Martin H. Krag

Human Lumbar Vertebrae: Quantitative Three-dimensional Anatomy

This study details the quantitative three-dimensional surface anatomy of human lumbar vertebrae based on a study of 60 vertebrae. The two lower vertebrae (L4 and L5) appeared to be transitional toward the sacral region, whereas the upper two vertebrae (L1 and L2) were transitional toward the thoracic region. Means and standard errors of the means for linear, angular, and area dimensions of vertebral bodies, spinal canal, pedicle, pars interarticularis, spinous and transverse processes were obtained for all lumbar vertebrae. This information provides a better understanding of the spine, and allows for a more precise clinical diagnosis and surgical management of spinal problems. The information is also necessary for constructing accurate mathematical models of the human spine.

Thoracic Human Vertebrae Quantitative Three-dimensional Anatomy

This study details the quantitative three-dimensional surface anatomy of thoracic vertebrae based on a study of 144 vertebrae. The thoracic spine was found to have three distinct regions: upper, middle, and lower segments. The two end segments appear to be transitional zones toward cervical and lumbar regions. The middle zone (T3 to T9) is of utmost importance due to the presence of the combination of narrow spinal canal and critical vascular supply. Means and standard errors of the means for linear, angular, and area dimensions of vertebral bodies, spinal canal, pedicle, pars articularis, spinous and transverse processes, and rib articulations are provided for all thoracic vertebrae. This information is necessary for constructing accurate mathematical models of the human spine. It will also provide a better understanding of the spine, and allow for a more precise clinical and surgical management of spinal problems.

Articular facets of the human spine. Quantitative three-dimensional anatomy.

This study provides the quantitative three dimensional surface anatomy of the articular facets for the entire human vertebral column based on a study of 276 vertebrae. Means and standard errors of the means for linear, angular, and area dimensions of the superior and inferior articular facets were measured for all vertebrae from C2 to 1.5. Facet orientations were described as angles with respect to the sagittal and transverse planes and also a card angles. The piano angles are similar to the angles seen on traditional radiographic views—radiographs and computed tomographic scans. The card angles, a new concept, are better at helping visualize the three-dimensional orientations of the facets. Excluding the superior C2 facet, the following minimum and maximum dimensions were found for the facets from C3 to L5: width = 9.6–16.3 mm; height = 10.2–18, 4; surface area =72.3–211.9 mm2; interfacet width = 20.8–40.6; interfacet height - 12.2–33.0 mm; transverse plane angle = 41.0–86.0; sagittel plane angle 67.4–154.8; X-card angle = 41.0–86.0; and Y-card angle = 5.8–66.1. The quantitative anatomy of the facets may improve the understanding of the spinal anatomy, help improve the clinical diagnosis and treatment, and provide the necessary data for constructing more realistic mathematical models of the spine.

Morphometry of the Thoracic and Lumbar Spine Related to Transpedicular Screw Placement for Surgical Spinal Fixation

Vertebral transpedicular screws provide secure attachment for posterior spinal fixation devices. Screw design details, biomechanics, and implantation safety depend upon anatomic constraints, especially from the pedicle and body. Previous morphometric data were limited; thus, a retrospective study was undertaken using computerized axial tomograms (CT) of 91 vertebrae (T9–L5). In addition, eight cadaver vertebrae were CT scanned and then cut transversely to compare x-ray measurements with direct physical measurements. Measured parameters included pedicle width, pedicle length, angle of pedicle axis to sagittal plane, and transpedicular cortex-to-cortex chord length. Good correlation is shown to occur between CT scan and direct physical measurements of human vertebrae. Implications for spinal implant screw dimensions and safety of implantation are discussed. Comparison with previously available data is made.

