| Sign In to gain access to subscriptions and/or personal tools. |
Stereometric Assessment of TMJ Space Variation by Occlusal SplintsClinic for Masticatory Disorders, Removable Prosthodontics and Special Care Dentistry, Center for Oral Medicine, Dental and Maxillo-Facial Surgery, University of Zürich, Switzerland, Plattenstrasse 11, CH-8032 Zürich, Switzerland Correspondence: * corresponding author, dominik.ettlin{at}zzmk.uzh.ch
Occlusal splints are used for the management of temporomandibular disorders, although their mechanism of action remains controversial. This study investigated whether insertion of an occlusal splint leads to condyle-fossa distance changes, and to mandibular rotation and/or translation. By combining magnetic resonance images with jaw tracking (dynamic stereometry), we analyzed the intra-articular distances of 20 human temporomandibular joints (TMJs) before and after insertion of occlusal splints of 3 mm thickness in the first molar region. For habitual closure, protrusion, and laterotrusion in the contralateral joint, occlusal splints led to minor—yet statistically significant—increases of global TMJ space and to larger increases at defined condylar areas. Condylar end rotation and translation in habitual closure were reduced. Hence, the insertion of a 3-mm-thick occlusal splint led to a change in the topographical condyle-fossa relationship, and therefore to a new distribution of contact areas between joint surfaces.
Key Words: biomechanics • Magnetic Resonance Imaging • stress • mechanical • occlusal splints • temporomandibular joint disorders • temporomandibular joint
Signs and symptoms of myoarthropathies of the masticatory system (MAP), i.e., of craniomandibular disorders, are quite common. In approximately 3.6 to 7% of affected persons, the symptoms are severe enough to require treatment (Okeson, 1996). The MAP etiology appears to be multifactorial and includes structural overload and lack of tissue adaptation (Milam, 2006; Stegenga and de Bont, 2006). In clinical practice, oral splints are one of the most common treatment modalities, in addition to physical therapy, medication, psychological treatment, and, ultimately, surgery (Dionne et al., 2006). Splint treatment is non-invasive and reversible if correctly applied; however, its mechanism of action has not been elucidated, and controversial ideas exist about its clinical indication and functioning. These include placebo effect, changes in occlusion, improvement of jaw-muscle function, modification of oral behavior, recruitment of different motor units, behavioral intervention, and new positioning of the condyle and/or the articular disc (Dao and Lavigne, 1998; Al-Ani et al., 2004; Forssell and Kalso, 2004; Nilner, 2004; Turp et al., 2004; Schindler et al., 2006). Regardless of its mode of action, some randomized clinical trials and literature reviews have documented its therapeutic effectiveness (Kreiner et al., 2001; Ekberg et al., 2002; Kuttila et al., 2002). The literature is not conclusive on whether an occlusal splint increases the condyle-fossa distance, as shown by measurements performed on radiographs of the temporomandibular joint (TMJ) (Moncayo, 1994; Ekberg et al., 1998; Kuboki et al., 1999; Lau and Green, 2003). Dynamic stereometry facilitates a non-invasive, in vivo, three-dimensional, and person-specific analysis of the condyle-fossa distance during static positions and mandibular movements. The technique combines the three-dimensional reconstruction of the TMJ anatomy obtained by magnetic resonance (MR) images with jaw motion data obtained by tracking the jaw with 6 degrees of freedom (Palla et al., 2003; Gossi et al., 2004; Gallo, 2005). The aim of this study was to analyze, by dynamic stereometry, the effect of the insertion of an occlusal splint of 3-mm thickness on (1) the minimum condyle fossa-distance, (2) the condyle-fossa distance closest in maximum intercuspation, and (3) condylar rotation and translation.
Participants Ten healthy asymptomatic individuals (five males and five females; age range, 18 to 31 yrs) participated in the study after giving informed consent. They were free of signs and symptoms of a myoarthropathy, as revealed by history and clinical examination. Exclusion criteria were tenderness to palpation of masticatory muscles and TMJ area, painful jaw movements (maximum mouth opening < 40 mm, laterotrusion and protrusion < 7 mm), TMJ pathological signs on MR images, systemic diseases, or contraindications for MR recordings (implanted metal or medical devices, claustrophobia, tattoos). The local ethics committee approved the study protocol.
