Protocol for the Placement of the Zygomatic Implant: A Graftless Approach for Treatment of the Edentulous Maxilla
8.1 Historical Perspective
The edentulous maxillae have a unique anatomic presentation that limits the installation of the appropriate number and distribution of implants within the alveolar ridge. The maxillary sinuses bilaterally and the position of the nasal floor in the premaxillary region may limit the vertical volume of alveolar bone available for placement of implants in the maxilla for a fixed, implant-supported prosthesis. The palatal and posterior resorption pattern of the edentulous maxillae may also limit the horizontal bony volume necessary to house endosseous implants.
The typically large pneumatized sinuses in this group of patients require extensive bone grafting if conventional implant placement is envisioned. Various procedures to reconstruct bony volume have been reported in the literature. Adell,  Breine, and Branemark  used composite grafts to provide a tooth anchorage system. LeFort I osteotomy  and iliac block grafts [4, 5] have also been used to re-establish bony volume for placement of implants. To create bone mass in the posterior maxillae, sinus-lift grafting procedures have also been advocated. Survival of implants with sinus-lift procedures was reported as having a 90% success at the “Sinus lift consensus conference ” of 1996 . The conclusion reached in the consensus conference was similar to Tolman’s report in 1995 which stated that due to the “multivariate and multifactorial protocols and materials reported, it was difficult to draw definitive conclusions and therefore, these must await controlled prospective studies” . Keller and Tollman have also described grafting of the maxillae with delayed implant placement and delayed loading, with 87 and 95% implant and prosthetic respective survival rates . Rasmusson using autogenous inlay, onlay, and/or LeFort I procedures  presented a success rate of 80 and 77% for grafting with delayed implant placement and for grafting with simultaneous implant placement, respectively. Other authors in order to recreate the alveolar volume and topography needed for stabilization of dental implants to support a fixed prosthesis have also reported using various bone grafting techniques to reconstruct the resorbed maxillae [2–5].
In more contemporary literature, to avoid extensive bone grafting procedures, tilted implants [10–12] have been recommended for the reconstruction of this group of patients.
For patients with significant anterior extension of their maxillary sinuses, extending forward into the bicuspid region, the zygomatic implant has become commonly treatment planned and utilized. The zygomatic implant allows for establishing posterior maxillary support by using the limited crestal alveolar bone in the bicuspid region to stabilize the platform of the implant and the os zygomaticum for establishing initial stability of the apical portion of the implant, thus allowing for a “quad-cortical” stabilizationof the zygoma implant (Fig. 8.1). The use of the zygomatic implant and its success rate have been studied internationally, with reported success rate between 94 and 100% [13–18].
Branemark in 2004 reported on the cumulative survival rate (CSR) of 94% in their initial retrospective study of 52 patients treated with the zygoma implant following the two-stage, delayed loading protocol . Bedrossian et al., in their 2010 article, reported on the 7-year prospective follow-up of patients treated using the immediate load protocol with a CSR of 97.2% . Aparicio et al. in 2014 also reported on ten prospective studies using the immediate load protocol with a similar 97.71% CSR .
Bedrossian in 2011 introduced the “Rescue Concept” using the zygoma implant as part of a decision-making algorithm for cases of failed sinus grafting procedures or in cases of failed tilted implants. This concept allows for continuity of care of the patient without major interruption in their treatment .
In recent years, the popularity of the use of the zygoma implants has also increased due to favourable systematic reviews by Chrcanovic et al. and Goito et al. [22, 23]. Chrcanovic et al. reported a 96.7% success rate over a 12-year follow-up period. Goito et al. in turn reported a 97.86% success rate following 1541 zygoma implants.
