Intradiscal Pressure Changes during Manual Cervical Distraction: A Cadaveric Study.

The objective of this study was to measure intradiscal pressure (IDP) changes in the lower cervical spine during a manual cervical distraction (MCD) procedure. Incisions were made anteriorly, and pressure transducers were inserted into each nucleus at lower cervical discs. Four skilled doctors of chiropractic (DCs) performed MCD procedure on nine specimens in prone position with contacts at C5 or at C6 vertebrae with the headpiece in different positions. IDP changes, traction forces, and manually applied posterior-to-anterior forces were analyzed using descriptive statistics. IDP decreases were observed during MCD procedure at all lower cervical levels C4-C5, C5-C6, and C6-C7. The mean IDP decreases were as high as 168.7 KPa. Mean traction forces were as high as 119.2 N. Posterior-to-anterior forces applied during manual traction were as high as 82.6 N. Intraclinician reliability for IDP decrease was high for all four DCs. While two DCs had high intraclinician reliability for applied traction force, the other two DCs demonstrated only moderate reliability. IDP decreases were greatest during moving flexion and traction. They were progressevely less pronouced with neutral traction, fixed flexion and traction, and generalized traction.

Primary and coupled motions after cervical total disc replacement using a compressible six-degree-of-freedom prosthesis.

This study tested the hypotheses that (1) cervical total disc replacement with a compressible, six-degree-of-freedom prosthesis would allow restoration of physiologic range and quality of motion, and (2) the kinematic response would not be adversely affected by variability in prosthesis position in the sagittal plane. Twelve human cadaveric cervical spines were tested. Prostheses were implanted at C5-C6. Range of motion (ROM) was measured in flexion-extension, lateral bending, and axial rotation under ± 1.5 Nm moments. Motion coupling between axial rotation and lateral bending was calculated. Stiffness in the high flexibility zone was evaluated in all three testing modes, while the center of rotation (COR) was calculated using digital video fluoroscopic images in flexion-extension. Implantation in the middle position increased ROM in flexion-extension from 13.5 ± 2.3 to 15.7 ± 3.0° (p < 0.05), decreased axial rotation from 9.9 ± 1.7 to 8.3 ± 1.6° (p < 0.05), and decreased lateral bending from 8.0 ± 2.1 to 4.5 ± 1.1° (p < 0.05). Coupled lateral bending decreased from 0.62 ± 0.16 to 0.39 ± 0.15° for each degree of axial rotation (p < 0.05). Flexion-extension stiffness of the reconstructed segment with the prosthesis in the middle position did not deviate significantly from intact controls, whereas the lateral bending and axial rotation stiffness values were significantly larger than intact. Implanting the prosthesis in the posterior position as compared to the middle position did not significantly affect the ROM, motion coupling, or stiffness of the reconstructed segment; however, the COR location better approximated intact controls with the prosthesis midline located within ± 1 mm of the disc-space midline. Overall, the kinematic response after reconstruction with the compressible, six-degree-of-freedom prosthesis within ± 1 mm of the disc-space midline approximated the intact response in flexion-extension. Clinical studies are needed to understand and interpret the effects of limited restoration of lateral bending and axial rotation motions and motion coupling on clinical outcome.

Restoration of spinal alignment and disk mechanics following polyetheretherketone wafer kyphoplasty with StaXx FX.

