Treatment Plans Related to Key Implant Positions and Implant Number

Treatment Plans for Key Implant Positions and Implant Number

Introduction

  • Historically, implant dentistry treatment planning was bone-driven, focusing on existing bone volume.
  • Implants were placed where bone was available, not necessarily in ideal prosthetic positions, leading to biomechanical issues.
  • A second phase emerged, prioritizing esthetics and ideal biomechanics.
  • Implant positions are now dictated by the teeth (prosthesis) being replaced.
  • Bone augmentation is used when necessary to achieve ideal implant positioning.
  • Implants placed in abundant bone with sufficient integration time have a surgical success rate exceeding 98%.
  • This high surgical success rate is generally independent of implant position, number, size, or design.
  • However, prosthesis success is significantly affected by these factors.
  • Occlusal loading of the implant prosthesis can increase the failure rate three to six times compared to surgical failure.
  • A meta-analysis showed a 15% failure rate (some reports exceeding 30%) when implants are loaded in softer bone.
  • Early loading failure, occurring within the first 18 months, is primarily due to biomechanical factors.
  • Excessive biomechanical stress or weak bone are the main culprits.
  • Reducing biomechanical stress is crucial for implant clinicians and can be achieved with:
    • Eliminating cantilevers.
    • Ideal implant positioning.
    • Adequate implant number.
    • Splinting implants.
  • Mechanical complications (abutment screw loosening, uncemented prostheses, material failure) are more frequent than surgical failures.
  • Parafunctional habits, unfavorable opposing dentitions, and improper occlusal schemes exacerbate these complications.
  • Misch developed a treatment plan sequence to minimize biomechanical overload:
    1. Prosthesis design.
    2. Patient force factors evaluation.
    3. Bone density determination at edentulous sites.
    4. Key implant positions and number determination.
    5. Implant size determination.
    6. Available bone determination at edentulous sites.
  • This chapter focuses on key implant positions and their treatment planning principles to reduce biomechanical stress.

