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PubMed2005

Heterogeneous three-dimensional strain fields during unconfined cyclic compression in bovine articular cartilage explants.

Articular cartilage provides critical load-bearing and tribological properties to the normal function of diarthrodial joints. The unique properties of cartilage, as well as heterogeneous deformations during mechanical compression, are due to the nonuniform microstructural organization of tissue components such as collagens and proteoglycans. A new cartilage deformation by tag registration (CDTR) technique has been developed by the authors to determine heterogeneous deformations in articular cartilage explants. The technique uses a combination of specialized MRI methods, a custom cyclic loading apparatus, and image processing software. The objective of this study was to use the CDTR technique to document strain patterns throughout the volume of normal bovine articular cartilage explants during cyclic unconfined compression at two physiologically-relevant applied normal stress levels (1.29 and 2.57 MPa). Despite simple uniaxial cyclic compressive loading with a flat, nonporous indenter, strain patterns were heterogeneous. Strains in the thickness direction (E(yy)) were compressive, varied nonlinearly with depth from the articular surface from a maximum magnitude of 11% at the articular surface, and were comparable despite a 2-fold increase in applied normal stress. Strains perpendicular to the thickness direction (E(xx) and E(zz)) were tensile, decreased linearly with depth from the articular surface from a maximum of 7%, and increased in magnitude 2.5-fold with a 2-fold increase in applied normal stress. Shear strains in the transverse plane (E(xz)) were approximately zero while shear strains in the other two planes were much larger and increased in magnitude with depth from the articular surface, reaching maximum magnitudes of 2% at the articular cartilage-subchondral bone interface. In general, strain patterns indicated that cartilage osteochondral explants exhibited depth-dependent nonisotropic behavior during uniaxial cyclic loading. These results are useful in verifying constitutive formulations of articular cartilage during cyclic unconfined compression and in characterizing the micromechanical environment likely experienced by individual chondrocytes throughout the tissue volume.

Animals↗

PubMed2006

Surface detail, compressive strength, and dimensional accuracy of gypsum casts after repeated immersion in hypochlorite solution.

STATEMENT OF PROBLEM: One method used to disinfect a dental cast is immersion in a disinfecting solution. Repeated immersion of a cast in disinfectant may affect surface detail as well as other physical properties. PURPOSE: The purpose of this study was to evaluate the changes in surface detail quality, dimensional accuracy, and wet compressive strength of dental casts as a result of repeated disinfection in slurry containing 0.525% sodium hypochlorite solution. MATERIAL AND METHODS: Two different test specimen configurations were used, one for evaluating surface detail quality and dimensional accuracy and the other for compressive strength. Sixty specimens each were made of type III and type IV dental stone (Excalibur). Thirty specimens were immersed in slurry, a supernatant solution of calcium sulfate in distilled water (control casts), and 30 in slurry with sodium hypochlorite (test casts) solutions for 30 minutes and air dried for 24 hours. This process was repeated 7 times before testing. Linear dimensional change, surface detail quality, and wet compressive strength were determined according to American National Standards Institute/American Dental Association (ANSI/ADA) standards. The data were analyzed using 2-way analysis of variance and 2-sample independent t tests (alpha=.05). RESULTS: For specimens prepared with type III and type IV stone, treatment by immersion in slurry (control casts) and in slurry with sodium hypochlorite (test casts) resulted in an increase (P<.0001) in linear dimensional change. The type IV casts treated with slurry with sodium hypochlorite showed significantly less (P<.0001) mean linear dimensional change (0.025%) compared with type III stone casts (0.063%), and the values remained within the ANSI/ADA specified standards. Both test solutions caused some degree of erosion or damage to the surface quality of casts made from type III and type IV stones. However, the difference between the control and test casts was not significant. Slurry with 0.525% sodium hypochlorite solution decreased the compressive strength of both types of stones significantly (P<.001) compared with distilled water slurry. However, the values remained close to ANSI/ADA standards. CONCLUSION: The results demonstrated that repeated immersion of type III and type IV stone specimens in slurry with distilled water and slurry with 0.525% sodium hypochlorite, along with drying in air, caused a significant increase in linear dimension and a significant decrease in wet compressive strength. Although both the solutions caused some degree of damage to surface details for type III and type IV stones, the difference was not significant.

Calcium Sulfate↗

PubMed2006

Three-level bilateral pediculolysis following osteoporotic lumbar compression fracture.

