Comparison between Distal Extension Attachment-retained Removable Partial Prostheses with Integrated and Conventional Reciprocation Designs: A Clinical Trial

INTRODUCTION

Removable partial prostheses (RPP) become the only choice when conventional or implant-supported fixed restorations are contraindicated as in case of long edentulous spans, difficulty to gain proper retention for a fixed restoration, insufficient number and/or unfavorable distribution of abutments as well as alveolar bone defects. There are main objectives to be achieved when planning dental treatments for partially edentulous patients in need of RPP: (1) restoring masticatory and speech functions, (2) restoring facial and dental appearances, and (3) preserving the remaining dentition with the supporting periodontium.

The masticatory function is fulfilled when correct occlusion with non-destructive harmonious articulation is provided by the newly restored dentition of the RPP. Proper speech is ensured with the correct design of the prosthesis as it relates to neighboring soft tissues.¹

Restoring appearance is often the primary request for these patients. Artificial teeth of compatible shade, shape, and size, as well as the correct positioning of these teeth, greatly improve dental esthetics. In addition, normal facial contours are only achieved by proper contouring and extension of the base of RPP. The objective of preserving the remaining supportive tissues and dentition is best fulfilled with proper mouth preparation, accurate designing and fabrication of RPP framework, proper tooth arrangement and correct size of denture base, periodic maintenance and follow-up, and continuous home care by the patient.¹

Kennedy’s classes I and II are the most problematic regarding the support of the RPP. They need special care when designing and restoring them. The support issue is problematic because of differences in resiliency between the teeth and the denture-bearing area. Such differences cause rotational movement at the distal extension base and produce stress on the terminal abutments and the alveolar ridge that eventually lead to progressive and irreversible resorption of alveolar ridges with adverse loading of abutments.^2–5

Treatment modalities other than conventional clasp-retained RPP are available for restoring distal extension bases with RPP. These modalities that aim to reduce stresses on residual ridge and terminal abutment teeth include attachment-retained and implant-retained or supported RPPs.^6–10

Conventional RPPs are barely accepted by patients due to either the unaesthetic appearance of clasps, improper design, poor retention, or manufacturing errors. Extracoronal attachments provide a successful alternative to clasps as it allows better retention and stability as well as superior esthetic appearance which renders attachment-retained RPP a considerable substitute for the conventional RPP. There is a vast diversity of extracoronal attachments that could be utilized for distal extension bases and that provide better stress distribution in addition to enhanced retention and high esthetics.^11,12

Prosthetic rehabilitation aims to improve patients’ quality of life and overall health and to preserve oral tissue.^13,14

Several techniques and methods which apply engineering and mechanical concepts to explain pathophysiological changes in the oral and maxillofacial complex are available and they permit stress and strain analysis which is important to understand the mechanics of oral components.^15,16

Clasps have many disadvantages as they deform in the long run, therefore affecting direct retention. They may also be subjected to failure due to fatigue under repeated loading. The unesthetic appearance of the clasp sometimes leads to placing it closer to the gingival margin. Utilizing deeper undercuts induces greater stresses on the abutments, in addition to abrasion lesions at the abutment cervix. Frictional scratching of the enamel of the abutments because of repeated removal and insertion of RPP was also reported, with deeper abrasion for more rigid materials.^17,18

Therefore, functional and esthetic demands for retainers possessing greater flexibility, in order to induce less stress with enhanced retention of the RPP are still great.¹⁹ An alternative rehabilitation option that achieves these demands is a RPP I combination with a fixed prosthesis joined by attachment. Attachment-retained RPP is viewed as a modern treatment modality for treating partially edentulous patients.^20,21

The retention force applied by the retentive element of RPP should be as nearest as possible to the abutment long axis and a reciprocal component of the RPP framework should oppose the retentive element. Such design is meant to withstand lateral forces applied on abutments by the retentive element.²² Rigid extracoronal attachments require deep understanding and awareness if they are to be utilized as they apply unfavorable stresses on abutments in distal extension bases, with similarity to horizontal cantilevers.²³ As well, they complicate oral hygiene and plaque control daily care by the patient.^24,25 On the other hand, resilient extracoronal attachments usually supply insufficient reciprocation owing to their resilient nature. So, they necessitate the integration of a reciprocation element in the RPP design.

