Clinical History: An 84-year-old male presents with lateral knee pain and swelling. Coronal (1A), axial (1B, 1D), and sagittal (1C) fat-suppressed proton-density-weighted images of the knee are provided. What are the findings? What is your diagnosis?
Gout with involvement of the intra-articular popliteus as well as distal quadriceps tendons.
Although the popliteus tendon is an integral component of the posterolateral corner of the knee as well as a site of chronic and insidious pathology, this long and strong tendon with its unique anatomy is oftentimes overlooked. Acute injuries to the popliteus tendon typically occur in conjunction with other injuries to the posterolateral corner structures of the knee, the cruciate ligaments and menisci and, rarely, in isolation. Chronic pathology of the popliteus tendon, which may be underappreciated, includes tendinosis, crystalline deposition diseases and pigmented villonodular synovitis. This Web Clinic will delve into the anatomy and function of the popliteus tendon with case examples of both traumatic and chronic disease.
Anatomy, Function and Normal MRI Appearance
The relatively small, triangular and obliquely oriented popliteus muscle ascends superolaterally from a broad attachment at the posteromedial tibia just above the soleal line, continues as a tendon deep to the lateral collateral ligament and biceps femoris tendon through the popliteal hiatus, and inserts at the popliteal sulcus of the lateral femoral condyle. The tendon is intra-capsular but extra-synovial and extra-articular.1 The popliteus musculotendinous complex, composed of the musculotendinous unit and popliteofibular ligament, provides static and dynamic resistance to external rotation and acts as a secondary restraint to posterior translation (Figure 3).1,2 The tendon and muscle are typically best evaluated on axial and coronal images (Figure 4).
Abnormalities of the Popliteal Hiatus
Assessment of the popliteal hiatus on sagittal images is important as pathology such as meniscal flaps and loose bodies occasionally extend into this area (Figure 5). Of note, the popliteus bursa is a normal fluid-filled and synovial-lined structure along the extra-articular tendon and musculotendinous junction which, on occasion, contains pathology and should not be confused for a musculotendinous injury (Figure 6).
Traumatic Popliteus Injuries
Acute popliteus tendon tears typically occur in the setting of multi-ligament and meniscal tears and, as such, careful attention should be paid to the adjacent posterolateral corner structures as well as the cruciate ligaments and menisci1-3 (Figure 7). Tears of the popliteus tendon should be characterized as partial or complete, intra-articular or extra-articular, and with regards to the proximity to the popliteal sulcus and the extent of retraction. Partial tears may be treated conservatively whereas complete tears retracted beyond the popliteal hiatus require an open or combined approach. The presence of an avulsed osseous fragment should also be noted as these can be fixated back into place (Figure 8). Rarely, isolated tendinous avulsions of the popliteus may occur (Figure 9).1
Popliteus tendinosis results from chronic repetitive stress, may be seen with downhill runners and is characterized by tendon enlargement and intrasubstance intermediate signal. It is not uncommon to see interstitial splitting and partial tearing superimposed upon tendinosis (Figure 10).
Crystalline Deposition Disease
The popliteus tendon is a common intra-articular location for gout in the knee. The tendon appears enlarged, diffusely infiltrated and intermediate in signal and erosive or cystic change may be present at the popliteal sulcus as in the initial case. Correlation with radiographs to assess for associated calcifications may be helpful. Rarely, hydroxyapatite deposition occurs at the popliteus tendon.4 Radiographs demonstrate a calcification or calcifications in the expected location of the popliteus tendon and if an MRI is obtained, associated edema and fluid should be noted.
Pigmented Villonodular Synovitis
Synovial proliferative diseases such as pigmented villonodular synovitis also may extend along the popliteus tendon (Figure 11).
The popliteus musculotendinous complex is a critical stabilizer of the posterolateral corner of the knee and resists both external rotation and posterior translation. The popliteus tendon is a strong but often underappreciated tendon and an important site for traumatic and chronic pathology. Routine assessment of the tendon and its hiatus in all imaging planes should be performed to ensure that relevant pathology is not overlooked.
- Jadhav S, More S et al. Comprehensive Review of the Anatomy, Function, and Imaging of the Popliteus and Associated Pathologic Conditions. RadioGraphics 2014;34:496-513. ↩
- Rosas H. Unraveling the Posterolateral Corner of the Knee. RadioGraphics 2016;36:1776-1791. ↩
- Vinson E, Major N et al. The Posterolateral Corner of the Knee. American Journal of Roentgenology 2008 Feb;190(2):449-58. ↩
- Doucet C, Gotra A et al. Acute Calcific Tendinopathy of the Popliteus Tendon: A Rare Case Diagnosed Using a Multimodality Approach and Treated Conservatively. Skeletal Radiol. 2017;46)7):1003-6. ↩