1. The thumb's MP and CMC joints abduct and adduct in a plane perpendicular to the palm. Some therapists also refer to abduction as "palmar abduction."
2. The thumb's MP and CMC joints flex and extend in a plane parallel to the palm. Some therapists refer to extension as "radial abduction," because the thumb moves toward the hand's radial side.
Why do we name CMC movements in this manner?
The CMC joint is biaxial, not triaxial. However, its loose capsule permits rotation, and the metacarpal rotates automatically when it moves in the other two planes. Specifically:
The diagram (Norkin & Levangier, 1992, p. 287) depicts the plane for CMC abduction/adduction. Compare it with the figure in your text (Hertling & Kessler, 1996, Fig. 11-7).
The saddle-shaped trapezium is:
However, because the thumb attaches to the hand at a different angle than do the other digits, the thumb's AP and lateral axes are oriented differently than in the other digits.
The thumb or first ray, which comprises the metacarpal, proximal phalanx, and distal phalanx, attaches to the trapezium and the rest of the hand at the first carpo-metacarpal (CMC) joint. This attachment is at nearly a right angle to those of the second, third, fourth, and fifth digits.
To appreciate this, examine the orientation of the fingernails on your relaxed hand as you point your fingertips toward yourself. The thumbnail is oriented at a right angle to the nails of the other fingers.
The familiar lateral axes, around which the fingers flex and extend, are turned 90 degrees at the thumb's CMC and MP joints. These joints flex and extend in a plane that is parallel to the palm.
The familiar AP axes, around which the MP joints abduct and adduct, are also turned 90 degrees at the thumb's CMC and MP joints. These joints abduct and adduct in a plane that is perpendicular to the palm.
Norkin, C.C., & Levangie, P.K. (1992). Joint structure and function (2nd ed.). Philadelphia: F.A. Davis.