This work aimed to understand the effect of integrated reciprocation in extracoronal A-RPP due to a deficiency of previously done research on their biomechanical effect and to compare it with the conventional design of parallel reciprocation.

MATERIALS AND METHODS

This research followed CONSORT 2010 and was planned as a randomized clinical trial.²⁶ Institutional Review Board (IRB) committee at Beirut Arab University approved the design of the research (Code: 2014-H-002-D-P-0011).

A convenient sample of fourteen patients was randomly picked up from the diagnosis clinic, in the School of Dental Medicine, at Beirut Arab University. The size of sample was calculated utilizing version 3.3.1 of the R statistical package. Proper size of the sample was revealed utilizing the T-test power calculations. Calculations for sample size were established on mean difference of 90% power and 5% two-sided level of significance with identical distribution to two arms.

The participating patients were those who attended in 2014 and refused an implant-retained RPP and accepted RPP retained by extracoronal attachments on two posterior teeth at each end of the arch that were splinted together by crowns.²⁷ These patients were informed about their rights and obligations in participating in this study and they signed a consent. Each patient was followed up for at least 9 months after the treatment was fulfilled.

Participants were chosen according to the following basis:

• Cooperative and well-motivated patients with good oral hygiene and proper manual dexterity to handle an attachment-retained RPP.²⁸

• Patients with bilateral distal extension bases posterior to 1st or 2nd premolars.

• Patients with Class-I jaw relationship.

• Patients having sufficient maxillary dentition to maintain stable occlusion.

• The four abutments have a crown-to-root ratio of at least 1:1.²⁹

• The width of the abutment teeth buccolingually at least 6 mm.³⁰

• The space between the edentulous ridge and opposing occlusal surfaces is more than 7 mm.³⁰

• The functional depth from the free gingival margin of mandibular anterior teeth is at least 8 mm with no undercuts.

CLINICAL PROCEDURE

The terminal abutments were prepared for splinted ceramo-metallic crowns on each side of the mandibular jaw with sufficient occlusal clearance and a deep chamfer finish line of 1.2 mm depth that was placed 0.5 mm subgingivally. To produce an even and smooth finish line, a retraction cord to enlarge the gingival sulcus was used. A two-step impression of the prepared teeth was made using A- silicone (Express, 3M ESPE, Minnesota, USA) and was poured with type IV dental stone. Provisional restorations were made and cemented on the preparations to protect them (Figs 2A and B).

On the produced cast, wax patterns for splinted crowns were fabricated. Castable plastic patrices of the attachments were connected to the proximal surfaces of the wax patterns as nearest as possible to the preparation, on the crest of the ridge, and parallel to the withdrawal and insertion path of RPP. For group II, a guiding plane was placed against the lingual aspect of the primary abutments to place a bracing arm and occlusal rest components.

The crown patterns of the two groups were sprued, invested and cast. After trying the resultant copings intraorally (Fig. 3), porcelain was applied to gain the required morphology and occlusal contacts. The crowns were re-examined intra-orally for proper margins and contact areas. Occlusion was refined and crowns were glazed.

The selective pressure impression technique was done over the splinted crowns, using monophase A-silicon (Hydrorise maxi monophase, Zhermack, Badia Polesine, Italy). The overall impression was done in a special tray after peripheral molding. The special tray had a 2–3 mm spacer over the teeth but was in close adaptation to the edentulous ridge.

RPP was waxed up on the refractory cast, the lingual bar was selected as a major connector in the two groups. Reciprocation in the RPP in group I was incorporated within the design of attachment, while in group II, it was gained from the lingual reciprocal arm within the design of the RPP. The wax pattern of RPP was cast in chrome cobalt alloy (Fig. 4) and checked for fit and passive insertion. Mounting the casts was done on semi-adjustable articulator by a face-bow and centric relation records. Semi-anatomic acrylic teeth were chosen, arranged, and tried intra-orally and then A-RPP was processed.

