Improved assessment of lumbar vertebral body strength using supine lateral dual-energy x-ray absorptiometry.

Clinical and biomechanical investigations indicate that assessment of vertebral body bone mineral density (BMD) by anteroposterior dual-energy x-ray absorptiometry (DXA) is a useful index of vertebral body strength and fracture risk in osteoporosis. However, inclusion of non-force-bearing and small-force-bearing mineralized structures, such as the posterior elements and aortic calcifications, in the measurement of anterior BMD obscures the assessment of vertebral body mass by this technique. Indeed, such interference is particularly severe in the presence of posterior element degeneration or previous spinal surgery. Recent anatomic studies illustrate that the lateral view provides unobstructed visualization of the L3, L4, and possibly L2 vertebral bodies, suggesting that supine lateral BMD may more accurately assess vertebral body fracture risk. We evaluated this hypothesis in a blinded using human cadaver spines to compare the value of supine lateral and anteroposterior BMD in assessing vertebral body fracture force, average compressive stress, maximum stored strain energy, and strain at failure. Both measures of BMD significantly correlate with these biomechanical measures. However, statistical comparison of the methods using multiple and stepwise regression reveals that supine lateral BMD provides a better assessment of the vertebral body fracture properties than anteroposterior BMD. The enhanced predictive value of supine lateral BMD occurs because of the variable contribution of posterior element mineral to the anteroposterior BMD measurement. Evaluation to test the utility of supine lateral BMD for the assessment of fracture risk and a fracture threshold in patients with osteoporosis is therefore recommended.

Mechanisms of basilar skull fracture.

Basilar skull fractures comprise a broad category of injuries that have been attributed to a variety of causal mechanisms. The objective of this work is to develop an understanding of the biomechanical mechanisms that result in basilar skull fractures, specifically focusing on mandibular impact and neck loading as potential mechanisms. In the characterization of the injury mechanisms, three experimental studies have been performed. The first study evaluated the response of the base of the skull to midsymphysis loading on the mental protuberance (chin) of the mandible. Five dynamic impacts using a vertical drop track and one quasi-static test in a servohydraulic test frame have been performed. In each test, clinically relevant mandibular fractures were produced but no basilar skull fractures were observed. The second study assessed the fracture tolerance of the base of the skull subject to direct loading on the temporomandibular joint in conjunction with tensile loading imposed locally around the foramen magnum to simulate the effect of the ligaments and musculature of the neck. Among four specimens that sustained either complete or incomplete basilar skull ring fractures remote from the sites of load application, the mean load at fracture was 4300 +/- 350 N. Energy to fracture was computed in three of those tests and averaged 13.0 +/- 1.7 J. Injuries produced were consistent with clinical observations that have attributed basilar skull ring fractures to mandibular impacts. In the third series of experimental tests, loading responses resulting from cranial vault impacts were investigated using unembalmed human cadaver heads and ligamentous cervical spines. Multiaxis load cells and accelerometers, coupled with high-speed digital video, were used to quantify impact dynamics. The results of these experiments suggest that while there is a greater probability of cervical spine injury, basilar skull ring fractures can result when the head is constrained on the impact surface and the inertia of the torso drives the vertebral column onto the occiput.

Laparoscopic heller myotomy and anterior fundoplication for achalasia results in a high degree of patient satisfaction.

HYPOTHESIS: Laparoscopic Heller myotomy with anterior fundoplication will alleviate the symptoms of achalasia and result in excellent patient satisfaction. DESIGN: Retrospective study of consecutive patients who underwent laparoscopic Heller myotomy with anterior fundoplication for achalasia between October 1995 and July 1999. A telephone survey assessed symptoms and satisfaction. Patients were asked to quantitate their symptoms on a scale of 0 to 3 (0 = none; 1, mild; 2, moderate; and 3, severe). SETTING: University referral center. PATIENTS: Twenty-four patients who underwent laparoscopic Heller myotomy with anterior fundoplication for achalasia. MAIN OUTCOME MEASURES: Postoperative symptoms and satisfaction. RESULTS: Twenty-one patients (88%) were successfully contacted. Mean follow-up was 16.5 months. The laparoscopic approach was successful in all but 3(88%). The mean dysphagia score was 2.81 preoperatively and 0.81 postoperatively (P<.000). The mean chest pain score was 1. 57 preoperatively and 0.86 postoperatively (P<.015). The mean supine regurgitation score was 2.10 preoperatively and 0.57 postoperatively (P<.000). The mean upright regurgitation score was 1.57 preoperatively and 0.52 postoperatively (P<.000). The mean heartburn score was 1.57 preoperatively and 0.57 postoperatively (P<.000). Postoperatively, 18 (86%) of 21 patients could swallow bread without difficulty and 17 (89%) of 19 patients could eat meat without difficulty (2 were excluded as they were vegetarians). Twenty (95%) of 21 patients reported improvement after the operation. CONCLUSIONS: Laparoscopic Heller myotomy with anterior fundoplication significantly relieves the symptoms of achalasia without causing the symptoms of gastroesophageal reflux disease. This procedure results in excellent overall patient satisfaction.

