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Surg Radiol Anat (2010) 32:519523 DOI 10.1007/s00276-009-0571-2
ANATOMIC VARIATIONS
Congenital absence of the semimembranosus muscle: case report
Valeria M. Moncayo Walter A. Carpenter Claude Pierre-Jerome Robert D. Smitson Michael R. Terk
Received: 8 July 2009 / Accepted: 25 September 2009 / Published online: 8 October 2009 Springer-Verlag 2009
Abstract Congenital anomalies of the muscles of the lower extremity are rare. A case of complete absence of the semimembranosus muscle incidentally found with magnetic resonance imaging is reported. The patient was a 55-year-old female presenting with knee pain and no previous history of trauma. Clinical and imaging Wndings were consistent with meniscal tears. Two cases of uncommon distal insertion of the semimembranosus tendon are also described to illustrate the anatomical variations in this area. The anatomical and biomechanical relations between the semimembranosus muscle and the posteromedial corner of the knee are examined in this case report.
Keywords Knee Hamstring muscles
Congenital absence Anatomical variation Semimembranosus muscle Semimembranosus tendon Magnetic resonance imaging
Introduction
Absence of the semimembranosus muscle detected by magnetic resonance imaging (MRI) has not been reported before. Human anatomical variations of the skeletal muscle are not uncommon. However, muscle absences in the lower extremity speciWcally variants of the hamstring muscles are rare. They can represent an incidental Wnding in radiologic examinations or in postmortem studies. Skeletal muscle variations include: congenital absence, aplasia, duplications, aberrant muscles and anomalies in origin or insertion. Usually they are unilateral and limited to a particular muscle or muscle group [2, 5].
The semimembranosus muscle so-called by its membranous tendon, originates in the posterior and lateral aspect of the ischial tuberosity above and lateral to the biceps femoris. It is the largest tendon in the popliteal fossa, and it inserts primarily in the horizontal groove of the posterome-dial aspect of the medial condyle of the tibia and meniscocapsular structures of the knee [5, 7]. The functions of the semimembranosus muscle as part of the hamstring muscles are to extend the hip, Xex the knee and medially rotate the lower leg when the knee is bent [5]. Furthermore, the semi-membranosus provides stability to the posteromedial aspect of the knee by preventing anteromedial rotatory instability.
There are Wve tendinous expansions in the meniscocapsular complex of the distal semimembranosus insertion: (1) the anterior...