Histoiogic Characteristics of the Lateral Pterygoid Muscle Insertion to the Temporomandibular Joint Geber T, Bittar. DDS, MS This study used low-power light microscopy to examine the histo- Researcfi Scholar logie organization of tbe laterai pterygoid muscle interface with the Carol A. Bibb, PhD, DDS temporomandibular joint. Tbe sample inciuded parasagittal sec- Adjunct Associate Professor tions of 20 intact temporomandibular joints from young adults Section of Orofacial Pain and Occlusion ¡mean age 26-2 years) at autopsy. Tbe lateral pterygoid muscle sbowed no consistent divisions into separate anatomic muscle Andrew G, Pullinger, DDS. MSc beads at tbe insertion. Tbe muscie fibers attached to tbe pterygoid Professor fouea of tbe condyie immediately inferior to tbe articular surface in Seotion of Orofacial Pain and Occlusion all cases. Some additional fibers inserted superiorly into the more and Dental Research Institute anterior part of tbe articular disc in a minority of cases (31%). School of Dentistry Vibers inserting into tbe disc represented only 2.4% to 6% ofthe University of California total superior-inferior length of the muscle insertion. It is hypothe- Los Angeles, California sized that the muscular force exerted by these few fibers inserting Gorrespondence to: into the disc would not be sufficient to displace tbe disc anteriorly Dr Carol A. Bibb to tbe condyle. Tbere were two bistoiogic types of insertion of tbe Section of Orofacial Pain and Occlusion lateral pterygoid muscle to tbe condyle. Tbe superior part of tbe School of Dentistry insertion was characterized by an identifiable tendon inserting OHS 43-009 through fibrocartilage. In tbe inferior part of tbe insertion, tbe University of California muscle attacbed to periosteum without an obvious tendon. Tbe Los Angeles, California 90024 presence of this tendon must be recognized in interpretation of soft tissue temporomandibular joint imaging. J OROFACIAL PAIN 1994;8:243-249, natomic and physiologic investigations of the lateral ptery- goid muscle insertion to the temporomandibular joint (TMJ} Ahave generated considerable controversy. While studies have consistently shown the pre,sence of two distinct muscle origins at the pterygoid plate of the sphenoid hone, the insertion of the muscle to the cotnponents of the TMJ remains unclear. The unresolved issues include whether there are anatomically and functionally distinct upper and lower muscle heads at the insertion site and the relation- ship of the most superior muscle fibers to the articular disc. The insertion of the superior fihers of the lateral pterygoid mus- cle to the TMJ has been the source of much discussion in the litera- ture. Early dissection studies reported that the superior fibers insert solely to the articular disc' or to the disc and anteriot capsule.^ More recent histologie studies have demonstrated that the majority of the superior fihers insert to the condyle with only a few fibers inserting to the articular disc,^"* The latter studies have emphasized the variability in the relationship of the superiot fibers to the condyle and disc, but they have not attempted to measure the pro- portion of fibers insetting directly into the disc or considered an anatomic relationship to disc position, Electromyographic ¡EMG| studies have been similarly inconsis- tent; some have tepotted that the lateral pterygoid is a single func- Journaf of Orofacial Pain 243 Bittar rional unit," but the majority have reported func- majority of the lateral pterygoid muscle fihers are rionally separate superior and inferior heads."-'^ reported to insert ro the anreromedial aspect of the Juniper^ has proposed that the superior head joint.'°--" Projection microscopy permitted discus- shotild be considered a separate nitiscle called the sion and consensus judgment by three examiners superior pterygoid muscle. observing a complete field of view at low magniri- These conflicting studies have led co rhe com- carion. A confidence score with rankings of 1 = mon usage of a diagram depicting a separate supe- possible, 2 = probable, 3 = confident, and 4 = defi- rior head of the lateral pterygoid attachitig entirely nite was applied to each evaluation. to the anterior portion of the articular disc."""' More detailed descriptive and histomorphomet- Together with a functional model for the lateral ric assessments of the lateral pterygoid insertion pterygoid muscle, in which the upper head func- tissues were carried out under low-power light tions independently and antagonistically to the microscopy using eentral (n = 18) and medial (n = lower head of the muscle during tnandibular 16) sections. Higher magnification was used to movements," the idea has heen proposed that positively identify muscle tissue by the presence of