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Since its introduction to dental literature in 2003,1 the All-on-4® (Nobel Biocare, www.nobelbiocare.com) treatment concept has proven to be a predictable2,3 and cost-effective4 method used for full-arch dental implant rehabilitation. True adherence to this manner of treatment involves restoring an arch with at least 4 dental implants, the distal of which are tilted up to 45 degrees, and immediately loading a screw-retained provisional prosthesis.5 Various articles have documented both short- and long-term success rates for All-on-4–style treatment, with dental implant and prosthetic survival rates consistently exceeding 98%.1-12 While these studies document the success rates for this treatment concept and others discuss variations on surgical design to accommodate specific anatomic situations,13-16 few studies examine complications associated with the procedure beyond standard infections or mechanical fracture of the prosthesis.1-20
Accordingly, the purpose of this article is to present obstacles and complications that may be encountered before, during, and after All-on-4–style treatment and how to manage them. The author draws on his experiences with treating more than 500 arches and more than 2000 dental implants using All-on-4–style treatment.
Presurgical Complications
Teeth Exfoliating With Initial Impressions
Many patients requiring All-on-4–style treatment have extremely compromised teeth that may be unintentionally removed during the initial impression process. This is typically due to severely decayed teeth or extremely mobile teeth secondary to advanced periodontal disease (Figure 1). Such situations must be recognized and these patients advised of the potential for tooth loss during the impression process.
When encountering teeth that have been exfoliated with impressions, the author typically cleans the exfoliated tooth and adjusts the length of the root so that the tooth may be temporarily bonded back into place using composite resin in a Maryland-bridge fashion. Occlusion is adjusted on the bonded tooth so that it makes no contact with opposing teeth.
On some occasions, the author has bonded patients’ natural exfoliated teeth to their existing removable partial prostheses. In select cases in which no adjacent teeth or removable prostheses are available for bonding of an exfoliated tooth, the author has used a transitional mini dental implant with a chairside-fabricated temporary crown as in interim restoration prior to the patient’s surgical procedure.
Intrasurgical Complications
Arterial Disruptions During Mucogingival Flap Reflection
Following the administration of local anesthesia, the first step in All-on-4–style treatment is full-thickness mucogingival flap reflection. Depending on anatomic variability, vasculature such as the greater palatine artery or sublingual artery, or various branches thereof, may be disrupted. Should this occur, excessive hemorrhaging may obscure visibility of the surgical field in addition to becoming a hazard for the patient.
Initial treatment in these situations involves administration of local anesthetic with at least 1:100,000 epinephrine.21 In most cases, the vasoconstrictor effect of epinephrine is enough to stop the hemorrhaging. Should the vasoconstrictor fail to alleviate the problem, an electrosurgical generator or laser with a coagulation setting may be applied to the source of hemorrhage. A third option for containing the bleed is to use a resorbable suture to tie off the vessel posterior to the site of hemorrhage (Figure 2).
Exposed Nerves
In cases of severe mandibular atrophy, supracrestal exposure of the mental foramen or inferior alveolar nerve (Figure 3) may be possible.8,22 When encountered, these situations require special consideration for incision design, mucogingival flap reflection, bone reduction, dental implant placement, soft-tissue reduction, and suturing. With exposed nerve tissue, each of these steps has the potential for causing damage.
When faced with an exposed inferior alveolar nerve, the author has placed dehydrated human amnion-chorion membrane (DHACM) directly onto the nerve fibers based on the positive outcomes that have been reported with DHACM nerve wraps around the prostatic neurovascular bundle in prostatectomy surgeries.23 In 4 cases of DHACM that the author used inferior alveolar nerve wraps, patients reported no adverse outcomes, such as permanent or transient anesthesia, paresthesia, hypoesthesia, or dysesthesia.
Pneumatized Maxillary Sinus Extending Below Bone Reduction Plane
It is well established that maxillary sinus pneumatization may occur following posterior maxillary tooth loss.24 In select cases, pneumatization of the maxillary sinus may extend beyond the anticipated horizontal plane of bone reduction required for All-on-4–style treatment (Figure 4). Failure to adequately reduce bone in these areas may lead to restorations of inadequate thickness that are at increased risk for fracture.25 Conversely, standard bone reduction in these areas may lead to significant damage to the Schneiderian membrane.
