Leg length inequality

There are a number of different factors affecting the perceived length of the leg before, during and after hip replacement.

As the hip joint wears out there is physical loss of material from the acetabulum and the head of the femur, this is a slow process and may go unnoticed. There is a natural tendency for arthritic hips to become stiff and fixed towards the mid line, causing apparent shortening of the limb due to pelvic obliquity. After surgery it can take a year or so for the pelvis to level out.


Fixed adduction causing pelvic obliquity	Image rotated to level the pelvis

In some cases a larger proportion of the femoral head may collapse, usually due to secondary avascular necrosis. This is often fairly rapid, over a few weeks, and very painful.


Significant shortening due to advanced arthritis and secondary AVN	Image rotated to allow calculation of true shortening

During surgery there are two factors that affect the true leg length. Firstly the position of the acetabular cup in the pelvis. Using the transverse ligament of the hip joint as a reference point it is usually possible to accurately recreate the normal anatomy. If there is a significant anatomical abnormality (for example Developmental Dysplasia of the Hip) it is sometimes better to fix the acetabular cup in the best bone, rather than the best place.

The second variable is the height of the femoral stem with it's head. Alterations in the position of this component can be used to compensate for any necessary compromises made in the position of the acetabular cup. Malposition of the femoral stem is the commonest cause of unwanted shortening or lengthening.


Accidental Leg Lengthening	Revision to restore leg length

Diagnosis

It is important to accurately measure the true length of the legs before considering any intervention. This is best measured using a CT scanogram [ Measurement of leg length discrepancy after total hip arthroplasty. The reliability of a plain radiographic method compared to CT-scanogram. Kjellberg M, Al-Amiry B, Englund E, Sjödén GO, Sayed-Noor AS. Skeletal Radiol. 2012 Feb;41(2):187-91. ]. Many patients are mis-diagnosed with true leg length difference because of a failure to account for pelvic obliquity - causing unnecessary anxiety, usually by therapists who do not have access to view x-rays.

Treatment

Postoperative leg length difference of up to 1 cm is usually not symptomatic [ Leg Length Discrepancy After Total Hip Arthroplasty William J. Maloney, MD, and James A. Keeney, MD. The Journal of Arthroplasty Vol. 19 No. 4 Suppl. 1 2004. http://www.robodoc.com/docs/Maloney%202004.pdf ]. Larger discrepancies, if symptomatic, may require the use of a shoe raise or revision surgery [ The importance of leg length discrepancy after total hip arthroplasty. Konyves A, Bannister GC. J Bone Joint Surg Br. 2005 Feb;87(2):155-7. http://www.bjj.boneandjoint.org.uk/content/87-B/2/155.full.pdf ].