Functional correlation of abutments of subperiosteal implant dentures

FUNCTIONAL SUBPERIOSTEAL IMRE FGLDV~~RI,

CORRELATION IMPLANT

OF ABUTMENTS DENTURES

OF

M.D.Doz.*

Medical University, Budapest, Hungary

A

of subperiosteal implant dentures are not equal to natural teeth or roots serving as abutments for prosthetic restorations. The resistance afforded by the natural teeth is entirely different from that of the implanted abutment. Consequently, it is necessary to evaluate the limits of the stress components that affect abutments of implant dentures. The height of abutments for implant dentures must conform with the individual interarch distance. High abutments are not useful for removable prosthetic restorations, for the movements of the prosthesis transfer increased and unbalanced forces on the abutments and create leverage. The abutments and the prosthesis form a single unit, and it makes no difference whether a unit is composed of one or two parts in terms of the effects of stress. The problem of the transfer of masticatory stresses by implant abutments is not in the length of the abutments but in their correct position and direction as well as their relation to the other abutments and their antagonists. Whether a fixed or removable restoration is planned, most often four abutments are used in the corresponding places of both canines and first molars. The four abutments are parallel and form a right angle to the Frankfort plane, so that the restoration can be inserted from a vertical direction (Fig. 1, A). A revision of this customary procedure is necessary for proper usage of the restoration. BUTMENTS

PRINCIPLES

FOR

ARRANGING

ARTIFICIAL

TEETH

The general rules of construction are similar whether the prosthesis is supported by an implanted framework or whether it rests on the mucosa. However, certain dynamic and static points must be considered in the arrangement of the artificial teeth. For complete dentures (no implant), the teeth are arranged over the crest of the residual ridge. Consequently, the occluding molars are usually positioned with opposing inclinations. A straight line across the occlusal surfaces of the occluding molars intersects a line through the crest of the ridge at an angle of 80 degrees when the teeth are in centric occlusion. Therefore, in lateral occlusion, the line crossing the cusps intersects the line through the crest of the ridge at a 90 degree angle, which is the optimal position. Thus, the restored molars transmit masticator-y stresses to the denture base in a manner that prevents tilting of the restoration. *Head,

Department

of Odontology. 796

ABUTMENTS

OF SUBPERIOSTEAL

IVIPLANT

DENTURES

Fig. 1 .-A, The four abutments are parallel. B, Improper nonphysiologic horizontal are transferred to bone as a consequence of the defective interrelation of the abutments. abutments are in harmony with the interridge crest line.

PRINCIPLES

FOR ARRANGING

ABUTMENTS

FOR IMPLANT

707

stresses C, The

DENTURES

Afore-mentioned principles for arrangement of artificial teeth cannot be followed for implant dentures. Abutments usually are not positioned by these principles and are not adapted to the direction of the masticatory pressure lines because the abutments stand vertically and are perpendicular to the Frankfort horizontal plane (Fig. 1, B) . Such a position transfers nonphysiologic tilting effects, and consequently, a suitable transfiguration of the underlying bone structure or even its preservation cannot be expected. Obviously, the inclination of the crest of the residual ridge of each abutment and the interridge crest lines must be taken into account, and the abutments must be situated in the appropriate direction to that inclination when the mandible is in centric relation. The abutments of the lower implant denture should be in two groups, one on the right and one on the left. Thus, the abutments of the lower implant on the right and left sides will, according to the inclination of the right anti left residual ridge crests, incline toward each other. The abutmerts are neither vertical nor parallel but are positioned to permit a transfer of phy&logic masticatory stressesto the bone (Fig. 1, C) . Masticatory pressure, in most instances, is largely manjfested in the molar region. Therefore, one of the abutments, usually the canine abutment, must bc placed distally so that both abutments on one side are parallel but still inclined toward the crest of the residual ridge and the interridge crest line. A one piece restoration cannot be inserted upon divergent abutments. Therefore, the prosthesis is formed from two parts, which enables insertion of each part (Fig. 2). The parts can be joined by one of many mechanical systems, e.g., a screw for a fixed restoration, or similarly to the fixed partial dentures that art: inserted in two parts because of extremely divergent or convergent ahutnwnts (Figs. 3 and 4).

798

J. Pros. Den. July-August, 1962

FijLDVARI

Fig.

Fig.

3.

Fig. 2.-A framework with properly correlated abutments divided. into two parts. Fig. 3.-Correctly inclined abutments for implant dentures prosthesis because of lack of parallelism of the abutments. Fig. 4.-The fit of the two units is such that when they are tory stresses are directed as though there were only one unit.

supports often in place

a prosthesis require

a two

in the mouth,

that section mastica-

is

Volume Number

12 4

ABUTMENTS

OF SUBPERIOSTEAL

IMPLANT

DENTURES

799

SUMhlARY

Abutments for implant dentures should be positioned so that they are inclined according to the inclination of the interridge crest line. The abutments are not parallel, so the prosthesis must be divided into two parts. In this functionally correlated position, masticatory stresses will be transferred to the supporting bone in a physiologic rather than a pathologic manner. MARIA BUDAPEST,

u. 52 HUNGARY