Unveiling the Influence of the Curve of Spee on Bite Force and Chewing Ability: A Comparative Study

Aim To investigate the impact of the curve of Spee (CS) accentuation on bite force, chewing ability, and additionally, other factors that influence chewing ability and bite force such as restorations, caries, gender, habits, and TMJ problems. Materials and Methods 231 participants (118 male and 113 female, mean age = ±27.96 years) were recruited for this cross-section study. Participants completed a data collection sheet in which age, gender, Angle's classification of malocclusion, overjet, overbite, TMJ problems, habits, restorations, and caries experience were recorded. Two examiners made a lower impression, chewing ability test and measured the bite force for each participant. Measurement of the CS was obtained digitally from the poured dental cast, and the categorization was as follows: flat (<1 mm), normal (1-2 mm), or deep (>2 mm). Results The mean maximum bite force (MBF) was 468.722 N for participants with flat CS, 389.822 N for normal CS, 647.08 N for deep CS, and 384.667 N for reverse CS. The average MBF was higher for participants with deep CS (p < 0.05). MBF force was higher in males. However, BMI was not significantly related to MBF values. Participants with normal and flat CS have comparable chewing capacity (p > 0.05). Also, a significant difference in bite force and chewing ability was found between the three categories of CS (p < 0.05). Conclusion Bite force variations across various CS types were linked to gender and habits. Chewing ability showed no differences concerning gender, habits, TMJ problems, caries, or restorations, emphasizing CS's significant impact on bite force while showcasing the unchanged nature of chewing ability amidst diverse factors.


Introduction
Te human jaws are complex structures that serve a variety of purposes.Tey are an important component of the stomatognathic system because they chew and break down food before transferring it to the alimentary canal.Maximizing masticatory efectiveness while minimizing stress on the masticatory system and teeth is one of the primary objectives of all dental treatments [1].Muscles of mastication work efciently when their action is in harmony with the alignment of teeth.Detentions are arranged according to diferent curves: the curve of Monson, Wilson, and Spee [2].Te curve of Spee (CS) is a signifcant component that contributes to establishing the natural occlusion.It allows for defning the normal functional protrusive movement of the mandible.It has been previously discussed how the CS afects force distribution in the oral cavity and how modifcations to the CSCS are necessary after extensive orthodontic and prosthetic therapy [3,4].Te removal of stresses from the condyle, as well as the maxilla and mandible, has been said to be essential for improving masticatory efciency and lowering total strain on a patient's oral cavity [5].
Tere was a notion that CS served a biomechanical purpose in food processing by enhancing the crush-shear ratio between the posterior teeth and the efectiveness of the occlusal force.Te forward inclination of the posterior teeth increased the crush surfaces of the molars, according to Osborn and Bragar 1987.Tey related the depth of the CS to the bite force by a mathematical model.Tey concluded that as the average occlusal plane is raised from 6 to 0 below the parallel plane passing through the condyles, the sum of the muscle tensions required to produce a given force along the axis of any given tooth decreases [6].Ten, Osborn in 1993 continued his investigation to determine if a relationship could be established between the crush-shear force and the sagittal plane of the anterior-posterior teeth.According to his theory, humans' superfcial masseter muscle is positioned correctly for crushing food between the posterior molars, which have occlusal surfaces that tilt forward and serve as the best illustration of the CS [7].
Masticatory efciency, or masticatory ability, refers to the placing of food in the mouth, and the subsequent biting, crushing, and grinding activities that mix it with saliva to form it into a bolus before it is swallowed [8][9][10].Tis complex task is subject to a number of variables such as bite/occlusal forces, mandibular kinetics, occluding teeth, salivary activation, and tongue function.Numerous methods exist for assessing masticatory function.It should be noted that the bite force is a biological variable, which is potentially associated with masticatory efciency; however, it can be employed as a measure against which proper masticatory function is determined [11,12].Essentially, it is the force produced by the activity of the masticatory muscles and the jaw's elevator muscles, which are modifed by craniomandibular mechanics [13], temporomandibular dysfunction [14], restorations [15], caries [16], and diferent stages of periodontitis [17,18].Recently, a number of studies on age-related masticatory efciency have decided there is no direct link between aging and mastication [19].Moreover, age-related decline in occlusal force, salivary fow, medication, and how many natural teeth a person retains have also been linked to reduced masticatory efciency [12,20,21].Studies indicated that gender diferences in the masticatory function of elderly individuals with complete natural dentition emphasize the importance of considering these distinctions in masticatory function evaluations [22][23][24].
One of the crucial therapeutic procedures involved in the rehabilitation of numerous long-span posterior restorations concerns the re-establishment of the occlusal plane.Tis re-establishment aims to maximize masticatory efciency while minimizing any load on the masticatory system.Indeed, the foundation of the optimal tooth arrangement is dependent on the restoration of compensatory curves, which promotes harmony between the anterior teeth and condylar guidance [25].However, previous studies did not demonstrate obvious relation between the accentuated and reverse CS and its efect on bite force and chewing ability [5].
Terefore, the aim of the study was to investigate the impact of CS accentuation on bite force, chewing ability, and additionally, other factors that infuence chewing ability and bite force such as restorations, malocclusion, caries, gender, habits, and TMJ problems.

