Vertical alveolar ridge augmentation

The use of titanium osseointegrated implants requires a minimum amount of supporting bone to achieve high success rate. Notwithstanding there is not a scientific demonstration, the generally accepted minimum vertical dimension is 7 mm (^{Lekholm and Zarb, 1985} ; ^{Adell et al., 1981}). In case of anatomic limitations such as presence of expanded maxillary sinuses or limiting position of the mandibular canal, the only possibility for implant placement is the use of regenerative procedures.

Vertical ridge augmentation with guided bone regeneration techniques has been proposed for the first time by ^{Simion et al. (1994)}. These authors showed the possibility of vertical augmentation in humans using a pure regenerative technique (blood clot and titanium reinforced membrane) and obtained supracrestal regeneration up to 3 to 4 mm. Similar results were shown by ^{Jovanovic et al. (1995)} in an experimental study in dogs. ^{Jensen et al. (1995)} created a vertical ridge augmentation model using the inferior border of the dog mandible and stated that autogenous bone in conjunction with a e-PTFE membrane gave the maximum regenerative result, whereas the autogenous bone graft alone underwent major resorption (70-80 %).

^{Tinti et al. (1996)} investigated the limit of regeneration using small autogenous bone grafts and titanium reinforced membranes : the combination of the bone and graft technique could regenerate up to 7 mm of bone in humans.

^{Simion et al. (1998)} published a study to analyze the effectiveness of autogenous or freeze-dried-demineralized bone grafts in conjunction with titanium reinforced e-PTFE membranes, obtaining mean regeneration of 5.02 and 3.14 mm, respectively.

The rationale for using a complex therapy to rehabilitate a patient is represented by the increase of implant support, improvement of biomechanics, aesthetics and oral hygiene procedures.

The increase of support is the most obvious reason for vertical ridge augmentation. The reconstruction of the lost bone offers the possibility to use longer implants and the use of longer implants is associated with higher success rates in many scientific long term reports (^{Adell et al., 1981}).

The second reason to undertake regenerative procedures is the biomechanic factor (^{Rangert et al., 1989}). The vertical regeneration in coronal direction offers in fact the possibility to reduce the length of the prosthetic crowns. Shorter crowns signify a reduction of the crown-implant ratio. Since the point of application of the occlusal load is constant, i.e. the occlusal plate, the possibility to move the fulcrum of the biomechanic system in a coronal direction offers the possibility to have a more advantageous lever arm. In this sense, vertical ridge augmentation is a more favorable therapy when compared to sinus lift procedures in the treatment of the atrophic maxilla, because in case of comparable regenerative result the biomechanic situation is more advantageous. Sinus lift procedures in fact augment bone in apical direction and does not improve the crown-implant ratio.

Vertical ridge augmentation offers also the possibility to improve aesthetics in areas of aesthetic interest. An altered crown-implant ratio creates, in fact, not only a biomechanical, but also an aesthetic problem. The possibility to restore correctly both hard and soft tissues is the only possibility to offer the patient ideal aesthetics and that can be achieved only by a correct three-dimensional positioning of the implants, regardless of the conditions of the pre-existing bone. To reduce aesthetic compromise, regenerative procedures are the only solution available for clinicians.

Surgical procedures

Treatment planning

The surgical technique for vertical ridge augmentation is considered one of the most challenging procedures in implant dentistry. In fact, even though the placement of implants in a supracrestal position does not appear difficult, the membrane adaptation, the bone graft positioning, and the flap management represent technically demanding procedures.

Due to the high risk of complications, like membrane exposures or infections, it is necessary to perform an atraumatic and delicate surgical procedure following all the steps and the surgical details necessary for uneventful wound healing.

The first, even if obvious, consideration is that only a correct treatment planning has a chance to be successful, especially when sophisticated treatments are needed. The use of a surgical stent to correctly position the implants can be the preventive solution to untreatable biomechanic and aesthetic problems that must be anticipated in the diagnostic wax up. Especially in case of severe atrophy and severe resorption of the alveolar process the perfect positioning of an implant could be difficult without the guide of a stent.

In some cases, especially when aesthetic is of primary importance, a staged approach offers the possibility to minimize the risk of failure in case of complication. In fact, if an exposure of the membrane occurs when implants are already in the site and the regeneration results incomplete, we could have to deal with implants extruding from the bone and sometimes from soft tissues. In this cases, the final aesthetic results are real disasters. For this considerations the multi-stage approach must be always considered in cases of aesthetic concern. Moreover, the positioning of the implants after bone regeneration simplifies the surgical approach, since the second surgery is similar to implant positioning in non regenerative cases.

Another obvious, but sometimes forgotten, aspect is the absolute necessity to ensure a healthy oral environment when undergoing a complex regenerative procedure. With the development of the surgical learning curve, we observed a few cases in which the regenerative result has been invalidated by infections occurring without membrane exposures. The site contamination probably derived from infected adjacent teeth. The first step to obtain success in this kind of therapy is therefore represented by a correct presurgical periodontal treatment.

