Sex differences in perceived discomfort during seated static posture holding.

This study investigated how sex modifies postural discomfort perception during a sagittally-symmetric, seated static posture holding (SPH) task. Ten male and 10 female participants performed SPH and conducted subjective discomfort ratings in a total of 108 task conditions. A regression analysis found that the impacts of the body joint reactive moments on perceived discomfort were larger for the female group than the male whereas that of the shoulder joint angle was more pronounced for the male than the female. Also, some of the 108 task conditions were found to be more uncomfortable for the male group, while some others, for the female. The observed sex impacts are thought to be due to the sex differences in physical work capacities (muscular strength and joint flexibility). The results suggest that new posture analysis tools allowing sex-specific analyses are needed as they would improve the accuracy and precision of ergonomics posture analyses. Practitioner summary: This study empirically investigated how sex modifies postural discomfort perception during a seated posture holding (SPH) task. Sex was found to modify the impacts of joint reactive moments and the shoulder joint angle. The study results seem to reflect the sex differences in muscular strength and joint flexibility.

Obesity impacts on task performance and perceived discomfort during seated foot target reaches.

This study examined obesity impacts on task performance and perceived discomfort during seated foot target reaches. Three independent variables, participant group, movement distance, and, movement direction, were considered. The task performance measures employed were reaction time, movement time and task performance time. Perceived discomfort was measured using a modified Borg CR-10 scale. Statistical analyses revealed that: obesity was associated with increases in movement time, reaction time and performance time; movement distance significantly affected the three task performance measures and discomfort rating; and, movement direction significantly affected movement time, performance time and discomfort rating. The obesity impacts observed are thought to reflect the decelerating effects of the extra fat mass in the obese body during foot reaches and possibly obesity-related physiological and cognitive changes. Design improvements of foot-operated systems, such as reducing distances to targets, increasing target sizes and avoiding forward foot reaches, may help counteract the observed obesity impacts. Practitioner Summary: This study empirically investigated the obesity impacts on task performance and perceived discomfort during seated foot target reaches. Obesity was found to be associated with increases in movement time, reaction time and performance time. The observed obesity impacts seem attributable to the anthropometric, motor and cognitive characteristics of the obese.Abbreviations: ANOVA: analysis of variance; BMI: body mass index; ROM: range of motion; Borg CR-10: Borg's category ratio 10 scale.

Relationship between Tongue Temperature Estimated by Infrared Thermography, Tongue Color, and Cold-Heat Pathological Patterns: A Retrospective Chart Review Study.

Tongue diagnosis is a technique used to determine cold-heat pathologic patterns (CHPPs). Herein, we reviewed electronic medical records of tongue temperature measured using infrared thermography (IRT), tongue color (luminance, green/red, and blue/yellow balance), cold-heat pattern questionnaires (CHPQ), and body temperature for 134 women with gynecological problems (age, 38.97 ± 11.49 years). The temperatures of seven tongue regions (root, center, tip, and both sides of the center and root) were determined, and the effects of age, regional differences, and their correlations with color parameters were examined. Factor analysis was conducted separately with the 10 cold pattern (CP) and 10 heat pattern (HP) items. Tongue temperature showed an age effect (ß; -0.198 to -0.210) and regional differences (both sides of the root > center and root > tip). Tongue temperature was positively correlated with luminance (r: 0.236-0.246), indicating that a higher temperature was associated with a brighter color. The factor analysis extracted two factors (cold sensitivity-pain and discharge-complexion factors) from the CP items and three factors (heat sensation-pain, discharge-breath, and cold preference-thirst factors) from the HP items. Tongue temperature was negatively correlated with the discharge-complexion factor of CP and the discharge-breath factor of HP (r: -0.171 to -0.203), indicating that a lower tongue temperature may be a consequence of emission of excessive heat in HP and a lower blood perfusion in CP. Body temperature did not correlate with the CHPQ factor scores. In conclusion, tongue temperatures measured using IRT may be a partial indicator of CHPPs.

Effect of screw position on load transfer in lumbar pedicle screws: a non-idealized finite element analysis.

