Oral Health Group
Feature

IMPLANTOLOGY: Growing Bone: Clinical Considerations

September 1, 2001
by David M. Vassos, DDS


Implant practitioners need to create additional bone in many situations. With advanced atrophy of the alveolar ridge, insufficient bone width or height may make placement of implants impossible. Alternatively, trauma or acute periodontal disease may remove bone that must be replaced before additional therapy can proceed. In other instances, patients may have enough bone to enable implant placement but not enough for optimal aesthetics to be achieved.

As more and more bone-grafting materials have become available, arguments for the superiority of one source over another have been advanced by various proponents. The author’s extensive clinical experience suggests, however, that no current alternative meets all the criteria for the ideal bone-grafting material. Instead, differing clinical situations dictate the use of different materials. Furthermore, stabilizing the graft material and achieving adequate soft-tissue coverage of the site are at least as important to the ultimate success of the graft as the source of the graft material. When an extensive bone defect must be grafted, these considerations become even more challenging. Several case studies provide insight into techniques useful in solving the challenges.

THE IDEAL BONE-GRAFT MATERIAL

New bone is created by means of both osteoinduction and osteoconduction, and both processes are generally necessary for any graft to succeed.1,2 The ideal bone-graft material should foster both osteoconduction and osteoinduction in the following ways:

It should provide an inorganic matrix that can be resorbed as new bone is formed osteoconductively, moving from the patient’s host bone inward into the graft site.3 The matrix also serves as a scaffold for fibrin, fibronectin, and blood vessels, all of which play a role in the osteoinductive pathway.

It should contain bone morphogenetic protein (BIVIP) and other factors that cause primitive mesenchymal cells to be transformed into chondroblasts, osteoblasts, and hemopoietic tissue4-6 necessary for osteoinductive bone formation.

A third criteria for the ideal bone-graft material relates to its source. Optimally the material should not come from the patient. Harvest of autogenous bone always carries some risk. For example, complications reported after acquisition of bone from the iliac crest (a popular donor site) include persistent post-operative pain, nerve injury, arterial injury, cosmetic deformity, hemorrhage, and infection.7

Performing a second surgery adds time and complexity to the treatment plan, and creates inconvenience for the patient. Moreover, access to an exogenous source of graft material ensures that the doctor need not worry about harvesting the precise amount of graft material needed.

Finally, the ideal bone-graft material should contain no potential for bringing disease into the recipient’s body or being rejected by it.

Bone-graft material falls into four major categories.

CURRENT OPTIONS

Autogenous bone, long considered to be the “gold standard” among graft materials, is collected from the patient’s own body. The most common intraoral harvest sites are the ramus, symphysis, tuberosity, and exostoses. Furthermore, small quantities of autogenous bone may be gleaned with the use of bone traps while creating osteotomies. Outside the mouth, autogenous bone may be harvested from the iliac crest, rib, posterior ilium, tibia, and cranium.

Fresh autogenous bone material provides the graft site with many important components. Hydroxylapatite (HA) is the primary inorganic material, but the inorganic autogenous bone matrix also contains osteocytes, osteoclasts, osteoblasts, and various proteins.8-11 The death of these cells at the graft site releases BMP and components13,14 that work to transform the stem cells into osteogenic cells.

The disadvantages of autogenous bone include the fact that bone retrieved from osteotomy sites is not sufficiently stable to resist overlying pressure and thus does not retain the space required for successful augmentation. Harvest from other locations imposes risks of mortality and morbidity and complicates the treatment plan. A certain amount of surgical expertise also is required to obtain autogenous bone material of the required shape and quantity. Furthermore, patients typically dislike the idea of having a second surgery.

Three categories of grafting material constitute the alternative to autogenous bone: allogenic bone, xenografts, and alloplasts.

Allograft material comes from a human source other than the patient. Most commonly, such bone is harvested from cadavers. After harvest, the bone may be processed in several ways. All allografts pose some risk of disease contamination, however, conscientious screening and processing can mitigate much of that risk. It is essential that the doctor work with a well-established and trustworthy bone bank.

