Okajimas Folia Anat. Jpn., 65 (1): 29-34, May, 1988

Case Report

An Anomalous Case of the Risorius Arising from the Masseter Tendon

By

Johji SEKINE, Noriyasu HAMADA, Hidetoshi TOH and Tadao OHMORI

Second Department of Oral Anatomy, Fukuoka Dental College, Sawara-ku, Fukuoka 814-01, Japan *Undergraduate of Fukuoka Dental College

-Received for Publication, April 4, 1988-

Key words: Anomaly, Risorius, Masseter tendon, Origin, Facial dimple

Summary: The present report describes an anomalous case of the risorius arising from the masseter tendon of an 83-year-old Japanese male used for student dissection practice. In this case, the risorius appeared to be almost of the typical type. However, one part of the risorius arose not from the fascia or skin over the masseter but directly from the masseter tendon itself. Such a situation is considered rare, since only one similar case has been reported by Eisler (1912).

The risorius, one of the facial muscles, However, we found a striking example of arises in the fascia or skin over the masseter the risorius in which the origin was the firm and inserts into the skin or the depressor masseter tendon. This paper describes and anguli oris at the angle of the mouth. The discusses the detailed anatomical findings risorius retracts the angles of the mouth, or and one of the possible functions of the produces a facial dimple. risorius in relation to a facial dimple. Many descriptions of the morphology of the risorius have given since that in 1724 by Santorini, the first reporter of the risorius. Materials and Methods Ruge (1885, 1887, 1896) and Bluntschli (1910) classified the morphology of the The present anomaly was found in the risorius into 3 types: Triangularis-risorius, left cheek of an 83-year-old male who died Platysma-risorius and Zygomaticus-risorius. of inferior pharynx cancer and was used for Nevertheless, the risorius is said to vary student dissection practice at Fukuoka much in its size and form (Bluntschli, 1910; Dental College in 1986. The dissection of Eisler, 1912). In general, the border between the risorius including its innervation and the risorius and other facial muscles tends to surrounding muscles was performed care- be clear, and the risorius and its surrounding fully. After making macroscopic observa- facial muscles are not fused together tions, the junction of the risorius and (Jitozono, 1957a,b). masseter tendon was removed and stained

