Extratemporal facial nerve anatomical variations in cadavers at kenyatta national hospital mortuary
BACKGROUND: The facial nerve (FN) is the seventh cranial nerve (CN). It is a mixed nerve with motor supply to the facial muscles being most crucial. It exhibits diversity in its course, dimensions and anatomic relations especially in the extratemporal part. An intimate knowledge of its anatomy is critical to avoid its inadvertent injury during rhytidectomy, parotidectomy, maxillofacial trauma surgery and ideally in any surgery of the head and neck region. METHODOLOGY: Dissection of fresh cadavers in Kenyatta National hospital mortuary during post mortem examination. STUDY OBJECTIVE: To establish the anatomic relationships and variability of the extratemporal FN trunk and its branches with emphasis on the intraparotid connections between the divisions and relations to various surgical landmarks. STUDY DESIGN: This was a descriptive cross-sectional study design using quantitative techniques of data collection on cadavers. The data includes morphometry of the FN as well as the various patterns of its distribution. STUDY AREA AND POPULATION: The study was conducted at the Kenyatta National Hospital (KNH) mortuary. The study population included cadavers that were presented for post mortem examination. A special chart was used to collect data. DATA ANALYSIS AND PRESENTATION: Data were coded and analyzed using the SPSS version 18.0 software. Descriptive analysis was done and presented using frequency diagrams, tables and graphs. Statistical tests included the Mann Whitney U, Wilcoxon signed rank, Spearman and Pearson coefficient frequency tests. The results were presented in the form of tables and figures. RESULTS: Twenty fresh cadavers were dissected left and right sides among which 12(60%) were males while 8(40%) were females (40FNs). The frequency of the various branching patterns using the Davis et al.1956 classification was as follows: types I 10(25%), II-9(22.5%), III- 7(17.5%), IV 6 (15%), V 2 (5%) and IV 6 (15%). The FN was noted to bifurcated in 32 (80%) and trifurcated in 8(20%) cases. However there was no significant difference in the branching patterns (p=0.509) and furcation types (p=0.414) between the right and left sides and between the genders. Regarding the morphometric data of the FN, the length of the FN was 16.14mm (+/- 3.28),the distance from the FN trunk to the tragar pointer (TP) was 9.87mm(SD+/-2.41), tympanomastoid suture( TMS) 5.81mm(+/- 1.28), external auditory meatus ( EAM) 15.64mm(+/- 2.74), posterior belly of the digastrics muscle ( PBDM) 8.09mm(+/-1.78), styloid process 16.48mm(+/- 5.47), temporomandibular joint(TMJ) 22.55mm(+/-1.99) and angle of the mandible 37.98(+/- 4.45). The styloid process was missing in 9 (22.9%) of the hemifacial dissections. The Mann Whitney U test did not elicit a statistically significant difference of the right side length of the trunk between genders (p=.238) and also the independent t test of means of the landmarks did not show any significant difference between the male and female cases. There was a positive correlation (Pearson’s correlation test) between the right side and the left side branching patterns (p=.002), length of the FN trunk(p=.000), TP(p=.003), TMS(p=.000), EAM(p=.000), PBDM(p=.003), the styloid process(p=.004) and angle of the mandible(p=.001) which was significant. CONCLUSION: The current study establishes variations of anatomical patterns of the extracranial FN in a Kenyan population. It shows that type I (Davis et al. 1956 classification) branching pattern as the commonest. The TMS and PBDM were the most accurate landmarks in FN trunk identification. RECOMMENDATIONS: The study strongly shows that the TMS and PBDM can be used as landmarks for the identification of the FN during surgery.