TREATMENT PLANS RELATED TO KEY IMPLANT POSITIONS AND

IMPLANT NUMBER

By,Dr. Smijal GMMDS 2016

INTRODUCTION

• In the past, treatment planning for implant dentistry was primarily driven by the existing bone volume in the edentulous sites and esthetics and both these schemes have resulted in complications related to biomechanics.

Hence, Misch developed a treatment plan sequence to decrease the risk of biomechanical overload, consisting of following.

1. Prosthesis design2. Patient force factors3. Bone density in the edentulous sites4. Key implant positions and number5. Implant size6. Available bone in the edentulous sites7. Implant design

ABUTMENT OPTIONS• Several treatment options are available for the adequate

restoration of an edentulous segment• As a general rule, implant supported prosthesis independent

from adjacent natural teeth are designed whenever possible.

ADVANTAGE OF IMPLANT RESTORATION

1. Reduces the risk of decay on the natural tooth margin next to the adjacent pontic or abutment teeth-supported fixed prosthetic restorations

2. Independent implant prostheses reduce or eliminate pontics, while simultaneously increasing the number of abutments and distributing forces more effectively.

3. The increase in abutment number decreases the risk of an unretained restoration

• However, when an implant restoration is joined to a natural tooth, an increased risk of abutment screw loosening, implant marginal bone loss, and unretained restoration occurs

• The ideal treatment plan for a partially edentulous patient includes an independent implant restoration

KEY IMPLANT POSITIONS

• Some implant positions are more critical than others in regard to force reduction. There are four general guidelines to determine key implant positions

• There are four general guidelines to determine key implant positions

• No cantilevers• No three adjacent pontics• Canine-molar site rule• Key arch positions

NO CANTILEVERS-RULE 1

• The first rule for ideal key implant positions is that no cantilever should be designed in the prosthesis.

• Cantilevers are force magnifiers to the implants, abutment screws, cement or prosthesis screws, and implant-bone interface.

• The length of the cantilever is directly related to the amount of the additional force placed on the abutments

• More the length of the cantilever , greater the additional force placed on the prosthesis abutments.

• A cantilever on two implants may be considered a Class 1 lever.

• When the center of each implant is 10 mm apart, with a 20-mm cantilever, a mechanical advantage of 2 is created.

• Therefore the load on the cantilever will be multiplied by 2 on the far implant, and the implant close to the cantilever receives the total stress of the two loads

Hence, Key implant position• 1-2 adjacent teeth missing:one implant per tooth.

• 3-4 adjacent teeth missing:two terminal abutments.

• 5-14 adjacent teeth missing:2 terminal abutments with additional pier abutments.

A, When three adjacent teeth are missing, the terminal abutments are key implant positions. When all patient force factors are low and bone density is good, these abutments may be adequate to replace the three missing teeth. B, When four adjacent teeth are missing, the terminal abutments are key implant positions. Rarely are these two implants sufficient to replace four posterior teeth.

• Some clinical conditions where cantilever is the most prudent treatment option.

1. When the available bone in the posterior regions may be insufficient for root form implant placement, without advanced procedures.

2. When cantilever on a prosthesis is represented by only a lateral incisor with canine as abutment.

Considerations when opting for a cantilever prosthesis:• Force factors of parafunction, crown height, masticatory

dynamics, implant location and opposing arch need to be scrutinized.

• Should not extend beyond A-P distance. • When force factors are unfavorable, cantilever length

must be reduced, implant number increased, implant design surface areas increased.

• No more than 2 pontics for a posterior cantilever.

A-P SPREAD

• The anteroposterior distance (A-P spread) of implants is measured from the distal of the last implants to the mid position of the most anterior implant.

• Because these splinted implants form an arch, the cantilever may extend up to 2.5 times the A-P distance (when patient force factors are low and bone density is good).

Arch shape affects the A-P distance. • The ovoid arch form (A) oftenA-P

distance of 6 to 8 mm. • A square arch form (B) A-P

dimension of 2 to 5 mm. • A tapered arch form (C) A-P

distance, larger than 8 mm

SOME VARIATIONS IN TREATMENT PLAN TO INCREASE A-P SPREAD & REDUCE CANTILEVER

Additional implant placed over the mental foramen

One posterior segment connected to anterior segment

Bilateral implants not splinted together

NO 3 ADJACENT PONTICS-RULE 2• In implant prosthesis designs, three adjacent pontics are

contraindicated since they are subjected to considerable additional force and flexure especially in posterior regions of the mouth.

