Histoiogic Characteristics of the Lateral PterygoidMuscle Insertion to the Temporomandibular Joint

Geber T, Bittar. DDS, MSResearcfi Scholar

Carol A. Bibb, PhD, DDSAdjunct Associate ProfessorSection of Orofacial Pain and Occlusion

Andrew G, Pullinger, DDS. MScProfessorSeotion of Orofacial Pain and Occlusionand Dental Research Institute

School of DentistryUniversity of CaliforniaLos Angeles, California

Gorrespondence to:Dr Carol A. BibbSection of Orofacial Pain and OcclusionSchool of DentistryOHS 43-009University of CaliforniaLos Angeles, California 90024

This study used low-power light microscopy to examine the histo-logie organization of tbe laterai pterygoid muscle interface with thetemporomandibular joint. Tbe sample inciuded parasagittal sec-tions of 20 intact temporomandibular joints from young adults¡mean age 26-2 years) at autopsy. Tbe lateral pterygoid musclesbowed no consistent divisions into separate anatomic musclebeads at tbe insertion. Tbe muscie fibers attached to tbe pterygoidfouea of tbe condyie immediately inferior to tbe articular surface inall cases. Some additional fibers inserted superiorly into the moreanterior part of tbe articular disc in a minority of cases (31%).Vibers inserting into tbe disc represented only 2.4% to 6% ofthetotal superior-inferior length of the muscle insertion. It is hypothe-sized that the muscular force exerted by these few fibers insertinginto the disc would not be sufficient to displace tbe disc anteriorlyto tbe condyle. Tbere were two bistoiogic types of insertion of tbelateral pterygoid muscle to tbe condyle. Tbe superior part of tbeinsertion was characterized by an identifiable tendon insertingthrough fibrocartilage. In tbe inferior part of tbe insertion, tbemuscle attacbed to periosteum without an obvious tendon. Tbepresence of this tendon must be recognized in interpretation of softtissue temporomandibular joint imaging.J OROFACIAL PAIN 1994;8:243-249,

Anatomic and physiologic investigations of the lateral ptery-goid muscle insertion to the temporomandibular joint (TMJ}have generated considerable controversy. While studies have

consistently shown the pre,sence of two distinct muscle origins at thepterygoid plate of the sphenoid hone, the insertion of the muscle tothe cotnponents of the TMJ remains unclear. The unresolved issuesinclude whether there are anatomically and functionally distinctupper 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 insertsolely to the articular disc' or to the disc and anteriot capsule.^More recent histologie studies have demonstrated that the majorityof the superior fihers insert to the condyle with only a few fibersinserting to the articular disc, "* The latter studies have emphasizedthe variability in the relationship of the superiot fibers to thecondyle and disc, but they have not attempted to measure the pro-portion of fibers insetting directly into the disc or considered ananatomic relationship to disc position,

Electromyographic ¡EMG| studies have been similarly inconsis-tent; some have tepotted that the lateral pterygoid is a single func-

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rional unit," but the majority have reported func-rionally separate superior and inferior heads."-'Juniper^ has proposed that the superior headshotild be considered a separate nitiscle called thesuperior pterygoid muscle.

These conflicting studies have led co rhe com-mon usage of a diagram depicting a separate supe-rior head of the lateral pterygoid attachitig entirelyto the anterior portion of the articular disc."""'Together with a functional model for the lateralpterygoid muscle, in which the upper head func-tions independently and antagonistically to thelower head of the muscle during tnandibularmovements," the idea has heen proposed thathyperactivity of the muscle could cause the articu-lar disc ro dislocate anteriorly."

The ptirpose of the present study was to presenta detailed description of the microatiatomy of thelateral pterygoid tnuscle interface with the TMJ inyoung adults. It is anticipated that the results willhave implications for TMJ soft tissue itnaging andinternal derangement etiology.

