Prevention of Lingual Nerve Injury in Third Molar Surgery: Literature Review

Accepted Manuscript Prevention of lingual nerve injury in third molar surgery: literature review Roberto Pippi, MD, DDS, Associate Professor, Andrea Spota, MD, DDS, Researcher, Marcello Santoro, DDS, PhD student PII:

S0278-2391(16)31328-3

DOI:

10.1016/j.joms.2016.12.040

Reference:

YJOMS 57602

To appear in:

Journal of Oral and Maxillofacial Surgery

Received Date: 7 August 2016 Revised Date:

23 November 2016

Accepted Date: 24 December 2016

Please cite this article as: Pippi R, Spota A, Santoro M, Prevention of lingual nerve injury in third molar surgery: literature review, Journal of Oral and Maxillofacial Surgery (2017), doi: 10.1016/ j.joms.2016.12.040. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT REVISED TITLE PAGE Prevention of lingual nerve injury in third molar surgery: literature review Authors: Roberto Pippi, MD, DDS, Associate Professor, Department of Odontostomatological and Maxillo Facial Sciences, “Sapienza” University of Rome.

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Andrea Spota, MD, DDS, Researcher, Department of Odontostomatological and Maxillo Facial Sciences, “Sapienza” University of Rome.

Marcello Santoro, DDS, PhD student, Department of Odontostomatological and Maxillo Facial

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Sciences, “Sapienza” University of Rome.

Corresponding author:

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Marcello Santoro, DDS, PhD student, Department of Odontostomatological and Maxillo Facial

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Sciences, Via Caserta 6, 00161 Rome, +393334025038, [email protected].

ACCEPTED MANUSCRIPT Prevention of lingual nerve injury in third molar surgery: literature review Abstract Purpose. To identify any factors which may aid the surgeon in preventing or minimizing the risk of lingual nerve injury during third molar surgery.

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Methods. Electronic research was carried out on the correlation between lingual nerve damage and lower third molar surgery (topographic anatomy, surgical technique, and regional anesthesia) using PubMed, Scopus and Cochrane central databases. The research only included articles published in English up to February 2016.

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Results. Lingual nerve anatomy varied greatly: direct contact between the lingual nerve and the third molar alveolar wall was reported in a wide range of cases (0-62%) and the nerve was located at the same level or above the top of the ridge in 0-17.6% of cases. No detailed data was found on the actual incidence of lingual nerve injury resulting from local anesthesia by injection. Permanent lingual nerve damage did not show statistically significant differences between the simple buccal approach and the buccal approach + lingual flap retraction, although the latter was significantly associated with an increased risk of temporary damage. As far as permanent damage was concerned, no statistically significant differences were found between the lingual split technique and the buccal approach with or without lingual flap retraction. As opposed to tooth sectioning, ostectomy was strongly statistically associated with permanent lingual nerve damage.

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Conclusions. Results should be interpreted with extreme caution due to the considerable heterogeneity of the data as well as the considerable influence of several anatomical and surgical variables, all closely related, but which are difficult to analyze independently. It seems preferable to avoid lingual flap elevation, except in selected cases in which the presence of more than one unfavourable surgical variable predict a high risk of nerve injury. Tooth sectioning may reduce the extent of ostectomy or even, in some cases, prevent it, potentially acting as a protective factor in lingual nerve injury.

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Introduction Lingual nerve injury in oral-maxillofacial surgery is rather uncommon, but it is a highly disabling complication with a high risk of medico-legal consequences.1-4 Lower third molar extraction is one of the most common procedures in oral surgery and is undoubtedly the most frequent cause of lingual nerve damage. Several etiological factors, either anatomical or surgical, have been addressed; however, nerve damage is often unexpected by the surgeon.5-7 The effect of certain surgical procedures on lingual nerve injury is also still unclear. The wide use of surgical instruments for lingual nerve protection during lower third molar extraction is still debated. There are no studies in the literature which unequivocally prove that lingual flap retraction can cause more harm than benefit.8-14 The aim of the present study was to identify any factors which may help the surgeon prevent or minimize the risk of nerve injury in lower third molar surgery.

