INTRODUCTION
The infraorbital nerve is the terminal branch of the maxillary division of the trigeminal nerve. Exiting the skull base through the foramen rotundum, the maxillary nerve enters the orbit via the inferior orbital fissure. It then follows the infraorbital groove on the orbital floor and emerges through the infraorbital foramen (IOF) into the soft tissues of the face. In the facial region, the infraorbital nerve branches into the inferior palpebral, nasal, and superior labial branches [1].
Extensive research has been conducted on the cutaneous innervation pattern of the infraorbital nerve [2–4]. The inferior palpebral branch supplies sensation to the skin of the lower eyelid, the upper cheek, and the conjunctiva. The nasal branch innervates the lateral aspect of the nose skin and the mobile portion of the nasal septum. The superior labial branch provides sensory input to the skin of the cheek and upper lip, as well as the corresponding region of the oral mucosa.
Fractures of the maxilla account for approximately 15% of all facial bone fractures. Common symptoms accompanying maxillary fractures include facial swelling, altered facial contour due to depression, malocclusion, and infraorbital nerve dysfunction, with the latter being reported in 24% to 84% of cases [5,6]. Sensory dysfunction of the infraorbital nerve can be attributed to several factors, such as avulsion, dislocation, or distraction of the nerve, as well as compression resulting from soft tissue edema, hematoma, or the displacement of fractured bone segments [7,8].
Studies have been conducted on infraorbital nerve injury and sensory recovery after maxillary fractures, with a focus on evaluation points, methods, and the duration of symptoms [9,10]. However, there is a lack of research specifically examining infraorbital nerve injury and recovery in relation to the type of maxillary fracture. In this study, we explored the impact of IOF involvement in maxillary fractures on the recovery of infraorbital nerve function.
METHODS
Patient selection and exclusion criteria
From 2020 to 2022, a prospective study was conducted on patients who underwent surgical treatment for maxillary fractures in the plastic surgery department. Exclusion criteria included patients with bilateral maxillary fractures, those with concurrent fractures such as orbital wall blowout fractures or nasal bone fractures, concomitant soft tissue injuries including abrasions, lacerations, or avulsions in the facial area, a history of previous facial surgery, loss of 1-year follow-up, and individuals receiving treatment for neuropathy. Patient information was collected, including age, height, weight, sex, time from injury to surgery, the presence of hypertension or diabetes mellitus, and smoking status. Maxillary fractures were categorized into three types based on the location of the fracture line as seen on the patients’ facial computed tomography (CT) scans. Type 1 fractures were identified by fracture lines that did not intersect the IOF. Type 2 fractures had fracture lines that intersected the IOF. Type 3 fractures were characterized by fracture lines that intersected the IOF, along with additional fractures that did not intersect the IOF (Fig. 1).
Surgical method and postoperative care
All patients underwent open reduction and internal fixation (ORIF) under general anesthesia. Access to the fracture site was achieved via an upper gingivobuccal sulcus incision. Additional subciliary or lateral eyebrow incisions were made when necessary. ORIF was accomplished utilizing either absorbable or titanium plates. To prevent the accumulation of blood or fluid, a silicone Barovac drain (Sewoon Medical Co., Ltd.) was placed through the upper gingivobuccal sulcus. The Barovac drain was typically removed by postoperative day 3.
Evaluation of infraorbital nerve injury
For sensory evaluation, both the two-point discrimination test and monofilament test were conducted. The two-point discrimination test assesses the smallest distance at which an individual can perceive two distinct points of contact. This is done by gradually increasing the distance between two points on a caliper by 1 mm increments. We performed this test with the Lafayette Aesthesiometer (Lafayette Instrument Co.). The monofilament test evaluates the smallest diameter of a monofilament that a person can feel when it is applied to the skin with enough pressure to bend it slightly. This test was conducted using the Baseline Tactile Monofilament Evaluator complete set.
Evaluation points were established at four locations within the sensory distribution area of the infraorbital nerve, with measurements recorded on both the affected and unaffected sides. P1 (lower eyelid) was measured 1 cm below the center of the lower eyelid margin, P2 (nose) at the midpoint of the nasofacial sulcus, P3 (upper lip) 1 cm above the oral commissure, and P4 (zygoma) 3 cm below the lateral canthus (Fig. 2).
Sensory evaluations were conducted preoperatively and at postoperative intervals of 1 week, 1 month, 3 months, 6 months, and 1 year. Two plastic surgeons independently performed the sensory evaluations, and the mean of their assessments was recorded. The examiners were blinded to the fracture classification when performing the assessments to minimize potential bias. At each evaluation point, measurements were taken after one-minute interval. This process was repeated until two consistent measurements were obtained, which were then recorded as the final result.
The mean values of measurements taken from four points on the side with the maxillary fracture were compared with the mean values from four corresponding points on the unaffected side. A larger discrepancy between these two mean values was interpreted as indicative of more severe infraorbital nerve dysfunction at the site of the maxillary fracture. Furthermore, a reduction in the difference between the two mean values over time was considered evidence of improved infraorbital nerve function recovery.
