Lower Limb Length Discrepancy

                                Dr. KS Dhillon

Introduction

Anatomical limb length discrepancy (LLD) can be acquired or congenital. Acquired LLD is usually due to an insult to the growth plate by trauma, radiation, infection, or tumor.  LLD can also occur secondary to fracture of the long bones with malunion or nonunion. Dislocations of the hip and dysplasia of the hip can also cause LLD.

Some causes of congenital LLD include tibial hemimelia, fibular hemimelia, congenital femoral deficiency, hemihypertrophy or other limb hypoplasias. 

Functional or non-structural shortening can also occur. It is a unilateral asymmetry of the lower extremity without any osseous shortening of the lower limb. Functional LLD may be caused by an alteration of lower limb mechanics, such as joint contracture, static or dynamic mechanical axis malalignment, and muscle weakness.

Limb length inequality is commonly associated with compensatory gait abnormalities and can lead to degenerative arthritis of the lower extremity and lumbar spine if the inequality is significant.

Besides clinical evaluation, there are several imaging modalities that are used to quantify LLD. The currently available imaging modalities include plain radiography, computed radiography, ultrasonography, CT, microdose digital radiography, and MRI.

Methods used for Assessing Leg-length Difference

1. Clinical Techniques

Tape measure: A tape measure is typically used to measure the length of the lower extremity by measuring the distance between the anterior superior iliac spine (ASIS) and the medial malleolus. Differences in the girth of the two limbs, and difficulty in identifying bony prominences as well as angular deformities can contribute to errors when we use this measurement tool.  There are certain causes of LLD such as fibular hemimelia and posttraumatic bone loss involving the foot where a significant portion of the limb shortening is distal to the ankle. Such shortening has to be measured from the pelvis to the bottom of the heel. There are cases where the lengths of the appendicular skeleton is equal, but apparent shortening can result from pelvic obliquity or contractures around the knee and hip joints. The apparent leg length can be measured from the umbilicus to the medial malleoli of the ankle.

One must not solely rely on one clinical assessment of LLD. An average value of two separate measurements is more reliable.

Standing on Blocks: The other method to measure LLD is by placing blocks of known height under the short limb to level the pelvis of the erect patient. This is known as the “indirect” clinical method for measuring LLD. This method takes into account the disparity in foot height between the two limbs. It also aids in determining the functional LLD by using varying heights of the block to establish the additional length required for the patient to feel level.

2. Imaging Methods

Plain Radiography. There are three distinct techniques for assessing LLD using standard radiography. These include orthoroentgenogram, scanogram, and teleroentgenogram. 

Orthoroentgenogram: The orthoroentgenogram was initially described by Green in 1946 [1]. An orthoroentgenogram utilizes three radiographic exposures centered over the hip, knee, and ankle joints in order to minimize magnification error. A single large cassette is placed under the patient who remains lying still between the three exposures [1]. This imaging method differs from a scanogram in that a longer cassette is required for the orthoroentgenogram. 

Scanogram: The scanogram is made with the lower limbs positioned with both patellae pointing towards the ceiling and a radio-opaque ruler taped to the table between the limbs. The patient-to-tube distance is typically 101 cm. Three separate AP images are obtained centered over the hip, knee, and ankle joints, using three separate 35 × 43-cm cassettes. The film cassette is moved under the patient between exposures while the patient remains motionless between the three exposures.

Teleroentgenogram: The teleroentgenogram is a full-length standing AP radiograph of the lower extremity. It consists of a single radiographic exposure of both lower limbs, with the x-ray beam centered at the knee from a distance of approximately 6 feet (180 cm) while the patient stands erect with both patellae pointing directly anteriorly. The pelvis is levelled with an appropriately sized lift placed under the short limb. If both iliac crests are at the same level, indicating equalization of LLD, one can simply measure the height of the lift under the short limb to calculate the LLD.

CT Scanogram:  Digitalized images obtained with a CT scan can also be used for measuring LLD [2,3]. Usually, an anteroposterior (AP) scout view of both femurs and tibias are obtained. Cursors are placed over the superior aspect of the imaged femoral head and the distal portion of the medial femoral condyle [3] with the distance between these two cursors representing the length of the individual femur. The tibial length is similarly measured between cursors placed at the medial tibial plateau and the tibial plafond. 

