Masseter muscle activity in denture wearers with superior and poor masticatory performance

Masseter musc le act ivity in denture wearers wi th superior and poor mast icatory performance

N e a l R . G a r r e t t , P h D , a M i c h a e l K a u r i c h , D D S , b P a u l P e r e z , D M D , c a n d K r i s h a n K. K a p u r , D M D , M S d

Department 0f Veterans Affairs Medical Center, West Los Angeles, and University of California, Los Angeles, Calif.

A c r o s s - s e c t i o n a l s t u d y t e s t e d the h y p o t h e s i s t h a t d e n t u r e w e a r e r s w i t h s u p e r i o r a n d p o o r c h e w i n g ab i l i t y u s e s imi lar m a s s e t e r m u s c l e e f fort a n d b i t i n g forces d u r i n g m a s t i c a t i o n . M a s t i c a t o r y p e r f o r m a n c e t e s t s on t h e p r e f e r r e d c h e w i n g s ide a n d s w a l l o w i n g t h r e s h o l d t e s t s w e r e c o n d u c t e d w i t h p e a n u t s a n d c a r r o t s in 70 d e n t u r e w e a r e r s , 35 w i t h s u p e r i o r (SP) ( i 46.3%) a n d 35 w i t h p o o r (PP) ( i 30.7%) m a s t i c a t o r y p e r f o r m a n c e . R i g h t a n d lef t m a s s e t e r m u s c l e e l e c t r o m y o g r a p h i c (EMG) a c t i v i t y w a s r e c o r d e d d u r i n g t h e m a s t i c a t o r y t e s t s a n d p e a k b i t e force d u r i n g c h e w i n g w a s e s t i m a t e d f rom t h e b i t e f o r c e - - E M G rat ios o n g u i d e d m a x i m a l b i t i n g trials . B i t e f o r c e u n d e r m a x i m a l p r e s s u r e d id n o t d i f fer s i g n i f i c a n t l y b e t w e e n th e t w o groups . N e i t h e r the to ta l m e a n EMG a c t i v i t y o f the p r e f e r r e d a n d n o n p r e f e r r e d s ide m a s s e t e r m u s c l e s n o r th e m e a n p e a k b i t i n g f o r c e s e x e r t e d b y the t w o g r o u p s d i f f e r e d s i g n i f i c a n t l y (p > 0.05). Th i s w a s t r u e w h e n d e n t u r e w e a r e r s r e s t r i c t e d c h e w i n g to the i r p r e f e r r e d s ide for a g i v e n n u m b e r o f s t r o k e s or c h e w e d t h e t e s t f ood f r e e l y u n t i l r e a d y to s w a l l o w . T h e o n l y s i g n i f i c a n t d i f f e r e n c e s (p < 0.05) w e r e e v i d e n t in t h e ra t io s o f t h e p r e f e r r e d to n o n p r e f e r r e d s ide m a s s e t e r EMG a c t i v i t y d u r i n g c h e w i n g . The r a t i o s w e r e 1.2 for p e a n u t s a n d 1.3 for carro t s in t h e S P g r o u p c o m p a r e d to 1.8 for bo th f o o d s in th e P P group. S imi lar p a t t e r n s o f b i l a t era l a c t i v i t y in th e S P g r o u p a n d u n i l a t e r a l a c t i v i t y in t h e P P g r o u p w e r e e v i d e n t for the s w a l l o w i n g t h r e s h o l d tes ts . The r e s u l t s i n d i c a t e d t h a t a p p l i c a t i o n o f m o r e e q u i v a l e n t force b y the r i g h t a n d lef t m a s s e t e r m u s c l e s d u r i n g u n i l a t e r a l c h e w i n g is c o n s i s t e n t w i t h i m p r o v e d c h e w i n g ab i l i ty in d e n t u r e w e a r e r s . (J PROSTHET DENT 1995;74:628-36.)

C l i n i c i a n s and researchers have observed wide var ia t ions in the chewing abil i t ies of denture wearers . Re- sults of s tandard ized mast ica tory performance tests t ha t assessed food comminution have indicated tha t the average mast ica tory performance of a denture wearer is sim- i lar to tha t of a person wi th six anter ior maxi l la ry and six

Presented in part at the International Association for Dental Re- search Meeting, Cincinnati, Ohio, 1990.

