Relationships between diffusion tensor parameters and measures of skeletal muscle fiber size and strength after anterior cruciate ligament injury.

Alterations in skeletal muscle morphology after an anterior cruciate ligament (ACL) tear are a major contributing factor to protracted quadriceps muscle weakness limiting person's return to function. Microstructural changes in skeletal muscle are difficult to assess noninvasively, but understanding these changes is vital to provide clinicians with additional information to assess injury and guide recovery. The purpose of this study was to evaluate the potential of magnetic resonance diffusion tensor imaging to provide noninvasive metrics of quadriceps muscle morphology and strength. Following a primary ACL tear and prior to surgical reconstruction, 44 individuals underwent bilateral isometric knee extension testing, vastus lateralis muscle biopsies, and diffusion tensor magnetic resonance of the thighs. Significant between limb differences were identified for quadriceps strength, fractional anisotropy, mean diffusivity, radial diffusivity, pooled fiber cross-sectional area, and pooled fiber minimum feret diameter. Fractional anisotropy and radial diffusivity were significantly associated with isometric knee extension peak torque for both limbs, after adjusting for mass. Fractional anisotropy was also significantly associated pooled cross-sectional area in the ACL-deficient limb only, after adjusting for mass. Our findings suggest that fractional anisotropy, mean diffusivity, and radial diffusivity can differentiate between the vastus lateralis of an ACL-injured limb and the healthy limb. Additionally, fractional anisotropy exhibited relationships with both measures of vastus lateral muscle fiber size and quadriceps strength for the ACL-deficient limb. These finding suggest that measures of diffusivity may be used cross-sectionally to infer between limb differences after an ACL-injury and that FA may be used to infer knee extensor strength.