Summary

The Generation of Closed Femoral Fractures in Mice: A Model to Study Bone Healing

Published: August 16, 2018
doi:

Summary

The murine closed femoral fracture model is a powerful platform to study fracture healing and novel therapeutic strategies to accelerate bone regeneration. The goal of this surgical protocol is to generate unilateral closed femoral fractures in mice using an intramedullary steel rod to stabilize the femur.

Abstract

Bone fractures impose a tremendous socio-economic burden on patients, in addition to significantly affecting their quality of life. Therapeutic strategies that promote efficient bone healing are non-existent and in high demand. Effective and reproducible animal models of fractures healing are needed to understand the complex biological processes associated with bone regeneration. Many animal models of fracture healing have been generated over the years; however, murine fracture models have recently emerged as powerful tools to study bone healing. A variety of open and closed models have been developed, but the closed femoral fracture model stands out as a simple method for generating rapid and reproducible results in a physiologically relevant manner. The goal of this surgical protocol is to generate unilateral closed femoral fractures in mice and facilitate a post-fracture stabilization of the femur by inserting an intramedullary steel rod. Although devices such as a nail or a screw offer greater axial and rotational stability, the use of an intramedullary rod provides a sufficient stabilization for consistent healing outcomes without producing new defects in the bone tissue or damaging nearby soft tissue. Radiographic imaging is used to monitor the progression of callus formation, bony union, and subsequent remodeling of the bony callus. Bone healing outcomes are typically associated with the strength of the healed bone and measured with torsional testing. Still, understanding the early cellular and molecular events associated with fracture repair is critical in the study of bone tissue regeneration. The closed femoral fracture model in mice with intramedullary fixation serves as an attractive platform to study bone fracture healing and evaluate therapeutic strategies to accelerate healing.

Introduction

Fractures are among the most common injuries occurring to the musculoskeletal system and are associated with a tremendous socioeconomic burden, including treatment costs that are projected to surpass $25 billion annually in the United States1,2. Although the majority of fractures heal without incident, healing is associated with substantial downtime and loss of productivity. Approximately 5 – 10% of all fractures result in a delayed healing or non-union, due to age or other underlying chronic health conditions, such as osteoporosis and diabetes mellitus3,4,5. No FDA-approved pharmacological treatments are currently available to promote efficient bone healing and shorten recovery time.

Fracture healing is a complex and highly dynamic process involving the coordination of multiple cell types. Hence, a comprehensive understanding of the cellular and molecular events associated with bone regeneration is crucial to the identification of therapeutic targets that accelerate this process. As with other human diseases, the establishment of a highly amenable and reproducible animal model is crucial in the study of bone healing. Larger animals, such as sheep and swine, have bone remodeling properties and biomechanics similar to humans, but are expensive, require substantial healing time, and are not readily amenable to genetic manipulation6. On the other hand, small animal models, such as rats and mice, offer many advantages, including an ease of handling, low costs of maintenance, short breeding cycles, and a shorter healing time7. Furthermore, the mouse genome is fully sequenced, allowing for the rapid manipulation and generation of genetic variants. Thus, the mouse is a powerful model system to study human disease, injury, and repair8. In humans, comorbidities like osteoporosis and diabetes mellitus increase the likelihood of a delayed healing. A number of existing mouse models are available to study the effects of comorbidities such as osteoporosis and diabetes mellitus on bone injury and healing. Patients suffering from osteoporosis have a markedly decreased bone formation during the later stages of a fracture healing9. Ovariectomized (OVX) mice exhibit rapid bone loss and delayed bone healing similar to that observed in postmenopausal osteoporosis10,11. Additionally, many mouse models of type I and type II diabetes mimic the low bone mass phenotypes and impaired fracture healing seen in humans11. Moreover, murine fracture models serve as a versatile platform to study the complex biological processes occurring in the callus and explore novel therapeutic strategies that accelerate bone tissue regeneration.

