Integrated versus Parallel Interlock Designs of an Extra-Coronal Attachment Retained Distal Extension Hybrid Partial Prosthesis: Radiographic Study

Running head: Integrated versus Parallel Interlock Designs of an Extra-Coronal Attachment

Objective: Different techniques and materials have been used to fabricate removable partial prosthesis for patients with distal extension ridges but with barely satisfactory aesthetic and functional results. This study has been done to compare radiographically between parallel and integrated interlock designs of reciprocation of a semi-precision extra-coronal attachment removable partial prosthesis (RPP).

Material and methods: 14 patients were randomly allocated. Each group consisted of 7 patients RPP with one of the two types of attachments was constructed and evaluated.

Group (I) received integrated interlock type of reciprocation. Group (II) parallel interlock type of reciprocation. Cone beam computerized tomography was made at time of denture insertion, and at 6 and 9 months of denture use to assess marginal bone level around the crowned abutment teeth.

Results: Comparing the MBL values at the mesial and distal abutments within each group showed no statistical difference at time of delivery, as well as at 6 & 9 months after RPP use.

After 6 and 9 months of RPP use, group-1 showed statistically significant lower mean amounts of bone loss than group-2.

Conclusions: Distal extension attachment-retained RPP with integrated and parallel interlock designs of reciprocation are associated with an increase in bone loss, integrated interlock RPP design is associated with a lesser amount of bone loss than parallel interlock RPP design and it is preferable to use the attachment-retained RPP with integrated interlock instead of parallel interlock design.

Keywords: Distal Extension, Attachment, Hybrid Partial Prosthesis, Cone beam computerized tomography.

Introduction

Different techniques and materials have been used to fabricate removable partial prosthesis (RPP) for patients with distal extension ridges but with barely satisfactory aesthetic and functional results (1). When designing and constructing an RPP, numerous biological and mechanical problems concerning the principles of retention, bracing and support are faced. In distal extension RPP, underlying supporting structures are used together with the teeth to accomplish these biomechanics and the RPP should be carefully designed in order to protect them from damage (2). On the other hand, attachments are designed to replace the occlusal rests and conventional clasps. Their use in the construction of RPD improved both retention and aesthetics of the RPP (3), on condition of careful placement to permit effective retention and to prevent tooth movement (4). In addition to improving retention and aesthetics, the availability of attachments had also lead to a more flexible designing of RPP where many cases with retention and aesthetic challenges were solved with proper use of suitable attachment (5).

The fundamental requirement for a physiologic prosthesis is stress control. Numerous studies were still being directed to evaluate the effect of Kennedy’s class-I RPP on soft and hard supporting tissues. Stresses on the terminal abutments can be decreased by using extra-coronal resilient attachments that direct more loads on the edentulous ridge. The level of loading influences the amount of reduction (6). In some cases, the extra-coronal attachment offers aesthetic advantage that outweighs its biologic as well as its mechanical disadvantages (7).

This study has been done to compare radiographically between the parallel and the integrated interlock designs of reciprocation of a semi-precision extra-coronal attachment RPP due to deficiency in research on their effect on the abutments and alveolar bone.

Materials and methods

This comparative study was designed as a randomized clinical trial and followed the CONSORT 2010 (8). A convenient sample of fourteen patients were randomly picked up from the outpatient clinic, of Faculty of Dentistry, at Beirut Arab University. The design of this study was accepted by Institutional Review Board (IRB) committee at BAU.

The participating patients were those who refused an implant-retained RPP and accepted an RPP retained by extra-coronal attachments on two posterior abutments on each side of the arch and that were splinted together by crowns (9). These patients were informed about their rights and obligations in participating in this study and they signed a consent.

Participants were selected according to the following inclusion criteria:

Cooperative and well-motivated patients with good oral hygiene and having proper manual dexterity to handle an attachment-retained RPP (10,11). Patients with Kennedy’s Class-I mandibular ridges posterior to first or second premolar. Patients with Angle’s Class-I jaw relation, having enough number of maxillary teeth to maintain a stable occlusion. Crown-root ratio of the four abutments at least 1:1 (12) and width of the abutment teeth bucco-lingually at least 6 mm (13). The space between the edentulous ridge and opposing occlusal surfaces not less than 7 mm (13). Distance between the functional depth and the free gingival margin of mandibular anterior teeth at least 8 mm with no undercuts.

Exclusion criteria included:

Patients with an unstable systemic condition, such as untreated hypothyroidism uncontrolled diabetes, or a malignancy in mid-treatment .Patients with metabolic bone disease. Dental history of para-functional habits.

Presence of mobility of the abutment teeth of more than grade I (12).