• The greater the span between abutments, the greater the flexibility of the metal in the prosthesis. The greater the load, the greater the flexure

• The greater the flexure, the greater the risk of porcelain fracture, uncemented prostheses, and abutment screw loosening.

Metal flexure is related to the cube of the distance. A, When one pontic is present, the metal flexes (x). B, When the fixed prosthesis has two pontics (2x), the metal flexes 2 x 2 x 2 = 8 times more. C, When three pontics exist (3x), the metal flexes 3 x 3 x 3 = 27 times more than a one-pontic restoration.

• The span of pontics in the ideal treatment should be limited to the size of 2 premolars i.e, 13.5-16mm.

• When a molar is one of the teeth missing between existing teeth, the missing molar space may be 10- to 14-mm long. Therefore when the span is greater than 14 mm, two pontics should be considered to replace the molar.

• Missing tooth span is often related to the missing number of • roots in the mandible• buccal roots in the maxilla.

• A five- to seven-unit fixed prosthesis has three key positions for the abutments.

• The terminal abutment follows Rule 1 (no cantilever)

• a one-pier abutment is positioned following Rule 2 (no three adjacent pontics). Rarely are these three abutments sufficient to support the prosthesis in the long term.

• Additional abutments are required when force factors are moderate to severe or bone density is poor around the implant.

• An edentulous arch missing 14 natural teeth may have 18 potential implant sites.

CANINE AND FIRST MOLAR SITES-RULE 3

• The most ideal prosthetic option to replace a missing canine is an implant.

• Implants are required whenever the following adjacent teeth are missing in either arch:

• (1) the first premolar, canine, and lateral incisor; • (2) the second premolar, first premolar, and canine; and • (3) the canine, lateral, and central incisors

• Whenever these combinations of teeth are missing, implants are required to restore the patient because:

• (1) the length of the span is three adjacent teeth, • (2) the lateral direction of force during mandibular excursions

increase the stress• (3) the magnitude of the bite force is increased in the canine region

compared with the anterior region. • Therefore, under these conditions, at least two key implant

positions are required to replace these three adjacent teeth, usually in the terminal positions of the span (especially when one of the terminal abutments is the canine position)

When the canine, first premolar, and second premolar teeth are missing, key implant positions are in the canine and second premolar

When the canine, lateral, and central incisor teeth are missing, key implant positions are in the central and canine positions

When the first premolar, canine, and lateral incisors are missing, the key implant positions are the first premolar and canine position.

• The 1st molar is also a key implant position when 3 adjacent posterior teeth are missing.

• The bite force doubles in the molar position compared with the premolar position in both the maxilla and mandible

• When 3 or more adjacent teeth are missing, including a 1st molar the key implant positions include the terminal abutments and the 1st molar position.

A, When the central, lateral, canine, and first premolar are missing, the ideal key implant positions are the central and first premolar (Rule 1, no cantilever) and the canine position (Rule 3, the canine and first molar position). B, When the central, lateral, canine, first premolar, second premolar, and first molar are missing, the three key implants positions are the central and first molar sites (Rule 1), and the canine site (Rules 2 and 3, no three adjacent pontic and canine and first molar position).C, When the central, central, lateral, canine, first premolar, and second premolar are missing, there are three key implant positions: the central and second premolar (Rule 1, no cantilever) and the canine position (Rule 3, the canine and first molar position).

D, When eight adjacent teeth are missing from second premolar to the opposite canine, there are four key implant positions: The canine and second premolar position (Rule 1), the opposite canine (Rule 3), and one of the central incisor positions (Rule 2). E, When 10 adjacent teeth are missing from second premolar to second premolar, there are 5 key implant positions: the 2 second premolar sites (Rule 1), the 2 canine sites (Rule 3), and one of the central incisor positions (Rule 2).

RULE 1

RULE 3

RULE 1

RULE 3

RULE 2

KEY ARCH POSITIONS-RULE 4An arch may be divided into five segments, similar to an open

pentagon.• 2 centrals + 2 lateral incisors: one segment.• the canines are independent segments,• premolars and molars on each side form a segment.

When multiple missing teeth extend beyond one of the open pentagon segments, a key implant needs to be positioned within each segment.

• An arch may be considered as an open pentagon: the two premolar and molar sites, the two canine sites, and the central and lateral incisors represent the five sides.

12 3

4 5

IMPLANT NUMBER• The key implant positions are often not enough support for the implant

restoration, unless all patient force factors are all low and the bone density is good (D1, D2).

• Therefore additional implants are added to the treatment plan according to patient force factors or bone density of edentulous sites.

