Proceedings of the Institution of Mechanical Engineers,Part H: Journal of Engineering in Medicine

The anatomy and biomechanics of the metacarpophalangeal (MCP) joint are briefly described. Hinge, flexible and surface designs of past and current MCP prosthetic joints are reviewed, outlining their respective advantages and disadvantages. Although existing prostheses can restore cosmetic appearance and relieve pain, none can equal the stability and versatility of the natural joint. Delayed reconstructive surgery may be partly responsible for the mediocre results experienced, since the later the surgery the worse will be the condition of the muscles and ligaments surrounding the joint. These are the structures responsible for strength, movement and stability of the joint. From a mechanical viewpoint it may be desirable to operate at an earlier stage of the disease than is currently indicated, but this is a clinical decision. Some design aspects, namely fixation and wear, require a different approach when designing an MCP prosthesis from that adopted in the case of prosthetic hips and knees.

A lycra glove has been instrumented with novel low-profile electrogoniometers to measure index and middle finger metacarpophalangeal (MCP) flexion/extension motion. It is lightweight and comfortable to wear, enabling portable, unobtrusive measurement of joint usage about the home or work environment. Preliminary results have shown it to be of comparable accuracy to existing clinical measurements. Many applications are envisaged in the fields of ergonomics, orthopaedics and rehabilitation.

This paper describes an experimental method for determining the minimum number of degrees of freedom of a human joint. Application of this technique to the wrist suggests that the normal, intact wrist joint uses only two degrees of freedom to move in a plane that is not aligned with the anatomic planes. The technique may be useful in identifying emerging joint pathologies and in simplifying kinematic models of joint function.

By developing a measurement method based on the palpation of bony landmarks, the three-dimensional positions of the scapula and clavicle can be measured at several angles of humerus elevation. An analysis of these measurements shows the interaction between all joints of the shoulder mechanism. With the help of a biomechanical shoulder model the role of some of the extracapsular ligaments in the motion pattern of scapula and clavicle can be derived. In addition, the interaction between the rotations in the acromioclavicular and sternoclavicular joints is shown, and the possible implications for the treatment of joint problems in the shoulder are discussed.

A six-channel instrumented load transducer has been developed for measuring hand and upper limb loading (maximum design load, 2 kN). Modular in design, the transducer can be adapted to a variety of formats, thereby increasing the range of activities to which it can be applied. When used with a motion analysis system, hand loading can be determined with respect to a laboratory coordinate system irrespective of transducer orientation and position. Measurement accuracy of approximately ± 1 per cent of applied loading has been determined through a system calibration.

Increasing application of expensive medical technologies, including joint replacement, is of concern in the current economic climate. Successful upper limb joint replacements (ULJRs) are being performed, but few detailed data about their utilization are available. To explore the resource implications of such surgery, the utilization of total shoulder replacement (TSR), total elbow replacement (TER) and total wrist replacement (TWR) were examined from the time these procedures first became available at the Mayo Clinic up to 1990. The age- and sex-adjusted utilization rate for TSR among Olmsted County, Minnesota, residents was 1.8 per 100000 person-years (p-y), 0.8 per 100000 p-y for TER and 1.1 per 100000 p-y for TWR. Overall utilization of primary ULJR among Olmsted County residents during the 19-year study period was 3.8 per 100000 p-y. The commonest indication for TSR was osteoarthritis (46 per cent) and for TER and TWR was rheumatoid arthritis (50 and 81 per cent respectively). There was no consistent trend in the utilization of these procedures among Olmsted County residents, but the number of referral patients receiving a ULJR at the Mayo Clinic increased steadily during the study period. The Mayo Clinic experience suggests an increasing demand for ULJR, particularly TSR and TER. With continued advances in, and diffusion of, the technology this demand may increase further.

The asperity lubrication in human joints is examined in the present paper, with particular reference to the tertiary undulation with wavelengths of around 20–45 μm. It was found that, under dynamic physiological loading conditions, the secondary waviness of the cartilaginous surface (typically 0.5 mm wavelength) could be effectively flattened to sustain a fluid film of 0.1–0.3 μm thick, while the tertiary waviness could be squashed to sustain a much thinner fluid film of 0.01 μm (10 nm) thick with normal synovial fluid as the lubricant. The calculated film thickness for the tertiary undulation was less than 5 nm when the ankle joint was lubricated by Ringer's solution or pathological synovial fluids, or when only quasi-static loading conditions were considered, while a sufficiently thick fluid film could still be formed when the secondary undulations were considered alone. It was thus suggested that the fluid film lubrication mechanism was operative for human joints with normal synovial fluid as the lubricant under physiological dynamic loading conditions and the mixed lubrication mechanism could take over when static loading conditions prevailed or when watery lubricants (n ≈ 0.001 Pas) were used.

The difficulty of achieving good distal contact between a cementless hip endoprosthesis and the femur is well established. This finite element study investigates the effect on the stress distribution within the femur due to varying lengths of distal gap. Three-dimensional anatomical models of two different sized femurs were generated, based upon computer tomograph scans of two cadaveric specimens. A further six models were derived from each original model, with distal gaps varying from 10 to 60 mm in length. The resulting stress distributions within these were compared to the uniform contact models. The extent to which femoral geometry was an influencing factor on the stress distribution within the bone was also studied.

Lack of distal contact with the prosthesis was found not to affect the proximal stress distribution within the femur, for distal gap lengths of up to 60 mm. In the region of no distal contact, the stress within the femur was at normal physiological levels associated with the applied loading and boundary conditions. The femoral geometry was found to have little influence on the stress distribution within the cortical bone. Although localized variations were noted, both femurs exhibited the same general stress distribution pattern.