A cross-over Randomised Controlled Trial of selectivepressure impressions for lower complete dentures

T.P. Hyde a,*, H.L. Craddock a, A. Blance b, P.A. Brunton a

aOperative Dentistry, Leeds Dental Institute, Clarendon Way, University of Leeds, Leeds LS2 9LU, United KingdombDivision of Biostatistics, Leeds Institute of Genetics, Health & Therapeutics, Clarendon Way, University of Leeds, Leeds LS2 9JT,

United Kingdom

j o u r n a l o f d e n t i s t r y 3 8 ( 2 0 1 0 ) 8 5 3 – 8 5 8

a r t i c l e i n f o

Article history:

Received 11 May 2010

Received in revised form

7 July 2010

Accepted 8 July 2010

Keywords:

Randomised

Trial

Denture

Impression

RCT

a b s t r a c t

Objectives: The objective of the research was to assess patient preference for a selective

pressure impression technique for complete dentures by means of a Randomised Controlled

Clinical Trial (RCT).

Methods: A literature review revealed no reported RCT’s of selective pressure impressions

for complete dentures. A cross-over, randomised, controlled, clinical trial was performed

comparing selective pressure impressions with a placebo and an alternative method of re-

distributing pressure. A sample size calculation yielded 65 patients. Sixty-nine patients, who

had a superficial mental foramen on the lower denture bearing area, were recruited for the

study. The primary outcome was the patients’ preference of the dentures provided. The

hypothesis under investigation was that one of the dentures would be preferred by the

patients.

Results: Sixty-six participants completed the trial, 33 (50%) preferred the denture from the

selective pressure impression, 19 (29%) the denture with traditional pressure relief, and 14

(21%) the placebo control. 95% confidence intervals showed that the preference for the

selective pressure impression was greater than that of the other two techniques. The null

hypothesis was rejected.

Conclusions: The participants in this trial showed a preference for the denture constructed

from the selective pressure impression technique, providing dentists with useful evidence

for a clinical impression technique.

# 2010 Elsevier Ltd. All rights reserved.

avai lab le at www.sc iencedi rec t .com

journal homepage: www.intl.elsevierhealth.com/journals/jden

1. Introduction

The dental academic literature has many papers, case

histories and textbooks written on the subject of impressions

for complete dentures. Four broad concepts for impressions

have been described: Muco-displacing,1 Mucostatic,2,3 Selec-

tive Pressure4–8 and Functional9,10 Impressions. These im-

pression techniques have been suggested and developed to

record the denture bearing tissues in various states of

displacement, and to manage particular clinical situations.

* Corresponding author. Tel.: +44 0113 343 8515; fax: +44 0113 343 623E-mail address: T.P.Hyde@Leeds.ac.uk (T.P. Hyde).

0300-5712/$ – see front matter # 2010 Elsevier Ltd. All rights reserveddoi:10.1016/j.jdent.2010.07.003

The different states of tissue displacement are said to be

achieved by variations in pressure within the impression. The

evidence for pressure variation with impressions comes from

the laboratory experiments by Frank,11 Komiyama,12 Masri,13

Al-Ahmad14 and Hyde15 who have all have shown variation in

the pressure of impressions in vitro. However, the level of in

vivo evidence and in particular the clinical evidence for

particular impression techniques is poor and needs careful

consideration.

Within the prosthodontic literature, there is a paucity of

high quality evidence in the form of Randomised Controlled

6.

.

Fig. 1 – Resorption of the mandible in trial patients. (Left) The typical clinical appearance and (Right) the typical radiological

appearance.

Fig. 2 – Traditional relief for the mental foramen obtained

by the use of metal foil spacers.

j o u r n a l o f d e n t i s t r y 3 8 ( 2 0 1 0 ) 8 5 3 – 8 5 8854

Trials (RCTs) for any specific impression technique.16,17 There

have been only two reported Randomised Controlled Trials, by

Firtell18 and McCord,19 which have investigated clinical

impressions for complete dentures. Neither study investigated

‘selective pressure’ impressions. Firtell’s18 study found no

evidence of a statistically significant preference for an

impression material. McCord reported problems with rando-

misation in his study and with just 11 patients looking at three

impression techniques the study was underpowered. The

evidence for the clinical benefits of impressions for complete

dentures is therefore based on expert opinion backed by the

anecdotal evidence of case histories. These form a lower level

of evidence than an RCT. One such case history was presented

by Hyde.5 He purported to show how to distribute pressure in a

‘selective pressure’ impression using modern materials. Since

there have been no previously reported RCTs of ‘selective

pressure’ impressions for complete dentures, a Randomised

Controlled Trial was indicated.

