Photoelastic Analysis of Stress Patterns on Teeth and Bone With Attachment Retainers for Removable Partial Dentures

Photoelastic analysis of stress patterns on teeth and bone with attachment retainers for removable partial dentures

F. James Kratochvil, D.D.S.,* Wayne D. Thompson, D. D.S.,** and Angelo A. Caputo, Ph.D. *** University of California, School of Dentistry, Los Angeles, Calif., and Veterans Administration Medical Center, Long Beach, Calif.

T he two basic types of direct retainers used for extension removable partial dentures are the extracoronal and the intracoronal. The most commonly used extracoronal retainer designs have been reported in previous articles.2. 3 The purpose of this study was to evaluate attachment retainers. Photo- elastic analysis was used to compare forces resulting on supporting structures by three commonly used attachment retainers based on different design principles.

LITERATURE REVIEW

Descriptions and discussions of attachment retainers have been reported in the literature by many.*-*I Chayes12 devised the first internal attachment. It was basically the same as attachments currently manu- factured, such as Stemgold (APM-Stemgold, San Mateo, Calif.), McCollum (APM-Stemgold), Ney (J. M. Ney Co., Bloomfield, Conn.), and Baker (Engel- hard Industries, Baker Dental Division, Newark, NJ.).*-. 13. * In this study, the Stemgold type 7 precision attachment was tested.

Another design is a European concept developed by Hans Dalla Bona in Switzerland, and n&keted as the Dalbo attachment (APM-Stemgold).. lo. I. I5 It has been described and classified as an extracoronal stressbreaker by Mensor15 and others.O

Thompson*B described an attachment retainer which is intracoronal, semiprecision, and has been

Read hefore the Pacific Coast Society of Prosthodontists, Orcas Island, Wash.

*Professor and Director of Postgraduate Prosthodontics, Universi- ty of California, School of Dentistry, Los Angeks, Calif.

**Chief, Restorative Section, Veterans Administration Medical Center, Long Beach, CaIif.

***Professor and Chairman, Biomaterials Science Section, Uni- versity of California, School of Dentistry, Los Angeles, Calif.

THE JOURNAL OF PROSTHETIC DENTISTRY

q A fm# q IC

Fig. 1. Plastics used to form test model were type A, photoelastic plastic for entire tooth; type B, photoelastic plastic for periodontal ligament; and type C, photoelastic elastic for bone.

---

Fig. 2. Stemgold attachment type 7 is basic H-shaped slide attachment.

described in the literature by many authors.-** Koper2* stated, The versatility of design, combined with retention, stressbreaking features, and an effec- tive method of indirect retention make this semiprecision retainer the one of choice for mesial- and

21

KRATOCHVIL. THOMPSON, AND CAPUTO

Fig. 3. Dalbo MK attachment showing cross-section of adjustable female housing (with retention for resin), with steel coil spring which rests on ball portion of male attachment.

Fig. 4. Dalbo MK attachment with male portion soldered to distal abutment crown.

Fig. 3. Axis of rotation of Thompson dowel attachment used in study.

distal-extension removable partial dentures. Many authors through the years have suggested splinting two abutments for each extension base.8. y. 23 Dyke- ma et a1.24 wrote, . . . if an attachment is used to retain a partial denture with a free-end extension base (Class I or II), fixed splinting of the abutment

Fulcrum Line -__________________________________

Fig. 6. Relation to fulcrum line of receptacle and retention dimple of Thompson dowel. A, Distal view. B, Occlusal view.

Fig. 7. Bilateral lingual retention dimples of Thompson dowel are in line with rotation axis established by rest shelf.

teeth is even more important than for clasp retained partial dentures. . .

MATERIAL AND METHODS

A photoelastic model of a mandibular cast was fabricated to record and study the forces transmitted to supporting structures by three types of attachments used with distal-extension removable partial dentures. The mandibular model included the six anterior teeth and the first premolar on each side. Fabrication of the model duplicated the procedures used in previous projects which evaluated extracoronal types of removable partial dentures.*. 3 The teeth, including the roots, were formed with one type of plastic, type A (PLM-lZ, Photolastic, Inc., Mal- vern, Penn.). Periodontal ligaments were formed by coating the root surfaces with a second plastic, type B (Solithane, Thiokol Chemical Corp., Trenton, N.J.). The remainder of the model, which simulates bone, was of a third plastic, type C (PL-2, Photolas-

22 JULY 1981 VOLUME 46 NUMBER 1

PHOTOELASTIC ANALYSIS OF STRESS PATTERNS

Fig. 8. Occlusal view of Dalbo MK attachment with load bar in position for testing and gold crowns secured to abutment teeth with threaded screws.

tic, Inc.) (Fig. 1). The teeth, roots, and tissue contours of the model were of average size and shape. The periodontal ligament was formed to an approx- imate 0.2 mm thickness..

The coronal portions of both canines and premolars were prepared for complete crowns. The three attachment designs tested were the (1) Sterngold type 7, (2) Dalbo MK, and (3) Thompson dowel. Each type tested was attached to the first premolars bilaterally. The premolars and the canines were then splinted by soldering and the series of tests repeated.