Key Implant Positions for a Fixed Implant Prosthesis

  • Implant position within the arch is critical for long-term success; some positions are more important for force reduction.
  • Four general guidelines for treatment planning fixed prostheses:
    1. Reduce or eliminate cantilevers, especially in the maxilla; terminal abutments are key positions.
    2. Limit adjacent pontics to a maximum of three.
    3. Canine and first molar sites are key positions, especially when adjacent teeth are missing.
    4. Place at least one implant in each segment when replacing multiple segments of an arch (arch divided into five segments).
  • These rules can be summarized as:
    • No cantilevers
    • Maximum of three adjacent pontics
    • Canine rule
    • Molar rule
    • Arch dynamics
Rule # 1: Minimize Cantilevers
  • Cantilevers should be minimized to avoid force magnification on implants, abutment screws, and the implant-bone interface.
  • Cantilevers increase force to the system.
  • Fixed partial dentures with cantilevers supported by natural teeth have higher complication rates than those with terminal abutments.
  • This is particularly relevant in cases of parafunction or reduced crown height space.
  • Ideal key implant positions are terminal abutment positions when adjacent teeth are missing.
  • Cantilever length is directly proportional to the force on the abutments.
    • Example: A 2525-lb force along the long axis of an implant results in a 2525-lb load on the system.
    • The same force on a 1010-mm cantilever increases the moment force on the abutment to 250250 lb-mm.
  • This increased force elevates the risk of:
    • Porcelain fracture.
    • Uncemented prosthesis.
    • Abutment screw loosening.
    • Crestal bone loss.
    • Implant failure.
    • Implant component or body fracture.
  • A cantilevered restoration on multiple implants acts as a class I lever.
  • The cantilever extension is the effort arm.
  • The last abutment next to the cantilever acts as the fulcrum.
  • The distance between the last abutment and the farthest abutment from the cantilever is called the anteroposterior distance or A-P spread, which represents the resistance arm.
  • Mechanical advantage calculation: Cantilever length (effort arm) / Resistance arm (A-P spread).
    • Example: Two implants 1010 mm apart with a 2020 mm cantilever have a mechanical advantage of two (2020 mm / 1010 mm = 2).</li><li>A).</li> <li>A25−lbforceonthecantileverresultsina-lb force on the cantilever results in a50−lbforceonthefarthestabutment(-lb force on the farthest abutment (25lb×2=50lb × 2 = 50 lb).
    • The abutment closest to the cantilever (fulcrum) receives a force equal to the sum of the other two forces, or in this example, 7575 lb (2525 lb + 5050 lb).
  • Ideal treatment plans should minimize cantilevers.
  • In some cases, cantilevers are necessary due to insufficient posterior bone in edentulous mandibles.
  • Alternatives include nerve repositioning or iliac crest bone grafts.
  • Cantilevers may be planned from anterior implants in such scenarios.
  • When cantilevers are unavoidable, compensate with:
    • Careful consideration of force factors (parafunction, bone density, crown height, masticatory dynamics, implant location, opposing arch).
    • Adequate A-P spread (distance between distal and anterior implants).
  • When implants are in one plane, cantilever length should rarely exceed the A-P distance, regardless of patient force factors.
  • Unfavorable force factors necessitate:
    • Reduced or eliminated cantilever length.
    • Increased implant number.
    • Increased implant size.
    • Increased implant surface area.
  • Square arch forms are least desirable due to minimal A-P spread.
  • Tapered arch forms are most ideal due to the greatest distance between anterior and posterior implants.
  • Ovoid arch forms fall between square and tapered.
Rule # 2: Limit the Number of Adjacent Pontics
  • Generally, more than three adjacent pontics are contraindicated on implants, similar to natural teeth.
  • Exception: Very low force factors and favorable implant conditions (bone density, available bone).
  • Multiple adjacent pontics increase force, especially in posterior regions.
  • Supporting three missing teeth places considerable additional force on adjacent abutments.
  • Pontic spans flex under load, with greater spans resulting in greater flexure.
  • This flexure induces shear and tensile loads on the abutments.
  • Increased flexure elevates the risk of:
    • Porcelain/zirconia fracture.
    • Uncemented prostheses.
    • Abutment screw loosening.
  • Material flexure is more problematic for implants than natural teeth.
  • Natural tooth roots offer mobility, acting as stress absorbers and reducing material flexure.
  • Implants are more rigid and have a greater modulus of elasticity than natural teeth.
  • Long-span flexure complications are greater for implant prostheses.
  • Angled forces in maxillary anterior prostheses amplify force on the implant system.
  • Ideal treatment plans limit pontic span by:
    • Reducing occlusal table width.
    • Reducing cusp height.
  • Replacing a molar-sized space with two premolar-sized teeth reduces damaging forces.
  • Narrowing the occlusal table and decreasing cusp height minimizes shear and off-axis forces.
Rule # 3: Implant Positioned in Canine Site
  • Restorations replacing a canine are at higher risk than most others in the mouth.
  • Adjacent incisors are weak, and the first premolar is often one of the weakest posterior teeth.
  • Traditional prosthetics discourage replacing a canine and two or more adjacent teeth with a fixed prosthesis on natural teeth.
  • Implants are required when force factors are unfavorable and the following adjacent teeth are missing:
    1. First premolar, canine, and lateral incisor.
    2. Second premolar, first premolar, and canine.
    3. Canine, lateral, and central incisors.
  • Implants are necessary in these cases due to:
    1. Span length (three adjacent teeth).
    2. Lateral force during mandibular excursions.
    3. Increased bite force in the canine region.
  • At least two key implant positions are needed to replace these three adjacent teeth; usually in the terminal positions.
  • When the first premolar, canine, and lateral incisor are missing, the key implant positions are the first premolar and canine.
  • This creates an anterior cantilever for the lateral incisor, which is acceptable because:
    • It is the smallest tooth in the arch.
    • It experiences the least bite force.
    • The canine implant is typically larger for esthetics.
  • Occlusion is modified to eliminate occlusal contact on the lateral incisor pontic in centric occlusion or excursions.
  • If force factors are high, a small-diameter implant may support the lateral incisor, with three implants eliminating the cantilever.
  • When the canine edentulous site is a pier abutment, the canine position is a key implant position to disocclude posterior teeth during mandibular excursions.
  • When four or more adjacent teeth are missing, including a canine and at least one posterior premolar, key implant positions include:
    • Terminal abutments.
    • Canine position.
    • Additional pier abutments to limit pontic spans to no more than two teeth.
  • The canine site is crucial for ideal occlusion.
  • Implant-protected occlusion (canine guidance) is often recommended.
  • Williamson and Lundquist's electromyographic studies showed that mutually protected occlusion reduces temporalis/masseter muscle fiber activity by two-thirds.
  • Positioning an implant in the cuspid position significantly reduces activity in the temporalis and masseter muscles.
Rule # 4: Implant Positioned in Molar Site
  • The first molar is a key implant position when three adjacent posterior teeth are missing.
  • Bite force in the molar position is double that of the premolar position.
  • The edentulous span of a missing first molar is 1010 to 1212 mm, compared to 77 mm for a premolar.
  • When three or more adjacent teeth are missing, including a first molar, key implant positions include the terminal abutments and the first molar position.
  • Example: Missing second premolar, first molar, and second molar requires three key implant positions: second premolar and second molar terminal abutments and the first molar pier abutment.
  • When one implant replaces a molar (span less than 1313 mm), it should be at least 55 mm in diameter.
  • Smaller-diameter implants require considering the molar as two premolars.
Rule # 5: Implant Positioned in Each Arch Segment
  • An arch may be divided into five segments, similar to an open pentagon:
    • Two central and two lateral incisors (one segment).
    • Canines (independent segments).
    • Premolars and molars on each side (one segment each).
  • Each segment is essentially a straight line, with limited biomechanical advantage against lateral forces.
  • Connecting two or more segments creates a tripod effect and an A-P distance.
  • When multiple adjacent missing teeth extend beyond one segment, a key implant position should be within each segment.
  • Example: Edentulous from first premolar to first premolar requires key implant positions at the two first premolars (terminal abutments), two canines, and either central incisor position.
  • These positions adhere to the rules of:
    • No cantilever.
    • No three adjacent pontics.
    • Canine position.
    • At least one implant in each edentulous segment.

