Clinical History
A 41 year-old woman was referred to our hospital because of pain in the medial aspect of her knees of three months duration. Clinical evaluation addressed a possible tear of the medial menisci. An MR examination of both knees was requested.
Imaging Findings
In her right knee the MRI revealed a lobulated ganglion located between the plantaris muscle (PM) and the popliteal vessels, communicating with the joint [Fig. 1]. The PM, on both knees, originated from the supracondylar ridge. Additionally a fusiform structure in both knees was revealed connecting the iliotibial band and the plantaris muscle. This structure originated from the iliotibial band just below the tendon of vastus lateralis muscle. Its course was parallel to the lateral femoral condyle, deep to the iliotibial band anteriorly and biceps muscle posteriorly. It merged with the belly of the PM. The morphology and signal intensity of this structure were reminiscent of muscle tissue. The findings (course, morphology, signal intensity, bilaterality) are consistent with the diagnosis of an accessory plantaris muscle on both knees [Fig. 1, 2].
Discussion
In cadaveric studies the PM may originate either from the lateral supracondylar ridge, the lateral condyle of the femur, the oblique popliteal ligament, the posterior capsule of the knee joint, or the posterior surface of the femur. When the PM has a bicipital origin, two areas are combined [1, 2]. In the most common bicipital type (prevalence of 35.41%), the PM originates from the supracondylar ridge of the femur and the posterior surface of the lateral femoral condyle [2]. A number of rare bicipital types of origin are reported in the literature as an incidental finding during cadaveric dissections [3, 4].
A retrospective review of 1000 knee MR examinations, performed because of acute or chronic knee symptoms, revealed unusual origin of PM from the iliotibial band, the lateral patellar retinaculum, or the iliotibial tract, with an overall prevalence of 6.3% [5]. In our case we report an accessory PM that had a usual origin from the lateral supracondylar ridge, and an unusual one from the caudal part of the iliotibial band.
An accessory PM may play a role in the patellofemoral pain syndrome if it originates from the lateral surface of the patella or the oblique popliteal ligament [6]. Moreover an accessory PM originating from the iliotibial band may play a role in pathologic conditions, such as patellar tracking disorders or iliotibial band friction syndrome [5]. In our case the patient had no radiologic signs of such pathologic conditions. The incidental posteriorly located ganglion cyst wasn’t considered as the cause of the patient’s symptoms. It is of interest that this anatomic variation was found on both knees.
Differential Diagnosis List
Accessory plantaris muscle
Accessory plantaris muscle
Accessory fibrous band
Final Diagnosis
Accessory plantaris muscle
