2.4. Effects of occlusal factors on temporomandibular disorders and masticatory function

2.4.1. Temporomandibular disorders (TMD): general view

Temporomandibular disorders (TMD) refer to a variety of medical and dental conditions affecting the temporomandibular joint (TMJ) and/or the masticatory muscles as well as contiguous tissue components. Although specific etiologies, such as degenerative arthritis and trauma, underlie some TMD, these conditions as a group have no common etiology or biologic explanation and comprise a heterogeneous group of health problems whose signs and symptoms overlap but are not necessarily identical. The term ‘TMD’ has commonly been used to characterize a wide range of conditions diversely presented as pain in the face or the jaw joint area, headaches, earaches, dizziness, masticatory muscle hypertrophy, limited mouth opening, closed or open lock of the TMJ, abnormal occlusal wear, clicking or popping sounds in the jaw joints and other complaints. (National Institutes of Health Technology Assessment 1997). The prevalence of at least one sign of TMD in non-patient adult populations has varied between 40% and 85% (Kuttila 1998), but the prevalence of severe dysfunction, according to Helkimo’s clinical dysfunction index (1974), is lower than 1% (Salonen 1990). Rauhala et al. (2000) found, in an epidemiologic study of the Northern Finland 1966 Birth Cohort, that 12% of men and 18% of women had suffered from facial pain during the past year, while clicking of the TMJ was present in 21% of men and 28% of women, and the prevalence of more severe symptoms was 13% or less. Distal occlusion was one of the factors related to TMD. Fluctuation of signs and symptoms of TMD is usual, but awareness of these problems seems to increase with age (Kuttila 1998). Magnusson et al. (2000) examined, in a longitudinal study, the status of the masticatory organ during a 20-year period and noticed substantial fluctuation of both reported and clinical signs and symptoms of TMD, but progression to severe pain or dysfunction was rare.

Sipilä et al. (2002) also subjected a subpopulation of the Northern Finland 1966 Birth Cohort to double-blinded case-control, anamnestic and clinical examinations. Facial pain associated significantly with TMD, and most of the cases belonged to the myogenous subgroup of TMD. Anamnestically, stress was the most often reported provoking factor for facial pain.

The reasons for headache may be numerous, and some of them have been associated with occlusal factors and masticatory dysfunction. Significant associations have been reported with clenching or grinding of teeth, malocclusion, tenderness to palpation of the masticatory muscles or impaired mobility and the frequency and intensity of headache (Lous & Olesen 1982, Wanman 1987, Sonnesen et al. 1998). Abnormal tonic hyperactivity in the masticatory muscles and the neck may be an important source of pain in these patients.

Placebo-controlled studies with occlusal adjustment with or without splint therapy in patients with muscle contraction headache (MCH), common migraine (CM) and combination headache (MCH+CM) have shown a decrease in headache frequency and a correlation with the decrease of the mandibular dysfunction index (Forssell et al. 1985, 1986, 1987). Patients with MCH and CM benefited most from the therapies.

At present, TMD is generally considered to have a multifactorial etiology. The current diagnostic classifications of TMD are based on signs and symptoms rather than etiology, in line with conditions of the lumbo-sacral spine, such as low back pain (National Institutes of Health Technology Assessment 1997). Greene (2001) has provocatively suggested that not only are the old mechanistic etiologic concepts incorrect, but two of the most popular current concepts (biopsychosocial and multifactorial) are also seriously flawed. Nevertheless, he continues to add that even in the absence of full understanding of etiology, good conservative care can be provided, but aggressive and irreversible treatments should be avoided.

As for etiology and diagnosis, a number of treatment modalities have been presented: supportive patient education; pain control with medication; physical therapy; various intraoral appliances; occlusal therapy; and surgical approach (National Institutes of Health Technology Assessment 1997). Forssell et al. (1999) found in their systematic review of the literature several weaknesses in many of the studies dealing with occlusal adjustments and splint therapies: small sample sizes (lack of power); short follow-up periods; lack of blinding of examiners; numerous and varying outcome measures. The results suggested that controlled clinical trials give some evidence of the use of splints, but sufficient evidence for occlusal treatment is lacking. Vallon & Nilner (1997) studied in a controlled 2-year follow-up survey the effects of occlusal adjustments on TMD and concluded that only single patients improved from counseling alone, a few more improved if one other kind of treatment, e.g. occlusal adjustment, was added to counseling, but the majority required a comprehensive treatment program.

