Existing in vitro methods for testing denture adhesives do not fully replicate the complex oral geometries and environment; and in vivo methods are qualitative, prone to bias and not easily reproducible. The purpose of this study was to develop a novel, quantitative and more accurate model to test the effect of adhesives on the retentive force of mandibular free end saddle partial dentures.
An in vitro model was developed based on an anatomically accurate cast of a clinical case. Experimentally, the amount of adhesive was varied (0.2g–1 g) and the tensile force required for displacement was measured. Different commercially available adhesives were then tested at the optimum volume using the in vitro model. A 3D finite element model of the denture was used to assess how the forces to induce denture displacement varied according to the position of the force along the saddle length.
The mass of adhesive was found to significantly alter retention forces, with 0.4–0.7 g being the optimum range for this particular scenario. Use of adhesives significantly improved mandibular free end saddle partial denture retention with the worst performing adhesive increasing retention nine-fold whilst the best performing adhesive increased retention twenty three-fold. The finite element model revealed that 77% more force was required to displace the denture by positioning forces towards the mesial end of the saddle compared to the distal end.
An in vitro denture adhesive model was developed, which demonstrated that mass of adhesive plays a significant role in enhancing denture retention and supported the design principle of placing as few teeth as clinically necessary on the distal end of the free end saddles.
Limiting the position of teeth on free end saddles to the mesial and mid portion of the saddle will reduce displacements caused by mastication. The movement of mandibular free end saddle partial dentures can be restricted with the use of denture adhesives. Altering the mass of adhesive used can further improve the retention of mandibular free end saddle partial dentures for patients.
The ability of a denture to resist vertical displacement away from tissues, also known as denture retention, is an important factor for the success of the prosthesis. Excessive movement, induced by vertical tensile forces, can cause pain, tissue damage and discomfort to the denture wearer. A review by Darvell et al. highlighted the importance of surface tension, viscosity, time, base adaptation, border seal, seating force and soft tissue quality in the retention of dentures, whilst other factors, such as atmospheric pressure vacuum, adhesion, cohesion, wettability, surface roughness, gravity and muscular control were not considered important in retention. Previous studies however have highlighted the benefits that denture adhesives have in preventing the movement and enhancing the retention of complete dentures .
Denture adhesives have been available since the 1930s . These materials were marketed exclusively to complete denture wearers to increase their confidence in wearing dentures by improving the level and consistency of retention. The 2009 Adult Dental Health Survey showed that the number of partially endentulous patients is increasing in the UK and this is leading to an increase in the number of people wearing partial dentures. A 10 year clinical evaluation of removable partial dentures however highlighted a high failure rate of partial dentures ranging from 66.7% for clasp-retained removable partial dentures to 33.3% for conical crown-retained dentures . Furthermore a patient satisfaction study on partial removable dentures demonstrated the most frequently encountered complication in association with partial removable dentures is the loss of retention, causing dissatisfaction of patients related to chewing ability . This recent increase in denture wearers and patient dissatisfaction has led to adhesive manufacturers releasing products specifically aimed at partial denture wearers.
The subject of denture retention has been the focus of a number of in vitro studies and in vivo clinical investigations . In vitro methods for testing denture adhesives however are over simplified and do not fully replicate the complex in vivo geometries and environment, which may lead to inaccurate predictions of clinical performance. Similarly in vivo testing employs performance indexes ( e.g. Kapur Index) and functional tests which are qualitative, prone to bias and not easily reproducible.
Previous literature has focused on investigating retention of complete dentures and in particular maxillary complete dentures . It is only recently that mandibular denture retention has been investigated . This was a logical step in light of recent evidence that a high percentage of patients are dissatisfied with the retention of their mandibular complete denture . Given the recent shift in marketing focus of denture adhesives towards partial dentures, it is vital to investigate whether any significant retention can be gained from their use.
The cases that suffer from the most problems with support and retention are removable partial dentures . This is due to the differences in compression of the mucosa and the periodontal ligament and the differences in retention forces between the clasp on the abutment and the peripheral seal around the saddle that allows rotation around the distal abutment tooth when forces away from the tissue are applied. Currently, adhesive manufacturers have limited quantifiable methods or evidence to estimate how effective their adhesive products could be for the retention of mandibular free end saddle partial dentures.
When planning to restore a mandibular free end saddle it is important to consider a shortened dental arch (SDA). SDA therapy will not be suitable for all patients and case selection is important because failure can compromise future treatment options . If the patient does require a removable partial denture, maximum retention can be achieved by using a tooth and mucosa borne denture design including an RPI system (mesial rest, distolingual guide plate, I-bar) and indirect retention where applicable . In cases where retention is not optimal, the free end saddle has a tendency to drift and rotate around the implant/abutment causing discomfort and potentially injuring soft tissues . There is evidence to suggest that 20% of free end saddle partial denture wearers are dissatisfied due to comfort and functional problems and in some cases this has led to patients not using the dentures over prolonged periods .
An argument could be made that, in a worst case scenario situation which combines unfavourable anatomy, poor ridge form and decreased neuromuscular control of the patient denture, an adhesive may prove to be an acceptable solution for the patient and clinician.
Denture adhesives are made up of synthetic polymers, antibacterial agents, preservatives, fillers, wetting agents and flavouring agents . The synthetic polymers hydrate when they come into contact with saliva. This increases their volume which helps to fill the voids between the denture and mucosal tissues. The difference in viscosity between the hydrated polymer and saliva helps to increase the denture’s retention . The synthetic polymers also form molecular cross-links which increase the cohesive forces within the adhesive . Denture adhesives are potentially less effective in poorly fitting dentures because these dentures will require a thicker layer of adhesive to fill the void between the denture and mucosal tissues and this, in turn, will allow a faster ingress of saliva and air .
