INTRODUCTION

The temporomandibular joint (TMJ) is a synovial joint with unique anatomical features (^{Gray & Lewis, 2000}). A precise description and knowledge of its anatomy is essential for clinical diagnosis of related pathological conditions and treatment planning (^{Brown et al., 2012}; ^{Matsumoto et al., 2014}; ^{Legemate et al., 2016}; ^{Xu et al., 2017}).

Magnetic resonance imaging (MRI) is described as the standard imaging test for assessing soft tissues such as muscles, adipose tissue, ligaments and joint discs among others. It is useful both in static and dynamic position (^{López López et al., 2005}), and has a specificity of 96 % and a sensitivity of 98 % (^{Bermejo, 1998}).

According to literature reports, the location of the mandibular condyle (MC) in the sagittal plane presents great variability with respect to its position, indicating that the central position of the condyle-disc-fossa is not directly related to the presence of pathological conditions (^{Alves et al., 2014}; ^{Coombs et al., 2019}; ^{Fan et al., 2019}). Temporomandibular disorders (TMD) are a group of pathologies that affect the TMJ, among which internal disorders may include, disc displacement and degenerative bone conditions, such as osteoarthritis. The use of minimally invasive techniques as corticosteroid injection, hyaluronic acid, or arthrocentesis, have been prescribed.

Due to its complex anatomy, TMJ imaging is particularly useful for intra-articular injection, providing accurate guidance for puncture site and to reduce possible complications such as a neurovascular injury, penetration of the mid-cranial cavity, or injury to the joint (^{Bjørnland et al., 1994}; ^{Sugisaki et al., 1995}). In order to determine exact depth of the TMJ for the best puncture technique, detailed knowledge of the topographic anatomy is required.

However, according to the evidence reviewed in this study, there is no reported record of this depth. Hence, the aim of this study was to measure the depth of the TMJ through magnetic resonance imaging (MRI) in patients with temporomandibular disorders (TMD).

MATERIAL AND METHOD

Design and ethical considerations. A cross-sectional study was carried out, randomly selecting 150 MR images of patients who attended the TMD and Orofacial Pain Polyclinic of the Dentistry Faculty, Universidad de La Frontera, Chile, from 2015 to 2018. This study followed Helsinski statement guidelines on medical and ethical protocols. All patients included in this study were anonymized and no data were obtained from their clinical records.

Variables studied. The variables analyzed were: 1) Depth of MC in the right and left TMJ; 2) Age of the patients; and 3) Sex of the patients. Depth of the MC was obtained by tracing a tangential line from the skin to the outermost area of the lateral pole of the MC, with frontal view of the TMJ; this distance was measured through the OsiriX® Program for MacOS, in T1 window. Patients´ sex and age were obtained from the data recorded in each MR.

Statistical analysis. The Shapiro-Wilk Test was performed to contrast the normal distribution. For the analysis of descriptive statistics, central tendency measures were calculated. For the inferential analysis, the non-parametric Mann Whitney U test and the Pearson linear correlation coefficient (r) were used. The analyses were performed using the IBM / SPSS Statistics 20.0 MacOS software, considering a p-value less than 0.05 as significant.

RESULTS

In reference to the total MRI analyzed, 114 corresponded to women, median age of 23 years. The median MC depth of the right TMJ was 17.16 mm and left side median was 16.98 mm. There was no statistically significant difference between the two (p> 0.05), however, a strong correlation was noted (r = 0.842).

There were no differences between the depths of the MC and the sex of the patients in both the right and left TMJ. There was also no correlation between the age of the patients and the depth of the MC.

DISCUSSION

In-depth knowledge TMJ anatomy is essential during diagnostic-therapeutic procedures such as intra-articular substance injection or arthrocentesis. Nowadays, these diagnostic tools are the methods of choice, given the advances in the understanding of intra-articular diseases, and the therapies available for these disorders. There has been important short and long-term development in this area, from a clinical point of view, as well as with the scientific evidence supporting the results.

MRI is considered a valuable tool, both during the diagnostic and therapeutic stages, as it allows identification of the soft structures of TMJ (^{Emshoff et al., 2002}). Furthermore alterations in position and composition are the triggering factors for pathological conditions, with the ensuing painful symptoms and functional impairment. By assessing both soft and hard anatomical components through MRI, an anatomical analysis can be obtained, providing access and allowing minimally invasive clinical procedures with a greater degree of safety, thus reducing the risk of postoperative complications and improving therapeutic results.

Arthrocentesis technique consists of marking the insertion points of the needle (s) on the skin (^{McCain, 1988}). The posterior entry point is located along the canto tragal line, 10 mm from the middle of the tragus, 2 mm below the line. The anterior entry point is located at a 10 mm distance along the line, and 10 mm below the line. These marks on the skin are the reference points that indicate the location of the joint cavity and articular eminence of the TMJ. There are other techniques involving the use of a needle, which underscores the relevance of thorough anatomical knowledge of the TMJ. However, morphology and position of the condyle may vary according to age and sex (^{Liu et al., 2018}), the presence of certain pathologies (^{Santos et al., 2019}), after orthodontic (^{Wang et al., 2018}), or dentomaxilofacial orthopaedics treatments (^{Ghoussoub et al., 2018}).

The aforementioned becomes vitally important when minimally invasive treatments are planned. These are carried out considering pathological conditions, alterations in anatomical characteristics and/or location of the intra-articular structures. In those cases, it is important to consider alteration of inter and intra articular pressure in cases of joint effusion, as these may change the position and relationship between condyle, cavity and joint disc.

In reference to these circumstances and whenever possible it becomes critical to know the exact anatomical location of joint structures prior to performing the various procedures. Since all minimally invasive procedures require intra-articular access, it is essential to have precise information as to anatomical depth at which the TMJ is expected to be found, in order to determine the proper choice of instruments and/or materials for treatment.

According to the literature researched, this is the first study reporting the depth of the mandibular condyle using MRI, evaluating the differences by sex, age or by right and left side, which provides important anatomical knowledge for the correct performance of minimally invasive intra-articular procedures. Furthermore, it is useful for the preferred choice of materials and equipment for these procedures, prompting fewer possible side-effects, procedural complications and possible surgical mishaps.

In conclusion, the depth of the right and left TMJ are highly correlated, being approximately 17 mm in the population studied. There is no association between patient age and condylar depth, and there are no differences in averages according to sex.