MRI Scan: MRI is traditionally used for soft tissue imaging but has now become an increasingly popular method to evaluate bony abnormalities as well. MRI images are obtained using a T1 weighted spin-echo sequence and the best coronal images are selected for standardized assessment of femoral length using the classic bony landmarks of the femoral head and medial femoral condyle [4].

Leitzes et al. [4] compared MRI scanogram with CT and radiographic scanogram using 12 cadaveric femoral specimens to assess the potential for assessing LLD. Three orthopaedists performed two separate measurements using each technique. Accuracy was assessed by comparing the measurements obtained with the imaging techniques and true measurement of the femoral length using an electronic caliper. The intraobserver and interobserver reliability was very high for all three techniques and all examiners. Compared to the true length of the femur, the mean absolute difference was 0.52 mm for the radiographic scanogram, 0.68 mm for the CT scanogram, and 2.90 mm for the MRI scanogram.

Radiological limb length measurement

The radiological length of the lower limb is measured from the proximal end of femoral head to the center of the tibial plafond on each side and the difference (LLD) calculated in millimeters [5].

Another way to measure the limb length is by adding up the lengths of the femur and tibia. This is called the “composed leg” limb length measurement [6]. The length of the femur can be measured from the proximal end of the femoral head to the centre of the knee joint axis. The length of the tibia can be measured from the centre of the knee joint axis to the centre of the ankle joint axis [7].

The radiological length of the femur cannot be measured from the anterior superior iliac spine (ASIS) or from the tip of the greater trochanter to the distal femur because neither the ASIS nor the greater trochanter contribute to the length of the femur. Excision of ASIS or the greater trochanter will not produce shortening of the femur.

Similarly the radiological length of the tibia cannot be measured from the medial knee joint line to the tip of the medial malleolus simply because the medial malleolus does not contribute to the length of the tibia. Excision of the medial malleolus will not produce shortening of the tibia.

Treatment

Treatment of limb length discrepancy will depend on the magnitude of discrepancy:

-    0 to 2 cm:  No treatment

-    2 to 6 cm:  Shoe lift, epiphysiodesis, shortening

-    5 to 20 cm:  Lengthening (may or may not be combined with other procedures)

-   More than 20 cm:  Prosthetic fitting

Shoe Lift:  Though many believe it is less desirable than surgical correction of a LLD up to 6 cm, a shoe lift is a satisfactory option for patients who do not desire or are not suitable for surgery.  For cosmetic reasons, up to 2 cm of the lift can be put inside the shoe with the remainder, if necessary, on the outside.   Lifts of more than 5 cm are poorly tolerated because the muscles controlling the subtalar joint have difficulty resisting inversion stresses. In patients who require a higher lift, an ankle-foot orthotic extension up the posterior calf can be added for stability. 

Epiphysiodesis: Epiphysiodesis is the treatment of choice for surgical correction of LLD in children with discrepancy from 2 to 6 cm [8,9,10,11]. Epiphysiodesis has low morbidity and low complication rate.

When carrying out surgical treatment of LLD, it is the discrepancy at maturity that is corrected and not the discrepancy in a growing child. The prediction of the effect of epiphysiodesis can be made accurately within 1 cm in almost all cases [12]. It is better to err on the side of undercorrection because there is an advantage to being tall [13,14,15,16].  Slight discrepancies are well tolerated. It is best to aim for 0.5 to 1.0 cm of undercorrection by performing the procedure slightly later. This will  allow for additional length to accommodate a brace or stiffness in the short limb.

Percutaneous epiphysiodesis is considered to be the treatment of choice when compared to open Phemister epiphysiodesis. Percutaneous epiphysiodesis is usually performed through one medial and one lateral incision under image intensifier control. The entire epiphysis can also be drilled through a single incision. Approximately 50% of the area of the plate is removed leaving a strong periphery.  Postoperative immobilization is not necessary.  Tibial epiphysiodesis should be accompanied by arrest of the proximal fibular physis in patients where more than 2.5 cm shortening is anticipated [17]. 

Open Phemister’s surgical technique can also be used. It involved removal of a rectangular block of bone from the medial and lateral physes that spanned two-thirds of the metaphyseal and one-third of the epiphyseal side of the plate. The rectangular block of bone is then reinserted in a reverse position and that would ultimately produce a bar across the growth plate.