hyperactivity of the muscle could cause the articu- striations to differentiate muscle tissue from the lar disc ro dislocate anteriorly." adjacent fibrous connective tissue of the disc or tendon. The ptirpose of the present study was to present a detailed description of the microatiatomy of the lateral pterygoid tnuscle interface with the TMJ in young adults. It is anticipated that the results will Results have implications for TMJ soft tissue itnaging and internal derangement etiology. Anatomic Divisions ofthe Lateral Ptetygoid Insertion Materials and Methods None of the 16 medial TMJ sections showed an obvious division into a distinct upper and lower muscle head ar rhe insertion site. In three of these Sample specimens (19%), loose connective or fatty rissue The autopsy material used for the current study was interposed between bundles of muscle fibers was collected and prepared by Solberg et al.'* The close to the insertion to rhe condyie, which pro- sample consisted of intact joints that were macro- duced partial separation bur wirh no consistent scopically cut into lateral, central, and medial organization. In these three specimens, two had thirds prior ro rhe preparation of histologie sec- three such divisions, and the other had two divi- tions from each third. The sections were prepared sions (Fig 1). The mean confidence score for both at 90 degrees to the long axis of the condyie, the presence of these divisions as well as the which would on average represent a 20-degree cor- absence of divisions in the other specimens was 1.5 rection from the true parasagittal plane. This pro- on the 4-point scale. tocol was designed to minimize oblique sectioning and erroneous presentation of soft tissue rhickness, which were critical to other studies using this Relationship Between the Lateral Pterygoid material. It is recognized that this angular correc- Muscle and the Disc tion does not provide ideal longitudinal orienta- Eleven of the 16 medial sections studied (69%) tion of the fibers of the lateral pterygoid muscle. A were characterized by a parallel arrangement of the total of 16 medial and 18 central sections were lateral pterygoid muscle fibers with rhe disc. In this selected for the present study. The mean age of the configuration, muscle fibers traveled parallel and sample was 26.2 years, range 16 to 35 years; 30% inferiorly to the disc to insert entirely into the were female and 70% male. pterygoid fovea of rhe condyie (Fig 1). There was a fibrous mesh of loose connective tissue between the disc and superior limit of the muscle. These fibers Histoiogic Assessment connected with the lining tissue of the anterior-infe- Projection microscopy was used ro exatnine the rior joint recess, which in turn was continuous with anatomic divisions of the lateral pterygoid muscle the articular fibrous connective tissue at rhe anteri- at the insertion site and the relationship of the or-superior aspect of the condyie. muscle to the articular disc. Medial sections (n = In the remaining five specimens (31%), there 16) were selected for rhis purpose because rhe were some muscle fibers interdigitating directly 244 Voiume 8, Number 3. 1994 Bittar into the anterior band of the disc (Figs 2 and 3) and 2) and confirmed by the presence of muscle while the majority of the muscle showed the paral- striations visible at higher magnification (Fig 4). A lel configuration inferiorly. The distinction be- layer of fibrocartilage was identified between rhe tween muscle and che fibrous connective tissue of tendon fibers and rhe compact bone of rhe ptery- the disc was confirmed by the identification of goid fovea (Figs I and 5). tnitscle striations at high magnification. The inter- Inferior to the tendinous insertion, muscle fibers digication oi'the muscle with the disc and the mus- inserted into the periosreum of the compact bone cle with the tendon appear identical at this magni- surface without an intermediate tendon or fibro- fication (Fig 3), The mean confidence score for the assessment of the muscle insertion relationships was 3,5, Table 1 shows the relative amount of muscle fibers inserting to the anterior band of the disc as a proportion of both the superior-inferior length of the total muscle insertion and the disc thickness ar thar sire. For the five joints wich muscle fibers inserting directly to the disc, muscle fibers occupied 20% to 30% of the thickness of che anrerior band and consrirnred 2.4% to 6.3% of the rotal lareral prerygoid inserrion lengrh. Four joints in the sam- ple had anteriorly displaced discs- All four showed the parallel configurarion in which rhe lareral ptery- goid inserted only ro the condyie (Table 1). Histologic Features of the Lateral Pterygoid Insertion Fig 1 Parasagittai histologie section frotii the medial