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Biomedical subjects

M M Panjabi

Publications and source records attributed to M M Panjabi.

At least 37 records · Page 2Linked to original sources
PubMed2000

The role of bone graft force in stabilizing the multilevel anterior cervical spine plate system.

STUDY DESIGN: The role of bone graft force in stabilizing an instrumented cervical spine was evaluated for one-level and three-level corpectomy models using in vitro experiments. OBJECTIVES: To investigate the role of bone graft force in enhancing stability of anterior cervical plate, and to study effects of fatigue loading. SUMMARY OF BACKGROUND DATA: The anterior cervical plate system is used widely in stabilizing the cervical spine after spinal corpectomy and grafting. Many factors such as applied screw torque, screw pullout force, plate strength, plate geometry, and type of bone graft have been studied. However, the role of bone graft in stabilizing the anterior plate system has not been explored. METHODS: Two models (one-level and three-level) incorporating corpectomy, strut graft, and anterior plate were constructed from eight human spine specimens (C2-T1). The flexibility of an intact specimen and two constructs with graft forces of 0 N and 100 N was determined. A flexibility test, simulating physiologic loads, consisted of pure moments of flexion, extension, lateral bending, and axial torques up to 1 Nm. For each moment, range of motion and neutral zone were determined. The stability potential index was defined as the decrease in motion caused by instrumentation, as compared with intact motion. A larger stability potential index indicates a more stable spinal construct. Repeated measures analysis of variance was used to determine the significant changes. RESULTS: In both models, bone graft force increased during extension, decreased during flexion, and showed minor changes during axial torsion and lateral bending. Higher bone graft force increased stability potential index-neutral zone and stability potential index-range of motion in the three-level model in all directions, but only in flexion-extension in the one-level model. Fatigue loading decreased bone graft force to a greater extent in the three-level model. CONCLUSIONS: In the corpectomy-graft-anterior-plate model, graft force decreased in flexion and increased in extension. Higher graft force increased and fatigue decreased stability of the spinal construct in the three-level model.

Adolescent↗
PubMed2000

Internal morphology of human cervical pedicles.

STUDY DESIGN: The internal architecture of cervical spine pedicles was investigated by thin sectioning and digitization of radiographic images. OBJECTIVES: To provide quantitative information on the internal dimensions and cortical shell thicknesses of the middle and lower cervical pedicles. SUMMARY OF BACKGROUND DATA: Although there have been a number of studies presenting data on the external dimensions of the cervical pedicle, little is known regarding its internal architecture and cortical shell thickness along the pedicle axis. METHODS: Twenty-five human cervical vertebrae (C3-C7) were secured to a thin-sectioning machine to produce three 0.7-mm-thick pedicle slices along its axis. Plain radiographs of the pedicle slices were scanned and digitized to facilitate measurement of the internal dimensions. Computer software was specifically developed to determine the external dimensions (i.e., pedicle height and width) and the internal dimensions (i.e., cortical shell thicknesses of the superior, inferior, lateral, and medial walls and the cancellous core height and width) of cervical pedicles. RESULTS: Superior and inferior wall cortical thicknesses of pedicle thin slices were similar, whereas the lateral wall cortical thickness was significantly smaller than the medial wall thickness. The medial cortical shell (average value range: 1.2-2.0 mm) was measured to be 1.4 to 3.6 times as thick as the lateral cortical shell (average value range: 0.4-1.1 mm). When medial and lateral cortical thicknesses were normalized for external dimensions, the combined cortical shell thickness was thinnest at C7 (average value range: 18. 6-25.6% of the external width), and this result was statistically significant when compared with other vertebral levels. CONCLUSIONS: The cervical pedicle is a complex, three-dimensional structure exhibiting extensive variability in internal morphology. Characteristics of the cervical pedicle at different spinal levels must be noted before transpedicular screw fixation.

Bone Screws↗
PubMed2000

Muscle response pattern to sudden trunk loading in healthy individuals and in patients with chronic low back pain.

