## 4 Discussion
A wide variety of materials were employed for the regenerative procedures, and all studies gave information about the materials used: for GBR procedures, one study [15] used a deproteinized bovine bone mineral (Bio-Oss), one study [17] used a freeze-dried mineralized bone allograft and the other two studies [19; 20] used demineralized freeze-dried bone particles (Dembone); for bone grafting, one study [18] used a Puros block allograft rehydrated with platelet-rich plasma and additional Puros particulate mineralized bone graft material, and the other two studies used autogenous bone grafts from symphysis [25] and from the retromolar area [16]; and the studies [24; 27] performing alveolar distraction osteogenesis used the same type of distraction device (Modus distractor, Medartis). All studies performing GBR or bone graft procedures used barrier membranes: three studies [15; 16; 18] used resorbable membranes and the other four studies [17; 19; 20; 25] used non-resorbable membranes. Regarding the timing of implant placement, two studies [15; 19] performed a regenerative procedure with a simultaneous implant placement, two studies [17; 25] performed a regenerative procedure with a simultaneous or delayed implant placement, depending on each patient clinical condition and five studies [16; 18; 20; 24; 27] performed a regenerative procedure with a delayed implant placement in the regenerated bone. All studies except two [16; 20] gave details about the type of implants used. The follow-up period ranged from a minimum of 6 months [15] to a maximum of 3 years [16; 24; 25], depending on each study. All studies reported good outcomes, with an enhancement of hard tissue, but few of them gave quantitative data about the amount of regenerated bone; therefore it is not possible to evaluate the efficacy of the different techniques in bone regeneration. Regarding soft tissue changes, no study reported data, except one [24]. Each regenerative surgical procedure showed some limits; one randomized study [15] reported that GBR with resorbable membranes and particulate bone substitutes undergoes some horizontal volume reduction during the healing stage. Greater reduction occurs at the coronal region and the use of different implant healing strategies (transmucosal or submerged) does not make a significant difference. This volume instability could result in the shrinkage of the grafted area and alteration of the ridge contour. To compensate for the volume reduction during the healing stage of GBR, over augmentation has been recommended by some clinicians [40]. In the coronal region, new bone formation may be predictably expected only within the bony envelope of the defect, but further studies are needed to confirm this result. The regenerated hard tissue, determined by the radiographic observation of a radiopacity, may not be true new bone in a histological sense, because not always the biomaterial is resorbed and replaced by new bone, but it can remain as a \"foreign body\" within the graft [41; 42; 43; 44; 45]. Another difficulty with GBR techniques is associated to the use of membranes: non-resorbable membranes have good mechanical properties but they also have a lower biocompatibility, request a second surgery for their removal and there is always a risk of early exposure during the healing process, which can compromise the result. Resorbable membranes have higher biocompatibility and do not request a second surgery, but they have unfavorable mechanical properties, volume instability due to their soft consistency and low resistance to pressure from the surrounding tissues, which may result in membrane collapse and the compromise of the new bone formation due to the loss of tissue volume during the healing stage of the GBR procedure. Finally, the GBR technique can only regenerate hard tissue, to ease an implant placement, but the soft tissue volume is often inadequate to provide an esthetic result, and it is difficult to simultaneously graft the ridge-augmentation site with soft tissue because of poor vascular support over a GBR membrane.