Epidemiology, classification, mechanism, and tolerance of human cervical spine injuries.

A review of published research is presented to examine human cervical spine injury epidemiology, classification, mechanism, and tolerance. Synthesis of the literature identifies several areas of cervical spine injury biomechanics in which the current understanding is greater than that suggested by individual investigations. Specifically, epidemiologic studies show an age dependent variation in the location of cervical spine injury. A classification scheme is developed on the basis of published work, in which the classes are defined by the resultant force acting at the site of injury. Further, for compression injuries it appears that a compression force tolerance criterion exists, and that eccentricity of the compressive force can be used to predict the type of cervical injury produced. However, to date, prediction of location of injury within the cervical spine has not been attempted. In particular, a compressive tolerance criterion is suggested between 2.75 and 3.44 kN for the adult cervical spine. In contrast, tolerance criteria for cervical injuries in other forms of loading are less well characterized. Review of the literature on spinal cord injury biomechanics and pediatric cervical spine injury reinforces the need for continued investigation in these areas.

The biomechanics of cervical spine injury and implications for injury prevention.

Most catastrophic cervical spinal injuries occur as a result of head impacts in which the head stops and the neck is forced to stop the moving torso. In these situations the neck is sufficiently fragile that injuries have been reported at velocities as low as 3.1 m/s with only a fraction of the mass of the torso loading the cervical spine. Cervical spinal injury occurs in less than 20 ms following head impact, explaining the absence of a relationship between clinically reported head motions and the cervical spinal injury mechanism. In contrast, the forces acting on the spine at the time of injury are able to explain the injury mechanism and form a rational basis for classification of vertebral fractures and dislocations. Fortunately, most head impacts do not result in cervical spine injuries. Analysis of the biomechanical and clinical literature shows that the flexibility of the cervical spine frequently allows the head and neck to flex or extend out of the path of the torso and escape injury. Accordingly, constraints which restrict the motion of the neck can increase the risk for cervical spine injury. These observations serve as a foundation on which injury prevention strategies, including coaching, helmets, and padding, may be evaluated.

Experimental impact injury to the cervical spine: relating motion of the head and the mechanism of injury.

The purpose of this study was to analyze, with use of an impact model, the relationships among motion of the head, local deformations of the cervical spine, and the mechanisms of injury; the model consisted of the head and neck of a cadaver. Traditionally, the mechanisms of injury to the cervical spine have been associated with flexion and extension motions of the head and neck. However, the classification of the mechanisms is not always in agreement with the patient's account of the injury or with lacerations and contusions of the scalp, which indicate the site of the impact of the head. Eleven specimens were dropped in an inverted posture with the head and neck in an anatomically neutral position. Forces, moments, and accelerations were recorded, and the impacts were imaged at 1000 frames per second. The velocity at the time of impact was on the order of 3.2 meters per second. The angle and the padding of the impact surface varied. Observable motion of the head did not correspond to the mechanism of the injury to the cervical spine. Injury occurred 2.2 to 18.8 milliseconds after impact and before noticeable motion of the head. However, the classification of the mechanism of the injuries was descriptive of the local deformations of the cervical spine at the time of the injury. Accordingly, it is a useful tool in describing the local mechanism of injury. Buckling of the cervical spine, involving extension between the third and sixth cervical vertebrae and flexion between the seventh and eight cervical vertebrae, was observed. Other, more complex, buckling deformations were also seen, suggesting that the deformations that occur during impact are so complex that they can give rise to a number of different mechanisms of injury.

