Are the Two Heads of the Human Lateral Pterygoid Separate Muscles? A Perspective Based on Their Supply

M. Ashraf Azir Based on biomechanic and eiectromyographic studies, it has been Associate Professor argued that tbe two heads of the human lateral pterygoid muscle Department of Anatomy (LPt) are reciprocally active during the masticatory cycle. Thus, it has heen proposed that the heads he considered separate muscles. Robert J. Cowie However, questions about the accuracy of these data have arisen. Associate Professor Department of Anatomy The authors hypothesized that partition cannot be complete without an independent nerve supply. To test this, complete unilateral lateral Cécile E. Skinner pterygoidectomies were performed on 20 dissection room cadavers. Associate Professor A novel approach, using an en bloc method, proved optima! to Department of Anatomy expose the detailed nerve supply to the LPt heads. In the two most Department of Fixed Prosthodontics frequently observed patterns OS of the 20 specimens), the heads were supplied from a common source that was derived from either Tsion S. Abdi the long huccal or , or from a loop that arose Research Assistant between the long buccal and lingual . In a third pattern, inde- Department of Anatomy pendent branches to either head arose from the deep temporal, long huccal, or mandibular nerve. In only 20% of the specimens did the Gavin Orza me Research Assistant two heads receive exclusive innervation from separate sources. The Department of Anatomy most significant finding of the present study is that hoth LPt heads in humans are usually supplied hy a common proximate source, hut Howard University College of Medicine each head also receives independent nerves in every case. In the Washington. DC absence of precise information ahout the functional components in each nerve branch, these data appear to support Juniper's proposal Correspondence to; to regard the two LPt heads as entirely separate muscles. M A Aziz, PfiD J OROFACIAL PAIN 1993;M:226-239. Departmont of Anatomy iHoward University College of Medicine key words: lateral pterygoid heads, mandibular nerve, independent 520 W Street Northwest Washington. DC 20059 innervation,

ypically the human lateral pterygoid muscle (LPt) has two dis- tinct heads of origin separated by a fascial plane that conducts Tthe long .'"^ Although there is agreement regard- ing the separation of the two heads at their origins, the degree of separation at their insertion is in dispute. Generally, the inferior head (ILPt) insens on the pterygoid fovea, while the superior head (SLPc) is reported to be variably attached to the disc (meniscus), condyle, fovea, and/or the capsule of the temporomandibular joint (TMJ).^"^* Based on phylogenetic,^'** anatomic,' biomechanic,^^"^' and eiec- tromyographic (EMG)'''-3"5 evidence, it has been suggested that the two LPt heads be regarded as separate muscles, in fact, it has been recommended that the SLPt be designated the "sphenomems- cus,"^'^ "protrudens menisci,"-'^ or "superior pterygoid"^' muscle. Juniper^' has proposed that the designation "lateral pterygoid" be retained for the ILPt only. Numerous biomechanic and physiologic (EMG) studies suggest that the two LPt heads are reciprocally active during the masticatory cycle.^"-^^.-,« However, because of

