Topographic Anatomy of the Mandibular Nerve Branches Distributed on the Lateral Pterygoid Muscle

Kang, Hyo Chang; Kwak, Hyun Ho; Park, Hyun Do; Kang, Min Gyu; Kim, Hee Jin

Abstract

According to the electromyographic study, the superior and inferior heads of lateral pterygoid, one of the masticatory muscles, are known to perform the reciprocal actions in mandibular movements; the superior head is active in closing movement, whereas the inferior head is active in opening of the jaw. Through these two reciprocal actions of these two heads, the articular disk of TMJ is seated in its resting position. Many reports regarded the superior and inferior heads of the lateral pterygoid as functionally independent muscles, but from a morphological point of view, the origin of the independent nerve innervation and intramuscular branching patterns of the mandibular nerve are unclear. Twenty -four adult hemi -sectioned heads were dissected to clarify the topography of the nerve distribution on two heads of lateral pterygoid and also to determine the anatomico -clinical relevance related with temporomandibular disorder. Most buccal nerves were found to run between the superior and inferior heads of the lateral pterygoid (21 cases, 87.5 %). In 3 cases, buccal nerves passed through the inferior head of the lateral pterygoid (12.5%). In front of the ascending ramus region, most buccal nerves ran in front of the temporalis without being entrapped within the temporalis (16 cases, 66.7%). However, in 8 cases buccal nerve passed in front of the temporalis being entrapped within the anterior fiber of the temporalis (33.3%). In this study, the mandibular nerve trunk was located intimately to the lateral pterygoid. Both heads of the lateral pterygoid muscles were innervated from the mandibular nerve branches, but the patterns of nerve distribution were various. Nerves innervated to the superior head of the lateral pterygoid had different origin. Only in 45.8% (11 cases), they originated from the buccal nerve. In 16.7% (4 cases) the nerve branches originated from the anterior deep temporal nerve only, and in 12.5% (3 cases) from the buccal and anterior deep temporal nerve. In contrast, nerves innervated to inferior head of the lateral pterygoid showed different pattern of distribution. In 58.3 %, nerves distributed on the inferior head originated from both the buccal and mandibular nerve trunk. In 20.8%, they originated from the buccal nerve only, in 12.5% from the mandibular nerve trunk only. In seven categories of the distribution of mandibular nerve branches, in only 20.8% (5 cases), both the superior and inferior heads of the lateral pterygoid had the common source of nerve innervation, the buccal nerve. In contrast, in 45.9% (11 cases) additional nerve twigs from the mandibular nerve trunk were distributed on the inferior head of the lateral pterygoid muscle. Author observed the pterygoid loop (ansa pterygoidea) located between the mandibular nerve trunk and the nerve innervating to the lateral pterygoid in 4 cases. In addition, the intramuscular nerve loop within the inferior head of lateral pterygoid was observed in 5 cases. Summarizing these results, besides the buccal nerve mentioned in an anatomical textbook, nerve twigs originating directly from the mandibular nerve trunk innervated to the lateral pterygoid and the ones originating from the anterior and middle deep temporal nerves distributed on the lateral pterygoid muscle in various manners. Considering the various patterns of nerve distribution on the lateral pterygoid, author concluded that the two heads of the lateral pterygoid are controlled by independent innervation. Furthermore, we found out that nerve entrapments and nerve communications are related with symptoms of temporomandibular disorder and with possible collateral route of motor innervation to the facial expression muscles, respectively.