Supported by Department of Veterans Affairs Medical Research and National Institute Of Dental Research grant No. R01DE06638.

aDirector, Oral Biology Research Laboratory, Department of Vet- erans Affairs Medical Center, West Los Angeles, and Adjunct Assistant Professor of Removable Prosthodontics, School of Dentistry, University of California, Los Angeles.

bLecturer of Periodontics, University of California, Los Angeles, School of Dentistry, and Consultant, Department of Veterans Affairs Medical Center, West Los Angeles.

cStaff Dentist, Department of Veterans Affairs Medical Center, Sepulveda, Calif., and Adjunct Assistant Professor of Hospital Dentistry, School of Dentistry, University of California, Los Angeles.

dprofessor of Removable Prosthodontics, School of Dentistry, University of California, Los Angeles, and Consultant, Depart- ment of Veterans Affairs Medical Center, West Los Angeles.

10/1/67592

anter ior mandibu la r teeth. 1 Some denture wearers can function as well as an average denta te person with 22 to 26 teeth. 2 The reason for this functional d ispar i ty is not fully understood and a series of invest igat ions has revealed only l imited influence of denture factors, the clinical excel- lence of dentures or r idge character is t ics on mast icatory performance. 3.s The large var iabi l i ty in chewing abil i ty among denture wearers may stem from the differences in oral sensorimotor functions involved in masticat ion.

Several sensory deficits and motor function al terat ions have been reported in den ture wearers. These include increased tact i le thresholds of denture tee th and difficulty in discerning the relat ive hardness and thickness of objects. 9 Low bu t significant correlations 1~ were found between oral tact i le thresholds and mast ica tory performance. Several researchers have compared maximal bi t ing pressures and electromyographic act ivi ty (EMG) dur ing mast icat ion in den ta te persons and denture wearers . Denture wearers exert significantly less force wi thin a nar row range when chewing foods of different toughness. 11 The EMG activity of the i r j aw muscles dur ing the closing phase of the chewing s t roke was found to be 50% less than tha t of denta te persons. 12 Although bite force and EMG activi ty during chewing differ considerably between denta te individuals and denture wearers , the i r impact on mast ica tory performance is not known. Similarly, no specific comparisons

6 2 8 VOLUME 74 NUMBER 6

GARRETT ET AL THE JOURNAL OF PROSTHETIC DENTISTRY

Fig . 1. In ter leaving beam s t ra in gage bite force t ransducer wi th autopolymerizing acrylic resin occlusal splints.

of oral sensorimotor function have been made among denture wearers wi th superior and poor mast ica tory performances.

This s tudy was designed to tes t the null hypothesis tha t the masse te r muscle activi ty and bi t ing forces exerted dur ing chewing do not differ between denture wearers wi th superior and poor mast ica tory performance.

M E T H O D S

S t u d y d e s i g n

Seventy male denture wearers from the Sepu lveda and Wes t Los Angeles Veterans Affairs (VA) Medical Centers in California, wi th an age range of 39 to 85 years (mean 63.9 -+ 7.8 years) par t ic ipa ted in this study. A br ief medical h is tory was t aken and an oral examinat ion screening was given to exclude denture wearers wi th significant sys- temic problems, mouth dryness, soft t issue disorders, chronic t issue t rauma, poor t issue support for dentures , and those who could not chew with the i r dentures.

The re tent ion and s tabi l i ty of each denture was scored on a four-point ordinal scale and the centric occlusion and vert ical dimension of occlusion (VDO) of the two dentures together on a three-point scale. The high score represented the most favorable and the low score represented the leas t favorable condition. A s imi lar approach was used to ra te the r idge shape and size, mucosal consistency, and border t issue a t tachments . Detai led scoring cri teria have been described. 13

The EMG activi ty of r ight and left masse te r muscles was

recorded while par t ic ipants exerted guided maximal bite forces and performed a preferred side mast ica tory performance tes t and a swallowing threshold test wi th each of the two foods, raw carrots and peanuts . Whereas the preferred side tests required par t ic ipants to chew the tes t food a given number of t imes on their preferred chewing side, swallowing threshold tests pe rmi t ted them to chew as they wished unt i l the tes t food was ready to be swallowed. The l a t t e r tes t was included to assess whe the r the activit ies of

the two masse te r muscles dur ing the uni la te ra l preferred side chewing are reasonable representa t ions of normal chewing.