BACKGROUND AND PURPOSE: EPFs sustained during VCFs degrade the disk's ability to develop IDP under load. This inability to develop pressure in combination with residual kyphotic deformity increases the risk for adjacent vertebral fractures. We tested the hypothesis that StaXx FX reduces kyphosis and endplate deformity following vertebral compression fracture, restoring disk mechanics. MATERIALS AND METHODS: Eight thoracolumbar, 5-vertebrae segments were tested. A void was selectively created in the middle vertebra. The specimens were compressed until EPF and to a grade I-II VCF. PEEK wafer kyphoplasty was then performed. The specimens were then tested in flexion-extension (±6 Nm) under 400-N preload intact, after EPF, VCF, and kyphoplasty. Endplate deformity, kyphosis, and IDP adjacent to the fractured body were measured. RESULTS: Vertebral body height at the point of maximal endplate deformity decreased after EPF and VCF and was partially corrected after StaXx FX, remaining less than intact (P = .047). Anterior vertebral height decreased after VCF (P = .002) and was partially restored with StaXx FX, remaining less than intact (P = .015). Vertebral kyphosis increased after VCF (P < .001) and reduced after StaXx FX, remaining greater than intact (P = .03). EPF reduced IDP in the affected disk in compression-flexion loading (P < .001), which was restored after StaXx FX (P = 1.0). IDP in the unaffected disk did not change during testing (P > .3). CONCLUSIONS: StaXx FX reduced endplate deformity and kyphosis, and significantly increased anterior height following VCF. Although height and kyphosis were not fully corrected, the disk's ability to pressurize under load was restored.

Effect of annular incision type on the change in biomechanical properties in a herniated lumbar intervertebral disc.

The technique used to incise the disc during discectomy may play a role in the subsequent healing and change in biomechanical stiffness of the disc. Several techniques of lumbar disc annulotomy have been described in clinical reports. The purpose of this paper was to study the influence of annulotomy technique on motion segment stiffness using a finite element model. Four incision methods (square, circular, cross, and slit) were compared. The analyses showed that each of the annular incisions produced increase in motions under axial moment loadings with circular incision producing the largest change in the corresponding rotational motion. Under shear loading mode, cross and slit-type annular incisions produced slightly larger changes in the principal motions of the disc than square and circular incisions. All other incision types considered in the current study produced negligibly small increase in motion under rest of the loading conditions. In addition to annulotomy, when nucleotomy was also included in the analyses, once again cross and slit incisions produced larger change in motion under shear loading mode as compared to the other two incision types. A comparison between the four types of annular incisions showed that cross incision produced an increase in motion larger than those produced by the other three incisions under flexion/extension and lateral moment loading and both shear force loadings. Circular incision produced the largest increase in motion under axial moment load in comparison to those produced by square, cross, and slit incisions. Sagittal plane symmetry was influenced by the incision injury to the motion segment leading to coupled motions as well as increased facet loads. From the study it can be concluded that the increase inflexibility of the disc due to annulotomy depends on the type of annulotomy and the annulotomy also produce asymmetrical deformations leading to increased facet loading.

A frontal plane model of the lumbar spine subjected to a follower load: implications for the role of muscles.

Compression on the lumbar spine is 1000 N for standing and walking and is higher during lifting. Ex vivo experiments show it buckles under a vertical load of 80-100 N. Conversely, the whole lumbar spine can support physiologic compressive loads without large displacements when the load is applied along a follower path that approximates the tangent to the curve of the lumbar spine. This study utilized a two-dimensional beam-column model of the lumbar spine in the frontal plane under gravitational and active muscle loads to address the following question: Can trunk muscle activation cause the path of the internal force resultant to approximate the tangent to the spinal curve and allow the lumbar spine to support compressive loads of physiologic magnitudes? The study identified muscle activation patterns that maintained the lumbar spine model under compressive follower load, resulting in the minimization of internal shear forces and bending moments simultaneously at all lumbar levels. The internal force resultant was compressive, and the lumbar spine model, loaded in compression along the follower load path, supported compressive loads of physiologic magnitudes with minimal change in curvature in the frontal plane. Trunk muscles may coactivate to generate a follower load path and allow the ligamentous lumbar spine to support physiologic compressive loads.

Comparison of posterior and transforaminal approaches to lumbar interbody fusion.

