Dynamic load response of human dura mater at different velocities.

Despite of its assumed role to mitigate brain tissue response under dynamic loading conditions, the human dura mater is frequently neglected in computational and physical human head models. A reason for this is the lack of load-deformation data when the dura mater is loaded dynamically. To date, the biomechanical characterization of the human dura mater predominantly involved quasi-static testing setups. This study aimed to investigate the strain rate-dependent mechanical properties of the human dura mater comparing three different velocities of 0.3, 0.5 and 0.7 m/s. Samples were chosen in a perpendicular orientation to the visible main fiber direction on the samples' surface, which was mostly neglected in previous studies. The elastic modulus of dura mater significantly increased at higher velocities (5.16 [3.38; 7.27] MPa at 0.3 m/s versus 44.38 [35.30; 74.94] MPa at 0.7 m/s). Both the stretch at yield point λf (1.148 [1.137; 1.188] for 0.3 m/s, 1.062 [1.054; 1.066] for 0.5 m/s and 1.015 [1.012; 1.021] for 0.7 m/s) and stress at yield point σf of dura mater (519.14 [366.74; 707.99] kPa for 0.3 m/s versus 300.52 [245.31; 354.89] kPa at 0.7 m/s) significantly decreased with increasing velocities. Conclusively, increasing the load application velocity increases stiffness and decreases tensile strength as well as straining potential of human dura mater between 0.3 and 0.7 m/s. The elastic modulus of human dura mater should be adapted to the respective velocities in computational head impact simulations.

Detection of subtle cartilage and bone tissue degeneration in the equine joint using polarisation-sensitive optical coherence tomography.

OBJECTIVE: To explore the ability of polarisation-sensitive optical coherence tomography (PS-OCT) to rapidly identify subtle signs of tissue degeneration in the equine joint. METHOD: Polarisation-sensitive optical coherence tomography (PS-OCT) images were systematically acquired in four locations along the medial and lateral condyles of the third metacarpal bone in five dissected equine specimens. Intensity and retardation PS-OCT images, and anomalies observed therein, were then compared and validated with high resolution images of the tissue sections obtained using Differential Interference contrast (DIC) optical light microscopy. RESULTS: The PS-OCT system was capable of imaging the entire equine osteochondral unit, and allowed delineation of the three structurally differentiated zones of the joint, that is, the articular cartilage matrix, zone of calcified cartilage and underlying subchondral bone. Importantly, PS-OCT imaging was able to detect underlying matrix and bone changes not visible without dissection and/or microscopy. CONCLUSION: PS-OCT has substantial potential to detect, non-invasively, sub-surface microstructural changes that are known to be associated with the early stages of joint tissue degeneration.

The mechanical influence of bone spicules in the osteochondral junction: A finite element modelling study.

While much has been done to study how cartilage responds to mechanical loading, as well as modelling such responses, arguably less has been accomplished around the mechanics of the cartilage-bone junction. Previously, it has been reported that the presence of bony spicules invading the zone of calcified cartilage, preceded the formation of new subchondral bone and the advancing of the cement line (Thambyah and Broom in Osteoarthr Cartil 17:456-463, 2009). In this study, the morphology and frequency of bone spicules in the cartilage-bone interface of osteochondral beams subjected to three-point bending were modelled, and the results are discussed within the context of biomechanical theories on bone formation. It was found that the stress and strain magnitudes, and their distribution were sensitive to the presence and number of spicules. Spicule numbers and shape were shown to affect the strain energy density (SED) distribution in the areas of the cement line adjacent to spicules. Stresses, strains and SED analyses thus provided evidence that the mechanical environment with the addition of spicules promotes bone formation in the cartilage-bone junction.

Macroscopically healthy articular cartilage with fibrillar-scale early tissue degeneration subject to impact loading results in greater extent of cell-death.