BACKGROUND CONTEXT: Osteoporotic compression fractures frequently occur at the thoracolumbar junction as a result of anterior column failure. Fractures of the pedicles are much less common and are not known to be associated with a prior compression fracture. Bilateral pedicle fractures over several consecutive lumbar levels in an osteoporotic elderly patient have not been previously reported. PURPOSE: To draw attention to this unusual case and to review the relevant literature. STUDY DESIGN: A clinical case report of bilateral fractures of the pedicles from L3 through L5 in an 83-year-old male 2 years after an osteoporotic L3 compression fracture presenting with low back pain. METHODS: An 83-year-old male presented with low back pain two years after sustaining osteoporotic compression fracture at L3 due to a fall. He had another minor fall and his radiographic workup revealed bilateral fractures of the pedicles of the L3, L4, and L5 vertebrae. The patient was treated nonoperatively. RESULTS: The patient's symptoms improved without surgical intervention. Subsequent radiographic evaluation with plain films, computed tomography, and bone scan demonstrated union of the fractured pedicles. CONCLUSIONS: In this uncommon case of bilateral lumbar pedicle fractures over three consecutive levels, isolated failure of the posterior rather than the anterior column occurred. This unusual fracture pattern may have been precipitated by the previous vertebral compression fracture. Nonsurgical management may result in acceptable clinical outcome.

Aged, 80 and over↗

PubMed2006

Clinical evaluation of traumatic central cord syndrome: emphasis on clinical significance of prevertebral hyperintensity, cord compression, and intramedullary high-signal intensity on magnetic resonance imaging.

BACKGROUND: We evaluated the prognostic and clinical value of radiological findings including prevertebral hyperintensity (HI), cord compression, intramedullary high-signal intensity (IMHSI) and instability in patients with traumatic central cord syndrome without evidence of fracture and dislocation. METHODS: The radiological and clinical findings of 23 patients who had undergone surgery between 1996 and 2002 were reviewed retrospectively. All of the patients underwent dynamic motion study and magnetic resonance (MR) imaging after trauma. Neurologic status was evaluated with American Spinal Injury Association motor score pre- and postoperatively and compared with the radiological findings. Anterior decompression and fusion were performed in 12 patients with 1- or 2-level lesions, and posterior decompression was done for 11 cases of multilevel lesions. RESULTS: Prevertebral HI was found in 17 patients. Among them, instability was revealed in 11 patients. There was significant correlation between prevertebral HI and instability (P = .014). Cord compression was found in varying degrees in all patients on MR imaging. Intramedullary high-signal intensity was found preoperatively in 19 (83%) of 23 patients, and it was revealed at the most compressed level of the spinal cord in all cases. The neurologic level was consistent with the level of instability (100%), IMHSI (95%), and cord compression (87%). Mean American Spinal Injury Association motor scores in patients with instability were lower than in those without instability (P < .05). CONCLUSIONS: The presence of prevertebral HI, IMHSI, and cord compression influenced the neurologic status of the patients. The instability was significantly associated with poor prognosis for neurologic outcome. Prevertebral HI on T2 MR imaging may be a possible indicator of instability in patients with central cord syndrome after hyperextension injury.

Adult↗

PubMed2005

Texture analysis in digitally-acquired echocardiographic images: the effect of JPEG compression and video storage.

The analysis of texture in video-stored echocardiographic images is an established method to characterize myocardial pathologies. We investigated whether or not texture parameters calculated from video-stored images and those derived from the joint photographic expert group (JPEG) format compressed data are equivalent to those calculated from uncompressed digital images. Texture parameters were calculated using uncompressed digital data, images stored on videotape, and three forms of compressed digital data (baseline JPEG, JPEG 2000 and lossless JPEG 2000). Video storage heavily affected most texture parameters. Although first-order texture parameters derived from JPEG-compressed images were generally equivalent to those derived from the uncompressed data, several second-order parameters differed significantly. We conclude that texture of video-stored images is not comparable to that of digitally-stored images and that JPEG compression changes important second-order texture parameters. This observation should be taken into account when analyzing texture of modern image data (uncompressed or compressed) and comparing the results with earlier studies utilizing video-stored data.

Data Compression↗

PubMed2003

Optical determination of anisotropic material properties of bovine articular cartilage in compression.