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Materials and methods Electronic research was performed using PubMed, Scopus and Cochrane central databases, including only articles published in English up to February 2016. The following keywords were used as search criteria: lingual nerve injury, lingual nerve and anatomy, lingual nerve and prevention, lingual nerve and third molar or wisdom tooth, lingual nerve and lingual flap, lingual nerve and lingual split, lingual nerve and mandibular block, lingual nerve and paraesthesia, and lingual nerve and anaesthetics. The research was carried out individually by all authors. The abstract of each article was viewed and the full text was downloaded if it was considered relevant to the following selection criteria of the study: for the anatomy section - articles which reported data about topographical relationships between the lingual nerve and the neighboring anatomical structures; for the section related to local anesthesia - articles which reported data on the incidence, mechanisms and preventive standards of nerve damage; for the surgical section - articles which reported clear and complete information regarding the technique used, the number of procedures, the number of extracted third molars and relative incidence of temporary and permanent nerve damage with at least 6 months of follow-up. Studies in which data of interest were indirectly obtainable by extrapolating them from other findings were also selected. Cross-checking was also carried out using the references of all collected articles and the missing studies that met the above-listed selection criteria were added. No exclusion criterion was adopted in relation to the study type. No attempt was made to identify unpublished material or to contact the authors of published studies to obtain more information or to go back to the missing data. Due to the limited number of studies, as well as the considerable differences in their study design, it was not possible to apply any statistical method to determine the risk of bias and the degree of inconsistency (heterogeneity). The statistical association between nerve damage and the surgical technique used was tested by the Pearson's chi-square test (Chi2). A first statistical analysis was performed with specific focus on the surgical technique (lingual flap, buccal approach, buccal approach + lingual flap retraction), by combining only data of "similar studies" for the analyzed primary variable, such as studies in which it was possible to identify differences between two adopted techniques in terms of nerve injury: temporary lingual nerve injury in the buccal approach + lingual flap retraction vs buccal approach alone, permanent lingual nerve injury in buccal approach + lingual flap retraction vs buccal approach alone, temporary 2

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lingual nerve injury in the lingual split technique vs buccal approach + lingual flap retraction, and permanent lingual nerve injury in lingual split technique vs buccal approach + lingual flap retraction. Subsequently, the results of all selected studies were resumed for the analyzed interest variable in a "mixed group" of studies in which it was possible to go back to the result of a single surgical technique (buccal approach, lingual split technique, buccal approach + lingual flap retraction) in terms of nerve injury in order to assess the possible discrepancy with the analysis conducted only among similar studies as well as to identify differences between surgical approaches which were not directly compared in the literature (temporary and permanent lingual nerve injuries in lingual split technique vs buccal approach). Since it was not possible to assess the heterogeneity of studies in the mixed group due to the incompleteness of their data in relation to interest variables, the Chi2 test was performed. In the present analysis studies in which it was not possible to clearly distinguish the number of cases related to each of the three analyzed surgical techniques were not considered. To evaluate the association between nerve damage and tooth sectioning or ostectomy, only articles which reported clear information on the number of cases in which these techniques were performed and the relative incidence of nerve damage were taken into account. All articles in which these data were missing or could not be deduced were excluded. Even in this case it was not possible to apply any statistical method to determine both the risk of bias and the degree of inconsistency (heterogeneity).

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Results Lingual nerve in the lower third molar region Out of the 228 articles analyzed, 9 were selected for this topic with a total number of 964 cases (Table 1).15-23 The average horizontal distance between the lingual nerve and the third molar lingual alveolar wall was about 3.05 mm (SE = 0.48; range: 0.57-9.3). The average vertical distance between the lingual nerve and the top of the alveolar ridge was 7.24 mm (SE = 0.95; range: 2.2816.8). Direct contact between the lingual nerve and the alveolar wall was reported in a wide range of cases (0-62%). The nerve was located at the same level or above the top of the ridge in 0-17.6% of cases. Lingual nerve minimum and maximum diameters in the third molar region were 1.86 and 3.45 mm, respectively. Lingual nerve injury and regional anesthesia No detailed data was identified for the actual incidence of lingual nerve injury resulting from local anesthesia by injection.24-39 Lingual nerve injury and surgical technique Out of the 1,028 analyzed articles, 28 were selected to evaluate the influence of surgical techniques on lingual nerve damage (Table 2).40-67 When combining the results of similar articles51,54,56,58,62,63,66,67 for the variable “temporary lingual nerve injury in the buccal approach + lingual flap retraction vs buccal approach”, for a total of 4,830 cases, buccal approach + lingual flap retraction were more significantly associated (p<0.01; RR:12.22; 1.71-15.07, CI 95%) to lingual nerve injury than to the buccal approach.