Before an RCT of selective pressure impressions could be

carried out, it became necessary to identify a relatively

homogeneous group of patients, who would benefit from a

selective pressure impression technique. There are particular

problems providing dentures for patients who have been

edentulous for many years. These problems are associated

with severe alveolar bone resorption20 (see Fig. 1). In some

patients, as the mandible resorbes down to basal bone the

mental foramen becomes involved in the denture bearing

area.21 The subsequent crushing of the emerging nerve by a

functioning lower denture can cause pain and discomfort.22

These patients may benefit from the use of a selective pressure

impression which reduces impression pressure in the area of

the mental foramen and ultimately reduces the pressure from

occlusal load under the subsequent denture in this area.

Experience at Leeds Dental Institute from an informal audit of

common clinical problems in complete denture patients had

suggested that the problem of the mental foramen on the

denture bearing area is under diagnosed.

The McGill consensus23 and York statement24 reflect the

current ‘gold standard’ in the care of edentulous patients.

They state that the minimum acceptable standard of care for

the treatment of the edentulous mandible should be the

provision of Osseo-integrated dental implants to support and

retain the complete lower denture. This approach was fully

supported by the clinical team (with co-authorship to the York

statement). However, there are barriers to treatment with

implants; notably cost and surgical risk.25,26 Patients who have

a mental foramen within the denture bearing area, necessarily

have very little height to their mandible. This lack of bone

would usually indicate bone enhancement prior to implant

provision. Medical and social problems may contraindicate

autogenous bone transplants for many of the more frail older

patients. Others may refuse because they do not accept the

cost25 or surgical risk.26 This leaves a cohort of patients for

whom implant treatment is contraindicated or refused; in

these patients, conventional complete dentures remain the

best option for treatment.

If conventional dentures are to be provided; a traditional

way of relieving pressure over the mental foramen has been

advocated in the literature.27 The traditional method involves

placing a metallic foil on the working cast, over the area of the

mental foramen (Fig. 2). The placing of the foil provides space

under the finished lower denture. The space created is said to

provide local relief from pressure on occlusal loading of the

finished dentures.

This study investigated the effectiveness of a selective

pressure impression15 against the traditional clinical method

of re-distributing pressure27 and a control denture from a

j o u r n a l o f d e n t i s t r y 3 8 ( 2 0 1 0 ) 8 5 3 – 8 5 8 855

relatively mucostatic standard impression (placebo). The study

aimed to provide evidence of best practice, as indicated by

patient preference, for clinical impressions in the treatment of

patients with the problem of a prominent mental foramen.

The null hypothesis was that none of the dentures would

be preferred by the patients. The working hypothesis was that

one of the dentures would be preferred by the patients.

2. Materials and methods

A sample size calculation based on the difference in propor-

tions of preferring the selective impression technique com-

pared to not preferring it, yielded a required sample size of 65.

Sixty-nine patients were recruited for the study. The trial was

conducted at Leeds Dental Institute, University of Leeds, UK.

Ethical approval was sought and obtained from the appropri-

ate Medical Ethical Committee. All participants gave written,

signed, informed consent. Participants were recruited be-

tween November 2006 and November 2008 from the waiting

lists at the Institute. The last assessment (a three monthly

follow up OHIP questionnaire) was completed and the trial

finished in June 2009. The inclusion criteria were subjects who

are able to attend, were edentulous in the lower arch, and had

the mental foramen apparent clinically or radiographically on

the denture bearing area of the lower ridge. Exclusion criteria

were subjects who are allergic to acrylic or silicone rubber.

Immediately after a patient had consented to treatment and

before treatment commenced, the patient was asked to

Fig. 3 – A selective pressure impression in preparation. (Top left)

scalpel and lifted out, (Top right) the acrylic custom impression t

with light bodied silicone (Express 3M) and (Bottom right) the fi

complete an OHIP-1428 questionnaire to identify a baseline

for the secondary outcome measure of the impact of new

dentures on their quality of life.

