Keys To Recognizing And Treating Limb Length Discrepancy
Some practitioners prefer radiographic assessment with the most accurate method being the scanogram in which a series of views examines the central ray aimed at the femoral head, tibial plateau and ankle mortise respectively.14 An alternative is to direct the central ray parallel to the femoral heads. No matter the chosen method, the subtalar joint must be neutral to prevent asymmetrical pronation from being considered a structural discrepancy. Additionally, one should evaluate the femoral neck angles since differences from left to right will result in limb asymmetry.
Indirect assessment of limb length occurs by placing boards of a specific thickness under the foot until the pelvis is level. Please note that none of these methods assesses the pathomechanical impact of the LLD during ambulation or whether the individual is functioning asymmetrically.
Understanding The Pathomechanics
Whatever the cause of the discrepancy, there is an asynchronous placement of the foot during the gait cycle, resulting in pelvic position asymmetry. The body may compensate in a number of ways and the problems that arise are usually the sequelae of these compensatory mechanisms.
Asymmetrical gait patterns are obvious in those with discrepancies greater than 2 cm.7 These individuals may exhibit plantarflexion at the ankle with secondary Achilles contracture on the shorter limb.15 When it comes to discrepancies from 1 to 1.5 cm, even though the shorter limb carries a greater risk of knee osteoarthritis, it is the longer limb that is subject to greater ground reactive forces and isometric torque.16 A study of military recruits revealed the occurrence of stress fractures to be 73 percent in the longer limb, 16 percent in the shorter limb and 11 percent in limbs of equal length.17 The hip on the longer side is also subject to increased stresses and resulting degenerative pathology. In a study of 100 patients presenting for hip replacement, 84 percent had osteoarthritis on the longer limb.18
The foot on the longer side is relatively pronated to functionally decrease the length of the limb.19 This is evident in increased midstance, stance and single support phases of gait.15,19 The foot on the shorter limb is relatively supinated with increased lateral calcaneal pressure in an attempt to extend its length during the contact and adaptive phases of gait.15,19 Accompanying this longer limb pronation is anterior migration of the talus, forward displacement of the tibia and concomitant knee flexion.20,21 Additionally, on the longer limb, there is increased medial calcaneal pressure.19 If there is inadequate range of motion in the subtalar and midtarsal joints, then compensation occurs more proximally.15 At the genicular level, there may be compensatory flexion or hyperextension in the sagittal plane, and genu valgum or varum in the frontal plane in an attempt to shorten the longer extremity. In the frontal plane at the pelvis, there is a lowering to the shorter limb with secondary lumbosacral scoliotic compensation.1,11,14,15,20,21
If the scoliosis is present with the spine flexed and extended, it indicates a structural deformity and any limb discrepancy would be secondary. If the scoliosis is present only when the patient is standing (spine extended) and disappears when seated (flexed), then it is a functional scoliosis secondary to the primary LLD.1,14 With the pelvis dropped toward the shorter limb, there is a compensatory tightening of the iliopsoas muscle contralateral to the scoliotic convexity in an attempt to counteract lateral spinal deviation.1 This process flexes the hip, consequently flexing the knee, internally rotating the femur and increasing subtalar pronation, all effectively shortening the longer limb.