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The same result was obtained by combining data from the mixed group44-49,51,52,54-67 for the “temporary lingual nerve injury in the buccal approach + lingual flap retraction vs buccal approach” variable for a total of 27,785 cases (p<0.01; RR:2.38; 2.60-22.97, CI 95%). As far as “permanent lingual nerve injury” was concerned no statistically significant differences (p=0.0743, 0.74-6.62, CI 95%) were found between the two techniques in the group of similar articles,51,54,56,58,62,63,66,67 and the analysis of the mixed group44-49,51,52,54-67 of studies seems to further confirm this finding (p=0.9808; 1.08-26.26, CI 95%). As for the “temporary lingual nerve injury in the lingual split technique vs buccal approach + lingual flap retraction” variable, in the group of similar studies (1,83046,48 cases) the lingual split technique was more significantly associated (p<0.01; RR:2.7; 0.26-0.47 CI 95%) to lingual nerve injury than to the buccal approach + lingual flap retraction technique, whereas in 9,49440-43,46,4851,54,56,58,62,63,66,67 cases of the mixed group, the association was significantly lower (p<0.01; RR:0.67; 0.48-4.11, CI 95%). As for permanent damage, the group of similar studies46,48 for “lingual nerve damage in the lingual split technique vs the buccal approach + lingual flap retraction” variable showed no statistically significant difference between the two techniques (p=0.2259; 0.73-6.59, CI 95%), as opposed to the mixed study group40-43,46,48-51,54,56,58,62,63,66,67 which showed a higher risk of permanent damage for the lingual split technique (p=0.0116; RR: 0.23; 0.13-6.11, CI 95%). When comparing the lingual split technique vs the buccal approach, no similar study group was found and the mixed study group40-48,50-67 (32,471 cases) showed a significantly higher risk of temporary and permanent lingual nerve damage for the lingual split technique (p<0.01; RR: 1.58; 10.06-42.59, CI 95%), whereas a greater risk of permanent lingual nerve damage was observed in the buccal approach (p<0.01; RR: 0.22; 4.43-64.65, CI 95%). In only 4 articles58,62,66,68 (6,064 cases, Table 3), information regarding the number of cases in which tooth sectioning was or was not performed and the relative incidence of nerve damage were clearly set out or they could be indirectly identified. Tooth sectioning was not statistically associated with permanent lingual nerve damage (p=0.523; 0.52-4.95, CI 95%). In only 4 articles46,58,66,68 (7,021 cases, Table 4), information regarding the number of cases in which ostectomy was or was not performed and the relative incidence of nerve damage were clearly set out or they could be indirectly identified. Ostectomy was strongly statistically associated with permanent lingual nerve damage (p<0.01; RR=6,92; 0.90-9.36, CI 95%).

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Discussion Lingual nerve in the lower third molar region Thorough knowledge of the lingual nerve anatomy and topography is crucial in order to avoid iatrogenic nerve damage or at least to reduce the risk of nerve injury. Actually, lingual nerve spatial localization ranges widely from individual to individual as well as in the same individual.15-23, 69-71 Although several studies15-23 have attempted to define topographical relationships between the lingual nerve and lower third molar in both the horizontal and vertical planes (Table 1), comparison among studies was quite difficult since the reference points used for measurements were different. In addition, most studies often neglected important parameters, and thus provided inaccurate information regarding race, age, gender, edentulism type and degree of bone atrophy of sample subjects. In addition, most investigations were based on studies performed on cadavers, for which relatively inaccurate information was often provided as far as time and method of preservation of anatomical preparations were concerned. Moreover, study sample dimensions were often too inadequate to obtain significant results. For example, discrepancies found in space measurements in the Mendes at al.22 and Bhenia et al.18 studies are not only related to the different landmarks used and the number of analyzed anatomical preparations, but to the storage time as well. Actually, although stored in the same way, Bhenia et al.18 refer to an extremely larger number of anatomical preparations analyzed within a maximum of about 24 hours of death, thus ensuring lower dimensional tissue alterations compared to the study of Mendes et al.22 in which the preparation times varied from 2 to 5 years. Kiesselbach and Chamberlain15 and Progel et al.16 results also came from rather small samples and might be susceptible to errors due to the possible lingual nerve dislocation from its actual position during fixation and preparation of anatomical specimens.18 Actually, although anatomical dissection is rather easy to perform, it is well known to differently alter the lingual nerve spatial position even more when not properly executed.69 Therefore, different methods have been proposed to evaluate lingual nerve running, such as MRI,17 ultrasounds,72 or radiographic imaging with radiopaque landmarks placed inside or alongside the lingual nerve.20,69,73 Although these indirect methods were intended to overcome possible spatial lingual nerve alterations due to the dissection, they were not able to accurately identify and measure anatomical relationships between the lingual nerve and neighboring structures.69 Other factors which make it even more difficult to interpret the results of different studies involve the high variability of lingual nerve position in relation both to the alveolar process morphology and to the existing space between the third molar and the ascending mandibular branch. Actually, the lingual nerve position may be influenced by both alveolar process prominence and a marked inclination of its lingual surface in the lower third molar region. A small distance between the lower third molar and the mandibular ascending branch may also justify a more cranial lingual nerve position.23,70 Although the influence of the degree of mandibular atrophy on lingual nerve topographical relationships has often been overlooked by most studies, some authors16,19 have suggested that the loss of both muscle tone and connective tissue tension, secondary to aging, may partially compensate the lingual nerve superficial position due to bone atrophy resulting from tooth loss through an easier nerve deepening within mouth floor, which possibly leaves nerve spatial relationships with mandibular alveolar ridge unchanged. 5