Implant Number

  • Historically, implant number was determined by available bone in the mesiodistal dimension.
  • Edentulous arches often used five to six implants in abundant bone or four in moderate-to-severe resorption.
  • This approach doesn't account for force magnifiers or the A-P spread.
  • Completely edentulous arches usually require a 12-unit fixed prosthesis (first molar to first molar).
  • Second molars are rarely replaced unless present in the opposing arch.
  • Implant positions may not always follow the four key implant position rules, leading to cantilevers or multiple pontics.
  • Treatment plans should avoid the minimum number of implants, as there is no safety factor if an implant fails.
  • Example: In 25 patients receiving four implants each (100 total), losing one implant per patient leaves only three, risking overload failure for all prostheses.
  • A 20% implant failure rate would result in only 5 of 25 patients having adequate support (20% prosthesis success).
  • While initially cheaper, this approach places the prosthesis at considerable risk.
  • Key implant positions alone are often insufficient unless:
    • Patient force factors are low.
    • Bone density is good (D1, D2).
  • Additional implants are added to the treatment plan to increase surface area and decrease stress.
  • Increasing implant number is an efficient method to decrease stress.
  • Terminal abutments alone for a four-unit prosthesis are inadequate unless force factors are low, bone density is ideal, and implant size isn't compromised.
  • Three implants to replace four missing teeth is often ideal.
  • High force factors and poor bone density (posterior maxilla) may require four implants to replace four teeth.
  • Three abutments for a five-tooth span distribute stress better than two.
  • An additional implant can:
    • Decrease implant reaction force by two times.
    • Reduce metal flexure fivefold.
    • Reduce moment forces.
  • Full-arch prostheses with six implants show better distribution and reduced stress compared to four-implant systems.
  • The treatment plan begins with implants in ideal key positions, with additional implants added based on patient force factors and bone density.
  • Young, large men with severe bruxism and excessive crown height space may require one implant per missing root (two implants per molar).
  • Patients with moderate force factors and poor bone density may also need this many implants.
  • Replacing all mandibular teeth typically requires five to nine implants, with at least four between the mental foramina.
  • Fewer than six implants necessitate a cantilever due to mandibular flexure.
  • Cantilevers should be projected in only one posterior quadrant to maximize A-P distance and reduce force.
  • Placing implants in four of five mandibular open pentagon positions reduces cantilever overload risk.
  • Seven or more implants allow fabricating two separate restorations without posterior cantilevers, accommodating mandibular flexure and torsion.
  • Second molars are usually not replaced in the edentulous mandible.
  • The maxilla generally requires more implants (77 to 1010) due to lower bone density and unfavorable biomechanics, with at least three from canine to canine.
  • Implants should be at least 1.51.5 mm from adjacent natural teeth and 33 mm from adjacent implants.
  • A 44-mm-diameter implant requires 77 mm of mesiodistal space.
  • The maximum number of implants between adjacent teeth can be calculated by adding these dimensions.
  • Example: A 2121-mm edentulous span is needed for three 44-mm implants, and 2828 mm for four.
  • It is better to err on the side of safety and add an additional implant if in doubt.
  • Implant-supported crowns in the posterior are often premolar-sized, allowing two implants to replace a molar if the span is at least 1414 mm for 44-mm implants.
  • When the missing molar is most distal, a 12.512.5-mm span is required for two 44-mm implants.
  • Advantages of 77- to 88-mm-wide premolar and molar-sized crowns:
    • More implants can be used.
    • Implants can range from 44 to 55 mm in diameter.
    • Adequate buccolingual bone dimension is available.
    • Crown contours allow sulcular probing.
    • Occlusal table width decreases moment forces.

