stress area of the mandibular alveolar mucosa under complete denture with linear occlusion at...

8/7/2019 Stress area of the mandibular alveolar mucosa under complete denture with linear occlusion at lateral excursion

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Chinese Medical Journal 2010;123(7):917-921 917

Original article

Stress area of the mandibular alveolar mucosa under complete

denture with linear occlusion at lateral excursionLÜ Ya-lin, LOU Hang-di, RONG Qi-guo, DONG Jian and XU Jun

Keywords: complete denture; linear occlusion; stress area distribution; finite element analysis

Background The rocking and instability of a loaded complete denture (CD) during lateral excursion reduce the bearing

area under the denture base, causing localized high stress concentrations. This can lead to mucosal tenderness,

ulceration, and alveolar bone resorption, and the linear occlusion design was to decrease the lateral force exerted on the

denture and to ensure denture stability. But it is not known how the bearing areas of linear occlusal CDs (LOCDs) and

anatomic occlusal CDs (AOCDs) differ. The purpose of this study was to analyze and compare the distributions of the

high and low vertical stress-bearing areas in the mandibular alveolar mucosa under LOCDs and AOCDs at lateral

excursion.

Methods Computerized tomography (CT) and finite element analysis were used to establish three-dimensional models

of an edentulous maxilla and mandible with severe residual ridge resorption. These models were composed of maxillary

and mandibular bone structure, mucosa, and the LOCD or AOCD. Lateral excursion movements of the mandible were

simulated and the vertical stress-bearing areas in the mucosa under both mandibular CDs were analyzed using ANSYS

7.0.

Results On the working side, the high stress-bearing (–0.07 to –0.1 MPa) area under the LOCD during lateral

excursion was smaller than that under the AOCD, while the medium stress-bearing (–0.03 to –0.07 MPa) area under the

LOCD was 1.33-fold that under the AOCD. The medium stress-bearing area on the non-working side under the LOCD

was 2.4-fold that under the AOCD. Therefore, the overall medium vertical stress-bearing area under the LOCD was 20%

larger than that under the AOCD.

Conclusions During lateral excursion, the medium vertical stress-bearing area under a mandibular LOCD was larger

and the high vertical stress-bearing area was smaller than that under an AOCD. Thus, the vertical stress under the LOCD

was distributed more evenly and over a wider area than that under the AOCD, thereby improving denture stability.Chin Med J 2010;123(7):917-921

complete denture (CD) is a type of mucosa-supported prosthesis. Only with good

stability does a denture have a good border seal, which isprerequisite for the retention and protection of theresidual ridge. A variety of factors can affect denturestability, including the supporting tissues and the dentureitself. For the supporting tissues, a well-rounded residualridge with a large bearing area is important for denturestability. The bearing area of an edentulous lower jaw isabout half that of the upper jaw,1 and will bear the largerstress when each is loaded with an equal occlusal force.This is particularly true after a mandibular residual ridgehas atrophied, which decreases the bearing area.Therefore, research on the stress distribution on themandibular residual ridge is much more meaningful thanthat performed on the maxilla. With regards to the dentureitself, properly arranged artificial teeth, good occlusion,and a well-shaped polished denture base surface allcontribute to denture stability. The rocking and instabilityof a loaded CD during lateral excursion reduce thebearing area under the denture base, causing localizedhigh stress concentrations. This can lead to mucosal

tenderness, ulceration, and alveolar bone resorption,which can further deteriorate denture stability.

The goal of the linear occlusion design described by

Frush2 was to decrease the lateral force exerted on thedenture and to ensure denture stability. Many clinicalstudies3,4 have shown that linear occlusal completedentures (LOCDs) are more stable and exhibit less boneresorption than anatomic occlusal complete dentures(AOCDs). Theoretically, the potentially larger bearingarea under an LOCD compared to an AOCD may allow amore uniform distribution of stress, although this has notbeen proven. Furthermore, it is not known how thebearing areas of LOCDs and AOCDs differ.

