Simplifying the assessment of the recovery from surgical injury to the lingual nerve.

OBJECTIVE: To determine the sensitivity of conventional sensory assessment in monitoring lingual nerve recovery subsequent to third molar surgery and to evaluate if the assessment methods can be predictive of injury outcome. METHOD: A prospective case series of 94 patients presenting with lingual nerve injuries evaluated using objective mechanosensory and subjective methods during the recovery period of up to 12 months. RESULTS: The conventional tests were often unable to diagnose the presence of injury due to variability and they were not predictive of outcome. As a result of this study, we are able to identify patients more likely to have permanent rather than temporary lingual nerve injury at four to eight weeks post injury, using patient reported subjective function. The subjective function test also minimises the requirements for specialist training or equipment providing an ideal method for general dental practice. CONCLUSIONS: The development of these simple subjective tests may enable us to identify which patients are at risk of permanent lingual nerve injuries in the early post injury phase, thus allowing expeditious therapy when indicated.

Objective evaluation of iatrogenic lingual nerve injuries using the jaw-opening reflex.

The extent of reflex inhibition of masseteric electromyographic activity, after an electrical stimulus applied to lingual mucosa, was used as a test of the ability of the lingual nerve to conduct nerve impulses and this was compared with the results of standard clinical tests. Two groups of subjects were assessed: healthy subjects (n=10) and patients with lingual nerve injuries (n=17). The patients were tested 8-9 weeks after their injury and retested 6 months later when they were retrospectively allocated to either a temporary injury or a permanent injury group. The group measure of reflex inhibition after stimulation of the tongue on the opposite side to the injury was no different from the same measure in controls, whereas two-point discrimination did differ. Group measures of inhibition and of subjective function after stimulation on the side of the injury were significantly different from controls whereas light touch and two-point discrimination were not. There was good agreement between quantified masseteric inhibition and subjective function, but it was not possible at 8-9 weeks after the injury to differentiate between those that would recover and those that would be permanent.

New method for the objective evaluation of injury to the lingual nerve after operation on third molars.

Existing tests of function of the lingual nerve are either subjective or, when they elicit the jaw-opening reflex, are dependent on the cooperation of the subject. We report a study in 12 healthy volunteers and 12 patients with iatrogenic injury to the lingual nerve. A bite block (containing stimulating electrodes) was held between the teeth and the tongue was held on to the electrodes by suction. When the lingual nerve was intact, an electrical stimulus elicited brief inhibition of masseteric electromyographic activity. Local analgesia and iatrogenic injury to the lingual nerve altered nerve conduction and caused a reduction in reflex inhibition. Two methods, compatible with limited numbers of applications of the stimulus, were used to quantify responses. One used an indirect measurement of intervals between action potentials of muscle and the other used a measurement of rectified signals falling below the mean amplitude before and after the stimulus. Both methods gave values that correlated with subjective sensations. The first gave an estimate of the probability of defining major malfunction of the nerve objectively; the second gave a linear measurement that allowed recovery of the nerve to be followed.

Rhythmic digastric activity in the naloxone-treated decerebrate rabbit pup.

In decerebrate rabbit pups (greater than 1 and less than 7 days postnatal), exhibiting a digastric reflex with a latency of 20-25 ms, the administration of naloxone was followed by depression of the 25 ms latency reflex and the appearance of rhythmic EMG activity (10-18 Hz) with a latency of 45-90 ms. Such a frequency is not consistent with the normal rhythms of feeding but may relate to tooth grinding behaviour. In contrast, spontaneous rhythmic activity occurring after naloxone had a lower frequency, consistent with the normal rhythm of feeding.

Recovery of the jaw-opening reflex after lesions of the lingual nerve in the rat.

In three groups of rats, lesions were produced in the right lingual nerves near the base of the tongue; the three types of injury inflicted (cryogenic, crush, and stretch) are reputed to spare the epineurium but produce different degrees of intraneural damage. In regular assessments of recovery, an electrical stimulus (sufficient to elicit the jaw-opening reflex) was applied to either side of the tongue in turn; the amplitude of the reflex was measured as the isometric force of jaw opening. The size of the reflex response to stimulation of the injured side was followed up to 4 months post-lesion, with the response elicited from the control side used as the reference level. The reflex was absent when the experimental side was stimulated immediately after creation of a lesion; the first sign of reflex recovery was found at about 15 days post-operative. Subsequently, in 84% of the animals, the reflex activity elicited from the experimental side increased until it exceeded that elicited from the reference side; this relative hyperreflexia started 1-4 months post-lesion and had a highly variable duration. There was no difference in the incidence, latency, or duration of the hyperreflexia following any of the three types of lesion. The hyperreflexia found in this study is not readily explained by existing hypotheses of the mechanisms underlying post-lesion hyperesthesia or central neuronal hyperexcitability.

