Purpose: The efficacy of occlusal splints for treating temporomandibular disorders (TMDs) remains controversial. This study aimed to evaluate and compare the effectiveness of flat-plane splints (FPSs) with a vertical thickness of 3 mm (VT3) and 5 mm (VT5) in treating disc displacement without reduction (DDWOR). Materials and Methods: This retrospective matched-cohort study selected the study participants from 400 TMD patients treated in the Department of Oral and Maxillofacial Surgery of Tainan Sin-Lau Hospital, Tainan, Taiwan, between August 2013 and July 2015. The thickness of occlusal splints was the predictor variable. The outcome variables included joint crepitus sound, deviation of the mandible during opening, TMD-associated headache, myofascial pain with referral, temporomandibular joint (TMJ) arthralgia, and maximum assisted opening. The case and control groups were matched 1:1 by propensity scoring to ensure that there were no statistical differences in the categorical variables and continuous variables. The analysis used χ2 tests, t tests, and multiple regression analyses. Results: We allocated 162 patients into 2 groups, with 81 patients each: VT3 group and VT5 group. Both VT3 FPSs and VT5 FPSs effectively improved the DDWOR. At 12 months after treatment, the VT5 group showed remarkable improvement in joint crepitus sound and TMJ arthralgia compared with the VT3 group. No statistical differences in other TMJ-associated symptoms such as deviation of the mandible during opening, TMD-associated headache, and myofascial pain with referral were observed between the 2 groups. Conclusions: Both VT3 and VT5 FPSs can effectively improve various clinical symptoms of DDWOR. We suggest that VT5 FPS treatment for at least 1 year is a suitable option for DDWOR patients with joint crepitus sound and TMJ arthralgia. For DDWOR patients without joint crepitus sound and TMJ arthralgia, there are no differences between the VT3 and VT5 FPSs.
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