Head Posture in the Pathogenesis of Postural Imbalance, the Role of Dental Occlusion and the Visual System

Head posture in the pathogenesis of postural imbalance, role of dental occlusion and the visual system

Dr. Piero Silvestrini Biavati
Dentist - PhD University of Siena - Master in Posturology University of Rome

Posturology, a very current branch in the therapeutic panorama, aims to investigate the causes of imbalance in order to seek remedy. It is clear to everyone that an adaptive attitude of the body will carry compensatory phenomena. If these exceed an individual threshold, compensation turns into pathology, first painful then functional, up to anatomical changes. The head, due to its spatial position (very high compared to the center of gravity) and its specific weight (the highest of the body), induces important phenomena on the body scheme of balance. In postural physiology, there are various causes of imbalance. Every sensory receptor is potentially able to create disturbances in the postural system.
We will not enter the fine postural system in this article, but only deal with macro bodily adaptations. Easily investigable by a careful objective examination by each of us.
Our body is in continuous adaptation, due to environmental problems that occur from time to time.
Statistically, the most important influences on head posture are gnathological disorders (dental occlusion) and ocular system disorders (phorias). Sometimes the disorders are closely related.
To define an occlusal postural imbalance caused by a heterophoria, we must first clarify these terms.
Heterophoria: an imbalance of the visual system, in general an alteration of the "synergy" of binocular vision: these are ocular mobility disorders that are kept latent by the intervention of cerebral fusion; therefore binocular vision is always admitted and they are defined according to the type of deviation, compared to the orthophoric position of the eyes. In practice, they are strabismus that can still be corrected reflexively.
It is obvious that such problems induce, or perhaps better said force, our body to adapt to compensate as much as possible for problems caused by the visual system.
Occlusal postural syndrome: an interaction between mandibular position and cervico-cranial posture. This term does not mean that only the upper body district influences the gnathological system, but simply the close correlation. Other districts can influence cranio-cervical posture, and from this create gnathological alterations and vice versa. It is difficult that a gnathological problem does not involve cranio-cervical adaptation. The two adaptations are finely correlated.
Mandibular adaptations to head posture.
The complex muscular system responsible for controlling mandibular position can be simplified into three subgroups: the elevator muscles, the depressor muscles, and the neck muscles, aimed at different functions (Fig. 2, 3). In fact, for mandibular posture we must necessarily distinguish two different positions: rest position and maximum intercuspation.
The muscles used for the rest position (RP) are mainly the hyoid muscles. They use the hyoid bone as a pulley, determining its position as the result of the tension of all muscles connected to it (Fig. 1). The suprahyoid, infrahyoid muscles and muscles related to clavicle, cervical spine, pharynx, and tongue actively participate.

Fig. 1 Spatial relationships 

between skull, mandible and hyoid (Tillmann) 

Fig. 2 Muscular relationships of the mandibular RP 

Fig. 3 Cranio-cervical-hyoid mandibular posture 

The muscles used for the maximum intercuspation position (intercuspal position ICP) are mainly the elevators, namely temporals, masseters, pterygoids etc.
Naturally these are only anterior muscle chains, they could not work without adequate control of head position by the neck muscles, primarily sternocleidomastoids and trapezii. In fact, without their intervention, the tension of the submandibular muscles would bring the mandible as low as possible, while simultaneously the supramandibular muscles would bring the mandible into ICP, resulting in a head in maximum forward flexion and clenched teeth. The neck muscles antagonize this effect and are therefore closely correlated. This is why variations in mandibular posture rarely do not involve changes in cranial posture. All this occurs both sagittally and frontally (Fig. 4).
We can therefore imagine the hyoid-mandibular system as a double pulley system: an extension of the head will lead to a more open mandibular rest position (more stretched mandibular muscles), while flexion leads to a more closed position (shorter mandibular muscles).
In frontal view, flexion of the head to the right inevitably causes shortening of the mandibular muscles on the same side, causing a right lateral deviation of the mandible (Fig.5).
In frontal view, rotation of the head to the left will drag on the same side, relative to a midline of the body, both mandible and hyoid (albeit in a proportionally smaller measure) but relative to a cranio-mandibular median axis, the mandible will be laterally deviated to the right. So with a gnathological disorder of right deviation (Fig. 6), the opposite of the lateralization of the head.
Note the displacement of the ocular visual axes in the three different head positions. Green muscles: more contracted; light gray: more stretched
Any misalignment between RP and ICP causes asymmetric work of the muscles and joints to reach ICP, potentially becoming iatrogenic in neuromuscular painful dysfunctions and TMJ.

Fig. 4 Orthogonal posture 

Fig. 5 Head flexed to the right 

Fig. 6 Head rotated to the left 

If the patient has correspondence between RP and ICP (clencher), the problems are further amplified both muscularly and articulately, due to the overload of work to which these structures are subjected.
The reason for this whole discussion on the influence of head posture on the mandible can be summarized in a simple postulate: everything that can influence head posture can consequently cause gnathological problems (adaptation).
But the problem can also be seen in the opposite way, generating a second postulate: mandibular malposition (gnathological problem) can influence head posture and consequently cause other adaptive phenomena.
Here come the phorias….
Any visual defect can induce adaptive phenomena of the head, aimed at functional compensation of the pathology. These adaptations are called Ocular Torticollis (OT) (Fig. 8).
From this it can be deduced that visual alterations that generate an OT force the mandible to adapt to the new cranial posture. The misalignment is aggravated by the counterreaction of the hyoid bone and the hyoid muscles, which in turn induce a further adaptation of the head. Often these compensations are "helped in their stabilization" by dental clenching: a way to close a muscular chain and allow the mandible to stay "close" to the occlusion. Clenching, in a vicious adaptive circle, worsens all dysfunctional gnathological parameters. In these cases, it could be hypothesized that nocturnal bruxism is a way to allow reoxygenation of the elevator muscles blocked all day in isometric situation.

Fig. 7 The ocular muscles and their individual action (Guidetti)          

Fig. 8 Ocular torticollis

Fig. 9 The phoria

Fig. 10 Ocular adaptation to head posture (Guidetti)

The opposite is also possible. From my experience I can say that phorias can be modified by changing occlusion. The means is always the same: adaptation. If occlusion determines a pathological head position, then the visual system must adapt to this position by modifying the habitual ocular axis (Fig. 10, 4, 5, 6).
This causes an asymmetric strengthening of the ocular muscles. This imbalance, in a visual dissociation analysis, i.e. with the eyes not able to simultaneously aim at the same target, brings the axes into a situation of non-orthogonality: the phoria. (Fig. 10) Such functional asymmetries of the ocular muscles (Fig. 7) can also induce corneal surface distortions: astigmatism (Fig. 19, 20, 21, 22), in turn a possible cause of OT.
Differential diagnosis is neither easy nor simple. Considerable experience in the gnathological, postural and visual fields is necessary. Diagnostic equipment and close collaboration with visual specialists: ophthalmologists, strabologists, orthoptists, behavioral optometrists are necessary. Conversely, in physiotherapeutic treatment, the opposite problem arises, analyzing occlusal influences and evaluating their importance.