Ultrasound assessment of synovial hypertrophy as a potencial functional biomarker in temporomandibular joint arthralgia: a pilot correlation study

Point-of-care ultrasound (POCUS) enables real-time visualization of synovial and capsular changes in temporomandibular (TMJ) disorders. This cross-sectional pilot study evaluated whether ultrasound-measured synovial membrane thickness was more strongly associated with pain-related functional limitation and pressure pain thresholds (PPT) than condyle–capsule distance in TMJ arthralgia. Nine participants (five with arthralgia and four asymptomatic controls) were included; the participant was the statistical unit. The symptomatic joint was used for unilateral arthralgia; otherwise, the mean of both joints was used. Synovial membrane thickness showed a strong inverse association with pain-free mouth opening (Pearson r = − 0.923, 95% CI −0.984 to −0.669; p = 0.0004; Spearman ρ = −0.803; p = 0.009). Condyle–capsule distance showed a weaker association ( r = −0.502, 95% CI −0.875 to 0.243; p = 0.168). A secondary exploratory ROC analysis showed complete separation in this dataset, yielding an apparent AUC of 1.00 and a sample-derived threshold of 1.70 mm, not interpretable as diagnostic accuracy. Right and left TMJ PPTs were strongly associated ( r = 0.880, 95% CI 0.519 to 0.975; p = 0.002), without permitting attribution to central sensitization. Synovial membrane thickness should be regarded as a candidate ultrasound-derived correlate of functional limitation requiring validation in larger cohorts.