Surface friction in near-vertex head and neck impact increases risk of injury.

A computational head-neck model was developed to test the hypothesis that increases in friction between the head and impact surface will increase head and neck injury risk during near-axial impact. The model consisted of rigid vertebrae interconnected by assemblies of nonlinear springs and dashpots, and a finite element shell model of the skull. For frictionless impact surfaces, the model reproduced the kinematics and kinetics observed in near-axial impacts to cadaveric head-neck specimens. Increases in the coefficient of friction between the head and impact surface over a range from 0.0 to 1.0 resulted in increases of up to 40, 113, 9.8, and 43% in peak post-buckled resultant neck forces, peak moment at the occiput-C1 joint, peak resultant head accelerations, and HIC values, respectively. The most dramatic increases in injury-predicting quantities occurred for COF increases from 0.0 to 0.2, while further COF increases above 0.5 generally produced only nominal changes. These data suggest that safety equipment and impact environments which minimize the friction between the head and impact surface may reduce the risk of head and neck injury in near-vertex head impact.

Characterization of the passive responses of live skeletal muscle using the quasi-linear theory of viscoelasticity.

The tensile viscoelastic responses of live, innervated rabbit skeletal muscle were measured and characterized using the quasi-linear model of viscoelasticity. The tibialis anterior (TA) and extensor digitorum longus (EDL) muscles of anesthetized New Zealand white rabbits were surgically exposed and tested under in vivo conditions. Rate sensitivity of the force-time history was observed in response to constant velocity testing at rates from 0.01 to 2.0 Hz. Average hysteresis energy, expressed as a percentage of maximum stored strain energy, was 39.3 +/- 5.4% and was insensitive to deformation rate. The quasi-linear model, with constants derived from relaxation testing, was able to describe and predict these responses with correlation exceeding the 99% confidence interval for the 132 constant velocity tests performed (rmean = 0.9263 +/- 0.0373). The predictive ability of this model was improved when compressive loading effects on the muscle were neglected, rmean = 0.9306 +/- 0.0324. The rate insensitivity of hysteresis energy was predicted by the model; however, the absolute value of the hysteresis was underestimated (30.2 +/- 4.0%). Both muscles demonstrated strikingly different elastic functions. Geometric normalization of these responses (stress and strain) did not result in a single elastic function capable of describing both muscles. Based on these results, the quasi-linear model is recommended for the characterization of the structural responses of muscle; however, further investigation is required to determine the influence of muscle geometry and fiber architecture on the elastic function.

The role of strength in rising from a chair in the functionally impaired elderly.

Rising from a chair is a task essential for independent living. Many elderly persons have difficult with this task. Previous studies have drawn conflicting conclusions as to the role of strength in limiting the ability to rise from a chair. The purpose of this study is to determine the role of knee extensor strength in rising from a chair in the functionally impaired elderly. It is hypothesized that knee extensor strength limits the minimum chair height from which a subject can rise in the functionally impaired elderly, but not in the young. Studying both young healthy adults and functionally impaired elderly showed that required joint moment increased monotonically with decreasing chair height. Further, the elderly used significantly more of their available strength to rise from any chair height, and their mean required knee moment was 97% of the available strength when rising from the lowest chair height from which they could successfully rise. These data suggest that strength is a limiting factor in determining the minimum chair height from which the functionally impaired elderly may rise.

The viscoelastic responses of the human cervical spine in torsion: experimental limitations of quasi-linear theory, and a method for reducing these effects.

The dynamic torsional viscoelastic responses of the human cadaver cervical spine were measured in vitro. The quasi-linear formulation of time dependent behavior was used to describe and predict the resultant torque as a function of applied angular deflection and time. The performance of the quasi-linear model was good, reaching correlation at the 99% confidence level; however, it tended to underestimate hysteresis energy (mean relative deviation = -19.1%) and observed stiffness. This was in part due to difficulties in establishing the physical constants of the quasi-linear model from finite rate relaxation testing. An extrapolation deconvolution technique to enhance the experimentally derived constants was developed, to reduce the detrimental effects of finite rate testing. The quasi-linear model based on this enhanced derivation showed improved predictive ability and hysteresis energy determination.

Dynamic responses of the head and cervical spine to axial impact loading.

