Relationship of occlusal vertical dimension to the health of the masticatory system

Warren C. Rivera-Morales, D.M.D., MS.,* and Norman D. Mohl, D.D.S., Ph.D.** Medical College of Georgia, School of Dentistry, Augusta, Ga., State University of New York at Buffalo, School of Dental Medicine, Buffalo, N.Y.

Changes in occlusal vertical dimension have been claimed to cause masticatory system disorders. Early articles on this subject were mainly limited to clinical case reports, and the more recent clinical studies have been flawed by the lack of control groups, blind evaluation, and by poor definition of criteria for evaluating the health of the masticatory system. Research with humans and animals has shown that if increases in occlusal vertical dimension are not extreme and the appliance used covers most of the dentition, there is a good possibility of adapta- tion. Current scientific knowledge does not support the hypothesis that moderate changes in occlusal vertical dimension are detrimental to the masticatory system. (JP~os~a~~D~~~1991;65547-63.)

T he evaluation and establishment of the occlusal group of authors. He concluded from clinical observations vertical dimension (OVD) is regarded as particularly im- made on 11 patients that mandibular overclosure caused portant in the treatment with complete where no posterior displacement of the condyles.‘* The condyles, in teeth are present to aid the dentist in the establishment of turn, were believed to exertpressure on the chorda tympani a therapeutic OVD. It may also be of special concern for and auriculotemporal nerves, and on the eustachian tubes. dentulous on partially edentulous patients who may have According to Costen,14 pressure on these anatomic struc- a reduced OVD due to wear or loss of the posterior teeth. tures caused headache, sinus pain, earache, tinnitus, a Many controversial claims have been made about the stuffy sensation in the ears, burning tongue, and metallic role of OVD on the health of the masticatory system. This taste and deafness, a group of symptoms that became review assesses the relative scientific merit of those claims known as “Costen’s syndrome.” These authors’ simplistic and provides a scientifically based conclusion about the and erroneous cause-and-effect deductions were based ei- relationship of OVD to the health of the masticatory sys- ther on inaccurate interpretations of anatomic and radio- tem. logic finding& 4s6* g or a misinterpretation of clinical “success” in relieving some of the symptoms involved by EARLY PATIENT REPORTS “opening the bite.“% %5,7s&11-14 An “overclosed or collapsed” OVD due to wear or loss of Although this hypothesis was widely accepted at the teeth has been claimed to cause atrophy and perforation of time, it was brilliantly challenged as early as 1925.15 Addi- the temporomandibular joint (TMJ) “meniscus” and per- tional evidence against it was offered in the late 1930s and foration of the tympanic plate, deafness, tinnitus, prob- early 1940s,r6-rg and it was proved wrong on anatomic lems in respiration and deglutition, earache and paresthe- grounds by Zimmerman as mentioned in Sarnat,20 and sia of the tongue and pharynx, facial and glossopharyngeal Sicher21 in the 195Os,and on clinical grounds by Schwartz22 neuralgia, vertigo, xerostomia, loss of tone of muscles of in 1959. Schwartz22 reported his work of 10 years with 2500 mastication, sleeplessness, lack of concentration, and dis- TMJ patients and observed that “the symptoms that we eases of the ear, nose, and throat.r-r3 Although he was not found were not those emphasized by Costen . . . we were the first, Costenr4 is perhaps the best known among this unable to relate them to ‘bite closure’.” Nonetheless, Cos- ten’s hypothesis led to the restoration of many dentitions at an “increased” OVD to treat or prevent the so-called This work was supported by U.S. Public Health Service grants Costen’s syndrome. This approach prompted a new series DE070309 and DE04353 of writings describing problems that were presumably *Assistant Professor and Coordinator of , School of caused by this practice, especially if the increase in OVD Dentistry, Medical College of Georgia. **Professor, Department of Oral Medicine, School of Dental obliterated the interocclusal distance (IOD) from postural Medicine, State University of New York at Buffalo. rest position or “freeway space.” 10/l/27034 The problems that were described included increased

