Cervical sagittal balance: a biomechanical perspective can help clinical practice.

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.

Biomechanical evaluation of a low profile, anchored cervical interbody spacer device in the setting of progressive flexion-distraction injury of the cervical spine.

INTRODUCTION: Anterior cervical decompression and fusion is a well-established procedure for treatment of degenerative disc disease and cervical trauma including flexion-distraction injuries. Low-profile interbody devices incorporating fixation have been introduced to avoid potential issues associated with dissection and traditional instrumentation. While these devices have been assessed in traditional models, they have not been evaluated in the setting of traumatic spine injury. This study investigated the ability of these devices to stabilize the subaxial cervical spine in the presence of flexion-distraction injuries of increasing severity. METHODS: Thirteen human cadaveric subaxial cervical spines (C3-C7) were tested at C5-C6 in flexion-extension, lateral bending and axial rotation in the load-control mode under ±1.5 Nm moments. Six spines were tested with locked screw configuration and seven with variable angle screw configuration. After testing the range of motion (ROM) with implanted device, progressive posterior destabilization was performed in 3 stages at C5-C6. RESULTS: The anchored spacer device with locked screw configuration significantly reduced C5-C6 flexion-extension (FE) motion from 14.8 ± 4.2 to 3.9 ± 1.8°, lateral bending (LB) from 10.3 ± 2.0 to 1.6 ± 0.8, and axial rotation (AR) from 11.0 ± 2.4 to 2.5 ± 0.8 compared with intact under (p < 0.01). The anchored spacer device with variable angle screw configuration also significantly reduced C5-C6 FE motion from 10.7 ± 1.7 to 5.5 ± 2.5°, LB from 8.3 ± 1.4 to 2.7 ± 1.0, and AR from 8.8 ± 2.7 to 4.6 ± 1.3 compared with intact (p < 0.01). The ROM of the C5-C6 segment with locked screw configuration and grade-3 F-D injury was significantly reduced from intact, with residual motions of 5.1 ± 2.1 in FE, 2.0 ± 1.1 in LB, and 3.3 ± 1.4 in AR. Conversely, the ROM of the C5-C6 segment with variable-angle screw configuration and grade-3 F-D injury was not significantly reduced from intact, with residual motions of 8.7 ± 4.5 in FE, 5.0 ± 1.6 in LB, and 9.5 ± 4.6 in AR. CONCLUSIONS: The locked screw spacer showed significantly reduced motion compared with the intact spine even in the setting of progressive flexion-distraction injury. The variable angle screw spacer did not sufficiently stabilize flexion-distraction injuries. The resulting motion for both constructs was higher than that reported in previous studies using traditional plating. Locked screw spacers may be utilized with additional external immobilization while variable angle screw spacers should not be used in patients with flexion-distraction injuries.

AOA Critical Issues Symposium: Mind the Gap: Addressing Confidence, Imposter Syndrome, and Perfectionism in Surgical Training.

ABSTRACT: Orthopaedic surgeons in training and in their careers can experience a lack of confidence and imposter syndrome. Confidence is built early through continuous improvement, accomplishments, support, and reinforcement. Although it is normal to lack confidence at times, the goal is to recognize this issue, work on visualizing success, and know when to seek help. Mentors can help mentees to build confidence and to normalize thoughts of insecurity and imposter syndrome. It is critical to develop and to maintain resilience, grit, emotional intelligence, courage, and vulnerability during training and throughout one's entire orthopaedic career. Leaders in the field must be aware of these phenomena, be able to talk about such issues, have methods to combat the harmful effects of imposter syndrome, and create a safe, supportive environment conducive to learning and working. Leading well builds not only confidence in oneself but also self-confidence in others. Leaders who are able to build the confidence of individuals will enhance team dynamics, wellness, and overall productivity as well as individual and organizational success.

Would an anatomically shaped lumbar interbody cage provide better stability? An in vitro cadaveric biomechanical evaluation.

