Intrinsic and extrinsic wrist ligaments: biomechanical and functional differences.

The human wrist is a complex arrangement of bones and ligaments and functions to position the hand and transfer forces between the hand and forearm. The carpal ligaments (extrinsic - connect carpal bones and forearm; and intrinsic - interconnect carpals) are vitally important for wrist function and still incompletely understood biomechanically. Our previous investigations have described the rate-dependent viscoelastic behavior of the scapholunate (intrinsic) ligament. This study demonstrates significant differences in the biomechanical behavior between the intrinsic and extrinsic classes of ligaments. Bone-ligament-bone preparations were obtained by dissecting fresh and fresh-frozen cadaveric specimens. Two intrinsic (IL) [scapholunate (SL, 7) and lunotriquetral (LT, 9)] and two extrinsic (EL) [radiolunate (RL, 2) and radioscaphocapitate (RC, 3)] ligaments were extensively tested on a Monsanto Tensometer testing apparatus in axial loading at 0.5, 1, 5, 10, 50, and 100 mm/min. Significant differences in behavior were noted between the intrinsic and extrinsic ligaments in (1) maximum stress at ultimate yield (IL greater than EL), (2) maximum strain at ultimate yield (IL greater than EL), (3) strain level for onset of plasticity (IL greater than EL), and (4) rate and amount of stress relaxation (IL greater than EL). These results indicate that the intrinsic ligaments are biomechanically quite different than the extrinsic ligaments. These distinctions have important implications for wrist function and pathomechanics.