Guided bone regeneration, autogenous bone graft : limits and indications

Introduction

The use of osseointegrated implants requires a minimum bone volume. However, the loss of one or several teeth is followed by a tridimensional bone resorption.

This resorption is centripetal in the maxillae and centrifugal in the mandibule. These changes are of greater or lesser severity and might modify the quality of bone anchorage and the placement of osseointegrated implants (^{Kopp, 1989}).

These post-extraction changes can be characterized by :

- a knife edged residual crest ;

- a reduction of the height of the crest ;

- an increase of bone concavities ;

- an alteration of the occlusal relationship ;

- a facial soft tissue collapse.

^{Bahat (1997)} pointed out the different surgical and prosthetic consequences resulting from those modifications :

- decrease in the number of favourable implant sites ;

- use of short or narrow implants which will not be able to withstand mechanical loading ;

- inappropriate angulated implants ;

- fenestration of the cortical bone plate during first surgical procedure ;

- unsatisfactory prosthetic results leading to esthetic and phonetic problems.

Four methods of bone reconstruction have been described in the literature, aiming at increasing the bone volume :

- use of appropriate growth factors for osteoinduction (^{Reddhi, 1981} ; ^{Reddhi et al., 1987} ; ^{Urist, 1965}) ;

- osteoconduction with graft materials (^{Buch et al., 1986}) ;

- osteogenesis by distraction principles (^{Illizarov, 1989a}, ^b) ;

- guided bone regeneration (^{Dahlin et al., 1988}, ¹⁹⁹¹).

Today only bone graft and guided bone regeneration have been found reliable for these clinical applications. The use of growth factors and osteogenesis by distraction are still experimental.

Autogenous bone graft

Used for decades in maxillo-facial surgery, these techniques were used essentially in severe bone resorption of edentulous ridges. These extended bone reconstructions have been undertaken with iliac, costal, or cranial donor sites (^{Breine and Brånemark, 1980} ; ^{Teissier, 1982} ; ^{Tulasne et al., 1990}).

Sometimes implants have been placed simultaneously, participating in the fixation and the immobilization of the bone blocks (^{Kahnberg et al., 1989} ; ^{Isaksson, 1994}).

^{Bloomquist and Turvey (1992)} recalled that autogenous grafts are considered the material of choice :

• Availibility

All the authors have agreed on the nature and the quantity of the donor bone graft. Studies have shown that the membraneous bone (cranial, ramus, symphysis) is preferable to the enchondral bone (iliac crest). The rate of resorption is less severe with bone grafts of membraneous origin (^{Smith, 1974} ; ^{Kusiak, 1985}).

• Osteogenicity

Two phenomena can be observed :

- autogenous grafts hold a large quantity of living cells (osteoblasts) giving an osteogenic potential ;

- autogenous grafts also play a role in stimulating the recipient site (osteoconduction) leading to osteoblastic bone formation (^{Burchardt, 1987}).

• Mineral matrix

The presence of the mineral matrix is the basis for the osteoconduction process. ^{Burwell (1969)} defined osteoconduction as a passive property allowing colonization of the graft by vascular and cellular elements coming from the recipient bed.

• Mechanical stability of the graft

The mechanical stability of the graft (immobilization) helps the quality of the graft blood supply through the recipient site and the periosteum. The post-operative resorption will then be less. The cortical bone plate allows the immobilization by mechanical means (miniscrews) and isolates the cancellous bone portion from all the surrounding cells but not from those coming from the recipient site.

According to ^{Boyne (1974)} only the cortico-cancellous bone graft will fulfil the four criteria for an excellent prognosis.

Guided bone regeneration

The biological principles of guided bone regeneration originate in those described by ^{Nyman et al. (1980)} and ^{Karring et al. (1984)}.

In animal studies (^{Dahlin et al., 1988}, ¹⁹⁹⁰) and clinical human studies (^{Buser et al., 1990} ; ^{Lang et al., 1994}) several authors have demonstrated the reliability of guided bone regeneration (pre-implanted sites). ^{Becker and Becker in 1990}, ^{Dahlin et al. in 1991}, used guided bone regeneration at the time of implants placement. In a recent consensus report, ^{Mellonig and Nevins (1995)} indicated that e-PTFE membrane is the material of choice for guided bone regeneration procedures.

This membrane placed between the gingival connective tissue and the bone, will create a space in which the blood clot will stabilize. The bone tissue cells will slowly invade the space and the blood clot will stay free from all mechanical stress (^{Selvig et al., 1980}), encouraging bone regeneration.

^{Buser et al. (1995)} defined the different conditions for success in guided bone regeneration :

- healing by complete primary closure to avoid membrane exposure ;

- creation and maintenance of a space to avoid membrane collapse using artificial space maintainer, titanium armed membranes, or autogenous graft ;

- adaptation and stabilization of the membrane against the bone tissue, using fixation devices to avoid proliferation of non osteogenic cells ;

- long healing time period to obtain complete and mature bone regeneration.

