Augustus A. White

Prediction of vertebral body compressive fracture using quantitative computed tomography.

We performed quantitative computed tomography in vitro on the first and third lumbar vertebrae in human cadavera using a dibasic potassium phosphate phantom for calibration. The quantitative computed-tomography numbers exhibited a significant positive correlation (R2 = 0.89, p less than 0.0001) with direct measurements of the apparent density of the vertebral trabecular bone. We also conducted uniaxial compression tests to failure of the vertebral bodies after removal of the posterior elements, and found that vertebral compressive strength was also correlated at a high level of significance (R2 = 0.82, p less than 0.0001) with direct measurement of the trabecular apparent density. These findings suggested the possibility that the quantitative computed-tomography values might be directly predictive of vertebral compressive strength. However, when we correlated the quantitative computed-tomography values directly with vertebral compressive strength, the results (R2 = 0.46, p less than 0.061) were suggestive but not quite significant. All vertebral bodies failed by compression of the end-plate, suggesting only a modest structural role for the cortical shell under these loading conditions. This was confirmed by comparing the compressive load to failure of twenty additional pairs of vertebrae that were tested with and without an intact vertebral cortex. Removal of the cortex was associated with approximately 10 per cent reduction in vertebral load to failure.

A Biomechanical Analysis of the Clinical Stability of the Lumbar and Lumbosacral Spine

Eighteen Functional Spinal Units (FSU), representing three levels of human lumbar and lumbosacral spine, were tested using preload forces that corresponded to the clinical situation of a person lying supine or standing while subjected to maximum physiologic flexion or extension forces. Sagittal plane displacements were measured using linear variable differential transformers (LVDTs) and a MINC-11/03 minicomputer. Sequential transection of components in the posterior-to-anterior and anterior-to-posterior directions until failure occurred allowed measurements of the displacement sagittal plane translation and rotation of the intact and transected FSU.

Three-dimensional flexibility and stiffness properties of the human thoracic spine

Abstract Mathematical models of the human spine structure are useful in predicting biomechanical behaviour of the spine where actual experiments may never be performed. These models, to be valid, should be based upon experimentally determined mechanical properties of the spine. Three dimensional flexibility and stiffness properties, including the coupling effects, were determined for all levels of the human thoracic spine. Fresh cadaver spines were used in a 100% humid atmosphere at 22°C to preserve physiological environment. Bottom vertebra of a two vertebrae construct was fixed. Twelve forces and moments, one at a time, were applied to the top vertebra. Vertebral displacement was accurately measured in three dimensional space. Load displacement curves for the main as well as the coupled motions were plotted. Flexibility and stiffness matrices were calculated for the center of the vertebral body. Values for the matrix coefficients, the load-displacement diagrams and the variation of the mechanical properties with the spine level are presented.

Importance of bone mineral density in instrumented spine fusions.

The effect of equivalent mineral density on pedicular screw fixation strength was investigated. The equivalent mineral density of human vertebral bodies was correlated highly with the pullout force of Kluger screws (r2 = 0.61, P less than 0.02). A moderate to high correlation existed between density and vertical force (r2 = 0.42 for Kluger screws, r2 = 0.55 for Steffee screws, P less than 0.02). In calf vertebral bodies of higher density (146 +/- 14 mg/cc), the forces were significantly higher than in the human vertebral bodies (P less than 0.05). Human lumbosacral spines were instrumented with three different fixators: Steffee plates, AO fixateur interne, and Kluger fixateur interne. Of five specimens with a mean density of 88 +/- 11 mg/cc, one screw loosened. More than one screw loosened in six specimens with a mean density of 63 +/- 12 mg/cc, and no screw loosened in four specimens with a mean density of 114 +/- 38 mg/cc. Measurement of equivalent mineral density correlates with the fixation strength of the intrapedicular screws in vitro and should be considered in patients with signs of osteopenia before using pedicular screws for spinal fusions. It is also concluded that calf spines are a good model for testing implants because they tend to focus failure processes in the implant rather than in the implant-bone interface.

Effect of screw diameter, insertion technique, and bone cement augmentation of pedicular screw fixation strength.

This study investigated (1) the effect of screw diameter and insertion technique in lumbar vertebrae, and insertion site in the sacrum, on the axial pullout force and transverse bending stiffness of pedicle screws, and (2) the effect of bone cement augmentation using polymethylmethacrylate (PMMA) and the biodegradable composite, poly(propylene glycol-fumarate) on axial pullout force and transverse bending stiffness of pedicle screws inserted into lumbar vertebrae. The axial pullout force and transverse bending stiffness of a 6.25-mm Steffee screw and a 6-mm Kluger screw did not differ significantly in vertebral bodies of similar equivalent bone mineral density. The axial pullout force of Schanz screws was significantly increased with a 1-mm increase in screw diameter. However, there was no significant increase in transverse bending stiffness. In the sacrum, an approach through the S1 facet produced significantly higher axial pullout forces and transverse bending stiffness than the approach described by Harrington and Dickson. PMMA and a biodegradable composite bone cement poly(propylene glycol-fumarate) both increased the axial pullout force. PMMA also increased the transverse bending stiffness.

