ARTÍCULO ESPECIAL

 

Preprosthetic surgery. A critical analysis

Cirugía Preprotésica. Análisis crítico

 

 

J.I. Salmerón Escobar

Médico Adjunto
Servicio de Cirugía Oral y Maxilofacial
Hospital Universitario Gregorio Marañón. Madrid, España

Correspondence

 

 

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ABSTRACT

One of the greatest problems in dental implantology is the various defects of the jaw bones, which can hinder the placement of implants or that can lead to faulty placement. The methods for bone augmentation have been, and still are, diverse and sometimes magical products appear for regenerating bone that are of dubious efficiency.
The aim of this article is to analyze the principal methods of preprosthetic surgery such as bone grafts and alveolar distraction, and to carry out a critical analysis based on personal experience and the current literature.

Key words: Preprosthetic surgery; Bone graft; Alveolar distraction; Dental implants.

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RESUMEN

Uno de los mayores problemas en la implantología dental son los diversos defectos de los huesos maxilares, que pueden impedir o hacerlo en forma defectuosa, la colocación de los implantes. Han sido y son diversos los métodos de aumento óseo y a veces aparecen productos mágicos para la regeneración ósea de dudosa eficacia.
El propósito de este artículo es analizar los principales métodos de cirugía preprotésica, como son los injertos óseos y la distracción alveolar y hacer un análisis crítico de la misma, basado en la experiencia personal y en la literatura vigente.

Palabras clave: Cirugía preprotésica; Injerto óseo; Distracción alveolar; Implantes dentales.

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Introduction

The principal causes of bone loss are: tooth loss -caries, periodontal disease, dental trauma, extractions-, facial trauma and tumors, in addition to other reasons such as systemic disease.

Being edentulous generates a progressive loss of bone tissue, which produces changes in the facial skeleton and face leading to ageing in the more extreme cases.¹ This makes placing the implants in an optimal position for subsequent prosthetic rehabilitation somewhat difficult, and sometimes this may even not be possible. Not only has the structural defect to be corrected, but also functional and aesthetic problems have to be addressed. And, the expectancies of the patient have to be met,² which is quite challenging in the more complex cases.

The principal methods of bone regeneration in preprosthetic surgery are: alveolar distraction and bone grafts, and their alternatives, mainly bovine-derived xenografts (Bio-Oss^®) and ceramic alternatives such as tricalcium phosphate (Cerasorb^®).

When the scientific presentations are reviewed together with articles in books and journals on preprosthetic surgery (around 1,000 references), the results and the cases are very good, but most of these lack a critical approach and scientific rigor. In a recent review of bone regeneration by Cochrane, Esposito and cols.,³ only 13 RCT’s were selected that were rigorous and from which conclusions could be extrapolated, but even these suffered in general from limited follow-ups and patient numbers.

 

Type of bone graft

Autografts

Mandible-chin-symphysis

This permits obtaining cortical-spongy bone grafts (especially cortical grafts) of a medium size. When using this technique, care should be taken not to damage the mental nerve and dental roots.^4,5

It is an ideal graft for small and medium defects. It provides bone of a membranous type and, as a result, there is less resorption than in endochondral bone and, as it is a bone that is basically cortical, there is less resorption than with the spongy bone type due to slower revascularization taking place over months, while in spongy bone this takes place in weeks.^6,7

The chin graft has the advantage of being obtained from an intraoral site, and being an uncomplicated procedure it can be carried out with local anesthesia.

The disadvantages are that there is morbidity and possible damage to the mental nerve or to incisor roots.

Ascending ramus (Fig. 1)

From the ascending ramus purely cortical grafts can be obtained that are of a small size. Only the external cortical bone is obtained in order to avoid any damage to the dental nerve. There is noticeably less morbidity than with the chin graft, with regard to possible nerve damage.⁸ When deciding from which area the graft should be obtained, the height of the dental nerve in the retromolar area should be assessed, as in most cases the area with most availability is where we will find the nerve, and transitory hypoesthesia may arise, or anesthesia if the technique is not carried out with care. This graft is membranous and therefore the rate of resorption is low. Its advantages are the ease with which the technique is carried out, as it can be carried out with local anesthesia through an incision that is similar to that used for a third molar, and its low morbidity.

Calvarial bone

This graft is obtained from external calvarial plate. It is membranous and of the cortical type. Resorption is therefore low.⁹ It provides a large volume of bone, which makes it appropriate for reconstructing large defects. Its advantages are low postoperative discomfort, hidden scar in hair, and the bone volume that can be obtained. The inconveniences are that the technique requires appropriate training, the graft is difficult to mold, general anesthesia is needed and there are possible complications which, although very infrequent, can be serious: Epidural hematomas, brain damage and cephalorrhachidian fistulas.^4,6

Iliac crest

This is the most used graft in maxillofacial reconstruction, ¹⁰ as a large volume of cortical-spongy bone is provided which is suitable for large reconstructions. The trap door technique is usually used, which avoids an unaesthetic depression of the crest. Releasing the gluteal muscles should be avoided in order to prevent any problems when walking.