Among xenografts, bovine bone has been grafted to the human skeletal system since the late 1950s,15 but concerns about the antigenicity of such material limited its use for many years.16 With the development of more effective techniques for deproteinating xenographic materials, a number of bovine-derived grafting materials have become available. The majority of these, while serving as a scaffold for osteoconduction, have lacked the organic cellular components required to foster osteoinduction. Recently a bone-substitute material has been introduced that may solve this deficiency by enhancing bovine bone mineral with a synthetic peptide that mimics a component of Type I collagen responsible for migration and cell attachment. However, clinical assessment of this product is still in progress.

The final category of bone-grafting materials are the alloplasts -synthetic bone substitutes including hydroxyapatite (HA) and hard tissue replacement (HTR). Used in combination with autogenous bone, such materials provide valuable support, for example, when an improved aesthetic profile is desired or a severely resorbed mandible must be strengthened to resist fracture. However, since alloplastic materials are not converted into bone, they cannot be used exclusively when grafting a site in preparation for implant placement.

From this discussion, it can be seen that the perfect bone-graft material does not yet exist. However, excellent results can be obtained by utilizing the existing alternatives in combination. Conditions at the graft site dictate which materials will work best for each individual patient. The following cases illustrate a variety of successful approaches.

CASE STUDIES

Case # 1

The patient, a 48-year-old male, presented with a missing first right lower molar (Fig. 1). Concave topography of the ridge made it obvious that, in the absence of grafting, only enough bone existed to permit placement of a very small implant lingual to the existing teeth. To achieve an appropriate emergence profile and avoid off-axis loading, a decision was made to place the implant in line with the other dentition and cover any exposed threads with an onlay graft composed of hard-tissue replacement mixed with autogenous bone.

A standard crestal incision was made and an osteotomy was created, employing bone traps to collect all the bone from the osteotomy. The cortical plat was also perforated at several points to induce bleeding from the medullary bone, an important source of osteogenic components (Fig. 2). A 6 mm-diameter, 1 6 mm-long Steri-Oss Replace implant (Nobel Biocare, Yorba Linda, California) was placed, achieving stable fixation. Figure 3 shows the implant in position, with several threads exposed buccally.

The autogenous bone salvaged from the osteotomy was then mixed with HTR and liquid antibiotic (Fig. 4). The resulting gelatinous mixture was applied to the exposed implant threads, simultaneously covering them and building up the ridge (Fig. 5). Biogide collagen material (Osteohealth, Shirley, NY) was positioned over the onlay graft (Fig. 6), and primary closure was achieved (Fig. 7).

Three months later, the healing screw was removed (Fig. 8). The final restoration can be seen in Figure 9.

Case #2

This 38-year-old female presented with missing mandibular first and second bicuspids and molars. As Figure 10 illustrates, the width of the posterior portion of the ridge was sufficient, but the anterior portion had experienced severe atrophy that required augmentation before any implant placement could proceed. Since a substantial quantity of bone was needed, the decision was made to extend a crestal incision distally, into the ramus (Fig. 11), where a block of bone roughly 14 mm by 5 mm by 2 mm was harvested (Fig. 12).

Osteotomies were created at the two molar sites, and bone traps were utilized to preserve the material removed. Two 5 mm-diameter, 10 mm-long Replace implants were placed in the first and second molar positions. A piece of sterile titanium foil was then positioned over both of the implants as well as the ungrafted anterior portion of the ridge to assess the amount of stabilizing material that would be needed to cover the ridge. Using the foil as a template, a piece of sugar mesh with a .12 mm-diameter hole size was cut to size and positioned over the two implants, extending mesially over the area to be grafted. Healing screws were screwed into the two implants through the mesh (Fig. 13), securing it.

To augment the atrophied portion of the ridge, the ramus bone was placed on the buccal face of the ridge, adjacent to the cuspid. A tapered fissure bur was utilized to pre-tap holes for stabilizing screws in the donor bone (Fig. 14). Defects around the donor bone were filled with a mixture of autogenous bone retrieved from the osteotomies, irradiated bone, and liquid antibiotic. The titanium mesh was then folded down over both sides of the ridge, and a stabilizing screw was inserted into the pre-tapped hole on the buccal (Fig. 15). A fastening screw secured the mesh on the lingual.