STUDY DESIGN: A study of the transforaminal lumbar interbody fusion and the posterior lumbar interbody fusion techniques was performed. OBJECTIVES: To describe the transforaminal lumbar interbody fusion technique, and to compare operative data, including blood loss and operative time, with data from posterior lumbar interbody fusion technique. SUMMARY OF BACKGROUND DATA: The evolution of posterior lumbar fusion combined with anterior interbody fusion has resulted in increased fusion rates as well as improved reductions and stability. The transforaminal lumbar interbody fusion technique pioneered by Harms and Jeszensky offers potential advantages and provides a surgical alternative to more traditional methods. METHODS: In 13 consecutive months, two spinal surgeons performed 40 transforaminal lumbar interbody fusions and 34 posterior lumbar interbody fusion procedures. Data regarding blood loss, operative times, and length of hospital stay were recorded. These data were analyzed using analysis of variance to show any significant differences between the two techniques. To determine whether differences in measured variables were dependent on patient gender or number of levels fused, epsilon(chi2) analysis was used. RESULTS: No significant differences were found between transforaminal and posterior lumbar interbody fusions in terms of blood loss, operative time, or duration of hospital stay when a single-level fusion was performed. Significantly less blood loss occurred when a two-level fusion was performed using the transforaminal approach instead of the posterior approach (P < 0.01). Differences in measured variables for the two procedures were independent of patient age, gender, and the number of levels fused. There were no complications with the transforaminal approach, but the posterior approach resulted in multiple complications. CONCLUSIONS: In this comparison of patients receiving transforaminal lumbar interbody fusion versus posterior lumbar interbody fusion, no complications occurred with the transforaminal approach, whereas multiple complications were associated with the posterior approach. Similar operative times, blood loss, and duration of hospital stay were obtained in single-level fusions, but significantly less blood loss occurred with the transforaminal lumbar interbody approach in two-level fusions. The transforaminal procedure preserves the interspinous ligaments of the lumbar spine and preserves the contralateral laminar surface as an additional surface for bone graft. It may be performed at all lumbar levels because it avoids significant retraction of the dura and conus medullaris.

Pedicle morphology of the immature thoracolumbar spine.

STUDY DESIGN: Human vertebral morphologic data were compiled from anatomic skeletal collections from three museums. OBJECTIVES: To quantify the morphometric characteristics of the pedicles of the immature thoracolumbar spine. SUMMARY OF BACKGROUND DATA: Little is known of pedicle morphology of the immature spine as related to pedicle screw fixation. METHODS: A total of 75 anatomic skeletal specimens were acquired from C1 to L5 in the age range of 3 to 19 years. The data were collected and analyzed using a computerized video analysis system. Each vertebral pedicle was measured in the axial and sagittal planes. The measurements included the minimum pedicle width, the pedicle angle, the distance to anterior cortex, and anteroposterior and interpedicular spinal canal diameters. RESULTS: Wide variation in pedicle morphology between specimens at each vertebral level was found in the young population. In general, compared with the average adult data, a younger spine demonstrated a near uniform reduction in the linear pedicle dimensions at each vertebral level. Pedicles from the lower lumbar vertebrae attained dimensions acceptable for standard screw sizes at an earlier age than in the thoracic vertebrae. CONCLUSIONS: The data in this study indicates that pedicle screws may be used in the adolescent spine. However, care should taken to accurately ascertain pedicle size before surgery so that improper use of screws can be avoided. Growth of the pedicles in relation to the spinal canal indicates that the increase in pedicle size is lateral to the spinal canal.

The biomechanical effect of postoperative hypolordosis in instrumented lumbar fusion on instrumented and adjacent spinal segments.

