Clinical history: A 58 year old woman presents for MRI with right knee and leg pain and swelling medially for 3 weeks, without a specific injury. Sagittal proton density images with fat saturation at the medial aspect of the knee (1a,1b) are provided. What are the findings? What is the diagnosis?
1: Acute medial gastrocnemius muscle strain.
2: Chronic degenerative medial gastrocnemius tendinosis, with partial tearing of medial tendon fibers, and with femoral bone marrow edema.
3: Joint effusion, with fluid within the gastrocnemius-semimembranosus bursa.
Abnormalities involving the medial and lateral gastrocnemius tendons are often not considered among the many possible causes of knee pain. The medial gastrocnemius is rarely affected in traumatic injuries to the knee, and symptomatic chronic degenerative changes are also relatively uncommon. However, case reports in the literature1,2, describe gastrocnemius tendon degenerative interstitial, longitudinal or partial-thickness tearing, and note that such abnormalities may cause clinical symptoms mimicking symptoms from more common lesions such as meniscal tears or collateral ligament strains. The medial and lateral gastrocnemius tendons proximal myotendinous regions are included in the field-of-view of knee MRI exams. Familiarity with the normal gastrocnemius tendon MRI features, as well as the spectrum of MRI findings with different types of pathology that may involve these structures is relevant for accurate interpretation of knee MRI.
Anatomy and Function
The medial and lateral gastrocnemius tendons together with the soleus muscle form the calf. The gastrocnemius muscles extend half-way down the lower leg, distally contributing to the Achilles tendon. The gastrocnemius muscles but not the soleus cross the knee joint, and they all distally cross the tibiotalar joint (and the subtalar joint as well). Innervation is by the tibial nerve. Vascular supply to the gastrocnemius muscles is by stout short branches from the adjacent popliteal artery and vein; detailed studies of the venous vascular configuration has demonstrated 4 main patterns, with 2 to 12 branches per muscle head3. The proximal tendons of the gastrocnemius muscles originate at the medial and lateral femoral epicondylar regions; the medial tendon is thicker along its medial margin and closer to midline contain small amounts of interspersed fat.
The gastrocnemius bursa is located deep to the tendon in the proximal myotendinous region, and usually communicates with the semimembranosus bursa. Fluid in the bursa is frequent when there is gastrocnemius tendinopathy, but is non-specific as it also occurs with internal derangement of the knee joint and multiple other pathologies. When the gastrocnemius-semimembranosus bursa is distended, this is termed a Baker’s cyst, frequently communicating with the knee joint through a 15-20 mm slit between the two, thought to occur at a region of frequent capsular insufficiency, and sometimes with a one-way valve function leading to continuous filling of the cyst4.
The main gastrocnemius muscle function is plantar flexion of the foot, but it also provides flexion of the non-weight-bearing knee, and acts as an agonist for the PCL. The gastrocnemius muscles have a higher proportion of fast twitch fibers than the soleus, reflecting their function in jumping and running, while the soleus, with a higher proportion of slow twitch fibers, mainly functions in walking and postural control. In addition, all the calf muscles have a role as a venous pump for the lower limb (the “calf muscle pump” function)5.
Of interest, the gastrocnemius muscle bellies are known by bodybuilders to be quite difficult to hypertrophy. This has been attributed to the already high repetitive use of these muscles during daily activities.
Mechanism of Injury and Clinical Findings
Gastrocnemius tendon degeneration may result from prior gastrocnemius injury, chronic overuse, or underlying posteromedial or posterolateral knee joint instability. Gastrocnemius tendinosis may progress to an interstitial tear, longitudinal split tear, partial tear, or very rarely a complete tear. Clinical symptoms usually develop gradually and may include local pain and tenderness, as well as swelling at the posteromedial or posterolateral knee and proximal lower leg, weakness, and limited range of motion.
