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Novel Analysis Of Role Of Mechanical Versus Nerve Induced Immobilization On Destructive Ectopic Bone Formation
Shailesh Agarwal, MD1, Shawn J. Loder, BS1, Cameron Brownley, BS1, Mohammed Abdul Sattar Khan, PhD2, Mohammed Abdul Sattar Khan, PhD2, Jeeva Martyn, MD2, Shoshana Woo, MD1, Paul S. Cederna, MD1, Benjamin Levi, MD1.
1University of Michigan, Ann Arbor, MI, USA, 2Massachusetts General Hospital, Boston, MA, USA.

PURPOSE Retrospective studies report conflicting effects of immobilization on heterotopic ossification after large surface-area burns. We hypothesized that immobilization of the hindlimb in our mouse model would alter HO formation.
METHODS C57BL/6 mice underwent Achilles’ tenotomy with 30% TBSA burn followed by no immobilization, continuous mechanical immobilization at the ankle/knee using a custom splint, or nerve induced immobilization by hindlimb sciatic nerve transection. At 5 weeks post-injury, mice were analyzed by µCT and histology for HO. Mesenchymal stem cells (MSCs) from burned mice were cultured in osteogenic differentiation medium on plates with axial strain to simulate mobilization.
RESULTS Splint-immobilized mice developed less HO than non-immobilized mice (0.75 v. 1.61 mm^3) (Fig 1). While a majority of the HO formed in non-immobilized mice was at the calcaneus, immobilized mice did not form calcaneal HO (0 v. 0.97 mm^3) (Fig 1). Separately, immobilization secondary to sciatic nerve injury resulted in more HO formation (3.17 v. 2.24 mm^3) with robust HO formation at the calcaneus (Fig 1). MSCs exposed to axial strain had significantly increased osteogenic differentiation compared to control based on alizarin red stain.
CONCLUSIONS Here we show that mechanical immobilization of the hind limb decreases HO following burn/tenotomy injury. Nerve induced HO, however, developed more HO compared to control mimicking the fact that patients with extremity paralysis are at increased risk of HO formation.


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