The primary aim of this study was to determine the angle and depth range most likely to facilitate entry into the TMJ cavity, a topic that has not been adequately addressed in the literature. This determination was made using 3D analysis in conjunction with the classical Holmlund Hellsing line and a two-needle approach. Particularly for blind procedures, a standardized puncture technique can significantly help the operator. However, the findings of this research indicate that there is considerable variance in the angle measurements for both anterior and posterior needles, making it challenging to propose a standard range, even though the variance is lower in depth.

Arthrocentesis is considered a minimally invasive procedure, but care must still be taken to avoid vascular and nerve injuries and to carefully assess the roof of the glenoid fossa, which separates the upper joint space from the neurocranial structures above. Fracture of these structures can lead to some significant complications that may require immediate hospitalization of the patient for monitoring and appropriate treatment [21]. It is therefore of great importance to identify approximately safe points and depths to access the upper joint space [24].

Arthrocentesis requires needle insertion into the upper space of the TMJ. Traditionally, this point is estimated using the surface anatomy of the head. The most commonly used references for entering the TMJ are the Holmlund-Hellsing line and its associated specific points [16, 25]. Many authors have used these points to perform TMJ arthrocentesis [4, 26]. However, failures during entry attempts due to anatomical variations among different patients have also been reported with this method [20]. Palma et al. reported very low success rates, varying between 0 and 37.5%, in cadaver studies on fresh or formalin-treated cadavers. [27] To facilitate access to the joint cavity, the researchers offered various reference points and entry techniques [28, 29]. However, the entry angle was rarely reported in these studies. Our study revealed that when a standard entry point is used, there is significant variation in the angles.

This study's age-related analyses revealed a weak correlation between the axial angle of the posterior needle and age. No correlation was found between other angles, depth measurements and age. Additionally, it was observed that there was a difference in needle depth between genders, with males exhibiting a higher anterior needle depth. Münevveroğlu et al. examined the classical Holmlund Hellsing points and two-needle method on MRI images. [30] Their findings indicated that there is no significant correlation between age and needle depth. Needle depths were higher in males than females, similar to our study.

One crucial aspect of arthrocentesis is mouth opening, which assists in expanding the joint space and facilitates easy access to the TMJ [2, 4]. Various studies recommend that the mouth be opened as wide as possible during needle placement [4, 13, 28]. Considering that some patients indicated for arthrocentesis have limited mouth opening, this should be taken into account in three-dimensional planning for the entry position. The reliability of the conventional point in the closed mouth position has been supported by Palma and colleagues. They concluded that the conventional point is not a correct reference for needle placement when maximum mouth opening is not achieved [27]. Virtual access to the joint space with the mouth open and closed was achieved in this study based on 3D images. However, no significant difference in entry angle or depth was observed between the closed- and open-mouth positions. A possible reason for this difference could be that the assessment was based solely on hard tissues. Therefore, extrapolating this result to a clinical scenario may not be entirely accurate.

It is well recognized that accessing the superior joint space with minimal trauma is necessary for effective irrigation [22]. For blind procedures such as arthrocentesis, physicians are encouraged to practice the procedure on cadavers or simulate it on plastic models before applying it to patients [31]. Training on cadavers provides good precision [16]. However, due to feasibility challenges, this is not always possible. An unforeseen benefit of this study is that the researcher(SAE), who was experienced in planning but inexperienced in the clinical application of arthrocentesis, reported feeling more comfortable performing arthrocentesis on subsequent patients after planning with CBCT. However, this statement is based only on personal experience with few patients. High-quality clinical studies are needed to evaluate the accuracy of these findings.

Strengths of this study include the use of verifiable 3D data assessed for reliability using intra-observer reliability tests and the absence of postmortem effects commonly found in cadaveric studies. Compared to manual measurements, 3D program not only precisely measures distance and angle but also allows for simultaneous investigation of various aspects of facial patterns. However, the main limitations of this study stem from its retrospective cross-sectional design and the nature of the imaging modalities used. This study is based on CBCT images from a single hospital, which limits the generalizability of the results. Additionally, the ability of CBCT to evaluate only hard tissues, creating a gap in the assessment of soft tissue structures that could be crucial in TMJ arthrocentesis.

The study's findings indicated that there were differences between the participants, particularly in the angles. Despite the fact that arthrocentesis is basically a method applied based on clinical examination and needle angles are personalized for each patient, planning and implementation based on CBCT data can make the procedure safer. Although CBCT does not offer as accurate information about soft tissues as MRI, it can provide useful information on needle depth and angles prior to arthrocentesis by estimating skin thickness at the appropriate threshold value. CBCT can serve as an auxiliary imaging tool before an arthrocentesis procedure.