Engineering in Medicine

Organised by The Bioengineering Group for the Study of Human Joints, University of Leeds, In Association with The Biological Engineering Society.

The stresses to which articular cartilage is subjected during its normal function undoubtedly influence its useful life. A simple yet rigorous approximate analysis of cartilage loading has been developed which allows cartilage stresses, strains and deformations to be determined from the thickness and material properties of the cartilage and its surface loading. Using this analysis to examine the probability of cartilage failure suggests that the pressure gradients and distribution in a joint are probably as important as the absolute values of pressure. The analysis also helps to explain certain characteristics of the cartilage and certain types of cartilage failure.

The frequent occurrence of degenerative joint disease (DJD) pathology and the high degree of functional prejudice associated with it, justify the search for experimental models with a view to a better understanding of this pathology and its treatment. The models most commonly used resort to a sudden mechanical degradation by shock (post-contusive osteoarthritis) or to a kinematic disruption of the joint through more or less extensive menisco-capsulo-ligamentary lesions or lesions induced by toxic substances.

Cartilaginous lesions met with in human pathology are often secondary to a morphological anomaly at the origin of a hyperpressure. Maquet (1977) has suggested the bringing forward of the anterior tibial tubercle (ATT) in order to reduce the retro-patellar pressure. The drawing back by hollowing of this ATT appeared to us a suitable model for experimental osteoarthritis by hyperpressure.

Clinical research into the pathology and treatment of knee disorders is greatly enhanced by the availability of objective data on knee function. A three-dimensional measurement system, using television cameras and force platforms interfaced to a computer, is used to make a combined kinetic and kinematic assessment of the knee. Information is provided on the standing knee alignment, the general gait parameters, and the knee angle and moment when walking, in both sagittal and coronal planes. An example is given of the use of these data in the assessment of unicompartmental knee replacement arthroplasty.

The range of observed movement at a given joint varies with a gaussian distribution throughout the population. Individuals whose joints display an unusually high degree of laxity are susceptible to a variety of rheumatic and orthopaedic symptoms. The impression that such individuals develop premature osteoarthrosis is a strong one, but hard to prove. In addition, certain individuals with inherited abnormalities of connective tissue are undoubtedly at risk of accelerated osteoarthrosis.

The observed range of movement at a joint is determined by a variety of factors. The main ones are the muscular tone, the laxity of the collagen that contributes to the joint capsule and surrounding structures, and the shape of the articulating bony surfaces. The first of these is probably neurologically mediated, the latter two are usually genetically determined. In a majority of cases it is probably the mechanical joint instability that produces the articular damage, though the alternative hypothesis is that the osteoarthrosis is initiated by genetic aberrations in the collagen that coincidentally cause lax joints. Clinical and family studies using the Leeds Hyperextensometer will help to clarify this and it remains a possibility that genetic markers may point to those individuals who will be at particular risk of osteoarthrosis should joint hyperlaxity develop.

160 patients (231 knees) with an angulation deformity of the knee have been examined. Varus deformity was uncommon except in patients with osteoarthritis. Osteoarthritis subsets, such as pyrophosphate arthropathy tend to cause a valgus deformity, as does rheumatoid disease. An analysis of 50 consecutive patients presenting with idiopathic knee osteoarthritis supports the impression that focal disease of the medial (common) or lateral (uncommon) compartment can occur. Different disease processes, with different associations, may be responsible. The biomechanics of the knee joint in relation to the development of angulation deformities is discussed.

Five patients were examined by quantitative computed X-ray tomography of the proximal tibia immediately after total knee joint replacement, and examinations were repeated after 6 to 26 months. The relative attenuation coefficient derived from the digital images was transformed to compressive yield strength according to earlier work (Bentzen et al., 1985a). Significant deviations from the normal distribution of bone strength were noted at the first examination with high strength medially in varus knees and laterally in valgus knees, and low bone strength at the unloaded condyle. After six months these abnormalities had changed towards a more normal pattern, bone strength being reduced where it had been high at the time of operation and slightly increased where it had been low. One tibia showed no signs of remodelling.

An experiment was performed to determine the effects of stiffness of cruciate ligament prostheses upon joint function and degeneration. Twenty four sheep were allocated into four groups. One group had the ligament excised with no replacement and subsequent groups underwent substitution procedures with either a stiff, matched, or lax prosthesis. After six months the animals with a stiff or matched prosthesis were significantly less lame than those with a lax prosthesis or no prosthesis. However, at post mortem examination the animals with a stiff prosthesis or no prosthesis showed significantly more widespread cartilage damage than the matched or lax groups. The results indicate that either a stiff or matched prosthesis may restore functional activity, but that the stiff prosthesis may predispose to increased degenerative changes.