David W.L. Hukins

Relation Between the Structure of the Annulus Fibrosus and the Function and Failure of the Intervertebral Disc

A simple model is presented that explains the observed function and failure of the intervertebral disc in compression, torsion, and bending; this model is based upon the observed arrangement of collagenous fibers in the annulus fibrosus. The fibers are considered to have the same mechanical properties as tendon; thus the stresses required to produce a given deformation and which irreversibly damage the fibers can be predicted. Predictions of the mechanical behavior of the disc are in good agreement with published results for compression and torsion; no comparable experiments have been performed for bending. It is further predicted that torsion and bending are likely to cause annular failure and protrusion. Failure is likely to occur posteriorly because of the effect of forward bending and because in the flattened and reentrant discs of the lumbar spine, torsional stress is concentrated at the points of maximum curvature. The structure of the disc tends to protect the collagenous fibers in forward bending and torsion. Compression is predicted to cause end-plate fracture rather than annular failure.

Composition and properties of connective tissues

Abstract Connective tissues consist of a complex system of interacting macromolecules. The mechanical, and hence biological, properties of this system can be related to its chemical composition using ideas which have been developed to analyse the behaviour of fibre-reinforced composite materials.

Migration of the nucleus pulposus within the intervertebral disc during flexion and extension of the spine

Study design Magnetic resonance images were obtained of the lumbar spines of three volunteers in neutral, flexed, and extended postures. Objectives To measure migration of the nucleus pulposus within the intervertebral disc during flexion and extension of the spine in living people. Summary of Background Data Results of experiments on bisected cadaveric spines have indicated that the nucleus migrates posteriorly during flexion and anteriorly during extension in nondegenerate discs. Degenerate discs may have faults or fissures that result in abnormal motion of the nucleus. Methods Proton density weighted, sagittal, magnetic resonance images were obtained from the lumbar spines of three volunteers. Measurements of the positions of the anterior and posterior margins of the nucleus and of flexion and extension angles were made on tracings of the images corresponding to neutral, flexed, and extended postures. Results The observed frequency (22 of 24 measurements) at which the margins of the nucleus migrated in the directions predicted by results of cadaveric studies was significantly greater than the frequency that would be expected by chance (P < 0.001). The two exceptions may be a result of disc degeneration. There was a significant (P < 0.05) linear correlation between the migration of the anterior margin and the flexion‐extension angle and a highly significant (P < 0.001) correlation for the posterior margin and the flexion‐extension angle. Conclusions Flexion of an intervertebral disc in a living person tends to be accompanied by posteriorly directed migration of the nucleus pulposus within the disc. Extension tends to be accompanied by an anteriorly directed migration.

Collagen polymorphism: Its origins in the amino acid sequence☆

Abstract The polymorphic forms of ordered collagen aggregation in vitro and in vivo are reviewed. The axially projected structures of a class of fibrils known as fibrous long spacing (FLS) collagen are solved using simulated positively stained banding patterns based on the amino acid sequence. This method is also used to solve the axial projection of a 670 A ( D ) periodic structure with a symmetrical banding pattern (DPS) re-precipitated from skin collagen. The relation between the obliquely striated and 110 A periodic forms of collagen is discussed. The specificity for the formation of FLS, DPS and segment long spacing (SLS) collagen is shown to be in the distributions of various amino acids in the sequence. Different residues are important for each type of structure, their importance being dependent on the chemical conditions and the presence of other macromolecules. The interaction of collagen fibrils with proteoglycans in vivo is discussed in terms of the amino acid sequence. Also the factors which affect collagen morphology in the presence of mucopolysaccharides and proteoglycans in vitro and in vivo are discussed. Some insight is gamed into the principles which govern the self-assembly of molecules into ordered fibrous aggregates.

Infection of catheterised patients: bacterial colonisation of encrusted Foley catheters shown by scanning electron microscopy.

SummaryThe surfaces of 32 encrusted urinary catheters were examined by scanning electron microscopy to investigate the association of bacteria with the encrusting deposits. Deposits consisted of struvite crystals surrounded by aggregates of very small crystallites of hydroxyapatite. Underneath these minerals there was a layer of closely packed bacteria. Impressions of bacteria were also observed in hydroxyapatite. Crystals were often engulfed by the bacterial layer, which thus appeared to bind the crystals to each other and to the catheter surface. This thick layer of bacteria associated with crystals may protect both the bacteria from antibiotics and the crystals from acidic bladder washout solutions intended to dissolve them. Furthermore, the existence of this sessile population explains why urease-producing bacteria are not invariably detected in the urine of patients with encrusted catheters. The observation of this bacterial layer (or “biofilm”) by scanning electron microscopy provided direct evidence for infection being implicated in catheter encrustation.