The bone is of an endochondral origin and the material is basically spongy. The resorption process will therefore be much greater¹¹ than with membranous and cortical type grafts.

The great advantage of this graft lies in the large volume of cortical and spongy bone that can be obtained either in blocks or chips. The disadvantages are the need for general anesthesia and the postoperative discomfort when walking. Other types of complications such as paralytic ileus are exceptional.

Tibia

The tibia graft provides spongy bone chip which can be obtained by either accessing the tibial tuberosity either laterally or medially,^12,13 and by the posterior trephination of the cortical bone and curettage of the spongy bone. The graft is basically indicated as filling material for sinus elevation and for cystic cavities. The advantages are the ease of the technique and the lack of morbidity. Complications such as fracture of the tibial plateau are exceptional.

Xenografts

Porous resorbable bovine hydroxyapatite (Bio-Oss^®) is mainly used. This has osteoconductive properties, which allows bone growth among the particles, and which has long term resorption. The final rate of resorption of the graft is low. It is mainly indicated for filling cavities especially in sinus floor elevation.¹⁴ It is also used in postextraction immediate implants and in fenestrated implants.

Alveolar distraction

This technique is based on Ilizarov´s¹⁵ principles, who observed patients with fracture consolidation problems, and whose fracture callus was submitted to compression using orthopedic apparatus. This resulted in the accidental turning of the screws in the opposite direction, and finally to the lengthening of the lower limbs. It showed how bone could be formed by carrying out an osteotomy and posterior distraction, and that bone would finally be formed in the gap of the bone callus. The first studies were carried out in long bones, and it was Block¹⁶ who in 1996 carried out the first alveolar distraction in dogs, using the implants as distractors, practically at the same time as Chin¹⁷ published the first human alveolar distraction osteogenesis with the intraosseous distractor that he designed.

The principal indication of alveolar distraction is for the vertical lengthening of the anterior portions of the mandible and maxilla. This achieves a lengthening of bone and soft tissue with very predictable and stable results.^2,18

Bone resorption of grafts and success rate

A very important problem associated with bone grafts is the resorption rate, and this is lower in membranous and cortical grafts as previously indicated. When we refer to horizontal augmentation, the influence of resorption rates in the final result is relative, as usually this does not compromise the subsequent placement of the implants. However, in vertical augmentation cases of certain relevance the results are worse, especially in the mandible where the problems associated with soft tissues, dehiscence and saliva contamination of the graft, can lead to the procedures failing or to associated morbidity.

There are various studies in the literature that analyze or compare the rate of resorption of grafts used in preprosthetic surgery.Verhoeven,^11,19 placed iliac crest grafts in the mandibles of 30 patients to facilitate the placement of two implants and the later rehabilitation with bars. The patients were followed for 3 years and a resorption rate was observed of 36% in the grafts. The analysis of their later article ten years after loading the grafts is very interesting, as the long term results of the prospective study are observed. Only 51% of the height of the graft remained stable and during this time various interventions were subsequently necessary for associated peri-implantitis. However, none of the implants were lost.

The rate of resorption of membranous bone is lower. Ikuza⁹ hardly noticed any resorption when calvarial bone grafts were used. This information coincides with that of Lenzen²⁰ who experienced a 10% resorption rate.

Proussaefs and Lozada²¹ reported a reduction in volume of 17.58% in grafts in the ascending ramus that were used for vertical augmentation in 8 patients. The interval before implant placement varied considerably (4 to 8 months) and they reported graft exposure in 3 out of the 8 cases. In one case there was early exposure leading to necrosis of the graft.

Stellingsma²² reviewed the literature in order to observe the different solutions for extremely resorbed mandibles: transmandibular implants, short implants, bone grafts and alveolar distraction. The bone grafts were placed using the onlay and inlay methods, and the implants were placed in one or two phases. The resorption of the graft was lower with the onlay technique and when the implants were placed in two phases, at three or four months of graft placement. On the other hand, if the implants are placed in a single phase, surgical time is shortened, but the prosthetic phase is more difficult because the implants are not always adequately placed, in addition to there being unpredictable resorption around the implant.²³ The rate of implant survival as reflected by Stellingsma varied between 88 and 100%. The associated local complications were: dehiscence of the wound, infection and sensory disturbance of the mental nerve.

