Classic subperiosteal implants (CSI) have been around for decades in the implant dentistry helping patients and surgeons especially with atrophic ridges. Since its introduction, there has been significant amount of development and researches around this technique. With the implementation of digital technology in oral surgery, there has been growing interest in revisiting CSI and developing patient-specific implants (PSI). Our goal is to provide a comprehensive overview of the evolution of subperiosteal implants and to address the treatment planning, design principles, and the fundamentals required for successful practice of PSI placement and restoration.
Key points
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This article will review the fundamentals of patient-specific implants (PSI) in dental rehabilitation of atrophic jaws.
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This article recounts the evolution of the classic subperiosteal implants and PSI and the important innovations which have regained popularity in dental implantology.
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This article will discuss the applications of PSI and provide clinical exams of successful cases.
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This article will analyze current challenges and limitations regarding PSI and propose future trends and innovations within the field.
Introduction
History of Subperiosteal Implant and Evolution of Patient-specific Implants
Classic Subperiosteal Implants (CSI) was first introduced in Sweden in 1943 by Dr. Dahl and was subsequently incorporated in the United States in 1949 by Goldberg and Gershkoff. They were initially designed as a custom cast framework that rests directly on the surface of the bone under the periosteum to distribute the forces of occlusion through a series of struts in a snowshoe like framework ( Fig. 1 ). Their emerging popularity was largely because of the fact that they offered a new restorative option for patients with severely atrophic ridges. Although it provided certain advantages and had specific clinical indications, suboptimal outcomes were initially commonplace. High failure rate was attributed to limited options for materials used for implant bodies and prosthetic appliances, rudimentary surgical technique, and high complication rates involving infection and soft tissue dehiscence. , ( Fig. 2 ). In 1952, Lew and Berman introduced direct bone impressions using improved materials like polysulfide, silicones, and polyether, resulting in better load distribution and enhanced implant survival rates. Then, in the 1970s, Bob James described a new concept that took advantage of the osteoinductive effect of tensile forces on the bone by changing the support system of the mandibular CSI from the horizontal surface to the lateral body of the ramus and symphysis. Additionally, he advocated the removal of lingual struts, which reduced lingual dehiscence complications and improved CSI survival rates. In 1971, he also pioneered the use of computed tomography (CT) data to create digital bone models for implant placement, moving away from direct bone impressions. In the late 1970s to 1980s, emphasis was placed on the mesostructure/mesobar to enhance implant rigidity and force distribution. Dr Linkow and Mr. Dan Root then went on to develop a tripoidal mandibular CSI, which connected the lateral ramus wings to the anterior substructure, providing effective tensile loading and achieving increased survival rate ( Fig. 3 ). In the last 2 decades, the CSI and patient-specific implants (PSI) have evolved to minimize compressive loads and optimizing tensile forces, with a focus on smooth contours and limited crossover struts to avoid dehiscence.
The history and design considerations for maxillary subperiosteal implant (SI) highlight notable challenges compared to mandibular SI. Maxillary CSI faced higher complication rates, with Linkow reporting 65% success rate on 600 maxillary SI from 1965 to 1985. The major obstacles that the maxilla presented included thin membranous bone and insufficient cortical bone for adequate stress distribution and support. He advocated for innovative designs, such as multiple single finger-like buccal struts and perforated palatal straps for loading, which led to improved success rate for subsequent cases from 1985 to 1997. ( Fig. 4 ). Despite these efforts, the continued use of older techniques and misconceptions of failure mechanisms led to continued poor outcomes overshadowing the improved success rates with newer techniques. Ultimately this led to the traditional CSI falling out of favor.
In the past 10 years, with the robust integration of digital technology within oral surgery, there has been resurgence and increasing interest in PSI as a solution for the treatment of partial or complete edentulism in patients with severe atrophic maxilla and mandible. PSI underwent extensive research, development, modifications, and publications in university and private practice settings. Computed tomography (CT) and cone beam computed tomography (CBCT) availability led to digital modeling and analog fabrication of the implant and eliminated the first stage direct impression surgery required for the first iterations of the CSI. Eventually full digital records design and fabrication were used in the fabrication of PSI. The advancements in CT, intraoral scanning, virtual surgical planning software, and three dimensional (3D) printing technology have led to superior designs that have enabled providers to overcome previous challenges leading to more predictable outcomes. ( Fig. 5 ).
Indications of Patient-Specific Implants in Maxillofacial Surgery
Indications for PSI most commonly include severely atrophied dental arches resulting from various reasons including prolonged edentulism, failed all-on-4 or zygomatic implants, pathology, trauma, and congenital defects. Before the reintroduction of the subperiosteal implant (SI), these cases faced significant challenges and limited treatment options. PSI serves as an excellent alternative to zygomatic implants, all-on-4, and extensive bone grafting procedures. Overall, the use of PSI allows patients with atrophic jaws to receive a fixed, custom-designed prosthesis, which provides improved form and function without the need for invasive and time-consuming grafting procedures.
Fundamentals of patient-specific implants
Anatomy of Subperiosteal Implants
Terminology for PSI can be divided into 3 main structures: supramucosal, transmucosal, and substructures ( Fig. 6 ). Supramucosal structure consists of mesostructure , abutment, and superstructure . Transmucosal structure consists of transmucosal post or permucosal post . Finally, substructure consists of struts or plates.