Normal MRI anatomy
The proximal tendons of the gastrocnemius muscles are flat, and originate at the medial and lateral femoral epicondylar regions in a linear fashion towards midline, slightly proximal to the condylar articular margin. At the medial gastrocnemius tendon origin, there are 2 features of importance for MRI evaluation: (1) the origin extends from the epicondyle (about 1 cm distal to the adductor magnus tendon insertion) and runs obliquely so that it is more proximal near the femoral posterior midline, and (2) the tendon is thicker and more defined medially, and more attenuated closer to femoral midline (4a,4b).
A small amount of fat tissue is normally present deep to the tendon fibers towards the midline, and often a small amount of fluid is present normally at the adjacent recess at the posterior capsular femoral insertion (6a).
On cross-sectional images, the low-signal tendon is seen at the medial to central aspects of the medial gastrocnemius, while closer to midline there is muscle to near the bony origin, with attenuated tendinous fascicles. The tendon remains well defined for only 1-2 cm distal to the medial joint line (7a).
A common finding at the medial gastrocnemius involves the muscle adjacent to the origin extending partly into the most proximal aspect of the intercondylar notch (8a). This is not known to be related to symptomatology.
Abnormalities of the Gastrocnemius Tendon on MR images
It has been noted that symptomatic abnormalities at the proximal gastrocnemius tendon and myotendinous junction are rare. This is in contrast to the well-described more distal gastrocnemius muscle injury (“tennis leg”). The clinical significance of proximal tendon injury has also been questioned6. It is therefore not surprising that the MRI features of proximal gastrocnemius tendon injuries have rarely been reported in the literature1,2,7. The MRI findings associated with abnormalities of the proximal medial gastrocnemius tendon have been described to include high signal within the tendon on proton density and T2-weighted images, poorly defined tendon margins, and sometimes a contour defect usually involving the most medial tendon fibers, at or near the femoral origin (9a).
Tendon thickening which can be focal or diffuse (10a,10b), or tendon attenuation may also be present. Intratendinous interstitial longitudinal tearing has also been reported2. Usually there is no associated proximal myotendinous muscle edema.
A complete gastrocnemius tendon tear is quite rare in the setting of chronic tendinosis, and is also rare in cases of acute knee trauma. Fluid in the gastrocnemius bursa, located deep to the proximal tendon and immediately superficial to the posteromedial joint capsule, is associated with medial gastrocnemius tendon pathology but is non-specific as it is commonly found with many types of knee joint pathology.
It is important to consider the “magic angle” MRI artifact as a common cause of intratendinous intermediate signal within especially the medial gastrocnemius tendon. This artifact can be seen on short TE images (such as T1s) in regions where a tendon is oriented at 55 degrees to the main magnetic field, such as the medial gastrocnemius tendon about 2 cm distal to the femoral origin; correlation with T2 weighted images will help to exclude focal pathology (11a).
Focal symptoms from the proximal gastrocnemius tendon at its femoral origin may also be caused by HADD, the rarely reported acute gastrocnemius avulsion fracture8, or fabellar fracture with or without a related tendon tear9,10. Symptoms from the adjacent proximal myotendinous region may relate to retracted longitudinal partial tendon tears2, rare intramuscular dissecting Baker’s cysts11,12, or venous thrombosis limited to the calf muscle veins13.
Hydroxyapatite deposition disease (HADD) is a rare but well-described cause of acute knee pain14. The medial margin of the medial gastrocnemius tendon is characteristically involved (12a), with a low signal center and surrounding soft tissue edema. There is often adjacent mild reactive bone marrow edema and synovitis at adjacent joint or bursa. Correlation with radiographic exams is often useful, and resolution of the calcific deposit can be followed radiographically after therapy.
In calcium pyrophosphate crystal deposition disease (CPPD), in addition to chondrocalcinosis there is often calcification involving tendons. Around the knee, calcification of the medial gastrocnemius tendon has been reported15. Acute gastrocnemius avulsion fracture has rarely been described in the literature, and would result from an acute injury; if there is significant displacement, surgery may be indicated.