Materials and Methods

Sample

The autopsy material used for the current studywas collected and prepared by Solberg et al.'* Thesample consisted of intact joints that were macro-scopically cut into lateral, central, and medialthirds prior ro rhe preparation of histologie sec-tions from each third. The sections were preparedat 90 degrees to the long axis of the condyie,which would on average represent a 20-degree cor-rection from the true parasagittal plane. This pro-tocol was designed to minimize oblique sectioningand erroneous presentation of soft tissue rhickness,which were critical to other studies using thismaterial. It is recognized that this angular correc-tion does not provide ideal longitudinal orienta-tion of the fibers of the lateral pterygoid muscle. Atotal of 16 medial and 18 central sections wereselected for the present study. The mean age of thesample was 26.2 years, range 16 to 35 years; 30%were female and 70% male.

Histoiogic Assessment

Projection microscopy was used ro exatnine theanatomic divisions of the lateral pterygoid muscleat the insertion site and the relationship of themuscle to the articular disc. Medial sections (n =16) were selected for rhis purpose because rhe

majority of the lateral pterygoid muscle fihers arereported to insert ro the anreromedial aspect of thejoint.'°--" Projection microscopy permitted discus-sion and consensus judgment by three examinersobserving a complete field of view at low magniri-carion. A confidence score with rankings of 1 =possible, 2 = probable, 3 = confident, and 4 = defi-nite was applied to each evaluation.

More detailed descriptive and histomorphomet-ric assessments of the lateral pterygoid insertiontissues were carried out under low-power lightmicroscopy using eentral (n = 18) and medial (n =16) sections. Higher magnification was used topositively identify muscle tissue by the presence ofstriations to differentiate muscle tissue from theadjacent fibrous connective tissue of the disc ortendon.

Results

Anatomic Divisions ofthe Lateral PtetygoidInsertion

None of the 16 medial TMJ sections showed anobvious division into a distinct upper and lowermuscle head ar rhe insertion site. In three of thesespecimens (19%), loose connective or fatty rissuewas interposed between bundles of muscle fibersclose to the insertion to rhe condyie, which pro-duced partial separation bur wirh no consistentorganization. In these three specimens, two hadthree such divisions, and the other had two divi-sions (Fig 1). The mean confidence score for boththe presence of these divisions as well as theabsence of divisions in the other specimens was 1.5on the 4-point scale.

Relationship Between the Lateral PterygoidMuscle and the Disc

Eleven of the 16 medial sections studied (69%)were characterized by a parallel arrangement of thelateral pterygoid muscle fibers with rhe disc. In thisconfiguration, muscle fibers traveled parallel andinferiorly to the disc to insert entirely into thepterygoid fovea of rhe condyie (Fig 1). There was afibrous mesh of loose connective tissue between thedisc and superior limit of the muscle. These fibersconnected with the lining tissue of the anterior-infe-rior joint recess, which in turn was continuous withthe articular fibrous connective tissue at rhe anteri-or-superior aspect of the condyie.

In the remaining five specimens (31%), therewere some muscle fibers interdigitating directly

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into the anterior band of the disc (Figs 2 and 3)while the majority of the muscle showed the paral-lel configuration inferiorly. The distinction be-tween muscle and che fibrous connective tissue ofthe disc was confirmed by the identification oftnitscle striations at high magnification. The inter-digication oi'the muscle with the disc and the mus-cle with the tendon appear identical at this magni-fication (Fig 3), The mean confidence score for theassessment of the muscle insertion relationshipswas 3,5,

Table 1 shows the relative amount of musclefibers inserting to the anterior band of the disc as aproportion of both the superior-inferior length ofthe total muscle insertion and the disc thickness arthar sire. For the five joints wich muscle fibersinserting directly to the disc, muscle fibers occupied20% to 30% of the thickness of che anrerior bandand consrirnred 2.4% to 6.3% of the rotal lareralprerygoid inserrion lengrh. Four joints in the sam-ple had anteriorly displaced discs- All four showedthe parallel configurarion in which rhe lareral ptery-goid inserted only ro the condyie (Table 1).

