Biomechanical analysis of fixation for vertically unstable sacroiliac dislocations with iliosacral screws and symphyseal plating.

OBJECTIVES: To examine the effects of various iliosacral screw configurations with and without symphyseal plating on sacroiliac (SI) motion and hemipelvis stability in the vertically unstable pelvic model. DESIGN: Biomechanical, human cadaver. SETTING: Level 1 trauma center. INTERVENTION: Hemipelvis and SI motion were analyzed on a Materials Testing System before and after creation of a vertically unstable APC III pelvic injury. Posterior fixation constructs consisted of iliosacral screws: (1). one into S1, (2). two into S1, or (3). one into S1 and one into S2. Results were obtained for all posterior constructs with and without a two-hole symphyseal plate. MAIN OUTCOME MEASUREMENT: Hemipelvis and SI motion with axial loading. RESULTS: There was no statistically significant difference between one or two iliosacral screws when hemipelvis rotational or linear displacement was examined at the SI joint. The two-hole symphyseal plate significantly increased the stability of the fixation construct in resisting linear displacement in all three planes. Without the symphyseal plate, an abnormal loading response was seen at the SI joint, resulting in paradoxical posterior translation and sagittal plane rotation. The addition of the plate restored the normal response, and anterior rotation and translation were observed as in the intact state. CONCLUSIONS: Anterior symphyseal plating for the vertically unstable hemipelvis significantly increases the stability of the fixation construct and restores the normal response of the hemipelvis to axial loading. A significant benefit to supplementary iliosacral screws in addition to a properly placed S1 iliosacral screw was not shown.

Electromagnetic field-based image-guided spine surgery part one: results of a cadaveric study evaluating lumbar pedicle screw placement.

STUDY DESIGN: Human cadaveric. OBJECTIVES: Compare the accuracy of electromagnetic field (EMF)-based image-guided lumbar pedicle screw insertion to conventional techniques using anatomic landmarks, and fluoroscopy. BACKGROUND: Image-guided surgical systems that aid in spinal instrumentation seek to minimize radiation exposure and improve accuracy. EMF tracking-based image-guidance was developed in the hopes of eliminating line-of-sight restrictions seen with other systems. MATERIALS AND METHODS: Sixteen fresh-frozen human cadavers were randomly allocated into three groups. Pedicle screws were inserted from L1 to L5 using only anatomic landmarks in group 1, fluoroscopy in group 2, and image-guidance in group 3. Insertion and total fluoroscopic time were recorded. Anatomic dissections were performed to assess screw placement. RESULTS: Accuracy was 83%, 78%, and 95% for groups 1, 2, and 3, respectively. However, image-guided pedicle screw placement resulted in a 5% critical perforation rate whereas anatomic and fluoroscopic placement resulted in a 15% and 22% critical perforation rate, respectively. The average degree of perforation was 1.5 mm with image guidance, and 3.8 mm with fluoroscopic guidance (P < 0.05). Fluoroscopy time and insertion time per screw were not improved using image guidance. CONCLUSIONS: Our study has shown that when EMF tracking was used for image-guided lumbar pedicle screw placement, accuracy was improved and the incidence and degree of cortical perforations that may place neurovascular structures at risk was also reduced. Current system requirements for set-up and image acquisition, however, do add time to the procedure, and when factored in, do not yet result in a decrease in the use of fluoroscopy or screw insertion time.

Electromagnetic field-based image-guided spine surgery part two: results of a cadaveric study evaluating thoracic pedicle screw placement.

