Patients and methods

In order to explore the factors that play a role in the incidence of facial nerve injury following TMJ surgery, this study recruited patients who had been diagnosed with a TMJ disorder that mandated a surgical intervention. This prospective study included 46 consecutive patients (32 females and 14 males) who underwent surgery for the treatment of TMJ disorders between the years 2000 and 2007. All study patients were recruited from the outpatient clinics of the oral and maxillofacial surgery departments of Al-Azhar University hospitals in Cairo, Egypt, between the years 2000 and 2007. The institutional ethics committee of the host institution reviewed and approved the study. After obtaining written patient consent, appropriate surgical procedures for the treatment of TMJ disorders were performed following careful clinical and radiographic evaluation to determine the nature of the disorder. The same surgical team performed all of the operations.

The reference surgery for this study was the surgical procedure undertaken for the treatment of the TMJ following the patient’s recruitment into the study. In order to be included in the study, the reference surgery needed to be either the patient’s first surgery to treat a TMJ disorder, or if she/he had undergone prior surgery, she/he was required to have no residual facial nerve dysfunction or injury following the prior procedure(s). Patients who had made an incomplete recovery from a facial nerve injury following previous TMJ or other maxillofacial surgeries and patients who presented with facial nerve palsy were excluded. Patients with a history of previous surgical treatment of the TMJ were screened preoperatively by electromyography (EMG) for motor unit action potentials (MUAPs) of the frontalis and orbicularis oculi muscles and underwent a facial nerve latency test (FNLT) to determine whether there was complete recovery of the facial nerve function. Other parameters of this study that were examined for correlation with the incidence and degree of facial nerve injury following the reference surgery were the type of surgery, surgical approach, and duration of surgery.

The House–Brackmann facial nerve injury grading system was used to grade facial nerve recovery following the reference surgery ( Table 1 ). The system involves a six-point scale, with grade I being normal function and grade VI being total paralysis.

Electrodiagnostic studies were performed using EMG or MUAP of the frontalis and orbicularis oculi muscles and FNLT. With regard to MUAP, all patients were classified into one of three groups in accordance with Danielides et al. : (1) group A were those in whom the muscle response was 51–95% of the response of the healthy muscle; (2) group B were those in whom the muscle response was 25–50% of the healthy muscle; (3) group C were those in whom the muscle response was <24% of the healthy muscle. For FNLT, all patients was classified into one of three groups in accordance with Danielides et al. : (1) group 1 were those in whom latency varied from 2.4 to 4 ms; (2) group 2 were those in whom latency varied from 4 to 6 ms; (3) group 3 were those in whom there was an absence of action potential (no response).

Postoperative evaluation of facial nerve function was performed at 24 h, 1 week, 1 month, 3 months, and 6 months. The reference values for ‘normal function’ were established for both MUAPs and FNLTs in each patient. In the case of patients undergoing unilateral joint surgery, the other ‘non-operated’ side of the face was used for reference values. In the case of patients undergoing bilateral joint surgery, preoperative MUAPs and FNLTs were collected and served as the reference values.

All statistical analyses for this study were performed using PASW Statistics version 18.0 software (SPSS Inc., Chicago, IL, USA). Bivariate tests of association and correlation between categorical variables (i.e., FNLT and MUAP outcomes, prior surgery, surgical approach, type of surgery, and facial nerve injury) were performed using Pearson correlation χ ² goodness-of-fit tests. The χ ² goodness-of-fit tests were performed to determine which categorical variables correlated with facial nerve injury following the reference surgery. Since the dataset contained a variety of ‘types of surgery’, an omnibus χ ² was not valid due to the small expected frequencies for many cells. Thus subsequent χ ² goodness-of-fit analyses tested each particular type of surgery compared to all others in order to determine if any particular type made a difference in the likelihood of facial nerve injury, using Yates’ continuity correction for instances that still had cells with small expected frequencies. Additionally, the Bonferroni corrected P -value used for these individual surgery types vs. other surgeries was employed to correct for multiple comparisons. Thus for these individual comparisons, the critical P -value was 0.008 (i.e., 0.05/6 comparisons). Bivariate tests for differences in the duration of surgery were performed using independent samples t -tests and/or one-way analysis of variance (ANOVA). Tukey’s post hoc test was used for pair-wise comparisons when the ANOVA test was significant. In order to determine if there was a difference in surgical duration for surgeries that resulted in a facial nerve injury and those that did not, an independent samples t -test was performed.

Results

The sample originally included 62 patients who provided consent to participate in this research study and who ultimately underwent surgery for the treatment of a TMJ disorder between the years 2000 and 2007. Of these 62 patients, seven did not meet the study inclusion criteria and nine were lost to follow-up. Thus 46 patients remained in the sample; 32 were females and 14 were males, and they ranged in age from 22 to 57 years (mean age 29 years). For these 46 patients, 66 TMJs were surgically treated during the study period.

