HIP ARTHROPLASTY ALTERS THE FUNCTIONAL CONNECTIONS OF THE BRAIN'S PAIN PROCESSING REGIONS

Hip osteoarthritis (OA) is characterized by chronic pain, which is seldom matched by radiological findings. Using brain MRI, we recently demonstrated that functional connections of the brain's pain processing regions [e.g., secondary somatosensory cortex (S2), posterior insula] are altered in hip OA patients, and the connections of several regions [e.g., thalamus, periaqueductal grey matter (PAG)] are modulated by pain-eliciting activity. In other chronic pain conditions, pain processing in the brain remains altered even when pain is alleviated. Although hip replacement surgery has a high success rate, it is unknown if the brain's pain processing networks are normalized, which could have implications for the clinical management of recurring pain. Resting-state fMRI was performed on 14 hip OA patients (43–72 years of age; 9 males), before and after (>18 months) hip replacement surgery. Patients underwent two fMRI scans per session, before and after stair climbing to elicit acute pain. Pain was assessed using a 10-pt visual analog scale. Data were analyzed using a region-of-interest (ROI) approach, whereby the average temporal signal from S2, PAG, thalamus and posterior insula were determined for each patient, and Pearson's temporal cross-correlation coefficient (r) was considered as the strength of functional connection (i.e., connectivity) between each possible pair of ROIs. A repeated-measures analysis of covariance was performed on the Fisher-transformed r values, with SURGERY (pre, post), STAIRS (before, after) and the interaction between SURGERY and STAIRS (i.e., if surgery impacted the change in functional connectivity evoked by stair climbing) as within-subject factors, SEX (male, female) as a between-subjects factor, and presurgical pain score as a covariate. The median presurgical pain score was 4.5, with a median increase of 2 after stair climbing. The median postsurgical pain score was 0, with a median increase of 1 after stair climbing. SURGERY significantly increased connectivity between S2 and posterior insula [F(1,11) = 5.95; p = 0.033], and higher presurgical pain scores were associated with a greater connectivity increase [F(1,11) = 5.48; p = 0.039]. The interaction between SURGERY and STAIRS was significant for connectivity between a) S2 and PAG [F(1,11) = 5.09; p = 0.045] (STAIRS decreased connectivity presurgically, but increased connectivity postsurgically); b) S2 and thalamus [F(1,11) = 14.10; p = 0.003] (STAIRS decreased connectivity presurgically, but did not alter connectivity postsurgically); and c) posterior insula and thalamus [F(1,11) = 15.41; p = 0.002] (STAIRS did not alter connectivity presurgically, but increased connectivity postsurgically). Hip replacement surgery significantly changes the functional connections between brain regions that process pain and how they respond to pain provoking stimuli. Further studies are planned to include healthy control participants, to determine if hip replacement normalizes pain processing in the brain. A chronically altered pain processing network could have clinical implications for postsurgical pain management.

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