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Biomedical subjects

M H Krag

Publications and source records attributed to M H Krag.

At least 37 records · Page 2Linked to original sources
PubMed1989

Pull-off strength of gardner-Wells tongs from cadaveric crania.

Failure of attachment ("pull-off") of Gardner-Wells tongs from the cranium occasionally occurs, and may cause problems, especially in cases of significant cervical spinal instability. The optimal method for Gardner-Wells tongs attachment is not well defined: the effect on pull-off strength of stem protrusion of the spring-loaded pin, and user accuracy in setting stem protrusion do not appear to be documented in the medical literature. The authors measured pull-off strength from five fresh cadaveric crania, for each of five stem protrusion settings. In a related experiment, staff surgeons were asked to follow the standard instructions for applying clinically available Gardner-Wells tongs (smooth, unmarked indicator stems on the spring-loaded pins), after which the actual stem protrusion lengths were recorded. When the indicator stem protruded 1.0 mm (manufacturer's recommendation), the mean pull-off strength (+/- standard deviation [SD] was 137 +/- 34 pounds (610 +/- 151 N). Even with stem protrusions as low as 0.25 mm, the lowest pull-off strength was 60 pounds. Failures of fixation occurring with traction loads of 35 to 50 pounds are almost surely associated with stem protrusions of less than 0.25 mm. Even without an indicator line on the stem at 1 mm, all of the 13 orthopaedists tested produced an actual stem protrusion of 0.37 mm or more. Secure attachment of larger cranial traction loads requires careful attention to pin tightening. Proper location on the skull and the risk of penetration through the inner table must also be kept in mind.

Biomechanical Phenomena↗
PubMed1989

Placement of transpedicular vertebral screws close to anterior vertebral cortex. Description of methods.

Strengthening of the screw-vertebra interface has been shown to occur with implantation of longer transpedicular screws, the tips of which are placed closer to the anterior cortex of the vertebral body. Such implantation probably results in increased risk for anterior cortex penetration and associated vascular or pulmonary damage. Typically recommended lateral and posterior-anterior radiography is shown here to provide potentially misleading visualization during implantation. To reduce the risk of anterior cortical penetration, presented here are 1) a radiographic method ("near approach view") that avoids this problem and allows direct visualization of the relationship between drill bit or screw tip and anterior vertebral cortex, and 2) a surgical technique ("mallet method") that provides both an audible and a palpable change when the drill bit contacts the anterior cortex.

Bone Screws↗
PubMed1989

The design and use of a microcomputerized real-time muscle fatigue monitor based on the medial frequency shift in the electromyographic signal.

We have designed a real-time microcomputerized muscle fatigue monitor based on the median frequency shift of the electromyographic signal, computed via the fast Fourier transform. For the ten subjects performing an isotonic and isometric trunk extension task on two separate days, preliminary results suggest a repeatable linear decrease in median frequency as a function of time.

Electromyography↗
PubMed1989

The balance point of the intervertebral motion segment: an experimental study.

A loading or "balance" point was sought that could serve as a functional reference for mechanically testing spinal motion segments. This point is located above the in-vitro motion segment where, when an axial compressive load is applied, the segment exhibits minimal coupled rotation. The balance point is a reliable indicator of the mechanical characteristics of the segment. Segments exhibited increasing rotation as axial compressive loads were applied further and further away from the balance point. The location of the balance point was significantly affected by sustained static or cyclic flexion-compression loading and by brief flexion-compression overloads.

Biomechanical Phenomena↗
PubMed1988

Depth of insertion of transpedicular vertebral screws into human vertebrae: effect upon screw-vertebra interface strength.

Improvement in the strength of the transpedicular screw-vertebra interface by increasing the depth of screw insertion may provide improved performance of spinal implants using such screws. Within human cadaveric vertebrae, we measured the failure strength of Vermont Spinal Fixator (VSF) screws under flexion or torsion loads and of Schanz screws under pull-out loads (along the screw axis). Comparisons between opposite pedicles of vertebral specimens were made at 50 vs. 80% and 80 vs. 100% of maximum available insertion depth. Mean failure strength of VSF screws at 50% depth was 75-77% (depending upon load type) of that at 80% depth; strength for screws at 100% ("to-cortex") depth was 124-154% of that at 80%. Reanalysis of the data from Lavaste shows, contrary to his conclusion, a 26% increase in strength from a 5-mm increase in screw depth of insertion. All these differences were significant (p less than 0.05) by the matched-pairs t test. Benefit from the increased strength of deeper screw placement must be balanced against possible increased operative risk. A "near-approach" x-ray view is suggested here to decrease that presumed operative risk.

Bone Screws↗
PubMed1988

Cranial morphometry related to placement of tongs in the temporoparietal area for cervical traction.

The optimal placement of cervical traction tong pins to avoid cranial penetration or tong pull-off is not well established. Cranial thickness and transverse diameter in the temporoparietal area we believe to be important variables: The former was measured on 10 embalmed cadaveric specimens, the latter on 20 cleaned skeletal specimens. For each specimen, measurements were obtained at 29 points arranged within a 5 x 9-cm rectangular region above the external auditory canal. Within this region, no significant thickness variations occurred, except for one point each at the extreme posterocephalad and posterocaudad corners. Transverse diameters were constant in the area just above the pinna and gradually decreased in the anterior, posterior, and cephalad directions. No outward-flaring temporal ridge was found. We recommend pin placement just above the pinna. Anteroposterior position should probably be determined by the amount, if any, of flexion or extension desired.