THE JOURNAL OF PROSTHETIC DENTISTRY 547 RIVERA-MORALES AND MOHL

tooth mobility, alveolar bone resorption, trauma and in- patients using cephalometric radiographs. They found a trusion or depression of teeth, fatigue, undue strain on large intrasubject and intersubject variability and that muscles of mastication, clenching and bruxism, increase in postural rest position can be affected by head posture, ab- the stress on the supporting tissues or excessive stresses on sence of teeth, absence or presence of a prosthesis, and the periodontium, recession and atrophy of bone and soft tis- emotional state. These findings have since then been sues, and myofascial pain. 23-30According to these authors, extensively confirmed.4g-5g an increase in OVD beyond postural rest position caused an increase in muscle activity in the jaw-closing muscles, “as LABORATORY STUDIES WITH ANIMALS the musculature attempts to recover the individual’s orig- The second hypothesis, that is, that an increase in OVD inal interocclusal clearance.“27 Unfortunately, no scientif- beyond postural rest position will produce an increase in ically compelling evidence was provided to support these masticatory muscle activity leading to pain and affecting claims, since clinical observations and patient reports ex- the TMJs, muscles of mastication, periodontal tissue, and clusively could not be used to establish cause-and-effect teeth has been tested in monkeys, rabbits, and rats.60-6s relationships. However, these clinical observations gave The methodologies have included histologic or histochem- rise to two closely related testable hypotheses: (1) interoc- ical studies of the TMJ; morphologic, histologic, and his- clusal distance from postural rest position is fixed and un- tochemical studies of masticatory muscles; histologic stud- changing throughout life, and (2) if OVD is increased be- ies of the periodontium; clinical dental examination; and yond postural rest position, it produces muscle hyperac- radiographic-morphologic studies.ec-68OVD has been in- tivity in an effort to reestablish the previous IOD with creased 5 mm interincisally in monkeys by use of gold possible symptomatic sequelae in the TMJs, muscles of crowns on lower left first molars,‘j” 3 mm by use of cast mastication, periodontal tissues, and teeth. splints covering all posterior teeth in both dental arches but occluding only on the distal aspects of terminal molar&; 2, EARLY CLINICAL STUDIES 3,5,10, and 15 mm with cast maxillary splints covering the Several early studies provided apparent support for the canines and all posterior teeth62; 2 mm with cast mandib- first hypothesis, that is, the claim for constancy of IOD. ular splints63; 7 mm64 and 15 to 18 mm65 with cast maxil- Niswongersl reported having found an interocclusal dis- lary splints covering all posterior teeth; and 0.25 mm with tance of 3.175 mm (l/s inch) in 85% of 400 subjects by use a unilateral left maxillary posterior splint.@ The increase of a soft tissue instrument he called a “jaw relator,” mea- in OVD was 5 to 7 mm interincisally in the experiment with suring between a point on the chin and the junction of the rabbits67 and 1 to 2 mm between the molars in the exper- philtrum and septum of the nose. This procedure provided iment with rats.@ The OVD-increasing devices were in a “one-shot” cross-sectional study, and the author implied place from 7 days66up to 3 yea@; sometimes for varying from his results that the IOD was the same for most sub- amounts of time for different subjects within each exper- jects. These results have not been replicated in subsequent iment. studies, which showed soft tissue facial measurements to be None of these studies directly tested the first part of the relatively inconsistent and inaccurate,32*33 accounting for second hypothesis, that an increase in OVD will cause hy- as little as only half of the skeletal movement involved, in peractivity of the masticatory muscles. The studies did at- measuring known alterations in OVD of up to 6 mm.33 tempt to test the second part of the hypothesis, that an in- Brodie,34 in studying cranial development in children crease in OVD will produce pain and affect the TMJs, from 3 months to 8 years of age, reported a “constancy of muscles of mastication, periodontium, and teeth. position of the mandible, which position is maintained by It is hard to measure objectively whether these monkeys muscle tensions.” Thompson and Brodie35 and Thomp- had any discomfort due to an increase in OVD, although son36conducted long-term follow-up studies (up to 8 years) the authors of one study reported the monkeys were in which the postural rest position was measured by use of initially irritated and had “gnashing” of their teeth until cephalometric radiographs. The authors reported postural they became used to the situation.60 Other authors also re: rest position to be stable and “not affected by presence or ported slight, if any, initial difficulty of the animals with absence of teeth,“35 with an “average” value of 2 to 3 mm eating, no significant problems, resumption of normal for the IOD.36 Based on these findings, the clinical tech- dietary behavior shortly after the experimental interven- nique of measuring vertical dimension at postural rest po- tion, no weight loss or any other apparent health prob- sition and reducing that measurement by 2 to 4 mm to al- lem62*63* 65* 66 and even a total gain of weight for the exper- low for IOD has been used and is still advocated for estab- iment and no apparent distress.64 Although objective mea- lishment of OVD.a7-42 surement of pain is difficult even in humans, the overall Thompson and Brodie’s findings34-36were not replicated clinical picture seems to imply their situation was not ex- in subsequent research by Olsen,43 who conducted a one- tremely uncomfortable. session study, and Atwood44-46and Tallgren47148 who con- These experiments confirmed that increasing OVD may ducted longitudinal studies of complete denture-wearing affect the different components of the masticatory system