A preferred side chewing tes t wi th peanuts was also conducted without the recording electrodes in place. The purpose of this test was to (1) assess any effects of the muscle recordings on mast ica tory performance and (2) use the performance score for classifying 70 denture wearers into two equal groups, superior performers (SP) with a score of 35 or grea ter and poor performers (PP) with a score of less than 35.

Masticatory performance tests

Peanuts and raw carrots were used as tes t foods. Three 3-gram portions of each test food were used. For preferred side mast ica tory tests , par t ic ipants were instructed to chew each port ion of peanuts for 20 s trokes and carrots for 40 strokes on thei r preferred chewing side only. For swallowing threshold tests , par t ic ipants were asked to chew normally, wi thout regard to side, unt i l they were ready to swallow. The chewed food for all three portions for a tes t was pooled. A 12-mesh sieve (1.7 mm opening) for peanuts and a five-mesh sieve (2.0 mm opening) for carrots were used to separa te coarse and fine part icles. Mast icatory performance was determined by dividing the volume of fine part icles by the total volume of the tes t food and ex- pressed as a percent. The rel iabi l i ty and val idi ty of these tes ts have been es tabl ished in previous studies. 14, 15

Bite force measurements

An inter leaving beam s t ra in gauge t ransducer 16 was used to measure bite force on the preferred chewing side of the par t ic ipants . The bite force t ransducer at)proxi- ma ted an a rea equivalent to a single molar and was placed in the second premolar and first molar location on the preferred chewing side. An autopolymerized acrylic resin occlusal spl int a t tached to the bite force t ransducer provided a constant posit ion dur ing tests (Fig. 1). The par t ic ipant

DECEMBER 1995 6 2 9

THE J O L ~ 4 A L OF PROSTHETIC DENTISTRY GARRETT ET AL

T a b l e L Comparisons of age and denture characteristics between denture wearers with superior and poor masticatory performance

S P g r o u p P P g r o u p

SD X SD P

Age (yrs) 63.9 8.4 63.8 7.7 0.99 Maxillary denture

Denture experience (yrs) 23.0 10.5 22.8 11.4 0.96 Retention 2.9 0.9 2.7 0.9 0.45 Stability 2.8 1.1 2.8 1.0 0.99

Mandibular denture Denture experience (yrs) 22.8 10.5 21.8 11.5 0.71 Retention 1.8 0.8 1.4 0.8 0.08 Stability 3.0 1.2 2.7 1.3 0.45

Both dentures Vertical dimension 1.9 0.9 1.9 0.9 0.70 Centric occlusion 2.4 0.7 1.9 0.9 <0.01

Total score for both dentures Overall rating 14.9 4.1 13.4 4.0 0.13

Table IL Comparisons of denture-support ing tissue characteristics between denture wearers with superior and poor masticatory performance

S P g r o u p P P g r o u p

,X SD X SD P

Maxilla Ridge shape 3.1 0.7 3.1 0.7 0.73 Tissue resilency 2.5 0.8 2.5 0.7 0.75 Border tissue attachment 2.1 0.6 2.3 0.8 0.06

Mandible Ridge shape 2.4 0.9 2.3 0.8 0.60 Tissue resilency 211 0.8 2.0 0.7 0.35 Border tissue attachment 1.7 0.7 1.7 0.8 0.87

was instructed to bite hard for 3 seconds on the t ransducer without causing pain. Three bite force registrations were made at 15-second intervals for each participant. Each of the three bite force trials was analyzed for peak ampli tude from the preferred side rectified EMG signal, and the max imum bite force was calculated from the bite force t ransducer signal simultaneously. The ratio of peak pounds to peak microvolts was calculated for est imating the maximal force dur ing chewing from the peak EMG activity.

