The purpose of this study was to present a descriptive analysis of eight cases of congenital agenesia of the anterior cruciate ligament (ACL). Congenital absence of the ACL is rare. The few series reported in the literature have been limited to sporadic cases, all associated with diverse malformations. We report a series of eight cases of knees in five patients presenting congenital knee laxity. These patients were all members of the same family, raising the question of an inherited condition. We describe the characteristic features and natural history of ACL agenesia compared with post-traumatic loss of the ACL.

This descriptive study included five patients (eight knees) with agenesia of the ACL. There were four men and one woman, with a mean age of 46 years. All five patients had a common ancestor.

The main symptom was medial femorotibial and patellofemoral pain, present in all patients. Physical examination revealed major anterior laxity (positive pivot test, Trillat Lachman test) involving the posterior capsule and ligament structures and an abnormal knob on the anterior tibial tubercle. Plain x-rays demonstrated an abnormal aspect of the tibial spines, suggestive of ACL agenesia. The spines were flat in three knees, smooth in three, and dome-shaped in two. On the lateral view, the femoral condyles presented an abnormal rounded posterior curvature in all of the affected knees. The tibial slope was increased in all knees, 20.6° on average. MRI and arthroscopy confirmed the diagnosis in three knees.

Examining our cases and the data in the literature led to the following remarks. The family pedigree in our patients suggested autosomal dominant inheritance. Episodes of serious laxity are rare in subjects with congenital absence of the ACL compared with post-trauma patients, probably because of adaptation since infancy. Hypoplasia of the tibial spine and the lateral femoral condyle are characteristic consequences of ACL agenesia. In comparison with post-trauma cases, the natural history of ACL agenesia is characterized by better functional tolerance and inevitable progression to osteoarthritis due to the permanent anterior laxity. Degenerative disease may develop late and more slowly than after traumatic injury of the ACL. Unlike common degenerative arthritis, the lateral femorotibial and patellofemoral compartments are preserved longer.

Though rare, congenital absence of the ACL should be evoked as a possibility in patients with chronic anterior laxity without trauma. The radiological aspect is highly suggestive of the diagnosis which can be confirmed by MRI or arthroscopy. Study of the present series enabled a description of the natural history of ACL agenesia, which is different from that of traumatic ACL tears because of the lack of secondary meniscal lesions and the later progression of osteoarthritic degeneration.