Moments and forces during pedicle screw insertion. In vitro and in vivo measurements

STUDY DESIGN

Moments and forces during pedicle screw insertion were measured in vivo and in vitro and were correlated to several parameters of the screw-bone interface.

OBJECTIVES

To compare the in vitro and in vivo screw insertion loads and to relate these measurements to bone mineral density, pedicle size, and other screw parameters (material, diameter).

SUMMARY OF BACKGROUND DATA

The in vitro screw insertion torque has been correlated to the screw pullout forces and the number of cycles to ultimate interface failure. However, there are no comparable in vivo data.

METHODS

One hundred three pedicle screws were included in the study, 43 in vivo and 60 in vitro. Duel-energy x-ray absorptiometry boen mineral density data were available for 20 in vivo and 32 in vitro specimens. A custom-made sterilizable six-axis load cell was integrated into a torque wrench, enabling the recording of the applied moments and forces during screw insertion. Statistical analysis was performed to detect differences and correlations.

RESULTS

The mean in vivo insertion torque (1.29 Nm) was significantly greater than the in vitro value (0.67 Nm). The linear correlation between insertion torque and bone mineral density was significant for the in vitro data but not for the in vivo data. No correlation was observed between insertion torque and pedicle diameter. Two patterns of torque were observed during the insertion process.

CONCLUSIONS

There is a significant difference between the insertion loads measured in vivo and those measured in vitro. Additional research is needed to verify whether this method provides an indication of screw fixation quality.

Constrained testing conditions affect the axial rotation response of lumbar functional spinal units

STUDY DESIGN

Human cadaveric spine specimens were tested in axial rotation using constrained and unconstrained methods.

OBJECTIVES

To determine the degree to which constrained methods affect the response of the functional spinal unit in axial rotation at lumbar and lumbosacral levels.

SUMMARY OF BACKGROUND DATA

A substantial controversy exists in the literature regarding the appropriateness of different testing methods. No study has been found in which the effect of constraint on axial rotation behavior was objectively examined.

METHODS

Ten human cadaveric spine specimens (five L3-L4, five L5-S1) were tested in axial rotation, using both constrained and unconstrained methods. In the unconstrained test, pure moments were applied to the upper vertebra, and its complete three-dimensional motion was measured using an optoelectronic camera system. In the constrained test, the specimens were loaded in a fixed-axis servohydraulic test machine individually around five rotational axis positions within the vertebral body, and the rotational motion was measured.

RESULTS

The rotational angles in the constrained tests were not different among the five rotational axis positions. However, the maximum rotation from the five axis positions was approximately 40% greater than the minimum rotation, a significant difference. The axial rotational motion of the unconstrained tests was always less than the maximum rotation measured in the constrained test. However, the total rotational angle using the helical axis of motion was not significantly different from the constrained angles.

CONCLUSIONS

The large differences between maximum and minimum rotation angles demonstrate that the behavior of the functional spinal unit in axial rotation is sensitive to the axis's position but the location of the axis is not repeatable. This supports the use of unconstrained methods in spinal testing.

Compressive strength of interbody cages in the lumbar spine: the effect of cage shape, posterior instrumentation and bone density

One goal of interbody fusion is to increase the height of the degenerated disc space. Interbody cages in particular have been promoted with the claim that they can maintain the disc space better than other methods. There are many factors that can affect the disc height maintenance, including graft or cage design, the quality of the surrounding bone and the presence of supplementary posterior fixation. The present study is an in vitro biomechanical investigation of the compressive behaviour of three different interbody cage designs in a human cadaveric model. The effect of bone density and posterior instrumentation were assessed. Thirty-six lumbar functional spinal units were instrumented with one of three interbody cages: (1) a porous titanium implant with endplate fit (Stratec), (2) a porous, rectangular carbon-fibre implant (Brantigan) and (3) a porous, cylindrical threaded implant (Ray). Posterior instrumentation (USS) was applied to half of the specimens. All specimens were subjected to axial compression displacement until failure. Correlations between both the failure load and the load at 3 mm displacement with the bone density measurements were observed. Neither the cage design nor the presence of posterior instrumentation had a significant effect on the failure load. The loads at 3 mm were slightly less for the Stratec cage, implying lower axial stiffness, but were not different with posterior instrumentation. The large range of observed failure loads overlaps the potential in vivo compressive loads, implying that failure of the bone-implant interface may occur clinically. Preoperative measurements of bone density may be an effective tool to predict settling around interbody cages.

