LIMB DEFORMITY CORRECTION IN THE 21ST CENTURY. IMAGING THE LOWER LIMB.

Abstract

Pre-operative planning for limb deformity correction involves detailed imaging of the lower limb to define the level, magnitude and direction of deformity. This is then used to plan the correction by defining the centre of rotational alignment (CORA). The method as described by Paley and Hertzenberg involves the use of orthogonal radiographs of the lower limbs using long cassettes (130 cm) taken from a distance of 305 cm to minimize magnification. This method requires special equipment, trained radiographers and multiple doses of radiation even when each radiograph was perfectly positioned first time every time.

We present a work in progress replacing the radiographs with a “virtual 3D” CT dataset of the lower limb which we hope will improve the ability to pre-operatively plan deformity correction, but at a lower cost in terms of skill, equipment and dose. Whole limb CT is too costly in terms of time and radiation dose for this to be suitable. New multislice CT systems allow a single coherent study to include segments of unscanned data. Thus it is possible to run a single series through a lower limb to include the articular surfaces, but excluding the diaphyseal segments (gaps). This reduces the radiation exposure to the patient. Such data when entered into suitable DICOM image manipulation software allows the Radiologist or Surgeon to measure and assess the deformity with great precision. Such software is available on the diagnostic radiology workstations but is also available for personal computers, allowing the Surgeon to perform preoperative planning in a numerical modeling setting. Allowing the elements of length, rotation, translation and angulation of the deformity to be measured and corrective surgery tested on the mathematical model.

We have compared the measurements taken from a deformity model using this new CT approach and compared it to standard radiographs and found that the above method is no less accurate. Rotational deformities are easier to estimate. However the advantage of our method is that the dataset can be manipulated to determine other technical aspects of deformity correction such as calculating the mounting parameters of the Talyor Spatial Frame.

We present worked examples of the methodology showing how this technique improves deformity appraisal.