Proprioception and osseoperception in prosthodontics – A review

Meaning of proprioception

Proprioception (otherwise known as kinesthesia) is defined as an unconscious sense that imparts an individual with the knowledge of their body’s position and physiologic state of their joints, muscles, limbs, etc. A proprioceptive receptor is the receptor which is affected by stimuli within the tissues and organs that convey deep pressure, orientation, tension, as well as awareness to change in position and movements.^[1]

Components of proprioception

The proprioceptive performance is dependent on proprioceptive information provided by the various receptors (which is the hardware component) and proprioceptive ability (which is the software component).^[2,3]

Joint mechanoreceptor

Mechanoreceptors of low threshold are located in temporomandibular joint (TMJ). They play a protective role in signaling movements and positions of joints,^[4] for example, Free nerve endings and Golgi-Mazzoni end organs (GTOs) and Corpuscular end organs.^[1]

Muscle mechanoreceptors

The major mechanoreceptors present in the muscles are GTOs. GTOs are present at the musculotendinous junction along with extra fusal muscle fibers. GTOs are involved in sensing the intramuscular tension produced during voluntary contractions (e.g., mastication). The muscle spindles transmit the sensory feedback (length and tonus of muscles) and thus act as proprioceptors.^[1,4]

Mucosal mechanoreceptors

In dentulous conditions, periodontal mechanoreceptors are pivotal for precise interdental discrimination. Recently, implant-supported prostheses are replacing conventional complete dentures, but they pose the problem of activation of mucosal receptors under the prosthesis leading to proprioception. Examples of mucosal mechanoreceptors include Meissner’s corpuscules, Merkel cells, glomerular endings, free nerve endings, and Ruffini-like endings.^[4]

Periosteal mechanoreceptors

Examples include complex unencapsulated endings, encapsulated endings, and free nerve endings. They are activated by the exertion of pressure on the periosteum by masticatory muscles and the mucosa.

Central neural representation

There is extensive representation within the central nervous system from mechanoreceptors (cutaneous and mucosal), muscle spindles, GTOs (from jaw muscles), and mechanoreceptors (from TMJ and periodontal ligament (PDL). These representations are at various levels of the trigeminal sensory nucleus, the orofacial somatosensory afferent pathway, the primary somatosensory cortex, and the ventroposteromedial nucleus of the thalamus.^[5]

Activation of periodontal sensory endings following stimuli in and around the tooth is conveyed to the trigeminal neurons in the trigeminal ganglion. Then, signals are transmitted to the ventral posterior nucleus (thalamus) and sensory cortex causing the neurosensory pathway feedback. In contrast to this in implants, there is deficiency in PDL mechanoreceptive input causing hampered peripheral feedback^[6] [Figure 1].

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Figure 1:

Sensory pathway for tooth/implant stimuli (red stars)-sensory nerve endings, (blue and red solid lines) neurosensory pathway feedback.

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Chewing center concept

A recent concept in masticatory control is that the mastication is regulated by a “chewing center.” Similar to the presence of respiratory center for rhythmic breathing, there is a chewing center for rhythmic mandibular movements.

The chewing center is activated by either peripheral sense organs (intraoral and extraoral sensory organs) or higher brain centers (cerebral cortex, extrapyramidal system, and midbrain-pontine nucleus). A diagram representing the somatotopic organization within the somatosensory cortex of mechanoreceptive afferent input from the surface of body is given in [Figure 2].

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Figure 2:

Somatotopic organization within the somatosensory cortex and primary motor cortex.

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Teeth’s tactile sensitivity to masticatory load

Anterior teeth exhibited 10 times higher sensitivity to axial load than posterior teeth. Teeth exhibited more sensitivity to lateral load compared to axial load.

Canine response

Canines have higher proprioceptive capabilities when compared to other teeth. The last natural teeth to be lost are the canines. Retaining the canine teeth for a conventional prosthesis or overdentures to retain the sensory feedback are of great importance.

Directional sensitivity

PDL is directionally sensitive. The receptors around the teeth receive the directional stimuli. PDL showed less directional sensitivity in axial direction when compared to that of along the long axis of the teeth.

Dimensional proprioception

It can be divided into two categories: (1) Discrimination of changes in the thickness of a material and (2) determination of minimal thickness. The size judgment is established by proprioceptive sensations from the masticatory muscles and TMJ. The ability to judge texture was considerably less in majority of denture wearers when compared to dentulous patients.

Perception of oral cavity

A complete edentulous state leads to total loss of the sensory input that had been provided by the PDL. The similar sensory mechanism which maintains the mandibular posture in the infant (i.e., in the edentulous state) also maintains it in the edentulous. The sensory inputs from the TMJ and mucosal receptors compensate for the loss of tooth associated proprioception.