The autogenous bone graft and guided bone regeneration procedures have demonstrated their effectiveness in reconstruction techniques. The clinical studies related to volumetric augmentation of the bone anchorage described three possible protocols :

- guided bone regeneration technique ;

- autogenous bone grafts used with a membrane ;

- autogenous bone grafts without other biomaterials.

However, the respective indications for one of these techniques is not easy. The choice for the more appropriate procedure needs to be established. The following is necessary :

- to analyze the dimensions of the defects, future bed of the reconstruction ;

- to evaluate, according to the defect morphology, the healing potential of the recipient site.

However, it is also necessary to quantify the number of and the access to the donor sites when an autogenous bone graft is indicated.

Defects classification

The description of the mesio-distal, bucco-lingual or palatal, and vertical components of the defect allows us to evaluate the number of progenitor cells present in a particular defect :

• The mesio-distal dimension (length) is represented by the number of missing teeth : the more teeth that are missing the more difficult the reconstruction will be.

• The bucco-lingual or palatal dimension (width) corresponds to the number of residual bone walls (1, 2, 3, or 4) : the success rate of reconstruction is inversely proportional to the number of bone walls left.

• The vertical dimension is the amount of resorption in the vertical direction, we will use the ^{Lekholm and Zarb classification (1985)} :

- type A : almost no resorption of the alveolar crest ;

- type B : moderate resorption of the alveolar crest ;

- type C : total resorption of the alveolar crest ;

- type D : basal resorption ;

- type E : extreme basal resorption.

The vertical bone resorption is an essential parameter for the evaluation of the primary stability of the implant ; if the quantity of bone resorption increases from A to E, the possibility of primary osseointegration of the implant will decrease in the same proportion.

We will describe, according to these parameters, three classes of defects :

• Class I defect : they correspond to the closing of the alveolar defect and are characterized by :

- a mesio-distal component limited to 1 or 2 teeth ;

- a bucco-lingual or palatal component made of 3 or 4 bony walls ;

- a large vertical component (A, B, C, D).

The Class I defects do not allow primary stability of the implants but possesses a large number of progenitor cells. The defect shape is rather an envelope-like shape that will allow an easy fixation and self-spaced membrane (^{fig. 1}).

• Class II defect : they correspond to the correction of the knife edged crest with vertical and/or horizontal bone reconstruction and are characterized by :

- a mesio-distal component of 2, 3 or 4 teeth ;

- a bucco-lingual or palatal component limited to 1 or 2 bony walls ;

- a vertical component of type B or C.

The Class II defects do not allow primary stabilization of the implants. Furthermore, because of the small number of bony walls left and the fact that the defect is extended longitudinally, the potential of cell activity is reduced.

The correction of Class II defects requires lateral bone volume augmentation of the residual crest : the more the mesio-distal and vertical components will be extended, the more the creation of the space will be difficult to manage (^{fig. 2}).

• Class III defects : they correspond to small defects involving a moderate by resorbed crest and are characterized by :

- a variable mesio-distal component (one or several teeth) ;

- a bucco-lingual or palatal component made of 2, 3, or 4 walls ;

- a reduced vertical component (A, B).

The Class III defects do not allow primary stabilization of the implants ; the number of bone walls left and the implant position will determine the spacing (^{fig. 3}).

The Class I and Class II defects will involve a pre-implant placement procedure. The Class III defects will be treated by peri-implant reconstruction at the time of implant placement.

Discussion

Healing potential (table I)

The tridimensional analysis of the bone defects pointed out the potential of regeneration : the number of bone walls left gives an excellent source of bone cells that will be directly involved in the healing process.

This potential appears to be important for Class I and III defects that do not differ in their vertical component. The class II defects that correspond to the knife-edged ridge have a poor cell activity.

Choice of the reconstruction technique (table II)

Class I defects

Their treatment needs the use of a e-PTFE membrane (^{Hammerle et al., 1996}).

The natural self-spacing creation could be improved by the placement of miniscews or autogenous particles if the number of bone walls is limited to three (^{fig. 4a}, ^4b, ^4c and ^4d).

Class III defects

They resemble fenestrations, dehiscences, extraction sockets. If anatomical conditions help in the self-spacing, guided bone regeneration is the technique of choice (^{Dahlin et al., 1991} ; ^{Gelb, 1993}) (^{fig. 5a}, ^5b and ^5c).

However, if the number of walls is less than three, an autogenous bone graft (particles) stabilized by a e-PTFE membrane will be a possible treatment option. (^{Jovanovic et al., 1992} ; ^{Buser et al., 1992}).

Class II defects

If the principles of treatment of Class I and Class III defects are well established, it is still difficult to give a rational method for the treatment of Class II defects.

