Crack growth of medical-grade silicone using pure shear tests

Investigating the potential advantages of a new design metacarpophalangeal joint

This paper investigates a new metacarpophalangeal joint design aimed at treating patients with moderate to severe forms of arthritis affecting the index, long, ring, and little fingers. Current small joint arthroplasty designs, including those for the metacarpophalangeal joint of the hand, have had limited success owing to mechanical failures and can be divided into two main families: single-piece elastomer implants and surface articulating implants. The design proposed in this study involves combining the principles of a surface weight-bearing articulating implant with those of a spanning elastomeric implant. The design consists of metacarpal and proximal phalangeal articulating housings and a central flexible spanning elastomer rod that maintains the alignment of the metacarpal and proximal phalangeal components. A preliminary finite element analysis was used to investigate the stresses in the design, the wear of the articulating bearing surfaces for different material combinations, and the stresses between the central elastomer rod and the articulating housings. This preliminary analysis shows that the design should withstand the probable loading conditions experienced within the human body and that the wear rates of the articulating surfaces and the central elastomer are acceptable.

Viscoelastic properties of bovine articular cartilage attached to subchondral bone at high frequencies

Background

Articular cartilage is a viscoelastic material, but its exact behaviour under the full range of physiological loading frequencies is unknown. The objective of this study was to measure the viscoelastic properties of bovine articular cartilage at loading frequencies of up to 92 Hz.

Methods

Intact tibial plateau cartilage, attached to subchondral bone, was investigated by dynamic mechanical analysis (DMA). A sinusoidally varying compressive force of between 16 N and 36 N, at frequencies from 1 Hz to 92 Hz, was applied to the cartilage surface by a flat indenter. The storage modulus, loss modulus and phase angle (between the applied force and the deformation induced) were determined.

Results

The storage modulus, E', increased with increasing frequency, but at higher frequencies it tended towards a constant value. Its dependence on frequency, f, could be represented by, E' = Alog_e (f) + B where A = 2.5 ± 0.6 MPa and B = 50.1 ± 12.5 MPa (mean ± standard error). The values of the loss modulus (4.8 ± 1.0 MPa mean ± standard deviation) were much less than the values of storage modulus and showed no dependence on frequency. The phase angle was found to be non-zero for all frequencies tested (4.9 ± 0.6°).

Conclusion

Articular cartilage is viscoelastic throughout the full range of frequencies investigated. The behaviour has implications for mechanical damage to articular cartilage and the onset of osteoarthritis. Storage modulus increases with frequency, until the plateau region is reached, and has a higher value than loss modulus. Furthermore, loss modulus does not increase with loading frequency. This means that more energy is stored by the tissue than is dissipated and that this effect is greater at higher frequencies. The main mechanism for this excess energy to be dissipated is by the formation of cracks.