All-on-4 Immediate Implant Concept

1. Overview

2. Indications

Patient-Level Indications

• Complete edentulism - most classic indication
• Failing dentition requiring full-arch extraction (terminal dentition)
• Severe periodontal disease with hopeless remaining teeth
• Atrophic maxilla or mandible where bone grafting is not desired or contraindicated
• Patients preferring to avoid bone grafting - reduced morbidity and cost
• Patients with moderate bone resorption (Cawood & Howell Classes III-V) where conventional axial implant placement is limited by anatomy

Jaw-Specific Indications

Bone Volume Minimum Requirements

• Minimum bone width: 5 mm
• Minimum bone height (maxilla): 10 mm (canine to canine region)
• Minimum bone height (mandible): 8 mm (interforaminal region)
• Residual ridge difficulty classification (Lopes et al.):
  • Low difficulty: ridge width >5 mm
  • Moderate difficulty: irregular ridge 4-5 mm wide
  • High difficulty: irregular ridge <4 mm wide (may need modification)

3. Implant Position Criteria

Implant Configuration (per arch)

Anterior Implants

• Placed vertically (axially) in the anterior region
• Positioned bilaterally between the canines
• Standard placement without angulation

Posterior (Tilted) Implants

• Tilted 30°-45° - the most common angulation described is 30°, with up to 45° depending on anatomy
• In the maxilla: tilted anteriorly (mesially), emerging anterior to the maxillary sinus
• In the mandible: emergence just anterior to the mental foramina
• Screw access holes for tilted posterior implants should emerge at the occlusal surface of the 1st molar, 2nd premolar, or 1st premolar
• The distal position of posterior implants reduces cantilever length, lowering biomechanical stress

Biomechanical Rationale for Tilting

• Tilting the distal implant by 30° decreases stress in compact bone by 52% and in cancellous bone by 47.6% compared to vertical implants with longer cantilevers (Bevilacqua et al.)
• Cross-arch splinting with immediate provisional distributes forces across all 4 implants
• Load is shared between mesial and distal implants through the prosthesis, avoiding overloading

4. Immediate Loading Criteria

Primary Stability (Most Critical)

• Insertion torque: 30-50 Ncm (minimum 30 Ncm to qualify for immediate loading)
• The most commonly reported range is 32-37 Ncm
• Under-preparation of the osteotomy (under-drilling, avoiding countersink) is used to maximize torque
• ISQ (Implant Stability Quotient via Osstell) assessment is an additional option, though not uniformly reported

Surgical Technique to Enhance Primary Stability

• Alveolectomy (removal of some alveolar bone) for 3 reasons:
  1. To use standard implants in basal bone (denser) for better stability
  2. To conceal the prosthesis-mucosa transition line aesthetically
  3. To create adequate restorative space for the prosthesis and multiunit abutments
• Under-drilling the osteotomy site

Prosthetic Requirements for Immediate Loading

• The 4 implants must be splinted by a rigid cross-arch provisional prosthesis within 48 hours
• Splinting provides cross-arch stabilization and controls micromotion (<150 μm threshold)
• The provisional should be screw-retained, not cement-retained

Occlusal Scheme (Immediate Provisional)

• Avoid cantilevers or minimize their length
• Simultaneous bilateral point contacts on all teeth - excluding teeth distal to implant emergence
• In lateral movements: group function or guidance with flat linear pathways and minimal vertical superimposition - no balancing contacts
• In protrusive movements: guidance distributed on all anterior teeth including canines
• If opposing a removable full denture: avoid balancing contacts in excursive movements

Occlusal Scheme (Definitive Prosthesis)

• Full prosthesis typically replaces teeth up to the 1st or 2nd premolar (with a 1-2 premolar cantilever at most)
• Second molars are generally not included

5. Contraindications

• Inadequate primary stability (<30 Ncm insertion torque) - should convert to delayed loading
• Severe uncontrolled systemic disease (uncontrolled diabetes, immunosuppression, bisphosphonate therapy with active disease)
• Bone width <4 mm (very narrow ridge - consider grafting or zygomatic implants)
• Active infection at implant sites
• Uncontrolled bruxism (high occlusal forces - relative contraindication)
• Heavy smoker (relative contraindication, not absolute - increased failure risk)
• Radiation therapy to the jaws (relative contraindication)

6. Implant Specifications Typically Used

• Length: Commonly 13-18 mm (longer implants preferred for posterior tilted positions)
• Diameter: 3.5-5 mm (standard platform)
• Surface: Roughened/sandblasted acid-etched surfaces for osseointegration
• Abutment: Angled multiunit abutments (17°, 30°) used to compensate for implant tilting and create a parallel emergence profile

Summary Table