226 Volume 12. Number 3. 1998 Aziz et al

their incomplete separation, variable insertion, deep Table I Specimen Profile location in the masticator space, and interdigitation Estimated with fibers of the temporalis and medial pterygoid Specimen Race Gender Side age (y) muscles, they can be differentiated only with con- HU 121 C siderable difficulty. Since the LPt is often com- F Right 50-60 HU 122 C F Left 50-60 pletely covered by the temporalis laterally and by HU 123 Wi M Right 60-70 the medial pterygoid medially, the precise place- HU 126 c F Left 50-60 ment of recording electrodes in the LPt heads is HU 127 A.A M Left 50-60 problematic. Therefore, at least some of the func- HU 129 C M Left 50-60 tional data is equivocal.'^•-'•^^•^^ HU 130 A-A M Right 60-70 HU131 C M Right 50-60 Because of the intimate topographic relationships HU 131D A-A M Right 50-60 of the LPt heads to the TMJ and adjacent nerves HU 132 A-A M Left 50-60 and blood vessels, the muscle has been implicated m HU 133 C M Right 50-60 temporomandibular disorders.-'"-''-'^ It has been HU 134 c M Left 5CU60 HU 139 claimed that spasms of or trauma to the SLPt alone c M Left 50-60 HU 140 A-A F Left 50-60 are involved in the anterior dislocation of the HU141 c M Right 50-ßO .3i but several recent studies argue agamst HU143 c F Left 50-60 this view.'*'-^'^'*'^- HU 144 A-A M Left 60-70 Each of the arguments to regard the two LPt HU 146 C F Right 60-70 HU 148 c M Left 50-60 heads as separately operating muscles would be HU155 c F Left 50-60 strengthened if it could be shown that the LPt heads receive independent innervation from the mandihu- l-Amertcan; C = Caucasian^ Mi ^ Mixed, lar nerve. Despite cursory observations on the nerve supply to the human LPt^-^'*^' there have been no studies specifically targeted to show the precise ori- reflection''^; ¡2) freeing the to reflect gins and distribution of the nerves that innervate the the heads laterally near their insertions at or close to LPt heads. the disc-condyle assembly"*-; (3} careful removal of The objective of the present study was to rest the the floor of the middle cranial fossa by osteotomy to hypothesis that separate muscles with independent expose the proximal branches of the mandibular functions are supplied by separate nerves. If the two nerve from its superior aspect; and (4) excision of a heads of the LPt act as separate muscles in any part tissue block that contained the intact lateral ptery- of the chewing cycle, then they must be supphed goid with the entire mandibular nerve inferior to the independently. A delineation of the actual course trigeminal ganglion, the disc-condyle assembly, and and distribution of the nerves that supply the two the associated blood vessels. The first and second heads also has potential implications for the differ- approaches proved unsatisfactory for the present ential diagnosis and treatment of TMJ dysfunaion. purposes because when the muscle heads or the Because some variability in the innervation of the condyle were reflected laterally, most of the small and delicate LPt nerves were unavoidably detached LPt is to be expected, the authors also aimed to doc- from either their sources or their targets. Although ument the extent of the variability. Preliminary promising, the third approach was laborious and reports of this work have heen previously time consuming, and it did not always yield an presented.''^•''•' unobstructed view. The en bloc lateral pterygoidec- tomy provided the most accurate record; this approach is described here in some detail because, Materials and Methods to the authors' knowledge, it has never been reported. The protocol was as follows. Unilateral lateral pterygoidectomies were performed on 20 dissection room cadavers; the opposite sides The superficial structures (eg, the parotid gland of the cadavers had been used by medical and den- and duct, the , etc) were removed. tal students for a deep dissection of the masticator The maxillary and the distal portions of the space in accordance with the technique of masseteric and long buccal nerves were carefully Sauerland.**** The sources of the LPt specimens are identified and retained in situ throughout the proce- detailed in Table 1. dure to establish topographic relationships. Several approaches were tested in an effort to The was entirety removed by completely expose the LPt nerve branches: (¡) mobi- osteotomy. The buccal fat pad over the buccinator lization of the two heads at their origins with lateral muscle and its extension into the infratemporal

Journal of Orofacial Pain 227 Aziz et al

Fig 1 Procedure for removal of intact rissue block from deep masticator space. (A) Three planei for osteotomy of the mandibular ramus thar are required to expose the lateral pterygoid muscle. (B) Lateral view of LPt tissue block disc-condyle assembly with its nerve branches, ADT - anterior deep temporal nerve; IA = inferior alveolar nerve; L - ; LB - long buccal nerve; M - ; PDT - posterior deep temporal nerve.

fossa were removed. Osteotomy of the mandibular ramu.s between these cuts was eased out to expose ramus wa.s performed along the three planes indi- tbe masticator space. cated in Fig 1. The coronoid process and the inser- The exposed connective tissue was carefully tion of the temporalis were mobilized hy means of cleaned to delineate the lingual, inferior alveolar, a cut that was made along line 1. Section 2 was and mylohyoid nerves and tbeir associated vessels. made approximately 1.75 cm inferior to the Tbe coronoid process and temporalis tendon were mandibular condyle to preserve the LPt insertion reflected superiorly after the long buccal nerve was on the fovea, while cut 3 completely excised the freed from its passage through the temporalis ten- mandibular ramus from its body to sever the itife- don. The nerve was followed medially to the connec- rior alveolar nerve and vessels. The portion of the tive tissue gap between the LPt heads, but no further.