548 APRIL lSS1 VOLUME 65 NUMBER 4 OVD AND HEALTH OF MASTICATORY SYSTEM

at a histologic and morphologic level. Nonetheless, the re- moderate increase in vertical dimension of occlusion does sponse is not one of collapse and breakdown, as the not seem to be a hazardous procedure, provided that hypotheses would predict, but one of general compensation ~cclusal stability is maintained.70 Besides the subjective and adaptation. symptoms, Christense# reported an increase of tender- These experiments resulted in relatively minor changes ness to palpation in all masticatory muscles and concluded and eventual adaptation in the masticatory system of the that increasing OVD “apparently deranged the function of animals as long as the increases in OVD were not extreme muscles and joints.” and the devices used covered all or most of the teeth. The A group of 22 complete denture-wearing patients also more severe responses, for example, tooth mobility, intru- participated in Christensen’s experiment, having their sion or extrusion,60T64 TMJ remodeling,61 and other oc- OVD increased to the clinically determined postural rest clusal, skeletal, and muscular changes62,65* 67 occurred only position. Rosas et a1.71conducted a similar experiment with when the increases in OVD were extreme62*65 or when 15 complete denture-wearing patients, in which five served the device used did not cover the dentition complete- as controls and two groups of five subjects each had the l~.~, 61p64* 67 The nature and severity of these experimental OVD in their dentures either increased or decreased by 7 changes are quite different from those used in routine den- mm. Their results and conclusion were similar to Chris- tal practice. The result of relatively large increases in OVD, tensen’s; that is, subjects in both experimental groups de- with only partial coverage of the dentition, cannot be gen- veloped significant symptoms that the authors reported as eralized to any increase in OVD or occlusal change. Inter- supporting the “critical value of properly established ver- estingly enough, in spite of the severity of these changes, tical dimension in .“71 The length of most of the experimental animals seemed to adapt to them these experiments was 6 to 8 days.7os71 successfully. We do not think that these results should be There are several problems with Christensen’s study. interpreted as implying that changes in occlusion or OVD First the dentulous subjects were all dental students. Be- are irrelevant to the health of the masticatory system. sides not being an appropriate control group to compare However, they do not prove that small or moderate changes with edentulous patients, the dental students may have in OVD as a result of clinical restorative dental treatment had preconceived notions about the experimental proce- are a major cause of pain or other symptoms of temporo- dure in which they were participating. This problem raises mandibular disorder (TMD). the possibility of serious confounding factors. Second, the finding of increased tenderness to palpation, defined as CLINICAL STUDIES WITH HUMAN “blink reflex elicited by moderate pressure”6g is of ques- SUBJECTS tionable value in light of the vagueness of the definition. Human studies in which OVD has been experimentally The lack of a double-blind evaluation design raises the increased have been few since ethical concerns limit the possibility of unconscious researcher bias as an additional type of intervention and amount of information that can be confounding factor. obtained. Nonetheless, findings of these studies are more Unfortunately, not enough details about the experimen- clinically relevant, because pain and other subjective tal design are given in the abstract by Rosas et a1.71Sub- symptoms can be more easily evaluated in humans. ject expectation may affect the outcome in an experiment Christensen6g and Carlsson et a1.70increased OVD in of this type. The way the experimental procedure is dentulous subjects by means of bilateral splints covering all presented to the subjects may influence their reaction. In molars6g and all teeth posterior to the canines70 over a 3- addition, for the use of a control group to be significant, the to 7-day experimental period. There was no control group examining clinician or researcher must be unaware of the in either of these experiments. As mentioned, results group to which the examined subject belongs; for example, obtained by increasing OVD with partial-coverage restora- control or experimental. Otherwise, unconscious researcher tions may not necessarily be the same as those obtained bias may play a factor in the evaluation of the results. In with complete coverage of the dentition. The extent of addition, control and experimental groups should be opening was beyond the interocclusal distance6g and ap- matched for age, sex, and other confounding factors. Un- proximately 4 mm interincisally.70 Both experiments re- fortunately, without appropriate clarification of these ported initial symptoms such as headaches, clenching and issues, the conclusions presented by the authors of this grinding, muscle and joint fatigue, soreness of teeth cov- study are unsubstantiated and questionable. ered by the splint, cheek biting, and problems with chew- These three studies6g-71do not provide sufficient evi- ing and speech articulation. Carlsson et a1.70reported that dence to support the hypothesis that increases in OVD may all symptoms diminished in intensity after 1 to 2 days for cause symptoms of TMD. Indeed, the study by Carlsson et all six subjects except one. They tested the hypothesis that a1.70suggested the possibility of human adaptability, sim- increased OVD would cause masticatory muscle hyperac- ilar to that found in animals studies, to increases in OVD. tivity by electromyographic recordings. They found no In addition to these purely experimental procedures, evidence to support this hypothesis, and concluded that a OVD has also been therapeutically changed in many

