Despite improvements in bone preservation following tooth extraction, edentulous sites are often deficient in bone volume for conventional dental implant therapy. Missing bone volume is often recaptured by surgery and grafting. This article discusses noninvasive alternatives to bone grafting. Part I of this topic discussed the use of short and narrow diameter implants. Part II discusses three additional alternatives: the use of tilted implants, the use of four or fewer tilted and axially-loaded implants to support a full-arch fixed-dental-prosthesis (FAFDP), and the use of zygomatic implants to restore the severely-atrophic edentulous maxillae lacking adequate bone for conventional treatment.
Alternatives to invasive site development for dental implant therapy include the use of tilted dental implants, zygomatic implants, and fewer numbers of implants. These alternatives have the advantages of reduced morbidity, fewer complications, shorter treatment time, lower costs, and better patient acceptance.
Tilted implants often eliminate the need for bone grafting.
Use of tilted and upright implants in combination allows clinicians to predictably rehabilitate edentulous arches with a fixed prosthesis supported by as few as 4 dental implants.
Zygomatic implants, either alone or in combination with upright and tilted maxillary implants, can provide a graftless solution to restore the atrophic edentulous maxilla when there is inadequate bone for conventional or all-on-4 treatment.
Edentulous sites are often characterized by moderate or advanced ridge resorption and irregular crestal bone topography, which compromises proper 3-dimensional implant positioning. If a given site cannot be functionally or esthetically restored with a dental implant, bone grafting is often needed. Bone grafting often prolongs treatment time and increases morbidity and expense. For these reasons, patients prefer less invasive treatment alternatives.
One alternative to bone grafting is the use of tilted implants that engage native bone with adequate volume, density, and structure to support a fixed restoration. For the edentulous arch, a second alternative is to use a combination of four or fewer tilted and upright implants to support full-arch fixed dental prostheses (FAFDPs). This chapter will address three questions: (1) Are tilted implants a sound alternative to axially loaded implants placed in grafted edentulous sites? (2) Are 4 or fewer axially loaded and tilted implants adequate to support a cross-arch fixed prosthesis? (3) Are zygomatic implants a viable alternative to invasive bone grafting of severely atrophic edentulous maxillary ridges?
Are Tilted Implants a Sound Alternative to Axially Loaded Implants Placed in Grafted Sites in Edentulous Arches?
Historically, the use of upright implants (implants placed perpendicular to the plane of occlusion) was the standard procedure for the rehabilitation of completely edentulous arches with implant-supported fixed prostheses. However, this is often not achievable without extensive bone grafting and avoiding vital anatomic structures (eg, the inferior alveolar nerve, the maxillary sinus, and the nasal cavity). The use of tilted implants is an alternative to upright implant placement in grafted bone. Tilted implants engage the maximum available residual bone in remote sites for enhanced anchorage and stability.
Guidelines and history
Tilted implants allows fixtures to be placed in any direction targeting native bone. For the edentulous maxilla, these sites include the anterior and posterior walls of the maxillary sinus, the nasal floor, the nasal spine, the pterygoid process, and the maxillary tuberosity. The use of tilted implants to avoid bone grafting in the edentulous maxilla was first described by Mattson et al. This study utilized 86 axial and tilted implants to treat 15 patients who needed bone grafting for conventional axial implant placement. In the edentulous mandible, a combination of axially loaded and tilted implants can be placed in the interforaminal region to avoid bone grafting in the posterior region. In addition, angulated implants can optimize anchorage, minimize cantilever length, and maximize interimplant distance. Maló et al restored the edentulous mandibles of 44 patients with FAFDPs supported by 176 axially and distally inclined Brånemark implants. Maló et al called this the “all-on-4” treatment concept.
Advantages of tilted implants
Implant angulations, lengths, and diameters are generally selected to fit within the boundaries of available bone. A minimum angulation that defines a tilted implant has not been established. However, a range of 15° to 45° off the vertical axis has been used to describe tilted implants by several authors. Longer implants are generally preferred to improve primary stability and engage as much residual bone as possible. Most tilted implant diameters fall within the range of 3.3 to 5.0 mm.
For implant rehabilitation of edentulous arches with inadequate bone volume, tilted implants have several advantages over axially loaded implants. The use of longer fixtures improves primary stability by engaging more bone. The increased distance between the anterior and posterior restorative platforms created by tilting implants reduces cantilever length and improves load distribution. Tilted implants are placed in residual native bone, eliminating the need for nerve lateralization and bone grafting, thereby reducing morbidity, expense and treatment time. Finally, tilted implants with good primary stability and cross-arch distribution can generally be immediately loaded with a fixed interim prosthesis, while allowing fewer numbers of implants to be used ( Figs. 1–4 ).
