Development and evaluation of multiple tendon injury models in the mouse

Abstract

The mouse has proven to be an advantageous animal model system in basic science research focused on aiding in development and evaluation of potential treatments; however, the small size of mouse tendons makes consistent and reproducible injury models and subsequent biomechanical evaluation challenging for studying tendon healing. In this study, we investigated the feasibility and reproducibility of multiple mouse tendon injury models. Our hypothesis was that incisional (using a blade) and excisional (using a biopsy punch) injuries would result in consistent differences in tendon material properties. At 16 weeks of age, 17 C57BL/6 mice underwent surgery to create defects in the flexor digitorum longus, Achilles, or patellar tendon. Each animal received 1–2 full-thickness, central-width incisional or excisional injuries per limb; at least one tendon per limb remained uninjured. The injuries were distributed such that each tendon type had comparable numbers of uninjured, incisionally injured, and excisionally injured specimens. Three weeks after injury, all animals were euthanized and tendons were harvested for mechanical testing. As hypothesized, differences were detected for all three different tendon types at three weeks post-injury. While all models created injuries that produced predictable outcomes, the patellar tendon model was the most consistent in terms of number and size of significant differences in injured tendons compared to native properties, as well as in the overall variance in the data. This finding provides support for its use in fundamental tendon healing studies; however, future work may use any of these models, based on their appropriateness for the specific question under study.

Introduction

Musculoskeletal disorders were reported by more than 100 million adults in the United States in 2005 alone (Jacobs et al., 2008). More specifically, tendon and other soft tissue injuries are known to cause considerable pain and disability. A better understanding of the healing response of these injuries will improve the ability to treat those affected as well as to better prevent injuries. Investigations into tendon mechanical properties and healing mechanics can provide valuable information as the field ventures further into the realm of tissue engineered constructs.

Animal models are often used to help understand human physiology and disease and to aid in development and evaluation of treatment modalities. The mouse has become a popular model system due to its similar physiology with humans, the ability to alter the mouse genome to mimic human disease, and the availability of a large number of biologic assays (Bedell et al., 1997). Despite these advantages, the small size of mouse tendons makes consistent and reproducible injury models and subsequent biomechanical evaluation challenging. Therefore, the objective of this study was to investigate the feasibility and reproducibility of multiple mouse tendon injury models by evaluating the hind flexor digitorum longus (FDL), Achilles, and a previously utilized patellar tendon injury model (Lin et al., 2006). With the goal of studying fundamental tendon healing processes at the basic science level, we aimed to develop and compare models that did not require suture repair as that might add to, or alter, the normal biological processes at the injury site (e.g., inflammation) and this study was aimed at developing and evaluating model systems for natural healing of these injuries. This technique was chosen rather than a complete laceration so that we would be able to assess the biomechanical efficacy of these injury models in a basic science context so that these fundamental studies might be used in a targeted fashion in future studies to address directly clinically relevant scenarios such as a complete laceration or other acute critical defect. Our hypothesis was that partial width laceration (incisional) and punch (excisional) injuries would result in consistent biomechanical differences in tendon material properties, providing multiple tendon injury models for use in our field.