Abstract
Purpose
The measurement of radial head translation about the capitellum (in percent): the radio-capitellum ratio (RCR) has proven to have excellent inter- and intra-observer reliabilities when measuring the RCR on a lateral radiological view of elbows at 90° of flexion and in the neutral position of the forearm. However, in the clinical setting, radiographs may be taken with the elbow in different positions. However, the purpose was to validate the RCR measurement method on elbows in different positions in flexion-extension and in different positions of the forearm in pronation-supination.
Method
Fifty-one healthy volunteers were recruited to evaluate the RCR in different elbow positions. Lateral elbow radiographs were taken with the elbow in different magnitude of ROM: maximal extension, maximal flexion, elbow at 90° and forearm in neutral, elbow at 90° and forearm in supination and elbow at 90° and forearm in pronation. The measurements of the RCR were done using the software SliceOmatic. ANOVA and paired T-test were used to assess the difference of the RCR depending on the position of the elbow and of the forearm. Pearson coefficients were calculated to obtain the correlation between the RCR in each different position.
Results
The mean RCR for each position were the following: elbow in maximal extension: −2%±7%, elbow in maximal flexion: −5%±9%, elbow at 90° and forearm in neutral: −2%±5%, elbow at 90° and forearm in supination: 1%±6% and elbow at 90° and forearm in pronation: 1%±5%. According to the Anova results, a significant difference exists between the RCR in different elbow positions (p=0.01) and in the different forearm positions (p<0.001). Paired T-test confirmed a significant difference between maximal elbow flexion and elbow flexion at 90° (p=0.003), as well as for maximal elbow extension and maximal elbow flexion (p=0.034). According to the Pearson coefficient, significant correlations exist between: elbow flexion at 90° and in maximal flexion (r=0.19, p=0.050); the forearm in neutral and in supination (r=0.34, p<0.001); the forearm in neutral and in pronation (r=0.42, p<0.001).
Conclusion
The RCR method is dependent on elbow (flexion-extension) and forearm (pronation-supination) positions. At both maximal elbow positions in flexion and extension, the measurements of the RCR have a higher standard deviation. In order to decrease its variability, we recommend as a convention measuring the RCR on lateral radiographs with the elbow at 90° and the forearm in neutral position. However, 95% of the values of RCR (except in maximal flexion which is unusual in trauma) are included in the normal range of RCR from −5% to 13%. Thus a value outside this range in any elbow positions (except maximal flexion) or any forearm positions must raise doubt on elbow alignment. Then, with a capitellum of 25 mm of diameter, the translation of the radial head must be less than 1 mm posterior and less than 3 mm anterior from the center of the capitellum.
