If it looks like bone, if it feels like bone, if it smells like bone…. is it bone?

Dr. Darko Veljanovski, oral surgeon

INTRODUCTION
The anatomical conditions of the implant recipient site are very often less than ideal.
To achieve our primary biologic goal – osseointegration and to have long term success, we need to place our implants in vital, vascularized bone of adequate volume and quality.
As a solution to this clinical issue , the guided bone regeneration (GBR) was proposed . Its roots lie in the concept of guided tissue regeneration in periodontology set by Nyman in the early eighties.

DEFINITION
How is “regeneration “ defined?
“The action or process of regenerating or being regenerated, in particular the formation of new tissue after injury or as a natural process”.
How is, in particular, “guided bone regeneration” defined?
“Guided bone regeneration or GBR, and guided tissue regeneration or GTR are dental surgical procedures that use barrier membranes to direct the growth of new bone and gingival tissue at sites with insufficient volumes or dimensions of bone or gingiva for proper function, esthetics or prosthetic restoration.”

RECIPE FOR SUCCESS
Can humans regenerate?
The simple answer is yes. But the answers are not always simple. So the correct answer to this question would be: yes, under certain conditions.
Another, as for me, important question is: what is the final result of bone regeneration in dentistry, meaning: do we always get real bone? This is where the title of this article comes from.
Daniel Buser, one of the pioneers of GBR, divides this procedure into: functional regeneration and reparative regeneration. In the first one, the bone is completely histologically restored, while in the second one, reparative tissue cells are also found.
Which type and in which case, is enough to meet our clinical expectations?
To answer this question, we need to understand the bone biology.
The proven recipe for successful guided bone regeneration( GBR) is the PASS principle:
1.Primary closure ( meaning: when we open surgical flaps, we should always close them without tension)
2.Angiogenesis ( meaning: to make way for blood vessels to deliver blood supply. There are three sources of blood supply in the alveolar bone: periosteum, periodontal ligament and medullary blood vessels)
3.Space (meaning: to provide space for the bone to grow)
4.Stability ( meaning: no movement, no migration of bone graft particles).

The ingredients we need for this recipe are:
1. Bone graft
2. Membrane

Although for the most of us these might be “ old news” , I would like to revisit the basics of GBR again.

ANATOMY
The bone of the upper and lower jaw consists of basal bone and alveolar bone. The alveolar bone loss may be a result of disease, trauma or extensive post-extraction bone modeling. This poses a therapeutic problems in reconstructive and/or implant dentistry with the lack of sufficient bone volume. The bone is a specialized connective tissue that is mainly characterized by its mineralized organic matrix. The bone organic matrix is comprised of collagenous and non-collagenous proteins. Within this matrix, ions of calcium and phosphate are laid down in the ultimate form of hydroxyapatite. This
composition allows the bone tissue to: 1.resist load, 2. protect highly sensitive organs (e.g. the central nervous system) from external forces, and 3. participate as a reservoir of minerals that contribute to systemic homeostasis of the body.
Bone is important in implant dentistry since it form the recipient site for the implant. Having in mind that our goal is the prosthetic rehabilitation of the patient, the diagnosis and planning should always be made from a prosthetic (restorative) point of view.
So If we need bone, we should regenerate it, because we should always remember the rule of 5P: Proper Planning Prevents Poor Performance.

BONE LOSS
What is the main problem after tooth extraction? During the first three months we lose between 1-3 mm of height and 3-5 mm of width of bone under normal circumstances.

These factors have large impact on post extraction bone modelling:
1. atraumatic (minimally invasive) extraction
2. alveolar ridge preservation
3. timing of implant placement
4. wound healing understanding
5. understanding the properties of bone graft materials
6. adherence to standardized surgical protocols

Structurally, the alveolar bone is divided into: compact bone, trabecular bone, lamellar bone, woven bone, bundle bone. The loss of the bundle bone, which embriologically belongs to the periodontium is responsible for post extraction bone loss.

MECHANISMS OF BONE REGENERATION
The bone consists of competent cells (osteoblasts, osteoclasts), matrix and proteins.
The cells are responsible for osteogenesis, which means formation of new bone “de novo”, with transplantation of the living cells within the graft. The osteoconduction is the property of the graft to induce differentiation of the progenitor cells into bone forming cells. Osteoconduction is the ability of the graft to act a scaffold for new bone formation.

DYNAMIC NATURE OF BONE
The bone cells: osteoprogenitor, osteoblasts, osteoclasts are all within a cell signal matrix. When it is activated we have fully regenerated bone within 4-12 months depending on different factors.
The bone is a living tissue with a dynamic nature:

Two phenomenons occur in it:
1. Bone remodelling, which is an internal turnover of bone with more-less balanced osteoclastic resorption and osteoblastic formation. This phenomenon is responsible for development and long term maintenance of healthy bone at the bone implant contact.
2. Bone modeling, which is a response to stimulus or physical force, in which the bone may change in its size and shape. This phenomenon is responsible for the post extraction bone loss. So the dynamic mechanical loading within physiological limits results in bone remodelling, while the mechanical overload results in bone modelling and bone loss.