From previous investigations it has been shown that there exists healthy-appearing articular cartilage that contains collagen fibril network destructuring. It is hypothesised that such sub-micron scale destructuring not only presents an increased vulnerability to tissue scale damage following impact loading, but an increase in cell death as well. Cartilage-on-bone blocks from 12 patellae, six healthy (G0) and the other six with sub-micron fibrillar destructuring (G1), were obtained and subject to 2.3 J impact loading. Two sets of sub-samples were obtained for each block tested. One set was used to examine for the live/dead cell response using calcein-AM and propidium iodide staining, imaged with confocal microscopy. The tissue microstructural matrix was imaged from the other matched set, unstained and in its fully hydrated state, using differential interference contrast optical light microscopy. High speed imaging of the impact was used to calculate the velocity changes or coefficient of restitution (COR) and used as a proxy of energy that the tissue absorbed. A previously defined tissue matrix damage score was used to quantify the extent of fracturing and cracking in the matrix. The cell death (PCD) was counted and presented as a percentage against all cells live plus dead. The energy absorbed was 36.5% higher in G1 than in G0 (p = 0.034). However, the damage score and PCD of samples in the G1 group was much larger than the G0 group, ~300% and 161% respectively. Microscopy showed that cell death is associated to both matrix compaction and further fibrillar destructuring from the ECM to the territorial matrix regions of the chondron. Following impact loading, cartilage tissue that appears normal but contains sub-micron fibrillar matrix destructuring responds with significantly increased cell death.

A viscoelastic two-dimensional network model of the lung extracellular matrix.

The extracellular matrix (ECM) comprises a large proportion of the lung parenchymal tissue and is an important contributor to the mechanical properties of the lung. The lung tissue is a biologically active scaffold with a complex ECM matrix structure and composition that provides physical support to the surrounding cells. Nearly all respiratory pathologies result in changes in the structure and composition of the ECM; however, the impact of these alterations on the mechanical properties of the tissue is not well understood. In this study, a novel network model was developed to incorporate the combinatorial effect of lung tissue ECM constituents such as collagen, elastin and proteoglycans (PGs) and used to mimic the experimentally derived length-tension response of the tissue to uniaxial loading. By modelling the effect of collagen elasticity as an exponential function with strain, and in concert with the linear elastic response of elastin, the network model's mechanical response matched experimental stress-strain curves from the literature. In addition, by incorporating spring-dashpot viscoelastic elements, to represent the PGs, the hysteresis response was also simulated. Finally, by selectively reducing volume fractions of the different ECM constituents, we were able to gain insight into their relative mechanical contribution to the larger scale tissue mechanical response.

How changes in interconnectivity affect the bulk properties of articular cartilage: a fibre network study.

The remarkable compressive strength of articular cartilage arises from the mechanical interactions between the tension-resisting collagen fibrils and swelling proteoglycan proteins within the tissue. These interactions are facilitated by a significant level of interconnectivity between neighbouring collagen fibrils within the extracellular matrix. A reduction in interconnectivity is suspected to occur during the early stages of osteoarthritic degeneration. However, the relative contribution of these interconnections towards the bulk mechanical properties of articular cartilage has remained an open question. In this study, we present a simple 2D fibre network model which explicitly represents the microstructure of articular cartilage as collection of discrete nodes and linear springs. The transverse stiffness and swelling properties of this fibre network are studied, and a semi-analytic relationship which relates these two macroscopic properties via microscopic interconnectivity is derived. By comparing this derived expression to previously published experimental data, we show that although a reduction in network interconnectivity accounts for some of the observed changes in the mechanical properties of articular cartilage as degeneration occurs, a decrease in matrix interconnectivity alone do not provide a full account of this process.

Multi-scalar mechanical testing of the calcified cartilage and subchondral bone comparing healthy vs early degenerative states.