Flap management

In vertical ridge augmentation, the importance of an atraumatic and delicate approach will never be sufficiently stressed. The possibility of a successful treatment starts, in fact, from the correct flap elevation. A full thickness, middle crestal incision is commonly used. A correct incision and an atraumatic flap elevation gives the possibility of primary closure and immediate healing, which is fundamental to avoid membrane exposures. The respect of the delicate periosteal structures must be another goal of the surgeon approaching this technique. The correct extension of the flap gives the possibility to have enough tissue to cover graft and membrane, maintaining a sufficient blood supply. It is necessary to extend the incision two to three teeth further the limits of the atrophic ridge.

If the vertical augmentation is limited to 1-2 mm the use of a self space making barrier, such as titanium reinforced membranes, and the blood cot is sufficient. Within this limit, in fact, the blood clot stabilized by the membrane offers the staminal precursors the possibility to differentiate towards the osteoblastic line to regenerate the bone (^{Simion et al., 1994}).

When regeneration exceeds the limit of 3-4 mm in vertical direction, the use of a pure regenerative approach does not seem to be predictable enough and the regenerative technique must be completed with the adjunction of a bone graft (^{Jensen et al., 1995} ; ^{Tinti et al., 1996}).

Barriers and grafts

Most of the published studies (^{Jensen et al., 1995} ; ^{Simion et al., 1994} ; ^{Buser et al., 1998}) confirm that autogenous graft is the gold standard when approaching regenerative procedures. Bone chips or blocks obtained from intra oral donor sites offer the possibility to treat most cases. Areas such as chin or retro-molar areas are generally suitable to collect the necessary material to graft the atrophic areas.

The use of allografts or xenografts (^{Hammerle et al., 1998} ; ^{Valentini et al., 1998}) is considered as a second choice, as substitutes of autografts, and the literature as demonstrated the effectiveness of these materials (^{Simion et al., 1998} ; ^{Nevins and Memmonig, 1992}).

One of the most important aspects that can turn a failure into a success is the perfect stability and immobility of both clot and graft. This can be achieved using some devices to support the membrane, such as pins or the implants themselves.

The use of titanium reinforced membranes as a self space making device is another step for being successful in vertical ridge augmentation.

Membrane and underlying graft must then be secured to the residual bone by means of fixing devices, such as mini-screws or tacks. The membrane-graft complex has to be carefully fixed at least at every line-angle and this could be difficult in areas of limited accessibility. In fact, if the graft is mobile under the membrane, osteogenic cells can not produce bone but only fibrous tissue. For this reason, after respecting all surgical rules, it is also mandatory to avoid any gingival supported provisional prosthesis during the healing period.

Suture

The last aspect to consider is the importance of a tension free suture. This is possible only by using generous periosteal release.

Because of the necessity to augment the residual ridge, the lack of soft tissues to cover membrane and graft is an major problem the surgeon as to deal with. For generous periosteal release, we mean a continuous partial thickness incision that conjuncts mesial and distal vertical releasing incisions of the primary flap and permits an overlap of at least 5 mm of the vestibular and lingual aspect of the flap. Mattress and interrupted sutures only secure the tension free flap in such a way to ensure a rapid healing. Sutures are generally mono-filaments and are removed after an average period of 10-14 days.

In case of vertical ridge augmentation the patient is obliged not to wear any removable gingival supported posthesis. The pressure of the provisional can in fact produce membrane exposure and dehiscences of the flap, compromising the regenerative result.

Complications

As any delicate and complicate technique, surgeons who perform vertical ridge augmentation procedures must undergo a step by step learning curve. As the learning curve progresse, the most feared complication, i.e. early exposure, diminishes. The average percentage of early exposure must be considered about 16 % actually (^{Simion et al., 1998}) and this is an higher rate of complications when compared to horizontal regenerative procedures (2.5-7 %) (^{Buser et al., 1998} ; ^{Simion et al., 1994}).

As already mentioned, with the diminishing of early exposures there is a relative increment of different type of complications, such as un-exposed infections derived from adjacent teeth. Another complication is delayed exposure, generally deriving from a scarce compliance of patients and from an inappropriate use of the provisional.

Limits

In case of uneventful healing with the association of membranes and an autogenous bone graft the regenerative result is generally complete. The limits of vertical ridge augmentation are related to the chosen surgical approach. In case of pure regenerative approach, blood clot stabilized from a titanium reinforced membrane, the accepted limit is 3 to 4 mm (^{Simion et al., 1994}). When a combined regenerative and graft approach is used, the limit of the regenerative result are higher, up to 7-8 mm (^{Jensen et al., 1995} ; ^{Tinti et al., 1996}) and even more. In conclusion the recent development in both merceologic and surgical technique can transform a complex risky challenge into a predictable and relatively safe approach. The regenerative result is surely satisfactory for both patient and dental team.

An important issue is that the goal of the surgical procedure is the rehabilitation of the patient mouth : the regenerative result is only a step to give the patient the best functional and aesthetic result. This means the necessity of developing an integrated approach, to treat both hard and soft tissue defects, often present in complex prosthetic rehabilitations.

The consciousness of a possible predictable result must not eliminate the consciousness of the complexity of the procedure. With all the limitation of the case vertical ridge augmentation is becoming a procedure suitable for clinical and not only research cases as in the immediate past.

BIBLIOGRAPHIE

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