The Tibial Fracture-Pin Model: A Clinically Relevant Mouse Model of Orthopedic Injury

Orthopedic trauma accounts for 25% of all injuries sustained by nearly 500 million people each year worldwide^1,2,3. Orthopedic trauma can be associated with complex muscle, bone, nerve, and connective tissue damage, necessitating hospitalization and surgery to ensure adequate recovery^3,4. Acute and chronic pain after orthopedic trauma can result in significant physical, psychological, and financial burdens that affect a patient's quality of life^1,4. Additionally, orthopedic surgery to stabilize and fix fractures is also associated with severe acute and chronic post-surgical pain^5,6,7,8,9.

The mechanisms underlying acute and chronic trauma-related pain need to be better understood to develop better treatments. To achieve this, reliable, reproducible, and clinically relevant preclinical models are required. Since most animal models of orthopedic trauma did not involve simultaneous injury to multiple tissue types (bone, muscle, nerves), they were not truly representative of human complex orthopedic trauma, for example, trauma after falls, motor vehicle crashes, or war-related injuries^10,11. Therefore, we developed the tibial fracture-pin mouse model to examine the major manifestations of such injury, including bone and muscle tissue damage and acute and chronic pain¹¹. The tibial fracture-pin model consists of a unilateral open tibial fracture with IMN internal fixation and simultaneous TA muscle injury. Histological sections of the TA show injury to the muscle in which dense fibrosis develops with associated loss of large, mature muscle fibers as early as 2 weeks post injury. Moreover, the fracture callus is apparent on microcomputer tomography (microCT) 4 weeks post injury and continues to undergo remodeling¹¹.

Various reflexive and nonreflexive behavior assays can be used to evaluate the sensory and affective components of pain in the tibial fracture-pin model. For example, one can use the Von Frey filaments to demonstrate mechanical hypersensitivity in this model. In fact, mice develop long-lasting mechanical hypersensitivity in the ipsilateral hind paw after tibial fracture-pin surgery¹¹. Another particularly useful behavioral paradigm is the hot plate assay, which traditionally measures the latency to paw withdrawal to a thermal stimulus. While this assay has been used for decades¹², truly a gold standard in preclinical pain research, measuring reflexive behavior alone has its limitations¹³. As a result, we have developed a modified hot plate paradigm that can capture elements of both reflexive and nonreflexive responses in the setting of a thermal stimulus¹⁴.

This modified hot plate assay determines the initial response latency as in the original hot plate test and an extended observation period to record additional nocifensive behaviors. By categorizing these extended behaviors into distinct categories (flinching, licking, guarding, jumping), the nonreflexive response to the thermal stimulus can be captured. Flinching is the rapid removal of the paw and/or splaying of digits, but the limb is quickly returned to the hot plate. Licking and biting of the hind and front paws are both defined as licking for analysis. Guarding is the continued raising of the limb beyond when afferent nociceptive information ends. Finally, jumping is the removal of all four limbs from the hot plate surface. These behaviors can be analyzed individually and grouped together with special care to still note the initial response latency.