• One of the most efficient methods to increase surface area and decrease stress is to increase the implant number.

• The number of implants to replace all mandibular teeth range from 5 – 9, with at least 4 between the mental foraminae.

• Usually the second molar is not replaced in the edentulous mandible.

• Cantilevers in the mandible should ideally be projected in only one posterior quadrant to increase the A-P distance and reduce the force to the implants

When implants are positioned in four of the five open pentagon positions in the mandible, a cantilever is at a reduced risk of overload because of favorable dynamics of an arch, increased large A-P distance, and favorable bone density.

When seven or more implants are placed in an edentulous mandible, two separate prostheses with no posterior cantilever may be designed. The mandible flexure and torsion is free to occur when the separation between the two prostheses is between the mental foraminae. A-P, Anteroposterior distance

• A greater number of implants are generally required in the maxilla to compensate for the less dense bone and more unfavourable biomechanics of the premaxilla, and range from 7 to 10 implants with at least 3 implants from canine to canine.

Ideal implant sites in Maxillary Edentulous arch• One central incisor position, • Bilateral canine positions, • Bilateral second premolar sites• Bilateral sites in the distal half of

the first molars

• In most situations, an implant should be positioned at least • 1.5mm from an adjacent natural tooth • 3mm from and adjacent implant.

• Using these guidelines, each 4mm diameter implant requires 7mm of mesiodistal space.

• Therefore the maximum number of implants between adjacent teeth can be calculated by taking the crest module of an implant (e.g., 4.0 mm) and adding these dimensions

The minimum mesiodistal dimension for two standard 4-mm diameter implants is 1.5 mm + 4 mm + 3 mm + 4 mm + 1.5 mm = 14 mm.

When three adjacent teeth are missing (two premolars and first molar), the mesiodistal dimension averages 7.1 mm + 6.6 mm + 10.4 mm = 24.1 mm. In this situation, planning 4-mm-diameter implants to fabricate two premolars of 7 mm each (1.5 mm + 4 mm + 1.5 mm) and one 5-mm-diameter implant for the first molar allows more bone around each implant

ADVANTAGES OF A 7- TO 8-mm-WIDE PREMOLAR OVER A MOLAR-SIZED CROWN.

• More implants may be used to restore the missing teeth. • Implants may range from 4 to 5 mm in diameter (most common

sizes) and often the available bone has adequate buccolingual bone dimension in this region.

• The emergence of the crown contours on implants of this dimension permit sulcular probing.

• The occlusal table width decreases mesial and distal moment forces compared with a molar-sized crown.

SPLINTED IMPLANTSImplants that are splinted to each other or adjacent natural teeth for force distribution.

ADVANTAGES OF INDEPENDENT IMPLANTS OVER SPLINTED IMPLANTS:

1. Easier to maintain interproximal oral hygiene.2. Ability to replace a single unit to repair porcelain fracture.3. A single crown has a caries risk of <1% within 10 years. However,

when natural teeth are splinted together, decay at the interproximal margin often occurs at a rate of approximately 22%

4. A single crown has an endodontic risk of 3% to 5.6%. Splinted teeth have an endodontic risk of 18%.

5. Does not impair the periodontal health of adjacent teeth.

ADVANTAGES OF SPLINTED IMPLANTS OVER INDEPENDENT IMPLANTS:

1. Decreased risk of porcelain fracture at porcelain to metal crown interface.

2. Increased functional area of support to resist lateral loads and decrease implant component fracture.

3. Reduced risk of abutment screw loosening.4. When an implant fails in a multiple splinted implant prosthesis, the

affected implant may be cut below the crown, and implant site converted to a pontic using the same prosthesis.

5. The splinted implants distribute less force to the implant bodies, which decreases the risk of marginal bone loss or implant body fracture.

• The exception to the splinted implant rule is a full arch mandibular implant prosthesis

• The body of the mandible flexes distal to the foramen on opening and has torsion during heavy biting.

• Full-arch mandibular restorations should have a cantilever or be made in two or three sections to accommodate the mandibular dynamics during function.

• The concept of flexion and torsion does not affect maxilla where all implants are splinted together, regardless of their positions in the arch.

SUMMARY• A biomechanical concept based treatment plan reduces

complications after implant loading with the prosthesis.

• Guidelines to reduce stresses are :1. No cantilevers2. No 3 adjacent pontics3. Canine molar site4. Key arch positions

• When more than 1 segment is being replaced, at least 1 implant must be placed in each segment.

• After key implants are selected, additional implants are indicated to reduce the risk of overload from patient force factors or reduced bone density.

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