Transmucosal post or permucosal post is the vertical component of CSI or PSI that transitions or attaches the mesostructure or MUA through the tissue to the underlying substructure.
In CSI, the mesostructure is the metal frame above the tissue that connects the transmucosal posts. It provides rigidity and distributes the occlusal forces to the underling substructure. Abutment is the connector for the prosthesis. PSI may be designed without the mesostructure. Muli-unit abutment(s) (MUA) or attachment system is placed on top of the transmucosal post. This MUA allows direct attachment of the prosthesis in a fixed detachable system similar to conventional all-on-X. This attachment system serves the same purpose as the transmucosal post in CSI and transmits forces of occlusion to the underlying substructure. The designs of PSI transmucosal abutments and prosthetics should accommodate the requirement of rigidity and load transfer to the substructure.
The CSI substructure consists of struts, which are designed in a snowshoe like fashion to dissipate occlusal loading over large area of basal or dense cortical bone in areas remote from transmucosal post. CSI’s primary struts connect the vertical transmucosal post to the underlying frame struts. Secondary struts are attached to the primary struts and provide further dissipation of forces.
In PSI, these struts are replaced with screw retained plates or wings. PSI’s primary plate connects the transmucosal abutment to the substructure. PSI may be limited to 1 primary plate or multiple secondary plates. It is a common practice for these plates to be perforated to allow placement of multiple fixation screws to facilitate osseointegration and rigid fixation.
Superstructure is a part of CSI that is incorporated or attached to the mesostructure to provide the retentive mechanism for the prosthetic. This superstructure can be designed to be replaced for wear. PSI superstructure is typically the MUA attachment system for the prosthetic and should be replaceable. This allows replacement and repair if abutment screws break or strip and cannot be retrieved.
Materials and Surface Coatings
Several factors influence osseointegration; the areas which have garnered the most attention related to the successful integration of SI are materials, surface coatings, substructure designs, manufacturing techniques, and surgical technique. In the early days of SI in the 1940s to 1970s, most were made of chromium-cobalt molybdenum alloys (Vitalium) and tantalum. It was not until the 1980’s that titanium root form implants were popularized. At that time, these materials were selected owing to their electric inertness, mechanical strength, resistance to corrosion, insolubility, and high biocompatibility. At that time, the leading theory suggested that the metal substructure leaked ions to the surrounding tissue causing localized soft tissue damage and inflammation. , Because of this, surface coatings, such as carbon and hydroxyapatite, were developed to minimize this phenomenon. Carbon coatings were chosen because of their excellent biocompatibility, which was thought to minimize the formation of a granuloma or connective tissue capsule around the SI framework. Though this theoretic benefit was initially well received, carbon coating did not gain much ground as these benefits were not supported clinically, and a study by Hess and colleagues identified several cases of carbon fragmentation leading to implant failure. In the 1980s, hydroxyapatite coatings were developed in the continued pursuit of improvement of osteointegration because of their reported osteogenic properties. This technique combined the favorable mechanical properties of metal materials with the bone-bonding characteristics of hydroxyapatite coatings. , , ( Fig. 7 ). Clinically, hydroxyapatite coatings posed issues with separation from the metal frame and decontamination issues when exposed by fenestrations. Because of mixed evidences, surface coating selections are currently subject to surgeon preference as only varying degrees of long-term success rates of different surface treatments are supported in the literature. Additionally, as current iterations of the SI are retained with fixation screws, it is possible that this discussion is now obsolete in many clinical applications today.
Design and Manufacturing Techniques
In recent decades, thorough analysis of the design and manufacturing process for SI has become possible with high accuracy with the advancement of imaging techniques, computer-aided design/manufacturing (CAD/CAM) technology, segmentation, and 3D modeling interfaces. This level of preoperative analysis allows the surgeon and engineer to minimize mechanical stress on the metal substructure and underlying basal bone and maximize soft tissue adaptation around transmucosal posts. The design of the SI is centered on the transfer and distribution of stress from the denture to the posts and subsequently from the posts to the metal framework with minimal to no point stresses to the patient. , The routine incorporation of finite element analysis , coupled with CAD/CAM software enables the virtual surgical planning team to evaluate the effects of mechanical stress upon loading. This information drives decision-making including plate thickness, substructure design, and fixation hole placement ( Fig. 8 ). Designers should maximize PSI support and pathways for plates over the dense cortical bone of the zygoma, palate, canine eminence, and piriform aperture. They should avoid thin fenestration prone areas such as the lateral sinus walls, which do not segment, as well as the denser regions. Transmucosal abutments should be sufficient in size and quantity to meet load transfer requirements. Primary plates should be recessed subcrestal, and should not be larger or bulkier than necessary to adequately transfer occlusal loading to the substructure. Shapes should avoid sharp angles and have convex surfaces allowing periosteum to adapt and reattach to the surrounding bone. Following design, the metal framework can be milled or 3D printed based on material selection. Postfabrication manufacturing techniques have evolved to include polishing, sandblasting, and acid etching based on desired surface characteristics. , For example, for intaglio surfaces, which lie in close adaptation to the bone, increasing the surface area through sandblasting or acid etching may be preferred, whereas polished surfaces are preferred in areas where bacterial and debris accumulation could lead to infection and device failure ( Fig. 9 ).
