Mechanical allodynia during ambulation in osteoarthritis (OA) animal models can be assessed as decreased extent of loading or decreased duration of loading. We propose to measure gait adaptation to pain by both mechanisms with the development of Limb Idleness Index (LII) in a rat model of knee OA.

Rats were assigned to anterior cruciate ligament transection (ACLT), Sham, or Normal group (n = 6). Gait data were collected at pre-injury, 1, 2, 3 and 6 months post-injury. Ratios of target print intensity, anchor print intensity, and swing duration were combined to obtain LII. The association of gait changes with pain was assessed by buprenorphine treatment at 3 and 6 months post-injury. At 6 months, OA-related structural changes in knee joints were examined by μCT and results from histological scoring were correlated with LII.

As compared to pre-injury level (range 0.75–1.20), LII in ACLT group was increased at 6 months post-injury, which was significantly higher than that in Sham and Normal groups (P = 0.024). The increase in LII in ACLT group was effectively reversed by buprenorphine treatment (P = 0.004). ACLT group exhibited a significantly higher maximum Osteoarthritis Research Society International (OARSI) score as compared to Sham (P = 0.005) and Normal (P = 0.006) groups. Significant correlation was found between LII and side-to-side difference in OARSI score (r = 0.893, P < 0.001).

LII presents a good measurement for OA-related knee pain in rat model.