Inferior Alveolar Nerve Injury in Trauma-Induced Mandible Fractures

Accepted Manuscript Inferior Alveolar Nerve Injury in Trauma-Induced Mandible Fractures Dr. Andrew Ban Guan Tay, BDS, MDS, FDS RCSEd, FAMS, Dr. Juen Bin Lai, BDS, MDS, FRACDS, FAMS, Dr. Kok Weng Lye, BDS, MDS, FRACDS, FAMS, Dr. Wai Yee Wong, BDS, MDS, FRACDS, FAMS, Dr. Nivedita V. Nadkarni, PhD, Dr. Wenyun Li, PhD, Dr. Dianne Bautista, PhD PII:

S0278-2391(15)00113-5

DOI:

10.1016/j.joms.2015.02.003

Reference:

YJOMS 56659

To appear in:

Journal of Oral and Maxillofacial Surgery

Received Date: 17 December 2014 Revised Date:

4 February 2015

Accepted Date: 4 February 2015

Please cite this article as: Tay ABG, Lai JB, Lye KW, Wong WY, Nadkarni NV, Li W, Bautista D, Inferior Alveolar Nerve Injury in Trauma-Induced Mandible Fractures, Journal of Oral and Maxillofacial Surgery (2015), doi: 10.1016/j.joms.2015.02.003. 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.

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INFERIOR ALVEOLAR NERVE INJURY

Dr. Andrew Ban Guan Tay BDS, MDS, FDS RCSEd, FAMS

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IN TRAUMA-INDUCED MANDIBLE FRACTURES

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Senior Consultant & Head Department of Oral & Maxillofacial Surgery National Dental Centre, Singapore 5, Second Hospital Avenue Singapore 168938 Tel: (65) 6324 8817 Fax: (65) 6324 8899 E-mail: [email protected]

Dr. Juen Bin Lai

BDS, MDS, FRACDS, FAMS

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Consultant Department of Oral & Maxillofacial Surgery National Dental Centre, Singapore 5, Second Hospital Avenue Singapore 168938 Tel: (65) 6324 8817 Fax: (65) 6324 8899

Dr. Kok Weng Lye

BDS, MDS, FRACDS, FAMS

Senior Consultant Department of Oral & Maxillofacial Surgery National Dental Centre, Singapore 5, Second Hospital Avenue Singapore 168938 Tel: (65) 6324 8817 Fax: (65) 6324 8899 E-mail: [email protected]

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Dr. Wai Yee Wong BDS, MDS, FRACDS, FAMS

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Dr. Nivedita V. Nadkarni PhD Assistant Professor Center for Quantitative Medicine Duke-NUS Graduate Medical School 20 College Road Singapore 169856 Tel: (65) 6576 7372

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Oral & Maxillofacial Surgeon Parkway Dental Practice 9 Scotts Road #12-02, Pacific Plaza Singapore 228210 Tel: (65) 6836 9808 Fax: (65) 6836 1193 E-mail: [email protected]

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Dr. Wenyun Li PhD Instructor Center for Quantitative Medicine Duke-NUS Graduate Medical School 20 College Road Singapore 169856

Dr. Dianne Bautista PhD Assistant Professor Center for Quantitative Medicine Duke-NUS Graduate Medical School 20 College Road Singapore 169856 Tel: (65) 6576 7366 & Senior Biostatistician Singapore Clinical Research Institute 31 Biopolis Way, Nanos #02-01 Singapore 138669 Tel: (65) 6508 8329

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Abstract

Purpose:

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This prospective observational cohort study sought to determine the prevalence of inferior alveolar nerve (IAN) injury after mandible fractures before and after treatment, and to elucidate factors associated with the incidence of post-treatment IAN injury and time to normalization of

Methods:

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sensation.

Consenting patients with mandibular fractures (excluding dentoalveolar, pathologic, previous fractures or mandibular surgery) were prospectively evaluated for subjective neurosensory disturbance (NSD) and underwent neurosensory testing (NST) before treatment, then 1 week, 1.5,

Results:

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3, 6 and 12 months after treatment.

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Eighty patients with 123 mandible sides (43 bilateral) were studied: 83.8% were male; mean age was 30.0 years (SD 12.6). Injury etiology included assault (33.8%), falls (31.3%), motor vehicle

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accidents (25.0%) and sports injuries (6.3%). Half (49.6%) of fractures involved the IAN-bearing posterior mandible; all condylar fractures (13.0%) had no NSD. Treatment included open reduction and internal fixation [ORIF] (74.8%), closed reduction and fixation (22.0%) or no treatment (3.3%). The

overall prevalence of IAN injury was 33.7% (95% CI: 24.8-42.6) before treatment and 53.8% (95% CI: 46.0-61.6) after treatment. In the IAN-bearing mandible, this was 56.2% (95% CI: 43.2-69.2) before treatment and 72.9% (95% CI: 63.0-82.7) after treatment. In contrast, this

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prevalence in the non-IAN bearing mandible was 12.6% (95% CI: 4.1-21.1) before treatment and 31.6% (95% CI: 20.0-43.3) after treatment.

Factors associated with development of post-

treatment IAN injury included fracture site and gap distance (a 1-mm increase was associated

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with 27% increase in odds of post-treatment sensory alteration). Time to normalization after treatment was associated with type of treatment (ORIF inhibited normalization) and fracture site

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(IAN-bearing sites took longer to normalize).

Conclusion:

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IAN injury was 4 times more likely in IAN-bearing posterior mandible fractures (56.2%) than in non-IAN bearing anterior mandible fractures (12.6%). After treatment, IAN injury prevalence

Introduction

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(in 12 months) was higher: 72.9% in the posterior mandible, 31.6% in the anterior mandible.

