Characterization of Pterygomaxillary Suture Morphology: A CBCT Study

Abstract

One of the most challenging procedures during maxillary osteotomy is pterygomaxillary suture (PMS) separation, due to the difficulty of directly inspecting this suture and the anatomical complexity of the adjacent structures. Knowing the precise anatomical dimensions and the position of the PMS, namely, the angle at which to approach this structure, may help in determining the proper osteotome. It will also help the oral and maxillofacial surgeon to perform this sensitive procedure more precisely and to minimize surgical complications (e.g., internal maxillary artery injury and unfavorable fracture during separation). The current study aimed to evaluate the morphology of PMS in an adult Israeli population using CBCT scans of the maxilla. Fifty CBCTs of healthy males (n = 27) and females (n = 23) were collected and analyzed. The vertical height, maximal thickness, and angulation of the PMS relative to the midsagittal plane of the skull were evaluated on both the right and left sides of the patient. An independent samples t-test was carried out to determine PMS morphological differences between males and females. A related samples t-test was conducted to determine the PMS morphological differences between the right and left sides. No significant differences in PMS parameters were found between sides in males and females (p > 0.225). Both males and females exhibited similar height and angulation of the PMS (p > 0.486). Interestingly, the PMS thickness was statistically greater in males (p = 0.029); however, this difference was clinically insignificant. The mean dimensions of the PMS in a healthy Israeli adult population are presented and discussed, as well as the clinical and methodological implications.

1. Introduction

The method for correcting dentofacial deformations of the midface involves Le Fort I maxillary osteotomy, during which a controlled fracture to separate the maxilla from the base of skull is carried out. The pterygomaxillary suture (PMS) is one of the synostoses that must be separated during this surgical procedure. This suture is a complex three-dimensional structure that connects the maxilla and the pterygoid plates of the sphenoid bone; it can be characterized by its thickness, height [1,2,3], and the angle created by the connection of the bones relative to the midsagittal plane of skull. The inferior part of the suture begins at the connection between the maxillary tuberosity and the pterygoid plate, and its superior part creates the base of the pterygomaxillary fissure (Figure 1).

The PMS is surrounded by numerous vascular and important anatomical structures, e.g., the posterior wall of the maxillary sinus is anterior to the PMS; the pterygoid plexus is posterior to it; its medial part is adjacent to the greater palatine foramen; and in its upper part, the internal maxillary artery enters the pterygomaxillary fissure [3,4,5]. The fissure includes the opening of the pterygopalatine fossa, which contains many important neuro-vascular structures and constitutes an important central junction between the nasal and oral cavities, the orbit, the infratemporal fossa, and the nasopharynx [4,5,6]. The PMS is separated using a designated angulated osteotome, because it is performed in a “blind” indirect fashion. The main challenge while separating the PMS is to find its position from the lateral aspect. This is usually done by identifying the convexity in the pterygomaxillary fissure using a curved osteotome. Some patients have less clear and defined anatomy, which makes it difficult to identify the precise location of the separation [7,8]. PMS separation is the final stage of maxillary release from the base of skull during Le Fort I osteotomy [9] (Figure 2).

The main reason that the PMS is separated last during maxillary osteotomy is its proximity to the pterygoid plexus vein and the maxillary artery [7,10,11,12]. Thus, if blood vessel damage occurs during osteotomy, the surgeon can directly locate the bleeding vessels and treat them. The most vulnerable source for significant bleeding during surgery is the internal maxillary artery and its branches, especially the descending palatine artery [3,4,9,10,11]; however, the pterygoid plexus can also be a significant cause of venous bleeding [11]. After arterial damage, there may be impaired maxillary perfusion, followed by loss of teeth, gingival necrosis, periodontal vulnerability, or even complete necrosis and loss of the dento-alveolar segment [10,13]. An additional complication that can occur as a result of PMS separation is an unfavorable bone fracture [14,15]. Several studies reported [6,11,15] that using excessive force while performing PMS separation resulted in unfavorable fractures of the sphenoid bone or other bones of the cranial base, which sometimes resulted in cranial nerve damage. Previously, three types of fractures were observed during PMS separation: (a) a fracture line in the desired plane, (b) a fracture line passing through the posterior wall of the maxillary sinus, and (c) a fracture line passing through the upper part of the pterygoid bone. When the fracture runs through the pterygopalatine fossa, additional damage to its contents occurs. A previous study showed that the complication rate due to a Le Fort I osteotomy ranges between 6.1% and 9.0% [16]. Most of the complications are transitory and are not life-threatening [13].