The research team at Leeds has developed a method of

producing lower dentures that are very similar to each other

apart from a single pre-determined difference.29 For this cross-

over study, three very similar lower dentures were produced for

each patient. The clinician, dental nurse and the participants

were blind to the identity of the dentures. The first denture was

a control, constructed ona castmadeforma standard, relatively

mucostatic, impression procedure. This standard impression

used a spaced acrylic custom tray and a medium bodied silicone

material (Express 3M) followed by a light bodied silicone wash

(Express 3M). The second denture was constructed by a

traditional method of dealing with problems of the mental

foramen. This traditional method used metal foils, placed over

the area of the mental foramina and processed the finished

denture on the spaced cast (Fig. 2). The foil used in this trial was

0.6 mm thick, manufactured by Minerva.

The third denture was constructed from a selective

pressure impression technique (the technique reported in

Hyde’s5 case history was adapted to relieve pressure over the

area of the mental foramen). The impression technique

incorporates altering the standard silicone impression by

cutting out the silicone from the area above the mental

foramen, perforating the impression tray in this area and

taking a light bodied silicone wash impression (Fig. 3).

The three dentures were given to each patient; the order in

which the dentures were assessed by the patient was

The silicone in the area of the mental foramen is cut with a

ray is perforated in this area, (Bottom left) the tray is loaded

nal impression.

j o u r n a l o f d e n t i s t r y 3 8 ( 2 0 1 0 ) 8 5 3 – 8 5 8856

determined by a blocked randomisation procedure. The order

the encoded dentures were to be worn was revealed on the day

of delivery of the first denture by a designated research nurse,

without the prior knowledge of the clinician. They wore each

denture for 1 week and assessed the denture individually.

When they had assessed each denture individually they were

given all three dentures together and asked to assess their

preferred denture over a period of 1–2 weeks. The stated

preference by the patient for a denture was the pre-

determined primary outcome measure for the study. All the

outcome assessments were performed by the patients; they

were recorded by the research nurse and the dentist remained

blind to the choice. After the patients had worn their preferred

denture for 3 months they were asked to complete a post

treatment OHIP-14 questionnaire to assess the impact of their

new dentures on their quality of life following treatment.

Multinomial logistic regression was used to analyse the

effect size and significance of possible confounding variables.

Subsequent analysis of patient preference was performed

using bootstrapped 95% confidence intervals.30

3. Results

Sixty-six patients completed the trial (Fig. 4). Three patients

failed to complete; one deceased, one whose spouse deceased,

and one who was unable to continue because he required

unrelated surgery. The CONSORT flow diagram (Fig. 4) reports

the numbers in each arm of the trial.

The primary outcome measure was patient preference for

the denture. Thirty-three patients (50%) choose the denture

from the selective pressure impression; 19 (29%) choose the

denture from the traditional method of relieving pressure on

the mental foramen; and 14 (21%) preferred the control denture.

The possible confounding variables of age, sex, order of

delivery of the dentures, initial OHIP score, and final OHIP

score were investigated by multinomial logistic regression. No

evidence of a sizable or important effect of these potential

Fig. 4 – CONSORT flow diagram for the cro

confounding variables was indicated. Therefore it was

considered unnecessary to adjust the model for these

variables. Consequently, bootstrapped 95% confidence inter-

vals were preferred to the Null model and were used for

analysis. Bootstrapped 95% confidence intervals were used to

analyse patient preference for

1. the selective pressure method over the control;

2. the selective pressure over the traditional foil relief;

3. the traditional foil relief over the control.

The difference in preference for the selective pressure

method over the control method was 29% with a 95%

confidence interval of (9, 47)%. The difference in preference

for the selective pressure over the traditional foil method was

21% and of borderline significance with a 95% confidence

interval of (�1, 47)%. The preference of the traditional method

over the control was not statistically significant.

When the participants wore the preferred denture for 3

months they reported an improved oral health related quality

of life via the OHIP questionnaire. The OHIP-14 score

decreased from 29 to 21.

4. Discussion

The null hypothesis was that none of the dentures would be

preferred by the patients. The results of this study show 95%

confidence intervals indicating preference for the dentures

constructed from the selective pressure impression. The null

hypothesis is therefore rejected. In rejecting the null hypoth-

esis, the alternative hypothesis is proposed that in this patient

group there was a preference for the denture made by the

selective pressure impression.

In addition to the statistical significance, it is important to

consider the clinical significance of these results. Patient

satisfaction is an important aim for complete denture

ss-over Randomised Controlled Trial.

j o u r n a l o f d e n t i s t r y 3 8 ( 2 0 1 0 ) 8 5 3 – 8 5 8 857

prosthodontics. The primary outcome of this trial, patient

preference, is closely related to patient opinion on satisfaction.

