Abstract
Restoration of equal leg lengths is essential when performing total hip arthroplasty. Pre-operatively, surgeons should assess leg length discrepancy (LLD) on plain radiographs to help adequately plan their operation. Often the hips are in a minor windswept position on radiographs and this can make assessing LLD unreliable. We have devised a simple method for assessing LLD on plain radiographs in patients with windswept hips.
Keywords: Leg length discrepancy, Windswept hips, Total hip arthroplasty
Pre-operative planning is critical to performing a successful total hip arthroplasty (THA). It is important to measure leg length discrepancy (LLD) both clinically and radiologically. Restoration of LLD is one of the key goals when performing THA. Unequal leg lengths post-operatively can lead to gait disturbance, instability, back pain, sciatic nerve dysfunction and is a leading cause of litigation following THA.1,2
There are multiple techniques described for assessing LLD on plain radiographs of the hips.1,3 However, in arthritic hips, adduction or abduction contractures commonly cause a compensatory pelvic obliquity which leads to a windswept appearance on radiographs. A windswept pelvis poses a challenge when calculating LLD, as demonstrated in Fig. 1.
Fig. 1.
Measuring LLD using the lesser trochanter as a reference indicates LLD = 1 mm. Measuring LLD using the greater trochanter as a reference indicates LLD = 12 mm.
A horizontal line is drawn intersecting the inferior aspect of the tear drops. Perpendicular lines are then drawn to the proximal aspect of the lesser trochanters, showing a LLD of 1 mm. However, if perpendicular lines are drawn to the tips of the greater trochanters this shows a 12 mm LLD. This discrepancy occurs because the femoral landmarks used have different angular relationships to the pelvis on each side. We therefore devised a simple way to calculate LLD in patients with windswept hips on plain radiographs (Fig. 2).
Fig. 2.
Step 1. Identify the centre of rotation of each hip. Step 2. Draw a large circle, with the same centre point, that intersects the proximal aspect of the lesser trochanter. Step 3. Draw a horizontal line through the inferior aspect of the tear drops with two perpendicular lines to the most distal aspect of the larger circles. The difference between the lengths of these perpendicular lines is the LLD.
Firstly, identify the centre of rotation of each hip. It is important to emphasise the use of the current centre of rotation rather than attempt to identify any pre-arthritic centre of the femoral head. Using the centre of rotation as a reference, draw a larger circle, with the same centre point, that intersects the proximal aspect of each lesser trochanter. Then draw a horizontal line through the inferior aspect of the tear drops (or other standard horizontal reference line) with two perpendicular lines to the most distal aspect of the larger circles. The difference between the lengths of these perpendicular lines is the LLD.
Limitations to this technique are that it is a 2D representation of a 3D problem and may not be adequate in patients with severe windswept deformity which can be combined with significant rotational deformity or flexion contractures of the hip. In such instances the only reliable imaging modality is CT scanning. Also of note is that other factors aside from the hips can contribute to LLD. Nevertheless, in many mild cases of windswept deformity, this technique is quick, easy and negates the use of repeat radiographs, long leg views and CT scanograms; which are often not practical in routine clinical practice.
Declaration of competing interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Tim Board reports consultancy payments and patents from DePuy Synthes, grants from NIHR and Symbios, speaker payments from DePuy Synthes, Corin, and Symbios, and travel/accommodation/meeting expenses from DePuy Synthes, Corin, Symbios, and MatOrtho, and unpaid shares in Eventum Orthopaedics, all unrelated to the study. Tim Board is Vice President of the British Hip Society. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
References
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