STUDY DESIGN: A biomechanical cadaveric study of lumbar spine segments. OBJECTIVE: To compare the immediate stability provided by parallel-shaped and anatomically shaped carbon fiber interbody fusion I/F cages in posterior lumbar interbody fusion (PLIF) and transforaminal lumbar interbody fusion (TLIF) constructs with posterior pedicle screw instrumentation. SUMMARY OF BACKGROUND DATA: Few biomechanical data are available on the anatomically shaped cages in PLIF and TLIF constructs. METHODS: Twenty human lumbar segments were tested in flexion-extension (FE) (8 N m flexion, 6 N m extension), lateral bending (LB) (± 6 N m), and torsional loading (± 5 N m). Each segment was tested in the intact state and after insertion of interbody cages in one of 3 constructs: PLIF with 2 parallel-shaped or anatomically shaped cages and TLIF with 1 anatomically shaped cage. All cages received supplementary pedicle screw fixation. The range-of-motion (ROM) values after cage insertion and posterior fixation were compared with the intact specimen values using analysis of variance and multiple comparisons with Bonferroni correction. RESULTS: All constructs significantly reduced segmental motion relative to intact (P < 0.001). The motion reductions in FE, LB, and axial rotation were 85 ± 15%, 83 ± 18%, and 67 ± 6.8% for the PLIF construct using parallel cages, 79 ± 5.5%, 87 ± 10%, and 66 ± 20% for PLIF using anatomically shaped cages, and 90 ± 6.8%, 87 ± 12%, and 77 ± 22% for TLIF with an anatomically shaped cage. In FE and LB, the reductions in the ROM caused between the 3 constructs were equivalent (P > 0.05). In axial rotation, the TLIF cage provided significantly greater limitation in the ROM compared with the parallel-shaped PLIF cage (P = 0.01). CONCLUSIONS: The parallel-shaped and anatomically shaped I/F cages provided similar stability in a PLIF construct. The greater stability of the TLIF construct was likely due to a more anterior placement of the TLIF cage and preservation of the contralateral facet joint.

Spondylolisthesis and spondylolysis.

Spondylolisthesis is a common condition that can be managed both nonsurgically and surgically. More than 80% of children treated nonsurgically have resolution of symptoms. For those patients requiring surgical treatment, fusion in situ may provide adequate treatment for young patients. Patients with neural compression may require decompression to relieve symptoms, and fusion is also usually indicated. High-grade and degenerative spondylolisthesis require care that is unique to those conditions. Spondylolysis is a defect in the pars interarticularis that occurs in approximately 5% of the general population. Approximately 15% of individuals with a pars interarticularis lesion have progression to spondylolisthesis.

Does Resection of the Posterior Longitudinal Ligament Affect the Stability of Cervical Disc Arthroplasty?

BACKGROUND: The need for posterior longitudinal ligament (PLL) resection during cervical total disc arthroplasty (TDA) has been debated. The purpose of this laboratory study was to investigate the effect of PLL resection on cervical kinematics after TDA. METHODS: Eight cadaveric cervical spine specimens were tested in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) to moments of ±1.5 Nm. After testing the intact condition, anterior C5-C6 cervical discectomy was performed followed by PLL resection and implantation of a compressible, 6-degrees-of-freedom disc prosthesis (M6-C, Spinal Kinetics Inc, Sunnyvale, California). Next, a second prosthesis was implanted at C6-C7 with PLL intact. Finally, the C6-C7 PLL was resected while the disc prosthesis remained in place. Segmental range of motion (ROM) and stiffness in the high flexibility zone around the neutral posture were analyzed using repeated measures ANOVA. RESULTS: At C5-C6, following TDA and PLL resection, FE, LB, and AR ROMs decreased significantly. Anterior and posterior disc height, segmental lordosis, and flexion stiffness increased significantly. At C6-C7, TDA with the PLL intact resulted in a significant increase in anterior disc height and segmental lordosis with no change in posterior disc height. FE, LB, and AR ROMs all decreased significantly, while flexion stiffness increased significantly compared to intact. PLL resection at C6-C7 did not result in a notable change compared to TDA with PLL intact. At the same level, flexion stiffness decreased following PLL resection compared to TDA with a value closer to intact. Two-level TDA (C5-C7) with PLL resection did not result in a loss of segmental stability. CONCLUSION: PLL resection did not significantly affect motion segment kinematics following cervical TDA using a prosthesis with inherent stiffness. Motion segment stiffness loss after PLL resection can be compensated for by a TDA design that can provide resistance to angular motion.

Intestinal ileus as a possible cause of hypobicarbonatemia.

