Swallowing threshold parameters of subjects with complete dentures and overdentures

doi:10.4236/ojst.2011.13012

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Open Journal of Stomatology, 2011, 1, 69-74

doi:10.4236/ojst.2011.13012 Published Online September 2011 (http://www.SciRP.org/journal/OJST/

OJST

Published Online September 2011 in SciRes. http://www.scirp.org/journal/OJST

Swallowing threshold parameters of subjects with complete

dentures and overdentures

Dick J. Witter1, F. Anneke Tekamp1, Ad P. Slagter2, Cees M. Kreulen1, Nico H. J. Creugers1*

1Department of Oral Function and Prosthetic Dentistry, College of Dental Science, Radboud University Nijmegen Medical Centre,

Nijmegen, The Netherlands;

2Department of Special Dental Care, Leeuwarden Medical Centre, Leeuwarden, The Netherlands.

Email: *[email protected]; [email protected]; [email protected]; [email protected]; [email protected]

Received 29 June 2011; revised 15 August 2011; accepted 27 August 2011.

ABSTRACT

Aim: To compare the chewing process and swallow-

ing threshold parameters of subjects with complete

dentures and overdentures with data obtained from

subjects with complete natural dentitions. Method-

ology: The chewing process in terms of swallowing

threshold parameters of four groups of subjects with

complete dentures (all females) was quantified by

sieving particles after chewing of an artificial test

‘food’ and compared with that of subjects with com-

plete natural dentitions as a reference group (33 sub-

jects). All subjects (except those of the reference

group) had a complete denture in the upper jaw. Re-

garding the lower jaw two groups with complete

dentures (with high (24 subjects), respectively low

mandible (12 subjects)) and two groups with over-

dentures (implant-retained (22 subjects), respectively

natural root supported (19 subjects)) were composed.

Results: The ‘overdenture-implants’ group needed

significantly more chewing cycles and time (mean: 45

cycles in 32 seconds) until ‘swallowing’ compared to

the group with complete natural dentitions (mean: 26

cycles in 19 seconds until ‘swallowing’). Also the

‘complete denture-low mandible’ group needed sig-

nificantly more cycles and time (mean: 52 cycles in 44

seconds) until ‘swallowing’ than the complete denti-

tion group. In the ‘overdenture-natural roots’ group

these outcomes (33 cycles in 24 seconds) were not

significantly different compared with the complete

dentition group. The ‘complete denture-high mandi-

ble’ group (32 cycles in 26 seconds) needed not sig-

nificantly more cycles until ‘swallowing’, however

time until ‘swallowing’ was significantly longer com-

pared to the complete dentition group. All denture

groups had significantly larger mean particle sizes

when ‘swallowing’ (sizes in the order of 3 mm) than

the natural dentition group (about 2 mm). Conclu-

sion: Despite efforts to compensate for a reduced

chewing efficiency, subjects with complete dentures

(including overdentures) had 50% larger median par-

ticle sizes when ‘swallowing’ compared to subjects

with complete natural dentitions.

Keywords: Swallowing Threshold; Chewing Efficiency;

Complete Denture; Overdenture; Oral Implant

1. INTRODUCTION

Chewing efficiency can be defined as the capacity to

pulverize food particles during a given number of chew-

ing cycles, for instance to half of the original particle

size [1]. Regarding particle size reduction during chew-

ing, it has been stated that “denture wearers reach only

25% of dentate chewing performance” [2], and “chewing

efficiency of denture wearers is less than one-sixth that

of subjects with a dentition” [3]. However, chewing effi-

ciency is only one parameter in the chewing process and

depends of variables such as bite force, salivary flow and

dental status including prosthodontic status. In order to

describe the chewing process as a whole, the number of

chewing cycles and the time needed until swallowing are

also relevant parameters together with the result of the

chewing process, i.e. the particle size reduction until

subjects feel the urge to swallow. These parameters have

been referred as swallowing threshold parameters [4,5].

Swallowing threshold parameters together with the

chewing frequency as derived from the number of ch-

ewing cycles and the time until swallowing are consid-

ered appropriate to describe how people manage their

chewing process [4,5]. Therefore, swallowing threshold

parameters outcomes can be used to indicate to what

extent people adapt to loss of teeth or prosthodontic de-

vices, e.g. by more chewing cycles until swallowing or

by swallowing larger food particles.