STUDY DESIGN: Change in lumbar lordosis was measured in patients that had undergone posterolateral lumbar fusions using transpedicular instrumentation. The biomechanical effects of postoperative lumbar malalignment were measured in cadaveric specimens. OBJECTIVES: To determine the extent of postoperative lumbar sagittal malalignment caused by an intraoperative kneeling position with 90 degrees of hip and knee flexion, and to assess its effect on the mechanical loading of the instrumented and adjacent segments. SUMMARY OF BACKGROUND DATA: The importance of maintaining the baseline lumbar lordosis after surgery has been stressed in the literature. However, there are few objective data to evaluate whether postoperative hypolordosis in the instrumented segments can increase the likelihood of junctional breakdown. METHODS: Segmental lordosis was measured on preoperative standing, intraoperative prone, and postoperative standing radiographs. In human cadaveric spines, a lordosis loss of up to 8 degrees was created across L4-S1 using calibrated transpedicular devices. Specimens were tested in extension and under axial loading in the upright posture. RESULTS: In patients who underwent L4-S1 fusions, the lordosis within the fusion decreased by 10 degrees intraoperatively and after surgery. Postoperative lordosis in the proximal (L2-L3 and L3-L4) segments increased by 2 degrees each, as compared with the preoperative measures. Hypolordosis in the instrumented segments increased the load across the posterior transpedicular devices, the posterior shear force, and the lamina strain at the adjacent level. CONCLUSIONS: Hypolordosis in the instrumented segments caused increased loading of the posterior column of the adjacent segments. These biomechanical effects may explain the degenerative changes at the junctional level that have been observed as long-term consequences of lumbar fusion.

Load-carrying capacity of the human cervical spine in compression is increased under a follower load.

STUDY DESIGN: An experimental approach was used to test human cadaveric cervical spine specimens. OBJECTIVE: To assess the response of the cervical spine to a compressive follower load applied along a path that approximates the tangent to the curve of the cervical spine. SUMMARY OF BACKGROUND DATA: The compressive load on the human cervical spine is estimated to range from 120 to 1200 N during activities of daily living. Ex vivo experiments show it buckles at approximately 10 N. Differences between the estimated in vivo loads and the ex vivo load-carrying capacity have not been satisfactorily explained. METHODS: A new experimental technique was developed for applying a compressive follower load of physiologic magnitudes up to 250 N. The experimental technique applied loads that minimized the internal shear forces and bending moments, loading the specimen in nearly pure compression. RESULTS: A compressive vertical load applied in the neutral and forward-flexed postures caused large changes in cervical lordosis at small load magnitudes. The specimen collapsed in extension or flexion at a load of less than 40 N. In sharp contrast, the cervical spine supported a load of up to 250 N without damage or instability in both the sagittal and frontal planes when the load path was tangential to the spinal curve. The cervical spine was significantly less flexible under a compressive follower load compared with the hypermobility demonstrated under a compressive vertical load (P < 0.05). CONCLUSION: The load-carrying capacity of the ligamentous cervical spine sharply increased under a compressive follower load. This experiment explains how a whole cervical spine can be lordotic and yet withstand the large compressive loads estimated in vivo without damage or instability.

A follower load increases the load-carrying capacity of the lumbar spine in compression.

STUDY DESIGN: An experimental approach was used to test human cadaveric spine specimens. OBJECTIVE: To assess the response of the whole lumbar spine to a compressive follower load whose path approximates the tangent to the curve of the lumbar spine. SUMMARY OF BACKGROUND DATA: Compression on the lumbar spine is 1000 N for standing and walking and is higher during lifting. Ex vivo experiments show it buckles at 80-100 N. Differences between maximum ex vivo and in vivo loads have not been satisfactorily explained. METHODS: A new experimental technique was developed for applying a compressive follower load of physiologic magnitudes up to 1200 N. The experimental technique applied loads that minimized the internal shear forces and bending moments, made the resultant internal force compressive, and caused the load path to approximate the tangent to the curve of the lumbar spine. RESULTS: A compressive vertical load applied in the neutral lordotic and forward-flexed postures caused large changes in lumbar lordosis at small load magnitudes. The specimen approached its extension or flexion limits at a vertical load of 100 N. In sharp contrast, the lumbar spine supported a load of up to 1200 N without damage or instability when the load path was tangent to the spinal curve. CONCLUSIONS: Until this study, an experimental technique for applying compressive loads of in vivo magnitudes to the whole lumbar spine was unavailable. The load-carrying capacity of the lumbar spine sharply increased under a compressive follower load, as long as the load path remained within a small range around the centers of rotation of the lumbar segments. The follower load path provides an explanation of how the whole lumbar spine can be lordotic and yet resist large compressive loads. This study may have implications for determining the role of trunk muscles in stabilizing the lumbar spine.

