yogabook / movement physiology / leg length difference

Leg length difference

Overview

Leg length differences are functional or anatomical differences in the length of the two legs. In about 5% of cases they are anatomical, i.e. there are length differences in at least one of the bones involved (femur, tibia), but in the majority of cases they are functional. In the case of functional leg length discrepancies, a distinction must be made between real and apparent discrepancies. The apparent ones result from structures of the pelvis appearing unequal in height when the affected person is standing straight and upright (!), giving the impression that the legs are unequal in length. The stance subjectively assumed to be straight and symmetrical does not necessarily have to be such objectively, as the person usually aligns them according to his proprioceptive body perception and will thus align himself asymmetrically in such a way that he feels equal lateral tensions, evenso in the case of laterally unequal muscle tensions.
Apparent differences in leg length are mostly caused by muscular imbalances, usually in the muscles covering the hip joint.
Functional leg length discrepancies also include genuine but fundamentally reversible differences, which are usually caused by subluxations in one of the joints involved.
A distinction must therefore be made between the following cases:

• Anatomical real – (5% of cases)
• Functionally real (usually subluxation-related): basically reversible
• Functional apparent (false): usually caused by muscular imbalances in the hip joint.

Anatomical leg length difference

In order to definitively prove the anatomical difference, an X-ray or more sophisticated imaging procedures that can quantify lengths are required. These differences cannot, of course, be corrected conservatively; they can be corrected surgically by corrective osteotomies, which may well be indicated depending on their extent. Without the use of an X-ray, they can be assessed on the basis of various anatomical structures. These are primarily

• Malleolus medialis (the medial malleolus)
• Malleoelus lateralis (the lateral malleolus)
• Inner joint line of the knee joint
• Outer joint line of the knee joint
• Greater trochanter of the femur
• Lower edge of the tibial tuberosity (subordinate because it is not in the lateral or medial bone aspect like the other landmarks).

These osseous „landmarks“ can be used to estimate differences in bone length. To do this, they are compared with each other or their height from the ground is measured in standard anatomical position (objectively, not subjectively! See above). Note, however, possible sources of error that are present if the measurement is not taken between two directly neighbouring landmarks in a proximal-distal line, such as a laterally unequal angle of the knee joint in the frontal plane, e.g. laterally unequal knock-knees or bow-legs. The external angle of the femur and tibia, which form the knee joint, is 174° in adults, but this is difficult to measure without an X-ray, as merely the above-mentioned landmarks are available for good orientation, but the bone shaft itself is almost impossible to measure in the case of the femur. In the case of the tibia, at least its front edge is prominent enough because it is not covered by muscles and allows for good orientation. The femur, on the other hand, is covered by muscles, especially medially (adductors), ventrally(quadriceps) and dorsally (ischiocrural group), to such an extent that the shaft is inaccessible, but it is also so largely covered by the lateral vastus that it can hardly be palpated.

If there were now a laterally unequal angle (seen in the frontal plane ) in the knee joint, this would lead to a side discrepancy in the measurement between the lateral malleoli and the trochanters if the bones were of equal length, or a real difference in the length of the bones could be masked by a distance between them measured as equal.

Other possible sources of error include unilateral or unilaterally more pronounced unilateral gonarthrosis or collateral ligament damage, which can lead to an asymmetrical stance. Uneven extension of the knee joint, whether due to inaccurate execution of standard anatomical position or a one-sided extension deficit of the knee joint, can also falsify the measurement.
For this reason, measurements between:

• Malleolus medialis (medial malleolus) and inner joint line of the knee joint, whereby – apart from the possibly laterally unequal depth of the joint space in pathological cases – only the length of the tibia actually plays a role
• Malleolus lateralis and outer joint line of the knee joint, whereby it should be noted that the length of the fibula is measured here, not that of the tibia, which is far less meaningful
• Greater trochanter of the femur and the outer joint line of the knee joint, which in turn only involves the femur and allows a statement to be made about the length of its shaft

Causes of anatomical side differences include, among others, inadequate care such as rickets or other growth disorders, as well as osteoporosis.

Functional leg length discrepancies

When measuring leg length discrepancies, the covered joints can create or simulate a leg length difference due to various effects. This will be discussed below for each joint.