The possible occurrence of metabolic acidosis in patients with intestinal ileus is not well recognized. We describe a patient with acute alcohol-induced pancreatitis and a large transverse colon ileus in which plasma bicarbonate dropped rapidly in the absence of an increase in the plasma anion gap. The urinary anion gap and ammonium excretion were consistent with an appropriate renal response to metabolic acidosis and against the possibility of respiratory alkalosis. The cause of the falling plasma bicarbonate was ascribed to intestinal bicarbonate sequestration owing to the enhancement of chloride-bicarbonate exchange in a dilated paralyzed colon.

Biomechanical Stability Analysis of a Stand-alone Cage, Static and Rotational-dynamic Plate in a Two-level Cervical Fusion Construct.

OBJECTIVE: To test the following hypotheses: (i) anterior cervical discetomy and fusion (ACDF) using stand-alone interbody spacers will significantly reduce the range of motion from intact spine; and (ii) the use of a static or a rotational-dynamic plate will significantly augment the stability of stand-alone interbody spacers, with similar beneficial effect when compared to each other. METHODS: Eleven human cadaveric subaxial cervical spines (age: 48.2 ± 5.4 years) were tested under the following sequence: (i) intact spine; (ii) ACDF at C4 -C5 using a stand-alone interbody spacer; (iii) ACDF at C5 -C6 and insertion of an interbody spacer (two-level construct); and (iv) randomized placement of either a two-level locking static plate or a rotational-dynamic plate. RESULTS: Insertion of stand-alone cage at C4 -C5 and C5 -C6 caused a significant decrease in the range of motion compared to intact spine (P < 0.05). Placement of both the locking and the rotational dynamic plate further reduced the range of motion at C4 -C5 and C5 -C6 compared to stand-alone cage (P < 0.01). No significant differences in range of motion restriction at either C4 -C5 or C5 -C6 were found when the two plating systems were compared (P > 0.05). CONCLUSIONS: Cervical stand-alone interbody spacers caused significant restriction in the range of motion. Both plates significantly augmented the stability of stand-alone interbody spacers, with similar stabilizing effect.

Cervical Spine Muscle-Tendon Unit Length Differences Between Neutral and Forward Head Postures: Biomechanical Study Using Human Cadaveric Specimens.

BACKGROUND: Forward head posture (FHP) may be associated with neck pain and poor health-related quality of life. Literature describes only qualitative muscle length changes associated with FHP. OBJECTIVE: The purpose of this study was to quantify how muscle-tendon unit lengths are altered when human cadaveric specimens are placed in alignments representing different severities of FHP. DESIGN: This biomechanical study used 13 fresh-frozen cadaveric cervical spine specimens (Occiput-T1, 54±15 y). METHODS: Specimens' postural changes simulating increasing FHP severity while maintaining horizontal gaze were assessed. Specimen-specific anatomic models derived from computed tomography-based anatomic data were combined with postural data and specimen-specific anatomy of muscle attachment points to estimate the muscle length changes associated with FHP. RESULTS: Forward head posture was associated with flexion of the mid-lower cervical spine and extension of the upper cervical (sub-occipital) spine. Muscles that insert on the cervical spine and function as flexors (termed "cervical flexors") as well as muscles that insert on the cranium and function as extensors ("occipital extensors") shortened in FHP when compared to neutral posture. In contrast, muscles that insert on the cervical spine and function as extensors ("cervical extensors") as well as muscles that insert on the cranium and function as flexors ("occipital flexors") lengthened. The greatest shortening was seen in the major and minor rectus capitis posterior muscles. These muscles cross the Occiput-C2 segments, which exhibited extension to maintain horizontal gaze. The greatest lengthening was seen in posterior muscles crossing the C4-C6 segments, which exhibited the most flexion. LIMITATIONS: This cadaver study did not incorporate the biomechanical influence of active musculature. CONCLUSIONS: This study offers a novel way to quantify postural alignment and muscle length changes associated with FHP. Model predictions are consistent with qualitative descriptions in the literature.

One-year follow-up of a randomized clinical trial comparing flexion distraction with an exercise program for chronic low-back pain.