Interbody cage stabilisation in the lumbar spine: biomechanical evaluation of cage design, posterior instrumentation and bone density

We performed a biomechanical study on human cadaver spines to determine the effect of three different interbody cage designs, with and without posterior instrumentation, on the three-dimensional flexibility of the spine. Six lumbar functional spinal units for each cage type were subjected to multidirectional flexibility testing in four different configurations: intact, with interbody cages from a posterior approach, with additional posterior instrumentation, and with cross-bracing. The tests involved the application of flexion and extension, bilateral axial rotation and bilateral lateral bending pure moments. The relative movements between the vertebrae were recorded by an optoelectronic camera system. We found no significant difference in the stabilising potential of the three cage designs. The cages used alone significantly decreased the intervertebral movement in flexion and lateral bending, but no stabilisation was achieved in either extension or axial rotation. For all types of cage, the greatest stabilisation in flexion and extension and lateral bending was achieved by the addition of posterior transpedicular instrumentation. The addition of cross-bracing to the posterior instrumentation had a stabilising effect on axial rotation. The bone density of the adjacent vertebral bodies was a significant factor for stabilisation in flexion and extension and in lateral bending.

Interbody cage stabilisation in the lumbar spine

We performed a biomechanical study on human cadaver spines to determine the effect of three different interbody cage designs, with and without posterior instrumentation, on the three-dimensional flexibility of the spine. Six lumbar functional spinal units for each cage type were subjected to multidirectional flexibility testing in four different configurations: intact, with interbody cages from a posterior approach, with additional posterior instrumentation, and with cross-bracing. The tests involved the application of flexion and extension, bilateral axial rotation and bilateral lateral bending pure moments. The relative movements between the vertebrae were recorded by an optoelectronic camera system.

We found no significant difference in the stabilising potential of the three cage designs. The cages used alone significantly decreased the intervertebral movement in flexion and lateral bending, but no stabilisation was achieved in either extension or axial rotation. For all types of cage, the greatest stabilisation in flexion and extension and lateral bending was achieved by the addition of posterior transpedicular instrumentation. The addition of cross-bracing to the posterior instrumentation had a stabilising effect on axial rotation. The bone density of the adjacent vertebral bodies was a significant factor for stabilisation in flexion and extension and in lateral bending.

Significant roentgenographic parameters for evaluating the flexibility of acute thoracolumbar burst fractures. An in vitro study

Plain lateral radiographs in a neutral position were studied in ten acute thoracolumbar burst fractures produced by high speed impact on three vertebrae human cadaveric spine segments. Six linear geometric parameters were measured on each film. The ratio of each value in the neutral injured to the intact condition was correlated linearly with the motion parameters obtained from post-traumatic three-dimensional flexibility data (neutral zone NZ; range of motion ROM). Anterior unit height (vertebra+adjacent discs) had the highest correlation with the neutral zone and flexibility in all directions, especially flexion-extension (NZ, R2 = 0.93; flexion ROM, R2 = 0.86; extension ROM, R2 = 0.79) lateral bending (NZ, R2 = 0.83; ROM, R2 = 0.90) and right axial rotation (NZ, R2 = 0.53; ROM, R2 = 0.86). The deformation ratio (average height to depth) correlated most with the neutral zone in left axial rotation (R2 = 0.91) and right lateral bending (R2 = 0.92). Due to the high correlations obtained, these parameters should be evaluated in clinical situations to assess their effectiveness in predicting the instability of burst fractures. Ultimately, prospective clinical studies are required to verify their clinical utility.