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Mandible fractures may result in injury to the inferior alveolar nerve (IAN) with sensory disturbances to the lower lip and chin. Sensory disturbance of the lower lip and chin may occur

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because of the mandible fracture itself or from surgical reduction and fixation of the fracture. Thurmuller et al.1 reviewed the literature on nerve injuries in facial trauma and reported the overall incidence of IAN injury in mandible fractures with associated paresthesia to be 5.7 to 58.5% after injury without treatment; that for fractures in the mandibular angle and body region was 46 to 58.5%. The incidence of IAN injury in treated mandible fractures ranged from 0.4 to 91.3%. The reported postoperative incidence of IAN injury in fractures of the nerve bearing area of the mandible (angle, body) was 76.1 to 91.3%. However, the majority of studies reporting on

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nerve injuries in mandible fractures were retrospective, or focused on outcomes or complications of internal fixation methods. Some publications lacked post-traumatic, pre-treatment sensory data, did not give adequate details of neurosensory tests used or did not provide sensory testing

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data. Some studies reported the incidence of sensory disturbance by number of patient instead of by jaw side, without any information of cases with bilateral mandible fractures, making

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comparison of results difficult.

A review of studies on neurosensory disturbance in mandible fractures in the English literature2showed that the reported incidence of IAN sensory deficit after injury before treatment was 5.4

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to 81.4%, that soon after intervention was 0.4 to 91.3%, and after 1 year was 0 to 46.6%. However, including only prospective observational studies with at least 6 months follow-up, designed with neurosensory testing with stated post-treatment duration and data by fracture side

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(not patients), the incidence of IAN sensory deficit post-injury was 46.2 to 53.1%, that after intervention was 76.9 to 91.3%, and after 1 year was 7.7 to 46.6%.10,12,17

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Various factors have been reported to influence the incidence of IAN injury in mandible fractures,1 including edentulousness of patients,10 site and type of mandible fracture,11,16 extent

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of displacement of the fracture,10,16 and type of treatment, in particular the use of internal fixation.10

Aim

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The primary aim of this prospective observational cohort study was to determine the prevalence of IAN injury after mandible fractures before and after treatment in the Department of Oral & Maxillofacial Surgery (OMS), National Dental Centre of Singapore (NDCS).

The secondary

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aims were (1) to elucidate the factors associated with the incidence of post-treatment IAN injury in mandible fractures, (2) to elucidate the factors associated with the time to normalization of sensation, and

(3) to determine the time to normalization of sensation after treatment of

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mandible fractures.

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Methods

This study was conducted in accordance with the principles of the International Declaration of

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Helsinki. Ethical approval was granted by the Institutional Review Board.

Patient eligibility criteria and recruitment

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Consecutive patients of any age, gender, race and nationality with mandible fractures referred to the OMS Department of the NDCS, were invited to participate in this study. Inclusion criteria

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included clinical evidence (malocclusion, step deformity, mandible discontinuity or mobility of the segments) and/or radiographic evidence of mandible fracture. Exclusion criteria included isolated dentoalveolar fractures, pathologic fractures, fractures with significant tissue loss, (e.g. gun-shot or avulsive wounds), soft tissue injury resulting in mental nerve injury, previous mandible fractures or surgery (orthognathic surgery, implant surgery, surgery for mandibular pathology likely to have involved the mandibular canal, and impacted lower third molar surgery

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that resulted in numbness of the lower lip and/or chin), and inability to undergo neurosensory testing (NST) because of preexisting mental retardation or disease, or head injury with unconsciousness, incoherence or inability to follow instructions. Written informed consent to

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participate in the study was taken from each patient or their parent or legal guardian if they were under 21 years of age. Recruitment was done between April 2005 and March 2010.

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Data Collection

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At presentation, each patient’s bio data, cause of injury and details of the mandible fracture (including fracture site, comminution and gap distance) were recorded on a study form. The site of mandible fracture was classified as follows (see Figure 1):

Fractures in the symphyseal/parasymphyseal region, i.e. anterior mandible between the canines

B.

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A.

Fractures in the anterior body, i.e. mandible body from the canine region up to 5 mm

C.

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anterior to the mental foramen

Fractures in the posterior body, i.e. mandible body in the molar/premolar region

D. E.

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including the mental foramen and 5 mm anterior to it Fractures in the mandible angle Fractures in the lower ramus, i.e. mandible ramus below and including the mandibular

foramen

F.

Fractures in the upper ramus and coronoid process, i.e. mandible ramus above and excluding the mandibular foramen

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G.

Fractures of the condyle from the sigmoid notch to the condylar head.

Regions C, D, and E comprise the IAN bearing area of the mandible. The anterior mandible

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(regions A and B) carries the incisive branch of the IAN, but for the purposes of this study, was regarded as an area of no consequence for IAN injury resulting in numbness in the mental nerve distribution. Regions A, B, F and G were non-IAN bearing sites. Mandible fractures were also

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classified into incomplete, undisplaced, displaced or comminuted fractures. In all cases, any displacement on the preoperative radiograph were measured with a ruler or periodontal probe in

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a direction parallel to the mandibular canal and the measurement adjusted for magnification using the mesio-distal width of the crown of a nearby tooth. Measurements were rounded to the nearest whole number.

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The mandible fractures were treated according to the patient’s choice in the light of the recommendations of the attending surgeon.

Treatment was categorized into conservative

treatment (no surgical intervention), closed reduction and fixation (CRF), and open reduction and

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internal fixation (ORIF). The method of internal fixation was decided by the attending surgeon in the best interests of the patient; the preferred method of internal fixation in the OMS

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Department in NDCS was two 2.0-mm miniplates per fracture using at least four monocortical screws per plate. Treatment details recorded included the type of treatment and intraoperative findings (fracture displacement distance before and after reduction, and any exposure of the IAN or mental nerve in the fracture site). Any injury seen in an exposed IAN was noted.

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Neurosensory Testing

The sensory function of the IAN was tested at diagnosis of a mandible fracture before treatment,

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and after treatment at 1 week, 6 weeks, 3 months, 6 months and 1 year. The patient was asked if there was numbness without stimulus and on stroking the lower lip skin with a finger. The patient was asked to grade the sensation present on an analog scale from 0 (no sensation) to 10

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(completely normal sensation); this formed the patient-reported neurosensory disturbance (NSD). English was the main language of communication, with translation into other languages were

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provided as necessary.