OBJECTIVE: Flexion distraction is a commonly used form of chiropractic care with chiropractor utilization rates of 58%. However, no previous randomized clinical trial has assessed the effectiveness of this form of care. The objective of this investigation was to compare the pain and disability during the year after active care based on treatment group allocation (Flexion Distraction versus Exercise Program). STUDY DESIGN: Randomized clinical trial, follow-up. SUBJECTS: Two hundred and thirty-five (235) subjects who were previously randomized to either chiropractic care (flexion distraction) or physical therapy (exercise program) within a clinical trial. OUTCOME MEASURES: Subjects were followed for 1 year via mailed questionnaires to assess levels of pain (Visual Analog Scale) and dysfunction (Roland Morris). RESULTS: Study subjects had a decrease in pain and disability after intervention regardless of which group they attended (p < 0.002), however, during the year after care, subjects who received chiropractic care (flexion distraction therapy) had significantly lower pain scores than subjects who received physical therapy (exercise program) (p = 0.02). CONCLUSIONS: In this first trial on flexion distraction care, flexion distraction was found to be more effective in reducing pain for 1 year when compared to a form of physical therapy.

Amount of health care and self-care following a randomized clinical trial comparing flexion-distraction with exercise program for chronic low back pain.

BACKGROUND: Previous clinical trials have assessed the percentage of participants who utilized further health care after a period of conservative care for low back pain, however no chiropractic clinical trial has determined the total amount of care during this time and any differences based on assigned treatment group. The objective of this clinical trial follow-up was to assess if there was a difference in the total number of office visits for low back pain over one year after a four week clinical trial of either a form of physical therapy (Exercise Program) or a form of chiropractic care (Flexion Distraction) for chronic low back pain. METHODS: In this randomized clinical trial follow up study, 195 participants were followed for one year after a four-week period of either a form of chiropractic care (FD) or a form of physical therapy (EP). Weekly structured telephone interview questions regarded visitation of various health care practitioners and the practice of self-care for low back pain. RESULTS: Participants in the physical therapy group demonstrated on average significantly more visits to any health care provider and to a general practitioner during the year after trial care (p < 0.05). No group differences were noted in the number of visits to a chiropractor or physical therapist. Self-care was initiated by nearly every participant in both groups. CONCLUSION: During a one-year follow-up, participants previously randomized to physical therapy attended significantly more health care visits than those participants who received chiropractic care.

Is Cervical Sagittal Imbalance a Risk Factor for Adjacent Segment Pathomechanics After Multilevel Fusion?

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.

Restoring geometric and loading alignment of the thoracic spine with a vertebral compression fracture: effects of balloon (bone tamp) inflation and spinal extension.

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.

Postural Consequences of Cervical Sagittal Imbalance: A Novel Laboratory Model.

STUDY DESIGN: A biomechanical study using human spine specimens. OBJECTIVE: To study postural compensations in lordosis angles that are necessary to maintain horizontal gaze in the presence of forward head posture and increasing T1 sagittal tilt. SUMMARY OF BACKGROUND DATA: Forward head posture relative to the shoulders, assessed radiographically using the horizontal offset distance between the C2 and C7 vertebral bodies (C2-C7 [sagittal vertical alignment] SVA), is a measure of global cervical imbalance. This may result from kyphotic alignment of cervical segments, muscle imbalance, as well as malalignment of thoracolumbar spine. METHODS: Ten cadaveric cervical spines (occiput-T1) were tested. The T1 vertebra was anchored to a tilting and translating base. The occiput was free to move vertically but its angular orientation was constrained to ensure horizontal gaze regardless of sagittal imbalance. A 5-kg mass was attached to the occiput to mimic head weight. Forward head posture magnitude and T1 tilt were varied and motions of individual vertebrae were measured to calculate C2-C7 SVA and lordosis across C0-C2 and C2-C7. RESULTS: Increasing C2-C7 SVA caused flexion of lower cervical (C2-C7) segments and hyperextension of suboccipital (C0-C1-C2) segments to maintain horizontal gaze. Increasing kyphotic T1 tilt primarily increased lordosis across the C2-C7 segments. Regression models were developed to predict the compensatory C0-C2 and C2-C7 angulation needed to maintain horizontal gaze given values of C2-C7 SVA and T1 tilt. CONCLUSION: This study established predictive relationships between radiographical measures of forward head posture, T1 tilt, and postural compensations in the cervical lordosis angles needed to maintain horizontal gaze. The laboratory model predicted that normalization of C2-C7 SVA will reduce suboccipital (C0-C2) hyperextension, whereas T1 tilt reduction will reduce the hyperextension in the C2-C7 segments. The predictive relationships may help in planning corrective strategy in patients experiencing neck pain, which may be attributed to sagittal malalignment. LEVEL OF EVIDENCE: N/A.

