Since the time Costen (1934) reported this syndrome until now, many aspects in the diagnosis and treatment of TMD have been controversial and continue to be an area for debate. Etiology of TMD is still poorly understood (Sherman and Turk 2001). Although several defined terms are based on different etiology (Costen 1934, Laskin 1969, Thompson 1971, McNeill 1990) similar signs and symptoms are presented, comprising a number of clinical problems that mainly involves two aspects: the masticatory muscles and/or the temporomandibular joint (Westesson 1993).
Within the last 20 years, improvements of imaging methods in TMJ using arthrography and MRI have led to increasingly accurate diagnosis of disc position (Wilkes 1978a, Katzberg et al. 1979, Westesson et al. 1980, Harms et al. 1985, Schellhas et al. 1986). Studies on TMD are in many aspects profound. Disc displacements in TMJ are considered to be factors which may lead to disc deformities, osseous changes and clinical symptoms of TMD (Westesson 1985, Farrar & McCarty 1979, Katzberg et al. 1996, Yoshida et al. 2000, Bertram et al. 2001). These imaging findings have been proved right by surgical observations (Wilkes 1978 a, b, Schellhas 1989a) as well as histological and anatomical studies in autopsy specimens (Westesson & Rohlin 1984, Larheim et al. 1999, Leonardi 2001). All these studies have provided more and more evidence in the diagnosis of TMD. It should be noticed that these studies focus mainly on disc displacements.
In comparison, research on the masticatory muscles concerning TMD is still under development (Helkimo 1974a, b, Solberg 1986a, De Laat 1998, Greene 2001). Among the masticatory muscles, the LPM is considered to play an important role in TMD procedures (Juniper 1984, Lafreniere et al.1997, Raustia et al. 1998). However, the LPM is a deep located muscle and it is difficult to detect by either clinical examination or EMG (Stratmann et al. 2000, Hiraba et al. 2000). Normal and abnormal functions of the LPM have been discussed in EMG studies, but they still remain unclear (Hiraba et al. 2000). More visible evidence is needed to diagnose pathological changes of the muscle.
Clinical studies concerning the skeletal muscles have shown that MRI is a reliable imaging method for diagnosing muscle diseases (Weinreb et al. 1985, Schellhas 1989b, Shellock & Fleckenstein 2000). The main questions explored in the study reported here areas follows: whether MRI can detect pathological changes of the LPM; what kinds of pathological changes of the LPM may appear in TMD; the characteristics of these pathological changes and associated clinical symptoms. No reports on these appear in previous studies.
The results of this study show that the visibility of the LPM in MRI is influenced by imaging projections (Paper I). Imaging diagnosis of pathological changes of the LPM should be based on images that can show both bellies clearly and be less affected by shifting of the scanning plane. According to the anatomic characteristics of the LPM, the more nearly parallel the scanning plane is to the long axis of the LPM, the clearer the observation of the muscle that can be obtained (Paper I, Quemar et al. 1993). A new imaging projection-CLPM projection for TMJ and the LPM is reported in this study, going through the condyle and parallel to the long axis of the LPM (Paper I). It is able to show the LPM constantly in either mouth closed or mouth opening images (Paper I). It appears that most of the CLPM and oblique sagittal images of TMJ can show the LPM and joint structures clearly. The characteristics of CLPM images need to be discussed with a more extensive patient material in further studies (Paper I, Quemar et al. 1993).
Normal imaging of the LPM, hypertrophy, atrophy and contracture of the muscle in patients with have been described in the studies reported here (Papers I-III). It appears that pathological changes of the superior belly and hypertrophy of the inferior belly combined with various abnormalities of the superior belly are the most frequently findings in the LPM of the patients with TMD (Papers II and III). These findings may provide imaging evidences for diagnosis of pathological changes of the LPM in TMD.
Muscular diseases may lead to pain symptoms, and abnormal functional movements of the related joints have been confirmed in studies concerning the leg, shoulder and back (Bertorini et al. 1994, Jacobs et al. 1994, Chen et al. 1998, Thyagarajan et al. 1998, Katz & Ropper 2000, Kader et al. 2000). Similarly, close associations are found between the imaging abnormalities of the LPM and the clinical symptoms of TMD in the studies reported here (Papers II and III). This indicates that the imaging findings of the LPM may explain the clinical symptoms of TMD and their alterations.
There are two other important findings in this study. One is that condyle hypermobility and related pathological changes of the LPM may play an important role in giving rise to the clinical symptoms in TMJs with disc in normal position (Papers II and IV). The other is that imaging abnormalities of the LPM are significantly reduced in condyle hypomobility cases in TMJs with ADDnr even osteoarthritis and disc deformities may occur in these cases (Paper III). Imaging abnormalities may be found in TMJs with disc in normal position, but normal imaging of the LPM may be observed in TMJs with severe disc displacement, osteoarthritic changes and disc deformity (Fig. 11). These findings may give some imaging evidence for understanding clinical symptoms that cannot be totally explained by disc displacements. The recognition of muscle alterations may lead to a correct diagnosis and improved understanding of the clinical symptomatology and disease pathophysiology under investigation (Schellhas 1989b). MRI findings of the LPM may provide more evidence for etiological study of pain symptoms in TMD.
Many treatments have been reported to release muscle pain in TMD, e.g. splint treatment (Dao & Lavigne 1998, Williamson & Rosenzweig 1998), manual therapy (Nicolakis et al. 2001), biofeedback and cognitive-behavioural therapy (Sherman & Turk 2001). However, proof of pathological changes of the masticatory muscles still seem to be lacking in these studies (Nicolakis et al. 2001, Williamson & Rosenzweig 1998). MRI findings of the LPM may offer a useful tool for diagnosis and give important information to evaluate the clinical efficacy of these different treatment procedures.
Obviously, histological confirmation of the imaging pathological findings in the LPM is very important and further research is needed in future.