E l e e t r o m y o g r a p h i c r e c o r d i n g s

Bipolar surface electrodes were placed over the left and right masseter muscles to record EMG activity during maximal guided bite force and masticatory tests given in a shielded chamber. The proper placement of the electrodes was verified by monitoring the EMG activity while the subject clenched and relaxed his teeth. The raw masseter activity was amplified and filtered by a polygraph (Coulbourne Ins t ruments , Allentown, Pa.) and recorded on

an FM tape recorder (A. R. Vetter Co., Rebersburg, Pa.) for off-line signal analysis. The EMG activity was band-pass filtered from 0 Hz to 400 Hz to remove the high-frequency component of the signal above 400 Hz. The EMG activity was acquired by computer at 1000 samples per second, per channel. The digitized wave forms were then full wave rectified and stored on disk. Trained technicians viewed

the digitized signals on the computer monitor and identi- fied the onset and offset of the masseter EMG burst with a cursor for each chewing stroke. The computer calculated the durat ions in milliseconds for each masseter burst and chewing cycle, total durat ion of the test, total number of chewing strokes, and the peak (maximal) EMG amplitude

(microvolts) for each stroke. The muscle activity was sum- marized by use of methods similar to those used by an ear- lier investigator. 17 The digitized values for each closing

burs t were smoothed with a three-point average, then summated and divided by 1000 to provide an estimate of masseter effort in arbi t rary uni ts expended on each stroke.

6 3 0 VOLUME 74 N t a ~ E R e

GARRETT ET AL THE JO U RN A L OF PROSTHETIC DENTISTRY

24-

22-

20 -

18-

o 16. ILl

14

SP GROUP Preferred Non-preferred

PP GROUP Preferred Non-preferred

--4~5 . . . . . ~---

- E - - - - - - - -

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -o

! ! ! !

5 10 15 20

STROKE INTERVALS Fig . 2. Mean preferred side and nonpreferred side EMG act ivi ty per s troke (a rb i t ra ry units) averaged for each five-stroke interval for preferred side mas t ica tory performance tes t wi th peanuts .

Averages were made for each five consecutive strokes to reduce the d imensional i ty of the variables.

R E S U L T S

Chi-square comparisons and t- tests were used to ascer- ta in the comparabi l i ty of the two groups for age, denture experience, denture qual i ty (Table I), and denture sup- por t ing t issues (Table II). Differences in these var iables between the groups could in terac t with masse te r muscle function and bite force. No significant differences were found between the SP and PP groups in mean age or denture experience (p > 0.05). Similarly, the t issue support for maxi l la ry and mand ibu la r dentures was comparable for the two groups with one exception: the mean border t issue a t t achmen t in the maxi l la ry arch was sl ightly h igher or more favorable in the PP as compared with the SP group (• 5.95; p 0.06).

Mean maxi l la ry and mand ibu la r denture s tabi l i ty and maxi l la ry denture re tent ion were s imi lar for the two groups. The mand ibu la r denture re tent ion was margina l ly be t te r in the SP than in the PP group (• 3.03;p 0.08). The VDO score was unsat is fac tory in approximate ly 50% of denture wearers in both groups. Gross centric occlusal p r ema tu r i t y was noted in 17 denture wearers with poor performance but only in three denture wearers with supe- r ior performance. This frequency difference between the two groups was s ta t is t ical ly significant (• 14.2; p < 0.01). The combined score of retention, stabil i ty, and horizontal and vert ical re la t ionship for the maxi l la ry and mandibu-

la r dentures ranged from 6 to 21 among the 70 partici- pants . A score of 16 or grea ter was considered indicative of clinically sat isfactory dentures. Only 29% of the dentures were assessed to be clinically sat isfactory or bet ter in the PP group, compared with 54% in the SP group. This difference was significant (• 5.23; p < 0.03).

Masticatory performance The mean performances of the preferred side mastica-

tory tests wi thout recording electrodes (2 36.9) and wi th electrodes (2 38.5) were not s ignif icantly different (p > 0.05). A highly significant correlat ion between performance with and without electrodes in place (r 0.7; p < 0.0001) indicated tha t the muscle activity recordings did not affect chewing behavior. S imi lar ly high correlations (r 0.7; p < 0.0001) were seen between the preferred side and swallowing threshold performance scores with both peanuts and carrots.

Mean mas t ica tory performance scores and s tandard de- viat ions for the two performance groups are i l lus t ra ted in Table III. Mul t ivar ia te analysis of var iance (MANOVA) of the preferred side performance scores revealed significant differences between the SP and PP groups with both peanuts and carrots (MANOVA, F 6.65; p < 0.0002). S imi lar differences were found for the swallowing threshold performances (MANOVA, F 4.84; p < 0.0004). This was ex- pected because the group ass ignment was based on the preferred side mast ica tory performance with peanuts . Comparisons of the individual var iables (Ryan-Einot-

DECEMBER 1995 6 3 1

THE JOURNAL OF PROSTHETIC DENTISTRY GARRETT ET AL

26

I,LI ~ ~--~ 24

22

b- I-- O ~ 18

14

S P G R O U P PP G R O U P

P r e e n e d Non-p re fe~ed P m f e ~ e d N o n - p r e ~ e d

+ ---~ . . . . ~__

. . . . . . . . . . . . . . D ........... ~ ........... D ............ []-. ~ "-'~ .... . . .......