Effect of compressive follower preload on the flexion-extension response of the human lumbar spine.

Traditional experimental methods are unable to study the kinematics of whole lumbar spine specimens under physiologic compressive preloads because the spine without active musculature buckles under just 120 N of vertical load. However, the lumbar spine can support a compressive load of physiologic magnitude (up to 1200 N) without collapsing if the load is applied along a follower load path. This study tested the hypothesis that the load-displacement response of the lumbar spine in flexion-extension is affected by the magnitude of the follower preload and the follower preload path. Twenty-one fresh human cadaveric lumbar spines were tested in flexion-extension under increasing compressive follower preload applied along two distinctly different optimized preload paths. The first (neutral) preload path was considered optimum if the specimen underwent the least angular change in its lordosis when the full range of preload (0-1200 N) was applied in its neutral posture. The second (flexed) preload path was optimized for an intermediate specimen posture between neutral and full flexion. A twofold increase in flexion stiffness occurred around the neutral posture as the preload was increased from 0 to 1200 N. The preload magnitude (400 N and larger) significantly affected the range of motion (ROM), with a 25% decrease at 1200 N preload applied along the neutral path. When the preload was applied along a path optimized for an intermediate forward-flexed posture, only a 15% decrease in ROM occurred at 1200 N. The results demonstrate that whole lumbar spine specimens can be subjected to compressive follower preloads of in vivo magnitudes while allowing physiologic mobility under flexion-extension moments. The optimized follower preload provides a method to simulate the resultant vector of the muscles that allow the spine to support physiologic compressive loads induced during flexion-extension activities.

Compressive preload improves the stability of anterior lumbar interbody fusion cage constructs.

BACKGROUND: Insertion of an anterior lumbar interbody fusion cage has been shown to reduce motion in a human spine segment in all loading directions except extension. The "stand-alone" cages depend on compressive preload produced by anular pretensioning and muscle forces for initial stabilization. However, the effect that the in vivo compressive preload generated during activities of daily living has on the construct is not fully understood. This study tested the hypothesis that the ability of the cages to reduce the segmental motions in flexion and extension is significantly affected by the magnitude of the externally applied compressive preload. METHODS: Fourteen specimens from human lumbar spines were tested intact and after insertion of two threaded cylindrical cages at level L5-Sl. They were subjected to flexion and extension moments under progressively increasing magnitudes of externally applied compressive follower preload from 0 to 1200 N. The range of motion at level L5-S1 after cage insertion was compared with the value achieved in the intact specimens at each compressive preload magnitude. RESULTS: The cages significantly reduced the L5-S1 flexion motion at all preloads (p < 0.05). They decreased flexion motion by 29% to 43% of that of the intact specimens for low preloads (0 to 400 N) and by 69% to 79% of that of the intact specimens under preloads of 800 to 1200 N. In extension, in the absence of an externally applied preload, the cages permitted 24% more motion than the intact segment (p < 0.05). In contrast, they reduced the extension motion at preloads from 200 to 1200 N. Under preloads of 800 to 1200 N, the reduction in extension motion after cage placement was 42% to 48% of that of the intact segment (p < 0.05). The reduction of motion in both flexion and extension after cage placement was significantly greater at preloads of 800 to 1200 N compared with the motion reductions at preloads of < or =400 N (p < 0.05). CONCLUSIONS: In contrast to the observed extension instability under anular tension preload only, the two-cage construct exerted a stabilizing effect on the motion segment (a reduction in segmental motion) in flexion as well as extension under externally applied compressive preloads of physiologic magnitudes. The external compressive preload significantly affected the stabilization provided by the cages. The cages provided substantially more stabilization, both in flexion and in extension, at larger preloads than at smaller preloads. CLINICAL RELEVANCE: The study suggests that the segment treated with an anterior lumbar interbody fusion cage is relatively less stable under conditions of low external compressive preload. The magnitude of preload required to achieve stabilization with stand-alone cages may be only partially achieved by anular pretensioning. Since the magnitude of the preload across the disc space due to muscle activity can vary with activities of daily living, supplemental stabilization of the cage construct may provide a more predictably stable environment for lumbar spine fusion.

