Key points

Introduction

In recent decades, endosseous dental implants have become an increasingly popular solution to the problem of replacing lost or missing teeth. Because of the near-constant strides in research relating to dental implants, advances in effectiveness, efficiency, and cost have made dental implants a viable option for a large percentage of the population. However, limitations still exist. Sufficient bony height and width are still required for successful placement and retention of functional dental implants. In some patients, this can present a challenging scenario for the oral surgeon and restorative dentist, both in the placement and in the restoration of the dental implants.

Osseointegration of implants in even a healthy and robust maxilla has been shown to have lower success rates when compared with the mandible. This is especially true for implants placed in the posterior maxilla. In patients with a severely atrophic maxilla, successful dental implant placement is especially difficult. Various solutions have been proposed for restoring the maxillary dentition in a patient with an atrophic maxilla. Options such as onlay bone grafting with autogenous iliac crest bone, sinus lift procedures, ridge split procedures, and even LeFort I surgical downfracture with interpositional bone grafting have been utilized. However, the zygomatic implant may present a far simpler approach to restoring the atrophic maxilla.

Overview of maxillary bone quality

As referenced previously, the quality and quantity of maxillary bone are often less than that of the mandible. This is mainly because of the type of bone present in the posterior maxilla. Lekholm and Zarb (1985) described 4 main bone types in the maxilla and mandible:

• 1.

  Type 1: bone comprised mostly (if not all) of homogenous cortical bone

• 2.

  Type 2: bone that contains a core of densely packed cancellous bone, surrounded by at least 2 mm of cortical bone

• 3.

  Type 3: bone that contains a core of densely packed cancellous bone, but surrounded by only a thin layer of cortical bone.

• 4.

  Type 4: bone comprised of mostly nondense cancellous bone, surrounded by only a thin layer of cortical bone

The higher the overall bone density, the greater the success rate of osseointegration. This is likely because of higher levels of bone-to-implant contact and stabilization, which occurs readily in dense cortical bone as compared with the more loosely organized cancellous bone. Generally, healthy maxillae have been shown to have type 3 bone in the anterior and type 4 in the posterior. In resorbed maxillae, the bone density is often even further diminished. This diminishment in the quality and quantity of maxillary bone is only exacerbated the longer a patient remains edentulous. It has been documented 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.

Treatment options for the atrophic maxilla

In order to solve the problem of low quality and quantity of maxillary bone for placing implants, various treatments have been proposed. For many years, the gold standard of treatment was considered to be bone grafting procedures. This included techniques such as crestal onlay grafts, sinus lifts, and, as mentioned previously, Lefort I osteotomy with interpositional bone grafting. It is beyond the scope of this article to go into these techniques in detail, other than to mention that bone grafting procedures are not always a viable or desirable option for many patients. For patients who have undergone maxillary resection and/or radiation therapy for cancer treatment, bone grafting may not be a viable option, because of compromised vasculature. The same can be said for patients with certain metabolic disorders, congenital deformities, or those in an immunocompromised state. Even in a healthy edentulous patient, factors such as graft donor site morbidity, increased healing time, longer surgical time, and increased chance of infection can all be factors that would deter an individual from wanting to undergo extensive grafting procedures. It has also been reported that there is a lower implant survival rate for areas in the maxilla that have been grafted, compared with native bone. Because of this, a nongrafting option to restore the resected or atrophic maxilla could be of great value to surgeons and patients.

Zygomatic implants

As early as 1988, the Branemark System described a standard surgical technique for intrasinus placement of zygomatic implants. Many have documented the placement of these implants for use as supports for an obturator or other larger maxillofacial prostheses for patients who have undergone maxillectomies. Since then, Branemark and others have described various surgical techniques and approaches for placement of zygomatic implants that could be used to restore a patient’s dentition. To this end, there have been 2 main treatment designs utilized.

First, for patients with sufficient anterior maxillary bone for traditional implant placement, 1 zygomatic implant should be placed on each side of the posterior maxilla. Two or more traditional endosteal implants should be placed in the anterior maxilla. For patients without sufficient anterior maxillary bone, 2 or more zygomatic implants should be placed on each side of the posterior maxilla.

Both of these designs have been shown to have high rates of success when used to support a fixed dental prosthesis or overdenture. The success of zygomatic implants as supports for full arch restorations has been documented by some to be at 100%. Other studies have shown some failures, but most authors agree that zygomatic implants are successful well over 95% of the time.

It should be noted that the success of zygomatic implants is likely not due to the bone quality of the zygoma, itself. On the contrary, the zygoma is comprised mainly of loose cancellous bone that is not favorable for implant success. The stability of zygomatic implants is thought, instead, to stem from 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. This will be explained.

Indications and contraindications

The obvious indications for zygomatic implants, as mentioned previously, include those patients with a severely resorbed posterior maxilla who require an implant-supported prosthesis. These patients may include those with systemic diseases that cause resorption of the maxilla, patients who had undergone maxillary resection or radiation therapy, immunocompromised patients, or those with congenital deformities such as severe cleft palate. More routine indications could include patients for whom bone grafting would not be desirable because of possible donor site morbidity, increased pain, longer surgical time, or even cultural/religious aversion to foreign bone material.

Contraindications and relative contraindications for zygomatic implants would be similar to those for normal dental implant placement, such as tobacco smoking addiction, head and neck radiotherapy, and bisphosphonate therapy. Additionally, some have stated that because zygomatic implants often pass through the maxillary sinus, their placement may increase the risk of chronic maxillary sinusitis if a patient contracts an upper respiratory tract infection that closes off the sinus ostium. For patients prone to these infections, zygomatic implants may be contraindicated.

Although not a contraindication, it is worthwhile to state a possible drawback to zygomatic implants. Because of their angulation, the head of zygomatic implants will often emerge more pallatally than traditional dental implants. This may make the dental prosthesis excessively bulky in this area, which can cause discomfort to selected patient. The extreme angulation of the zygomatic implant necessitates the use of angled abutments, which come in standard 25°, 35°, 45°, and 55° angulations.