OBJECTIVE: The calcified cartilage layer is thought to be integral to force transmission between the compliant articular cartilage (AC) above and underlying stiff bone. This study aims to determine how such a stiffness gradient across the calcified cartilage and bone changes with joint degeneration and how different scalar levels of testing are correlated. METHOD: Using a bovine model of early osteoarthritis (OA), multiple samples of calcified cartilage on subchondral bone (SB) from sixteen bovine patellae, displaying a range of cartilage states from intact (healthy) to moderately degenerate, were tested using macroscopic three-point bending, microhardness indentation, and nanoindentation. Mechanical properties were correlated to cartilage health and microstructural morphometric measurements obtained from high resolution imaging using Differential Interference Contrast (DIC) Microscopy. RESULTS: There was a significant decrease in the moduli obtained from tests done at increasing scalar levels. The macroscale average modulus obtained from three-point bending showed that the SB was 10 times stiffer than the calcified cartilage in healthy tissue, 5 times in tissue displaying mildly degenerate AC and 8 times with moderate degeneration. Microhardness testing of multiple points from the calcified cartilage to the SB revealed that there was a monotonic gradual increase in the mean modulus. The moduli obtained from nanoindentation testing indicated that the SB was about twice the stiffness of the calcified cartilage. CONCLUSION: The mechanical transition from calcified cartilage to SB involves a graded continuum of increasing material stiffness. This stiffness gradient is altered in association with early degenerative change in the overlying AC, detectable only at the macro level.

The bovine patella as a model of early osteoarthritis.

The bovine patella model has been used extensively for studying important structure-function aspects of articular cartilage, including its degeneration. However, the degeneration seen in this model has, to our knowledge, never been adequately compared with human osteoarthritis (OA). In this study, bovine patellae displaying normal to severely degenerate states were compared with human tissue displaying intact cartilage to severe OA. Comparisons of normal and OA features were made with histological scoring, morphometric measurements, and qualitative observations. Differential interference contrast microscopy was used to image early OA changes in the articular cartilage matrix and to investigate whether this method provided comparable quality of visualisation of key structural features with standard histology. The intact bovine cartilage was found to be similar to healthy human cartilage and the degenerate bovine cartilage resembled the human OA tissues with regard to structural disruption, cellularity changes, and staining loss. The extent of degeneration in the bovine tissues matched the mild to moderate range of human OA tissues; however, no bovine samples exhibited late-stage OA. Additionally, in both bovine and human tissues, cartilage degeneration was accompanied by calcified cartilage thickening, tidemark duplication, and the advancement of the cement line by protrusions of bony spicules into the calcified cartilage. This comparison of degeneration in the bovine and human tissues suggests a common pathway for the progression of OA and thus the bovine patella is proposed to be an appropriate model for investigating the structural changes associated with early OA.

New insights into the role of the superficial tangential zone in influencing the microstructural response of articular cartilage to compression.

OBJECTIVE: The purpose of this study was to characterize the microstructural response of healthy cartilage in a perturbed physical environment to compressive loading with a novel channel indentation device. Manipulation of the cartilage physical environment was achieved through (1) removal of the superficial tangential zone (STZ) and (2) varying the saline bathing solution concentration. DESIGN: Cartilage-on-bone blocks were subjected to creep loading under a nominal stress of 4.5 MPa via an indenter consisting of two rectangular platens separated by a narrow channel relief space to create a specific region where cartilage would not be directly loaded. Each sample was fixed in its near-equilibrium deformed state, after which the cartilage microstructure was examined using differential interference contrast (DIC) optical microscopy and scanning electron microscopy (SEM). The cartilage bulge in the channel relief space was studied in detail. RESULTS: STZ removal altered the indentation response at the macro- and microstructural levels. Specifically, the strain in the directly compressed regions was reduced (P=0.012) and the bulge height in the channel relief space was greater (P<0.0001) in the STZ-removed compared with the surface-intact samples. The bulge height in the STZ-removed group was always less than the preloaded cartilage thickness. There was intense shear in the non-directly-loaded regions of intact-cartilage but not in STZ-removed cartilage. Bathing solution concentration influenced only the STZ-removed group, where lower concentrations produced significantly abrupt transitions in matrix continuity between the directly compressed and adjacent non-directly-loaded cartilage (P=0.012). CONCLUSIONS: This study showed that while the surface layer was important in distributing loads away from directly-loaded regions, so were other factors such as the matrix fibrillar interconnectivity, swelling potential, and tissue anisotropy.