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In this regard, a recent study23 stressed the strong discrepancy in lingual nerve location in relation to edentulism type, alerting the possible greater lingual nerve proximity to the mandibular alveolar crest in edentulous subjects. A marked bone loss may therefore be an important risk factor for lingual nerve injury during routine oral surgery involving lingual access flap management in the posterior region of the mandible.19 Actually, almost all anatomical studies highlight the risk of the lingual nerve being close to the lingual surface of the alveolar process (0-62%) or to an even higher level than the mandibular bone crest (0-17.6%), almost directly laying on the retromolar region (Table 1).15-23 All these findings are considerably important in the prevention of lingual nerve damage during most routine oral surgeries. Therefore, in lower third molar extraction, the distal release incision should not be directed towards the lingual aspect of the ascending branch of the mandible, but instead should be inclined vestibularly 45° in respect to the lower third molar distal surface.12,14,19,52,74 Lingual nerve injury and regional anesthesia The lingual nerve is also susceptible to injury through common local anesthetic injection procedures although the true incidence of this event and the actual causative mechanisms are still being debated. Damage to the lingual nerve may be directly caused by the syringe needle24-26 during needle penetration as the nerve cannot be easily moved away during needle advancement since it is stabilized within the inter-pterygoid fascia when the mouth is fully open. Alternatively, the nerve can be directly damaged by the needle during its retraction from the tissues24,27 especially when common chamfered needles are used since their tips are easily susceptible to distortion and irregular curvatures, particularly as a result of bone contact or in the case of multiple injections which can break the perineurium and induce both endoneurium herniation and interruption of multiple nerve fibers or entire fascicles.27-29 However, since the lingual nerve average diameter is 1.86 mm and the diameter of the most commonly used needle in dental practice (25-gauge) is about 0.45 mm, it seems impossible that this type of trauma can be capable of breaking the nerve bundle, thus resulting in neurotmesis.24-27, 29,30 Furthermore, needle penetration can be responsible for direct trauma to the intraneural vessels with formation of an intraneural hematoma able to exert a certain degree of compression to the nerve fibers, leading to a focal block of nerve transmission without any interruption of both the axonal and perineural connective continuity. The return to normality may take several weeks as the progressive reduction of hematoma pressure allows nerve remyelination.24-27,29,30 It has also been suggested that local anesthetics are able to cause a localized chemical injury to the nerve if deposited in the intra-fascicular space or inside the nerve during needle retraction.24,26 Chemical trauma may be responsible for demyelination, axonal degeneration, inflammation and edema of the endoneural fibers.31 The more easily the ester or amide group hydrolysis occurs, the greater the amount of bioavailable alcohol group is; therefore, the stronger the anesthetic power is, the greater the possibility of nerve cell damage. In this regard, articaine and prilocaine have shown a greater cytotoxic power than lidocaine.24,26,29,32 High doses of these anesthetics are therefore not recommended due to easy production of neurotoxic metabolites. High concentrations of lidocaine also showed neuropathic effects after perineural or intrafascicular injections.24,26,29,32-34