228 Volume 12, Number 3, 1998 AZIZ et al

Right side

Masseteric nerve

Posletiot tleep tempotai nerve Anterior deep témpora nerve

Fig 2 Template used to record precise distribution of mandibular nerve branches to rhe rwo LPt heads, hi rhis and all similar figures the structures are shown in a modified frontal view: the specimen faces the observer.

The lateral surfaces of the LPt and the medial was pushed inferiorly, which caused the anterior, pterygoid were cleaned of the pterygoid venous posterior, and medial parts of the TMJ capsule to plexus and fascia, and ail of the branches of the become taut and exposed. They were then cut hori- and mandibular nerve were identi- zontally to partially free the disc-condyle assembly fied. The portion of the maxillary artery that cov- from its assoctation with the mandtbular fossa. ered the LPt and the sphenomandibular bead of the Under a dissecting microscope, the areas medial to temporalis was removed to completely expose the rhe lingual and inferior alveolar nerves were carefully lateral surface of the LPt, A sharp-ended probe was cleaned to mobilize each nerve from the mandibular inserted between the periostium that covered the nerve to its origin. This procedure allowed careful bony SLtrfaces of the tnftatemporal crest, the greater exploration wirh in the fascial plane hetween the LPt wing of the spbenoid, and the SLPt, This muscle and medial pterygoid without fear of disruption to was cleanly elevated from its ortgins, which allowed the small nerves that supply these muscles. all of the nerves and vessels related to the supenor 1 he origins of the ILPt were freed from the lateral surface (masseteric and ) to pterygoid plate and maxtlla wtth a sharp scalpel. A remain attached. hlunt prohe was carefully retnserted between the The lateral temporomandibular ligament and the infratemporal surface of the sphenoid and the SLPt capsule were incised horizontally to open the TMJ. to ease the etitire block of tissue down and expose A probe was inserted between the disc and the the trunk of the mandtbular nerve (anteriorly) and and the disc-condyle assembly the (posteriorly). The

Joumal of Orofaciai Pain 229 Aziz st ai

Table 2 Overview of Nerve Branches That Supplied Lateral Pterygoid Heads Independent Common Independent superior LPt nerves LPt nerves inferior LPc nerves Specimen Pattern V3 LBN Other V3 LBN V3 LßN Other

HU 121 A-B 1 1 ioop 2 loop HU 122 C 1 DT 2 HU 123 A 2DT 1 2 HU 126 A 1 1 DT 1 2 HU127 A 1 1 1 1 HU 129 C 1 1 1 HU 130 A 1? 1 1 HU 131 A 1 1 2 HU 131D B 1 1 1 2 loop HU 132 A 2 1 DT 1 1 HU133 A 1 2DT 1 1 1 Lng HU 134 A 1 1 2 HU139 c 1 2 2 mma HU 140 c 1 1 1 HU 141 A 1 ! 1 HU 143 A 2 2 1 1 HU 144 A 1 1 1 1 HU 146 A 1 1 HU148 A 1 1 2 HU 155 A 1 1

Total Specimens 11 10 6 3 13 8 12 3 Branches 13 11 8 3 13 13 16 5 LPt = lateral pterygoid rerve. V3 = mandibukir nerve. LBN ^ long buccal nerve. DT = deep temporai nen/e. Lng = lingjal rlervei loop = ansa. mandibular nerve and the middle meningeal artery number of nerves from their respective origins to the were cut as close to their respective foramina as pos- two LPt heads was counted; this numeric data is sible. The intact block was lifted out of the mastica- presented in Table 2. tor space. The block included the LPt, the TMJ disc- condyle assembly, rhe mandibular nerve with its truncated major branches, and the attached blood Results vessels. The outhne of the tissue block that was removed is shown m Fig L Individual records of our observations were grouped The tissue block was identified, dated, and in similar patterns of nerve distribution and are washed thoroughly. A dissecting microscope was shown in Table 2 and Figs 3 to 6. In the most preva- used to further clean the fascia and expose the lent pattern, each LPt head received a branch from a nerves. The long buccal nerve was cleaned carefully common source that originated either from the long in the cleft between the two LPt heads to expose the buccal or mandibular nerve (Fig 3). This pattern was branches that were freqtiently fount! to arise from it. designated as the common lateral pterygoid nerve, To locate most other nerve branches, it was neces- and it was observed in 15 of 20 specimens (75%). sary to carefully clean tbe connective tissue between This configuration is show^n as pattern A in Fig 4a. the mandibular nerve and the LPt heads. By dissec- In 12 of these 15 cases, the common nerve origi- tion of the superior aspect of the tissue block the nated directly from the long buccal nerve in the cleft twigs that supply the SLPt from tbe deep temporal between tbe two heads. In two more of these 15 or masseteric nerve were exposed. cases, the common nerve arose independently from To record the observations of each specimen, the the inferior division of the mandibular nerve. One of precise distribution of mandibular nerve branches the specimens (HU 155] that was included in the and their topographic relations to each LPt head first group of 12 also sbowed a common nerve that were drawn on a standardized coronal view as arose from the inferior division of the mandibular depicted in Fig 2. Photographic records of each nerve (Fig 3). In anotber case, which is described specimen at critical junctures were kept, in addition below as pattern B, a common LPt nerve was also to notes made throughout each dissection. The found in a specimen with a nerve loop.