Sample distribution:

According to the design of attachment-retained RPP, patients were distributed randomly into two groups: Group-1: Seven patients received an extra-coronal semi-precision attachment-retained mandibular Class-I RPP with integrated interlock design (Vario-Stud-Snap with shear distributor 1.7mm diameter)[footnoteRef:1] connected to splinted crowns on the two terminal abutments on each side (Fig 1). [1: Bredent company, GmbH & COKG.Weissenhorner Str.2.89250 Senden, Germany]

Group-2: Seven patients received an extra-coronal semi-precision attachment-retained mandibular Class-I RPP with parallel interlock design (Vario-Stud-Snap 1.7mm diameter)[footnoteRef:2] connected to splinted crowns on the two terminal abutments on each side with a reciprocal arm on the primary abutment (Fig 2). [2: Bredent company, GmbH & COKG.Weissenhorner Str.2.89250 Senden, Germany]

Clinical procedure

The terminal abutments (canine and first premolar or the premolars) were prepared for splinted porcelain fused to metal crowns, on both sides of mandibular arch, with sufficient occlusal clearance, and a heavy chamfer finish line of 1.2 mm width that was placed 0.5 mm sub-gingival. To produce an even and smooth finish line, retraction cord to enlarge the gingival sulcus was used. An impression with addition silicon[footnoteRef:3] was made for the prepared abutments by the two-step impression technique and was poured with type IV dental stone. Provisional restorations were made and cemented on the preparations to protect them. [3: Express; 3M ESPE]

On the produced master cast, splinted crowns wax pattern was fabricated. The castable plastic patrices of the attachments were connected to the distal surface of the wax pattern 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-2, a guiding plane was placed against the lingual surface of the primary abutment to accommodate a reciprocal arm and occlusal mini-rest components.

The crown patterns of the two groups were sprued, invested and casted. After trying the resultant copings in patient’s mouth, porcelain was applied to gain the required morphology and occlusion. The crowns were checked again intra-orally for proper margins and contact areas. Occlusion was refined and crowns were glazed. With the splinted crowns in place, selective pressure impression technique was done, using regular body addition silicon[footnoteRef:4]. The pick-up impression was made in a special tray after border 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 used as a major connector for both groups. Reciprocation in the RPP of group-1 was integrated within the design of attachment, while for group-2, it was gained from the lingual reciprocal arm within the design of the RPP. The wax pattern of RPP was casted in chrome cobalt alloy 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 selected, set up and tried in the patient’s mouth and the RPP was processed. Crowns were cemented on day of delivery of the attachment-retained RPP using Glass Ionomer Cement[footnoteRef:5] . Attachments were painted with a thin layer of petroleum jelly (Vaseline) in order to easily remove RPP that was seated during cementation to reduce occlusal errors. Patients were told not to remove the RPP for 24 hours to allow for complete setting of the cement. [4: 3M ESPE Monophase; 3M ESPE Dental, United States] [5: Ketac Cem 3M ESPE]

Radiographic evaluation:

Cone beam computerized tomography (CBCT) was made at time of denture insertion, and at 6 and 9 months of denture use to assess marginal bone level (MBL) around the crowned abutment teeth. For each tooth, the coronal and sagittal planes passing through the most coronal point and through the root apex were defined in the oblique slicing. Each tooth was oriented so that it was upright when visualized in these planes. The teeth were also visualized in the axial plane as well. (fig. 3). Following orientation, the distance from a determined reference point to the alveolar bone crest was measured. The determined reference point was the margin of the crown as viewed in coronal and sagittal planes after orientation of the teeth. Two measurements in each plane were taken for the four abutment teeth. Marginal bone level mesial and distal to every abutment tooth was measured in the coronal plane while buccal and lingual marginal

bone level was measured in the sagittal plane, (fig. 4) Data collected from radiographic readings was organized in tables and statistically analyzed within and in between the two types of attachments. Significance level was set at P ≤ 0.05. Statistics was done using the IBM® SPSS® Statistics Version 20 for Windows.

Results

Comparison of MBL between mesial and distal abutments in each group:

Comparing the MBL values at the mesial and distal abutments within each group showed no statistical difference at time of delivery, as well as at 6 & 9 months after RPP use. The mean of MBL was used for comparison between the two groups.

Comparison of MBL between the two groups:

Group-1 showed a significantly lower mean of MBL than group-2 during the whole time of the study. (Table 1)

Effect of time on MBL within each group:

In group-1 and in group-2, there was a significant increase in mean MBL after 6 and 9 months. (Table 2)

Comparison of amounts of bone loss between the two groups:

After 6 and 9 months of RPP use, group-1 showed statistically significant lower mean amounts of bone loss than group-2. (Table 3).

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. Recognizing the nature of the different tissues supporting an RPP and their behaviour is important for continuing success of the prosthesis. This difference as well as the functional load create great stresses on the teeth and the edentulous ridge supporting the prosthesis (14). Clinical researches in this field have been mainly done to evaluate the effect of the different components and designs on the stresses transmitted to the abutment teeth (15). This study was done to compare and evaluate the effect of two different reciprocation designs, the integrated and the parallel interlocks, in extra-coronal semi-precision attachment-retained RPP on conservation of periodontium of abutment teeth with distal extension bases. The selected participants had 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 (16). The crown root ratio of selected abutments was diagnosed radiographically to be not less than 1:1. In addition, abutments with mobility more than grade-1 were excluded from the study (12).