The incision was approximated and allowed to heal. Figure 16 shows the ridge after five months, with the mesh just visible through the mucosa. A vertical incision was made mesial to the cuspid, along with a horizontal incision along the crest of the ridge. The screws securing the titanium mesh were removed, and the mesh was dissected off the ridge. Figure 17 shows the augmented ridge, now wide enough to accommodate the placement of one or more implants.

As an economy measure, this patient elected to have just one implant placed in the first bicuspid position. A 4.3 mm-diameter Replace implant was inserted and allowed to heal with an exposed healing screw for an additional three months.

Figure 18 shows the third implant in place with the healing collar to be left exposed. After another three-month healing period, the healing screws were removed, and transfer pins were inserted for an impression. Abutments were placed without complication (Fig. 19), and the finished restoration can be seen in Figure 20.

In this case, the larger defect combined with the proximity to the ramus dictated the use of the harvested ramus bone, in conjunction with bone collected from the osteotomies and irradiated bone. The titanium mesh provided extra stability for the donor bone, a valuable benefit in achieving osseointegration.

Case #3

A cannister of oxygen had exploded in close proximity to the face of this patient, a 36-year-old male firefighter. In addition to breaking his jaw in several places, the trauma knocked out his four lower incisors, as well as much of the underlying bone (Fig. 21).

Due to the complexity of the case and the large amount of missing bone, a decision was made to undertake the grafting in stages. First an incision was made on the crest of what remained of the alveolar ridge, and a mucoperiosteal flap was reflected (Fig. 22).

As in the previous case, sterile titanium foil was used as a template for determining how much titanium grid mesh would be required (Fig. 23) to act as a scaffold and provide stability to the graft material.

Cortical and medullary bone was harvested from both sides of the symphysis (Fig. 24), and the donor sites were filled with irradiated bone and covered with Biogide. A piece of titanium grid mesh was cut and positioned within the cavity, forming a hollow structure to contain the graft material (Fig. 25).After trying in this structure, it was removed, lined with Biogide collagen material, then filled with a mixture of irradiated bone and ground cortical and medullary bone. Once repositioned over the void, the titanium mesh structure was screwed into place (Fig. 26) and covered with another layer of Biogide.

The periosteum was scored with releasing incisions, and primary closure was obtained over the grafting architecture. Nonethe-less, the tissue was under tension, and after three months, dehiscence had become obvious (Fig. 27).The exposed portion of the titanium mesh was removed with a fissure bur, and new collagen was found to have formed over the underlying graft. The patient was allowed to heal for two more months, at which time the tissue was dissected away. All the remaining titanium mesh was removed (Fig. 28), revealing that despite the loss of some bone, a significant amount of new hard tissue had formed.

Because still more bone was needed to achieve a proper emergence profile, a second graft was undertaken after six more weeks of healing. At this point, achieving adequate soft-tissue coverage became of paramount concern. To avoid further dehicense, two vertical incisions were made distal to the cuspids, down into the vestibule. A poncho flap was created, and the loose tissue was pulled up over the ridge toward the lingual (Fig. 29). Performations to establish bleeding were created in the newly formed bone.

A second titanium support structure was constructed and lined with collagen (Fig. 30). This was filled with irradiated bone only (Fig. 31), screwed into position over the ridge (Fig. 32), and covered with more collagen.

Figure 33 reveals significant improvement in the ridge, apparent after a five-month healing period. A crestal incision was made along the ridge, the unattached tissue was peeled away (Fig. 34), and the titanium mesh was removed (Fig. 35).

Although the patient had lost four anterior teeth, the original dentition had been crowded, so a decision was made to restore the incisors with a three-unit bridge placed over two implants. Osteotomies were created, taking care to position them so as not to encroach on the limited blood supply (Fig. 36). Figure 37 shows the two 4.3mm Replace implants in position. The abutments have been placed in Figures 38 and 39 illustrate the final restoration.

