A novel mouse model of polytrauma with spinal cord injury–associated heterotopic ossification

Abstract

Heterotopic ossification (HO) refers to abnormal bone formation in soft tissues, which is a common complication of traumatic spinal cord injury (SCI). Current therapeutic approaches are non-specific, have limited efficacy and pose serious off-target effects due to insufficient knowledge of HO mechanisms. In the absence of clinically relevant models of traumatic SCI-associated HO, a novel mouse model of polytrauma with concomitant SCI and musculotendinous injury (MTI) was developed. SCI was induced by T9-T10 spinal cord transection, and muscle crush/tenotomy was performed on left quadricepses (SCI+MTI), leaving right limbs intact as internal controls. Age and sex-matched control mice underwent left hindlimb MTI alone or SCI with left quadriceps crush only (SCI+MI). High-resolution micro-computed tomography revealed variable amounts of ectopic mineral deposits only in injured hindlimbs of SCI+MI and SCI+MTI mice. Median ectopic mineral volumes in SCI+MTI mice were higher than SCI+MI mice at 1 and 4 weeks postoperative. Histology of serial sections confirmed von Kossa-positive mineral colocalizing with ALP-positive osteoblast activity and TRAP-positive osteoclast activity, with a significant increase in ALP between both timepoints. Immunofluorescence indicated F4/80-positive macrophages infiltrating ectopic mineral in SCI+MTI mice at 1 week postoperative, and Osterix-positive osteoblasts lining trabecular bone in HO at 4 weeks postoperative. Orthotopic bone loss manifested as cortical thinning and trabecular bone loss in femurs of SCI mice at 4 weeks postoperative. This study thus identifies SCI+MTI mice as a clinically relevant small animal model of polytrauma with SCI in which to study pathogenic mechanisms and assess novel therapeutic approaches to SCI-associated HO.

Keywords

• polytrauma
• spinal cord injury
• musculotendinous injury
• ectopic mineral
• heterotopic bone
• osteopenia