Musculoskeletal
A new model to analyze metaphyseal bone healing in mice

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Abstract

Background

Despite the increasing clinical problems with metaphyseal fractures, most experimental studies investigate the healing of diaphyseal fractures. Although the mouse would be the preferable species to study the molecular and genetic aspects of metaphyseal fracture healing, a murine model does not exist yet. Using a special locking plate system, we herein introduce a new model, which allows the analysis of metaphyseal bone healing in mice.

Methods

In 24 CD-1 mice the distal metaphysis of the femur was osteotomized. After stabilization with the locking plate, bone repair was analyzed radiologically, biomechanically, and histologically after 2 (n = 12) and 5 wk (n = 12). Additionally, the stiffness of the bone-implant construct was tested biomechanically ex vivo.

Results

The torsional stiffness of the bone-implant construct was low compared with nonfractured control femora (0.23 ± 0.1 Nmm/° versus 1.78 ± 0.15 Nmm/°, P < 0.05). The cause of failure was a pullout of the distal screw. At 2 wk after stabilization, radiological analysis showed that most bones were partly bridged. At 5 wk, all bones showed radiological union. Accordingly, biomechanical analyses revealed a significantly higher torsional stiffness after 5 wk compared with that after 2 wk. Successful healing was indicated by a torsional stiffness of 90% of the contralateral control femora. Histological analyses showed new woven bone bridging the osteotomy without external callus formation and in absence of any cartilaginous tissue, indicating intramembranous healing.

Conclusion

With the model introduced herein we report, for the first time, successful metaphyseal bone repair in mice. The model may be used to obtain deeper insights into the molecular mechanisms of metaphyseal fracture healing.

Introduction

Bone healing in cancellous bone is of great importance because of a high incidence of metaphyseal fractures in the elderly population. Clinically, metaphyseal bone fractures still represent a major challenge, particularly because many of these fractures occur in patients suffering from osteoporosis [1]. So far, metaphyseal bone healing is poorly understood. In spite of this, most experimental studies addressing fracture healing in osteoporotic bone have used diaphyseal fracture models [2], [3]. However, the bone structure with a thin cortical shell and mainly cancellous bone in the metaphysis is different from the primarily cortical bone in the diaphyseal region, which may influence the mechanisms of fracture healing. The few studies analyzing metaphyseal fracture healing were performed in sheep [4], dogs [5], and rats [6] but not in mice.

During the last years, mouse models have become the preferred tools in fracture healing research [7] because of the increasing availability of specific antibodies and gene-targeted animals. However, the small size of mice represents a major challenge for the development of fracture models, especially in the metaphysis of the bone and, so far, an appropriate mouse model did not exist. Using a special locking plate system, we herein introduce a new model, which allows, for the first time, the analysis of metaphyseal bone healing in mice.

Section snippets

Animals and surgical procedures

All animal procedures were performed according to the National Institute of Health guidelines for the use of experimental animals. A total number of 30 female CD-1 mice (12 to 14 weeks old) were used. At 2 wk after osteotomy and stabilization 12 of these mice were killed for radiological, biomechanical (n = 6), and histomorphological (n = 6) analyses. After 5 wk another 12 mice were killed, serving also for radiological, biomechanical (n = 6), and histomorphological (n = 6) analyses. Additional six

Handling of the metaphyseal MouseFix implants

In all 30 animals the plate could be successfully fixed to the ventrolateral aspect of the femur in a standardized manner. According to the CT measurements, the osteotomy could be performed in the metaphyseal part of the distal femur. The complex surgical procedure required four hands (two surgeons). The operation time was ∼30 min for all procedures. The critical step during the surgical procedure was the exact placement of the plate parallel and ventrolateral to the femur. Therefore, two

Discussion

We herein introduce a new mouse model which allows for the study of metaphyseal bone healing. We could show that the specially designed locking plate can be positioned in the metaphyseal part of the distal mouse femur. We further demonstrated that the metaphyseal femur osteotomy heals within 5 wk, as indicated by a complete radiological bridging and a torsional stiffness of 90% of the control femora. The histological analysis confirmed new woven bone within the osteotomy gap without external

Acknowledgments

The authors appreciate the excellent technical assistance of Janine Becker. This study was supported by a grant of the German Section of the AO.

Conflict of interest statement: R.M. is an employer of the AO Foundation. None of the other authors have any affiliation or financial arrangement with an organization or company that has financial interests in the subject matter discussed in the manuscript.

References (19)

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