In summary, the severity of this defect made it necessary to approach the restoration in stages. The location made the use of autogenous chin bone a logical choice, but due to the size of the defect, a supplementary source of grafting material also was necessary. Irradiated bone filled this function well, and the use of titanium mesh enabled the graft mixture to be stabilized well enough to ensure that osteogenesis occurred. The case also illustrates how unattached vestibular tissue can be used to achieve adequate closure over even a very extensive graft.

Case #4

A 38-year-old male presented with four missing maxillary incisors. Since both the height and the width of the ridge were significantly compromised (Figs. 40 & 41), a staged grafting procedure was planned.

As in the previous case, two substantial pieces of autogenous bone were harvested from either side of the symphysis. After making a crestal incision and reflecting a mucoperiosteal flap, the donor bone was positioned on the buccal face of the ridge and secured with stabilizing screws (Fig. 42). Bleeding from the medullary bone above the graft material also was established (Fig. 43).

All voids around the donor bone were filled with a mixture of irradiated bone and medullary bone from the chin (Fig. 44). The entire graft site was then covered with Biogide collagen, and primary closure was achieved after releasing incisions were made in the periosteum.

After five months, a vestibular incision was made and the graft site was inspected (Fig. 45). The stabilizing screws were removed (Fig. 46), and sterile titanium foil was utilized to assess the amount of titanium mesh needed to stabilize the graft material needed in the next phase. After being cut to size, the titanium mesh was formed into a structure and lined with Biogide, filled with irradiated bone and placed in position and fastened with screws on both the buccal and lingual aspect (Fig. 47). The vestibular incision was then closed (Fig. 50).

After five months, the bulbous vestibular tissue remained substantially intact, with only a tiny dehiscence beginning to form (Fig. 51). The loose, unattached tissue was then removed with a scalpel (Fig. 52), and the titanium mesh was dissected off the ridge.

With sufficient height and width now available (Fig. 53), attention turned to ensuring the presence of sufficient attached tissue to enable primary closure over the implants to be placed. Alloderm, an acellular freeze-dried dermal graft material (Life Cell Corp., The Woodlands, TX) was rehydrated (Figs. 54 & 55), positioned over the ridge (Fig. 56), and sutured into place (Fig. 57). During the subsequent healing period, the Alloderm filled the same function as the bone graft material–acting as a scaffold for the formation of new mucosa. Figure 58 shows the ridge approximately two weeks after placement of the Alloderm.

Six weeks later, a crestal incision was created, and four 4.3 mm diameter, 13 mm-long Replace implants were placed (Fig. 59). Primary closure was then achieved with the new mucosal tissue. After six more months, the implants were restored (Figs. 60-63).

Case #5

In this case, the patient, a 44-year-old female, had already had implants and a Dolder bar placed in her edentulous mandible, and she desired to have her maxillary denture stabilized in similar fashion (Fig. 64). Although exposure of the maxillary ridge revealed that significant atrophy had occurred, enough width remained to make splitting and grafting the ridge feasible.

Care was taken to reflect the tissue enough to allow for adequate scrutiny of the cortical plates during the ridge-splitting procedure (Fig. 65). A tapered fissure bur was used to bisect the buccal and lingual cortical plates (Fig. 66), penetrating to the nasal spine, just short of the level of the sinus. A flat spatula was then employed to begin wedging apart the two cortical plates (Fig. 67). Replace Select Osteotomes (Nobel Biocare, Yorba Linda, CA) were also utilized at the implant placement sites to further assess the space available for the implants (Fig. 68).

Six 3.8 mm HA-coated Steri-Oss implants (Nobel Biocare, Yorba Linda, CA) were immediately screwed into the desired positions along the bisected ridge (Fig. 69). No osteotomies were necessary, since the inward pressure of the two sides of the ridge held the implants firmly in position. Voids around the implants were then filled with irradiated bone mixed with liquid antibiotic and covered with collagen (Fig. 70).

