Articular cartilage provides the smooth bearing surfaces in freely moving (synovial) joints. Its mechanical properties are important because structural failure of cartilage is closely associated with joint disorders, including osteoarthritis. Some mechanical properties of cartilage are well characterized, but little is known about its compressive strength. A technique for measuring cartilage compressive strength is evaluated, and an overview of experiments which relate strength to stiffness and tissue hydration is given. Specimens of bovine articular cartilage-on-bone, approximately 15 mm square, were loaded on a hydraulic materials testing machine using flat impermeable indentors. Linear-ramp loading/unloading cycles of 1 s duration, and of increasing severity, were applied until failure was evident on force-displacement graphs. Some specimens were tested following a 30 min period of creep loading. Inkstaining and histology were used to locate the site of initial damage to each specimen. Specimen failure occurred first in the cartilage surface layer at a nominal applied stress of 14-59 MPa (mean 35.7 MPa). Mechanical properties were little affected by specimen or indentor size, provided both remained within defined limits, and compressive strength could be measured to an accuracy of approximately +/- 5 per cent. Compressive stiffness was a significant predictor of strength, but only if it was measured at high levels of stress. Strength increased following creep-induced water loss, and initial mechanical damage could propagate under moderate cyclic loading. This technique for measuring cartilage compressive strength has potential for investigating the causes of cartilage failure in vivo.

The unique biomechanical properties of healthy cartilage ensure that articular cartilage is able to transmit force between the joints while maintaining almost friction-free limb movement. In osteoarthritis, the biomechanical properties are compromised, but we still do not understood whether this precedes the onset of the disease or is a result of it. This review focuses on the physical changes to cartilage with age, disease, and mechanical loading, with specific reference to the increased collagen cross-linking that occurs with age (nonenzymatic glycation), and the response of chondrocytes to physiological and pathological loads. In addition, the biomechanical properties and matrix biosynthesis of cartilage from various joint surfaces of the knee and ankle are compared to elucidate reasons why the ankle is less affected by progressive osteoarthritis than the knee.

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Circadian Technologies, Inc. (CIRCADIAN) is an international consulting firm that for over 20 yr has helped employees better cope with shiftwork, while ensuring their employers reduce costs and maximize profits. This paper describes two of the methods employed to reduce accidents, absenteeism, and turnover, while improving morale and productivity in extended hour operations (shiftwork). Shiftworkers rarely receive on-the-job training to help them cope better with the rigors of working nights, evenings, and early mornings. A pre-and post-training study by CIRCADIAN demonstrated improvements in health and fatigue indices, and an increase in daytime sleep length. Companies who provide shiftwork lifestyle training also have lower rates of absenteeism and turnover compared to companies that do not provide training. The second intervention described in this paper is a novel method of shift scheduling. There are thousands of potential schedules, so how does a company choose the best schedule for their facility? The answer lies in allowing employees to be involved in the process of selection. This results in a workforce that experiences less fatigue, turnover and absenteeism, and higher morale.