Histologic Features of the Lateral PterygoidInsertion

In the specimens studied, rwo histologie types ofmsertion of the lateral prerygoid to rhe pterygoidfovea were idennfied. Superiorly, muscle insertedinto a well-orgamzed fibrous connective tissue ren-don that in turn inserted inro the pterygoid fovea(Figs 1 and 2), The distinction between muscle andtendon was based on differential scainmg (Figs 1

and 2) and confirmed by the presence of musclestriations visible at higher magnification (Fig 4). Alayer of fibrocartilage was identified between rhetendon fibers and rhe compact bone of rhe ptery-goid fovea (Figs I and 5).

Inferior to the tendinous insertion, muscle fibersinserted into the periosreum of the compact bonesurface without an intermediate tendon or fibro-

Fig 1 Parasagittai histologie section frotii the medialthird of a TMJ from a 33-year-old female. The lateralpterygoid muscle runs inferiorly to the disc (which isanteriorly displaced) to itisert into the prerygoid füvea ofthe condyie. Note that the most superior muscle fibersinsert to the condyie via a tendon. Loose connective tis-sue (arrow) has produced partial separation of the mus-cle into two divisions close to the insertion,(Hematoxylin-ttlosin stain in all figures).

Fig 2 Parasagittai histologie section from the centralthird of a TMJ from a 34-year-old male, A few tnusclefibers insert in the anterior aspect of the disc (arrow)with the majority of the muscle Inserting inferiorly mtcthe pterygoid fovea of the condyie.

Fig 3 Higher magnification of the disc-muscle inser-tion identified in Fig 2. Note the interdigitation ofmuscle fibers (M) with the disc (D) superiorly, whileinferiorly muscle fibers interdigitate with the tendon (T).(Bar = 0.05 mm.)

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Fig 4 I'hotomiLTiigr.ipli of the tnLiscle-tendon interfacein rhe medial third of a TMJ from a lé-year-old female.Note that the distinction between muscle (M| and ten-don (T) is based on differential staining and the presenceof muscle striations (arrow), (Bar - Ü,02 mm.)

Fig 5 Photomicrograph of the lateral pterygoid inser-tion in the medial third of a TMJ from a 16-year-oldfemale. Superiorly, the tendon (T| of the lateral ptery-goid inserts to the compact bone (B) by a layer of fibro-cartilage (FC¡. Inferiorly, the muscle (M) inserts directlyto periosteum (P|. (Bar - 0.02 mm.)

Table 1 Relative Amount of Lateral Pterygoid Muscle (LPM) Fibers Inserting to Disc"

Joint

12345

6-16**

•Medial s-[ficludes

Disc: Anteriorband thickness

(mm)

1,91,51,51.11.5

ectionsall jomls with drsc displacenlent(n = •

Total LPMinsertion

length (mm)

8,810,06,47,2

12.3

Length of LPMinserting todisc (mm)

0.40.50,40,20,3

Proportion ofanterior band with

LPM fibers (%)

2030252020

0

Proportion ofLPM insertedto disc (%)

4,55,06,32,82,40

cartilage layer (Figs 1, 2, and 5). There were smallsecondary tendinous insertions Inferior to rhe maintendon in 3 of IS (17%) of the central sectionsand 4 of 16 (25%) of the medial sections (Fig 2).The tendinous insertion occupied 48.4% (range18% to 80%) of the overall superior-inferiorlength of the insertion in the central sections and44.5% (33% to 72%) in the medial sections.

Discussion

The present study examined the histologie organi-zation of the insertion of the lateral pterygoid mus-

cle to the components of the TMJ in an effort toresolve the controversy over the relationship of themuscle to rhe joint. The autopsy material studied isbelieved to he clinically relevant because the agerange of the sample (ló to 35 years) is consistentwith the population likely ro seek treatment forTMJ disorders-' and especially the segment of thepopulation with internal derangements.-- Four ofthe joints in the sample had obvious disc displace-ments. However, it must be emphasized that inthis cross-sectional autopsy study no history ofpain or mechanical dysfunction was available forcorrelation with the anatomic findings. The study

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is also limited hy the small sample size and theavailability of only one technically acceptable sec-tion from each joint region. However, it is difficultto acquire material from this clinically importantage group.