Design and evaluation of a device for measuring three-dimensional micromotions of press-fit femoral stem prostheses

Implant micromotion is considered to be a major factor in the loosening of cementless total hip replacements. Translational micromotion at the bone-implant interface generally occurs in all three spatial directions. Under physiological loading, the interfacial micromotion consists of a cyclic amplitude and changes in the mean, which, in the cranio-caudal direction, represents subsidence of the prosthesis. Existing measurement strategies, which are based on dial gauges, extensometers, LVDTs, hall-effect transducers or strain gauge techniques provide information about only one component of the general three-dimensional micromovement. Moreover, in the majority of the studies, the data are difficult to interpret due to the measured motions being composed of interfacial micromotion and femoral strains. A new transducer was designed that allows the accurate measurement of all three isolated components of micromotion. An optoelectronic approach, based on silicon position-sensitive detectors (PSD) in combination with high precision mechanical parts, was chosen. To exclude thermodrifts during long-term testing, a thermistor was integrated in the sensor. Validation experiments on a precision positioning table indicated the high precision and resolution of the developed sensors. Furthermore, in-vitro tests on a standard press-fit prosthesis demonstrated the easy handling and reliability of the system.

The relative importance of vertebral bone density and disc degeneration in spinal flexibility and interbody implant performance. An in vitro study

STUDY DESIGN

An in vitro biomechanical investigation in the human lumbar spine focuses on the functional significance of vertebral bone density and intervertebral disc degenerations.

OBJECTIVE

To determine that interrelationship between vertebral bone density and intervertebral disc degeneration, their effect on normal spine motion, and their significance in the biotechnical performance of interbody fixation techniques.

SUMMARY OF BACKGROUND DATA

A relationship between vertebral bone density and intervertebral disc degeneration has been suggested, but a definitive relationship has not been established. The effect of vertebral bone density and intervertebral disc degeneration on interbody stabilization remains unknown despite the rapidly increasing use of this surgical method for patients with chronic low back pain.

METHODS

The vertebral bone density and intervertebral disc degeneration of 72 functional spinal units were determined using dual energy x-ray absorptiometry scans and macroscopic grading, respectively. A three-dimensional flexibility test was performed on 24 functional spinal units in the intact and stabilised conditions. The compressive behavior of the bone-implant interface was evaluated in 48 functional spinal units.

RESULTS

The vertebral bone density in moderately degenerated disc was significantly lower than at all other levels of intervertebral disc degeneration. Increasing intervertebral disc degeneration resulted in more axial rotation and less lateral bending. In flexion-extension and lateral bending, better vertebral bone resulted in significantly better stabilization. This trend was observed also in axial compression in which higher failure loads were observed with greater bone densities.

CONCLUSION

The authors conclude a significant relationship exists between bone density and disc degeneration, bone density is a highly important factor in the performance of interbody stabilization, and disc degeneration, is of moderate importance in signal motion.

Effects of posture and structure on three-dimensional coupled rotations in the lumbar spine. A biomechanical analysis

STUDY DESIGN

A biomechanical lumbar spine model was constructed to simulate three-dimensional spinal kinematics under the application of pure moments. Parametric analysis of the model allowed for the estimation of how much of the coupled motions could be predicted by the lumbar lordosis and the intrinsic mechanical properties of the spine.

OBJECTIVES

To evaluate the relative effects of lordosis and intrinsic mechanical spine properties on the magnitude and direction of coupled rotations.

SUMMARY OF BACKGROUND DATA

Clinical evidence suggests that abnormal coupled motion in the lumbar spine may be an indicator of low back disorders.

METHODS

The biomechanical lumbar spine model consisted of five vertebrae separated by intervertebral joints that provided three rotational degrees of freedom. In vitro experimental data, obtained from nine fresh-frozen (L1-S1) cadaveric specimens, were used to establish the mechanical properties of the intervertebral joints. Two different submodels were considered in simulating the three-dimensional intervertebral rotations in response to the applied moments. In the first, it was assumed that the coupled motions were generated solely as a result of the vertebral orientation caused by lordosis. In the second, additional intrinsic motion coupling was assumed.

