Depressor Muscles Research Articles

Jumping performance of planthoppers (Hemiptera, Issidae)

The structure of the hind limbs and the kinematics of their movements that propel jumping in planthopper insects (Hemiptera, Auchenorrhyncha, Fulgoroidea, Issidae) were analysed. The propulsion for a jump was delivered by rapid movements of the hind legs that both move in the same plane beneath the body and parallel to its longitudinal axis, as revealed in high-speed sequences of images captured at rates up to 7500 images s(-1). The first and key movement was the depression of both trochantera about their coxae, powered by large depressor muscles in the thorax, accompanied by rapid extension of the tibiae about their femora. The initial movements of the two trochantera of the hind legs were synchronised to within 0.03 ms. The hind legs are only 20% longer than the front and middle legs, represent 65% of the body length, and have a ratio of 1.8 relative to the cube root of the body mass. The two hind coxae have a different structure to those in frog- and leafhoppers. They are fused at the mid-line, covered ventrally by transparent cuticle, and each is fixed laterally to a part of the internal skeleton called the pleural arch that extends to the articulation of a hind wing. A small and pointed, ventral coxal protrusion covered in microtrichia engages with a raised, smooth, white patch on a dorsal femur when a hind leg is levated (cocked) in preparation for a jump. In the best jumps by a male Issus, the body was accelerated in 0.8 ms to a take-off velocity of 5.5 m s(-1), was subjected to a force of 719 g and was displaced a horizontal distance of 1.1 m. This performance required an energy output of 303 microJ, a power output of 388 mW and exerted a force of 141 mN, or more than 700 times its body mass. This performance implies that a catapult mechanism must be used, and that Issus ranks alongside the froghopper Philaenus as one of the best insect jumpers.

A model for insect locomotion in the horizontal plane: Feedforward activation of fast muscles, stability, and robustness

We develop a neuromechanical model for running insects that includes a simplified hexapedal leg geometry with agonist–antagonist muscle pairs actuating each leg joint. Restricting to dynamics in the horizontal plane and neglecting leg masses, we reduce the model to three degrees of freedom describing translational and yawing motions of the body. Muscles are driven by stylized action potentials characteristic of fast motoneurons, and modeled using an activation function and nonlinear length and shortening velocity dependence. Parameter values are based on measurements from depressor muscles and observations of kinematics and dynamics of the cockroach Blaberus discoidalis; in particular, motoneuronal inputs and muscle force levels are chosen to approximately achieve joint torques that are consistent with measured ground reaction forces. We show that the model has stable double-tripod gaits over the animal's speed range, that its dynamics at preferred speeds matches those observed, and that it maintains stable gaits, with low frequency yaw deviations, when subject to random perturbations in foot touchdown and lift-off timing and action potential input timing. We explain this in terms of the low-dimensional dynamics.

A biomechanical model of feeding kinematics forDunkleosteus terrelli(Arthrodira, Placodermi)

Biomechanical models illustrate how the principles of physics and physiology determine function in organisms, allowing ecological inferences and functional predictions to be based on morphology. Dynamic lever and linkage models of the mechanisms of the jaw and skull during feeding in fishes predict function from morphology and have been used to compare the feeding biomechanics of diverse fish groups, including fossil taxa, and to test ideas in ecological morphology. Here we perform detailed computational modeling of the four-bar linkage mechanism in the skull and jaw systems ofDunkleosteus terrelli, using software that accepts landmark morphological data to simulate the movements and mechanics of the skull and jaws during prey capture. The linkage system is based on the quadrate and cranio-thoracic joints: Cranial elevation around the cranio-thoracic joint forces the quadrate joint forward, which, coupled with a jaw depressor muscle connecting the jaw to the thoracic shield, causes the jaw to rotate downward during skull expansion. Results show a high speed transmission for jaw opening, producing a rapid expansion phase similar to that in modern fishes that use suction during prey capture. During jaw closing, the model computes jaw and skull rotation and a series of mechanical metrics including effective mechanical advantage of the jaw lever and kinematic transmission of the skull linkage system. Estimates of muscle cross-sectional area based on the largest of five specimens analyzed allow the bite force and strike speed to be estimated. Jaw-closing muscles ofDunkleosteuspowered an extraordinarily strong bite, with an estimated maximal bite force of over 6000 N at the jaw tip and more than 7400 N at the rear dental plates, for a large individual (10 m total length). This bite force capability is among the most powerful bites in animals. The combination of rapid gape expansion and powerful bite meant thatDunkleosteus terrellicould both catch elusive prey and penetrate protective armor, allowing this apex predator to potentially eat anything in its ecosystem, including other placoderms.