29 30 J. Sekineet al. with hematoxylin-eosin. Light microscopy connective tissue reached the skin over the was then undertaken to elucidate the platysma (Figure 3, arrow). detailed features of the junction. Light microscopic observation of the In the cadaver, no scar was found out in junction of the risorius and the masseter the left facial skin. tendon revealed the typical findings of a muscle-tendon junction. Findings Discussion In this case, the risorius was apparently of an almost typical type (Figures 1-3). It Morphological variations of the risorius consisted of some narrow bundles of fibres have been observed and described by (width, 1-2 mm). The bundles of the risorius Santorini (1724), Ruge (1885, 1887, 1896), could be divided into two parts, an upper Bluntschli (1910), Eisler (1912) and others, part (UP) and a lower part (LP), according and recently in Japan by Kojima (1953), to their origins (Figure 2). Okeya (1954), Jitozono (1957a) and others. The upper part of the risorius arose Ruge (1885, 1887, 1896) and Bluntschli directly from where the four masseter (1910) classified the variations of the tendon fibres were attached beneath the risorius into 3 types; Triangularis-risorius, platysma (about 5 mm backwards from the Platysma-risorius and Zygomaticus-risorius, anterior border of the masseter, and about according to the details of its derivation. 10 mm below the parotid duct). Away from Kojima (1953), who surveyed the Japanese the origin, the bundle of fibres divided into risorius, indicated that Triangularis-risorius two and passed through the platysma. On occurred at a frequency of 77.5%. In the the platysma, the bundles of fibres passed case of defect of Triangularis-risorius,fibres forwards about 15 mm dividing into four, of the platysma or the zygomaticus major and then reached the depressor angli oris. form Platysma-risorius or Zygomaticus- The lower part of the risorius arose in the risorius. However, because the risorius varies masseteric fascia about 10 mm below the so much in its form (Bluntschli, 1910), it is origin of the upper part on the platysma. The considered difficult and imprecise to classify top bundle of fibres arose in the skin over the risorius according to its morphology. the masseter, passing forwards about 15 mm To resolve this problem, Kojima (1953) parallel to the base of the mandible, and attempted to classify the variational patterns then attached to the bundles of fibres of the of the risorius according to the innervation: upper part. Another bundle of fibres, which Triangularis-risorius is innervated by the was fan-shaped, arosejust beneath the origin buccal branches of the facial nerve, of the top bundle of fibres, passing forwards Platysma-risorius by the mandibular branch, about 20 mm, and then reached the and Zygomaticus-risorius by the zygomatic depressor angli oris (Figure 3). branches. According to Kojima (1953), the The risorius, including both the upper present risorius, innervated by the buccal and lower parts, was supplied entirely by the branches of the facial nerve, would thus buccal branches of the facial nerve (Figure 2, belong to Triangularis-risorius. arrow). In some textbooks, the origin of the The junction of the risorius and the risorius is given as the masseteric fascia or masseter tendon was surrounded by dense skin over the masseter. However, in the connective tissue. Furthermore, the dense present case, one part of the risorius arose Anomalous Case of the Risorius 31 directly from the firm masseter tendon, and single action of the risorius but also by the the junction of the risorius and the masseter cooperative action of the muscle and its tendon showed the typical features of a surrounding connective tissue. muscle-tendon junction. Eisler (1912) noted that the risorius References sometimes originated in the masseteric aponeurosis. However, there is something 1) Argamaso, R.V.: Facial dimple: Its formation unclear in Eisler's description of the by a simple technique. Plast. Reconstr. Surg., aponeurosis. According to PNA, the super- 48: 40-43, 1971. ficial portion of the masseter arises in a 2) Bluntschli, H.: Beitrage zur Kenntnis der Variation beim Menschen, I und II. strong tendon from the anterior two-thirds Morpholog. Jahrbuch, Bd.XL, 195-261, 1910. of the lower border of the zygomatic arch. 3) Boo-Chai, K.: The facial dimple: Clinical From this standpoint, the aponeurosis study and operative technique. Plast. described by Eisler (1912) would appear to Reconstr. Surg., 30: 282-288, 1962. be a tendon by PNA. Thus, the situation in 4) Eisler, P.: Die Muskeln des Stammes. Bardelebens Handb. d. Anat. d. Menschen, which the risorius arises from the masseter Abt. II, Teil 1, Gustav Fischer, Jena, 1912. tendon does not seem to have been reported 5) Fujita, T. and Terada, H.: Textbook of except by Eisler (1912), so that the present somatology, 12th ed., pp.57, Nanzando case is very rare. Company Ltd., Tokyo, 1976 (in Japanese). Concerning the function of the risorius, it 6) Jitozono, C.: Morphological and anthropo- logical study of mimic muscle. Part 1. is said not only to retract the corner of the Anthropological study on the facial muscle in mouth but also to produce a facial dimple: a South Kyushu. J. Kagoshima Med. Soc., 9: round depression of the buccal skin which 427-440, 1957a (in Japanese). occurs in conjunction with smiling (Nakao, 7) Jitozono, C.: Morphological and anthropo- 1958). Fujita et al. (1976) reported that a logical study of mimic muscle. Part 2. The study on differention and growth of the dimple would be produced by retraction of mimic muscle. J. Kagoshima Med. Soc., 9: the risorius that was attached just below the 441-458, 1957b (in Japanese). skin. 8) Kojima, Z.: M. risorius kaj kia nervo. Acta Recently, in the field of plastic surgery, it Anat. Nippon., 28: 157-167, 1953 (in has become possible to form an artificial Japanese). facial dimple by surgical techniques (Boo- 9) Nakao, Y.: Somatological study on the dimples in Japanese adult woman. Acta Anat. Chai, 1962; Argamaso, 1971). The method Nippon., 33: 317-329, 1958 (in Japanese). of Boo-Chai (1962) is to connect the skin 10) Okeya, T.: On the facial muscle in the and the active facial muscle (usually the Japanese fetus. Bulletin of Dept. of Anat., buccinator) using exogenous suture material. Kanazawa Univ. Faculty of Med. (Author's trans.), 47: 49-87, 1954 (in Japanese). The tissue condition created by this simple 11) Ruge, G.: Uber die Gesichtsmuskulatur der method appears to resemble that of the Halbaffen. Morpholog. Jahrbuch., Bd.XL: present case, i.e. connection of the risorius, 243-315, 1885. the masseter tendon and the platysma by 12) Ruge, G.: Untersuchungen ilber die dense connective tissue. We are attempting Gesichtsmuskulatur der Primaten, W. to determine whether the man, the cadaver Engelmann, 8 Taf. Leipzig, 1887 (Cited from 2). of this case, had a facial dimple on the left 13) Ruge, G.: Uber des peripherische Gebiet des cheek during his life. Nervus facialis bei Wirbeltieren. Festsch., Bd. In summary, it is suggested that a facial III: 183-348, 1896 (Cited from 2). dimple may be produced not only by the 14) Santorini, G.D.: Observationes anatomicae, Cap. I., pp.32, 1724 (Cited from 2). 32 J. Sekine et al.

Explanation of figures

Plate I

Fig. 1. Facial muscles on the left side.

Fig. 2. Origins and innervation of the risorius. Arrow shows the buccal branches of the facial nerve.

Fig. 3. Whole appearance of the risorius. Arrows show dense connective tissue reaching the skin over the platysma.

Abbreviations: B: Buccal branches of the facial nerve, D: Parotid duct, DA: Depressor angli oris, LP: Lower part of the risorius, M: Masseter, MT: Masseter tendon, P: Platysma, R: Risorius, UP: Upper part of the risorius. Anomalous Case of the Risorius 33 Plate I