Two questions remain for clinicians :

- are the principles of guided bone regeneration available in these situations ?

- in which way will the autogenous bone graft with or without a e-PTFE membrane, correct those defects ?

Class II defects present two major problems compared to defects that create a self-maintained space :

- the amount of reconstruction is inversely proportional to the cellular activity ;

- the anatomy of those defects (fewer of walls left) does not allow enough space underneath the membrane.

^{Buser et al. (1993)} reported better results at sites treated with guided bone regeneration and screws used as tents compared to sites treated with a similar technique but without screws as space holders. These authors observed a partial membrane collapse, laterally to the screws, reducing considerably the amount of regenerated bone.

According to these results, the use of autogenous cortico-cancellous grafts to simultaneously support the membrane, and to serve as a scaffold to ostseoconduction process with stimulation of bone regeneration seem to be the next logical therapeutic proposal.

It is important to mention that these authors stabilize the cortico-cancellous bone blocks 3 mm apart from each other. This design will allow the extension of the space over the entire length of the defect. The residual gaps around the screwed bone blocks are then filled with bone particles. The perfect immobilization of the autogenous particles between the autogenous blocks can only be obtained by covering them with a e-PTFE membrane. The membrane will protect the bone graft and will limit its resorption (^{Buser et al., 1995}).

The post-operative resorption of the autogenous graft is a well known phenomenon. It is clear that the nature of the graft is an important factor in the rate of resorption. The cortico-cancellous donor material taken from the chin or the ramus are membraneous in origin and do not seem to be subject to a high rate of resorption compared to that from the iliac crest which is of endochondral in origin (^{Misch et al., 1992} ; ^{Jensen et al., 1994}) (^{fig. 6a}, ^6b, ^6c and ^6d and ^{fig. 7a}, ^7b and ^7c).

^{Misch (1997)} suggested that the use of membrane will not be necessary as long as the graft size is sufficient (the dimension of the graft and of the defect will be identical), the graft adaptation to the recipient bed will be perfect and the immobilization of the graft will be done with appropriate means (^{Phillips and Rahn, 1996}).

On the other hand, studies have shown that post-operative resorption becomes very important when the particles of the autogenous graft are reduced (^{Fonseca et al., 1980} ; ^{Dado and Izquierdo, 1989}).

We can consider that the association « autogenous graft-membrane » is indicated only in the following situations :

- the cortico-cancellous graft is smaller than the recipient bed ;

- in certain situations, the cortico-cancellous graft can be harvested only in chips or particles and only a membrane will assure the adaptation and stabilization of the particles (^{fig. 8a}, ^8b, ^8c, ^8d and ^8e).

In fact, the choice of the reconstruction technique for Class II defects depends not only on the shape and the size of the lost part, but also on the possibility of harvesting one or several autogenous grafts (^{table III}).

The healing time necessary to obtain bone anchorage varies according to the technique (^{table III}) :

- the period is about 9 to 10 months when e-PTFE membrane alone and spacing screws are used (^{Buser et al., 1995} ; ^{Jovanovic and Nevins, 1995}) ;

- if an autogenous bone graft is used with a membrane, 7 months will be sufficient. The characteristic fibrous layer generally observed underneath the e-PTFE membrane was not noticed (^{Jovanovic and Nevins, 1995}) ;

- when a graft of membraneous origin can be secured alone the healing time can be reduced to 4 to 6 months (^{Misch et al., 1992} ; ^{Jensen et al., 1994}).

Clinically the autogenous bone graft used in the treatment of Class II defects will result in the acquisition of a type I to type II bone (^{Lekholm and Zarb classification, 1985}).

^{Listrom and Symington (1988)} suggested, during the implant placement, the use of drilling instruments identical to those used on the non grafted sites. The delayed implant insertion will also stimulate the preservation of the bone volume (^{Lew et al., 1994}).

The acquisition of particularly dense grafted sites and the possibility of bicortical anchorage will be very important for good biomechanical conditions of loading (^{Misch, 1990}).

Conclusions

If the technical considerations are essential to the choice of the treatment, they are only a small part of the decision tree. The patient factors, the pre, per, and post surgical conditions should be evaluated cautiously (^{Mellonig and Nevins, 1995}).

It is important to take into account the psychological aspects of the patient, the risk factors (systemics, tobacco, stress…) as well as the maintenance of good plaque control. The treatment plan will take into account the pre-surgical condition and create a controlled environment for the appropriate surgical procedure.

The clinical success depends on the control of infection risk, increased by the use of a large membrane. If all of the risk factors can be managed, and if the possibilities of bone harvesting are sufficient, our clinical choice for treatment of Class II defects will be oriented to autogenous graft.

Demande de tirés à part

Jean-Pierre GARDELLA, Résidence Prado Palace, Immeuble Hermès, 131, avenue du Prado, 13008 MARSEILLE - FRANCE.

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