The precise nature of the material symmetry of articular cartilage in compression remains to be elucidated. The primary objective of this study was to determine the equilibrium compressive Young's moduli and Poisson's ratios of bovine cartilage along multiple directions (parallel and perpendicular to the split line direction, and normal to the articular surface) by loading small cubic specimens (0.9 x 0.9 x 0.8 mm, n =15) in unconfined compression, with the expectation that the material symmetry of cartilage could be determined more accurately with the help of a more complete set of material properties. The second objective was to investigate how the tension-compression nonlinearity of cartilage might alter the interpretation of material symmetry. Optimized digital image correlation was used to accurately determine the resultant strain fields within the specimens under loading. Experimental results demonstrated that neither the Young's moduli nor the Poisson's ratios exhibit the same values when measured along the three loading directions. The main findings of this study are that the framework of linear orthotropic elasticity (as well as higher symmetries of linear elasticity) is not suitable to describe the equilibrium response of articular cartilage nor characterize its material symmetry; a framework which accounts for the distinctly different responses of cartilage in tension and compression is more suitable for describing the equilibrium response of cartilage; within this framework, cartilage exhibits no lower than orthotropic symmetry.

Animals↗

PubMed1987

Cervicomedullary compression in young patients with achondroplasia: value of comprehensive neurologic and respiratory evaluation.

We studied prospectively 26 young patients with achondroplasia to test two hypotheses: that respiratory problems may be the result of occult spinal cord compression, and that achondroplastic patients with cord compression might have occult respiratory abnormalities. Respiratory abnormalities were present in 85%, the majority caused by a primary problem of the pulmonary system, such as small thoracic cage or obstructed airway. Three patients had hypoxemia, recurrent cyanotic spells, and episodes of respiratory distress explainable only by cervicomedullary cord compression; in each patient, respiratory problems were alleviated by decompressive surgery. Another six patients with cervicomedullary compression had, in addition, at least one primary pulmonary cause of respiratory problems. After decompressive surgery the respiratory problems improved in three and were unchanged in three. Reconstructed sagittal CT images proved the most sensitive technique for detecting craniocervical stenosis as a cause of cervicomedullary cord compression, although some degree of stenosis was present in nearly all of the patients.

Achondroplasia↗

PubMed2001

Compressive properties of cancellous bone defects in a rabbit model treated with particles of natural bone mineral and synthetic hydroxyapatite.

A rabbit model was developed to evaluate the compressive mechanical properties of cancellous bone defects treated with particles of selected bone graft substitute materials. A novel feature of the model was the precise retrieval of the site of implantation. A notable finding was a 9-fold increase in the modulus of elasticity of the defect implanted with a synthetic hydroxyapatite material after 26 weeks when compared to the modulus of the trabecular bone normally at the site. The compressive modulus of lesions treated with particles of a natural bovine bone mineral (anorganic bovine bone) was closer to the normal modulus of the cancellous bone at the site. While the compressive strength of the anorganic bone particles was less than that of normal bone, the site implanted with the bone mineral particles achieved compressive strength greater than normal after 6 weeks. Moreover, the anorganic bone particles accelerated the increase in strength of the lesion, at 6 weeks exceeding the strength achieved by the untreated defect after 26 weeks. The potential problem associated with the disparity in the compressive modulus between sites implanted with the synthetic HA particles and surrounding bone is discussed.

Animals↗

PubMed1989

Bilateral optic nerve compression as a mechanism for the Foster Kennedy syndrome.

An 11-year-old boy with a juvenile nasopharyngeal angiofibroma developed optic disc pallor in one eye and optic disc edema in the other eye (Foster Kennedy Syndrome [FKS]). The mechanism was believed to be bilateral, asymmetric optic nerve compression. Review of the 36 previously reported cases of FKS revealed that 12 cases (33%) were probably also caused by bilateral optic nerve compression. Only eight (22%) of the cases satisfied Foster Kennedy's original hypothesis for the pathogenesis of his syndrome, namely, direct compression of one optic nerve causing atrophy and increased intracranial pressure causing contralateral papilledema. In 15 (41%) reported cases of FKS, descriptions were inadequate to determine a mechanism, while two (5%) were probably caused by long-standing increased intracranial pressure without direct optic nerve compression. The authors believe that as sophisticated imaging permits earlier diagnosis and more precise localization, most future cases of FKS caused by mass lesions will be found to result from bilateral direct optic nerve compression.

Child↗

PubMed2003

Characterization and in vivo evaluation of ocular bioadhesive minitablets compressed at different forces.

