Chapter 1. Introduction

Bony defects in the cranio-maxillofacial skeleton may arise as a result of congenital areas of failed development such as in patients with cleft lip and palate, the results of ablative surgery in which segments of bones are resected to treat tumours, and due to trauma in which case osseous tissue may have been traumatically avulsed. Such osseous defects can be reconstructed by bone grafts or hopefully, in the future, using bone graft substitutes or by modulating bone regeneration using a variety of osteoactive agents.

In the future, the ideal clinical scenario would have a surgeon identify an osseous defect, reach up to a shelf for a container of a reliable bone graft substitute, obviating the need for a second surgical site, and sparing the patient a donor site defect. However that day has not yet arrived, as autogenous bone still remains the gold standard for maxillofacial osseous reconstruction (Clokie et al. 2000). Bone grafting studies have shown, that autogenous cancellous bone produces the most successful and predictable results (Marx 1994). Therefore, the ability to harvest and graft autogenous bone using a minimally morbid technique would greatly enhance the success and patient acceptance of oral and maxillofacial reconstructive surgery by reducing morbidity.

Sources of non-vascularized autogenous bone for grafting can be broadly divided into local and distant sites, and their successful application to maxillofacial reconstructive surgery is well documented (Marx 1993). If the defect requiring a graft is small, often local or intra-oral donor sites are sufficient (Kainulainen et al. 2002a). When a moderate to substantial amount of bone is required, the distant or extra-oral sites are usually employed. Of these distant sources, the iliac crest has become a favoured donor site because of the relative ease of surgical access and the quantity of bone available (Dingman 1950, Converse & Campbell 1954, Flint 1964, Levy & Siffert 1969, Crockford & Converse 1972, Mrazik et al. 1980, Hall & Smith 1981).

The anterior ilium provides an adequate volume of bone for many of the cranio-maxillofacial reconstructive procedures that require grafting. While a variety of techniques have been devised with the intent to reduce morbidity (Wolfe & Kawamoto 1978, Mrazik et al. 1980, Grillon et al. 1984, Tiley & Davis 1984, van der Wal et al. 1986), the most commonly employed and least complex technique is to harvest a corticocancellous block through either a medial or lateral approach to the anterior ilium. In 1994 Tayapongsak et al. found no significant difference in morbidity in a comparative study of lateral versus medial surgical approaches to the anterior ilium. However, these very routine standard approaches can still produce significant morbidity for the patient (Cocklin 1971, Wolfe and Kawamoto 1978, Marx & Morales 1988, Tayapongsack et al. 1994). Because of this degree of morbidity clinicians have been hesitant to adopt bone grafting as a treatment option and patients have been reluctant to accept such treatment. Thus, there is an advantage to developing a method for obtaining bone from the anterior ilium, which creates less morbidity for the patient than the traditional method.

The use of trephines in other fields of surgery has been shown to be safe (Duncan et al. 1980, Kreibich et al. 1994). The technique of using a trephine to harvest bone specimens from the anterior iliac crest for the purpose of biopsy has been used with minimal morbidity (Waldman & Kleinfeld 1970, Smirnov & Baranov 1971, Johnson, Kelly & Jowsey 1977, teVelde et al. 1978, Schuyt, Meulmans & van Eek 1979, Minns & Sher 1983, Faugere & Malluche 1983). The use of a power-driven trephine to procure bone from the anterior iliac crest is a simple technique, and should be adaptable to a minimally invasive surgical approach for the purposes of bone graft procurement with the intent to minimize morbidity.

Alternatively, a bone graft substitute would be ideal in minimizing post surgical morbidity by eliminating the donor site, thereby decreasing post-operative discomfort and saving the surgeon the time required to harvest bone. The ideal bone graft substitute should be biologically inert, readily available, easily adaptable to the site in terms of size and shape, biodegradable and replaceable by host bone (Bajpai 1983).

Many materials have been used as bone graft substitutes in the past. One formerly popular material, hydroxyapatite (HA), had been used extensively. Studies have demonstrated that the porous form of HA allows rapid fibrovascular tissue ingrowth which may stabilize the graft and help resist micromotion (Jarcho 1986, Alexander et al. 1987, Kenny, Lekovic & Caranza 1988, El Deeb & Holmes 1989, Ricci et al. 1989). However, HA may not undergo appreciable resorption. Furthermore, histological studies have shown that HA does not completely ossify, but rather, becomes encapsulated with fibrous tissue (Rosen & McFarland 1990, Byrd, et al. 1993).

On the other hand, coral-derived granules (CDG) do exhibit some of the characteristics described by Bajpai in 1983, including being completely resorbable and replaceable by host bone (Chiroff et al. 1975, Guillemin et al. 1987, Roux et al. 1988a).

CDG consist of natural coral skeletons from the genera Acropora, of the group Madrepora, collected from the French part of the Great Barrier Reef in New Caledonia (Guillemin et al. 1987).The process of coral resorption has been shown to be related to the action of carbonic anhydrase (Chétail & Fournie 1969, 1970), an enzyme contained in osteoclasts (Simasaki & Yagi 1960 and Gay & Miller 1974), which may act on the calcium carbonate in the coral skeleton. CDG are completely resorbable and replaceable by bone (Ouhayoun et al. 1991). Implanted coral is well tolerated in a variety of animal models (Issahakian et al. 1987a, Shabana et al. 1991), and also in humans (Souyris et al. 1985, Issahakian & Ouhayoun 1988, Ouhayoun et al. 1992). Prior to this present work, longer term experience and follow-up in augmenting defects of the human cranio-maxillofacial skeleton with CDG had been lacking.

One other novel application of CDG might be the preservation of the alveolar ridge after tooth-loss in a paediatric population. In 1994, Ostler and Kokich investigated changes in alveolar ridge width after removal of retained primary molars in patients who were congenitally missing mandibular second premolars. They have shown that the alveolar ridge width decreases 25% within 3 years after extraction of the retained primary molars, and diminishes a further 4% over the next 3 years (Ostler & Kokich 1994). Our own experience at the Hospital for Sick Children in Toronto suggests that the premature loss of permanent maxillary incisors due to trauma, removal of retained ankylosed primary molars, and other ankylosed and submerged teeth results in the development of alveolar ridge defects. Alveolar ridge defects compromise the suitability of these sites for future restoration with dental implants, or impair the aesthetics of the restorative solutions.

Augmenting these types of alveolar defects with CDG may preserve bone volume until such time as the patient is ready to undergo definitive restoration with a dental implant when skeletal growth has ceased (Kurol & Ödman 1996). An alveolar sparing procedure using a bone graft substitute could be beneficial to such patients. The time required for wound healing and incorporation of the coral granules could advantageously allow for the completion of jaw growth, all the while preserving the dimensions of the residual alveolar ridge. The goal would be to allow the placement of a dental implant, in an uncomplicated manner, without the need to harvest a bone graft from a second anatomic location, thereby minimizing post-operative morbidity.

The present study therefore focuses on two aspects of minimizing the morbidity of cranio-maxillofacial osseous reconstruction. Firstly, the development of more conservative but effective harvesting techniques should reduce patient post-operative morbidity in those situations where autogenous bone graft material is deemed necessary. Secondly, the use of an effective bone graft substitute, where deemed appropriate, should diminish post-operative morbidity by eliminating the bone graft donor site while accomplishing a satisfactory reconstructive result.