Although numerous studies in the literature show successful outcomes of the GBR procedure [6, 31], the most common clinical complication in GBR procedures is early membrane exposure . There is a general clinical impression that the ridge augmentation results are compromised in the case of early membrane exposures [32, 33]. In this case-controlled study, which was based on a patient subset from our previous randomized clinical trial, the clinical effect of exposure of a bioresorbable matrix membrane was evaluated . Based on clinical ridge width dimension measurements, a mean ridge width gain of 1.4 and 2.6 mm were calculated for the test and the control group, respectively. On the other hand, a reduction of 4.7 and 3.1 mm of the initially augmented ridge width was measured for the test and control group, respectively. Together, these results clearly indicated that the early membrane exposure in lateral ridge augmentation procedure resulted in significantly lower ridge width gain probably due to a significant higher resorption of the augmented graft during the healing process.
Still, the ridge width gain in both groups was sufficient to allow for the successful placement of dental implants in all 14 subjects without any complication. The exposed matrix barrier degraded within 6-7 weeks or was covered by soft tissue without any further complications. This observed degradation time is markedly longer than that of collagen membrane, which is reported to be completely resorbed 1 to 2 weeks after exposure [18, 34]. The prolonged degradation time of matrix barrier seems to provide prolonged protecting of the underlying graft supporting the bone regeneration process. During this healing process, all exposures did resolve within 6-7 weeks and no membrane had to be extracted. During this period, the exposed bioresorbable matrix barrier became covered with keratinized tissue over time. The secondary healing in exposed area lead to a subsequent increase in the width of keratinized tissue superior to the band of keratinized tissue observed in the control group (Fig. 1a). This shows the epithelization nor the subsequent keratinization process was not altered by an inflammatory situation that could have been triggered by the presence of the matrix barrier or its degradation product. This demonstrated the good healing properties of this barrier membrane. However, the gain of keratinized tissue was not quantitatively measured; thus, this is a clinical observation rather than a documented outcome. The predictability of gaining both keratinized tissue and horizontal ridge dimension simultaneously needs further investigation to confirm this observation. The other main advantage using a bioresorbable matrix barrier over non-resorbable PTFE membrane in the GBR procedures was that all exposures did resolve within 6-7 weeks without any complications and without the need of second surgery to extract the barrier. This might be an important advantage in the patient showing a thin biotype and in situations where primary closure is difficult to achieve in the GBR procedures.
The microbial contamination of the matrix barrier during exposure could be another important factor that might hamper bone formation within the underlying graft. This factor has not been investigated in the present study. However, it has been reported by other groups that the resorbable matrix barrier per se might be less prone to bacterial contamination and can be better cleaned using disinfectant agents such as chlorhexidine rinse than PTFE membranes . Matrix membrane presents an outer and inner surface. The external surface is more occlusive (the pore sizes are bigger than those of internal surface) to allow gingival tissue penetration. The internal layer, smaller pores, prevents further penetration of the gingival tissue through the barrier, thus protecting new bone formation underneath the barrier. From clinical observation, the space between the two layers seemed already occupied by connective tissue protecting the inner layer and leaving only the outer layer of the matrix exposed to the oral cavity and subsequent degradation. Still, this clinical observation has to be confirmed in further studies.
The results from this study suggest that primary flap closure over the matrix barrier is preferable leading to better ridge width gain than when the matrix is left exposed or early exposures happen. However, exposures were not completely detrimental to the lateral ridge augmentation and sufficient ridge width gain could be achieved allowing for successful implant placement. In critical cases, where 1 or 2 mm less bone would affect the esthetic results, the matrix barrier should not be left exposed and due care should be taken to avoid any exposures during healing after primary closure was achieved.