========== ANNALS Of ANATOItIY ========== Accessory fasciculi of the human pronator teres muscle: Incidence, morphological characteristics and relation to the median nerve J. Nebot-Cegarra, F. Reina-de la Torre and J. Perez-Berruezo Universitat Autonoma de Barcelona, Facultat de Medicina, Departament de Ciencies Morfologiques. Unital d'Anatomia i Embriologia, E - 08193 Bellaterra, Spain Summary. The purpose of this study is to determine the incidence of the accessory fasciculi of the pronator teres muscle and to evaluate the predispositional role of its mor- phological characteristics to median nerve entrapment. The accessory fasciculi of the pronator teres muscle have been found in 8.3 % of 60 upper extremities which were studied. The se fasciculi arose from the tendon of the brachialis muscle (5.0 %), from the radial tendon of the biceps brachii muscle (3.3 %), from the Gantzer's muscle ( 1.6 % ) or from the flexor digitorum superficialis muscle ( 1.6 %) . They were slips of origin of the deep head of the pronator teres muscle. They represented the deep head, when there was no other origin slip, or they were addi- tional origin slips when the deep head also had a coronoid origin. Only the accessory fasciculi that represented the deep head and the accessory fasciculi located inside to the coronoid fasciculus were related directly to the median nerve . Three characteristics of these accessory fasciculi adjacent to the median nerve could be considered to be factors that predispose to the nerve entrapment: I) the location dorsal to the superficial head of the pronator teres muscle, because it increases the area of contact with the median nerve ; 2) the location between the superficial head and the coronoid fasciculus, because it reduces the space occupied by the median nerve; and 3) the tendinous nature of considerable segments of short accessory fasciculi because they are less extensible. Key words: Pronator teres muscle - Median nerve - Entrapment neuropath y - Anatomical variations Correspondence to: J. Nebot-Cegarra Ann Anat (1994) 176: 223 - 228 Gustav Fischer Verlag lena Introduction The role of the accessory fasciculi of the pronator teres muscle in the entrapment of the median nerve has been demonstrated surgically (Bell and Golner 1956; Kopel! and Thompson 1958; Nigst and Dick 1979; Hartz et al. 1981; King and Dunkerton 1982). In order to evaluate the possible significance of the characteristics of the accessory fasciculi of the pronator teres muscle in this compressive syndrome, data about their location , tendinou s or/and mus- cular fibrillary composition , morphometry and relationship to the median nerve should be provided. These anatomical data have to be compared with the known clinical data. However, neither the above quoted surgical observations nor the anatomi cal studies focused on this subject (MacAlister 1866; MacAlister 1868; MacAlister 1875 ; Testut 1884; Le Double 1897 ; Kopsch 1955; Lanz and Wachsmuth 1959; John son et al. 1979; King and Dunker- ton 1982; Dellon and MacK innon 1987; Bergman et al. 1984) have provided an adequate basis for this analysis, because they only provided partial information. Furthermore, surprising divergences have been reported in their statistical incidence (Wood 1867 ; Wood 1868; Beaton and Anson 1939; Johnson et al. 1979; King and Dunkerton 1982; Dellon and MacKinnon 1987). In the present study, we have analyzed the incidence of the accessory fasciculi of the pronator teres muscle as well as their morphology and neurovascular relation s. Further- more, we discuss the possible predispositional role of these anatomical characteristics to the entrapment of the median nerve.