Study on effect of graded facetectomy on change in lumbar motion segment torsional flexibility using three-dimensional continuum contact representation for facet joints.

Facet joints provide rigidity to the lumbar motion segment and thus protect the disk, particularly against torsional injury. A surgical procedure that fully or partially removes the facet joints (facetectomy) will decrease the mechanical stiffness of the motion segment, and potentially place the disk at risk of injury. Analytical models can be used to understand the effect of facet joints on motion segment stability. Using a facet joint model that represents the contact area as contact between two surfaces rather than as point contact, it was concluded that a substantial sudden change in rotational motion, due to applied torsion moment, was observed after 75 percent of any one of the facet joints was removed. Applied torsional moment loading produced coupled extension motion in the intact motion segment. This coupled motion also experienced a large change following complete unilateral facetectomy. Clinically, the present study showed that surgical intervention in the form of unilateral or bilateral total facetectomy might require fusion to reduce the primary torsion motion.

Human growth hormone transgene expression increases the biomechanical structural properties of mouse vertebrae.

STUDY DESIGN: Caudal vertebrae were obtained from male and female mice from two transgenic lines expressing an erythroid-specific human growth hormone transgene construct, and gender-matched, age-matched, non-transgenic control mice. OBJECTIVE: To characterize the effect of human growth hormone transgene expression on the biomechanical structural properties of caudal vertebrae in compression. SUMMARY OF BACKGROUND DATA: An increase in trabecular and cortical bone deposition caused by erythroid-specific human growth hormone transgene expression was demonstrated previously. METHODS: Compression tests were performed on individual caudal vertebrae (Ca4, Ca5, Ca6) obtained from male and female mice from two transgenic lines (TG420 and TG450) and nontransgenic control mice. Two age groups were evaluated: 12 weeks old and 6 months old. The data were used to obtain axial stiffness, maximum load, and energy to failure. RESULTS: Vertebrae from male TG420 transgenic mice produced significantly larger values for maximum load, energy to failure, and axial stiffness at both 12 weeks and 6 months in comparison with their age-matched non-transgenic male controls. Vertebrae from female TG420 transgenic mice produced similar responses at 6 months. Vertebrae from male TG450 transgenic mice showed significant increases in maximum load and energy to failure at 6 months. In general, the biomechanical properties of vertebrae were significantly larger in the 6-month age group than in the 12-week age group, and this increase was significantly greater in the transgenic mice than in the gender-matched control mice during the same time span. This process was also influenced by transgenic genotype and gender. CONCLUSIONS: Erythroid-specific production of human growth hormone in transgenic mice resulted in significant increases in biomechanical properties of their caudal vertebrae in compression. The changes in the biomechanical properties were influenced by genotype, age, and gender.

Tibial segmental defect repair: chondrogenesis and biomechanical strength modulated by basic fibroblast growth factor.