Jumping strategies and performance in shore bugs (Hemiptera, Heteroptera,Saldidae)

The jumping movements of the hemipteran shore bug (Saldula saltatoria, sub-order Heteroptera, family Saldidae) were analysed from sequences of images captured at 5000 frames s(-1). Adult Saldula weigh approximately 2.1 mg and are approximately 3.5 mm long. The hind legs that propel jumping are 180% longer than the front legs and 90% of body length, but non-jumping species in the same family have longer hind legs relative to the lengths of their bodies. Jumps were powered by large trochanteral depressor muscles in the thorax in two different strategies. In the first (used in 24% of jumps analysed), a jump was propelled by simultaneous extension of the two hind legs powered by rapid depression movements about the coxo-trochanteral joints, while both pairs of wings remained closed. In the second strategy (74% of jumps), the wings were opened before the hind legs began to move. At take-off, the position of the wings was variable and could be 8-21 ms into either elevation or depression. When the hind legs alone propelled a jump, the body was accelerated in 3.97+/-0.111 ms at a take-off angle of 52+/-6.5 degrees to a take-off velocity of 1.27+/-0.119 m s(-1); when the wings also moved, the body was accelerated in 3.86+/-0.055 ms at a take-off angle of 58+/-1.7 degrees to a take-off velocity of 1.29+/-0.032 m s(-1). These values are not different in the two jumping strategies. In its best jumps the take-off velocity reached 1.8 m s(-1) so that Saldula experienced an average acceleration of 529 m s(-2), equivalent to almost 54 g, expended 3.4 microJ of energy, while exerting a force of 1.1 m N. The power requirements for jumping indicate that a catapult mechanism must be used in which the trochanteral depressor muscles contract and store energy in advance of a jump. These jumps should propel it to a height of 105 mm or 30 times its body length and distances of 320 mm. The two jumping strategies achieve the same jumping performance.

Resilin and chitinous cuticle form a composite structure for energy storage in jumping by froghopper insects

BackgroundMany insects jump by storing and releasing energy in elastic structures within their bodies. This allows them to release large amounts of energy in a very short time to jump at very high speeds. The fastest of the insect jumpers, the froghopper, uses a catapult-like elastic mechanism to achieve their jumping prowess in which energy, generated by the slow contraction of muscles, is released suddenly to power rapid and synchronous movements of the hind legs. How is this energy stored?ResultsThe hind coxae of the froghopper are linked to the hinges of the ipsilateral hind wings by pleural arches, complex bow-shaped internal skeletal structures. They are built of chitinous cuticle and the rubber-like protein, resilin, which fluoresces bright blue when illuminated with ultra-violet light. The ventral and posterior end of this fluorescent region forms the thoracic part of the pivot with a hind coxa. No other structures in the thorax or hind legs show this blue fluorescence and it is not found in larvae which do not jump. Stimulating one trochanteral depressor muscle in a pattern that simulates its normal action, results in a distortion and forward movement of the posterior part of a pleural arch by 40 μm, but in natural jumping, the movement is at least 100 μm.ConclusionCalculations showed that the resilin itself could only store 1% to 2% of the energy required for jumping. The stiffer cuticular parts of the pleural arches could, however, easily meet all the energy storage needs. The composite structure therefore, combines the stiffness of the chitinous cuticle with the elasticity of resilin. Muscle contractions bend the chitinous cuticle with little deformation and therefore, store the energy needed for jumping, while the resilin rapidly returns its stored energy and thus restores the body to its original shape after a jump and allows repeated jumping.