Previous finite element analyses (FEAs) have typicallyused the peak stress value for the stress distribution on the

A

DOI: 10.3760/cma.j.issn.0366-6999.2010.07.028Center of Stomatology, Beijing Anzhen Hospital, Capital MedicalUniversity, Beijing 100029, China (Lü YL and Dong J)College of Engineering, Peking University, Beijing 100871, China(Lou HD and Rong QG)Department of Prosthodontics, School & Hospital of Stomatology,Peking University, Beijing 100081, China (Xu J)Correspondence to: Dr. XU Jun, Department of Prosthodontics,School & Hospital of Stomatology, Peking University, Beijing100081, China (Tel: 86-10-62179977 ext. 2347. Fax:86-10-64416242. Email: [email protected])LÜ Ya-lin and LOU Hang-di contributed equally to this article.This study was supported by a grant from the Beijing NaturalScience Foundation, China (No. 3073018).



Chin Med J 2010;123(7):917-921 918

Figure 1. Three-dimensional solid geometric models of denture of two occlusal schemes. A, B: AOCD. C, D: LOCD.Figure 2. Three-dimensional solid geometric models of skull and denture of two occlusal schemes. A: AOCD. B: LOCD.

mucosa under the denture base. By now, no study hasdetermined what area of the mucosa under a CD base isstressed. We hypothesized that different vertical stressdistributions are found in the mandibular mucosa underLOCDs and AOCDs. To test this, we used computerizedtomography (CT) scans and FEA software to establish threedimensional (3-D) FEA models of a maxilla and mandible

with severe residual ridge resorption (RRR). These modelsconsisted of maxillary and mandibular bone structure,mucosa, and either an LOCD or an AOCD. Lateralexcursion was simulated and the vertical stress areas of themucosa under the LOCD and AOCD were analyzed.

METHODS

Establishment of 3-D FEA models

A 76-year-old female volunteer with an edentulous jawand a class IV mandibular residual alveolar ridge wasenrolled after informed consent was obtained.5 The

interocclusal record of the patient was taken and wastransferred to an articulator. An LOCD and an AOCDwere fabricated on the same articulator, usinginterchangeable artificial teeth, so that the two dentureswere highly identical except for the occlusal schemes of the artificial molars. The AOCD was composed of GoldNail porcelain teeth with a 30° cusp inclination (ShanghaiDental, China), while the LOCD had linear porcelainteeth (Geneva 2000TM, Switzerland).

After inserting the LOCD or AOCD properly, the subjectwas asked to bite in centric occlusion. The patient laid ina supine position with the orbitomeatal plane parallel tothe horizontal plane and with the cephalosome fixed.Successive ultrathin transverse CT scans (AQUILION 64slice spiral CT, Toshiba, Japan) were obtained from the

mandible inferior plane to the superior infratemporal fossaplane. A 0.5-mm slice thickness and a 0.3-mm spacingwere used, and a total of 412 CT units were obtained.

Boundary information of the bone tissue was extractedfrom the CT images, based on the threshold of the bonetonal value. The data were transferred to the FEA

software ANSYS 7.0 (ANSYS, USA) and were processedto establish 3-D solid models of the skull (with maxillaand mandible, including the cortical and cancellous bones)and the LOCD/AOCD. Using a Boolean operation, thealveolar mucosa were modeled between the denture baseand the bone structure. The mandibular complex (Figures1 and 2), including the mandible, alveolar mucosa, andCD, was simulated to the lateral excursion position forthe AOCD or the LOCD. The geometric models wereutilized for meshing using the FEA 10-node tetrahedronmodule, and were comprised of a number of nodes (LA:255452; LL: 253475) and elements (LA: 167136; LL:

165629). The model materials were regarded ascontinuous and isotropic linear elastic materials, withmechanical properties (Table 1) obtained from literature.6

Table 1. Mechanical properties of materialsMaterials Elastic modulus (MPa) Poisson’s ratios

Cortical bone 13700 0.30

Cancellous bone 1370 0.30

Mucosa 3 0.45

Denture base 2352 0.30

Porcelain tooth 82800 0.28

In ANSYS, the mandibular complex models were set tothe lateral excursion position, with the CD at a balancedocclusion and the anterior artificial teeth not in contact.The degrees of freedom of the maxilla’s upper nodes of and the nodes at the rotation axis of the bilateral condyle