Despite differences in bone structure and metabolism, the overall process of bone fracture healing remains very similar in mice and humans, involving a combination of endochondral and intramembranous ossification followed by bone remodeling. Endochondral ossification involves the recruitment of progenitor cells to less mechanically stable regions surrounding the fracture gap, where they differentiate into chondrocytes that hypertrophy and mineralize the cartilage to produce a soft callus. The second wave of progenitor cells infiltrate the callus and differentiate into mature osteoblasts that secrete new bone matrix12,13,14,15. During intramembranous ossification, progenitors on the periosteal and endosteal surfaces directly differentiate into matrix secreting osteoblasts and facilitate the bridging of the fracture gap9,11,12,13. Together, the endochondral and intramembranous ossifications result in the development of a hard callus, which is further remodeled over time to form a strong secondary bone capable of supporting mechanical loads13,14,15. In healthy humans, the healing process takes approximately 3 months, compared to only 35 days in mice16.

Fracture healing has commonly been studied using either open or closed surgical models17. Open surgical approaches, such as the generation of a critically sized defect or complete osteotomy, standardize the injury location and geometry to reduce deviations caused by comminuted fractures. Osteotomies serve as an excellent model to study the underlying mechanism behind a non-union because healing is often delayed compared to closed fractures. Furthermore, a rigid external fixation is required to stabilize the osteotomized bone, meaning the regeneration will primarily depend on the intramembranous ossification. Open surgical approaches use devices such as locking nails, pin-clips, and locking plates to provide axial and rotational stability to the fractured limb; however, such devices are expensive and require significantly more time in surgery18,19,20,21. On the other hand, closed models are stabilized with a simple intramedullary fixation device, allowing for enough instability to stimulate endochondral healing. As a result, closed fracture models do not readily mimic the conditions of a non-union. Internal fixation techniques, such as intramedullary pins, nails, and compression screws, are advantageous as they are cheap, easy to use, and minimize the time in surgery21,22,23. In some cases, intramedullary pins are inserted prior to the fracture, but the bending of the intramedullary pin can lead to the angulation or displacement of the fractured femur, contributing to a variable callus size and healing. The fracture location and geometry are more difficult to standardize in closed models, as they are generated using a three-point bending device, wherein a weight is dropped on the diaphysis. However, with the proper technique, this surgical approach offers rapid and consistent results. Moreover, the closed fracture model serves as a clinically relevant tool to study fractures caused by high force impact or mechanical stress22.

This surgical protocol was adapted from previously described methods using an intramedullary pin to stabilize fractured femurs in rats and mice22,24,25. First, an intramedullary needle of a small diameter is inserted through the intracondylar notch to establish a point of entry, and a guidewire is introduced prior to generating a transverse fracture at the femoral midshaft using a gravity-dependent three-point bending device. Following the successful generation of a closed femoral fracture, an intramedullary rod of a larger diameter is incorporated over the guide wire to stabilize the fractured femur. This method avoids the risk of delayed healing caused by the angulation of the intramedullary pin during the fracture, as the placement of the rod post-fracture allows for the repositioning and optimized stabilization of the injured femur.

Protocol

The following procedure was performed with approval from the Indiana University School of Medicine Institutional Animal Care and Use Committee (IACUC). All survival surgeries were performed under sterile conditions as outlined by the NIH guidelines. Pain and risk of infections were managed with proper analgesics and antibiotics to ensure a successful outcome. 1. Anesthesia and Preparation Weigh the mouse and anesthetize it with a mixture of ketamine (100 mg/kg) and xylazine (10 mg/…

Representative Results

The successful implementation of the surgical procedure was monitored with radiographic imaging. Key steps include the insertion of an intramedullary needle, the placement of a guide wire, the induction of a transverse fracture at the femoral midshaft, and the proper stabilization with an intramedullary rod (Figure 2Ai – 2Aiv). The healing progression of the fracture callus was monitored with weekly radiographic images up to …

Discussion

The goal of this surgical procedure is to generate standardized closed femoral fractures in mice. A key advantage of this model is that the internal fixation takes place after the generation of the fracture, thereby avoiding an angulation of the intramedullary rod. Perhaps the most critical aspect of this protocol is the generation of a standardized transverse fracture at the femoral midshaft, as the fracture geometry is dependent on the applied bending force and the positioning of the hind limb. Improper positioning of …

Disclosures

The authors have nothing to disclose.