The influence of the compression force on the physical properties, the in vitro release and the in vivo behavior of ocular minitablets is evaluated in the present study. The bioerodible minitablets (Ø 2 mm, 6 mg) were produced at different compression forces. The crushing strength, friability, water uptake, hydration and swelling of the minitablets both in vitro as well as in vivo after application in the cul-de-sac were evaluated. The friability remained below 1% only for the minitablets made at 0.500 and 0.750 kN. The crushing strength measured was 3.53+/-0.98, 12.34+/-1.69 and 18.64+/-2.37 N for minitablets made at 0.250, 0.500 and 0.750 kN, respectively. The full hydration time equalled 20 and 30 min for minitablets compressed at 0.250 kN and 0.500-0.750 kN, respectively. Increasing the compression force resulted in a decreased swelling capacity. The in vivo release was evaluated in healthy volunteers using a non-invasive method to measure the apparent sodium fluorescein concentration in the tearfilm-cornea compartment as a function of time. The longest residence time of the fluorescent tracer at the administration site was obtained by the minitablets compressed at 0.750 kN. The in vitro release was evaluated with three different dissolution methods: the reciprocating cylinder method, vials in an oscillatory shaking bath and a static method with vials. The best correlation with the in vivo behavior of the matrix minitablets was obtained with the shaking bath method.

Adhesives↗

PubMed2001

Intervertebral disc herniation: studies on a porcine model exposed to highly repetitive flexion/extension motion with compressive force.

OBJECTIVE: To determine whether repeated motion with low magnitude joint forces, and flexion/extension moments consistently produce herniation in a non-degenerated, controlled porcine spine motion segment. DESIGN: Combined loading (flexion/extension motions and compressive forces) was applied to in vitro porcine functional spinal units. Biomechanical and radiographic characteristics were documented. BACKGROUND: While most studies performed in vitro have examined uniaxial or fixed position loading to older specimens, there have been few studies that have examined whether 'healthy' intervertebral discs can be injured by low magnitude repeated combined loading. METHODS: Porcine cervical spine motion segments (C3-C4) were mounted in a custom jig which applied axial compressive loads with pure flexion/extension moments. Dynamic testing was conducted to a maximum of 86400 bending cycles at a rate of 1 Hz with simultaneous torques, angular rotations, axial deformations recorded for the duration of the test. RESULTS: Herniation (posterior and posterior-lateral regions of the annulus) occurred with relatively modest joint compression but with highly repetitive flexion/extension moments. Increased magnitudes of axial compressive force resulted in more frequent and more severe disc injuries. CONCLUSIONS: The results support the notion that intervertebral disc herniation may be more linked to repeated flexion extension motions than applied joint compression, at least with younger, non-degenerated specimens. Relevance. While intervertebral disc herniations are observed clinically, consistent reproduction of this injury in the laboratory has been elusive. This study was designed to examine the biomechanical response and failure mechanics of spine motion segments to highly repetitive low magnitude complex loading.

Animals↗

PubMed2003

The effect of cyclic compression on the mechanical properties of the inter-vertebral disc: an in vivo study in a rat tail model.

OBJECTIVE: To assess the changes in the mechanical properties of inter-vertebral discs in vivo following static and cyclic compressive loading of different frequencies. DESIGN: An in vivo biomechanical study using a rat-tail model of the inter-vertebral disc.Background. Mechanical loading has been suggested as playing a major role in the etiology of disc degeneration, but the relationship is still not fully understood. METHODS: Sixty Sprague-Dawley rats were subject to daily compressive stress via pins inserted in the 6th and 7th caudal vertebrae over a two-week loading period. Animals were randomly divided into a sham group (pin insertion, no loading), a static loading group, or cyclic loading groups of 0.5, 1.5, or 2.5 Hz. Loading was applied for 1 h each day from the 3rd to 17th day following pin insertion, and the angular compliance, angular laxity, and inter-pin distance were measured in vivo at days 0, 3, 10 and 17. RESULTS: Changes in the inter-vertebral disc height depended on the frequency of loading, with the decrease in disc height in the static compression group significantly greater than that in all other groups, whereas the decrease in the 1.5 Hz cyclic compression group was significantly smaller than that in all other compression groups. CONCLUSIONS: Changes in disc properties depend on both the total load exposure and the frequency of loading. Cyclic loading in general produced less marked changes than static loading, but loading at particular frequencies may result in more severe changes. RELEVANCE: Previous studies have shown the in vivo changes in the mechanical properties of inter-vertebral discs to depend on the magnitude and duration of loading. In this study, a frequency dependent response to cyclic loading is also demonstrated.

