Unequal Leg Length After Total Hip Replacement

“…and strength by limping sway disabled”
Shakespare. Sonnett 66

In one study the operated leg was a mean of 9mm longer three months after the operation in 62% of patients and 43% were aware of the lengthening.¹ Even in patients who do not have significant leg length discrepancy there may be a perception that this is so (apparent leg lengthening). In some cases there may be a period of postoperative perceptual adjustment if a leg previously shortened by hip joint pathology is restored to normal length by arthroplasty. Indeed, in 1979 Sir John Charnley, the father of modern hip surgery, stated that lengthening of up to 10mm was common and acceptable and may actually improve function by slightly tightening the abductor muscles. Lengthening significantly greater than 10mm risks producing a poor functional outcome, with pain due to tissue stretching around the joint, backache due to secondary scoliosis, gait abnormalities and easy fatigue after walking.

Unfortunately, patient dissatisfaction is a potential concern when there is significant postoperative leg inequality. It is not acceptable to some patients to be told postoperatively that they need to wear a raised shoe on the previously normal opposite limb. As a result, unexpected lengthening of the operated leg is now one of the most common reasons for litigation against orthopaedic surgeons in the United States. A similar frequency of claims for leg inequality may be emerging in Australia. Over the past three years approximately 20% of MDA National Insurance’s orthopaedic incidents and claims related to total joint replacement surgery arose as a result of postoperative leg length inequality. However, the numbers are still too small to be able to measure the trend accurately.

What causes leg lengthening?

The cause of postoperative leg lengthening is multifactorial. The surgical task in hip arthroplasty calls for a balance between tissue tension and stability of the prosthesis in order to restore normal biomechanical function and achieve a good range of pain-free movement, while at the same time aiming for final leg length equality. These multiple goals are sometimes conflicting. Additionally, scar tissue contraction and abductor muscle action, factors beyond the surgeon’s control, may sometimes cause early postoperative lengthening, which resolves in the majority of patients.

If the prosthesis feels loose or dislocates easily in flexion and internal rotation when first inserted the surgeon needs to adjust for this by increasing the neck length of the modular prosthetic hip, thereby tightening the soft tissues and imparting stability, but almost certainly at the cost of lengthening the leg.

Intraoperative judgment of leg length is easier and clinically more accurate if the patient is supine, when length can be measured from the iliac spine to the level of the ankle malleoli or the knees. But this is not the most common operative approach. Most hip replacements are performed through a posterolateral or sometimes direct lateral approach, with the patient in the lateral position, when accurate tape measurement of leg length is not possible.

How do we manage the risk of lengthening and avoid postoperative complaints?

Consent

Fully informed preoperative patient consent is very important. Patient expectations of the operation should be anchored in reality. Because leg length equality can never be guaranteed after hip joint replacement it should be fully discussed as a potential complication. Those discussions should be carefully documented.

The patient’s preoperative leg lengths should be measured and recorded in the patient’s notes; common methods involve tape measurement from the anterior superior iliac spine to the medial ankle malleolus, or by progressively inserting blocks under the foot of the standing patient until the pelvis is level. In most preoperative patients with arthritic hips the legs still feel of equal length to the patient.

True preoperative shortening may occur with:

• congenital dislocation;
• severe acetabular dysplasia;
• as a result of Perthes disease;
• avascular necrosis or collapse of the femoral head;
• post-traumatic arthritis; and
• rarely as a result of a previous osteotomy;

Apparent shortening may be secondary to:

• idiopathic or degenerative scoliosis of the spine;
• fixed pelvic obliquity; or
• flexion and adduction contractures of the arthritic hip.

In such cases restoring leg length to normal will cause initial apparent lengthening but this should settle by 6 to 12 weeks postoperatively.

Paradoxically (and rarely) an arthritic hip may stiffen and cause preoperative apparent lengthening because of fixed hip abduction due to contracture of the capsule and the gluteal muscles.

