The zygoma anatomy-guided approach (ZAGA) for rehabilitation of the atrophic maxilla

In patients with a resected or atrophic maxilla, placement of traditional endosseous implants is not possible due to inadequate bone. The management of a severely atrophic maxilla using zygomatic implants is different and more complicated than the placement of conventional endosseous implants from a surgical perspective. In most cases, patients with extreme maxillary atrophy have only one chance for oral prosthetic rehabilitation using zygomatic implants. The learning curve of the procedure may be long due to the reduced number of patients to be treated in a single office. Hence, the management of maxillary atrophy using zygomatic implants should preferably be performed by an experienced clinician.

ZAGA, the acronym for zygoma anatomy-guided approach, is the concept used to place zygomatic implants in a prosthetically driven manner and according to the anatomy of the patient. The ZAGA method aims at promoting a patient-specific therapy by adapting the osteotomy type and implant design to the patient´s structure. The technique relies on the recognition of the existence of the inter-individual anatomical differences as well as the intra-individual variations (Fig. 1). In most cases, as explained further, this method avoids the opening of a window or slot into the lateral wall of the maxillary sinus before implant placement. Instead, a mucoperiosteal flap, including the posterior maxillary wall and the superior zygomatic rim, is raised to allow visual control of the complete surgical field (Fig. 2a, b).

The essence of the ZAGA philosophy is to individualize the surgical procedure that is guided by the anatomy of the patient according to specific prosthetic, bio-mechanic, and anatomic criteria. The ZAGA Concept helps the surgeon to understand the anatomic variations and, accordingly, to determine the ideal prosthetic implant position together with optimized anchorage, proper load distribution, and to achieve crucial long-term objectives of proper bone sinus sealing and stable soft tissue around the implant.

As an evolution of the extra-sinus approach, the relationship of the zygomatic buttress–alveolar crest area was classified into five different types in 2011 (Fig. 1). This classification was used to establish an ideal patient-specific implant path and to develop the zygoma anatomy-guided approach (ZAGA) protocol. The ZAGA Concept provides surgeons with a standardized guiding system to achieve an ideal prosthetic implant position for anchorage, load distribution, and implant head emergence along with the achievement of long-term soft tissue stability and appropriate sinus sealing. The rationale for the ZAGA technique is to give customized treatment to every patient depending on their individual anatomical presentation. The variations of the anatomy make necessary the use of different types of implant designs. Accordingly, two types of new zygomatic implant design and section are introduced: the ZAGA Round used for the ZAGA Tunnel-Type osteotomy and the ZAGA Flat indicated for the ZAGA channel-type osteotomy (Fig. 1).

In the ZAGA technique, the anatomy of the zygomatic bone, maxilla, and maxillary sinus governs the implant site preparation. The anatomical, biomechanical, and prosthetic factors determine the coronal entry point at the level of the alveolar process to achieve an optimal prosthetic outcome. The zygomatic bone anatomy and the required number and design of implants govern the zygomatic entrance point. These two points define the implant trajectory, which in turn determines the preparation and path of the implant body. In the ZAGA technique, the path of the implant body can range from a total extra-sinus to a total intra-sinus path. Indeed, the type of the implant path will determine the election of the implant design (Fig. 1).

The pre-surgical evaluation for the ZAGA protocol is the same as that for the original surgical technique. General anesthesia or intravenous sedation with local anesthesia can be used while treating the patient. One hour before the surgical procedure, antibiotics (amoxicillin 35 mg/kg) are administered to the patient in a single dose. In the presence of mandibular teeth, the patient is prescribed additional amoxicillin 750 mg every 8 h from the day of the surgery for 7 days.

A slightly beveled palatal incision is made starting from the posterior buccal aspect of the maxillary tuberosity till the midline. An ascending vertical vestibular-releasing incision is performed till the nasal spine. The palatal positioning objective is the prevention of eventual soft tissue dehiscence by displacing the adequate volume of connective tissue from the palatal to the buccal (Fig. 2a,b). A mucoperiosteal flap (Fig. 2b) is elevated with modified dissectors (Fig. 2c,d) (Salvin Instruments, ZAGA kit) to expose the alveolar crest, the infraorbital nerve, and the posterior and lateral maxillary walls up to the superior rim of the zygomatic arch. For a less traumatic insertion of unilateral zygomatic implants, a hemi-maxillary flap is advised. When the placement of regular implants is planned below the nasal cavity, the floor of the cavity should be raised to protect the integrity of its mucosa. A modified retractor (Salvin Instruments, ZAGA kit) with a distal hook is anchored onto the superior rim of the zygomatic arch for the following reasons (Fig. 2b–d):