Evidence of connective tissue involvement in acupuncture

Acupuncture needle manipulation gives rise to “needle grasp,” a biomechanical phenomenon characterized by an increase in the force necessary to pull the needle out of the tissue (pullout force). This study investigates the hypothesis that winding of connective tissue, rather than muscle contraction, is the mechanism responsible for needle grasp. We performed 1) measurements of pullout force in humans with and without needle penetration of muscle; 2) measurements of pullout force in anesthetized rats, with and without needle rotation, followed by measurements of connective tissue volume surrounding the needle; 3) imaging of rat abdominal wall explants, with and without needle rotation, using ultrasound scanning acoustic microscopy. We found 1) no evidence that increased penetration of muscle results in greater pullout force than increased penetration of subcutaneous tissue; 2) that both pullout force and subcutaneous tissue volume were increased by needle rotation; 3) that increased periodic architectural order was present in subcutaneous tissue with rotation, compared with no rotation. These data support connective tissue winding as the mechanism responsible for the increase in pullout force induced by needle rotation. Winding may allow needle movements to deliver a mechanical signal into the tissue and may be key to acupunctures therapeutic mechanism.

Effects of disc injury on mechanical behavior of the human spine.

The effects of injury to the intervertebral disc were investigated using three-dimensional flexibility and creep measurements of functional spinal units from fresh cadaver lumbar spines. The techniques utilized were accurate and the data had a high degree of reproducibility. An injury to the annulus and a removal of the nucleus significantly altered the mechanical properties of the spinal unit. Not only were the main motions affected but also the coupled motions. Sagittal plane symmetry was disturbed, resulting in asymmetric facet joint movements. These effects of injury could be measured because of the three-dimensionality of the experiments. Previous studies, utilizing only axial compression loading, claimed to observe no changes due to the disc injuries and are, therefore, in conflict with the present findings.

Biomechanics of thoracolumbar spinal fixation. A review.

Extensive development of spinal instrumentation has occurred recently, benefitted by improved biomechanical knowledge. Reviewed here are various devices and the major biomechanical issues relevant to them. The devices are categorized by site of attachment. The major emphasis is on the most recently developed category: devices attached by transpedicular screws. Aspects of this last category reviewed here include screw design, screw placement, longitudinal linking devices (rods, plates), and transverse connectors (cross-linking). Emphasis is placed not only on current knowledge, but also on unresolved issues.

Repair of failed transpedicle screw fixation. A biomechanical study comparing polymethylmethacrylate, milled bone, and matchstick bone reconstruction.

In a random, controlled laboratory study, pedicle screws placed in human cadaveric vertebrae were axially loaded to failure. Three repair methods were tested. Use of low pressure polymethylmethacrylate yielded 149% of the original pullout strength, milled bone yielded 70% of the initial pullout strength, and matchstick bone yielded 56% of the initial pullout strength. Two incidents of cortical penetration during reinsertion in the matchstick group occurred. The results were statistically analyzed.

Repeatability of Four Clinical Methods for Assessment of Lumbar Spinal Motion

Spinal motion usually is recorded from subjective observation of the fully flexed trunk using a goniometer or the distance from the fingertips to the floor. To quantify functional improvement in the low-back pain patient, the repeatability of four clinical techniques was studied: the common fingertip-to-floor distance; the modified Schober; the two-inclinometer method, and a photometric technique. Ten normal subjects (five men, five women), ages 24 to 34 years old, were examined in full flexion, full extension, and the erect position, both standing and sitting. Repeatability was poor for the fingertip-to-floor method in all postures and for the two-inclinometer method in full flexion. Although other methods for various postures had good repeatability, the modified Schober method of determining lumbar spinal motion was the most repeatable and is recommended for a routine, noninvasive, clinical evaluation of lumbar spinal motion.

Internal Displacement Distribution from in Vitro Loading of Human Thoracic and Lumbar Spinal Motion Segments: Experimental Results and Theoretical Predictions

Small metal markers are implanted within intervertebral discs and their displacements in response to a complex load (flexion, compression, and anterior shear) are measured radiographically. These are contrasted to the displacements predicted by a finite element model (FEM) that uses 20 constant-strain triangular elements and is based upon a linear elastic isotropic material. The central portion of the disc (nucleus pulposus) moves posteriorly and oppositely to the FEM prediction. The anterior and posterior portions of the disc agree more closely with the FEM than the central portions of the disc. In general, the FEM predicts much more accurately the up-down displacement components than it does the anterior-posterior components. The measured displacements provide a new class of information concerning the function of the interior of the disc, and also provide a new basis for validation of FEMs that attempt to mimic real intervertebral disc behavior. Implications for understanding of disc function and pathology are discussed.