BONE GRAFTS
The role of the bone graft is to:
1. Support the membrane and prevent its collapse
2. Be a matrix for bone formation through the mechanisms of osteogenesis, osteoinduction and/or osteoconduction.
3. Prevent soft tissue ingrowth
4. Prevent migration and resorption of the graft itself which is why they are sometimes used as composite grafts, e.g. xenograft with BondBone (Fig. 1)

The bone graft material can be classified by their source: autogenous or autograft, allograft, xenograft and alloplast.
They can be also divided by their mechanism of action into : osteogenic, osteoinductive and osteoconductive.
Only the autograft has the three properties whereas the other ones have at least one of them.
The mostly used donor sites for autograft are intraoral ( mandibular ramus, symphysis, maxillary tuberosity and palatal block bone graft – Gluckman). Autograft particles can also be collected upon implant site preparation ( Anitua, “biological
drilling”).

MEMBRANES
The role of the membranes in GBR is to:
1. prevent the migration of the epithelial cells into the defect
2. stabilize the graft material
3. secure the space for bone formation.

The membranes are divided into:
1.Non resorbable membranes : (e.g. e-ptfe.) These membranes need to be secured with pins, need to be covered with soft tissue and have good capability of contour formation. But, they have frequent dehiscences, they are expensive and demand second surgical procedure for removal. To overcome these procedures, resorbable membranes are widely adopted option.
2. Resorbable membranes. They can be made of polymers, collagen or liquid bio resorbables. The collagen membranes are most frequently used. They are made of collagen type I or combination of type I and III, and are mostly made of porcine or bovine
pericardium.

The advantages of the collagen membranes are:
1. Hemostatic capabilities
2. Chemotaxic for the periodontal and gingival fibroblasts
3. Low immunogenic characteristics
4. Easy manipulation
5. Direct influence on the bone formation

The disadvantages of the collagen membranes are:
1. Easily degraded by the leukocytes
2. Poor mechanical characteristics and lack of contour forming capabilities
3. Possible soft tissue dehiscences, especially in cross linked membranes.

The resorbable membranes are divided into cross linked and non cross linked membranes:
Cross linked membranes are good for larger bone graft volumes and are resorbed within a longer period of time. However, they tend to have lower tissue integration and are prone to tissue dehiscences.
Non cross linked membranes have better vascularization from the periosteum and surrounding tissues, are more easily resorbed and integrated within the tissues, which makes them a viable option for most GBR cases.

BONE DEFECTS
There are many classifications od alveolar ridge defects mostly based on their anatomy. GBR can be used for repair of horizontal, vertical and/or combined defects of the alveolar ridge. Its final result is an interplay of three factors:
1. Anatomy and type of the bony defect -horizontal defect related GBR is more predictable than vertical and combined defects. GBR in maxilla is more predictable than in mandibula, which also refers to the frontal vis – a vis posterior regions.
2. Patient related factors (general health status, oral hygiene habits, cooperation)
3. Clinician related factors (choice of graft and membrane types, surgical technique, experience.)

BONE REGENERATION – CLINICAL USE
The clinical research and experience has brought several modifications of the principles of guided bone regeneration.
New bone is formed in a post extraction gap ( socket) without any bone graft as a result of natural wound healing. As a proof of principle, Dennis Tarnow found new histologically determined bone on the surface of immediately placed implants even in
large non grafted facial gaps.
New bone is also formed in post extraction grafted sockets without use of membrane. Some authors (Zuhr, Hürzeler) find this post extraction bone graft socket filling problematic because of encapsulation of coronal graft particles with connective
tissue.
Therefore they advocate the socket filling with graft “ socket preservation” (probably “ridge preservation“ is more adequate term ) mostly for pontic sites of fixed partial dentures. (Fig.2)

The dual zone concept ( Chu, Tarnow) uses demineralized bovine bone matrix for grafting both the gap and the soft tissues when placing immediate implants. They have achieved terrific results using bone graft and immediate provisionals.
Some clinicians use xenografts without membrane for buccal soft tissue contouring around implants since xenografts are close to non resorbable materials.
(Pikos, “ veneer grafting”.)

Misch explains the exclusion of soft tissue cells from the graft particles by the “barrier by bulk” concept.
Another concept – “Sticky bone” (D.S.Sohn) bypasses the need for membrane when mixing bone particles with platelet rich blood-derived products.
The main question, however, remains the same. What type of bone do we achieve to get? The only way to determine this (apart from postoperative CBCT scan) is to make a histological analysis. Again, in some clinical scenarios it really this is not
that important. For example, when we do horizontal bone regeneration at the time of implant placement. If we get more than 3 mm of total labial thickness it is close to irrelevant if it is bone, unresorbed graft and/or soft tissue.
But, when we do staged surgery, we need a living bone to place our implants in.

What happens to the graft in situ?
The natural bone turnover should eventually have it replaced by native bone tissue. But this is quite nature – of – graft- dependent.
Autografts incorporate all three mechanism of bone regeneration but become resorbed quickly.
Xenografts are clinically treated as close to non resorbables. They are a great option for ridge preservation since they have the potential of minimizing the effect of post extraction bone collapse.
Allograft and alloplasts have properties of both.
This is why it is a common practice to use composite grafts as mixtures of two or more grafts or mixtures of grafts and blood-derived products.
The future research should reveal which is the most appropriate graft for each clinical scenario.

CONCLUSION
There is a very popular phrase among us dentists lately that goes ”It works in my hands”. It is not a crime to have different opinions to an issue since to every issue there is always more than one solution. It is also good to have a personal preference. But we must not end up with the “ Whatever Works” phrase ( by the way, Woody Allen’s great movie from 2009).
We have to be and stay evidence – based. We have to respect science and biology . Because behind every single thing in medicine, there is a biological rationale.
This is the only way to work for the best of our patients.
Or as Dennis Tarnow finished one of his latest lectures, with the take – home message “Think Biologically”.

2018-10-26T00:03:30+00:00