## 4 Discussion  
The aim of the present systematic review was to assess the level of evidence behind the clinical use of type I collagen-based devices to treat nerve injuries after oral surgery procedures. Based on the available literature, only four studies on 65 patients, all of them with a low-level of evidence, fulfilled the inclusion criteria and could be included. Microsurgical peripheral nerve repair has been extensively investigated especially with respect to hand nerve, wrist and brachial plexus [17]. Historically, autogenous nerve grafts of secondary importance have been used as autografts with good results in term of sensibility regeneration [23; 24; 25]. However, due the high percentage of postoperative morbidity, loss of function at the donor site and the limited handling related to the grafted nerve sizes [26], alterative treatment options such as the use of nerve conduits have become popular in augmentative nerve grafting [27]. Indeed, the use of a conduits allowed a tension-free direct neurornraphy besides avoiding complicated nerve harvesting procedures [28]. The mechanism behind their use is to create a 3-D chamber to promote nerve regeneration and at the same time isolate the injured nerve from the surrounding tissues [29; 30]. Among all the various investigated materials (i.e., silicons, PGAs and collagens), the scientific interest on the use of collagen devices have increased due to their characteristics such as maximal biocompatibility, the ability to stimulate different cell types adhesion and to guarantee a long-term cell proliferation [31; 32]; nonetheless its use has been proved to be effective mainly in small nerve injury defects (i.e., up to 20 mm) [17]. Despite the level of evidence gathered from the animal model models [32; 33], only few clinical studies have tested its reliability in a clinical model with positive promising short-mid-term results in term of functional sensory recovery [34; 35]: in particular, [PERSON] et al. [19] (i.e., 8/9 patients) and [PERSON] et al. [21] (i.e., 22/22 patients) reported significant improvement in terms of functional sensory recovery. In the literature there is very little evidence of retrospective/prospective studies comparing different grafting materials: on this topic, [PERSON] and coworkers [23] reported clinical data from a retrospective study on 43 patients treated whether with a collagen conduit (NeuraGen) or with a porcine small intestinal submuc conduit (AxoGuard) after LN injury. At the latest follow-up examination, no statistically significant differences in terms of achieved functional sensory recovery (FSR) were recorded, which leads to the conclusion that both treatment modalities might be reliable options. Irrespective to the selected surgical intervention, one aspect that has been proved to be crucial to increase the chances of sensory recovery is the timing: indeed, as reported by [PERSON] and [PERSON] [9], surgery is indicated if neurosensory deficit does not show any improvement 3 months after injury. When evaluating the results obtained from the included studies, despite the wide reported time range (i.e., 2-36 months), all treated patients underwent surgical intervention at least 2 months after injury: therefore, it seems recommended to postpone the decision to surgically treat these injuries after this time lapse. Due to its promising results in the oro-facial district, the use of type I collagen devices has gained interested in the in orthopedic surgery with a high interest in the rehabilitation field. To date, [PERSON] and coworkers [36] evaluated the effect of wrapping bioabsorbable nerve conduit around primary suture repair on motor nerve regeneration in 40 rats. These latter were randomly allocated into two experimental groups according to the type of repair of the rat sciatic nerve: group I (primary suture repair) and group II (primary suture repair plus bioabsorbable collagen nerve conduit (NeuraGen(r) 1.5 mm, Integra LifeSciences Corp., Plainsboro, NJ, USA) wrapped around the repair).