I I I I I I I I

5 10 15 20 25 30 35 40

STROKE I NTE RVALS Fig. 3. Mean preferred side and nonpreferred side EMG activity per stroke (arbitrary units) averaged for each five-stroke interval for preferred side masticatory performance test with carrots.

Table III. Comparisons of masticatory performance, chewing time, and number of chewing strokes for masticatory performance and swallowing threshold tests

SP g r o u p P P g r o u p

SD X SD F ' v a l u e

Preferred side Peanuts

Percent performance 43.6 14.5 30.7 11.8 24.39* Chewing time (sec) 13.8 3.3 12.9 2.2 1.74

Carrots Percent performance 62.4 19.7 41.8 21.7 17.31" Chewing time (sec) 27.2 5.3 27.0 4.6 0.20

Swallowing threshold Peanuts

Percent performance 61.6 14.0 45.9 15.0 19.50" Chewing time (sec) 22.0 7.3 24.7 7.8 2.79 Strokes (n) 34.8 12.9 40.5 13.4 3.16

Carrots Percent performance 65.2 19.9 55.9 22.0 3.40 Chewing time (sec) 27.2 9.2 31.1 11.3 3.21 Strokes (n) 43.3 17.0 49.4 18.5 1.96

*REGW corrected p <0.05.

Gabriel-Welsch, [REGW]) demonstrated that the SP group had significantly greater preferred side performance scores for both peanuts and carrots (F 24.39 and 17.31 respectively; corrected p < 0.05). For the swallowing threshold tests, significant differences were found in performance with peanuts, but not carrots (F 19.5;p < 0.05, and F 3.40; p > 0.05 respectively). No significant differ-

ences were found between the two groups in the mean time and number of strokes taken to prepare each test food for swallowing.

T o t a l m a s s e t e r m u s c l e a c t i v i t y

Means and standard deviations for the total preferred side and nonpreferred side EMG activity and their ratios


GARRETT ET AL THE JOURNAL OF PROSTHETIC DENTISTRY

Table IV. Comparisons of preferred side and nonpreferred side masse te r EMG activi ty for preferred side mast ica tory performance tests

SP group PP group

SD X SD F - v a l u e

Peanuts Total preferred side EMG 454.2 317.2 459.1 252.0 0.01

activity (arbitrary units) Total nonpreferred side EMG 397.7 240.4 325.4 238.4 1.60

activity (arbitrary units) Ratio of preferred 1.24 0.60 1.78 1.00 7.62*

side/nonpreferred side EMG Carrots

Total preferred side EMG 876.1 579.5 919.0 477.8 0.11 activity (arbitrary units)

Total nonpreferred side EMG 722.5 421.5 647.6 435.3 0.54 activity (arbitrary units)

Ratio of preferred 1.32 0.63 1.77 0.93 5.62* side/nonpreferred side EMG


for the prefer red side tes ts wi th peanuts and carrots are presented in Table IV. MANOVA comparison for the combined six var iables between the two groups revealed a sig- nif icant difference in masse te r response (F 2.73; p < 0.02). Comparisons between the two groups of the means for each of the six var iables revealed tha t only the rat io of preferred to nonpreferred side muscle act ivi ty was s ta t is t ical ly sig- nif icant for both peanuts and carrots. The rat ios of 1.24 to 1.32 were significantly lower and closer to 1 in the SP group than the rat ios of 1.78 and 1.77 found for the PP group (REGW corrected p < 0.05). Al though the total mean nonpreferred side EMG activi ty in the SP group was 22.2% higher with peanuts and 11.5% higher with carrots t han the respective score for the PP group, the differences were not s ta t i s t ica l ly significant (REGW corrected p > O.O5).

The resul ts of the muscle act ivi ty comparisons for the swallowing threshold tests (Table V) were s imilar to those for the preferred side tes ts (MANOVA, F 2.29; p < 0.05). Only the rat ios of the preferred and nonpreferred side muscle activit ies were significantly lower for the SP than those for the PP group (REGW corrected p < 0.05).

