Nonunion is defined as the permanent failure of a fractured bone to heal, often necessitating surgical intervention. Atrophic nonunions are a subtype that are particularly difficult to treat. Animal models of atrophic nonunion are available; however, these require surgical or radiation-induced trauma to disrupt periosteal healing. These methods are invasive and not representative of many clinical nonunions where osseous regeneration has been arrested by a “failure of biology”. We hypothesized that arresting osteoblast cell proliferation after fracture would lead to atrophic nonunion in mice. Using mice that express a thymidine kinase (tk) “suicide gene” driven by the 3.6Col1a1 promoter (Col1-tk), proliferating osteoblast lineage cells can be ablated upon exposure to the nucleoside analog ganciclovir (GCV). Wild-type (WT; control) and Col1-tk littermates were subjected to a full femur fracture and intramedullary fixation at 12 weeks age. We confirmed abundant tk+ cells in fracture callus of Col-tk mice dosed with water or GCV, specifically many osteoblasts, osteocytes, and chondrocytes at the cartilage-bone interface. Histologically, we observed altered callus composition in Col1-tk mice at 2 and 3 weeks post fracture, with significantly less bone and more fibrous tissue. Col1-tk mice, monitored for 12 weeks with in vivo radiographs and microCT scans, had delayed bone bridging and reduced callus size. Following sacrifice, ex vivo microCT and histology demonstrated failed union with residual bone fragments and fibrous tissue in Col1-tk mice. Biomechanical testing demonstrated a failure to recover torsional strength in Col1-tk mice, in contrast to WT. Our data indicates that suppression of proliferating osteoblast-lineage cells for at least 2 weeks after fracture blunts the formation and remodeling of a mineralized callus leading to a functional nonunion. We propose this as a new murine model of atrophic nonunion.

中文翻译：

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骨折后增殖的成骨细胞系细胞的消融导致小鼠模型中的萎缩性骨不连

骨不连被定义为骨折骨无法愈合，通常需要手术干预。萎缩性骨不连是一种特别难以治疗的亚型。有萎缩性骨不连的动物模型；然而，这些需要手术或辐射引起的创伤来破坏骨膜愈合。这些方法是侵入性的，并不代表许多临床骨不连，其中骨再生因“生物学故障”而停滞。我们假设骨折后阻止成骨细胞增殖会导致小鼠萎缩性骨不连。使用表达由 3.6Col1a1 启动子 (Col1-tk) 驱动的胸苷激酶 (tk)“自杀基因”的小鼠，可以在暴露于核苷类似物更昔洛韦 (GCV) 后消融增殖的成骨细胞谱系细胞。野生型（WT；对照）和 Col1-tk 同窝仔猪在 12 周龄时进行了全股骨骨折和髓内固定。我们证实了在用水或 GCV 给药的 Col-tk 小鼠的骨折愈伤组织中存在丰富的 tk+ 细胞，特别是软骨-骨界面处的许多成骨细胞、骨细胞和软骨细胞。在组织学上，我们在骨折后 2 周和 3 周观察到 Col1-tk 小鼠的愈伤组织成分发生改变，骨骼明显减少，纤维组织增多。Col1-tk 小鼠，监测 12 周 我们在骨折后 2 周和 3 周观察到 Col1-tk 小鼠的愈伤组织成分发生了改变，骨骼明显减少，纤维组织增多。Col1-tk 小鼠，监测 12 周 我们在骨折后 2 周和 3 周观察到 Col1-tk 小鼠的愈伤组织成分发生了改变，骨骼明显减少，纤维组织增多。Col1-tk 小鼠，监测 12 周体内X 光片和显微 CT 扫描显示骨桥延迟和愈伤组织尺寸减少。牺牲后，体外显微 CT 和组织学证明与 Col1-tk 小鼠中残留的骨碎片和纤维组织结合失败。与 WT 相比，生物力学测试表明 Col1-tk 小鼠无法恢复扭转强度。我们的数据表明，在骨折后抑制增殖的成骨细胞谱系细胞至少 2 周会减弱矿化愈伤组织的形成和重塑，从而导致功能性骨不连。我们提出这是一种新的小鼠萎缩性骨不连模型。