Occlusal Splint The same casts were also used to construct the metal frames used to attach the frames carrying the light-emitting diodes (LED) to the dental arches (Mang, 2006).
Data Acquisition
Experimental Procedure
Data Analysis Condyle-Fossa Distance The effect of the occlusal splint on the condyle-fossa distance was assessed by two parameters: (1) the global minimum condyle fossa-distance (hmin), defined as the mean of the 30 minimal distances between the polygon vertices of the condyle and fossa surfaces (Fig. 1C  ); and (2) the condyle-fossa distance (h0) at the condylar point P0, defined as the point on the condylar surface closest to the fossa in maximum intercuspation, i.e., without the person wearing the splint (Fig. 2A ). In this situation, hmin was identical to h0. The spatial coordinates of hmin in relation to the condylar surface were determined for the recordings without and with splint obtained in closed position. The parameters hmin and h0 were averaged over all time steps during each recording. The condyle-fossa distance during laterotrusion was calculated only for the balancing-side joint.
Condylar Translation and Rotation To analyze the effect of occlusal splint insertion on condylar position, we computed the spatial translation (mm) and rotation (°) around the condylar main axis (Fig. 2B ). The reference point for the rotation was the mid-point of the condylar main axis connecting the 2 condylar poles. A detailed description of the materials and methods has been published elsewhere (Mang, 2006).
Statistical Analysis
Condyle-Fossa Distance Global Minimum Condyle-Fossa Distance (hmin) The insertion of the occlusal splint led to a slight, yet statistically significant, increase of hmin for all positions: habitual closure (median/mean of 0.3/0.3 mm; interquartile range, 0.2 to 0.6 mm), protrusion (median/mean of 0.4/0.3 mm; interquartile range, 0.0 to 0.5 mm), and laterotrusion (median/mean of 0.3/0.2 mm; interquartile range, 0.0 to 0.6 mm) (Table ). The increase in hmin was found in 16 TMJs in closed position and in 15 TMJs during protrusion and laterotrusion. Splint insertion led to a change in the position of the global minimum joint space in relation to the condylar surface dorsoventrally (median of 1.7 mm, range –7.5 to 5.4 mm), lateromedially (median of 1.8 mm, range –2.1 to 7.7 mm), and craniocaudally (median of 2 mm, range –1 to 4.6 mm) (example of one joint in Fig. 3A). The displacement vectors for all left and right joints are represented in Fig. 3B.
Condyle-Fossa Distance (h0) at Point P0 The insertion of the occlusal splint produced a statistically significant increase of h0 for all positions: habitual closure (median/ mean of 1.5/1.6 mm; interquartile range, 0.9 to 2.1 mm), protrusion (median/mean of 1.7/1.8 mm; interquartile range, 1.3 to 1.9 mm), and laterotrusion (median/mean of 1.6/1.6 mm; interquartile range, 1.1 to 2.1 mm) (Table ). The increase was observed in all joints.