STUDY DESIGN: Human cadaveric. OBJECTIVES: Compare the accuracy of electromagnetic field (EMF)-based image-guided thoracic pedicle screw insertion to conventional techniques using anatomic landmarks and fluoroscopy. BACKGROUND: Image-guided surgical systems that aid in spinal instrumentation seek to minimize radiation exposure and improve accuracy. EMF image guidance was developed as an alternative to optical tracking to eliminate potential line of sight issues. MATERIALS AND METHODS: Four fresh-frozen human cadavers were randomly allocated into two groups. Pedicle screws were inserted from T1 to T12 using anatomic landmarks and fluoroscopy in group 1 and image guidance in group 2. Insertion and fluoroscopy time were recorded. Anatomic dissections were performed to assess screw placement. RESULTS: Image guidance placed 92% of thoracic pedicle screws safely, and conventional fluoroscopy placed 90% safely. The average degree of perforation was 2.4 mm with conventional fluoroscopy and 1.7 mm with image guidance (P = 0.055). Fluoroscopic time per screw was 5.9 seconds for conventional fluoroscopy and 3.6 seconds for image guidance (P = 0.045). Insertion time per screw was 4.35 minutes for conventional fluoroscopy and 2.98 minutes for image guidance (P = 0.007). However, when set-up time and image capture time were taken into account, the total insertion time per screw was not significantly different between the two groups. CONCLUSIONS: Our study has shown that EMF image-guided thoracic pedicle screw placement results in a similar incidence of safely placed screws as does conventional fluoroscopy. When set-up time and image-capture time were factored in for image guidance, the average time to insert a pedicle screw was equal for both techniques. The use of EMF image guidance significantly reduced fluoroscopic time and thus radiation exposure per screw compared with conventional fluoroscopic techniques.

Peak stresses observed in the posterior lateral anulus.

STUDY DESIGN: The stress distributions within cadaveric lumbar intervertebral discs were measured for a range of loading conditions. OBJECTIVES: To examine the distribution of stress across the area of the intervertebral disc and to compare regional variations in peak stress during compression loading with various flexion angles. SUMMARY OF BACKGROUND DATA: The rate of disc degeneration and the occurrence of low back disorders increase with higher mechanical loading of the spine. The largest peak stresses occur in the anulus. METHODS: Human lumbar L2--L3 and L4--L5 cadaver functional spinal units were obtained and tested. The distribution of disc stress was measured using a pressure probe with loads applied, pure compression and compression with 5 degrees of either flexion or extension. RESULTS: Stress profiles were recorded across the intervertebral disc at a compressive force of 1000 N and each of the three flexion-extension angles. The highest values (2.99 +/- 1.31 MPa) were measured during extension-compression lateral to the midline of the disc in the posterior anulus. The pressure in the nucleus was relatively unchanged by flexion angle remaining about 1.00 MPa for a 1000-N compression. CONCLUSIONS: Pressure measurements of the cadaveric nucleus have been used to validate models of lumbar spine loading and to evaluate the risk of low back injury and disc herniation. Previous observations limited to midsagittal measurements of the nucleus did not identify the regions of highest stress. The highest values observed here within the posterolateral anulus correspond to common sites of disc degeneration and herniation.

Effects of the Positive Action program on achievement and discipline: two matched-control comparisons.

This paper reports on the effectiveness of an integrated comprehensive school model for character development, problem behavior prevention, and academic achievement enhancement. The Positive Action program consists of a school curriculum, together with schoolwide climate, family, and community components. As evaluated here, the yearly K-6 curriculum consists of over 140 fifteen-to-twenty-minute lessons per year delivered in school classrooms on an almost daily basis. The program is based on theories of self-concept, learning, behavior, and school ecology. We use a matched control design and school-level achievement and disciplinary data to evaluate program effects on student performance and behavior in two separate school districts. The program improved achievement by 16% in one district and 52% in another, and reduced disciplinary referrals by 78% in one district and 85% in the other. We discuss implications of these replicated findings for the prevention of substance abuse and violence, the improvement of school performance, and the reform of American schools.

Nicotine dependence, withdrawal symptoms, and adolescents' readiness to quit smoking.

The purpose of this study was to characterize nicotine dependence and nicotine withdrawal symptoms among adolescent smokers and to investigate associations between these key factors and adolescents' readiness to quit smoking. A total of 5624 high school students participated in a school-based survey. Of 1111 adolescents who were current or former smokers, the following stage-of-change distribution for smoking cessation was observed: precontemplation, 52.5%; contemplation, 16.0%; preparation, 7.5%; action, 13.2%; and maintenance, 10.8%. Among current smokers, 18.1% were substantially dependent on nicotine, 45.2% had moderate dependence, and 36.7% had no dependence. Higher proportions of current smokers than successful quitters reported withdrawal symptoms with their most recent quit attempts. Precontemplators exhibited significantly higher mean nicotine dependence scores than did students in the contemplation or preparation stages (F(2,837) = 12.03; p < 0.0001). A similar trend was observed for withdrawal-symptom scores across the stages of change. The nicotine dependence and withdrawal-symptom scores were significantly correlated (r = 0.44, p < 0.001). Nicotine dependence and nicotine withdrawal appear to interfere with adolescents' abilities and readiness to quit smoking, suggesting a potential role for nicotine replacement therapy in the treatment of tobacco use and dependence among adolescents.