BACKGROUND: The effect of recombinant human basic fibroblast growth factor (bFGF) on cartilage development and bone biomechanical strength during healing of a tibial segmental defect was studied in the rat. Two reports on the effect of basic FGF administration during fracture healing and several reports on the effects of acidic FGF have documented different responses of callus cartilage to this important growth factor. This is the first report of the effect of bFGF on cartilage formation in the healing of a grafted segmental defect in the rat. METHODS: The tibiae of 80 male rats underwent segmental resection of the mid-diaphyseal region. One-half of this group consisted of controls that received insertion of an intramedullary wire with a coralline hydroxyapatite graft and Gelfoam without bFGF. The tibiae of the other half were treated identically but had the Gelfoam impregnated with 1 microgram bFGF. Animals were killed at 2, 4, and 8 weeks postoperatively. Histological sections were stained with toluidine blue to differentiate the cartilage. Areas of metachromatically stained extracellular matrix, cell areas, cell size, and cellularity were quantified by using image analysis. Unfixed treated and control tibiae were tested for bone failure strength by using four-point bending on an Instron machine. RESULTS: Control bone failure strength was significantly greater than bFGF-treated bones at 2 weeks and energy-to-failure was significantly decreased in treated bones at 2 weeks. Although strength increased with time in all groups, treated groups at 4 and 8 weeks did not differ from controls. Basic FGF treatment promoted an increase in the development of normal hyaline cartilage and vasculogenesis at 2 weeks as compared with controls. Total cartilage declined over time in all groups. Average cell size and cell number did not change with either treatment or time. Bone formation and healing was equivalent in treated and control groups at 8 weeks. CONCLUSIONS: The results indicate that bFGF released directly and initially but not continuously exerts a transient positive effect on hyaline cartilage formation at the expense of repair site strength and does not accelerate healing.

Heterozygous oim mice exhibit a mild form of osteogenesis imperfecta.

The oim strain of mice is one of several rodent models that exhibit an osteogenesis imperfecta (OI) phenotype. These mice have a mutation in the gene encoding alpha-2 chain of type I procollagen that prevents proper assembly of this propeptide with alpha-1 propeptides. Homozygous oim mice experience multiple bone fractures under standard laboratory animal housing conditions and are representative of moderate to severe forms of OI. Because fractures are not typically experienced by heterozygous oim mice, they have not been studied extensively. The present studies show that the organization of cortical bone is deficient in heterozygotes, exhibiting a morphology intermediate to specimens from homozygotes and wild-type mice. The biomechanical properties of femurs isolated from heterozygous oim mice are also intermediate to homozygotes and wild-type mice when tested in four-point bending. Although it is not possible to distinguish visually between heterozygous oim and wild-type mice, the quality and biomechanical properties of bone in heterozygotes is significantly reduced by twelve weeks of age. Heterozygous oim mice are useful as a model for a mild form of OI.

Geometric analysis of coronal decompensation in idiopathic scoliosis.

STUDY DESIGN: Frontal plane geometry of postoperative curves was analyzed using a geometric model to investigate the relationship between coronal decompensation and postoperative apical shifts from the center sacral line for various thoracic and lumbar Cobb angles. OBJECTIVE: To determine if a balanced spinal configuration is possible when the postoperative lumbar curve is larger than the thoracic curve, and to determine the limits on the postoperative magnitude of the lumbar curve relative to the thoracic curve beyond which a spinal configuration with acceptable balance cannot be achieved. SUMMARY OF BACKGROUND DATA: Previous studies have suggested that overcorrection of the primary thoracic curve may be the principal cause of coronal decompensation after selective thoracic correction and fusion in King Type II curves. Also, other causative factors, such as inappropriate selection of fusion levels and hook patterns, have been implicated as possible reasons for decompensation after Cotrel-Dubousset instrumentation for idiopathic scoliosis. METHODS: Postoperative thoracic curves of 20 degrees, 25 degrees, and 30 degrees were simulated on a model spine. For each thoracic Cobb angle, three left lumbar curves were simulated with the lumbar curve larger than thoracic by 5 degrees, 10 degrees, and 15 degrees. For each combination of thoracic and lumbar Cobb angles, spinal configurations corresponding to different lateral shifts of the thoracic and lumbar apical vertebrae from the center sacral line were obtained. RESULTS: For a given combination of postoperative thoracic and lumbar Cobb angles, there is an optimal range of postoperative lateral distance between the thoracic and lumbar apices (relative apical distance) that will maintain acceptable balance (decompensation < or = 10 mm). Smaller values of the relative apical distance will decompensate the spine. For a constant postoperative thoracic Cobb angle, the postoperative distance between the thoracic and lumbar apices needed to maintain a balanced spine increases with increasing postoperative lumbar Cobb angle. Similarly, for a constant difference between the postoperative thoracic and lumbar Cobb angles, the postoperative distance between the thoracic and lumbar apices needed to maintain a balance spine increases with increasing postoperative thoracic Cobb angle. For postoperative thoracic curves of 20 degrees-30 degrees, acceptable balance can be achieved when the magnitude of the postoperative lumbar curve is up to twice the thoracic curve as long as adequate postoperative relative apical distance can be maintained. CONCLUSIONS: Decompensation does not appear to be caused by the relative magnitudes of the postoperative thoracic and lumbar curves, but is a result of inadequate relative distance between the thoracic and lumbar apical vertebrae in the postoperative geometry.