Bite force, chewing efficiency and their relationship

were previously analyzed for subjects with complete

(over) denture prostheses and for natural dentitions [1].

D. J. Witter et al. / Open Journal of Stomatology 1 (2011) 69-74

Also, swallowing threshold parameters were previously

analyzed, however, only for subjects with implant-retained

overdentures after chewing variable portion sizes [4].

A direct comparison of swallowing parameters for

subjects with different denture prostheses status and sub-

jects with complete natural dentitions has not been pub-

lished so far. This paper aims to quantify the swallowing

threshold parameters of subjects with complete dentures

on low and high mandibles, of subjects with implant-

retained overdentures and of subjects with natural roots-

supported overdentures, and to compare the outcomes

with those of subjects with complete natural dentitions

as a reference.

2. MATERIALS AND METHODS

2.1. Subjects

The study sample (n = 110) was partially drawn from

participants in a longitudinal clinical trial on implant-

retained mandibular overdentures and partially from

patients of the Nijmegen dental school [1,4]. Because a

gender effect has been demonstrated for chewing effi-

ciency and only few males were included in the clinical

trial [4] and analyzed for bite force and chewing effi-

ciency [1], the present study only comprised females.

Four denture groups were defined (Ta b l e 2 ). All par-

ticipants of the denture groups had a complete denture in

the upper jaw. Subjects in the ‘complete denture-low

mandible’ group (n = 12) and subjects in the ‘overden-

ture-implants’ group (n = 22) all fulfilled the clinical

trial criteria for inclusion as described elsewhere [1,4]

and therefore had a mandibular symphysial bone height

between 8 and 15 mm as measured on a standardized

lateral cephalogram. Subjects of the ‘complete den-

ture-high mandible’ group (n = 24) had a mandibular

symphysial bone height of 16 mm or more. Subjects in

the ‘overdenture-implants’ group had two bar-connected

cylindric IMZ implants in the interforaminal region [1,4].

Subjects in the ‘overdenture-natural roots’ group (n = 19)

had two functional natural roots without additional at-

tachments. All subjects of the denture groups had their

appliances for periods that made habituation or adapta-

tion plausible. The complete dentition group as a refer-

ence comprised of 33 subjects with complete natural

dentitions with or without 3rd molars. In this group 2

subgroups were defined (an older (n = 14) and younger

group (n = 19; Tab le 1) in order to detect possible age-

effects.

No participant indicated to suffer from pain related to

temporomandibular disorders. The ethics committee of

the Nijmegen University Medical Centre had given ap-

proval for the study and informed consent was obtained

from all participants.

2.2. Swallowing Thr eshold Tests

Swallowing threshold tests were performed with cubic

(edges 5.6 mm) silicone particles (Optocal Plus based

upon the silicone component OptosilR Plus (Bayer Den-

tal, Leverkusen, Germany)) [1,4]. Subjects were asked to

chew portions of 17 particles (approximately 3 cm3) in

“a way they normally do”, and to spit out the chewed

particles when they felt the urge to swallow. Number of

chewing strokes until ‘swallowing’ as well as time (in

seconds) until swallowing were registered. Particles

Table 1. Swallowing threshold parameters (means (s.d.)) of the older and younger complete natural dentition sub-samples.

Complete natural

dentition sub-samples n Age Mean

(SD) (yrs)

No. of chewing cycles

until swallowing

Time until

swallowing (sec)

Chewing frequency

(cycles/min)

Median particle size

(X50swal)* (mm)

Older 14 54.1 (6.4) 26.5 (11.2) 19.1 (8.0) 84.5 (13.2) 2.0 (0.9)

Younger 19 22.7 (1.5) 26.4 (10.8) 18.7 (7.4) 84.8 (12.1) 2.1 (0.8)

(X50swal)* is the aperture (mm) of a theoretical sieve through which 50% of the weight of the particles pass when ‘swallowing’.

Ta b le 2 . Swallowing threshold parameters (mean (s.d.)) of the dental groups; denture groups had a complete denture in the upper

jaw.

