression, and flexion revealed that the fusions performed in conjunction with spinal instrumentation were more rigid (P<.05). Quantitative histomorphometry showed that the volumetric density of bone was significantly lower (ie, device-related osteoporosis occurred) for fused versus unfused spines. In addition, a linear correlation occurred between decreasing volumetric density of bone and increasing rigidity of the spinal implant (r=.778); ie, device- related osteoporosis occurred secondary to Harrington, Cotrel-Dubousset, and Steffee pedicular instrumentation. Oophorectomized dogs became more osteoporotic than their surgically matched controls (posterolateral bone grafting alone, Cotrel-Dubousset pedicular instrumentation, and Steffee pedicular instrumentation); device-related osteoporosis added to the degree of hormonally induced osteoporosis (t=5.0, P<.0001). This is the first study to date documenting the occurrence of stress shielding in the spine secondary to spinal instrumentation. Device-related osteoporosis occurred secondary to pedicular instrumentation and to a lesser degree with conventional Harrington and Luque instrumentation. Overall, vertebral osteoporosis secondary to spinal fixation does not appear deleterious. There was an increased chance of achieving a successful arthrodesis, and the biomechanical rigidity of the fusion was more favorable because of an increased cross-sectional area of the posterolateral fusion mass.

Cotrel-Dubousset instrumentation in adults: a preliminary report

In an attempt to assess the value of the Cotrel-Dubousset (CD) system for adult spinal disorders, the first 50 adult cases performed at Johns Hopkins were reviewed. Treatment of adult scoliosis with the CD system yielded results comparable to standard techniques. Curve correction was directly proportional to the preoperative flexibility with no loss of correction in any case. All patients went on to a solid arthrodesis, with only three patients requiring the use of postoperative orthoses. Operative time was initially prolonged during the phase of acquiring expertise with the system; however, blood loss and hospitalization were comparable. In both the scoliosis and kyphosis groups instrumentation and fusion incorporated the same number of levels as would have been required for conventional instrumentation systems. In the spondylolisthesis, tumor and trauma groups a total of 88 transpedicle screws was used in 18 patients without neurologic complications. Pedicle screws provided a fixation alternative in cases requiring laminectomies. In the 25 cases with tumors, spondylolisthesis, and trauma, CD instrumentation reduced the number of vertebral levels required for fixation. Compared to Harrington or Luque systems, the average number of motion segments spared per patient was 1.3 in the spondylolisthesis group, 2 in the tumor group and 2.1 in the trauma group. This study suggests that the CD system, although initially developed for Idiopathic adolescent scoliosis, is versatile and can be safely and effectively applied to a variety of adult spinal conditions. In cases of spinal pathology due to neoplasm, spondylolisthesis, and trauma, CD instrumentation with the option of transpedicle fixation appeared to offer significant advantages over conventional methods, and an average of 1.6 lumbar motion segments could be preserved per case.

Predicting the need for tracheostomy in patients with cervical spinal cord injury.

BACKGROUND Approximately 75% of hospitalized patients with a cervical spinal cord injury (CSCI) will require intubation and mechanical ventilation (MV) because of compromised respiratory function. It is difficult to predict those CSCI patients who will require prolonged ventilation and therefore will most benefit from early tracheostomy. This study intended to show the benefits of tracheostomy, particularly early, and to identify predictors of prolonged MV after CSCI. METHODS A retrospective review of patients aged 16 years and older with acute CSCI admitted to London Health Science Center from 1991 to 2010 was performed. Demographic data and clinical parameters were extracted from medical records and the trauma registry. Regression analysis was used to identify predictors of prolonged MV. RESULTS There were 66 eligible patients of which 42 (62%) had a tracheostomy performed. Five patients (7.6%) remained ventilator dependent and seven (10.6%) died more than 7 days after injury secondary to sepsis. After adjusting for the number of ventilator days after injury, patients who had a tracheostomy had fewer pulmonary complications than those who did not have a tracheostomy (p = 0.001). Early tracheostomy resulted in fewer days on the ventilator and a shorter hospital stay. Clinical parameters that predicted MV to be required longer than 7 days were Injury Severity Score > 32, complete SCI, and a PAO2/FIO2 ratio < 300 3 days after MV was initiated. CONCLUSION We recommend early tracheostomy if the Injury Severity Score is >32, the patient has a complete SCI, and the PAO2/FIO2 ratio is <300 3 days after MV was initiated. LEVEL OF EVIDENCE Prognostic study, level III.