Posterior lumbar interbody fusion using posterolateral placement of a single cylindrical threaded cage.

STUDY DESIGN: An in vitro biomechanical study of posterior lumbar interbody fusion (PLIF) with threaded cages was performed on 18 bovine lumbar functional spinal units. OBJECTIVES: To compare the segmental stiffness among PLIF with a single long posterolateral cage, PLIF with a single long posterolateral cage and simultaneous facet joint fixation, and PLIF with two posterior cages. SUMMARY OF BACKGROUND DATA: In most cases, PLIF with threaded cage techniques needs bilateral facetectomy, extensive exposure, and retraction of the cauda equina. Posterior element deficiency is detrimental to postoperative segmental stiffness. METHODS: All specimens were tested intact and with cage insertion. Group 1 (n = 12) had a long threaded cage (15 x 36 mm) inserted posterolaterally and oriented counter anterolaterally on the left side by posterior approach with left unilateral facetectomy. Group 2 (n = 6) had two regular-length cages (15 x 24 mm) inserted posteriorly with bilateral facetectomy. Six specimens from Group 1 were then retested after unilateral facet joint screw fixation in neutral (Group 3). Similarly, the other six specimens from Group 1 were retested after fixation with a facet joint screw in an extended position (Group 4). Nondestructive tests were performed in pure compression, flexion, extension, lateral bending, and torsion. RESULTS: The PLIF procedure involving a single cage (Group 1) had a significantly higher stiffness than PLIF with two cages (Group 2) in left and right torsion (P < 0.05). Group 1 had higher stiffness values than Group 2 in pure compression, flexion, and left and right bending, but differences were not significant. Group 3 had a significant increase in stiffness in comparison with Group 1 for pure compression, extension, left bending, and right torsion (P < 0.05). For Group 4, the stiffness significantly increased in comparison with Group 1 for extension, flexion, and right torsion (P < 0.05). Although there was no significant difference between Groups 3 and 4, Group 4 had increased stiffness in extension, flexion, right bending, and torsion. CONCLUSIONS: Posterior lumbar interbody fusion with a single posterolateral long threaded cage with unilateral facetectomy enabled sufficient decompression while maintaining most of the posterior elements. In combination with a facet joint screw, adequate postoperative stability was achieved.

Characteristics of unicortical and bicortical lateral mass screws in the cervical spine.

STUDY DESIGN: A biomechanical study evaluating the safety and efficacy of unicortical versus bicortical lateral mass screws in the cervical spine. OBJECTIVES: To analyze the safety, pullout strength and radiographic characteristics of unicortical and bicortical screws placed in cadaveric spines and to evaluate the influence of level of training on the positioning of these screws. SUMMARY OF BACKGROUND DATA: Lateral mass plating for posterior cervical spine fusion is an effective method for the treatment of traumatic and degenerative instability. The initial description of the technique included bicortical screw purchase. The added benefit of bicortical purchase must be weighed against the increased risk of injury to nerve roots and the vertebral artery. METHODS: In 21 cadaveric spines (mean age 78.9 years), 3.5-mm anterior oblique lateral mass screws were placed bilaterally from C3 to C6 (n = 168) using a modification of the Magerl technique. In the right side (unicortical) only 14-mm screws (effective length of 11 mm) were used, whereas on the left side, bicortical purchase was obtained. All screws were evaluated clinically and radiographically for safety and zone placement. Pullout force was determined for all screws. RESULTS: Most screws (92.8%) were rated satisfactory. There were no injuries to the spinal cord. On the right side (14 mm) 98.9% of the screws were satisfactory, and on the left side (bicortical) 68.1% were satisfactory. There was a 5.8% incidence of direct artery injury (compression of vessel wall) and a 17.4% incidence of direct nerve root injury by the bicortical screws. There were no direct injuries with the unicortical screws. Most of the direct-injury bicortical screws were placed by the surgeon with the least experience. The mean pullout force for all screws was 542.9 +/- 296.6 N. There was no statistically significant difference between the pullout force for unicortical (519.9 +/- 286.9 N) and bicortical (565.2 +/- 306 N) screws (P < 0.05). There were no significant differences in pullout strengths in association with zone placement. CONCLUSIONS: Fourteen-millimeter lateral mass screws (effective length, 11 mm) placed in a superolateral trajectory in the adult cervical spine provide an equivalent strength with a much lower risk of injury than the longer bicortical screws placed in a similar orientation.