Preoperative planning

This is essential in achieving length equality and entails:

• Measurement of leg length difference clinically, accepting that observer error may be up to 10mm,
• Estimation of the correction needed to achieve equality.
• Good quality radiographs – AP pelvis, including both hips and upper half of femora,
• Templating with overlays to gauge the predicted desirable level of the femoral neck osteotomy and offset, as well as
• Ensuring the components to be used have the modularity to restore as closely as possible normal anatomy.

However, digital images are now variable in magnification or reduction. Although templates (usually visualised radiologically with a 10% magnification factor) cannot be absolutely accurate at least they provide some guidance.

Fixed neck-shaft angle stems with limited neck lengths will not accurately fit every patient. The surgeon needs to be particularly careful of the patient with a long femoral neck and significant femoral offset. If the offset is not corrected, achieving stability will inevitably lead to lengthening, often to an unacceptable extent i.e. 15mm or greater.

Intraoperative management

• The patient should be carefully positioned and ideally held with a frame or supports to fix the position of the hips perpendicular to the table if the most common lateral/posterolateral approach is used.
• Intraoperative X-rays are sometimes recommended if the patient is supine. i.e. in an anterolateral approach, but are very difficult with the more common lateral approach.

It is strongly suggested that prior to dislocation and femoral neck transection a measuring device be used. Simply palpating the level of the patella or using the diathermy lead to estimate leg length is not sufficient. Similarly, palpating the tightness of the anterior capsule or feeling how much give or “shuck” occurs on traction with trial components is not accurate even in the hands of an experienced orthopaedic surgeon.

Computer navigation may eventually lead to more accurate cup and stem positioning but currently the most reliable gauging method is to use a pin in the pelvic brim, a second pin in the greater trochanter and a frame device as shown, which is tightened and set aside for estimation after the trial components are positioned. This provides the best possible estimate of leg length and offset.

Failure to document some reasonably accurate method of intraoperative leg measurement may make defence of a leg lengthening claim more difficult.

What is acceptable lengthening?

Even with use of an intraoperative leg length guide, lengthening of up to 10mm is common at 6 weeks postoperatively due to scar and abductor tightness and will usually resolve over 6 to 12 weeks and will not require any shoe modification. At 3 months, even if x-rays suggest offset and length are within 10mm of equality, if the patient feels the leg is functionally longer that perception is likely to be permanent.

A leg lengthening of greater than 15mm may cause tightness, possible pain, a limp and long-term low back pain, with a significantly poorer overall functional status.² An allegation that this outcome is due to negligence is difficult to defend if preoperative warnings were inadequate or not documented, if there is no documentation of any preoperative leg length inequality and if there is no record of intraoperative measurement to estimate as closely as possible leg length equality and stability of the hip prosthesis.

Summary

• Preoperative informed consent should include documented discussion of leg length inequality as a possible complication.
• Careful preoperative leg measurements should be recorded.
• It cannot be assumed that one type of joint component or prosthesis fits all patients.
• Preoperative planning is essential.
• Recognition that more modular prosthetic components, particular in regard to length and angle of femoral neck and therefore offset, maybe needed.
• Intraoperative measurement prior to dislocation of the hip joint should lead to greater accuracy in postoperative leg equality.

Postoperative patient dissatisfaction should be lessened by such means and claims for alleged negligent or inappropriate treatment in what is generally regarded as a highly successful operation may be minimised.

Dr J Stephen Quain
MB BS, FRACS, FRCS(Orth)
Consultant Orthopaedic Surgeon, 
St Vincent’s Hospital and St Vincent’s Clinic, Sydney 
Member, Eastern Cases Committee,  
MDA National Insurance

References

1. Konyves A, Bannister GC. The importance of leg length discrepancy after total hip arthroplasty. J Bone Joint Surg (Br) 2005;87(2) :155-7.
2. Wylde V et al. Prevalence and functional impact of patient-perceived leg length discrepancy after hip joint replacement. Int Orthop. Published online 25 April 2008.