THE JOURNAL OF PROSTHETIC DENTISTRY 549 RIVERA-MORALES AND MOHL

patients by orthognathic surgical procedures. Wessberg et treatment of TMD or that “improper” OVD plays no eti- a1.72evaluated neuromuscular adaptation to orthognathic ologic role on TMD. It appears, therefore, that the issue is superior repositioning of the maxillae in 15 subjects, with still an open one and further research is justified. clinical and radiographic features of “maxillary vertical excess.” They had a control group of 10 adult women with CRITICAL REVIEW OF SOME normal dentofacial morphology. The subjects in both TECHNIQUES TO EVALUATE OVD groups were evaluated before and 3 months after surgery OVD has been long regarded as important in dentistry as by computerized morphometry, mandibular kinesiography evidenced by the long list of methods and techniques pro- (MKG), and EMG measurements. posed to establish it.76 None of the proposed techniques has The authors found no statistically significant differ- been proved scientifically accurate or superior, and scien- ence between presurgical and postsurgical trials, with tific research has only succeeded in destroying old myths. respect to mean interocclusal distance from postural rest Regretably, alternative concrete, practical, and accurate position or masticatory EMG activity @ > 0.05). This guidance for clinicians has not been provided although finding provides further evidence for the possibility of clinical realities demand methods that are predictable, sci- adaptability to changes in OVD in the human masticatory entifically tenable, and universally applicable. A brief ex- system. amination of a few alternate approaches should help to Additional evidence along those lines has been provided substantiate our conclusions. by clinical treatment studies in TMD. Manns et al.73stud- ied the influence of OVD in the reduction of myofascial OVD AND BITE FOriCE pain-disorder (MPD) symptoms and masseter muscle The influence of OVD on bite force has been studied77*I8 EMG activity74 in subjects with TMD. For the first of these while testing Boos’ hypotheses7g that there is an optimum studiesI they had 75 subjects with range in age of 13 to 53 vertical dimension for development of masticatory force, years, with MPD symptoms as described by Laskin,75 and one which occurs close to the postural rest position of the no previous use of occlusal splints or removable partial mandible which should be used as a therapeutic OVD. dentures. They used full-coverage “,” that Mackenna and Turker’s results with 23 cats,78 and the is, nonanterior repositioning acrylic resin splints with ca- Manns et aly7 results in eight humans agreed with the first nine disclusion and anterior guidance, for 3 hours daily and part of Boos’ hypothesis, but their findings for optimal all night for 3 weeks. length were 15 to 20 mm of opening77 and when the mouth The subjects were randomly assigned to three groups. In was “almost completely open,“78 disagreeing with the sec- the first group, OVD was increased by 1 mm with the splint. ond part. In the second group, OVD with the splint was increased Although interesting, these findings provide no evidence halfway between OVD in centric occlusion and vertical for the third and most critical part of the hypothesis, that dimension of minimal masseter EMG activity. The mean the jaw opening where maximal closing force is developed increase in OVD with the splint was 4.42 mm. For the third is the most appropriate for a therapeutic OVD. There is no group, OVD was increased to that of minimal EMG clinical indication for the use of this technique at this time. activity in the masseter muscles. The mean increase in OVD with the splint was 8.15 mm. The experiment THE COMFORT ZONE APPROACH OF lasted 3 weeks. All of the subjects had a gradual de- ESTABLISHING OVD crease in symptom intensity, but the decrease was faster Tryde et a1.80v81 used a different approach for the deter- for groups 2 and 3, up to twice as fast for some symp- mination of OVD in edentulous patients. They had an ad- toms. justable screw jack inserted with wax occlusal rims. The For the second study,74 they used a similar methodology, screw jack was closed down until the reduced height was with 60 subjects randomly divided into three groups. In this perceived by the subjects as being too low; then it was study, they evaluated masseter EMG activity at postural opened, until the increased height was perceived by the rest position and with “slight tooth contact” on the splints. subjects as being too high. This procedure was repeated to Both of these readings gradually decreased for groups 2 and obtain 22 pairs of high and low readings during one 3, but remained about the same for group I. session,81or two pairs of high and low readings during each Replication of these results by other researchers is not of 15 separate recording sessions done over a 3-week available; however, these results do not support the tradi- period.sc The term “comfort zone” for the range betwen the tional belief that an increase in OVD beyond postural rest high and low readings was used. This zone measured position will produce masticatory muscle hyperactivity and approximately 1 mm on the one-session experiments1 and symptoms of TMD. These results are, indeed, the complete 1.3 mm on the multiple session experiment.80 opposite of what the hypothesis would predict. These experiments were well designed, especially the The evidence from these studies is not sufficient to prove multiple-session experiment@ in which two different in- that thicker splints work better than thinner splints in the vestigators made separate measurements, and the order of