230 Volume 12, Number 3, 1998 Aziz et si

Fig .î Nerve supply to the LPt heads in individua specimens. Note that each specimen displays a com- mon LPt nerve that arises from the long buccal nerve (for example.^ see Figs 5 and 9).

Journal of Orofacial Pam 231 Aziz et al

Right

Fig 4a Innervation pattern A, Fig 4b Innervation pattern B, Fig 4c Innervation pattern C, which shows common LPt nerve which shows branches from an which shows sources of indepen- sources to both heads. ansa, or loop, that includes a com- dent nerve supplies. mon LPt nerve.

Fig 5 ¡left and right) Nerve sup- ply to LPt heads in two specimens that show a loop, or ansa ptery- goidea, which provides branches to the inferior head (compare with Fig 9). Note that the superior head in specimen HU 121 also receives innervation from a common LPt nerve that arises from the ansa.

Fig 6 Nerve supply to LPt heads in specimetis that display variable sources of distribution (see Fig 10 for example

232 Volume 12, Number 3, Aziz et ai

Within the main group of 14 specimens with a that bridges the long buccal and lingual nerves (pat- common nerve, the superior head received an addi- terns A and B, Fig 4), Thus, our most significant tional independent branch or branches directly from finding is that both LPt heads in humans are usually another source in 12 tissue blocks (Figs 3 and 7). stjpplied by a common proximate source. Based on This additional superior lateral pterygoid nerve otir hypothesis of the necessity of separate nerve (SLPtN] originated from either the mandibular, long supplies to support independent muscle actions, this buccal, or deep temporal nerve. Similarly, the infe- finding could lead to the conclusion that the two rior head was supplied by an independent branch LPt heads should function as a single unit. (ILPtN) or branches, which arose either from the However, this conclusion is unwarranted because mandibular or long buccal nerve; this phenomenon each head receives a separate branch from the com- was also encountered in 12 of these 15 cases. mon source. Additional independent branches to each head were almost universally seen (17 of 20 Independent nerves in addition to the common cases) to arise from sources such as the deep tempo- source that supplied both LPt heads were observed ral, long buccal, and/or mandibular nerves. These in 12 instances with pattern A (Fig 8). data seem to support Juniper's^^ proposal to regard A second pattern, pattern B (Fig 4b), was distin- tbe two LPt heads as entirely separate muscles. The guished by the presence of a nerve loop (the ansa anatomic, functional, and clinical implications of pterygoidea), which communicated between the these findings, including contrary viewpoints, are long buccal nerve and the lingual nerve. It always further explored below. gave rise to branches that supplied the inferior LPt head (Fig 5). This pattern was found in 2 of the 20 Without knowledge of the exact functional com- specimens (10%). Similar to those cases with a com- ponents of the axons that comprise each nerve mon LPt nerve, specimen HU 121 displayed a com- branch (sensory, motot, or mixed), no categoric mon branch from the loop that supplied the supe- detetmination of the degree of muscle separation can rior and inferior heads (Fig 9), Also similar to most be made. The ultimate answer lies in neuronal trac- of the specimens that exhibited pattern A, each head ing studies designed to label the populations of received additional branches either from the long motorneurons and/or primar;' afferent neurons that buccal or the mandibular nerve. supply each LPt head. While studies of this nature The remaining four cases (20%) were distin- cannot be conduaed in humans, it may be possible to obtain this information in nonhuman primate ot guished by the absence of a common nerve source suid models. Juniper's^^ proposal will be unequivo- and the