3

Bone Grafts

Bone has a pronounced healing potential, which is still present even if the bone is temporarily cut off from the blood supply, ie, avascular grafting. This is due to the bone morphogenetic proteins (BMPs) and growth factors stored in bone and the principle of slow remodeling of bone from the inside out. Because the bone structure is constantly being functionally remodeled throughout life, grafts can be remodeled into patients own bone, and bone injuries heal over time without scarring.

3.1 Biologic Effect of Bone Grafts

Autogenous bone grafts bring three factors necessary for tissue regeneration: cells, matrix, and growth factors (Fig 3-1).

Cells

Vital, human, divisible, and osteogenically differentiated bone cells survive various forms of processing in cell culture experiments (Figs 3-2 and 3-3), with laboratory studies showing that the autologous iliac crest graft offered by far the highest yield of vital osteogenic cells, setting the gold standard for others to follow as a reference¹ (Fig 3-4). Among the osteotomy methods, the bone scraper and bone mill yielded higher counts of culturable vital cells than piezoelectric devices and drills.² It is still relatively unclear how long these detected vital cells survive after bone grafting. The surviving cells have relevance to the osteoinductive effect of autogenous bone, as studies in goats have shown that bone grafts containing vital cells showed a faster and 30% higher yield of regenerated bone than the same grafts containing nonvital cells.³

Fig 3-1 Three factors necessary for tissue regeneration.

Fig 3-2 a. Bull bone mill (Mondeal). b. Interior view with scraped bone chips. c. R. Quétin bone mill.

Fig 3-3 a. Reusable bone filter for insertion into the suction hose (Schlumbohm). b. Opened filter.

Growth factors

Even cell-poor compact bone can also be grafted because an important part of the osteoinductive effect of the autogenous bone graft is due to its content of bone growth and differentiation proteins such as BMPs. BMPs are relatively stable proteins that are produced from fresh bone in active form and can be extracted with a yield of about 1 mg per kg bone.⁴ They can be exposed and made bioavailable from the bone by opening the matrix (eg, via a scraper or mill).

Matrix

The third part of the graft effect is based on the osteoconductive effect of the autogenous bone matrix. When osteoprogenitor cells differentiate, they require a solid substrate to attach to in order to become osteoblasts. The cells grow on the matrix as if on a scaffold. But osteoclasts are also activated, which resorb the matrix and release BMP.

Fig 3-4 a. Human bone chips with different origin and processing were placed in cell culture medium. b. The growth of osteogenically differentiated cells was best in native iliac cancellous bone, but even intraoral bone ground with a steel bur with subsequent collection in the bone filter still yielded vital and divisible bone cells. (Adapted from Springer et al.¹)

A bone substitute material that only fulfills the matrix function can acquire osteoinductive properties and be improved in its efficiency by mixing it with autogenous bone chips as a mixed graft (Fig 3-5). Bone scrapers and bone filters are suitable for this purpose. The speed of healing depends on whether all three properties are present. A hydroxyapatite-based bone substitute lacks the cellular component and growth factors, and a pure cortical graft also lacks the cellular component. Accordingly, the healing of these materials is slower and less predictable compared to autogenous pelvic bone blocks. In challenging defects, eg, vertical augmentation for implant placement, vital autogenous grafts with all three properties should be used if possible.