Neurosensory testing (Table 1, Figure 2) included direction sense and 2-point discrimination (Level A), contact detection (Level B) and pain sensation (Level C), as described by Zuniga and

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Essick (1992).32-34 Preoperative testing utilized Level A and C (modified) but omitted Level B; the rationale was that Level A detected the presence of at least mild sensory impairment, while Level C indicated a possible significant nerve injury. For Level C testing, a thermode was the

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instrument of choice; however, a thermode is bulky and expensive, and could not be used if the patient unable to come to the dental clinic. Therefore, at preoperative testing, a sharp probe was

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used instead of thermode testing for Level C.

Although two different sensory modalities

(pinprick pain with the sharp probe; temperature pain with the thermode) are being assessed, they can be ‘equated’ in the sense that loss of either pinprick pain or temperature pain is indicative of severe sensory impairment. Pinprick pain was reported by Zuniga and Essick (1992) as an alternative method to temperature pain for level C testing.32 Temperature pain testing has the advantage of avoiding false positives that may arise from the pressure applied via a sharp

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probe on the lower lip overlying a fractured mandible. Post-treatment neurosensory testing was performed using the complete Zuniga-Essick protocol as described. From clinical use of this neurosensory testing protocol in nerve injuries from third molar surgery, it is known that early

testing was performed after 3 months from nerve injury.

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testing with this protocol may indicate a more severe level of sensory impairment than if the

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Results from both sides of the lower lip and chin, as well as the area and character of paresthesia were recorded in the study form. The result of neurosensory testing was categorized depending

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on whether the result was normal at Level A testing (i.e. “No sensory impairment”), abnormal at Level A (“Mild sensory impairment”), abnormal at Level B (“Moderate sensory impairment”) or abnormal at Level C (“Severe sensory impairment”).

Neurosensory testing results were

compiled according to the various time points (before surgery, after surgery at 1 week, 6 weeks, Post-treatment assessments continued until

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3 months, 6 months, 1 year) for analysis. normalization or up to 1 year.

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Study Outcomes

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The primary outcome of interest, IAN injury (before or after treatment) was defined as patientreported sensory disturbance in the lower lip and chin (mental nerve distribution) with a VAS score of less than 9/10. A secondary outcome, time to normalization of sensation was defined as the earliest time after treatment when NST = 0 combined with a VAS score of 9/10 and a nil area of numbness. An NST score of 1 or more was suggestive of some degree of nerve injury depending on the time after injury; an NST score of 3 or higher from 3 months post-injury would be an indicator of significant nerve injury, i.e. Sunderland IV or V degree nerve injury. If a

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participant withdrew consent before the end of the study, dropped out, or failed to attain normalization at 1 year, the observation was censored. For these cases, the time to normalization

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was defined as the number of months from the date of treatment to the date of the last visit.

Statistical Analysis

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For the primary objective, the prevalence of IAN injury after mandible fracture before and after treatment was estimated using the method of generalized estimating equations (GEE) which

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accounts for the longitudinal and clustered binary structure of the data.

Details and

implementation of the method have been described by Hanley et al.35 The prevalence of IAN injury was estimated separately for an IAN-bearing site, for a non-IAN bearing site and for the entire cohort.

To elucidate the factors associated with the incidence of IAN injury after

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treatment, the GEE method was performed with IAN injury as the response variable and fracture site (IAN bearing versus non-IAN bearing), comminution (incomplete and undisplaced and displaced versus comminuted), gap distance (in mm) and treatment (ORIF versus CRF) as

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independent variables. The logit link function was used. The association between the response and independent variables was quantified by adjusted odds ratios with corresponding 95%

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confidence intervals. Due to sparsity of jaw sides undergoing conservative treatment (n = 4, 3.3%), these were excluded from the analyses (Table 2). The variable ‘comminution’ was also dichotomized owing to sparsity of cases at the two extreme categories. To elucidate the factors associated with time to normalization (in months) from date of treatment with CRF or ORIF, Cox proportional hazards (PH) regression was performed. To account for dependence arising from bilateral fractures, the model variance was calculated using a jackknife approximation and

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the standard errors of estimated hazard ratios were obtained using the sandwich estimator. The set of independent variables was identical to the set used in the preceding GEE analysis. Adjusted hazard ratios were used to quantify the association between time to normalization and

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the independent variables with corresponding 95% confidence intervals. Finally to determine the time to normalization of sensation after treatment, Kaplan-Meier (KM) estimation was used. The time to normalization was obtained for the entire cohort and by subgroup according to the

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fracture site (IAN bearing versus non-IAN bearing). The cumulative incidence of normalization of sensation after treatment at 1 week, 6 weeks, 3 months, 6 months, and 12 months is reported.

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The jaw side was used as the unit of analysis. The level of significance was set at 5%. The SAS 9.2 software and R software were used to run the GEE (proc genmod) and time-to-event analyses (km, coxph packages) respectively.

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Results

Over a period of 5 years from April 2005 to March 2010, 197 patients with mandible fractures

Singapore.

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were managed by the Department of Oral & Maxillofacial Surgery, National Dental Centre of Of these, 155 patients were screened and 81 patients were enrolled with 124

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fractured mandible sides (see Figure 2). Information of 1 patient with 1 fracture side could not be obtained after enrollment, hence there were 80 patients and 123 sides that were available for analysis.

The sample was predominantly male (83.8%). The mean age was 30.0 years (SD 12.6 years), with median of 25 years and age range of 14 to 73 years. The ethnic distribution was roughly

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similar to local ethnic proportions: Chinese 38 (47.5%), Indian 20 (25.0%), Malay 17 (21.3%), Caucasian 3 (3.8%) and others 2 (2.5%). The majority were Singaporean citizens (78.8%) [see

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Table 2].

The etiologies for mandible fractures were: assault 33.8%, falls 31.3%, motor vehicle accidents (MVA) 25.0%, sports accidents 6.3%, industrial accidents 2.5%, military accidents 1.3%.