Statistical analysis
Continuous variables are presented as means with standard deviations, while categorical variables are expressed in terms of frequencies and percentages. To compare two groups, we employed the independent sample t-test and the Wilcoxon rank-sum test. For analyses involving three groups, one-way analysis of variance and the Kruskal-Wallis rank test were utilized. The chi-square test and Fisher exact test were used for categorical variables. A p-value less than 0.05 was considered to indicate statistical significance. All statistical analyses were conducted using SPSS version 27.0 (IBM Corp.).
RESULTS
Patient demographics
A total of 267 patients were treated for maxillary fractures, and from this cohort, 60 patients were selected for a prospective study. These 60 patients were categorized into three groups based on the type of fracture, and their demographic data were subsequently analyzed (Table 1). The study population comprised 54 men and six women, with mean age of 36.85±14.85 years. The mean height was 170.18±6.92 cm, and the mean weight was 66.00±10.49 kg. The mean time from injury to surgery was 12.16±5.84 days. There was one patient with diabetes and 28 smokers. The type 1 group consisted of 19 individuals, the type 2 group comprised 18 individuals, and the type 3 group included 23 individuals. There were no significant differences between the groups.
Two-point discrimination test
After surgery, statistically significant differences were observed between the type 3 and type 1 groups at 1 month (p=0.023), 3 months (p<0.001), 6 months (p=0.001) postoperatively and throughout the entire study period (p<0.001). Additionally, at 1 year post-surgery, the type 3 group showed statistically significant differences when compared to both the type 1 and type 2 groups (p=0.015) (Table 2, Fig. 3). Examination values were compared between two groups based on whether the maxillary fracture involved the IOF: the “no IOF involvement” group (type 1) and the “IOF involvement” (types 2 and 3). Significant differences were observed between these groups at 1 month (p=0.026), 3 months (p=0.002), 6 months (p=0.002) and 1 year (p=0.049) post-surgery, as well as over the entire follow-up period (p=0.004) (Table 3, Fig. 4).
Monofilament test
Three months after surgery, the type 3 group exhibited a significantly larger difference in mean measurements compared to the type 1 group (p=0.038). Furthermore, throughout the entire duration, the type 3 group demonstrated a significantly greater difference in mean measurements compared to both the type 1 and type 2 groups (p=0.005) (Table 4, Fig. 5). To assess differences in nerve recovery based on IOF involvement in maxillary fractures, we compared the type 1 group (no IOF involvement) with the type 2 and type 3 groups (IOF involvement). At 3 months post-surgery, a significant difference in mean measurements was observed (p=0.021) (Table 5, Fig. 6).
DISCUSSION
The zygomaticomaxillary complex is a prominent feature of the facial skeleton, and its fractures are the third most common among facial bone fractures, following those of the nasal bones and the mandible [11–13]. Dubron et al. [14] found that fracture lines passing through the infraorbital canal, along with orbital floor fractures and dislocations of the zygomaticomaxillary complex, classified as Zingg B type fractures, significantly increase the risk of infraorbital nerve damage. Sakavicius et al. [5] observed that recovery from mild infraorbital nerve injuries typically occurs within 3 months, moderate injuries require about 6 months for recovery, and severe injuries may need over 12 months for nerve function to return. In our study, the involvement of the IOF in cases of maxillary fractures was clearly identifiable using facial CT scans. This allowed us to assess infraorbital nerve dysfunction and recovery levels by comparing cases with and without the fracture line involving the IOF. However, the path of the infraorbital nerve within the orbital floor is not clearly discernible on facial CT scans. Consequently, cases with accompanying blowout fractures were excluded from the study.
All patients participating in this study underwent ORIF, which is the primary treatment for maxillary fractures. Several studies have suggested that ORIF surgery in patients with facial fractures may reduce neural sheath edema and compression, thus aiding in symptom relief and positively influencing recovery speed. However, Yoon et al. [15] reported that ORIF might also lead to postoperative nerve dysfunction. Concerns exist that ORIF could cause swelling or hematoma around the infraorbital nerve or direct nerve injury during bone reduction, which could potentially impede the improvement of hypoesthesia symptoms. According to Shin et al. [7], a study involving 26 patients with isolated unilateral fractures of the anterior wall of the maxillary sinus indicated that while ORIF shortened the duration of symptoms in patients with severe maxillary fractures, it did not significantly affect hypoesthesia recovery in the majority of cases. In our study, we excluded patients with severe fractures involving soft tissue injuries, as these could potentially influence sensory evaluation. Nevertheless, our results indicated that all patients experienced recovery from infraorbital nerve dysfunction following surgery.