On new bone formation in the pre-osteoarthritic joint.

OBJECTIVE: This study investigated the structural alterations in the osteochondral junction, traversing the intact-to-lesion regions, with the aim of elucidating the way in which the pre-osteoarthritic (pre-OA) state progresses to fully developed osteoarthritis (OA). METHOD: Thirty bovine patellae showing varying degrees of degeneration, with lesions located in the distal-lateral quarter, were used for this study. Cartilage-on-bone blocks were cut along the lateral facet to include both the lesion site in the distal end and the intact site in the proximal end. The blocks were formalin-fixed, mildly decalcified and microtomed to obtain 30 microm - thick osteochondral slices. Using differential interference contrast optics, the tissue microstructure was captured at high resolution in its fully hydrated state. RESULTS: There were structural changes in the osteochondral junction beneath the still-intact articular cartilage adjacent to the lesion site. The changes observed in traversing from the intact to the lesion site exhibited characteristics that were strikingly similar to those associated with primary bone formation. The evidence suggests that disruption of the cartilage continuum by a lesion has wider mechanobiological consequences at the osteochondral junction. CONCLUSION: The progression of OA appears to involve new bone formation adjacent to lesion sites. We hypothesise that the new bone spicules that appear in regions beneath intact cartilage adjacent to lesion sites provide a snapshot of the elusive pre-OA state.

On how degeneration influences load-bearing in the cartilage-bone system: a microstructural and micromechanical study.

OBJECTIVE: This study investigated the microanatomical response to compression of intact and degenerate cartilage-on-bone samples with the aim of elucidating the functional consequences of articular surface disruption and related matrix changes. METHOD: Two groups of mature bovine patellae were identified at the time of harvest; those with intact cartilage and those with cartilage exhibiting mild to severe degeneration. Cartilage-on-bone samples were statically compressed (7 MPa) to near-equilibrium using an 8-mm diameter cylindrical indenter, and then formalin-fixed in this deformed state. Following mild decalcification full-depth cartilage-bone sections, incorporating the indentation profile and beyond, were studied in their fully hydrated state using differential interference contrast optical microscopy (DIC). RESULTS: Differences in matrix texture, degree of disruption of the articular surface layer (or its complete absence), number of tidemarks and absence or presence of vascularization of the calcified cartilage zone were all observable features that provided clear differentiation between the normal and degenerate tissues. Under load a chevron-type shear discontinuity characterized those samples in which the strain-limiting surface layer was still largely intact. The extent to which this shear discontinuity advanced into the adjacent non-directly loaded cartilage continuum was influenced by the integrity of the cartilage general matrix. For those tissues deficient in a strain-limiting articular surface there was no shear discontinuity, the cartilage deformation field was instead shaped primarily by its osteochondral attachment and a laterally-directed compressive collapse of a much weakened matrix. In the degenerate samples the altered matrix textures associated with different regions of the deformation field are interpreted in terms of an intrinsic fibrillar architecture that is weakened by two fundamental processes: (1) a de-structuring resulting from a reduction in connectivity between fibrils and (2) subsequent aggregation of these now disconnected fibrils. CONCLUSION: DIC microscopy provides a high-resolution description of the integrated osteochondral tissue system across the full continuum of matrices, from normal to severely degenerate. Our study demonstrates the important functional role played by the strain-limiting articular surface, the consequences associated with its disruption, as well as the loss of effective stress transmission associated with a 'de-structured' general matrix. The study also provides new insights into the integration of cartilage with both its subchondral substrate and the wider continuum of non-directly loaded cartilage.

Mechanical properties of articular cartilage covered by the meniscus.

