Journal of Oral Rehabilitation 2002 29; 484±491

Changes in occlusal force and occlusal contact area

after active orthodontic treatment: a pilot study using
pressure-sensitive sheets
M. H. SULTANA, K. YAMADA & K. HANADA Division of Orthodontics, Department of Oral Biological Science,
Section for Oral Life Science, Graduate School of Medical and Dental Sciences, Niigata University, Niigata, Japan

SUMMARY The aim of this study was to investigate retainer. The increases of OcFr and OcAr were larger
functional changes in occlusion during retention. in the molar region, especially at the second molar.
Data on occlusal force (OcFr) and occlusal contact At 1 year after removal of retainer, OcFr and OcAr
area (OcAr) was obtained using the pressure-sensi- in the second molar were signi®cantly larger in the
tive sheet, from a treated group (20 female patients) treatment group than in the control sample, and a
who had had four premolar extractions and treat- similar distribution pattern of OcFr and OcAr to
ment with standard edgewise appliances, and a those in normal occlusion was seen. These results
control sample who matched the treated group of suggested that balanced OcFr and OcAr might be
retainer for sex, age and Angle classi®cation at obtained during and after retention, due to the
1 year after removal. A repeated measures analysis settling of molars that had been discluded by active
of variance showed that the mean values of total orthodontic treatment.
OcFr and OcAr in the treatment group gradually KEYWORDS : occlusal contact area, occlusal force,
KEYWORDS:
increased during retention and were 669á3 N and orthodontic treatment, pressure-sensitive sheets,
15á1 mm2, respectively, at 1 year after removal of retention

been one Japanese report showing changes in OcFr and

Introduction
OcAr over a short-term (Nakajima et al., 1994). There-
Many orthodontic practitioners believe that interdigi- fore, long-term longitudinal changes in OcFr and OcAr
tation of the teeth generally improves after orthodon- during and after retention remain unclear.
tic appliances are removed and the teeth are allowed In order to evaluate functional changes in occlusion
to settle. The occlusion acquired through active during and after retention, we measured OcFr and
orthodontic treatment can change as a result of the OcAr of the lower dentition using the pressure-sensitive
growth, and remodelling of soft and hard tissues sheet.
during the period of post-treatment settling after the
removal of active appliances. Concerning these occlu-
Subjects and methods
sal changes, an increased number of occlusal contacts
in maximum intercuspation have been reported as an
Case selection
improved interdigitation of the teeth, using rubber
impression materials (Durbin & Sadowsky, 1986) and Twenty subjects were randomly selected from patients
photo-occlusion techniques (Gazit & Lieberman, who were treated with the extraction of their ®rst
1985). premolars, second premolars or a combination of ®rst
However, concerning the occlusal force (OcFr) and and second premolars at the Department of Orthodon-
occlusal contact area (OcAr), which are indicators of tics, Niigata University Dental Hospital. These cases
how occlusal contact acts functionally, there has only were treated to an optimum occlusion, with the

