Being born without an anterior cruciate ligament (ACL) is uncommon but possible. The ACL is one of the four main ligaments in the knee, connecting the thigh bone (femur) to the shin bone (tibia). Its primary role is providing stability to the knee joint by limiting excessive forward movement of the tibia and preventing hyperextension. This ligament helps maintain knee function during various movements, from walking to sports.
Understanding Congenital ACL Absence
Congenital absence of the anterior cruciate ligament means the ACL does not form during fetal development. This condition is also known as ACL agenesis or aplasia. This developmental anomaly typically occurs around the seventh to tenth week of pregnancy when the knee’s internal structures are forming.
This condition is rare, with a reported prevalence of about 1.7 per 100,000 live births. While it can occur in isolation, it is often associated with other developmental irregularities of the lower limb. These associated conditions can include hypoplasia (underdevelopment) of the lateral femoral condyle, abnormalities of the intercondylar eminence or notch, absent or atypical menisci, or congenital short femur.
Researchers categorize congenital ACL deficiency into types based on the presence and development of other cruciate ligaments. Type I involves an underdeveloped or absent ACL with a normal posterior cruciate ligament (PCL). Type II describes an absent ACL combined with an underdeveloped PCL. Type III indicates the absence of both cruciate ligaments.
Recognizing the Signs and Diagnosis
Individuals born without an ACL may experience various signs or symptoms, though some remain asymptomatic, relying on compensatory mechanisms from surrounding muscles and ligaments. When symptoms appear, they often include knee instability, a sensation of the knee “giving way,” or pain, particularly as children become more active. These symptoms can sometimes be mistaken for a traumatic injury, making accurate diagnosis important.
Diagnosis typically begins with a physical examination, where a healthcare provider assesses knee laxity using tests like the Lachman test, pivot shift test, and anterior drawer test. Imaging techniques confirm the absence of the ligament. Magnetic Resonance Imaging (MRI) is useful as it shows the absence of the ACL and any associated bony or soft tissue abnormalities within the knee. Radiographic findings, such as an underdeveloped tibial spine or a narrow intercondylar notch, can also suggest a congenital origin.
Living with Congenital ACL Absence
Managing congenital ACL absence involves strategies tailored to the individual’s symptoms, activity level, and any other knee abnormalities. Non-surgical approaches are often the initial course of action, particularly for those with minimal or no symptoms. This conservative management includes physical therapy focused on strengthening the muscles surrounding the knee, especially the quadriceps and hamstrings, to enhance joint stability. Activity modification to avoid movements that provoke instability, alongside the use of braces, is also beneficial.
For individuals experiencing persistent symptomatic instability despite conservative measures, surgical intervention may be considered. Ligament reconstruction, using grafts from other parts of the body or donors, can create a new ACL. This surgery in congenital cases can be more challenging than for traumatic ACL tears due to altered knee anatomy, such as an underdeveloped intercondylar notch or tibial spines. In younger patients, specialized techniques that avoid damaging growth plates may be used.
Outcomes for individuals born without an ACL vary significantly. Some adapt well and lead active lives without surgery, while others may experience ongoing instability or develop early signs of joint degeneration. The decision for surgery is made on a case-by-case basis, considering the patient’s specific needs and the potential benefits and risks.