RESULTS

Intervertebral coupling was partially predicted by lumbar lordosis; however, the inclusion of intrinsic mechanical coupling dramatically improved the simulation of the intervertebral rotations (root mean square error < 1 degree). Comparison of the results from the two models demonstrated that the lumbar lordosis and intrinsic mechanical properties of the spine had about an equal effect in predicting the coupling between axial rotation and lateral bending. In contrast, coupled flexion, associated with lateral bending, was almost fully accounted for by the presence of lumbar lordosis.

CONCLUSIONS

The lumbar lordosis and intrinsic mechanical properties of the spine were equally important in predicting the magnitude and direction of the coupled rotations.

Subfailure injury of the rabbit anterior cruciate ligament

Ligamentous injuries range in severity from a simple sprain to a complete rupture. Although sprains occur more frequently than complete failures, only a few studies have investigated the phenomena of these subfailure injuries. The purpose of our study was to document the changes in the load-deformation curve until the failure point, after the ligament has been subjected to an 80% subfailure stretch. Thirteen paired fresh rabbit bone-anterior cruciate ligament-bone preparations were used. One of the pairs (control) was stretched until failure; the other (experimental) was first stretched to 80% of the failure deformation of the control and then stretched to failure. Comparisons were made between the load-deformation curves of the experimental and control specimens. The nonlinear load-deformation curves were characterized by eight parameters: failure load (Ffail), failure deformation (Dfail), energy until failure (Efail), deformations measured at 5, 10, 25, and 50% of the failure load (D5, D10, D25, and D50, respectively), and stiffness measured at 50% of the failure force (K50). There were no significant differences in the values for Ffail, Dfail, and Efail between the experimental and control ligaments (p > 0.33). In contrast, the deformation values were all larger for the experimental than the control ligaments (p > 0.01). The deformations D5, D10, D25, and D50 (mean +/- SD) for the control were 0.36 +/- 0.13, 0.49 +/- 0.23, 0.81 +/- 0.35, and 1.23 +/- 0.41 mm. The corresponding deformations for the experimental ligaments were, respectively, 209, 186, 153, and 130% of the control values. K50 was also greater for the experimental ligament (125.0 +/- 41.7 N/mm compared with 108.7 +/- 31.4 N/mm, p < 0.03). These findings indicate that even though the strength of the ligament did not change due to a subfailure injury, the shape of the load-displacement curve, especially at low loads, was significantly altered. Under the dynamic in vivo loading conditions of daily living, this may result in increased joint laxity, additional loads being applied to other joint structures, and, with time, to joint problems.

Validity of the three-column theory of thoracolumbar fractures. A biomechanic investigation

STUDY DESIGN

This study validated the three-column theory of fractures by correlating the multidirectional instabilities and the vertebral injuries to each of the three columns, using a biomechanic trauma model.

OBJECTIVES

The objective was to validate the three-column theory as applied to the thoracolumbar fractures.

SUMMARY OF BACKGROUND DATA

The widely used three-column theory of fractures for classification and evaluation was based on retrospective analysis of radiographs. No biomechanic study, using realistic spinal fractures and multidirectional instability measurements, was available.

METHODS

Using 16 fresh cadaveric thoracolumbar human spine specimens, two groups of burst fractures were produced by either simple axial compression or flexion-compression, using a high-speed trauma model. Multidirectional flexibility was measured before and after the trauma, thus quantifying the instability of the burst fracture. Computed tomography scans were taken after the fracture, and a newly developed injury scoring scheme quantified the injuries to the anterior, middle, and posterior columns. Statistical correlations were obtained between the flexibility parameters and injuries to each of the three columns.

RESULTS

In the axial compression group, the middle column injury, compared with the other two columns, showed the highest correlations to eight of the nine flexibility parameters (average R2 = 0.77). In the flexion-compression group, again the middle column injury showed the highest correlations to eight of the nine flexibility parameters (average R2 = 0.85).

