Load-transfer characteristics of mandibular subperiosteal implants

Load-transfer subperiosteal

characteristics implants

S. D. Jones, D.D.S.,*

A. A. Caputo, Ph.D.,**

University

School

of California:

of Dentistry,

Los

of mandibular D. Benson, D.D.S.,

Angeles,

M.S.,***

and E. Borggrebe****

Calif.

M

andibular subperiosteal implant dentures have been used successfully for over 25 years in the treatment of edentulous, atrophied mandibles. Histologic studies of the human mandible have shown that a complete mandibular subperiosteal implant denture may be tolerated and the adjacent surrounding oral tissues maintained in a relatively healthy condition for 10, 15, and more years.le3 Mandibular subperiosteal implant denture substructures utilize the principle of cross-arch stabilization with a positive substructure seated on a large area of dense cortical mandibular bone for maximum distribution of masticatory forces (Fig. 1). Minimum bone injury is produced with denture function, which accounts for the favorable prognosis of a well-constructed subperiosteal implant complete denture.* Occasionally the dense cortical mandibular bone supporting a subperiosteal implant may slowly resorb over a period of years as a result of excessive forces of occlusion (Fig. 2). If a local region of bone receives too large a workload, resorption which could endanger the implant may take place. Bodine and Mohammed,‘-3 Bodine,4 and other investigators used a bar to connect the anterior and posterior posts of the conventional subperiosteal implant to improve the distribution of occlusal forces. The purpose of this investigation was to compare the load-transfer characteristics of the conventional subperiosteal implant designs with the occlusal connected bar design. MATERIALS

AND

METHODS

Photoelastic stress analysis was employed in this study. This reveals the distribution and concentra*Lecturer, Removable Prosthodontics. **Professor and Chairman, Biomaterials Science. ***Associate Clinical Professor and Chairman, Removable thodontics. ****Dental student.

0022-3913/79/080211

+ 06$00.60/O

Q 1979 The C. V. Mosby

Co

Pros-

Fig. 1. A radiographshowsa conventionalsubperiosteal

implant after 5 years in function.

Fig. 2. A radiographshowsa conventionals&per&teal implant after 14 yearswith boneresorption under the left molar post and peripheral frame. tions of stress within a model under load. Stress concentrations will generally develop where (1) geometric discontinuities exist (Fig. 3), (2) bearing loads are operative (Fig. 3), and (3) a modulus mismatch exists between two portions of a model. *PL-2, Photolastic Inc., Malvern, Pa. tsubperiosteal Frameworks, Buttress Angeles, Calif.

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Fig. 5. The photoelastic beam indicates stress concentrations caused by bearing loads and geometric discontinuities.

ET AL

Fig. 6. A subperiosteal implant with integral bilateral mesostructure and extended peripheral frame on a model of a human atrophied mandible.

Fig. 4. A conventional subperiosteal implant framework on a model of a human atrophied mandible.

Fig. 7. The experimental test setup for a subperiosteal framework subjected to occlusal loading by way of a complete denture.

Fig. 5. A subperiosteal implant with integral bilateral mesostructure on a model of a human atrophied mandible.

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A typical human edentulous, atrophied mandible was duplicated in an epoxy-based photoelastic material* with a modulus of approximately 30,000 psi. Three subperiosteal frameworks were fabricated for this mandible from surgical vitallium using standard techniques.t One framework was a conventional implant design employing vertical posts that were not connected occlusally (Fig. 4). The other two frameworks incorporated the integral bilateral poste-

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Fig. 8. Schematic of the circular polariscope

arrangement

used to record photoelastic

data.

Fig. 9. Concentration of stress under the posterior portion of a conventional implant due to vertical loads. Abooe, Load applied to the second molar. Below, Load applied to the first molar. rior mesostructure (Figs. 5 and 6). One of these frameworks included a distally extended peripheral frame (Fig. 6). All other aspects of the frameworks were identical. Complete dentures were constructed as superstructures for the frameworks. The mandible was supported by the condyles in a specially constructed tank (Fig. 7). The tank was filled with cedar-wood oil to facilitate visualization of stress patterns which develop under load. Identical vertical loads were applied to the first and second molar regions of the left side of the mandible by means of a frame situated above the tank. Subsequently, identical lateral loads were applied to the

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first molar region of the left side of the mandible. A fiber optic light bundle, together with polarizer and quarter-wave plate, was arranged lingually so that it could be rotated to view all areas of the mandible. Stresses developed within the mandible were recorded photographically in the field of a circular polariscope (Fig. 8). RESULTS Examination of the model mandible prior to and after placement of implant frameworks and dentures revealed a stress-free condition. The conventional framework caused stresses to be

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Fig. 10. Decreased stresses anterior vertical loads. Above, load applied molar.