Recruitment in a heterogeneous population of motor neurons that innervates the depressor muscle of the crayfish walking leg muscle

According to the size principle the fine control of muscle tension depends on the orderly recruitment of motor neurons from a heterogeneous pool. We took advantage of the small number of excitatory motor neurons (about 12) that innervate the depressor muscle of the crayfish walking leg to determine if the size principle applies to this muscle. We found that in accordance with the size principle, when stimulated by proprioceptive input, neurons with small extracellular spikes were recruited before neurons with medium or large spikes. Because only a small fraction of the motor neurons responded strongly enough to sensory input to be recruited in this way, we extended our analysis to all neurons by characterizing properties that have classically been associated with recruitment order such as speed of axonal conduction and extracellular spike amplitude. Through a combination of physiological and anatomical criteria we were able to identify seven classes of excitatory depressor motor neurons. The majority of these classes responded to proprioceptive input with a resistance reflex, while a few responded with an assistance reflex, and yet others did not respond. Our results are in general agreement with the size principle. However, we found qualitative differences between neuronal classes in terms of synaptic input and neuronal structure that would in theory be unnecessary, according to a strict interpretation of the size principle. We speculate that the qualitative heterogeneity observed may be due to the fact that the depressor is a complex muscle, consisting of two muscle bundles that share a single insertion but have multiple origins.

Neurons controlling jumping in froghopper insects

The neurons innervating muscles that deliver the enormous power enabling froghopper insects to excel at jumping were revealed by backfilling the nerves from those muscles. The huge trochanteral depressor muscle (M133) of a hind leg consists of four parts. The two largest parts (M133b,c) occupy most of the metathorax and are innervated by the same two motor neurons that have small, laterally placed somata in the metathoracic ganglion and axons in nerve N3C(2). They are also supplied by three dorsal unpaired median (DUM) neurons with the largest diameter somata in the central nervous system. A small metathoracic part of the muscle (M133d) is supplied by two motor neurons with lateral somata and by common inhibitory motor neuron CI(1), all with axons in nerve N3C(3) The motor neuron with the larger soma has a thick primary neurite that projects across the midline of the ganglion so that its branches overlap those of its symmetrical counterpart,innervating the same muscle of the other hind leg. The fourth coxal part of the muscle (M133a) is innervated by two motor neurons (one with a ventral and the other with a dorsal and lateral soma), by CI(1), and by a DUM neuron with a small soma. All have axons in nerve N5A. The two trochanteral levator muscles of a hind leg are contained within the coxa and are separately innervated by nerves N3B and N4, respectively. The properties of the different motor neurons are discussed in the context of the neural patterns that generate jumping.

Interaction between descending input and thoracic reflexes for joint coordination in cockroach: I. Descending influence on thoracic sensory reflexes

Tethered cockroaches turn from unilateral antennal contact using asymmetrical movements of mesothoracic (T2) legs (Mu and Ritzmann in J Comp Physiol A 191:1037-1054, 2005). During the turn, the leg on the inside of the turn (the inside T2 leg) has distinctly different motor patterns from those in straight walking. One possible neural mechanism for the transformation from walking to inside leg turning could be that the descending commands alter a few critical reflexes that start a cascade of physical changes in leg movement or posture, leading to further alterations. This hypothesis has two implications: first, the descending activities must be able to influence thoracic reflexes. Second, one should be able to initiate the turning motor pattern without descending signals by mimicking a point farther down in the reflex cascade. We addressed the first implication in this paper by experiments on chordotonal organ reflexes. The activity of depressor muscle (Ds) and slow extensor tibia muscle (SETi) was excited and inhibited by stretching and relaxing the femoral chordotonal organ. However, the Ds responses were altered after eliminating the descending activity, while the SETi responses remain similar. The inhibition to Ds activity by stretching the coxal chordotonal organ was also altered after eliminating the descending activity.

