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Purpose: In this article, we summarize our work on understanding the influence of cervical sagittal malalignment on the mechanics of the cervical spine., Methods: Biomechanical studies were performed using an ex vivo laboratory model to study the kinematic and kinetic response of human cervical spine specimens in the setting of cervical sagittal imbalance. The model allowed controlled variations of C2-C7 Sagittal Vertical Alignment (C2-C7 SVA) and T1-Slope so that clinically relevant sagittally malaligned profiles could be prescribed, while maintaining horizontal gaze, and their biomechanical consequences studied., Results: Our results demonstrated that increasing C2-C7 SVA caused flexion of lower cervical (C2-C7) segments and hyperextension of suboccipital (C0-C1-C2) segments to maintain horizontal gaze. An increase in C2-C7 SVA increased the lower cervical neural foraminal areas. Conversely, increasing T1-slope predominantly influenced subaxial cervical lordosis and, as a result, decreased cervical neural foraminal areas. Therefore, we believe patients with increased upper thoracic kyphosis and radicular symptoms may respond with increased forward head posture (FHP) as a compensatory mechanism to increase their lower cervical neural foraminal area and alleviate nerve root compression as well as reduce the burden on posterior muscles and soft and bony structures of the cervical spine. Increasing FHP (i.e., increased C2-C7 SVA) was associated with shortening of the cervical flexors and occipital extensors and lengthening of the cervical extensors and occipital flexors, which corresponds to C2-C7 flexion and C0-C2 extension. The greatest shortening occurred in the suboccipital muscles, suggesting considerable load bearing of these muscles during chronic FHP. Regardless, there was no evidence of nerve compression within the suboccipital triangle. Finally, cervical sagittal imbalance may play a role in exacerbating adjacent segment pathomechanics after multilevel cervical fusion and should be considered during surgical planning., Conclusions: The results of our biomechanical studies have improved our understanding of the impact of cervical sagittal malalignment on pathomechanics of the cervical spine. We believe this improved understanding will assist in clinical decision-making.

Background: A lateral approach with open reduction and internal fixation with a plate is a very effective technique for the majority of distal fibular fractures. However, this open approach for ankle fixation may be complicated by wound dehiscence and infection, especially in high-risk patients. An alternative to plating is an intramedullary implant, which allows maintenance of length, alignment, and rotation and which allows for decreased soft tissue dissection. While there has been clinical data suggesting favorable short-term outcomes with these implants, there is no current biomechanical literature investigating this technology in this particular fracture pattern. This study sought to biomechanically compare an emerging technology with an established method of fixation for distal fibular fractures that traditionally require an extensive exposure., Methods: Ten matched cadaveric pairs from the proximal tibia to the foot were prepared to simulate an Arbeitsgemeinschaft für Osteosynthesefragen/Orthopaedic Trauma Association (AO/OTA) 44C2 ankle fracture and randomized to fixation with a distal fibular locking plate or intramedullary fibular rod. A constant 700-N axial load was applied, and all specimens underwent testing for external rotation stiffness, external rotation cyclic loading, and torque to failure. The syndesmotic diastasis, stiffness, torque to failure, angle at failure, and mode of failure were obtained from each specimen., Results: There was no significant difference in syndesmotic diastasis during cyclic loading or at maximal external rotation between the rod and plate groups. Post-cycle external rotation stiffness across the syndesmosis was significantly higher for the locking plate than the fibular rod. There was no significant difference between the rod and plate in torque at failure or external rotation angle. The majority of specimens had failure at the syndesmotic screw., Conclusions: In the present cadaveric study of an AO/OTA 44C2 ankle fracture, a modern fibular rod demonstrated less external rotation stiffness while maintaining the syndesmotic diastasis to within acceptable tolerances and having similar failure characteristics.

Study Design: A biomechanical study using human spine specimens., Objective: The aim of this study was to assess whether the presence of cervical sagittal imbalance is an independent risk factor for increasing the mechanical burden on discs adjacent to cervical multilevel fusions., Summary of Background Data: The horizontal offset distance between the C2 plumbline and C7 vertebral body (C2-C7 Sagittal Vertical Axis (SVA)) or the angle made with vertical by a line connecting the C2 and C7 vertebral bodies (C2-C7 tilt angle) are used as radiographic measures to assess cervical sagittal balance. There is level III clinical evidence that sagittal imbalance caused by kyphotic fusions or global spinal sagittal malalignment may increase the risk of adjacent segment pathology., Methods: Thirteen human cadaveric cervical spines (Occiput-T1; age: 50.6 years; range: 21-67) were tested first in the native intact state and then after instrumentation across C4-C6 to simulate in situ two-level fusion. Specimens were tested using a previously validated experimental model that allowed measurement of spinal response to prescribed imbalance. The effects of fusion on segmental angular alignments and intradiscal pressures in the C3-C4 and C6-C7 discs, above and below the fusion, were evaluated at different magnitudes of C2-C7 tilt angle (or C2-C7 SVA)., Results: When compared with the pre-fusion state, in situ fusion across C4-C6 segments required increased flexion angulation and resulted in increased intradiscal pressure at the C6-C7 disc below the fusion in order to accommodate the same increase in C2-C7 tilt angle or C2-C7 SVA (P < 0.05). The adjacent segment mechanical burden due to fusion became greater with increasing C2-C7 tilt angle or SVA., Conclusion: Cervical sagittal imbalance arising from regional and/or global spinal sagittal malalignment may play a role in exacerbating adjacent segment pathomechanics after multilevel fusion and should be considered during surgical planning., Level of Evidence: N/A.