Video-assisted thoracic diskectomy and anterior release: a biomechanical analysis of an endoscopic technique.

This study evaluates the residual biomechanical stability of the spine following multilevel anterior diskectomies and anterior longitudinal ligament release using video-assisted thoracoscopic surgery (VATS). Eighteen domestic pigs were randomly divided into three groups of six pigs. Group 1 underwent thoracic anterior release from T4-T9 using a left-sided VATS approach, group 2 underwent thoracic anterior release from T4-T9 via a traditional left thoracotomy (open), and group 3 did not undergo surgery and served as a control. After surgery, the animals were euthanized, and the thoracic spinal columns were harvested for biomechanical testing. Nondestructive testing was performed on all specimens in pure compression, flexion, extension, right lateral bending, and torsion. Specimens from group 1 had significantly lower stiffness values (P<.05) than the control group for all five test modes. These data demonstrate that adequate anterior release of the thoracic spine can be obtained with the VATS technique. Further prospective clinical studies on VATS are required before the widespread application of this technique.

Cervical flexion, extension, protrusion, and retraction. A radiographic segmental analysis.

STUDY DESIGN: A lateral radiographic analysis of the cervical spine was performed on 20 asymptomatic volunteers. OBJECTIVES: To quantify the contribution of each cervical segment to each of four sagittal cervical end-range positions: full-length flexion, full-length extension, protrusion, and retraction. SUMMARY OF BACKGROUND DATA: Recent clinical research supports the relevance of cervical protrusion and retraction in symptomatic patients. Currently, few quantitative studies are available regarding cervical protrusion and retraction. METHODS: Lateral cervical radiographs of 20 asymptomatic volunteers for four test positions and a neutral position were collected. Mean angular measurements and available ranges of motion were calculated from the occiput to C7. RESULTS: Retraction consists of lower cervical extension and upper cervical flexion, whereas protrusion consists of lower cervical flexion and upper cervical extension. Full-length cervical flexion produced more flexion at lower segments than did protrusion, and full-length cervical extension produced more extension at lower segments than did retraction. With both full-length flexion and retraction, upper cervical segments are positioned in the flexion portion of their total range, but only retraction takes Occ-C1 and C1-C2 to their full end-range of flexion. Similarly, with both full-length extension and protrusion, upper cervical segments are positioned in the extension portion of their total range, but only protrusion takes Occ-C1 and C1-C2 to their end-range of extension. CONCLUSION: A greater range of motion at Occ-C1 and C1-C2 was found for the protruded and retracted positions compared with the full-length flexion and full-length extension positions. Effects on cervical symptoms reported to occur in response to flexion, extension, protrusion, and retraction test movements may correspond with the position of lower cervical segments.

Fracture toughness of CoCr alloy-PMMA cement interface.

An unsymmetric cantilever geometry was used experimentally to determine the critical energy release rate values for cobalt chromium alloy-polymethylmethacrylate cement (CoCr alloy-PMMA) interfaces with satin finished, grit blasted, and plasma sprayed surface treatments applied to the CoCr alloy. Critical energy release rates of 0.013, 0.181, and 0.583 N/mm were found for the satin finish, grit blasted, and plasma sprayed CoCr alloy-PMMA interfaces, respectively. A finite element model of the experimental test specimen was used to determine the crack tip phase angles (-8.73 degrees to -27.1 degrees) that indicated that the tensile load applied to the specimens resulted in a tensile (mode I) and in-plane shear (mode II) loading at the crack tip. The experimental data suggest that a satin finish CoCr alloy-PMMA interface has minimal resistance to crack propagation when compared to grit blasted or plasma sprayed surface treatments.

Cervical sagittal range-of-motion analysis using three methods. Cervical range-of-motion device, 3space, and radiography.

