Entrapment of the index finger radial collateral ligament in the metacarpophalangeal joint: A case report.

Introduction: Digital collateral ligament injuries are common hand injuries that can cause significant pain and functional impairment. Ultrasonography can be useful in the evaluation of these ligamentous injuries, as it is both cost-effective and allows for easy, dynamic evaluation during imaging. Case report: We report a rare sonographic finding of an index finger radial collateral ligament injury that was found to have a flap of the ligament entrapped within the metacarpophalangeal joint, which to our knowledge has not been described previously. We correlate this finding with an intraoperative image of the entrapment of the collateral ligament. We also report on the novel application of superb microvascular imaging to aid in the diagnosis of digital collateral ligament injury. Discussion: This particular injury pattern has not been reported in the literature previously and likely explains the patient's lack of improvement with nonoperative management. Our finding is similar to a Stener lesion seen in a thumb ulnar collateral ligament injury in which the ligament is unable to heal due to entrapment. In addition, using superb microvascular imaging (SMI), we were able to visualize hyperemia to surrounding structures and the ligament itself which suggested an acute injury. Conclusion: We anticipate that this case report will provide sonographers with knowledge and images of this specific injury pattern to the digital collateral ligaments.