relative exclusivity of their nerve supply to cally upheld only if it is found tbat the motorneurons the LPt heads (pattern C, Fig 4c). As illustrated in or primary afférents that supply each head belong to Fig 6, within this group the superior head received different populations within the trigeminal motor its innervation from the anterior deep temporal (HU nucleus and/or the mesencephalic trigeminal nucleus. Î22), long buccal (HU 139 aud HU 140), or mandibular nerve (HU 129 and HU 140) (Figs 6 Given the intimate relationship of the LPt heads and 10). The inferior head was supplied by to the proximal portion of the mandibular nerve branches from the long buccal (HU 129, HU 139, (SLPt in particular), it is evident that if one or both and HU 140) and/or the mandibular nerve (HU of the heads were injured as a result of latrogenic 122, HU 129, and HU 139). events, ttaumatic force, vascular accident, or infec- tious agents, one or mote of the following adverse In a finding not directly related to the innervation outcomes could occur (Fig 11): of the LPt, 2 of the 20 cases (HU 140 and HU 146) showed that the anterior deep temporal nerve coursed through the substance of the superior LPt 1. Entrapment neuropathy of the long buccal head but did not supply it (Figs 3 and J). nerve, which passes between the two LPt heads or medial to them and includes common and independent LPt branches 2. Fntrapment neuropathies of the deep temporal Discussion and masseteric nerves, which pass within the The above observations lead to several important SLPt head or between it and the bony roof of implications about the nerve supply to the two LPt the heads. In an over^vhelming number of cases (1^ of 3. Fntrapment neuropathy of the inferior root of 20) the two heads were supplied from a cominon the mandibular nerve itself as it passes between LPt nerve that was derived from the long buccal or the SLPt and the cartilaginous wall of the mandibular nerve, or from the ansa pterygoidea nasopharynx (torus tubarius)

Journal of Orofacisl Pain 233 Aziz et ai

Fig 7 Lacerai view of tissue block HU 132 that shows Fig 8 Medial aspect of tissue block HU 141 thar shows the common LPr nerve ¡B) that arises from the lottg buc- the cotnmon LPt nerve (B) and an independent branch to cal nerve (A) and supplies both heads. Inferior LPt head the inferior LPt head (N) that arises from the mandibular (F) is retracted wirh forceps (compare with Fig 3], C = nerve (M), It should be emphasized diat the divisions of superior LPt nerve; D = branch to inferior head; E = supe- tbe mandibitlar nerve intimately conform ro the contours rior LPt bead; G = disc-condyle assembly. of tbe sphenoid and the cartilaginous wall of the nasopharynx (compare with Fig 3). A - long buccal nerve; C = superior LPt nerve; D - branch to inferior head; F = superior LPt head; F = inferior LPr bead; O = nerve ro masseter; P = posrerior deep temporal nerve.

Fig 9 Right lateral view of the nerve loop (X), which Fig 10 Superomedial aspect of rissue block HU 129 supplies rhe inferior LPt head in tissue block HU 1.Î1D, that shows independent branches (C and D) to both LPt The entire LPt muscle was removed from around the loop heads in the absence of a common LPr nerve. Here, it to expose the mandibular nerve at its entrance to the can be appreciated that all branches rhat pass superior to infratemporal fossa via the foramen ovale (compare with the SLPt are in direct contact witb rhe infraremporal sur- Fig 5). A = long bttccal nerve; D - branch to inferior face of the sphenoid, Tbe mandibular nerve (M) was head; lA - inferior alveolar nerve; L = lingual nerve; M - truncated (compare with Fig 6), A = long huccal nerve; E mandibular nerve; 0 = nerve to massesrer; P = posterior = superior LPt head; F = inferior LPt head; G - disc- deep temporal nerve. condyle assetnbly; O = nerve to masseter; P = posterior deep temporal nerve.

