The reliability and validity of identifying trigger points within the masticatory musculature has not been established in patients with TMD.²¹ In one controlled study of trigger points that generated referred pain when needled, researchers found that only 62 percent generated referred pain when palpated.¹⁸ The natural fluctuation of trigger point irritability also may cause an inconsistent ability to generate referred pain over time.²²

It has been demonstrated to apply pressure directly to the trigger point to generate the patient’s referred pain. Palpating over a muscle band that contains a trigger point or an adjacent band within the same muscle often is all that is needed to reproduce the patient’s referred pain; however, it does require greater palpating pressure.²² Locating a trigger point sometimes is difficult, especially when there is no firm structure behind the muscle to support it when the muscle is palpated. It also has been found that palpating nodules of spot tenderness is a highly reliable method for generating the patient’s referred pain, if the examiner is skilled at detecting these nodules.^4,6,11,23

Masticatory and cervical muscles are not the only structures that can refer pain to the orofacial region. It has been demonstrated that other structures—including the temporomandibular joint, or TMJ, sinuses and teeth—also can produce referred orofacial pain.^6,7,10,12

I speculate that referred pain patterns generated from controlled palpations of a population of patients with TMD may be quite similar to the referred pain patterns that practitioners treating these patients are likely to observe. In this prospective study, I assessed a large group of patients with TMD for the occurrence of referred pain elicited by palpating trigger points, nodules of spot tenderness, and selected masticatory structures within the head and neck region.

	SUBJECTS AND METHODS

TOP ABSTRACT SUBJECTS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
I evaluated all patients referred to a TMD specialty clinic at Lackland Air Force Base, Texas, using a medical history and a clinical examination. A total of 230 patients with TMD met the TMD research diagnostic criteria,²¹ or RDC. I evaluated them for referred pain in the craniofacial region.

During the clinical examination, I identified myofascial trigger points and nodules of spot tenderness within the head and neck region by sustaining finger pressure while sliding my fingers along the muscle’s length. I applied firm pressure, up to the subject’s tolerance, for approximately five seconds to the identified trigger points, nodules and other selected masticatory structures.⁶

I palpated the following muscles for trigger points and nodules of spot tenderness in the following locations:

– temporalis muscle—the entire portion of the muscle superior to the zygomatic arch;

– masseter muscle—the entire body;

– trapezius muscle—the upper portion within the region of the neck (this muscle has many other cervical muscles beneath it, which are nearly impossible to differentiate, so I made no attempt to distinguish between it and the underlying muscles⁶);

– splenius capitis muscle—the upper portion between the trapezius and sternocleidomastoid muscles (this muscle has many layers of cervical muscles below it, so I made no attempt to distinguish it from the underlying muscles);

– sternocleidomastoid muscle—the full length of the muscle (by squeezing it between my thumb and index finger);

– anterior digastric muscle—the full length medial to the inferior border of the mandible;

– posterior digastric muscle—the portion accessible from behind the angle of the mandible (by sliding my finger toward the earlobe along the anterior border of the sternocleidomastoid muscle).

I palpated the following selected masticatory structures as indicated:

– TMJ—the lateral pole and posterior aspect with the mouth wide open;

– lateral pterygoid area—the lateral side of the alveolar ridge above the maxillary molars (using my fifth digit);

– medial pterygoid muscle—the portion just posterior to the typical site for an inferior alveolar injection intraorally (using my index finger);

– coronoid process—the medial and anterior portion intraorally (using my index finger); this is where the temporalis muscle’s tendon inserts into the mandible.

As I was applying firm pressure against these structures, I asked subjects whether pain was developing or intensifying in a different location than that being palpated. For subjects who felt pain in locations other than the areas being palpated, I recorded the referred pain site and point of palpation on drawings such as those shown in Figures 1 and 2 (pp. 1310–11).

View larger version (38K): [in this window] [in a new window]   Figure 1. Map of referred pain generated by palpation of labeled anatomic areas.

View larger version (45K): [in this window] [in a new window]   Figure 2. Map of palpation locations that generated referred pain to the labeled anatomic areas. The boxes below some of the figures represent intraoral sites.

	RESULTS

TOP ABSTRACT SUBJECTS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

Among the 230 subjects, 73 (32 percent) reported having a history of neck pain at least once a month, and 40 (17 percent) were aware of at least monthly neck tightness. Once their referred pain was generated, 115 subjects (50 percent) reported that this pain occurred or intensified when their TMD symptoms were aggravated, and 65 subjects (28 percent) were aware that this pain occurred or intensified when their neck symptoms were aggravated.

The site for referred pain generation reported most often was the cheek area, followed by the ear and forehead. Subjects reported feeling referred pain in these areas on 260, 178 and 170 occasions, respectively. Table 1 (p. 1312) gives the frequency distribution for the evaluated sources and perceived sites of referred pain.