Ankle joint

Since the two malleoli of the lower leg bones are located very distally in the bones and beyond the epiphyseal plates, it also makes sense to measure the distance of the two malleoli from the ground. This should give the same distance from the ground for the two medial malleoli and for the two lateral malleoli, whereby it should be noted that physiologically the distance of a medial malleolus from the ground is less than that of a lateral malleolus, so they cannot be compared with each other. If there are unequal distances between the same (medial or lateral) malleoli from the ground, it is necessary to check for disorders in the talocrural joint or subtalar joint. In the latter case, there may be a unilateral or more pronounced unilateral kinked foot, i.e. a hindfoot valgus, or more rarely a varus position of the calcaneus instead of a valgus position.

Hip joint

If anatomical landmarks proximal to the hip joint are compared laterally with those distal to the hip joint, there is always a risk that a laterally unequal position of the hip joints will falsify the measurement. The most important topographical landmarks proximal to the hip joint are certainly:

• ASIS, the anterior superior iliac spine
• PSIS, the posterior superior iliac spine as a prominent dorsal mediocranial edge (or also: corner) of the hip bone

Distal to the hip joint, the points mentioned above are particularly relevant.
There are several possible distorting factors in the hip joint:

• unequal angle of abduction/adduction in the hip joints. In this case, the two hip joints with the two talocrural joints (or also the calcanei) do not form a symmetrical trapezium, but a distorted one. This distortion causes the above-mentioned ASIS and PSIS landmarks on the side of the leg with greater abduction (or less adduction) to move further down, which distorts all distance measurements to points further distal, beyond the hip joint.
• Unequal angle of flexion/extension in the hip joint. If the feet are parallel and exactly next to each other, this case should only occur in conjunction with a slight flexion of one knee joint or a pelvic torsion, see below. When a knee joint is flexed (regardless of whether this is due to carelessness or lack of extensibility), the above-mentioned reference points also move further down, which in turn distorts the measurement towards points further distal, beyond the hip joint.
• Pelvic torsion, i.e. an unequal nutation angle of both SI joints. This means that one ASIS is further down (distally) than the other and the laterally equal PSIS is further up (cranially), so that the measurement is distorted towards points further distal, beyond the hip joint. Indications of this are, for example, a perceived unequal position of the pelvis in the supine position (savasanna), a conspicuous standing bendover test or a tendency to subjectively perceptible discomfort in one or both SI joints. This is often caused by side discrepancies in the muscles that move in the sagittal plane of the hip joint, i.e. the hip flexors and hip extensors.

Knee joint

In these considerations, the knee joint proves to be less complicated than the other joints. But here, too, there may be factors that distort the measurement:

• A one-sided or one-sided greater extension deficit. In standard anatomical position, this can be a soft-elastic extension deficit if the biceps femoris caput breve is very severely shortened, which is likely to be the preserve of older runners with otherwise sedentary activities. Firm-elastic extension deficits are more likely to be traumatic or iatrogenic, i.e. occurring after serious injuries to the knee joint and their surgical treatment. However, other serious diseases of the knee joint such as RA can also cause extension deficits. If there is an extension deficit and the feet are aligned in exactly the same way next to each other, the extension deficit of the knee joint must also result in a changed angle in the ankle joint (in the direction of dorsiflexion) and in the hip joint (in the direction of flexion). This configuration means that the hip joint – talocural joint – distance is less than contralateral.
• Only rarely will a laterally unequal terminal rotation distort measurements across the knee joint
• A unilateral sabre sheath tibia also distorts the result

Intervention

Functional differences, unlike anatomical differences, respond to appropriate conservative intervention, which may be the reduction of a subluxated/dislocated joint or the restoration of correct upright posture (e.g. standard anatomical position) and a laterally equal gait by addressing muscular side discrepancies. For example, if the tension of one adductor group is higher than contralateral, this leg will always tend to be adducted or less abducted in relation to the other.

Survey of page differences

First of all, there are the possibilities described above for determining anatomical bone length differences. In addition, there are other ways of recognising leg length discrepancies, whereby it should be borne in mind that two different disorders can influence or even cancel each other out, depending on the test:

Assessment of the pelvic reference points (SIAS, SIPS) in tadasana (standard anatomical position)

In the case of differences in the abductors/adductors, the pelvis is lower on one side in the upright stance assumed by the person being examined to be straight, so that the two reference points (ASIS, spina iliaca posterior superior) and SIPS are lower and the contralateral leg appears shorter. The pelvis is then also shifted towards one side and there is presumably an dysbalance-induced false leg length difference. If one ASIS is higher and the lateral PSIS is lower than contralateral in a position where the legs are exactly symmetrical to the vertical, a pelvic torsion must be assumed.