The effect of food consistency upon jaw movement in the macaque: a cineradiographic study.

Variation in the form of masticatory cycles in individuals is often assumed to be limited. The contrary hypothesis, that jaw cycles vary widely but systematically with food consistency, was tested in macaques fed similarsized pieces of monkey chow, apple, and banana. With the animals under general anesthesia, radiopaque markers were inserted into the jaw, tongue, and hyoid. Oral movements were recorded by cineradiography at 100 frames/sec in lateral projection synchronously with frontal view cinephotography (50 frames/sec). The films were examined for the events that subdivide each jaw movement cycle into its constituent phases (fast closing, slow closing, intercuspal, slow or early opening, final opening). The frame numbers at which these events occurred were used to define phase durations. The numbers of cycles preceding a swallow increased with the hardness of the ingested food item. Regardless of the test food, every feeding sequence (initial ingestion to final clearance of mouth) contained multiple swallows, each of which defined the end of a sub-sequences when the animals were feeding on chow, the sub-sequences were initially long (20 cycles or more), but when they were feeding on banana, the sub-sequences were short (10 cycles or fewer). Although the form of individual cycles (defined by phase durations) was often unrelated to that of neighboring cycles, the general cycle characteristics in a sub-sequence typified a particular food. Chow feeding cycles were characterized by slow-closing (SC) phases of long duration with slow-opening (SO) phases of short duration; the characteristics of banana feeding cycles were the reverse. SC duration correlated directly and SO duration correlated inversely with food hardness (p < 0.001). The evidence supports the view that the centrally generated pattern of movement is highly dependent upon intra-oral sensory feedback.

A computer model of jaw and hyoid movement in the cat.

A digital model was used to study the varying movements of the feline jaw and hyoid in the sagittal plane. The effect of mandibular movement on the path of movement of the hyoid relative to the palate was investigated for model paths of movement of the hyoid, i.e. paths of movement that were always elliptical relative to the mandible. Major effects upon the path of movement of the hyoid relative to the palate were produced by two factors: (1) the presence of a fast open/fast close complex within the profile of jaw movement; (2) the time relationship of the backwards movement of the hyoid relative to that of the fast open/fast close complex. The average antero-posterior position of the hyoid in the mandibular plane and the angle of the major axis of the ellipse to that plane were less important factors. Within limits (relative to the mandible), the overall amplitude of the hyoid movement, its vertical position and the direction in which the ellipse was travelled were all unimportant factors in determining the general nature of hyoid movement relative to the palate.

The electromyographic activities of jaw and hyoid musculature in different ingestive behaviours in the cat.

Electromyographic (EMG) activity in the muscles moving the jaw, hyoid and tongue of the cat was recorded during the intake of solid and liquid foods; the nature of the movements of the jaw, hyoid, tongue and food were recorded and identified cineradiographically. Synergy was evident in muscles with similar anatomical orientation. However, most muscles were activated more than once during each jaw cycle and some of these additional periods of activation occurred at times not predicted by the anatomical arrangement of the muscles. The pattern of EMG activity was the same during all lapping cycles (excluding lap/swallow cycles) but was characteristically different from that occurring during the ingestion of solid food. With solid food the EMG pattern changed during the course of the ingestive sequence and was characteristic for each of the four successively different types of jaw cycle, i.e. transport cycles moving food back from the front of the mouth to the cheek teeth, chewing cycles, transport cycles moving food through the fauces and, following the accumulation of a bolus in the vallecula, swallowing cycles. In these data, provided that the EMG activities of a complete ingestive sequence were available (from food pick-up to swallow), the cycle type could be identified from the intracycle timings and amplitudes of the bursts of EMG activity occurring in the fibres of temporal, posterior digastric and geniohyoid muscles alone. Two variable components of the cyclical EMG pattern could be identified, one relating to tongue movement, the other to jaw movement.

Tongue movement in the cat during the intake of solid food.