Angled screw insertion has been advocated to enhance fixation strength during posterior spine fixation. Stresses on a pedicle screw and surrounding vertebral bone with different screw angles were studied by finite element analysis during simulated multidirectional loading. Correlations between screw-specific vertebral geometric parameters and stresses were studied. Angulations in both the sagittal and axial planes affected stresses on the cortical and cancellous bones and the screw. Pedicle screws pointing laterally (vs. straight or medially) in the axial plane during superior screw angulation may be advantageous in terms of reducing the risk of both screw loosening and screw breakage.

Biomechanics of Cervical "Skip" Corpectomy Versus Standard Multilevel Corpectomy.

STUDY DESIGN: In vitro biomechanical study of flexibility with finite-element simulation to estimate screw stresses. OBJECTIVE: To compare cervical spinal stability after a standard plated 3-level corpectomy with stability after a plated 3-level "skip" corpectomy where the middle vertebra is left intact (ie, two 1-level corpectomies), and to quantify pullout forces acting on the screws during various loading modes. SUMMARY OF BACKGROUND DATA: Clinically, 3-level cervical plated corpectomy has a high rate of failure, partially because only 4 contact points affix the plate to the upper and lower intact vertebrae. Leaving the intermediate vertebral body intact for additional fixation points may overcome this problem while still allowing dural sac decompression. METHODS: Quasistatic nonconstraining torque (maximum 1 N m) induced flexion, extension, lateral bending, and axial rotation while angular motion was recorded stereophotogrammetrically. Specimens were tested intact and after corpectomy with standard plated and strut-grafted 3-level corpectomy (7 specimens) or "skip" corpectomy (7 specimens). Screw stresses were quantified using a validated finite-element model of C3-C7 mimicking experimentally tested groups. Skip corpectomy with C5 screws omitted was also simulated. RESULTS: Plated skip corpectomy tended to be more stable than plated standard corpectomy, but the difference was not significant. Compared with standard plated corpectomy, plated skip corpectomy reduced peak screw pullout force during axial rotation (mode of loading of highest peak force) by 15% (4-screw attachment) and 19% (6-screw attachment). CONCLUSIONS: Skip corpectomy is a good alternative to standard 3-level corpectomy to improve stability, especially during lateral bending. Under pure moment loading, the screws of a cervical multilevel plate experience the highest pullout forces during axial rotation. Thus, limiting this movement in patients undergoing plated multilevel corpectomy may be reasonable, especially until solid fusion is achieved.

Biomechanical analysis of a novel hook-screw technique for C1-2 stabilization.

UNLABELLED: The Food and Drug Administration has not cleared the following medical devices for the use described in this study. The following medical devices are being discussed for an off-label use: cervical lateral mass screws. OBJECT: As an alternative for cases in which the anatomy and spatial relationship between C-2 and a vertebral artery precludes insertion of C-2 pedicle/pars or C1-2 transarticular screws, a technique that includes opposing laminar hooks (claw) at C-2 combined with C-1 lateral mass screws may be used. The biomechanical stability of this alternate technique was compared with that of a standard screw-rod technique in vitro. METHODS: Flexibility tests were performed in 7 specimens (occiput to C-3) in the following 6 different conditions: 1) intact; 2) after creating instability and attaching a posterior cable/graft at C1-2; 3) after removing the graft and attaching a construct comprising C-1 lateral mass screws and C-2 laminar claws; 4) after reattaching the posterior cable-graft at C1-2 (posterior hardware still in place); 5) after removing the posterior cable-graft and laminar hooks and placing C-2 pedicle screws interconnected to C-1 lateral mass screws via rod; and 6) after reattaching the posterior cable-graft at C1-2 (screw-rod construct still in place). RESULTS: All types of stabilization significantly reduced the range of motion, lax zone, and stiff zone compared with the intact condition. There was no significant biomechanical difference in terms of range of motion or lax zone between the screw-rod construct and the screw-claw-rod construct in any direction of loading. CONCLUSIONS: The screw-claw-rod technique restricts motion much like the standard Harms technique, making it an acceptable alternative technique when aberrant arterial anatomy precludes the placement of C-2 pars/pedicle screws or C1-2 transarticular screws.