The biomechanical and histomorphometric properties of anterior lumbar fusions: a canine model.

An in vivo model was developed to compare the biomechanical stability, incidence of radiographic union, bone formation rate, and bone graft remodeling parameters of anterior interbody fusions. Eighteen 1-year-old beagles underwent anterior and posterior spinal destabilization procedures at L5-L6 to produce a reproducible amount of spinal instability--resection of the anterior longitudinal ligament, L5-L6 intervertebral disk, L5 and L6 lamina, spinous processes, zygoopophyseal joints, and ligamentum flavum. Group I (N = 6) were surgically destabilized controls; Group II (N = 6) underwent anterior L5-L6 interbody fusion with iliac crest bone graft; and Group III (N = 6) underwent anterior stabilization with a longitudinal fibular strut graft in addition to the same operative procedure as Group II. Six months postoperatively the group with the highest incidence of successful radiographic L5-L6 arthrodesis was Group III, anterior interbody fusion and fibular stabilization (p less than .10). The rank order of biomechanical stability was the same for the three groups for both torsional and axial compressive stiffness, with Group I (destabilized controls) being the least rigid, then Group II (anterior fusion with iliac crest grafting only), and the most rigid to both torsion and axial compressive loading was Group III (anterior fusion with fibular stabilization and iliac crest bone graft). The bone formation rate [mm3/(mm3 x year) x 10(3)], which was derived from the distance between fluorochrome markers, revealed that the more stable the individual spinal construct, the lower the bone formation rate. In summary, the beagle provided a successful model for studying in vivo the response of anterior bone grafts over a 6-month interval and provided comparative biomechanical and histomorphometric data on spinal interbody fusion techniques.

The effect of soft-tissue restraints after type II odontoid fractures in the elderly: a biomechanical study.

Study Design. A biomechanical analysis of soft-tissue restraints to passive motion in odontoid fractures. Objective. To quantify the role of the C1–C2 facet joint capsules and anterior longitudinal ligaments (ALLs) in the setting of a type II odontoid fracture in the elderly. Summary of Background Data. The odontoid process itself is the primary stabilizer at the C1–C2 level; however, little is known about the role of the soft-tissue structures that remain intact in the setting of an odontoid fracture after a low-energy mechanism. Methods. Ten cadaveric C0–C2 spinal segments were studied. Specimens were tested under simulated axial rotation with an applied moment of ±1 Nm and with an application of a 10 N anteriorly directed force to the body of C2 to induce sagittal translation. Optical motion data were initially collected for the intact state and after a simulated dens fracture. The specimens were then divided into 2 groups, where 1 group underwent unilateral and then bilateral C1–C2 facet capsular injuries followed by an ALL injury. The second group underwent the ALL injury before the same capsular injuries. Changes in axial range of motion and C1–C2 translation were analyzed using 2-way repeated measures analyses of variance and post hoc Student-Newman-Keuls tests (&agr; = 0.05). Results. In axial rotation, there was an increase in range of motion by approximately 13%, with the fracture of the dens compared with the intact state (P < 0.05). An increase was also present for each subsequent soft-tissue injury state compared with the previous (P < 0.05); however, there was no difference found between the 2 sectioning protocols. For sagittal translation testing, it was found that the odontoid fracture alone showed an increase of 3 mm of C1–C2 translation compared with intact (P < 0.05). Further soft-tissue injuries did not show an increase until the complete injury state. Conclusion. This study identifies that type II odontoid fractures without associated soft-tissue injury may be stable under certain loading modes.

