Theoretically, popliteus tendinitis means localized inflammation in the tendon of the popliteus6, 9. As a kind of tendinitis, popliteus tendinitis also has some common symptoms of tendinitis itself. The main complaint of patient is the pain in the posterolateral corner of the knee. In some cases, there is significant tingling accompanied by sharp pain and joint stiffness, which limits the movement of the affected joints3, 9. In the cases that we included, most of the patients were middle-aged and elderly individuals. The clinical manifestations of the preoperative patients affected their daily living. At present, many researchers have proposed that repeated minor trauma, repetitive application of knee rotation in turnout, exercise, overuse and poor blood supply to tendons are the most common causes of popliteus tendinitis2, 3, 10.
Based on cadaveric study, LaPrade et al. reported that the popliteus tendon rested proximal to the popliteus sulcus on the lateral femoral condyle from full knee extension to 112° of flexion. At knee angles of 112° or more, the popliteus tendon engaged in the popliteus sulcus11, 12. However, to date, there have been no reports in the literature about the intra-articular trajectory of the popliteus tendon. According to our arthroscopic findings, the popliteus tendon could be comprehensively observed arthroscopically via an auxiliary extreme lateral approach. We found that when the knee flexed from full extension, the popliteus tendon stretched properly and externally rotated against the lateral tibial plateau and the lateral femoral condyle. Conversely, when the knee from approximately 90° of flexion angle to extension, the popliteus tendon begins to internally rotated relative to the lateral tibial plateau and the lateral femoral condyle (Video 2). Consequently, we speculated that the mechanism of popliteus tendon injury might be related to repeated rolling friction against the lateral tibial plateau and the lateral femoral condyle, and traction injury to its collagen fiber tissue. Previous research reported that the popliteus tendon was in tension when the proximal tibia was externally rotated and became relaxed when the tibia was internally rotated13, 14. Due to the clinically relevant anatomy of the popliteus tendon, Marc et al. found that downhill running or other deceleration activities was prone to lead to popliteus tendinitis. This might stem from the popliteus acting to prevent excessive posterior tibial translation relative to the femur15. Ferrari et al. indicated that inflammation of the popliteus was associated with overuse or fatigue of the quadriceps4. When the fatigued quadriceps cannot adequately resist forward displacement of the femur on the tibia, undue stress occurs on the secondary restraints, overwhelming the relatively small popliteus muscle. Moreover, the blood supply of the popliteus muscle is from the medial inferior genicular branch of the popliteus artery and the muscular branch of the posterior tibial artery16. It is different from the posterior cruciate ligament, which has abundant blood vessel and synovial coverage. There was no obvious capillary and synovial coverage on the surface of the popliteal tendon under the arthroscopic examination. Thus, it occurs with some physical injuries and degeneration over time.
Popliteus tendinitis is not a common cause of knee pain. Many doctors are not knowledgeable about popliteus tendinitis and readily misdiagnose it. Therefore, it is essential that it is diagnosed correctly. First, a detailed understanding of the patient's case history is required. Patients with popliteus tendinitis usually have posterolateral pain in the knee joint, especially when running downhill and participating in strenuous activities. Upon physical examination, the main finding is tenderness along the posterolateral joint line. Palpating while the patient is sitting up with the leg crossed in a "4" position is recommended. For the two specific physical examinations, the patient is asked to resist the examiner's external rotation force on the tibia. These actions result in pain if the popliteus tendon is inflamed. Although more common medially, a popliteus cyst, lateral meniscus tear or lateral compartment articular damage can present a similar pattern of symptoms. If the physician is not sure about the diagnosis, the imaging examination (radiographs and MRI) may be helpful17. In our study, there were 11 and 12 patients with increased intratendinous or myotendinous signals and fluid signals around the popliteus tendon in MR images, respectively. MRI has a vital role in the diagnosis and treatment management of popliteus tendinitis due to inherent difficulties in visualizing this region with arthroscopy and the challenge in detecting these injuries clinically.
Generally, the patients with popliteus tendinitis can choose conservative therapy for their management. These patients require a reduction in sports activities, getting more rest, avoiding trauma or injury, and even wearing knee braces with 30° of knee flexion for 3 weeks18. When popliteus tendinitis is accompanied by severe symptoms or injuries in other posterolateral side structures, there will be different management depending on the specific circumstance. In Eric’s study, a patient with a popliteus tendon injury received a steroid injection and conservative management with physical therapy9. Petsche et al. suggested that most patients respond well to physical therapy and NSAIDs5. Recalcitrant cases may require a local corticosteroid injection. In Blake’s study of patients with popliteus tendon tenosynovitis, the sheath was injected with 40 mg of methylprednisolone under arthroscopic guidance after joint irrigation3. However, repeated corticosteroid injections result in a series of unwanted events, including a loss of tensile strength of the popliteus tendon, septic arthritis, a postinjection flare and suppression of the HPA axis7, 8, 19, 20.
For most popliteus tendinitis patients, we recommend non-surgical therapy for those who have minor injuries. However, for patients whose symptoms are severe and not responsive to conservative therapy, the patients could undergo arthroscopic intervention. In our study, we chose an extreme lateral approach to obtain a better view of the relations of the popliteus tendon and tibial plateau and their trajectories. In addition, we also clean the loose body that is beside the popliteus hiatus with this approach (see the details on our previous studies)21. In our cohort of patients, compared with the preoperative data, there were significant improvements in knee joint scores (Lysholm, Tegner, IKDC and VAS scores) after surgery. Hence, arthroscopic treatment is positive and effective clinical management in popliteus tendinitis patients.
In this article, we introduced a new extreme lateral approach to observe and manage popliteus tendonitis, which is accompanied by deformation of the popliteus hiatus. From our intraoperative findings, we observed that long-term chronic abrasion of the knee joint can cause tissue degeneration injuries and inflammation of the popliteus tendon articular cavity. The patients felt discomfort and pain with this degeneration. Through arthroscopic observation and ablation, local inflammation and wear were effectively controlled. The postoperative symptoms were significantly relieved. After follow-up, the patients’ life quality was improved, and the knee function score increased significantly. From 2014 to 2018, we collected 123 registered cases of popliteus tendinitis. However, only 15 patients required arthroscopic intervention. This is also proved that the incidence of popliteus tendinitis is low and conservative treatment can also play an important role in treatment. Arthroscopic surgery is an effective method in recurrent patients or even patients with a more serious condition.