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Most patients were treated at one of two main hospitals: the Singapore General Hospital 50.0% or the Changi General Hospital 41.3%, with 8.8% treated at NDCS itself; most patients were

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followed at NDCS.

Of 123 mandible fracture sides, 43 were bilateral. The side of fracture was evenly distributed: left 50.4% and right 49.6%. About half (49.6%) of mandible fracture sides involved the IAN

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bearing body/angle/lower ramus of the mandible. Treatment rendered included open reduction and internal fixation (ORIF) 74.8%, closed reduction and fixation 22.0%, and conservative

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treatment 3.3% (see Table 2).

Regarding complications of treatment, the following were recorded: 1 patient (1 condylar

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fracture side) had facial (VII) nerve weakness after ORIF, 1 patient (bilateral condylar fractures) had an anterior open bite without intermaxillary fixation, and 1 patient (1 angle fracture with a contralateral symphyseal/parasymphyseal fracture) required re-plating 4 days after ORIF. Regarding exposure of the IAN in the operation site, the following were noted: 2 nerves in separate patients were noted to be “lacerated”; in 39 sides, the nerve was seen intact, of which in

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28 was recorded “nerve manipulated” intraoperatively; in 82 sides, no nerve was seen in the fracture site.

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The overall prevalence of IAN injury (sensory alteration) before and after mandible fracture treatment was 33.7% (95% CI: 24.8 – 42.6) and 53.8% (95% CI: 46.0 – 61.6), respectively. For a fracture in an IAN-bearing site, the prevalence of IAN injury before and after treatment was

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56.2% (95% CI: 43.2 – 69.2) and 72.9% (95% CI: 63.0 – 82.7), respectively. For a fracture in the non-IAN bearing mandible, the prevalence before and after treatment was 12.6% (95% CI:

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4.1 – 21.1) and 31.6% (95% CI: 20.0 – 43.3), respectively. Thus compared with fractures in a non-IAN bearing site, the prevalence of IAN injury in IAN-bearing sites is higher by 43.6% (95% CI: 29.1 – 58.2) before treatment and by 41.2% (95% CI: 24.8 – 57.7) after treatment.

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Of 82 sides that were of normal sensation before treatment, 36.6% developed IAN injury after treatment with CRF or ORIF. Fracture site and gap distance were found to be significantly associated with the development of IAN sensory disturbance after treatment. Particularly, the

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odds of IAN injury for a fracture located at an IAN-bearing site were 3.13 (95% CI: 1.06 – 9.28) times the odds when compared to a non-IAN bearing site. Furthermore, a 1 mm increment in the

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gap distance before treatment was associated with a 1.27 (95% CI: 1.04 – 1.55) increase in the odds of disturbance after treatment holding the values of the remaining variables constant. The adjusted effects of treatment and comminution were not statistically significant (see Table 3).

Time to normalization from the date of CRF or ORIF treatment was significantly associated with treatment and fracture site in a model that also included gap distance and comminution. A non-

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IAN bearing site was found to be favorable to normalization compared to an IAN-bearing site (HR = 2.15, [95% CI: 1.14 – 4.04]). Treatment with ORIF was found to be associated with

These results were consistent with expectation (see Table 4).

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inhibited normalization relative to treatment with CRF (HR = 0.40, [95% CI: 0.24 – 0.65]).

Within each category of fracture site, fractures treated with CRF generally took a shorter period

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to normalize compared to fractures treated with ORIF. For fractures in a non-IAN bearing site, 80% (95% CI: 56% - 91%) of sides normalized by 6 months after treatment. For fractures in

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IAN-bearing sites, 51% (95% CI: 29% - 66%) of sides normalized by 12 months. In the combined cohort, 50% (95% CI: 37% - 67%) of sides normalized sensation by 3 months, and only 63% (95% CI: 48% - 74%) of sides attained normalization of sensation (see Figure 3).

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Severe sensory impairment in the best NST result after 3 months to either 12 months or the last attendance before 12 months, was compiled as follows: for IAN-bearing fracture sides, there were 6 to 7 (including 1 drop-out before 12 months) of 61 sides (9.8% - 11.5%) with severe

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sensory impairment; for non-IAN bearing fracture sides, there were 1 to 4 (including 3 drop-outs

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before 12 months) of 46 sides (2.2% to 8.7%).

Discussion

Mandible fractures result in pain, bleeding, malocclusion with impairment of mastication and speech, often predispose to infection owing to communication of the fracture site with the oral cavity, and may sometimes result in noticeable facial disfigurement. The current methods of

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ORIF allow predictable and safe realignment and repositioning of fractured segments, and immobilization for optimum bone healing. Unless there had been significant tissue loss resulting from injury, e.g. in avulsive wounds, both oral function and facial esthetics can be predictably

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preserved or restored. The loss of sensory function of the IAN in mandible fractures after intervention has been reported previously, but IAN sensory disturbance before intervention and its long term outcomes have not been as closely studied. Loss of sensory function in the IAN,

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besides being bothersome to the individual, has been associated with functional disturbances such as being unaware of fluid drooling or food escape during eating, accidental lip biting, and

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interference with shaving, applying makeup and kissing.