STUDY DESIGN: A quick-release method in four directions of isometric trunk exertions was used to study the muscle response patterns in 17 patients with chronic low back pain and 17 matched control subjects. OBJECTIVES: It was hypothesized that patients with low back pain would react to sudden load release with a delayed muscle response and would exhibit altered muscle recruitment patterns. SUMMARY OF BACKGROUND DATA: A delay in erector spinae reaction time after sudden loading has been observed in patients with low back pain. Muscle recruitment and timing pattern play an important role in maintaining lumbar spine stability. METHODS: Subjects were placed in a semiseated position in an apparatus that provided stable fixation of the pelvis. They exerted isometric contractions in trunk flexion, extension, and lateral bending. Each subject performed three trials at two constant force levels. The resisted force was suddenly released with an electromagnet and electromyogram signals from 12 trunk muscles were recorded. The time delay between the magnet release and the shut-off or switch-on of muscle activity (reaction time) was compared between two groups of subjects using two-factor analysis of variance. RESULTS: The number of reacting muscles and reaction times averaged over all trials and directions showed the following results: For healthy control subjects a shut-off of agonistic muscles (with a reaction time of 53 msec) occurred before the switch-on of antagonistic muscles (with a reaction time of 70 msec). Patients exhibited a pattern of co-contraction, with agonists remaining active (3.4 out of 6 muscles switched off) while antagonists switched on (5.3 out of 6 muscles). Patients also had longer muscle reaction times for muscles shutting off (70 msec) and switching on (83 msec) and furthermore, their individual muscle reaction times showed greater variability. CONCLUSIONS: Patients with low back pain, in contrast to healthy control subjects, demonstrated a significantly different muscle response pattern in response to sudden load release. These differences may either constitute a predisposing factor to low back injuries or a compensation mechanism to stabilize the lumbar spine.

Adult↗
PubMed2000

Equivalence of single and incremental subfailure stretches of rabbit anterior cruciate ligament.

Experimental models are often used in the laboratory to produce incomplete soft-tissue injuries simulating those observed clinically. Single and incremental stretch protocols have been utilized. The latter has many advantages over the former. This study was designed to determine if incremental and single ligamentous stretches are biomechanically equivalent. Eleven paired fresh rabbit bone-anterior cruciate ligament-bone preparations were used. One of each pair (single-stretch protocol) was stretched to 88% of the average failure deformation and then stretched to failure. The other ligament (incremental-stretch protocol) was stretched to 55, 66, 77, and 88% of the average failure deformation and then stretched to failure. All stretches were performed at 1.2 m/sec. Stress-relaxation tests were performed before and after the 88% stretch for both stretch protocols. Relaxation curves were parameterized as forces at six time points and were also fitted to a three-element model. Load-deformation curves recorded during stretch to failure were characterized by eight parameters. Each incremental stretch step produced a significant increase in deformation, indicating alteration in the mechanical properties of the ligament. Both groups of ligaments, when intact, exhibited no differences in relaxation curves (p > 0.2). The 88% stretches, produced by each of the two stretch protocols, significantly altered the viscoelastic behavior of the ligaments (p < 0.002). However, after the 88% stretch, there were no differences in either viscoelastic (p > 0.1) or load-deformation (p > 0.1) parameters of the two stretch protocols. In conclusion, the 88% subfailure stretch significantly altered the mechanical properties of ligament, and the incremental and single stretches were biomechanically equivalent.

Animals↗
PubMed2000

Biomechanical evaluation of the New Zealand white rabbit lumbar spine: a physiologic characterization.

Physiologic motions of the human, sheep, and calf lumbar spines have been well characterized. The size, cost, and ease of care all make the rabbit an attractive alternative choice for an animal lumbar spine model. However, comparisons of normal biomechanical characteristics of the rabbit lumbar spine have not been made to the spines of larger species. The purpose of this study was to establish baseline physiologic kinematic data for the rabbit lumbar spine. Ten skeletally mature New Zealand white rabbit osteoligamentous spines were obtained. L4-L7 spine segments were harvested and mounted. Multi-directional flexibility testing was performed by applying pure moments up to 0.27 Nm. Resulting rotations were measured using an Optotrak system. Data were analyzed for each intervertebral level in the three planes of rotation. The three levels tested had roughly similar range of motion (ROM). The mean (SD) angular ROMs in flexion for L4-L5, L5-L6, L6-L7 were 12.10 degrees (2.59 degrees), 12.38 degrees (2.70 degrees), and 15.17 degrees (3.22 degrees), respectively. The ROMs in extension were 5.86 degrees (1.21 degrees), 5.58 degrees (1.48 degrees), and 6.13 degrees (2.03 degrees). Lateral bending and axial rotation were roughly symmetric due to the symmetric nature of the spine. For right lateral bending, the ROMs were 8.25 degrees (2.44 degrees), 4.96 degrees (1.70 degrees ), and 4.25 degrees (1.20 degrees). For left axial rotation, the ROMs were 1.23 degrees (1.16 degrees), 0.35 degrees (0.61 degrees), 0.87 degrees (0.64 degrees ). Neutral zone (NZ) was on average 60% (29%) of ROM for the motions studied. The physiologic ROM of the New Zealand white rabbit lumbar spine was found to be similar between the rabbit and human. This relatively conserved physiologic flexibility supports the use of the rabbit as a model of the lumbar spine for kinematic studies. However, the overall NZ was found to be a greater percentage of ROM in the rabbit than the corresponding percentage in the human (60% as compared to 25%). This suggested that the rabbit lumbar spine has a greater laxity than that of the human.