ã 2002 Blackwell Science Ltd 484

 CHANGES IN OCCLUSAL FORCE AND CONTACT AREA 485

treatment objectives generally satis®ed. Cases requiring and which are coated with PET resin ®lms and wrapped
prosthetic replacement of missing teeth were not with PET ®lm on both sides. Each sheet is 0á097 mm
included. Just after the subjects' active orthodontic thick.
treatment (removal of appliance), their OcFr and OcAr
were examined using pressure-sensitive sheets after
Analysing computer system
their informed consent was obtained. These cases were
classi®ed as the treatment group. The treatment group A computer-assisted analysing system (FPD-703)*
consisted of 20 females with a mean age of 21á6 years, visualizes the distribution of the load on a display.
and a range of 18±46 years. Prior to treatment ®ve The magnitude of OcFr and OcAr is estimated from the
subjects had Class I malocclusions, seven had Class II colour density identi®ed on the pressure-sensitive
malocclusions, and eight had Class III malocclusions. A sheet, using an application program (Occluzer Graph
control sample was also investigated to compare the M)².
differences of OcFr and OcAr between treated and
untreated subjects. The OcFr and OcAr in the treatment
Methods for measuring OcFr and OcAr
group at 1 year after removing retainer was compared
with those of a control group of 20 subjects who were A pressure-sensitive sheet ®xed in a holder was posi-
matched with the treated group at 1 year after remov- tioned at the appropriate place in the subject's mouth.
ing retainer for sex, age and Angle classi®cation. The The holder prevented the sheet from being deformed by
untreated control sample consisted of 20 female ortho- the buccal mucosa. Each subject was seated comfort-
dontic patients with an average age of 21á6 years and a ably in a chair with a head support such that the
range of 18±46 years. Frankfort plane was parallel to the ¯oor. The subjects
were then instructed to clench their teeth with maxi-
mum bite force on the dental prescale sheet in
Retainers
intercuspal position for 2 s. Data was always taken
All patients received a maxillary circumferential type three times to verify their reproducibility. After testing,
retainer and a mandibular ¯exible spiral wire (FSW) the sheets were stored in a lightproof box for at least 3 h
retainer. The use of circumferential and FSW retainers to stabilize the intensity of colour.
was completed over a period of 2 years after their Occlusal contact areas were veri®ed with a black
setting retainer. Patients were instructed to wear the silicone (Bite checker)³. The black silicone was mixed
removable retainers at all times except during meals for according to the manufacturer's instructions and
the ®rst 18 months, and nightly during the remaining applied with a syringe to the occlusal surface of the
half year. mandibular teeth, and subjects were instructed to bite
®rmly in intercuspal position and hold this position for
approximately 2 min. This bite frame was used as a
Methods for measuring occlusal force
guideline for checking the contact points of each tooth
Occlusal records were taken using methods similar to on the pressure-sensitive sheet.
those in Hassan et al. (1997) and Yamada et al. (2000) Recording of OcFr and OcAr was longitudinally
using the Dental Prescale System. This system consists of performed, within 30 min of debanding (T1), 1 (T2),
a pressure-sensitive sheet (Dental Prescale SystemTM, 6 (T3), 12 (T4), 18 (T5) and 24 (T6) months after initial
50H R type)* and an image scanner (Occluzer FPD- setting of retainer and 12 months (T7) after removal of
703)* (Watanabe et al., 1995; Hattori et al., 1996; Hassan retainer.
et al., 1997; Hidaka et al., 1999; Yamada et al., 2000).
The sheet can measure OcFr through a wide range of
Method of analysis
pressures (5±120 kg mm±2). It contains colour-devel-
oping materials and various sizes of microcapsules The pressure-sensitive sheet was digitized by a scanner
containing colour-producing ingredients which display attached to the analysing system. After scanning, to
different colourations depending on the loaded pressure
²
Scimolex Co., Tokyo, Japan.
³
*Fuji Film Co., Tokyo, Japan. GC Corporation, Tokyo, Japan.

ã 2002 Blackwell Science Ltd, Journal of Oral Rehabilitation 29; 484±491

486 M . H . S U L T A N A et al.

evaluate the distribution of OcFr and OcAr on the lower

Results
arch, pixels of the lower arch were displayed on the
monitor using a software (Occluzer Graph M)². The
Angle classi®cation
positions of the lower teeth were determined by
comparing the pixels on the monitor with the black To determine whether Angle classi®cation relations
silicone test for each subject. might have an undue effect on results, the subjects
Data from the lower arch was collected and the were separated into Class I, II and III. No signi®cant
means and standard deviations (s.d.) of OcFr and OcAr difference was found between the means for the
in the total dental arch and molar, premolar and variables in any retention periods of the treated group
anterior teeth regions were calculated from the three and the control sample (Table 2).
tests performed on each subject. Reproducibility of
OcFr in the total dental arch was assessed by a
Occlusal data
coef®cient of variation [CV ˆ (s.d. of OcFr/n)* 100]
(Table 1). The CV of OcFr ranged from 1á3 to 19á1% (mean
11á3 ‹ 9á1%) in the total dental arch during all stages.

