Subacromial Ketamine Injection for Pain in Rotator Cuff Tendinopathy: A Randomized, Double-Blind, Placebo-Controlled Crossover Trial
Abstract
<title>Abstract</title> <p> <bold>Purpose:</bold> Emerging evidence indicates that peripheral N-methyl-D-aspartate receptor (NMDAR) activation may contribute to pain perception in individuals with rotator cuff tendinopathy (RCT). This study investigated whether subacromial injection of ketamine, an NMDAR antagonist, could reduce pain in patients with RCT. <bold>Methods:</bold> In a randomized, double-blind, placebo-controlled crossover trial participants received ultrasound-guided subacromial injections of either ketamine or saline, separated by a 2–14 week washout period. The primary outcome was the difference in pain intensity at rest, assessed using the visual analogue scale (VAS) 15 minutes post-injection with follow-up limited to 60 minutes post injection. Secondary outcomes included VAS scores at rest at additional time points, VAS during 90° abduction, shoulder abduction range (degrees), as well as pressure pain threshold and tolerance. Data were analyzed using mixed-effects models. <bold>Results:</bold> Fourteen patients (7 males, 7 females) were included. Pain at rest 15 minutes post-injection was statistically significantly lower following ketamine compared with saline (VAS 2.1, 95% CI 1.2, 3.0 vs. 2.9, 95% CI 2.1, 3.8), with an estimated treatment effect of -0.8 (95% CI -1.5, -0.1, p=0.021). Over the 60-minute follow-up, ketamine was associated with consistently lower pain scores at rest and during 90° abduction, with significant differences at some time points, and small, non-significant improvements in shoulder abduction range of motion. Pressure pain threshold and tolerance showed only minimal, non-significant differences between groups. <bold>Conclusion:</bold> Subacromial ketamine was associated with a small, short-term reduction in pain and a favourable safety profile in patients with RCT. However, the magnitude of the effect was below commonly accepted thresholds for minimal clinically important differences. These findings should therefore be interpreted as hypothesis-generating and support further investigation in larger, adequately powered studies with longer follow-up, repeated dosing, and sex-specific analyses. <bold>Level of evidence: I</bold> Trial registration: EudraCT No. 2012-002782-35 (Norwegian Medicines Agency approval dated 20/09/2016); ClinicalTrials.gov, NCT02967640 (first submitted 16/11/2016) </p>
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