[Translated article] It's possible to know tibial torsion in a X ray of the limbs?

INTRODUCTION Rotational deformities are a common reason for consultation and are usually assessed clinically using the Staheli Rotational Profile and/or torsional CT. The authors present a new method, using a simple AP X-ray of the lower limbs to measure tibial torsion, comparing this method with two other common methods: the thigh-foot angle and CT.

MATERIAL AND METHOD The radiological measurement is based on the constant relationship between the epiphyseal and metaphyseal centers of the tibia and fibula, independent of the degree of rotation. If we express this relationship as the sum of the radii of the tibial and fibular epiphyses, we can see that as tibial torsion increases, the distance between the epiphyseal centers decreases when we observe the ankle in an AP projection. Expressing this as the trigonometric function Cosine of Alpha=Adjacent Leg/Hypotenuse, we obtain values between 0 and 1 and applying the inverse function to the Cosine of Alpha, Arc Cosine of Alpha, we can determine how many degrees of rotation α value <1 corresponds to. This measurement is performed on an AP TeleRx of the MMII with the patellas in front and is compared with the thigh-foot angle and the condyle-malleolar angles measured tibio-malleolar by CT.

RESULTS The medical records of 70 patients (39 women/31 men) were reviewed, comprising 140 limbs. Clinical measurements, CT scans, and Arc Cosine Alpha measurements were recorded on plain radiographs of the lower limbs (MMII). The mean patient age was 12.5 years (r: 10-16). The mean thigh-foot angle was 23° (r: -15°-55°). According to the CT scan, the mean condyle-malleolar angle measured by the radiologist was 44.3° (r: 13°-72°), and that measured by the surgeon was 38.6° (r: 11°-66°). The mean tibial torsion on the lower limbs radiograph according to Arc Cosine Alpha was 40.4° (r: 0°-68°).The interobserver variability between the CT measurements of the radiologist and surgeon for the condylo-malleolar angle was 6° (ICC 0.8). The variation between the CT measurements and the radiological measurement was 4° (ICC 0.65). Comparisons of the clinical measurement (thigh-foot angle) with the CT and radiological measurements were inaccurate and highly variable.

CONCLUSIONS A simple X-ray of the limbs allows tibial torsion measurement. The results are very similar to the values condylar-maleolar angle obtained by CT. It is a way to minimize exposure to ionizing radiations, mainly in children. It is a quick tool to evaluate external rotation in pre or post-surgical treatment.