Osteoarthritis

TKR needed 8 years earlier in obesity-related knee OA


A rethink is needed on how abnormal mechanical loading accelerates the progression of obesity-related knee OA, Australian researchers say.

Pressure from extra weight on the knee in obese patients leads to horizontal fissuring at the osteochondral unit interface and much earlier total knee replacement (TKR), a West Australian study has found.

Scientists at the  University of Western Australia and Fiona Stanley Hospital have shown that obese patients undergo first TKR around eight years earlier than those who are a regular weight.

The researchers analysed the correlation between BMI and the age at which patients undergo TKR in 41 023 patients from the Australian Orthopaedic Association National Joint Replacement Registry. They also investigated the effect of BMI on pathological changes of the tibia plateau of knee joint.

Published in the Annals of Rheumatic Diseases their study showed that 58% of patients in Australia who had TKR were obese. Compared to people of normal weight, those who were overweight, obese class I & II or obese class III underwent a TKR 1.89, 4.48 and 8.08 years earlier, respectively.

In a representative subset of 88 cases from the registry, histopathology investigation showed that patients with higher BMI had less disruption and matrix loss in the articular cartilage surface in the predominant compartment, as well as overall in both medial and lateral compartments, when compared with patients with lower BMI.

Microscopic examination also showed that people with obesity had horizontal fissures with cartilage erosion at the osteochondral interface with subchondral bone, characterised by irregular cartilage erosion, fibrogranulation tissue infiltration, the presence of cartilage/bone debris and rupture of microcapillaries.

The frequency of horizontal fissure was strongly associated with increased BMI: an increase in one unit of BMI (1 kg/m2) increased the odds of horizontal fissures by 14.7%.

Reduced cartilage degradation and alteration of subchondral bone microstructure were also associated with increased BMI.

Lead author Professor Ming-Hao Zheng, Associate Dean of UWA’s Faculty of Health and Medical Sciences, noted that 84.4% of the horizontal fissures in the predominant compartment were attributable to obesity.

“The data revealed that 80 per cent of the obese patients had a knee replacement due to horizontal fissuring,” he said.

“This was different to the reason regular weight patients sought knee replacements – instead they underwent surgery mainly due to cartilage damage from normal wear and tear to a joint.

“This means obese patients are most likely to require further replacement of prosthetic implant as the lifespan of the prosthesis is less than maximum of 15 years.”

Speculating on a mechanism, Professor Zheng noted that hyaline cartilage, calcified cartilage and subchondral bone have different mechanical properties and that high compression loading would generate excessive shear forces and thus induce fissuring at the interface.

“The structural deterioration enhances the metainflammatory reactions, attracts the ingrowth of vessels and nerves into the cartilage and further aggravates the symptoms of patients with obesity during joint motion,” they wrote.

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