OBJECTIVE: To investigate the mechanical properties and morphological characteristics of articular cartilage on the tibial plateau of human knees, including the region covered by the meniscus. DESIGN: Using a 1-mm diameter flat-ended cylindrical probe to apply a constant load (0.6 MPa) at specific sites on the tibial plateau, the mechanical properties of articular cartilage were studied using seven cadaver knees. Comparison was made between data obtained by the cartilage covered by the meniscus and that not covered. This was done for both the medial and lateral plateaus. Histological sections of the articular cartilage were also performed to study differences between cartilage from these regions of the tibial plateau. RESULTS: Compared to cartilage that was not covered by the meniscus, the articular cartilage beneath the meniscus showed a significantly (P<0.05) larger modulus by as much as 70%, and was less thick by about 40%. Also, the subchondral bone quantity and calcified layer thickness were observed to be significantly lesser in the regions covered by the meniscus. CONCLUSIONS: Our findings revealed a significant difference between the mechanical properties and associated structures of articular cartilage in the region covered by the meniscus compared with the articular cartilage not covered by the meniscus.

Acetabular morphometry for determining hip dysplasia in the Singaporean population.

PURPOSE: To assess and evaluate the usefulness of 7 morphological measurements of the acetabulum in establishing the prevalence of acetabular dysplasia in the Singaporean population. METHODS: Standardised plain anteroposterior radiographs of 522 hip joints of 261 asymptomatic patients (mean age, 60 years; range, 16-99 years) were evaluated. The 7 morphological measurements were centre-edge angle, acetabular angle, depth-to-width ratio, roof obliquity, extrusion index, lateral subluxation, and peak-to-edge distance. RESULTS: 19 (7.3%) patients were acetabular dysplastic (centre-edge angle of <20 degrees). The mean centre-edge angle was 31.2 degrees (range, 5-52 degrees), acetabular angle 39.46 degrees (range, 10-58 degrees), depth-to-width ratio 0.32, roof obliquity 7.86 degrees, extrusion index 0.18, lateral subluxation 9.9 mm, and the peak-to-edge distance 15.65 mm. CONCLUSION: Centre-edge angle was the most useful measurement and correlated significantly with acetabular angle, extrusion index, peak-to-edge distance, and roof obliquity. These preliminary results show a relatively higher rate (7.3%) of acetabular dysplasia in the Singaporean population, compared with other similar but larger Asian studies performed in Hong Kong (1.1%) and Korea (1.8%).

Mechanical contribution of the fibula to torsion stiffness in the lower extremity.

The role of the fibula in rotational stability of the lower extremity, taking into account the intact knee joint and tibia, to the best of our knowledge, has not been investigated. A cadaver study was designed to determine the torque transfer down the lower extremity, with and without the fibula. Six fresh, frozen human cadaver legs were used. The knee joint was left intact and the foot disarticulated. An external rotation up to 5 degrees , coupled with axial compression, was applied to the femur passing through the knee joint in extension and down the lower extremity to the fixed distal end, where torque (N m) was measured via a load cell. Each specimen was further tested with the fibula, cut 4 cm from the distal end, and finally with the entire fibula disarticulated at the proximal end and removed. To achieve 5 degrees rotation, torques reached 1.82 N m (SD, 0.66 N m). When the fibula was cut, the torque reduced a small yet significant amount of 5%. With the entire fibula removed, the torque reduction was significant and more pronounced at 11%. The fibula provided a small yet significant role in torsion stability. From this study, where the biomechanical characteristics of the fibula is explicated, it is hoped that this information will have use in further understanding the biomechanical role of the fibula, especially, in relation to the altered mechanics associated with lower limb pathology involving a deficient fibula.

Biomechanical strength of deep-frozen versus lyophilized large cortical allografts.

