Thorecolumlbar fascia can increase the efficiency of the erector spinae muscles

A model is developed which demonstrates that the thoracolumbar fascia can increase the axial stress generated by active contraction of the erector spinae muscles by restricting their radial expansion (bulging). The radial stress in the muscles was calculated from their radii, the radius of the abdominal cavity, and the intra-abdominal pressure generated during lifting. Radii were measured from magnetic resonance images and values of intra-abdominal pressure obtained from the literature. The resulting radial stress was used to calculate the increase in axial stress developed as a result of restricting radial expansion. Values for the axial stress generated by contraction of an unrestricted muscle, which are also required for this calculation, were obtained from the literature. The results indicate that restriction of radial expansion of the erector spinae muscles by the thoracolumbar fascia may increase the stress generated during their contraction by up to about 30%. This would lead to a proportional increase in the extensor moment exerted by these muscles.

Stress in collagen fibrils of articular cartilage calculated from their measured orientations

Articular cartilage may be considered as a form of pressure vessel in which the internal swelling pressure is balanced by tensile stress in the collagen fibrils. This stress is calculated by analysing the tissue as a series of microscopically small pressure vessels. The previously measured orientations of the collagen fibrils describe the structure necessary for this calculation. The stresses and strains developed in the fibrils are shown to be well within physiological limits.

Large cartilage proteoglycan (PG-LA) influences the biosynthesis of macromolecules by isolated chondrocytes

Bovine articular and tracheal chondrocytes were cultured at high density in multilayers. Intact or fragmented large aggregating proteoglycans (PG-LA) from cartilage were added to the cultures and the biosynthetic response studied by the incorporation of [3H]-leucine and [35S]-sulfate for proteins and proteoglycans respectively. Incorporated radiolabel and patterns of synthesized macromolecules were compared with control cultures without additives and cultures containing either of the synthetic polymers dextran or dextran sulfate. All proteoglycans and derivatives containing globular protein structures had a stimulatory effect on the biosynthesis of both proteins and proteoglycans as did the highly polyanionic polymer dextran sulfate. Distribution of the radiolabeled material between the cell- and medium pools were however different in the various cultures. A radiolabeled protein, migrating as a triplet band at a position of approximately 140 kDa after reduction, was detected by SDS-PAGE and fluorography. The protein was present in all cell extracts and in the media of cultures stimulated with proteoglycans and proteoglycan fragments, except chondroitin sulfate side chains. The protein was shown to be collagenous in nature by collagenase digestion and identified as procollagen II by immunoprecipitation.

Structure and mechanical properties of the longitudinal ligaments and ligamentum flavum of the spine

Stress-strain curves were recorded from anterior and posterior longitudinal ligaments and ligamenta flava dissected from pig lumbar spines. Ligaments were examined during extension by light microscopy, to observe crimp structure, and by X-ray diffraction, to determine collagen fibril orientations. Scanning electron microscopy (SEM) was used to examine ligaments fixed at high and low strains. Initial stages of ligament extension involved alignment of collagen fibrils. Collagen fibrils in unstrained ligamentum flavum were much more disoriented than in the longitudinal ligaments. Thus, fibril alignment, and consequent stiffening, occurred at much higher strains than for longitudinal ligaments, allowing ligamentum flavum to exploit the extensibility of its elastin.

The spine as an arch. A new mathematical model

A new model is presented for the static behavior of the human spine that considers it to work as an arch rather than the traditional view of a cantilever. This theory is based on limit criteria, derived from plasticity theory, which determine bounds within which the structure is mechanically stable and thereby enables the prediction of failure when these criteria are not satisfied. It is shown that theorems developed for the plastic analysis of masonry arches can be simply adapted for the spine. An analysis is performed of three postures and associated loads described in the literature. The forces and intradiscal pressures are calculated and shown to be in good agreement with published measurements. The results show that compressive stresses in the spine are not as high as was previously calculated and that the curvature of the spine is necessary for its load-bearing function. Preservation of the lumbar lordosis, in conjunction with intra-abdominal pressure, strengthens the spine and is crucial to protect the spine from injury when lifting heavy loads.

A new mathematical model of the spine and its relationship to spinal loading in the workplace

A knowledge of the loads imposed on the spine during manual work and of the postures in which the spine is least stressed while performing a particular task are of considerable importance for the design of a safe and comfortable working environment. Spinal loads are intrinsically very difficult to measure and so are generally calculated from mathematical models of the spine. These models are based on assumptions of how the spine functions mechanically, and the forces calculated are dependent on the particular model used. A new model, which assumes that the spine functions in a similar way to an arch, is discussed. This model shows that spinal stresses are not as great as previously calculated using the traditional cantilever model and that, even when no external loads are being carried, the stress is strongly dependent on posture. The arch model re-interprets the role of intra-abdominal pressure and shows that it acts together with the lumbar lordosis to strengthen the spine. Maintaining a lumbar lordosis is then important when lifting heavy loads.

