Mechanical resonance spectra in human cancellous bone.

The dynamic mechanical response of fresh human cancellous bone at low audio frequencies contains two resonance spectra. The spectral frequencies in each series have the ratios 1:4:9:16 ... n(2). The frequencies are in quantitative agreement with the concept of momentum wave modes of calcium and phosphorus atoms in the lamellae, with no variable parameters.

"Stiction-friction" of total hip prostheses and its relationship to loosening.

The static friction, or "stiction-friction", in McKee-Farrar and Charnley-Müller prostheses in a hip joint simulator was compared with dynamic friction determined while the prostheses were oscillated. Under physiological conditions stiction-friction differed little from dynamic friction in both metal-on-metal and metal-on-plastic prostheses, and was affected very little by the lubricant as long as some fluid was present. Stiction-friction increased significantly only after relatively long stationary periods and high loads. However, the frictional forces generated in total hip-replacement prostheses were at least forty times higher than those generated in normal joints and may well be enough to cause late loosening of acetabular components by fatigue failure.

Changes in the bone-cement interface after total hip replacement. An in vivo animal study.

UNLABELLED: In order to study the temporal sequence of radiographic, histological, mechanical, bacteriological, and chemical changes around the femoral component following total hip replacement, a model was created by implanting plastic-on-metal total hip replacements in sheep and walking the animals on a concrete surface beginning six weeks postoperatively. This model demonstrated a decreased torsional rigidity between the prosthesis and the femoral cortex in all sheep. Failure of bonding occurred at the bone-cement interface and appears from our results to be most probably due to alterations in the functional stress of the proximal end of the femur following insertion of the femoral component rather than exothermic polymerization, toxicity of free monomer residue, or infection. CLINICAL RELEVANCE: An understanding of the causes of loosening of cemented metal femoral components in total hip replacement has been thwarted by a lack of specimens at sequential times in the loosening process. Since a patient is not operated on until the prosthetic components are completely loose, such specimens are difficult to obtain clinically. For this reason a model for examining the bone-cement interface, measured by decreased torsional rigidity of the prosthesis relative to the bone, was developed in sheep. Of all the parameters studied, those inherent in the effects (mechanical or vascular, or both) of insertion of the prosthesis itself appeared to be the most likely cause of the change in the mechanical properties of the interface. This suggests that degenerative changes of the bone-cement interface may be inevitable.

Effect of prolonged walking on concrete on the knees of sheep.

Adult sheep were subjected to prolonged activity on hard surfaces by walking them daily on concrete and housing them on tarmac. Control sheep were walked on compliant wood chip surfaces and pastured. After two and a half years significant changes were seen in both the distal femoral articular cartilage and subchondral trabecular bone of the knee joints of the hard surface walkers. The hexosamine content of the articular cartilage in the hard surface walkers was lower and this decrease was more marked in the weight-bearing than in the non-weight-bearing areas of the knee. The trabecular pattern of the subchondral bone became significantly altered, with a marked change in trabecular structure acting to stiffen the tibio-femoral joint at th expense of the patello-femoral articulation. There was a substantial increase in the contiguity ratio of bone in the tibio-femoral area. The cortical thickness of the subchondral plate was increased in both the tibio-femoral and patello-femoral areas. We concluded that significant changes occur in both cartilage and bone as a result of prolonged walking on hard surfaces.

Factors influencing articular cartilage wear in vitro.

The in vitro wear of articular cartilage in oscillating and impulsively loaded bovine metatarsophalangeal joints was studied. Articular cartilage scarification had little effect on cartilage wear, but stiffening of the subchondral bone with methyl methacrylate greatly increased the rate of cartilage loss, whether or not it had previously been scarified. Glutaraldehyde treatment of articular cartilage for 30 minutes decreased is wear rate. Guanidinium chloride extraction of the cartilage before wear testing caused it to peel off its subchondral bed. This effect could be spared by prior glutaraldehyde fixation.

Effect of repetitive impulsive loading on the knee joints of rabbits.

To simulate the forces from hopping, the right foot of adult rabbits was subjected to 1 1/2 the animal's body weight 40 times a minute for 20-40 minutes per day. During these brief periods of repetitive impulsive loading the legs were held in short-leg splints to eliminate the natural shock-absorbing mechanism associated with ankle dorsiflexion and calve muscle stretching. Under these conditions subchondral bone stiffening occurred and was associated with the earliest metabolic changes of cartilage damage. When bone stiffening returned to normal the effect on the cartilage did not completely disappear, although these effects diminished. The results suggested that subchondral bone stiffening accompanies the earliest metabolic changes in osteoarthrosic chondrocytes and suggests that trabecular microfracture may occur very early in this sequence of events.