The overall prevalence of patient-reported sensory disturbance after mandible fracture before intervention was 33.7% (95% CI: 24.8 – 42.6); in the IAN-bearing posterior mandible this was

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56.2% (95% CI: 43.2 – 69.2) compared to 12.6% (95% CI: 4.1 – 21.1) in the anterior mandible. The prevalence for posterior mandibles was similar to previously published data of pre-treatment sensory disturbance in posterior mandible fractures by Thurmuller et al. (46% to 58.5%),1 Iizuka

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and Lindqvist (50.7%),10 and Schultze-Mosgau et al. (46.2%).17 The overall prevalence of sensory disturbance after treatment of mandible fractures was 53.8% (95% CI: 46.0 – 61.6); that

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in the posterior mandible was 72.9% (63.0 – 82.7) and in the anterior mandible was 31.6% (20.0 – 43.3). The prevalence for posterior mandibles was similar or slightly lower to that reported by Thurmuller et al. (76.1% to 91.3%)1 and Schultze-Mosgau et al. (76.9%).17

Compared with

fractures in a non-IAN bearing site, the prevalence of sensory alteration (i.e. IAN injury) in IANbearing sites is higher by 43.6% (95% CI: 29.1 – 58.2) before treatment and by 41.2% (95% CI: 24.8 – 57.7) after treatment. These data confirm the clinical impression that fractures of the

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IAN-bearing posterior mandible are associated with a higher risk of preoperative and postoperative sensory alteration. In fact, among the incident cases (i.e. normal sensation before but developed injury after treatment,) the odds of IAN injury for a fracture in the IAN-bearing

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posterior mandible were three times [3.13 (95% CI: 1.06 – 9.28)] the odds compared to that in the non-IAN bearing site. The prevalence of preoperative sensory alteration in the non-IAN bearing anterior mandible may be related to the presence of post-traumatic swelling; the

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prevalence of postoperative sensory alteration in the anterior mandible after ORIF may be explained by plating in close proximity to the mandibular foramen with the inherent risk of

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manipulation or injury of the mental nerve.

Fracture site and gap distance were significantly associated with the development of postoperative IAN sensory disturbance. A 1-mm increment in the gap distance before treatment

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was associated with a 1.27 (95% CI: 1.04 – 1.55) increase in the odds of disturbance after treatment, i.e. a 1-mm increase in gap distance was associated with a 27% increased odds of IAN sensory disturbance. However, there are two pitfalls in correlating risk of IAN injury directly

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with fracture gap. The first is that the degree of IAN injury is logically correlated to the maximum separation of the ends of the posterior fracture segments that was sustained at the time

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of injury, rather than the fracture gap that is visualized radiographically; there is no way to accurately estimate this clinically.

The second is that imaging the mandibular canal is a

surrogate method of assessing the physical state of the IAN, and may not reflect this reliably. The number of cases where the exposed mental nerve or IAN was noted to be injured (“lacerated”) was very small (only 2 sides) and insufficient for analysis.

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The treatment modality (ORIF, CRF or conservative) may be expected to have an effect on postoperative IAN sensory disturbance, with ORIF being more likely to be in association, since it is more likely to be indicated in more displaced or complex fractures. ORIF allows for better

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visualization, control and fixation of the fracture in three dimensions, while CRF is more likely to be performed for minimally displaced mandible fractures (rarely for reasons of avoiding a general anesthetic or reducing treatment costs). Conservative treatment was applied where

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mandible fractures were incomplete or undisplaced and likely to remain stable. In this study, however, due to the very small number (n = 4) of jaw sides that underwent conservative

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treatment, its impact (relative to more invasive treatments) on the likelihood of post-operative IAN injury was not estimated from data. Comminuted fractures intuitively suggest higher force trauma with concurrent increased risk of IAN injury. However, the adjusted effects of treatment and comminution were not statistically significant (see Table 3) after accounting for the effects

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of fracture site and gap distance.

Nonetheless, fracture site and treatment were associated with the time to normalization of IAN

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sensation. A non-IAN bearing site was found to be favorable to normalization compared to an IAN bearing site (HR = 2.15 [95% CI: 1.14 – 4.04]). The time to normalization for IAN-bearing

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fractures were longer than non-IAN bearing fractures. ORIF was found to be associated with inhibited normalization relative to treatment with CRF (HR = 0.40 [95% CI: 0.24 – 0.65]). The time to normalization curves of CRF on an IAN-bearing site and ORIF on a non-IAN bearing site did not appear to differ greatly, while CRF on a non-IAN bearing site showed the fastest time to normalization of sensation. As expected, ORIF in an IAN-bearing site showed the longest time to normalization of sensation. The proportion of cases normalizing sensation after

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treatment in general was 32% at 1 week, 42% at 6 weeks, 50% at 3 months, 60% at 6 months and 63% at 12 months.

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In fractures of the IAN-bearing posterior mandible, correct realignment and repositioning of the fracture segments should result in realignment of the IAN in the mandibular canal. However, the process of manipulating the fracture segments may result in further inadvertent stretching or

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compression of the IAN, and drilling in preparation for internal fixation may result in partial or complete transection of the IAN. ORIF screws should preferably be directed away from the

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likely position of the mandibular canal, but this can be difficult in practice. Even if the screw is located short of the mandibular canal, there is still the possibility of the drill having penetrated deeper than the screw depth to encroach the mandibular canal. Nagadia et al. showed from CT imaging that the mean distance from the outer buccal cortex to the mandibular canal in the

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second molar region in Chinese mandibles was 6.97 mm, with a minimum of 4.8 mm;36 the use of 7 mm monocortical screws in most cases in our series may have contributed to the increased incidence of IAN sensory alteration postoperatively. Levine et al. reported using CT imaging

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that the distance of the buccal cortical margin to the buccal aspect of the mandibular canal was 4.9 mm.37 Al-Jandan et al. using CBCT also showed that the distance of the buccal outer cortex

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to the IAN was 4.6 mm at the first molar region and 5.75 mm at the second molar region.38 The average thickness of the buccal bone in the mandible also decreases from the molar region towards the anterior. Fernandes et al. reported that the thickness of buccal cortical bone around the mental foramen in dry dentate mandibles was greater at the level of the foramen than at a level 1.25 mm superior and below the foramen border; the mean buccal cortical bone thickness in these areas was less than 3 mm.39

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The current standard of care does not require routine post-ORIF 3 dimensional imaging, e.g. cone beam computed tomography (CBCT), but this may be helpful in showing if the ORIF

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screws were indeed a factor in cases with persistent sensory disturbance and moderate to severe sensory impairment on NST. Taking a CBCT after mandible fracture ORIF may be reasonable, particularly if there is sensory disturbance of the IAN that was absent before fixation, to

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determine if the fixation screws are impinging on the mandibular canal. If so, replacement of the impinging screw with a shorter screw may have to be considered, but has to be weighed against

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the considerations of repeat surgery in the light of the extent of the patient’s injuries and clinical condition.