Statistics
The OcFr and OcAr in total dental arch
T1 describes the period when the retainer was set,
T2, T3, T4, T5 and T6 describe the times 1, 6, 12, 18 Table 3 presents OcFr and OcAr in the total dental arch.
and 24 months after setting the retainer, respectively, The OcFr and OcAr gradually increased during retent-
and T7 describes the time 12 months after removal of ion; a repeated measures analysis of variance in OcFr
the retainer. Means and s.d. were obtained for the and OcAr during retention showed signi®cant increases.
occlusal data. One-factor repeated measures analysis
of variance was used to examine the signi®cance of
OcFr and OcAr in anterior, premolar and molar regions
the differences from T1 to T7. The Mann±Whitney
U-test for independent samples was used between the There was no signi®cant change in OcFr and OcAr in
control sample and T7, to compare the control anterior and premolar regions between T1 and T7
sample and treated group at 1 year after removal of (Table 4). On the other hand, in the molar region, a
the retainer. The level of signi®cance was pre- repeated measure analysis of variance, indicated a
determined at P < 0á05. P > 0á05 was considered not signi®cant linear trend over time of increasing OcFr
signi®cant (NS). and OcAr during retention.

Table 1. Occlusal data measures

Characteristic Formula

CV of OcFr (%) [s.d. of OcFr/n] * 100

Occlusal force (N)
Total OcFr Sum of all teeth OcFr
Molar OcFr Sum of ®rst and second molars OcFr on both sides
Premolar OcFr Sum of ®rst and second premolars OcFr on both sides
Anterior OcFr Sum of incisors and canines OcFr on both sides
First molar OcFr Sum of ®rst molar OcFr on both sides
Second molar OcFr Sum of second molar OcFr on both sides

Occlusal contact area (mm2)

Total OcAr Sum of all teeth OcAr
Molar OcAr Sum of ®rst and second molars OcAr on both sides
Premolar OcAr Sum of ®rst and second premolars OcAr on both sides
Anterior OcAr Sum of incisors and canines OcAr on both sides
First molar OcAr Sum of ®rst molar OcAr on both sides
Second molar OcAr Sum of second molar OcAr on both sides

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 CHANGES IN OCCLUSAL FORCE AND CONTACT AREA 487

Table 2. Occlusal force (OcFr) and occlusal contact area (OcAr) in each angle classi®cation

Total OcFr (N) Total OcAr (mm2)

Class I Class II Class III Class I Class II Class III

Average s.d. Average s.d. Average s.d. P Average s.d. Average s.d. Average s.d. P

T1 494á5 170á1 470á5 103á4 300á0 90á1 NS 9á0 2á2 8á9 3á7 6á3 2á6 NS
T2 491á6 190á1 471á7 188á5 319á1 100á2 NS 9á7 2á2 8á8 3á6 6á4 1á5 NS
T3 595á6 170á1 493á3 139á8 390á0 120á1 NS 11á5 3á2 9á7 3á1 7á0 2á8 NS
T4 692á1 150á2 627á6 248á4 440á0 170á1 NS 14á6 4á3 13á3 6á4 7á0 3á3 NS
T5 700á5 166á5 643á2 201á7 565á1 190á1 NS 15á3 7á4 14á0 4á6 11á6 3á8 NS
T6 727á1 185á9 695á8 256á2 610á5 180á5 NS 15á6 7á2 15á1 7á6 14á4 6á5 NS
T7 728á2 120á1 696á9 167á9 605á5 150á2 NS 16á0 3á5 15á7 5á6 14á0 4á3 NS
Control 585á6 286á4 618á8 290á1 456á5 148á9 NS 12á9 5á6 15á2 8á0 11á0 3á3 NS

NS: Not signi®cant.