Biomechanics of a fixed-center of rotation cervical intervertebral disc prosthesis.

BACKGROUND: Past in vitro experiments studying artificial discs have focused on range of motion. It is also important to understand how artificial discs affect other biomechanical parameters, especially alterations to kinematics. The purpose of this in vitro investigation was to quantify how disc replacement with a ball-and-socket disc arthroplasty device (ProDisc-C; Synthes, West Chester, Pennsylvania) alters biomechanics of the spine relative to the normal condition (positive control) and simulated fusion (negative control). METHODS: Specimens were tested in multiple planes by use of pure moments under load control and again in displacement control during flexion-extension with a constant 70-N compressive follower load. Optical markers measured 3-dimensional vertebral motion, and a strain gauge array measured C4-5 facet loads. RESULTS: Range of motion and lax zone after disc replacement were not significantly different from normal values except during lateral bending, whereas plating significantly reduced motion in all loading modes (P < .002). Plating but not disc replacement shifted the location of the axis of rotation anteriorly relative to the intact condition (P < 0.01). Coupled axial rotation per degree of lateral bending was 25% ± 48% greater than normal after artificial disc replacement (P = .05) but 37% ± 38% less than normal after plating (P = .002). Coupled lateral bending per degree of axial rotation was 37% ± 21% less than normal after disc replacement (P < .001) and 41% ± 36% less than normal after plating (P = .001). Facet loads did not change significantly relative to normal after anterior plating or arthroplasty, except that facet loads were decreased during flexion in both conditions (P < .03). CONCLUSIONS: In all parameters studied, deviations from normal biomechanics were less substantial after artificial disc placement than after anterior plating.

Biomechanics of one-level anterior cervical discectomy and plating using two screws versus four screws.

BACKGROUND CONTEXT: Most one-level anterior cervical plates use two screws per vertebra (four screws in total). No study has addressed whether a simplified plate using one screw per vertebra is adequate for one-level fixation. PURPOSE: To compare stability achieved by four-screw and two-screw plates after discectomy and placement of interbody spacer. STUDY DESIGN: Nondestructive multidirectional flexibility tests were performed in three independent groups of cadaveric spines to assess spinal stability after instrumentation. METHODS: Human cadaveric C4-C7 specimens were tested intact and after discectomy followed by placement of a polyetheretherketone interbody graft and an anterior plate. Rigid two-screw (n=8), semiconstrained four-screw (n=8), and rigid four-screw (n=8) plates were compared. Nonconstraining pure moments were applied under load control (maximum 1.5 Nm) to induce flexion, extension, lateral bending, and axial rotation, whereas vertebral motion was measured optoelectronically. Mean range of motion (ROM) was compared among groups. RESULTS: All three plates significantly reduced ROM relative to normal in all directions of loading (p<.003). Mean ROMs±standard deviation (and corresponding intergroup p value) for rigid two-screw, semiconstrained four-screw, and rigid four-screw plates, respectively, were as follows: flexion: 2.6±2.0°, 1.8±1.1°, 1.8±0.8° (p=.46); extension: 2.5±2.6°, 2.1±1.3°, 1.4±1.3° (p=.45); lateral bending: 1.8±1.0°, 1.3±1.0°, 1.1±0.5° (p=.29); axial rotation: 2.9±1.9°, 1.6±0.9°, 1.5±0.7° (p=.08). Despite a tendency for the rigid two-screw plate to allow more motion than the four-screw plates, there was no significant difference among groups during any loading mode. CONCLUSIONS: In terms of immediate postoperative cervical stability after one-level discectomy and placement of an interbody spacer, the rigid two-screw plate performed comparably to conventional rigid four-screw and semiconstrained four-screw plates. Further research on relative fatigue endurance of the different plate types is also needed.

Dynamic lumbar pedicle screw-rod stabilization: in vitro biomechanical comparison with standard rigid pedicle screw-rod stabilization.