To overcome this complication, the author carefully reduces the residual maxillary crestal bone until crestal exposure of the Schneiderian membrane is achieved (Figure 5). On crestal visualization of the Schneiderian membrane, the author uses curettes to lift the membrane apically from the anticipated plane of bone reduction (Figure 6). The maxillary bone is then reduced apically, and the process is repeated until the desired amount of bone reduction is achieved. When performed in this manner, the Schneiderian membrane remains intact while crestal areas of reduced maxillary bone will have openings of varying degrees depending on the amount of bone reduction (Figure 7). The author does not augment these openings with bone graft and simply closes the surgical site with primary closures of thick mucogingival flaps. To date, patients receiving treatment from the author in this fashion have had no adverse outcomes (Figure 8).
Hemorrhage From Osseous Nutrient Canals
All-on-4–style treatment typically involves reduction of alveolar and/or basal bone (Figure 9) to achieve adequate restorative space for both the interim and final prostheses.25-27 During reduction of this bone, exposure of intraosseous nutrient canals28,29 may lead to hemorrhaging that may obscure visualization of the surgical field. If left uncontrolled, hemorrhaging from nutrient canals can become significant, resulting in excessive blood loss. It is not uncommon to encounter multiple concurrently hemorrhaging intraosseous nutrient canals, especially during mandibular bone reduction.
When faced with this situation, the author uses the blunt end of a handheld instrument, such as a Woodson instrument (Figure 10), to put heavy pressure on bone directly adjacent to the bleeding nutrient canal. This pressure typically deforms the adjacent bone to a point at which it occludes the hemorrhaging. The author has encountered hundreds of hemorrhaging intraosseous nutrient canals and has never had this technique fail to achieve the desired results.
Accessory Mental Foramina
The prevalence of accessory mental foramina is well documented in dental literature with findings from 2.7% to 13% (Figure 11).30-32 Accessory mental foramina pose a particular obstacle for All-on-4–style treatment because the mental foramen is the landmark used to determine the posterior limits of tilted implant placement (Figure 12).9 The presence of accessory mental foramina often requires more mesial placement of dental implants, thus limiting the anterior-posterior spread of the All-on-4–style prosthesis. Furthermore, in cases in which accessory mental foramina are located superior to the main mental foramen,30 the amount of bone reduction may be compromised, increasing the risk for prosthetic fracture.25 Presurgical cone-beam computed tomography (CBCT) scans and radiographs for proposed All-on-4–style surgeries should be scrutinized for accessory mental foramina (Figure 13). Notably, dental literature has shown that even with careful evaluation of CBCT scans, it is not possible to identify 100% of accessory mental foramina cases.31 As such, care should be taken during mucogingival flap reflection to search for this anatomic anomaly.
When encountering accessory mental foramina, the author has performed modifications to bone reduction and/or more mesial placement of posterior dental implants; on a few occasions, he has carefully dissected the nerve from the accessory mental foramen and relocated it to a more distal position. To date, the author has had no adverse outcomes in patients, such as permanent or transient anesthesia, paresthesia, hypoesthesia, or dysesthesia, resulting from this technique.
Sublingual Artery Insertion in the Anterior Mandible
Because All-on-4–style treatment in the mandible requires dental implant placement anterior to the mental foramina, special consideration should be given to the vasculature in this region. A number of articles have reported potentially life-threatening complications when dental implant placement has damaged vasculature in the anterior mandible. A 2015 literature review33 on immediate or immediate postoperative bleeding complications in implant dentistry noted that the most common cause of bleeding complications was damage to the sublingual artery. Although this literature review noted an absence of fatalities, advanced airway management by intubation was needed in 41% of patients and tracheostomy was required in 47% of patients. Certainly, avoidance of the sublingual artery during implant placement in the anterior mandible is paramount.
Cadaveric34,35 and CBCT36 assessments of vascular supply to the anterior mandible have found that lingual foramina for insertion of the sublingual artery most commonly occurs in the midline of the mandible. While the location of sublingual artery insertion in the anterior mandible is typically beyond the depth of bone reduction required for most All-on-4–style surgeries, it may be encountered in cases of severely resorbed mandibles. Furthermore, while dental implant placement is typically not required in the mandibular midline for All-on-4–style treatment, it is occasionally necessary to place a dental implant in this location. When faced with this situation, the author has modified the total amount of bone reduction in the anterior mandible to allow for safe placement of a dental implant that avoids vascular impingement while still enabling adequate prosthesis thickness.