Animals↗
PubMed2000

The anatomic variability of human cervical pedicles: considerations for transpedicular screw fixation in the middle and lower cervical spine.

Transpedicular screw fixation has recently been shown to be successful in stabilizing the middle and lower cervical spine. Controversy exists, however, over its efficacy, due to the smaller size of cervical pedicles and the proximity of significant neurovascular structures to both lateral and medial cortical walls. To aid the spinal surgeon in the insertion of pedicle screws, a number of studies have been performed to quantify the gross dimensions and angulations of the cervical pedicle. Notwithstanding these quantitative studies, there has been a conspicuous absence of research reporting the qualitative characteristics of the cervical pedicle. The purpose of our study was to provide comparative graphical data that would systematically document the anatomic variability in cervical pedicle morphology. Such information should better elucidate the complexity of the pedicle as a three-dimensional structure and provide the spinal surgeon with a more complete understanding of cervical pedicle architecture. Twenty-six human cervical vertebrae (C3-C7) from six fresh-frozen spines were secured to a thin sectioning apparatus to produce three 0.7-mm-thick pedicle slices along its axis. Radiographs taken of these pedicle slices were scanned, digitized, and traced to facilitate visual comparison. The pedicle slices were found to exhibit substantial variability in composition and shape, not only between individual spines and vertebral levels, but also within the pedicle axis. However, the lateral cortex was consistently found to be thinner than the medial cortex in all samples. These physical findings must be noted by surgeons attempting transpedicular screw fixation in the cervical spine.

Bone Screws↗
PubMed2000

Superiority of incremental trauma approach in experimental burst fracture studies.

OBJECTIVE: To compare the incremental and single trauma approaches in experimental spinal trauma production. DESIGN: An in vitro study to produce experimental burst fractures in human spine specimens. BACKGROUND: Experimental burst fractures have been produced by researchers for various purposes using two approaches: single and incremental traumas. Both the experimental trauma approaches use drop weight technique. There have been no studies to compare these two markedly different methods. Showing clear advantages of one approach over the other may significantly affect the design of future experimental trauma studies, not only of the spine. METHODS: Using human spine specimens and drop weight technique, burst fractures of varying degrees of severity (defined by canal encroachment) were produced. Impact energies needed for the initial burst fracture and for the progression of the injury, i.e. increased canal encroachment, were studied using regression analyses. RESULTS: Poor correlation was found between the impact energy and the canal encroachment of the initial burst fracture (R(2)=0.27). A much higher correlation was found when the initial burst fracture points (energy-encroachment) were initialized to zero values and only the progression of the injury was studied (R(2)=0.84, p<0.001). The two regressions represent respectively single and incremental approaches. CONCLUSIONS: Incremental trauma approach was found to be superior to the single trauma approach, in producing a burst fracture with the desired canal encroachment in the human spine specimens, in spite of their inherent variability in size and strength of human vertebrae. RELEVANCE: The design of experimental traumas studies will benefit from the results of the present comparative evaluation of single and incremental trauma approaches. The quality of the experiment may be significantly improved.

Adult↗
PubMed2000

A biomechanical comparison of multistrand flexor tendon repairs using an in situ testing model.

