Estimation of tendon moment arms from three-dimensional magnetic resonanceimages

New three-dimensional (3D) magnetic resonance imaging (MRI) methods for mea suring the tendon moment arm were created and were evaluated on the tendon moment arm of the flexor digitorum profundus at the third metacarpophalange al joint. Using an open magnet MRI system and a hand holder, a series of st atic images were acquired at four joint angles and analyzed using specially created computer programs. Three methods were evaluated: (1) a 3D tendon e xcursion method that extended the method of Landsmeer; (2) a 3D geometric m ethod whereby the moment arm was the perpendicular distance between the joi nt axis of rotation and the tendon path, and (3) a two-dimensional (2D) geo metric method whereby single image slices were analyzed. Repeating the imag ing and measurement processes, the 3D tendon excursion method was more repr oducible (6% variation) than the 3D geometric method (12%), and both were m uch more reproducible than the 2D geometric method (27%). By having three o perators analyze a single set of image data, we found that the precision of the 3D tendon excursion method was much less affected by segmentation erro r than the 3D geometric method. With the 3D imaging methods, tendon bowstri nging and a displacement of the joint center of rotation toward the dorsal side of the hand were evident, leading to as much as a 60% increase in mome nt arm with joint flexion. Because of the dependence on flexion and variati on between subjects, we recommend patient-specific measurements for target applications in functional neuromuscular stimulation interventions and tend on transfer surgery. (C) 1999 Biomedical Engineering Society. [S0090-6964(9 9)01402-2].