8.2 Patient Selection
In planning patients with the zygoma implant, a “systematic preoperative evaluation protocol” is required before the surgical treatment . Both surgical and prosthetic needs must be considered before initiation of the surgical treatment to allow for a predictable outcome. The preoperative evaluation protocol takes into consideration the available alveolar bone in the different zones of the maxilla (Fig. 8.2). The zygomatic implant is considered in the group of patients who demonstrate bone in zone 1 only. The systematic preoperative treatment planning protocol also evaluates and determines whether the final prosthesis is a ceramo-metal bridge or profile/hybrid prosthesis. The determination of the type of prosthesis is based on the lack or the presence of “composite defect”. If hybrid prosthesis is considered, recognizing the level of the transition linepreoperatively allows for an aesthetic outcome of the final prosthesis. The “transition line” is defined as the junction between the hybrid prosthesis and the residual edentulous crestal soft tissues (Fig. 8.3) that should always be apical to the extreme “smile line” for an aesthetic outcome.
8.3 Radiographic Evaluation
Although computerized and conventional tomography can be used, the Panorex radiograph is critical in the initial evaluation of the patient [1, 9, 16, 18]. The presence of alveolar bone in the premaxilla, zone 1, and lack of bone in the bicuspid and the molar regions, zones 2 and 3, respectively, are the indications for considering the zygomatic concept . In 2003, Nkenke et al.  evaluated the anatomy of the zygomatic bone, describing that reformatted frontal images in 2 to 3 mm cuts afford the less experienced operator more information for planning the surgery (Fig. 8.4).
Frontal and axial 3-dimensional slices can also be obtained to further evaluate the maxillary sinus, naso-ethmoidal complex, bony osteomeatal complex as well as contour of the lateral maxillary walls allowing for the adoption of the ZAGA classification concept . The width of the residual maxillary alveolar bone and the width and height of the zygomatic body can also be visualized in frontal reformatted sections. The presence of sinus pathologic conditions including, but not limited to, thickening of the Schneiderian membrane and air fluid levels may also be ruled out using 3-dimensional radiographic studies.
8.4 Primary Load-Bearing Bone Under Function
The indication for the placement of the zygoma implant is lack of bone in zones 2 and 3 (bicuspid and the molar region, respectively). For patients where the zygoma implant is considered there is less than 1 mm of residual maxillary alveolar bone remaining in the bicuspid and molar regions. Therefore, after placement of the zygoma implant, minimal bone-to-implant contact is achieved at the alveolar crest. Knowing that bone-to-implant contact (BIC) is limited at the maxillary crest, the questions become these: How important is BIC at the maxillary crest in distribution of the occlusal forces? Is there a benefit for having crestal maxillary bone at the implant platform?
Ujigawa and colleagues  in an elegant finite element analysis described the weight-bearing bones associated with the use of the zygoma implant. In their 2007 Finite Element Analysis paper, they reported that the zygoma bone, and not the maxilla alveolus, supports the simulated occlusal loads. However, in 2013 as well as 2015, Freedman et al. [28, 29] corrected the observation in the Ujigawa study. The highlighted area over the lateral cortex of the zygoma reported as the load-bearing area in the Ujigawa study was instead the origin of the masseter muscle under activation (Fig. 8.5). Freedman et al. demonstrated in their papers that the maxillary crest plays a role in the distribution of the occlusal loads and the preservation and anchoring of the implant in the maxillary crest are recommended when possible.
8.5 Deformation Under Function
Past clinical reports  support the observation and highlight the importance of cross-arch splinting zygoma implants with other stable implants in the premaxilla to allow for a more favourable force distribution under function.
In 2013, the objective of the Freedman et al. study was to investigate the influence of maxillary alveolar bone on the stress distribution and the deformation of zygomatic implants under function. Two study models were created. The first model had the zygomatic implants placed in the skull supported by the zygoma and the maxillary alveolar bone with rigid cross-arch stabilization by a fixed bridge. The second model duplicated the first model; however the area of the maxillary alveolar bone supporting the zygomatic implants was removed. Occlusal and lateral forces were applied to both models and the maximum von Mises stresses were recorded. In the model lacking alveolar support higher maximum stresses were noted. Occlusal stresses were higher than lateral stresses in the model with no alveolar support. The zygoma bone demonstrated low stresses in both models. Therefore, the authors concluded that the maxillary alveolar bone support is beneficial in the distribution of forces for zygomatic implants.