Acknowledgements

This work was supported by grants from the Department of Defense (DoD) US Army Medical Research and Materiel Command (USAMRMC) Congressionally Directed Medical Research Programs (CDMRP) (PR121604) and the National Institutes of Arthritis and Musculoskeletal and Skin Diseases (NIAMS), NIH R01 AR068332 to Uma Sankar. Justin Williams is supported through a Comprehensive Musculoskeletal T32 Training Program from NIAMS/NIH (AR065971).

Materials

Oster Minimax Trimmer Animal World Network 78049-100
POVIDONE-IODINE Thermo Fisher Scientific 395516
OPHTHALMIC OINTMENT Thermo Fisher Scientific NC0490117
Styker T/Pump Warm Water Recirculator Kent Scientific Corporation TP-700
1ml Sub-Q Syringe Thermo Fisher Scientific 309597
ENCORE Sensi-Touch PF Moore Medical LLC 30347 Latex, powder-free surgical glove
PrecisionGlide 25G Hypodermic Needles Thermo Fisher Scientific 14-826-49
Ultra-High-Temperature Tungsten Wire, McMaster-Carr 3775K37 0.005" Diameter, 1/16 lb. Spool, 380' Long
304 stainless steel, 24G thin walled tubing Microgroup Inc 304h24tw-5ft
#15 Scalpel Blades Fine Science Tools 10015-00
#10 Scalpel Blades Fine Science Tools 10010-00
Narrow Pattern Forceps Fine Science Tools 11002-12 Serrated/Straight/12cm
Iris Forceps Fine Science Tools 11066-07 1×2 Teeth/Straight/7cm
Dissector Scissors Fine Science Tools 14081-09 Slim Blades/Angled to Side/Sharp-Sharp/10cm
Fine Scissors Fine Science Tools 14058-11 ToughCut/Straight/Sharp-Sharp/11.5cm
Olsen-Hegar Needle Holder with Suture Cutter Fine Science Tools 12002-12 Straight/Serrated/12cm/with Lock
Crile Hemostat Fine Science Tools 13004-14 Serrated/Straight/14cm
Tungsten Wire Cutter ACE Surgical Supply Co., Inc. 08-051-90 ACE #150 Wire Cutter, tungsten carbide tips
3-0 VICRYL Suture Ethicon Suture J423H 3-0 VICRYL UNDYED 27" FS-2 CUTTING
piXarray 100 Digital Specimen Radiography System Bioptics, Inc Cabinet x-ray system
Einhorn 3-Point Bending Device N/A N/A Custom Built