A variety of assessments are used to evaluate the sensory function of the infraorbital nerve, including the visual analogue scale, two-point discrimination test, pressure thresholds, blink reflex test, vibration threshold test, thermography, and gross temperature assessment [5,10]. However, some studies have indicated that the two-point discrimination test and the monofilament test, which were used in our study for sensory evaluation, may have limited sensitivity. This could result in an underestimation of the severity of nerve damage. Despite this limitation, these tests are advantageous for clinical use because they are simple to perform with equipment that is commonly available, yield quick results, and are widely recognized for sensory assessment due to their standardized methodologies [16,17].
Nerve injuries are classified into three main types. The mildest form, neurapraxia, is characterized by a failure of nerve conduction while the axon and surrounding tissues maintain their structural integrity. The second type, axonotmesis, involves structural damage to the axon, but the surrounding fascicular connective tissue remains intact. The third type, neurotmesis, results in damage to both the axon and the surrounding fascicular connective tissue [18]. Neurapraxia typically results from compression, while axonotmesis is often due to more severe crush or stretch injuries. Neurotmesis is usually caused by lacerations. In the central nervous system, nerve recovery is not possible; however, some functional recovery may occur due to the plasticity of surrounding nerves. By contrast, the peripheral nervous system can support the recovery of the damaged nerve itself [19]. In this study, the IOF was accessed during ORIF using subciliary and upper gingivobuccal incisions. No complete disruption of the infraorbital nerve connection was observed visually, and the nerve injury was determined to be either neurapraxia or axonotmesis.
The recovery mechanism of peripheral nerves involves processes such as remyelination, collateral sprouting from surviving axons, and axonal regeneration [20]. When 20%–30% or fewer axons are damaged, nerve recovery primarily occurs through collateral sprouting by surviving axons, a process that typically takes about 2–6 months. In cases where more than 90% of axons are damaged, axonal regeneration at the injury site becomes the main recovery mechanism. Following axonotmesis or neurotmesis, the distal portion of the injured nerve undergoes Wallerian degeneration—a process of degeneration and disintegration aided by the phagocytic activity of macrophages. Subsequently, new nerve fibers grow from the proximal site of injury. In cases of axonotmesis, where axons are severed, the distal nerve degenerates due to Wallerian degeneration, but Schwann cells within the intact fascicular connective tissue act as guides for nerve regeneration and axonal regrowth. However, when neurotmesis occurs and the continuity of the entire nerve trunk, including surrounding tissues, is disrupted, achieving functional nerve recovery is challenging and often requires additional measures such as surgery [21–23]. The recovery mechanism of the infraorbital nerve includes regeneration from the proximal injury site and collateral sprouting from nearby sensory nerves, such as the zygomatico-facial, buccal, and external nasal nerves [24]. In our study, we categorized patients based on whether maxillary fractures involved the IOF, and we observed significant differences in the recovery of the infraorbital nerve among these groups. This indicates that maxillary fractures involving the IOF may lead to more severe nerve damage.
In this study, we calculated the differences between the mean measurements taken at four evaluation points on the affected side and those on the unaffected side. These values represented the degree of sensory deficit, with a reduction in these values over time indicating sensory recovery. A transient worsening of sensory deficit was noted 1 week after surgery, with recovery consistently observed from one month onward. This suggests that postoperative swelling, which persisted 1 week after surgery, likely affected sensory evaluations in the surgical area. Furthermore, no significant differences were observed when comparing measurements between the groups.
Maxillary fractures involving the IOF in the types 2 and 3 groups exhibited higher levels of sensory deficit compared to those without IOF involvement in the type 1 group at all evaluation time points and over the total period (Tables 3 and 5, Figs. 4 and 6). Among IOF-involved cases, the type 3 group tended to show greater deficits than the type 2 group. Severe trauma that results in bony fractures can lead to nerve injuries in the affected area, with nerves in the vicinity commonly impacted in both upper and lower limb fractures [25]. Therefore, the involvement of the IOF in maxillary fractures is believed to influence nerve dysfunction.
This study has several limitations. First, it was restricted to patients who underwent surgery, which precludes comparisons regarding the effects of surgery on nerve recovery levels. Second, the CT image analysis was limited to detecting fractures at the IOF, without assessing fractures in adjacent structures along the entire path of the infraorbital nerve. Third, sensory testing of the upper gingiva and maxillary teeth—areas innervated by the infraorbital nerve—was not performed due to the absence of objective measurement techniques and the challenges associated with such measurements. Notably, many patients experienced reduced sensation in the maxillary teeth, and some had delayed recoveries or did not recover sensation at all.
Maxillary fractures are a frequent type of facial bone fracture resulting from facial trauma and are often accompanied by injury to the infraorbital nerve. The degree of dysfunction and subsequent recovery of the infraorbital nerve following a maxillary fracture is influenced by whether the IOF is involved. Consequently, identifying involvement of the IOF via CT imaging can help predict the extent and prognosis of sensory dysfunction after surgery in patients with maxillary fractures.
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