Owing to the complexity of the PMS structure and the difficulty in viewing it directly during Le Fort I maxillary osteotomy, it is of major importance to acquire accurate information about the PMS morphology. Previous studies examined the anatomy of the pterygomaxillary region with respect to the vessels’ position, and the bone size, and described the normal and pathological anatomy of the pterygopalatine fossa [5,8,17]; however, exact PMS suture measurements were not presented. Since the PMS separation is the most sensitive part of the Le Fort I osteotomy, oral and maxillofacial surgeons must be well acquainted with PMS morphology and must diagnose the best angle of access to this structure. Moreover, knowing the precise size and angulation of PMS may aid in properly designing osteotomes, which could minimize the risk and complications involved during the separation procedure.

Therefore, our main goal was to evaluate the morphology of the PMS (its thickness, height, and angulation) in an Israeli adult population. Additionally, we wanted to determine whether age, side, and sex-related differences exist in the PMS dimension and position.

2. Materials and Methods

2.1. Study Sample

The current study investigated the morphology of the PMS from the CBCT scans of the maxilla (Ortophos 3D SL device, Sirona Dental Systems Gmb Bensheim, Germany). All of the CBCT scans were taken for various dental purposes unrelated to this study as a part of the dental treatment conducted at the School of Dental Medicine, Tel Aviv University. A total of 50 CBCT scans (n = 27 males, n = 23 females) were randomly selected from the list of scans from the Department of Radiology, the School of Dental Medicine, Tel Aviv University, Israel. The inclusion criteria were as follows: (1) the patient’s age ranged between 18 and 80 years old, (2) a lack of pathologies in the scanned area (e.g., tumors, cysts), (3) the absence of previous trauma to the head and neck, (4) a clear medical history, (5) no history of previous orthognathic surgery, and (6) the scans present the complete anatomy of the maxillary bone and the pterygoid process of the sphenoid bone. Exclusion criteria included the following: (1) patients under the age of 18 years, or over 80 years old; (2) technically aberrant CBCT images; (3) a scan that does not include bi-lateral anatomical structures of the maxilla and the sphenoid bone; (4) a history of previous head and neck trauma or orthognathic/orthofacial surgery; and (5) the patient was diagnosed with a syndrome affecting the development of the skull. The current research was approved by the institutional ethical review board of Tel Aviv University.

2.2. Measurement Method

The data were stored in the Digital Image Communications in Medicine (DICOM) templates. The thickness of the slice used was 1 mm. The measurements were performed using Randi Ant Dicom Viewer software version 5.5 (Medixant, Poznan, Poland). The measurements were taken directly from the CBCT scans and included one angular and two linear parameters that characterize PMS morphology. All of the measurements were established on the axial sections of the maxilla separately for the right and left side of the patient. These measurements included the following:

• PMS thickness (mm): The maximal distance measured between the innermost points on the lateral concave and medial convex surfaces of the suture (Figure 3). The axial cross section in which the PMS thickness was greatest was evaluated separately for each side since the height of this section could vary slightly between the right and left sides of the same patient, depending on the anatomical differences between the sides.
• PMS angulation (°): The angle created between the PMS thickness line and the midsagittal plane (Figure 3).
• PMS height (mm): The suture height was calculated based on the number of axial sections counted from the suture initiation (the section where the maxillary lump connects to the pterygoid hamulus) and the suture termination (the last section before the suture is opened into a pterygomaxillary fissure) multiplied by the slice thickness (Figure 4).