Conversion of amorphous calcium phosphate into hydroxyapatite investigated by EXAFS spectroscopy

Abstract Amorphous calcium phosphate (ACP) was precipitated from solution at pH 10. Some samples were allowed to transform to poorly crystalline hydroxyapatite (HAP), at this pH, for periods up to 120 h. All samples were stabilised by freeze-drying and characterised by extended X-ray absorption fine structure (EXAFS) spectroscopy as well as by chemical analysis, infra-red spectroscopy and X-ray powder diffraction. EXAFS spectra, recorded above the K absorption edge of Ca, were interpreted using a model developed previously to explain the features of the EXAFS spectrum of fully crystalline HAP. Eight shells of atoms surrounding Ca out to 0.57 nm were required to explain the appearance of poorly crystalline HAP. In contrast, only the innermost three of these shells were required to interpret the spectrum of the initial ACP. Moreover, these three shells had almost identical radii and Debye-Waller factors as in the poorly crystalline HAP and so the process of crystallisation involves only the development of longer-range order without changing the immediate environment of Ca.

Structure of bovine milk calcium phosphate determined by X-ray absorption spectroscopy.

Calcium in cows milk is mainly in the form of calcium phosphate-phosphoprotein complexes known as casein micelles. These micelles, in contrast to other phosphoprotein complexes in bone and other tissues, can be readily isolated and studied, but conventional techniques have given ambiguous and conflicting evidence on the structure of milk calcium phosphate. Extended X-ray absorption fine structure and near-edge structure measurements at the newly commissioned Synchrotron Radiation Source at Daresbury indicate that it closely resembles brushite, CaHPO4 X 2H2O. This result, and chemical analysis, requires that phosphate groups from the matrix phosphoproteins be incorporated in the brushite lattice, probably in the surface, suggesting that these organic phosphate groups act as heterogeneous nucleation sites for phase separation of the calcium phosphate from solution.

Spinous process strength.

Study Design. Mechanical testing of cadaveric lumbar spines and dual energy radiograph absorptiometry scanning were performed. Objectives. To devise a technique to measure the strength of lumbar spinous processes and to determine the bone mineral density of the vertebrae used. Summary of Background Data. The spinous process has been identified as the weakest part of the anatomy to which a flexible fixation device can be attached. It was unknown if the spinous processes could withstand the forces applied by the device. Methods. A hook was fitted to the spinous process of 32 lumbar vertebrae. A custom-built rig was designed to secure a vertebra to a materials testing machine. A loop of cord was passed over a bar mounted on the crosshead of the machine and around the two bollards of the hook. As the crosshead was raised, a tension was applied to the cord. Each vertebra was tested to failure. The bone mineral density of each vertebra was then measured using dual energy radiograph absorptiometry. Results. Failure of the specimens occurred by failure of the spinous process, pedicles, or vertebral body. The logarithm (base 10) of the load (N) at which failure occurred was 2.53 ± 0.3, which corresponded to a mean failure load of 339 N. The bone mineral density of each vertebral body varied between 0.263 and 0.997 g/cm2. A significant linear correlation was found between bone strength and bone mineral density (P < 0.0001). Conclusions. Specimens with a bone mineral density in the range of 0.263–0.997 g/cm2 failed at a mean load of 339 N when the load was applied through the spinous process hook of a flexible fixation device.

Morphology of mineral deposits on encrusted urinary catheters investigated by scanning electron microscopy.

Struvite and hydroxyapatite were precipitated from artificial urine onto the surfaces of catheter materials by the controlled addition of urease. They were precipitated both together and separately (by omitting components of the artificial urine), and with and without the inclusion of albumin (which was intended to mimic the proteinaceous debris found in infected urine). Precipitates were identified by X-ray powder diffraction and the artificially encrusted surfaces examined by scanning electron microscopy. In the presence of protein, hydroxyapatite was precipitated as a poorly crystalline form which aggregated to form a crust. Struvite crystals could be easily identified under the scanning electron microscope by their relatively large size and characteristic appearance. Fifteen encrusted catheters from patients were also examined by scanning electron microscopy, and a further six using X-ray microanalysis. Their appearance was very similar to that of the materials encrusted in vitro. Encrustation involves the formation of hydroxyapatite and the growth of struvite crystals, intimately associated with bacteria.

Collagen Fibrils as Examples of Smectic A Biological Fibres

Abstract Collagen fibrils provide a biological example of smectic A liquid crystals. They demonstrate spiralling of their constituent chiral molecules, about the normal to their layers, when they undergo a transition to smectic C. Under tension, perpendicular to the planes of the layers, their molecules are tilted and some of them rearrange so as to describe a lattice whose unit cell has a square base. Novel features of the collagen fibril are that the layer thickness is dictated by the amino acid sequence of its collagen molecules, and not by their length, and that extra stability is conferred on the structure by covalent cross-links.