CONCLUSIONS

The results of this study supported the three-column theory of the thoracolumbar fractures and bolstered the concept of the middle column being the primary determinant of mechanical stability of this region of the spine.

Dynamic canal encroachment during thoracolumbar burst fractures

In the burst fractures seen clinically, often poor correlation exists between the neurological deficit and the canal encroachment measured on post-trauma radiographic images. The purpose of the present study was to determine whether the dynamic canal encroachment during the trauma is greater than the static canal encroachment posttrauma. We successfully produced burst fractures in nine of 15 fresh human cadaveric thoracolumbar spine specimens (T11-L1). The specimens were incrementally impacted in a high-speed trauma apparatus until fracture occurred. During the trauma, dynamic canal encroachments were measured using three specially designed transducers placed in the canal at the levels of the superior end-plates of the T12 and L1 and the T12/L1 disk. After the trauma, residual static spinal canal encroachments were measured from the radiographs of the specimens that were prepared with 1.6-mm diameter steel balls lining the canal in the midsagittal plane. We found that the average canal diameter was 16.6 +/- 1.3 mm and the static canal encroachment was 18.0% of the canal diameter. The corresponding dynamic canal encroachment was 33.3%. Thus, the dynamic canal encroachment was 85% more than the static measurement. The clinical significance of this study lies in providing awareness to the clinician that the dynamic canal encroachment is significantly greater than the static canal encroachment seen on posttrauma radiographs or computed tomography scans. The finding may also explain the clinical observation of poor correlation between the canal encroachment measured radiographically and the neurological deficit.

Disc degeneration affects the multidirectional flexibility of the lumbar spine

STUDY DESIGN

An in vitro biomechanical investigation using human lumbar cadaveric spine specimens was undertaken to determine any relationship between intervertebral disc degeneration and nonlinear multidirectional spinal flexibility.

SUMMARY OF BACKGROUND DATA

Previous clinical and biomechanical studies have not established conclusively such a relationship.

METHODS

Forty-seven discs from 12 whole lumbar spine specimens were studied under the application of flexion-extension, axial rotation, and lateral bending pure moments. Three flexibility parameters were defined (neutral zone (NZ), range of motion (ROM), and neutral zone ratio (NZR = NZ/ROM)) and correlated with the macroscopic and radiographic degeneration.

RESULTS AND CONCLUSIONS

In flexion-extension, the ROM decreased and NZR increased with degeneration. In axial rotation, NZ and NZR increased with degeneration. In lateral bending, the ROM significantly decreased and the NZR increased with degeneration. In all three loading directions, the NZR increased, indicating greater joint laxity with degeneration.

Axes of motion of thoracolumbar burst fractures

The neurological injury associated with thoracolumbar burst fractures may be due to the acute trauma event or due to chronic instability. For functional diagnosis and appropriate treatment, knowledge of the altered motion patterns of burst fractures may be helpful. Thirteen human cadaveric spine specimens were impacted at high speed in axial compression, resulting in 10 clinically relevant burst fractures. The specimens were subjected to a three-dimensional flexibility test (flexion, extension, bilateral lateral bending, and bilateral axial torque) before and after trauma. The vertebral motion across the burst fracture was described in terms of the helical axis of motion (HAM), a set of parameters that concisely and completely describes the three-dimensional motion. The vertebral rotations about the HAM increased significantly with burst fracture in all loading directions: flexion 8.1-17.7 degrees, extension 7.2-12.5 degrees, lateral bending 8.5-20.6 degrees (to one side), and axial torque 3.6-12.6 degrees (to one side). The HAM shifted significantly in a posterior direction with burst fracture in flexion (11-mm shift), extension (15-mm shift), and axial torque (11-mm shift). No other significant shifts in the HAM position were observed. The translation along the HAM and the orientation of the HAM did not change significantly with injury in any of the loading directions. The results provide clinically relevant information regarding the optimal treatment of thoracolumbar burst fractures. Specifically, fixation methods for burst fractures must be particularly stiff in lateral bending and axial rotation, the directions of greatest instability.

