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INVITED REVIEW
Year : 2015  |  Volume : 7  |  Issue : 3  |  Page : 113-118

Synergy of hard and soft tissue augmentation around implants


Clarus Dental Specialities, Pune, Maharashtra, India

Date of Web Publication31-Dec-2015

Correspondence Address:
Neel Bhatavadekar
Clarus Dental Specialities, Pune - 411 001, Maharashtra
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/2231-0754.172925

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   Abstract 

With the advent of dental implants, alveolar defects with insufficient bone height and width have emerged as an area of great concern. Since the overlying soft tissue forms the chief component of any esthetic effect, the maintenance and augmentation of the soft tissue emerged an area of concern and focus. This review discusses the synergy of hard and soft tissue augmentation around implants. The reader is introduced to the fundamental concepts of bone grafting,and guided to the sequence of treatment and treatment options that exist for ensuring optimum peri-implant esthetic and functional results.

Keywords: Implant augmentation, Connective tissue grafts, bone grafts


How to cite this article:
Bhatavadekar N. Synergy of hard and soft tissue augmentation around implants. J Int Clin Dent Res Organ 2015;7, Suppl S1:113-8

How to cite this URL:
Bhatavadekar N. Synergy of hard and soft tissue augmentation around implants. J Int Clin Dent Res Organ [serial online] 2015 [cited 2019 Dec 5];7, Suppl S1:113-8. Available from: http://www.jicdro.org/text.asp?2015/7/3/113/172925


   Introduction Top


With the advent of dental implants, alveolar defects with insufficient bone height and width have emerged as an area of great concern. Several factors such as developmental defects/clefts, congenitally missing teeth, trauma, odontogenic cysts and tumors, tooth extractions, dehiscence or fenestration defects, and advanced periodontal disease contribute to alveolar ridge deficiencies.[1],[2] Alveolar defects arising from tooth extraction or loss are considered to be the most common, with the greatest bone loss occurring in the first year after extraction. An estimated 25% volume loss in the first year increasing to about 40% loss in 3 years has been reported.[3],[4],[5],[6],[7] Alveolar ridge resorption undergoes several stages that have been categorized by Atwood [8] (1971) under six categories, ranging from initial to severe ridge resorption.

Since implants are a treatment of choice with well-documented long-term results,[9] the option of restoring a deficient edentulous ridge with dental implants has been investigated by several clinicians over the past two decades, with an evolution of different techniques using bone grafts, membranes, titanium meshes, and growth factors. In this review, we look at the underlying principles of guided bone regeneration (GBR), clinical ways to achieve a predictable result while highlighting some important hard tissue and soft tissue augmentation techniques with proven long-term results.

Tissue engineering and the principles of GBR

The concept of GBR stems from the fundamental concept of tissue engineering where if the appropriate cells, a scaffold, and a relevant signaling molecule are available, tissue regeneration has been demonstrated to occur, given the time and environment factor [Figure 1].
Figure 1: tissue engineering concept as it applies to guided bone regeneration

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Signaling molecules are biological agents that activate genes in the cells and thus, bring about a change in cellular activity. Thus, in one sense, they are biological mediators. These signaling molecules act by initiating a cascade of events, such as chemotaxis and gene expression, that bring the necessary cells to the area and then make them regenerate the required tissue structure. Growth factors, such as platelet-derived growth factor (PDGF), bone morphogenetic protein (BMP), and tumor growth factor (TGF) are examples of signaling molecules that recruit necessary cells to the surgical site so as to bring about regeneration of lost tissue. When a surgical site is created, the body essentially reacts in a similar way as it does to injury, that is, it leads to the formation of a platelet plug and releases growth and differentiation factors that in turn recruit cells to regenerate lost tissue. This cascade is quite complex and out of scope of the review. However, the important factor to remember is that these signaling molecules act as very powerful biological mediators and inductive agents. BMP-2, for instance, leads to osteoinductive action, superseding the conventional osteoconduction obtained by most graft materials. (BMP-2 has been discussed in greater detail toward the end of this review.)