Assessment of the large trochanters in tadasana (standard anatomical position)

When standing upright, look between the legs at a vertical structure as a reference, such as a door frame, to ensure that the leg positions are symmetrical. If the legs then produce an exactly mirror-symmetrical figure to the known vertical, the height (perpendicular to the floor) of the large trochanters is measured and compared. A difference may indicate an anatomical leg length difference in the tibia or femur or, only rarely, in the tarsal bones or a genuine functional leg length difference in one of the covered joints: Knee joint, OSG, subtalar joint.

Assessment in supta dandasana (supine position)

In a supine position with the hips bent at 90°, i.e. legs stretched towards the ceiling as in supta dandasana, with the legs in a symmetrical position (centre line between the legs vertical, which is ensured by looking through the legs towards a structure known to be vertical, such as a door frame), the height of the heels is raised. To do this, a solid flat object such as a shoulderstand plate can be pressed onto the feet from above so that they assume 0° plantar flexion. If there is a side discrepancy in the height of the heels, i.e. if the object is not horizontal but leans to one side, there is a difference in length without specifying the type, which can be caused by any of the bones and joints between the floor and the calcaneus.

Assessment in savasana with bent knees

In the supine position with the knee joints bent at approx. 120° and the feet positioned symmetrically next to each other, the height of the knees is compared onunder the additional condition that the legs form an exactly symmetrical figure, i.e. the degree of adduction in the two hip joints does not differ. Visible differences in height can then indicate disorders in the ankle (talocural or subtalar joint) or hip joint or bones of unequal length (tibia, femur).

Assessment in uttanasana

In an uttanasana where the legs are exactly symmetrical, a difference in the length of the legs in the area of the pelvis and also the lower back can be recognised by the different heights that both sides show. The prerequisite for this is, of course, that the legs are positioned exactly symmetrically to the vertical, which can be ensured by looking through them against a vertical structure. A different height of the pelvis on both sides can indicate an anatomical leg length difference in tibia or femur, only rarely in the tarsal bones, or a genuine functional leg length difference in all covered joints: Knee joint, talocrural joint, subtalar joint.

Assessment of the linea alba in tadasana (standard anatomical position)

A pelvic posture that is tilted to one side in the frontal plane due to a difference in leg length (different degree of abduction/adduction in the hip joints, see above) typically also leads to a non-vertical course of the linea alba when standing upright, at least in its caudal area. If there is no scoliosis, this also applies to its entire course. The older the tendency to take a tilted pelvic posture and the more frequently it has been adopted, the more likely it is that a scoliosis hat developed.
Here too, the prerequisite is that the legs are at the same angle to the vertical. A non-vertical caudal linea alba can indicate anatomical or genuine functional leg length discrepancies, which can be based in all joints and bones distal to the linea alba.

Assessment of the height of the greater trochanters with shifting

In front of a (preferably adjustable) horizontal structure that is at the height of a major trochanter, the other major trochanter is checked in standard anatomical position with closed feet to see whether it is at the same height. If this is not the case, the pelvis may be moved sideways along the structure until the second greater trochanter has reached the height of the structure. On the one hand, (a) is checked for any deviation of the first greater trochanter from this height that has occurred in the meantime, and on the other hand (b) is checked to see whether the legs are still symmetrical. If (a) or (b) are given, there is a side discrepancy.

Allis test

The Allis test, in which the feet are placed exactly equidistant from the pelvis in a supine position with the knee joint flexed rather far (e.g. 120°), if one knee is higher than the other, this indicates unequal lengths of the femur or tibia with a certain probability that has not yet been determined. However, the specificity is reduced by possible misalignments in the ankle and hip joint.

Modalities

Among the factors for functional leg length discrepancies are some inconstant ones that can appear and disappear depending on the posture and movement. For example, a slight misalignment in the hip joint is often corrected in trikonasana or ardha chandrasana. A slight misalignment of the knee joint is often repositioned by bending the knee joint widely under load, such as when assuming a full squat, sometimes also when assuming poses with one leg in a baddha konasana– or lotus-like pose by turning out the thigh when bending the knee joint. Common repositioning triggers can also be identified for malpositions of the subtalar joint, such as wide supination of the foot with a certain degree of plantar flexion. Of course, the Weber-Barstow manoeuvre is also known to reduce a subluxed hip joint in particular.