Fig 3-5 Use of autogenous bone chips. a. Fenestration defect on an implant in a maxillary left central incisor site. b. Harvesting of bone with a scraper in the defect area. c. The SafeScraper Twist (Geistlich) is a disposable instrument. d. Opened collection container with bone chips in the form of flakes, which occupy a large volume. e. Bone particles obtained during implant osteotomy in the bone filter. f. Bone substitute material is mixed with sterile autologous blood. g. After all particles are soaked with the sterile blood, the filter bone is added. This will induce blood clotting. h. The titanium surface is covered with the bone chips from the scraper as the innermost layer of the construct. i. Trimming of the collagen membrane (Bio-Gide, Geistlich). The middle section forms a tongue that is clamped under the palatal flap. The right section serves as a double layer. j. The membrane has been fixed under the palatal flap. After coagulation, the bone substitute material mixture is placed in a 4-mm-thick layer; the contour should flow out harmoniously to the sides. The coronal portion is overcorrected. k. The second membrane section is placed transversely as a double layer and is intended to increase the durability of the barrier and cushion the soft tissue flap from sharp particles. The membrane is moistened with saline after placement. l. The flap is held in place with a single hook, and the 15c scalpel is used to slit the periosteum for flap mobilization. m. Suture closure is performed in a coronally overcorrected position using interdental sutures and single interrupted sutures (Supramid 4-0 and 5-0, Resorba).

3.2 Graft Bed

If an augmentation material lacks one or two of the three properties of the autologous graft, the graft bed must compensate for them. Conversely, the poorer the graft bed, the more complete the augmentation material must be. The probability of success decreases if the graft bed is weakened by soft tissue scarring or general disease. In patients with risk factors, the use of vital autogenous material may be a safer option than bone graft substitute. The success of bone grafting depends largely on the quality of the tissue at the recipient site. It is critical whether the site has good bony support (eg, a cyst cavity) that provides bone cells and BMP, or rather poor support limited to soft tissues (eg, a mandibular continuity defect with minimal connection to the residual jaw). A well-perfused graft bed is referred to as a strong graft bed. The more scarring and circulatory disturbances are present, the weaker a graft bed is in terms of replacement. Full irradiation of tumor site leads to a graft bed that is no longer capable of taking the graft.

The choice of material in a bone defect depends critically on the defect wall thickness of the graft site, as shown by the sequence of inlay, interpositional, appositional, and onlay grafts in chapter 2.

3.3 The Gold Standard: The Autogenous Iliac Bone Graft

The autogenous monocortical iliac crest block graft is referred to as the gold standard of bone grafting in the international literature across disciplines^5–9 in terms of its clinical predictability and healing potency compared with materials from other sources. The autogenous iliac bone graft has cells, growth factors, and interconnecting porosity of the matrix in physiologic proportion. Autogenous bone grafting is associated with a high degree of predictability of graft take with minimal complications in the shortest possible time. Note that the term gold standard comes from economics, with gold being a reference point for currency but not necessarily the most valuable material. Likewise, in this case, calling the autogenous iliac bone graft the gold standard means it is a reference point for biologic potency, and does not necessarily indicate that it is the best material of its group. For example, recombinant BMPs have been shown in some studies to have a higher regenerative performance than the iliac bone graft.⁶

3.4 Donor Sites: Quality and Harvest Morbidity of Autogenous Bone Grafts

A variety of autogenous bone grafts from different intraoral donor sites are available for bone regeneration (Fig 3-6). If larger quantities are required, bone can be harvested from the cranium and the anterior and posterior iliac crests (Fig 3-7). The various sites of origin and harvesting techniques differ in their invasiveness and patient burden and in their biologic effectiveness. Because cancellous bone grafts contain vital osteogenic cells, they are suitable for treating critical defects even in poor graft beds. Cortical blocks, on the other hand, better resist the surface resorption of the healing phase than particulate grafts. However, they place higher demands on the graft bed and on the patient’s behavior. Because of the higher risk of dehiscence compared to particulate grafts, soft tissue coverage should be ideal and the patient should not chew food in that area, for example. On the other hand, using bone mills or drills reduces the cellularity in the grafts.¹

Chips from scrapers, mills, and piezoelectric devices

Scraper chips are often the best alternative for restoring the smallest bone defects and as fillers. Scraper chips are easy to obtain, have a high surface area with exposed BMPs, and have a high volume. According to an in vitro study, chip extraction with the piezoelectric device versus the manual scraper showed minimal, nonsignificant differences in terms of cell viability.⁷ Milled chips can be obtained relatively atraumatically with implant systems that use low-speed twist drills (Fig 3-8

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