Figure 3. Three-dimensional finite element models used here, including the maxilla, mandible, alveolar mucosa, and complete dentures.Figure 4. Vertical stress distribution area of the mucosal surface under the base of a complete denture on lateral excursion. A: AOCD. B:LOCD.

were set to zero. The left and right masticatory forces(Table 2) were assumed to be unequal at lateral excursion.Forces due to muscle attachment on the mandible areawere calculated according to Rues et al7 and Schindler etal8 in the anatomic direction (Table 3).

Table 2. Muscle forces due to muscle attachment on the mandiblearea (n)

Muscles Working sides (right) Non-working sides (left)

Masseter 30 15

Medial pterygoid 45 10

Temporalis 30 15

Table 3. Anatomic direction of muscle forcesMuscles Starting positions Pointings

Masseter Angulus mandibulae masseteric

tuberosity

Zygomatic arch margo

inferior median part

Medial pterygoid Angulus mandibulae interior

pterygoid tuberosity

Descending lamina of

sphenoid bone interior

Temporalis Mandibular coracoid process tip Temporal bone surface

center

Model analysis

The vertical (Z-axis) stress distribution of the mucosaunder each denture base was calculated using ANSYS.The overall stress range was divided into eight successiveintervals and was inputted as a parameter to search thenodes of each respective value. The nodular areas werecalculated to obtain the stressed area in each specificrange. The mucosal surface was categorized according todifferent stress ranges, and the distribution areas of thedifferent ranges were estimated. The verticalstress-bearing areas of the mucosa under both the LOCD

and the AOCD were analyzed.

RESULTS

Three-dimensional FEA models (Figure 3), whichincorporated the maxilla, mandible, alveolar mucosa, andLOCD or AOCDs at lateral excursion, were successfullyestablished. The cranial bones were fixed by a constraintat the upper areas, and the muscular forces were loadedon the mandible at the muscle attachment sites. Theocclusal force was generated by the contact of theocclusal surface with the artificial teeth on the dentures.

The mucosal region under either the AOCD or the LOCD

in the stress range of –0.1 to –0.2 MPa was constituted bydispersed nodes from which the stress-bearing area couldnot be calculated (Figure 4). On the molar region of theworking side, the AOCD and LOCD high stress-bearing(–0.07 to 0.10 MPa) areas were 38 mm2 and 0,respectively. The LOCD medium stress-bearing (–0.03 to

0.07 MPa) area was 1.33-fold compared with that underthe AOCD. On the non-working side, the LOCD highstress-bearing area was 0.92-fold that under the AOCD,and the LOCD medium stress-bearing area was 2.4-foldthat under the AOCD (Table 4).

Table 4. Vertical stress-bearing area (mm2) of the mucosa under thebase of complete dentures at lateral excursion

Ranges of stress (Mpa)Variables

0 to –0.03 –0.03 to –0.07 –0.07 to –0.1 Total

Working side (right)

AOCD 265 178 38 481

LOCD 310 237 0 547

LOCD/AOCD 1.17 1.33 0 1.14Non-working side (left)

AOCD 280 70 12 362

LOCD 305 167 11 483

LOCD/AOCD 1.09 2.39 0.92 1.33

Both sides

AOCD 545 248 51 844

LOCD 615 404 11 1030

LOCD/AOCD 1.13 1.63 0.22 1.22

Because the vertical stress-bearing area under the LOCDwas 0 on the working side, we compared the total verticalstresses on both the working and non-working sides. Thetotal high vertical stress-bearing area of the mandibularmucosa under the LOCD was 0.22-fold compared withthat under the AOCD, while the total medium verticalstress-bearing area under the LOCD was 1.63-foldcompared with that under the AOCD. Considering thetotal vertical stress-bearing area of the mandibularmucosa under the CDs, the area under the LOCD was1.2-fold that under the AOCD.