The USS pedicle hook system : A morphometric analysis of its safety in the thoracic spine

The Universal Spine System (USS) pedicle hook design includes a fixation screw that passes obliquely in the anterocranial direction in the pedicle. The addition of the fixation screw was to address concerns with rotation of the hook and hook disengagement. This study was designed to evaluate the safety of the USS screw locked pedicle hook. Eleven cadaveric thoracic spines were instrumented posteriorly with USS pedicle hooks from T1 to T12. Spinal instrumentation was performed by a spinal surgeon experienced with the USS system. Spinal deformity was created prior to instrumentation, ranging from 0 to 55 degrees in the horizontal plane (rotation) and from 0 to 50 degrees in the frontal plane (scoliosis). Radiographs, computed tomography (CT), and segmental dissection were used for data acquisition. Morphometric CT analysis before instrumentation demonstrated that the transverse pedicular diameter was the smallest at T5 with a mean of 3.7 mm. The transverse pedicular angle (TPA) was found to always point toward the midline. The largest TPA was observed at T1 with a mean TPA of 28.4 degrees. The pedicle with the least angular deviation from the midline was T11 with a mean TPA of 7 degrees. Postinstrumentation CT analysis and segmental dissection revealed perforations of the pedicle cortex by the fixation screw in 15% of instrumented pedicles (26/172). There were 6 medial and 20 lateral perforations. Medial perforations occurred exclusively in the three most proximal spinal segments, whereas the lateral perforations occurred throughout the thoracic spine. The mean encroachment of the fixation screw was 1.67 mm medially and 1.95 mm laterally. This study demonstrates the variation in caliber and direction of the thoracic pedicles. Medial and lateral perforations of the pedicle can occur with the USS pedicle hook instrumented system.

Remote inflammatory response in liver is dependent on the segmental level of spinal cord injury.

BACKGROUND: Traumatic spinal cord injury (SCI) triggers a systemic inflammatory response (SIR) that contributes to a high incidence of secondary organ complications, particularly after a cervical or high-level thoracic injury. Because liver plays a key role in initiating and propagating the SIR, the aim of this study was to assess the effects that SCI at differing segmental levels has on the intensity of the inflammatory response in the liver. METHODS: Using male Wistar rats, clip compression SCI was performed at the 4th thoracic (T4 SCI; high-level SCI) or the 12th thoracic (T12 SCI; low-level SCI) spinal cord segment. Sham-injured rats had a partial laminectomy, but no SCI. Leukocyte recruitment to the liver, hepatic blood flow, and hepatocellular injury/death were assessed using intravital microscopy and histology. Chemokine and cytokine concentrations were assessed in the liver. Outcomes were measured at 1.5 hours, 12 hours, and 24 hours after SCI. RESULTS: At 12 hours after injury, T4 SCI caused a threefold increase in hepatic leukocyte recruitment compared with T12 SCI (p < 0.05). T4 SCI induced 50% more hepatocyte injury than T12 SCI at 12 hours (p < 0.05). Hepatic blood flow decreased after SCI, but not after sham injury, and stayed decreased only after T4 SCI at 24 hours after injury. The T4 SCI-induced changes were accompanied by increases in the hepatic concentrations of interleukin-1&bgr;, leptin, interleukin 10, and cytokine-induced neutrophil chemoattractant-1 at 1.5 hours. CONCLUSIONS: Our findings indicate that traumatic SCI triggers an acute SIR that contributes to hepatocellular injury. SCI-induced remote injury/dysfunction to the liver appears to be transient and is more robust after an upper thoracic SCI compared with a lower thoracic SCI.

Comparing the fixation of a novel hollow screw versus a conventional solid screw in human sacra under cyclic loading.