STUDY DESIGN: Cervical flexion, extension, protrusion, and retraction were comparatively measured in volunteers using a cervical range-of-motion device (Performance Attainment Associates, Roseville, MN), a 3space system (Polhemus, Colchester, VT), and lateral radiographs. OBJECTIVES: To compare the outcomes of three methods of measurement of cervical flexion, extension, protrusion, and retraction. SUMMARY OF BACKGROUND DATA: Few studies compare cervical range-of-motion measurement devices with radiographic measurements, and no studies have compared methods of measurement for cervical protrusion and retraction measurement. METHODS: In 20 asymptomatic volunteers, four end-range sagittal cervical positions (flexion, extension, protrusion, and retraction) were measured simultaneously using a cervical range-of-motion device, a 3Space and lateral cervical radiographs. Measurements were compared, and differences were analyzed. RESULTS: There were no significant differences for flexion and extension measurements between the cervical range-of-motion device and that radiographic angle determined by an occipital line and the vertical, nor were there any between the 3Space and that radiographic angle between this same occipital line and C7. The cervical range-of-motion device and the 3Space measurements for flexion and extension, however, differed significantly from one another (P < 0.05). For protrusion and retraction, there was no significant difference between the 3Space and radiographic measurements, but these two both differed significantly from the cervical range-of-motion device (P < 0.05). CONCLUSIONS: Available methods of measurement differ as to whether the cervical spine is isolated or includes upper thoracic motion. Protrusion and retraction can be measured reliably with all three methods studied, but without measurement consistency between devices. Because end-range cervical flexion and extension-cannot occur in isolation from upper thoracic motion, true cervical motion can be measured only with an internally referenced, or landmark-based, methodology such as the 3Space. Even though the cervical range-of-motion device cannot measure isolated cervical flexion and extension, it is nevertheless a reliable clinical tool in measuring flexion and extension as well as protrusion and retraction as long as patient thoracic positioning is standardized to minimize the upper thoracic contribution.

Video-assisted thoracic corpectomy and spinal reconstruction: a biomechanical analysis of open versus endoscopic technique.

The purpose of this study is to evaluate the technique of video-assisted thoracic surgery (VATS) to perform corpectomy and spinal reconstruction with the Harrington rod and polymethylmethacrylate construct, and to compare the biomechanical stability of the constructs created with both open thoracotomy and VATS technique. Fourteen farm-raised pigs were divided into two groups. Group I underwent thoracic corpectomy using a VATS approach and group II had the same procedure performed through a traditional thoracotomy. The stiffness (newtons per millimeter) for the non-destructive tests for each loading mode are flexion-compression (open 124.4 +/- 124.9; VAT 75.8 + 29.9); extension-compression (open 165.8 + 41.8: VATS 96.5 + 31.2); pure compression (open 231.4 + 126.4; VATS 264.6 + 184.3). The difference between group I (VATS) and group II (open) in extension compression is statistically significant. Although not statistically significant, the results also show the group II (open) specimens to be stiffer in flexion-compression and the group I (VATS) specimens to be stiffer in pure compression. Although corpectomy and spinal reconstruction can be performed with the VATS technique, the constructs obtained endoscopically may not have the same strength as those constructs obtained via an open procedure.

Interference screw fixation of cervical grafts: a biomechanical study of a new method of cervical fixation.

The dislodgement of anterior bone graft in the cervical spine is a frequent complication of attempted fusion following discectomy or corpectomy. It has been hypothesized that fixation augmented with interference screws may increase the pull-out strength of the construct and decrease the rate of these complications. Mechanical tests were conducted to compare interference screw fixation methods for enhancing the fixation between the bone graft and the adjacent vertebra. The anterior pull-out strengths of cervical bone grafts were compared using fixation with and without the addition of interference screws. Both discectomy and corpectomy graft models were examined in vitro. The mean pull-out force for a Smith-Robinson type bone graft alone was 58.1 N (SD +/- 11.4 N); for the graft augmented with two 3.5 mm cancellous bone screws, 153.9 N (+/- 58.9 N); for the graft with four 3.5 mm screws, 217.1 N (SD +/- 69.9 N). The pull-out strengths of the two and four 3.5 mm screw constructs were significantly greater than the strength of the graft alone (p less than 0.05). Similarly placed 2.7 mm cortical screws of the same length provided increased pull-out strength (123.7 N +/- 38.6 N and 142.5 N +/- 38.2 N for two and four screws, respectively); however, in comparison to the graft alone, these differences were not statistically significant. For both screw types, the four screw fixations were stronger than the two-screw fixations, although these differences were not statistically different.(ABSTRACT TRUNCATED AT 250 WORDS)