M a s s e t e r a c t i v i t y a c r o s s t h e c h e w i n g s e q u e n c e

The averaged muscle activi ty for the whole chewing cycle could mask group differences t ha t might exist a t the s troke level in the chewing sequence. Therefore, plots were made of the mean muscle act ivi ty for each interval of five chewing strokes for the preferred side performance test wi th peanuts (Fig. 2) and carrots (Fig. 3) for comparisons between the two groups. The mean act ivi ty of the preferred

side muscle at each interval was s imi lar in both the SP and PP groups for both tes t foods. The nonpreferred side EMG act ivi ty was consis tent ly grea te r a t all in tervals in the SP than tha t in the PP group for both tes t foods: However, the repeated measures analysis of var iance revealed tha t the differences between the two groups were nonsignificant (p > 0.05). These resul ts are s imi lar to those presented in Table IV with the total EMG activit ies for the preferred and nonpreferred side muscles. A sl ight interact ion between the chewing interval and the preferred side activi ty was noted for the peanuts . The SP group exhibited the grea tes t preferred side effort dur ing the first five strokes and the leas t effort dur ing s trokes 10 to 15 of the chewing cycle. The reverse was t rue in the PP group with the leas t preferred side effort dur ing the first five strokes and the grea tes t effort dur ing strokes 10 to 15 of the chewing cycle.

Plots of the ra t ios of the preferred side to nonpreferred side muscle act ivi ty at five-stroke intervals for both peanuts and carrots are i l lus t ra ted in Fig. 4. For peanuts , the rat ios ranged from 1.11:1 to 1.32:1 in the SP group and from 1.52:1 to 1.78:1 in the PP group. The repeated- measures analysis of var iance revealed significant differences between the two groups (p < 0.03). Bonferonni cor- rections for mul t ip le comparisons were used to compare the rat ios between two groups at each of the five-stroke intervals. The rat io was significantly smal ler for the SP group than for the PP group a t each interval when peanuts were chewed. S imi la r resul ts were found with carrots, but the rat io was significantly grea te r in the PP compared with the SP group only for the first 35 of the 40 strokes (p < 0.05).

DECEMBER 1995 6 3 3


3

2.5- " o

1.5-

0.5

SP GROUP PP GROUP

PEANUTS CARROTS PEANUTS CARROTS - -n~ - ~ - --E~ . . . . . ~--

I I I I I I I I

5 10 15 20 25 30 35 40

STROKE INTERVALS Fig . 4. Ratios of preferred side to nonpreferred side EMG activi ty averaged for each five- s troke in terval for preferred side mast ica tory tests wi th peanuts and carrots.

Table V. Comparisons of EMG activi ty for the preferred side and nonpreferred side masse te r dur ing tests of swallowing threshold

SP g r o u p P P g r o u p

'X SD ~, SD F - v a l u e

Peanuts Total preferred side EMG 709.5 519.7 846.9 544.2 1.17

activity (arbitrary units) Total nonpreferred side EMG 658.8 371.0 678.7 533.9 0.03

activity (arbitrary units) Ratio of preferred 1.11 0.58 1.52 0.75 6.62*

side/nonpreferred side EMG Carrots

Total preferred side EMG 877.4 577.5 1063.8 664.7 1.57 activity (arbitrary units)

Total nonpreferred side EMG 820.1 430.5 881.2 658.4 0.21 activity (arbitrary units)

Ratio of preferred 1.10 0.57 1.46 0.67 5.75* side/nonpreferred side EMG


Maximal bite force and preferred s ide c h e w i n g force

The guided maximal bite force of 14:43 _+ 6.60 lb in the SP group was only 3% less than the 14.91 _+ 8.76 lb for the PP group. The dis t r ibut ions were s imi lar for both groups. The peak forces appl ied dur ing each chewing stroke for the

preferred side test were es t imated by mult iplying the preferred side peak EMG ampl i tude by the rat io of peak bite force to peak EMG activi ty recorded during the guided maximal bite force test. The means per stroke for the five- s troke intervals are i l lus t ra ted in Fig. 5. The mean peak forces dur ing chewing of carrots were sl ightly less than those wi th peanuts within both groups. The peak force ap-


GARRETT ET AL THE JO U RN A L O F PROSTHETIC DENTISTRY

-~ 16-

od 15"

14"

13, O 12- LL

~( 11 < LLI 10 12.