Denture groups n Age (y) No. of chewing cycles

until swallowing

Time until

swallowing (sec)

Chewing frequency

(cycles/min)

Median particle

size (X50swal)*(mm)

Complete denture-low mandible

Overdenture-implants

Complete denture-high mandible

Overdenture-natural ro ots

Complete natural dentition

57.8 (6.6)

56.7 (7.1)

58.3 (8.8)

59.9 (8.5)

36.0 (16.3)

51.6 (31.8)a

45.0 (17.1)a

32.3 (12.6)b

33.3 (18.2)b

26.5 (10.8)b

44.5 (33.0)a

32.0 (12.0)a

25.7 (12.0)b

24.4 (14.0)bc

18.9 (7.5)c

74.0 (13.4)a

84.4 (12.1)b

77.3 (10.6)b

84.6 (15.3)b

84.7 (12.4)b

3.8 (1.4)a

2.8 (0.8)a

3.2 (1.1)a

3.0 (1.1)a

2.1 (0.8)b

abc: same characters indicate no significant difference (Mann-Whitney tests comparing the subsequent dental groups); (X50swal)* is the aperture (mm) of a theo-

retical sieve through which 50% of the weight of the particles pass when ‘swallowing’.

D. J. Witter et al. / Open Journal of Stomatology 1 (2011) 69-74 71

were air-dried for at least one week and sieved for 20

minutes in a stack up to 12 sieves, with square meshes

decreasing from 5.6 mm at the top to 0.5 mm at the bot-

tom and a bottom plate (Laboratory Sieving machine

VS1000; F. Kurt Retsch, Haan, Germany). Frequent in-

termittent eccentric movements of the sieves gave ir-

regularly shaped particles (as after chewing) opportuni-

ties for favorable positions to fall through the square

meshes. After the drying period, the sizes of the chewed

particles were assessed by determining the aperture of a

theoretical sieve through which 50% of the weight of the

particles can pass and this was expressed as the median

particle size (X50swal) [4, 5].

Tests were carried out twice.

2.3. Statistical Analyses

To determine the added value for repeating the chewing

tests, Spearman’s correlation coefficient was used to

assess the relation between the two measurements in

each subject. All correlations (for the number of chewing

cycles, the time until ‘swallowing’, X50swal , and chewing

frequency) were 0.85 or higher. This indicates a very

high correlation between the first and second test. In the

main analysis, the number of chewing cycles, and the

time until ‘swallowing’, and X50swal were averaged. The

chewing frequency was calculated from these mean val-

ues as the number of chewing cycles per minute.

The two subgroups ‘complete dentition-older’ and

‘complete dentition-younger’ were compared in order to

detect possible age effects (Mann-Whitney tests). For

reason of irregular distributions of the outcomes, non-

parametric tests were used: Kruskal-Wallis tests to detect

group effects comparing all five dental groups. If the

groups differed significantly according to the Kruskal-

Wallis test, Mann-Whitney tests were used to analyze

differences between the five subsequent dental groups

with the following sequence: complete denture-lo w mandi-

ble, overdenture-implants, complete denture-high man-

dible, overdenture-natural roots, and complete natural

dentition. Next, the relation between the number of

chewing cycles until ‘swallowing’, the time until ‘swal-

lowing’, and X50swal was assessed (Spearmans’s Rho).

For the analyses, SPSS program, version 16.0 was

used. P-values ≤ 0.05 were considered significant.

3. RESULTS

Swallowing threshold parameters of the older and

younger complete natural dentition sub-samples did not

differ significantly (Table 1; all 4 tests: P ≥ 0.392).

Therefore, the sub-samples were combined to the com-

plete natural dentition group for statistical analyses (n =

33; Table 2).

Descriptive statistics and presence of statistical sig-

nificant differences between the groups regarding the

four outcome variables are given in Table 2.

Concerning the number of chewing cycles until ‘swal-

lowing’ the Kruskal-Wallis test revealed a significant

difference (P < 0.001). Amongst the denture groups a

dichotomy can be observed: a high number of chewing

cycles for the ‘complete denture-low mandible’ and the

‘overdenture-implants’ groups compared to the other two

denture groups (‘high mandible’ and ‘natural-roots’) and

the control group.

Regarding the time until ‘swallowing’, the Kruskal-

Wallis test again revealed a significant difference (P <

0.001). An analogous dichotomy for the denture groups

can be distinguished: the ‘complete denture-low mandi-

ble’ and ‘overdenture-implants’ groups chewed longer

than the other two denture groups. However, all denture

groups with the exception of the ‘overdenture-natural

roots’ group used significantly longer time until ‘swal-

lowing’ than the complete natural dentition group (con-

trol).