This trial presents results which show how a simple clinical

technique can significantly influence patient preference (and

related patient satisfaction) in a group of patients who have

been given a choice of three lower complete dentures. This

group of patients had difficult and multifaceted problems with

their lower dentures. It is clear that no single technique can be

the sole answer to these multifaceted problems. However, the

evidence for patient preference from this RCT is clear and

significant. Clinicians should consider adding this useful

impression technique to their prosthodontic repertoire. It is a

new technique for consideration when dealing with these

difficult clinical problems; the level of clinical evidence for the

impression technique from this RCT is strong.

The casts on which the three dentures were constructed

were labelled 1, 2 and 3 during the construction process. The

dentist constructing the dentures was initially blind to which

cast was which. Throughout the trial the dentist was not

informed which denture was which, but because the casts were

visibly different it was not possible to be certain the dentist

remained blind during the denture construction process of the

trial. This is a potential source of bias; this was anticipated and

safeguards put in place. The safeguards were threefold. Firstly,

the trial was designed so that the completed dentures were re-

coded and re-labelled (A–C). Followingthis recoding, thedentist,

research nurse and patients remained blind to this coding of the

processed dentures. Secondly, the patients, who preformed all

the assessments, were blind throughout the trial. Thirdly, the

methodology used in this trial to produce very similar

dentures29 deliberately reduces the chance that potentially

biased actions by the dentist can affect denture shape. It is

therefore considered unlikely that intentional or unintentional

bias could have been introduced by the dentist constructing the

dentures.

The denture produced from the selective pressure tech-

nique5 is preferred in this study. It is reasonable to postulate

that the designed selective pressure of the impression

translates into preferential loading under the resultant

denture, and explains the patient preference for that denture.

This study provides evidence from an RCT of a preference for

a particular impression technique for complete dentures in this

patient group. The evidence is statistically and clinically

significant. This is the first Randomised Controlled Trial to

provide such evidence. Given the history and size of the

academic literature for impressions for complete dentures this

is remarkable. The failure of previous RCT’s to produce a

positive outcome may be explained by the confounding

influence of the multitude of variables involved in denture

construction. Scientific methodology normally requires the

elimination or control of potential confounding variables. This

is a particular problem in prosthodontic research because of the

‘artistry’ of the discipline. In prosthodontic research the

application of the principles embodied in the scientific method

requires a particularly strict adherence to a disciplined protocol.

In the opinion of the authors there are three factors which have

been particularly important in providing a clear outcome for

this trial; firstly the methodology used in the duplication of the

shape and occlusion of the three trial dentures, secondly the

selection of a group of patients who all had a single specific

clinical condition and third, the use of a single operator. These

three factors reduced the confounding variables. Further, the

blocked random allocation process safeguards against any

unknown (or unexpected) confounding. The sample size was

sufficiently large to allow the detection of the difference that

exists within this sample. Finally in the opinion of the authors

the positive outcome of the trial was helped by the selection of

patient preference as the primary outcome. Patient preference

as an outcome measure was sufficiently sensitive and specific

in this study. Future RCTs in this area of study may benefit from

adopting similar strategies.

5. Conclusions

It is concluded that this particular patient group had a

preference for the selective pressure impression technique.

Thisadvocatestheselective pressure impressiontechnique and

provides dentists with good evidence for the clinical procedure.

Acknowledgment

This trial was funded by a grant from Dunhill Medical Trust.

r e f e r e n c e s

1. Fournet SC, Tuller CS. A revolutionary mechanical principleutilized to produce full lower dentures surpassing instability the best modern upper dentures. Journal of theAmerican Dental Association 1936;23:1028–30.

2. Page HL. Mucostatics—a capsule explanation. Chronicle of theOmaha District Dental Society 1951;14:195–6.

3. Addison PI. Mucostatic impressions. Journal of the AmericanDental Association 1944;31:941.

4. Boucher CO, Hickey JC, Zarb GA. Prosthodontic treatmentfor edentulous patients. 9th ed. St. Louis: Mosby; 1990. p.185–93, 229–42.

5. Hyde TP. Case report: differential pressure impressions forcomplete dentures. European Journal of Prosthodontic andRestorative Dentistry 2003;1:5–8.

6. Duncan JP, Raghavendra S, Taylor TD. A selective-pressureimpression technique for the edentulous maxilla. Journal ofProsthetic Dentistry 2004;92:299–301.