Contrary to common belief, anatomically sepa-rate superior and inferior lateral pterygoid bellieswere not found. While there was variahility in theinsertion of the most superior fibers of the muscleto the joint, the most frequent model (69%] wasthe parallel configuration in which muscle fibersinserted only into the pterygoid fovea on theanteromedial surface of the condyle rather than tothe articular disc. This finding is consistent withWilkinson's hypothesis that the articular discattaches to the roof of the superior lateral ptery-goid fibers by a "foot" of connective ttssue.'

In the five specimens in which a few of the supe-rior muscle fibers inserted directly into rhe articu-lar disc, this muscle insertion occupied less thanone third of the thickness of the disc at that siteand at most represented 6% of the overall inser-tion length. This represents the first attempt tomeasure the proportion of lateral pterygoid musclefibers inserting directly to the disc. It seems unlike-ly that the proportion of fibers inserting into rhedisc in comparison to those inserting into thecondyle would change markedly from section rosection,

A disc displacement etiology by spasm of the lat-eral pterygoid muscle is nor supported hy thisanaromic study. This does not mean that the later-al pterygoid does not exert any action on the disc,but rather that hypertonic contraction of these fewfibers would be unlikely to cause the articular discto shift independently to an anterior position. Thisconclusion is consistent with reports from previousdissection studies stating that anterior tension onthe lateral pterygoid muscle did not pull the discanteriorly." Furthermore, the presence of lateralpterygoid muscle fibers inserting directly into thedisc apparently does not predispose to disc dis-placement, because none of the fiye joints withmuscle fibers inserting directly to tbe disc had dis-placed discs (Table 1).

Variation in lateral pterygoid muscle insertionrelationships has also been described in other his-tologie invesrigations, iMeyerberg et aP examined25 TMJs and reported that 60% had a few lateralpterygoid muscle fibers inserting into the articulardisc. In a sample of 15 TiVIJs, Moritz and Ewers"reported that the inferior fibers of the lateral ptery-goid always attached to the pterygoid fovea, andin 12 of the cases (80%), some superior fibersattached anteromedially to the disc. In 9 of the 15

joints (60%), an additional insertion of superiorfibers into the medial joint capsule was also found.

This study shows that dissection alone is notsufficient to accurately display the anatomy of thelateral prerygoid/TMJ interface. Preyious studiesshowing distinct muscle heads at rhe insertion sitemay have been based on artifacts of blunt dissec-tion. Furthermore, histologie sections are requiredfor study under sufficiently high magnification todifferentiate muscle tissue from the adjacentfibrous connective tissue of the disc, tendon, andcapsule. These technical issues may account forsome of the conflicting interpretation by earlierauthors.

The classical literature describes rhe existence oftwo distinct types of tendinous insertion of muscleto bone,-'''* In a direct insertion, a layer of fibro-cartilage, called a "plug," is present between thetendon fibers and the compact bone. In an indirectinsertion, muscle fibers blend into the compactbone without the presence of a defined intermedi-ate fibrocartilage layer. The layer of fibrocartilagein the direct insertion makes the distinction be-tween the two types of insertion easily identifiableunder light microscopy.

In the present study, both types of insertionwere identified in the interface ofthe lareral ptery-goid muscle ro the condyle. A direct insertion witha dense organized fibrous connective tissue tendonwas observed in the more superior part of theinsertion, and fibrocartilage was present at the ten-don/bone interface. An indirect insertion waslocated inferiorly and did not have an organized,dense, fibrous tendon. It is interesting to speculatethat these upper and lower insertion types mayrepresent separate functional muscle bellies report-ed by EMG studies and certainly imply some dif-ferencial stimulus to these sites.

The results of this study have important clinicalimplications for the interpretation of magnetic res-onance imaging (MRI) of rhe TMJ, The clinicianshould not expect to see separate upper and lowermuscle bellies at the insertion site. The consistentpresence of a dense, fibrous connective tissue ten-don at rhe superior aspect of the muscle insertionto the condyle must also be recognized and caremust he taken to differentiate the low signal fromthe tendon in this location from anterior disc dis-placement, Beltran" has commented similarly thatthe tendon of the inferior belly of the lateral ptery-goid should not be confused with the disc onMRIs. However, he complicates the issue by stat-ing that the superior belly is attached to the disc bya tendon that was not observed in the presentstudy. It is noteworthy that some recent textbooks

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are presenting a more anatomically accurate dia-gtam of the relationship between the lateral ptery-goid muscle and the TMJ."