This study explores the inertial effects of the head and torso on cervical spine dynamics with the specific goal of determining whether the head mass can provide a constraining cervical spine end condition. The hypothesis was tested using a low friction impact surface and a pocketing foam impact surface. Impact orientation was also varied. Tests were conducted on whole unembalmed heads and cervical spines using a drop track system to produce impact velocities on the order of 3.2 m s-1. Data for the head impact forces and the reactions at T1 were recorded and the tests were also imaged at 1000 frames s-1. Injuries occurred 2-19 ms following head impact and prior to significant head motion. Average compressive load a failure was 1727 +/- 387 N. Decoupling was observed between the head and T1. Cervical spine loading due to head rebound constituted up to 54 +/- 16% of the total axial neck load for padded impacts and up to 38 +/- 30% of the total axial neck load for rigid impacts. Dynamic buckling was also observed; including first-order modes and transient higher-order modes which shifted the structure from a primarily compressive mode of deformation to various bending modes. These experiments demonstrate that in the absence of head pocketing, the head mass can provide sufficient constraint to cause cervical spine injury. The results also show that cervical spinal injury dynamics are complex, and that a large sample size of experimentally produced injuries will be necessary to develop comprehensive neck injury models and criteria.

Inertial properties and loading rates affect buckling modes and injury mechanisms in the cervical spine.

Cervical spine injuries continue to be a costly societal problem. Future advancements in injury prevention depend on improved physical and computational models which, in turn, are predicated on a better understanding of the responses of the neck during dynamic loading. Previous studies have shown that the tolerance of the neck is dependent on its initial position and its buckling behavior. This study uses a computational model to examine the mechanical factors influencing buckling behavior during impact to the neck. It was hypothesized that the inertial properties of the cervical spine influence the dynamics during compressive axial loading. The hypothesis was tested by performing parametric analyses of vertebral mass, mass moments of inertia, motion segment stiffness, and loading rate. Increases in vertebral mass resulted in increasingly complex kinematics and larger peak loads and impulses. Similar results were observed for increases in stiffness. Faster loading rates were associated with higher peak loads and higher-order buckling modes. The results demonstrate that mass has a great deal of influence on the buckling behavior of the neck, particularly with respect to the expression of higher-order modes. Injury types and mechanisms may be substantially altered by loading rate because inertial effects may influence whether the cervical spine fails in a compressive mode, or a bending mode.

An improved method for finite element mesh generation of geometrically complex structures with application to the skullbase.

An automated method has been developed to generate finite element meshes of geometrically complex structures from CT images using solely hexahedral elements. This technique improves upon previous voxel-based mesh reconstruction approaches by smoothing the irregular boundaries at model surfaces and material interfaces. Over a range of mesh densities, RMS error in surface Von Mises stress was higher in the unsmoothed circular ring models (0.11-0.24 MPa) than in the smoothed models (0.080-0.15 MPa) at each mesh density. The element-to-element oscillation in surface element stress, as measured by the average second spatial derivative of Von Mises stress along the outer surface of the ring, was higher in the unsmoothed models (11.5-15.0 kPa mm-2) than in the smoothed models (4.0-6.8 kPa mm-2). Similarly, in a human skullbase model, the element-to-element oscillation in surface Von Mises stress was higher in the unsmoothed model (5.52 kPa mm-2) than in the smoothed model (1.83 kPa mm-2). Using this technique, finite element models of complex geometries can be rapidly reconstructed which produce less error at the surface than voxel-based models with discontinuous surfaces.

The effects of padded surfaces on the risk for cervical spine injury.

