True and Apparent Leg Length Testing – The Lying/Sitting Test
The lying/sitting test for evaluation of sacroiliac dysfunction was briefly discussed here in a recent post. At that time I said we would come back and look again in more detail considering the mechanics involved and how we can make rational clinical interpretations. The lying/sitting test is a variation of other well- described Kinetic tests for sacroiliac dysfunction.
These most commonly described are:
1) Hip flexion test in standing (Stork, Fowler or Gillet test),
2) Forward flexion in standing or sitting,
3) Hip extension in standing,
4) Lateral flexion in standing
5) Rotation in sitting or standing.
The purpose of these tests is to provide a battery of procedures to quantify which sacroiliac joint shows the greatest degree of motion impairment indicative of a “so called” positive kinetic test. The advantage of performing these tests in non weight-bearing means that other reference points involving the legs can be used to assist in the positional diagnosis. The principle of the lying/sitting test is that it can be easier clinically to monitor changes in leg length than to monitor specific pelvic bony landmarks during the standing flexion test and therefore provide an additional layer of evaluation to quantify the pelvic position.
The first principle to establish is whether there is a true leg length difference present. This is best established in a supine position where the iliac crests and ASIS are monitored on the pelvis together with the position of the medial malleoli of both legs. If the pelvic alignment is symmetrical and there is a difference in the position of the medial malleoli then there is a suspicion of a true leg length difference. This can be further explored by the patient flexing their knees and evaluating the level of the superior patellas from a side view. If this isn’t clearly visible placing a book or flat object across the top of the knees will demonstrate any inclination and thus alteration in leg length using this test. Of course the critical issue in determining true versus apparent leg length difference is whether the clinical strategy is to use some form of orthotic device to compensate for the leg length difference and optimise stress distribution.
Patients with a true leg length difference may well need this type of corrective action but patients with an apparent leg length difference do not and the treatment strategy needs to be directed towards optimising lumbosacral and pelvic alignment. This is a common pitfall in clinical practice and one reason why establishing true versus apparent leg length difference is such an important consideration.
The apparent change in leg length which occurs as a consequence of sacroiliac or lumbo pelvic dysfunction appears to arise from a number of biomechanical factors. In supine an anterior innominate rotation carries the acetabulum forwards and down (relative to the axis of the SI joint) and therefore makes the leg on that side appear longer. Conversely a posterior innominate rotation draws the acetabulum backwards and posteriorly (relative to the axis of the SI joint) apparently shortening the leg on the side of the posterior innominate rotation.
The mechanical effects of moving from supine lying to upright are based upon our current understanding of the kinematics of this movement. In supine lying the acetabular lie anteriorly and craniad relative to the ischial tuberosities. On moving to the long sitting position from supine flexion occurs initially in the thorax and then the lumbar spine at which point the pelvis starts to rotate forwards and eventually pivots over the tuberosities as one unit. The acetabular are therefore moved further anteriorly and also downwards so that the legs appear to lengthen equally. On returning to supine the reverse pattern occurs when the pelvis rotates backwards as a unit with the acetabular moved upwards and posteriorly drawing the legs equally with this movement pattern.
When sacroiliac joint motion is compromised (for whatever reason) these normal mechanics do not apply. It is thought that the interference with Sagittal plane motion in this test is compensated for by transverse plane rotation whereby the side with the positive kinetic test (dysfunctional movement side) forces a swivelling action to occur about the opposite ischial tuberosity with the blocked side of the pelvis now moving / rotating backwards (transverse plane twisting – probably because the other side continues to rotate forward to end range) effectively shortening the leg on that side. The reverse is true for a posterior rotated Ilium in which the leg will appear to lengthen in the long sitting position.
We must remember of course that these alterations in joint mechanics may not be primarily indicative of an articular dysfunction but may be a consequence of myofascial restriction distorting movement patterns. The clinical prioritisation of targets therefore comes down to a question of judgement on the primary mechanism, treating an apparent articular dysfunction and reassessing within a treatment session or expanding the repertoire of techniques to incorporate myofascial components if clinically indicated. Myofascial components in the form of mechanical restrictions are easily evidenced by alterations in length / tension with appropriate length tests and in the absence of these findings the implication is of a motor control /strength deficit. Of course the real challenge is quantifying a mixed pattern of dysfunction related to both true and apparent components of a leg length difference but that’s for another day. So the lying sitting test is a useful assessment tool to add to the battery of kinetic tests when evaluating pelvic dysfunction
Enjoy the clinical challenge.