Ole T. Jensen, DDS, MS, and Mark W. Adams, DDS, MS
We present a technical note and 3 case reports of all-on-4 treatment of highly resorbed maxillas. The use of 4 angled implants, placed at as much as a 30° off axis, that engage the lateral nasal wall bone provide high torque fixation for immediate temporization. The technique is proposed as an alternate to sinus grafting and for use with multiple implants or zygomatic implants.
© 2009 American Association of Oral and Maxillofacial Surgeons
J Oral Maxillofac Surg 67:1739-1744, 2009
Twenty years ago a biomechanically based site classification suggested that dental implants must have a length of 10 mm in bone and must be placed in the axial plane in order to obtain and maintain sufficient osseointegration.1 The treatment planning idea during that time period was to place implants “in line”, that is, perpendicular to occlusal forces and inferred that angulated implants would lose bone and eventually “de-osseointegrate” if placed otherwise than straight up and down. Implied also from with this classification was the thinking that at least 10mm of bone was required in order for there to be long term confidence in an implant restoration.2-4
In addition to these intuitive concepts the number and distribution of implants was studied for full arch restorations when placement was limited to the anterior zone, that is, anterior to the sinus cavities. Biomechanical analysis found the distribution of implants was more important than the number of implants placed.5-7 For a fixed detachable bridge the concept of an “anterior posterior spread” was born with more favorable outcome suggested by a greater A-P spread.8 And finally, with the advent of bone graft augmentation, implants began to be employed in the posterior maxilla so cantilever restorations could be avoided.9-12 But because bone graft sites were thought to have less biomechanical strength than native bone both increased length and number of implants were suggested for use in the posterior zone.13,14
The net treatment planning effect of all of these ideas was for the surgical-prosthetic team to prescribe the greatest number with the greatest length and best anterior posterior spread of axially aligned implants as possible for full arch implant restorations.15 This usually meant 6 to 8 implants for the edentulous maxilla and sometimes more. If the length of implants, or number of implants, or A-P spread were deemed insufficient the final prosthesis was considered compromised by inadequate biomechanical support and treated accordingly, usually with a removable prosthesis or if fixed, truncated cantilevers.17-19
In the last two decades, due to improved materials science as well as improved surgical – prosthetic techniques, these “intuitive concepts” are slowly going by the wayside, though some remain as treatment planning vestiges of over engineering.
About ten years ago further biomechanical analysis and technical advancement led to the introduction of the”all on four” full arch restoration strategy which evolved from the six implant supported fixed detachable bridge.20,21 Biomechanical analysis determined that a fifth or sixth implant was unnecessary in terms of mechanical support.23 In fact, as few as two well angled implants of sufficient length was calculated to be able to hold an entire arch restoration strictly based on mathematical analysis.24
All on four posterior implants are placed in the maxilla angled anteriorly at 30° just missing the anterior wall of the sinus cavity. This gives increased length, greater A-P spread and most often eliminates the need for bone grafting the sinus floor.25,26
All-on-four technique anterior implants are generally placed straight up and down with straight abutments, or angled slightly anteriorly when required, but still placed axially.23
Reported here, using the all-on-four technique, are anterior placed implants, also placed at up to 30° angles, but angled posteriorly in the axial plane sometimes engaging the apex of the posterior implants (see Figure 1). This convergence creates an “M” shape as seen on panorex. Anterior implant angulations provide the same advantage as found with the posterior angled implants, that is, increased length, increased A-P spread, a decreased bone grafting requirement and most importantly increased insertion torque for primary fixation.
Designated the “M-4” technique, this approach enables an anterior placement emergence strategy even when there is only 5 to 8mm of bone available sub-nasally. By angling the apex of the fixture posteriorly, just missing the nasal aperture, the spread and implant position is still maintained sub-nasally, but 10 to 13mm fixtures are able to be used in what otherwise would be a very limited space requiring 7mm length fixtures or less.
After angled abutments are placed (17° or 30°) the implant distribution and abutment inclinations appear consistent with a typical all-on-4 case.
The main advantage of the M-4 technique is the increased biomechanical advantage that occurs both for the immediate load provisional phase and for long term.
When immediate load is contemplated a 10 to 13mm fixture is much more likely to provide adequate insertion torque that will be sustainable during the provisional phase.
The use of 7mm fixtures may lower the confidence for both short and long term stability.
Tilted anterior fixtures require that there be at least five millimeters of bone available. With 5mm of vertical bone there is enough vertical alveolar height to angle an implant into bone at the lateral pyriform rim sometimes deriving a net implant length of up to 13mm or more.