234 Volume 12, Nutnber 3, 1998 Aziz et al

Fig 11 Mandibular nerve and its distribution. Anteromedial view emphasizes the close and inriinate topography of the region with its numerous sites of possible nerve injury and entrapment. Note tbat ncitbcr the bony and cartilaginous superior and medial walls of the infratemporal fossa nor tlic vascular network are sbown so that cbe complex neural distri- bution may be appreciated. TTn = tensor tympani neri'e^ TVPn - tensor veli palatini nerve.

Such conditions might easily result in the various organism. Since the LPt is involved in so many sites of pain and paresis associated with temporo- critical behaviors (opening, closing, and transla- mandibular disorders. Once begun, these specific tion of tbe during the masticatory cycle; conditions within the rigid and confined mastica- protrusion during incisive biting; and derived func- tor space could affect (and be aggravated by) oro- tions such as panting, yawning, vomiting, vocaliza- facial behaviors other than mastication. Figure 11 tion, and facial signaling), it appears reasonable provides a summary perspective of the topographic rhat natural selection would favor the observed relations of the mandibular nerve to the LPt heads. adaptation for several independent sources of Since there was no independent source to a innervation for eacb head. This organization is given LPt head in addition to the common LPt insurance against noniatrogenic injury and im- nerve in only three cases, one wonders why the paired function. heads were supplied from diverse origins. From an The implications of our results mnst also be evolutionary perspective, nerves that arise from a viewed in relation to additional anatomic and func- source other than a common one would ensure the tional criteria for the separation of the LPt heads. continued function essential for the survival of the To be considered independent, a muscle sbould

Jojrnai of Orofacial Pain 235 Aziz et al show several traits in addition to an independent LPt beads are only differentiated by the presence nerve supply and a definable set of central nervous of type IIB fibers (which show high threshold, system refle.x controllers. Thus, each independent fast twitch, and fatigue resistance). The SLPt has muscle "belly" should: (I) have a clearly discernible a greater proportion of these than its inferior origin and insertion and be enclosed in its own fas- counterpart. cial compartment; (2) display a consistent myoar- The lateral pterygoid is also characterized by a chitecture and muscle fiber type; and (3) exhibit low incidence of intrafusal muscle fibers (afferent definable ontogenetic and phylogenetic origins. spindles), which are chiefly found in its inferior There has been general consensus for many years part.'"'"'^ The SLPt appears to be mostly bereft of that the LPt muscle originates from two distinct these stretch receptors.^^"^^ Thus, whereas the ILPt heads. Recently, Abe et al'** confirmed this in 79 has neural feedback during jaw opening, protru- human samples; tbey found that 70% of LPt mus- sion, and contralateral excursion, the superior cles originate from two completely distinct bellies. bead is activated by some as yet unknown modal- However, there were instances where the two heads ity during ¡aw closing and clenching.^° These find- were incompletely separated even at their origins. ings support the previously discussed idea that the Despite numerous macroscopic and microscopic primary afférents to either head may be different studies over many years, the precise insertion, and in topography and/or type. therefore the degree of separation of the two Embryologie studies indicate the early direct heads, remains controversial. Investigators have attachment of the SLPt onto the developing reported that the insertion of the SLPt is: (1) not ¿¡g^26,58-69 j^Qj. g contrary view see Baume and attached to the disc,*"* (2) exclusively attached to Holtz^"). The discomalleolar ligament (with its the disc,^''^"'-'**^ (3) attached to tbe disc and the potentially associated "laxator tympam" muscle^') condyle,'--i---'^^'"'*'''" (4) attached to the disc and and the anterior ligament of the malleus are con- pterygoid fovea,^^'^^'^^"'"' (5) attached to the disc sidered to be "atavistic" posterior extensions of and the TMJ capsule,'**'^'•-^•''^ (6) attached to the the SLPt^"'^^'™ (atavism refers to the retention of disc, condyle, and fovea,'^''^ and (7) attached evolutionary relics). It has also been argued that exclusively to the fovea.^- the disc itself is derived at least partially from the To address these discrepancies, careful investiga- 51_Pf60,fi 1,66,68,72 (f^^ dissenting views see tions by Wilkinson and Maryniuk,'^ Flatau and Furstman,*^ Youdelis,^'' and Baume and Holtz^°). Klineberg,-^' and Klineberg''^ have revealed that the Based on tbe comparative myology of the LPt in SLPt inserts on the condyle and/or the pterygoid different mammals, Prentiss'^'^ has suggested that fovea but that a connective tissue bridge anchors the the two heads probably originated at different times "sole of the foot" of the disc to the SLPt. Bittar et and have evolved at different rates. Fie regarded tbe al^^ concur with this view, but also observed that the SLPt as homologous to the sphenomeniscus muscle two heads inserted into tbe fovea by different mecha- of ancestral mammals. The common innervation is nisms when examined histologically. Thus, it likely to be a derived characteristic in primates. The appears that the differences in the detailed manner of fact that in 20% of the present cases the two heads connective tissue attachment of the LPt heads leads were exclusively supplied by their own nerve may to the observed differences in the action of the mus- bolster Premiss''-'* contention. cles and movements of the disc-condyle assembly. This work summarized the differences between With reference to their myoarchitecture, the two LPt heads with reference to their attach- Schumacher^ and Widmalm et aP' have stated that ments and fascial enclosure, myoarchitecture and the fibers of the SLPt exhibit a parallel configura- histochemistry, and ontogenetic and phylogenetic tion, while the ILPt displays a pennate arrange- criteria. The present data also appear to argue for ment. Schumacher^ also found that the Sl.Pt and the partition of the lateral pterygoid muscle. the ILPt are rotated medially 37 degrees and 33 However, a careful examination of these data degrees, respectively, from the parasagittal plane; shows that they are limited and as yet preliminary. they lie 20 degrees above and 20 degrees below the It is necessary to collect and more critically evaluate occlusal plane, respectively. Schumacher thus con- information in all of the above categories with cluded that the vector of the pull of the two heads larger sample sizes before formally splitting the "probably will be different" (see also McDevitt^ muscle. As indicated previously, if neural mapping and Aziz and Cowie'*-'"^). were to show the unequivocal segregation of Based on histochemical fiber analysis, Eriksson centrally located neurons, the partition of the mus- and coworkers"'^-' have recorded that the two cle should be sanctioned.