Outcomes for tilted versus upright implants
There is a scarcity of randomized controlled trials (RCTs) comparing outcomes for patients treated with upright implants placed in grafted sites with similar groups restored with tilted implants in nongrafted sites. One of the first forays into this area was by Widmark et al., who reported that in the maxilla, implants placed in native bone had a higher success rate after five years (87%) than implants placed in grafted sites (74%). A limitation of this study is that although trial implants were positioned to avoid bone grafting, not all were tilted.
Another early study restored maxillary posterior quadrants of 25 partially-edentulous patients with partial fixed dental prosthesis (PFDP) using 101 upright and tilted implants in nongrafted sites. After five years, investigators reported a cumulative survival rate (CSR) of 95.2% for 42 tilted implants and 91.3% for 59 axial implants. Prosthesis survival at five years was 100%. A more recent study reported three-year outcomes for 37 patients restored with a mandibular FAFDP supported by either four upright implants, or two upright anterior and two tilted posterior implants. There were no reported differences in outcomes between the 2 groups with respect to implant survival, prosthesis complications, marginal bone loss (MBL), or periodontal probing depth. Recent systematic reviews pooling data from a number of clinical studies using tilted implants reported similar findings.
In summary, tilted implants, placed in tandem or in combination with upright implants, can reduce or eliminate the need for bone grafting, reduce complication risk, and result in high levels of patient satisfaction when used to restore partially and fully edentulous jaws.
Are Four or Fewer Implants Adequate to Support a Full Arch Fixed Prosthesis?
An implant load analysis investigation concluded that as few as four implants may be adequate for a full-arch fixed prosthesis when the implants are properly positioned. An early application of this finding was the introduction of the all-on-4 immediate-function treatment concept for the mandible in 2003, and the maxilla in 2005. The all-on-4 immediate-function treatment concept consists of 3 principles: the use of a combination of upright and tilted implants to avoid bone grafting, 4 implants in each edentulous jaw to support a fixed prosthesis, and immediate loading.
The all-on-4 treatment protocol uses four upright and tilted implants in the anterior edentulous jaw to support an immediately loaded, cross-arch fixed prosthesis. Two anterior implants are axially oriented and placed in the incisor region, while posterior implants are tilted distally with the implant platform in the second premolar/first molar area. The goal is to create a large inter-implant distance that minimizes cantilever length. To facilitate immediate loading, a modified surgical protocol can be followed which achieves an adequate minimum insertion torque depending on the quality and volume of available bone.
All-on-4 treatment outcomes
Studies comparing the effectiveness of the all-on-4 treatment concept to 5 or more upright implants in grafted bone are lacking. One retrospective study reported outcomes for 380 patients restored with 482 immediately loaded FAFDPs supported by 4, 5, and 6 upright and tilted implants. The 7-year CSR for the 2081 implants was 97.0%. Implant survival rates for restorations supported by 4, 5, and 6 implants were 96.5%, 96.6%, and 99.7%, respectively. However, the study protocol dictated that if an insertion torque of ≥30 Ncm was achieved for 4 implants, no additional implants were placed. This suggests that when five or six implants were placed they may have been used in arches with lower bone volume or quality.
Two systematic reviews reported favorable all-on-4 treatment outcomes, although some of the included studies were of limited quality and short duration. A long-term clinical study followed 245 patients with edentulous mandibles treated with the all-on-4 protocol for 10 years. The 10-year implant CSR was 94.8%, with a prosthesis survival rate of 99.2%. Another clinical research group treated 200 maxillary and mandibular arches with 800 implants following the all-on-4 treatment protocol. After five-years, an, implant CSR of 97.3% was reported for both axial and tilted implants. The prosthetic survival rate was 99.0%. Similar outcomes were reported in another study where 111 patients were treated and followed for up to 7 years. The 7-year implant CSR was 94.5%, while prosthetic survival rate was 97.8%. The average MBL at 5 years was 1.27 mm for tilted implants and 1.34 mm for axial implants. A study that followed 12 patients for 7 years reported 100% implant and prosthetic survival rates, but documented several prosthetic complications. These complications included tooth fracture (6.25%), loosening of prosthetic and abutments screws (18.75%), and replacement of denture teeth (18.75%)
Full-arch fixed dental prostheses with 2 or 3 implants
Recent preliminary investigations report 6- and 12-month outcomes for FAFDPs supported by 2 (fixed-on-2) or 3 (fixed-on-3) implants in the mandible and maxilla, respectively. A 6-month cohort study placed machined surface implants as parallel as possible using a flapless surgical approach to treat 25 patients. If a minimum insertion torque could be achieved, an immediate, interim fixed prosthesis was placed. Investigators reported 100% implant and prosthesis survival for both arches after 6 months of loading.
