Cortically Supported Custom-Made Subperostealis Implant – A Case Report

In the following case report, we present a custom-made implant type that was manufactured 6 years ago but went through continuous development during the years to achieve its current geometry, surface treatment, and fixation point design. Development trends were determined by implantation experiences on patients with bone deficiency. Our purpose was to create a long-lasting, mechanically stable baseplate profile of ideal size and geometry that has correction possibility for the dimensional changes of overlaying tissues by defining patient-specific articulation position and considering occlusal forces and articulation movements. Implant material had to fulfil necessary osseointegration requirements and subgingival management of soft tissues. We present a case of a female patient who was 69 years old at the time of implant insertion. She has significant bone deficiency at maxillary areas. Conventional osteoplasty would not have been enough to provide the necessary bone volume for the insertion of standard cylindrical dental implants.


Introduction
Dental implants are used to replace missing teeth. Implants have different types based on their operating principles and insertion methods. We report a case with a subperiosteal implant, which belongs to the family of custom-made implants as it is impossible to entirely standardize these implants and it is also very difficult to just partly conventionalize them. From a historical point of view, first subperiosteal implants appeared at the end of the 1930s. Dahl was the first to publish a study about them in the beginning of the 1940s [1]. This implantation method provides a fixed denture solution for edentulous patients. Early surgical technique consisted of two operations. During the first surgery, gum tissue was opened, and an impression was made to model existing bone anatomy. Implant baseplate insertion required another surgical procedure. This often resulted in posttraumatic effects and complications. Nickel-cobaltchrome metallic alloy was used as implant material, and dentures were fixed to posts protruding from the gum tissue. Although, Dahl's implantation method had several disadvantages: the baseplate was not fixed to mandibular and maxillary bone regions, its fixation was provided only by being embedded in connective tissue. Bacterial and destructive inflammatory processes often occurred due to the surgical procedure, implant material, lack of surface treatment, and lack of design experience with fixation pillars of the implant [2,3]. They resulted in high risk of complications, which lead to destructive resorption and relatively high number of inserted implants had to be removed. As a result of this, the implantation procedure had a bad reputation.
However, we completely revised subperiosteal implants thanks to the availability of medical grade titanium, CBCT imaging possibility to substitute physical impressions, and use of virtual design and analysis software. It became possible for us to create permanent functional bone replacements to restore chewing ability of patients with insufficient bone volume for the insertion of cylindrical dental implants. Nowadays, different types of innovation and technological development made it possible to positively change public assessment as well [4][5][6].

Case Report
CBCT images of the patient were processed virtually by generating an STL file. The STL file contained data of the precise bone surface based on adjusted parameters. It is a general problem that thin details of cortical bone ridge do not provide sufficient x-ray shadow which can lead to deficiencies on the bone surface [7][8][9].  Figure 1 shows the highlighted area of the corrected surfaces at the extension of the baseplate and the corrected model. The corrected surface generated this way was ready for further design steps. Design started with the virtual replacement of missing teeth based on the relation of articulation and antagonistic teeth. Here in this design step, previous denture of the patient was studied as well to precisely adjust trans-occlusal space, tooth sizes, and later periodontal relations. During the expansion of the baseplate, anatomic details of the bone surface were considered to calculate and minimize necessary expansion dimensions using a simulation software [6]. Boundaries of the expanded baseplate were perforated to increase mechanical stability and provide space for fixation points. When designing these fixation points, thickness of the underlying cortical plate was taken into consideration. Figure 2 shows the virtual image of the denture and its supporting subperiosteal plate.   These pieces of information are necessary for the design of threaded denture fixation points in the trans-occlusal direction.
In this phase, a positioning ring was created on the plate that facilitated pillar insertion. Its role was to achieve correct positioning and insertion of the threaded sleeve after printing. The virtually designed plate structure was inspected and approved by the oral surgeon. Figure 3 shows the virtually designed baseplate with its fixation points, which was sent for examination. In the previous steps, a dual threaded multi-body pillar structure had been fixed in the positioning rings of the baseplate. Component no. 1 was the threaded sleeve, which fitted to the baseplate and was later fixed with laser welding. Component no. 2 was the abutment, which was adjustable to different gum tissue levels. It could be replaced to compensate 1 to 3-mm-thick gum tissue levels depending on the change of gum thickness during the healing period after surgery. Component no. 3 was the interface, which was bonded into the denture, while the denture could have been fixed with threaded detachable joint. Component no. 4 was a micro screw that provided denture fixation. All structural components were machined with CNC technology and made from Grade 5 titanium material [10].

Subperiosteal framework was physically implemented from
Grade 23 Ti-6Al-4V material with LMF additive manufacturing technology using a Sisma MySint 100-type 3D-printer. Figure 5 presents the raw 3D-printed titanium implant baseplate after heat treatment. Assembly of the ready baseplate and pillar structure was carried out by welding the threaded sleeve into its previously defined correct position [11]. Afterwards, surface treatment of the weld joint and the framework was conducted with sand-blasting the entire structure and polishing pillar neck areas in a 1.5-mm-wide line. The last step was acid etching of the plate surface to promote osseointegration. Abutments could be changed after implantation as well to readjust red white aesthetics depending on the extent of gum volume change and to sustain cleanability. The custom-made assembled implant structure went through packaging process after ultrasonic cleaning.
Its final sterilization took place at the dental clinic before surgical implantation. Figure 7 shows the implant ready for insertion.     Figure 9 shows the construction of this dual membrane envelope. It was important during suturing of the implant to have tight gum tissue around the epithelial protrusion points. It was vital to achieve long-term implant survival. If this prerequisite is not met, another surgery will be necessary just to create tight gum tissue. Before this, plate insertion is not possible at all. Figure 10 shows tight gum tissue fitting around protrusion points of the pillars and closed but stressfree sutures.  An improperly fixed removable denture could have caused great damage in the inserted plate. Thus, an immediate load fixed temporary denture was created. Cleanability of the temporary denture is an important factor to achieve irritation-free healing.
It is also very important to precisely adjust temporary denture articulation in order to achieve well-balanced baseplate loading. Figure 11 shows the immediate load temporary denture fitted to the inserted implant structure. During suture removal, temporary bridge replacement was corrected according to epithelial impression. It is shown in Figure 12 At 3-weeks follow up, control CBCT images were made after examining cleanability of the temporary denture. CBCT images can be seen in Figure 13

Conclusion
Good quality CBCT images that show all details of cortical bone volumes are indispensable for implant design. According to our experiences, at least half a year must be passed after possible dental extractions or any other dental surgeries e.g. corrective jaw surgery in order to achieve adequate healing of bone surfaces.