OBJECT: It is unclear how the biomechanics of dynamic posterior lumbar stabilization systems and traditional rigid pedicle screw-rod systems differ. This study examined the biomechanical response of a hinged-dynamic pedicle screw compared with a standard rigid screw used in a 1-level pedicle screw-rod construct. METHODS: Unembalmed human cadaveric L3-S1 segments were tested intact, after L4-5 discectomy, after rigid pedicle screw-rod fixation, and after dynamic pedicle screw-rod fixation. Specimens were loaded using pure moments to induce flexion, extension, lateral bending, and axial rotation while recording motion optoelectronically. Specimens were then loaded in physiological flexion-extension while applying 400 N of compression. Moment and force across instrumentation were recorded from pairs of strain gauges mounted on the interconnecting rods. RESULTS: The hinged-dynamic screws allowed an average of 160% greater range of motion during flexion, extension, lateral bending, and axial rotation than standard rigid screws (p < 0.03) but 30% less motion than normal. When using standard screws, bending moments and axial loads on the rods were greater than the bending moments and axial loads on the rods when using dynamic screws during most loading modes (p < 0.05). The axis of rotation shifted significantly posteriorly more than 10 mm from its normal position with both devices. CONCLUSIONS: In a 1-level pedicle screw-rod construct, hinged-dynamic screws allowed a quantity of motion that was substantially closer to normal motion than that allowed by rigid pedicle screws. Both systems altered kinematics similarly. Less load was borne by the hinged screw construct, indicating that the hinged-dynamic screws allow less stress shielding than standard rigid screws.

Biomechanical effects of laminoplasty versus laminectomy: stenosis and stability.

STUDY DESIGN: In vitro human cadaveric study simultaneously quantifying sagittal plane flexibility and spinal canal stenosis. OBJECTIVE: To compare biomechanical stability and the change in cross-sectional area during flexion and extension after laminectomy and open-door laminoplasty. SUMMARY OF BACKGROUND DATA: Spinal canal stenosis has been quantified in vitro but has not been quantified in studies of laminectomy or laminoplasty. METHODS: Cadaveric specimens were loaded in physiologic-range flexion and extension using nonconstraining pure moments while recording segmental angles optoelectronically. Custom flexible tubing was placed within the spinal canal, and water was continuously pumped through the tubing while measuring upstream pressure. Spinal canal cross-sectional area correlated to water pressure, allowing continuous monitoring of the smallest cross-sectional area of the canal. Specimens were tested (1) normal, (2) after modeling stenosis by inserting hemispherical wooden beads in the spinal canal at 3 levels, (3) after open-door laminoplasty at 5 levels, and (4) after expanding laminoplasty to laminectomy. RESULTS: Range of motion (ROM) in the normal, stenotic, and laminoplasty conditions did not differ significantly. However, laminectomy increased ROM significantly more than other conditions. ROM after laminectomy was 13% greater than after laminoplasty. After modeling stenosis, the cross-sectional area decreased to 52% +/- 12% of normal. Laminoplasty restored the cross-sectional area to 70% +/- 12% of normal whereas laminectomy restored cross-sectional area to 101% +/- 4% of normal. Among all conditions, areas differed significantly except normal versus laminectomy. CONCLUSION: Laminoplasty leaves the spine in a significantly more stable condition than laminectomy. However, laminoplasty failed to relieve stenosis completely. In this study, stenosis was modeled as about 50% occlusion of the spinal canal. The degree of stenosis should be considered in clinical decisions of whether laminectomy or laminoplasty is more appropriate.

Biomechanics of C-7 transfacet screw fixation.

