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4 Flap Design and Soft Tissue Considerations

4 Flap Design and Soft Tissue Considerations

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84



J. Pi-Anfruns and B. Le



Fig. 4.2  Flap design for

GBR. Extension of the flap

to one adjacent tooth. The

placement of vertical

releasing incisions at the

distobuccal line angles and

wide-based trapezoiddesigned flap are crucial to

successful GBR



vertical releasing incisions should be placed away from the aesthetic zone whenever

possible and at the distobuccal line angles of the adjacent teeth. In aesthetic areas,

papilla-sparring incisions should be used to prevent papilla loss around the adjacent

teeth. The flap should be trapezoid-shaped with a wide base to ensure adequate

blood supply (Fig. 4.2).



4.5



Preparation of the Recipient Site



Preparation of the recipient site is another key factor in successful GBR procedures.

During guided bone regeneration, vascular channels (several small perforations of

the cortical bone at the recipient site) are recommended. The biological rationale for

this is to access the bone marrow space, increasing blood supply and facilitating the

migration of cells and nutrients into the defect site [6–8]. In an animal study utilizing a GBR model, Lee and co-workers showed that perforations of the recipient bed

improved neo-angiogenesis in bone grafts and increased osteoneogenesis, especially in the early healing phase. Moreover, the vascular channels will in turn

increase the number of osteoclastic tunnels or cutting cones [9, 10]. These cutting

cones are essential in bone remodeling because they are immediately followed by

capillaries and osteoblasts, which lay down the osteoid to fill the cutting cone.

These vascular channels can be rounded perforations of 1–2 mm in diameter or

longitudinal channels and can be created using a wide range of instruments. Bone

scrapers, fissure burs, and ultrasonic cutting devices have all been utilized for this

purpose. Regardless of the instrument used or shape of the perforation, the depth of

the channels must penetrate into the marrow space, allowing for blood flow into the

grafted site (Fig. 4.3).

When creating the vascular channels, care should be taken not to damage the

adjacent teeth, nerves, vessels, or flap itself during preparation.



4.6



Graft Materials and Selection



An ideal bone graft material possesses the three characteristics of bone formation:

osteogenesis, osteoinduction, and osteoconduction. Osteogenesis refers to the



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85



Fig. 4.3  Vascular channels

created with a #1 round bur

to provide additional blood

supply to the grafted site.

Note the active bleeding

from within the medullary

space. With simultaneous

implant placement,

vascular channels should

be created prior to implant

placement to avoid damage

to the implant surface



natural process of bone formation. Osteoinduction is the process by which osteogenesis is induced, and it implies the recruitment of immature cells and the stimulation of these cells to develop into preosteoblasts. Osteoconduction refers to the

provision of a scaffold which bone can grow into.

As with normal bone repair, bone grafts heal in three major phases: inflammation, proliferation, and remodeling [11–15]. The inflammatory phase begins immediately following the injury and will continue for about 5  days. Inflammation is

caused by blood vessel injury and triggers hemostasis. During this phase, the action

of the platelets and vasoconstriction will allow a blood clot to form, initiating the

healing process. At this stage, neutrophils and macrophages debride the wound site,

reducing the bacterial load. The proliferation phase of graft healing will last about

3 weeks and result in the production of a disorganized osteoid or new bone woven

by the bone graft cells. In the remodeling phase that follows, the immature bone is

resorbed and replaced by organized lamellar bone. The entire process may take

months, depending on the patient and the characteristics of the bone graft material

used for regeneration.

Bone grafts can be classified according to the source from which they were

obtained. Autogenous grafts are composed of bone harvested from the patient being

treated, allografts are taken from another individual of the same species (cadaver),

xenografts are from a different species than the recipient (animal), and alloplastic

grafts are composed of synthetic materials (Table 4.1).

Bone graft substitutes vary in origin, composition, structure, particle size, and

resorption time. A single bone graft material can be selected, or a combination of materials can be used to maximize specific properties of each material [16, 17]. Ideally, the

resorption rate of any bone graft substitute should be similar to that of new bone formation. Resorption rates will be influenced by certain properties of the material such as

crystallinity, particle size, surface area, and porosity but also by local factors like pH,

presence of cells, water content, and the overall healing capacity of the patient [18].