The aim of this study was to develop a novel, quantitative and more accurate model to test the effect of adhesives on the retentive force of removable partial dentures. This will be achieved by: assessing if mass of adhesive plays a role in retention of mandibular free end saddle partial dentures; assessing how effective different commercially available denture adhesives are at improving the retention of these dentures; and establishing the tensile load required to induce a 1 mm vertical displacement from the ridge at different positions along the saddle. Therefore the null hypotheses are: the mass of adhesive will not affect vertical displacement force; the use of commercially available adhesives will make no significant difference to the vertical force required to displace the mandibular free end saddle from the ridge; and the tensile load to induce a 1 mm vertical displacement will not significantly vary as the position along the saddle length changes.
Materials and methods
A clinical case with the minimum level of retention was chosen (Kennedy’s class II), where there was only one guide plane and a negligible amount of undercut on the abutment tooth. For this study, no direct retention (clasp) was employed to simplify the model and to be able to measure the effect of the adhesive on retention alone. The denture framework used was composed of acrylic only, with no cobalt chrome frame. This allowed the results to estimate the percentage improvement in retention that adhesives can potentially achieve in this worst-case scenario situation. The clinical case mandible master impression was poured using Labstone Dental Stone (Dentsply, Surrey, UK, Fig. 1 ). The cast was duplicated to avoid wear-related issues arising from repeated testing and cleaning protocols. The mandibular free end saddle baseplate was constructed from Selectaplus standard acrylic (Dentsply, Surrey, UK). A displacement loop was placed into the mid-point of the free end saddle area in order to connect the saddle to the displacing force ( Fig. 1 ). Two more identical mandibular free end saddle baseplates were constructed with displacement loops at the mesial and distal ends of the saddle.
To assess the mechanical retention force, a Lloyd LF-Plus materials testing machine (Ametek, West Sussex, UK) was used to apply a vertical tensile force at a cross-head speed of 50 mm/min. Previous literature has determined that this is approximately the speed which occurs clinically in normal function and is the speed widely employed by other studies allowing for direct comparisons to be made with other results . The vertical tensile force was applied to the baseplate through rigid steel wires connected to the displacement loop ( Fig. 2 ). Rigid steel wires ensured negligible elastic deformation when loading the baseplate. In this experiment the force was placed at the mid-point of the saddle.
To determine whether adhesive mass had an effect on denture retention, the displacement force was measure when using 0.2–1.0 g of Polygrip ® adhesive (GlaxoSmithKline, Waterford, Ireland). The starting amount was approximately equal to that indicated by the visual representation on the manufactures instructions (0.2 g). To simulate the oral environment, 1 mL of water was applied evenly to the free end saddle area of the cast prior to applying the adhesive according to the manufacturer’s instructions. The baseplate was pressed into place with even pressure for 10 s; this force was not measured but was applied by the same operator each time. The adhesive was then left for a further 5 min before the displacement force was applied. Sample size was estimated using a power calculation (Eq. (1) ), where SD was the standard deviation obtained from a pilot study testing five samples using the manufacturer’s recommended amount (Polygrip ® adhesive, 0.2 g) of adhesive (±2.3 N); Z α/2 is 1.96 arising from a 95% confidence of avoiding a type I error (based on Z-table); Z φ is 0.842 corresponding to 80% power (based on Z-table); and d, the effect size, is 3N (considered a clinically significant change in retention force based on a study by Kumar et al. ). Based on this calculation, ten measurements were taken for each mass of adhesive and the mean values calculated to establish statistically significant differences.
Sample size = 2 × S D 2 ( Z α / 2 + Z φ ) 2 d 2
Vertical retention was assessed for 5 different experimental groups; the control (without adhesive) and four different commercially available adhesives ( Table 1 ). For the control group the baseplate and cast were soaked in water for 10 min and an additional 1 mL of water was applied to the free end saddle area prior to firmly attaching the baseplate into place and loading. The following methodology was used for each of the 4 adhesives. In order to simulate the oral environment the optimum mass of adhesive (0.6 g) was premixed with half its weight in water and left for 5 min before being applied to the baseplate. One mL of water was applied evenly to the cast’s free end saddle area. The baseplate was pressed into place with an even pressure for 10 s. The adhesive was then left for a further 5 min before the displacement force was applied.
|Polygrip ® for Partials||Paste||GlaxoSmithKline, Stafford Miller Ltd, Dungarvan Co. Waterford, Ireland)||Calcium/Sodium, PVM/MA copolymer, Petrolatum, Cellulose Gum, Paraffinum, Liquidum|
|Polygrip ® Ultra||Paste||GlaxoSmithKline, Stafford Miller Ltd, Dungarvan Co. Waterford, Ireland)||Calcium/Sodium, PVM/MA copolymer, Petrolatum, Cellulose Gum, Paraffinum, Liquidum, Aroma, Cl 45430|
|Fixodent Neutral Taste||Paste||Procter and Gamble UK, Weybridge, Surrey, KT13 OXP||Calcium/Zinc, PVM/MA copolymer Petrolatum, Cellulose Gum, Paraffinum, Liquidum, Silica|
|Boots Smile||Paste||The Boots company PLC Nottingham England NC2 3AA||Calcium/sodium, PVM/MA copolymer, liquid paraffin, white soft paraffin, cellulose gum, purified water, Flavour, Bisabolol, BHT, Cl 73360|