Myectomy and Botulinum Toxin for Paralysis of the Marginal Mandibular Branch of the Facial Nerve: A Series of 76 Cases

Paralysis of the marginal mandibular branch of the facial nerve is frequently seen in patients with oromandibular reconstructions or facial palsy. However, deformities caused by overpulling of the depressor muscles of the contralateral lower lip without being antagonized by the diseased counterpart is often quite conspicuous. Depressor myectomy of the contralateral lower lip therefore provides a method for correcting the dynamic deformity. Seventy-six patients with paralysis of the marginal mandibular branch of the facial nerve were treated with either surgical depressor myectomy of the lower lip (25 patients), depressor myectomy with subsequent botulinum toxin injection (eight patients), or only chemical depressor myectomy with botulinum toxin injections (43 patients). Good to fair results were always achieved, with near balance of the lower lip during mouth opening and in facial expressions. Surgical myectomy may still result in recurrence in eight patients (24 percent), which will necessitate further treatment with botulinum toxin injections. Using myectomy for paralysis of the marginal mandibular branch of the facial nerve can be an effective treatment for this significant deformity. Chemical myectomy with botulinum toxin injection is a safe and convenient mode of treatment; however, the disadvantage is that it needs repeated injections and costs more.

Anatomy of anuran tadpoles from lentic water bodies: systematic relevance and correlation with feeding habits

I studied anatomy, gut content, and the relationship among these traits in a set of anuran tadpoles. Larval stages (mainly Gosner stages 31–36) of nineteen species from various lentic environments were selected. Morphological characters from the skeleton, musculature, oral apparatus and buccopharyngeal cavity were recorded, and a gut content analysis was performed, with emphasis on food size distribution. Ordination techniques were applied in order to find patterns of similarity in morphology and gut content. Canonical ordination methods were used to investigate the relationship among gut content, morphology, and phylogeny in the species considered. The results show that several skeletal, muscular, and buccal characters are relatively maintained within genera. Other features, which have appeared independently in different lineages, reflect convergence phenomena in some cases related to ecological aspects. The configuration of the hyobranchial skeleton, the development of the buccal floor depressor and levator muscles, and mouth gape width correlate with prey size. In some species, morphology is clearly related with feeding. Tadpoles that ingest large food particles relative to their body length present morphological traits attributable to macrophagy. Taxonomically unrelated tadpoles of Dendropsophus nanus, D. microcephalus and Ceratophrys cranwelli possess hyobranchial skeletons with robust, rostrocaudally long ceratohyals and reduced branchial baskets with short ceratobranchials devoid of lateral projections and spicules. Lepidobatrachus llanensis tadpoles have laterally extended ceratohyals which, along with the lateral extension of the jaws, result in a very wide oral apparatus and an ample buccopharyngeal cavity that allows the tadpole to ingest large and whole prey; the branchial basket, although its ceratobranchials lack lateral projections and spicules, is slightly reduced in area. The four species mentioned have a noticeable development of the buccal floor depressor muscles, and buccal cavities with scarce filtering and entrapping structures. In Elachistocleis bicolor, Dermatonotus muelleri, Chiasmocleis panamensis, and Xenopus laevis tadpoles, the branchial basket occupies >70% of the total hyobranchial skeleton area, and the hypobranchial plates are highly reduced; the buccal floor levator muscles are well-developed, with an increased site of attachment on the ventral expansion of the lateral process of the ceratohyal; the scarcity of the filtering structures in the buccopharyngeal cavity are balanced with the great development of the branchial filters and secretory zones; all these features relate to a diet based on small particles not significantly different from those of most other species; however, experimental studies show that species with similar hyobranchial apparatus and muscles are the most efficient when retaining minute particles. Finally, a large group of species present generalized morphological characters, such as a branchial basket occupying about 50% of the total hyobranchial apparatus, intermediate values of mouth gape width and buccal floor levator / depressor muscles ratio, and abundant filtering structures in the buccopharyngeal cavity; these species feed frequently on food particles between 1–30% of the tadpole body length; however, in some of the species, macrophagous diets are also reported in the literature, indicating that this morphology is flexible in more ample prey size ranges.

