Proximal Femoral Focal Deficiency 

Proximal femoral focal deficiency (PFFD) is an uncommon but complex problem. PFFD was commonly grouped with other disorders, such as coxa vara and short bowed femurs, which led to confusion and misunderstanding.^[1]

Recent studies

Herzenberg et al reported on congenital femoral deficiency in children treated with femoral lengthening. According to the authors, a common complication is fracture soon after removal of the external fixator, often despite prophylactic hip spica cast application, presenting special challenges because of tight soft tissues and sclerotic intramedullary canals. The investigators treated 9 such fractures (in 8 children; average age, 5.4 years), most of which were spontaneous events resulting in transverse fracture through regenerate bone or pin sites. All the fractures were stabilized with intramedullary Rush pins using special insertion techniques, and union was achieved (average, 6 weeks) without occurrence of significant complications.^[2]

Chorniak et al studied the vascular supply of lower extremities with CT angiography in 21 patients (age range, 1 to 19 years) to identify vascular changes in patients with proximal femoral focal deficiency. In Pappa types I through IV, the external iliac, femoral, and deep femoral arteries were found to be substantially reduced in length and diameter, and the deep femoral artery arose more proximally than that in the contralateral extremity. In 2 patients with type III disease, there was an atypical anatomy of the vessels: the anterior part of the thigh and the pseudarthrosis were supplied through the femoral artery (the external iliac artery) as a terminal branch, while the remainder of the extremity was supplied from the internal iliac artery.^[3]

Aston et al performed a retrospective review of 30 femoral lengthenings in 27 patients with congenital short femurs using the Ilizarov technique. On the basis of their findings, the authors recommended that in this group of patients, lengthening of the femur with an Ilizarov construct be carried out through a proximal osteotomy over a Rush nail. They also recommended that lengthening be limited to a maximum of 6 cm during one treatment, or 20% of the original length of the femur, to reduce the risk of complications. The mean increase in length was 5.8 cm (3.3 to 10.4 cm), with a mean time in the frame of 223 days. In the proximal osteotomy group, preplacement of a Rush nail reduced the rate of regenerate deformation or fracture requiring open reduction and internal fixation from 100% without a nail to 0% with a nail.^[4]

Using knee arthroscopy, one study sought to identify the changes in cruciate ligaments and their relation to the different types of proximal femoral focal deficiency. In the majority of patients, radiologic testing showed a posterior shift of the tibia on the affected side; anterior and posterior drawer test results were increased but did not directly correlate to the presence or absence of cruciate ligaments. While these changes were not clinically relevant in most of the patients studied, nor were they related to the type of Pappas classification, imaging of cruciate ligaments prior to lengthening of the extremity was recommended in order to avoid knee dislocation.^[5]

In general, in individuals with proximal femoral focal deficiency, the proximal femur is partially absent, and the entire limb is overall shortened. A few main biomechanical abnormalities are present in children with PFFD, as well as in adults with limb deficiencies. These include limb length discrepancies, malrotation, proximal joint instability, and inadequacy of the proximal musculature.

Frequency

The incidence of the deficiency ranges from 1 case per 50,000 population to 1 case per 200,000 population.^[6]

The etiology of proximal femoral focal deficiency is not known exactly, but certain theories have been proposed and agents implicated. Sclerotome subtraction is one such theory that has been offered to explain several different limb deficiencies. Specifically, this theory states that injury to the neural crest cells that form the precursors to the peripheral sensory nerves of L4 and L5 results in PFFD.^[7]

A second theory, advanced by Boden et al, states that PFFD may be the result of a defect in proliferation and maturation of chondrocytes in the proximal growth plate.^[8] Agents implicated in causing such injuries include anoxia, ischemia, irradiation, bacterial and viral infections and toxins, hormones, mechanical energy, and thermal injury.^{[7, 9]} Thalidomide, when taken by the mother between the fourth and sixth weeks of gestation, has been demonstrated to be a definite cause of PFFD in humans.^[9] Currently, no evidence indicates a genetic etiology.^{[6, 10]}

The appearance of proximal femoral focal deficiency is not subtle, so it is easily recognized. The femur is shortened, flexed, abducted, and externally rotated.^{[7, 9, 11]} Gillespie noted that, in his patients, the hips were never normal and the knees were dysfunctional.^{[1, 12]} Flexion contractures of the hip and knee are also present. The bulbous proximal thigh quickly tapers to the knee. Because of the short femur and bulbous thigh, examination of the hip can be difficult. As a result of hip instability, pistoning may be present. The knee is uniformly unstable in an anteroposterior plane secondary to absent cruciate ligaments. Additionally, generalized knee hypoplasia has been reported.^[1]