2.4.2. TMD and malocclusion

The cause-and-effect relationship between malocclusions and TMD is controversial. One of the first practitioners to assume a relationship between TMJ and occlusion was Costen (1934), an otolaryngologist who noticed that many of his patients with pain in the TMJ region benefited from alteration of their occlusion, especially in the vertical dimension.

Associations between certain features of occlusion and TMD have been mentioned in many reports. Relationships have been found between open bite and TMD in some studies (Riolo et al. 1987, Henrikson et al. 1997, Sonnesen et al. 1998) and between deep bite and TMD (Kerstens et al. 1989). A significant association of TMD with unilateral crossbite and midline displacement has also been reported (Sonnesen et al. 1998). Abnormal overbite and overjet may be associated with more extensive deviation in the temporal and condylar form, particularly when combined with age, which has been interpreted as evidence to support the idea that longer exposure to malocclusion may be associated with more extensive TMJ changes (Solberg et al. 1986). O´Ryan & Epker (1984) have also presented that dentofacial deformities and malocclusions may lead to adaptive changes within the TMJ. Schellas (1989) hypothesized, interestingly, based on his MR image study that TMJ pathology may be the cause of malocclusion rather than vice versa. He concluded that it is critically important to diagnose significant TMJ pathology before attempting permanent occlusal adjustment, including orthognathic surgery.

Several studies have reported more TMD in skeletal Class II (or excessive overjet) than in other dentofacial deformities, e.g. skeletal Class III (Upton et al. 1984, Riolo et al. 1987, Magnusson et al. 1990, White & Dolwick 1992, Le Bell et al. 1993, Fernandez Sanroman et al. 1997, Sonnesen et al. 1998). A tendency towards more TMD in patients with normal or low mandibular plane angles compared to patients with high mandibular plane angles has been observed (Kerstens et al. 1989, White & Dolwick 1992).

There are also a number of studies reporting no significant association between occlusal relationships and TMD. There are studies that have failed to confirm significant relationships between TMJ or muscle tenderness and Angle´s classification or any occlusal contact relationships, or between functional occlusal relationships and TMD (Sadowski & Beyole 1980, Bush 1985, Egermark-Eriksson et al. 1987). In their review articles, Reynders (1990) and Seligman & Pullinger (1991) concluded that there existed no scientific evidence for a causal relationship between occlusion and TMD. Wadhwa et al. (1993) studied three patient groups, one with normal occlusions, one with untreated malocclusions and one with orthodontically treated malocclusions. They concluded that the role of orthodontic treatment in either the precipitation or the prevention of TMD remains questionable. Although considerably high rates of prevalence of uni- or bilateral disc displacements, clicking of the joints and pain in the TMJ or masticatory muscles have been found in pre-orthognathic surgery patients examined clinically and with arthrography, no association could be seen between signs or symptoms and the type of dentofacial deformities (Roberts et al. 1987, Dahlberg et al. 1995). Although Kirveskari and Alanen (1993) believe that there is insufficient evidence to warrant the rejection of the hypothesis that occlusal factors are part of the causal complex of TMD, it seems that, with the present weak and varied epidemiologic data, there is little predictive value in the attempts to relate a specific dentofacial malocclusion to an individual´s risk for developing TMD.

The available evidence seems insufficient to warrant prophylactic modalities of therapy. The statement of National Institutes of Health Technology Assessment (1997) suggests that surgical interventions (TMJ) should be considered in the small percentage of patients with persistent and significant pain and dysfunction who show evidence of pathology or suggest that an internal derangement of the TMJ is the source of their pain and dysfunction, and for whom more conservative treatment has failed.

