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.
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 70
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
12
22
24
19
33
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’.
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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
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D. J. Witter et al. / Open Journal of Stomatology 1 (2011) 69-74 72
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
e ’ 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
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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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