Serum 25-Hydroxyvitamin D Status and 12-Week Functional Outcomes After Extracorporeal Shock Wave Therapy for Lateral Epicondylitis: A Retrospective Cohort Study

Background/Objectives: Serum 25-hydroxyvitamin D [25(OH)D] is a clinically used biomarker of vitamin D nutritional status, although it is also influenced by sunlight exposure, supplementation, season, and other host factors. Short-term functional status after extracorporeal shock wave therapy (ESWT) for lateral epicondylitis varies. We evaluated whether serum 25(OH)D level and status were associated with 12-week functional outcomes among ESWT-treated patients. Methods: This single-center retrospective cohort included 62 adults with lateral epicondylitis who received outpatient ESWT and had baseline and 12-week assessments. Baseline variables included grip strength ratio, Quick Disabilities of the Arm, Shoulder and Hand (QuickDASH) score, serum 25(OH)D measured using the Architect 25-OH D vitamin kit, common extensor tendon (CET) thickness, age, sex, and body mass index. Multivariable linear regression was used in an analysis-of-covariance framework. Serum 25(OH)D was assessed continuously and as <20 versus ≥20 ng/mL in exploratory threshold analysis. Results: Serum 25(OH)D was 21.0 ± 8.4 ng/mL; 30 patients (48.4%) had <20 ng/mL, 22 (35.5%) had 20–29.9 ng/mL, and 10 (16.1%) had ≥30 ng/mL. QuickDASH decreased from 42.0 ± 17.4 to 27.0 ± 13.7, and grip strength ratio increased from 0.58 ± 0.14 to 0.76 ± 0.14. Higher serum 25(OH)D was associated with lower 12-week QuickDASH after adjustment (β per 10 ng/mL = −4.04, 95% CI −7.17 to −0.91; p = 0.012). Additionally, 25(OH)D <20 ng/mL was associated with higher 12-week QuickDASH (β = 6.43, 95% CI 1.17 to 11.69; p = 0.017). Serum 25(OH)D was not clearly associated with 12-week grip strength ratio. Conclusions: Lower serum 25(OH)D, interpreted as a vitamin D nutritional-status marker rather than as a nutrition-specific causal exposure, was associated with worse 12-week patient-reported function, but not grip strength ratio. The <20 ng/mL threshold analysis was exploratory and was not powered for subgroup inference. These findings should be interpreted as observational and hypothesis-generating.