BIOMECHANICS, TREATMENT PLANING, AND PROSTHETIC

CONSIDERATIONSChapter 76

Biomechanical considerations

• Osseointegrated implant provide predictable means of replacing missing teeth

• Load-bearing capacity of implants > anticipated loads during function

• If applied loads > load-bearing capacity

Failure : Mechanical, Biologic

• Implant failure• Mechanical failure : screw

loosening, bend, fracture of the implants

• Biologic failure : resorption-remodeling response of bone around implants Progressive bone loss

Biomechanical considerations

• Teeth are suspended within the supporting alveolar bone by periodontal ligament

Physiologic movement

Teeth movement: orthodontic

Implant- VS Tooth-retained restoration

Excessive forces

• Osseointegrated dental implants : direct contact with alveolar bone

No periodontal ligament

Destructive alveolar bone

Implant- VS Tooth-retained restoration

Excessive forces

Load-bearing capacity

• The bone appositional index : percentage of bone-to-implant contact

• Several factors influence load-bearing capacity• Number and size of implant• Angulations of implant position• Quality of the bone-to-implant interface

Load-bearing capacity• Posterior maxilla : less dense

trabecular, thin cortical plate layer

poor bone quality • Anterior mandible : dense

trabecular, thick cortical plate layer

• Surface modification : altered microtopography achieve higher bone apposition index

• Lack of bone height ≠ long implant -> reduce bone-to-implant contact

Use of short, wide implants

Increase load-bearing capacity

Load-bearing capacity

Angulation and arrangement

• Angulation of implants in relation to the plane of occlusion and the direction of the occlusal load

• Axial loads are well-tolerated • Implant at an angle of 20 degrees or more -

>resorptive remodeling response of bone

• Nonaxial loads concentrated stresses around the neck of the implant

Angulation and arrangement

Mechanism of implant failure• Excessive occlusal loads• Load resulting in microdamage: fracture, cracks, and

delaminations• Resorption-remodeling response of bone• Loss of bone at the bone-to-implant interface as a

result of remodeling• Vicious cycle of continues loading, additional

microdamage, and bone loss progressing to implant failure

• Distal cantilivered pontics were used to replaced missing maxillary posterior teeth

Nonaxial occlusal forces

Angulation and arrangement

• Distal angulation and the curve of Spee

Nonaxial loads

Angulation and arrangement

Treatment planing with dental implants

• Anterior-posterior (A-P) implant spread • Distance from the middle

of the most anterior implant to the distal edge of the most posterior implant

Edentulous maxilla

• Implant-supported prosthesis : provide stability comfort and restore confidence • Poor ridge form with a marginally stable

conventional maxillary denture : 2 or 4 implants provide greater stability and security of a maxillary denture in function

Edentulous maxilla• Implant-supported prosthesis : provide stability

comfort and restore confidence • Lack of posterior support with an intact mandibular

anterior dentition : Combination syndrome “Hammer and anvi”

• Implant-supported prosthesis : provide stability comfort and restore confidence • Palatal coverage is not tolerated

Minimum of four implants with adequate A-P spread

Edentulous maxilla

• Implant-assisted maxillary overdenture is preferred over an implant-supported fixed prosthesis -> labial flange can provide lip support

Edentulous maxilla

Edentulous mandible

• Mandibular complete denture is more problematic especially for pt with severely atrophic mandibular ridge

• Two-implant-assisted overdenture -> treatment of choice

• Implant-supported fixed prosthesis • 4,5, or 6 implants arranged in an appropriate arc

of curvature with at least 1 cm of A-P spread • distal extension cantilevers up to twice the A-P

spread

Edentulous mandible

Partially edentulous patients• Multiple tooth sites• The number of implants used will influence the

load-bearing capacity• Posterior maxilla : one implant for every missing

tooth• Posterior mandible : three-unit bridge supported

by two implants is widely accepted

• Single tooth sites

Partially edentulous patients

• Implant: Adequate number, size and position to sustain the occlusal loads

• Residual ridge and site permits : wide diameter implants should be used for molar replacement

Strategies to avoid implant overload

Strategies to avoid implant overload

• Place implants perpendicular to the occlusal plane• Place implants in tooth positions• Use an implant for each unit being replaced• Avoid the use of cantilevers in linear configurations• Avoid connecting implants to teeth

Strategies to avoid implant overload

• If connecting implants to teeth, use a rigid attachment

• Control occlusal factors such as cusp angles and width of occlusal table

• Restore anterior guidance if possible

• Occlusal design• Limit the width of occlusal table• Flattening the cusp angles• Avoid Cantilevered restoration

Strategies to avoid implant overload

• Connecting implant to teeth• Keep implant-supported restorations separate

from natural teeth

• Implants and teeth function differently

• Connect with the rigid system

Strategies to avoid implant overload

Conclusions

• The biomechanics must be factored into the planning at the beginning of any implant treatment

• To achieve long-term, predictable success