OBJECTIVE: To compare biomechanical strength of deep-frozen versus lyophilized large cortical allografts. DESIGN: In vivo transplantation studies performed in tibia of adult cats using 4 cm deep-frozen and lyophilized, gamma-irradiated allografts to bridge large cortical defect model. BACKGROUND: Bridging large cortical bone defect is a challenging problem. Options include autografts, allografts, bioceramics and prostheses. Allografts provide a suitable option. METHODS: Forty mature cats were used. A large defect (4 cm) was created in mid-diaphysis of right tibia. In 16 cats, cortical defect was reconstructed using deep-frozen allografts (-80 degrees C) with intra-medullary rodding. In another 16 cats, lyophilized, gamma-irradiated allografts were used. Observation periods include 8, 12, 16 and 24 weeks. The specimens were procured together with unoperated legs as controls. Mechanical testing was performed using a materials testing machine with torsion test device of up to 500 Nm at speed of 0.18 rpm. Parameters studied included maximum torque, torsional stiffness and energy of absorption. RESULTS: Deep-frozen allografts did not reach 100% strength, achieving only 64% at 6 months. In marked contrast, lyophilized allografts were significantly weaker with only 12% maximum torque strength at 6 months. Lyophilized allografts were significantly weaker than deep-frozen allografts in all observation periods (p < 0.05). CONCLUSION: Deep-frozen allografts did not reach 100% normal strength and were significantly weaker than non-vascularised autografts. Lyophilized allografts were significantly weaker than deep-frozen allografts. RELEVANCE: For the reconstruction of massive cortical bone defects, only deep-frozen cortical allografts should be used. Lyophilized allografts are not suitable.

Gait adaptations in patients with longstanding hip fusion.

PURPOSE: To examine the long-term effects of hip arthrodesis in terms of gait adaptations. METHODS: Motion analysis was performed on 9 patients who underwent unilateral hip arthrodesis between 1979 and 1991. A standard clinical gait analysis 3-dimensional model for the lower limb was used to calculate the effect of the fused hip on walking, compared with the contralateral normal hip. RESULTS: Significant (p<0.05) gait adaptations noted in the fused side were, compensatory hip hiking during the swing-phase, a 24% reduction in hip adduction moment, a 37% decrease in genu-varus moment, 80% reduced hip power, and excessive pelvic tilt. CONCLUSION: It appears that the excess pelvic tilt observed was to achieve relative hip extension via increased relative lumbar lordosis, while the decreased coronal plane moments of the hip and knee observed were to reduce joint loading on the affected side.

The natural history of spontaneous osteonecrosis of the medial tibial plateau.

The natural history of spontaneous osteonecrosis of the medial tibial plateau remains controversial and incomplete. We have studied 21 patients (aged between 53 and 77 years) with clinical and scintigraphic features of spontaneous osteonecrosis of the medial tibial plateau who were observed prospectively for at least three years (37 months to 8.5 years). The mean duration of follow-up was 5.6 years. The mean duration of symptoms at presentation was 4.7 weeks (3 days to 12 weeks). Radiographs of the affected knee at the first visit were normal in 15 patients and mildly arthritic in six. The characteristic radiographic lesion of osteonecrosis was noted at presentation in five of the mildly arthritic knees and during the evolution of the disease in eight of the radiographically normal knees. During the follow-up, subchondral sclerosis of the affected medial tibial plateau was noted in 16 knees. There are three distinct patterns of outcome: 1) acute extensive collapse of the medial tibial plateau in two knees within three months of onset; 2) rapid progression to varying degrees of osteoarthritis in 12 knees, in eight within a year, in all within two years and deterioration of the pre-existing osteoarthritis in three; and 3) complete resolution in four knees, two of which were normal at presentation and two mildly osteoarthritic. The two patients with acute extensive collapse and three who had rapid progression to severe osteoarthritis required total knee arthroplasty. We conclude that osteonecrosis of the medial tibial plateau progresses in most cases to significant degenerative disease of the knee.

Biomechanical study on the effect of twisted human patellar tendon.

