X-rays Taken With External Fixator on Leg May Be Unreliable
A significant difference in leg length from one leg to the other may require surgical correction. A special device called a circular external fixator is often used to maintain a correct position of the leg as it heals.
Standing full-length X-rays are used to guide the surgeon in recreating normal alignment of the lower extremity. Correcting the deformity and equalizing the leg length are important outcomes of this procedure.
In this study, the accuracy of X-ray assessment of limb alignment is reviewed. All patients included had surgery to realign the leg. Most subjects were children but a few adults were also included.
Everyone had an external fixator applied during the operation. X-rays were taken during the early phase of bone healing with the fixator in place and again after the fixator was removed.
Alignment of the bones, angles, and rotation of the knee were measured and analyzed using special computer technology. The data generated was used to evaluate the effect of the external fixator on the leg length measurements. In fact, the effect of the fixator on both legs was evaluated. Measurements obtained for alignment, rotation, and leg length were compared to values known to be normal.
The authors found that standing full-length X-rays may not be a reliable way to assess limb length. Not only does the fixator affect the leg it's on, it also changes the alignment of the other leg.
Using the X-rays, surgeons may overestimate the malalignment and discrepancy between the two legs. The result may be undercorrection or overcorrection of limb alignment. The authors describe the various positions of the knee and how these might change the accuracy of measurements taken radiographically.
The most common error was a position of external rotation of the knee in the fixator device when the X-ray was taken. This led to an overestimation of varus (bow-legged) alignment. The difference in angle measurements also increased from before to after the device was removed. The more external rotation occurred in the device, the more the varus angle (and malalignment) increased.
It appears that there is a tendency for a leg in an external fixator to externally rotate at the knee. X-ray measurements taken before and after the fixator is put on and taken off may be affected by this malalignment of the leg. This can be a problem for the surgeon who is depending on the X-rays to guide treatment.
Surgeons using external fixators to correct leg length discrepancy or reconstruct the limb after trauma should be aware of this problem. Full-length standing radiographs to assess (and correct) limb alignment and limb length may be inaccurate. Clinical results may be negatively affected.
Sanjeev Sabharwal, MD, et al. The Effect of Circular External Fixation on Limb Alignment. In Journal of Pediatric Orthopaedics. May 2008. vol. 28. No. 3. Pp. 314-319.