Table 3. Total occlusal force (OcFr) and occlusal contact area Discussion
(OcAr)

Method
Total OcFr (N) Total OcAr (mm2)
Evaluation of the number of occlusal contacts (Gazit
Mean s.d. Mean s.d.
& Lieberman, 1985; Durbin & Sadowsky, 1986; Razdolsk
T1 416á7 167á7 8á0 3á6 & Sadowsky, 1989; Haydar et al., 1992 Sauget et al.,
T2 421á3 180á1 7á9 3á6
1997) has been mainly used for assessing changes in
T3 460á2 154á9 8á8 3á5
T4 564á0 180á5 11á9 4á0
teeth function after orthodontic treatment. Concerning
T5 620á1 219á4 13á3 5á5 OcFr and OcAr, however, there has only been one short-
T6 670á1 216á5 15á9 7á2 term report on three cases (Nakajima et al., 1994) and
T7 669á3 133á8 15á1 4á6 the details of long-term changes in OcFr and OcAr
P* < 0á0001 < 0á0001 remain unclear.
*Repeated measures analysis of variance from T1 to T7. In the present study, the recently developed dental
prescale system (Watanabe et al., 1995; Hattori et al.,
Concerning changes in OcAr and OcFr between ®rst 1996; Hassan et al., 1997; Hidaka et al., 1999) was used
and second molars, a signi®cant linear trend was to evaluate OcFr and OcAr by the colours appearing in
discovered during retention (P < 0á0001) showing that the pressure-sensitive ®lm. It has been reported that
both OcFr and OcAr increased gradually, their increase OcFr in maximum intercuspation can be recorded
being larger in the second molar than in the ®rst repeatedly with little error using a similar methodology
molar (Table 5). (Matsui et al., 1996; Hassan et al., 1997). The ®lm is
unaffected by intraoral humidity and temperature
(Noguchi et al., 1983) and the velocity and duration
Comparison of OcFr and OcAr between T7 and control sample
of the forces applied to it have negligible in¯uences on
Total OcFr and OcAr at T7 tended to be larger than colour formation (Suzuki et al., 1994).
those of the control group (Table 6). The OcFr and In the present study, the CV of OcFr was 11á3 ‹ 9á1%.
OcAr in second molar at T7 were signi®cantly larger This result suggested that OcFr determined by the
than those of the control group (Table 6). present study showed adequate reproducibility.

Distribution of OcFr and OcAr in dental arch Changes of OcFr and OcAr

The ®rst molar showed the largest OcFr and OcAr from The number of occlusal contacts has been widely
T1 to T3, but the second molar showed the largest considered as a functional evaluation of occlusion
ones from T4 to T7 and in the control sample (Fig. 1). during retention (Gazit & Lieberman, 1985; Durbin

ã 2002 Blackwell Science Ltd, Journal of Oral Rehabilitation 29; 484±491

488 M . H . S U L T A N A et al.

Table 4. Occlusal force (OcFr) and occlusal contact area (OcAr) in anterior, premolar and molar regions

Occlusal force (N) Occlusal contact area (mm2)

Anterior Premolar Molar Anterior Premolar Molar

Mean s.d. Mean s.d. Mean s.d. Mean s.d. Mean s.d. Mean s.d.

T1 28á3 26á8 70á0 60á5 318á4 168á2 0á5 0á5 1á4 1á5 6á0 3á4
T2 25á9 23á9 62á1 37á9 333á3 201á9 0á5 0á5 1á1 0á7 6á1 4á0
T3 29á3 24á7 53á4 42á4 366á9 158á7 0á6 0á6 1á1 0á9 7á5 3á5
T4 26á0 24á6 50á6 36á3 513á1 252á5 0á6 0á6 1á0 0á8 11á0 6á2
T5 35á9 29á6 60á3 41á7 520á2 216á3 0á7 0á5 1á3 1á0 11á3 5á2
T6 32á9 21á9 68á2 49á4 568á8 183á9 0á7 0á3 1á4 1á1 14á5 6á4
T7 17á8 20á6 68á2 49á4 587á3 150á6 0á4 0á5 1á4 1á1 14á1 4á5
P* NS NS < 0á0001 NS NS < 0á0001

*Repeated measures analysis of variance from T1 to T7.