Study Population.
Te cross-sectional study took place at the Department of Prosthodontics, Dental College, Jouf University in Sakaka, Saudi Arabia, spanning from June 2021 to January 2023.Ethical approval was granted by the University's Local Committee of Bioethics (9-04/43).Te sample size was calculated using G Power 3.1.9.2 software from Heinrich-Heine-Universität Düsseldorf, Germany, employing a 95% confdence interval, 80% power (α � 0.05).Participants were recruited from screening clinics, with enrollment limited to those meeting the specifed inclusion criteria.Prior to any examinations, each participant provided informed consent by signing a consent form and received easily comprehensible information about the study's objectives.Participants were assured of the voluntary nature of their involvement and their right to withdraw at any time.Of 751 volunteers screened, 239 met the inclusion criteria, while 5 declined participation, and three individuals were excluded due to a history of peanut allergy.

Participant Characteristics.
Te inclusion criteria for participation in the research encompass individuals possessing fully erupted upper and lower teeth from the second molar to the central incisor on both the right and left sides, aged 18 or above, and demonstrating mental ftness.Individuals with patients with medical conditions that could afect their chewing ability or a history of previous trauma to the TMJ area, extensive dental restorations, ongoing orthodontic treatment, mixed dentition, noncarious tooth wear, periodontal problems, endodontically treated tooth #6, food allergies related to the study, missing opposing teeth to the lower frst molar, and orofacial pain were excluded from the study.

Data Collection.
Upon enrollment in the study, participants underwent a comprehensive assessment to gather current medical and dental histories alongside demographic details, including age, gender, education, marital status, and occupation.Te primary investigator (ZAK) and the supervisor (MGS) conducted a clinical examination using a dental mirror (15/16 inch, Hahnenkratt, Königsbach-Stein, Germany) and a dental explorer probe (0700-9, anatomical handle single ended; ASA Dental, Bozzano, Italy) in a dental unit.Bitewing X-rays were taken for investigating 2 International Journal of Clinical Practice participants when proximal carious lesions were anticipated on the maxillary and mandibular frst molars and for patients displaying grade I mobility to assess periodontal status.Independent variable data such as age, gender, Angle's classifcation of malocclusion, overjet, overbite, TMJ problems, habits, restorations, and caries experience and the dependent variable data such as bite force and chewing ability were collected using a customized data collection form and transferred onto Microsoft Excel software.

Measurements of CS and Depth.
Te study casts were positioned in a base former aligning the right posterior teeth parallel to the former's border.After sizing and aligning the occlusal plane, the casts were set on holders to ensure parallelism.Using a Nikon Coolpix S4000 camera at a 90 °angle, photographs of the casts were taken and analyzed on a computer.Corel DRAW X5 software, with 800% enlargement, electronically measured the CS from the digital images.Reference planes were established from canine buccal cusps to the distobuccal cusp tips of the second molars, enabling precise measurement using a "dimension tool" to determine perpendicular distances.Perpendicular lines were drawn from this reference line to key dental landmarks, such as the frst molar, mesiobuccal cusp of the second molar, and premolar cusp tips, defning the depth of the CS as the greatest length observed [26].Te CS depth categories were based on Lie's previous study: fat (<1 mm), normal (1-2 mm), or deep (>2 mm) [27].

