The unique anatomy of the thumb carpometacarpal joint allows it to be a stable and yet incredibly versatile joint which according to many maybe what sets us apart from the rest of the primate family.
Osseous: The trapezium is has a saddle shaped articular surface (hence a “saddle” joint) which is concave when seen laterally and convex when seen anteroposteriorly. The metacarpal base has the converse anatomy. In addition, there is also a volar beak at the metacarpal base which locks into a small defect in the trapezium to give the joint more stability in opposition.[1, 2]
Capsule and ligaments: There is a thin capsule that surrounds the carpometacarpal joint, which is thickened in a few areas, giving rise to the stabilizing ligaments. There are four ligaments that one needs to keep in mind. Controversy still exists in regards to which ligament is the primary stabilizer.
1) Anterior oblique ligament runs from the palmar surface of the metacarpal base (ulnarly) to the palmar aspect of the trapezium (radially). It was previously thought to be the primary stabilizer of the thumb. 
2) Posterior oblique ligament runs from the dorsal ulnar tubercle of the trapezium to the ulnar tubercle of the thumb metacarpal base. A little later, it was debated that this along with the intermetacarpal ligament were the primary stabilizers. [4, 5] This can be difficult to see on MRI.
3) The dorsal radial ligament that connects the two bones dorsally at the radial aspect (reinforced by the abductor pollicis longus tendon). This is currently felt to be the primary stabilizer to dorsal dislocation, (the most common kind) as in a cadaveric study it was found that there was least instability when all other ligaments were cut but this one was intact and most instability when this one alone was cut. 
4) The intermetacarpal ligament which connects the radial aspect of the second metacarpal to the ulnar aspect of the thumb metacarpal base.
5) Capsular anatomy: The capsule the weakest dorsally and radiovolarly, except along the dorsal radial ligament. It is strongest volarly, where it is reinforced by the anterior oblique ligament at it’s ulnar aspect and reinforced by the abductor pollicis longus tendon.
Biomechanics of injury: Thumb carpometacarpal injuries account for less than 1% of hand injuries and are usually the result of axial loading in flexion. [7-9] As a result of this load there is dorsal dislocation because of the relative weakness of the dorsal structures and often an associated avulsion injury at the volar attachment of the ligament/capsule at the metacarpal base (Bennett fracture).
MRI technique: Rather than position the patient for a dedicated thumb MRI, the position for a wrist MRI seems to give more information. A small surface coil is probably best and once the scout is obtained, coronal and axial images centered at the first carpometacarpal joint are obtained. Sagittal images are not really helpful most of the time. In my training, we preferred high resolution 3 plane PD without fat suppression, a GRE sequence and a STIR sequence. Connell et al recommend the following: 
1) Coronal FSE with TR/TE 4000/45, matrix 512 x 320, NEX 2, FOV 8 cm and 2.5 cm thick sections with no skip (I would go down to 1.5 but there is some effect on SNR), ETL of 8 and bandwidth 20-30
2) Axial FSE with similar parameters
3) Coronal MPGR TR/TE 460/20, flip 45, matrix 256 x 256, NEX 3, FOV 8 and section thickness of 3 skip zero
4) Coronal STIR with TR/TE/TI 3500/45/120, matrix 256 x 224, NEX 4, FOV 10 and 3mm section thickness with no gap.
Key points in interpretation:
- Anatomic alignment can be difficult to assess on MRI and may vary with position so be careful in calling subluxations and dislocations without appropriate clinical history or significant ancillary findings of soft tissue injury or appropriate marrow edema pattern. (see the coronal images in Figure 1. They are of the same patient obtained at different times. In figure 1A there does not seem to be any subluxation while in figure 1B, there is apparent mild subluxation. Clinically, there was no instability.)
- Use the STIR image to identify any bone marrow edema pattern which may relate to an acute injury or subchondral reactive change from osteoarthritis (which is also common at this location)
- Assess the articular cartilage, which could be worn diffusely as in osteoarthritis or more focally from an acute injury.
- Assess the four ligaments (see images) and the associated abductor pollicis longus tendon. Grade the injury based on the percentage of fibers disrupted. Occasionally, the capsule and ligaments may appear stretched.
Two major abnormalities are associated with the first carpometacarpal joint. The first and more common is osteoarthritis where, in advanced cases, arthrodesis maybe performed.
The less common one is dislocation as described earlier. A recently proposed management plan outlined by Bosmans et al is as follows :
- Closed reduction with stable clinical appearance: Plaster cast and manage conservatively
- Closed reduction with unstable appearance: K-wire fixation, plaster cast and close radiographic/CT followup
- Closed reduction with unstable appearance and not responding well to wire fixation, consider capsullorraphy/ligament reconstruction
Open reduction, Kwire fixation, capsulorrhaphy, and/or ligament reconstruction with a plaster cast are mandatory.
1. Pieron, A.P., The mechanism of the first carpometacarpal (CMC) joint. An anatomical and mechanical analysis. Acta Orthop Scand Suppl, 1973. 148: p. 1-104.
2. Napier, J.R., The form and function of the carpo-metacarpal joint of the thumb. J Anat, 1955. 89(3): p. 362-9.
3. Eaton, R.G. and J.W. Littler, Ligament reconstruction for the painful thumb carpometacarpal joint. J Bone Joint Surg Am, 1973. 55(8): p. 1655-66.
4. Pagalidis, T., K. Kuczynski, and D.W. Lamb, Ligamentous stability of the base of the thumb. Hand, 1981. 13(1): p. 29-36.
5. Harvey, F.J. and W.D. Bye, Bennett’s fracture. Hand, 1976. 8(1): p. 48-53.
6. Strauch, R.J., M.J. Behrman, and M.P. Rosenwasser, Acute dislocation of the carpometacarpal joint of the thumb: an anatomic and cadaver study. J Hand Surg [Am], 1994. 19(1): p. 93-8.
7. Mueller, J.J., Carpometacarpal dislocations: report of five cases and review of the literature. J Hand Surg [Am], 1986. 11(2): p. 184-8.
8. Shah, J. and M. Patel, Dislocation of the carpometacarpal joint of the thumb. A report of four cases. Clin Orthop Relat Res, 1983(175): p. 166-9.
9. Chen, V.T., Dislocation of the carpometacarpal joint of the thumb. J Hand Surg [Br], 1987. 12(2): p. 246-51.
10. Connell, D.A., et al., MR imaging of thumb carpometacarpal joint ligament injuries. J Hand Surg [Br], 2004. 29(1): p. 46-54.
11. Bosmans, B., M.H. Verhofstad, and T. Gosens, Traumatic thumb carpometacarpal joint dislocations. J Hand Surg [Am], 2008. 33(3): p. 438-41.
Filed under: Radiology