STUDY DESIGN: This is an in vitro study comparing cervical spine injuries produced in rigid head impacts and in padded head impacts. OBJECTIVES: To test the hypothesis that deformable impact surfaces pose a greater risk for cervical spine injury than rigid surfaces using a cadaver-based model that includes the effects of the head and torso masses. SUMMARY OF BACKGROUND DATA: It is widely assumed that energy-absorbing devices that protect the head from injury also reduce the risk for neck injury. However, this has not been demonstrated in any experimental or epidemiologic study. On the contrary, some studies have shown that padded surfaces have no effect on neck injury risk, and others have suggested that they can increase risk. METHODS: Experiments were performed on 18 cadaveric cervical spines to test 6 combinations of impact angle and impact surface padding. The impact surface was oriented at -15 degrees (posterior impact), 0 degree (vertex impact), or +15 degrees (anterior impact). The impact surface was either a 3-mm sheet of lubricated Teflon or 5 cm of polyurethane foam. RESULTS: Impacts onto padded surfaces produced significantly larger neck impulses (P = 0.00023) and a significantly greater frequency of cervical spine injuries than rigid impacts (P = 0.0375). The impact angle was also correlated with injury risk (P < 0.00001). CONCLUSIONS: These experiments suggest that highly deformable, padded contact surfaces should be used carefully in environments where there is the risk for cervical spine injury. The results also suggest that the orientation of the head, neck, and torso relative to the impact surface is of equal if not greater importance in neck injury risk.

The role of torsion in cervical spine trauma.

A dynamic servocontrolled torsion machine has been used to characterize cervical injury due to pure rotation of the head. Resultant force moment, torque, and applied rotation have been measured. Torque applied to the base of the skull resulted in injury to the atlantoaxial joint. No evidence of lower cervical injury was observed by computed tomography, magnetic resonance imaging, in situ fluoroscopy, or visual inspection. Torque applied directly to the lower cervical spine induced ligamentous injury and unilateral facet dislocation; however, the torque to injure the lower cervical spine was significantly greater than the torque to injure the atlantoaxial joint. It was concluded that pure rotation of the head does not mediate lower cervical ligamentous injury because of the comparative weakness of the atlantoaxial joint.

The cervical facet capsule and its role in whiplash injury: a biomechanical investigation.

STUDY DESIGN: Cervical facet capsular strains were determined during bending and at failure in the human cadaver. OBJECTIVE: To determine the effect of an axial pretorque on facet capsular strains and estimate the risk for subcatastrophic capsular injury during normal bending motions. SUMMARY OF BACKGROUND DATA: Epidemiologic and clinical studies have identified the facet capsule as a potential site of injury and prerotation as a risk factor for whiplash injury. Unfortunately, biomechanical data on the cervical facet capsule and its role in whiplash injury are not available. METHODS: Cervical spine motion segments were tested in a pure-moment test frame and the full-field strains determined throughout the facet capsule. Motion segments were tested with and without a pretorque in pure bending. The isolated facet was then elongated to failure. Maximum principal strains during bending were compared with failure strains, by paired t test. RESULTS: Statistically significant increases in principal capsular strains during flexion-extension loading were observed when a pretorque was applied. All measured strains during bending were significantly less than strains at catastrophic joint failure. The same was true for subcatastrophic ligament failure strains, except in the presence of a pretorque. CONCLUSIONS: Pretorque of the head and neck increases facet capsular strains, supporting its role in the whiplash mechanism. Although the facet capsule does not appear to be at risk for gross injury during normal bending motions, a small portion of the population may be at risk for subcatastrophic injury.

Mechanical evidence of cervical facet capsule injury during whiplash: a cadaveric study using combined shear, compression, and extension loading.

STUDY DESIGN: A comparison of cervical facet capsule strain fields in cadaveric motion segments exposed to whiplash-like loads and failure loads. OBJECTIVES: To compare the maximum principal strain in the facet capsular ligament under combined shear, bending, and compressive loads with those required to injure the ligament. SUMMARY OF BACKGROUND DATA: The cervical facet capsular ligament is thought to be an anatomic site for whiplash injury, although the mechanism of its injury remains unclear. METHODS: Motion segments from seven female donors were exposed to quasi-static flexibility tests using posterior shear loads of 135 N applied to the superior vertebra under four compressive axial preloads up to 325 N. The right facet joint was then isolated and failed in posterior shear loading. The Lagrangian strain field in the right facet capsular ligament was calculated from capsular displacements determined by stereophotogrammetry. Statistical analyses examined the effect of axial compression on motion segment flexibility, and compared maximum principal capsular strain between the flexibility and failure tests. RESULTS: Capsular strain increased with applied shear load but did not vary with axial compressive load. The maximum principal strain reached during the flexibility tests was 61% +/- 33% of that observed in subcatastrophic failures of the isolated joints. Two specimens reached strains in their flexibility tests that were larger than their corresponding strains at subcatastrophic failure in the failure tests. CONCLUSIONS: The cervical facet capsular ligaments may be injured under whiplash-like loads of combined shear, bending, and compression. The results provide a mechanical basis for injury caused by whiplash loading.

