1
Background
Although tendinopathy popliteus is a common injury in athletes, it is often misdiagnosed because of the anatomical and biomechanical particularities of the popliteus muscle.
Overuse of the muscle-tendon junction unit has been incriminated in the pathogenesis of the popliteus tendinopathy. It usually occurs in sportsmen who run or train on hills or irregular surfaces [ ]. Athletes with a history of other knee injuries are also more likely to develop popliteus tendinopathy. This condition is relatively rare in the general population of nonathletes without a history of knee trauma [ , ].
Anatomically, the popliteus muscle is a small muscle located on the posterolateral corner of the knee. It has three origins: the lateral femoral condyle, the fibular head, and the lateral meniscus.
Biomechanically, this muscle performs tibial internal rotation during walking especially during the swing phase (open chain), also called endorotation of the lower leg. The most important role of the popliteus is to provide advancing stabilization of the knee and the stabilization of the lateral meniscus during knee flexion. It is considered as a flexion starter as it allows the knee to be flexed from full extension. Due to this function, the popliteus muscle is often seen as the key to unlock the knee [ , ]. The third function prevents forward displacement of the femur on the tibia and limits knee valgus when the foot is fixed to the ground (closed chain) [ ]. The fourth function is to tighten the posterior capsule and help the anatomically weak capsular-ligamentous elements of the posterolateral side of the knee [ ].
Several risk factors for popliteus tendinopathy have been found. These include downhill running, pace lengthening, increased distance, and over pronation of the foot or the use of bad footwear. Downhill running has been reported to be the predominant cause of this injury [ ].
Clinically, this condition is characterized by pain localized to the posterolateral aspect of the knee that appears on weight-bearing activities requiring knee flexion. Specific clinical tests are needed for the diagnosis.
Clinical findings are not always typical, and in these cases, clinicians may resort to ultrasound (US) and magnetic resonance imaging (MRI) which are of great interest in diagnosing acute popliteus tendinopathy.
Surgery is an effective treatment that should be reserved for patients in which conventional therapy has failed.
2
Synonyms
Popliteus tendinitis.
Popliteus tendon tenosynovitis.
3
Clinical Study
3.1
Symptoms
Popliteus tendinopathy is often diagnosed in professional runners and triathletes [ ]. It is uncommon in a nonathletic person without a history of knee trauma [ ].
The characteristic symptom for this condition is pain localized in the posterolateral aspect of the knee that appears in the early degrees of knee flexion on weight bearing activities [ ].
If the patient continues his physical activities, pain will get worse impairing activities that require knee flexion [ ].
Walking, running, and going up stairs can be impaired especially in the acute stages of the injury [ ]. Downhill running or walking can exacerbate symptoms by causing increased stress on the popliteus muscle-tendon unit, since this activity requires a deceleration in which the popliteus muscle is involved [ ].
3.2
Physical Examination
During examination, tenderness on palpation of the posterolateral side of the lateral femoral epicondyle is found in popliteus tendinitis [ ]. This pain can be associated with inflammatory signs such as localized swelling, redness, and marked tenderness at the insertion of the popliteus tendon [ ]. A crepitation sound is also frequently heard when the tendon is moved [ ] ( Fig. 7.1 ).
When evaluating range of motion (ROM), the knee cannot be fully extended because of the muscle tightness. while knee flexion remains normal [ ]. Both knees should be examined while paying attention for asymmetry.
To test the popliteus tendon, the patient is asked to sit in “a 4 figure” position with the affected side crossed over the healthy side, the hip flexed, abducted, and externally rotated, and the knee flexed, while the therapist palpates the posterolateral corner looking for tenderness [ ].
To test the popliteus muscle in closed chain, the patient stands on both feet with the knees flexed at 30 degrees. The examiner applies a valgus stress on the affected knee, while the patient opposes this movement. The test is positive if it reproduces the patient’s pain ( Fig. 7.2 ).
Another test for the popliteus muscle can be performed with the patient in supine position on the table and instructed to perform foot adduction and knee flexion, while the examiner opposes this movement [ ] ( Fig. 7.3 ).
4
Differential Diagnosis
The differential diagnosis of popliteus tendinopathy includes:
4.1
Posterior Horn Tear of the Meniscus
Symptoms of a posterior horn medial meniscus tear include pain localized at the posterior aspect of the knee, swelling, stiffness, catching, or locking and a sensation of instability that are aggravated with deep squatting.
4.2
Osteochondritis Dissecans
Pain and swelling of the knee often triggered by sports or physical activity are the most common initial symptoms of osteochondritis dissecans. Advanced cases may present with joint catching or locking.
4.3
Iliotibial Band Syndrome
Pain in the lateral aspect of the knee that is worsened with running downhill, cycling, or rowing is suggestive of iliotibial band syndrome (ITBS).
5
Imaging
5.1
Standard X-rays
In some cases, conventional radiographs may show radiodensities in the region involving the popliteus tendon.
5.2
Ultrasound
The main ultrasonographic finding in patients with popliteus tendinopathy is an increase in muscle and tendon thickness.
Thickening of the popliteus muscle compared to the unaffected side can be observed on dynamic US examination during both knee flexion and extension. Increased muscle thickness during lower leg internal rotation has been reported to be present in 90% of subjects with popliteus tendinitis [ ].
5.3
Magnetic Resonance Imaging
MRI is an interesting technique for detecting acute popliteus tendinopathy. Intratendinous signal alteration and abnormal thickening have been described. In fact, thickening was found in 33% in an MRI study of knees with popliteus tendinitis [ ] ( Fig. 7.4 ).