NS: Not signi®cant.

Table 5. Distribution of occlusal force (OcFr) and occlusal contact area (OcAr) in molar region

Occlusal force (N) Occlusal contact area (mm2)

First molar OcFr Second molar OcFr First molar OcAr Second molar OcAr

Mean s.d. Mean s.d. Mean s.d. Mean s.d.

T1 186á2 108á1 127á9 79á9 3á6 2á2 2á4 1á6

T2 195á8 108á0 138á2 101á2 3á7 2á1 2á5 2á0
T3 197á5 88á2 169á4 96á8 4á1 2á0 3á4 2á3
T4 240á7 120á4 272á4 165á4 5á2 2á9 5á7 4á2
T5 210á1 90á3 306á7 153á8 4á5 2á3 6á8 3á7
T6 230á1 125á6 338á3 92á8 6á0 3á6 7á9 3á4
T7 219á3 83á5 368á0 117á6 5á5 2á7 8á6 3á4
P* < 0á0001 < 0á0001 < 0á0001 < 0á0001

*Repeated measures analysis of variance from T1 to T7.

Table 6. Comparison of occlusal

T7 Control
force (OcFr) and occlusal contact
Mean s.d. Mean s.d. P area (OcAr) between T7 and control
group
Occlusal force (N)
Total OcFr 669á3 133á8 529á1 255á0 NS
Molar OcFr 587á3 150á6 431á1 236á3 NS
Premolar OcFr 68á2 49á4 65á5 48á7 NS
Anterior OcFr 17á8 20á6 32á5 21á8 NS
First molar OcFr 219á3 83á5 196á8 118á5 NS
Second molar OcFr 368á0 117á6 234á3 150á0 < 0á05
Occlusal contact area (mm2)
Total OcAr 15á1 4á6 13á1 6á0 NS
Molar OcAr 14á1 4á5 10á0 5á7 NS
Premolar OcAr 1á4 1á1 1á6 1á2 NS
Anterior OcAr 0á4 0á5 1á5 1á0 NS
First molar OcAr 5á5 2á7 4á5 3á2 NS
Second molar OcAr 8á6 3á4 5á5 3á5 < 0á05

NS: Not signi®cant.

ã 2002 Blackwell Science Ltd, Journal of Oral Rehabilitation 29; 484±491

 CHANGES IN OCCLUSAL FORCE AND CONTACT AREA 489

Fig. 1. Distribution of occlusal force and occlusal contact area in dental arch.