Estimation of Chewing Ability.
Te study employed three standardized food portions (carrot, apple, and peanut) to assess participants.Carrot and apple were each cut into 1 cm cubes, and the peanuts (averaging 1 g) were utilized.Participants, seated upright without head support, chewed three pieces of each test food.Te test foods were randomly distributed using a lottery system to ensure patient anonymity.Five calculations were recorded for the masticatory strokes employed by the participants to chew the food portions, namely, food mastication included time until initial swallow, time until mouth empty, number of masticatory actions before the frst swallow, number of masticatory actions until mouth empty, and number of swallows before the mouth was free of food.To prevent muscular fatigue, a 4--5-minute break allowed participants to drink and rinse their mouths.Two observers, situated in front of the participants, recorded time and counted masticatory strokes.Te mean values from the three recordings were used to determine chewing ability [28].
2.6.Bite Force Registration.Bite force was assessed using GM10 Occlusal Force Meter, a device equipped with a hydraulic pressure gauge and a vinyl biting element, displaying digital Newton units.Participants were familiarized with the equipment and instructed to exert maximum bite force upon the transducer's placement on the lower frst molar while seated in an upright position in a dental chair, maintaining the Frankfort plane parallel to the foor.Average left-and right-side bite force values were recorded for participants meeting the inclusion criteria on both sides.Measurements were conducted before the chewing test to prevent muscle fatigue.
To assess reliability, intraexaminer and interexaminer measurements were performed on 10 participants.Intraexaminer reliability, evaluated with an AK value of 0.87, indicated excellent agreement, while interexaminer reliability, determined by a well-trained intern dentist, showed a BK value of approximately 0.86, signifying satisfactory agreement.An independent-sample t-test and a MANOVA test were used to compare the signifcant diference in chewing ability and bite force male for various independent variables.

Results
In total, 231 (118 males and 113 females) participants were assessed for the outcome variables such as chewing ability and bite force.Te age group of the participants ranged from 18 to 54, with a mean age of 27.96 (±7.22).Te bite force of the participants ranged from 147.5 to 952.5 N, with a mean bite force of 491.62 (±187.64).Te chewing ability of the participants ranged from 3.13 to 17.63, with a mean chewing ability of 8.03 (±2.26) (Tables 1 and 2).
Te MANOVA test results revealed no signifcant differences in bite force among diferent Angle's classifcations of malocclusion (Wilks' lambda � 0.643, p > 0.05) (Table 4).Te mean bite force values for class I malocclusion, class II division 1, and class II division 2 were 495.645, 462.833, and 469.818, respectively.Similarly, for chewing ability, no signifcant diferences were found among the diferent malocclusion classifcations (Wilks' lambda � 0.332, p > 0.05).Te mean chewing ability scores for class I malocclusion, class II division 1, and class II division 2 were 7.984, 9.088, and 7.719, respectively.Tese fndings suggest that, in this study, malocclusion classifcations did not exert a statistically signifcant infuence on either bite force or chewing ability.
Te MANOVA test (Table 5) revealed a signifcant diference between reverse, fat, normal, and deep CS when considered jointly on the variables bite force and chewing ability (Wilks' lambda �0.00, p < 0.01).Post hoc comparison was conducted using the Tukey test to determine the individual group diferences with respect to bite force and chewing ability (Table 6).In case of bite force, there was a statistically signifcant diference in deep CS in comparison with reverse, fat, and normal CS (p < 0.05).In case of chewing ability, there was a statistically signifcant diference in fat and deep CS (p < 0.05).