2.3. Statistical Analysis

The data were summarized and analyzed using IBM SPSS statistics software (version 20, IBM Corp., Armonk, NY, USA). A normal distribution of the variables was examined using the one-sample Kolmogorov–Smirnov test. Descriptive statistics (the mean, standard deviation, and range) were carried out for all variables. An independent samples t-test was performed to compare the age and suture parameters between males and females. A related samples t-test was performed to compare the PMS measurements between the right and left sides. A Pearson correlation analysis was carried out to determine whether an association exists between the PMS parameters and age. The level of statistical significance was set at p < 0.05.

2.4. Reliability

In order to determine the intra-tester’s ability to replicate the studied measurements, a researcher (S.D.) evaluated twice, using a blind method, the CBCT of 10 cases (totaling 20 sets of measurements, including the right and left sides). For determining the inter-tester reliability, the same cases were evaluated by another independent researcher (L.K.) in a one-month period. The reliability analysis was performed using an ICC test.

3. Results

3.1. Reliability Test

The intra- and inter-tester reliability analysis revealed a high reproducibility level for all PMS measurements (0.789 ≤ ICC ≤ 0.987) (p < 0.001).

3.2. Study Sample

The sample included 50 CBCT images: 27 male individuals (54%) and 23 female individuals (46%). The mean age of the sample was 46.46 ± 16.411 years; it ranged between 19 and 79 years. Females were significantly younger when compared with males (41.8 years vs. 51.04 years, respectively) (p = 0.048) (

Table 1. Descriptive statistics of the study sample.

Sex	n (%)	Mean Age (Years)	± SD	Min	Max	p-Value *
Males	27 (54)	51.04	16.211	20	79	0.048
Females	23 (46)	41.88	15.742	19	74
Total	50 (100)	46.46	16.411	19	79

* p-value for the age difference between the sexes. The significant value is denoted in bold. SD: standard deviation; min: minimum; and max: maximum.

).

3.3. PMS Morphology by the Right and Left Sides in Males and Females

No significant differences were found in any of the measured PMS parameters between the right and left sides of the patient, in both males and females (p > 0.225) (

Table 2. Comparison of PMS morphology between the sides, males only.

PMS Measurement	Side	Mean	± SD	Min	Max	p-Value *
Thickness (mm)	Right	1.02	0.139	0.75	1.25	0.225
	Left	0.98	0.144	0.70	1.25
Height (mm)	Right	16.67	2.569	11.40	22.50	0.392
	Left	17.10	2.744	12.80	22.90
Angulation (°)	Right	57.86	5.212	67.20	49.10	0.380
	Left	58.74	5.259	66.70	49.20

* p-values for the differences between the sides. PMS: pterygomaxillary Suture; SD: standard deviation; min: minimum; and max: maximum.

and

Table 3. Comparison of PMS morphology between the sides, females only.

PMS Measurement	Side	Mean	± SD	Min	Max	p-Value *
Thickness (mm)	Right	0.91	0.178	0.51	1.18	0.738
	Left	0.90	0.179	0.57	1.36
Height (mm)	Right	16.35	2.748	11.40	22.50	0.973
	Left	16.37	3.467	12.50	24.20
Angulation (°)	Right	58.42	6.210	67.60	47.90	0.975
	Left	58.46	5.305	69.90	49.20

* p-values for the differences between the sides. PMS: pterygomaxillary Suture; SD: standard deviation; min: minimum; and max: maximum.

). Descriptive statistics and p-values are presented in Table 2 and Table 3.