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It is not possible to provide accurate information about the true incidence of lingual nerve injury during local anesthesia since it is often difficult to extrapolate it from that of the inferior alveolar nerve, and since it is often not clearly specified whether local anesthesia is followed by non-surgical procedures to exclude other possible causes of lingual nerve damage. Sambrook and Goss35 estimated a 1:27.415 risk of lingual nerve and inferior alveolar nerve damage during truncal mandibular anesthesia. Similarly, Pogrel and Thamby24 reported a nerve damage incidence which ranged from 1:160.571 to 1:26.762, claiming that during the professional life of each practitioner it is very likely to have at least one patient with nerve trauma due to local anesthesia. Harm and Durham25 reported approximately a 3.62% chance of lingual nerve trauma whenever mandibular nerve block anesthesia is performed while Krafft and Hickel36 found a 0.15% incidence of lingual nerve injury (18/12,104) during nerve block anesthesia not performed for surgery. Moreover, nerve damage due to local anesthesia more frequently affected the lingual nerve (more than 2/3 of cases) rather than the inferior alveolar nerve.24,34,37 Actually, although the diameter of the two nerves was found to be approximately the same at the injection site, the lingual nerve seems to be more exposed to needle trauma and also possesses less nerve fascicles compared to the inferior alveolar nerve.38 Pogrel al.38 showed that in approximately 33% of cases the lingual nerve is formed by only a single fascicle at the injection point, as opposed to 7-39 fascicles in the lower third molar area. This might explain why nerve alterations resulting from local anesthesia most frequently involve the entire lingual nerve distribution area.24,36,38 Furthermore, dysesthesia, which is the most disabling sensorial alteration, oddly occurs more frequently after nerve injury resulting from local anesthesia procedures (34%) rather than after surgical procedures (8%).24 Therefore, patients not only face an increased risk of being in greater discomfort, but they almost always are also poorly-informed about the possible risk of lingual nerve damage during local anesthesia compared to that related to surgery.24 Although sensory alteration recovery occurs within 8 weeks in 85-94% of cases,24,36 preventive measures for lingual nerve injury should be applied as soon as at the time of local anesthesia, a procedure which is often underestimated by dentists and surgeons. Due to the considerable variability of lingual nerve size and anatomical position, little can be done to predict and prevent its injury at this stage.39 However, it seems preferable to use a small-diameter needle while high concentrations of local anesthetics and multiple injections should be avoided.32,34,37,40 In particular, 4% prilocaine and 4% articaine appear to be associated to a 7.3 and 3.6 times greater probability, respectively, of causing paresthesia during inferior alveolar nerve block anesthesia than expected on the basis of the total local anesthetic use by U.S. dentists.72 Moreover, these substances, compared to other common local anesthetics, may more easily result in neuropathic pain although the true incidence of these phenomena still remains unclear.23,24,75 Lingual flap and nerve protection in third molar surgery Routine use of a surgical instrument for lingual nerve protection during lower third molar extraction is still debated. Actually, although the use of a lingual flap retractor can apparently provide greater operating safety, it is considered a possible etiologic factor of lingual nerve injury.13,14,46,49,51,52,54 However, there are no studies in the literature which unquestionably prove that lingual flap retraction may more easily lead to damage rather than benefit. In most studies, the different levels of experience of surgeons was not taken into account, the surgical technique was not standardized, and the information describing the surgical steps and variables (tooth eruption degree, inclusion 7

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depth, patient age, type and drug for anesthesia, surgical flap, type of retractor, type of ostectomy and tooth section, etc.) was not provided in a clear and comprehensive way. In addition, most studies were small-sized and observational so they were subjected to the lower statistical power of this study type (Table 2).40-67 Lingual nerve protection undoubtedly requires a great deal of caution. Although different types of lingual retractors have been proposed, including those of Howarth, Ward, Meade, Hovell, Walters and Rowe, there is no scientific evidence supporting the superiority of one over other instruments, the choice of which seems strongly influenced by surgeon’s personal experience.8,11,13,14,50,76 However, a large and smooth rather than narrow and pointed active retractor surface has been suggested to be less traumatic on the lingual nerve during flap retraction.8 Some authors assert that the lingual flap detachment for retractor placement, although associated with a high possibility of temporary nerve damage, may reduce the incidence of permanent nerve injury through improved access to and better visibility of the surgical area, thus ensuring safer use of both manual and rotating instruments.8,12-14 On the contrary, the results of the present analysis seem to confirm the conclusions reached by Pichler al.7, that is, although lingual retraction may be associated with a higher incidence of temporary nerve damage, it neither protects nor harms the lingual nerve permanently. Actually, the present review does not show any statistically significant difference in permanent lingual nerve damage between simple buccal approach and the combined approach (buccal approach + lingual flap retraction; p=0.98), although the latter is significantly associated with an increased risk of temporary damage (p<0.01). However, the lingual split technique seems to be statistically associated with an increased risk of temporary (p<0.01) nerve damage compared to the buccal approach with or without lingual flap retraction (p<0.01), whereas no statistically significant difference was found between the lingual split technique and the buccal approach + lingual flap retraction in permanent lingual nerve damage. However, these results and more so those derived from the “mixed groups”, should be interpreted with extreme caution due to the considerable heterogeneity of data from which they arise as well as to the presence of many confounding factors. Avoiding lingual flap detachment as much as possible in order to reduce the incidence of lingual nerve damage is therefore highly recommended. Actually, lingual nerve protection through the lingual flap should be restricted to selected cases in which the presence of more unfavorable surgical variables predict a high risk of nerve injury.9,10,13,14,46,49,51,52,54,77 Tooth sectioning and ostectomy in lower third molar surgery Although tooth sectioning is one of the possible risk factors for lingual nerve injury during lower third molar surgery,9,54,78-80 the present review did not find any statistically significant differences between the incidence of lingual nerve damage in surgeries in which tooth sectioning was performed and that of surgeries in which it was not. However, the present results (Table 3) refer to an extremely limited sample (6,064 lower third molars) since most of the studies analyzed did not clearly specify the number of cases in which tooth sectioning was actually performed or they omitted information on nerve damage incidence in cases in which tooth sectioning was performed or not. Furthermore, it is almost always impossible to correlate cases in which tooth sectioning was performed and in which lingual nerve damage occurred with lower third molar position or number and type of sections. The lack of statistically significant differences can also be related to confounding factors such as the surgeon’s skills and experience and therefore variables related to the surgical technique (anesthesia, 8