Simple computer controlled apparatus for in vitro mechanical testing of connective tissues

Connective tissues are viscoelastic and so a series of experiments, at carefully controlled loading rates, is required to determine their mechanical properties. This paper describes a simple, inexpensive apparatus for performing these experiments; it is based on a ball screw driven by a microcomputer controlled stepper motor.

Dimensional measurements on images from a video camera or cassette recorder using a BBC microcomputer

We have developed a system that enables a BBC microcomputer to make dimensional measurements on images from a video camera or cassette recorder without a framestore. The RGB output of the computer is synchronized to the output of the camera or recorder and the two displayed simultaneously on the same monitor. Measurements can be made directly from the displayed image using points indicated by the computer graphics cursor via its keyboard. Distances can be measured with a precision of about 0.4%. The system is very versatile but is especially useful for measuring the freeze-frame output from a Philips standard videotape.

Collagen organisation in the cervix and its relation to mechanical function

The organisation of collagen fibrils within the cervices from non-pregnant humans and rats and day 22 pregnant rats was measured using X-ray diffraction. This technique yields the direction of preferred orientation of the collagen fibrils and an orientation distribution function. In the human cervix there are three distinct zones which blend smoothly into each other on passing radially outward from the canal. Adjacent to the canal and in the outermost zone the fibrils are oriented predominantly longitudinally, that is parallel to the canal. In the middle zone the fibrils have a preferred orientation in a circumferential direction. A similar structure is seen in the non-pregnant rat cervix. The collagen fibrils in the day 22 pregnant rat showed no preferred orientation and are therefore randomly oriented within the tissue. The directions in which the collagen fibrils are oriented determine the directions in which the tissue can best withstand tensile stress.

The theory of fibre-reinforced composite materials applied to changes in the mechanical properties of the cervix during pregnancy

A theory is presented that relates changes in the mechanical properties of the uterine cervix during pregnancy and the puerperium to changes in biochemical composition. The cervical connective tissue is considered as a fibre-reinforced composite material in which collagen fibres are embedded in a gel-like ground substance. Theories derived from synthetic fibrous composites show that changes in the collagen concentration and organization, alteration of the relative proportions of molecular species in the ground substance and changes in the water content can account for the marked alteration in mechanical properties observed by the end of pregnancy.

Collagen organisation in the interspinous ligament and its relationship to tissue function

X-ray diffraction and determination of optical anisotropy show that collagen fibres in pig and human lumbar interspinous ligaments tend to be orientated parallel to the spinous processes. There is a distribution of fibre orientations about this preferred direction. Flexion of the spine does not change the direction of preferred orientation but the angular spread of fibres increases. This pattern of collagen fibre orientations is consistent with the interspinous ligaments being able to transmit tension from the thoracolumbar fascia to the vertebrae. Since the collagen fibres tend to be aligned antero-posteriorly, they provide a high efficiency of reinforcement in this direction. The lack of fibres orientated perpendicular to the spinous processes will lead to low strength in the cranio-caudal direction so that the ligament can provide little resistance to flexion of the spine.

Intra-abdominal pressure and its role in spinal mechanics

Intra-abdominal pressure has been suggested as a mechanism for reducing the compressive load on the spine. The cantilever model of the spine, on which the mechanism is based, is described and the nature of the intra-abdominal pressure response examined within this context. This model is also the basis for the proposal to use intra-abdominal pressure as an index of compressive stress on the spine, although the dependence on the model is not always acknowledged, and the validity of this approach is discussed. Experimental evidence relating intra-abdominal pressure to posture and loading is examined and compared with the predictions of the model and these are often found to disagree. A fundamental problem in spinal mechanics, that of explaining why the vertebrae are not crushed during the lifting of even relatively small loads, is shown to be unresolved. This suggests that a new model of the spine is required, within which there is a useful role for intra-abdominal pressure with the described characteristics. A model based on an analysis of the spine as an arch is proposed.

Analysis of magnetic resonance images from normal and degenerate lumbar intervertebral discs

Degenerate discs can be identified quantitatively by measurement of magnetic resonance (MR) relaxation times. MR images have been recorded from 16-year-old and 82-year-old cadaveric L3-4 discs at the highest resolution attainable with a Picker International MR Imaging System operating at 0.26 Tesla. By recording images with a series of spin-echo and/or saturation-recovery sequences of differing time intervals, the values for sample magnetization, M infinity, and the T1 and T2 relaxation times, have been calculated from each pixel in the MR image. The distribution of M infinity values shows the relative degrees of hydration in different regions of the disc while the corresponding T1 and T2 values are sensitive to the chemical environment of the water molecules. Images from cadaveric discs allowed the nucleus pulposus and annulus fibrosus to be distinguished clearly, and the laminated structure of the annulus could be seen. Loss of water from the nucleus during aging was demonstrated by a reduction and change in the distribution of the M infinity values for an 82-year-old disc, as compared with a 16-year-old disc. Values of T1 and T2 indicated a difference in the chemical environment of water molecules in the nucleus pulposus and annulus fibrosus; the extent of this difference was much greater for younger than for older discs. High-resolution MR images from discs of living subjects showed almost as much detail as those from experimental specimens, but in the latter, the laminated structure of the annulus was resolved.(ABSTRACT TRUNCATED AT 250 WORDS)

A method for the clinical measurement of relaxation times in magnetic resonance imaging

A method for the determination of relaxation times in clinical magnetic resonance images is described. Three components are measured: the spin-lattice (T1) and spin-spin (T2) relaxation times and the proton density (M infinity). These components are separated in the algorithm to give increased tissue discrimination. Multiple data points are used to minimise error and increase reproducibility. Errors that arise in imaging data because of the short sequence repetition periods are considered and a technique for their reduction described. Clinical results obtained using the method are reviewed. These results demonstrate the clinical utility of the technique.