Osteoconduction refers merely to the availability of a scaffold that directs tissue (and bone) growth, whereas osteoinduction refers to a powerful mechanism wherein cells are actively recruited and at times, morphodifferentiated into the desired cell types so as to bring about the formation of a certain type of tissue (that in the case of osseoregeneration would be bone).[10],[11]

GBR is a technique based on the principles of guided tissue regeneration (GTR), in which barrier techniques are used in the hope of regenerating lost tissue.[12] The objective of GBR is thus the formation of new bone to reconstruct a deficient ridge prior to or in conjunction with implant placement, by using the primary principle of differential cell growth. Cell-occlusive barrier membranes have been used for horizontal or vertical ridge augmentation with or without bone graft, with varying degrees of success. The principle of GBR is based on the creation and maintenance of space and the exclusion of soft tissue cells from the adjacent bone to allow the preference for bone-forming cells to populate and regenerate the defects. GBR procedures have been shown to provide a fairly predictable outcome if strict adherence to the proper surgical technique and compliance with its principles are used.[12]

Clinical principles for predictable bone regeneration

When we apply the tissue engineering principles to regenerate bone, we derive a set of clinical principles to guide us. The “PASS” principles for predictable bone regeneration [13] provide some basic principles we need to bear in mind for any surgical site where bone augmentation is desired [Figure 2].
Figure 2: after tooth extraction, the buccal wall dehiscence is observed. using principles of predictable bone regeneration, bone has been regenerated, to enable implant placement 4 months later

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P:Primary wound stability.

A:Angiogenesis.

S:Space maintenance.

S:Stability of wound.

In addition to the above principles, a fourth principle ought to be the presence of tension-free closure and minimal stress on the underlying graft. Existence of stress on the flap or the graft can lead to early flap dehiscence, change in graft shape, and an eventual compromised result. Hence, the PASS principle could be modified to include “T” for tension-free closure (PASST).

Although it is hard to quantify what “flap passivity” would be, many clinicians are of the opinion that tension-free flap closure should mean at least a 2-mm passive overlap of the flaps after perioesteal release.

Another hypothesized principle that provides for predictable bone regeneration is the concept of “rapid acceleratory phenomenon” (RAP).

RAP: Frost first proposed this concept that relates to an observation that the rate of remodeling in an area of bone defect is faster than at a normal site.[14] It is postulated that this RAP is induced when a surgical wound is created. It is hypothesized that decortication with burs prior to graft placement, in addition to opening direct access to cancellous native bone, also elicits RAP response, thus leading to faster healing and better bone regeneration. Hence, for most hard tissue augmentation procedures, bone decortication is performed prior to graft placement.

Bone remodeling and choice of graft material

Bone remodeling is a very complex and crucial component of the hard tissue augmentation process. The bone remodeling would use the triad of cell, scaffold, and signaling molecule to achieve bone regeneration. The primary cell type targeted are the osteoblasts; however, unless the bone graft scaffold that is provided is completely degraded, the space cannot be filled up with vital bone. Another issue that can come up with a bone graft scaffold is the potential effect(s) that the degradation products can have on the formation of new bone. The key is to have the optimum graft resorption time. This, in other words, is when a graft that stays long enough to provide the necessary physical support for osteoblast migration and bone growth but which resorbs and does not interfere with continued bone growth. Depending on the clinical situation, the clinician would choose a graft material with varying resorption times.

Alveolar bone is lost when a tooth is removed, and this process is initiated by the loss of bundle bone at the crestal part of the alveolar process where the periodontal ligament (PDL) fibers from the tooth insert. With loss of the tooth and absence of PDL fibers, this bundle bone starts getting resorbed. The rest of the ridge undergoes remodeling such that the socket heals and the cross-dimensional shape of the alveolar bone changes. In a prospective multicenter study, 596 implants in 192 patients over 5 international test centers were observed over a 5-year period.[15] It was observed that the crestal bone change around the implants was significant from implant placement until the point of prosthesis placement but was not significant from then on up to 5 years. These results suggest that the factors that influence early healing around implants are significantly different from those that affect later marginal bone remodeling.

Another interpretation is that bone loading is important from a bone preservation standpoint. If we extrapolate this logic to hard tissue augmentation prior to implant placement, it seems logical that we cannot wait for an extended period of time after horizontal ridge augmentation with either a block or a particulate graft. This is because the bone resorption starts in the absence of loading from the implant, after osseointegration. There is thus, an optimal window for implant placement after bone augmentation that would be 4-6 months after particulate graft augmentation and 6-8 months after block graft placement, beyond which the ridge dimension could be compromised due to resorption.

Type of bone grafts

The four main types of bone grafts available for hard tissue augmentation can be divided into four groups – autografts, allografts, xenografts, and alloplasts, depending on their source.