Orthotic treatment (braces) for scoliosis is best understood in light of the natural history of idiopathic scoliosis and the mechanics of spine stability. Idiopathic scoliosis is a spinal deformity confronted often by spine surgeons. With prompt recognition, treatment can be effective.

Background Context: The anatomy of the atlantoaxial joint makes stabilization at this level challenging. Current techniques that use transarticular screw fixation (Magerl) or segmental screw fixation (Harms) give dramatically improved stability but risk damage to the vertebral artery. A novel integrated device was designed and developed to obtain intra-articular stabilization via primary interference fixation within the C1-C2 lateral mass articulation., Purpose: To assess the atlantoaxial stability achieved with a novel integrated device when compared with the intact, destabilized, and stabilized state using the Harms technique., Study Design: A biomechanical study of implants in human cadaveric cervical spines., Methods: Six human cadaveric specimens were used. Biomechanical testing was performed with moment control in flexion-extension, lateral bending, and axial rotation. Range of motion (ROM) was measured in the intact state, after both destabilization by creation of a Type II odontoid peg fracture and sequential stabilization using the integrated device and the Harms technique., Results: Mean flexion-extension ROM of the intact specimens at C1-C2 was 14.1°±2.9°. Destabilization increased the ROM to 31.6°±4.6°. Instrumentation with the Harms technique reduced flexion-extension motion to 4.0°±1.4° (p<.01). The integrated device reduced flexion-extension motion to 3.6°±1.8° (p<.01). In lateral bending, the respective mean angular motions were 1.8°±1.1°, 14.1°±5.8°, 1.4°±0.7°, and 0.4°±0.3° for the intact destabilized Harms technique and integrated device. For axial rotation, the respective mean values were 67.3°±13.8°, 74.2°±16.1°, 1.4°±0.7° and 0.9°±0.7°. Both the Harms technique and integrated device significantly reduced motion compared with the destabilized spine in flexion-extension, lateral bending, and axial rotation (p<.05). Direct comparison of the Harms technique and the integrated device revealed no significant difference (p>.10)., Conclusions: The integrated device resulted in interference fixation at the C1-C2 lateral mass joints with comparable stability to the Harms technique. Perceived advantages with the integrated device include avoidance of fixation below the C2 lateral mass where the vertebral artery is susceptible to injury, and access to the C1 screw entry point through the blade of the integrated device avoiding extended dissection superior to the C2 nerve root and its surrounding venous plexus., (Copyright © 2012 Elsevier Inc. All rights reserved.)

Background Context: Lumbar fusion is traditionally used to restore stability after wide surgical decompression for spinal stenosis. The Total Facet Arthroplasty System (TFAS) is a motion-restoring implant suggested as an alternative to rigid fixation after complete facetectomy., Purpose: To investigate the effect of TFAS on the kinematics of the implanted and adjacent lumbar segments., Study Design: Biomechanical in vitro study., Methods: Nine human lumbar spines (L1 to sacrum) were tested in flexion-extension (+8 to -6Nm), lateral bending (+/-6Nm), and axial rotation (+/-5Nm). Flexion-extension was tested under 400 N follower preload. Specimens were tested intact, after complete L3 laminectomy with L3-L4 facetectomy, after L3-L4 pedicle screw fixation, and after L3-L4 TFAS implantation. Range of motion (ROM) was assessed in all tested directions. Neutral zone and stiffness in flexion and extension were calculated to assess quality of motion., Results: Complete laminectomy-facetectomy increased L3-L4 ROM compared with intact in flexion-extension (8.7+/-2.0 degrees to 12.2+/-3.2 degrees, p<.05) lateral bending (9.0+/-2.5 degrees to 12.6+/-3.2 degrees, p=.09), and axial rotation (3.8+/-2.7 degrees to 7.8+/-4.5 degrees p<.05). Pedicle screw fixation decreased ROM compared with intact, resulting in 1.7+/-0.5 degrees flexion-extension (p<.05), 3.3+/-1.4 degrees lateral bending (p<.05), and 1.8+/-0.6 degrees axial rotation (p=.09). TFAS restored intact ROM (p>.05) resulting in 7.9+/-2.1 degrees flexion-extension, 10.1+/-3.0 degrees lateral bending, and 4.7+/-1.6 degrees axial rotation. Fusion significantly increased the normalized ROM at all remaining lumbar segments, whereas TFAS implantation resulted in near-normal distribution of normalized ROM at the implanted and remaining lumbar segments. Flexion and extension stiffness in the high-flexibility zone decreased after facetectomy (p<.05) and increased after simulated fusion (p<.05). TFAS restored quality of motion parameters (load-displacement curves) to intact (p>.05). The quality of motion parameters for the whole lumbar spine mimicked L3-L4 segmental results., Conclusions: TFAS restored range and quality of motion at the operated segment to intact values and restored near-normal motion at the adjacent segments.