Design I-Sterngold type i attachment (Fig. 2)

Complete gold crowns were fabricated on the first premolars with the female portions of the attachments positioned parallel to each other and to the distal aspect of the abutment teeth. A gold framework was constructed to which the male portions of the attachments were soldered.O The crowns were finished so that they fitted passively on the prepared abutment teeth. The removable partial denture attachments were adjusted until they could be placed and removed smoothly.

Design 2-Dalbo MK attachment (Fig. 3)

Complete gold crowns were waxed and cast for the first premolars. The Dalbo attachments were soldered to the distal portion of the crowns according to the manufacturers recommendations (Fig. 4). A gold framework was cast to join the denture bases bilaterally. The female portions of the Dalbo attachments were secured with acrylic resin (Duralay,

s FIBER OPTIC LIGHT SOURCE

P POLARIZATION

Q QUARTER WAVE PLATE - CIRCULAR POLARIZED LIGHT

A ANALYZER - PGLARUER M MODEL - BIREFRINGENT PLASTIC

Fig. 9. Schematic drawing illustrating position of light, lens, and camera utilized in testing.

Fig. 10. Test apparatus with photoelastic model and attachment in position for testing.

Reliance Dental Mfg. Co., Worth, Ill.) to the denture base in proper relationship to the male attachments as recommended by the manufacturer.

Design 3-Thompson dowel attachment (Fig. 5)

During fabrication of the complete gold crowns on the premolar abutment teeth, the receptacle portion was formed as shown in Fig. 6. Dimples for retentive retainers were placed into the lingual areas directly in line with the rotation axis established by the rest seat floorP (Fig. 7). The removable partial denture

THE JOURNAL OF PROSTHETIC DENTISTRY 23

KRATOCHVIL, THOMPSON, AND CAPUTO

IA

511

Fig. Ilk Photoelastic stress distribution resulting from tests of Sterngold attachment type 7 with single abutment. Fig. 1ZA. Photoeleastic stress distribution resulting from tests of Sterngold attachment type 7 with abutments splinted. Fig. 13A. Photoelastic stress distribution resulting from tests of Dalbo MK attachment with single abutment. Fig. 14A. Photoelastic stress distribution resulting from tests of Dalbo MK attachment with abutments splinted. Fig. 15A. Photoelastic stress distribution resulting from tests of Thompson dowel attachment with single abutment. Fig. 16A. Photoelastic stress distribution resulting from tests of Thompson dowel attachment with abutments splinted.

2A

4A

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118 12B

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148

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Fig. 1lB. Diagrammatic sketch of forces resulting from tests of Sterngold attachment type 7 with single abutment. Fig. 12B. Diagrammatic sketch of forces resulting from tests of Sterngold attachment type 7 with abutments splinted. Fig 136. Diagrammatic sketch of forces resulting from tests of Dalbo MK attachment with single abutment, Fig. 14B. Diagrammatic sketch of forces resulting from tests of Dalbo MK attachment with abutments splinted. Fig. 13B. Diagrammatic sketch of forces resulting from tests of Thompson dowel attachment with single abutment. Fig. 168. Diagrammatic sketch of forces resulting from tests of Thompson dowel attachment with abutments splinted.

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framework was cast and adjusted to the abutment receptacles allowing free rotation without binding or torquing.

Assembly procedures

The gold crowns were secured to their respective abutment teeth with threaded screws (Fig. 8). This arrangement allowed for ease of placement and removal of the crowns when testing the three types of removable partial denture attachment retainers, while assuring fixation comparable to a crown cemented in place clinically. Acrylic resin denture bases were attached to each prosthesis. A uniform thickness of 2 mm silicone material (Sir, Stern Dental Co., Mt. Vernon, N.Y.) was positioned between the denture base and the model. This resilient silicone layer simulated oral mucosa.

A metal bar was positioned between the right and left denture bases at the level of the occlusal plane in the region of the mesial cusp of the first molars (Fig. 8). A matrix was used to duplicate the bar position on all frameworks. The load was directed against the center of this bar. The removable partial denture on the photoelastic model was positioned in the center of a straining frame. The frame could be turned to present all parts of the model to a fixed camera (Fig. 9). A fiber optic light source was positioned at the rotational center of the photoelastic model, in a fixed relation to the camera. Results were recorded photographically. The load cell was positioned over the center of the bar between the right and left edentulous regions. A vertical force of 33 pounds was applied and monitored by an XY recorder (Fig. 10).

The first premolar abutments were then splinted to the canines with solder to form a double abutment. The entire sequence of testing was repeated to evaluate differences between single and double abutments.

RESULTS

Examination of the model before and after the placement of the removable partial denture frameworks on the photoelastic model revealed no signifi- cant stresses. Similar responses to the applied force were observed on both sides of the arch. Therefore, to simplify data presentation, only results from the right side will be analyzed. To facilitate presentation and interpretation of the photoelastic data, schematic representations of stress intensity were prepared. Areas of darker shading represent higher stress. It is

to be emphasized that these diagrams do not include actual isochromatic fringe lines.

