Maxillary All-On-Four® Surgery: a Review of Intraoperative Surgical Principles and Implant Placement Strategies
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Maxillary All-on-Four® Surgery: A Review of Intraoperative Surgical Principles and Implant Placement Strategies David K. Sylvester II, DDS Assistant Clinical Professor, Department of Oral & Maxillofacial Surgery, University of Oklahoma Health Sciences Center Private Practice, ClearChoice Dental Implant Center, St. Louis, Mo. Ole T. Jensen DDS, MS Adjunct Professor, University of Utah School of Dentistry Thomas D. Berry, DDS, MD Private Practice, ClearChoice Dental Implant Center, Atlanta, Ga. John Pappas, DDS Private Practice, ClearChoice Dental Implant Center, St. Louis, Mo. residual bone. Advocates for additive treatment BACKGROUND attempt to procure the bone volume necessary for implant support through horizontal and vertical augmentation techniques. Graftless Implant rehabilitation of full-arch maxillary approaches seek to offer full-arch implant edentulism has undergone significant changes support through creative utilization of angled since the concept of osseointegration was first implants in existing native bone. introduced. Controversy over the ideal number of implants, axial versus angled implant Biomechanical analysis of the masticatory placement, and grafting versus graftless system repeatedly demonstrated that the treatment modalities have been subjects of greatest bite forces are located in the posterior continuous debate and evolution. Implant jaws. Anatomic limitations of bone availability supported full-arch rehabilitation of the maxilla due to atrophy and sinus pneumatization make was originally thought to be more difficult than maxillary posterior implant placement its mandibular counterpart due to lower overall challenging. The resulting controversy with bone density. regards to full-arch rehabilitation was whether prostheses with long distal cantilevers could be The foundation for any implant supported full- tolerated. If tilting posterior implants could arch rehabilitation is the underlying bone. The circumvent anatomic limitations while dilemma faced by most surgeons is whether to maximizing the use of residual bone, cantilevers treat this residual bone in an additive fashion could be reduced. through bone augmentation, or a graftless fashion utilizing angled implants secured in Nonaxial placement and loading of implants 4. Angled implants provide secondary was thought to be biomechanically unfavorable resistance to vertical displacement by preventing osseointegration and leading to virtue of their nonaxial, oblique position increased peri-implant bone loss.1 Limitations in the archform which is perpendicular to imposed by residual maxillary bone availability occlusal forces. This stabilization is and the requirement for axial placement of separate from insertional torque and is implants meant cantilever length often needed only enhanced by cross-arch stabilization to be 20 mm or greater in order to provide 5. Distal cantilevers are minimized when a molar occlusion. Conversely, literature at the more posterior emergence of distal time reported a direct association between implants is achieved resulting in greater cantilever length and marginal bone loss/failure biomechanical stability. of underlying implants.2 It had been shown that 6. Anteroposterior spread (AP spread) is prostheses with cantilevers less than 15 mm enhanced. survived better than those with cantilevers 7. An alternative treatment option for greater than 15 mm.3 patients with severe systemic conditions which may render them poor candidates Today, Maxillary All-on-Four® is a common for bone grafting. treatment modality utilized by clinicians all over the world for immediate full-arch rehabilitation. The technique evolved as a graftless solution ALL-ON-FOUR® HISTORY which sought to reduce treatment time and avoid complications associated with bone In 1991 and 1992, Bruggenkate published grafting. Potential bone grafting complications reports of angled implant placement in the include: increased cost, increase treatment time, posterior maxilla used to support donor site morbidity, significant limitations overdentures.4,5 In 1995, a one-year primate imposed by chronic systemic medical study performed by Celletti et. al. demonstrated conditions, and unpredictable reduction in bone osseointegration of both straight and angled graft volume as a result of resorption. implants. This study provided both clinical and histologic evidence of implant osseointegration Tilted implants were proposed as a method to irrespective of implant angulation.6 avoid anatomic structures while achieving sufficient biomechanical support. By tilting In 1995, Brånemark published a 10-year survival implants, dense cortical bone of the natural study of fixed prostheses retained