Condylar Translation and Rotation
The objective of this study was to analyze, by means of dynamic stereometry, the effect of the insertion of a 3-mm-thick occlusal splint on the condyle-fossa distance in asymptomatic persons while mandibular teeth were kept occluding in various positions, such as habitual closure, during protrusion, and during laterotrusion. This study is therefore the first to provide an analysis of changes in the condyle-fossa distance, not only three-dimensionally and dynamically, but also over the entire joint surface, in contrast to other studies, where the condyle-fossa distances were measured statically and only on one joint section (Kuboki et al., 1997, 1999). The results revealed that the global minimum condyle-fossa distance hmin increased significantly after the insertion of a 3-mm-thick occlusal splint for effortless habitual closure, as well as during sliding movements. The increase was small, however, and it is arguable whether this difference translates into joint load reduction. Due to the incongruent shapes of both the condyle and the fossa, the location of hmin shifted in all joints. Analysis of our data further revealed a slight ventral and caudal shift of the condyle and a simultaneous jaw opening in persons wearing the occlusal splint. Also, the minimum condyle-fossa distance h0 at point P0 (i.e., the point on the condylar surface closest to the fossa in maximum intercuspation with the person not wearing the splint) increased significantly with splint insertion, both in habitual closure as well as during sliding movements. The variation of h0 was much larger than the variation of hmin. This change might be caused either by the rotation/translation of the condyle-disc complex and/or by a different disc thickness that would coincide with P0 at the new condylar position. It is worth noting that P0 was never located in the most posterior condylar region. As for the measurement of hmin, notable inter-individual differences have also been observed for h0, possibly due to differences in joint anatomy. It has been postulated that during clenching as well as jaw movements, forces between the condyle and the articular eminence are compressive and tangential in nature and undergo different magnitude levels (Kuboki et al., 1997, 1999). It can be assumed that the intra-articular distance is mainly influenced by the disc’s poroviscoelastic properties (Beek et al., 2003). These allow the disc to serve as the primary element for stress distribution and shock absorption. When its properties are altered, the functional effectiveness of the disc is reduced, presumably resulting in increased joint failure (Chen et al., 1999). Hence, degenerative joint disorders may be the result of mechanical joint overload, and there seems to be an association between the intensity and frequency of mechanical stress and the reaction of the cartilage matrix (Milam and Schmitz, 1995). Investigating joint load in vivo, however, requires invasive direct monitoring of joint forces, which has never been successfully performed due to technical challenges. Therefore, rather than investigating intra-articular distances of the TMJ under load, this study focused on joint space changes when joint forces were minimized by keeping the mandibular teeth occluding with only minimal pressure in effortless habitual closure. We also observed that the path of the minimum condyle-fossa distance during movements varied, likely due to person-specific TMJ morphologies (data not reported). This effect, as well as the shift of the area of minimum condyle-fossa distance in habitual closure after occlusal splint insertion, is an indirect indicator of changes in contact areas, and therefore contact stresses, between and among the different joint components. This contact area shift may contribute to an altered TMJ mechanical environment, a concept well-established for other load-bearing joints, such as the knee, where wedge osteotomies are performed to redistribute contact stresses for the management of osteoarthritis (Dahl et al., 2005; Brouwer et al., 2006; Ramsey et al., 2007). However, the findings of this study must be interpreted with caution regarding pain relief following splint therapy in the case, for instance, of osteoarthritis. First, our data were collected on a sample of asymptomatic persons, and therefore cannot be extrapolated to pain patients. Second, dynamic stereometry is based on the reconstruction and animation of the TMJ by means of anatomical and kinematic data. Although the technique is currently the most suitable method to study variations of the intra-articular space under static and dynamic conditions, it is still unknown whether mandibular deformation affects condyle-fossa measurements under larger bite forces, as might occur during parafunction. Nevertheless, it has been reported that bilateral biting in the canine area, even with a force up to 150 N, does not produce a mandibular deformation in the premolar area (Jiang and Ai, 2002). Since the participants in this study held the mandible effortlessly in contact with the opposing arch (without and with splint), it can be reasonably assumed that negligible deformation occurred in the TMJ area. With the same method, results of experiments in which the person was asked to clench on a force transducer placed in different areas of the dental arch showed that the minimum intra-articular distance decreased with increasing bite forces (unpublished observations). It is therefore likely that the augmented global minimum intra-articular distance found in this study could be neutralized if the person were to clench or brux with high force. Splint insertion, however, is likely to cause a shift in the position of the minimum intra-articular distance (and therefore of the contact stress areas), when habitual closure as well as sliding movements occur with high force. This will be the focus of future studies. In conclusion, dynamic stereometry showed that the insertion of a 3-mm-thick occlusal splint led to a redistribution of the condyle-fossa distances in the TMJ, both during habitual closure as well as during sliding movements, thus redistributing contact areas between joint surfaces.
This work was supported by the standard financial plan of the University of Zürich and by Grant #325200-110067 of the Swiss National Science Foundation, Berne, Switzerland. Received for publication November 23, 2007. Revision received May 8, 2008. Accepted for publication June 16, 2008.
Journal of Dental Research, Vol. 87, No. 9,
877-881 (2008)
|
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 |
|
|
 |
|
Sign In to gain access to subscriptions and/or personal tools.
TOP
ABSTRACT
INTRODUCTION