Mechanical behavior of the human lumbar and lumbosacral spine as shown by three-dimensional load-displacement curves

The lumbar region is a frequent site of spinal disorders, including low-back pain, and of spinal trauma. Clinical studies have established that abnormal intervertebral motions occur in some patients who have low-back pain. A knowledge of normal spinal movements, with all of the inherent complexities, is needed as a baseline. The present study documents the complete three-dimensional elastic physical properties of each lumbar intervertebral level from the level between the first and second lumbar vertebrae through the level between the fifth lumbar and first sacral vertebrae. Nine whole fresh-frozen human cadaveric lumbar-spine specimens were used. Pure moments of flexion-extension, bilateral axial torque, and bilateral lateral bending were applied, and three-dimensional intervertebral motions were determined with use of stereophotogrammetry. The motions were presented in the form of a set of six load-displacement curves, quantitating intervertebral rotations and translations. The curves were found to be non-linear, and the motions were coupled. The ranges of motion were found to compare favorably with reported values from in vivo studies.

Thoracolumbar burst fracture. A biomechanical investigation of its multidirectional flexibility

Assessment of clinical instability of thoracolumbar burst fractures remains controversial and subjective. The purpose of the study was to obtain objective measures of acute instability of these fractures. Thirteen fresh cadaveric human spine specimens (T11-L1) were subjected to high-speed axial trauma, resulting in burst fractures in 10 specimens. Multidirectional flexibilities were measured when the specimen was intact and after the trauma. The average ranges of motion of the burst fractures, measured as percentages of the corresponding intact values at 7.5 Newton-meters, were 202%, 403%, 266%, and 462% for flexion/extension, axial rotation, lateral bending, and tension/compression, respectively. For the neutral zone motion parameter, the motions of the burst fracture were even greater: 670%, 1650%, 779%, and 650%, respectively. All of the increases were significant (P < 0.05). The clinical significance of the study lies in its finding of high multidirectional acute instability of the thoracolumbar burst fracture, especially in axial rotation.

Quantitative discomanometry and acute disk injuries: an experimental model

Quantitative discomanometry is a study of intradiscal pressure changes during quantitative injection. The purpose of this study was to determine if correlations exist between discomanometric parameters and disk injuries. Sixteen three-vertebrae porcine spine segments, with two intervening disks, were subjected to standardized high-speed trauma. The injuries were documented by a radiographic injury score (RIS), using pre- and posttrauma lateral radiographs. An anatomic injury score (AIS) also was obtained, based on an anatomic dissection and mid-sagittal plane cuts of the frozen specimen. Before the cutting, each of the disks was subjected to quantitative discomanometry, providing pressure/volume curves. Significant negative correlations were found between the RIS and the maximum pressure sustained (R = -0.60, p < 0.001), and pressure/volume slope (R = -0.60, p < 0.001). Similar relationships were found between the AIS and the maximum pressure (R = -0.71, p < 0.001), and pressure/volume slope (R = -0.63, p < 0.001). This study suggests that quantitative discomanometry can be used to quantify disk injuries. Because the intradiscal pressurization mimics the physiology with weight bearing, its use as a measure of integrity of the end-plate-annulus-end-plate enclosure might be justified.

Functional radiographs of acute thoracolumbar burst fractures. A biomechanical study

The geometric changes of acute thoracolumbar burst fractures under extension and traction loadings were analyzed using functional radiographs. The injuries were produced in an in vitro high-speed impact model. The changes in nine geometric parameters (three angular and six linear) were analyzed from neutral posture to extension and traction positions. In the extended position, all parameters, except the posterior vertebral height and vertebral diameter, were significantly different from the neutral posture values. Also in extension, the posterior vertebral height, vertebral diameter, and posterior unit height were significantly different from their intact values. In the traction position, all nine geometric parameters changed significantly from the neutral posture, whereas only the vertebral diameter remained significantly different from its intact value. These findings demonstrated the treatment advantages of applying traction force to acute burst fractures in contrast to extension moments. Further, changes in the angular parameters due to motion from neutral to extension posture demonstrated that the acute flexibility of the three-vertebrae segment was contributed almost equally by the upper disc (35%), lower disc (27%), and fractured vertebra (38%).