Instability of the lumbar burst fracture and limitations of transpedicular instrumentation.

STUDY DESIGN: This study analyzed the changes in the load-displacement behavior of lumbar spine segments caused by burst fractures that were experimentally produced in fresh human cadaveric spines. The effect of three transpedicular surgical constructs on stability was investigated in each specimen. OBJECTIVES: To quantify the loss of mechanical stiffness caused by the injury, and to evaluate the stiffness of three transpedicular surgical constructs. SUMMARY OF BACKGROUND DATA: Although various investigators have studied the biomechanical characteristics of the burst fracture and surgical stabilization techniques, few have reported quantitative data on the three-dimensional biomechanical instability of these fractures. METHODS: Load-displacement data were acquired in flexion, lateral bending, and axial rotation for intact specimens, after the L1 burst fracture was created and after the T12-L2 segments were stabilized using Luque plates, VSP plates, and Isola rods with one transverse connector. RESULTS: Spines with burst fractures showed a bilinear load-displacement behavior with significant instability (loss of stiffness relative to intact) at low loads (up to 3 N.m) in flexion, lateral bending, and axial rotation. The loss of stiffness was greatest in axial rotation over the entire load range (up to 10 N.m). If posterior element injury also was present, a significantly larger loss of stiffness was observed in flexion and axial rotation. The three transpedicular constructs improved the stability of the injured spine beyond that of the intact spine in flexion and lateral bending at low loads. At high loads, they restored the stiffness to intact levels. However, in axial rotation they did not restore the stiffness to pre-injury level, particularly when the posterior column was disrupted. CONCLUSIONS: Reduction of the burst fracture returns the spine to its position of greatest inherent instability, essentially requiring the transpedicular instrumentation to be load bearing. To enhance mechanical stability, it may be necessary to augment the transpedicular construct, particularly when the posterior column is disrupted.

Limitations of the standard linear solid model of intervertebral discs subject to prolonged loading and low-frequency vibration in axial compression.

The purpose of this study was to answer the following questions: (1) Can the standard linear solid model for viscoelastic material simulate the influence of disc level and degeneration on the ability of a disc to withstand prolonged loading and low-frequency vibration? (2) How well does the SLS model explain the relationship between the ability of a disc to resist prolonged loading and its ability to resist dynamic loads and dissipate energy when subjected to low-frequency vibration? Responses of human thoracic and lumbar discs were measured in axial compression under a constant load, and for cyclic deformations at three frequencies. Parameters of the SLS model for each disc were determined by a least-squares fit to the experimental creep response. The model was subsequently used to predict the disc's response to cyclic deformations. The SLS model was able to qualitatively simulate the effects of disc level and degeneration on the ability of an intervertebral disc to resist both prolonged loading and low-frequency vibration. However, the model underestimated the stress relaxation, dynamic modulus and hysteresis of thoracic and lumbar discs subjected to low-frequency vibration. The SLS model was unable to explain the relationship between the ability of a disc to resist prolonged loading and its ability to resist dynamic loads and dissipate energy when subjected to low-frequency vibration. Although in the lumbar discs the steady-state predictions of the SLS model were significantly correlated to the experimental response, the strength of model predictions decreased with increasing frequency, particularly for hysteresis.

The role of the interosseous membrane and triangular fibrocartilage complex in forearm stability.