A biomechanical, radiologic, and clinical comparison of outcome after multilevel cervical laminectomy or laminoplasty in the rabbit.

STUDY DESIGN: A rabbit model was used to compare clinical outcome, radiographic changes, and biomechanical flexibility after cervical laminectomy and open-door laminoplasty. OBJECTIVE: This study tested the hypothesis that radiographic changes and biomechanical flexibility could explain the differences in clinical outcome after cervical laminectomy and laminoplasty. SUMMARY OF BACKGROUND DATA: Although multilevel cervical laminoplasty is thought to have advantages over cervical laminectomy, clinical outcome studies have been contradictory, and no experimental study has examined the possible mechanisms for the differences after healing. METHODS: Twenty-four New Zealand White rabbits were randomized into four groups: normal, sham, C3-C6 wide laminectomy, and C3-C6 open-door laminoplasty. Clinical, radiographic, and biomechanical data were collected and compared up to 3 months after surgery. RESULTS: Laminectomy had a statistically significant poorer clinical outcome when compared with laminoplasty after 3 months of healing. Radiologic analysis showed statistically significant angular deformity in the laminectomy group compared with laminoplasty and control groups at 3 months. In contrast, biomechanical measures of flexibility, neutral zone, and range of motion showed only small differences between any of the groups at any time. CONCLUSIONS: The presence of deformity, and not a change in flexibility, is responsible for the differences in clinical outcome observed after laminectomy compared with laminoplasty in this model.

Strength and stability of posterior lumbar interbody fusion. Comparison of titanium fiber mesh implant and tricortical bone graft.

STUDY DESIGN: A paired comparison was done of the bending flexibility and compression strength of tricortical bone graft and titanium fiber mesh implants in a human cadaver model of posterior lumbar interbody fusion. OBJECTIVES: To test the hypothesis that a titanium fiber mesh implant and a tricortical bone graft provide adequate and equal mechanical strength and stability in posterior lumbar interbody fusion constructs. SUMMARY OF BACKGROUND DATA: Although studies of posterior lumbar interbody fusion constructs have been performed, the authors are unaware of any study in which the strength and stability of a titanium fiber mesh implant are compared with those of tricortical bone graft for posterior lumbar interbody fusion in the human cadaver lumbar spine. METHODS: Changes in neutral zone and range of motion were measured in a bending flexibility test before and after placement of posterior lumbar interbody fusion constructs. Tricortical bone graft and titanium fiber mesh implant construct stability than were compared in a paired analysis. The constructs than were loaded to failure to evaluate construct strength as a function of graft material and bone mineral density. RESULTS: The posterior lumbar interbody fusion procedure produced statistically significant decreases in neutral zone when compared with the intact spine. No statistically significant differences in neutral zone, range of motion, or strength were detected between the two implants. Construct strength correlated strongly with bone mineral density. CONCLUSIONS: Posterior lumbar interbody fusion procedures result in equal or improved acute stability for titanium fiber mesh implants and tricortical bone graft implants when used without additional posterior stabilization.

Measurement of vertebral cortical integrity during pedicle exploration for intrapedicular fixation.

STUDY DESIGN: This study determined the predictive ability of electrical impedance measurement in detecting cortical perforation in a porcine model of pedicular exploration. OBJECTIVE: This study tested the hypothesis that a large decrease in electrical impedance would occur as a result of perforation of the vertebral cortex by the pedicle probe. SUMMARY OF BACKGROUND DATA: The resistivity of cortical bone has been reported to be 25 to 100 times greater than that of soft tissues. METHODS: A total of 42 pedicles of the lumbar spines of six swine were explored using the instrumented pedicle probes. RESULTS: Using a 1 microAmp 30-Hz current source, measurement of electrical impedance predicted cortical rupture with a sensitivity, specificity, and accuracy of 95%. Maximum applied voltages of 2.8 mV did not result in myogenic stimulus. CONCLUSIONS: Electrical impedance measurement provides an accurate real-time measurement of cortical perforation. This technique is adapted readily for use with pedicular screws and screw tape. Further investigation to determine the clinical use of this technique is recommended.