4. Discussion
In previous studies, authors have diverged about the number of insertion patterns of the upper head of the LPM. While some authors (Omami and Lurie, 2012; Taskaya-Yilmaz et al., 2005) report two types of insertion – a single insertion in the articular disc, or simultaneous insertion in the disc and in the head of the mandible – others claim there are three types of insertion (Dergin et al., 2012; Imanimoghaddam et al., 2013; Mazza et al., 2009). After detailed evaluation of the MRI files, we indeed find that three types of insertion can be described for the upper head ofthe muscle (Fig. 2) (Imanimoghaddam et al., 2013; Mazza et al., 2009). Based on the interpretation of parasagittal MR images of the LPM in the open- and closed-mouth positions, we observed that type II represented the most frequent insertions, whereas type I represented the least frequent (Table 2). Other studies reported similarly higher frequencies for type II insertions (Dergin et al., 2012). Besides, some authors (Imanimoghaddamet al., 2013) reported the highest prevalence for type I(63.8%) and the lowestfor type III(12.5%)insertions. Despite studies reporting only two types of insertion (Omami and Lurie, 2012; Taskaya-Yilmaz et al., 2005), in one of them (Omami and Lurie, 2012), 62.5% of patients had the upper head ofthe muscle connected to both the disc and the head of the mandible, corroborating our findings. In the other study (Taskaya-Yilmaz et al., 2005), the authors reported no type I insertions, possibly resulting from differences in methodology. These authors only evaluated MR images in the closed-mouth position, meaning that they were not able to visualize the entire upper head of the muscle, which can only be seen with the contraction in the open-mouth position (Mazza et al., 2009).