DISCUSSION

Here, we compared the stressed mucosal areas under anLOCD and an AOCD. At the working side of lateral

excursion, the high stress-bearing (–0.07 to –0.1 MPa)molar area of the mucosa under the AOCD was larger



Chin Med J 2010;123(7):917-921 920

than that under the LOCD, suggesting that the AOCDcaused higher stress concentrations due to an unevenpattern of mucosal stress. The medium stress-bearing(–0.03 to –0.07 MPa) areas under the LOCD were1.33-fold and 2.4-fold under the AOCD on the workingand non-working sides, respectively. This result indicated

that the stress distribution of the LOCD was evenly morethan that of the AOCD, and the LOCD could distributethe stress to both sides of the CD, thereby enhancing itsstability.

Only with good stability does a CD have an integratedborder seal and is the stress even distributed across themucosa. Various factors influence the stability of a CD;however, the residual ridge, as the primary stress-bearingarea, is one of the most important factors. The fullness of a residual ridge influences the area of denture base, andtherefore potentially influences the capability of retention.

Low or bladed residual ridges are not good for theretention, stability, or support of a CD. The retention andstability are the structural and mechanical bases of mastication and phonetic/aesthetic function of a denture.The stability of a CD is enhanced when the primarystress-bearing area, particularly the dynamic area, isincreased.

Abundant research has shown that there is a significantcorrelation between the occlusal scheme chosen and theresultant horizontal force, which influences dentureretention and stability.9-14 In order to maintain the healthof the supporting tissue and decrease RRR, a vertical

occlusal force on the residual ridge is sometimes madeusing teeth with reduced cusp inclinations or usingcuspless teeth. Clinical research has also shown that CDswith occlusion modified in such a way present betterstability.4 For cases with severe RRR, LOCDs exhibitbetter stability and less bone resorption than AOCDs.However, it has not been studied that to what extent theAOCD and LOCD affect the stressed area.

Previous studies investigating the stress distribution onthe mucosa under a CD base have typically used the peakstress value as the parameter for detecting focal overload,

which can cause mucosa lesions and bone resorption.15-18

However, singularities during FEA setup can interferewith the definition of this peak value, thus result indiscrepancies between such models and the clinicalsituation. Stress distribution characteristics are notstraightforward enough to describe the relationshipbetween an increased bearing area and the improvementof denture stability when CDs with different occusalschemes are being compared. In that sense, thestress-bearing area, which has not been previously used,could be a more straightforward quantitative parameterfor mucosal stress distribution under a CD.

Proper simulation is the key to a useful FEA model. Inthis study, ultrathin CT images were used to obtain highgeometric similarity between the model and the clinical

situation. Loading through the muscle forces is muchcloser to the actual clinical situation than loading throughthe denture or alveolar residue, enhancing the predictiveaccuracy of the calculation.

We used the dental arch switching technique to ensure

that the same tissue surface area was found under eachdenture base. By efficiently increasing the functionalstress-bearing area under a mandibular CD, the totalstress is evenly distributed and the loading capability of the supporting tissue is utilized sufficiently. This not onlyprotects the supporting tissue, but can also increasedenture stability. Although the AOCD used here showedbalanced occlusion, it also displayed a smallerstress-bearing area on the non-working side of themucosa, indicating that this side did not sufficiently sharethe occlusal burden. The total vertical stress-bearingmucosal area under the LOCD was 1.2-fold compared

with that under the AOCD, indicating that thestress-bearing area of linear occlusion was 20% largerthan that of anatomic occlusion. This is beneficial for thesimultaneous and even distribution of occlusal force onthe entire stress-bearing area. The results of this studyprovide scientific data that supports former research onLOCDs.

At lateral excursion, the vertical stress-bearing area of themandibular mucosa under an LOCD was larger and undera lower stress than that under an AOCD. This indicatedthat an LOCD can distribute stress more evenly and overa larger area than an AOCD, which is beneficial for

improving denture stability and maximally utilizing theloading ability of the supporting tissue.

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Edited by PAN Cheng

stress area of the mandibular alveolar mucosa under complete denture with linear occlusion at...

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