When subjects reported that pain was referred to the teeth, they frequently reported that they also perceived some degree of pain in the gingiva and alveolar process of the corresponding teeth. The frequency distribution of referred pain to the teeth is given in Table 2 (p. 1313).

The referred pain sources for the different craniofacial regions are shown in Figure 2. The more common sources for each region are designated by orange, and the less common sources (with at least two subjects reporting this source) are designated by blue. In some muscles, identified sources were evenly distributed throughout the muscle, while in other muscles the identified sources were concentrated in a particular portion of the muscle. Because of this distribution variation, some muscles with a higher frequency of referred pain generation are designated by blue, while a specific portion of another muscle with an overall lower frequency of referred pain generation is designated by orange. Since minimal information would be trayed for the soft palate/retromolar and throat figures, I did not include them in Figure 2.

Occasionally, referred pain was generated on the side contralateral from the structure being palpated. Of the 89 subjects who reported referred forehead pain—sometimes from multiple sources—18 reported it was generated bilaterally from unilateral palpation. Of the 51 periorbital pain—sometimes from multiple sources—five reported it was generated bilaterally from unilateral palpation.

	DISCUSSION

TOP ABSTRACT SUBJECTS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
During the 18 months I conducted this study, 85 percent of the subjects reported referred pain generation from my palpating the evaluated head and neck regions. Referred ear and maxillary and mandibular tooth pain were generated from masticatory structures on 133 and 138 occasions, respectively (Table 1). This common referral pattern may help explain the clinical observations in which ear or tooth pain have concomitantly resolved with TMD therapy.^24,25

A recent study showed that if practitioners tell patients that they expect to observe referred pain, patients will perceive referred pain in a certain location on palpation; this biases patients to more frequently perceive referred pain.²⁶ Among their unbiased patients with TMD, 45 percent reported referred pain from 15 seconds of pressure applied to the focal site of maximum masseter muscle tenderness. In the present study, I was careful not to bias the subjects; only 29 percent of the subjects reported referred pain from the masseter muscle. Both studies reported a similar referred pain pattern. The present study’s less frequent referred pain generation could have resulted from the shorter time I applied pressure (approximately five seconds), lower palpation pressure (up to patient’s tolerance) or possibly less symptomatic patients with TMD.

Simons and colleagues⁴ and Fricton and colleagues¹¹ reported muscle trigger point referred pain patterns very similar to the ones I observed in this study. All three studies found that referred pain was generated from the temporalis muscle to the maxillary teeth; masseter muscle to the teeth, ear and TMJ; lateral pterygoid muscle to the maxillary sinus area and TMJ; and sternocleidomastoid muscle to oral structures and the forehead, which occasionally crossed the midline.

In both this study and that of Fricton and colleagues,¹¹ referred pain was found to be generated frequently from the cervical protuberance area of the trapezius muscle to the forehead and from the lateral pterygoid and posterior digastric muscles to the ear. Both studies observed that referred pain frequently was generated from the splenius capitis muscle to the forehead, but in this study, I also observed it commonly referred to the ear. Simons and colleagues⁴ did not report observations from palpating these posterior cervical areas.

Both Simons and colleagues⁴ and this study found that referred pain was generated from the anterior digastric muscle to the throat and the posterior digastric muscle to the ear. In addition to evaluating the muscles palpated in these other studies, this study also evaluated referred pain generation from the TMJ and coronoid process.

The mechanism for the referred pain generated in this study is not fully understood by researchers, and no single mechanism is able to explain all aspects of referred pain.⁷ The underlying feature for referred pain is the convergence-projection theory. It is well-known that there are more nerves to carry information into the central nervous system, or CNS, than there are neurons to transfer the information to the higher centers of the CNS. This difference necessitates that information from multiple nerves traveling into the CNS has to converge on fewer neurons to carry this information to the higher centers. It is theorized that the higher centers receive the information in a way such that two or more regions may be perceived as its source.^1,2,7,10

There is extensive convergence of sensory nerves that serves the orofacial region.^1,19 It has been demonstrated in cat brains that the areas of the higher center receiving input from the TMJ also receive input from facial skin and intraoral sites.²⁷ Similarly, in rat brains, it has been observed that 80 percent of the neurons from the TMJ and masseter muscle converge in the trigeminal sub-nucleus caudalis.²⁸

It also has been demonstrated that the trigeminal sensory complex receives pain input from the trigeminal nerve, as well as converging input from the facial, glossopharyngeal, vagus and hypoglossal cranial nerves and the upper cervical nerves.¹⁹ It has been shown that at least one-half of the pain-carrying neurons that normally are activated by the trigeminal nerve can be activated by electrical stimulation outside their normal receptive field.²⁹

