The arcuate sign is often a subtle but important finding on knee x-rays and represents an avulsion fracture of the proximal fibula at the site of insertion of the arcuate ligament complex, and is usually associated with cruciate ligament injury (~90% of cases) 2. The fracture fragment is attached to the lateral (fibular) collateral ligament, the biceps femoris tendon or both.
The fracture results most often from a direct blow to the anteromedial tibia when the knee is extended, resulting in posterolateral subluxation of the tibia in external rotation 1,3. Alternatively, sudden hyperextension of the knee with the tibia internally rotated may cause the same injury 1.
The importance of this injury is that if it is not diagnosed acutely, posterolateral instability may develop which is challenging to correct, and may result in failed cruciate ligament reconstruction 1,3.
The shape and size of the avulsed fracture vary according to which ligaments are responsible.
The avulsion fracture is usually small, less than 1 cm in size, and involves the styloid process of the fibula. It is displaced superiorly and medially. Occasionally the fracture may be larger and extends to include the lateral aspect of the proximal fibula.
Slight internal rotation AP films are usually best to radiographically demonstrate this injury 3.
MRI can delineate the soft tissue component of the injury as well as to evaluate associated injuries that are common. Associated findings include 2:
cruciate ligament injuries
- especially posterior cruciate ligament 3
- bone bruises
- anteromedial femoral condyle: 50%
- anteromedial tibial plateau: 28%
- medial meniscus: 28%
- lateral meniscus: 22%
- popliteus muscle injury: 33%
- medial collateral ligament injury: common 3
- avulsion is from the tibia
- fragment more medial and anterior
The knee is a complex synovial joint that can be affected by a range of pathologies:
- bone and cartilage
- distal femoral condyle fracture
- tibial plateau fracture (classification)
- patella fracture
avulsion fractures of the knee
- Segond fracture
- reverse Segond fracture
- anterior cruciate ligament avulsion fracture
- posterior cruciate ligament avulsion fracture
- arcuate complex avulsion fracture (arcuate sign)
- biceps femoris avulsion fracture
- iliotibial band avulsion fracture
- semimembranosus tendon avulsion fracture
- Stieda fracture (MCL avulsion fracture)
- patella fracture
- chronic avulsion injuries
- chondromalacia patellae
- osteoarthritis of the knee
- osteochondral defects
- osteochondritis dissecans of the knee
- pattern of bone contusion in knee injuries
- knee fractures
- meniscal lesions
- synovial lesions
- fat pad
- popliteal fossa
- 1. Strub WM. The arcuate sign. Radiology. 2007;244 (2): 620-1. doi:10.1148/radiol.2442042160 - Pubmed citation
- 2. Juhng SK, Lee JK, Choi SS et-al. MR evaluation of the "arcuate" sign of posterolateral knee instability. AJR Am J Roentgenol. 2002;178 (3): 583-8. AJR Am J Roentgenol (full text) - Pubmed citation
- 3. Huang GS, Yu JS, Munshi M et-al. Avulsion fracture of the head of the fibula (the "arcuate" sign): MR imaging findings predictive of injuries to the posterolateral ligaments and posterior cruciate ligament. AJR Am J Roentgenol. 2003;180 (2): 381-7. AJR Am J Roentgenol (full text) - Pubmed citation