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.
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].
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.
Accessory plantaris muscle
1. On the bilateral knee MRI sequences, there is a visible additional tendon shadow in the posterior region, suggesting a variant course of an extra gastrocnemius or plantaris muscle. Based on the literature, this is more likely an "Accessory Plantaris Muscle."
2. This extra muscle exhibits a “double-headed” origin, with one head arising from the supracondylar ridge of the lateral distal femur and the other seeming to continue from the distal portion of the iliotibial band.
3. Adjacent to the lateral side, the local signal of the iliotibial band near the additional muscle appears normal, without obvious inflammatory edema or thickening.
4. No obvious abnormal signal is seen in the semimembranosus or semitendinosus tendons and their attachment sites, nor are there any clear signs of meniscal tear.
5. A small cystic lesion is observed posteriorly with signal characteristics consistent with either a tendon sheath cyst or a synovial cyst adjacent to the joint capsule. It is relatively small in volume, and there is no significant inflammatory reaction in the surrounding tissue.
6. No other obvious abnormality is noted in the remaining soft tissues, cortical bone, or articular cartilage. The articular cartilage signals and thickness of the femoral condyles and proximal tibia are generally within normal limits, and there is no significant abnormality in the shape or signal of the medial or lateral meniscus.
Considering the patient’s age, symptoms, physical examination, and MRI findings, the primary imaging features in this case include bilateral accessory plantaris muscles (Accessory Plantaris Muscle) as an anatomical variation and a small posterior cystic lesion. Currently, there is no clear evidence of meniscal tear or any other significant soft-tissue abnormality. In most cases, an accessory plantaris muscle is an incidental finding, and in the absence of other structural damage or inflammatory signs, it is not typically considered the main cause of knee pain. There is no imaging evidence supporting a definite meniscal tear in this case, and the cystic lesion appears mildly changed. Thus, it is more likely an “accessory plantaris muscle anatomical variation” with a small accompanying cyst, without definitive imaging alterations explaining medial knee pain.
1. Conservative Treatment and Observation:
Given no obvious meniscal tear or significant soft tissue pathology on imaging, and that accessory plantaris muscles are usually discovered incidentally, a conservative approach is recommended first. This may include nonsteroidal anti-inflammatory drugs (NSAIDs) to relieve pain, reducing excessive joint load, and strengthening the quadriceps and hamstring muscles through rehabilitation exercises. If symptoms improve, no further invasive treatment may be necessary.
2. Physical Therapy and Exercise Prescription:
- Initial Phase (Weeks 1-2): Focus on range-of-motion exercises and low-intensity muscle strength training, avoiding excessive load and large-range flexion/extension of the knee. Exercises such as seated knee extensions, straight leg raises, and small-range knee bends can be performed for 10-15 minutes, 3-4 times per week.
- Intermediate Phase (Weeks 3-6): As symptoms permit, gradually increase lower extremity strengthening exercises, such as resistance band activities, partial squats, or leg press exercises in a seated position. Perform these for 15-20 minutes, 3-4 times per week, monitoring knee pain and progressing gradually.
- Rehabilitation Phase (After 6 weeks): If knee pain significantly decreases during weight-bearing and activities, low-impact aerobic exercises (e.g., stationary cycling, elliptical training) can be introduced to maintain muscle strength and cardiovascular fitness. Each session can last around 30 minutes, 3-5 times per week, adjusting intensity based on subjective fatigue and knee joint response.
3. Other Interventions:
- If symptoms recur or functional impairment of the knee develops later, arthroscopic evaluation may be considered to rule out hidden meniscal injuries or other soft tissue lesions.
- In rare cases where it is confirmed that the accessory plantaris muscle is the core cause of symptoms (e.g., mechanical friction or fascia tension has been definitively identified), targeted injection therapy or surgical intervention can be considered, though this is uncommon.
This report is based solely on the current clinical and radiological information for academic and reference purposes and does not replace an in-person consultation or the treatment plan provided by a qualified physician. If symptoms persist or worsen, it is recommended to seek immediate in-person evaluation by a specialist for further examination and treatment.
Accessory plantaris muscle
