Role of the mechanic muscular injury in the development of heterotopic ossifications. A study on an animal model

Abstract

Optimising the first model developing heterotopic ossification (HO) on paraplegic mouse requires the study of cardiotoxin intramuscular injection substitution by a mechanical injury. Two lines of research were developed: pressure by extended crush and brief high-energy physical impact, with and without IM injection of LPS (purified bacterial lipopolysaccharides). The first kind of mechanical injury evaluated was the crush with a pressure manometer Jamar ® . The hamstring of the hind limbs of five groups of paraplegic mice underwent pressure forces from 2.5 to 10 kg with compression length going from 5 to 25 seconds. The second involved a brief high-energy physical impact on thigh muscles through an impactor, making possible a defined and reproducible soft tissue contusion. The hamstring of hind limbs of 2 subgroups of paraplegic mice underwent 2 different forces of muscular impact (energy for impact 1 = 0.19 J & for impact 2 = 0.32 J). In these two experiments, a spinal cord injury of T6 level was made, associated or not to LPS injection. The presence and the volume of the HO was measured by μCT after fixation and decalcification. There was no HO: (1) in the different groups after muscular crush and in the control groups with and without injection of LPS in the hamstring. (2) Neither in the brief high-energy impact without LPS groups. On the other hand, we found HO in the hamstrings of the brief high-energy impact with LPS injection group ( n = 7 mice) (mean volume: 0.355 mm 3 for impact 1 and 0.76 mm 3 for impact 2; mean density: 209.75 mg HA/cm 2 for impact 1 and 265.5 mg HA/cm 2 for impact 2). A brief high-energy impact could contribute to the damage mechanism favouring HO development. Extended muscular crush doesn’t seem to be involved. LPS role needs to be specifically study because it injection has to be associated with the brief high-energy impact.