An Anatomical Comparison of Transpalpebral, Endoscopic, and Coronal Approaches to Demonstrate Exposure and Extent of Brow Depressor Muscle Resection

Plastic and Reconstructive Surgery: April 1, 2007 - Volume 119 - Issue 4 - p 1374-1377 doi: 10.1097/01.prs.0000255174.47522.91

Escape flight initiation in the fly

Visually evoked escape flight initiation in Drosophila, according to the accepted account, involves a rapid extension of the middle legs that propels the fly into the air while the wings are still folded. This description has remained unchallenged and is accounted for in terms of the activation of a simple neural circuit, the Giant fibre (GF) system. The accepted description of escape is however inconsistent with the sequence of events recorded when the GF system is stimulated. Specifically, previous electrophysiological recordings have shown that the wing depressor muscles are activated before the wings are in a position to be depressed because they have not yet been elevated. Here we show that the accepted behavioural description is wrong. Escape flight initiation actually begins with wing elevation. The current model of the GF system is revised to account for the actual sequence of events that occur when a fly escapes.

Eyebrow Height after Botulinum Toxin Type A to the Glabella

Botulinum toxin type A (BTX-A) has demonstrated impressive safety and efficacy for the treatment of dynamic facial rhytides, particularly in the upper face. Numerous reports have cited an associated brow lift with BTX-A injections in the glabellar complex, presumably caused by deactivation of the brow depressor muscles. Few analyses examining this phenomenon more closely exist, however. The objective was to examine objective changes in eyebrow and eyelid height following BTX-A treatment for glabellar rhytides. A retrospective analysis of subjects' photographs taken during a single-center, dose-ranging, parallel-group, double-blind, randomized trial with 1-year follow-up in which women with moderate-to-severe wrinkles at maximum frown received a total of 10, 20, 30, or 40 U BTX-A in seven sites in the glabella alone. Photographs of the eyes and forehead region were taken in repose at baseline and every 2 weeks after treatment for up to 20 weeks. Eyebrow height was measured at midpupillary line ("a"), outer edge ("b"), and medial canthus ("c"). Changes in eyebrow height between baseline and after treatment were recorded for each subject. Brow lift was considered successful if measurements "a" and "b" increased after treatment. A total of 79 women were assessed. Central injections of 20 to 40 U BTX-A into the glabella alone (with the most lateral injection at the midpupillary line) led to an immediate lateral eyebrow elevation, followed by a central and medial eyebrow elevation that peaked at 12 weeks after treatment. The lowest dose of BTX-A (10 U) produced an initial mild brow ptosis and the weakest response. Doses of 20 to 40 U BTX-A produced dramatic changes in eyebrow position that may be due to diffusion of BTX-A into and partial inactivation of the medial fibers of the frontalis, with resulting increased muscle tone in the lateral and superior muscle fibers of the frontalis.

An Anatomical Comparison of Transpalpebral, Endoscopic, and Coronal Approaches to Demonstrate Exposure and Extent of Brow Depressor Muscle Resection

An Anatomical Comparison of Transpalpebral, Endoscopic, and Coronal Approaches to Demonstrate Exposure and Extent of Brow Depressor Muscle Resection

Retrograde labeling reveals extensive distribution of genioglossal motoneurons possessing 5-HT 2A receptors throughout the hypoglossal nucleus of adult dogs

Inspiratory hypoglossal motoneurons (IHMNs) innervate the muscles of the tongue and play an important role in maintaining upper airway patency. However, this may be reduced during sleep and by sedatives, potent analgesics, and volatile anesthetics. The genioglossal (GG) muscle is the main protruder and depressor muscle of the tongue and contributes to upper airway patency during inspiration. In vitro data suggest that serotonin (5-hydroxytryptamine, 5-HT), via the 5-HT 2A receptor (5-HT 2AR) subtype, plays a key role in controlling the excitability of IHMNs. The distribution of GG motoneurons (GGMNs) within the hypoglossal (XII) nucleus has not been studied in the adult dog. Further, it is uncertain whether the 5-HT 2AR is located on GGMNs in the adult dog. We therefore used the cholera toxin B (CTB) subunit as a retrograde tracer to map the location of GGMNs in combination with immunofluorescent labeling to determine the presence and colocalization of 5-HT 2AR within the XII nucleus in adult mongrel dogs. Injection of CTB into the GG muscle resulted in retrogradely labeled cells in a compact column throughout the XII nucleus, extending from 0.75 mm caudal to 3.45 mm rostral to the obex. Fluorescence immunohistochemistry revealed extensive 5-HT 2AR labeling on CTB-labeled GGMNs. Identification of the 5-HT 2AR on GGMNs in the XII nucleus of the adult dog supports in vitro data and suggests a physiological role for this receptor subtype in controlling the excitability of GGMNs, which contribute to the maintenance of upper airway patency.