ã 2002 Blackwell Science Ltd, Journal of Oral Rehabilitation 29; 484±491

490 M . H . S U L T A N A et al.

& Sadowsky, 1986; Razdolsky & Sadowsky, 1989). tissue and continuous vertical eruption over the life of
These researchers showed that the number of occlu- the tooth (Sicher, 1965). Therefore, OcFr and OcAr in
sal contacts increased 1á26% at 3 months (Durbin the molar region increase with stabilization of the
& Sadowsky, 1986) and 56% at 1 year (Gazit & periodontium (Reitan, 1959, 1967, 1969) and adapta-
Lieberman, 1985) over those contacts at setting of the tion of craniofacial morphology (Hassan et al., 1997)
retainer. Concerning the long-term changes in occlusal during retention, leading to an ideal distribution of
contacts, Razdolsky and Sadowsky (1989) reported that OcFr and OcAr.
continued settling of occlusion occurred until Concerning the changes in OcFr and OcAr in the
21 months after setting retainer. The ®ndings of these premolar regions, as there were no changes during the
studies support the clinical observations of practitioners retention period and after retention, elongation and/or
that the interdigitation of the teeth generally improves eruption in the premolar region after active orthodontic
after the removal of orthodontic appliances and the treatment should not be expected to occur. This
teeth are allowed to settle. suggests the importance of establishing occlusal contact
On the other hand, it has also been reported that in the premolar region through active orthodontic
OcFr is related to craniofacial morphology (Hassan treatment.
et al., 1997; Yamada et al., 2000), and OcFr has been As for the anterior region, the ratios of OcFr and
used as an evaluation of jaw function. Investigating OcAr in the anterior region to those in the total dental
changes in OcFr and OcAr during retention could be arch decreased to 2á7% after removing retainer, which
meaningful in terms of examining how the increase in were close to the ratios (1á5%) in Japanese females with
number of occlusal contacts previously reported is normal occlusion (Hassan et al., 1997). This might be
related to occlusal function (Yamada et al., 2000). because of the upper and lower anterior teeth kept by
However, concerning these changes in OcFr and retainer having moved to a balanced position in the
OcAr during retention, although Nakajima et al. (1994) total dental arch. However, the ratio of OcFr and OcAr
examined three cases over a period of 3 months after in the anterior region was higher in the control sample
setting retainer and reported that more change than in the treated group, which might be due to an
occurred at 1 month after setting retainer, changes imbalance in OcFr and OcAr caused by malocclusion.
over the long-term have not yet been reported on and As for differences in OcFr and OcAr between
details of variation in OcFr and OcAr during retention untreated and treated subjects (during and after retent-
have remained unclear. ion), although the present study did not do a full
The present study showed that OcFr and OcAr in the longitudinal comparison for OcFr and OcAr between
total dental arch signi®cantly increased during retent- both these two groups, Throckmorton et al. (1996)
ion, with signi®cant increases in OcFr and OcAr in the reported that OcFr and OcAr did not vary greatly over
molar region, especially the second molar, which time in adult females with Class I malocclusion.
supported previous reports (Gazit & Lieberman, 1985; However, in the present study, a cross-sectional com-
Razdolsky & Sadowsky, 1989) which showed an parisons between the control sample and the treated
increase in number of occlusal contacts in the molar groups (at 1 year after removing retainer) revealed that
region from 6 months after setting retainer, the distri- OcFr and OcAr in the treated group were larger than
bution of OcFr and OcAr showed the same pattern as in those in the control sample and close to those of
normal occlusion (Hassan et al., 1997), in which the Japanese female subjects with normal occlusion. These
OcFr and OcAr of individual teeth increases posteriorly. results suggested that active orthodontic treatment
Reorganization of the periodontal ligaments occurs might bring functional, balanced OcFr and OcAr
over a 3- to 4-month period after treatment (Reitan, (Roth, 1981; McHorris, 1979; Beyron, 1964; McNamara
1959, 1967, 1969), whereas the gingival collagen-®bre & Henry, 1974) as well as an improvement of esthetics
network typically takes 4±6 months to be remodelled; and in the number of occlusal contacts.
the elastic supracrestal ®bres remain deviated for more Orthodontic diagnosis of occlusion is mainly mor-
than 232 days (Reitan, 1967). It seems that molars, phologic and static, and is not directly related to normal
especially the second molar which is discluded by active function or functional disorders. Measuring OcFr and
orthodontic treatment, might settle after the bands are OcAr as the present study may serve as a permanent
removed, with remodelling of periodontal and gingival pre- and post-treatment record of occlusal functional

ã 2002 Blackwell Science Ltd, Journal of Oral Rehabilitation 29; 484±491

 CHANGES IN OCCLUSAL FORCE AND CONTACT AREA 491

relationship, and may be useful for assessing functional RAZDOLSKY , B.Y. & SADOWSKY , C. (1989) Occlusal contact
change after active orthodontic treatment. following orthodontic treatment: a follow-up study. Angle
Orthodontist, 59, 181.
REITAN , K. (1959) Tissue rearrangement during the retention of
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