The fabella is a small sesamoid located within the lateral gastrocnemius at the level of the lateral femoral condyle. It is present in 10-20% of the population, and is usually ossified with a small facet of hyaline cartilage towards the joint, or may be fully cartilaginous (13a). It might show participation in osteoarthritis of the knee joint, with cartilage loss, subarticular marrow edema, and marginal osteophytes causing overall enlargement of the fabella which occasionally leads to localized pain related to impingement during knee flexion. Fabellar fracture9 or displacement10 are rare causes of acute posterolateral knee pain, while localized discomfort labeled “fabellar syndrome” has been described as a cause of insidious chronic symptoms, accessible to manual fabellar mobilization therapy16.
A dissecting Baker’s cyst11 may present with tender fullness at the posteromedial aspect of the knee just below the joint; these lesions represent rare instances where a Baker’s cyst enters through the muscle fascia and is present often both outside and within a muscle compartment. Normally a Baker’s cyst enlarges in the direction of least resistance, most commonly along the medial gastrocnemius muscle belly distally. If a focal fascial defect occurs, or at a pre-existing weak region, the Baker’s cyst can enter the muscle compartment. As with other Baker’s cysts, treatment of the underlying cause of the joint effusion is indicated, as the cysts are manifestations of fluid decompressing out of the joint due to higher intra-articular pressure.
Venous thrombosis of the lower leg most commonly involves the deep vessels and often also the calf muscle veins, and is often thought to originate at these muscle veins. A small percentage of cases demonstrate thrombosis limited to the calf muscle veins; these cases may present with symptoms similar to post-exertion calf muscle pain, and may be clinically mistaken for a muscle strain13.
Other abnormalities involving the gastrocnemius tendons or proximal myotendinous region include an accessory gastrocnemius muscle belly, post-surgical changes related to the use of the gastrocnemius for a vascular flap at a soft tissue defect around the knee, the rare use of a gastrocnemius silicone implant for cosmetic procedures, and soft tissue masses such as a lipoma.
Accessory gastrocnemius muscle bellies (“third head of the gastrocnemius”) have been associated with vascular claudication due to extrinsic vascular functional compression or entrapment17, though the majority of these congenital lesions are asymptomatic (Figure 14). MR angiography has proven useful for evaluation of suspected popliteal artery entrapment syndrome18.
Unusual distribution of muscle tissue around the knee (Figure 15) may be related to prior reconstructive surgical procedures. It has been stated that the medial gastrocnemius muscle flap is the workhorse of all muscle flaps around the knee19, due to the proximal entry of the main arterial and venous (sural) branches and the tibial nerve supply, the considerable size and length of the muscle belly, and that its transfer does not significantly impair knee joint function.
Surgical release procedures with tenotomy or myofascial recession have been developed for the gastrocnemius tendons in cases of resistant contracture, or spasticity related to cerebral palsy. Another surgical procedure involving the proximal gastrocnemius region involves muscle augmentation using silicone gastrocnemius implants placed in the submuscular plane between the gastrocnemius and the soleus20.
Soft tissue masses including intermuscular lipoma (16a,16b) and intramuscular lipoma may involve the proximal gastrocnemius tendon and muscle, and may be asymptomatic or present as a non-tender mass.
The treatment for symptomatic medial or lateral gastrocnemius proximal tendinosis is usually conservative, including rest from strenuous activities. Physical therapy may be indicated, aiming to reduce inflammation and improve range of motion. Rarely, surgical repair has been described in cases of retracted acute tendon tears.
Degenerative proximal tendinosis at the medial or lateral gastrocnemius may develop into interstitial or partial-thickness tendon tearing. These abnormalities may co-exist with the more common causes of knee pain such as meniscal tear or osteoarthritis, or may be the only abnormality demonstrated on an MRI examination of the knee. Gastrocnemius tendons may demonstrate MRI signal changes attributed to magic angle artifact, but when there is high T2 signal within the tendon, contour change or defect, and associated edema at bone marrow and soft tissues, gastrocnemius tendinopathy can be diagnosed on knee MRI exams, and may well be contributing to the patient’s clinical symptoms.
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