The causes of leg length inequality, or anisomelia are numerous. Minor discrepancies are seen in clubfeet, hip dysplasia, and Perthes disease. Major differences are seen in tibial, fibular, or femoral agenesis.
The course of anisomelia is determined by the cause. The inhibition or acceleration that causes progressive forms of anisomelia varies according to the etiology. Growth inhibition from congenital defects is usually constant and makes predicting the final disparity feasible. Inhibition or acceleration from vascular, infectious, or neoplastic disorders are variable. For example, growth acceleration may be associated with chronic diaphyseal osteomyelitis. The acceleration occurs only when the infection is active.
The effect on gait depends on the magnitude of the discrepancy and the age of the patient. Children compensate for discrepancies by flexing the knee on the long side or by standing in equinus on the shortened limb. These compensations level the pelvis. Discrepancies are compensated by altered function. The long limb may be circumducted during the swing phase or by “vaulting” over the long limb. This vaulting results in a rise and fall of the body and consumes more energy than a normal gait.
The adverse effects of anisomelia have been overstated. Limb length difference in childhood does not lead to an increased risk of structural scoliosis or back pain in adults.
During evaluation, calculate the projected height of the patient and the degree of shortening at skeletal maturity if untreated. This evaluation requires a screening examination, a search for the cause, clinical and radiographic assessment of severity, and a determination of bone age. Serial evaluations are necessary during growth to improve the accuracy of the evaluation. From the history, determine if the child has been injured or has experienced any musculoskeletal diseases.
Screening examination Note any asymmetry and alterations in body proportions. Does the asymmetry involve only the lower limbs? Is the long side the normal or abnormal side? Sometimes overgrowth makes the long side the abnormal one. Is it a hemihypertrophy or hemihypoplasia? Hemihypertrophy is important to recognize because it is sometimes associated with Wilm’s tumor. The finding of hemihypertrophy should prompt an abdominal ultrasound evaluation. Hemihypoplasia is usually due to hemiparesis from cerebral palsy. Often these underlying problems are more significant than the length discrepancy itself. Observe the child walking. Is equinus, vaulting, circumduction, or abductor lurch present? Assess the abnormal limb to determine the site or sites of the discrepancy. Are the feet of equal length? Are the tibial and femoral segments equal? Are the forearms of equal length? Are any associated abnormalities present? Is joint motion symmetrical? Assess to determine whether the difference is in the femur, tibia, or a combination.
Clinical measures of discrepancy Assess leg length difference by placing blocks of known thickness under the short side until the pelvis is level. The patient will often sense when symmetry is established. By this method, all segments, including foot and pelvis, are assessed.
Imaging methods Image to measure discrepancies and determine any associated bone or joint deformities. Radiographic measures include the teleroentgenogram with a single exposure or orthodiagrams requiring multiple exposures on the same film. The orthodiagrams may be full length on a 36-inch film or telescoped on a 17-inch film. For the infant and young child, order a teleroentgenogram because it provides an excellent screen for other problems such as hip dysplasia, it requires only one exposure, and it does not require patient cooperation. Enough serial studies should be made to provide adequate documentation to accurately predict the discrepancy at maturity and to time the epiphysiodesis. These need not be done yearly. If the discrepancy is detected in the infant, obtain the baseline study early and repeat at about 3, 6, and 9 years of age.
Bone age Bone age is the most inaccurate of the measurements. Often measurements are given with a ±2 year qualification. The standard for assessment is the Gruelich-Pyle atlas. It is wise to sample bone ages over a period of several years and average any differences from the chronological age to improve reliability.
Body height at skeletal maturation The projected height at skeletal maturation is sometimes useful in planning correction of anisomelia. Shortening is more feasible for the tall individual, whereas lengthening may be more acceptable for those of short stature. The estimation can be made by comparing the child’s height with the bone age to determine a percentile. This percentile is projected to maturity to estimate adult height.