Given the possibility of wound dehiscence, contamination and the possibility of graft infection and loss in extremely resorbed mandibles, the submental extraoral approach has been described with submucosal dissection and dissection of the mental nerve. There is, in addition, another risk with these mandibles, which is the fracture of the mandibular body. Bell²⁴ used this method in extremely resorbed mandibles with a height that was less than 7 mm, using iliac crest as a graft, reconstructing the mandibular body and symphysis and placing the implants in two phases in the symphyseal region. The gain in bone height varied between 9 and 22 mm. Bone resorption before placing the implants at 3 to 4 months of graft placement was 33% . At 12 months of loading it was inappreciable in the symphyseal region with the loaded implants, and 11% in the mandibular body. In none of the cases was there dehiscence or infection of the graft. We have used this approach in some of the patients with considerable mandibular resorption, using calvarial bone as graft material, achieving good results with no associated local complications.

Comments on the grafts

From my point of view and based on personal experience and the bibliography reviewed, bone grafts are the best alternative for horizontal bone augmentation. The best options are grafts from the chin or the ascending ramus for medium and small defects respectively, and iliac crest for larger defects with horizontal deficiencies of the maxilla and or mandible. In cases of small vertical defects, ramus or chin grafts are an alternative. In cases of larger vertical defects, especially in the anterior portion of the maxilla and mandible we should look for alternatives that are more predictable such as alveolar distraction.

Sinus elevation: type of grafts to be used and success rate (Fig. 2)

Sinus floor elevation is considered the method of choice for the vertical augmentation of the posterior maxilla. The filling material to be used in sinus floor elevation has changed over time. From the beginning in which autogenous bone was considered the gold standard, initially from spongy hip bone and spongy tibia bone, to the mixtures of autogenous bone with PRP and hydroxyapatite in cocktails with different recipes depending on the author, to the use of biomaterials on their own, or even the use of cellulose (surgicel ^®) or just the clot itself; everything has been tried in the maxillary sinus. In a recent review on bone filling in maxillofacial surgery, Ochandiano²⁵ was of the opinion that the maxillary sinus is a non-critical defect and that biomaterials on their own stabilize the clot and they can be sufficient for the ossification of the sinus.

Hallman²⁶ carried out 36 sinus elevations that were augmented using autogenous bone chip, bovine hydroxyapatite and a 80/20 mix of hydroxyapatite-autogenous bone. The implants were placed after a period of 6-9 months. The short term results showed no significant differences in the three groups.

Following a split mouth study involving 10 beagle dogs with autogenous bone filling in one side and Bio-Oss^® in the other, Schlegel²⁷ concluded that Bio-Oss^® is a good filling material and that it undergoes less resorption that autogenous bone.

Szabó²⁸ used, in a multicentric prospective split mouth study of 20 patients, autogenous bone in one side and tricalcium phosphate (Cerasorb^®) in the other. Significant differences were not found between the groups.

Gray²⁹ achieved sinus regeneration using just oxidized cellulose (surgicel^®). Lundgreen³⁰ carried out sinus elevation by suspending the raised membrane and by placing the implants on the residual alveolar ridge (4-10 mm) in order to maintain the elevation. Ossification of the sinus was achieved simply by stabilizing the clot that was formed in this way.

In his review of RCTs on bone augmentation techniques, Esposito³ concluded that bone substitutes (Bio-Oss, Cerasorb) were perhaps as efficient for augmenting very atrophic sinuses as autogenous grafts.

Comments

According to the studies reviewed and my own experience, there is currently no reason for using autogenous bone in sinus elevation. Autogenous bone has the disadvantage of requiring a donor site and it undergoes more resorption than Bio-Oss^®. The only disadvantage of the latter is that the healing period before implant placement is delayed 6 to 9 months.

Alveolar distraction and success rate

Alveolar distraction represents a predictable method for the vertical augmentation of bone and the complication rate is very low.³¹ The major difficulty lies in suitably directing the distraction vector.^2,4,32,33 Herford³² described various methods of orthodontic support for carrying out distraction osteogenesis with an adequate vector.

Bilbao³³ carried out a total of 44 alveolar distractions and 39 were considered a success because placing the implants correctly was possible and because bone was taken to the desired position, with no need for any complementary technique.

Stellingsma²² considered distraction osteogenesis as an efficient method for the augmentation of the anterior portion of extremely resorbed mandibles, in combination with endosseous implants, although it is pointed out that more long term studies are needed.