OBJECTIVE: To compare the stiffness and maximum strength between the untwisted and twisted free-tendon. DESIGN: 22 twisted and untwisted sectioned-specimens of human cadaver patellar tendons were used and pulled to failure to obtain load-deformation profiles from which stiffness, maximum load to failure and elastic elongation limit were derived. BACKGROUND: In the reconstruction of the deficient anterior cruciate ligament, the use of the central one-third of the patellar tendon is a well-established procedure in which, prior to insertion, the tendon graft may be twisted to mimic the natural orientation of the anterior cruciate ligament in the knee joint. RESULTS: The untwisted tendons had a mean stiffness of 36.5 kg/mm (SD, 16.6 kg/mm) and maximum load of 165.9 kg (SD, 86.8 kg). With a 90 degrees twist, the average stiffness of the twisted tendon was 66.5 kg/mm (SD, 25.4 kg/mm), with maximum load at 364.5 kg (SD, 109.9 kg), an increase of over 100%. The elastic elongation limit, or allowable elongation before permanent deformation or failure, was significantly larger in twisted tendons by 35%. CONCLUSION: Twisting increased the resistance to deformation of the tendon in this study. Relevance The finding supports the surgical practice of pre-twisting tendon grafts for anterior cruciate ligament reconstruction, based on the premise that a stronger and stiffer graft provides a more favourable outcome.

Influence of PLIF cage size on lumbar spine stability.

STUDY DESIGN: In vitro biomechanical testing on functional spine units with posterior lumbar interbody fusion cage implants of progressively larger sizes. OBJECTIVES: To determine the influence of increasing cage size on the restoration of spine stability after total facetectomy. SUMMARY OF BACKGROUND DATA: Bilateral insertion of cages in posterior lumbar interbody fusion commonly involves facetectomy. To restore stability with no additional instrumentation, the cages must provide sufficient distraction of the vertebrae and adequate tension in the anulus. The size of cages is therefore an important consideration in posterior lumbar interbody fusion. METHODS: Eight human lumbar functional spine units were obtained and divided into two equal groups; one group underwent bending tests and the other twisting. The functional spinal units were tested intact, after total bilateral facetectomy and with three sets of cages that were progressively larger in size. RESULTS: After facetectomy, the functional spine unit's stiffness reduced significantly from that of the intact spine in extension (48% of intact), lateral bending (25%), and torsion (39%). With the posterior insertion of small cages into the facetectomized functional spine units, only extension stiffness was restored to the intact level, whereas flexion stiffness reduced significantly (41% of intact). The medium cages restored the lateral bending stiffness of the facetectomized functional spine units; only the large cages managed to restore the torsional stiffness. Flexion stiffness of the facetectomized functional spine units with cages remained significantly less than that of the intact spine, regardless of cage size. CONCLUSION: In the facetectomized lumbar spine unit, cage size influences lateral bending and torsional stability.

Effects of varying backpack loads on peak forces in the lumbosacral spine during walking.

OBJECTIVE: To compare the differences in lumbosacral spine forces under varying backpack loads. DESIGN: A biomechanical model was used to determine the changes in peak forces in the L5/S1 joint with increasing backpack loads during level walking. BACKGROUND: Most studies involving varying external backpack loads have been concerned mainly with kinematic and physiological measurements. To the author's knowledge, there has been no investigation of the change in peak forces in the lumbosacral joint during the carriage of such loads. METHOD: Data acquisition was carried out using a 5-camera Vicon motion analysis system and two Kistler force plates. Ten male subjects with similar weights, height and age were recruited for this study. Three different backpack loading conditions were studied, that is walking with no load, with 15% BW and with 30% BW. RESULTS: It was observed that all the ten subjects while walking with heavier backpack load adopted a compensatory trunk flexion posture. However, kinematic gait parameters such as walking speed and stride length remained unchanged with the increasing loads. Walking with backpack load of 15%BW and 30%BW resulted in corresponding increase in lumbosacral force of 26.7% and 64% respectively when compared to walking without backpack load. CONCLUSION: In carrying a given packload during walking, it will give rise to a disproportionate force increase acting on the L5/S1 joint.