The osteoarthritis knee model: psychophysical characteristics and putative outcomes

The knee osteoarthritis (KOA) model is a convenient and coherent archetype that is frequently used in pharmaceutical trials of drugs with analgesic and/or anti-inflammatory properties; yet, little is known about its specific pathophysiology. The presumed chronic inflammatory etiology of osteoarthritis suggests that nociceptive processes and neurogenic inflammation predominate in this condition. However, most chronic pain conditions are associated with changes in peripheral and central processing. Recent data corroborate this as an important mechanism in KOA. We compared psychophysical characteristics (including thermal Quantitative Sensory Testing); thermal, mechanical, and functional wind-up; thermal and mechanical aftersensations; and pressure algometry of 37 subjects with KOA with 35 age- and sex-matched controls. A third of the KOA subjects demonstrated hypoesthesia to vibration and the 4.56 von Frey fiber, yet few showed allodynia in their worse knee. The majority of subjects had abnormalities to pinprick (41% were hyperalgesic and 27% were hypoesthetic). Compared to controls, the more painful knee was hypoesthetic to cold detection and had greater thermal wind-up, lower pressure-pain thresholds, thermal and mechanical aftersensations, and twice the pain ratings of controls after stair climb. Substantial intraindividual differences were found in KOA subjects and controls for mechanical wind-up and algometric thresholds.

These results develop the KOA model and suggest mechanistic hypotheses. Certain of these tests may ultimately prove to be responsive, quasi-objective, and quantitative outcomes for research and lend empirical support to the notion of measurable sensitization in osteoarthritis.