with either maxillofacial buttresses could be engaged, even four or six implants. This article suggested that, in cases of severe maxillary atrophy. Theoretical although there was a tendency for increased advantages of tilting implants included: failure rates in patients with only 4 implants, the overall survival rate for both implants and 1. Avoidance of anatomic structures. prostheses was the same for both groups. Prior 2. Longer, tilted implants could be placed in to this publication, the tendency of some cases of vertical bone deficiency clinicians was to place as many maxillary obviating the need for bone grafting. implants as possible in cases of full-arch 3. Bicortical stabilization can be more easily rehabilitation.7 This publication was the first to achieved. Longer, tilted implants make demonstrate equal success with an implant residual cortical bone more accessible foundation consisting of only 4 implants. resulting in greater primary stability. observed time period only one maxillary implant A graftless surgical technique and medium- was lost. length study using angled implants was published by Mattsson et. al. in 1999.8 This In 2001, Aparicio et. al.11 used a combination of solution to maxillary full-arch rehabilitation was angled and axial implants as an alternative to postulated to increase treatment predictability, sinus grafting in patients with severe maxillary decrease patient cost, and decrease resorption. Implant surgery was two-stage with complication rates. 86 implants were placed burying of implant fixtures at the time of into 15 patients. Patients were followed for an surgery. Final restorations consisted of fixed average of 45 months. During this time, one partial prostheses. Twenty-five patients were implant was lost resulting in a 98.8% implant followed for an average of 37 months (up to 5 survival rate. All patients had a stable prosthesis years). Residual dense bone was engaged by at the end of the observation period. Bilateral angling implants parallel with the anterior and fenestrations were created into the maxillary posterior walls of the sinus. In these instances, sinuses for the purpose of anatomic exploration posterior implants found apical stabilization in and bone sounding of the anteromedial sinus the pterygomaxillary region, and anterior wall. Posterior implants were placed parallel to implants were apically stabilized at the piriform. the sinus walls. Anterior implants were placed In other instances, the curvature of the palatal axially. Eighty-eight percent of implants vault was apically engaged. A total of 101 installed had 2-5 exposed palatal threads which implants were placed. The overall prosthesis were not grafted. This finding did not seem to survival rate was 100%. Tilted implants had a lead to any mucosal problems or marginal bone survival rate of 100%. Axial implants had a resorption according to their report. Primary survival rate of 96.5%. closure was obtained at the time of surgery. In this study, patients were not allowed to wear a At the time, these studies represented a subset removable prosthesis for 2 weeks. Implants of those available in the body of literature that were uncovered at 6 months. The authors suggested axial and tilted implants had concluded that the maximum use of residual comparable success in short and medium- bone stock, angulation of implants, and exposed length follow-up periods. Still, surgeries were palatal implant threads may allow for full-arch two-stage with healing periods of 6-8 months rehabilitation of severely resorbed maxillas. before implant loading. Advances in implant design and surface coatings aimed to reduce In 2000, Krekmanov reported placement of healing time and increase bone-to-implant angled implants in severely resorbed contact. It was found that implant roughness, maxillas.9,10 An open sinus technique was porosity, topography, and surface energy were performed by way of a sinus fenestration, and a synergistic accelerators of osteointegration. straight probe was used to sound the anterior Anodization of implant surfaces as well as air and posterior walls of the sinus. Implants were powered particle abrasion followed by acid placed parallel to the anterior and posterior washings were two techniques manufacturers walls at approximately 30-degree angulations. employed to increase microtexturing of implant A total of 75 maxillary implants were placed and surfaces.12-18 followed for 18 months. Nineteen implants were palatally inclined and placed tangential to In accordance with original principles of the curve of the palate at the molar regions osseointegration, implants were placed and engaging maximum cortical bone. During the buried beneath the soft tissue for a healing period to prevent excessive movement. Studies obtained by underpreparing implant osteotomy later emerged showing that limited