In 2015, Freedman addressed the extra-sinus placement of the zygoma implant. The first model had two zygomatic implants placed in the skull in an extra-sinus position connected by a rigid cross-arch splinted bridge. The second model was the duplication of the first with the area of the maxillary alveolar bone supporting the implants removed, which is an inherent consequence of this type of implant placement (Fig. 8.6). Occlusal and lateral forces were applied with higher maximum stresses noted in the model with no alveolar support (Fig. 8.7a). Occlusal stresses were higher than lateral stresses in the model with no alveolar support (Fig. 8.7b). Occlusal stresses were lower than lateral stresses in the model with alveolar support. Low stresses were noted in the zygomatic bone in both models. They concluded that “Maxillary alveolar bone support is beneficial in the distribution of forces for zygomatic implants placed in an extra-sinus position”.
8.6 Preoperative Considerations
The surgical procedure is usually performed in the office setting under IV sedation . All patients are premedicated 1 h before the surgical procedure. The proper administration of sufficient local anaesthesia is critical in the management of these patients under IV sedation. The various infiltrations and nerve blocks include circumvestibular infiltration of the maxilla, greater palatine blocks, and bilateral transcutaneous infiltration of the temporal areas over the zygomatic body. Bilateral inferior alveolar nerve blocks are considered to allow retraction of the lower jaw during the surgery without undue stimulation of the sedated patient. It is recommended that the path of the implant from the premolar area to the base of the zygoma be directly visualized whenever possible [15, 31]. Direct visualization of the base of the zygomatic body has also been advocated in clinicians who have used computer-assisted treatment planning and surgical templates for placement of the zygomatic implant . Having a clear view of the path of the instruments needed in establishing the osteotomy and visualizing the path of the implant during its insertion prevent disorientation and potential complications associated with placement of the zygomatic implant. Generally, three potential trajectories of the implant are possible. The proper axis/trajectory is a path extending from the bicuspid region through the maxillary sinus, entering the mid-portion of the zygomatic body. If the entry point in the zygomatic body is more anterior to the described trajectory, potential for penetration into the orbit exists. However, if the line axis is posterior to the described trajectory, the potential for entering the pterygomaxillary space, leading to soft-tissue enveloping and subsequent lack of osseointegration of the implant, as well as the potential for unexpected haemorrhage exist (Fig. 8.8). The zygomatic implant has a unique design. The diameter at the apical two-thirds of the implant is 4.0 mm; at the alveolar one-third, the implant widens to a diameter of 5.0 mm. The zygomatic implants are available in lengths ranging from 30 to 52.5 mm. A specialized series of long zygoma drills are used to prepare the osteotomy (Fig. 8.9).
8.7 Surgical Options
In reconstruction of the edentulous maxilla with the zygomatic implant, two approaches are considered. The first is the traditional two-stage protocol [13, 15]. Clinicians may also consider the immediate-load protocol [15, 31]. The two-stage protocol requires the immediate cross-arch splinting of the zygoma implants at the time of the Phase II procedure. This can be accomplished efficiently by using the CAL technique (Fig. 8.10) for the fabrication of a passive bar before the uncovering procedure . The patient’s denture is adjusted to accommodate the CAL bar and after healing of the soft tissues final impressions of the implants are taken for the fabrication of definitive fixed hybrid prosthesis. An alternative and more contemporary technique for the cross-arch splinting of the zygoma implants at the time of uncovering, in a delayed loading protocol, is to splint the zygomatic implants with the premaxillary implants by conversion of the patient’s existing denture into a fixed provisional bridge using the same direct or indirect conversion technique described for the conversion protocol used when immediate loading is considered.