Abutments had proper height of clinical crown to receive an attachment as the occlusal plane cannot be placed superiorly to accommodate the attachment (17). 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 (18). Cone beam computed tomography (CBCT) was chosen because it provides detailed anatomy of teeth, bone, and the changes in the dento-alveolar area in multiple planes. Data can be oriented so that the patient’s anatomic features are realigned which helps in standardization of images. In addition to cursor-driven measurements that provide the clinician with dimensional assessment that are free from distortion and magnification (19). CBCT technology provides advantages over conventional CT scans for maxillofacial imaging where it has reduced acquisition times which is comparable to panoramic radiography, with lower effective radiation doses, and decreased financial burdens (20). 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 extra-coronal resilient-attachments induce favourable stress patterns with minimal force transmitted to the abutments (21). These results were supported by Kumar (2011) who agreed that the use of extra-coronal resilient-attachments reduce the forces that are directed to the underlying periodontal tissues (16). As compared to group-1, there was a significantly higher loss of marginal bone level around abutment teeth of group-2 where there was reciprocal arm incorporated with parallel interlock design. This result was in-agreement with Saito et al (2003), who demonstrated in their work that attachment-retained RPP where a bracing arm was designed, produced more stresses on the abutment teeth as compared to an attachment-retained RPP without a bracing arm. It seems logical to conclude that with lingual bracing arm, there was more rigidity in the system than without it which could be due to the structure of the extra-coronal attachment connecting the denture to the abutment teeth at the distal surface of the terminal abutment (21). These results could also be supported by Wang et al (2011) who compared the biomechanical effects of rigid and resilient extra-coronal attachments on abutment teeth and alveolar ridges in distal extension RPP by finite element analysis. They found that stress distribution was similar but the magnitude was different. They also found that the most movement occurred between the matrix and patrix was when loading the RPP in a bucco-lingual 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 to the abutment teeth. (6)

Mechanically the bracing arm occupied most of the clinical height 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 abutment, similar to a conventional clasp (Saito et al, 2003). 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 (22). 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. From the standpoint of leverage, the prosthodontists advised the location of any component of RPP near to the center of rotation of the teeth. Grasso and Miller (1991) pointed out that the conventional clasps are more stressful to the abutments than the attachments. The base for this justification is that attachment is placed at the proximal surface of the abutment while the retentive clasp arm is placed on the facial or lingual surface, therefore the stresses run through the long axis and are resisted by most of the periodontal ligament fibers. Stresses directed in this manner are concentrated closer to the center of rotation of the abutment as compared to those with a conventional clasp (23). Moreover, the extra-coronal 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 abutment (24) which explains the clinically acceptable MBL level in the two groups. Reciprocation that is integrated in attachment design, includes a shear distributor within the vicinity of the attachment. This sheer distributes leverage through the long axis of the abutment omitting the need for a reciprocal arm. The integrated interlock design of reciprocation can restrain the lateral forces that have adverse effects on the terminal abutment in class-I RPP with universal hinge attachments (6). One of the advantages of integrated interlock is that it allows some indirect retention and simpler path of insertion. moreover, the location of integrated interlock nearer to the long axis of abutment tooth reduces the torque. Splinting of the abutments resists only antero-posterior forces. Saddles of the RPP being attached on both sides of the arch and connected with a lingual bar, can provide cross arch stabilization against forces acting in a bucco-lingual direction.

This explanation could be supported by the results of the present study which showed the lingual bracing arm was not essential for reciprocation in attachment-retained RPP and it negatively affected the health of abutment teeth while the integrated interlock design provided the necessary reciprocation without affecting the gingival and periodontal health of the abutments.

Conclusion

Within the limitations of this work, the following was deduced:

1- Distal extension attachment-retained RPP with integrated and parallel interlock designs of reciprocation are associated with an increase in bone loss.

2- Integrated interlock RPP design is associated with a lesser amount of bone loss than parallel interlock RPP design.

3- It is preferable to use the attachment-retained RPP with integrated interlock instead of parallel interlock design

Funding

No funding was received for this research.

Declaration of Competing Interest

The authors have no conflict of interest to declare.

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Figure legends:

Fig 1: Integrated interlock design for group-1

Fig 2: Parallel interlock design for group-2

Fig 3: Adjustment of coronal and sagittal planes of the abutment before measuring marginal bone level.

Fig 4: Measurement of the marginal bone level of the abutment tooth in sagittal plane at buccal and lingual surfaces and in coronal plane at mesial and distal surfaces

Tables :

Table 1: Bar chart demonstrating mean MBL in mm between the two groups

Table 2: Line chart demonstrating mean MBL in mm at different time periods for each group

Table 3: Bar chart demonstrating mean amounts of bone loss in mm between the two groups

Fig 2: Parallel interlock design for group-2

Fig 4: Measurement of the marginal bone level of the abutment tooth in sagittal plane at buccal and lingual surfaces and in coronal plane at mesial and distal surfaces.

Fig 3: Adjustment of coronal and sagittal planes of the abutment before measuring marginal bone level.

Fig 1: Integrated interlock design for group -1