Cervical spine mechanics as a function of transection of components

Knowledge of the biomechanics of the spine following injury is essential to the surgeon in his management of patients. However, there is not much information available on the alterations of the biomechanics after injury. This experiment was performed to collect basic information, with the purpose of establishing thresholds of stability of the cervical spine under normal physiologic loads. Seventeen motion segments of the human cervical spine were studied. (A motion segment consists of two adjacent vertebrae and the interconnecting soft tissues.) A constant load of 25 per cent of the body weight was applied to the upper vertebra while the lower vertebra was fixed. Various ligaments, the intervertebral disc, and facet joints of the motion segment, here referred to as components, were dissected in two predetermined patterns: anterior to posterior or posterior to anterior. The load was applied to simulate flexion or extension. Motion was measured in the sagittal plane at each stage of the component transection. Results were plotted for rotation and horizontal translation of the upper vertebra—as function of transection of the components. In general, the elastic deformation was small, even when a majority of the components were sectioned. Failure was sudden and there was no consistent pre-failure phase. These in vitro studies have provided accurate information, which is clinically useful in predicting stability of the motion segments as a function of ligament, disc and facet transection.

Relief of pain by anterior cervical-spine fusion for spondylosis. A report of sixty-five patients.

An extensive evaluation of the results of anterior spine fusion in sixty-five patients with cervical spondylosis showed that good results with respect to relief of pain were Obtained in 90 per cent. The factors predisposing to more favorable results were: presence of radicular symptoms preoperatively, presence of positive roentgenographic findings at only one vertebral level, presence of myelographic defects which correlated with the levels operated on, and achievement of a solid union without interspace collapse. None of these factors, however, were indispensable to a good result. Those factors which were associated with a bad result were: the presence preoperatively of long tract signs and the preoperative presence of subluxation of a vertebra. Psychological testing (Cornell Index) did not differentiate whether the results would be favorable or unfavorable.

The four biomechanical stages of fracture repair

Based on analysis of the torque-angle curves and roentgenographic findings in fifty-three healing tibial fractures in rabbits tested in torsion to failure, four biomechanical stages of fracture healing were defined, as follows: Stage I--failure through original fracture site, with low stiffness; Stage II--failure through original fracture site, with high stiffness; Stage III--failure partially through original fracture site and partially through intact bone, with high stiffness; and Stage IV--failure entirely through intact bone, with high stiffness. These stages correlated with the progressive increases in the average torque and energy absorption to failure as healing progressed and also with the average times since the original experimental fracture. It is hoped that this system of staging will provide both a standard by which important variables related to ultimate strength of healing fractures can be correlated and an objective way to predict delayed unions and non-unions and to determine the level of activity that is safe for patients with a healing fracture.

VARIATIONS OF STIFFNESS AND STRENGTH ALONG THE HUMAN CERVICAL SPINE

The load-displacement response and strength of the mid (C2-C5) and lower (C5-T1) cervical regions were determined for combinations of sagittal loads, in vitro. In unpaired t-test comparisons, the mid cervical region was significantly stiffer in compression and extension than the lower region. In tests to failure, failure in six out of seven mid cervical specimens resulted from flexion alone, while combined compression-flexion was required to fail five of the eight lower cervical specimens. Post-test dissections revealed no regional differences in the pattern of failure. In addition to sagittal tests, the load-displacement responses of three-vertebrae cervical specimens were measured with the upper body axially rotated with respect to the lower body. The effect of this pre-torsion was to diminish the zone of low slope near zero load for axial, shear, and flexion motion. Three of the four axially rotated specimens failed in flexion without added compression. These controlled load-displacement measurements of cervical spine specimens describe for the first time the continuous flexion-compression response up to failure, and suggest that consideration of the biomechanics of three apparently distinct mobile regions of the cervical spine (C1-C2, C2-C5, C5-T1) may facilitate the interpretation of hazardous conditions and the diagnosis of injury. These data also provide basic information for the in vitro investigation of passive cervical spine protection such as helmets and head-rests, suggesting that the head should be kept in a non-rotated position to reduce risk of injury.

Basic biomechanics of the spine

The purpose of this presentation is to provide basic biomechanical information concerning the spine, its components, and the spinal cord. It is shown that this information is helpful in understanding the fundamental functions of the spinal column. The experimentally determined physical properties of the vertebra, various spinal ligaments, the disc, and the spinal cord under many different loading conditions are described. The role of the special characteristics of the spine ligaments in allowing physiological motions of the spine, preventing excessive motions between vertebrae, and protecting the spinal cord during trauma are discussed. Movements of the spinal cord within the spinal canal and associated changes of its section during physiological movements of the spine are also described. The kinematics of the various regions of the spine are discussed and their clinical significance is presented. The problems of spinal trauma and is associated spinal instability are analyzed. Guidelines are recommended to assess spinal stability. The proper application of such guidelines will provide the basis for sound clinical judgments.