Lateral Pterygoid Muscle Anatomy: New Insights into Attachment Variations and TMJ Disorders
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MRI studies are revealing a more complex anatomy of the lateral pterygoid muscle (LPM) than previously understood, with implications for understanding temporomandibular joint (TMJ) disorders.
For decades, anatomical texts have described the LPM as consisting of two distinct heads: the superior (SLPM) and inferior (ILPM). However, recent research suggests a third, medial fiber component may be present in a significant number of individuals. A recent study confirmed the presence of these medial fibers in 9.8% of cases, aligning with findings from other investigations reporting prevalence rates of 20.22% and 29.6% based on MRI analysis. This anatomical variability presents challenges in understanding the muscle’s function and its role in TMJ pathology.
Variable Attachment Patterns and Classification Challenges
The literature consistently demonstrates that LPM attachment patterns are not uniform. Factors such as MRI signal quality, imaging sequences, slice thickness, and the angle of imaging slices can all influence the visibility of these different heads. While there is general agreement regarding the ILPM’s insertion point on the condyle, the SLPM’s attachment site remains a subject of debate.
“Our results confirm that the LPM demonstrates variable attachment patterns,” researchers noted. MRI evaluations in this study identified three distinct attachment types to the disc condyle complex (DCC). A key obstacle in the field is the lack of a standardized classification system. Existing systems vary, with some defining four attachment types and others utilizing a three-type classification. This study adopted the system proposed by Litko et al., providing a consistent framework for analysis.
Prevalence of Attachment Types in TMJ Dysfunction
Analysis of existing studies reveals a range of attachment configurations. Dergin et al. found that in nearly 30% of cases, the SLPM attached to the disc while the ILPM attached to the condyle. Approximately 41% exhibited SLPM attachment to both the disc and condyle, with the ILPM attaching to the condyle, and another 30% showed SLPM attachment to the disc alongside attachments of both middle and inferior heads to the condyle.
Cadaveric studies have also illuminated these variations. Kılıç et al. identified that in over 36% of cases, the SLPM attached to both the DCC and the condyle, while the ILPM attached only to the condyle. Antonopoulou et al., examining 18 cadavers, reported that the SLPM was attached to both the condyle and DCC in over 55% of cases. A large MRI-based analysis by Litko et al. of 382 joints revealed that in roughly 67% of cases, the SLPM attached to both the disc and condyle.
Further investigation into 272 TMJs—categorized by disc position (normal, DDwR, DDwoR)—showed a strong trend: in most cases, the SLPM was attached to the disc and the ILPM to the condyle. However, a significant minority (ranging from 14% to 43% depending on disc position) exhibited SLPM attachment to both the disc and condyle, alongside ILPM attachment to the condyle.
In the current study, Type 1 attachment (ILPM to the condyle and SLPM to the disc) was observed in 27.8% of cases, Type 2 (SLPM to both disc and condyle and ILPM to condyle) in 62.4%, and Type 3 (SLPM to the disc and ILPM and medial fibers to the condyle) in 9.8%.
The Complex Relationship with Disc Displacement
The connection between LPM attachment types and disc displacement remains contentious. Some studies, like those by Yeşiltepe et al., Taskaya-Yılmaz et al., Dergin et al., Imanimoghaddam et al., and Omami et al., have found no significant correlation. However, Litko et al. reported a significant correlation, potentially due to their comprehensive evaluation of disc position in multiple planes and their use of imaging slices aligned with the LPM.
Electromyographic studies have shed light on the functional roles of the SLPM and ILPM, with the SLPM stabilizing the disc and condyle and the ILPM primarily involved in mouth opening. A displaced disc can disrupt this balance, potentially leading to overactivation of the SLPM as a compensatory mechanism. Researchers have also observed pathological changes in both heads of the LPM in the presence of disc displacement. Signal intensity changes in the SLPM, specifically, have been correlated with the severity of displacement, suggesting edema and fatty degeneration.
Despite these findings, the current study did not find a statistically significant relationship between LPM attachment types and TMJ disc position, aligning with the results of several other investigations.
Beyond Attachment: A Multifactorial Picture
It’s increasingly clear that TMJ pathology is influenced by a multitude of factors beyond LPM attachment. Joint effusion, degenerative changes like osteoarthrosis, restricted motion, and morphological characteristics all play a role. Furthermore, lifestyle factors such as prone sleeping positions or prolonged dental procedures can contribute to muscle overactivity and TMJ symptoms. Tenderness in the LPM is a common finding in patients with temporomandibular disorders (TMDs), even in the absence of confirmed disc displacement.
Researchers hypothesize that LPM attachment type may contribute to the persistence of disc displacement once initiated by other factors like trauma or parafunctional habits. As one researcher suggested, the type of muscular attachment may not be decisive in causing disc displacement, but could influence its long-term stability.
Intra-articular changes, such as thinned discs and elongated retrodiscal tissues, were observed in joints with disc displacement, reinforcing the idea that the LPM’s anatomy contributes to a complex interplay of functional and biomechanical factors.
Future Directions and Therapeutic Implications
Further research incorporating functional imaging and longitudinal follow-up is warranted to fully elucidate the relationship between LPM anatomy and TMJ disorders. The LPM may represent a potential target for therapeutic interventions. Studies correlating TMJ and LPM MR images from different planes with clinical findings are also needed.
While the Research Diagnostic Criteria for Temporomandibular Disorders (RDC/TMD) were not applied in this study, future prospective studies that simultaneously evaluate both RDC/TMD diagnostic criteria and MRI findings could provide a more comprehensive understanding of these complex disorders. Including asymptomatic controls in future studies would also improve the reliability of findings. Additionally, employing higher-resolution imaging and standardized protocols could address limitations related to slice thickness and image quality.