During feeding, solid food in the mouth progresses towards the pharynx during transport cycles, but it does not do so in chewing cycles. In the cat, the two cycle types differ with respect to jaw and hyoid movement but it is not known if, or how, any differences in tongue movement arise. This study sought to quantify the tongue movements in the different cycle types. Radio-opaque markers were placed in the midline of the cat tongue. While the animals ate solid food, the marker movements (viewed in the sagittal plane) were recorded by cine-radiography. The movement of the tongue relative to the palate could be split into three components derived from (a) movement of the mandible, (b) movement of the hyoid, (c) movement produced within the body of the tongue itself. Although the differences in jaw movement between transport cycles and chewing cycles produced some differences in tongue movement relative to the palate, differences in the movements produced within the tongue itself were of greater significance. Transport cycles were characterized by rhythmic extensions of the tongue; the protracted tongue was about 60% longer than the retracted tongue. In chewing cycles, rhythmic length changes (viewed in the sagittal plane) were reduced and could be partly explained by an associated rotation of the tongue. In transport cycles the tongue, with food on it, was elevated to the palatal rugae as it extended, but when it shortened it was out of contact with the palate. It is suggested that these movements form the basis of a transport mechanism.

Tongue movement of the cat during lapping.

The movements of radio-opaque markers in the tongue were recorded cine-radiographically. The animals were fed bariumized milk, with or without a thickening agent to increase the viscosity. The movements of the tongue markers relative to the palate were roughly elliptical and resulted from the summation of at least three components: simple movement produced within the tongue, movement imposed on the tongue by hyoid movement and movement imposed on the tongue-hyoid complex by jaw movement. Relative to the palate, the anterior markers protracted high and retracted low, whereas the posterior markers did the reverse. The movements could be explained as having utility for the intra-oral transport of liquid by two mechanisms. The transported liquids then accumulated between the soft palate and the tongue prior to swallowing. The swallow appeared as a brief interruption in the jaw-opening phase of otherwise standard lapping cycles.

The effect of head position upon jaw reflexes in the rat.

The vestibular apparatus of the decerebrate rat was stimulated by tilting the head dorsally or ventrally, with or without an associated stimulation of neck afferents i.e. either the head alone, or the head and body together, was tilted. The effect of such conditions, upon short latency reflexes elicited in antagonistic jaw and hyoid muscles, was examined. Reflexes in the different jaw and hyoid muscles were affected differently when the head was tilted, whether or not neck movement was permitted. Variations in the pattern of response in different animals were explicable on the basis of the known variable recovery of neck reflexes opposing the influence of the vestibular afferents. Head position can therefore be an important but complex factor which may enhance or depress reflex activity differentially in the different masticatory muscles, at least in the experimental situation.

Hyo-mandibular relationships during feeding in the cat.

The movement of the hyoid was studied by cineradiography to obtain information on the movements of the tongue base. During feeding on solid foods, the path of movement of the hyoid in the sagittal plane was highly variable and often complex when referred to the palate. When the effect of jaw movement was excluded by referring hyoid movement to the line of the moving mandible, the path of movement of the hyoid became simpler and less variable, tending to an ellipse. The reversal of hyoid movement from backwards to forwards (relative to the mandible) occurred close to the time at which minimum gape was reached and the reversal from forwards to backwards occurred either (a) at low angles of gape during early jaw opening or (b) at maximum gape. Two different categories of stable state therefore existed together with some intermediate forms of relationship in which the periodicity of jaw movement and of hyoid movement differed. Although the mechanical linkage between jaw and hyoid was a major influence on hyoid movement related to the palate, it was not the sole source of variation in the pattern of that movement.

Correlation between intraoral pressures and tongue movements in the suckling pig.

The objective was to clarify the relationship between tongue movements during suckling and the pressures in different parts of the oral cavity. A modified teat allowed a miniature pressure transducer to be passed through into the mouth. Intraoral pressures were recorded in piglets suckling on the teat attached (1) to a non-vented bottle or (2) to an automated milk delivery system. The movements of the tongue, of the milk and the transducer position were recorded by cine-radiography. In both modes of feeding, waves of elevation on the tongue moved in a pharyngeal direction and rose to contact the mid-posterior palate. Each wave corresponded to a jaw (suck) cycle in which milk was moved into and through the oral cavity. After each wave passed the transducer in the anterior part of the mouth, cyclical negative pressures were recorded. In bottle feeding, the intraoral pressure fluctuations (+/-2 mmHg) occurred against a background of a gradually developing negative pressure but, when feeding on the automatic delivery system, the same or smaller fluctuations occurred as changes from atmospheric pressure. Where the elevations contacted the mid-posterior palate in each cycle, a seal was formed (contact pressure >40 mmHg), so producing two functional antero-posterior compartments within the mouth; in these compartments pressures were generated independently. With the transducer in the valleculae, no general increase in pressure was recorded as milk accumulated there in each suck cycle but large positive pressures were recorded during the less frequent cycles when the vallecular space was emptied.