Design I-Sterngold type 7 attachment

Single abutment (Fig. 11). The applied load produced a pronounced tendency to bend the premolar distally, as revealed by the pattern within the root. Pressure was observed at the alveolar crest distal to the premolar and progressed in an apical direction along the distal aspect of the root. Apical stresses developed in the structures supporting the premolar; interaction of these stresses with the canine apex were noted.

Double abutment (Fig. 12). When the premolar and canine were splinted, a substantial modification of the response to the load was observed. The roots of both the premolar and canine were uniformly stressed, indicating that the direction of force was along the vertical axis of the teeth. Some pressure occurred at the alveolar crest distal to the premolar and along the distal aspect of the root; this was of a smaller magnitude when compared to the unsplinted crowns. There was a reduction in apical stresses in the premolar region compared to the unsplinted crowns. An increase in apical stresses at the canine was observed. The improvement of the stress distribution over the unsplinted crowns was accompanied by an increased sharing of the applied force with the posterior edentulous region.

Design 2-Dalbo MK attachment

Single abutment (Fig. 13). The stresses within the premolar were uniform and of a low intensity. Some pressure resulted at the distal crest of the premolar and along its distal root. Low level stresses were observed at the apex of the premolar, which interacted with the root of the canine.

Double abutment (Fig. 14). The stress within both the premolar and canine was uniform and of a low intensity when compared to the unsphnted crowns. Reduced pressure was observed at the distal crest of the premolar and along its distal root. Lower apical stresses were observed in the premolar region, while increased apical stresses occurred at the canine. More force transfer to the edentulous region was observed than with the unsplinted crowns.

Design d--Thompson dowel attachment

Single abutment (Fig. 15). Evidence of distal bending of the premolar was present. Considerable pressure was observed at the alveolar crest distal to

26 JULY 1981 VOLUME & NUMBER 1


the premolar and along the distal aspect of the root. Apical stress developed which interacted with the distal pressure and with the canine apex.

Double abutment (Fig. 16). Splinting produced a pronounced change in the stresses within the premolar root. A uniform state of stress was noted in both the premolar and the canine. Pressure was reduced at the distal alveolar crest of the premolar and along the root in comparison to the unsplinted crowns. Stresses were also reduced at the apex of the premolar which interacted with the canine apex. Very low stresses were noted at the apex of the canine. Greater stress on the edentulous region was observed than with the unsplinted situation.

Comparing the attachments

Single abutment. The Dalbo attachment produced the lowest and most uniformly distributed stresses of all the designs tested. The Thompson dowel and Sterngold attachment presented similar stress intensities and distributions; both caused a pronounced distal bending of the premolar.

Double abutment. Splinting improved the stress distributions for all the designs. The Dalbo attachment produced the least stress to the abutments and the most stress to the edentulous region. The Thompson dowel produced less stress to the abutment teeth than did the Sterngold attachment, especially with respect to the pressure developed at the distal crest of the premolar.

DISCUSSION

These experiments indicate that fixed splinting of adjacent abutment teeth is an important factor when attachment retainers are used for an extension removable partial denture. When individual abutment teeth are used, undesirable horizontal forces are much greater than with splinted abutments.

The Dalbo philosophy basically advocates tissue support, with the attachment acting as a positioning control and the teeth providing only minimal support. The validity of this philosophy was verified by the results of this study. The Thompson dowel with splinted abutments appears to be a good example of the principles of broad distribution of stresses in that it utilizes both the teeth and edentulous regions. However, the distal rest rotation point on a single abutment tooth generates unfavorable distal forces according to these tests. The rigid Sterngold attachment produced the greatest forces to the supporting structures with both splinted and unsplinted abut-

ments. It is important to note that the forces evaluated were direct vertical forces. If lateral forces had been incorporated in the experiment, different stress patterns could have been demonstrated. Further investigation in this area is indicated.

SUMMARY

This investigation was designed to evaluate the forces developed in supporting structures by removable partial dentures with attachment retainers. The attachments tested were the (1) Sterngold type 7, (2) Dalbo MK, and (3) Thompson dowel. The study utilized a photoelastic model with stress areas recorded photographically.

The results showed that: 1. Splinted abutments are indicated when using

the tested attachment retainers. 2. With single abutments, the attachment retain-

ers induced distal force on the teeth which resulted in unfavorable horizontal bone forces.

3. The Dalbo MK attachments produced the most force on the edentulous regions and the least force on the abutment teeth.

4. The Thompson dowel and Sterngold type 7 attachment retainers induced similar stress patterns on single abutments and both produced distal abutment forces.

5. The Thompson dowel induced more favorable stress patterns when the abutments were splinted.

We would like to thank APM-Sterngold for their support of this project.

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Reprint requests to: DR. F. JAMES KRATOCHVIL

UNIVERSITY OF CALIFORNIA SCHOOL OF DENTISTRY Los ANGELES, CA 90024


Documents

Photoelastic Analysis of Stress Patterns on Teeth and Bone With Attachment Retainers for Removable Partial Dentures