Animals↗

PubMed2003

Axial compression force measurement acting across the strut graft in thoracolumbar instrumentation testing.

OBJECTIVE: Current recommendations for spinal implant testing do not consider the determination of axial compression forces of the overbridging implant on the strut graft. No direct data exist on the influence of load transfer through the strut graft and of the kind of instrumentation, especially in thoracolumbar corpectomy models. DESIGN: Therefore in this biomechanical in vitro study a method for measurement of the axial compression force acting across the strut graft in different thoracolumbar instrumentations was developed. METHODS: In this in vitro study, a corpectomy model was simulated and anterior, posterior and combined short fixation devices currently available were tested under pure moments to evaluate their biomechanical stabilizing characteristics. Range of motion, neutral zone and the axial compressive force acting on the strut graft were measured continuously in the three primary directions. RESULTS: Without loads, the combined stabilization and followed by anterior instrumentation created a higher axial compression force than the dorsal instrumentation on the strut graft. Especially during maximal extension there was no axial compression of the dorsal instrumentation on the strut graft, which resulted in an increase of the range of motion. CONCLUSION: The feasibility of the new method was demonstrated in this study. For the purpose of standardization and comparison it should be considered in spinal implant testing.

Aged↗

PubMed1997

Active compression-decompression cardiopulmonary resuscitation in standing position over the patient: pros and cons of a new method.

Active compression-decompression cardiopulmonary resuscitation (ACD-CPR) has been introduced to improve outcome of CPR after cardiac arrest. Usually, ACD-CPR is performed with the rescuer kneeling beside the patient (ACD-B), but ACD-CPR with the rescuer in standing position (ACD-S) has been taught and applied in some centres in addition to conventional ACD-CPR (ACD-B). The aim of this randomised and cross-over study was to evaluate the new technique of ACD-S and to compare it with conventional ACD-B. Twelve professional rescuers (aged 30.8 +/- 7.9 years) applied both methods of ACD-CPR on a manikin. We obtained the following results. (1) Duration of CPR performance was comparable for ACD-S (13.2 +/- 7.1 min) and ACD-B (15.5 +/- 10.2 min, P = 0.48). (2) Pain in the upper extremity and pain in the vertebral column were the main reasons for break-off by the rescuers. Exhaustion was judged to be similar during ACD-S (5.3 +/- 2.3) and ACD-B (6.2 +/- 2.1; on a rating scale with 1 = no and 9 = complete exhaustion). (3) Oxygen consumption was significantly higher during ACD-S (P < 0.005), whereas heart rate and lactate levels did not differ. (4) Decompression forces were lower than compression forces. The averaged decompression forces in both methods were similar during the first 2 min and the last min. Compression forces decreased in ACD-S from 55.1 to 48.9 kp (P = 0.002) and in ACD-B from 52.8 to 47.0 kp (P = 0.069). We conclude that ACD-CPR in standing position can be considered equal to ACD-B in view of maximal duration of CPR, exhaustion of the rescuers and decompression forces. The decrease of compression forces in ACD-S and ACD-B as well as the difference between compression forces in ACD-S and ACD-B seem to be of no clinical relevance, and exhaustion was judged to be similar despite oxygen consumption being higher in ACD-S than in ACD-B.

Adult↗

PubMed1981

Effects of graded compression on intraneural blood blow. An in vivo study on rabbit tibial nerve.

Compression applied to a peripheral nerve may easily interfere with intraneural blood flow. In the present experimental study, a vital microscopic technique was used to observe changes in intraneural microcirculation (intrafascicularly and extrafascicularly) when graded compression was applied to a rabbit's tibial nerve by a specially designed minicompression device. Interference with venular flow was observed already at a pressure of 20 to 30 mm Hg while arteriolar and intrafascicular capillary flow was impaired at about 40 to 50 mm Hg. At 60 to 80 mm Hg no blood flow could be observed in the nerve. Nerves observed 3 or 7 days after 2 hours of compression at 400 mm Hg showed no or very slow stagnant blood flow within the previously compressed segment. It is concluded that acute compression of nerve may cause persistent impairment of intraneural microcirculation due to mechanical injury to blood vessels.

Acute Disease↗

PubMed2003

A new index, the core erosion ratio, of compression-coated timed-release tablets predicts the bioavailability of acetaminophen.