References

  1. Schnell, S., Friedman, S. M., Mendelson, D. A., Bingham, K. W., Kates, S. L. The 1-Year Mortality of Patients Treated in a Hip Fracture Program for Elders. Geriatric Orthopaedic Surgery & Rehabilitation. 1 (1), 6-14 (2010).
  2. Burge, R., et al. Incidence and economic burden of osteoporosis-related fractures in the United States, 2005-2025. Journal of Bone and Mineral Research. 22 (3), 465-475 (2007).
  3. Cunningham, B. P., Brazina, S., Morshed, S., Miclau, T. Fracture healing: A review of clinical, imaging and laboratory diagnostic options. Injury. 48, S69-S75 (2017).
  4. Einhorn, T. A. Can an anti-fracture agent heal fractures?. Clinical Cases in Mineral and Bone Metabolism. 7 (1), 11-14 (2010).
  5. Hak, D. J., et al. Delayed union and nonunions: epidemiology, clinical issues, and financial aspects. Injury. 45, S3-S7 (2014).
  6. Decker, S., Reifenrath, J., Omar, M., Krettek, C., Muller, C. W. Non-osteotomy and osteotomy large animal fracture models in orthopedic trauma research. Orthopaedic Reviews (Pavia). 6 (4), 5575 (2014).
  7. Histing, T., et al. Small animal bone healing models: standards, tips, and pitfalls results of a consensus meeting. Bone. 49 (4), 591-599 (2011).
  8. Jacenko, O., Olsen, B. R. Transgenic mouse models in studies of skeletal disorders. Journal of Rheumatology Supplement. 43, 39-41 (1995).
  9. Nikolaou, V. S., Efstathopoulos, N., Kontakis, G., Kanakaris, N. K., Giannoudis, P. V. The influence of osteoporosis in femoral fracture healing time. Injury. 40 (6), 663-668 (2009).
  10. Bain, S. D., Bailey, M. C., Celino, D. L., Lantry, M. M., Edwards, M. W. High-dose estrogen inhibits bone resorption and stimulates bone formation in the ovariectomized mouse. Journal of Bone and Mineral Research. 8 (4), 435-442 (1993).
  11. Haffner-Luntzer, M., Kovtun, A., Rapp, A. E., Ignatius, A. Mouse Models in Bone Fracture Healing Research. Current Molecular Biology Reports. 2 (2), 101-111 (2016).
  12. Einhorn, T. A., Gerstenfeld, L. C. Fracture healing: mechanisms and interventions. Nature Reviews in Rheumatology. 11 (1), 45-54 (2015).
  13. Schindeler, A., McDonald, M. M., Bokko, P., Little, D. G. Bone remodeling during fracture repair: The cellular picture. Seminar in Cellular and Developmental Biology. 19 (5), 459-466 (2008).
  14. Ai-Aql, Z. S., Alagl, A. S., Graves, D. T., Gerstenfeld, L. C., Einhorn, T. A. Molecular mechanisms controlling bone formation during fracture healing and distraction osteogenesis. Journal of Dental Research. 87 (2), 107-118 (2008).
  15. Gerstenfeld, L. C., et al. Three-dimensional Reconstruction of Fracture Callus Morphogenesis. Journal of Histochemistry & Cytochemistry. 54 (11), 1215-1228 (2006).
  16. Marsell, R., Einhorn, T. A. Emerging bone healing therapies. Journal of Orthopaedic Trauma. 24, S4-S8 (2010).
  17. Lybrand, K., Bragdon, B., Gerstenfeld, L. Mouse models of bone healing: fracture, marrow ablation, and distraction osteogenesis. Current Protocols of Mouse Biology. 5 (1), 35-49 (2015).
  18. Garcia, P., et al. The LockingMouseNail–a new implant for standardized stable osteosynthesis in mice. Journal of Surgical Research. 169 (2), 220-226 (2011).
  19. Histing, T., et al. An internal locking plate to study intramembranous bone healing in a mouse femur fracture model. Journal of Orthopaedic Research. 28 (3), 397-402 (2010).
  20. Garcia, P., et al. A new technique for internal fixation of femoral fractures in mice: impact of stability on fracture healing. Journal of Biomechistry. 41 (8), 1689-1696 (2008).
  21. Holstein, J. H., et al. Advances in the establishment of defined mouse models for the study of fracture healing and bone regeneration. Journal of Orthopaedic Trauma. 23 (5 Suppl), S31-S38 (2009).
  22. Bonnarens, F., Einhorn, T. A. Production of a standard closed fracture in laboratory animal bone. Journal of Orthopaedic Research. 2 (1), 97-101 (1984).
  23. Holstein, J. H., Menger, M. D., Culemann, U., Meier, C., Pohlemann, T. Development of a locking femur nail for mice. Journal of Biomechistry. 40 (1), 215-219 (2007).
  24. McBride-Gagyi, S. H., McKenzie, J. A., Buettmann, E. G., Gardner, M. J., Silva, M. J. Bmp2 conditional knockout in osteoblasts and endothelial cells does not impair bone formation after injury or mechanical loading in adult mice. Bone. 81, 533-543 (2015).
  25. Williams, J. N., et al. Inhibition of CaMKK2 Enhances Fracture Healing by Stimulating Indian Hedgehog Signaling and Accelerating Endochondral Ossification. Journal of Bone and Mineral Research. , (2018).
check_url/58122?article_type=t

Play Video

Cite This Article
Williams, J. N., Li, Y., Valiya Kambrath, A., Sankar, U. The Generation of Closed Femoral Fractures in Mice: A Model to Study Bone Healing. J. Vis. Exp. (138), e58122, doi:10.3791/58122 (2018).

View Video