A high incidence of fibular deficiency and valgus feet is associated with PFFD.^[9] Fibular deficiencies are found in as many as 70-80% of persons with PFFD. Approximately 50% of patients with PFFD have other limb anomalies.^[13] However, Aitken reported almost a 70% incidence of other anomalies.^[14] Cleft palate, clubfoot, congenital heart defects, and spinal anomalies, although rare, occur as well. PFFD is bilateral in 15% of the cases.^[10]

Classification

Several classification systems describe congenital anomalies of the femur, but most have been based on radiographic appearances alone. The Amstutz and Pappas classifications provide detailed radiologic descriptions of the various forms of PFFD that these researchers encountered.^{[15, 16]} Hamanishi described a progressive reduction of the femur, ranging from simple shortening to total absence.^[17] Fixsen and Lloyd-Roberts divided their patients into stable and unstable categories.^[18]

Gillespie and Torode reviewed their patients from both a radiographic and, more importantly, a clinical viewpoint and found that most could be divided into 2 groups. The first group included persons with congenital short femurs, and the second group was composed of individuals with true PFFD. These 2 groups not only differed with respect to clinical and radiographic appearances but also were functionally unique and had different surgical and prosthetic requirements.^[1]

The Aitken classification, which is the most widely used classification, divides PFFD into 4 categories based on the radiographic appearance.^[14] Remember that late ossification may occur, whereby the bone may be present but not visualized radiographically. Occasionally, push-pull comparison radiographs, as well as abduction-adduction views, are necessary to distinguish between class A and class B. Arthrography also can be helpful.^{[7, 9]}

In individuals with Aitken class A, a shortened femur is present proximally, ending at or slightly above the level of the acetabulum. The femoral head is often absent but later ossifies; femoral head presence is indicated by a well-developed acetabulum. Additionally, there is a subtrochanteric defect, which eventually ossifies and thereby establishes bony continuity. After ossification, there is usually a residual subtrochanteric varus deformity.

Persons with class B have a more severe defect or absence of the proximal femur. This defect does not heal spontaneously. At skeletal maturity, there is no connection between the femoral head and proximal femur; the end of the proximal femur is above the acetabulum. The femoral head, although present, may have delayed ossification, and there is often a bony tuft on the proximal end of the shaft.

An individual with Aitken class C has an absent femoral head that does not ossify and a markedly dysplastic acetabulum. The class C femoral shaft is shorter than in a person with class B, in whom the entire proximal femur, including the trochanters, does not develop.

In a person with class D, the most severe form, there is a severely shortened shaft, which often has only an irregularly ossified tuft of bone proximal to the distal femoral epiphysis. No acetabulum is present because the lateral pelvic wall is flat.

In a symposium, Gillespie proposed a more functional classification system in which he divided his patients into 3 treatment groups from a surgical and prosthetic viewpoint.^[12] His first patient set, group A, consisted of possible candidates for limb lengthening. This group included individuals who had congenitally short femurs but clinically stable hips, had no significant knee flexion contractures, and had the ipsilateral foot at or below the level of the middle of the contralateral tibia. Gillespie's group B consisted of patients classified by Aitken as classes A, B, and C and who required prosthetic treatment. Therefore, any surgical procedure is designed to maximize prosthetic function. Gillespie's group C represented the same patients as Aitken's class D in that they had subtotal absence of the femur. Gillespie also recommended prosthetic treatment for his group C patients; however, these patients did not require knee fusions prior to prosthetic fitting.

A study of the efficacy of MRI in classifying PFFD also compared MRI findings to radiographic classification. The study, which used the Amstutz classification system, found that radiographic evaluation tends to overestimate the degree of deficiency and that, therefore, MRI is the better modality.^[19]

Indications for lengthening include a limb with a predicted discrepancy at maturity not exceeding 20 cm, a hip that is or can be made stable, and a relatively good knee, ankle, and foot. For other indications related to specific procedures, see Surgical therapy.

In proximal femoral focal deficiency, the proximal femur is partially absent, and the entire limb is overall shortened.

If the predicted discrepancy is greater than 20 cm, or if for any other reason the child is not suitable for limb lengthening, prostheses should be considered. For contraindications to specific surgical procedures, see Surgical therapy.