Shortening: The indications for acute shortening are the same indications as that for epiphysiodesis. It is offered to patients who are skeletally mature and also in patients who have conditions where the extent of the discrepancy at maturity cannot be confidently predicted. Excessive shortening can be harmful because it can lead to weakness to the muscles due to the shortened muscle length.  Although shortening of 7.5 cm has been reported without loss of function [18], usually  shortening of more than 5 cm in the femur and 3 cm in the tibia is not performed.  There are two techniques of carrying out shortening:

1.Proximal Shortening:  In proximal shortening, femoral shortening is performed  at the level of the lesser trochanter with blade plate fixation. Patients recover quadriceps strength quickly, but the surgery leaves a large scar on the lateral thigh. Postoperatively weight bearing is restricted, and sometimes a second operation for removal of the plate is required.  

2. Mid-Diaphyseal Shortening:  It can be performed with an intramedullary saw followed by intramedullary rod fixation. The major disadvantages of this technique are technical complications, risk of respiratory distress syndrome during reaming [19], and significant weakness of the quadriceps. This technique leaves a small scar, and can allow for earlier weight bearing. The procedure to remove the rod is simpler than that required to remove a blade plate.  

Limb Lengthening:  Lengthening is often a procedure of last resort and is usually  reserved for those situations in which other methods of correction are not appropriate.  The goal of lengthening for most patients is less than 8 cm for the femur and 5 cm for the tibia.  Patients who require larger corrections may require simultaneous lengthening of femur and tibia, repeated staged lengthening of the same bone [20], or a supplementary shortening procedure on the long leg.    

An Ilizarov type of external fixator is applied and a metaphyseal osteotomy is performed. After the surgery, the latency period starts when the bones are allowed to rest for five to seven days to begin the healing process. After the latency period, the distraction period starts when the patient will adjust the orthopedic device so that it slowly pulls apart the two bone segments. This can be done by turning a device on the fixator. As the bone segments are slowly pulled apart, new bone forms in the space between them. This new bone is called regenerate bone.

During the distraction phase, the bone segments are pulled apart at a slow rate of approximately 1 mm per day. Approximately 2.5 cm (1 inch) of length can be gained per month. Increasing the frequency of lengthenings without changing the rate promotes faster consolidation. Lengthening by 0.25 mm four times per day is better than lengthening by 1 mm one time per day. 

When the required length has been obtained the distraction is stopped and the consolidation phase begins, where the regenerate bone slowly gets stronger. Partial weight bearing is now permitted. After the regenerate bone has fully consolidated, the orthopedic lengthening device can be removed. To provide additional protection to the new bone, a cast is usually applied for 3 to 4 weeks. 

Complication rates can be extremely high following lengthening procedures. Many patients do not reach their anticipated lengthening goals without functional compromise. Deformity due to soft tissue tension, delayed union, pin tract infections, nerve or artery damage, mechanical failure due to broken or loosened pins, fracture through the lengthening gap or deformity through the gap, are some of the most frequently encountered complications.

Prosthetic Fitting:  Amputation is a treatment of last resort for patients with a very large limb length discrepancy and for those with deformed and functionless feet [21,22]. Following the amputation, the stump is fitted with a prosthesis.  Limb length discrepancies that are anticipated to become more than 15-20 cm and those involving a femoral length less than 50% of the contralateral femur may be treated in this way [23].  This method of treatment has the advantage of only requiring one definitive operation.  

Children with below-knee amputations (BKA) can function very well. Their gait is almost normal and they can participate in recreational and sporting activities. Those with an above-knee amputation (AKA) can also function well but not as well as those with a BKA. 

A Van Nes Rotationplasty (VNPR) can be carried out in some patients where limb saving shorting of bone is carried out by bone resection. It is a surgical procedure where shortening of the leg is carried out with a rotation of 180 degrees of the lower leg which is adapted to the remaining femur. This changes the ankle function into a new knee joint and allows a limb that would otherwise perform as an AKA to function as a BKA and it provides active control and motor power to the prosthetic knee [24].

Children who undergo surgery and prosthetic fitting early in life have the best outcome.  The best time for performing a Syme amputation is toward the end of the first year of life near walking age while the best timing for rotationplasty is at about 3 years of age.  

References

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