Anterior Cruciate Ligament (ACL) aplasia or hypoplasia is a rare congenital malformation, with a reported incidence of 0.017 per 1000 people. In 1956, Giorgi was the first author to describe such deformity. Since then, many other Authors have reported analogous cases. The deformity usually affects the anterior cruciate ligament, although agenesis of both cruciate ligaments and – recently – an isolated agenesis of the posterior cruciate ligament have been described. Anyhow, the absence of the only anterior cruciate ligament is more common than commonly suspected and it is often associated with abnormalities of other bones (patella, tibia, fibula, femur dysplasia i.e.) or soft tissue in the lower limb, as well as organ syndromes (i.e. thrombocytopenia absent radius syndrome (TAR), arthrogryposis). Such anomalies determine both morphological and biomechanical alterations that can be assessed through an accurate physical examination and an appropriate radiological evaluation.
Since the body adapts to the congenital defects, many patients may not experience joint instability until a knee twisting injury occurs. Consequently, with the exception of patients displaying proven leg length discrepancy or other organ diseases, past medical history is often silent and the patient is generally referred to the orthopedic due to the instability felt after a trauma.
The aim of the present paper is to classify the epidemiological, clinical and radiological features most frequently related to ACL aplasia, in order to understand if a correct diagnosis may be feasible before the surgical approach.
We searched Pubmed, Science Direct, Medline and Scopus databases, employing the keywords “Anterior”, “Cruciate”, “Ligament”, “ACL”, “Aplasia”, Hypoplasia”, “Agenesis”, “Absence”, “Congenital”, “Knee”. We included all articles reporting about ACL deformities, excluding studies focused on other types of knee anomalies. We also excluded papers in languages not comprehensible by the authors. We collected a total of 60 papers, both case reports and case series. Considering the abovementioned exclusion criteria, we excluded 8 works.
Our analysis included 117 patients and a total of 161 affected knees, being the malformation bilateral in 35 subjects (19 males). Average age at diagnosis was 14.2 (males 14.6, females 16.4). The most frequently associated congenital deformities were leg length discrepancy (18%), congenital hip dislocation (14%) and fibular hypoplasia (12%). Concerning the physical examination, a positive Lachman test (27%), Anterior Drawer test (24%), Pivot Shift test (12%) as well as anterior knee pain (6%) were the more frequently documented alterations.
The most common abnormalities documented with standard X-rays turned out to be the hypoplasia of the femoral condyles (32.27%), tibial spines hypoplasia (27.27%), hypoplasia of the intercondylar fossa (27.21%), whereas ACL absence (31.64%) and PCL absence (8.86%) were the most frequently reported in preoperative MRI.
Our results show that a preoperative diagnosis of ACL aplasia is challenging. In doubtful cases, based on a physical examination positive for knee instability, standard X-rays seem more objective than MRI. Treatment of ACL aplasia remains an open debate among Authors (conservative vs. surgical treatment), therefore an early diagnosis may help the physician in the proper management of these patients, especially those with organ syndromes.