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Soft tissue expansion: principles and inferred intraoral hydrogel tissue expanders

Henri J.J. Uijlenbroek

Department of Oral Implantology and Prosthetic Dentistry, Academic Centre of Dentistry Amsterdam (ACTA), University of Amsterdam and VU University Amsterdam, The Netherlands

Private Practice, Edisonweg 3, 8501 XG Joure, The Netherlands

E-mail : h.uijlenbroek@tandartsjoure.nl

YuelianLiu

Department of Oral Implantology and Prosthetic Dentistry, Academic Centre of Dentistry Amsterdam (ACTA), University of Amsterdam and VU University Amsterdam, The Netherlands

Daniel Wismeijer

Department of Oral Implantology and Prosthetic Dentistry, Academic Centre of Dentistry Amsterdam (ACTA), University of Amsterdam and VU University Amsterdam, The Netherlands

DOI: 10.15761/DOCR.1000140.

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Abstract

Background

Tension-free closure of theintraoral site is a prerequisite at bone augmentation procedures. Dental surgeons mainly use a technique called soft tissue mobilisation. The soft tissue expander has not yet been used in the daily practice. However, in plastic surgery, soft tissue expansion is often used to create a surplus of soft tissue. The experience gathered in plastic surgery can be extrapolated into intraoral surgery. Especially the new small-sized hydrogel soft tissue expanders can be used intraorallyin implant dentistry.

Purpose

 This paper presents systematically the principles of soft tissue augmentation. Its purpose is to summarize the general knowledge about soft tissue expanding and the characteristics of tissue expanders. Experience gathered in plastic surgery is transformed to intraoral use. It outlines the advances of tissue expansion and its possible role usinghydrogel soft tissue expanders in dental implant therapy.

Materials and methods

 The electronic data bases Science Direct, PubMed, High Wire and Google Scholar were searched for articles published until March 2015. The related terms were used.

Result and discussion

Intraoral tissue expansion seems useful and practicable, although complication rates are still high, which could be caused by poor clinical experience. A decision tool is given to decide whether intraoral soft tissue expansion is a treatment option. Although the studies for intraoral use are minimal, the technique is absolutely valid.

Introduction

Soft tissue expansion is a natural phenomenon seen in pregnancy, obesity, tumour growth and(ancient) tribal rituals.In reconstructive plastic surgery tissue expansion has become a common surgical procedure to grow extra skin through controlled mechanical overstretch. It creates skin that matches the colour, texture, and thickness of the surrounding tissue, while minimizing scars and risk of rejection [1]. The goal of tissue expansion is to develop donor tissue by tissue expansion adjacent to [2,3] or in the centre [4] of the site of reconstructive surgery.

In 1957 Neumann [5] introduced a rubber balloon to expand soft tissue in order to reconstruct an ear. In 1984 Radovan[2] placed an expander adjacent to the defect and doubled the size of the donor flap by intermittent injections of normal saline into the expander. The tissue expanders used had an external port to fill them. In 1982 Austadetal. [6] described a self-inflating tissue expander. He used a permeable silicone balloon filled with a saturated NaClsolution. The disadvantages were the long inflation period of 8 to 14 weeks and the risk of rupture, which could result in necrosis of the overlying tissue. Wieseet al.[7-9]researched the use of hydrogels for tissue expansion to overcome these problems. This led to the Osmed® hydrogel expander. Hydrogels can imbibe water or biological fluids and thus increase in volume. The smaller-sized Osmed® hydrogel expanders can be used intraorally.

Hydrogel based soft tissue expanders are used in plastic surgery with moderate [10,11] to good [12-15] results as well as in ophthalmology [9,16]. The use of intraoral soft tissue expanders was first described in 2007[17]as small hydrogel expanders became available. Animal studies[18,19]as well as human studies [20]have been done.

In this paper we present the history of tissue expansion, the surgery needed in soft tissue expansion procedures and isrelated problems. The intraoral applicationsare discussed with their limitations and an indication is provided for the future application in implant dentistry.