Calculating discrepancy at maturation The discrepancy at maturation is the sum of the current discrepancy and the discrepancy accumulated during the period of remaining growth. The current discrepancy is assessed by clinical and radiographic measures. The discrepancy created by remaining growth must be calculated based on the percentage of growth retardation (or acceleration).
Minimal acceptable height (MAH) The MAH is the shortest stature that would be acceptable to the family. This will be based on racial, social, cultural, individual, and family differences. As a starting point for discussion, set the MAH at 2 SD below the mean value, or about 65 inches for men and 59 inches for women. Establishing the MAH involves an integration of complex issues such as the value the family gives to preservation of height, balanced with the increased risks of limb lengthening over shortening.
The objective of management is to level the pelvis by equalizing extremity length without imposing excessive risk, morbidity, or height reduction. The severity of the discrepancy determines the general approach to management.
Severity Degrees of shortening can be categorized to aid in planning management. These values are influenced by the minimal acceptable height as determined during evaluation. For severe discrepancies, start with lengthening. Assess progress and then consider epiphysiodesis to complete the correction.
Lifts Lifts may be useful in discrepancies greater than 2–3 cm. Lifts cause problems for the child. They make the shoe heavier and less stable and are usually a source of embarrassment. Lifts make a clear statement, “I have a disability,” which may be harmful to the child’s self-image and status among piers. Because no immediate or late harmful effect of uncompensated anisomelia has been shown, the lift should improve function enough to compensate for inherent problems of wearing a lift. Walking without the lift will not damage the child. Lifts may be applied inside the shoe or on the heel. Make the lift as inconspicuous and lightweight as possible. More in-shoe correction can be placed in a high-top shoe.
Timing of Correction
The usual objective of management is to correct the leg length discrepancy to within 1.5 cm of the opposite side, with the long leg remaining as the longer leg. Because of its simplicity, effectiveness, and safety, epiphysiodesis remains the most effective means of correcting discrepancies between 2 and 5 cm.
The timing of epiphysiodesis determines the degree of correction, and five methods of timing are currently used.
The simplistic method is useful for giving a rough estimate of the discrepancy at maturation from discrepancies of congenital origin. This is based on the assumption that the growth retardation is consistent. For example, a child with a congenital discrepancy of 3 cm at age 2 years has reached roughly half of projected adult height. Thus, at skeletal maturation, the discrepancy is likely to be about 6 cm.
The straight-line graph method of Moseley requires a special graph for each patient. The method is graphic and has the advantage of averaging the bone ages.
The arithmetic method is based on average growth rates and chronological age. On average, the distal femur contributes 3/8 inch of growth per year, the proximal tibia contributes 1/4 inch per year. Girls complete growth at 14 and boys at 16 years of age. Use this method for long-term planning.
The Paley multiplier method allows prediction of eventual discrepancy and the appropriate time for correction by a contralateral epiphysiodesis. This timing is performed in steps:
Predict the disparity at maturation Apply the appropriate formula based on whether the deformity is congenital or acquired to determine the disparity at maturation. This makes use of the multiplier charts that are different for boys and girls.
Determine the timing for epiphysiodesis Apply a special formula to calculate the time for correction.
Mosca approach This method combines key elements of the arithmetic and multiplier methods.
Principles The vast majority of acquired arrests, the arrest is total and the effect can be calculated by the arithmetic method.
Be aware that a contralateral epiphysiodesis will only arrest the progressively increasing disparity and not reduce the disparity. Dealing with the existing disparity requires accepting the current disparity, adding another level to the epiphysiodesis, or performing a shortening or lengthening procedure.
Calculate disparity at maturity Determine the current discrepancy. Level the pelvis by placing blocks of known thickness under the foot of the short leg. Make a standing radiography of the pelvis. From this study, measure the difference in level of the femoral heads. Combine the block height and the radiographic measures to establish the total leg length difference. This difference includes the disparity in length of the femur, tibia, and foot. Determine the difference at maturity by multiplying this measured difference with the multiplier factor.
