Direct Mouse Trauma/Burn Model of Heterotopic Ossification

Summary

An Achilles tenotomy and burn injury model of heterotopic ossification allows for the reliable study of trauma induced ectopic bone formation without the application of exogenous factors.

Abstract

Heterotopic ossification (HO) is the formation of bone outside of the skeleton which forms following major trauma, burn injuries, and orthopaedic surgical procedures. The majority of animal models used to study HO rely on the application of exogenous substances, such as bone morphogenetic protein (BMP), exogenous cell constructs, or genetic mutations in BMP signaling. While these models are useful they do not accurately reproduce the inflammatory states that cause the majority of cases of HO. Here we describe a burn/tenotomy model in mice that reliably produces focused HO. This protocol involves creating a 30% total body surface area partial thickness contact burn on the dorsal skin as well as division of the Achilles tendon at its midpoint. Relying solely on traumatic injury to induce HO at a predictable location allows for time-course study of endochondral heterotopic bone formation from intrinsic physiologic processes and environment only. This method could prove instrumental in understanding the inflammatory and osteogenic pathways involved in trauma-induced HO. Furthermore, because HO develops in a predictable location and time-course in this model, it allows for research to improve early imaging strategies and treatment modalities to prevent HO formation.

Introduction

Heterotopic ossification (HO) is the formation of ectopic bone in which osteo-potent cells are aberrantly induced to form endochondral bone outside of the skeleton. While the details of the HO formation pathway are still largely unknown, an accepted paradigm includes three key factors: an inflammatory inciting incident, a permissive niche, and mesenchymal stem cells capable of forming bone.^1-3 HO is a common comorbidity complicating over 60% of major burn injuries, 65% of combat-related injuries, and 10% of invasive orthopaedic surgery cases.^4,5 However, it is often difficult to predict where HO will form because it can occur at sites of local....

Protocol

Ethics statement: All animal procedures were carried out in strict accordance with good animal practice as defined in the guidelines provided in the Guide for the Use and Care of Laboratory Animals: Eighth Edition from the Institute for Laboratory Animal Research (ILAR, 2011) and were approved by the Institutional Animal Care and Use Committee of the University of Michigan (PRO0001553).

1. Mouse Surgical Procedures

NOTE: Use 8-10 week old C57BL/6 mice. Other ages, backgrounds, and strains of mice can also be used to test different conditions or genetic make-up. For the tenotomy portion of the procedure, maintain st....

Results

In this present study, the protocol is described for the previously published mouse model of trauma/burn induced HO.^14-17 This involves the creation of concurrent localized musculoskeletal injury with an Achilles tenotomy and global inflammatory insult with a partial thickness burn injury. This results in the reliable formation of reactive bone at the tenotomy site which can be followed with serial imaging. To date, all mice (more than 50) that have sustained both Achilles tenotomy and burn injury have develop.......

Discussion

Heterotopic ossification represents a major functional impairment faced by patients that sustain trauma, burns, and invasive musculoskeletal procedures. The most at-risk population are soldiers in modern conflicts with major blast injuries from mechanisms such as improvised explosive devices (IED).¹⁸ Improved body armor and forward positioned medical units allows for improved survival of major extremity injury. After initial stabilization and repair of their extremity injury, these patients are at high risk.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
C57BL/6 mice	Jackson Laboratory	664	8-10 weeks old
Isoflurane – Fluriso	VET one, Boise, ID	V1 501017
Buprenorphine – Buprenex	Reckitt Benckiser Healthcare	NDC 12496-0757-1	0.3 mg/ml solution
Betadine	Owens and Minor, Mechanicsville, VA	2047PVP202
5-0 Vicryl sutures	Ethicon, Summerville, NJ	J493
Tegaderm Film, 6 cm x 7 cm	3M	1624W	Cut in half to properly cover burn site
µCT - GE eXplore Locus SP	GE Healthcare Pre-Clinical Imaging, London, ON, Canada
Microview 2.2 Advanced Bone Analysis Application	GE Healthcare Pre-Clinical Imaging, London, ON, Canada