ALL ON 4 TREATMENT OF THE HIGHLY ATROPHIC MANDIBLE WITH THE MANDIBULAR V-4: REPORT OF TWO CASES

Ole T. Jensen, DDS, MS, and Mark W. Adams, DDS, MS

This report contains a technical note and 2 case reports of the “all-on-4” treatment of the highly resorbed mandible. The use of 4 angled implants directed toward the midline of the mandible at 30° angles provides the advantage of increased implant length and adequate insertion torque for immediate temporization. The technique engages or perforates the inferior border with implants placed in a spaced distribution to avoid fracture of the mandible. The technique is proposed as an alternative to bone graft reconstruction.

© 2009 American Association of Oral and Maxillofacial Surgeons
J Oral Maxillofac Surg 67:1503-1509, 2009

The markedly atrophic edentulous mandible, the socalled Cawood Class IV-V mandible, in which the available vertical basal bone is 5 to 7 mm, can still be treated without bone grafting for dental implant placement and immediate loading in a modified “All-on-4” technique.1-6

A typical all-on-4 protocol uses posterior implants tilted at 30° (posteriorly) that pass just anterior to the mental foramen; anterior implants are placed vertically in the canine-lateral incisor area. In the case of marked atrophy, vertically placed anterior implants7,8 require placement of 8- to 10-mm implants that are allowed to perforate the inferior border.5,6 Four 4-mm holes placed through the anterior hoop of the mandible can lead to stress risers, especially when perforating holes converge or come too close together.7 This can lead to mandibular fracture.9-11

To avoid fracture complications, most clinicians prefer to place a bone graft to gain vertical height at implant placement12-14 or before implant surgery15-17 to have enough bone height to sustain at least 10-mm implants, especially if a favorable anterior-posterior spread is difficult to obtain.18,19

An early site classification system suggested that the bone mass requirement for sustained osseointegration should be no less than 10 mm.20 However, the use of this cutoff was arbitrary and not determined from biomechanical mathematics, which must take into account moment loads and cyclical loading for an extended period.21,22 Also, during that formative period, implant surfaces were not as improved as they are today,23,24 such that, given current technology and improved biomechanical understanding, the use of even a 4- or 5-mm-length implant is thought to be sufficient (mathematically) for load bearing in a welldistributed scheme.25,26 However, most clinicians have not been willing to accept the use of short implants that could lose 1 to 2 mm of bone support over time, leaving only a few millimeters of residual height for continued function.27 Therefore, the issue of implant length remains intuitively and experientially valid, rather than mathematically determined.

Given the dilemma of the markedly atrophic mandible in a patient group not highly amendable to bone graft reconstruction and a profession still in the developmental phase for the use of bone morphogenetic protein-alloplast augmentation,28-30 the question must be asked whether we can simply use the bone that is available and avoid a complicated grafting procedure. In the All-on-4 technique, tilting the posterior implant increases the length in the bone by 50%.31 If anterior implants are also tilted 30°, this same figure also applies. By tilting all 4 implants toward the midline in a V formation, 5-mm vertical bone can house 10-mm-length implants if the implants are allowed to slightly perforate inferiorly, a significant improvement for osseointegration potential.

This technique has been designated the “V-4” approach for the All-on-4 technique and has allowed immediate fixed provisionalization for even highly atrophic mandibles without bone grafting. The great advantage of this approach is that the greater bone mass generally found more toward the midline can be taken advantage of, and, despite implant convergence toward the midline, the holes perforating the inferior cortex remain well distributed and relatively far apart from each other, reducing the fracture potential. The parasymphyseal area, where a mandibular fracture is mostly likely to occur, is avoided altogether.32,33

Case Reports

Case 1

A 72-year-old woman had worn dentures for longer than 30 years and presented with severe mandibular atrophy with 8 to 10 mm of alveolar height as viewed on the Panorex (Fig 1). However, because of reverse architecture only 4 to 5 mm of vertical dimension was present in the mid-alveolar (axial) dimension. The
mental foramina were dehisced and relatively forward in the arch. The nerves were partially exposed posteriorly.

A full-thickness crestal incision was made anteriorly but only through the mucosa posteriorly to avoid cutting the nerves. Using blunt dissection, the nerves were located and deflected laterally, leaving the foramen free of neural tissue. Anteriorly, the mentalis muscle attachment was left undisturbed to prevent ptosis. The mandible appeared very fragile overall, but it had been especially resorbed in the parasymphyseal regions. Although all-on-4 fixture placement had been planned on the computer, the surgical placement criteria dictated placing the implant visually to not fracture the mandible. The first fixture was placed directly into the right mental foramen (Fig 2A) and angled forward 30°. The anterior implants were evenly spaced and also directed toward the midline at 30° (Fig 2B). This created an overall V-shape placement appearance on Panorex designated a “V-4” All-on- 4 placement (Fig 3). Additionally, the implant placement angles were tilted anteriorly to avoid lingual plate perforation (Fig 4). Finally, 30° abutments taking an oral bisphosphonate (Fosamax; Merck, Whitehouse Station, NJ) for 7 years for osteoporosis presented for dental implant rehabilitation. The results from a fasting C-terminal telopeptide study were satisfactory (315 pg/mL).