550 APRIL 1001 VOLUME 65 NUMBER 4 OVD AND HEALTH OF MASTICATORY SYSTEM

the investigator making the measurement and the height of studied scientifically, and remains an unproved assump- the screw jack were randomized. These results appear to tion. If optimum OVD for the edentulous state were differ- contradict the possibility of adaptability suggested by pre- ent-and this is certainly within the realm of possibility- vious experiments, because the average comfort zone the clinical establishment of OVD may have to be reeval- turned out to be a relatively narrow one, 1.3 mm. The clin- uated. It should be conceded, however, that traditional ical implication would be that complete dentures con- clinical wisdom in establishing OVD on a routine basis and structed at an OVD outside of this comfort zone may cause the clinical outcome achieved, appears to satisfy most den- “discomfort” of the masticatory system. tists. The authors’ conclusions are prudent ones as they Nonetheless, the absence of compelling scientific evi- observe that: “Further studies are needed before a clinical dence to prove or disprove the routinely employed clinical method applying a screw jack can be recommended.“@ In techniques should not be regarded as a justification for addition to acknowledged limitations in their experimen- careless or haphazard approaches to the issue. Considering tal approach, the authors recognize the questionable mer- the amount of work involved, expense, and relative irre- its of transferring the observed results to a clinical situa- versibility of extensive prosthetic treatment, the adequate tion, since the possibility of patient adaptation to complete establishment and evaluation of OVD continues to be an dentures constructed with an OVD outside of the comfort important issue. zone was never tested. This issue may be especially critical when fixed prosthe- The existence of a comfort zone measured under artifi- ses are prescribed, because in none of the studies reviewed cial laboratory conditions was well established, but whether was the OVD increased by use of fixed, individual, un- that comfort zone had any relationship to a physiologic or splinted restorations. The effect of increases of OVD by use therapeutic OVD used for clinical restorative dental of metal or acrylic resin splints may not necessarily be the treatment was not explored. This should be considered same as the effect of an increase by individual fixed resto- a separate hypothesis and could be tested by making rations. This issue has not been studied in a controlled dentures with OVD outside of the comfort zone and devel- manner. oping means for quantifying how well patients do with them. Research of this kind should be done if the concept SUMMARY of “comfort zone” is to be regarded as scientifically tena- A review of a large number of human and animal exper- ble. iments failed to provide substantial evidence that moder- ate changes in OVD cause hyperactivity of the masticatory DISCUSSION muscles and symptoms of TMD. Nor have published This review suggests that the state of the science in this studies demonstrated that the postural rest position of the field has been found wanting. However, because OVD is an mandible is fixed and unaffected by a variety of extrinsic integral part of routine treatment, a state- and intrinsic factors. of-the-art analysis is necessary. No technique to establish OVD has been shown to he The hypotheses that postural rest position is fixed and scientifically accurate or superior to any other. Most of the unchanging throughout life and that restorations or pros- assumptions on which popular techniques have been based, theses that encroach on the postural rest position may for example, “constancy of postural rest position,” have not cause pathologic sequelae appear to be untenable. been supported by controlled research. The belief that ap- It seems reasonable to accept an alternate hypothesis propriate OVD in the dentulous state is the same as OVD that the correct or physiologic OVD can better be described for the edentulous state also has not been shown. This is as a range instead of a fixed point or position for most sub- one of the many aspects in dental practice in which clini- jects. cians rely mainly on empirical knowledge. Nonetheless, the width of that comfort zone may vary It can be agreed that the application of this empirical among individuals and in a single individual at different knowledge has enabled and continues to enable the dentist times, since individual adaptive capacity is unknown. It is to provide successful treatment. After all, OVD, like other also unknown whether what constitutes the optimum range quantifiable aspects of a body or system function, such as for OVD in a fully dentulous state with diverse craniofacial blood pressure per weight, for example, is unlikely to be a morphofunctional features is carried over to, or modified rigid, specific, and unchangeable entity; hence, the con- by, different stages of partial edentulism or complete ventional clinical wisdom of regarding it as a variable range. edentulism. Nonetheless, research that reconciles clinical techniques Most techniques for clinical establishment of OVD in with basic science information should underscore the cho- complete denture-wearing patients are based on the sen methods of routine clinical application, OVD, like its premise that the OVD the patient had with natural teeth allied and perhaps inseparable horizontal maxillomandib- is likely to be the best one for the new edentulous state and ular considerations, deserves a more scientific approach to therefore should be reproduced. This premise has not been its understanding.