With a didactic intention, we differentiate three main zones of the zygomatic implant path (Fig. 2a):

The “zygomatic implant critical zone” (ZICZ) is the complex formed by maxillary bone, soft tissue, and the zygoma implant at the coronal level where the first contact with maxillary bone occurs. Residual alveolar bone and soft tissue preservation/augmentation at the coronal level of the zygomatic implant are critical to prevent late complications.

The zygomatic antrostomy zone (ZAZ) is the area where the drill penetrates into the maxillary sinus cavity. Depending on the maxillary anatomy, the zygomatic antrostomy zone will be located either at the internal side of the remaining alveolar bone (Tunnel Osteotomy) or apically from the ZICZ when there is not enough alveolar bone and the osteotomy trajectory is buccally offset (channel osteotomy). The location of the antrostomy will depend on the zygoma buttress curvature and on the position of the coronal entrance point.

The zygomatic anchoring zone (ZAZ) is the section of the zygomatic bone where the implant reaches its maximal primary stability. To maximize primary stability, the ZAGA Concept uses a tangential zygomatic bone-to-implant intersection, penetrating the four corticals of the maxillary zygomatic process and zygomatic bone to achieve optimum structural zygomatic stabilization.

The main aim of performing an osteotomy for conventional or zygomatic implant placement is to establish an implant bed that fits as closely as possible to the implant shape without injuring the adjacent structures. The requirement for a lateral window is eliminated when the osteotomy matches the implant direction (Fig. 3). This preserves maximum bone at the maxillary lateral wall and at the crest. It is the responsibility of the zygomatic implant to close the osteotomy. This osteotomy closure eliminates the need for grafts and makes the surgery minimally invasive. A three-step technique is used to select the ideal implant position immediately following the incision and elevation of the mucoperiosteal flap while adhering to the ZAGA guidelines.

Each step is performed as per the criteria mentioned below.

The initial step of implant placement is the identification of the intraoral coronal entrance point. It is governed by anatomic, biomechanical, and prosthetic considerations. As a natural prosthesis is the aim of implant rehabilitation, the starting point (implant head emergence) should be at or close to the top of the alveolar ridge crest (Fig. 4) to achieve a less bulky and esthetic prosthetic design that is easy to clean and does not interfere with the tongue.

To establish mid-crestal implant head emergence, placing the entrance point on the palatal aspect of the alveolar ridge is prevalent. The operator should take into consideration the anatomical criteria while determining the position of the implant head on the buccal–palatal orthoradial axis.

When two zygomatic implants are planned, a mesial–distal entrance at the level of the second premolar or first molar region is advised for balanced load distribution and minimal cantilever length. When four zygomatic implants are planned, the anterior implants should be positioned between the lateral teeth to prevent extensive mesial cantilevers (Fig. 5). The zygomatic implants should not be inserted too close or parallel to each other. Too wide implant designs (5 mm diameter) can result in close or parallel implant placement and prevent optimal implant distribution.

Zygomatic implants should be placed in regions with inadequate bone for the stabilization of conventional implants. The mesial–distal variations of zygomatic implant position should depend on pristine bone distribution. In case of anterior atrophy, the zygomatic implant must be placed in the anterior maxilla (Fig. 6). Two major factors determine the buccal–palatal location of the implant in association with the residual alveolar bone:

When there is adequate bone at the sinus floor level to house the implant neck, i.e., 4 mm high × 6–7 mm wide in a satisfactory alveolar architecture, such as described for original ZAGA types 0 and 1 and some ZAGA 2, the initial osteotomy was done from the palatal aspect of the crest (Fig. 6a). Efforts were made to place the implant through it using a tunnel-shaped osteotomy. The sinus membrane is then perforated when the antrostomy is completed. This implant emplacement, together with adequate implant stability and design, provided enough bone to implant contact (BIC) able to osseointegrate and be responsible for a long-term antrum sealing. Moreover, since alveolar bone buccal to the implant neck is maintained, the risk of late soft tissue complications is minimized. A “tunnel-type osteotomy” was also performed in ZAGA type 3 cases where the alveolar bone adopts a triangular, buccally inclined, profile and the maxillary anterior wall is concave. Then, a circular osteotomy of the alveolar bone leaves intact the sinus lining regardless of the maxillary wall curvature. In the case of a tunnel osteotomy preparation, Straumann ZAGA Round zygomatic implant design was chosen to seal the preparation.

