Carpometacarpal Joint Dislocation Thumb Imaging

Clinical suspicion is imperative in making this diagnosis. Patients present with pain and swelling of the hand. A dorsal lump, or in the case of the less frequent volar dislocation, a volar lump or thickening of the palm, is present. However, this may be masked by marked generalized swelling of the hand. In the case of a volar dislocation, careful examination of the median nerve is important due to the possibility of an acute carpal tunnel syndrome. In addition, the function of the motor branch of the ulnar nerve should be examined due to its proximity to the fifth car-pometacarpal joint. Dislocation of the fifth carpometacarpal joint may result in an ulnar deviation of the little finger. Also there is often impairment of the grasping strength due to pain as well as a loss of the mechanical advantage due to loss of the longitudinal alignment of the hand.

Radiographs are the key to this diagnosis. In an emergency room setting, this diagnosis is often missed due to an incomplete radiographic evaluation consisting of only an anteroposterior (AP) film. With significant clinical suspicion, the injury can be appreciated on an AP film. In a normal hand, clean joint spaces between the bases of the metacarpals and the carpus with parallel joint surfaces can be appreciated. In the event of a dislocation, the parallel nature of the joint spaces is lost and overlap is present. The importance of parallel M lines on AP radiographs has been stressed. A zigzag line is drawn along the distal curvatures of the trapezoid, capitate, and hamate. A parallel line is drawn along the bases of the index through the little metacarpals (Fig. 55—3). A break in the parallel relationship between the two lines suggests a carpometacarpal dislocation. There also will be shortening of the meta-carpal. These are more subtle findings, and the key to the diagnosis is the true lateral radiograph. This film demonstrates the true position of the dislocation (Fig. 55-4). Computed tomography is an adjunct diagnostic modality that can be useful in elucidating associated fractures such as metacarpal base fractures. The clinical approach to this injury is often unchanged with this information, as these fractures often reduce along with the reduction of the carpometacarpal joint due to their many ligamentous attachments.

Base The 5th Cmc Dislocation

Figure 55—3. ParallelM lines.

The width between these two lines should be uniform and when they are not, this can be diagnostic of a CMC dislocation.

Figure 55—3. ParallelM lines.

The width between these two lines should be uniform and when they are not, this can be diagnostic of a CMC dislocation.

Cmc Dislocation Ray
Figure 55—4. (A) Special view using a plain radiograph with the handpositioned to show a dislocation of the fifth CMC joint. (B) Dislocated fifth CMC joint, visible on the corresponding x-ray.
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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