For chewing frequency, the Kruskal-Wallis test again

showed a significant difference, but this outcome was

less clear (P = 0.046). The frequency was significantly

lowest in the ‘complete denture-low mandible’ group,

without a significant difference between the other (sub-

sequent) groups.

For X50swal, the Kruskal-Wallis test showed a clear cut

significant difference (P < 0.001). All denture groups

had similar X50swal scores in the order of 3 mm while the

complete natural dentition group showed a X50swal of

approximately 2 mm.

Within all dental groups, the number of chewing cy-

cles until ‘swallowing’ was highly significant correlated

with time until ‘swallowing’ (Spearman’s Rho’s ranged

from 0.87 to 0.95; all P-values < 0.001). As a conse-

quence the chewing frequency was more or less constant

within each group. Moreover, for all five groups the

number of chewing cycles until ‘swallowing’ was sig-

nificant negatively correlated with X50swal (Spearman’s

Rho’s ranged between –0.45 and –0.87; all P-values

0.037 or lower).

4. DISCUSSION

4.1. Study Design

The dental groups of this study were convenient samples

of subjects participating in other studies of the Nijmegen

dental school, and selected on the basis of availability.

As only a small number of males was included in the

clinical trial on implant-retained overdentures [4], males

were eventually not included in this study. It has been

demonstrated in that clinical trial that males chewed their

food more efficiently than females, as they achieved

D. J. Witter et al. / Open Journal of Stomatology 1 (2011) 69-74

greater particle size reduction using the same number of

chewing cycles [4]. Although gender was found to have

a small indirect effect on chewing function because of

bite force [6], it is plausible that the relationships found

in the present study also apply to males.

The criterion of a symphysial bone height of 16 mm

as applied in this study originates from the inclusion

criteria of the original clinical trial [4]. This cut-off cor-

responds with Cawood Class V: “a flat ridge form, in-

adequate in height and width” [7], which might com-

promise complete denture function and thus gives rele-

vance for using implants.

In the previous study investigating chewing efficiency

and bite force [1], subjects with complete dentitions

were slightly younger than those of the denture groups.

Therefore, a group of young adults was included in order

to detect possible age effects. In the present study age

effects were not found (Table 1), which is in accordance

with findings of others [6,8]. Therefore, both age groups

were combined for further analyses.

Analyses revealed a very high correlation between the

repeated measurements within each subject. This implies

a marginal added value of the second test. Consequently,

in similar studies a restriction to one test only can be

advised.

In cases the Kruskal-Wallis test showed an overall dif-

ference between groups, the Mann-Whitney was applied.

This test was chosen to minimize the number of post-hoc

tests. However, this test only compares subsequent (ad-

jacent) groups in a more or less logical sequence, for

instance starting from ‘poor’ to ‘good’ outcomes as ex-

pected beforehand. For the three primary outcomes this

appeared to be an appropriate approach because the out-

comes coincided with the sequence of the groups. How-

ever, for chewing frequency this was not the case. Here

two groups not being subsequent groups, i.e. the ‘com-

plete denture-low mandible’ and the ‘complete den-

ture-high mandible’ groups were found to deviate from

the other groups. Consequently, for chewing frequency

the labeling of the groups (Table 2) to indicate differ-

ences or similarities amongst non-adjacent groups is too

crude.

4.2. Chewing Test Outcomes

The relative similarity between the outcomes of the first

and the second test indicates that the chewing process

until ‘swallowing’ is fairly constant within a subject for

a specific type of food. On the other hand, the large dif-

ferences in outcomes together with large standard devia-

tions (Ta bl e 2 ) indicate wide variations among subjects.

This implies that the chewing process is an individually

determined and adapted process e.g. to denture function

[9,10].

It can be subject of debate whether chewing just a

single type of artificial test ‘food’ represents ‘real life’

chewing function. The difference in particle size reduc-

tion between ‘good’ and ‘bad’ chewers in such a test

depends partly on food consistency, and the firm artifi-

cial test ‘food’ as used in the present study might have

magnified differences between ‘good’ and ‘bad’ chewers

[5,11]. Moreover, it has been shown that larger portions

to chew until ‘swallowing’ increase X50swal, and the

number of chewing cycles until ‘swallowing’ increases

linearly with the volume [4,12], but it is unknown wh-

ether this increase is proportionally amongst dental

groups. Despite these shortcomings the outcomes of this

study are considered to be indicative for clinically rele-

vant swallowing parameters when comparing groups

with different dental or prosthodontic status.