A model for the function and failure of the meniscus

The meniscus has been modelled as a toroid of constant wedge-shaped cross-section resting on a compliant base and loaded in compression from above. This simple model allows the cross-sectional shape and the mechanical properties of the fibrocartilage to be varied systematically in order to determine the essential features of the response of the meniscus to compressive load. A finite element representation of this model shows that a circumferential tensile strain is developed, irrespective of the cross-sectional shape or the elastic moduli of the fibrocartilage, to balance the applied load. A horizontal boundary between regions of positive and negative shear strains and a region of radial tensile strain are also developed. These regions coincide with the locations of horizontal and longitudinal tears which are the most commonly observed lesions of the meniscus. The model shows that the posterior segment of the medial meniscus is most vulnerable to injury.

Collagen orientations in the meniscus of the knee joint

X-ray diffraction shows that the collagen fibrils of dog, pig and human meniscus tend to be oriented circumferentially in the bulk tissue and radially in the surface region. Individual fibrils are not all oriented in these directions but are distributed about the preferred orientations with a standard deviation of around 15 degrees in the bulk tissue. Polarised light microscopy yields results which are consistent with X-ray diffraction. This technique also shows that the fibrils are aggregated into crimped fibres, resembling those of tendon, and that some of these fibres pass from the outer region of the meniscus into the bulk tissue. Artificial split lines demonstrate the radial orientation of outer fibres over most of the superior and inferior surfaces of the meniscus. Split lines reorient as they pass into the bulk tissue which is consistent with the interpretation of X-ray diffraction and polarised light microscopy results of this study. The pattern of collagen orientations is also consistent with the meniscus transmitting compressive load in the knee joint.

Birefringence of articular cartilage and the distribution on collagen fibril orientations

Birefringence of the deep zone of pig tibial plateau cartilage has been measured by polarized light microscopy using monochromatic light and a Sénarmont compensator. Thin (20 micrometers) sections of the tissue were imbibed with mixtures of physiological saline and glycerol to separate the effects of form and intrinsic birefringence. Measured birefringence was plotted against liquid refractive index and the experimental points defined a parabola whose minimum corresponded to a refractive index for collagen of 1.48. When the tissue was imbibed with glycerol, the birefringence measurements provided a good estimate of intrinsic birefringence. The orientation distribution function, g(phi), of collagen fibrils in this zone was then determined by X-ray diffraction. Thus X-ray diffraction yielded the form of g(phi) which corresponded to the measured intrinsic birefringence. Using this result the intrinsic birefringence of a hypothetical cartilage site with perfectly oriented collagen fibrils was calculated to be (6 +/- 1) x 10(-4). Birefringence measurements on further sites could then be converted into standard deviations of g(phi), assuming a Gaussian distribution.

Collagen organization in articular cartilage, determined by X-ray diffraction, and its relationship to tissue function

X-ray diffraction has been used to measure the preferred orientation of the collagen fibrils, and their angular distribution within the tissue, as a function of depth from the articular surface in patellar cartilage. Measurements have been made at four different sites chosen to represent differing surface curvatures and regimes of wear. The orientation of fibrils in the surface layer allows it to oppose the swelling pressure exerted by the gel of hydrated glycosaminoglycans within the cartilage. An intermediate layer (where a bimodal distribution of fibrils is sometimes resolved) allows the orientation of the fibrils to change, with increasing depth, until they are roughly perpendicular to the articular surface. In this deep layer the fibrils can tie into the underlying calcified tissue so as to firmly anchor the cartilage. In the plane of the surface the fibrils tend to be aligned in the direction of stress caused by motion.

Determination of collagen fibril orientation in the cartilage of vertebral end plate

X-Ray diffraction and scanning electron microscopy show that collagen fibrils tend to be oriented parallel to the surface of the vertebral end plates in pig, rat and man. It is suggested that this orientation allows the end plate to withstand the pressure exerted on it by the nucleus pulposus. Scanning electron microscopy shows that lamellae occur in human end plates. Collagen fibrils also tend to be preferentially aligned in the direction of the ventral-dorsal axis.

The lamina splendens of articular cartilage is an artefact of phase contrast microscopy

The so-called lamina splendens of articular cartilage is shown to be a characteristic of phase contrast microscopy; this technique provides no evidence for an anatomically distinct surface layer. Fresnel diffraction occurs at edges separating regions of different refractive indices. These diffraction effects, when viewed under phase contrast, lead to the appearance of a bright line along the edge.