Crowns were cemented on the day of delivery of the attachment-retained RPP using GIC (Ketac Cem, 3M ESPE, Minnesota, USA). Attachments were painted with a fine layer of separating medium (Vaseline) for safe removal of the A-RPP that was seated during cementation to reduce occlusal errors. Participants were told not to take out the RPP for 24 hours to allow for the complete setting of the cement. They were given the following instructions:

• Brushing two times a day.

• Rub the attachments with gauze.

• Floss regularly with a super-floss between the splinted crowns.

• Clean the fitting side of A-RPP and the inner surface of attachments with non-abrasive toothpaste and brush.

• Come for monthly recall visits.

RESULTS

DISCUSSION

One of the major clinical problems facing the general practitioner is the choice and design of bilateral distal extension RPP. The equal distribution of forces is important to maintain healthy remaining alveolar ridges and abutment teeth and to provide the patient with improved comfort and function.³¹ Clinical research in this field have been mainly done to evaluate the influence of the different components and designs on the stresses directed to the abutment teeth.^32,33

This research was directed to compare and assess the influence of two different reciprocation designs, the integrated and the parallel interlocks, of extracoronal semi-precision A-RPP on the conservation of periodontium of abutment teeth with distal extension bases.

Extracoronal attachments were selected in this study because of their resilient attributes, so their alignment is not highly critical due to their motion in many planes. This provides the merit of multiple paths of insertion for the RPP.³⁴ Moreover, extracoronal attachment is a castable category with plastic retentive elements. Due to their elastic property, it is more feasible to control flexure and manufacture a shock-absorbing and resilient prosthesis.³⁰

Conventional sagittal ball attachment which was utilized for group II participants should always be utilized with a lingual bracing arm adapted on a milled shear distributor, to ascertain the optimum distribution of masticatory forces to the abutments. This design mandates deeper tooth preparations lingually to keep away from over-contouring of the splinted crowns.

On the other hand, reciprocation that was integrated within the attachment itself in the form of a built-in shear distributor that was used for group I patients does not require any additional reciprocation, therefore excluding the demand to make tooth preparation deeper on the lingual aspect and hence enabling tooth structure preservation.

In the current study, a red plastic female component (matrix) for high snap-in friction was used for all the cases. The retentive mechanism is based on a plastic female component (matrix) that sits in a metal housing.

The selected participants had maxillary natural dentition with Angle’s class-I jaw relation to avoid uncommon forces that might be present in class-II or III and might increase the torque on abutment teeth.³⁵

Mandibular class-I ridges were chosen for conducting this research instead of maxillary ridges as they present the lesser amount of support for RPP and are most critical to treat with satisfactory results. Patients with a firm covering mucosa on the edentulous ridges were chosen for better stability and support that they provide for the RPP. Ridges with displaceable flabby tissues allow undue movement of the denture base leading to stresses transmitted to the adjacent abutments.³⁶

The crown root ratio of selected abutments was diagnosed radiographically to be not less than 1:1. In addition, abutments with a mobility more than grade-1 were excluded from the study.²⁹ Abutments had a proper height of clinical crown to receive an attachment as the occlusal plane cannot be placed superiorly to accommodate the attachment. There should be enough length of attachments to keep maximum resistance to dislodgment and it should be placed as close as possible to the tooth to reduce the applied tipping forces.³⁷

In every A-RPP, specifically the distal extension prosthesis, splinting of the two terminal abutments on each side is recommended.³⁸ In this study, terminal teeth were joined with connected crowns at each edentulous span to provide better distribution of stress. The posterior splinted abutments selected on both sides were either two premolars or a canine and first premolar, to avoid splinting of all remaining mandibular teeth.

