Establishing wear patterns in the patient’s mouth is the best way to determine abnormal muscular involvement related to occlusal function. Do you see occlusal evidence of bruxing? When evaluating wear patterns, it is best and most accurate to have correctly mounted study casts with arc of closure registrations along with condylar guidance registration. The steepness of the condylar guidance helps determine the posterior tooth disclusion in lateral and protrusive movements. Prior orthodontic or iatrogenic grinding such as an occlusal equilibration, can influence visual evidence of wear. These factors need to be established before using wear patterns to determine muscle dysfunction.
Of course, the best articulator is the patient. There are limitations in using the patient, especially a patient in pain who will avoid areas of discomfort, which may give an inaccurate picture. This can be demonstrated in some patients who have posterior interferences that are mitigated by the use of an anterior deprogrammer. Limited lateral movements without the anterior deprogrammer are very restricted but will immediately free up with the placement of the anterior deprogrammer. Note any muscle trismus when the mandible is moved into a lateral or straight protrusive movement. This is an indicator that there may be interferences within the occlusal scheme that evoke a protective response from the muscles that function to allow this movement to take place.
If one single improper stimulus enters into this complex interdependent system, the entire mechanism is automatically restructured into an adaptive mode in an attempt to accommodate the stimulus. If this is a temporary stimulus, then the system will right itself after the stimulus is removed. The stimulus may be from the periodontal ligament receptors, a neuromuscular spindle cell in a muscle of mastication, iatrogenic tooth position, or muscles of the tongue or hyoid group. A change in the function of the mastication-hyoid complex changes the balancing requirements of the posterior cervical group of muscles.
In an attempt to balance the stomatognathic system, the labyrinthine reflexes can be affected. At times, stimulation of the proprioceptors in the periodontal ligament that fail to return to normal can have far reaching effects within this system. The chief complaint of the patient may not be the initiating factor in the observable response. This is where our powers of observation and knowledge of how the masticatory system works are invaluable in attempting to determine the cause and effect process.
Note the matching of wear patterns of the mandibular and maxillary teeth. When putting the patient in these positions, note any report of muscular pain. Remember the patient can put five to nine times more force on the teeth when bruxing as when consciously articulating the teeth together. There may be evidence of excursive interferences, especially after immediately leaving the rotational stage of condyle function. There may be evidence of hypo-occlusion on the same side or hyper-occlusion on the opposite side. Involvement of the deep masseter may be due to retrusive inclines and/or retrusive forces in lateral excursion.