Discussion
Tis study used the GM10 digital bite force transducer in order to accurately assess the participants' bite force.With the ability to measure a bite force of up to 1000 N, the GM10 dynamometer's key advantages are its portability, secure, and comfortable recording method.Furthermore, the dynamometer's accuracy and repeatability functions are well established, having been utilized efectively in a number of other studies to measure human bite force [3,25,29].Te bite force was measured on the frst molar on both sides for each participant.Te frst molar was selected due to the larger area and periodontal ligament surrounding the roots of posterior teeth and stronger bite force that can be tolerated.Moreover, diferent parts of the oral cavity have diferent ranges of bite force, and the highest bite force was found on the posterior teeth [30].Contrary to many other studies, we have used fve parameters instead of chewing gum or typical sieving techniques to evaluate masticatory function.We did evaluate a variety of natural test foods, though, including soft and hard food.We chose an efective and simple method that counts the number of chewing strokes and the time it takes to prepare the test food for swallowing.Tis approach did not require the participants to remove the masticated test material or pay particular attention to the food, which is what typically happens when people eat [31].Te test foods were ofered to the participants using a predetermined random order system, which helped standardize the methodology.As previously stated that occlusal force and chewing ability decline with tooth loss, only those with entirely natural full dentition were selected for this study [4,9,32].
It was found that the bite force mean values for the male and female participants were 585.8 and 397.4,respectively, which was statistically signifcant (p < 0.05).Tese results corroborate fndings from other research, which indicate that males possess signifcantly higher maximal bite force than females [10,33,34].One explanation for this is that female teeth tend to be smaller, which corresponds to smaller periodontal ligament regions and thus, less powerful bite force.In addition, males' larger muscular potential, which can be explained by anatomical diferences between the sexes, is likely to be a further contributing factor [34]. Te only research that could be found to refute this was conducted on durophagous species.Tat study examined whether variations in bite force could be a refection of the dimorphism in head size and shape of the durophages Malaclemys terrapin.Te fndings of that research showed that the female terrapins, whose heads are larger than their bodies, had a more powerful bite force than the males [35].However, that study was applied on animal subjects, while the current study was on humans.
In terms of chewing ability, despite the fact that there was a preference for men over women, the diferences were insignifcant (p > 0.05), and this result concurs with previous studies by Kosaka et al. [9] and Ikebe et al. [19].On the other hand, in 2003, a study by Ono et al. found that the preference for masticatory ability was related to age, gender, and dental condition and that masticatory ability was signifcantly superior in males.According to Poli et al. [12], the decline in bite force with age does not directly correlate with malnutrition in the elderly, as other factors like natural International Journal of Clinical Practice dentition, frailty, and comorbidities play signifcant roles in chewing capability.Providing a unique and broader perspective compared to existing literature predominantly focused on young and adult populations.Tese diferences could be explained by the fact that in their study, they used gummy gel as a test food which is infuenced by cognitive performance [24,36].Another study was undertaken in 2016 that compared masticatory characteristics and the ability to predict masticatory efciency in male and female adolescents.In that study, the participants were aged 14-17 years old, and the sample size was smaller than in the current study.Nevertheless, the results showed that boys chewed more frequently and possessed greater masticatory efciency than girls [24].Manzon et al. [37] indicated that patients using overdenture prostheses experience improved bite force, although not reaching the levels observed in fully dentate subjects.Notably, bite force does not exhibit a correlation with the body mass index despite the increased prevalence of obesity observed in edentulous subjects or those with prostheses.
Another study by Gudipaneni in 2020 also used juveniles to fnd the efect of all types of dental caries on the bite force of children aged 7-9 years, as it related to their frst permanent molars.Tese results suggest that bite force decreases as the incidences of caries increase [38].Te aforementioned discrepancies in this study may be accounted for by the inclusion of individuals with occlusal proximal caries involving enamel and/or dentin, whereas in the research under discussion, participants with caries that went beyond pit and fssure were not included.Another issue to be considered is the type of prosthetic restorations as reported by the study that the type of removable denture infuences bite forces and chewing efciency, with only CoCr-RPDs objectively restoring satisfying chewing function [39].
Te study examined bite force and chewing ability in individuals displaying signs but no symptoms of temporomandibular problems, fnding no signifcant diferences between those with or without TMJ issues (p > 0.05 for bite force, p > 0.05 for chewing ability).Contrasting fndings from Shimada et al. indicated that introducing pain into the masseter muscle did not notably impact masticatory efciency or bite force, aligning with Pizolato's study showing no substantial reduction in maximum bite force in young individuals with TMJ disorders and bruxism [40].However, research comparing afected and unafected sides in unilateral TMJ problems revealed markedly reduced bite force in afected areas, contrasting with no signifcant diferences in bite force in bilateral TMJ disorders.Notably, the absence of participants with painful TMJ in the current study's criteria may explain the discrepancies observed in these fndings.
Tis study examined biting and chewing habits like lip and nail biting and gum chewing, revealing that individuals with established habits exhibited higher occlusal bite forces (p < 0.05).Nakagawa et al. found similar results, showing improved occlusal force in the elderly after gum chewing exercises [41].Shirai et al.'s study, employing gum-chewing exercises, mirrored these outcomes across various facial morphologies [42].However, Castelo et al. discovered In spite of the apparent signifcance of deep occlusal curvature in chewing ability, other research suggests that this curve must be positioned in a way that absorbs occlusal stresses during maxillomandibular function [45].Te relative position of the condyle within the mandibular fossa was biomechanically linked to the CS.A fatter CS, for instance, is associated with more posteriorly placed condyles, while a steeper CS was observed to have more anteriorly placed condyles.It may be that a more posteriorly oriented condyle within the mandibular fossa is a contributing factor in cases of anterior displacement of the articular disc, which frequently causes TMJ noises [46].Tis infers that a fatter CS may be a risk factor when it comes to TMJ sounds, or that this may be the result of the condyle being more posteriorly placed during growth and maturation.Te diferences in occlusal curvatures and maxillary arch dimensions between people with symptoms of TMJ disorders and those who were asymptomatic have been investigated by Kanavakis and Mehta [47].Te conclusion was that participants with TMJ symptoms had a fatter CS, which indicates that deep occlusal curvatures are not connected to TMJ or muscular pain [47].
In this investigation, it was found that chewing ability improved in individuals with normal or deep curves, suggesting that masticatory efectiveness increases with the deepening of the occlusal curvature.Another study by Fueki et al. [48] examined whether a connection existed between occlusal curvature and the ability of young adults with permanent dentition to mix and comminute food.Tey concluded that the participants with fatter curves and a big sphere radius of the occlusal curvature were better at mixing and cutting food.In young adults with permanent dentitions, occlusal curves like the CS seemed to be associated with the ability to mix and cut food.Tis indicates that subjects with strong bite force possess a mild anteroposterior occlusal curvature [48].
4.1.Limitations.Te study did not examine participants' dietary impact on the CS and occlusion.Diet infuences jaw stress and orofacial bone stimulation, vital for typical occlusal curvature.Te CS classifcations' efect on occlusal interferences was not explored.Te limitations of chewing ability tests include potential issues with age of the sample, subjective assessments, and challenges in interpreting results, which may hinder generalizability and sensitivity to changes in chewing function.Further research is needed to gauge the depth of the CS impacting TMJ and investigate its relationship with occlusal interferences.Te study aimed to link CS accentuation with masticatory function, recommending broader research on diet's role, older age groups, diverse malocclusions, and facial morphologies to validate these fndings and explore occlusal interferences.