In patients with severe resorption and inadequate residual bone architecture at crestal level, as in ZAGA type 4 and some types 2, instead of penetrating the antrum through a thin bone layer, the coronal osteotomy was buccally shifted to prevent future sinus or nasal–oral communication/fistula. The initial preparation is done by placing the ZICZ on the buccal aspect of the residual ridge (Fig. 7). The antrostomy was usually established not at the zygoma itself, but at the zygomatic process of the maxilla, inferior to the zygomaticomaxillary suture, and separated as much as possible from the zygomatic critical zone located were the implant contacts for the first time with the alveolar bone. The final contour of the coronal entrance site is determined by the crestal anatomy and the degree of bone resorption. The ZAGA Concept aims to prevent possible complications. Hence, while performing the first osteotomy close to the crest, the risk of early or late sinus communication and soft tissue recession around the implant head or body should be considered. The integrity of the sinus lining at the crest at the ZICZ should be maintained. This type of osteotomy, not capable of providing a complete covering of the implant mid-body and neck, is known by the authors as “channel-type osteotomy''. It is a groove made on the coronal alveolar bone, and sometimes also in the lateral maxillary wall and zygomatic buttress. In the case of a channel osteotomy preparation, Straumann ZAGA Flat zygomatic implant design was chosen to avoid soft tissue compression and seal the preparation.

The position, condition, and nature of the initial preparation are strongly associated with both possibilities. An oral–sinus communication can develop over time after a palatal or crestal osteotomy is done through a thin bone of thickness < 2 mm. This lack or late loss of soft tissue or bone sealing around the implant entrance can also be due to:

The implant should not be inserted too far buccally or too separated from the crestal bone to prevent soft tissue recession due to compression by the implant body. A more buccal position of the implant can cause mucogingival dehiscence. In an extremely atrophic maxilla, where implant sealing is achieved only by soft tissue (at least on its buccal aspect), special attention should be paid to prevent mucogingival dehiscence (Fig. 8). If the above-mentioned complications are unavoidable, prevention of sinus infection should be the operator’s highest priority.

In a maxillary atrophy type IV ZAGA case, entry to the sinus is taken as close as possible to the apical entrance point of the zygoma. Additionally, at least 5–10 mm of residual bone and intact membrane at the crest level of the maxillary anterior wall should be maintained lateral to the implant neck (Fig. 9).

The ZAGA 0 to IV classification describes the association of the zygomatic buttress–alveolar crest regions including alternatives for the various ZAGA implant paths. It aids the dentist in understanding inter-individual and intra-individual anatomic variations. When the residual bone at the sinus floor level has adequate thickness and width (minimum: 4 mm height, 6 mm width) in a patient without a history of periodontitis, the position of the entry point should be close to the middle portion of the crest with an intra-sinus starting path of the implant if the maxillary wall is flat or convex. When the crestal bone height or thickness is inadequate, the alveolar entrance point should be shifted buccally, regardless of the maxillary wall curvature. Based on the maxillary wall concavity and the height of pristine bone, the osteotomy is shaped like a tunnel or canal (Figs. 5 and 10).

In cases where the alveolar architecture is insufficient, the main objective is to establish an implant trajectory that:

The zygomatic implant gains its main anchorage when the zygomatic bone is perforated twice on its facial cortical side. An oblique lateralized path of the implant should be aimed for whenever possible. As per the ZAGA protocol, the key factors while selecting the anterior maxillary or zygomatic cortical entry point of the implant are:

At this surgical stage, iatrogenic zygomatic bone fracture and interference with structures of the orbital fossa should be avoided (Fig. 11).