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flap design, need for ostectomy and its characteristics, suturing etc.), lingual nerve and lower third molar local anatomical variability, possible local pathological processes such as inflammatory adhesions as a result of recurrent pericoronitis affecting one or more surgical steps. During tooth sectioning, the lingual nerve may be involved by the bur. In the case of lower third molar bone impaction, predisposing conditions are both pre-existing bone fenestrations and iatrogenic intra-operative perforation of the alveolar wall at the level the lingual nerve running. For this reason, it would be a good rule to perform a subtotal lower third molar section and then complete the separation of fragments by using an elevator or another hand instrument.52 However, tooth sectioning may reduce the extent of ostectomy, or even help avoid it altogether in some cases, thus making it a possible protective factor for lingual nerve injury.9,52,62 Actually, in the present study removal of periradicular bone tissue, especially at the lingual and/or disto-lingual sites, was found to be strongly correlated to lingual nerve damage (p<0.0, Table 4). However, this finding also arises from a rather limited number of cases (7,021) since in most studies clear and comprehensive information regarding the number of cases in which ostectomy was actually performed, and/or the incidence of nerve damage in cases in which it was performed or not, is missing. Several authors claim that ostectomy should be carried out only when a total view of the periradicular bone surface is possible. Therefore, ostectomy should not be blindly performed, especially at the lower third molar distal and disto-lingual sites.81-83 If preoperative planning indicates the need to remove lower third molar distal or disto-lingual bone, a limited retraction of the lingual tissue may be appropriate both to improve surgical field visibility and to allow lingual nerve protection during bur use.74 Suture There is no data on the true incidence of lingual nerve damage caused by suture at the lower third molar fresh extraction site. Only a few studies cite suture as a possible risk factor for lingual nerve injury through a direct trauma of the needle or through "throttling" during knot closure.12,14,74 It is therefore recommended to avoid inserting the needle too apically in relation to the incision line. Chossegros et al.12 considered safe-inserting the needle at about 3 mm from the gingival margin of the lingual flap. Anatomical considerations in preventing lingual nerve injuries in other oral surgical procedures Even during implant or periodontal surgery in the posterior lingual mandibular region, it is preferable to perform intra-sulcular incisions, avoiding vertical relaxation incisions, especially in the molar region, and full-thickness flaps, avoiding any tearing or damage that may indirectly injure the lingual nerve.69 Moreover, the possibility of a relatively shallow position of the lingual nerve, even anteriorly to the lower third molar, should be always considered especially in cases of marked alveolar ridge atrophy.19 A study on cadavers (30 analyzed nerves) by Chan et al.69 analyzed the topographic lingual nerve anatomy in relation to the teeth mesial to the lower third molar region. Actually, these authors identified the lingual nerve on the vertical plane at about 9.6 mm, on average, below the meanlingual portion of the second molar cemento-enamel junction, until reaching an average of about 13 mm at the first molar level, and 15 mm in the second premolar area, without reporting any statistically significant differences between the left and right side of the jaw. The same authors53 also found that the first molar region was that in which the lingual nerve most frequently curved in a medial direction towards the tongue (41.60-58.33% of cases; 24 analyzed nerves, 12 per side), followed by the second molar (16.67-33.33% of cases) and finally, by the second premolar regions 9

