Metrical analysis of disc-condyle relation with different splint treatment positions in patients with TMJ disc displacement

Abstract Objective: To evaluate the effect of bite positions characterizing different splint treatments (anterior repositioning and stabilization splints) on the disc-condyle relation in patients with TMJ disc displacement with reduction (DDwR), using magnetic resonance imaging (MRI). Material and Methods: 37 patients, with a mean age of 18.8±4.3 years (7 male and 30 females) and diagnosed with DDwR based on the RDC/TMD, were recruited. MRI metrical analysis of the spatial changes of the disc/condyle, as well as their relationships, was done in three positions: maximum intercuspation (Position 1), anterior repositioning splint position (Position 2), and stabilization splint position (Position 3). Disc/condyle coordinate measurements and disc condyle angles were determined and compared. Results: In Position 1, the average disc-condyle angle was 53.4° in the 60 joints with DDwR, while it was −13.3° with Position 2 and 30.1° with Position 3. The frequency of successful "disc recapture" with Position 2 was significantly higher (58/60, 96.7%) than Position 3 (20/60, 33.3%). In Positions 2 and 3, the condyle moved forward and downward while the disc moved backward. The movements were, however, more remarkable with Position 2. Conclusions: Anterior repositioning of the mandible improves the spatial relationship between the disc and condyle in patients with DDwR. In addition to anterior and inferior movement of the condyle, transitory posterior movement of the disc also occurred.


Introduction
Temporomandibular joint (TMJ) disc displacement is the most common type of TMJ arthropathy and involves an abnormal relationship or misalignment of the articular disc relative to the condyle. The usual direction for displacement of the disc is anteriorly or anterior-medially 29 . In spite of their apparent efficacy and widespread use for treating TMD, the precise mechanisms of action of oral splints remain controversial 10 . Hypotheses proposed include repositioning of condyle and/or articular disc, reduction parafunctional behaviours, and changes in patient's occlusion 6 . Two common types of oral splints used in clinical practice are the stabilization and anterior repositioning splints.
Anterior repositioning splints (ARS) have been shown to be effective for the management of disccondyle disorders 14, 18,26 . Also known as anterior positioning appliances or mandibular orthopedic repositioning appliances, they serve to direct the mandibular condyle anteriorly in the glenoid fossa (i.e., protrusive mandibular position). The purpose of ARS treatment is not to alter the condylar position permanently, but to change it during the treatment to help the adaption of the retrodiscal tissues 24 . Based on clinical and MRI assessments, approximately 70% of reducing displaced discs was captured with the use of ARS 18 . ARS could also alter mechanical stresses on the TMJ arising from the immediate physiologic improvement in the disc-condyle relationship 4 and has been shown to facilitate regenerative remodeling of the TMJ 22 . Although the recaptured discs can occasionally move backward with successful ARS therapy, the amount of disc movement was found to be negligible 19 .
The improved condyle-disc relationship with ARS was thought to be achieved primarily by the anteroinferior movement of the condyle.
Stabilization splints (SS) cover all the maxillary and mandibular teeth and are used to treat both joint and masticatory muscle disorders 2,17 . In contrast to ARS, SS are permissive appliances (allows for teeth to glide unimpeded over the biting surface) and do not protrude the mandible. They serve to provide a temporary and removable ideal occlusion at increased vertical dimension and centric relation. The use of SS increases TMJ space 12 and allows for anteroinferior movement of the condyles 7,11,16 . SS are also used to manage disc-condyle disorders 3 . They are effective in eliminating the signs/symptoms of TMD, except TMJ clicking 28 . When compared to ARS for the treatment of TMJ DDwR, reduction in dysfunction and TMJ symptoms were found to be lower with SS therapy 5,25,27 .
The mechanism of action of both ARS and SS  inferiorly when compared to normal joints (Table 2).
In contrast to normal joints, the disc moved 2.

Discussion
In this study, we determined the spatial changes As with all studies, the current research has some limitations. Since actual oral splints were not on disc and condyle positions/relationships could not be ascertained. The long term effects of ARS and SS on disc-condyle-fossa relationships were also discrepancies, malocclusion, and rheumatic or degenerative joint diseases were excluded, which may also affect disc "recapture" in joints with DDwR.

Conclusions
In summary, anterior repositioning of the mandible improved the spatial relationship between the disc and condyle, increasing the prospect of disc reduction in patients with DDwR. In addition to anterior and inferior movement of the condyle, transitory posterior movement of the disc also occurs with the anterior mandibular repositioning.
Metrical analysis of disc-condyle relation with different splint treatment positions in patients with TMJ disc displacement