2.4.3. TMD and orthognathic surgery

The prevalence of various signs and symptoms of TMD prior to orthognathic surgery has varied between studies from 14% up to 97% of subjects (Laskin et al. 1986, Kerstens et al. 1989, DeClercq et al. 1995, Karabouta & Martis 1985, Scheerlinck et al. 1994, White & Dolwick 1992, Fernandez Sanroman et al. 1997, Schneider & Witt 1991, Link & Nickerson 1992). This great variation may be due to differences in referral patterns or motivations to seek treatment (Laskin et al. 1986) or selection of study subjects as in the study of Link & Nickerson (1992).

An important goal of orthognathic surgery is to improve the masticatory function and to minimize TMD. Most of the earlier studies seem to suggest that this goal can be achieved, but some controversy also exists. Most of the studies that have reported positive effects on TMD after orthognathic surgery report this association with skeletal Class II deformity (or mandibular retrognathia/hypoplasia or Angle Class II). Decrease of signs and symptoms by more than 50% compared to the preoperative state was reported to occur by Karabouta and Martis (1985), Kerstens et al. (1989), Magnusson et al. (1990), De Clercq et al. (1995) and White and Dolwick (1992), while subjects with skeletal Class III (or mandibular prognathia/hyperplasia) or patients with a high mandibular plane angle (> 32°) seem to benefit considerably less (Kerstens et al. 1989, White & Dolwick 1992, De Clercq et al. 1995). However, improvement of TMD even in Class III patients may be attained with orthognathic surgery, as shown by the studies of Ingervall et al. (1979), Magnusson et al. (1986, 1990), Le Bell et al. (1993).

Disc positions and internal derangements in orthognathic surgery patients have been assessed with TMJ imaging techniques (arthrography, MR) in some studies, and slight improvements in disc position, pain and joint sounds after treatments have been seen (Eriksson et al. 1990, Gaggl et al. 1999).

Egermark et al. (2000) studied 52 patients with malocclusions, who had undergone orthognathic surgery involving LeFort I and/or sagittal split osteotomy. Approximately 5 years after surgery, the patients were examined for TMD. Some of them had reported recurrent or daily headaches before treatment, but at the 5-year examination, only two patients reported having a headache once or twice a week, while the others suffered from headaches less often or had no headache at all. The authors concluded that orthognathic surgery results in improvement of TMD, including headaches. A few other studies have also found improvement in headache after orthognathic surgery, but they have similar shortcomings, as will be mentioned at the end of this chapter (Magnusson et al. 1986, 1990, LeBell et al. 1993, Nurminen et al. 1999, Westermark et al. 2001). It seems, however, that occlusal adjustment may have a favourable influence on headache, especially on muscular contraction type headache.

Orthognathic surgery may not have only beneficial effects on TMJ function, but also adverse consequences. The prevalence of TMD after orthognathic surgery among preoperatively asymptomatic patients has varied from 3.7% to 11.9% (Karabouta & Martis 1985, White & Dolwick 1992, Scheerlinck et al. 1994, Kerstens et al. 1989, De Clercq et al. 1995). As fluctuation of TMD is common (Kuttila 1998, Magnusson et al. 2000), this may also be part of normal variation. Thus, an investigation with a non-treatment control group would be indicated, as planned in the present study.

Table 1. shows some demographic data of the reports on the influences of orthognathic surgery on TMD.