Another 12-month study followed a similar treatment protocol (fixed-on-2 for the mandible, fixed-on-3 for the maxilla), with 40 patients randomly assigned to 1 of 2 treatment arms. Group 1 received machined surface implants, while the other received rough surface implants. For the mandible, implants were placed in both canine/first premolar positions, while in the maxillae, the 2 distal implants were placed as far posteriorly on each side of the arch as bone volume allowed. The third implant site was placed as centrally as possible, preferably in the central incisor region but, wherever bone volume allowed. Two implants were lost during the reporting period. Only minor prosthetic complications were reported, including several functional or esthetic complaints associated with the shortened dental arch prosthesis. Otherwise, significant differences were not detected between the two treatment groups.
Advantages of these treatment protocols include the ability to simultaneously treat both jaws in a single, relatively short surgical session, the comparative ease of fabricating prosthesis frameworks that precisely and passively seat on fewer numbers of implants, and lower expense associated with fewer implants. However, while these results suggest that an immediately loaded cross-arch fixed prosthesis can be supported by only 2 mandibular and 3 maxillary dental implants, it must be emphasized that this treatment approach has not been validated. For this reason, it should be considered preliminary and highly experimental at this time.
It remains unclear as to the optimal minimal number of implants needed to support a partial or cross-arch fixed prosthesis at this time. Two well-constructed recent reviews independently concluded that successful outcomes after 5 years and beyond should be expected for FAFDPs supported by 4 to 6 implants. However, the number of teeth replaced by an implant-supported FAFDP can be variable, making it difficult to make direct comparisons across studies. Since FAFDPs generally replace 10 to 12 teeth, the implant-to-replaced-units ratio may be a more reliable metric for reporting implant and prosthetic survival outcomes in the future.
Are Zygomatic Implants a Viable Alternative to Invasive Bone Grafting Severely Atrophic Edentulous Maxillary Ridges?
Planning the restoration of the edentulous maxilla with an implant-supported FAFDP can sometimes require addressing profound anatomic challenges. The combination of a severely atrophic maxillary ridge and large pneumatized maxillary sinus can lead to inadequate bone volume for vertical or tilted implants. In 1998, Branemark introduced zygomatic implants as an alternative anchorage solution for cancer patients who had undergone maxillectomy. Zygomatic implants are self-tapping, screw-shaped fixtures that engage the zygomatic buttress to gain primary stability and anchorage.
Zygomatic implants can be used to restore the severely atrophic edentulous maxilla without bone grafting and in reduced treatment time. Zygomatic implants can be used independently, or in combination with vertical and tilted implants placed in the anterior and posterior maxilla. A limitation of zygomatic implants is the surgical parameters dictated by the anatomy of zygomatic bone. Zygomatic implants have also been associated with increased sinusitis and peri-implant bleeding due to oral hygiene difficulties.
Zygomatic implants are available in 8 different lengths starting from 30 mm to 52.5 mm. They have a 45° angulated head to create a restorative platform that is accessible to the restorative dentist and parallel to the platforms of vertically placed implants. The apical section, which engages the zygomatic bone, is threaded with a diameter of 4 mm, while the head of the fixture, which engages the maxillary residual alveolar ridge, has a diameter of 4.5 mm.
General guidelines for zygomatic implants
Patient selection is determined following a detailed prosthetic and surgical evaluation. Bedrossian et al divided the maxilla into three zones, the anterior teeth (zone 1), the premolars (zone 2), and the molars (zone 3). The treatment concept is based on the volume of available bone in each of the 3 zones. If adequate bone is available in zones 1, 2, and 3 for axially loaded implants, a traditional approach can be followed using upright implants in the anterior and posterior maxilla. If adequate bone is only available in zones 1 and 2, the all-on-4 treatment concept can be implemented. The zygomatic implant concept was developed for patients with insufficient bone in zones 2 and 3, or lacking bone in all 3 zones. In cases where bone is only available in zone 1, prosthesis support can be derived from a minimum of 4 implants, 2 premaxillary implants and 2 zygomatic implants. The angulation of the head of the zygomatic implant allows the restorative platform of all implants to be in the same plane. Anterior and zygomatic fixtures can then be splinted to support a screw-retained fixed prosthesis. In situations whereby bone is lacking in all 3 zones, 4 zygomatic implants (quad zygoma), 2 in each buttress, can be used to support an FAFDP.
In summary, when posterior bone is unilaterally or bilaterally lacking in zones 2 and 3 for proper prosthesis support, zygomatic implants can complement upright or tilted conventional implants. It is also provides a graftless solution when all 3 zones are deficient in bone.
Treatment outcomes with zygomatic implants
A recent systematic review investigated the survival and complications of zygomatic implants over 12 years. The survival rate of zygomatic implants was 95.21%. Implant failure was only detected in the first six months following surgery, with sinusitis the main complication at a rate of 2.4%. In three additional reports, zygoma quad and two zygoma implants coupled with maxillary anterior axial implants have been shown to have similar survival rates.