Acknowledgment

This study was funded by ihe Section of Orofacial Pain andOcclusion, UCLA School of Dentisiry,

References

1. Porter MR, The attachment of the lacerai pterygoid mus-c!e to ihe meniscus, J Prosrhet Dent 1970; 24:55^-561,

2. Honne GLJM, The anatomy of the lateral pterygoid mus-de, Aaa Morphol Ncerl Scand 1972:10:331-340,

3. Meyenberg K, Kuhik S, Palla S, Relationships of the mus-cles of maitication to the articular disc of the temporo-mandibular joint, Helv Odont Acta 1986;30:S15-S34,

4. Moritz T, Ewers R, Der ansatz des musculub ptcrygoideuslaceralis am kiefergelenk des menschen, Dt5ch Zahnärztl Z1987;42:680-685.

5. Carpentier P, Yung J-P, Marguelles-Bonnet R, MeunissierM. Insertions of the lateral pterygoid muscle: An anatomicstudy of the human temporomandibular joint, J OralMaxillofac Surg 1988;26:477-^82.

6. Wilkinson TM, The relationship between ihe disk and thelateral pterygoid muscle in tbe human temporomandibulatjoint.] Prosthet Dent 1988;60:71i-724.

7. Basmajian JW, Primary Anatomy, ed 6. Baltimore:Williams and Wilkins, 1970:137,

S, McNamara JA, The independent functions of the twoheads of the lateral pterygoid muscle. Am J .i^nat19SO;US:197-2O6.

3. Juniper RP, The superior pterygoid muscle? Br J Oral ,SurgI98];I9:I21-128.

10, Mahan PE, Wilkinson TM, Gibbs CH, Mauderii A,Brannon LS, Superior and inferior bellies of the lateralpterygoid muscle: EMG activity at basic jaw positions, JProsthet Dent 1983;50:7]0-718,

11, Gibbs CH, Mahan PE, Wilkinson TM, Maudt,'rli A- EM^^activity of the superior belly of the lateral pterygoid mus-cle in relation to other jaw muscles, | Prostiiei lient1984;Sh691-701,

12, Widmalm SE, LillieJH, Ash Jr MM, An.m>i)iiL.ii and elec-tromyographic studies of the lateral ptcrygiiid muscle. JOral Rehabil 1987;14:429-t46,

13, Toller PA, Surgery of the temporomandibular joint. In:Moote JR (ed). Surgery of the Mouth and Jaws, Onford:Blackwell Scientific, 1985:601,

14, Jankelson RR, Nc-uromuscular Dental Diagnosis andTreatment, St Louis: Ishiyaku EuroAmerica, 1990:4,

15, Ide Y, Naka?.awa K, Anatomical Atlas of the Tem-poromandibular Joini, Quintessence: Chicago, 1991;7S.Beitran J, MRI Musculoskeletal System, Philadelphia:Lippincott, 1990:2,10-2.11,

Juniper RP, Temporomandibular )oint dysfunction: A the-ory based upon electromyographic studies of the lateralpterygoid muscle, Br J Oral Maxillofac Surg ]984;22:l-8,Solherg WK, Hansson TL, Nordstrom B, The temporo-mandibular joint in young adults at autopsy: A morpho-logic classification and evaluation, J Oral Rehabil 198S;12:303-321,

] lyiander WL, Functional anatomy. In: Sarnat BG, LaskinDM {eds). The Temporomandibular Joint: A BiologicalBasis for Clinical Practice, ed 4, Philadelphia: Saunders,

20, Assad LA. Functional anatomy. In: Kaplan AS, Assad LA,Temporomandibular Disorders: Diagnosis and Treatment,Philaddphia: Saunders, 1992:6,

21, Clark GT, Mulligan RA, A review of the prevalence oftemporomandibular dysfunction, Gerontol 1984;3:231-236,

22, Pulhnger AG, Seligman DA. TMJ osteoarthrosis: A differ-entiation of diagnostic subgroups by symptom history anddemographics, J Craniomandib Disord Facial Oral Painl987;l;2Sl-25é.

