The Rodent Tibia Fracture Model: A Critical Review and Comparison With the Complex Regional Pain Syndrome Literature

Abstract

Distal limb fracture is the most common cause of complex regional pain syndrome (CRPS), thus the rodent tibia fracture model (TFM) was developed to study CRPS pathogenesis. This comprehensive review summarizes the published TFM research and compares these experimental results with the CRPS literature. The TFM generated spontaneous and evoked pain behaviors, inflammatory symptoms (edema, warmth), and trophic changes (skin thickening, osteoporosis) resembling symptoms in early CRPS. Neuropeptides, inflammatory cytokines, and nerve growth factor (NGF) have been linked to pain behaviors, inflammation, and trophic changes in the TFM model and proliferating keratinocytes were identified as the primary source of cutaneous cytokines and NGF. Tibia fracture also activated spinal glia and upregulated spinal neuropeptide, cytokine, and NGF expression, and in the brain it changed dendritic architecture. B cell-expressed immunoglobulin M antibodies also contributed to pain behavior, indicating a role for adaptive immunity. These results modeled many findings in early CRPS, but significant differences were also noted.

Perspective: Multiple neuroimmune signaling mechanisms contribute to the pain, inflammation, and trophic changes observed in the injured limb of the rodent TFM. This model replicates many of the symptoms, signs, and pathophysiology of early CRPS, but most post-fracture changes resolve within 5 months and may not contribute to perpetuating chronic CRPS.

Keywords: Pain; autoimmunity; complex regional pain syndrome; cytokine; fracture; innate immunity; neuropeptide.

Figure 1

This cartoon illustrates the anatomic distribution of the neuropeptide signaling changes observed in the first several weeks after fracture in the tibia fracture model (TFM). CALCA: the CGRP calcitonin related polypeptide alpha gene, CALCB: the CGRP calcitonin related polypeptide beta gene, CGRP: calcitonin gene-related peptide, SP: substance P, TAC1: the SP tachykinin precursor 1 gene, NK1: the SP neurokinin 1 receptor, TACR1: the SP tachykinin 1 receptor gene.

Figure 2

Overview of innate and adaptive immune changes in skin and spinal cord after fracture in the TFM. The neuropeptides substance P (SP) and calcitonin gene-related peptide (CGRP) are released from sensory neurons and activate their cognate receptors (NK1, CRLR, and RAMP1) on cutaneous venules, causing plasma extravasation and vasodilatation. SP also stimulates NK1 receptors on dermal mast cells, causing proliferation and degranulation. Furthermore SP and CGRP activate NK1, CRLR, and RAMP1 receptors on keratinocytes and microglia, causing these innate immune cells to proliferate and express/secrete inflammatory mediators (TNF, IL-1, NGF) that sensitize sensory afferent neurons. Likewise, norepinephrine (NE) is released from sympathetic neurons and stimulates β2 adrenoceptors (β2-AR) on keratinocytes to induce IL-6 expression/secretion that further sensitizes sensory afferents. Antigen presenting cells (APCs, including Langerhans cells, macrophages, and microglia) are also activated by SP and CGRP released from sensory neurons. Apoptotic cells release autoantigens that are phagocytized by activated APCs and then attached to membrane bound MHC II molecules. Activated APCs migrate to lymph nodes, where they present autoantigens to T cell receptors (TCRs) and polarize T cells to the Th1/2-type immune responses associated with autoimmunity. The Th1/2 polarized T cells present autoantigens to B cells that then express IgM immunoglobulin. Norepinephrine (NE) released by sympathetic neurons activates beta 2 adrenoceptors on B cells, further stimulating immunoglobulin expression. IgM antibodies form autoantigen-antibody complexes that bind to C1 complement, initiating the classical complement activation pathway. This results in the expression of C3a and C5a complement, thus causing the attraction and activation of keratinocytes and microglial cells that express inflammatory mediators including TNF, IL-1, IL-6, and NGF, all of which contribute to nociceptive sensitization in the fracture model of CRPS. APC: antigen presenting cell, β2-AR: beta 2 adrenoceptor, CGRP: calcitonin gene-related peptide, CRLR: the CGRP calcitonin receptor-like receptor, IL-1: interleukin 1β, IL-6: interleukin 6, MHC II: major histocompatibility complex class II, NGF: nerve growth factor, NE: norepinephrine, NK1: the tachykinin 1 receptor, RAMP1: the CGRP receptor activity modifying protein 1 co-receptor, SP: substance P, TAC1: the SP tachykinin precursor 1 gene, TACR1: the SP tachykinin 1 receptor gene, TCR: T cell receptor, TNF: tumor necrosis factor α.