Absent ACL mimicking as ACL tear in knee

A young woman aged 24 years presented with right knee pain since 2 days after slipping from stairs while getting down. She was unable to stand and walk.

Radiograph of the knee joint was taken which showed shallow femoral notch and hypoplastic tibial spines (Figure 1). The joint space was normal. MRI later done revealed mild knee joint effusion in the supra patellar region. There was mild contusion at the lateral tibial condyle. Also noted absent ACL bundle (Figure 2) with visualization of only few of the lateral fibres adjacent to the femoral condyle. PCL was thin and showed buckling (Figure 3). There was no pericruciate fat oedema (Figure 4). On coronal T2 image, There was a small subchondral fracture along the lateral tibial condyle and empty femoral notch noted (Figure 5). No free fluid/subchondral oedema at the intercondylar notch and tibia. No significant meniscal injuries were seen. The patello-femoral compartment was normal.

Congenitally absent anterior cruciate ligament is very rare, prevalence of 0.017 per 1000 live births. It was first described by Giorgi in 1956. It can be an isolated finding or associated with other bone abnormalities like short femur, dysplastic tibia and also sometimes with hypoplastic/absent menisci [1].  Manner et al have classified it into three main types [2]. Type 1 is aplastic or hypoplastic ACL and normal PCL. Type 2 is aplastic ACL and hypoplastic PCL. Type 3 is when both are absent [3]. The clinical symptoms can vary widely from being asymptomatic to incapacitating pain and even dislocation. These patients have increased risk of early arthritis in future [4]. Treatment is typically ACL reconstruction using patellar graft or posterior tibial tendon allograft [4]. Since this can also lead to hip and knee osteoarthritis, periodic follow-up is essential and arthroplasty is reserved when secondary arthritic changes are leading to instability [5]. In this case, the patient had a trivial fall, however, there was no instability on clinical examination and was managed conservatively.