Autogenous bone has a higher and more unpredictable resorption rate than other materials and is not recommended for use alone in GBR procedures [19]. Xenografts, on the



86



J. Pi-Anfruns and B. Le



Table 4.1  Resorption rates of various particulate graft materials from fastest (green) to slowest (red)



AUTOGENOUS

same individual



ALLOGRAFT

different individual



ALLOPLAST

synthetic materials



XENOGRAFTS

different species



No one material is ideal for all clinical scenarios, and the material should be selected based on the

desired resorption rate



other hand, have shown to retain their overall volume stability for as long as 11 years

[19, 20], though this may not be desirable in all clinical situations.

The presence of pores in bone graft substitutes is essential for new bone formation because they allow migration and proliferation of cells, thus promoting angiogenesis. In addition, a porous surface improves mechanical interlocking between

the bone graft substitute and the surrounding natural bone, providing greater

mechanical stability at this critical interface [21]. The micro- and macroporosity of

the graft particle will influence the bioresorbability of the material. The recommended pore size range for a bone substitute is 100–400 μm [22]. It has been shown

that a higher porosity leads to faster osteogenesis, likely due to a larger surface area

[23]. The structure of the pores is another key factor that determines the effectiveness of the biomaterial. The pores may either be part of a network (interconnected)

or contained. Interconnected pores have been shown to be advantageous because a

continuous porous structure facilitates the ingrowth of new bone [24].

The selection of a bone graft material depends on several factors. The rate of

resorption of the graft material and the shape and size of the defect are important

factors to consider when deciding what type of graft material is best suited. As mentioned, autogenous bone alone should not be used for GBR because of its very fast

resorption rate. It does, however, provide viable nutrients when used in combination

with other graft materials. Xenografts, on the other hand, have no viable cells but

will provide the longest volumetric stability over time. Therefore, these materials

can be combined to provide the best environment for new bone formation based on

their combined ability to provide and recruit the necessary cells and to protect the

augmentation volume over time. In regard to the shape and size of the defect, a

circumferential or four-wall defect is more favorable than a two- or one-wall defect,

and a single tooth defect is more favorable than several adjacent missing teeth.



4.7



Barrier Membranes



The biologic principle of GBR relies on the use of a mechanical barrier to protect

the blood clot, isolate the bony defect, prevent penetration of connective tissue,

and allow bone precursor cells to populate the defect. Barrier membranes can be



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87



Table 4.2  Advantages and disadvantages of resorbable and non-resorbable membranes for use in

GBR

RESORBABLE MEMBRANES

Advantages

• eliminates need for removal surgery

• lower cost

• lower complication rates



Disadvantages

• unpredictable degree of resorption

• unfavorable mechanical properties

• not indicated for large grafts



NON-RESORBABLE MEMBRANES

Advantages

• space maintaining

• occlusive yet allow cell migration



Disadvantages

• require removal

• higher cost

• higher complication rates



Note that individual patient needs and clinician preferences will strongly influence material

selection



resorbable or non-resorbable, and their selection should vary depending on the

nature of the defect to be regenerated. For instance, in cases were a localized horizontal deficiency requires a minor GBR procedure, a resorbable membrane can be

utilized. If the defect is vertical or a combined vertical-horizontal defect, then a

non-resorbable reinforced membrane is recommended to provide additional support and prevent collapse. Barrier membranes should be trimmed and rounded to

remove any sharp edges in order to prevent perforation of the soft tissues. This is

especially important with non-resorbable membranes and titanium mesh

(Table 4.2).



4.7.1 Resorbable Membranes

Resorbable membranes currently used in GBR are made of natural or synthetic

polymers. The most common used are collagen and aliphatic polyesters (polyglycolide or polylactide). Collagen membranes are most commonly manufactured

from type I collagen of bovine or equine origin. These membranes can be treated

chemically or by radiation to strengthen the bonds of the collagen fibers, extending their resorption times. This process is known as cross-linking. Some collagen

membranes can be side-specific, where a specific side is intended to face the bone

or soft tissue. These are usually indicated by the manufacturer, and one should

ensure the proper application of these membranes. Resorbable synthetic membranes, as those made of polylactide or polyglycolide, have extended resorption

times and resorb via hydrolytic degradation. Their handling and clinical manageability are more complex, making them less user-friendly and therefore less popular than the above.