The tissue was approximated (Fig. 71), and the patient was sent home with a relined denture and instructions to adhere to a soft diet six months. After about seven months, pressure from the denture had nonetheless begun to cause some minimal exposure (Fig. 72). The implants were then uncovered and found to be immobile. Healing screws were placed and the denture was hollowed out and relined.

After six weeks, the healing screws were removed, transfer pins were placed (Fig. 73), and impressions were taken and sent to the laboratory for fabrication of a screw-retained Dolder bar. At the patient’s next visit, the healing collars were removed, and the newly fabricated Dolder bar was secured on the implants (Fig. 74). The denture was adapted to the bar by lining it with Molloplast-B soft denture liner (DETAX GmbH & Co. KG, Karlsruhe, West Germany), as well as adding metal retention clips to the denture. Figures 75 & 76 show the finished case.

CONCLUSION

For every case where bone augmentation is indicated, it would be ideal to utilize a graft material that came from a source other than the patient, posed no risk of disease or rejection, provided a reliable osteoconductive matrix, and supplied copious amounts of the factors required to stimulate vigorous osteoinduction. Unfortunately, no such ideal grafting material yet exists. However, the cases described in this article illustrate that it is nonetheless possible to obtain excellent results through judicious selection or combination of existing grafting materials.

While the choice of graft material is important, the often-problematic need to stabilize that material and achieve adequate soft-tissue coverage deserves equal attention. Consistent use of barrier and confinement materials as well as unattached vestibular tissue can provide a solution to both challenges. Finally, although successful treatment of large an d complex voids may not be possible in a single procedure, patient and thoughtful staging of the grafting procedures can yield outcomes that fully meet both the clinician’s and the patient’s expectations for success.OH

Dr. Vassos is Diplomate, American Board of Oral Implantology/Implant Dentistry; Fellow and Diplomate, International Congress of Oral Implantology; and Honored Fellow, American Academy of Implant Dentistry. He maintains a practice in Edmonton, AB.

Oral Health welcomes this original article.

REFERENCES

1.Roberts E. Bone physiology and metabolism. Calif Dent Assoc J 1987; October:54-67.

2.Roberts E, Simmons K. Bone physiology and metabolism in dental implantology: Rick factors for osteoporosis and other metabolic bone diseases. Impl Dent 1992:1 (1):11-21.

3.Smiler D. Bone grafting: materials and modes of action. Pract Periodontics Aesthet Dent 1996:8(4): 413-416.

4.Urist MR, Strates BS. Bone morphogenetic protein. J Dent Res 197 1;50:1392-1406.

5.Urist MR, DeLange RJ, Finerman GA: Bone cell differentiation and growth factors. Science 1983;220:680-686.

6.Urist MR, Silverman BF, Buring K, et al. The bone induction principle. Clin Orthop 1967;53:243-283.

7.Kurz LT, Garfin SR, Booth RE. Harvesting autogenous iliac bone grafts: a review of complications and techniques. Spine 1989; 14:1324-133 1.

8.Rosen V, Theis S. The BMP proteins in bone formation and repair. Trends in Genetics 1992;8(3):97-102.

9.Sampath TK, Reddi AH. Homology of bone-inductive proteins from human, monkey, bovine, and rat extracellular matrix. Proc Natl Acad Sci 1983;80:6591-6595.

10.Wittbjer J, Palmer B. Osteogenetic activity in composite grafts of demineralized compact bone and marrow. Clin Orthop 1983; 173:229-238.

11.Wozney JM. The bone morphogenetic protein family and osteogenesis. Molec Reproduct and Devel 992;32:160-167.

12.Urist MR, Mikulski A, Lietze A. Solubilized and insolubilized bone morphogenetic protein. Proc Natl Acad Sci 1979; 76:1823-1828.

13.Hurley, LA and Losee, FL. Anorganic bone — chemistry, anatomy, and biological reactions. J Military Med 1957; 121:101-104.

14.Krauser JT, McGrew RA, Tofe AJ. A spectroscopic study of grafting and augmentation materials. Acad Osseointegration Seventh Annual Meeting, February 27-29, 1992.