The mandible was highly atrophic with 3 to 4 mm of vertical bone in the right symphysis and 5 to 6 mm in the left. In preparing the implant sites, the vertical available bone was a maximum of 5 to 7 mm (Fig 5).

After reflection of a flap, taking care to avoid nerve injury and preserving the mentalis muscle attachment, posterior implants were placed through the foramen sites after deflection of the dehisced nerves. Because the mandible resorbs, the mental foramen often presents in a mid-crestal location. These implants were placed at 30°, angling forward (Fig 6).

The front implants were well-distributed and placed at somewhat less than 30° but still angled toward the midline (Fig 7). The overall distribution and display on the Panorex was a V shape (Fig 8). The patient was immediately provided, after placement of the 30° abutments, with a fixed provisional bridge.

Discussion

Patients who have worn dentures for 3 or more decades may seek implant reconstruction because of the pain from exposed inferior alveolar nerves owing to complete alveolar loss from atrophy. Denture compression of exposed nerves is best treated in this setting by dental implants; however, the lack of jaw bone height is a concern. Although a 10-mm vertical height may be present mid-symphysis, the parasymphyseal areas are often one half the height of the symphysis. Also, in this setting, the mid-alveolar area is often of a reverse architecture, such that the actual mid-axial alveolar height is much less than seems apparent on a Panorex. Although the lateral bone height can be relatively high, it cannot be accessed for implant placement; thus, often implants must be placed where bone is relatively deficient. Therefore, most experienced clinicians prefer to place implants with a careful minimal torque technique but still perforating through the inferior border. Using this approach, an 8- or 10-mm fixture is still placed into a 5- to 7-mm site.

The value of angulation of implants in a V-4 distribution strategy is that bone grafting can be avoided, because fixtures are favorably directed toward the location of maximal bone mass. This approach is also excellent to use without inferior border perforation if somewhat greater bone mass is available.

The V-4 technique is biomechanically favorable in 3 ways: 1) mandibular continuity preservation; 2) a greater length of implants; and 3) the V-shape is very stable biomechanically.

Mandibular Continuity Preservation

A 4-mm hole drilled into the anterior tibia, a weightbearing bone, reduces bone strength by 40%.34 A 4-mm hole drilled into the mandible, especially into a low-bone-volume atrophic mandible, may considerably weaken the jaw, even though it is not a weightbearing bone.35 The placement of 4 holes through the hoop of the mandible, especially if they are not centrally placed, risks a discontinuity fracture intraoperatively36 or during the demineralization phase of healing.37 At about 3 weeks after surgery, it is possible for a jaw fracture to occur under normal functional loading38owing to the relative weakening of the jaw caused by the regional acceleratory phenomenon.39 However, the area at the greatest risk of this is the parasymphysis, which is avoided using V-4 angulation.

The implants should be placed using a screw tap method, even using self-tapping implant protocols to decrease insertion torque values and not overload the bone.38,40

Greater Length Of Implants

What is important is not simply to have a greater implant length, but also to have the implant primarily fixated into compact bone.41 Bone grafting to gain implant length is an alternative strategy; however, vertical bone grafts are the earliest to fail under stress, and implants secured mainly by bone grafts can sometimes fail with time.42 The incidental elevation of inferior border periosteum to gain periosteal proliferative bone must also be considered as potential secondary support, although it does not always occur43 (Fig 9). Therefore, the most dependable bone for long-term osseointegration is compact bone, more of which is encountered by implant angulation using a V-4 strategy.

These compromised sites should probably use 4-mm diameter implants or less rather than trying to gain more surface osseointegration using shorter, wider (5-mm) implants, which considerably increases the risk of jaw fracture.

V-Shape Biomechanic

The reason the V shape is favorable biomechanically is the greater length of implants into more dense bone. Also, the angulated implant pull-out strength in a splinted configuration is intuitively greater for angled implants. In the V-4 strategy, this is multiplied by a factor of 4, although this has not been studied experimentally. Finite element analysis of the tilted implants that are splinted in a full-fixed prosthesis revealed a decreased peri-implant “bone strain” compared with vertical implants, supporting a cantilevered prosthesis and implying better load-bearing biomechanics.44 The highly atrophic mandible in the elderly patient can be treated with an all-on-4 technique without bone grafting with an immediate loading protocol by distributing the implants in a V shape, designated the V-4 technique. The V-4 is protective of mandibular continuity, derives increased implant length with acceptable insertion torque values, and maintains a standard all-on-4 pattern of prosthetic distribution despite the angulated placement. A splinted V-4 distribution has highly favorable biomechanics. Overall, the V-4 permits the use of a conservative nongrafting approach in what might otherwise require significant bone graft reconstruction in a commonly elderly population.

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