Although compression-coated tablets are a commonly used timed-release drug delivery technology, their utility is often limited by poor bioavailability. To try to improve the bioavailability of these tablets, the effect of their core composition of compression-coated tablet on in vivo pharmacokinetics was investigated. First, the extent of mass reduction of cores in different compression-coated tablet core formulations was used to establish a new index, the core erosion ratio. The data show that adding excipients with high water solubility to the core results in a greater core erosion ratio. Next, to elucidate the effect of core erosion ratio on in vivo acetaminophen (AAP) release, three compression-coated tablet formulations with similar in vitro AAP release profiles but different core erosion ratios were administered to four fasted dogs. The time for first appearance (TFA) of AAP in plasma did not differ significantly among formulations, indicating that the in vivo lag time was the same for all formulations. In separate experiments, necroscopy revealed that 3h after oral administration, the tablets were located in the ileum and colon and that all three formulations had identical GI transit times. However, the area under the AAP plasma concentration-time curve was greater in dogs given formulations with larger core erosion ratios. These results suggest that a formulation with a large core erosion ratio can significantly increase in vivo drug release from compression-coated tablets, leading to increased drug absorption from the lower GI tract.

Acetaminophen↗

PubMed2003

The level of compressive load affects conclusions from statistical analyses to determine whether a lateral meniscal autograft restores tibial contact pressure to normal: a study in human cadaveric knees.

This study addressed the question of whether the level of compressive load would affect the conclusions from statistical analyses aimed at determining how well a lateral meniscal autograft restores tibial contact (as indicated by the maximum contact pressure, mean pressure, and contact area) to that of the intact knee. If statistical analyses indicated that normal tibial contact was not restored with a higher, more physiologic load, then a secondary question was whether an autograft surgically implanted with bone plugs would improve tibial contact compared to that in a meniscectomized knee. Nine, fresh-frozen human cadaveric knees were subjected to a low, non-physiologic compressive load of 400 N and a higher, more physiologic compressive load of 1200 N under three conditions (lateral meniscus intact, lateral meniscus removed and reimplanted as an autograft, and lateral meniscus removed). Contact pressure on the lateral plateau was measured with pressure sensitive film at 0 degrees, 15 degrees, 30 degrees, and 45 degrees of flexion. At 400 N, p-values from statistical analyses indicated that both the maximum and mean pressures with the autograft were comparable to those of the intact knee (p> or =0.685). However, at 1200 N, p-values from statistical analyses indicated that both the maximum and mean pressures with the autograft were significantly greater than those of the intact knee (p< or =0.0001). Therefore studies designed to evaluate tibial contact pressure for a meniscal transplant should use a higher, more physiologic compressive load, because lower loads overestimate the transplant's effectiveness. Although none of the contact variables was restored to normal when the compressive load was increased to 1200 N, all of the contact variables were more normal than those of the meniscectomized knee. Thus, lateral meniscal allografts implanted using bone plugs can significantly improve contact pressure relative to a meniscectomized knee at the time of implantation.

Adult↗

PubMed2003

Effect of compressive follower preload on the flexion-extension response of the human lumbar spine.

Traditional experimental methods are unable to study the kinematics of whole lumbar spine specimens under physiologic compressive preloads because the spine without active musculature buckles under just 120 N of vertical load. However, the lumbar spine can support a compressive load of physiologic magnitude (up to 1200 N) without collapsing if the load is applied along a follower load path. This study tested the hypothesis that the load-displacement response of the lumbar spine in flexion-extension is affected by the magnitude of the follower preload and the follower preload path. Twenty-one fresh human cadaveric lumbar spines were tested in flexion-extension under increasing compressive follower preload applied along two distinctly different optimized preload paths. The first (neutral) preload path was considered optimum if the specimen underwent the least angular change in its lordosis when the full range of preload (0-1200 N) was applied in its neutral posture. The second (flexed) preload path was optimized for an intermediate specimen posture between neutral and full flexion. A twofold increase in flexion stiffness occurred around the neutral posture as the preload was increased from 0 to 1200 N. The preload magnitude (400 N and larger) significantly affected the range of motion (ROM), with a 25% decrease at 1200 N preload applied along the neutral path. When the preload was applied along a path optimized for an intermediate forward-flexed posture, only a 15% decrease in ROM occurred at 1200 N. The results demonstrate that whole lumbar spine specimens can be subjected to compressive follower preloads of in vivo magnitudes while allowing physiologic mobility under flexion-extension moments. The optimized follower preload provides a method to simulate the resultant vector of the muscles that allow the spine to support physiologic compressive loads induced during flexion-extension activities.

Adult↗