Magnetic resonance imaging has revolutionized the diagnosis of shoulder lesions, in many cases becoming the method of choice. However, anatomical variations, artifacts and the particularity of the method may be a source of pitfalls, especially for less experienced radiologists. In order to avoid false-positive and false-negative results, the authors carried out a compilation of imaging findings that may simulate injury. It is the authors' intention to provide a useful, consistent and comprehensive reference for both beginner residents and skilled radiologists who work with musculoskeletal magnetic resonance imaging, allowing for them to develop more precise reports and helping them to avoid making mistakes.

Keywords: Shoulder; Magnetic resonance imaging; Musculoskeletal; Artifacts; Pitfalls.

OBJECTIVE: To evaluate qualitative and quantitative magnetic resonance imaging (MRI) criteria for injury of the common peroneal nerve (CPN) in patients with acute or subacute injuries in the posterolateral corner (PLC) of the knee, as well as to evaluate the reproducibility of MRI evaluation of CPN alterations. MATERIALS AND METHODS: This was a retrospective study of 38 consecutive patients submitted to MRI and diagnosed with acute or subacute injury to the PLC of the knee (patient group) and 38 patients with normal MRI results (control group).Two musculoskeletal radiologists (designated radiologist A and radiologist B, respectively) evaluated the images.Nerve injury was classified as neurapraxia, axonotmesis, or neurotmesis.Signal strength was measured at the CPN, the tibial nerve (TN), and a superficial vein (SV).The CPN/TN and CPN/SV signal ratios were calculated.The status of each PLC structure, including the popliteal tendon, arcuate ligament, lateral collateral ligament, and biceps tendon, was classified as normal, partially torn, or completely torn, as was that of the cruciate ligaments.For the semiquantitative analysis of interobserver agreement, the kappa statistic was calculated, whereas a receiver operating characteristic (ROC) curve was used for the quantitative analysis. RESULTS: In the patient group, radiologist A found CPN abnormalities in 15 cases (39.4%)—neurapraxia in eight and axonotmesis in seven—whereas radiologist B found CPN abnormalities in 14 (36.8%)—neurapraxia in nine and axonotmesis in five.The kappa statistic showed excellent interobserver agreement.In the control group, the CPN/TN signal ratio ranged from 0.63 to 1.1 and the CPN/SV signal ratio ranged from 0.16 to 0.41, compared with 1.30–4.02 and 0.27–1.08, respectively, in the patient group.The ROC curve analysis demonstrated that the CPN/TN signal ratio at a cutoff value of 1.39 had high (93.3%) specificity for the identification of nerve damage, compared with 81.3% for the CPN/SV signal ratio at a cutoff value of 0.41. CONCLUSION: CPN alterations are common in patients with PLC injury detected on MRI, and the level of interobserver agreement for such alterations was excellent.Calculating the CPN/TN and CPN/SV signal ratios may increase diagnostic confidence.We recommend systematic analysis of the CPN in cases of PLC injury.

Keywords: Peroneal nerve/diagnostic imaging; Knee injuries/diagnostic imaging; Knee/innervation; Magnetic resonance imaging.