An in situ testing model was used to evaluate the performance of zone II flexor tendon repairs and to compare the biomechanical properties of 4-strand repairs with 2- and 6-strand repairs. Fifty digits from human cadaveric hands were mounted in a custom apparatus for in situ tensile testing. Intratendinous metallic markers were placed so that gap formation could be determined by fluoroscopy during tensile testing. Three 4-strand repairs (the 4-strand Kessler, the cruciate, and a locked modification of the cruciate repair) were compared with the 2-strand Kessler and the 6-strand Savage repairs. Ultimate tensile strength, load at 2-, 3-, and 4-mm gap formation, and work of flexion were determined. Work of flexion, while increased for the multistrand repairs, did not show a statistically significant correlation with the number of strands crossing the repair site. The tensile strength of the 6-strand repair was significantly greater than each of the 2- or 4-strand repairs. The tensile strength of all 4-strand repairs was significantly greater than the 2-strand repair. The 6-strand repair and the 2 cruciate repairs demonstrated a statistically increased resistance to gap formation compared with the 2-strand Kessler repair, but notably there was no statistically significant difference in gap resistance between the 2and 4-strand Kessler repairs. This in situ tensiletesting protocol demonstrated that 4- and 6-strand repairs have adequate initial strength to withstand the projected forces of early active motion protocols. Three of the 4 multistrand repairs demonstrated improved gap resistance compared with the 2-strand repair. The presence of the second suture in the Kessler configuration significantly increases its strength but not its gap resistance.

Analysis of Variance↗
PubMed2000

Radiographic parameters for evaluating the neurological spaces in experimental thoracolumbar burst fractures.

It is important to know the condition of neural spaces during the nonoperative treatment of thoracolumbar burst fractures. The goals of the current study were to identify the correlation between the degree of deformity of a fractured vertebra and the encroachment of neural spaces, and to determine how the encroachment and the deformity can be improved by the extension posture simulating the postural reduction. Experimental burst fractures were produced in L1 vertebrae of nine human thoracolumbar spine segments (T11-L3) with neural spaces lined with tiny steel balls. Lateral radiographs were taken in neutral and extended posture before and after the trauma. Anterior vertebral height, posterior vertebral height, vertebral height ratio, vertebral kyphotic angle, posterior vertebral body angle, and the cross diagonal angle were the geometric parameters used to describe the vertebral deformity. The canal diameter and superior and inferior intervertebral foramen areas were defined as the neural spaces. All parameters were measured on the scanned images of radiographs, as seen on the computer screen. Among the vertebral body parameters, the posterior vertebral height, posterior vertebral body angle, and cross diagonal angle showed significantly higher correlations with the canal encroachment. The extended posture did not improve the canal and intervertebral foramen encroachments. The kyphotic deformity (vertebral kyphotic angle and anterior vertebral height) was improved but the deformity of the vertebral posterior wall (posterior vertebral height and posterior vertebral body angle) was not improved because of the extended posture.

Adult↗
PubMed1999

Loosening at the screw-vertebra junction in multilevel anterior cervical plate constructs.

STUDY DESIGN: An in vitro biomechanical study of one-level and three-level corpectomy and anterior cervical plate models. OBJECTIVE: To investigate the failure of the screw-vertebra interfaces in one- and three-level corpectomy models. SUMMARY AND BACKGROUND DATA: Although there are several biomechanical studies of strength and stability of anterior cervical plating, there has been no investigation into clinically observed failures. METHODS: One- and three-level models (corpectomy, strut graft, and anterior plate) were constructed from eight cadaveric specimens (C2-T1). Multidirectional flexibility tests (1.0 Nm moments) performed before and after fatigue (1000 cycles, 1.0 Nm flexion-extension, 0.14 Hz) documented the screw-vertebra motions at upper and lower ends. Ranges of motion and neutral zones were determined. Analysis of variance was used to evaluate significant differences between the upper and lower ends of the plates and changes caused by fatigue loading (P < 0.05). RESULTS: Extension motion at the lower ends was more than at the upper ends in both models. Fatigue increased three-level model ranges of motion at the lower end by 171% in flexion, 164% in extension, 153% in lateral bending, and 115% in axial rotation. Similar increases were observed in neutral zones. Fatigue loading produced no significant changes in one-level models. CONCLUSION: There was excessive screw-vertebra motion caused by fatigue at the lower end of the three-level corpectomy model. These findings of loosening may explain clinically observed failures at the caudal end of long anterior cervical plate constructs. Longer screws, larger diameter screws, and supplemental posterior fixation may decrease screw-vertebra loosening.

