Congenital conditions like proximal femoral focal deficiency cause shortening by a combination of methods and, as a consequence, can lead to rather dramatic discrepancies.

LIMB LENGTH INEQUALITY is a common problem which is frequently discovered during the growing years. Although the cosmetic effects are well recognized, the more important mechanical and functional aspects of the problem and their potential association with adverse long term sequelae are less well appreciated. Discrepancies greater than 2 cm can be associated with pelvic obliquity, scoliosis, and alterations in the normal walking pattern. From a functional standpoint, there is strong, though not conclusive, evidence of an associated increase in the incidence of low back pain and of osteoarthritis of the hip.

Etiology
Limb length discrepancies result from processes that cause either asymmetric shortening or lengthening of bone and may be static or dynamic in nature. Shortening can result from an alteration in bone length as occurs following a fracture with overlapping or angulation of the bone ends. In this situation the effect on length is immediately evident but is usually non progressive. Alternatively, shortening may result from an interference with bone growth as classically occurs after an epiphyseal injury or infection in which case the effect on length is initially minimal, but increases progressively. Congenital conditions like proximal femoral focal deficiency cause shortening by a combination of methods and, as a consequence, can lead to rather dramatic discrepancies.

Limb length inequality can also result from overgrowth of an extremity. This phenomenon can occur in association with a number of processes such as chronic osteomyelitis, juvenile arthritis, or trauma which invoke a prolonged hyperemic state that presumably stimulates bone growth. The exact mechanism of this growth stimulation, however, remains poorly understood.

Table 1 lists the various causes of limb length discrepancy.

Evaluation
As with assessment of any problem, a complete history is mandatory. Specific to the problem of limb length discrepancy, the history should provide information as to the cause of discrepancy, previous treatment, and neuromuscular status of the limb. The etiology determines whether the discrepancy is more likely to remain static or to progress. For example, the presence of a unilateral neuromuscular deficit mandates that the shorter involved limb be under corrected slightly to facilitate clearance of the foot which may lack dorsi flexion, during walking.

Clinical Evaluation
The patient is evaluated in two legged stance in order to assess pelvic obliquity, relative height of the knees, presence of angular deformity, and foot size and heel pad thickness. Overall discrepancy can be assessed by having the patient stand with the short limb on graduated blocks until the pelvis is leveled. Leveling of the pelvis is more reproducibly assessed by comparing the position of fixed bony landmarks such as the top of the iliac crests or the posterior superior iliac spines. If a functional scoliosis is present, it will often be observed to straighten as the pelvis levels. (Fig. 1)

Examination is then performed with the patient prone, hips extended and knees flexed to 90 degrees. In this position the lengths of the femoral and tibia. segments of the two limbs can be compared, and the relative contribution of the difference within each segment to the overall discrepancy can be determined grossly. (Fig. 2)

The patient is next examined in supine position so that static measurements of the limb lengths can be performed. This is done with a tape measure. The distance from anterior superior iliac spine to medial malleolus represents the true length of the limb while that from umbilicus to medial malleolus the apparent length. (Fig. 3) Tilting of the pelvis due to a lumbar scoliosis or to soft tissue contracture about the hips can create the appearance of limb shortening on the elevated side and will be reflected by a difference in the apparent length measure. Differences in the true length however, are absolute and represent actual lengthening or shortening of the limb.

Radiographic Evaluation
Radiographic assessment of the leg lengths may be performed if there is clinical evidence of a discrepancy. It can be repeated at 6 to 12 month intervals in order to establish the growth pattern of the limbs. In children over 8 years old, the growth pattern must be adjusted to skeletal age, which has been shown to be a more accurate indicator of maturation than chronological age. Skeletal age is determined by obtaining a radiograph of the wrist and then comparing it to the standardized atlas of skeletal age, based on appearance of the hand and wrist, developed by Gruelich and Pyle.

When several determinations of limb length and skeletal age have been compiled, the remaining growth and the ultimate discrepancy between the limbs can be calculated, and a treatment plan selected. Thus, successful treatment of limb length inequality is predicated on the ability to predict future growth and discrepancy of the limbs. which is in turn dependent on an accurate record of past and present growth. Treatment is rarely rendered solely on the basis of a single determination of the existing discrepancy in a skeletally immature child.