PP GROUP Peanuts Carrots

___~ . . . . _~_

SP GROUP Peanuts Carrots

~ : : : : : : : : : : : : ~ . . . . . . . . ~ ......... - - ~ ........ __--~ ............. ~ ... . . . . . . . . . ~ ............

I i I i i I I I

5 10 15 20 25 30 35 40

STROKE INTERVALS Fig . 5. Es t ima ted peak bite force (lb) per s troke averaged for each five-stroke in terval for preferred side mast ica tory performance tests with peanuts and carrots.

plied to chew ei ther food was considerably higher in the SP group than t ha t appl ied by the PP group. The average peak force came to 12.9 -+ 10.9 lb and 14.08 _+ 14.06 lb respec- t ively for chewing carrots and peanuts in the SP group, and 10.2 + 9.07 lbs. and 10.38 -+ 8.87 lbs. in the PP group. Be- cause of the wide var ia t ions among par t ic ipants , the mean differences between the two groups were not significant (MANOVA, F 1.12; p > 0.05) for e i ther food. A t rend of de- creasing force can be noted in the SP group as chewing of both tes t foods progresses. However, these decreases were not significant ( repeated measures ANOVA, F 1.25; p > 0.05).

D I S C U S S I O N

This s tudy a t t empted to de te rmine whether denture weare r s wi th superior or poor mast ica tory performance appl ied different amounts o fmasse t e r muscle activi ty and bi t ing forces when they were ins t ructed to chew peanuts and carrots on the i r preferred chewing side or to chew normally.

Although s ta t is t ical ly significant differences between the SP and PP groups were not found for e i ther the total mean act ivi ty of the r ight and left masse te r muscles or mean peak bi t ing forces, significant differences (p < 0.05) were noted in the rat io of the EMG act ivi ty of the two muscles. This was t rue when denture wearers res t r ic ted the i r chewing to the i r preferred chewing side for a given number of s t rokes or chewed the tes t food freely unti l they were ready to swallow. With both peanuts and carrots, the ra t ios of the preferred side to nonpreferred side EMG ac- t ivi ty were more equivalent and closer to 1 in the SP group than the rat ios in the PP group. The rat io was 1.2:1 in the SP group and 1.8:1 in the PP group for chewing peanuts on

the preferred side. The pa t t e rn of b i la te ra l muscle activi ty was evident wi th both test foods and irrespective of whether den ture wearers chewed only on thei r preferred side or chewed normal ly for the swallowing threshold test. As can be noted in Figs. 2 and 3, the differences in the ratios between the two groups resul ted pr imar i ly from the marked d i spar i ty in the nonpreferred side muscle activity. The SP group applied considerably more effort on the nonpreferred side than the PP group. The rat ios of 1.5:1 to 1.8:1 seen in the PP group are s imilar to the 1.7 and 1.8:1 rat ios found dur ing the uni la te ra l chewing of the same tes t foods by denta te persons repor ted in a previous study. Is Other investigators19, 20 have also repor ted large ratios of ipsi- la te ra l to contra la tera l masse te r act ivi ty during uni la te ra l chewing in den ta te persons.

Although the SP and PP groups were comparable in terms of age, years of denture experience, and tissue sup- por t characteris t ics , 54% of the dentures in the SP group were r a t ed sat isfactory as compared with 29% in the PP group. This d i spar i ty resul ted main ly because of the gross centric p r e m a t u r i t y in 17 of the 35 denture wearers in the PP group and three in the SP group. Otherwise, the scores for retention, stabili ty, and VDO were s imi lar for the two groups. To de termine the impact of p remature occlusion, the rat ios of the EMG activi ty of the preferred side to nonpreferred side muscles in the two groups were further analyzed by use of analysis of covariance with centric occlusion as a covariate. The EMG rat ios remained significantly different (p < 0.05) between the PP and SP groups af ter factoring out the influence of the centric occlusion prema- turi ty. Comparisons were then made between 17 denture wearers wi th poor denture ra t ings and 18 with sat isfactory or bet ter ra t ings in the PP group. The mean mast ica tory

DECEMBER 1995 6 3 5


performance score of 26.3 with peanuts for 17 denture wearers with poor centric occlusion was similar to 26,8 of the remaining 18 with satisfactory centric occlusion in the PP group. The mean scores for the preferred side performance tests with carrots were also comparable for these two subgroups. Similarly, no significant differences were noted between these two subgroups in the EMG ratios of the preferred and nonpreferred side muscles. Further comparisons between the EMG ratios of 18 denture wearers with satisfactory dentures in the PP group and the SP group continued to reveal statistically significant differences as were seen for the two complete groups in Table IV. This indicates that the clinical quality of dentures did not significantly influence either the chewing ability or the ratio of the right and left masseter muscle activities during unilateral chewing of the two test foods.