OBJECT: The small diameter of the pedicle can make C-7 pedicle screw insertion dangerous. Although transfacet screws have been studied biomechanically when used in pinning joints, they have not been well studied when used as part of a C7-T1 screw/rod construct. The authors therefore compared C7-T1 fixation using a C-7 transfacet screw/T-1 pedicle screw construct with a construct composed of pedicle screws at both levels. METHODS: Each rigid posterior screw/rod construct was placed in 7 human cadaveric C6-T2 specimens (14 total). Specimens were tested in normal condition, after 2-column instability, and once fixated. Nondestructive, nonconstraining pure moments (maximum 1.5 Nm) were applied to induce flexion, extension, lateral bending, and axial rotation while recording 3D motion optoelectronically. The entire construct was then loaded to failure by dorsal linear force. RESULTS: There was no significant difference in angular range of motion between the 2 instrumented groups during any loading mode (p > 0.11, nonpaired t-tests). Both constructs reduced motion to < 2 degrees in any direction and allowed significantly less motion than in the normal condition. The C-7 facet screw/T-1 pedicle screw construct allowed a small but significantly greater lax zone than the pedicle screw/rod construct during lateral bending, and it failed under significantly less load than the pedicle screw/rod construct (p < 0.001). CONCLUSIONS: When C-7 transfacet screws are connected to T-1 pedicle screws, they provide equivalent stability of constructs formed by pedicle screws at both levels. Although less resistant to failure, the transfacet screw construct should be a viable alternative in patients with healthy bone.

Trajectory analysis and pullout strength of self-centering lumbar pedicle screws.

OBJECT: An experiment was performed to study the limits of the ability of screws designed to center themselves in the pedicle during insertion, and to study whether straight-ahead versus inward screw insertion trajectories differ in their resistance to pullout. METHODS: Forty-nine human cadaveric lumbar vertebrae were studied. Pedicle screws were inserted in trajectories starting 0 degrees, 10 degrees, 20 degrees, or 30 degrees from the optimal trajectory, either medially or laterally misdirected. The surgeon then inserted the screw with forward thrust but without resisting the screw's tendency to reorient its own trajectory during insertion. On the opposite pedicle, a control screw was inserted with the more standard inward-angled anatomical trajectory and insertion point. Cortical wall violation during insertion was recorded. Screws were then pulled out at a constant displacement rate while ultimate strength was recorded. RESULTS: Lateral misdirection as small as 10 degrees was likely to lead to cortical wall violation (3 of 7 violations). Conversely, medial misdirection usually resulted in safe screw insertion (1 of 21 violations for 10 degrees, 20 degrees, or 30 degrees medial misdirection). The resistance to pullout of screws inserted in a straight-ahead trajectory did not differ significantly from that of screws inserted along an inward trajectory (p = 0.68). CONCLUSIONS: Self-tapping, self-drilling pedicle screws can redirect themselves to a much greater extent during medial than during lateral misdirection. The cortical wall is more likely to be violated laterally than medially. The strength of straight-ahead and inward trajectories was equivalent.

Atlantoaxial rotatory subluxation with ligamentous disruption: a biomechanical comparison of current fusion methods.

OBJECTIVE: We evaluated the biomechanical effects of 4 instrumented configurations after induced atlantoaxial rotatory subluxation: transarticular screw fixation (T/A) and polyaxial C1 lateral mass and C2 pedicle screw and rod fixation (LC1-PC2) for atlantoaxial arthrodesis with unilateral and bilateral instrumentation. METHODS: Three-dimensional intervertebral motion was tracked stereophotogrammetrically while 14 human cadaveric spine specimens underwent nonconstraining pure moment loading. Nondestructive loads were applied quasi-statistically in 0.25-Nm increments to a maximum load of 1.5 Nm during flexion-extension, right and left axial rotation, and right and left lateral bending. Hyperrotation injuries were created using torsional loads applied during left axial rotation until visible failure occurred. RESULTS: In the normal condition, the values for angular range of motion, lax zone (zone of ligamentous laxity), and stiff zone (zone of ligamentous stretching) were similar in both groups in all directions of loading, with no significant differences (P > 0.05) between groups at C0-C1 or C1-C2. Both instrumentation systems (bilateral configurations) substantially stabilized angular motion at C1-C2 (P < 0.05) during all loading modes for the T/A group, and during all but right lateral bending (P = 0.072) for the LC1-PC2 group. The mean failure load for both intact and instrumented specimens was slightly greater, but not significant for the LC1-PC2 group compared with the T/A group (P > 0.14). CONCLUSION: Both methods fixated atlantoaxial subluxation equally well. Compared with unilateral instrumentation, a bilateral configuration with the LC1-PC2 technique significantly increased stability during extension (P < 0.05). During axial rotation, bilateral T/A screws significantly increased stability compared with unilateral fixation (P < 0.02).