Autograft

Autograft has been long considered as the “gold standard” for bone augmentation. Several previous reports have explored the use of autografts for sinus elevations, bone augmentation in edentulous sites, periodontal defect regeneration, and extraction site preservation,[16] with good results. Autografts have an advantage in that they theoretically contain vital autogenous cells that would provide the necessary growth factors, and an inherent scaffold for bone growth. However, the author is not aware of any articles that have been able to quantify the presence of these vital cells. Also, the use of autografts necessitates a second harvesting site (such as the iliac crest, ramus, and symphysis) that can add substantial morbidity. Often, the patient might complain of greater discomfort from the harvest site, compared to the surgical site. For example, it is estimated that about 50% patients suffer from combined major and minor postoperative complications after an iliac crest graft harvest.[17]

Xenograft

This refers to the bone derived from an animal source. Bovine bone is quite commonly used as a xenograft. Most xenografts are available in varying particle sizes and this has a bearing on their resorption rate. Xenografts have shown to have excellent clinical results in ridge augmentation as well as sinus augmentation.[18]

Allograft

These are grafts transferred between members of the same species that are genetically dissimilar.[18] The inherent advantages of an allograft are its availability in higher quantities compared to an autograft and the fact that it eliminates the need for a second donor site and morbidity. Although the risk of disease transmission is virtually nonexistent, concern still exists for some patients.[19] This, in part, led to research in alloplast development.

Alloplast

This is a synthetically processed agent that serves mainly as an osteoconductive agent, with no inductive property. Hydroxyapatite, calcium sulfate, and calcium phosphate are examples of an alloplasts, and have been used for ridge augmentation and sinus augmentation cases.

All these four groups of grafts (auto, allograft, alloplast, and xenograft) have varying resorption times, in part due to material content, and pore and particle size. Xenografts in general appear to have the longest resorption time and this factor needs to be taken into consideration depending on the planned treatment modality. For instance, placing a xenograft beneath a pontic site for a bridge might be a good idea but might not be indicated if the implant placement is planned within 3 months after tooth extraction since the graft will not have enough time to remodel. An allograft might perhaps be a better alternative in that case.

Membranes for hard tissue augmentation

In osseous reconstructive surgery, a membrane is used to prevent ingrowth of soft connective tissue into bone defects. In principle, the membrane is placed in direct contact with the surrounding bone surface, thereby placing the periosteum on the outer surface of the membrane. The mucoperiosteal flap is then repositioned and sutured so as to create a protected space beneath the membrane into which the osteoblasts will proliferate and start laying down the bone.[15] It is important to differentiate between GBR and GTR. GBR refers to regeneration of bone alone while GTR refers to regeneration of multiple tissues such as the periodontal ligament, cementum, and bone.[19] In this chapter, we will focus on GBR since we wish to regenerate the bone around implants and other tissues are not desired.

There are a wide variety of membrane materials available to clinicians today but it would behoove the clinician to understand the relevance of membrane factors such as cell occlusion, biocompatibility, space maintenance capability, and clinical handling before choosing a specific membrane for a specific osseous defect.

The biocompatibility of membranes is characterized by different parameters such as cytotoxicity, genotoxicity, histocompatability, and microbial effects. Membranes made of inert materials (i.e., materials that do not degrade under physiologic conditions) generally present a less complex safety situation compared to degradable materials that can release breakdown products into the surrounding tissues, leading to local adverse reactions.[20] Cell occlusion and tissue integration are two other important criteria for any membrane that will dictate the ability of the membrane to differentially promote a certain cell type. Earlier research on membranes focused on periodontal regeneration,[21] where soft connective tissue was excluded for the purpose of promoting regeneration around teeth. The theory of total cell occlusion has been controversial and some studies have found macroporous membranes to be more effective than nonpermeable membranes [19],[21] in the last few years since it is possible that nutrient transfer across the barrier might be necessary for successful regeneration.

Space maintenance

The ability of a membrane to resist collapse after placement can be defined as its space-maintaining ability. The stiffer the material, usually the better is the space maintenance ability. However, a stiffer material is not necessarily the best membrane material since stiffer materials are usually more difficult to handle clinically, and might not readily adapt to the required defect morphology, thus increasing the chances of a membrane perforation through the flap.

Nonresorbable membranes such as expanded polytetrafluroethylene (ePTFE) have shown good clinical results as barrier membranes but have their own limitations. Complications such as soft tissue dehiscence with membrane exposure and infection have been associated with the use of these membranes, with a subsequent impaired bone fill.[17] To eliminate some of these problems and overcome the need for a secondary surgery to remove the membrane, resorbable membranes were developed.