The similar distributions of the guided maximal bite force in the SP and PP groups indicate that denture wearers with poor performance have the capability to apply bite pressures equal to those of denture wearers with superior performance. The peak bite forces applied by both groups during chewing were 75% to 90% of their maximal guided bite force test. This is consistent with the observations 21 that penetration forces required for many hard foods are near but not greater than the maximal forces that denture wearers are capable of applying. The reduction in peak force as chewing progressed in the SP group (Fig. 5) may have resulted from the rapid decrease in particle size because of more efficient chewing as compared with the im- paired comminution in the PP group. This might also ex- plain the relatively constant peak forces noted throughout the chewing sequence in the PP group.

CONCLUSIONS AND CLINICAL IMPLICATIONS

The results show that the application of a more equivalent force by both the right and left masseter muscles during unilateral chewing seems to be related to the masticatory ability of denture wearers. This premise of superior performance with bilateral muscle effort is consistent with instructions given by dentists to denture patients to chew food bilaterally with a more up-and-down jaw motion for better stabilization of dentures during mastication and to avoid lateral force. It is plausible that denture wearers who retain the pronounced unilateral muscle activity pattern after becoming edentulous perform poorly. Others who develop a bilateral muscle activity pattern during unilateral chewing tend to perform better. Perhaps, this performance gain results from increased stabilization of their dentures. Additional support for this possibility is provided by the occurrence in the SP group of the most pronounced bilateral activity during the early phases of the tests, when the reduction of large size particles would tend to destabilize dentures. An evaluation of jaw movements needs to be in- corporated in future studies to further clarify the relation- ship between masticatory performance, EMG activity, and

chewing stroke patterns. These findings have prompted an evaluation of a behavioral approach for training denture wearers with poor masticatory performance to develop a chewing pattern with bilateral masseter muscle effort.

R E F E R E N C E S

1. Kapur KK. Management of the edentulous elderly patient. Gerodontics 1987;3:51-4.

2. Wafter AH, Muench ME, Kaput KK, Chauncey HH. Masticatory performance and food acceptability in persons with removable partial dentures, full dentures and intact natural dentition. J Gerontology 1984;39:284-9.

3. Manly RS, Vinton P. Factors influencing denture function. J PROSTHET DENT 1951;1:578-86.

4. Kapur KK, Soman S, Stone K. The effect of denture factors on masticatory performance. Part I. Influence of denture base extension. J PROSTHET DENT 1965;15:54-64.

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8. Kapur KK, Soman S, Shapiro S. The effect of denture factors on masticatory performance Part V: Food platform area and metal inserts. J PROSTH~T DENT 1965;15:857-66.

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14. Kapur KK, Soman S, Yurkstas A. Test foods for measuring masticatory performance in denture wearers. J PROSTHET DENT 1964;14:483-91.

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16. Clark GT, Carter MC. Electromyographic study of human jaw-closing muscle endurance, fatigue and recovery at various isometric force lev- els. Arch Oral Biol 1985;30:563-9.

17. Hannam AG, Scott JD, De Cou RE. A computer-based system for the simultaneous measurement of muscle activity and jaw movement during mastication in man. Arch Oral Biol 1977;22:17-23.

18. Perez P, Kapur KK, Garrett NR. Studies of biological parameters for denture design. Part III: Effects of occlusal adjustment and base retention and fit on masseter muscle activity and masticatory performance. J PROSTHET DENT 1985;53:69-73.

19. Moller E. The chewing apparatus. An electromyographic study of the action of the muscles of mastication and its correlation to facial mor- phology. Acta Physiol Scand 1966:280;1-229.

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636 VOLUME 74 NUMBER 6

Masseter muscle activity in denture wearers with superior and poor masticatory performance

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