Igor L. Paul

Response of joints to impact loading, III: relationship between trabecular microfractures and cartilage degeneration

Abstract The knee joints of adult rabbits were subjected to daily one hour intervals of impulsive loading equivalent to their body weight at 60 cpm. They developed changes in their knee joints consistent with those of degenerative joint disease. The cartilage destruction was preceded by stiffening of the underlying subchondral bone. Numerous healing trabecular microfractures were associated with this increased stiffening. Similar microfractures have been observed in human specimens. The implications of these findings to bone remodelling and osteoarthritis are discussed.

ROLE OF MECHANICAL FACTORS IN PATHOGENESIS OF PRIMARY OSTEOARTHRITIS

Abstract Studies of the relation between joint function and mechanical stress have led to a revival of the old concept that primary osteoarthritis is actually a wearing out of joints. Recent experimental evidence suggests that joints can wear out by repetitive impulsive loading, rather than by rubbing. This new mechanistic approach is compatible with the pathology of, and clinical experience with, the disease.

A Comparison of the Dynamic Force Transmitting Properties of Subchondral Bone and Articular Cartilage

Compression tests on plugs of bovine articular cartilage and of subchondral bone demonstrated that both are capable of deforming under pressure and thus attenuating peak dynamic forces applied to the plugs. Although cancellous bone is approximately ten times stiffer than articular cartilage per unit of thickness, subchondral bone in vivo is considerably thicker and would, therefore, seem capable of contributing to dynamic force attenuation as much as the cartilage does or more. Under physiological loads and loading rates, a film of synovial fluid on the cartilage added nothing to the ability of cartilage plugs to attenuate peak dynamic force; however, at very low load rates cartilage plugs coated with synovial fluid were consistently stiffer than specimens coated with veronate buffer. It is highly likely, from the evidence presented, that alterations in the quality of the subchondral bone could have a profound effect on the ability of a subchondral bone-articular cartilage system to withstand compressive dynamic forces.

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.

The response of joints to impact loading. II. In vivo behavior of subchondral bone.

The knee joints of live guinea pigs, subjected to repeated longitudinal impaction, developed obvious cartilage degeneration over a 3 week period. In vitro tests had previously shown that articular cartilage is particularly susceptible to injury from impact loading and that subchondral bone acts as a cushion to protect the overlying cartilage from damage during such loading. Associated with, and slightly preceding the earliest cartilage changes, as judged histochemically, was a stiffening of the underlying subchondral bone. The bone stiffness measurements returned to within the normal range as the cartilage degeneration progressed.

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 animals 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.

A Consolidated Concept of Joint Lubrication

Animal joints contain two systems which require lubrication: a soft-tissue system, involving the sliding of synovial membrane on itself or on other tissues, and a cartilage-on-cartilage system. The resistance to motion caused by soft-tissue stretching and friction is far greater than that due to cartilage rubbing on cartilage; in fact, soft-tissue resistance may be primarily responsible for clinical joint stiffness. Lubrication of the softtissue system depends on molecules of hyaluronate in the synovial fluid which stick to the synovium in a layer and keep moving surfaces apart. Cartilage-on-cartilage lubrication, by contrast, is independent of hyaluronic acid. A hyaluronate-free glycoprotein fraction which has been isolated from synovial fluid confers on a buffer solution a lubricating advantage equal to that of whole synovial fluid. Removal of this fraction from synovial fluid deprives the fluid of any lubricating advantage over buffer. The action of this glycoprotein on cartilage is a boundary phenomenon similar to that of hyaluronate on synovium. In addition to this boundary effect, cartilage surfaces are kept apart by a fluid squeeze film made up of joint fluid and interstitial fluid which weeps from the articular cartilage itself. The squeeze-film effect is probably potentiated by the undulations of the surface and the elasticity of the cartilage, which may lower frictional resistance by elastohydrodynamic effects.

Musculo-skeletal shock absorption: Relative contribution of bone and soft tissues at various frequencies

Abstract In order to study the relative contribution of bone and soft tissue in the attenuation of peak dynamic force as a function of frequency, the legs of rabbits with a force transducer implanted in their upper tibial shaft were impulsively loaded with a variety of cams. The effects of this loading were studied with the animals anesthetized and after removal of various soft tissue constituents of the lower extremities. The results were subjected to frequency analysis. There was no difference in results whether the animal was awake or asleep. A resonance frequency between 6–9 Hz existed in the bone of all animals tested. Soft tissue and bone attenuated peak forces most effectively at higher frequency ranges. These components tended to complement the previously described shock-absorbing mechanism involving the stretching of muscle under tension, which appears to be more effective at lower frequencies. The heel pad contributed to peak force attenuation over the entire measured frequency spectrum.

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.

A comparative study of total hip replacement prostheses

Abstract A comparative laboratory study has been made of three widely used types of total hip replacement prostheses. With the aid of hip joint simulators and a specially designed wear measuring gauge the friction and wear characteristics of the prostheses have been measured during 1000 hr laboratory tests. Scanning Electron Microscope studies have shown the wear mechanisms operating in the different types of prostheses during the simulator tests. Similar studies of prostheses removed from patients have shown the validity of drawing conclusions about the clinical performance of the prostheses from the laboratory test results.