Background Context: In patients with osteoporosis, changes in spinal alignment after a vertebral compression fracture (VCF) are believed to increase the risk of fracture of the adjacent vertebrae. The alterations in spinal biomechanics as a result of osteoporotic VCF and the effects of deformity correction on the loads in the adjacent vertebral bodies are not fully understood., Purpose: To measure 1) the effect of thoracic VCFs on kyphosis (geometric alignment) and the shift of the physiologic compressive load path (loading alignment), 2) the effect of fracture reduction by balloon (bone tamp) inflation in restoring normal geometric and loading alignment and 3) the effect of spinal extension alone on fracture reduction and restoration of normal geometric and loading alignment., Study Design/setting: A biomechanical study using six fresh human thoracic specimens, each consisting of three adjacent vertebrae with all soft tissues and bony structures intact., Methods: In order to reliably create fracture, cancellous bone in the middle vertebral body was disrupted by inflation of bone tamps. After removal of the bone tamps, the specimen was compressed using bilateral loading cables until a fracture was observed with anterior vertebral body height loss of >/=25%. Fracture reduction was performed under a compressive preload of 250 N first under the application of extension moments, and then using inflatable bone tamps. The vertebral body heights, kyphotic deformity of the fractured vertebra and adjacent segments and location of compressive load (cable) path in the fractured and adjacent vertebral bodies were measured on video-fluoroscopic images., Results: The VCF caused anterior wall height loss of 37+/-15%, middle-height loss of 34+/-16%, segmental kyphosis increase of 14+/-7.0 degrees and vertebral kyphosis increase of 13+/-5.5 degrees (p<.05). The compressive load path shifted anteriorly by about 20% of anteroposterior end plate width in the fractured and adjacent vertebrae (p=.008). Bone tamp inflation restored the anterior wall height to 91+/-8.9%, middle-height to 91+/-14% and segmental kyphosis to within 5.6+/-5.9 degrees of prefracture values. The compressive load path returned posteriorly relative to the postfracture location in all three vertebrae (p=.004): the load path remained anterior to the prefracture location by about 9% to 11% of the anteroposterior end plate width. With application of extension moment (6.3+/-2.2 Nm) until segmental kyphosis and compressive load path were fully restored, anterior vertebral body heights were improved to 85+/-8.6% of prefracture values. However, the middle vertebral body height was not restored and vertebral kyphotic deformity remained significantly larger than the prefracture values (p<.05)., Conclusions: The anterior shift of the compressive load path in vertebral bodies adjacent to VCF can induce additional flexion moments on these vertebrae. This eccentric loading may contribute to the increased risk of new fractures in osteoporotic vertebrae adjacent to an uncorrected VCF deformity. Bone tamp inflation under a physiologic preload significantly reduced the VCF deformity (anterior and middle vertebral body heights, segmental and vertebral kyphosis) and returned the compressive load path posteriorly, approaching the prefracture alignment. Application of extension moments also was effective in restoring the prefracture geometric and loading alignment of adjacent segments, but the middle height of the fractured vertebra and vertebral kyphotic deformity were not restored with spinal extension alone.

The analysis of current cervical collars (Aspen and Miami J collars) and cervical thoracic orthoses (CTOs) (Aspen 2-post and Aspen 4-post CTOs) in reducing cervical intervertebral and gross range of motion in flexion and extension was performed using 20 normal volunteer subjects. The gross sagittal motion of the head was measured relative to the horizon with the use of an optoelectronic motion measurement system. Simultaneous measurement of cervical intervertebral motion was performed with the use of a video fluoroscopy (VF) machine. Intervertebral motion was described as (1) the angular motion of each vertebra and (2) the translational motion of the vertebral centroid. We used surface electromyographic (EMG) signal data to compare subject efforts between the two collars and between the two CTOs. Each orthosis significantly reduced gross and intervertebral motion in flexion and extension (p < 0.05). No statistically significant differences were found between the Miami J and Aspen collars in reducing gross or intervertebral sagittal motion, except at C5-6. Both CTOs provided significantly more restriction of gross and intervertebral flexion and extension motion as compared to the two collars (p < 0.05). The Aspen 2-post CTO and 4-post CTO performed similarly in flexion, but the Aspen 4-post CTO provided significantly more restriction of extension motion (p < 0.05).