7. Weng BX, Khlevnoy V. Pressure control for completedenture impressions. Oral Health 1995;85. 21-3, 24 27.

8. Lynch CD, Allen PF. Management of the flabby ridge: usingcontemporary materials to solve an old problem. BritishDental Journal 2006;200:258–61.

9. Chase WW. Tissue conditioning utilizing dynamic adaptivestress. Journal of Prosthetic Dentistry 1961;11:804–15.

10. Vig RG. A modified chew-in and functional impressiontechnique. Journal of Prosthetic Dentistry 1964;14:214–20.

11. Frank RP. Analysis of pressures produced during maxillaryedentulous impression procedures. Journal of ProstheticDentistry 1969;22:400–13.

12. Komiyama O, Saeki H, Kawara M, Kobayashi K, Otake S.Effects of relief space and escape holes on pressurecharacteristics of maxillary edentulous impressions. Journalof Prosthetic Dentistry 2004;91:570–6.

13. Masri R, Driscoll CF, Burkhardt J, Von Fraunhofer A,Romberg E. Pressure generated on a simulated oral analog

j o u r n a l o f d e n t i s t r y 3 8 ( 2 0 1 0 ) 8 5 3 – 8 5 8858

by impression materials in custom trays of differentdesigns. Journal of Prosthodontics 2002;11:155–60.

14. Al-Ahmad A, Masri R, Driscoll CF, von Fraunhofer J,Romberg E. Pressure generated on a simulated mandibularoral analog by impression materials in custom trays ofdifferent design. Journal of Prosthodontics 2006;15:95–101.

15. Hyde TP, Craddock H, Brunton P. The effect of seatingvelocity on pressure within impressions. Journal of ProstheticDentistry 2008;100:384–9.

16. Harwood CL. The evidence base for current practices inprosthodontics. European Journal of Prosthodontic andRestorative Dentistry 2008;16:24–34.

17. Jokstad A, Esposito M, Coulthard P, Worthington HV. Thereporting of randomized controlled trials in prosthodontics.International Journal of Prosthodontics 2002;15:230–42.

18. Firtell DN, Koumjian JH. Mandibular complete dentureimpressions with fluid wax or polysulfide rubber: acomparative study. Journal of Prosthetic Dentistry 1992;67:801–4.

19. McCord JF, McNally LM, Smith PW, et al. Does the nature ofthe definitive impression material influence the outcome of(mandibular) complete dentures. European Journal ofProsthodontic and Restorative Dentistry 2005;13:105–8.

20. Budtz-Jørgensen E. Prosthodontics for the elderly: diagnosisand treatment. Chicago, London: Quintessence Pub. Co.;1999. p. 42.

21. Zarb GA, Bolender CL. Prosthodontic treatment foredentulous patients: complete dentures and implant-supported prostheses. 12th ed. St. Louis: Mosby; 2004.

22. Basker RM, Davenport JC. Prosthetic treatment of theedentulous patient. 4th ed. Oxford: Blackwell; 2002. p. 281.

23. Feine JS, Carlsson GE, Awad MA, Chehade A, Duncan WJ,Gizani S, et al. The McGill consensus statement onoverdentures. Montreal, Quebec, Canada, May 24–25.International Journal of Prosthodontics 2002;15:413–4.

24. Thomason JM, Feine J, Exley C, Moynihan P, Muller F, NaertI, et al. Mandibular two implant-supported overdentures asthe first choice standard of care for edentulous patients—the York Consensus Statement. British Dental Journal2009;207:185–6.

25. Esfandiari S, Lund JP, Penrod JR, Savard A, Thomason JM,Feine JS. Implant overdentures for edentulous elders: studyof patient preference. Gerodontology 2009;26:3–10.

26. Walton JN, MacEntee MI. Choosing or refusing oralimplants: a prospective study of edentulous volunteers for aclinical trial. International Journal of Prosthodontics2005;18:483–8.

27. McCord JF, Smith PJ, Grey N. Treatment of edentulouspatients. Edinburgh: Churchill Livingstone; 2004. p. 63.

28. Slade GD. Derivation and validation of a short-form oralhealth impact profile. Community Dentistry and OralEpidetniology 1997;25:284–90.

29. Dillon S, Hyde TP, Brunton PA. A technique to constructduplicate dentures for clinical research. Quintessence Journalof Dental Technology 2008;6:30–9.

30. Efron B, Tibshirani RJ. An introduction to the bootstrap. NewYork: Chapman & Hall; 1993.