Additional Treatment Planning Principles

Independent Prosthesis
  • Implant-supported prostheses should be independent from adjacent natural teeth to minimize marginal decay risk.
  • Splinting a tooth in a fixed partial denture causes decay in 22% of cases within 10 years, compared to less than 1% for individual crowns.
  • Unrestored natural teeth have a lower decay risk; implants don't decay.
  • Teeth-supported fixed prosthetic restorations have endodontic-related factors in approximately 15% of cases within 10 years.
  • Implant abutments don't require endodontic procedures, and unsplinted natural tooth crowns have fewer endodontic procedures.
  • Independent implant prostheses may reduce or eliminate pontics while increasing abutment number and force distribution.
  • This decreases the risk of unretained restorations.
  • Independent implant prostheses have fewer complications, greater long-term success, and greater survival rates of adjacent teeth.
  • Joining an implant restoration to a natural tooth increases risks of:
    • Abutment screw loosening.
    • Implant marginal bone loss.
    • Tooth decay.
    • Unretained restoration.
  • Occlusal force distribution is optimized with independent implant prostheses.
  • The ideal treatment plan includes an independent implant restoration for partially edentulous patients.
Splinted Implants
  • Splinting dental implants is controversial.
  • Teeth and implants respond differently to forces, impacting splinting decisions.
  • There are advantages to splinting implants:
    • Increased functional surface area.
    • Increased A-P distance to resist lateral loads.
    • Distributed force over a larger area.
    • Increased cement retention.
    • Decreased risk of abutment screw loosening.
    • Decreased risk of marginal bone loss.
    • Decreased risk of implant component fracture.
  • If an independent implant fails:
    • The implant is removed.
    • The site is grafted and reimplanted.
    • A new crown is fabricated.
  • If one implant of multiple splinted implants fails:
    • The affected implant can be sectioned below the crown.
    • The implant/crown site can be converted to a pontic using the same prosthesis.
  • This simplifies procedures compared to multiple surgeries and prosthetics for independent units.
  • Splinted implants distribute less force, reducing marginal bone loss and implant body fracture risk.
  • Sullivan reported a 14% implant body fracture rate for a 4-mm single implant replacing a molar compared to 1% for splinted implants.
  • Balshi and Wolfinger reported a 48% screw loosening rate over 3 years for single-tooth molar implants, reduced to 8% with splinted implants.
  • The exception to splinting is a full-arch mandibular implant prosthesis due to mandibular flexure and torsion.
  • Full-arch mandibular prostheses replacing first or second molars shouldn't be splinted to molars on the opposite side, requiring a cantilever or multiple sections.
  • Flexure and torsion don't affect the maxilla, where all implants are often splinted regardless of position.
  • Splinted implant crowns provide greater retention and transfer less force to the cement interface, reducing uncemented restorations, especially with short abutments or lateral forces.
  • Some clinicians dislike splinting implants due to technical complexities.
  • However, prosthetic and laboratory advancements are reducing these concerns.
  • Interproximal hygiene concerns are less significant with implants:
    • The general population flosses irregularly
    • Implants are usually 3mm apart.
    • Patients can be trained to use floss threaders, proxy brushes, or water-piks.
  • Splinted units may complicate restorative material fracture repairs.
  • However crown marginal ridges between splinted implants are supported by metal/zirconia connectors, placing porcelain/zirconia under compression.
  • Moreover the increased use of monolithic zirconia has decreased material fracture substantially.
  • Natural teeth may experience recurrent decay.
  • Therefore clinicians have the mindset that exists with natural teeth when considering the restoration of implants.
  • A single crown on a natural tooth has a caries risk of less than 1 with 10 years of service.
  • Splinted teeth decay at the interproximal margin around 22% of the time.
  • Single natural tooth crowns have an endodontic risk between 3% and 5.6%, while splinted teeth increase the rate to 18%.
  • Although implants do not decay, the interproximal area might still get infected if the patient is not practicing good oral hygiene.

Treatment Planning Should Not Be Dictated by Finances

  • Patients may have unrealistic expectations regarding treatment duration and cost.
  • Clinicians may compromise treatment (e.g., forgoing bone grafting) due to cost or complexity, opting for ultrashort implants, mini-implants, angled implants, or shortcuts.
  • Biomechanical complications often occur within the first few years of function.
  • Nonideal or shortcut procedures can increase clinician risk due to remediation costs.
  • Manufacturers are worsening the situation by providing unconventional treatment options that may have an increased failure rate.

Summary

  • Biomechanically-based treatment plans reduce post-loading complications.
  • Key implant positions for replacing missing teeth:
    1. Minimize cantilevers.
    2. Avoid greater than three adjacent pontics.
    3. Prioritize canine and first molar sites.
    4. Ensure at least one implant in each missing segment of the maxillary arch.
  • Increasing implant number increases surface area and reduces stress.
  • Additional implants are indicated to reduce overload risks from:
    • Patient force factors.
    • Reduced bone density.
  • When in doubt, add an additional implant.