Restoring the dentition of an edentulous patient is often challenging. Endosseous dental implants have allowed for far more versatility in this area but still require adequate maxillary and mandibular alveolar bone. Often, unless significant bone grafting techniques are used, true restoration of the dentition can be impossible with traditional endosseous implants. The advent of zygomatic implants, however, may provide a viable, predictable, and stable alternative for the restoration of the dentition in patients with severe maxillary alveolar bone loss.
Atrophic maxilla following extraction of teeth is a commonly seen dilemma faced by implant surgeons. Inadequate bone foundation requires bone augmentation which can be a lengthy and complex process. Zygomatic implants offer the implant surgeon a viable alternative.
Zygomatic implants can by themselves (if 2 implants are placed on either side of the maxilla, referred to as quad implants) offer a viable alternative for retention of dentures in an otherwise atrophic jaw.
Zygomatic implants can easily be prosthetically managed as any other endosseous implant for the capable restorative dentist.
Endosseous dental implants have quickly become the mainstay for restoration of edentulous spaces and an adjunct to the restoration of completely edentulous patients. The major limitation in the use of endosseous implants is the availability of sufficient alveolar bone. In patients with severely limited bone due to resorption, resection, or degenerative disease, for example, the process of implant placement and restoration can prove difficult, if not impossible.
The longevity of dental implants can be even more compromised in the maxilla. It has been shown that the osseointegration of implants in even a healthy and robust maxilla has lower success rates than the mandible. These success rates are even lower in the posterior maxilla compared with the anterior. In patients with a severely atrophic maxilla successful endosseous implant placement becomes even more challenging. Many solutions have been offered and practiced for treating such a maxilla, such as onlay bone grafting, sinus lift procedures, ridge split procedures, and even LeFort I surgical downfracture with interpositional bone grafting. In recent years, however, the advent of zygomatic implants may allow for a much simpler solution for the restoration of the dentition of a patient with an atrophic maxilla.
Treatment options for the atrophic maxilla
As mentioned earlier, various modalities have been used in order to aid in placement of implants into low quality and quantity of maxillary bone. The gold standard of treatment has long been bone grafting procedures, such as crestal onlay grafts, sinus lifts, and Lefort I osteotomy with interpositional bone grafting. However, bone grafting procedures are not always a viable option for many patients. Patients with cancer who have undergone head and neck radiation therapy, for example, may present with compromised vasculature, rendering bone grafting a procedure with high risk of failure. This same can be said for patients with certain metabolic disorders, congenital deformities, or those in an immunocompromised state.
Even without the aforementioned comorbidities, patients may choose not to undergo extensive bone grafting procedures due to factors such as graft donor site morbidity, increased healing time, longer surgical time, and increased chance of infection. Lower implant survival rates for grafted areas of the maxilla have also been reported, when compared with native bone. Zygomatic implants offer a nongrafting option to restore the resected or atrophic maxilla.
Overview of maxillary bone quality
As previously stated, the type of bone present in the maxilla is usually of lower quality and quantity than that of the mandible. Lekholm and Zarb (1985) described 4 main bone types in the maxilla and mandible, listed as follows in descending order of density:
Type 1: mostly or all homogenous cortical bones
Type 2: bone that contains a core of densely packed cancellous bone, surrounded by at least 2 mm of cortical bone
Type 3: bone that contains a core of densely packed cancellous bone, surrounded by only one layer of cortical bone less than 2 mm
Type 4: mostly nondense cancellous bone, surrounded by only a thin layer of cortical bone
In general, the success rate of osseointegration increases in proportion to the bone density. It is thought that increased bone-implant contact and stabilization occurs in dense bone, as opposed to the loosely organized cancellous bone. A healthy maxilla will usually have type 3 bone anteriorly, changing to type 4 in the posterior. Bone density is often even further diminished in a resorbed maxilla. Studies have shown that the maxilla resorbs, on average, 2 mm within the first year after tooth extraction, and then at a rate of 0.5 mm/y, compared with 0.2 mm/y in the mandible.
The Branemark System outlines a surgical technique for the intrasinus placement of zygomatic implants in 1988. Zygomatic implants have been shown to be successful as support for an obturator or other larger maxillofacial prostheses for patients who had undergone maxillectomies. Since then, the surgical techniques and approaches for placement of zygomatic implants have been expanded. Two main treatment designs are currently commonly used for the restoration of a patient’s dentition:
For patients with sufficient anterior maxillary bone for traditional implant placement: 1 zygomatic implant placed on each side of the posterior maxilla (2 total), and 2 or more traditional endosteal implants placed in the anterior maxilla.
For patients without sufficient anterior maxillary bone: 2 or more zygomatic implants on each side of the posterior maxilla.
Both designs have been shown to have high success rates when used to support a fixed dental prosthesis or overdenture. Some have documented success rates approaching 100%. Although some studies have shown occasional failures, most agree that zygomatic implants have success rates of greater than 95%.
Zygomatic implant stability
The stability and success of zygomatic implants is likely not due to the bone quality of the zygoma, itself. On the contrary, the zygoma is composed mainly of loose cancellous bone that, as explained earlier, is not favorable for implant osseointegration. Rather, the stability of zygomatic implants is thought to be due to the fact that the implant usually passes through 3 to 4 cortical layers of bone, compared with the single cortical layer that most traditional implants would pass through (explained later in this article).
Technique for placement of zygomatic implants
Many techniques have been described for zygomatic implant placement. One technique, known as the intrasinus approach, is the most commonly used, which is explained later (surgical steps adapted from Dental Clinics, Clinics Review Articles, April 2020). Other approaches are described as variations or modifications on the intrasinus approach .
Surgical incision and flap : a crestal incision is made on the palatal aspect of the maxillary crest from the area of the first molar to the opposing first molar. A flap is then elevated to expose the lateral surface of the maxilla until the zygomatic process is revealed, similar to the flap used in a LeFort 1 osteotomy. The infraorbital neurovascular bundle should also be visualized. At this point, due to the placement of the incision, both the buccal and palatal aspect of the alveolar crest should be fully exposed. One technique that the investigators use for this step includes an anterior midline vertical releasing incision. This allows for complete flap elevation unilaterally ( Fig. 1 A), which can be helpful for ease of flap elevation and hemostasis. Closure can be performed one side at a time or both sides simultaneously ( Fig. 1 B).
Lateral window : a window should be made into the lateral aspect of the maxillary sinus, close to the inferior border of the zygomatic crest. This can be performed with a rotary handpiece and round bur, Piezo, or any such instrument that would normally be used for the osteotomy of a lateral window approach for a maxillary sinus lift. The dimensions of the window should be such that it will facilitate easy visualization of the implant drill and zygomatic implant, itself ( Fig. 2 ). The suggested size is approximately 10 × 5 mm.