This previously unreported phenomenon is a vital treatment planning option that helps to avoid bone grafting and enables immediate temporization.
An edentulous 72 year old patient had a 25 year history of wearing dentures. Atrophy was very prominent in the maxillary anterior zone where implants were desired to be placed due to combination syndrome. Beneath the nasal floor only 4-5mms of very thin alveolus was present for implant placement (Figure 1). The posterior fixtures were placed angled forward 30° just avoiding the anterior projection of the sinus cavity (Fig 2a, b).
In order to avoid the nasal fossa and increase length of the anterior implants while still maintaining equalized distribution of implant emergence, the anterior implants were placed angled posterior toward the apex of the posteriorly placed implant (Fig 3). This enabled greater length and fixation for the implants. The implants converged creating an M shape when viewed on panorex designated the M-4 all on 4 technique (Fig 4).
A 48 year old female presented status post maxillary fracture with traumatic avulsion of the left anterior alveolar process and teeth and compromised remaining bone (Fig 5).
Fracture hardware was still in place. In the anterior region, where implants were desired for an all on four implant strategy bone mass was lacking. Consideration for placing zygomatic implants or doing sinus bone grafting was discussed. The patient declined bone grafting or extensive surgery desiring to avoid extensive surgical intervention if possible. The M-4 all on four4 approach was planned.
Despite the lack of alveolar height (4-5mm) the triangle of bone at the lateral pyriform rim provided enough bone mass for M-4 placement (Fig 6 M). Sufficient length (10.5mm and 15mm) of implants for good primary stability (50NcM) was obtained. The presentation on panorex post-operatively was of an M-shaped distribution (Fig 8). Two implant apices converged at bone found between the lateral nasal wall and anterior sinus wall above the nasal floor, designated as “M-point” in the M-4 nomenclature as it is the optimal apical position for fixture placement.
A failed upper dentition due to severe periodontal disease had nearly resorbed the entire maxillary bone. Zygomaticus and sinus bone graft reconstruction alternatives were considered (Fig 9). In this case following dental extraction the anterior bone was so poor a 30° angulation posteriorly was required in order to gain enough length for primary stability (Fig 10a, b). The apices of the implants engaged creating an M-4 pattern as seen on panorex (Fig 11). Despite the osseous deficiency, by using 30° angulation for all the implants, 50 NcM fixation could be obtained and adequate implant length (11mm, 13mm) was achieved.
The clinical use of an “all on four” strategy for implant dentistry has come into question when considering the typical resorbed edentulous maxilla – a situation most often treated with bone graft augmentation including sinus bone grafting followed by multiple implants. All on four in the maxilla, though reported to be as successful as in the mandible, is generally viewed by the dental implant professional with skepticism. One reason for this is the presumed need to use a greater number of implants and or the alternative of zygomatic implants. 27-30
The angled fixture approach, though, offers a significant mechanical advantage as long as a favorable distribution of implants is maintained.24, 31 This is not only because of the added length of implant, but the added anchorage from increased surface area at both the alveolar crest point of entry to the primary fixation bone at the lateral nasal wall.32
Angulation of fixtures that are well distributed and angled in opposing directions instead of simply parallel to each other provides a greater mechanical advantage than fixtures placed in parallel given the distribution is equivalent.
Four axially placed fixtures have less mechanical pull out strength and torsion resistance than 4 angled fixtures when splinted together in a single unit prosthesis. Therefore, the M-4 technique may actually have greater mechanical advantage than the standard all on four approaches not including the added length provided by angulation when considering resistance to load of the prosthesis.24
Presented here is a technical note and three case examples suggesting that four angled implants (M-4) fixated at the lateral nasal rim is favorable biomechanically for immediate and long term loading. The technique is of sufficient biomechanical strength to permit immediate provisional restoration even in the highly resorbed maxilla. The mathematical advantage of angulation occurs independent of the advantage of increased fixture length. The M-4 is indicated for use in the highly resorbed maxilla, the Cawood Class IV – V maxilla, where there is insufficient alveolar height for vertical implant placement.
The M-4 approach then, becomes an added treatment strategy for the oral and maxillofacial surgeon and prosthodontist to gain implant stability without subjecting the patient to the added morbidity and cost of bone graft reconstruction and multiple implants as well as the uncertainty of using short implants without grafting. And, the technique is a viable alternative to Zygomatic implants. For the prosthodontist, overall treatment remains standardized and simplified as angled abutments provide a standard prosthetic restorative scheme that can be easily addressed.