Inadequate Bone Density
Multiple studies indicate that a high degree of primary implant stability is a prerequisite for the successful immediate loading of dental implants37,38 and that high insertion torque values are correlated to high primary implant stability.39-42 Insertion torque and primary implant stability have proven to be substantially dependent on bone density with denser bone producing a more favorable situation.42 In cases of inadequate bone density, or “soft bone,” some authors have recommended underpreparing dental implant osteotomies to achieve improved implant stability.7,9 Although this technique is effective in many cases, select instances arise in which underpreparing the bone still results in low initial torque values.
When such instances occur, whether in the mandible or maxilla, the author first attempts to place a larger-diameter implant of the same length as the implant that failed to achieve high initial torque value. Should this adjustment prove ineffective, a longer implant of a larger diameter is then used. In the maxilla, the longer implant may engage the vomer or lateral piriform rims to achieve apical stability.13 In the mandible, the author achieves primary implant stability by engaging the cortical bone at the inferior border of the mandible with the apical portion of the dental implant (Figure 14 and Figure 15).
Removal of Dental Implants
All-on-4–style treatment sometimes requires the removal of previously placed dental implants (Figure 16 and Figure 17). Although some of these implants may have biologic complications such as peri-implantitis that may ease the removal process, most previously placed implants requiring explantation by the author have been firmly osseointegrated. Previously placed dental implants are not typically suitable for All-on-4–style surgeries because the platforms were placed to a level in accordance with the natural teeth. Although this is perfectly suitable when natural teeth are present, these platform positions are too coronal for All-on-4–style treatment. Furthermore, previously placed dental implants preclude the ability for adequate bone reduction. In the author’s experience, using previously placed dental implants for All-on-4–style treatment often results in restorations of inadequate thickness that have a propensity for fracture due to the coronal position of the platforms and the lack of bone reduction. As such, the author routinely removes previously placed dental implants when performing All-on-4–style treatment.
Dental implant removal for All-on-4–style surgery is particularly concerning because of the need for immediately loading a prosthesis. Dental implants requiring removal are typically the same diameter as the implants intended for use in the All-on-4–style surgery. If the implants can be removed with a high-torque dental implant retrieval tool (Figure 18), bone is preserved and a dental implant of at least the same diameter and length can be used for the All-on-4–style treatment. Should the implant retrieval tool fail to explant the previously placed dental implant, trephination is required. The bone loss subsequent to trephination of a dental implant may eliminate the possibility of placing a new implant and thus compromise immediate loading of a transitional prosthesis. As has been reported by others,43-45 the author has had a high degree of success in removing previously placed dental implants with high-torque retrieval tools. In cases in which trephination was required, the author has replaced the previously existing implants with new implants of both wider and longer dimensions.
Postsurgical Complications
Fracture of Provisional Restorations
Postsurgical biologic complications following All-on-4–style dental implant surgery are rare.1-12 The most common postsurgical complication following this surgery noted in the dental literature is fracture of the provisional restoration, with rates from 4.17% to 41%.46-48 Fractures of provisional All-on-4–style restorations during healing are concerning, because they eliminate cross-arch stabilization and disrupt stress distribution patterns. Furthermore, fractures of these prostheses are unsettling to patients because they impair masticatory function and marginalize esthetics. The most common causes of fractured All-on-4–style provisional restorations are prostheses of inadequate thickness due to under-reduced bone, processing errors, or improper occlusal adjustment.46 In some cases, however, fractures may occur because acrylic and polymethyl methacrylate materials are inadequate for extended periods of heavy occlusal use.
To reduce the incidence of postsurgical fracture of All-on-4–style provisional restorations, the author reinforces transitional prostheses with 1-mm-diameter interplaited steel wire (Figure 19 and Figure 20) based on the findings of research by Yamaguchi et al,49 Li et al,50 and others.51-53 Upon incorporating this reinforcement technique for All-on-4–style provisional restorations, the rates of fracture for the author’s provisional restorations decreased from 16.14% to 4.17%.46
Conclusion
Although All-on-4–style full-arch dental implant procedures have one of the highest success rates of any treatment in dental literature, the technique is also among the most difficult and can be fraught with obstacles. With careful planning and knowledge of potential pitfalls associated with performing the procedure, clinicians can incorporate All-on-4–style dental implant treatment into their practices with a high degree of confidence and less stress.
Disclosure
The author had no disclosures to report.
About the Author
Dan Holtzclaw, DDS, MS Consultant Faculty, US Navy Postgraduate Dental School, Department of Periodontics, Bethesda, Maryland; Private Practice, Austin, Texas
Queries to the author regarding this course may be submitted to authorqueries@aegiscomm.com.
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