Duration of symptoms is associated with conditioned pain modulation and somatosensory measures in knee osteoarthritis

Impaired descending pain inhibition has been linked to chronic pain and poorer patient outcomes. Vibration perception threshold (VPT) has also been observed and linked to radiographic stage of OA. However, it is not known how duration of symptoms may influence these neurophysiological measures. Our purpose was to evaluate the relationship between duration of symptoms and conditioned pain modulation (CPM), a measure of descending pain inhibition, and VPT, and to determine whether these relationships differed in men and women seeking orthopedic care. We evaluated 18 men and 27 women with moderate to severe knee OA. We assessed CPM using a submaximal-effort tourniquet test: Pressure pain threshold (PPT) at the symptomatic knee was evaluated before and after a noxious stimulus. CPM impairment was indicated by a ratio of pre-to-post stimulus PPT ≥1. VPT was assessed using a biothesiometer at the medial femoral condyle. We used chi-square, t-tests and Pearson correlations to address study questions. 72% of men, but only 44% of women had CPM impairment. Duration of symptoms was associated with CPM impairment in women (R = 0.566, p = 0.003) but not men (R = 0.366, p = 0.135). Duration of symptoms was also associated with VPT in both men (R = 0.580, p = 0.012) and women (R = 0.406, p = 0.039). These results suggest that longer duration of knee OA may predict more severe pain sensitization and that important sex differences exist in descending pain inhibition in people with chronic knee OA that may affect disease and course of treatment in male and female patients.