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(16.67-25% of cases). On the contrary, this finding was quite rare in the first premolar area (8.33%; 1/12 analyzed nerves, on the left side).69 Few studies, on the other hand, have assessed an intra-lingual distribution of the lingual nerve.84-86 Their results indicated that a wide median longitudinal portion of the dorsum of the tongue was poorly innervated by lingual nerve and that, therefore, its branches could be easily avoided such as during the excision of median pathological lesions or during the removal of tissue at the lateral margins of the tongue up to about 1 cm from the latter. Actually, lingual nerve terminal branches, which run on genioglossus muscle, can be excluded from iatrogenic injury if incisions involve the lateral margins of the tongue and tissues connected to the genioglossus muscle are carefully dissected.84-86 The anatomical relationship between the lingual nerve and the submandibular gland duct (Wharton duct) is also quite variable. Mendes et al.22 (24 analyzed nerves) reported that the glandular duct crossed the lingual nerve superiorly in 62.5% of cases, whereas it ran beneath the nerve in 37.5% of cases. Similarly, in a topographical study on atrophic mandibles, Holzle et al.19 reported 4/34 cases (11.7%) in which Wharton duct ran deeper than the lingual nerve. On the contrary, Erdogmus et al.21 asserted that the lingual nerve maintained the same spatial relationship beneath the submandibular duct in all 42 cases. Knowledge of such anatomical variables may therefore be fundamental in preventing lingual nerve injury through a careful blunt dissection of tissues during surgical procedures involving both the floor of the mouth and the tongue. Conclusion

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The present review does not show any statistically significant differences in permanent lingual nerve damage between simple the buccal approach and the buccal approach combined with lingual retraction in lower third molar surgery, although buccal approach + lingual flap retraction is significantly associated with an increased risk of temporary nerve damage. The lingual split technique is statistically associated with an increased risk of temporary nerve damage compared both to the buccal approach + lingual flap retraction and to the simple buccal approach, while it does not show any differences in permanent lingual nerve damage compared to buccal approach + lingual flap retraction. It seems therefore preferable to avoid lingual flap elevation, except in selected cases in which the presence of more unfavourable surgical variables predict a high risk of nerve injury. There seems to be no statistically significant difference between the incidence of lingual nerve damage in surgeries in which tooth sectioning was performed and those in which it was not. Tooth sectioning may reduce the extent of ostectomy or even, in some cases, avoid it altogether, thus potentially acting as a protective factor in lingual nerve injury. Actually, removal of peri-radicular bone tissue, especially at the lingual and/or disto-lingual sites, is strongly associated with lingual nerve damage. However, these results should be interpreted with extreme caution due to the considerable heterogeneity of the data as well as the substantial influence of several anatomical and surgical variables, all closely related to possible nerve damage, but which are difficult to analyze independently. Moreover, most studies are observational and include a very limited significant sample population compared to the rareness of lingual nerve damage, so long-term randomized large-scale prospective studies are necessary to identify risk factors for lingual nerve injury. 10

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15

ACCEPTED MANUSCRIPT Table 1. Lingual nerve location in the lower third molar region N. of nerves

Horizontal distance (mm)

Vertical distance (mm)

Mandibular bone contact (%)

At the level of or above the mandibular crest (%)

Diameter (mm)

Kiesselbach and Chamberlaind, 198415

C

34

0.59±0.90

2.28±1.96

62

17.6

1.86

Pogrel et al., 199516

C

40

3.45±1.48

8.32±4.05

Miloro et al., 199717

MR

20

2.53±0.67

2.75±0.97

Bhenia et al., 200018

C

669

2.06±1.10

3.01±0.42

Holzle and Wolff, 200119

C

68

0.86±1.00

7.83±1.65

Karakas et al., 200720

C

21

4.19±1.99

9.56±5.28

Erdogmus et al., 200821

C

42

Mendes et al., 201322

C

24

Dias et al., 201523

C

46

0

15

3.45

25

10

2.54

23.27

14.05

-

57.4

SC

8.82

2.74

0

4.7

2.56±0.59

9.3±2.1

7.06±1.3

0

0

2.04±0.4

4.40±2.4

16.8±5.7

0

0

-

0.57±0.56

9.15±3.87

26

7

-

AC C

EP

TE D

C=cadaver; MR=magnetic resonance

RI PT

Study type

M AN U

Authors, year

16

ACCEPTED MANUSCRIPT Table 2. Incidence of lingual nerve injury Surgical method

Anesthesia

Bone removal method

LTMs (number)