Morphology and action of the hind leg joints controlling jumping in froghopper insects

The morphology and movements of key joints of the hind legs that generate the rapid jumping of froghoppers were analysed. The movements of an individual hind leg during a jump occur in three phases. First, the trochanter is slowly levated about the coxa so that the femur moves anteriorly and engages with a lateral protrusion on the coxa. Second, both hind legs are held in this fully levated (cocked) position without moving for a few seconds. Third, both hind legs depress and extend completely in less than 1 ms. The critical, power-generating movement underlying a jump is the rapid and simultaneous depression of the trochantera about the coxae. The lever arm of the hind trochanteral depressor muscle is smallest at the cocked position, but does not appear to go over the centre of the pivot. It then increases to a maximum after some 80 degrees of depression movement. By contrast, the lever arm of the trochanteral levator tendon is similar over the range of joint movements and is exceeded by that of the depressor only after 40 degrees of depression. Three prominent arrays of hairs on the trochantin, coxa and trochanter are appropriately positioned to act as proprioceptors signalling key movements in jumping. In the fully levated position, a protrusion on the dorsal, proximal surface of a hind femur engages with a protrusion from the ventral and lateral part of a coxa. These structures are not present on the front and middle legs. Both protrusions are covered with a dense array of small projections (microtrichia) that both increase the surface area and may interlock with each other. To depress rapidly in a jump these protrusions must disengage. If the hind leg of a dead froghopper is forcibly levated, it will lock in its cocked position, from which it can depress rapidly by movement of the coxo-trochanteral joint and disengagement of the femoral and coxal protrusions. A prominent click sound occurs at the start of a jump that results either from the initial movements of the coxo-trochanteral joint, or from the disengagement of the microtrichia on the coxa and femur. Larval Philaenus, which do not jump, lack a femoral protrusion and have no microtrichia in equivalent positions on either the coxa or femur.

Neural Control and Coordination of Jumping in Froghopper Insects

The thrust for jumping in froghopper insects is produced by a rapid, synchronous depression of both hind legs generated by huge, multipartite trochanteral depressor muscles in the thorax and smaller levator muscles in the coxae. A three-phase motor pattern activates these muscles in jumping. First, a levation phase lasts a few hundred milliseconds, in which a burst of spikes in the trochanteral levator motor neurons moves the hind legs into their fully cocked position and thus engages a mechanical lock between a coxa and a femur. Second, a cocked phase lasts a few seconds, in which a trochanteral depressor motor neuron spikes continuously at a frequency gradually rising to 50 Hz, although the hind legs remain stationary. Levator motor spikes are sporadic. Third, the jump movement lasts <1 ms, in which the spikes in the depressors stop abruptly and the legs rapidly depress. This pattern may vary in the speed of the initial levation and in the duration of the cocked phase. Recordings from the depressor muscles on both sides showed remarkable synchrony of their motor spikes. In one 4.9-long cocked phase all 174 spikes were synchronous and in another 27 s period of continuous spiking all but one of 1,176 spikes were synchronous. When a single hind leg moves rapidly, these depressor spikes are nevertheless independent of those of the other leg. These features of the motor pattern and the coupling between motor neurons to the two hind legs ensure powerful movements to propel rapid jumping.

