Purpose: Congenital absence of the anterior cruciate ligament (ACL) is an uncommon finding. Cases reported in the literature have been sporadic. The aetiology remains obscure. The purpose of this study was to describe results obtained in five patients (eight knees) with agenesia of the ACL to determine the characteristic features and search for elements differentiating the natural course of agenesia and post-traumatic ACL rupture.
Material and methods: The patients were four men and one woman, mean age 46 years. All five patients had a common ancestor, raising the question of the hereditary nature of the condition.
Results: The cardinal sign was medial femorotibial and femoropatellar pain, found in all patients. Physical exam revealed major anterior laxity (pivot shift +, Trillat Lachman +) and an abnormal protrusion of the anterior tuberosity of the tibia. The standard x-rays revealed elements suggestive of ACL agenesia and the MRI confirmed the diagnosis in all cases.
Discussion: In comparison with reports in the literature, our patients illustrated a few specific points:
- the familial nature of the condition in our patients suggestive autosomal dominant inheritance with variable penetration;
- the rare occurrence of instability, probably due to adaptation since childhood;
- hypoplasia of the tibial spines and the lateral condyle, consequence of ACL agenesia.
Comparing the natural history of ACL agenesia with that of post-trauma rupture led to certain observations:
- clinical and radiological congenital laxity is well tolerated;
- degeneration appears inevitable in case of congenital anterior laxity. It occurs late, progresses slowly, slower than after traumatic rupture of the ACL;
- unlike trauma-induced rupture, the lateral femototibial and femoro-patellar compartments are preserved for a very long period.
The lack of genetic studies and the familial nature of the cases reported here suggest that genetic surveys would be useful to elucidate this condition.