Temporary LN injury (%)

Permanent LN injury (%)

Rud, 197040

PS

LST

L

C, D

718

0.83

0.14

Rood, 198341

PS

LST

G

C, D

1,400

6.64

0

Hochwald et al., 198342

PS

LST

L&G

C

Rud, 198443

PS

LST

G

C

Goldberg et al., 198544

PS

BA

L&G

D

Wofford and Miller, 198745

PS

BA

L

D

Mason, 198846

PS

BA+LF

G

C, D

Obiechina, 199047

PS

BA

Rood, 199248

PS

BA+LF LST

PS

BA+LF

Walters, 199550

PS

Robinson and Smith, 199651

PS

L

G

C, D

Moss and Wake, 199953

RS

AC C

Gargallo-Albiol et al., 200054

Benediktsdottir et al., 200455 Gomes et al., 200556

PS

0

609

3

0

500

0.6

0.2

576

0.71

0.17

747

8.3

0.4

293

19.8

1.02

367

0.27

0

406

3.2

2

384

12.8

0

791

1.9

0

LST

G

C

1,000

0.5

0

BA+LF

L&G

D

352

6.53

0.85

390

0.51

0.25

BA

L

D

504

0.2

0

LST

G

C

2,088

0.8

0

BA+LF

L

D

142

2.11

0

158

0.63

0

EP

PS

4.3

D

BA

Appiah-Anane and AppiahAnane199752

598

L

TE D

Schultze-Mosgua and Reich, 199349

M AN U

LST

RI PT

Study type

SC

Authors, year

BA

PS

BA

L

D

388

0.5

0

RCT

BA

L&G

D

55

0

0

55

9.1

0

BA+LF Jerjes et al., 200657

PS

BA

L

NS

1,087

5.52

1

Baquain et al., 201058

PS

BA

L

D

299

0.3

0

110

9.1

0

3,236

0.62

1.1

BA+LF Jerjes et al., 201059

PS

BA

L

17

NS

ACCEPTED MANUSCRIPT Janakiraman et al., 201060

PS

BA

Lata201161

PS

BA

L

CharanBabu et al., 201362

PS

BA

L

119

4.2

1.7

D

90

5.5

1.1

D

92

1.1

0

8

37.5

0

1,455

0.14

0.07

134

1.5

0

7,659

6.5

0

Smith, 201363

PS

BA

L&G

D

BA+LF

RI PT

BA+LF

RS

BA

G

D

Nguyen et al., 201465

RS

BA

L&G

D

10,160

0.069

0.079

Yadav et al., 201466

PS

BA

L

D

576

1.7

0.2

624

9.1

0.5

190

2.1

0.52

190

8.95

0

BA+LF RCT

BA BA+LF

L

NS

M AN U

Shad et al., 201567

SC

Guerrouani et al., 201364

AC C

EP

TE D

C=chisel; D=drill; PS=prospective study; G=general; L=local; BA=buccal approach; LF=lingual flap; LN=lingual nerve; NS=not specified; RS=retrospective study; LST=lingual split technique; LTMs=lower third molars; RCT=randomized controlled trial

18

ACCEPTED MANUSCRIPT

Table 3. Lingual nerve injury by tooth sectioning Total cases (number)

Cases with TS (%)

LN injury with TS (%)

LN injury without TS (%)

Baquain et al., 201058

426

27.5

5.7

1.3

Cheung et al., 201068

4,338

62

0.57

0.91

100

13

7.7

3.4

1,200

57.7

0.4

CharanBabu et al., 201362 Yadav et al., 201466

AC C

EP

TE D

M AN U

SC

LN=lingual nerve; TS=tooth sectioning

RI PT

Authors, year

19

0.2

ACCEPTED MANUSCRIPT Table 4. Lingual nerve damage by ostectomy Total cases (number)

Cases with O (%)

LN injury with O (%)

LN injury without O (%)

Mason, 198846

1,040

66

20.9

3.4

Baquain et al., 201058

443

68.6

3.6

0

Cheung et al., 201068

4,338

20

0.91

0.64

Yadav et al., 201466

1,200

93.5

0.3

AC C

EP

TE D

M AN U

SC

LN=lingual nerve; O=ostectomy

RI PT

Authors, year

20

0