Biomechanical Phenomena↗
PubMed1999

Stabilizing potential of anterior cervical plates in multilevel corpectomies.

STUDY DESIGN: An in vitro investigation of three-dimensional kinematics of cervical spine models of one- and three-level corpectomy with anterior plate fixation. OBJECTIVES: To evaluate the capability of an anterior plate to stabilize the reconstructed cervical spine under simulated physiologic motions, and to study the effects of fatigue loading. SUMMARY OF BACKGROUND DATA: Clinical studies have found high failure rates of multilevel anterior cervical plate fusions, indicating suboptimal stabilization. However, no biomechanical studies have been done to investigate the stabilizing capabilities of long-plate instrumentations in corpectomy models. METHODS: Seven fresh human cadaveric cervical spine specimens (C2-T1) were used. Flexibility tests consisted of flexion, extension, and bilateral torsion, and lateral bending, each with a pure moment of 0.25, 0.5, 0.75, and 1.0 Nm. Stabilizing potential indices [(MotionIntact-MotionInstrumented)/MotionIntact] for ranges of motion and neutral zones obtained from the flexibility tests, were measured when the specimen was intact and after one-level (C5) and three-level (C4, C5, and C6) corpectomies and anterior plate stabilizations). The stabilizing potential indices were re-measured after a 1000-cycle fatigue loading (1 Nm flexion and extension moments at C5 vertebra at 0.14 Hz). RESULTS: The differences in stabilizing potential indices of range of motion and neutral zone between one-level and three-level plates were not significant before fatigue. However, after fatigue, the stabilizing potential indices significantly decreased (P < 0.05) for the three-level model, but not for the one-level plate model. CONCLUSIONS: The capability of an anterior cervical plate to stabilize the spine after three-level corpectomy was significantly reduced with fatigue loading.

Adolescent↗
PubMed1999

Lumbar spine stability can be augmented with an abdominal belt and/or increased intra-abdominal pressure.

The increased intra-abdominal pressure (IAP) commonly observed when the spine is loaded during physical activities is hypothesized to increase lumbar spine stability. The mechanical stability of the lumbar spine is an important consideration in low back injury prevention and rehabilitation strategies. This study examined the effects of raised IAP and an abdominal belt on lumbar spine stability. Two hypotheses were tested: (1) An increase in IAP leads to increased lumbar spine stability, (2) Wearing an abdominal belt increases spine stability. Ten volunteers were placed in a semi-seated position in a jig that restricted hip motion leaving the upper torso free to move in any direction. The determination of lumbar spine stability was accomplished by measuring the instantaneous trunk stiffness in response to a sudden load release. The quick release method was applied in isometric trunk flexion, extension, and lateral bending. Activity of 12 major trunk muscles was monitored with electromyography and the IAP was measured with an intra-gastric pressure transducer. A two-factor repeated measures design was used (P < 0.05), in which the spine stability was evaluated under combinations of the following two factors: belt or no belt and three levels of IAP (0, 40, and 80% of maximum). The belt and raised IAP increased trunk stiffness in all directions, but the results in extension lacked statistical significance. In flexion, trunk stiffness increased by 21% and 42% due to 40% and 80% IAP levels respectively; in lateral bending, trunk stiffness increased by 16% and 30%. The belt added between 9% and 57% to the trunk stiffness depending on the IAP level and the direction of exertion. In all three directions, the EMG activity of all 12 trunk muscles increased significantly due to the elevated IAP. The belt had no effect on the activity of any of the muscles with the exception of the thoracic erector spinae in extension and the lumbar erector spinae in flexion, whose activities decreased. The results indicate that both wearing an abdominal belt and raised IAP can each independently, or in combination, increase lumbar spine stability. However, the benefits of the belt must be interpreted with caution in the context of the decreased activation of a few trunk extensor muscles.

Abdomen↗
PubMed1999

Subfailure injury affects the relaxation behavior of rabbit ACL.