The influence of age and dental status on elevator and depressor muscle activity

The objective of this study was to determine whether the muscle activity at various mandibular positions is affected by age and dental status. Thirty edentulous subjects (E), 20 young dentate individuals (G1) and 20 older dentate individuals (G2) participated in this study. Surface electromyographic (EMG) recordings were obtained from the anterior temporal (T), masseter (M) and depressor muscles (D). Muscle activity was recorded during maximal voluntary contraction (MVC), maximal opening (O(max)) and in six different mandibular positions. One way anova and the Bonferroni tests were used to determine the differences between groups. Significant differences between the three tested groups were found at MVC and O(max) for all examined muscles (P < 0.001). The differences in muscle activity in dentate subjects of different age were found in protrusion for depressor muscles (P < 0.05) and in lateral excursive positions for the working side temporal (P < 0.05) and non-working side masseter and depressor muscle (P < 0.05). There was a significant effect regarding the presence of natural teeth or complete dentures in protrusion and maximal protrusion for all muscles (P < 0.05) and in lateral excursive positions for non-working side temporal (P < 0.05) and working side masseter muscle (P < 0.05). Muscle activity at various mandibular positions depends greatly on the presence of the prosthetic appliance, as edentulous subjects had to use higher muscle activity levels (percentages of maximal EMG value) than age matched dentate subjects in order to perform same mandibular movement. Different elevator muscles were preferentially activated in the edentulous subjects when compared with dentate group in lateral excursive positions of the mandible. The pattern of relative muscle activity was not changed because of ageing.

Changes in the edentate mandible in the elderly

Resorption of alveolar bone is the best recognized feature of mandibular aging in the edentate subject. The other consequences of the loss of teeth in the elderly are less well known. An anthropometric study of the mandible by antero-posterior and lateral radiographs of subjects older than 70 years both dentate and edentate but without any maxillo-mandibular dysmorphosis has been done to demonstrate the differences, which exist between the dentate and edentate mandible. The edentate mandibles showed a diminution in the height of the symphysis and increase in the height of the mandibular incisure. A diminution in the height of the body and an increase in the gonial angle in the significant manner. No significant difference was seen for the height of the ramus and the length of the mandible, the minimum width of the ramus and the bigonial width. The diminution in the height of the mandibular symphysis and of the body is explained by the resorption of the alveoli part of the mandible. The increase in the mandibular angle and the diminution in the height of the mandibular incisure may be explained by disequilibrium between the elevator and depressor muscles of the mandible, as a function of the elevator muscles or by the absence of the molar buttress.

An Anatomical Comparison of Transpalpebral, Endoscopic, and Coronal Approaches to Demonstrate Exposure and Extent of Brow Depressor Muscle Resection

Approaches for exposure of the muscles of brow depression include transpalpebral, endoscopically assisted, and open coronal techniques. The purpose of this anatomical study was to compare the capacity for visualization and amount of brow depressor muscle resection with each technique. The corrugator supercilii, depressor supercilii, medial orbicularis oculi, and procerus muscles were studied by gross anatomical dissection carried out on 24 sides of 12 cadaver heads. First, all visible corrugator and depressor supercilii muscles were resected by means of an upper blepharoplasty incision. Subsequently, a subgaleal endoscopic approach was used to evaluate the extent of resection performed and remove the remaining muscle after transpalpebral corrugator resection. This was followed by coronal exposure to assess the anatomical location and extent of muscle resection accomplished by the two previously mentioned techniques. In all dissections, endoscopy demonstrated that up to one-third of the lateral aspect of the transverse heads of the corrugator supercilii remained following transpalpebral resection. Oblique corrugator head resections were complete, without significant residual muscle following transpalpebral corrugator resection. The procerus muscle was able to be blindly transected by means of the transpalpebral incision but exposed and ablated in all 12 specimens using endoscopy. Coronal exposure demonstrated that no significant amount of corrugator, depressor supercilii, or procerus muscle remained in any of the 12 heads following endoscopically assisted exposure and resection. The medial head of the orbital portion of the orbicularis oculi was visualized and accessible regardless of the technique used. In 24 anatomical dissections, transpalpebral corrugator resection failed to remove up to one-third of the transverse head of the corrugator muscle. Removal of the brow depressor muscles was accomplished with the endoscopic approach, as confirmed by coronal exposure.