Human wrist motors: biomechanical design and application to tendon transfers.

Moment arm, muscle architecture, and tendon compliance in cadaveric human forearms were determined and used to model the wrist torque-joint angle relation (i.e. wrist torque profile). Instantaneous moment arms were calculated by differentiating tendon excursion with respect to joint rotation. Maximum isometric tension of each wrist muscle-tendon unit was predicted based on muscle physiological cross-sectional area. Muscle forces were subsequently adjusted for sarcomere length changes resulting from joint rotation and tendon strain. Torque profiles were then calculated for each prime wrist motor (i.e. muscle-tendon unit operating through the corresponding moment arm). Influences of moment arm, muscle force, and tendon compliance on the torque profile of each motor were quantified. Wrist extensor motor torque varied considerably throughout the range of motion. The contours of the extensor torque profiles were determined primarily by the moment arm-joint angle relations. In contrast, wrist flexor motors produced near-maximal torque over the entire range of motion. Flexor torque profiles were less influenced by moment arm and more dependent on muscle force variations with wrist rotation and with tendon strain. These data indicate that interactions between the joint, muscle, and tendon yield a unique torque profile for each wrist motor. This information has significant implications for biomechanical modeling and surgical tendon transfer.

Muscle, joint, and tendon contributions to the torque profile of frog hip joint.

The relative contributions of muscle force, moment arm, and tendon compliance were determined as a function of joint angle in the frog semitendinosus-hip joint system. Muscle, joint, and tendon properties were individually measured and then combined to predict the torque generated at the hip joint as a function of joint angle (i.e., the hip torque profile). Predicted torques were then compared to experimentally measured torques using a stepwise regression model to quantify the relative importance of muscle, joint, and tendon contributions to the hip torque profile. Variation in moment arm accounted for 74% of the variability observed in the hip torque profile, while addition of the muscle's intrinsic sarcomere length-tension property accounted for an additional 19% of the torque profile variability. Tendon compliance, which permitted a small amount of sarcomere shortening, accounted for only about 4% of the torque profile variability. We conclude that in this muscle-joint system, the relative fiber length-to-moment arm ratio is the major determinant of the shape of the isometric joint profile. The fiber length-to-moment arm ratio in other mammalian systems is also discussed.

Transcript-specific mRNA trafficking based on the distribution of coexpressed myosin isoforms.

mRNAs encoding four myosin heavy chain (MHC) isoforms were localized in rat skeletal muscle fibers by in situ hybridization. The ratio of MHC transcript signal in the fiber core compared to the fiber periphery was quantified using image analysis. Two distinct patterns of subcellular localization were observed. Type 1 (beta-cardiac) and type 2A MHC mRNAs were located preferentially in the muscle fiber periphery, while type 2B and type 2X mRNAs were distributed homogeneously across the fiber cross section. Since most normal muscle fibers express only a single MHC isoform, this difference in mRNA distribution could reflect either variation in the localization of the synthetic apparatus across different fiber types or differences in the trafficking of different MHC transcripts. To examine the basis for the observed differential distribution in normal muscles, mRNA distribution was assessed in muscle fibers that coexpressed multiple isoforms of the fast MHCs (i.e. types 2A, 2X and 2B), which occurred either in the combination type 2A/2X or type 2X/2B. The quantitative mRNA distribution seen in muscle fibers expressing a single isoform was not significantly different compared to that observed for mRNAs coexpressed in the same fiber (p > 0.6). Given the size similarity and homology of our riboprobes, these data suggest that their subcellular localization may be determined by relatively small differences in the sequences of the mRNAs, perhaps by differential binding of RNA sequence motifs to cytoskeletal elements.

Intramedullary Kirschner wire fixation of open or unstable forearm fractures in children.

This retrospective review evaluates the efficacy of standard intramedullary Kirschner wires (K-wires) for the treatment of open or unstable diaphyseal forearm fractures in 32 children with a mean follow-up of 13 months. Thirty-one patients had an excellent result, and one patient had a good result. Average time to bridging cortex was 3 months. Four patients lacked full pronation and supination, with none lacking >20 degrees, and no patients had evidence of growth-plate arrest. Nine complications occurred in eight patients: lost reduction after K-wire removal (three), refracture (two), deep infection (one), pin-site infection (one), transient anterior interosseous nerve palsy (one), and skin ulcer over buried K-wire (one). Both infections occurred in cases in which the K-wire ends were left outside the skin. Each case of lost reduction occurred in single-bone fixation cases when the K-wires were removed before 4 weeks. In children, intramedullary fixation by using standard K-wires plus cast immobilization provides effective treatment for the problematic open or unstable diaphyseal forearm fracture when closed management has failed. Refinement of the technique may help to avoid complications. We now recommend burying the K-wires under the skin for 3-5 months and stabilizing both the radius and ulna with an intramedullary K-wire.