The human periodontal ligament contains various mechanoreceptor morphologies, which may all contribute to the exteroceptive function. Periodontal mechanoreceptors play the primary role in tactile function of teeth & are very sensitive to external forces applied to the teeth. The majority of the periodontal mechanoreceptor unit in man adapt slowly with low force threshold. This is in agreement with results from animal species. Periodontal mechanoreceptors also exhibit directional sensitivity which implies that they respond maximally to force applied in a particular direction. Furthermore, rate & magnitude of force applied to the tooth may modify the response characteristics of the mechanoreceptors.

Neurophysiological investigations on the sensory function of the trigeminal system in humans are scarce. Evidence can be found by non-invasive approaches for evaluation of oral tactile function. The first approach is the recording of the so-called trigeminal somato sensory evoked potentials (TSEP) after stimulation of receptors in the oral cavity. 7, 8 Another non-invasive method to assess sensory function is the visualization of brain activities by functional magnetic resonance imaging (fMRI). 9

Psychophysical studies on the oral sensory function are numerous and its major advantage is that these are simple non-invasive techniques that may be performed in a clinical environment. Psychophysics includes a series of well defined methodologies to help determining the threshold level of sensory receptors in man. Psychophysical methods allow connecting the psychological response of the patient to the physiological functions of the receptors involved. When performed meticulously and under standardized conditions, these studies may provide nearly as precise information as neurophysiological setups.                              

In the psychophysical approach, a clear distinction should be made between the “passive” threshold or detection of the force applied to the teeth and the “active” one, where interocclusal detection of the small objects such as strips or foils of varying thickness is performed. The large discrepancies between active and passive thresholds can be explained by the fact that several receptor groups may respond to active testing (active threshold determination provide a means to observe a parameter of jaw motor control), this may involve the activation of mechanoreceptors, mainly originating from the peridontium but also from the muscles, inner ear, and TMJ. The periodontal mechanoreceptors involved in the passive threshold determination although not in a very physiological situation. It should, however, be realized that the foil materials used for active threshold detection may have different thermal and mechanical properties, resulting in conflicting results (Jacobs et al. 1992). Foil materials with high thermal conductivity (e.g. steel, aluminium) may lower the threshold level by activation of thermal receptors. 10               

The active detection task is further divided into a static and a functional threshold determination. 11 Absolute as well as differential threshold determination can be performed in both cases. The absolute threshold (RL), for the German Reiz Limen) is the stimulus amplitude at which a subject detects the stimulus, whereas the differential threshold (DL, for the German Differenz Limen) can be expressed as the smallest increment of the stimulus which is just detectable by the subject. The endosseous implants are less sensitive than natural teeth for the passive DL level of forces but seem equally sensitive at force levels in the order of chewing forces. Different stimulating devices are proposed for the passive detection of forces applied to a tooth. The active threshold is seven to eight times higher for dentures but only three to five times higher for implants when compared with tactile function of natural dentition. For the passive detection of forces applied to upper teeth, thresholds increased 75 times for dentures and 50 times for implants.            

In psychophysical threshold determination, a threshold range rather than an absolute value exists. A subject uses his own criteria to discriminate between stimulus and noise. The reliability of the responses are affected by the subject’s attention and psychological attitude. Unfortunately, tactile threshold determination is often performed without using an appropriate psychophysical methodology. 12 Environmental factors should be well-controlled as background noise is distracting to patient and examiner. To minimize the effect of noise, testing should be done in a quiet room with stable background illumination.

Low-threshold mechanoreceptors are present in the TMJs and in other joints of the body. These receptors could potentially provide detailed information to jaw position and movement [described in the TMJs of rabbits; these have been classified as limited-range receptors (Lund and Matthews, 1981).

i. Golgi tendon organs: are found at the musculo-tendinous junction in series, with a small number of extrafusal muscle fibers, activated by the pull of the muscle fibers and with muscle contraction. Golgi tendon organs are the most appropriate mechanoreceptors for signalling during voluntary contractions such as biting.

ii. Muscle spindles: are the most complex somatosensory receptor in the body with sophisticated physiological properties and they provide detailed information on muscle length and rate of length change. These receptors help in the assessment of jaw position and movement.

Cutaneous mechanoreceptors in the facial skin are activated by skin stretching or contraction of facial muscles and may operate as proprioceptors involved in facial kinaesthesia and motor control.

When natural teeth are present, periodontal mechanoreceptors are important for refined interdental discriminative function. Whereas after tooth loss, in implant-supported prostheses opposing complete dentures, this oral kinesthetic perception could come from the activation of mucosal receptors beneath the complete denture and possibly periosteal and/or mucosal mechanoreceptors in the vicinity of the implant fixture (Jacobs and Van Steenberghe, 1991).