Anatomical and biomechanical analyses of the unique and consistent locations of sacral insufficiency fractures.

STUDY DESIGN: Correlation of locations of sacral insufficiency fractures is made to regions of stress depicted by finite element analysis derived from biomechanical models of patient activities. OBJECTIVE: Sacral insufficiency fractures occur at consistent locations. It was postulated that sacral anatomy and sites of stress within the sacrum with routine activities in the setting of osteoporosis are foundations for determining patterns for the majority of sacral insufficiency fractures. SUMMARY OF BACKGROUND DATA: The predominant vertical components of sacral insufficiency fractures most frequently occur bilaterally through the alar regions of the sacrum, which are the thickest and most robust appearing portions of the sacrum instead of subjacent to the central sacrum, which bears the downward force of the spine. METHODS: First, the exact locations of 108 cases of sacral insufficiency fractures were catalogued and compared to sacral anatomy. Second, different routine activities were simulated by pelvic models from CT scans of the pelvis and finite element analysis. Analyses were done to correlate sites of stress with activities within the sacrum and pelvis compared to patterns of sacral insufficiency fractures from 108 cases. RESULTS: The sites of stress depicted by the finite element analysis walking model strongly correlated with identical locations for most sacral and pelvic insufficiency fractures. Consistent patterns of sacral insufficiency fractures emerged from the 108 cases and a biomechanical classification system is introduced. Additionally, alteration of walking mechanics and asymmetric sacral stress may alter the pattern of sacral insufficiency fractures noted with hip pathology (P = 0.002). CONCLUSION: Locations of sacral insufficiency fractures are nearly congruous with stress depicted by walking biomechanical models. Knowledge of stress locations with activities, cortical bone transmission of stress, usual fracture patterns, intensity of sacral stress with different activities, and modifiers of walking mechanics may aid medical management, interventional, or surgical efforts.

A new stand-alone cervical anterior interbody fusion device: biomechanical comparison with established anterior cervical fixation devices.

STUDY DESIGN: A new anchored spacer-a low-profile cervical interbody fusion cage with integrated anterior fixation-was compared biomechanically to established anterior cervical devices. OBJECTIVE: To evaluate the fixation properties of the new stand-alone device and compare these properties with established fixation methods. The hypothesis is that the new device will provide stability comparable to that provided by an anterior cervical cage when supplemented with an anterior plate. SUMMARY OF BACKGROUND DATA: It is accepted that the use of anterior cervical plating increases the chance of achieving a solid fusion. However, its use may be associated with an increase in operation time and a higher postoperative morbidity caused by a larger anterior approach and disruption of the anterior musculature. This dilemma has led to the development of a new, low profile stand-alone cervical anterior cage device with integrated screw fixation. METHODS: Twenty-four human cadaveric C4-C7 cervical spines were loaded nondestructively with pure moments in a nonconstraining testing apparatus to induce flexion, extension, lateral bending, and axial rotation while angular motion was measured optoelectronically. The specimens were tested: 1. Intact (N = 24). 2. After discectomy and anterior stabilization. a. Interbody cage + locking plate (N = 8). b. Interbody cage + dynamic plate (N = 8). c. Anchored spacer (N = 8). 3. After ventral plate removal of group 2a and 2b (N = 16). RESULTS: All fixation techniques decreased range of motion (ROM) and lax zone (LZ) (P < 0.05) in all test modes compared with the intact motion segment and cage-only group. There were no significant differences between the anchored spacer and cage + locking plate or cage + dynamic plate. CONCLUSION: The anchored spacer provided a similar biomechanical stability to that of the established anterior fusion technique using an anterior plate plus cage and has a potentially lower perioperative and postoperative morbidity. These results support progression to clinical trials using the cervical anchored spacer as a stand-alone implant.

Biomechanical consequences of cervical spondylectomy versus corpectomy.