Latency of the jaw-opening reflex in the premature, preweaning and adolescent pig (hanford strain miniature pig, Sus scrofa).

The jaw-opening (digastric) reflex was elicited by electrical stimulation of oral mucosa in miniature pigs (Sus scrofa) varying in age from 5 days premature to 101 days post-term. The latency of reflex electromyographic activity varied between 12-14 ms in the most immature animals and 9-11 ms in the oldest animals. The very long-latency digastric responses found in the immature young of nesting mammals were not seen in the relatively precocious young of this species.

Mastication and swallowing: an overview.

The cycles of jaw and tongue movement during feeding produce not only the breakage of food but its intra-oral transport; which activity predominates depends upon the physical characteristics of the food. When hard food is eaten and tooth-food-tooth contact is made during jaw closure, the velocity of closing is suddenly reduced, producing two clearly different phases of closure; during the second phase the activity of the jaw closing muscles is much increased. Conversely, in cycles with a mainly transport function (eating soft food), the antero-posterior movements of the tongue are much greater; this alters the time and rate at which the jaw opens. The pattern of jaw movement during closing and during opening consequently varies with food consistency. The evidence suggests that sensory input controls the form of the cyclical tongue and jaw movements. However the basic plan of movement is produced by the activity of a brainstem pattern generator which receives input from both cerebro-cortical and peripheral sources. The swallow that occurs in normal feeding consists of the equivalent of the classical second stage of swallowing inserted into the occlusal or initial jaw opening phase of an otherwise standard cycle. Although leakage of traces of food or saliva into the vallecula appears to be a peripheral sensory input of major importance in inducing such a swallow, the execution of the swallow is due to a pattern generator in the brainstem.

The role of animal models in understanding feeding behavior in infants.

The common evolutionary history humans share with mammals provides us with a solid basis for understanding normal oropharyngeal anatomy and functions. Physiologically, feeding is a cycle of neurophysiologic activity, where sensory input travels to the CNS which sends motor signals out to the periphery. Research with animal models is valuable because it is possible to disrupt this cycle, and develop predictive models on the causal basis of deviation from normal. Based on work with animal models, normal mammalian infant feeding behavior consists of the tongue functioning as a pump. First, the tongue assists in acquisition of milk from the nipple into the oral cavity, and then it pumps milk from the oral cavity into the valleculae prior to the pharyngeal swallow. Starting with this basic model, feeding in infant pigs was manipulated to determine the impact of variation in sensory input on behavioral output. One set of experiments suggested that chemo- or liquid sensation, in the form of milk is necessary to elicit continuing rhythmic activity. However, the rates of rhythmic suckling are intrinsic to an animal, and variation in rate cannot be entrained. Another set showed that initiation of the swallow does not purely depend on the volume of milk delivered, but also on the sensory stimulation at the mouth. These results support the idea that feeding behavior involves complex sensory integration.

Quantitative thermosensory testing of the lingual and inferior alveolar nerves in health and after iatrogenic injury.

Quantitative testing of the sensory thermal thresholds testing was applied at sites innervated by the third division of the trigeminal nerve in 20 patients with no reported sensory impairment and in 20 subjects with iatrogenic nerve injuries after third molar surgery. In the control group sensitivity to cooling was significantly greater than the sensitivity to warming at all sites. The labial mucosa innervated by the inferior alveolar nerve was significantly more sensitive to thermal changes than either the mental region or the lingual mucosa. At sites supplied by nerves that had been injured, there were raised thresholds to both warming and cooling compared with the control group, and with uninjured contralateral sites. The results indicate that this test can identify iatrogenic lingual and inferior alveolar nerve injury with reference to a control group but because of spatial variation selection of control sites for comparison should be done cautiously.