The maxilla is a challenging area for dental implant restoration. Encroachment of anatomic structures such as the sinus and nasal floor make vertical placement difficult. Implants placed at an angle may be used to avoid these anatomic structures or eliminate the need for a bone grafting procedure. The question occasionally arises about the possible detrimental effects of placing implants at an angle. It should be noted that because of bone resorption numerous implants, especially in the maxillary anterior, have been placed at significant angles for many years. Anecdotally these tilted implants seem to work, but what evidence is available in the literature with regard to the efficacy of implants placed at an angle?

A literature search was conducted regarding the placement of off axis implants. It has been concluded by some using Finite Element Analysis, mathematical models, and mechanical testing that off-axis loading will produce more stress on the implant and implant/bone interface, although the 2 articles that speculated on the possible results of these forces believed the forces would be within the physiologic range for the most part. In other studies Finite Element Analysis concluded that, under many common clinical situations, no stress differences were apparent between tilted and nontilted implants. Two animal studies showed no apparent long-term differences in hard or soft tissue results around nonaxial implants, although one showed short-term differences in the healing mechanisms.

Although mathematical models, mechanical testing, and animal studies can provide useful information, long-term human clinical results are required to ensure a procedure is effective. There have been numerous studies and articles published regarding tilted implants in humans.

Implants placed into the pterygomaxillary regions were some of the first implants intentionally tilted. Such implants have been used for more than 20 years. Pterygomaxillary implants often allow for the placement of implants in the posterior maxilla without the use of sinus augmentation procedures or other types of bone grafts. This method decreases the cost of implant treatment and saves time, eliminating the need for cantilevers in many cases. Balshi and colleagues found the survival rate of these implants to be comparable to previous studies for implants placed in the maxillary arch. A subsequently published study by the same investigators using surface-roughened implants in the pterygomaxillary region showed excellent clinical results. Valerón and Valerón followed pterygomaxillary implants for a minimum of 5 years and up to 10 years. These investigators lost only 2 of 152 implants after functional loading, and concluded that despite the necessity for inclination, these implants easily supported functional load. It should be noted that these implants are often placed into the worst quality bone and under the highest forces possible. The majority of the implants in most studies were 4.0 mm or less in diameter. All articles on these off-axis implants in the pterygomaxillary region appear to endorse their use.

Another implant that is intentionally placed at an angle is the zygomatic implant. These implants have also been used for more than 15 years. Three studies concluded that these implants are a predictable alternative to extensive bone grafting. Two other articles found acceptable results but advocated further studies. None of these articles referenced concern regarding adverse outcomes due to the angulations of these implants.

Implants placed off-axis usually require angle-corrected abutments. Eger and colleagues concluded that implants placed at unfavorable angles may be restored with angled abutments without compromise of function or esthetics. Sethi and colleagues published 2 articles following 3100 angle-corrected restorations over 10 years, concluding that good esthetic and functional results can be achieved. Koutouzis and Wennström compared bone levels of fixed partial dentures restored on implants at 5 years that used both axial and nonaxial placed implants, and concluded that implant inclination had no effect on peri-implant bone loss.

Articles have been published using intentionally tilted implants in other locations. Krekmanov and colleagues followed cases for up to 5 years that involved the tilting of implants distally anterior to both the sinus and the mental foramen, and concluded that this method of treatment for edentulous arches represents an alternative or complementary technique to others mentioned in the literature. The investigators stated that this technique leads to an improved position of support, and allows for placement of longer implants and/or improved anchorage in dense bone. Biomechanical measurements showed that the tilting does not have a negative effect on the load distribution when it is a part of prosthesis support. The advantages are further extension of the prosthesis in a posterior direction, possible use of longer posterior implants, and improved bone anchorage. Krekmanov and colleagues concluded that the technique is relatively easy to perform in any outpatient setting by a surgeon who is not familiar with bone grafting of the maxillary sinus. Furthermore, it eliminates the need for such advanced techniques for some patients.

Maló and colleagues used implants in the maxilla and mandible in a similar manner to Krekmanov except that most implants were immediately restored. At 1 year Maló and colleagues concluded that this treatment modality was highly successful. Four additional studies used a similar technique, immediately restoring the maxilla and/or mandible with full-arch fixed prostheses. All 3 studies found similar bone levels, and all 3 concluded that tilted implants may be a viable treatment modality.

Rosén and Gynther followed implants in the maxilla for 8 to 12 years that were tilted to avoid grafting procedures, concluding that this was a successful alternative procedure to the more resource-demanding techniques involving bone grafting. Calandriello and Tomatis showed similar finding in a 1-year follow-up study. Krennmair and colleagues studied 62 patients with overdentures and analyzed the various angles of the implants placed for optimal restoration. It was concluded that sagittal inclination should be attributed more importance than axial loading of implants. Aparicio and colleagues followed fixed implant bridges supported by both axial and tilted implants for 21 to 87 months after insertion. Fortin and colleagues followed intentionally placed tilted implants using an image-guided system in the atrophic maxilla over 4 years. Both of these groups concluded that the use of tilted implants is an effective and safe alternative to maxillary sinus floor augmentation procedures.