Introduction
A discrepancy in the leg lengths may result from a variety of congenital or acquired conditions (Table 1), and while up to 25% of the American population may have a difference of greater than one centimeter, only a small percentage have more than a 2 centimeter difference in leg lengths. The main consequence is gait asymmetry. An increase in vertical pelvic motion is observed, and more energy must be expended during ambulation. While a small compensatory lumbar curvature may develop, there is little evidence to suggest that leg length discrepancy results in back pain, structural scoliosis, or degenerative arthritis. The goal of treatment is to have a discrepancy of less than 2 to 2.5 centimeters at skeletal maturity, and a variety of treatment methods are available to achieve this objective. A knowledge of the underlying etiology, coupled with regular followup to assess limb growth and skeletal maturity, allows the treating physician to project the discrepancy at skeletal maturity and to plan treatment. A subset of patients will have coexisting abnormalities in the viscera or musculoskeletal system which must be identified and treated as well.
Table 1: Common Causes of Lower Extremity Length
Congenital
- Proximal femoral focal deficiency
- Coxa vara
- Hemiatrophy-hemihypertrophy (anisomelia)
- Developmental dysplasia of the hip
Developmental
- Legg-Calvé-Perthes disease
- Neuromuscular
- Polio
- Cerebral palsy (hemiplegia)
Infectious
- Pyogenic osteomyelitis with physeal damage
Trauma
- Physeal injury with premature closure
- Overgrowth
- Malunion (shortening)
Tumor
- Physeal destruction
- Radiation-induced physeal injury
- Overgrowth
Diagnosis and Clinical Findings
Gait asymmetry is the most frequent complaint. The diagnosis is made on physical examination, and specialized radiographs help to quantify the discrepancy, and to follow the discrepancy over time. The discrepancy may relate to bony shortening and/or angular deformity (actual shortening), to soft tissue contracture at the hips, knees, or ankles (apparent shortening), or to a combination of these. Other contributing factors include joint subluxation or dislocation (hip), or a decrease in the height of the foot (congenital or neuromuscular). As such, a careful physical examination is required to identify all factors contributing to the discrepancy.
There are several clinical methods for measuring limb length. Our preference is to perform a standing examination, in which blocks of various sizes are placed under the short leg until the pelvis is leveled. An alternate method is to measure the lengths of each leg with the patient supine. A tape measure is used, and the distance between the anterior superior iliac spine and the medial malleolus is measured. These methods should be reasonably accurate in the absence of "apparent" causes of discrepancy. In addition to using one or both of these methods, the range of motion at the hip, knee, and ankle must be assessed to identify any causes of apparent discrepancy. For example, a 10 degree fixed abduction (or adduction) contracture of the hip will create an apparent leg length discrepancy of 2-3 centimeters. Similarly, a flexion contracture of the hip and/or knee will create apparent shortening of the extremity, while an equinus contracture at the ankle will create apparent lengthening of the extremity. A rigid lumbar scoliosis (suprapelvic contracture) will create pelvic obliquity and an associated limb length inequality. Once a discrepancy is quantified, it must be followed at regular intervals.
Assessments at six to twelve month intervals are most common. In addition to the physical examination, it is common to obtain a specialized radiograph to quantify the discrepancy, and an AP radiograph of the left wrist to estimate the degree of skeletal maturity. Multiple points of data, collected longitudinally, allow the most accurate prediction of the discrepancy at maturity.
Radiographic Evaluation
The radiologic evaluation complements the clinical exam, and both are typically employed when making treatment decisions. The same technique should be used longitudinally to maximize accuracy. Four different techniques are available. The teleoroentgenogram is a single exposure of both lower extremities (standing), and requires a long cassette. A ruler is placed on the film, and direct measurements are made, factoring in a 6% magnification error. One advantage is that angular deformities may be assessed. Its primary indication is for young children. The orthoroentgenogram consists of three separate exposures of the hips, knees, and ankles on a long cassette. The patient is supine, and a ruler is placed on the cassette for measurement of bone length. There is no magnification error. However, the patent must lie still for the three exposures, which is often difficult to achieve in younger children. The scanogram also consists of separate exposures of the hips, knees, and ankles on a cassette with a radiographic ruler, however a small film cassette is used. There is no magnification error, however patients must remain still for the three exposures, and angular deformities cannot be assessed. While CT is the most accurate technique, the assessment is time consuming, and the technique is not available in most centers. In addition to quantifying the discrepancy, it is essential to determine skeletal age (bone age). An AP radiograph of the hand and wrist is usually obtained at each visit, and compared with the standards in the Gruelich and Pyle Atlas in order to estimate skeletal age. While more accurate techniques are available, most are time consuming and impractical for routine clinical application. The range of variability using the atlas is approximately 9 months, so the method is most accurate when multiple data points have been collected.
