A new method of diagnosing metacarpophalangeal instabilities of the thumb

With a plain anteroposterior stress radiograph of the metacarpophalangeal joint of the thumb and observation of the displacement of the sesamoid bones, it is possible to interpret the injury mechanism and accurately diagnose the anatomical site of the ligament lesion. Fresh anatomical specimens were used to systematically investigate and document the role played in joint stability by each of the anatomical elements. Each ligament was sectioned and instability documented. A separate group (8) was used to replicate the different mechanisms responsible for ligament lesions. A radiological study was performed on 20 patients who had no history of trauma in their metacarpophalangeal joint and on another group of 17 patients with disease. Clinical instability was radiologically studied and showed a pattern of displacement of the sesamoid bones with respect to the metacarpal head, closely related to the different types of ligament lesions. When trauma occurs at the flexed metacarpophalangeal joint, the metacarpophalangeal ligament alone is torn and parallelism between the sesamoid bones and the metacarpal is not lost. In contrast, when trauma occurs with the joint extended, both the metacarpophalangeal and sesamoid metacarpal ligaments tear and the normal anatomical relationship between the sesamoid bones and the metacarpal head is lost. Therefore, if the radiograph shows no parallelism between the sesamoid bones and the metacarpal head, the injury is total and complete and requires surgical reparation. This highly reliable method can be implemented in any emergency department at low cost. It allows the extent and seriousness of the ligament lesion to be determined in an emergency.