When several determinations of limb length and skeletal age have been compiled, the remaining growth and the ultimate discrepancy between the limbs can be calculated, and a treatment plan selected.

Table 1: Classification of Causes of Leg-Length Discrepancy

Classification	By Growth Retardation	By Growth Stimulation
I. Congenital	Congenital hemiatrophy with skeletal anomalies(e.g., global aplasia, femoral aplasia, coxa vara), dyschondroplasia(Ollier's disease), dysplasia epiphysealis punctata, multiple exostoses, congenital dislocated hip, clubfoot	Partial giantism with vascular abnormalities (Klippel-Trenaunay, Parkes-Weber) Hemarthrosis due to hemophilia
II. Infection	Epiphyseal plate destruction due to osteomyelitis (femur, tibia), tuberculosis (hip, knee joint, foot), septic arthritis	Diaphyseal osteomyelitis of femur or tibia, Brodie's abscess Metaphyseal tuberculosis of femur or tibia (tumor albus) Septic arthritis Syphilis of femur or tibia Elephantiasis as a result of soft tissue infections Thrombosis of femoral or iliac veins
III. Paralysis	Poliomyelitis, other paralysis (spastic)
IV. Tumors	Osteochondroma, (solitary exostosis) Giant cell tumors Osteitis fibrosa cystica generalisata Neurofibromatosis (Recklinghausen)	Hemangioma, lymphangioma Giant cell tumors Ostitis fibrosa cystica generalisata Neurofibromatosis (Recklinghausen) Fibrous dysplasia (Jaffe-Lichtenstein)
V. Trauma	Damage of the epiphyseal plate (e.g., dislocation, operation) Diaphyseal fractures with marked overriding of fragments Severe burns	Diaphyseal and metaphyseal fractures of femur or tibia (osteosynthesis) Diaphyseal operations (e.g., stripping of periosteum, bone graft removal osteotomy)
VI. Others	Legg-Calve-Perthes' disease Slipped upper femoral epiphysis Damage to femoral or tibial epiphyseal plates due to radiation therapy

For patients with larger discrepancies or those of short stature in whom shortening or epiphysiodesis may be inappropriate, limb lengthening is an option.

Treatment
As a general guideline, discrepancies less than 2 cm do not require treatment. Discrepancies between 2 cm and 6 cm are usually treated by a procedure to shorten the longer limb. Lengthening of the shorter limb or a combined lengthening/shortening procedure are options for discrepancies greater than 6 cm. In very severe discrepancies, amputation of the deficient limb and prosthetic fitting may be indicated.

More specifically, shortening of the longer limb can be achieved by an appropriately timed arrest of the distal femoral and/or proximal tibial growth plates, provided the patient is skeletally immature and sufficient growth potential exists in the shorter limb to make up the discrepancy. Alternatively, if there is insufficient epiphyseal growth remaining, the longer limb can be shortened by excising a segment of the bone and then stabilizing it with a metal plate or rod.

For patients with larger discrepancies or those of short stature in whom shortening or epiphysiodesis may be inappropriate, limb lengthening is an option. The advantages of this approach are fairly obvious. By performing a corrective procedure on the abnormal, shorter limb rather than a compensatory procedure on the longer, "normal" limb, stature is not compromised and proportion is maintained. Although the results of lengthening can be quite dramatic (Fig. 4a & 4b) it is a much more complex procedure than either shortening or epiphysiodesis.

Current techniques are based on the principal of "tension stress" introduced in 1951 by a Russian surgeon named Ilizarov. The essence of this biologic principle is that bone and soft tissue will regenerate under conditions of gradual controlled distraction and mechanical stability following an osteotomy.

From a practical standpoint, lengthening is performed by applying an external fixation/distraction device to the bone to be lengthened and then cutting it by a minimally invasive technique, known as a corticotomy, which aims to minimally disturb the soft tissue envelope and maximally preserve the blood supply to the bone. After a latency period of 3 to 14 days, the bone ends are gradually distracted by manipulating the fixator. As the bone lengthens, the gap between the cut ends regenerates with a neo-osteogenic material. When the desired length has been achieved, the regenerated bone is allowed to mature and the fixator removed. Total treatment time using this method averages about one month per centimeter lengthened. Although this may seem somewhat protracted, the patient remains ambulatory throughout, can bear full weight on the extremity, and is encouraged to participate in non contact activities, including swimming.