Calculate age for epiphysiodesis Use the arithmetic method to determine the appropriate age for the epiphysiodesis. To increase the accuracy of timing, consider adjusting growth potential numbers slightly based on the height of the child and family. If the family is tall, the distal femur may grow 11 mm instead of 10 mm per year. Likewise, if the family is short, this growth may be only 8 or 9 mm.
Example In a child of average height with a calculated 3 cm disparity at maturation, perform a distal femoral epiphysiodesis at age 13 years for a boy or age 11 years for a girl.
Bone shortening is a relatively safe and effective method of correcting discrepancies in the patient beyond the age when correction by epiphysiodesis is possible. Closed shortening procedures are now the standard.
Stapling as a means of achieving an epiphysiodesis is appropriate only when calculating the appropriate timing for an epiphysiodesis is not possible due to difficulties in reading bone age and plotting growth.
Epiphysiodesis is the best method to correct most discrepancies between 2 and 5 cm. The traditional method leaves a long scar. Newer percutaneous methods use either a curette or a drill to remove the growth plate.
Lengthening as a means of correcting anisomelia has been practiced for 90 years. During the past two decades, the worldwide application of a 50-year-old method has reduced the risks and made the procedure more effective. This increased effectiveness is primarily due to the improved osteogenesis achieved by applying biological principles established through research.
Epiphysiodesis is the safest procedure for correcting limb length inequality. The procedure is usually performed in the distal femur or upper tibia, or at both levels. Epiphysiodesis may be achieved by many techniques. Newer techniques are percutaneous and sometimes performed from only one entry site. The described techniques are suitable for both femoral and tibial level procedures.
Modified Phemister Epiphysiodesis
Technique Position the child supine with imaging available. Prepare the skin and drape. The physis is usually at the mid-patella level. Under tourniquet hemostasis, make a 3-cm longitudinal incision centered over the physis. Incise the periosteum and identify the cartilagenous physeal line. With a straight osteotome, remove a 2–2.5-cm square block of bone centered over the physis. Curette about half of the physis. Extend the curettage anterior and posterior, leaving the cortex intact. Rotate the block and replace it in the defect. Repeat the procedure on the opposite side. Perform a subcuticular closure of both wounds. Inject marcaine in wound margins. Place a bulky compressive dressing.
Postoperative care The day following surgery, replace the compressive bulky dressing with a conventional one, replace the knee immobilizer, and discharge the patient. Maintain non-weight-bearing status for 4–5 weeks with knee immobilizer in place. Physical therapy is not necessary. Allow full activity in 6–8 weeks. Evaluate at 6 months and yearly to assess the effect of fusion. Check knee angle to identify asymmetrical fusion and limb lengths to assure complete physeal closure by a progressive reduction in length difference.
Technique Prepare and drape with the knee region exposed. Under tourniquet hemostasis with image guidance using a K wire marker, make a 3–5-mm skin incision directly over the epiphysis. Enter the epiphysis with a 6-mm (0.25-inch) drill or cannulated drill. Examine the drilling for cartilage fragments to confirm physeal penetration. Drill out the epiphysis by sweeping the drill from anterior to posterior. An alternative method is to make an array of holes in the physis from a single point of entry. Leave the cortex intact. Make certain the entire central portion of the epiphysis has been removed. The epiphysiodesis may be performed from only the medial side. Some surgeons consider it prudent to perform the procedures on both sides of the epiphysis, as the growth plate is curved and sometimes difficult to access from only one side.
Postoperative care Discharge home the same day or the day after as with the modified Phemister technique.
Hematoma formation Reduce risk by placing a bulky compressive dressing following the procedure.
Compartment syndrome following tibial procedures is very rare. Identify early and release compartment.
Failure of fusion is rare and probably due to an inaccurate level of the procedure.
Asymmetrical fusion results in a progressive increase in varus or valgus deformity. Identify early and immediately refuse the open side.