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Biomedical subjects

M H Krag

Publications and source records attributed to M H Krag.

At least 19 recordsLinked to original sources
PubMed1992

Screw fixation in the human sacrum. An in vitro study of the biomechanics of fixation.

A load-to-failure test was used to study the biomechanical properties of sacral screw fixation in human cadaveric specimens. The goals of this study were 1) to determine the effects of the two commonly chosen sacral screw orientations of fixation characteristics; 2) to determine the effects of selected screw-instrumentation linkages on the biomechanics of sacral screw fixation; 3) to correlate the biomechanical properties with a noninvasive assessment of sacral bone density; and 4) to correlate the torque during screw insertion with these biomechanical properties. The bone density of each specimen was measured with quantitative computed tomography. A screw was inserted from the dorsal surface either anteromedially or anterolaterally into the body of S1, and the torque needed to insert each screw was measured. The screw head was attached to a constrained or semiconstrained loading linkage. Force was applied to the screw in an inferior direction until the maximum load was achieved. The maximum load, screw translation, rotation at maximum load, and initial compliance of the bone-screw interface were determined. It was found that the anteromedial screw orientation, combined with a rigidly constrained loading linkage, resulted in the greatest maximum load to failure, the least screw rotation, and the least initial compliance of the four groups studied. The maximum load and the initial stiffness of bone-screw fixation increased significantly with bone density. Torque measurements correlated significantly with maximum load to failure, initial interface stiffness, and bone density. It was therefore concluded that bone density and torque measurements can be useful in assessing sacral screw fixation.

Aged↗
PubMed1992

Diagnosing instability.

The various definitions of instability are reviewed and preference is given to the definition of instability as a loss of stiffness. This definition fits with current laboratory observations. Roentgenographic changes, particularly those associated with degeneration, have no relationship to instability. Multiple roentgenographic images can be of use, but accuracy is limited, and often valuable information at midmotion range or in other planes is missing. Stereoroentgenography appears to offer some promise, but implanted metallic markers are necessary to attain adequate accuracy. Ionizing radiation dose levels are of concern in these techniques. External fixation techniques appear to be of use in some patients. Kinematic linkages and frames containing infrared light emitting diodes are extremely promising, because they give kinematic information in detail.

Humans↗
PubMed1991

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.

Biomechanical Phenomena↗
PubMed1991

Hole preparation techniques for transpedicle screws. Effect on pull-out strength from human cadaveric vertebrae.

In each of eight thoracolumbar human cadaveric vertebrae, a hole was made through one pedicle into the vertebral body with a drill bit and through the contralateral pedicle with a probe. Identical metal screws were implanted into the holes to equal depths, and maximum pull-out force was determined for each screw. Using a paired Student t test, no significant difference (P = 0.87) was found in pull-out strength between the screws implanted into drilled holes and those implanted into probed holes. In fact, the average pull-out strengths for the two groups differed by less than 2%. The pedicular cortex was broken through during hole preparation in 5 of the 16 pedicles: 3 as a result of drilling and 2 secondary to probing. The average pull-out strength of the screws in these five pedicles was 11.0% less than the average pull-out strength of the screws implanted into the contralateral intact pedicles. Although this does not represent a statistically significant difference (P = 0.15), it suggests that damaging the pedicular cortex may weaken pedicle screw fixation.

Aged↗
PubMed1990

Body height change during upright and recumbent posture.

Body height changes are known to occur in response to changes in loads acting along the spine, such as those produced by movement from a recumbent to an upright posture. Lack of previously available data is addressed here by body height measurements closely spaced in time of ten normal subjects during 8 hours upright and 4 hours recumbent, with particular attention paid to the initial changes. Experimental errors are carefully defined and determined. A Kelvin-unit model is fitted to the mean data for each phase and confidence limits provided for the model equation parameters. Rapid height changes are shown to occur: 26% of the 8-hour loss occurred in the first hour upright, and 41% of the 4-hour recovery occurred in the first hour recumbent.

Adult↗
PubMed1990

A patient registry for orthopedic surgery.

Identification of patient subpopulations for retrospective clinical studies, documentation of residents' clinical experience, and other administrative purposes can be difficult and time consuming. The problem of identification is exacerbated when a teaching program involves several hospitals or when the desired subpopulation is not adequately defined by standard diagnosis or procedure codes used by the institution. A useful patient registry system is reported here for the storage and retrieval of data on orthopedic patients treated by surgical residents at a major teaching hospital and its affiliates. The registry uses a simple, yet powerful encoding scheme to describe patient entries. In addition to a multidimensional encoded description based on SNOMED, the system supports the entry of free text to provide greater detail. This combination gives the patient registry both power and versatility.

Hospital Information Systems↗
PubMed1989

Internal deformations of intact and denucleated human lumbar discs subjected to compression, flexion, and extension loads.

Three rows of six evenly spaced 0.5 mm metal beads were implanted midsagittally into the discs of ten L4-5 human lumbar motion segments. The intradiscal bead displacements in response to compression, flexion, and extension loads were obtained by digitizing the bead positions from sagittal plane radiographs taken before and during the load application. Each disc was denucleated and the loading process was repeated. For the intact discs, in compression, the intradiscal bead displacements were predominantly anterior. In flexion, the beads in the center of the disc moved posteriorly whereas the beads closer to the periphery of the disc moved anteriorly. In extension, the central beads moved anteriorly and the beads closer to the periphery of the disc moved posteriorly. After denucleation, the bead displacements for compression and flexion implied an inward bulging of the inner wall of the annulus, despite outward bulging of the disc surface. We hypothesize that the inward bulging causes radial tensile stresses within the disc, leading to disruption of adjacent layers of annulus.

Biomechanical Phenomena↗
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↗