This study investigated the relative roles of the interosseous membrane (IOM) and triangular fibrocartilage complex (TFCC) in the transmission of force from the hand to the humerus. Our findings suggest a spectrum of forearm destabilizing injuries. The intact radius abutting the capitellum provides the primary restraint to proximal migration of the radius. After radial head excision, up to 7 mm of proximal radial migration can occur under axial compression. If the TFCC or the IOM alone is disrupted, little alteration in load or displacement is evident. When both the midportion of the IOM and TFCC are incompetent, however, further proximal radial migration occurs, the radial stump abuts the humerus, and load is shifted back to the radial column. These data suggest that the central portion of the IOM is the crucial structural subdivision within the IOM acting as a restraint to proximal radial migration. The TFCC also resists proximal radial migration and participates in load transfer. We propose that clinical migration of the radius under an axial load greater than 7 mm implies disruption of both the midportion of the IOM and TFCC.

Cross-sectional geometrical properties and bone mineral contents of the human radius and ulna.

The mechanical strength of the human radius and ulna depends on their geometrical and material properties. This study quantifies the cortical bone cross-sectional properties of the adult radius and ulna (cross-sectional area, thickness, centroids, area moments of inertia and section moduli) using computerized tomographic (CT) scanning coupled with image processing along the lengths of eight human cadaveric forearms. Bone mineral mass and apparent ash density were also quantified at serial locations. Sites of significant variation of selected geometric and mineral properties along the length of each forearm bone were determined. Our results show that interpolation of CT measurements made at 10 and 30% of the radial length in the radius and 30 and 90% of the radial length in the ulna can provide approximate geometric values over the 10-90% region. This information can be used to develop a protocol using the fewest sites to clinically assess changes in forearm bone geometry. Regression analyses did not show significant linear relationships between geometric properties and apparent cortical ash density. Thus, CT derived geometric properties are not helpful in estimating the extent of changes in bone density. Area moment of inertia results suggest that the junction of the middle and distal third of the radius, and the ulnar shaft region may have increased vulnerability to fractures. The former is likely due to the change in moment of inertia values, whereas the latter is due to the relatively small magnitude of cross-sectional moments along the ulnar shaft as compared to the proximal or distal ends. This is consistent with fracture patterns observed clinically when a single forearm bone is fractured: Galeazzi fracture of the radius and nightstick fracture of the ulna.

Acetabular micromotion as a measure of initial implant stability in primary hip arthroplasty. An in vitro comparison of different methods of initial acetabular component fixation.

Micromotion has been shown to affect bony ingrowth into cementless components. This study was designed both to quantitate initial micromotion at the prosthesis-periacetabular bony interface and to compare different methods of commonly employed acetabular component fixations, ie, a press-fit hemispherical titanium cup, a press-fit hemispherical titanium cup with one and two dome screws, a press-fit titanium hemispherical cup with three spikes, and a cemented chromium-cobalt cup. The press-fit component without screws demonstrated the greatest motion equaling 162 microns at the ilium, 97 microns at the publis, and 54 microns at the ischium. With one and two screws placed into the dome, the mean ileal displacement decreased by 28 microns (17%) and 36 microns (22%), respectively. Dome screw placement demonstrated a minimal effect at the pubis and ischium. Compared to the press-fit component without augmentation, the tri-spike motion was less at the pubis and ischium. The cemented prosthesis provided the least amount of motion in all three areas tested. This experiment demonstrates that the ilium provides the least amount of support to immediate acetabular fixation, while the pubis (anterior column) and ischium (posterior column) provide more stability. One dome screw does not affect the stability of a hemispherical prosthetic cup significantly. A two dome screw fixation provides an added method of support at the ilium, but fails to decrease motion at the pubis or ischium significantly. The tri-spike fixation does not restrict motion at the ilium to the extent as the dome screws, but its effect at the ischium and pubis is much more pronounced. The obvious difference between initial motion seen with cemented versus uncemented components may suggest that before surgery, patients may need a period of protected weight bearing until ingrowth has occurred.