Multidirectional instabilities of experimental burst fractures of the atlas

For the purpose of understanding the acute instability of a burst (Jefferson) fracture of the atlas, the authors produced the fractures experimentally and measured multidirectional flexibilities in seven cadaveric C0-C3 specimens. The flexibilities were measured by the authors' standardized method: they applied six types of physiologically pure moments (up to 1.5 Nm) and recorded the ensuing C0-C2 motions by stereophotogrammetry. The flexibility tests were performed before and after the production of the fracture. The greatest increase in flexibility due to the injury was in flexion-extension (+22.0 degrees, 41.7%). In lateral bending, the increase was 7.7 degrees, or 23.9%. The flexibility was mostly maintained in axial rotation (+4.8 degrees, 5.4%). The increase in motion was due to an increase in neutral zone in flexion-extension, and an increase in the elastic zone in lateral bending. These flexibility results of experimentally produced fractures reflect quite well the acute instabilities seen clinically.

Stress relaxation of the respiratory system in developing piglets

To characterize the effect of postnatal development on the viscoelastic behavior of the respiratory system, we quantified the amplitude and time course of stress relaxation in the lungs and chest wall of seven newborn and eight 8-wk-old anesthetized piglets. Stress relaxation was distinguished from other dissipative pressure losses by performing airway occlusions at various constant inspiratory flows and fitting the pressure decays that ensue during the occlusions to a double-exponential function. We found that the amplitude of stress relaxation related linearly to the increase in elastic recoil (and, by extension, in the volume) of the lungs, chest wall, and respiratory system during the inflations preceding the occlusions. On the average, the slope of this relationship was 38-44% lower in the 8-wk-old than in the newborn piglets for the lungs and was not different for the chest wall. The time course of stress relaxation, expressed as a time constant, was not influenced by age. Our results indicate that respiratory system viscoelasticity is sensitive to the geometric and structural changes experienced by the lungs during the period of rapid somatic growth that follow birth in most mammals.

The onset and progression of spinal injury: a demonstration of neutral zone sensitivity

Spinal injuries are a great cost to society and the afflicted individuals. It is well known that most spinal injuries are not bony fractures but rather soft tissue lesions falling in the 'subfailure' region. For the clinical diagnosis of spinal injuries, abnormal motion patterns under physiological loads are considered an important factor. The purpose of the present study was to determine the onset and progression of spinal injury, and compare the sensitivity of three motion parameters: neutral zone (NZ), elastic zone (EZ), and range of motion (ROM). Spinal injury was defined as a significant increase in any of the three motion parameters. A repeatable high-speed flexion-compression load vector was applied individually to six porcine cervical spine specimens. Several impacts of increasing severity were applied to each specimen. After each impact, flexion-extension motion was measured. Neutral zone was the residual deformation from the neutral position to the position under zero load at the start of the final load cycle. Elastic zone was the displacement from zero load to the maximum load on the final load cycle. Range of motion was the sum of the neutral and elastic zones. The first significant increase in motion was determined by the neutral zone parameter with few observable anatomic lesions on the specimens. This was the onset of spinal injury. The next significant motion increase was also determined by the neutral zone parameter. After this motion increase, termed the progression of injury, ligament ruptures were observed in some specimens. It was concluded that the neutral zone was the most sensitive motion parameter in defining the onset and progression of spinal injury.(ABSTRACT TRUNCATED AT 250 WORDS)