The periosteum contains free nerve endings, complex unencapsulated and encapsulated endings. By pressure or stretching of the periosteum through the action of masticatory muscles and the skin, free nerve endings gets activated. When applying forces to osseointegrated implants in the jaw bone, it might be assumed that the pressure build-up in the bone is sometimes large enough to allow deformation of the bone and its surrounding periosteum (Sakada, 1983; Capra and Dessem, 1992). 6

After activation of peripheral receptors or mechanoreceptors, nerve impulse generate action potential and motor response is triggered (Figure-1). 17

Buser et al (1990) 18 in their study placed titanium implants in the mandible of monkeys where apical root portions were retained. The histological examination revealed that a cementum layer with inserting collagen fibers was achieved around implants. These results concluded that dental implants with a true PDL can be accomplished.

Takata et al (1993) 19 studied that a new connective tissue attachment can occur on a hydroxyapatite surface(HA) when PDL-derived cells with the ability to form new connective tissue attachment are allowed to populate the surface of HA in an experiment.

Warrer et al (1993) 20 also found that a PDL can form on titanium dental implants in areas where a void is present between the surrounding bone and the implant at the time of insertion.

Choi (2000) 21 in study on dog mandible found that cultured PDL cells can form tissue resembling a true PDL around implants.

Jahangiri et al (2005)22 in animal study (beagle dog) succeeded to partially regenerate the periodontal ligament on an implant surface. In this study, orthodontic tooth movement was initiated following implant placement to tip the first pre-molar roots into contact with the implant; an animal model was established in which the proximity of tooth-to-implant contact lead to partial generation of PDL on a bioactive implant surface

It has been studied that existing mechanoreceptors in the periosteum may also play a role in tactile function upon implant stimulation in a study by Jacobs and van Steenberghe (1991). 6

Histologically, it has been seen that there may be some reinnervation around osseointegrated implants.

Tanaka et al (1996) 23 in immunocytochemical study of nerve fibres containing substance P in the junctional epithelium of rats found that substance P & free nerve endings respond to pain, touch and pressure.

Lambrichts (1998) 24 in animal study reported the presence of neural fibres in the immediate vicinity of implants in the cat’s jaw. Whereas Wang et al (1998) 25 reported that nerve fibres increase their density over time at the implant/ bone interface.In a study by Ysander et al (2001) 26, the nerve fibres were detected in the remodelled bone adjacent to the implants.

According to Klineberg & Murray (1999) 6, sensory response comes from the muscle spindle and joint receptors that substitute for the PDL of natural teeth.            

Bonte et al (1993) 27 in animal study on cat jaw muscle observed that when forces are applied to a tooth, periodontal mechanoreceptors, which evoke reflex inhibitions to motor units in the jaw-closing muscles, are stimulated; however, there is evidence that mechanoreceptors situated distant to the periodontium can also evoke such reflexes.    

It has been documented that reinnervation in association with controlled forces directed to implants results in proprioception. This is due to the regeneration of new nerve fibers around the implant with physiologic loading when osseointegration is near to woven bone.

Wada et al (2001) 28 observed the effects of occlusal forces on the distribution of neurofilament protein (NFP) positive nerve fibres around peri-implant bone in animal study. Indeed, it has been shown that there is degeneration of environing neural fibres by surgical trauma during endosseous implant placement but soon after there is sprouting of new fibres gradually during the first week of healing.                   

In an animal study by Saul Weiner et al (2004) 29, it was found that loading of implants elicit a sensory response in the inferior alveolar nerve observed in neurophysiologic recordings. In an animal study by Fujii et al (2003) 30, they found that the peri-implant epithelium shows the same innervations as that in normal junctional epithelium.       Similar results were seen by Suzuki et al (2005) 31.

According to Skada S (1974) 32, it is evident that oral implants offer another type of loading and force transfer than teeth, considering an intimate bone-to-implant contact with elastic bone properties different from viscoelasticity of the periodontal ligament in natural dentition. Thus, forces applied to osseointegrated implants are directly transferred to the bone and bone deformation may lead to receptor activation in the peri-implant bone and the neighbouring periosteum.

Yamashiro T et al (2001) 33, studied that bone strain, either compression or elongation, may serve to activate free nerve endings that project through the inferior alveolar nerve (IAN) to regions of the trigeminal system in the pons where jaw motor activity is mediated (Figure-2.). Schulte (1995) 34 studied tactile sensibility of implants versus natural teeth. Passive tactile sensibility seems to be less clearly localized in the case of implants when compared to natural teeth. He had found that the deformation of the peri-implant bone, which might cause stretching of the periosteum was responsible for tactile sensibility.