4.7.2 Non-resorbable Membranes

Currently available non-resorbable membranes consist of titanium mesh and

polytetrafluoroethylene (PTFE). First introduced in 1969 by Boyne et al. [25], titanium mesh has excellent mechanical properties for the stabilization of bone grafts,

but its rigidity, stiffness, and sharp edges lead to significantly higher incidences of

exposure. Similarly, its higher macroporosity makes it more challenging to remove,

as the soft tissue integrates with the pores and becomes embedded to the mesh.

High-density PTFE (d-PTFE) membranes have been well documented since

their introduction in 1993 [26, 27]. With a submicron pore size of 0.2 μm, bacterial



88



J. Pi-Anfruns and B. Le



Fig. 4.4 Non-resorbable,

titanium-reinforced

d-PTFE membrane secured

with two fixation pins for

horizontal and vertical

GBR in the anterior

maxilla prior to implant

placement



infiltration into the bone augmentation site is eliminated, protecting the underlying

graft material. Another advantage of this type of membrane is that soft tissue does

not attach to the surface, making it easy to remove by pulling on the membrane.



4.7.3 Fixation

Fixation of the membrane will be required in certain clinical situations where additional support is needed to stabilize the graft. In small, self-contained defects where a

resorbable membrane is utilized, stabilization may not be required (one- or two-teeth

defect). In contrast, larger defects where a non-resorbable membrane is need for support, fixation of the membrane is required in order to prevent it from moving during

healing. Stability of the graft material is crucial to successful GBR. Fixation can be

achieved with tacks, pins, or screws. Tacks and pins have similar designs and therefore

behave similarly. Some incorporate a retentive element that can provide additional

retention, ideal in cases where a non-resorbable membrane is required. Fixation

screws, on the other hand, are generally reserved for titanium mesh and can be harder

to remove as they may become partially buried by newly formed bone (Fig. 4.4).

Membranes utilized in GBR, regardless of their composition, must fulfill the following five features in order to achieve successful bone regeneration [28]:



4.8



Biocompatibility



Barrier membranes should not be harmful to the surrounding living tissues or the

patient.



4.9



Create and Maintain a Space for Bony Ingrowth



Maintenance of a suitable space for regeneration is a key factor to successful

GBR. Membrane thickness, composition, and stiffness will determine the support



4  Guided Bone Regeneration (GBR) for Implants in the Aesthetic Zone



89



provided to the site. If the membrane selected is not able to maintain the desired

space, it may affect the outcome of the grafting procedure. Flexible membranes

such as those made of resorbable collagen may need the support of fixation pins or

tenting screws to provide adequate space for successful regeneration.



4.10 Occlusivity

The occlusivity of the membrane is determined by its porosity and the ability to

avoid fibrous tissue formation within the site. On the other hand, membrane

pores facilitate the diffusion of critical fluids, oxygen, and nutrients from the

periosteum, which are vital for bone formation. Large pores will allow the penetration of faster-moving cells (epithelial cells or gingival fibroblasts) and provide an easier pathway for bacterial contamination. If the pores are too small, on

the other hand, it may inhibit nutrient exchange to the site. Currently available

membranes for GBR contain pores that allow nutrient exchange while excluding

soft tissue cells.



4.11 Tissue Integration

Tissue integration is key in stabilizing the healing wound process. The barrier membrane must be able to adapt to the borders of the neighboring bone. A membrane

that is too stiff is not able to adapt to the contours of the recipient site. Poorly

adapted membranes cannot fully prevent migration of soft tissues into the graft site

and are more likely to perforate through the flaps during healing.



4.12 Clinical Manageability

Barrier membranes need to be manageable during the surgical procedure. If a membrane is too stiff, it may not be easily contoured, and its sharp edges are more likely

to lead to perforation of the soft tissue. On the other hand, softer membranes are

more difficult to handle once hydrated.



4.13 Suturing Principles for GBR

Adequate approximation and closure of the wound edges is fundamental to successful GBR procedures and requires the use of sutures. Sutures should be biocompatible, retain adequate strength during the critical period of healing, and induce

minimal tissue reactions [29]. Sutures are made from resorbable or non-resorbable

materials and may be fabricated by either mono- or multifilament fibers [30]. Studies

have shown that multifilament, braided sutures (i.e., silk) are more likely to be contaminated by bacteria than monofilament sutures [31, 32], which induces increased

inflammation.



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