Effect of physiological loads on cortical and traditional pedicle screw fixation.

STUDY DESIGN: Human cadaveric biomechanical study. OBJECTIVE: To determine the fixation strength of laterally directed, cortical pedicle screws under physiological loads. SUMMARY OF BACKGROUND DATA: Lateral trajectory cortical pedicle screws have been described as a means of obtaining improved fixation while minimizing soft-tissue dissection during lumbar instrumentation. Biomechanical data have demonstrated equivalent strength in a quasi-static model; however, no biomechanical information is available comparing the fixation of cortical with traditional pedicle screws under cyclic physiological loads. METHODS: Seventeen vertebral levels (T11-L5) underwent quantitative computed tomography. On 1 side, a laterally directed, cortical pedicle screw was inserted with a traditional, medially directed pedicle screw placed on the contralateral side. With the specimen constrained in a testing apparatus, each screw underwent cyclic craniocaudal toggling under incrementally increasing physiological loads until 2 mm of head displacement occurred. Next, uniaxial pullout of each toggled screw was performed. The number of craniocaudal toggle cycles and load (N) required to achieve pedicle screw movement as well as axial pullout resistance (N) were compared between the 2 techniques. RESULTS: The mean trabecular bone mineral density of the specimens was 202 K2HPO4 mg/cm. Cortical pedicle screws demonstrated significantly improved resistance to toggle testing, requiring 184 cycles to reach 2 mm of displacement compared with 102 cycles for the traditional pedicle screws (P=0.002). The force necessary to displace the screws was also significantly greater for the cortical versus the traditional screws (398 N vs. 300 N, P=0.004). There was no statistical difference in axial pullout strength between the previously toggled cortical and traditional pedicle screws (1722 N vs. 1741 N, P=0.837). CONCLUSION: Laterally directed cortical pedicle screws have superior resistance to craniocaudal toggling compared with traditional pedicle screws. LEVEL OF EVIDENCE: N/A.

Biomechanical evaluation of a low-profile, anchored cervical interbody spacer device at the index level or adjacent to plated fusion.

STUDY DESIGN: In vitro biomechanical study. OBJECTIVE: To test the hypotheses: (1) an anchored spacer device would decrease motion similarly to a plate-spacer construct, and (2) the anchored spacer would achieve a similar reduction in motion when placed adjacent to a previously fused segment. SUMMARY OF BACKGROUND DATA: An anchored spacer device has been shown to perform similar to the plate-spacer construct in previous biomechanical evaluation. The prevalence of adjacent segment disease after fusion is well established in the literature.There is currently no evidence supporting the use of an anchored interbody spacer device adjacent to a previous fusion. METHODS: Eight human cervical spines (age: 45.1 ± 13.1 yr) were tested in moment control (±1.5 Nm) in flexion-extension, lateral bending, and axial rotation without preload. Flexion-extension was then retested under 150-N preload. Spines were tested intact and after anterior cervical discectomy and fusion (ACDF) at C4-C5 and C6-C7 with either a plate-spacer or anchored spacer construct (randomized). The specimens were tested finally with an ACDF at the floating C5-C6 segment using the anchored spacer device adjacent to the previous fusions. RESULTS: Both the plate-spacer and anchored spacer significantly reduced motion from the intact spine in flexion-extension, lateral bending, and axial rotation (P < 0.005). There was no statistically significant difference between the 2 fusion constructs in their abilities to reduce motions (P = 1.0). ACDF using the anchored spacer at the floating C5-C6 level (in between the plate-spacer and anchored spacer constructs) resulted in significant motion reductions in all modes of testing (P < 0.05). These motion reductions did not significantly differ from those of a single-level anchored-spacer construct or a single-level plated ACDF. CONCLUSION: The anchored spacer provided significant motion reductions, similar to a plated ACDF, when used as a single-level fusion construct or placed adjacent to a previously plated segment. LEVEL OF EVIDENCE: N/A.