Anthropometry↗
PubMed1988

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.

Adolescent↗
PubMed1988

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.

Adult↗
PubMed1988

A new halo-vest: rationale, design and biomechanical comparison to standard halo-vest designs.

The traditional halo-vest rigidly grips the cranium, but not the torso. Unexpectedly large motion and forces in the cervical spine have been shown by others to be present during halo-vest wear. In an effort to reduce these motions and forces, an experimental vest has been designed. Motion of the vest on the thorax has been measured on four normal volunteers, for each of nine load types, for each of seven commercially available vests as well as the experimental vest. Despite its lighter weight and less cumbersome structure, the experimental vest has the lowest mobility score of all the vests tested.

Biomechanical Phenomena↗
PubMed1987

A mechanical model for the human intervertebral disc.

Stress relaxation experiments were performed on specimens from a human intervertebral disc. Specimens were made from the nucleus pulposus and from the external lamellae of the anulus fibrosus in two different orientations. Tests were run with varying moisture content so as to develop a relaxation master curve. A model was developed based on the experimental data. It was found that the short term master curve for the lamellae of the anulus and nucleus are similar, whereas the long term rubbery plateau is different between the lamellae and the nucleus. It was also established that the master curves for different lamellae and the nucleus were shifted relative to each other in the time domain due to changes in water content. The average relaxation modulus of the whole disc was obtained by averaging the properties between the anulus and nucleus. This model was then used for studies of Schmorl's nodes, of degenerated discs and for circumstances in which hydration is considered to be important.

Body Water↗
PubMed1987

Water content in human intervertebral discs. Part I. Measurement by magnetic resonance imaging.

Tension-relaxation experiments were performed on human disc lamellae specimens. The water content was found to affect the viscoelastic behavior and a master relaxation curve was constructed from the experimental data. The water content of disc phantoms is measured by magnetic resonance imaging (MRI) techniques. MRI was used to compare the discs of patients of different ages. The possibility of obtaining cross-sectional water distribution in human intervertebral disc material using MRI techniques and its relation to the disc's mechanical properties was explored, with the goal of constructing a realistic mathematical model of the disc which takes into account the water content of the disc.

Body Water↗
PubMed1987

Water content in human intervertebral discs. Part II. Viscoelastic behavior.

Water content of intervertebral discs is a significant aspect of both viscoelastic behavior and age-related degenerative changes. Using water content as a dependent variable, stress-relaxation was measured using standardized anulus fibrosus specimens strained at various levels of strain. Synthesis of experimental data into a master relaxation curve allows prediction of specimen response over time intervals not readily accessible experimentally. A quantitative understanding of the role of water content may have important clinical application, since magnetic resonance imaging is a tool which should allow water content determination in vivo.

Biomechanical Phenomena↗
PubMed1987

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.

Aging↗
PubMed1986

An internal fixator for posterior application to short segments of the thoracic, lumbar, or lumbosacral spine. Design and testing.

A new spinal implant has been designed and biomechanical testing completed, intended for application to "short-segment" spinal defects such as disc degeneration, fracture, spondylolisthesis, or tumor. Major improvements over currently available devices include: only 2-3 vertebrae are spanned, not 5-7 as with Harrington rods; true three-dimensional fixation is achieved, preventing such problems as hook or rod dislocation; three-dimensional adjustment is easily accomplished, allowing fracture or spondylolisthesis reduction to be readily performed; attachment to vertebrae is by means of transpedicular screws eliminating deliberate encroachment into the spinal canal, such as Luque wires or Harrington hooks; no special alignment between screws is needed (such as with holes or slots in a plate), allowing screw placement to fully conform to anatomic structures; and laminectomy sites and lumbosacral junction are readily instrumented. Background investigations presented here for design of this device include: CT-defined pedicle morphometry showing that screws may be larger than those currently used; effect of pitch, minor diameter, and tooth profile on screw pull-out strength; mechanical testing of a compact, three-dimensionally adjustable, strong, nonloosening articulating clamp; and establishing of the relationship between depth of penetration and strength of fixation of transpeduncular screws.

Adolescent↗
PubMed1984

Thoracic spine centers of rotation in the sagittal plane.

The purpose of this study was to determine in vitro the centers of rotation of thoracic functional spinal units in the sagittal plane. The center of rotation is a convenient concept and part of a precise method of documenting the kinematics of a joint moving in a plane. Fresh cadaver functional spinal units from the thoracic region were utilized. Six load types were used that produced motions only in the sagittal plane, namely anterior and posterior shear forces, flexion and extension moments, and compression and distraction forces. The resulting motion with three degrees of freedom was measured with dial gauges. Statistical methods were used to analyze data from the viewpoint of vertebral level, load magnitude, and load type. Only the load type was found to be significantly related to the location of the centers of rotation. Although there was significant variability in the centers of rotation, there were definite locations related to each load type. The average center of rotation was 15-45 mm directly below the geometric center of the moving vertebra. The results of the present study may be helpful in the clinical interpretation of spinal kinematic studies.

Adult↗