The value of NST lies in its ability to assess the presence of a significant nerve injury

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(Sunderland IV to V degree injury) and hence support the option for nerve repair. The NST result of “severe sensory impairment” is not reliable in identifying a significant nerve injury before 3 months after injury, but a “mild” or “no sensory impairment” result even before 3

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months would logically exclude a significant nerve injury. Taking into account the drop-outs in this study, the proportion of fractures in the IAN-bearing mandible likely to involve a significant

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IAN injury ranged from 9.8 to 11.5% in this study.

The proportion of non-IAN bearing

mandible fractures likely to involve a significant IAN injury ranged from 2.2 to 8.7%. These may have been the result of either a stretch injury from tissue retraction or mechanical injury from internal fixation.

The potential demand for nerve repair after mandible fracture is

approximately 1 in 10 fractures in the IAN-bearing region.

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In instances where there is reasonable evidence of significant IAN injury in a mandible fracture (e.g. visually evident nerve injury during ORIF), several questions arise: firstly, should the nerve be repaired during ORIF of the mandible fracture? How should the nerve repair be undertaken,

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before, during or after fixation of the mandible fracture? It would be preferable to align and stabilize the fracture segments before undertaking nerve repair, but the aligned fracture segments are likely to block access to the nerve repair. If immediate nerve repair is not tenable, when The patient’s condition is the primary concern, for example, a

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should it be undertaken?

prolonged operation to repair the IAN in the context of significant wound contamination or

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polytrauma would be ill advised and should be postponed until the patient had recovered and the mandible fracture had healed sufficiently to permit the buccal corticortomy necessary to expose and release the injured nerve segments preparatory to repair, likely 3-6 months after ORIF. On the other hand, an otherwise healthy patient with a proven transection nerve injury, e.g. sharp

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transection of the mental nerve, with a mandible fracture, may benefit from immediate repair with concomitant mandible ORIF. The decision whether to fix the fracture or repair the nerve first will depend on the clinical situation and the availability of microsurgical expertise and

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resources. The difficulty is that IAN injuries in mandible fractures are unlikely to be single narrow site injuries but rather extended site or even multiple site nerve injuries, which may

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require nerve grafting. Another consideration is the risk of weakening the healing mandible by decortication of the mandible body to reach the IAN bundle for repair. Given the relatively low probability of a significant IAN injury with mandible fractures (approximately 1 in 10) and the very narrow conditions required, it appears that the indication for immediate nerve repair with ORIF of a mandible fracture is likely to be uncommon.

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Bagheri et al.40 presented 33 patients with IAN injury caused by mandible fractures (21 mandible angle, 12 mandible parasymphysis) along with 9 other infraobital, lingual and long buccal nerve injuries. The findings, surgical procedures and outcomes were reported without distinguishing

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those involving IAN injuries from other nerve injuries. The most common mechanism of nerve injury was compression (19), followed by partial transection (9). The most common surgical procedure was external decompression / internal neurolysis (20), followed by autogenous nerve

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grafting for a discontinuity defect (10). The average time from nerve injury to repair was 12.5 months (range 2-24). After at least 1 year follow-up with NST, 86% had regained “useful

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sensory function”, of which 13% had full recovery using the Medical Research Council scale. The remaining 14% showed inadequate or no recovery.

Bagheri et al. recommended pre-

treatment NST followed by simultaneous nerve repair during ORIF if there was significant NSD and microsurgical capability was available; otherwise serial NST should be performed over 3

NSD.

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months after fracture treatment and the patient referred to a microsurgeon if there was significant The authors recommended that if ORIF was performed, the nerve canal should be

enlarged to compensate for post-injury osseous proliferation, and to avoid excessive

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manipulation of the fracture segments and placing internal fixation screws near or at the nerve canal. The difficulty with enlarging the nerve canal is the restricted access in the posterior

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mandible and the risk of further nerve injury from a rotary instrument. Where delayed nerve repair is indicated, the authors recommended a delay of 10 weeks after treatment of a mandible fracture.

A limitation of this study is that patients with mandible fractures who were unconscious and unable to undergo NST pre-operatively, would have been excluded from the study. Such cases

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include head trauma patients who may also concomitantly have more severe mandible fractures, likely with significant IAN injuries; the true incidence of IAN injury in all mandible fractures may therefore be higher than shown in this study. There was also no postoperative 3-D imaging

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to determine if the plating screws were impinging on the IAN, as this was not the standard of care. Evaluation of the effect of plate fixation on IAN injury was not an objective in this study.

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Conclusion

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IAN sensory alteration after mandible fracture was more than 4 times more likely to present in fractures of the IAN-bearing posterior mandible (56.2%) than in fractures of the non-IAN bearing anterior mandible (12.6%). The (12-month period) prevalence of IAN sensory alteration after treatment was also higher in the posterior mandible (72.9%) compared to the anterior Factors associated with the prevalence of postoperative IAN sensory

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mandible (31.6%).

alteration included fracture site and fracture gap distance; a 1-mm increase in the gap distance was associated with a 27% increase in the odds of sensory alteration after treatment. Factors

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influencing the time to normalization of IAN sensation included fracture site and treatment, with ORIF inhibiting normalization relative to CRF. The majority of mandible fractures did not

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appear to be associated with significant IAN injury by NST.

Acknowledgements

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This study (04/03-04) was funded by the National Medical Research Council Enabling Grant, received through the National Dental Centre Research Fund. We also gratefully acknowledge

Surgery, National Dental Centre Singapore, towards this study.

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the contribution and support of all the surgeons in the Department of Oral & Maxillofacial

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An oral abstract of preliminary results of this study was presented at the 89th American Association of Oral & Maxillofacial Surgeons (AAOMS) Annual Meeting in October 2007 in Honolulu. lt was published in the JOMS September 2007 issue.

