Evaluating a scientific paper often starts with an assessment of the manuscript’s technical construction and the project design; these elements often characterize the reader’s “first impression.” From this perspective, the article by McKeon et al. makes a good first impression. In this cadaver dissection study, stability of the thumb metacarpophalangeal joint is assessed after progressive release of the proper and accessory collateral ligaments. The authors identify radiographic parameters that correlate with metacarpophalangeal joint instability, noting in particular that radial translation of the proximal phalanx relative to the thumb metacarpal head correlates with disruption of both collateral ligament structures.
This paper is easy to read, and the technical details of the experimental environment and anatomic dissection are detailed and clear. The radiographic reproductions, which are especially critical to the presentation of the data, are well prepared and simple to understand. The authors wrote a good Discussion section in which they properly frame the practical considerations related to a reliable radiographic finding that correlates with complete ulnar collateral ligament disruption. Furthermore, this project is important because ulnar collateral ligament injuries of the thumb are very common and general orthopaedic surgeons as well as hand surgeons will continue to see these injuries at the time of first presentation.
Beyond a first impression, however, are other considerations that enhance this project’s value. An accurate and prompt evaluation of thumb ulnar collateral ligament injury is important because many of these injuries represent structural damage to the ligament complex that is best treated with surgery. Missing a completely unstable injury can lead to an instability arthritis pattern that might have been otherwise prevented. Traditional assessment of thumb ulnar collateral ligament injury has relied primarily on stress testing of the thumb in both extension and flexion and comparing the findings with those of a comparison examination of the uninjured side1. The presence of valgus laxity of the thumb with the metacarpophalangeal joint positioned in flexion alone correlates with damage to the proper collateral ligament2. Valgus laxity that is present with the thumb in both extension and flexion correlates with loss of integrity of both the accessory and the proper collateral ligament and more likely is associated with proximal entrapment of the ligament behind the adductor aponeurosis as described by Stener3.
However, as anyone who has examined patients with acute thumb metacarpophalangeal injuries will attest, it is often not easy to perform precise physical examination of these injuries. The patients frequently have substantial swelling and pain, and unless they have a large tender mass just proximal to the ulnar joint line (which is typical of a Stener lesion), it can be quite difficult to diagnosis thumb instability with any reliability. What is even more difficult and awkward on examination is assessing differences in stability based on thumb metacarpophalangeal flexion and extension positioning. Also, slight changes in the rotational alignment of the thumb metacarpal can mimic valgus laxity, further complicating the interpretation of physical findings; patients will commonly try to rotate their hand slightly in an effort to make the physical challenge of stress testing less painful.
The difficulty of quantifying the results of the physical examination is one reason why magnetic resonance imaging (MRI) of this injury has become popular, although MRI is not a perfect modality. Because findings are influenced by magnet strength and the use (or lack of use) of a surface coil, MRI is often inconclusive. Furthermore, radiologists will offer an interpretation supporting ligament damage predicated on the lack of visualization of a normal ligament, but damage and lack of visualization are actually two very different things. Not being able to see something is not the same as seeing something that is abnormal. Additionally, MRI can be so expensive that its cost may actually exceed the cost of the care, whether it be operative or nonoperative.
For these reasons, this paper has extra value. The authors described a simple radiographic finding that correlates with disruption of both the proper and the accessory collateral ligament that would make additional expensive imaging unnecessary. Furthermore, the method by which radial translation of the proximal phalanx is elicited is quite simple; it can be done with a simple anteroposterior stress radiograph and the authors point out that use of a size-32 rubber band produces enough force to elicit the pathognomonic radial translation. Even if a new rubber band is used for each patient, in order to avoid the variability introduced by stretching out of material from repeated use, the rubber-band method is quite economical. A bag of 900 size-32 rubber bands costs less than eight dollars. Reliably identifying critical ligament instability in the thumb with a device that costs less than one penny sounds like a good deal.