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

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Are the Two Heads of the Human Lateral Pterygoid Separate Muscles? a Perspective Based on Their Nerve Supply Are the Two Heads of the Human Lateral Pterygoid Separate Muscles? A Perspective Based on Their Nerve 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 mandibular nerve, or from a loop that arose Research Assistant between the long buccal and lingual nerves. 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, temporomandibular joint ypically the human lateral pterygoid muscle (LPt) has two dis- tinct heads of origin separated by a fascial plane that conducts Tthe long buccal nerve.'"^ 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 joint capsule 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 masseter muscle, etc) were removed. tal students for a deep dissection of the masticator The maxillary artery 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 zygomatic arch 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 - lingual nerve; LB - long buccal nerve; M - masseteric nerve; 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.
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