Cone-beam computed tomography was used because it permits full anatomy of teeth and bone and allows a detailed view in multiple planes of the changes occurring in the dentoalveolar area.³⁹ Teeth can be aligned so that the anatomic features are re-oriented in standard images. Moreover, CBCT provides the clinician with cursor-driven measurements for assessing dimensions that are free from magnification and distortion.⁴⁰

In this study, the two groups were associated with some amount of bone loss with no significant difference between the mesial and distal abutment teeth of both designs. This loss is clinically acceptable as extracoronal resilient attachments induce favorable stress patterns with the minimal force transmitted to the abutments.⁴¹ These results were supported by many authors who agreed that the use of extracoronal resilient attachments reduces the forces that are directed to the underlying periodontal tissues.^35,42

As compared to group I, there was a significantly higher loss of marginal bone level around the abutment teeth of group II where there was a reciprocal arm incorporated with a parallel interlock design. This result was supported by the work of Satio et al. who demonstrated in their work that A-RPP where a bracing arm was designed, elaborated more stresses on the abutments as compared to A-RPP that lacked a bracing arm. It appears logical to conclude that lingual bracing arms add rigidity in the system which could be due to the nature of extracoronal attachments joining the RPP to the abutment teeth at the proximal surface of the primary abutment.⁴¹

These results could also be supported by Wang et al. who weighed up the biomechanical influence of resilient and rigid extracoronal attachment on abutment teeth and alveolar ridges in distal extension RPP by finite element analysis.³¹ They found that stress distribution was similar, but the magnitude was different. They also found that the most movement that occurred between the matrix and patrix was when loading the RPP in a buccolingual direction. This could explain our finding that there is more stress associated with parallel interlock design on the abutments from its lingual bracing arm that resisted the movement of RPP in the bucco-lingual direction and placed more loads on the abutment teeth.

Mechanically the reciprocal arm occupied most of the clinical length of the primary abutment closest to its center of rotation. As a result, forces transmitted by the RPP will be passing through the long axis of the abutment, similar to a conventional clasp.⁴¹ On the other hand, the creation of parallel milled surfaces in crowns of abutment teeth, in conjunction with RPP that have intimate contact with these milled surfaces resulted in a controlled path of insertion and removal.⁴³ The greater increase in stability and resistance that was added to the rigidity of the parallel interlock design could explain the greater amount of marginal bone loss.

Moreover, the extracoronal attachment decreases the lever arm with respect to the length of the root as the support for RPP is placed nearer to the bony support of the abutment which explains the clinically acceptable MBL level in the two groups.⁴⁴

Reciprocation which is integrated into attachment design, incorporates a shear distributor within the vicinity of the attachment. This sheer distributes leverage through the long axis of the abutment omitting the demand for a reciprocal arm. The integrated reciprocation design can restrain the lateral forces that have adverse effects on the terminal abutment in class-I RPP with universal hinge attachments.³¹

One of the advantages of integrated reciprocation is that it allows some indirect retention and a simpler path of withdrawal and insertion. Moreover, integrated reciprocation that is positioned nearer to the abutment’s long axis reduces the torque.

Splinting of the abutments resists only anteroposterior forces. Saddles of the RPP is attached on each side of the arch and connected with a lingual bar provides cross-arch stabilization against stresses acting in a buccolingual direction. Such an explanation could be supported by the outcome of this research which showed the lingual bracing arms were not essential for reciprocation in A-RPP and it negatively affected the periodontium of abutments while the integrated reciprocation design provided the necessary bracing without affecting the gingival and periodontal health of the abutments.

Limitations of this research include the sample size and the relatively short period of follow-up.

CONCLUSION

After conducting this research, it was deduced that distal extension A-RPP with integrated and parallel reciprocation designs was accompanied by an acceptable increase in bone loss with time. Integrated reciprocation design was accompanied by a lesser amount of bone loss than parallel interlock RPP design. Therefore, it is preferable to use integrated reciprocation instead of parallel reciprocation design while treating partial edentulism with A-RPP.

RECOMMENDATIONS

After conducting this study, the following may be recommended:

• Longitudinal studies with larger sample sizes and extended follow-up periods of attachment retained RPD.

• Comparing the retentive force of both designs of reciprocation.

• Bone density measurement and studies to analyze stresses induced by the attachments around the abutment teeth and alveolar ridge by finite element.

• Microbiological studies of dental plaque to detect the quality of subgingival microflora associated with attachment retained RPD.

• Study the relationship of crown root ratio and abutment survival time with attachment retained RPD.

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