Engineering in Medicine

An anthropometric model for human vertebrae in the mid-sagital plane is described, using parameters which were measured from lateral X-rays of the cervical and lumbar regions in normal, healthy men. The data was processed statistically and results are presented. The findings reported are new in the literature. They can be used in many ways, including modelling of the spine, and orthopaedics. Clinically significant parameters are the heights of the vertebral bodies and the intervertebral spacings. The relations between these parameters are presented and discussed.

A theoretical model of the skin is developed to model the results obtained from uniaxial in vivo tests. The model, which exhibits non-linear viscoelastic behaviour, is based on idealized representations of the skin's structure.

Although cartilage and bone are considered to be rigid tissues they are nevertheless living and changing throughout life. One of the retrogressive processes in cartilage is calcification, but calcification also occurs as a temporary strengthening during the replacement of cartilage by bone. Studies of the zone of calcification have shown that crystalline calcium phosphate is deposited alongside the collagen fibres.

A morphometric study of the zone has now been made using novel techniques in which the surface of an epoxy-embedded thick section of undecalicified tissue is polished, stained with toluidine blue, and inspected by reflection light microscopy. Measurement of the area of stain deposition in normal tissue has led to an estimate of the area ratio for the section and, hence, the volume of calcified cartilage in the femoral head. Studies on osteoarthritic heads have revealed smaller area ratios as well as differences in distribution in the boundary layer. The significance of the data is discussed in relation to physical properties and possible remedial treatments in severe arthritis.

An equivalent bearing was proposed to represent the normal human ankle joint. The geometry was based on measurements of dissected ankle joints and tissue properties were obtained from the work of previous investigators. Theoretical models were developed to estimate the cyclic variation in lubricant film thickness and coefficient of friction during repetitive activities such as walking. Solutions were obtained for various combinations of input parameters. For the conditions representing the walking cycle, film thicknesses of about 0.7 μm were calculated. Although this value was smaller than most previous measurements of the rms roughness of cartilage, it was not much smaller and suggested that transient elastohydrodynamic lubrication played a role in synovial joint lubrication. The possibility of full fluid film lubrication was supported only when a very high input viscosity was employed, based on values estimated from the previous experimental studies of the boosted lubrication mechanism. Also, an attempt was made to link the current findings to a published experimental study of whole joint lubrication.

The gel phase of articular cartilage is reinforced by collagen fibrils. These fibrils have low flexural and torsional stiffness, but are able to provide reinforcement if deformation of the tissue increases their tensile stress. An estimate suggests that the lengths of collagen fibrils in articular cartilage are at least of the same order as their critical length so that tensile stress in the tissue will increase the stress in the fibrils rather than simply pull them out of the gel. In the surface zone the collagen fibrils are oriented so that the efficiency of reinforcement, η, is about 0.6 tangential to the surface; tension in the fibrils is thus able to withstand swelling pressure within the tissue whose condition for stability resembles that of a pressure vessel. Swelling pressure allows the tissue to support applied pressure. An intermediate zone has a roughly isotropic η value of about 0.2, while in the deep zone collagen fibrils appear to tie the cartilage to the subchondral bone; in this deep zone η has a value of about 0.6 perpendicular to the surface direction. There is also some preferred orientation of collagen fibrils in the plane of the articular surface within the surface zone; in patellar cartilage the preferred orientations can be related to the direction of stress which could be generated by movement of the joint.