Study Design. The loosening rates of two monocortical pedicle screw designs (hollow and solid) were compared in a cadaveric sacrum model subjected to cyclic loading. Objective. To determine if a hollow screw would be more resistant to loosening than a solid pedicle screw when placed into the pedicles of S1 and tested under stair-cased cyclic loading. Summary of Background Data. Screw loosening is a clinical problem for lumbosacral fusions. No previous literature has evaluated the use of a monocortical hollow screw within the sacrum; however, in other vertebral bodies, results of using this screw have been varied. Methods. Seven fresh-frozen cadaveric sacra were thawed and stripped of soft tissues. Solid and hollow screws were inserted contralaterally into the pedicles of S1. A materials testing machine applied alternating flexion and extension bending moments at 1 Hz, to each screw independently, via a standard connecting rod. Flexion moments were applied starting at 0.5 Nm and increased by 0.5 Nm after every 1000 cycles until the screw had visibly failed. Extension moments were maintained at 0.5 Nm. Screw rotation (flexion) relative to the sacrum was recorded using a custom optical tracking system, and analyzed using 2-way repeated measures analyses of variances and post hoc Student-Newman-Keuls tests (&agr; = 0.05). Results. Screw rotation tended to gradually increase to six degrees, after which the screw was grossly loose. Overall, the hollow screw required fewer loading cycles (P = 0.004) and less applied moment (P = 0.003) to achieve the same magnitude of screw rotation as the solid screw. To achieve 6 degrees of screw rotation, the number of loading cycles were 6301 ± 2161 and 11151 ± 4221 for hollow and solid screws, respectively. The corresponding applied moments were 3.5 ± 1.0 Nm and 5.8 ± 2.0 Nm. Conclusion. The novel hollow screw was less resistant to loosening when compared with a conventional solid pedicle screw in this sacral model under cyclic loading.

Human Spinal Cord Injury Causes Specific Increases in Surface Expression of Beta Integrins on Leukocytes

Spinal cord injury (SCI) activates circulating leukocytes that migrate into the injured cord and bystander organs using adhesion molecule-mediated mechanisms. These cells cause oxidative damage, resulting in secondary injury to the spinal cord, as well as injury to bystander organs. This study was designed to examine, over a 6-h to 2-week period, changes in adhesion molecule surface expression on human peripheral leukocytes after SCI (9 subjects), using as controls 10 uninjured subjects and 6 general trauma patients (trauma controls, TC). Both the percentage of cells expressing a given adhesion molecule and the average level of its expression was quantified for both circulating neutrophils and monocytes. The percentage of neutrophils and monocytes expressing the selectin CD62L was unchanged in TC and SCI patients after injury compared to uninjured subjects. Concurrently, the amount of surface CD62L on neutrophils was decreased in SCI and TC subjects, and on monocytes after SCI. The percentage of neutrophils expressing α4 decreased in TC, but not in SCI, subjects. Likewise, the percentage of neutrophils and monocytes expressing CD11d decreased markedly in TC subjects, but not after SCI. In contrast, the mean surface expression of α4 and CD11d by neutrophils and monocytes increased after SCI compared with uninjured and TC subjects. The percentage of cells and surface expression of CD11b were similar in neutrophils of all three groups, whereas CD11b surface expression increased after SCI in monocytes. In summary, unlike changes found after general trauma, the proinflammatory stimulation induced by SCI increases the surface expression of adhesion molecules on circulating neutrophils and monocytes before they infiltrate the injured spinal cord and multiple organs of patients. Integrins may be excellent targets for anti-inflammatory treatment after human SCI.

Neuropathologic changes with experimental spinal instrumentation: transpedicular versus sublaminar fixation.

Fifty-six mature beagles underwent lumbar spine destabilization, followed by fusion using four techniques. Spinal cord neuropathologic analysis was carried out to determine the number of abnormalities within each group. Group I (n = 14) had posterolateral bone grafting without instrumentation. Group IIa (n = 14) had Cotrel-Dubousset (CD) pedicle screws and rods. Group IIb (n = 14) had Steffee pedicle screws and plates. Group III (n = 14) had sublaminar wires and rods. All of the animals remained clinically neurologically normal throughout the 6 months of the study. The incidence of moderate to severe neuropathologic changes was 21% in Group I, 18% in Group II, and 64% in Group III. Thus, a significantly higher percentage of neuropathologic abnormalities occurred with sublaminar instrumentation than with no instrumentation (p = 0.027), or with transpedicular instrumentation (p = 0.027). In this controlled animal study, the theoretical advantage of pedicle screws, which should not violate the spinal canal, over sublaminar devices, which must enter the canal, was confirmed.