Platform switch hybrid zygoma implants improve prosthetics and marginal bone protection after extra-sinus placement.

Abstract Purpose The aim of our studdy is clinical evaluation of Platform switch hybrid zygoma implants. Materials and Methods 117 zygomatic implants were followed up during this time. They included 55 Brånemark System zygoma implants, 38 Noris implants, and 24 novel iRES hybrid implants with platform switch. Results Bone quality and quantity are the prerequisite for successful implant treatment. Zygomatic implants are intended for patients with severely resorbed maxilla that cannot accommodate conventional implants without prior extensive bone grafting. Such regenerative procedures, like sinus lifts, prolong implant rehabilitation to several months (12–18). Furthermore, extensive grafts are less predictable showing varying degrees of graft resorption. Zygoma implants enable full, often immediate, reconstruction of the upper dental arch without the need for sinus lift treatment. The original zygoma protocol runs the implants through the sinus, requires general anesthesia, and positions the prosthetic platform of the implants on the palate, which makes prosthesis cumbersome. It also induces risk for post‐op sinusitis. Extra‐sinus approach with novel zygoma hybrid implants bypasses sinuses and positions the implant prosthetic platform on the crest allowing for same good prosthetics as on conventional dental implants. Furthermore, crestal threads and a platform‐switch, of the novel zygoma design, increase implant anchorage and minimize marginal bone loss. The study presents evolution of zygoma implant rehabilitation protocol and zygoma implant design in our clinical practice over 15 years (2004‐2019). Conclusion Extra‐sinus zygomatic implant placement lowers the risk of post‐op sinusitis and makes procedure possible to be done in local anesthesia.


| INTRODUCTION
Loss of teeth leads to bone atrophy of the alveolar crest 1-9 up to 1/3 of the original height within a few weeks after extraction. In the following years, atrophy progresses both from the crest and the sinus as a result of invasive proliferation of the maxillary sinus mucosa. 10 The shape and structure of the zygomatic bones presented good anchorage alternative for longer implants (zygomatic implants). The efficacy of rehabilitation with zygomatic implants in maxilla is well documented. [11][12][13] The limitations for the more comprehensive use of this method were invasive surgery under general anesthesia and prosthetic challenges with palatally positioned implant heads.
This study presents evolution of the protocol from intrasinus in general anesthesia into extra-sinus in local anesthesia 14  Zygoma implants reduced overall treatment (full upper arch rehabilitation) time and eliminated the need for bone grafting into maxillary sinus. 16,17 The protocol was then modified to four zygomatic implants two on each side 18 for patients who do not have enough bone in the front of maxilla.
The goal of our clinical research was first to facilitate prosthetics by moving the implant heads to the crestal ridge. Therefore, we began to place zygoma implants more mesially, in the front of maxilla. 19 The 30 mm implants went through the sinus cavity and the implant heads sticked out at the second molar site. Prosthetics became normal then and did not require any additional prosthetic elements towards the palate.
Then we wanted to bypass the sinus to make procedure less invasive and minimize the risk of post-op sinusitis. So we went with drills more buccally that the implant does not pass through the maxillary sinus but runs in the sinus wall or outside. Crestally we wanted to preserve a bony bridge around implant head as much as possible to prevent soft tissue recession around the prosthetic abutment. We used implants 40 or 45 mm 20,21 long. The implant head was at the position of second premolar or first molar but not exactly on the crest. In this protocol, however, prosthetic framework had to be thicker and palatal extensions were often necessary.  We also performed the procedure under general anesthesia at the patient's request.
F I G U R E 2 Patient with advanced periodontal disease rehabilitated with Noris zygomatic implants-x-rays before and after surgery. Extraoral images at 1 year follow up-visible F I G U R E 3 Zygomatic hybrid implant with "platform-switch" prosthetic connection F I G U R E 1 Evolution of zygoma implants and surgical protocol After a thorough physical examination, we qualify the patient for surgery according to the ASA scale. Due to the extent of the procedure and its duration, we use general anesthesia in a complex manner-intravenously and intratracheally. We collect a patient's consent each time after routine preanesthetic testing and risk assessment.
We place the patient in a prone position-with the option of using the Trendelenburg position. Then we use standard vital functions monitoring, that is, automatic periodic RR measurement, ECG recording from four precordial leads, pulse, and arterial blood saturation recording. We perform venipuncture with a 1.  Hybrid implants (C), with rough threaded apex and machined (nonthreaded) body and crestal threads accounted for 21% (n = 24).
General anesthesia was done to patients who received an intrasinus implant-35.04%. All extra-sinus procedures were performed under local anesthesia-64.96% (Table 1).
The crestal position of the prosthetic abutments was achieved in 71.79% ( Table 2).
The implants used in the study were from 30 to 50 mm long. The most commonly implanted zygomatic implants were 45 mm-32.48% (Table 3).
During follow-up visits, periodontal examination with a calibrated plastic tube, periimplantitis was found in around 3% of classic zygomatic implants (Table 4) and maxillary sinusitis was below 6% ( Table 5). The immediate loading was applied in 22 patients. Zygomatic implants in remaining 27 patients were loaded within 3-6 months after surgery.
In the study group, about six implants (11%) Brånemark System and six Noris implants (16%) were exposed. There was no mucosal recession around the prosthetic abutments of hybrid implants.
T A B L E 2 Position of the implant head/prosthetic abutment: on the crest or palatally    Therefore, extra-sinus placement of hybrid (rough/machined) surfaced implants with crestal threads and internal platform switch connection lowers the risk of sinusitis and implant failure. Furthermore, subcrestal placement and platform-switched abutments on the crestmake prosthetics more comfortable for the patient and predictable as with conventional dental implants. 22 The novel implant design reduces gingival recession around prosthetic abutments due to platform-switch applied. 23,24 Furthermore the results of our study indicate efficacy of immediate loading of extra-sinus zygomatic implants which is the major benefit for the patient and treating team. 25 Prosthetic loading 3-6 months after surgery is also very popular among authors doing similar research. The overall failure rate of zygomatic implants in our study does not differ from the reported by other authors and amounts to 1.7%. 26 According to our knowledge, no report on zygoma platform switch hybrid implants placed extra-sinus has been published yet and therefore our findings may be encouraging for other investigators to further examine and popularize this graft-less method of full and frequently immediate rehabilitation of highly compromised patients.

| CONCLUSION
Extra-sinus zygomatic implant placement lowers the risk of post-op sinusitis and makes procedure possible to be done in local anesthesia.
The use of hybrid implants lowers the risk of periimplntitis, sinusitis and implant failure. Crestal threads and internal platform-switch connection enable subcrestal placement and on-crest emerging of prosthetic abutment hence making prosthetics as good as on conventional dental implants. Soft tissue augmentation with fat-pads can be made in patients with a thin soft tissue biotype to avoid gingival recession.
The overall failure rate of zygomatic implants in our study does not differ from the reported by other authors and amounts to 1.7%. 26 The use of zygomatic implants is often a rescue procedure after complications in patients who have previously received conventional implant treatment.