Specific and shared mechanisms mediating pain intensity and functional outcomes in chronic neck pain

BACKGROUND: To date, very few studies have used mediation analysis to explore the reasons why people with chronic neck pain improve after physical therapy intervention involving manual therapy or exercise. OBJECTIVES: To evaluate which specific mechanisms (mobility, pain modulation and endurance) and shared mechanisms (self-efficacy, working alliance, engagement, fear, and positive coping) mediated the effects of treatment on clinical outcomes at 4-weeks and 6-months in individuals with chronic neck pain. METHODS: The study incorporated planned indirect‑only mediation analyses to evaluate both specific and shared mechanisms as mediator using data from a mechanistic clinical trial comparing manual therapy and exercise interventions, which demonstrated no between‑group differences. Baseline pain intensity was modeled as the independent variable, and pain interference and physical function at four-weeks and six-months were specified as outcomes. Direct, indirect, and total effects were estimated across four mediation models. RESULTS: None of the mediation analyses found specific or shared mechanisms that mediated the relationship of baseline pain intensity to long-term pain interference and physical function. Discussion/Conclusion: Our findings suggest that the absence of significant mediation implies that commonly targeted mechanisms may not be the primary drivers of the relationship of pain intensity to longer-term pain interference or physical function. CLINICAL TRIAL REGISTRATION: www.clinicaltrials.gov identifier is NCT05940012. Chronic neck pain is common, but we still do not fully understand why certain treatments help people improve. This study looked at whether different physical and psychological factors, called “mechanisms,” explain how people with chronic neck pain get better over time. The study included data from a clinical trial where adults with chronic neck pain received either manual therapy or strengthening exercises for 4 weeks. Since both treatments worked about the same, the team focused on whether changes in specific physical factors (like neck mobility, muscle endurance, or sensitivity to pressure) or shared psychological factors (like fear of movement, coping skills, self‑efficacy, or the therapist–patient relationship) explained improvements in pain and function. Across 16 statistical models (known as mediation models), most proposed mechanisms did not explain the link between starting pain levels and later outcomes at 4 weeks or 6 months. The only meaningful findings were that reductions in fear‑avoidance and improvements in coping during the first few weeks of treatment helped explain better physical function at 4 weeks. No mechanisms explained changes in pain interference, and none predicted longer‑term outcomes at 6 months. These results suggest that early psychological shifts, especially feeling less fearful about movement and using better coping strategies, may play a small role in short‑term functional improvement. However, many commonly assumed physical and psychological mechanisms did not show measurable influence. This highlights the complexity of chronic neck pain and the need for more precise tools to understand how and why people improve.