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A poster abstract of interim results of this study was presented at the 20th lnternational Conference of Oral & Maxillofacial Surgery |COMS) in November 2011 in Chile. lt was published in the IJOMS.October 2011 issue.

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Bagheri SC, Meyer RA, Khan HA, Steed MB.

Microsurgical repair of peripheral

Method

Test

Static 2-point discrimination

Use soft brush to stroke test area 15 times; note the number of times the patient gives correct direction of stroke. Less than 90% correct responses gives an abnormal result. Use modified sliding gauge to determine minimum distance that 2 points of contact is perceived; test in descending order twice for each interval; perform 5 trials, obtain mean of last 4 trials as result. Inability to feel 2 points at interval distance of 12 mm or more gives an abnormal result.

Use Semmes-Weinstein fibres; touch until fiber bends, hold for 1s then remove; test in ascending order until sensation for 2 consecutive fibers, record first positive ascending fiber; then in descending order to until sensation for 2 consecutive fibers, record the first negative descending fiber. The mean of the ascending and descending value gives the result. More than 2.83 gives an abnormal result for the lower lip.

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Level B Contact detection

Stroke both sides; ask patient to quantify how much sensation is present on the test side compared to the normal side (10 = normal, 0 = anaesthesia)

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Level A Brush stroke direction

(Patient’s eyes must be closed for all tests)

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Sensory analog scale

Neurosensory Testing Instructions.

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Table 1.

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trigeminal nerve injuries from maxillofacial trauma. J Oral Maxillofac Surg 67:1791, 2009.

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40.

Level C Pinprick pain

Thermal pain

Use a sharp probe to press to a depth of 4 mm, twice in each of 3 regions, one on the vermilion, one on the labiomental fold and one on the chin. Absence of pain in more than 1 out of 6 times is an abnormal result. Hold the probe of a Peltier-type thermode (runs the temperature at the probe tip from 35oC to 50oC at the rate of increase of 0.1oC per second) on the test area. Patient to indicate when pain is felt (threshold value) and when pain is unbearable (tolerance value). More than 47oC for threshold and 50oC for tolerance gives an abnormal result for the lower lip.

Table 2.

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Socio-demographic characteristics of subjects and clinical characteristics of mandible

Gender Race

No. of Sides

Side

Mean (±SD) Median (Range) Male Female Chinese Indian Malay Caucasian Others Unilateral Bilateral Left Right

Subjects (n = 80) 30.0 (±12.6) Nationality 25 (14, 73) 67 (83.8%) 13 (16.3%) 38 (47.5%) 20 (25.0%) Etiology 17 (21.3%) 3 (3.8%) 2 (2.5%) 37 (46.3%) 43 (53.8%) Mandible Sides (n = 123) 62 (50.4%) Treatment 61 (49.6%)

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Age

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fractures at presentation

Singaporean Malaysian Indian Bangladeshi Others Assault Fall Motor vehicle accident Sports accident Industrial accident Military accident

63 (78.8%) 6 (7.5%) 2 (2.5%) 2 (2.5%) 7 (8.8%) 27 (33.8%) 25 (31.3%) 20 (25.0%) 5 (6.3%) 2 (2.5%) 1 (1.3%)

ORIF CRF Conservative treatment

92 (74.8%) 27 (22.0%) 4 (3.3%)

N (%)

Gap (mm) Mean (±SD)

Incomplete

37 (30.1) 9 (7.3) 29 (23.6) 32 (26.0) 0 (0.0) 16 (13.0) 123 (100.0)

2.47 (±2.22) 2.89 (±2.62) 2.77 (±1.83) 4.47 (±4.66) 2.31 (±3.30) 3.07 (±3.20)

2 (5.5) 1 (11.1) 1 (3.4) 1 (3.1) 0 (0.0) 5 (4.1)

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Site Symphyseal / Parasymphyseal (A) Anterior Body (B) Posterior Body (C) Angle (D) Ramus (E, F) Condylar (G) Total

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Fracture characteristics (n = 123) Fracture Type, N (%) Undisplaced Displaced

7 (18.9) 2 (22.2) 5 (17.2) 6 (18.8) 5 (31.3) 25 (20.3)

22 (59.5) 5 (55.6) 19 (65.5) 24 (75) 11 (68.8) 81 (65.9)

Comminuted

6 (16.2) 1 (11.1) 4 (13.8) 1 (3.1) 0 (0.0) 12 (9.8)

Table 3.

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Factors associated with the incidence of IAN sensory disturbance after treatment

Estimate Treatment ORIF VS CRF IAN-bearing site Yes VS No Fracture Comminution Displaced/Comminuted VS Incomplete/Undisplaced

1.14 0.57 -0.17 0.24

p-value

Adjusted OR

95% CI

0.113

9.73

0.58 – 162.64

0.040

3.13

1.06 – 9.28

0.879

0.71

0.01 – 61.48

0.018

1.27

1.04 – 1.55

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Gap (mm)

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(N=82)

Table 4.

Treatment ORIF VS CRF IAN-bearing site No VS Yes Fracture Comminution Displaced/Comminuted VS Incomplete/Undisplaced Gap (mm)

Factors associated with time to normalization (N=112)

Estimate

p-value

Adjusted HR

95% CI

-0.92

0.0002

0.40

0.24-0.65

0.76

0.018

2.15

1.14-4.04

-0.38

0.320

0.68

0.32-1.45

-0.08

0.146

0.93

0.84-1.03

List of Figures

Classification of Mandible Fractures

Figure 2.

Neurosensory Testing

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Figure 1.

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Level A: Brush Stroke

(b)

Level A: 2-Point Discrimination

(c)

Level B: Contact Detection

(d)

Level C: Pinprick Pain

(e)

Level C: Thermode

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(a)

Participant Flow Chart

Figure 4.

Kaplan-Meier estimates of time to normalization by treatment and fracture site

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Figure 3.

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Classification of Mandible Fractures

A B

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Figure 1.

G

C

F

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E

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D

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Figure 3.