Resorbable membranes such as collagen membranes have good handling properties and are often hydrophilic in nature. They have shown excellent clinical results and a low risk of postoperative complications such as premature membrane exposure. At the same time, they have the inherent disadvantage of not being able to maintain space on their own and hence, always need an underlying graft material with a sufficiently long resorption time.

When grafting materials are used in combination with a resorbable membrane, the results of GBR have been shown to be comparable to the results achieved with nonresorbable ePTFE membranes.[22] In clinical use, noncross-linked collagen membranes are usually preferred. The author restricts the use of nonresorbable membranes for cases of vertical deficiency or where there is complete lack of bony support for space maintenance.

Treatment sequencing of hard and soft tissue grafting (concept of synergy)

The choice of treatment sequencing (whether soft tissue graft or hard tissue graft should be done first) depends on the following factors:

  1. Keratinized tissue.
  2. Tissue biotype.
  3. Volume deficiency.


If a ridge deficiency is such that the volume augmentation required is massive, it should be evaluated whether it is possible to mobilize and suture the flap adequately with the existing amount of keratinized tissue and the tissue biotype. It might often be necessary to augment the soft tissue earlier and then proceed with bone grafting. In such a situation, the time period that one should wait in between the two procedures is a minimum of 8-10 weeks to achieve good soft tissue stabililty.

Combining particulate grafting and soft tissue grafting

The concept of treatment synergy also extends to the choice of particulate grafting with soft tissue grafting. For tissue defects where both hard and soft tissue deficiencies exist, unless the treatment synergy is considered, the final result would be suboptimal.

Particulate bone grafting — The choice of doing either a block graft versus a particulate graft depends on factors such as patient preference, secondary donor site, and morbidity and anatomy of the ridge (e.g., a buccal concave defect responds quite well to particulate graft since it can be well-contained).

The surgical procedure for the particulate graft placement is similar to block grafting, except for the preparation of the donor site (symphysis/ramus). The graft material can consist of the autogenous bone, an allograft (such as demineralized freeze dried bone), alloplast, or a xenograft. Particulate grafts can often be layered, using different graft materials next to the implant and over the first layer. For instance, some clinicians, including the author, prefer using autogenous grafts next to the implant, given their osteogenic potential, and using allografts or xenografts over it. Since xenografts/allografts typically have a much longer resorption time compared to autografts and they can be obtained in large quantities, they serve as osteoconductive agents and help to maintain space, especially when harvesting a sufficient amount of autogenous bone is not possible. On similar lines, the “sandwich bone augmentation technique”[23],[24] suggests using mineralized human cancellous allograft (inner layer), mineralized human cortical allograft (outer layer), and coverage with barrier membrane to essentially “replicate” the natural structure of cancellous bone within cortical bone and take advantage of the resorption times of these graft materials [Figure 3].
Figure 3: siebert class III deficiency (combination horizontal and vertical ridge deficiency). decortication performed to elicit RAP phenomenon, and angiogenesis. particulate graft comprising a mix of autogenous and allograft placed with a Ti-reinformed gore-tex membrane for space maintenance. gore-tex sutures used for primary flap closure

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In some clinical scenarios, it might be necessary to combine soft and hard tissue augmentation (particulate grafting). Utilizing the same principles enlisted above, an improved aesthetic and functional appearance can be achieved [Figure 4].
Figure 4: baseline presentation shows the discrepancy between the right central incisor pontic and the left natural central incisors. existence of a vertical and horizontal ridge deficiency (siebert class III). horizontal bone augmentation to allow for implant placement. a papilla sparing incision is used. at implant recovery, a connective tissue graft is placed to add to the vertical soft tissue profile. note the improvement in vertical soft tissue height, the excess tissue can now be adequately contoured to create a matching gingival contour

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   Conclusion Top


Our concepts about hard and soft tissue management around implants have greatly changed over the past two decades. We are in an exciting era where we have a better understanding of the anatomical differences between periimplant soft tissue and the periodontium, a better ability to utilize conventional surgical techniques, and the eagerness to embrace promising bioengineering approaches for the benefit of our patients. The synergistic treatment combining hard and soft tissue augmentation promises the best chance of obtaining a desirable result. Although long-term follow-up and more randomized controlled trials are necessary to demonstrate long-term prognosis of some of the recent techniques, researchers have provided us good published data to enable an evidence-based approach to treatment decisions with regard to periimplant soft tissue enhancement.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
   References Top

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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4]



 

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