Three-dimensional mechanical properties of the thoracolumbar junction

The thoracolumbar junction region is a frequent site of spinal trauma. Accurate knowledge of the normal mechanical behavior of the intervertebral joints in this region is of importance to the clinician in treating the spinal injuries. The present study documented the complete three-dimensional motions of levels T11-T12 and T12-L1 in the thoracolumbar region. Pure moments of flexion/extension, bilateral axial torque, and bilateral lateral bending were applied to 11 three-vertebrae human cadaveric specimens (T11-L1) to a maximum of 7.5 Nm. Intervertebral motions were calculated using stereophotogrammetry and presented in the form of load-displacement curves, each containing three rotations and three translations at one intervertebral level. Average +/- SD flexion, extension, axial rotation, and lateral bending ranges of motion to one side were 2.7 +/- 1.3 degrees, 2.4 +/- 1.3 degrees, 1.8 +/- 0.7 degrees, and 3.5 +/- 1.1 degrees, respectively, at level T11-T12. The same ranges of motion at T12-L1 were 2.9 +/- 1.4 degrees, 3.9 +/- 1.4 degrees, 1.2 +/- 0.7 degrees, and 3.7 +/- 1.1 degrees, respectively. The extension and axial rotation ranges of motion at level T11-T12 were found to be significantly different than the same motions at T12-L1. The different geometry in the facet joints explains these observed differences in the mechanical behavior of T11-T12 and T12-L1.

Euler stability of the human ligamentous lumbar spine. Part II: Experiment

The lateral backing and postbuckling behaviour of the intact and injured whole human lumbar spine was experimentally studied using six fresh cadaveric specimens. The ligamentous lumbar spine was loaded in axial compression and the lateral rotation of each vertebra was recorded. At the point of the load application, the most superior vertebrae, the specimens were constrained to move in the frontal plane since sagittal plane buckling will not occur due to the lumbar lordosis. The average load required to buckle an intact whole lumbar specimen was 88 N, and significantly decreased with injury. Once the spines had buckled, the postbuckling behaviour was recorded. These results were compared to theoretical predictions of a model (see Part I). The model was demonstrated to be in excellent agreement with the experimental results.

The effect of injury on rotational coupling at the lumbosacral joint. A biomechanical investigation

The lumbosacral joint is frequently indicated as a source of low-back pain, a cause of which may be abnormal patterns of vertebral motions. The goal of this study was to describe the influence of injury on the coupled motions of the L5-S1 joint in a human cadaveric model. Nine whole lumbosacral spine specimens were studied under the application of flexion, extension, left/right axial torque and right/left lateral bending pure moments. Injuries to the posterior ligaments, intervertebral disc, and articular facets at L5-S1 were produced, and the motion at L5-S1 was determined after each sequential injury. No significant coupled rotations were observed under flexion or extension moments. Under axial torque, lateral rotation at L5-S1 occurred to the same side as the applied torque and increased significantly only after injury to the intervertebral disc. Also coupled to axial torque was flexion rotation in the intact specimen, which became extension rotation after facetectomy. Under lateral bending moments, coupled axial rotation was to the opposite side of the applied moment and increased significantly only after removal of the facets of L5. Based on these results, it was concluded that intervertebral disc most resisted the coupled motion of lateral rotation under the application of axial torque, whereas the articular facets most resisted the coupled axial rotation under the application of lateral bending at the lumbosacral joint. Also, the facets were the structures that produced the flexion rotation of L5 on S1 under axial torque loading.

Biomechanical stability of five pedicle screw fixation systems in a human lumbar spine instability model

In this study, the three-dimensional biomechanical stabilizing capabilities of five pedicle screw fixation systems and facet screw fixation were determined. These systems were the Ace device without and with transverse wiring, AO device, CD device, Steffee plate, Wiltse device with single and double rods, and facet screw fixation. All systems were applied to the L5S1 level in a human in vitro spine rendered unstable by transection of the posterior ligaments and transverse holes drilled through the intervertebral disc. There were no statistically significant differences in the biomechanical stability provided by any of the pedicle screw devices, where stability was defined as the average stiffness from the load-displacement curve. All devices restored motion to less than 50% of intact levels under flexion moments. In extension, all devices, except the facet screw method, restored motion to below intact levels. In lateral bending, all devices restricted motion to less than 50% of intact motion. Under axial torque, the CD device provided the least motion while the AO device did not restore motion to the intact level.