Mean Contact Stress Research Articles

Contact stress distributions in malreduced intraarticular distal radius fractures.

Residual articular incongruity of the distal radius following intraarticular fracture has been correlated with early osteoarthritis (OA) and a poor clinical outcome. We developed a simple in vitro fracture model of the distal radius to investigate the relationship between degree of articular incongruity and the resulting distribution of radiocarpal contact stress. Twelve fresh-frozen cadaver arms were dissected, packets of Fuji Pressensor film were inserted into the wrist, and the wrist was loaded through its flexor and extensor tendons. We created a simple intraarticular fracture that allowed controlled distal radius articular incongruity. Loading trials were performed for the intact distal radius, for a fully reduced case, and for step-offs of 0.4, 1, 2, and 3 mm. Mean contact stress was significantly greater than the anatomically reduced case at only 3 mm of step-off. Contact area was greater than the anatomically reduced case at 0.4, 1, and 2 mm of step-off. The elevations in contact stress that we observed were only modest, suggesting that other factors may be involved in the pathogenesis of radiocarpal OA in the presence of residual articular incongruity.

Displaced intra-articular fractures of the distal radius: The effect of fracture displacement on contract stresses in a cadaver model

Contact stresses in the wrist were measured after simulating displaced fractures of the lunate fossa in the distal radius of eight human cadaver arms. Osteotomies created displaced lunate fossa fractures of 0, 1, 2, and 3 mm. Contact stresses were measured with Fuji pressure-sensitive film after loads of 100 N were applied to the wrist through wrist flexor and extensor tendons. Mean contact stresses were significantly increased with step-offs of 1 mm or more. Maximum stresses and overloaded areas were significantly increased with step-offs of 2 mm or more. As the magnitude of the fracture displacement increased, there was a shift in the focus of the maximum stresses toward the fracture line. In this model, simulated displaced die-punch fractures created alterations in both the magnitude and location of contact stresses in the wrist joint.

Non-uniformity of contact stress on polyethylene inserts in total knee arthroplasty

This study examined tibiofemoral contact stresses in 15 commercially available TKR designs, using digitally imaged pressure-sensitive film. The objectives were (1) to determine the correlation between minimization of spatial mean contact stress and minimization of the amount of overloaded (>10 MPa) polyethylene contact area, and (2) to ascertain the difference in contact stresses for machined versus moulded polyethylene inserts. The data showed that there was no statistically significant (design rank-order) correlation of spatial mean contact stress with overloaded polyethylene area. The data also showed that machining cutter preparation of the polyethylene insert caused substantial local contact stress non-uniformities that corresponded to the pattern of grossly visible machining marks.

The Stiffness of Human Apophyseal Articular Cartilage as an Indicator of Joint Loading

The stiffness and thickness distribution of healthy lumbar apophyseal cartilage was measured in 25 lumbar motion segments (L1–4). The cartilage indentation and needling techniques of Swann and Seedhom (37) were suitably altered to cope with the low modulus and small size of the joint surfaces. A load of 3.12 ± 0.19 N (mean ± s.d.) was applied to the cartilage through a hemispherical indenter of 4.756 mm diameter. The stiffness was calculated using the displacement and instantaneous load 150 ms and 2 s after the indenter first contacted the surface, and using the equations of both Waters (46) and Hayes et al. (47). The mean stiffness of apophyseal joint cartilage was 2.8 M Pa ± 4 per cent (mean ± 95 per cent confidence limit), and thickness 1.02 mm ± 3 per cent. Peripheral apophyseal joint cartilage was softer than central cartilage. The stiffnesses of the centres of the superior and inferior joint surfaces were 3.01 MPa ± 12 per cent and 3.55 MPa ± 11 per cent. Inferior surfaces had a mean thickness of 0.93 mm ± 5 per cent and stiffness of 2.88 MPa ± 7 per cent. Superior surfaces had a mean thickness of 1.10 mm ± 4 per cent and stiffness of 2.74 MPa ± 5 per cent. It was found that the stiffness of cartilage calculated according to the formula of Hayes et al. (47), based on a constitutive analysis of a thin isotropic elastic layer, was directly proportional to both the stiffness calculated using the semi-empirical formula of Waters (46), derived to describe indentation of thin rubber sheets, and the nominal compressive creep modulus calculated by dividing the mean contact stress by the strain at the deepest point of indentation. The creep modulus calculated 2 s after contact was directly proportional to the creep modulus calculated 150 ms after indenter contact, implying that deformation behaviour was uniform between these points despite variation of cartilage stiffness.

In vitro contact stress distribution on the femoral condyles.

Contact stress distributions were measured across the tibiofemoral joints of 11 fresh-frozen normal cadaver specimens. Local stress magnitudes were sensed by arrays of miniature piezoresistive transducers inset superficially in the cartilage of the femoral condyles. Knee joints were tested at 0, 10, 20, and 30 degrees of flexion and later retested at 0 degrees following medial and dual meniscectomies. For the intact joint, both the spatial mean and peak local contact stresses rose approximately linearly with the joint resultant, reaching levels of 2.6 and 8.0 MPa, respectively, at 3 kN of applied loading. Flexion angle variations in the range studied failed to cause significant changes in the major contact parameters. Although meniscectomies involved contact area reductions and consequent stress increases, such effects were more moderate than results previously seen for static loading of the tibial plateau.

In vitro contact stress distributions in the natural human hip

Time variant distributions of intra-articular contact stress were assembled from direct measurement in seventeen grossly normal fresh cadaveric hips. Local stresses were sensed by arrays of 24 compliant miniature transducers inset superficially in the femoral head cartilage. Local contact stress magnitude was usually found to rise nearly linearly with applied joint loads in excess of about 1000 N. The sites of maximum local stress were found to underlie the general region of the acetabular dome. For a resultant joint load of 2700 N, the spatial mean contact stress and peak local contact stress averaged 2.92 MN m −2 and 8.80 MN m −2, respectively, for the 68 loading cycles analyzed. The full contact stress patterns were irregular and complex, but most commonly the general feature was a central band or ‘ridge’ of pressure elevation, oriented in an approximately anterior-to-posterior direction.

Mean contact stress in the densification rolling of powder metallurgy materials

Mean contact stress in the densification rolling of powder metallurgy materials