236 Volume 12. Number3. 1998 Aziz et al

Acknowledgments 20. Bertilsion O, Strom D. A literature survey of a hundred years of anatomic and functional lateral pterygoid muscle We thank Mr Arnold Miller and Mr Eugene Downing for pho- research. J Orofacial Pain 1995;9:17-23. tography. We are also indebted to Ms Bonnie Cobbs and Ms 21. Heylings DJA, Nielsen I, McNeill C. Lateral pterygoid Alyce Smith foi preparing an early draft of our manuscript. We muscle and the temporomandibular disc. J Orofacial Pain are grateful to Mr Romain Demarais and Dr John Young for 1995;9:9-16. their translations of the summary into French and Getman, 22. Naidoo LCD. Lateral pterygoid muscle and its relation- respectively. Finally, we thank Dr Blair Tumer for his helpful ship to the meniscus of the temporomandihuiar joint. Oral suggestions. We respectfully acknowledge the individuals who, Maxillofac Surg 199â;82i4-9. hy the donation of their remains, made this research possible. 23. Vaughan HC. The external prerygoid mechanism. 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Resumen Zusammenfassung

¿Son las Dos Porciones dei Pterigoideo Laterai en el Sind die Beiden Spitzen des Menschlichen Quei- Humano Mtjscuios Separados? Estudio Perspecrivo Pterygoid Separate Musiiein? Eine Perspective die auf Basado en su Surninistro Nervioso Ihrer Nervelichen Ausstattung Basiert

Estudios biomeeánicDs y electromiográfieos han generado discu- Auf dem Grund der biomechanische und eiectromyographische siones sobre si las dos porciones superiores del músculo Studien, ein abwechseind Wirkung der iwei Teilen der Muskei pterigoideo lateral en el humano, son activas reciprocamente pterygoideus iateralis (LPtl des Menschen durch die Kajzykius durante el ciclo masticatorio. Por io tanto, se ha propuesto que war vorgeschiagt. Aiso, die beide Teilen i^onnte Mann wie estas porciones superiores sean consideradas músculos separa- isolierte Musi

Joumal of Orofacial Pain 239