Pelvic Asymmetry and Leg Length Difference

The clinical challenge of differentiating true and apparent leg length difference is not frequently discussed in the literature and is fraught with difficulty.  In general we can classify pelvic asymmetry as;

1) Primary intrinsic pelvic ring dysfunction

2) Asymmetry secondary to lower limb leg length variation

3) Asymmetry secondary to spinal mal-alignment.

The principle of quantifying pelvic orientation is to eliminate the effect of the legs and assess bony pelvic landmarks in prone, supine and sometimes sitting.  This allows for direct comparison side to side and at least the ability to quantify asymmetry. The well described observations include:

Anterior innominate rotation

Posterior innominate rotation

Innominate upslip

Innominate downslip

Innominate inflare

Innominate outflare

Sacral torsion.

These types of classifications allow us to state the positional relationship of the innominate and sacrum but often pose a significant challenge in determining which is the side of asymmetry i.e. is the high side high or the opposite side low?  To evaluate this question the assessment needs to be supplemented with specific muscle length and movement tests to attempt to establish a pattern. The well recognised strategy of using the umbilicus as a reference point allows for easy visualisation and distance measurement but has the drawback of requiring some “normative” distance reference for which there is not a reliable baseline and the measurement error would likely be unacceptable.

Useful bony landmarks for reference are:

Iliac crests

ASIS

PSIS

Ischeal tuberosities

Sacral Sulcus

Sacral inferior lateral angle

Because alterations in pelvic alignment contribute to changes in leg length the clinical challenge of defining what is a real leg length difference , what is an “apparent “ or functional leg length difference and what is a “combined” lesion can be very taxing. This is compounded by the fact that apparent conflicts in findings hamper the reasoning process. For example an innominate upslip produces an apparent leg shortening on the same side but in standing the elevated pelvis can be misinterpreted as a consequence of a long leg on that side.

Anterior or posterior innominate rotation are perhaps the easiest of the pelvic asymmetries to quantify. To answer the question of which is anterior and which is posterior supplemental length / tension tests are really helpful. An anterior rotated innominate is frequently associated with restricted hip flexion either by posterior buttock tension or anterior hip impingement.SLR can also be restricted on the same side.

A posteriorly rotated innominate is frequently associated with restricted hip extension (the prone hip extension test), Lumboscaral facet impingement / Sacroiliac strain and Rectus femoris tightness.

Supplementary manual resistance tests may reveal weakness of the prime movers associated with the alignment asymmetry.

If we consider the inflare / outflare pelvic alignment scenario the most important point to recognise is that anterior innominate rotation is coupled with innominate outflare and posterior innominate rotation is coupled with innominate inflare. Therefore it is necessary to address the rotational mal-alignment as the first priority and having established alignment in the saggital plane then proceed to assessing the “flare” component. An outflared innominate is frequently associated with a restricted F / ADD test either due to posterior buttock strain or medial groin impingement. An inflared innominate is frequently coupled with a restricted FABER test and usually by adductor tightness.

In practise, the initial strategy is to align the innominates and subsequently assess for sacral position. Obviously Sacral mal-alignment may alter innominate position and visa versa but in order to provide a useful framework the above sequence is suggested.

One of the most widely applied differential tests is the lying/sitting test. This attempts to quantify alterations in leg length associated with a change in pelvic alignment and thus differentiate between true and apparent differences.

In a future post we will look in detail at the lying siting test and the factors which influence the test results.

Enjoy the clinical challenge
David

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