3.4. Comparison of the PMS Morphology between the Sexes

Since no significant differences were noted between the right and left sides of the patient for all the PMS measurements, the mean of both sides was calculated and compared between males and females. For a comparison between the sexes, the descriptive statistics and p-values are presented in

Table 4. Comparison of the PMS morphology between the sexes.

PMS Measurement	Sex	Mean	± SD	Min	Max	p-Value *
Thickness (mm)	Male	1.00	0.130	0.76	1.22	0.029
	Female	0.90	0.170	0.54	1.27
Height (mm)	Male	16.89	2.322	12.85	22.45	0.486
	Female	16.36	2.994	12.05	23.35
Angulation (°)	Male	58.30	4.557	66.85	50.35	0.915
	Female	58.44	4.755	67.35	49.95

* p-values for the differences between the sexes. Significant values are denoted in bold. PMS: pterygomaxillary suture; SD: standard deviation; min: minimum; and max: maximum.

. No statistically significant difference was found regarding PMS height and angulation between males and females (p > 0.486). Regarding PMS thickness, a statistically significant difference between the sexes was found (p = 0.029) when males were presented with a wider suture than were females (1.00 mm vs. 0.90 mm, respectively).

Although the difference in PMS thickness was statistically significant, the biological significance of such a difference (0.1 mm) seems to have only a minor effect.

3.5. PMS Morphology and Age by Sexes

No statistically significant difference was found between the PMS parameters and age in both males and females (p > 0.400) (

Table 5. Association between PMS morphology and age (Pearson correlation analysis) by sex.

PMS Measurement	Sex	r	p-Value *
Thickness (mm)	Male	0.17	0.400
	Female	0.04	0.858
Height (mm)	Male	−0.16	0.411
	Female	0.13	0.558
Angulation (°)	Male	0.16	0.439
	Female	0.14	0.516

* p-values for the correlation analysis between the PMS parameters and age.

).

4. Discussion

To date, controversy exists in the literature regarding the preferred method for pterygomaxillary separation [18,19,20]. Turvey and Fonseca [9] suggested that the position and orientation of the curved osteotome are critical to avoid complications during PMS separation, in particular, to avoid damage to the maxillary artery. In contrast, Robinson and Hendy [15] reported that use of a curved osteotome created unfavorable fractures in the pterygoid plate when used to separate the PMS. Precious et al. [19] compared the outcomes of PMS separations using CT when it was performed with and without an osteotome. They found that an ideal separation was successful in 27% of cases in which it was performed without an osteotome, and in 29% of cases in which it was performed with an osteotome. Others proposed a technique for pterygomaxillary separation performed anterior to the PMS in order to avoid vascular damage during the osteotomy [20,21]. A survey of 205 oral and maxillofacial surgeons investigated surgeons’ preferences for pterygomaxillary separation [22]. Importantly, 78% of the participants reported that they use an osteotome or a micro-oscillating saw for pterygomaxillary separation: 8% reported serious vascular complication outcomes. Others, who used leverage alone or tuberosity osteotomy, did not report any vascular complications. Although the literature presents diverse approaches for pterygomaxillary separation [12,23], the most popular approach is to use a curved osteotome through PMS separation. Therefore, knowing the PMS size and position relative to the midsagittal plane is essential for surgeons performing this procedure. Our study investigated PMS morphology in an adult Israeli population with respect to patients’ age, sex, and the measured side. The vertical height of the PMS, its maximal thickness, and its angulation relative to the midsagittal plane were measured based on the CBCT scans. We found that in an adult Israeli population, the PMS mean height was 16.9 ± 2.3 mm and 16.4 ± 3.0 mm, in males and females, respectively, with no significant difference between the sexes (Figure 5).