OBJECTIVE: To evaluate the differences in spinal stability and stabilizing potential of instrumentation after cervical corpectomy and spondylectomy. METHODS: Seven human cadaveric specimens were tested: 1) intact; 2) after grafted C5 corpectomy and anterior C4-C6 plate; 3) after adding posterior C4-C6 screws/rods; 4) after extending posteriorly to C3-C7; 5) after grafted C5 spondylectomy, anterior C4-C6 plate, and posterior C4-C6 screws/rods; and 6) after extending posteriorly to C3-C7. Pure moments induced flexion, extension, lateral bending, and axial rotation; angular motion was recorded optically. RESULTS: After corpectomy, anterior plating alone reduced the angular range of motion to a mean of 30% of normal, whereas added posterior short- or long-segment hardware reduced range of motion significantly more (P < 0.003), to less than 5% of normal. Constructs with posterior rods spanning C3-C7 were stiffer than constructs with posterior rods spanning C4-C6 during flexion, extension, and lateral bending (P < 0.05), but not during axial rotation (P > 0.07). Combined anterior and C4-C6 posterior fixation exhibited greater stiffness after corpectomy than after spondylectomy during lateral bending (P = 0.019) and axial rotation (P = 0.001). Combined anterior and C3-C7 posterior fixation exhibited greater stiffness after corpectomy than after spondylectomy during extension (P = 0.030) and axial rotation (P = 0.0001). CONCLUSION: Circumferential fixation provides more stability than anterior instrumentation alone after cervical corpectomy. After corpectomy or spondylectomy, long circumferential instrumentation provides better stability than short circumferential fixation except during axial rotation. Circumferential fixation more effectively prevents axial rotation after corpectomy than after spondylectomy.

Occipitocervical vertical distraction injuries: anatomical biomechanical, and 3-tesla magnetic resonance imaging investigation.

STUDY DESIGN: Biomechanical load-to-failure findings correlated with anatomic dissection measurements and intact (prefailure) 3-Tesla (3-T) magnetic resonance images (MRI). OBJECTIVE: To better understand why the same distractive force to the head can result in occipitoatlantal dislocation (OAD) in some individuals and atlantoaxial dislocation (AAD) in others. SUMMARY OF BACKGROUND DATA: Distraction injuries to the cranio-vertebral junction have been studied biomechanically but have not been studied relative to ligamentous anatomic variations. We theorized that morphologic variations in the ligaments should influence the injury pattern during axial distraction. METHODS: After obtaining 3-T MRI scans, 10 occiput-C2 specimens were loaded to failure under axial tension. Direct anatomic measurements were also obtained from the distracted and injured specimens. RESULTS: AAD was observed in 7 specimens (mean force +/- standard deviation 1229 +/- 181 N) at a significantly higher magnitude than OAD, which was observed in 3 specimens (823 +/- 127 N; P = 0.009, nonpaired t test). In OAD specimens, the superior cruciate ligament (SCL), which was smaller than the inferior cruciate ligament (ICL), failed. The apical ligament was unidentifiable in these 3 specimens. In 5 of the 7 AAD specimens, the ICL ruptured and was smaller than the SCL. In the remaining 2 specimens, both SCL and ICL ruptured. The apical ligament, which ruptured, was identifiable in all 7 specimens. CONCLUSION: Axial distraction across the cranio-vertebral junction can produce either OAD or AAD. The SCL and ICL dimensions, alar ligament orientations, and apical ligament presence may affect the injury site. Visualization with 3-T MRI allows better understanding of the injury mechanism and location, which is important clinically in selecting single- or multilevel fixation.

Biomechanical comparison of occipitoatlantal screw fixation techniques.