ET AL

to the posterior post of a conventional implant due to to the second molar. Below, load applied to the first

Fig. 11. Reduced concentration of stress under the posterior portion of an implant with a mesostructure due to vertical loads. About, Load applied to the second molar. Below, Load applied to the first molar. concentrated beneath the posterior molar post for vertical loads placed in the first and second molar regions (Fig. 9). Additionally, stress was concentrated in the retromolar pad and inferior border of the mandible. Obseryations in the more anterior portions of the mandible revealed a rapid decrease in the magnitude of transmitted stresses (Fig. 10). The framework with the mesostructure demon-

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strated a more uniform distribution of the vertical occlusal loads with ,a reduced intensity of stress under the posterior molar post (Fig. 11). Further, there was a reduction in stress magnitude in the region of the retromolar pad. As was observed for the conventional implant, minimum occlusal force was transmitted anteriorly. The mesostructure with the distally extended

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Fig.

12. Further reduction of stress under the posterior portion of the implant with a mesostructure and extended peripheral frame due to vertical load applied to the second molar.

peripheral frame produced the least concentration of stress beneath the posterior post and the region of the retromolar pad as a result of vertically applied loads (Fig. 12). A greater concentration of stress in the posterior segment of the implant frame was observed from lateral occiusal forces in a buccal direction from the conventional implant. The least amount of concentration of stress was observed with the extended mesostructure compared to the conventional and mesostructure implant. A lesser concentration of stress in the posterior segment of the implant frame was observed from lateral occlusal forces in a lingual direction compared to buccal directed forces in all three types of implant frameworks tested. Similar intensity was observed in all three types of frameworks, with the extended mesostructure framework providing more uniform distribution of stress.

DISCUSSION The designs of the mandibular subperiosteal implant play a prominent role in the distribution and concentration of occlusal forces to the mandible. Improvements of the stress state within the mandible were observed to occur with designs that include (1) a bar connecting the two posterior posts (mesostructure) and (2) an extension of the implant frame into the region of the retromolar pad and the external oblique ridge. With these prevailing conditions the concentration of stress is substantially reduced under the molar post area and along the retromolar pad and the external oblique ridge.

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Lingual-directed occlusal forces were shown to cause higher concentrations of stress in the posterior regions of the implant frameworks than buccaldirected forces. Consequently, the type of complete denture occlusion is an important consideration in the construction of subperiosteal implant dentures. Therefore it is recommended that posterior teeth be set in lingualized occlusion, since masticatory forces are exerted in a lingual rather than a buccal direction. A framework that distributes occlusal loads more uniformly and produces fewer localized stress concentrations will tend to reduce or eliminate bone resorption under the implant frame. If the problem of bone resorption under the molar post can be eliminated, then the possibility of epithelium invagination can be greatly reduced, thereby extending the longevity of the mandibular subperiosteal implant. Two additional observations of interest were: (1) All the frameworks shared some of the unilateral vertical occlusal loads with the nonworking sides of the mandible. (2) Vertical occlusal loads on the second molar tended to result in an instability of the denture, with a displacement of the denture on the nonworking side. Consequently, it is suggested that teeth on mandibular complete dentures for a subperiosteal implant not be placed posterior to the posterior post.

CONCLUSION This study suggests that the subperiosteal implant design with the integral bilateral mesostructure and

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extended periphery should sis for long-term survival.

provide

the best progno-

3.

Bodine, human

R. L., and Mohammed, C. I.: Histologic studies of a mandible supporting an implant denture. Part II. J

PROSTHET

REFERENCES 1.

Bodine, human

4.

R. L., and Mohammed, mandible supporting

THET DENT

2.

21:203,

C. I.: Histologic studies of a an implant denture. J PROS-

Bodine, R. L., and Mohammed, C. I.: Implant denture. histology-Gross and microscopic studies of a human mandible with a 12 year subperiosteal implant denture. Dent Clin North Am 14:145. 1970.

Stabilized D. Ray

Ph.D.,

occlusion

McArthur,

Bioassay J. Perry

Jr.,

Fabrication

K. Norling,

216

D.D.S.,

interarch

Ph.D.,

and

A review

W. J. Tarbet,

of the

D.D.S.,

Ph.D.

spaces

for composite H. Hembree,

jaw position

resin restorations

Jr.,

D.D.S.

registration

anterior

guide

table

D.D.S.

Ph.D.,

surfactants and

Morris

The effect of anteroposterior force H. Okane, Ph.D.

A. Heyd,

and stability:

D.D.S.

The effect of nonionic impression materials Barry

and John

of a custom

K. Naylor,

retention

Ph.D.,

liner

D.D.S.,

interocclusal

T. Moberg,

ISSUES

MS.

of an experimental

The lateral

Craig

OF CALIPORNIA OF DENTISTRY ,e.OS hGELES, &LIP. 90024

rims for small

D.D.S.,

McGinnis,

Clifton

J. Pawelchak,

seven to

SCHOOL

aspects of denture

R. E. Lindstrom,

1971.

U~rvrmsm

TO APPEAR IN FUTURE

Physiochemical literature

26:415,

Reprint requests to: DR. S. D. JONES

1969.

ARTICLES

Dmr

Bodine, R. L.: Implant dentures-Follow up after ten years. J Am Dent Assoc 67:352, 1963.

T. Yamashina,

on bubble

H. Reisbick,

inclination D.D.S.,

entrapment

in elastomeric

D.D.S.

of the occlusal

T. Nagasawa,

D.D.S.,

Ph.D.,

plane on biting and

H. Tsuru,

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