However, this finding differs from what was previously reported in the Thai (15.14 ± 2.5 mm), American (14.6 ± 3.1 mm), and Chinese (12.1 ± 2.0 mm) populations [7,9,24]. These differences could result from a different methodology used in these studies; however, they also may reflect ethnical anatomical differences [5]. The blade length of the osteotome used to separate the PMS should resemble the PMS height. Clinically, this measurement, based on the Israeli population, implies that when a curved osteotome with a cutting edge of about 15 mm is used to separate the PMS in a Le Fort I osteotomy, the mean safety margin from the pterygomaxillary fissure is 1.5 mm. However, it should be remembered that anatomical variations exist regarding the PMS height and that an individual CBCT evaluation is critical prior to the surgical procedure.

Regarding the PMS thickness, we found that in our sample the mean thickness was 1.0 ± 0.13 mm and 0.9 ± 0.17 mm in males and females, respectively. Although the biological difference between the sexes seems to be minimal, this difference was found to be statistically significant. Previous studies evaluated the thickness using a different method [8,17]; however, some sex-related differences were also found. Kanawaza et al. [25] found an association between PMS thickness and unwanted fractures in the pterygoid process. It was suggested that the force applied from the osteotome is easily dissipated when a thin pterygomaxillary junction is present, which may lead to an unwanted pterygoid plate fracture. Thus, the PMS thickness should serve as an approximate estimate for the surgeon regarding the amount of force that should be applied to the osteotome during PMS separation.

The ideal fracture line begins laterally to the PMS and progresses medially along the PMS between the maxilla and the pterygoid process. Knowing the angle between the sagittal plane and the PMS can help the oral and maxillofacial surgeon to determine at which angulation to place an osteotome for PMS separation and thus possibly minimize the incidence of unwanted fracture lines. In our study, we found that the PMS angulation, measured at the greatest thickness of the suture, was about 58 ± 4.6° for both males and females, regardless of the measured side (right and left) and the patient’s age. Therefore, it is reasonable to assume that the sharp edge of the osteotome used to separate the suture should be placed at this angle in order to create ideal separation (Figure 6).

Using 12 cadavers, Stajčić [26] compared frequency that a pterygoid plate fracture occurred when a regular osteotome (50 degrees) and a large-angle osteotome (80 degrees) were used during Le Fort I osteotomy. His findings suggest that although the incidence of pterygoid plate fracture was lower using a large angle osteotome, there was a greater risk of unwanted palatinal bone separation. The mean PMS angulation in an Israeli population was found to be 58 degrees; however, this angle may differ between patients due to anatomical variations. Owing to the “blind” approach of PMS separation, it is difficult to identify the instrument’s angle relative to the sagittal plane in the operating room. Therefore, we suggest estimating the PMS size and the position measurements using the patients’ CBCT as well as practicing the PMS angle on a printed 3D model prior to performing the surgical procedure.

5. Limitations of the Study

Although the study sample was randomly selected based on the inclusion criteria and showed a similar age range in male and female populations, the mean age difference was noted between the sexes. Nevertheless, no significant association was found between the age and the PMS morphometrics in both males and females; moreover, the results were presented separately for males and females. However, the association between the age and suture parameters may need further confirmation and validation from other populations of a similar mean age. Deducing data on the PMS morphology from the current study sample was limited to average facial type patients, which justifies a further investigation of different facial type populations. Additionally, the findings of the current study should be validated by a future clinical study providing CBCTs prior to and following the Le Fort I osteotomy.

6. Conclusions

Precise disjunction of the maxilla from the pterygoid process of the sphenoid bone requires recognizing of the pterygomaxillary suture morphology and position with regard to the patient’s sex and age. Our findings show that PMS morphology is similar on the right and left sides in both males and females. Only PMS thickness is statistically significantly greater in males than in females; however, such a difference is of little clinical relevance. No significant age-related changes are expected regarding PMS morphology in individuals above the age of 18 years in both sexes. The mean angle of the PMS was 58 degrees in an Israeli population. Our findings can help in developing the best fitted osteotome in order to provide a safe pterygomaxillary separation during Le Fort I osteotomy.