Congenital absence of the anterior cruciate ligament

Clinical History

This 5-year-old child of Polish origin was found at birth to have constriction bands over the right lower leg and foot. He underwent release of the constriction bands as a neonate, with only minor residual foot deformity. He developed progressive laxity of the right knee. No history of trauma.

Imaging Findings

Figure 1: MR T1-weighted TSE coronal image of the right knee (TR 328/ TE 9) shows a focal, well defined area of low signal in the intercondylar region of the distal femoral epiphysis, at the expected site of the insertion of the anterior cruciate ligament (ACL).
Figure 2: MR proton density TSE midline sagittal image (TR 3270/ TE 12) demonstrates a corresponding focal area of high signal at the expected site of the insertion of the ACL. The normal low signal fibres of the ACL are absent. No posterior cruciate ligament (PCL) fibres were seen.
Figure 3: Plain radiograph of the right knee shows that the right distal femoral intercondylar notch and epiphysis appear smaller compared to the left side, as does the right proximal tibial epiphysis.

Discussion

Congenital absence of the anterior cruciate ligament (ACL) is rare. The few series reported in the literature are sporadic. This case was detected at a young age during follow-up of distal congenital constriction bands. There is no known association described between the two conditions in the literature and they appear to be unrelated.

ACL formation has been observed in foetal development at 8 weeks. It originates as a ventral condensation of the foetal blastoma and migrates posteriorly with the formation of the intercondylar space [1]. The menisci are derived from the same ventral condensation, which accounts for the association of congenital absence of the ACL with meniscal agenesis [2]. Congenital ACL absence is also associated with hypoplasia of the posterior cruciate ligament, lower limb length discrepancy and predisposes to dislocation of the knee joint.

Reported symptoms include medial femorotibial and patellofemoral pain. The Lachmann and pivot test in this case were positive for marked anterior laxity, which has also been reported in the literature [3].

Imaging plays a pivotal role in making the correct diagnosis. Plain radiographs may demonstrate changes in the intercondylar notch width and height [4]. A narrow intercondylar notch suggests agenesis of the ACL. The tibial eminence may be also be abnormal, with only one protuberance, as opposed to the medial and lateral tibial spines in the normal knee. This hypoplasia of the tibial spine is thought to relate to lack of traction from the ACL during development [5]. A rounded shape of the femoral condyles has also been described, which is attributed to recurrent anterior subluxation leading to a disturbance in normal bone development [2].

Magnetic resonance imaging (MRI) is useful to show the absence of the ACL and its associated conditions. The advantages of MRI are the ability to obtain multiplanar sequences, to visualise hyaline cartilage and to avoid the use of ionising radiation [6].

Several therapeutic options have been discussed in the literature. Some authors report good results after ACL reconstruction, others prefer conservative treatment with physiotherapy and muscular training.

Congenital absence of the anterior cruciate ligament is extremely rare, but can be considered in cases of chronic anterior laxity.

Plain radiographic findings aid diagnosis, which can be confirmed with MR or arthroscopy. Radiological signs include a narrow intercondylar notch and hypoplasia of the tibial eminence and femoral condyle

Differential Diagnosis List

Final Diagnosis

Congenital absence of the anterior cruciate ligament

URL:	https://www.eurorad.org/case/9301
DOI:	10.1594/EURORAD/CASE.9301
ISSN:	1563-4086