Convergence is believed to occur to a greater degree among neurons carrying information from deeper structures such as muscles, joints or tooth pulp than from cutaneous structures. This has been hypothesized as the reason people with pain from deep structures have difficulty localizing it and often sense it as referred to regions distant from their source, while people with cutaneous pain can localize it with great accuracy.²

Another feature that appears to contribute to the referred pain mechanism is central sensitization.^6,30–32 It appears that a continuous barrage of painful input can activate specific receptors that increase the sensitization of neurons, altering normal processing to the higher centers, expanding the receptive field area and causing nonpainful information to be relayed as painful.^31,32 The referred pain observed in this study could be due to multiple CNS mechanisms, and it appears that the two most prominent mechanisms are convergence and central sensitization.

It was observed and previously reported that to generate a patient’s referred, pain practitioners must palpate the trigger point or nodule of spot tenderness with a force beyond what is needed to produce local pain.⁴ Additionally, the referred pain generally was not produced immediately but took a few seconds to develop.^{3,5–7,19,20}

As indicated in Figure 1, trigger points found in one vicinity may project referred pain to many different sites. I recommend that once practitioners determine a trigger point’s referred pain site, they should not assume other trigger points in the vicinity refer to the same site. Occasionally I observed that multiple trigger points in close proximity referred pain to dramatically different locations.

It also was not uncommon to observe that multiple pain sources distant from each other generated referred pain to the same site. If practitioners want to test how much impact a specific trigger point has on the identified referred pain site, they can attempt to inactivate it through methods such as a trigger-point injection, trigger-point compression, ice, heat, spray and stretch and massage.

It appears that palpating the muscle against a firm support such as an underlying bone lets practitioners better compress the suspected source and increase the likelihood of causing referred pain. This may be one reason that referred pain often was generated just below the skull’s insertion of the trapezius and splenius capitis muscles; this region was palpated against the base of the skull, providing a firm support against which the suspected source could be "pinched."

Once practitioners determine a trigger point’s referred pain site, they should not assume other trigger points in the vicinity refer to the same site.

As reported by other investigators, it was common for patients in this study to be minimally aware, or even not aware, of symptoms at the location of their referred pain’s source.^2,4–7,20 Practitioners searching for a patient’s referred pain source may find Figure 2 helpful in identifying the location, because referred pain patterns tend to be similar from patient to patient.^2,4,6–8

Twenty-eight percent of the subjects in this study were aware that their referred pain occurred or intensified when their neck symptoms were aggravated, but most subjects were not aware that this pain was referred from their necks. Once their pain was generated when I palpated their necks, all of the subjects were receptive to the referred pain explanation.

In addition, many subjects reported a correlation between their periorbital or forehead pain and their TMD symptoms, despite the fact that their periorbital or forehead pain could be generated only from their cervical musculature. Their observed correlation could be due either to secondary muscle splitting of their cervical muscles in response to an aggravation of their TMD symptoms, to the cervical muscles and TMD symptoms both being aggravated by similar nonspecific factors (for example, psychological tension, poor sleep and emotional stress), or to an unidentified masticatory referred pain source.

Among the subjects in whom referred forehead or periorbital pain was generated, I produced bilateral forehead or periorbital pain from unilateral palpation in 20 percent and 10 percent of these subjects, respectively. Referred pain normally is generated on the same side of the body as the source, but crossing the midline of the body has been reported by other authors.^4,6,11

	CONCLUSIONS

TOP ABSTRACT SUBJECTS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Practitioners should be aware that patients with TMD often report referred craniofacial pain generated from head and neck palpation, as well as consider the propensity for referred craniofacial pain. In this study, the pattern between the referred pain source and site typically was consistent and predictable. Practitioners can refer to Figure 2 to determine regions highly probable to be the patient’s referred pain source.

During TMD evaluations, I recommend that practitioners keep in mind that periorbital or forehead pain typically is not generated from the temporalis muscle but more commonly is produced from the cervical musculature. It is not uncommon for the cervical musculature to be the source of temple pain, and, if palpating the pain’s site does not generate the patient’s pain complaint, the pain may be referred from another location.

If a dental practitioner wants to recommend therapy to treat a patient’s cervical region and does not have an appropriate treatment protocol, I suggest that the practitioner refer the patient to a family physician. Family physicians commonly treat cervical disorders and usually are familiar with the patient’s medical insurance benefits.

I also recommend that if the patient does not have the anticipated response from initial should consider that the patient’s pain may be referred from a different source. Referred pain also should be considered if the patient experiences no pain relief after the pain’s source has been anesthetized; for example, a patient with a painful tooth that has been anesthetized for endodontic therapy does not report pain relief.