Two prevalent situations associated with the number of zygomatic implants to be inserted are:

Following the ZAGA Concept of prevention of complications, the authors prefer contemporary narrow implant designs with an apical diameter of 3, 4 mm. These designs need a smaller initial osteotomy and preserve more bone volume between implants, which in turn decreases the risk of zygoma bone fractures. Two main aspects to be taken into consideration while achieving zygomatic implant stability are:

As per the ZAGA protocol, the osteotomy at the zygomatic level should have two features:

The zygomatic path of the osteotomy requires maximum implant stabilization. This is achieved by perforating the facial aspect of the zygoma twice, once at the inferior entry point and then at the superior exit point. After deciding the alveolar implant position, the zygomatic entrance is decided according to the zygoma process curvature. The more curved the zygomatic process, the further from the alveolar bone will be the antrostomy position, whereas the less pronounced the facial zygomatic curvature, the closer to the alveolar process will be the antrostomy. The zygomatic osteotomy can include posterior perforation of the zygomatic bone into the temporal fossae, partial implant path on the posterior temporal fossae, zygomatic upper posterior entrance from the sinus cavity, or temporal fossae, and final zygomatic upper anterior cortical exit.

For prevention of a drilling-related zygoma fracture, it is essential to strategically position the zygomatic entry point to maintain at least 3 mm of residual bone above the apical end of the zygomatic implant. The narrower the implant diameter, the more conservative and less invasive will be the necessary osteotomy (Fig. 11).

After defining the coronal entry point on the crestal bone, the bur points toward the zygomatic bone. The ZAGA technique does not require an antrostomy before the zygomatic osteotomy in most cases. The aim of placing zygomatic implants is to establish an implant bed that complements the implant body. Matching the osteotomy to the implant configuration eliminates the requirement of a lateral window and preserves the maximum amount of bone at the crest and the maxillary lateral wall level (Fig. 10). During the surgery, no slot or window is created. Alternatively, a direct apical maxillary or zygomatic osteotomy is done that complements the implant path. The factors dictating the apical osteotomy site and the implant direction are:

When the coronal and apical points are connected by inserting an implant, the implant body complements and seals the osteotomy eliminating the requirement of bone grafts.

We are conscious that the proposal of not making a maxillary window prior to the osteotomy preparation can create the sensation of working blind. However, this is not the case for different reasons. Total control of the surgical field boundaries is obtained provided an adequate flat is raised (Fig. 2a, b). Then, a modified retractor instrument will be located on the angle formed by the temporal and frontal apophasis of the zygoma. Indeed, adequate flap extension and retractor position can satisfactorily expose the limits of the intervention. Precise knowledge of the specific zygomatic, maxillary and crestal anatomy is achieved thanks to the pre-surgical CBCT. Indeed, the entrance and emerging alveolar and zygomatic points are virtually planned to follow the ZAGA protocol before the surgery. Before the drilling procedure, the intended ZICZ and the antrostomy points are transferred to the surgical field using a marker. In the case of a ZAGA type 0 maxilla showing an adequate amount of remaining sinus floor (i.e.,  > 4 mm height), the zygomatic entrance cannot be directly seen since an intra-sinus path is planned. Then, a guiding line, externally joining the alveolar entrance with the planned zygomatic entrance, is drawn to provide drilling orientation to the surgeon. Ideally, a rehearsal surgery is performed in a 3-D model (3-D models can be printed by sending the DICOM images to ct@zagacenters.com and completing a form at www.​zagacenters.​com) (Fig. 12).

The implant osteotomy is achieved by joining the ZICZ and the AZ sites once they have been identified. Implant path is obtained by preparing a particular osteotomy in as less traumatic as possible manner with no necessity for a previous “window” osteotomy. As a result, maximal BIC is obtained along the residual alveolar crest, the maxillary wall, and the zygomatic bone. Moreover, the implant profile will totally seal the minimally invasive osteotomy.

The ZICZ is visually determined using the aforementioned parameters. The zygomatic entrance point can be directly seen in almost every case (unless we use an intra-sinus path). No matter how large the “window” may be, usually, through a maxillary “window osteotomy” it is not possible to see the outer side upper emergence point of the drill. The hypothetical direct visualization of the upper inner perforation of the drill has been claimed to be possible by placing a rectangular “window” at the level of the zygomatic process of the maxilla. However, what we “see” is the drill traversing the window but not the final upper spot to be perforated. Moreover, by using a rectangular upper window, we may lose some mechanical properties of the implant (i.e., exposed apical threads diminishing the stabilization power of the implant).