Methods

The electronic data bases Science Direct, PubMed, High Wire and Google Scholar -excluding patents and quotations- were searched for articles published until March 2015. The terms “expander”, “oral expander”, “dental expander”, “dental soft tissue expanders”, “oral soft tissue expanders”, ”intra(-)oral soft tissue expansion”, “Osmed expanders” and “Osmed soft tissue expanders” were used. Every article concerning intro-oral soft tissue expansion with Osmed® hydrogel expanders was included. To understand and illustrate soft tissue expansion in plastic surgery basic articles were consulted.

Tissue expanders

Tissue expansion as a treatment option

Reconstructive surgical procedures are used to restore or improve defective, damaged or missing structures. In case of a lack of tissue of the same texture and colour,donor tissue can be generated by the healthy tissue next to the reconstruction site by expanding this tissue before the reconstructive surgery takes place [21-23]. Because of their plasticity mucosa and skin can easily adapt to environmental changes [24] and are therefore tissues eminently suitable for being expanded. In intraoral implant surgery there is not always sufficient bone present to place an implant, so a bone augmentation procedure is called for. But for predictable bone regeneration primary wound closure is a prerequisite [25].If exposure of the augmented material is expected tooccur, asurplus of soft tissue may be created before augmentation. A (created) surplus of soft tissue will make tension-free closure of the augmented siteeasier, or possible at all. Tissue expansion should therefore be considered in the treatment planning in all those cases where a lack of soft tissue of the appropriate quality and / or quantity is to be expected.

Made of an inert material.

No toxic damage to the surrounding tissues.

Gradual and slow expanding process.

Less discomfort for the patient.

Less likely to perforate because the tissue has time to adapt.

Expanding process as short as possible.

Reduction of treatment time.

The ratio: volume after expansion / volume before expansion of the expander has to be as large as possible.

A maximum of tissue generated.

An option of fixation.

Keeping the expander in place during expansion.

A rounded shape of the expander.

Reduction of the risk of perforation during expansion.

The surface area of the side of the tissue expander adjacent to the underlying tissue should be: as large as possible and adaptable to form and size of the underlying tissue.

The pressure to the underlying structures will be better distributed so local peak pressure is minimized.

The expander - and its filling port, if applicable - should easily be recognisable in the operation field during placement and removal.

Comfort and effectiveness for the surgeon.

Table 1. Characteristics of an ideal tissue expander.

Characteristics of an ideal expander

An expander should have maximum effectiveness without any damage to the tissuesand with maximum comfort for patient and surgeon. The ideal characteristics are listed in Table 1.Any material which damages the biological system or its surrounding tissue will cause undesired reactions. Leunget al.[26] reported breast pathology in complications associated with polyacrylamide hydrogel (PAAG) mammoplasty.Less damage to the tissues is created by gradual and slow expansion. It will give the overlying tissues time to expand. Rapid expansion can lead to perforation of the expander [11]. However, a short expansion time reduces treatment time and discomfort for the patient. Less tissue damage will occur at insertion usingan expander with a small volume. Besides, a small-sized expander is easier to insert. After insertion and placement of the expander the expander volume will be increased to expand the overlying tissues. The more the expander increases in volume, the more tissue it will expand and generate. For this reason the ratio: volume after expansion / volume before expansion of the expander has to be as large as possible. Any expansion of a tissue expander creates a force. This force is exerted on overlying as well as underlying tissue. The tissue with the least resistance will move the most.If the underlying tissue is hard, such as bone, and the tissue to be expanded is stiff and difficult to expand, such as palatal mucosa, the expander may be dislocated by its own forces when it is not fixated. Therefore a possibility of fixation is a prerequisite. Damage to the surrounding tissues of the expander also depends on the shape of the expander. Peak pressure during expansionis caused by an angular part of the expander rather than by a rounded shape.Sothe tissue expander should have rounded shapes.Damage to the underlying tissue depends on the local peak pressure to which it is exposed. A large adaptable surface will distribute the pressure on the underlying tissues more evenly, so that excessive pressure at one point will decrease.Even in the best of circumstances tissue expansion,may damage the underlying tissue, and for instance result in bone resorption[27,28]. The surgical procedure will be unnecessarily complicated when the tissue expander or its filling port is difficult to find. Camilleriet al.[29] reported 7% failure in locating the filling port with the Becker tissue expander in reconstructive surgery of the breast after mastectomy. To overcome the use of ultrasound to find the filling port is described[30].

