and Prosthetic Biomechanical Considerations When Using the Zygoma Implant
Quad-cortical stabilization; the four points of BIC when placing the zygoma implant
5.3 The Trajectory of the Zygoma Implant
Preparing the trajectory of the osteotomy for placement of the zygomatic implant is a critical step. There is only one trajectory of the zygoma implant as described by PI Branemark [25, 26]. However, recently there has been a confusion of nomenclature and new recommended surgical protocol with the term “extra-maxillary” zygoma implant being used without long-term, multicentre data . Therefore, it is crucial to understand the proper anatomy and trajectory of the zygoma implant.
Establishing the proper trajectory of the implant following the Branemark protocol may take several attempts by the surgeon. After exposing the posterior maxillary alveolar ridge and the lateral wall of the maxilla, the distal portion of the lateral maxillary wall as it turns medially to form the posterior maxillary sinus wall is identified. This is the vertical “corner” of the posterior portion of the maxillary sinus as represented with the “black dotted line” (Fig. 5.2). To begin the osteotomy, a rectangular opening through the lateral maxillary wall is made into the maxillary sinus with the posterior trajectory of the opening paralleling the “black dotted” line.
The initial drill enters the maxillary sinus through the crestal osteotomy. The platform of the zygomatic implant is generally in the second bicuspid or the mesial half of the first maxillary molar (Fig. 5.3). Its path is directly visualized through the bony window established through the lateral wall of the maxillary sinus. The round drill creates a small indentation in the roof of the maxillary sinus allowing for the purchase of the 2.9 mm twist drill without chattering. To confirm the proper trajectory established by the long axis of the round drill, the shaft of the round drill should parallel the posterior wall of the maxillary sinus opening which in turn parallels the “black dotted” line (Fig. 5.4). After establishing a shallow osteotomy by the round bur to prevent the 2.9 mm drill from chattering, the round drill is removed and the 2.9 mm drill enters the floor of the sinus. It travels through the maxillary sinus, enters the zygoma bone via the roof of the maxillary sinus and eventually exits at the fronto-zygomatic notch. At this point, the shaft of the 2.9 mm drill is also parallel to the posterior wall of the maxilla (Fig. 5.5). Once the implant has been fully seated, its proper trajectory is confirmed by the distal portion of the zygomatic implant body being immediately adjacent and in intimate contact with the posterior wall of the lateral maxillary wall (Fig. 5.6).
It is important to discuss the various relationships that may exist between the mid-portion of the zygoma implant and the lateral wall of the maxillary sinus.
Aparicio in multiple publications has described the various relationships of the mid-portion of the zygoma implant with the lateral maxillary wall [28–31]. The lateral maxillary wall may be “straight”, ZAGA 0 or “concave” ZAGA 4. The classification by Aparicio describes the relationship of the lateral maxillary wall to the mid-portion of the zygoma implant. If the lateral maxillary wall is straight and without any concavity, then the entire implant is within the maxillary sinus.
However, with various degrees of lateral maxillary wall concavity, the mid-portion of the zygoma implant may be partially or totally outside of the maxillary sinus. It is important to appreciate that no matter what the anatomy of the lateral maxillary wall may be, the trajectory of the zygoma implant in all cases is “ad modum Branemark”.
Aparicio begins with ZAGA 0, referring to the lateral maxillary wall being straight with the mid-portion of the zygoma implant inside the sinus. He then describes varies levels of concavity of the lateral maxillary wall and ends with ZAGA 4 which is the most extreme resorption of the maxilla. In ZAGA 4 patients, the platform and the mid-portion of the zygoma implant are “outside the maxillary sinus” due to lack of bone as a consequence of either the existing “normal” concave topography of the patients’ lateral maxillary wall or severe resorption of the patients’ lateral maxillary wall as well as in patients having had oncologic resection of their maxilla (Fig. 5.7). It is once again emphasized by the authors that the trajectory of the osteotomy for the placement of the zygoma implant is to be followed as described by professor Branemark, regardless of the intra- or extra-sinus relationship of the mid-portion of the implant (Fig. 5.8). The intra-sinus trajectory of the implant is preferred in order to achieve a “quad-cortical” stabilization and the surgeon must recognize that the presence of the implant outside the maxillary sinus and directly beneath the vestibular soft tissues may lead to erosion of the mucosa and exposure of the implant shaft (Fig. 5.9). In an attempt to allow for ease of long-term hygiene maintenance of the exposed intraoral threads of the zygoma implant, few case reports have been published with limited success [32, 33]. The inclusion of the buccal fat pad graft either at the time of the initial surgery or as a “preventative” measure against soft-tissue dehiscence only adds unnecessary morbidity for the patients due to the poor prognoses of these type of soft-tissue defects.
In extreme cases such as ZAGA-3 and ZAGA-4 or in cases of total or partial maxillectomy, the zygoma implant will be immediately beneath the soft tissues of the vestibule and the occurrence of soft-tissue dehiscence cannot be prevented. In all other cases, the lateral contour of the maxillary sinus wall will dictate the degree of the zygoma implant exposed outside of the maxillary sinus. Attempt should be made to place the mid-portion within the maxillary sinus following the Branemark protocol as described earlier in order to minimize the long-term potential soft-tissue complications.
The indication for the placement of the zygoma implant is lack of bone in Zones 2 and 3 (bicuspid and molar region, respectively). In this group of patients, there is commonly less than 2–3 mm of residual maxillary alveolar bone remaining in the bicuspid and molar regions. Therefore, it is important to review the response of the maxillary crestal bone as well as the zygoma bone under functional loads.
Ujigawa and colleagues , in an elegant finite element analysis, described the distribution of forces when the zygoma implants are used to support a full-arch maxillary prosthesis. In the Ujigawa study, two different models were considered. Each model had two zygoma implants, one in each posterior quadrant as well as two anterior maxillary implants. In the first model, the implants were rigidly cross-arch splinted, whereas in the second model, all implants were lone standing without any cross-arch splinting under function.
Both models were subjected to simulate centric and lateral occlusal loading. Two areas were identified as load bearing along the implant: the abutment-implant platform interface (yellow arrow) and the mid-portion of the implant (red arrow) (Fig. 5.10). In the un-splinted model, the magnitude of the loads was significantly higher than in the splinted model. In lateral excursion, significant increase in the magnitude of the load distribution pattern was also observed with the highest loads seen in the un-splinted zygoma implant model which can lead to stress fractures at the neck of the zygoma implant, red arrow (Fig. 5.11). This FEA study is consistent with the past clinical reports  supporting the importance of cross-arch splintingin the zygoma implants with other stable implants within the arch to allow for a more favourable force distribution.