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24, Woo S, Maynard J, Butler D, Ligament, tendon and jointcapsule insertion to bone. In: Woo S, Buckwaltcr JA (eds).Injury and Repair iif tbe Musculoskeletal Soft Tissues,Illinois: American Academy of Orthopedic Surgeons,1987:134-137,

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Resumen

Características histoiógicss de ia inserción del músculopterigoideo lateral en la articulación teporomandibiilar

Este estudio examinó la organización de la irterfase histológicadei músculo pterigoideo iaterai con la articuiación lemporo-mandibular (ATM) utilizando microsoopia de \uz a bajo aumento.La muestra incluyó secciones parasagitales de autopsias de 20ATM intactias de adultos jóvenes (edad promedio: 26.2 años).El músculo ptengoideo lateral no mostró divisiones consistentesen cuanto a cabezas rnusculares anatómicas separadas en iainserción. En todos los casos ias fibras muscuiares estabaninsertadas en la fosa pterigoidea dei cóndiio inmediamente infe-rior a la superficie articular En algunos casos (31%) se obser-varon aigunas fibras adicionales insertadas en sentido superioren la parte mas anterior dei disco articuiar. Las fibras que seinsertaban en el disco representaban solo dei 2 4% al 6% de iaiongitud Supero-infenor tolai de inserción muscuiar. Se presentala hipótesis de que ia fjerza muscuiar ejercida por estas pocasfibras insertadas en ei disco no sen'a suficiente para desplazaral disco antenormente ai cóndilo. Se observaron dos tipos his-tológicos de inserción dei múscuio pterigoideo laterai ai cóndiio.La parte superior de la inserción estaba caracterizada por untendón aparente. La presencia de este tendón debe ser recono-cida en ia interpretación de las inágenes diagnósticas del tejidobiandodeiaATM,

Zusammenfassung

Histologische Eigensehaften det Ansatzstelle des M,pterygoideus lateralis am KiePergelenk

Diese Studie untersuchte die histologische Struktur derAnsalzsteiie des M pterygoideus lateralis am Kiefergelenk licht-mikroskopisch Die Probe umfasste 20 parasagittaie Schnittevon 20 intakten Kiefergeleni en ¡urger Erwachsener (mittleresAlter 26.2 Jahre) in Autopsie Der M. pterygoideus iateraiiszeigte an der Ansatzsteile keine deutiiche Aufteiiung in einzeineanatomische Musi<eikópfe. In ailen Falien setzten dieM u s ke i fasern an der Fovea pterygcidea des Kondyius urmittel-bar unterfialb der Gelenkflacher an Einige zusätzliche Faserninserierten oberhaib davon in den vorderen Teil des Diskus.aber nur bei einer Minderheit der Fäile 131 %). Vor dergesamten in superior-inferiorer Richtung gemessener Langeder Muskeiansatzstelle machten die in den Diskus einstrahlen-den Fasem nur gerade 2.4%-6% aus. Man nimmt an, dass dieMuskelkraft dieser wenigen in den Diskus inserierenden Fasernnicht ausreicht, um den Diskus vor den Kondyius zu ver-schieben. Der M. pterygoideus inserierte auF zwei histologischverschiedene Arten am Kondytus. Der obere Teii des Ansatzeswar gekennzeichnet durch eine erkennbare Sehne, die durchFaserknorpei hindurch ansetzte, im unteren Teii der Ansatzstelieinserierte der Muskel ohne sichtbare Sehe direkt am Periost.Das Vorhandensein der erwähnten Sehne muss bei derInterpretation von Weich teil bilde rn des Kiefergeienkes beachtetwerden.

Journal of Otofacial Pain 249