Treatment
Options for treatment include observation, a shoe lift or custom orthosis, a limb shortening procedure (acute shortening versus gradual shortening by growth arrest), a limb lengthening procedure, or more than one of these. In the congenital deficiencies (femur, tibia, fibula), an early foot amputation is often the best option to manage a severe predicted discrepancy and achieve the best functional outcome. In addition to the magnitude of discrepancy predicted at skeletal maturity, both the anticipated adult height of the patient (estimated from family members), and the desires of the patient and his or her family, are important considerations. General guidelines for treatment are as follows.
Discrepancies of up to 2.5 centimeters may be treated by observation or a shoe lift. Up to three eights of an inch may be placed within the shoe, and up to five centimeters may be placed on the outside of the shoe. Complete correction of inequality is not required, and the height of the lift should be adjusted based upon the patient's gait and comfort. An orthotic may be used as a temporizing measure prior to definitive treatment. For patients with a discrepancy between 2 and 5 centimeters, an epiphysiodesis is offered in skeletally immature patients, and an acute shortening may be performed in a skeletally mature patient. Epiphysiodesis refers to a temporary or permanent cessation of growth at one or more physes. A permanent growth arrest is most commonly performed as long as sufficient data is available with which to accurately predict when to perform the procedure. Approximately 65% of the growth of the lower extremity comes from the distal femur (37%, 9 mm/yr) and proximal tibia (28%, 6 mm/yr). Boys typically grow until 16 years of age, while girls grow until 14 years of age. As such, performing an epiphysiodesis of both the distal femur and the proximal tibia in a patient with three years of growth remaining should achieve approximately 4.5 centimeters of correction. Techniques used to determine the timing of epiphysiodesis are the Menelaus method ("rule of thumb"), the Green and Anderson method, the Moseley straight line graph, and the multiplier method. The most common surgical technique is the percutaneous epiphysiodesis, in which the physis is ablated with a drill and curets under image intensification. This is an outpatient procedure with few complications. Insertion of screws across the physis is an alternative. For patients in whom sufficient data is unavailable, or those in whom the underlying diagnosis is associated with an unpredictable pattern of growth, then a reversible technique may be considered. Physeal stapling involves the extraperiostial insertion of one or more staples across both sides of the growth plate. Once equalization has been achieved, the staples are removed, allowing growth to resume. When the patient is skeletally mature, or if it is deemed appropriate to wait until maturity before treatment, and acute shortening may be the best option. Acute shortening is typically performed at the femur (several techniques have been described), given the increased risk of complications (compartment syndrome, neurovascular problems) associated with shortening of the tibia and fibula.
For discrepancies greater than 5 centimeters, lengthening of the short limb is the procedure of choice. An exception would be a discrepancy secondary to overgrowth of one limb, in which limb shortening would be preferred in order to preserve body proportions. Patients with anticipated discrepancies greater than 8-10 centimeters often require one or more limb lengthening procedures (spread several years apart) with or without an epiphysiodesis. The most common technique used for limb lengthening involves placement of an external fixator, either a ring fixator such as the Ilizarov device, or a monolateral device. The bone is cut at the metaphyseal-diaphyseal junction, and lengthening is achieved gradually through distraction at the corticotomy. The usual rate of lengthening is one millimeter per day, and it takes approximately one month of time in the fixator for each centimeter of length gained. A maximum of 15-25% of the original length of the bone may be gained at each session. An advantage of the circular fixator is the ability to correct coexisting angular deformities at the same time. Although specialized training is required, and complications occur in nearly all patients (most are minor and easily treated), excellent results may be achieved in these difficult cases. Complications include pin tract infection (most common), wound infection, hypertension, joint subluxation, muscle contracture, premature consolidation, delayed union, implant related problems, and fractures after implant removal. Finally, early amputation and prosthetic fitting may provide the best long term function in patients with projected discrepancies in excess of 18-20 centimeters, especially when there are coexisting deformities or deficiencies of the ipsilateral foot. The alternative would be multiple reconstructive procedures throughout childhood and adolescence. The impact of multiple procedures on the child's psychosocial development must also be kept in mind when formulating the treatment plan in these complex cases.
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