The purposes of soft tissue management around dental implants are successful primary closure, papillary reconstruction, gain of keratinized tissue and preservation of ridge contour. Keratinized tissue is a specialized mucosa covered with keratin or parakeratin which includes the free and attached gingiva and extends from the gingival margin to the mucogingival junction. However, The need and significance of keratinized tissue around dental implants is a controversial issue. Wennstrom et al. [1] reported that there is no clinical difference between teeth with and without adequate keratinized tissue and no association between the width of keratinized tissue and the presence of bleeding on probing. Bengazi et al. [2] reported that the width of keratinized tissue was a poor predictor for occurrence of soft tissue recession. The recession was primarily the result of a remodeling of the soft tissue for establishing appropriate biological dimensions. Albrektsson et al. [3] reported that dental implants may have a high survival rate, irrespective of keratinized conditions. Although the significance of keratinized tissue is still controversial, it is certain that the attached gingiva provides increased resistance of the periodontium to external injury, contributes to the stabilization of the gingival margin position, and aids in the dissipation of physiological forces that are exerted by the muscular fibers of the alveolar mucosa on the gingival tissues [4]. Despite the observation that the lack of keratinized tissue does not influence long term implant survival, the preservation and the reconstruction of keratinized tissue around dental implants could be important. Many surgical techniques have been utilized to augment gingival tissue dimensions. Langer and Sullivan [5] suggested techniques to obtain adequate amounts of keratinized tissue around two stage implants, primarily based on the preservation of keratinized tissue over the edentulous ridge. At the time of implant exposure, apically positioned or laterally positioned flaps were proposed to reconstruct keratinized tissue of adequate width around implants. When the amount of keratinized tissue over the edentulous ridge was minimal, a free gingival graft (FGG) was suggested. These techniques however are associated with significant patient morbidity due to the need for creating a wound at the palatal donor site. Recently, many of the disadvantages of the classic procedure have been overcome by modification and the use of tissue engineering materials. Wei et al. [6] used an acellular dermal matrix allograft to achieve increased attached gingiva. Simion [7] used a collagen membrane for soft tissue healing. More recently, a collagen matrix has been proposed as a substitute for the palatal donor tissue to augment gingival tissue dimensions [8]. Here we performed APF only, APF combined with FGG, and APF combined with collagen matrix coverage at the second implant surgery and evaluated the width of keratinized tissue around dental implant.

The controversy regarding the need for an 'adequate' width of keratinized tissue around teeth in order to preserve periodontal health still exists. There are clinical situations where the presence of a certain width of keratinized tissue may be important in maintaining periodontal health and preventing soft tissue recession, such as in the areas around fixed prosthetic restorations [10]. In spite of the observation that the lack of keratinized tissue may not influence implant survival, the careful management of soft tissue around implants is considered essential by clinicians [11-15]. The increase of keratinized tissue has been achieved traditionally using the FGG [16]. Augmentation of keratinized tissue width and vestibular deepening with autogenous FGGs has been reported to be a predictable and effective method [17-19].

Although the incidence of complications is very low, discomfort and pain at the donor site are frequently observed. This technique causes other wounds at the palatal site, and increases the morbidity of the patient.

To avoid this morbidity, a substitute for palatal donor tissue has been studied. For example, acellular dermal matrix allograft, collagen membrane, and collagen matrix have been used, instead of palatal tissue. The acellular dermal matrix allograft showed good results at soft tissue augmentation, because this material is derived from human cadavers, however, it is associated with ethical concerns and the possible risk of disease transmission [20-22].

Collagen membrane has also been shown to have a positive effect on soft tissue augmentation and healing, but collagen matrix has a more porous layer meaning greater keratinized tissue can be achieved because of the space creating effect and blood clot formation [23]. Thus, the collagen matrix can be expected to be more effective at augmenting keratinized tissue. The main objective of this clinical study was to evaluate the changes in width of keratinized tissue following three surgical techniques, APF, APF combined with FGG, and APF combined with collagen matrix coverage. The result from this clinical study indicated that after 3-4 weeks, all of the cases achieved keratinized tissue. Furthermore, after 3-4 weeks, the three cases also achieved proliferation and maturation. The FGG cases showed a greater increase of keratinized tissue, and the APF combined with collagen matrix coverage cases showed more keratinized tissue increase than the APF only cases. The keratinized tissue after surgery in the APF combined with collagen matrix coverage cases were characterized by more physiologic and favorable morphology than the APF only cases. It is believed that the collagen matrix acts as a scaffold to prevent the mucosal relapse, and protection of the recipient bed.

Evidence of the advantages of collagen matrix is insufficient. Therefore, further studies are necessary to determine the influence of collagen matrix on the recipient bed, and on how to prolong the short absorption period of collagen matrix (10-14 days).