There are also studies that report only minimal or no specific change in TMD after orthognathic surgery. Sostmann et al. (1991) evaluated 86 orthognathic surgery patients with Helkimo´s anamnestic and dysfunction indexes and found no relationship between TMD and the type of malocclusion, the surgical approach and molar support before and after surgery, but concluded that the possible beneficial effect was achieved in certain symptoms, such as TMJ pain and sounds. A modification of Helkimo´s index was also used in a prospective study of 22 patients operated with BSSO (Smith et al. 1992). Subjectively, muscular pain, headache, joint noise and parafunctional habits decreased, but clinical dysfunction remained unchanged and partly even deteriorated. The prospective multicenter study of Rodrigues-Garcia et al. (1998) explored the relationship between severe Class II malocclusion and TMD before and 2 years after BSSO. The patients were evaluated with the Craniomandibular Index (CMI), the Peer Assessment Rating (PAR) Index and symptom questionnaires. The results showed significant improvement in occlusion, CMI and muscle pain, reduction in subjective pain and discomfort and decrease in clicking upon opening. On the other hand, crepitus in the TMJ increased and the magnitude in the change of muscular pain was not related to the severity of the pretreatment malocclusion, and the authors concluded that the results do not support the theory that TMD is related to Class II malocclusion. However, the subjects of this study were mostly not seeking treatment for TMD: only 28% of patients reported TMD as the reason for seeking treatment. The Craniomandibular Indexes (CMI) and Dysfunction Indexes (DI) used in the examinations of the patients showed preoperative mean scores of only 0.14 and 0.13 on a scale from 0 to 1.0. Onizava et al. (1995) investigated alterations in TMD after orthognathic surgery in a series of 30 patients and 30 healthy volunteers followed up for 6 months. They found no significant difference in TMD between the two groups and concluded that alterations of TMJ symptoms do not always result from the correction of malocclusion.

There appears to be a high range of variation in the prevalence of signs and symptoms of TMD in the orthognathic surgery population prior to treatment, but in several studies, a significant number of patients with dentofacial deformity and TMD have experienced improvement of their symptoms after orthognathic surgery, while, on the other hand, some preoperatively asymptomatic subjects may have developed TMD postoperatively. There are, however, many weaknesses in most of these studies: there are no non-treatment control groups or the patient samples are small, follow-up is short or the studies are retrospective, and IMF has often been used instead of contemporary internal rigid fixation.

2.4.4. Masticatory performance and malocclusion

Many of the orthognathic surgery patients may, before their treatment, exhibit masticatory performance and occlusal characteristics that differ from those seen in a normal population, including a decreased number and intensity of occlusal contacts and malocclusions with frequent interferences (Athanasiou et al. 1989b). Masticatory efficiency has been investigated in relation to occlusion, and a significant correlation has been reported. Patients with malocclusions have been shown to have reduced masticatory efficiency. (Luke & Lukas 1985, Kikuta et al. 1994). Masticatory efficiency may be related to many different factors, including the number of teeth present, the number of occluding tooth pairs, the occlusal contact area, the actions of soft tissues and the preferred chewing side (Magnusson T et al. 1990). Luke & Lukas (1985) found that even relatively minor variations from normal or ideal intercuspation in the buccal segments may reduce chewing efficiency. Electromyographic (EMG) studies of subjects with various dentofacial morphologies have provided support for the assumed association between muscle structure and function and skeletal malocclusion (Ingervall & Thilander 1974, Proffit et al. 1983, Dean et al. 1992). Individuals with vertical maxillary excess (long face) tended to have decreased EMG activity, whereas persons with a vertically short maxilla showed increased EMG activity compared to subjects with normal facial morphology. On the other hand, several factors may influence the generation of maximal occlusal force, including the size of the muscles, the distribution of different types of muscle fibers, the activity level of the muscles, the sensitivity of the teeth and the muscles and TMJ and the patient`s willingness to exert maximal force. (Ingervall & Thilander 1974, Proffit et al. 1989).

Alteration of the facial skeleton by means of orthognathic surgery may produce significant changes in the mechanical advantage of the masticatory muscles and also physiologically by altering the sensory and proprioceptive inputs (Proffit et al. 1989, Harper et al. 1997). A noticeable tendency towards normalization in the masticatory patterns has been recorded after orthognathic correction of severe Class II and Class III malocclusion (Ehmer & Broll 1992). Myofascial facial pain patients with more severe pain intensity are likely to reduce their intake of dietary fiber (Raphael et al. 2002). This is possiby due to an effort to decrease masticatory activity to avoid exacerbating facial pain and may increase the risk of constipation.

As a summary, research has provided evidence that masticatory forces and functional patterns in patients with dentofacial malocclusions are different from those in the normal population, but only a few studies have reported presurgical and long-term postsurgical follow-up data on functional alterations.