The clinical presentation varies based on the degree of deformity and the associated congenital conditions present. When associated with a congenital knee dislocation, the condition is often obvious at birth with a varying degree of hyperextension at the knee, which can often be quite striking. When combined with severe fibular hemimelia or other significant congenital deformity (PFFD, congenital short femur ) there is often obvious shortening of the limb with other abnormalities noted throughout the extremity. Older children/adolescents may present with more subtle findings. They may report increasing knee instability and exhibit increasing valgus subluxation and deformity about the knee (Fig. 23.4). They may present with issues related to a limb-length discrepancy or malalignment. It is important to recognize that congenital absence of the ACL may present as knee subluxation during limb-lengthening procedures (Fig. 23.5). Moreover, in what may be a common though under-recognized presentation, is the patient who presents with an “injury” to the knee and is found, on subsequent workup, to be ACL deficient, possibly not realizing it is a congenital absence.

Children with more severe disease and multiple anomalies may present at an earlier age with often obvious physical findings such as significant limb shortening or deformity consistent with their underlying condition. Patients with less severe involvement typically present to clinic with findings related to lower-extremity alignment and length. They may present with a gait abnormality, limp, or leg-length discrepancy. Rarely do they present with isolated instability of the knee.

Physical examination of the lower extremity should begin with the hip, with documentation of the range of motion compared to the opposite side. A rotational profile of the lower extremities should be performed. This includes the foot progression angle, internal/external rotation of the hip, thigh-foot-axis, and any abnormalities of the foot. Clinical leg-length discrepancy, if any, should be measured. This can be performed from the umbilicus to the ankle/foot, or from the greater trochanters to the ankle/foot. When measuring the clinical leg-length discrepancy it is important to include the size of the foot in the measurement, as measuring to the medial malleolus distally may underestimate the true discrepancy. As an alternative, wooden blocks can be placed under the shorter limb until the pelvis is leveled, which will show the global discrepancy of the extremity. Lower extremity examination should include coronal plane alignment, especially valgus of the knee joint. ACL laxity examination should include Lachman test, pivot shift test, and anterior and posterior drawer tests. Varus and valgus stability of the knee should be assessed in both full extension and 30° of flexion.

Roux and Carlioz in 1999 described the clinical examination and function of the cruciate ligaments in children with fibular hemimelia [17]. While they found a large percentage of patients to have physical findings of laxity (positive Lachman in 84%, anterior glide in 90%), only a much smaller portion of patients (16%) actually reported complaints of symptomatic instability. Valgus instability due to hypoplasia of the lateral femoral condyle was a common finding (83%).

Imaging of the lower extremity should include full-length standing anteroposterior (AP) alignment and knee radiographs. Common radiographic findings, which are typically indicative of fibular hemimelia, include hypoplasia of the lateral femoral condyle (98.5%), hypoplasia of the tibial spines (93%), patella hypoplasia (52%), lateral displacement of the patella (32%), and patella alta (12%) [17].

If indicated based on radiographic findings, magnetic resonance imaging (MRI) of the knee should be performed to further characterize the involvement of both the anterior and posterior cruciate ligaments, as well as the other associated structures of the knee (Fig. 23.6). Hypertrophy of the meniscofemoral ligaments could be seen on MRI and is thought to reduce symptomatic anterior instability [14, 15]. MRI will allow classification of the type of ACL deficiency and can help guide treatment options.

Given the extremely rare nature of this condition, there is no consensus regarding the optimal management strategy. Furthermore, there have been no comparative studies published regarding surgical and nonsurgical treatment of congenital absence of the ACL. The literature related to management of ACL deficiency has been summarized in Table 23.1. The frequent occurrence of other congenital anomalies in the extremity, often very complex and requiring multiple surgical procedures themselves, further complicates the development of an algorithmic approach to this condition. Most of the studies involving syndromes in which congenital absence of the ACL is a component have focused primarily on the correction of malalignment or limb-length inequality that is often initially performed, with minimal attention to the instability secondary to ACL deficiency [1, 2, 5, 11, 18, 19]. Since most congenital ACL deficiency patients have other associated anomalies or dysplasia, the activity demand on the knee may be considerably less, thus masking the symptoms and manifestations of ACL deficiency.