OBJECT: The stability provided by 3 occipitoatlantal fixation techniques (occiput [Oc]-C1 transarticular screws, occipital keel screws rigidly interconnected with C-1 lateral mass screws, and suboccipital/sublaminar wired contoured rod) were compared. METHODS: Seven human cadaveric specimens received transarticular screws and 7 received occipital keel-C1 lateral mass screws. All specimens later underwent contoured rod fixation. All conditions were studied with and without placement of a structural graft wired between the skull base and C-1 lamina. Specimens were loaded quasistatically using pure moments to induce flexion, extension, lateral bending, and axial rotation while recording segmental motion optoelectronically. Flexibility was measured immediately postoperatively and after 10,000 cycles of fatigue. RESULTS: Application of Oc-C1 transarticular screws, with a wired graft, reduced the mean range of motion (ROM) to 3% of normal. Occipital keel-C1 lateral mass screws (also with graft) offered less stability than transarticular screws during extension and lateral bending (p < 0.02), reducing ROM to 17% of normal. The wired contoured rod reduced motion to 31% of normal, providing significantly less stability than either screw fixation technique. Fatigue increased motion in constructs fitted with transarticular screws, keel screws/lateral mass screw constructs, and contoured wired rods, by means of 19, 5, and 26%, respectively. In all constructs, adding a structural graft significantly improved stability, but the extent depended on the loading direction. CONCLUSIONS: Assuming the presence of mild C1-2 instability, Oc-C1 transarticular screws and occipital keel-C1 lateral mass screws are approximately equivalent in performance for occipitoatlantal stabilization in promoting fusion. A posteriorly wired contoured rod is less likely to provide a good fusion environment because of less stabilizing potential and a greater likelihood of loosening with fatigue.

Biomechanics of unilateral compared with bilateral lumbar pedicle screw fixation for stabilization of unilateral vertebral disease.

OBJECT: An in vitro flexibility experiment was performed to compare the biomechanical stability of asymmetrical lumbar pedicle screw fixation (longer hardware attached ipsilaterally to a 1-sided lesion), short and long fixation, and fixation with and without interconnection to the involved vertebra. METHODS: Seven human cadaveric specimens (T12-S1) were studied intact; after simulated unilateral lesions were created at L2-3 and L3-4, the segments were stabilized by 1) L2-4 unilateral fixation (L-3 excluded), 2) L2-4 bilateral fixation (L-3 included contralaterally), 3) L2-5 unilateral fixation (L-3 excluded), 4) L2-5 fixation ipsilateral (L-3 excluded) and L2-4 fixation contralateral (L-3 included), 5) L2-5 bilateral fixation (L-3 included contralaterally), and 6) L2-5 bilateral fixation (L-3 excluded). The testing order varied among specimens. Angular range of motion (ROM) and lax zone were recorded optically while loading to 6.0 Nm was created with nonconstraining pure moments. RESULTS: Unilateral short fixation provided significantly worse stabilization than any other construct tested in all loading modes (p < 0.05, repeated-measures analysis of variance). There was a mean 56% reduction in ROM across the lesion after adding 1 additional level rostrally and caudally. Asymmetrical long/short stabilization provided similar stability to symmetrical long stabilization. Minimal additional stability was gained by including L-3 in the long bilateral fixation construct. CONCLUSIONS: Unilateral fixation is inadequate for stabilizing a 2-level unilateral lesion. Bilateral fixation, whether symmetrical or asymmetrical, provides good stabilization for this injury. It is not important for stability to include the level of the lesion within the long construct contralaterally.

Traumatic loading of the Bryan cervical disc prosthesis: an in vitro study.

OBJECTIVE: The Bryan disc prosthesis (Medtronic Sofamor Danek, Memphis, TN) relies on a precise fit between the device and the vertebral endplates to provide immediate stability after cervical arthroplasty. The safety of the cervical arthroplasty in the setting of trauma is unknown. We compare the segmental strength of the normal cervical spine and the cervical spine after single-level arthroplasty. METHODS: Fifteen cadaveric cervical spines with arthroplasty placed at the C5-C6 level were compared with 16 intact cadaveric controls. A pure moment was applied to induce flexion, extension, or axial rotation until the segment failed. RESULTS: The prosthesis provided 63, 45, and 69% of the strength of a normal spine during flexion, extension, and rotation, respectively. There were no cases of prosthesis expulsion. CONCLUSION: After insertion of the Bryan disc, the remaining ligamentous tissues provide adequate acute stability to the spine.