The "Empty Merchant Sign" in acute patellar tendon ruptures.

INTRODUCTION: We introduce a new sign on a Merchant view present in acute patellar tendon ruptures (APTR). We aim to determine the accuracy, sensitivity, and specificity of this new radiographic sign and measure the effect of a tutorial on these measures between trainees and non-trainees.

METHODS: Lateral and Merchant radiographs (22 images) of knees with four conditions (patellar instability, APTR, quadriceps tendon rupture, and controls) were randomly shown to 50 trainees and non-trainees who were asked to make a diagnosis based on radiographs. A brief tutorial was administered describing the "Empty Merchant Sign" and the same 22 images were randomly shown after the tutorial. Accuracy, sensitivity, specificity, and positive predictive value were calculated between the two image types and the effect of tutorial on these measures was assessed.

RESULTS: After the tutorial: 1. the "Empty Merchant Sign" had a higher specificity (100%) and positive predictive value (99%) compared to the lateral radiograph (81% and 64% respectively, P < 0.001), 2. There was significant improvement (from 56% to 95.3%; P < 0.001) in the accuracy of the Merchant view, making it as accurate as the lateral view (95.3% vs. 90.7%, respectively; P = 0.113). There was no difference in the accuracy of the Merchant view between trainees (97.2%) and non-trainees (90.5%) (P = 0.079).

CONCLUSION: The "Empty Merchant Sign" is a highly sensitive and specific diagnostic sign in cases of APTR. With very little training, physicians can identify this sign to diagnose APTRs on a Merchant view.