The effect of posterior decompressive procedures on segmental range of motion after cervical total disc arthroplasty.

STUDY DESIGN: We quantified the segmental biomechanics of a cervical total disc replacement (TDR) before and after progressive posterior decompression. We hypothesized that posterior decompressive procedures would not significantly increase range of motion (ROM) at the index TDR level. OBJECTIVE: To quantify the kinematics of a cervical total disc replacement (TDR) before and after posterior cervical decompression. SUMMARY OF BACKGROUND DATA: A reported yet unaddressed issue is the potential for the development of same-segment disease after implantation of a cervical TDR and the implications of same-segment posterior decompression on TDR mechanics. METHODS: Eight human cadaveric cervical spines C3-C7 were tested in flexion-extension, lateral bending, and axial rotation while intact, after C5-C6 TDR, C5-C6 unilateral foraminotomy, C5-C6 bilateral foraminotomies, and after C5 laminectomy in combination with the bilateral foraminotomies. Moment versus angular motion curves were obtained for each testing step, and the load-displacement data were analyzed to determine the range of angular motion for each step. RESULTS: Unilateral foraminotomy did not result in a statistically significant increase in flexion-extension ROM, and did not increase the ROM to a degree greater than normal. Although bilateral foraminotomies did increase flexion-extension ROM, motion remained within a physiological range. A full laminectomy added to the bilateral foraminotomies significantly increased ROM and was also associated with distortion of the load-displacement curves. CONCLUSION: With respect to segmental biomechanics as demonstrated, we think that for same-segment disease, a unilateral foraminotomy can be performed safely. However, the impact of in vivo conditions was not accounted for in this model, and it is possible that cyclical loading and other physiological stresses on such a construct may affect the behavior and lifespan of the implant in a way that cannot be predicted by a biomechanical study. Bilateral foraminotomies would require close observation and additional clinical follow-up, whereas complete laminectomy combined with bilateral foraminotomies should be avoided after TDR given the significant changes in kinematics. In addition, future disc replacement designs may need to account for changes after posterior decompression for same-segment disease. LEVEL OF EVIDENCE: N/A.

Compressive preload reduces segmental flexion instability after progressive destabilization of the lumbar spine.

STUDY DESIGN: Biomechanical human cadaveric study. OBJECTIVE: We hypothesized that increasing compressive preload will reduce the segmental instability after nucleotomy, posterior ligament resection, and decompressive surgery. SUMMARY OF BACKGROUND DATA: The human spine experiences significant compressive preloads in vivo due to spinal musculature and gravity. Although the effect of destabilization procedures on spinal motion has been studied, the effect of compressive preload on the motion response of destabilized, multisegment lumbar spines has not been reported. METHODS: Eight human cadaveric spines (L1-sacrum, 51.4 ± 14.1 yr) were tested intact, after L4-L5 nucleotomy, after interspinous and supraspinous ligaments transection, and after midline decompression (bilateral laminotomy, partial medial facetectomy, and foraminotomy). Specimens were loaded in flexion (8 Nm) and extension (6 Nm) under 0-N, 200-N, and 400-N compressive follower preload. L4-L5 range of motion (ROM) and flexion stiffness in the high-flexibility zone were analyzed using repeated-measures analysis of variance and multiple comparisons with the Bonferroni correction. RESULTS: With a fixed set of loading conditions, a progressive increase in segmental ROM along with expansion of the high-flexibility zone (decrease of flexion stiffness) was noted with serial destabilizations. Application of increasing compressive preload did not substantially change segmental ROM, but did significantly increase the segmental stiffness in the high-flexibility zone. In the most destabilized condition, 400-N preload did not return the segmental stiffness to intact levels. CONCLUSION: Anatomical alterations representing degenerative and iatrogenic instabilities are associated with significant increases in segmental ROM and decreased segmental stiffness. Although application of compressive preload, mimicking the effect of increased axial muscular activity, significantly increased the segmental stiffness, it was not restored to intact levels; thereby suggesting that core strengthening alone may not compensate for the loss of structural stability associated with midline surgical decompression. This suggests that there may be a role for surgical implants or interventions that specifically increase flexion stiffness and limit flexion ROM to counteract the iatrogenic instability resulting from surgical decompression. LEVEL OF EVIDENCE: N/A.