THE JOURNAL OF PROSTHETIC DENTISTRY 551 RIVERA-MORALESANDMOHL

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Power spectral analysis of electromyographic signal of masticatory muscles at rest position and habitual clench

Chong-Shan Shi, D.M.D.,’ Guan Ouyang, D.M.D.,Z’ and Tian-Wen GUO, D.M.D.“’ The Fourth Military Medical University, Stomatological College, Xian Shaanxi, P.R. China

Thirteen normal subjects were selected for this investigation. At the vertical dimension of rest position and habitual clench, their myoelectric activities of the masseter, anterior and posterior temporal, and digastric muscles on the left and right sides were simultaneously sampled and processed by computer, and the mean power frequency (MPF) and the mean amplitude (MA) of the myoelectric signal were calculated. At rest position, the temporal muscles were major muscles and the MPF differences of various masticatory muscles were not significant. At habitual clench, with the increase in MA, the MPF of various muscles increased. The results show that MPF may have application in describing a central tendency of myoelec- trical signal frequency distribution and in evaluating recruitment of motor units. MPF and MA are the quantitative indices reflecting masticatory muscle function. (J PROSTHET DENT 1991;66:553-6.)

Electromyogram (EMG) has been analyzed quanti- nal. Spectral analysis involves the breakdown of the tatively from duration to frequency domain. Frequency continuous EMG signal into a range of different frequency analysis is done by a digital computer using a fast Fourier components; the power spectrum is a type of histogram transform algorithm (FFT) to generate the power spectral showing the amount of power (activity) at each frequency.l density function to describe the power spectra of the sig- In reviewing power spectral analysis of the EMG of mas- ticatory muscles, many research results are of interest. First, muscle fatique during sustained isometric contrac- tion of the jaw muscles causes a shift to low frequencies in Supported by the National Natural Science Fund No. 3880830. the EMG frequency power spectrum. The shift is due to *Lecturer, Prosthodontic Department. **Professor, Prosthodontic Department. increase in the low frequency range and decrease in the high “‘Associate Professor, Prosthodontic Department. frequency range, and it induces the mean power frequency 10/1/27035 (MPF) to decrease.2* 4 Second, an increase in bite force

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