Table 2 displays the frequency distributions of the various categorical measures taken in this study. Surgical procedures consisted of TMJ reconstruction (12%, n = 8), gap arthroplasty (26%, n = 17), eminectomy (42%, n = 28), high condylar shave (9%, n = 6), meniscectomy (3%, n = 2), and disc plication and discoplasty (8%, n = 5). The surgical approaches used were either via modified endaural incision ( n = 33) or modified pre-auricular incision ( n = 33). Twelve of the patients, accounting for 20 joints (30%), had undergone prior surgical interventions, but exhibited no residual facial nerve dysfunction prior to the reference surgery used in this study. A preoperative EMG study for MUAPs of the frontalis and orbicularis oculi muscles and FNLT confirmed that there was complete recovery of the facial nerve function of this particular group of patients before the reference surgery. The duration of surgery ranged from 70 to 200 min, with a mean of 128.6 min (standard deviation 43.1 min).

Following the reference surgery, the incidence of initial facial nerve injury was found to be 71% ( n = 47). Table 3 displays the outcomes of the χ ² analyses that tested each particular type of surgery compared to all others in order to determine if any particular type made a difference in the likelihood of facial nerve injury. These analyses revealed that the incidence of facial nerve injury was higher for joints that underwent a modified pre-auricular approach (87.9%) than joints that underwent a modified endaural approach (54.5%; P = 0.003). Joints with prior surgeries also had a higher incidence of facial nerve injuries (100%) than joints with no prior surgeries (58.7%; P = 0.001). Among the various types of surgery, joints that underwent gap arthroplasty, TMJ reconstruction, and meniscectomy had a higher incidence of facial nerve injury (100%) than other procedures (61.2%; P = 0.006).

While the likelihood of an injury was higher when using a modified pre-auricular approach ( Table 3 ), there was also interest in whether or not the different approaches resulted in different grades of facial nerve function at different points in time. χ ² tests were used to determine if the House–Brackmann facial nerve grade differed between the surgical approaches at 1 week, 1 month, 3 months, and 6 months. Table 4 shows the results of these comparisons for the different time periods. The surgical approaches differed in grade of injury/function at 1 week ( P = 0.005), but not at 1 month ( P = 0.721) or 3 months ( P = 0.080), while at 6 months all joints in both groups were grade I functioning. At 1 week, joints that underwent the modified endaural approach were more likely to be back to normal functioning, while those that underwent the modified pre-auricular approach were more likely to have grade II, III, and IV injuries. There were no differences between the surgical approaches for the grade of injury at 1 month or 3 months, but there was a general trend over time for both groups to regain normal functioning.

Table 4

Comparisons of injury grade and FNLT and MUAP results at different time points according to the surgical approach; χ ² goodness-of-fit.

Time	Grade a	Modified endaural		Modified pre-auricular		P -value
		n	%	n	%
1 Week	I	15	45.5	4	12.1	0.005 *
	II	1	3.0	8	24.2
	III	7	21.2	6	18.2
	IV	10	30.3	15	45.5
1 Month	I	16	48.5	12	36.4	0.721
	II	1	3.0	2	6.1
	III	7	21.2	7	21.2
	IV	9	27.3	12	36.4
3 Months	I	24	72.7	14	42.4	0.080
	II	2	6.1	7	21.2
	III	5	15.2	8	24.2
	IV	2	6.1	4	12.1
6 Months	I	33	100.0	33	100.0	–
	II	0	0.0	0	0.0
	III	0	0.0	0	0.0
	IV	0	0.0	0	0.0
1 Week	FNLT 1	19	57.6	15	45.5	0.325
	FNLT 2	14	42.4	18	54.5
1 Month	FNLT 1	22	66.7	18	54.5	0.314
	FNLT 2	11	33.3	15	45.5
3 Months	FNLT 1	28	84.8	25	75.8	0.353
	FNLT 2	5	15.2	8	24.2
6 Months	FNLT 1	33	100.0	33	100.0	–
	FNLT 2	0	0.0	0	0.0
1 Week	MUAP A	16	48.5	12	36.4	0.319
	MUAP B	17	51.5	21	63.6
1 Month	MUAP A	21	63.6	16	48.5	0.215
	MUAP B	12	36.4	17	51.5
3 Months b	MUAP A	33	100.0	27	81.8	0.032 *
	MUAP B	0	0.0	6	18.2
6 Months	MUAP A	33	100.0	33	100.0	–
	MUAP B	0	0.0	0	0.0

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