Advantages of soft tissue expansion

Obtaining tissue to restore defects can be done in several ways. In maxillofacial reconstructive surgery after ablative surgery, vascularized osseous free flaps still represent the gold standard of restoration [31]. However, the donor site loses some quality. To obtain donor tissue, tissue engineering can also be used. The tissues are produced in vitro, which makes it possible to have sufficient tissue. However, incorporating engineered tissue in the host is not always easy, because of the absence of a vascular network [32]. Tissue expansion on the other hand can create tissue of the same colour, texture, hair-bearing characteristics and thickness adjacent to the restoration site[33]. It minimizes scars and risk of rejection. Tissue expansion is a less radical approach compared to vascularized osseous free flaps and at the moment less complicated thantissue engineering. Once enough donor soft tissuehas been gained adjacent to the restoration site, it can easily be harvested locally maintaining blood supply. Angiogenesis is a prerequisite for good wound healing [25]. A fortunate consequence of tissue expansion is a significant increase in vascularity [27,34].

Types of expanders

Several types of expander exist, varying in material, size, filling substitute, filling port and filling valve. Shapes may vary: round, rectangular, tubular or crescent-shaped. The expanded sizes vary from about 0.35 ml (cupola Dental Osmed® GmbH) to 1300 ml (Integra® breast expanders).

Most expanders consist mainly of a silicone balloon with an injection port. The silicone balloon is gradually filled with saline so that the expander increases in volume. In 1976 Radovan [2] designed the first silicone expander inflated by means of a sealed remote-injection port placed under the skin. The technique has been widely utilised. The expander is inflated regularly over a period of several weeks. Depending on the expander type and sizepatients have to visit their surgeon sometimes several times a week.This may be inconvenient to the patient and surgeon. Home inflation of a tissue expander has been developed[35] but is not widely spread. As the filling port of the silicone balloon expander can be localised on the expander, the latter may be damaged by the needle when it is being filled, because it is sometimes difficult to palpate the port. Percentages of inability to find the filling port vary between 2.8% [36] and 4% [37]. The filling port may also be connected to the expander by a tube, so that it can be placed at a distance from the expander and the expanding tissue, which reduces the risk of damaging the expander at filling. Very rarely however, the tube can be impenetrable to the saline, when it is twisted, for instance. Most tissue expanders have a subcutaneous self-sealing injection port placed through a separate incision in a pocket dissected at the time of expander placement. It can be fixed with sutures to prevent movement and to make it easier to find. Sometimes this type of filling port is hard to find.

The latest generation of expanders consists of hydrogels. In contrast to the balloon expander they are self-inflating and therefore have no filling port. Hydrogels are cross-linked polymers that have hydrophilic groups and can therefore absorb fluids. The material’s properties range from hard and tough to soft and weak, depending on the amount of inhibited fluids. Biomedical and pharmaceutical hydrogel applications include a wide range of systems and processes that utilize several molecular design characteristics [38]. In a review Drury etal.[39] classified hydrogel applications for tissue engineering in three categories: space filling, bioactive molecule delivery and cell/tissue delivery.Space filling hydrogels used as tissue expanders were developed in 1993 [7]. The Osmed® hydrogel tissue expander (Figure1) is based on an osmotically active hydrogel. The self-filling device is made of a cross-linked hydrogel consisting of co-polymers based on methyl-methacrylate (MMA) and N-vinylpyrrolidone. Preoperatively Osmed® hydrogel tissue expanders in their pre-expanded state are hard and easy to handle. After implantation Osmed® hydrogel implants start to absorb body fluid and grow consistently to a predefined form and size. The volume of the expander - dependent on the product type - increases between 3 to 12 timesand thus leads to an increase of soft tissue. Some of the Osmed®expanders are enveloped in a perforated silicone shell. Through the holes in the shell the body fluids are absorbed, thus inducing volume increase. A non-enveloped hydrogel expander will have faster expansion compared to a hydrogel tissue expander in a silicone envelope and is therefore more likely to lead to mucosa necrosis [40]. The perforated shell reduces the swelling speed and almost linear growth is achieved. The expansion of enveloped Osmed® hydrogel expanders is regular and predetermined by their osmotic properties and the permeability of their silicone envelopes, which both are predetermined by the manufacturer. The regular visits to the surgeon for inflation are no longer necessary.