Conclusion
Within the limitations of the study, it can be concluded that bite force exhibited variance across diferent CS types, demonstrating gender and habitual diferences.However, in contrast, chewing ability did not display variations concerning gender, habits, TMJ problems, caries, or restorations.Tus, the study underscores the impactful role of CS accentuation in bite force, elucidating its infuence among diverse factors, while revealing the unaltered nature of chewing ability in relation to these assessed variables.
Gamal Salloum was responsible for conceptualization, project administration, visualization, writing, reviewing, and editing.

Table 2 :
Descriptive statistics for the variables such as age, overjet, overbite, bite force, and chewing ability among participants (N � 231).

Table 3 :
Comparison of bite force and chewing ability among variables such as gender, restorations, TMJ problem, habits, and caries.

Table 4 :
Comparison of bite force and chewing ability in relation to various classes of malocclusion.

Table 5 :
Comparison of bite force and chewing ability in relation to diferent types of CS.

Table 6 :
[44] hoc comparison of bite force and chewing ability in relation to diferent types of CS.International Journal of Clinical Practice reduced bite force linked to thumb sucking in children with mixed dentition, difering from this study in age group, teeth condition, and bite force gauge used[43].Notably, despite habitual chewing practices, this study found no signifcant diferences in chewing ability, an aspect largely unexplored in existing literature.Tis study examined various malocclusions' impact on bite force and chewing ability suggesting no signifcant diferences across malocclusion types (p > 0.05).Singh et al. similarly found no substantial diferences in occlusal force among diferent malocclusion groups[44].However, contrasting views suggest a signifcant relationship between bite force and malocclusion severity, particularly on the right side.Yet, this study's comparability is limited due to diverse malocclusion severity and age demographics in the samples, difering from the present research, primarily comprising class I occlusions without class III malocclusion instances.Owens et al.'s fndings revealed decreased masticatory effciency in individuals with malocclusions, while conficting results exist regarding the impact of malocclusion treatment on chewing ability, with some studies suggesting limited improvement posttreatment due to diverse variables in test foods and assessment materials.
6 Te mean diference is signifcant at the 0.05 level.6