4.3. Compensation for Impaired Chewing

Capacity

It has been stated that chewing efficiency of complete

denture wearers is substantially inefficient compared to

subjects with complete natural dentitions: less than

one-sixth [3], or even one-seventh [13].

The present study on the other hand, focusing on

chewing until ‘swallowing’ reveals a less dramatic pic-

ture. Compensation for the reduced chewing efficiency

by more chewing cycles and longer time until ‘swallow-

ing’ in the denture groups resulted overall in a 40% to

50% larger X50swal than in the complete dentition group,

however subjects with ‘complete denture-low mandible’

had approximately twice larger X50swal (Table 2).

When people use more chewing cycles until ‘swal-

lowing’ and chew with lower frequency, however swal-

low larger particles, this reflects the effectiveness of

their chewing process. Compared to the controls, sub-

jects in the ‘complete denture-low mandib l

’ group used

a higher number of chewing cycles until ‘swallowing’

(factor 1.9), had lower chewing frequency (factor 1.1),

and had larger outcome X50swal (factor 1.8). Although

these are dependent variables, these figures can be inter-

preted as that the chewing effectiveness is 1.9 (higher

number of chewing cycles) × 1.1 (lower chewing fre-

quency) × 1.8 (larger X50swal) = 3.8 times lower than that

of the controls. Following this interpretation, in the

‘complete denture-high mandible’ group the chewing

effectiveness was 1.2 × 1.1 × 1.5 = 2.0 times lower; in

the ‘overdenture-implants’ group 1.7 × 1.0 × 1.3 = 2.2

lower, and in the ‘overdenture-natural roots’ group 1.3 ×

1.0 × 1.4 = 1.8 times lower than that of subjects with

complete natural dentitions.

With respect to interventions aiming to achieve a

more acceptable chewing process, the application of

implants (mostly in low mandibles) or the preservation

D. J. Witter et al. / Open Journal of Stomatology 1 (2011) 69-74 73

of natural roots are considered valid interventions.

However, improvement of chewing effectiveness is lim-

ited to the level of the ‘complete denture-high mandible’

group. This indicates that mandibular implant-retained

overdenture treatment is most effective in terms of im-

proved masticatory performance in persons with a less

than adequate mandibular ridge. This suggestion has

been stated also by others [14], irrespective different

mesostructure modalities including bar-clip (as in this

study), ball or magnet attachments [15].

As stated previously, the relatively large standard de-

viations in X50swal denote substantial individual variation

within groups. In other words, the differences within

groups are large compared to the mean differences

amongst groups. Just as an illustration, the subject with

the lowest number of chewing cycles before ‘swallow-

ing’ was found in the ‘complete denture-low mandible’

group. It should be noted that denture status is just one

of the chewing efficiency determinants. Other determi-

nants are saliva, muscles (influencing bite force), and

cultural habits and personality. In other words, also cul-

tural and personality traits determine ‘slow’ and ‘fast’

swallowers [9,10,16-18]. The multifactorial nature of the

chewing process undoubtedly explains the large indi-

vidual differences in the outcomes of this study on

swallowing threshold parameters. This individual varia-

tion is considered unproblematic since the assumed link

between chewing function and deficient dietary intake is

based only on relatively weak correlations and cannot

confer a causal relationship [19], as it is not clear to what

extent human digestion depends on chewing [20].

5. CONCLUSIONS

1) Despite efforts to compensate for their reduced chew-

ing efficiency by a larger number of chewing cycles until

‘swallowing’, subjects with mandibular (over)dentures

and maxillary complete dentures had approximately 40%

- 80% larger X50swal than subjects with complete natural

dentitions.

2) The outcomes of swallowing threshold parameters

of subjects with overdentures retained by implants or

natural roots were on average comparable to those of

subjects with complete dentures on a ‘high’ mandible.

3) Apart from dental and denture status, large indi-

vidual differences in outcomes of swallowing threshold

parameters were observed indicating ‘slow’ and ‘fast’

swallowers.

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