Electrognathographic and electromyographic observations on jaw depression during neck extension

Abstract

Albeit never substantiated through experimental and clinical evidence, the theoretical linchpin of the mechanics of a so-called whiplash injury of the temporomandibular joint (TMJ) is the postulate that a pre-existing depressor force (continual anchoring force), generated by the anterior suprahyoid (SH) muscles, will always act on the mandible and cause traumatic mouth opening (anterior acceleration of the TMJ condyles) when the neck is extended (posterior acceleration of the head). To test aspects of this postulate, six subjects assumed the positions of neutral (0 degrees ), medium (32 degrees ) and maximum (58 degrees ) neck extension while the mandible was in its postural positions of rest and light centric occlusion. By means of surface electromyography, it was shown that the relative contractile activities of the anterior SH muscles never exceeded 7.3% of the contractile activity required to anchor the mandible in a position of maximum depression. By means of electrognathography, it was shown that the maxillary and mandibular incisors were never separated by more than 2.6 mm during neutral, medium, and maximum extension of the neck. In other words, during neck extensions there was no evidence of a continual or induced voluntary or involuntary depressor force that would and could anchor the mandible in a position of traumatic mouth opening. Accordingly, and in agreement with other biophysical and biomedical evidence, it was concluded that there is no foundation for the pseudoscientific speculations and unsubstantiated opinions offered in support of a concept and diagnosis of a so-called TMJ whiplash injury. Additionally, this study found co-activation of cervical flexor muscles and mandibular elevator as well as depressor muscles.

Kinematic and kinetic observations on ballistic depression and elevation of the human mandible

“…In agreement with other studies (Forsberg et al, 1985;Clark et al, 1993;McKay & Christensen, 1999), this study showed co-activation of cervical flexor and mandibular depressor/elevator muscles. The onset of myoelectrical activity in the sternocleidomastoid and anterior suprahyoid muscles showed perfect synchronism (Fig.…”

“…We have previously discussed the tenuous diagnosis of a so-called whiplash injury of the temporomandibular joint, and have shown that the postulated diagnosis and postulated mechanism of injury have no foundation in basic and clinical science (Christensen & McKay, 1997;McKay & Christensen, 1998, 1999. That is to say, if a rapid injurious depression of the mandible were to occur during the first phase of a whiplash manoeuvre of the head and neck (posterior rotation of the head/extension of the neck), the motion would immediately be counteracted by a mandibular elevation, elicited through a stretch reflex in the mandibular elevator muscles (Christensen & McKay, 1997), and by the inability of the rather weak depressor muscles to anchor the mandible in space (McKay & Christensen, 1999). Nonetheless, we have recently and personally encountered the following postulate: during the first phase of a head/neck whiplash manoeuvre, the subject may voluntarily open his/her mouth to elicit a scream which, in turn, injures the jaw joints.…”

“…Finally, because cervical flexor muscles and mandibular depressor/elevator muscles show co-activation, or functional coupling of myoelectrical activity (Funakoshi & Amano, 1973;Forsberg et al, 1985;Boyd et al, 1987;Clark et al, 1993;Browne et al, 1993;McKay & Christensen, 1999), it seemed reasonable to apply surface electromyography of the sternocleidomastoid, anterior temporalis, and anterior suprahyoid muscles to study the phenomenon of co-activation.…”

“…For the myoelectrical observations, the model EMG-4 electromyograph* was used. Although the system records both the amplitude (mV) and the duration (ms) of surface myoelectrical activity, only the duration variable in synchrony with electrokinesiography was applied in this study (Christensen & McKay, 1997;McKay & Christensen, 1999). Bipolar silver/silver-chloride surface electrodes (interelectrode distance of 20 mm; impedance of 200 V at 60 Hz) were affixed over the right and left anterior temporalis muscles, the right and left anterior suprahyoid muscles (platysma, digastric, mylohyoid, geniohyoid muscles), and the right and left sternocleidomastoid muscles (level of the gonial angle of the mandible).…”

Global Postural Reeducation and Static Stretching Exercises in the Treatment of Myogenic Temporomandibular Disorders: A Randomized Study

“…They seem to be useful in the treatment of TMD with anterior disk displacements without reduction. 20 Manual therapy, muscle stretching, and strengthening exercises seem to be the most useful techniques. 21 We used 2 different stretching techniques.…”