OBJECTIVE: To study changes in viscoelasticity of a ligament due to an incomplete or a subfailure injury. DESIGN: An in vitro study of anterior cruciate ligament preparations. BACKGROUND: The viscoelastic properties are an inherent part of the physical characteristics of a ligament. An injury to a ligament alters both its elastic and viscous properties. Although the effects of several parameters on the mechanical properties of a ligament have been studied, there is no information in the literature concerning the effect of an incomplete or subfailure injury on its viscoelastic behavior. METHODS: Ten pairs of rabbit femur-anterior cruciate ligament-tibia specimens were used. A standardized relaxation (Relax) test was adopted to quantify the viscoelastic behavior, before and after a subfailure injury. One member of the pair was subjected to three sequential tests: Relax 1; Relax 2; and stretch to failure. The other member of the pair was subjected to other three tests: Relax 3; 80% subfailure injury, i.e. stretch of 80% of failure deformation; and Relax 4. RESULTS: We found that the relaxation test by itself (Relax 1 vs Relax 2), did not affect the viscoelasticity of the ligament. On the other hand, the 80% subfailure injury (Relax 3 vs Relax 4) affected the ligament viscoelastic behavior. The force was decreased by about 50% at time zero (10.46 vs 4.79 N, p = 0.014), and at 180 s (8.14 vs 4.11 N, p = 0.018). Fitting a three-element linear viscoelastic solid model to our data, we found the serial spring stiffness to decrease by about 50% (p = 0.01), the parallel spring remained unchanged, and there was a tendency for the dashpot coefficient to decrease (by 57%, p = 0.09). CONCLUSIONS: The 80% subfailure injury decreased the initial stiffness of the ligament, and tended to decrease its viscoelastic property. RELEVANCE: Subfailure or incomplete injuries of ligaments are more common than the complete injuries. The present study describes the effects, on both the elastic and viscous properties, of a ligament subjected to a subfailure injury.

Analysis of Variance↗
PubMed1999

A biomechanical comparison of different wrist external fixators with and without K-wire augmentation.

To compare stability of wrist external fixation, simulated unstable extra-articular distal radius fractures were created in 7 fresh-frozen cadaveric upper extremities and stabilized using 4 different external fixators. Physiologic muscle tension across the wrist was simulated by application of 40-N load distributed among the wrist tendons. Alternating loads of up to 100 N in flexion and extension of the wrist were applied during stability testing and 3-dimensional kinematics of the proximal and distal fracture fragments were determined using attached infrared light-emitting diodes and a 3-dimensional motion tracking system. Fracture stability was reassessed for each of the constructs after augmentation of the fracture fragments with a single dorsal transfixion K-wire. K-wire augmentation demonstrated a significant reduction in motion of the distal radial fragment of at least 40% in all 3 rotational planes. For flexion/extension, the reduction in motion was from 4.5 degrees to 2.6 degrees. For radial/ulnar deviation, the range of motion decreased from 3.0 degrees to 1.5 degrees. Rotational motion declined from an average of 3.2 degrees to 1.2 degrees. The addition of the single dorsal transfixion K-wire significantly improved stability of each of the 4 fixators in at least 1 of the 3 planes in which motion was measured. While we compared the most rigid with some of the least rigid external fixators, the data do not support an important difference in fracture fragment stability among the 4 fixators. The data much more strongly support the concept of K-wire augmentation for increasing stability of an unstable extra-articular distal radius fracture regardless of the type of external fixator that is used.

Biomechanical Phenomena↗
PubMed1999

Load-displacement behavior in a distal radial fracture model. The effect of simulated healing on motion.

External fixation of fractures of the distal end of the radius neutralizes external forces and maintains axial alignment during healing. As far as we know, there have been no biomechanical studies of the effects of early removal of the fixator in a partially healed fracture model. The purpose of the present study was to observe the load-displacement behavior of a distal radial fracture model in which we had simulated partial healing by injection of butyl-rubber caulk and augmented this simulated healing with Kirschner-wire fixation. Sixteen fresh-frozen hand-wrist-forearm specimens from cadavera were mounted in mid-rotation in resin pots, and a load was applied. An osteotomy was used to simulate the fracture. Relative motion at the site of the osteotomy was compared, with use of a three-dimensional Optotrak kinematic device, during physiological loading of six constructs with Kirschner-wire transfixion or outrigger fixation. In the experimental group, partial healing was simulated by injection of butyl-rubber caulk into the site of the osteotomy and testing with simulated muscle-loading was performed through a full range of motion of the wrist. No difference could be detected between the relative motion at the osteotomy sites that had been treated with standard fully augmented external fixation and that in the experimental group (p > 0.05). T test analysis revealed that motion was equivalent regardless of whether Kirschner-wire transfixion or outrigger fixation had been used (p = 0.62) and that all of the augmented constructs had significantly less relative motion than all of the nonaugmented constructs (p < 0.001).