Participant Flow Chart

Note on Participant Flow Chart: The sum of the number of patients (Np) in each treatment

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category appears to exceed the total number of patients because in some patients with bilateral fractures, each side was treated with different modalities, such that these patients were treated with up to 2 distinct modalities. The number of sides (Ns) gives the true

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denominator for calculations. At each time point, the number of patients or sides with

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abnormal NST forms a subset within the number of patients or sides with NSD.

ACCEPTED MANUSCRIPT Screen Failures: 74 patients

Total patients screened: 155 patients

Baseline nerve injury evaluation and NST before treatment: Np = 81; Ns = 124 Treated for mandible fracture: N = 80; Ns = 123

48 8 4 2 2 1 1 4

3 1

11 patient information patientnono

CRF: Np = 21; Ns = 27

Conserv: Np = 4; Ns = 4

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ORIF: Np = 66; Ns = 92

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Total patients eligible and consented: Np (No. of patients) = 81; Ns (No. of sides) = 124

Declined participation Injury to lip/chin Inadequate consciousness Psychiatric history Fracture site infected Previous fracture Patient intubated, sedated Unable to test before surg Case missed fr inclusion Unable to get consent

1week post treatment NSD: Np= 2 ; Ns = 2 NST: Np= 2; Ns = 2 Normal: Np= 18 ; Ns = 24 Default: Np= 0 ; Ns = 0 Lost to FU: Np= 1 ; Ns = 1

1week post treatment NSD: Np= 0 ; Ns = 0 NST: Np= 0 ; Ns = 0 Normal: Np= 2 ; Ns = 2 Default: Np= 0 ; Ns = 0 Lost to FU: Np= 2 ; Ns = 2

6 weeks post treatment NSD: Np= 31 ; Ns = 35 NST: Np= 11 ; Ns = 13 Normal: Np= 33 ; Ns = 37 Default: Np= 7 ; Ns = 8 Lost to FU: Np= 10 ; Ns = 12

6 weeks post treatment NSD: Np= 2 ; Ns = 2 NST: Np= 1 ; Ns = 1 Normal: Np= 18 ; Ns = 24 Default: Np= 0 ; Ns = 0 Lost to FU: Np= 1 ; Ns = 1

6 weeks post treatment NSD: Np= 0 ; Ns = 0 NST: Np= 0 ; Ns = 0 Normal: Np= 2 ; Ns = 2 Default: Np= 0 ; Ns = 0 Lost to FU: Np= 2 ; Ns = 2

3 months post treatment NSD: Np= 18 ; Ns = 20 NST: Np= 6 ; Ns = 6 Normal: Np= 36 ; Ns = 42 Default: Np= 8 ; Ns = 10 Lost to FU: Np= 17 ; Ns = 20

3 months post treatment NSD: Np= 2 ; Ns = 2 NST: Np= 0 ; Ns = 0 Normal: Np= 18 ; Ns = 24 Default: Np= 0 ; Ns = 0 Lost to FU: Np= 1 ; Ns = 1

3 months post treatment NSD: Np= 0 ; Ns = 0 NST: Np= 0 ; Ns = 0 Normal: Np= 2 ; Ns = 2 Default: Np= 0 ; Ns = 0 Lost to FU: Np= 2 ; Ns = 2

6 months post treatment NSD: Np= 16 ; Ns = 20 NST: Np= 7 ; Ns = 8 Normal: Np= 40 ; Ns = 47 Default: Np= 2 ; Ns = 2 Lost to FU: Np= 20 ; Ns = 23

6 months post treatment NSD: Np= 2 ; Ns = 2 NST: Np= 0 ; Ns = 0 Normal: Np= 18 ; Ns = 24 Default: Np= 0 ; Ns = 0 Lost to FU: Np= 1 ; Ns = 1

6 months post treatment NSD: Np= 0 ; Ns = 0 NST: Np= 0 ; Ns = 0 Normal: Np= 2 ; Ns = 2 Default: Np= 0 ; Ns = 0 Lost to FU: Np= 2 ; Ns = 2

12 months post treatment NSD: Np= 9 ; Ns = 10 NST: Np= 4 ; Ns = 4 Normal: Np= 42 ; Ns = 49 Default: Np= 0 ; Ns = 0 Lost to FU: Np= 28 ; Ns = 33

12 months post treatment NSD: Np= 0 ; Ns = 0 NST: Np= 0 ; Ns = 0 Normal: Np= 18; Ns = 24 Default: Np= 0 ; Ns = 0 Lost to FU: Np= 3 ; Ns = 3

12 months post treatment NSD: Np= 0 ; Ns = 0 NST: Np= 0 ; Ns = 0 Normal: Np= 2 ; Ns = 2 Default: Np= 0 ; Ns = 0 Lost to FU: Np= 2 ; Ns = 2

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1week post treatment NSD: Np= 48 ; Ns = 57 NST: Np= 32 ; Ns = 40 Normal: Np= 30 ; Ns = 31 Default: Np= 1 ; Ns = 1 Lost to FU: Np= 3 ; Ns = 3

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Figure 4.

Kaplan-Meier estimates of time to normalization

Fracture site

Time after treatment (CRF or ORIF)

KM1 Cumulative incidence of normalization

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1 week 6 weeks 3 months 6 months 12 months IAN-bearing 1 week 6 weeks 3 months 6 months 12 months Combined IAN1 week and Non IAN6 weeks bearing sites 3 months 6 months 12 months 1 KM = Kaplan-Meier; 2 Not estimable from the data

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Non IAN-bearing

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by treatment and fracture site

0.46 0.62 0.69 0.80 ----2 0.22 0.27 0.35 0.45 0.51 0.32 0.42 0.50 0.60 0.63

95% CI 0.27 – 0.60 0.41 – 0.75 0.48 – 0.82 0.56 – 0.91 ----2 0.10 – 0.33 0.13 – 0.38 0.19 – 0.48 0.27 – 0.59 0.29 – 0.66 0.22 – 0.42 0.30 – 0.51 0.37 – 0.60 0.46 – 0.70 0.48 – 0.74