Hindlimb Suspension and Subsequent Weight-Loading on Femoral Morphology, Knee Joint Alignment, and Patellar Tracking in Growing Rats

IntroductionMechanical stress is considered essential during bone formation.Decreased mechanical stress, such as that experienced during weight unloading, bed rest, or space flight, results in bone loss and increased bone fragility 1,2 .Hindlimb suspension (HS), constituting an experimental reduction of mechanical stress, causes reductions in bone mass 3 , mineral density 4 , and strength properties 5 and alterations of bone microstructure 6 .However, few studies have examined how decreased mechanical stress affects bone morphologies and joint alignment.Abnormalities in femoral morphology and knee joint alignment are associated with various musculoskeletal disorders of both the knee and hip, including patellofemoral instability, patellar dislocation, and osteoarthritis (OA) 7,8 .Alterations in bone morphology and alignment can induce the generation and accumulation of abnormal mechanical stress at joints, and consequently contribute to the development of musculoskeletal disorders.Indeed, abnormal femoral morphology is associated with abnormal locomotion patterns 9 and in concomitant with patellofemoral joint pathology in rat models 10 .Accordingly, preventing abnormal femoral morphology and knee joint alignment may have the potential to avert the onset of musculoskeletal diseases.Several experimental models have demonstrated the ability to alter femoral morphologies and knee alignment in growing animals.For example, surgically induced patellar