Bone Wires↗
PubMed1998

Cervical spine models for biomechanical research.

Biomechanical models have been used for the understanding of the basic normal function and dysfunction of the cervical spine and for testing implants and devices. Biomechanical models can be broadly categorized into four groups: 1) Physical models, made of nonanatomic material (e.g., plastic blocks), are often used for the testing of spinal instrumentation when only the device is to be evaluated. 2) In vitro models consisting of a cadaveric spine specimen are useful in providing basic understanding of the functioning of the spine. Human specimens are more suitable for these models than are animal specimens whenever anatomy, size (for instrumentation), and kinematics are important. Animal specimens are less costly, easier to obtain, and often have less variability but should be used with care because of the absence of anatomic fidelity with the human. 3) In vivo animal models provide the means to model living phenomena, such as fusion, development of disc degeneration, instability, and adaptive responses in segments adjacent to spinal instrumentation. Choosing the appropriate animal is important. The appropriate animal should have spinal loading, kinematics, kinetics, vertebral size, and healing-fusion rates as similar to those in humans as possible. For better interpretation of in vivo animal experimental results, in vitro biomechanical study using the same animal cadaveric specimen is useful but has not been used routinely. 4) Computer models are developed from mathematical equations that incorporate geometry and physical characteristics of the human spine and may be advantageously used for problems that are difficult to model by other means. Examples are the changes in disc and vertebral stresses in response to graded transection of facet joints and the study of changes in endplate loading caused by disc degeneration. Because these models are purely mathematical, their validation is essential. Validation is best achieved by first incorporating high-quality geometry and physical characteristics of the human spine and then comparing the model predictions with experimental observations. Sometimes an enthusiastic researcher may use a computer model beyond its validation boundary, making the model's predictions unreliable. In general, it is important to remember that a biomechanical model, similar to any other model, represents only a certain aspect of the real living human being. The aspect chosen for representation should be selected with great care. The model should be designed to answer specifically the question asked. Its predictions are valid only within the boundaries of assumptions and limitations that it incorporates.

Animals↗
PubMed1998

Stability potential of spinal instrumentations in tumor vertebral body replacement surgery.

STUDY DESIGN: The multidirectional stability potential of anterior, posterior, and combined instrumentations applied at L1-L3 was studied after L2 corpectomy and replacement with a carbon-fiber implant. OBJECTIVES: To evaluate the biomechanical characteristics of short-segment anterior, posterior, and combined instrumentations in lumbar spine tumor vertebral body replacement surgery. SUMMARY OF BACKGROUND DATA: The biomechanical properties of many different spinal instrumentations have been studied in various spinal injury models. Only a few studies, however, investigate the stabilization methods in spinal tumor vertebral body replacement surgery. METHODS: Eight fresh frozen human cadaveric thoracolumbar spine specimens (T12-L4) were prepared for biomechanical testing. Pure moments (2.5 Nm, 5 Nm, and 7.5 Nm) of flexion-extension, left-right axial torsion, and left-right lateral bending were applied to the top vertebra in a flexibility machine, and the motions of the L1 vertebra with respect to L3 were recorded with an optoelectronic motion measurement system after reconditioning. The L2 vertebral body was resected and replaced by a carbon-fiber cage. Different fixation methods were applied to the L1 and L3 vertebrae. One anterior, two posterior, and two combined instrumentations were tested. Load-displacement curves were recorded and neutral zone and range of motion parameters were determined. RESULTS: The anterior instrumentation provided less potential stability than the posterior and combined instrumentations in all motion directions. The anterior instrumentation, after vertebral body replacement, showed greater motion than the intact spine, especially in axial torsion (range of motion, 10.3 degrees vs 5.5 degrees; neutral zone, 2.9 degrees vs. 0.7 degrees; P < 0.05). Posterior instrumentation provided greater rigidity than the anterior instrumentation, especially in flexion-extension (range of motion, 2.1 degrees vs. 12.6 degrees; neutral zone, 0.6 degrees vs. 6.1 degrees; P < 0.05). The combined instrumentation provided superior rigidity in all directions compared with all other instrumentations. CONCLUSIONS: Posterior and combined instrumentations provided greater rigidity than anterior instrumentation. Anterior instrumentation should not be used alone in vertebral body replacement.

Aged↗