Both the silicone expanders and the hydrogel expanders have their advantages and disadvantages, which are summarized in Table 2.

Silicone  Balloon  Expander

Hydrogel  Expander

Soft, easy to adapt to the underlying tissues.

Hard, not adaptable to the underlying tissues.

Soft, not always easy to get unfolded in its pocket.

Hard, easy to push into its pocket.

Possibility of delayed expansion after placement of the expander.

Immediate expansion, starts as soon as body fluids touch the hydrogel.

Filling port.

No filling port.

Regular filling moments needed for expansion.

No filling moments.

Expansion speed can be handled at the filling moments.

Expansion speed cannot be influenced by the surgeon.

Expansion peaks.

More gradual expansion provided it is enveloped.

End volume can be handled by the filling.

End volume pre-defined.

Leakage of the expander can damage the surroundings tissues.

Leakage of the expander impossible.

Table 2. Comparison of silicone balloon expander and hydrogel expander.

Figure 1. Osmed ® Tissue Expander - Cylinder 0.7 ml: sizes before and after 1 h and 43 days swelling.

Disadvantages of soft tissue expanders

Just as in any treatment the use of tissue expanders has also a flip side. The treatment time for patients and clinicians is increased as the expander has to be placed in an additional procedure. After expansion and maturation of the expanded tissue the expander has to be removed. This can be carried out at the same time as the reconstructive surgical procedure, so explanationwill be no extra burden. If an expander is used which needs to be filled on a regular basis, the patient has to visit the surgery several times. It is not very time-consuming, but can be an extra load to the patient and surgeon. At filling,fluid is injected into the expander and the increased tension on the skin can cause some discomfort. Discomfort and pain both depend on the location of the expander and on the adaptability and compliance of the patient. The expander has to be in place for a certain time, which can be from several weeks to several months, depending on the expander used and the location of the expander. The skin over the expander frequently appears bluish or pink. This discoloration is caused by the revascularization process and fades after final reconstruction.Tissue expanders involve extra costs, which makes the procedure more expensive. On the other hand, using a soft tissue graft or engineered soft tissue also involves costs. As stated before, tissue expansion mayaffect its underlying structure. However, there is no concord in the literature on the behaviour of the tissues beneath the tissue expander after the expansion has been completed. Some researchers [18,27,28]reported bone resorption beneath the tissue expander, but others did not [41],and others again claimed bone gain[42].

Complications related to soft tissue expanders

Exposure of the tissue expander is a common clinical complication, reported in about 11.1%of the cases [36]. Fast expansion will not allow the overlying tissue to stretch, but maycause penetration by the expander[11,43]. Expander exposure has been attributed to several factors: too rapid expansion, dehiscence of the incision, a fold in the expander that eroded through the skin, inadequate tissue coverage or manipulation of the expander by a noncompliant patient [43]. Infection of the expander pocket, the expander or the filling port is possible [36,37,43,44]. Therefore aseptic techniques are to be followed during surgery and during inflation in the office. Hematoma should not be underestimated because it can lead to infection[37]. It has been reported that the filling port cannot be found when a tissue expander with an external filling port is used[29]. The disadvantage of the hydrogel expander if used without silicone envelope is its uncontrolled rapid swelling, which maylead to perforation[11]. To overcome this problem Anwanderet al.[40] introduced a silicone envelope with some holes in it in which the hydrogel is sealed. The expanding process of a hydrogel expander stays uncontrolled. The hydrogel expander cannot be deflated. Varying with the area in which hydrogel expanders were extra-orally, 0-25% complications have been reported [45].

Clinical applications and surgical procedure

Since the early 1980s tissue expansion has been used in a wide field of surgery: burns,head and neck surgery, the reconstructive treatment of face and scalp defects, breast surgery, congenital nevi, congenital anophthalmos, trauma and intraoral bone augmentation. [13,16,33,36,43,46,47].In well selected casesin which there is a lack of donor tissue the technique of tissue expansion can provide donor tissue with minimal donor area morbidity. Depending on the location of the tissue expansion the patient should be willing to undergo several surgeries and accept some discomfort for several weeks with sometimes extreme deformity, which however, seldom results in an unusual appearance. Smoking [29,48] and previous radiotherapy [29] are relative contraindicat