Chiapasco et cols,³⁴ carried out an extensive prospective multicentric study on alveolar distraction osteogenesis: 37 patients, 4 centers, 138 implants, bone gain of 9.9 mm (4- 15). The results obtained in this study were: accumulated success after 4 years of loading the implants of 94.2%, accumulated rate of implant survival of 100%, 1 partial failure with incomplete distraction, 5 cases of inclination of distraction fragment, 0 % infection, biopsies with intramembranous ossification of the distraction gap, bone resorption of the distracted fragment 0.3 mm, peri-implant resorption 1.4 mm after 4 years of loading. It was concluded that distraction is a very predictable and stable method, considerable gone gain is achieved with a low rate of morbidity, and that the success and survival rate of implants in generated bone through distraction osteogenesis is similar to that of normal residual alveolar bone.

Comments

Alveolar distraction is a predictable and stable method for vertical alveolar augmentation. The greatest difficulty lies in controlling the distraction vector, which can be difficult in some cases. It is most indicated for the anterior portion of the maxilla and mandible. In the posterior portion of the maxilla it is not indicated as sinus floor elevation would be the method of choice. In the mandibular body portion it is useful, although the distraction technique in this area is difficult.

 

Alternatives in preprosthetic surgery

Short implants

Short implants of 10 mm or under can represent a valid alternative for edentulous resorbed mandibles, in which 4 implants can be placed for one overdenture, and also in the posterior portion of the mandible, as other alternatives such as grafting or distraction are difficult and they do not always give the best results. In extremely resorbed mandibles, which are not very common, bone fractures may occur³⁵ which are very difficult to treat, as such a weak bone is submitted to the stress of a load. In these cases mandibular reinforcement should be assessed with grafts and with, probably, an extraoral submental approach.²⁴

Stellingsma,²² in a review of different methods for treating atrophic edentulous mandibles, pointed out that short implants for overdentures in the symphyseal area, have a survival rate that varies between 88 and 100%, according to the different series, and in general a lower rate of associated complications can be appreciated than when implants are combined with bone grafts.

Arlin³⁶ compared short implants of 6 or 8 mm with longer ones of 10 to 18 mm in partially edentulous areas. No statistically significant differences were found in survival rates at 2 years.

Esposito³ considered short implants, used for overdentures as an alternative of choice in very resorbed mandibles.

Cortical expansion with osteotomes (Fig. 3)

This consists in expanding maxillary cortical bone, by introducing instruments with an increasing diameter in order to separate both bone structures and to create a space that will permit the insertion of an implant with satisfactory primary stability. This is used in atrophic alveolar ridges with a 3-4 mm diameter. The use of bone grafts in cases with moderate maxillary horizontal alveolar defects is in this way avoided.

The two types of osteotome techniques that are used are those that are introduced by means of pushing or screwing. The latter have the advantage of avoiding the unpleasant repetitive trauma for the patient of the pushing system when introduced.

Angled implants for avoiding sinus elevation

For cases in which there is a sinus pathology of purulent sinusitis or chronic recurring sinusitis, and sinus floor elevation is not possible or it has failed, or if the patient does not desire sinus floor elevation, the situation can be salvaged by placing an angled implant in front of the sinus and another behind (pterygoid).

This technique should not be carried out blindly, using a sense of touch, so that accidentally penetrating the sinus is avoided. We carry this out using a planning system with Simplant (Materialise) software, obtained from the patient´s CAT scan, and an implant is placed virtually in front of the sinus and another behind (in the area of the pterygoid), and they are tilted between 25-30 degrees so as not to hinder prosthetic rehabilitation. This planning analysis is sent away in order to obtain custom-made splints using a CAD/CAM system. The 3 splints that are obtained permit placing the implants with three twists of the bur’s diameter. In the exact sense as was planned virtually in the patient. The implants are then placed, and the posterior portion of the maxilla is rehabilitated in those cases in which sinus floor elevation is not possible. (Fig. 4 and Fig. 5)

 

Conclusions (Table 1)

1. All that glitters is not gold. There are no magical or single remedies for generating bone, and each technique has its negative side (morbidity and complications).

2. Each case should be viewed individually and the most suitable method used, depending on the patient´s bone defect and personal characteristics.

3. In horizontal defects (transverse or width) bone grafts are the technique of choice.

4. In the posterior portion of the upper jaw, the technique of choice is sinus floor elevation and the filling material to be used should consist of biomaterials.

5. Alveolar distraction osteogenesis is the most predictable method for vertical augmentation, especially in the anterior portion.

6. In very resorbed edentulous mandibles, short implants should be considered as a rehabilitation method.

7. In extremely resorbed mandibles, placing grafts using a submental approach should be assessed in order to avoid mandibular stress fractures.

8. The mandibular body is the most unpredictable with regard to rehabilitation and the use of short implants should be considered.

Correspondence:
José Ignacio Salmerón
Clínica SP
C/ Rodríguez Marín 71.
28016 Madrid, España
Email: jisalmeron@clinicasp.com

Recibido: 03.01.07
Aceptado: 29.01.07

 

 

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