Biological and molecular profile of fracture non-union tissue: A systematic review and an update on current insights

Fracture non-union represents a common complication, seen in 5%-10% of all acute fractures. Despite the enhancement in scientific understanding and treatment methods, rates of fracture non-union remain largely unchanged over the years. This systematic review investigates the biological, molecular and genetic profiles of both (i) non-union tissue and (ii) non-union-related tissues, and the genetic predisposition to fracture non-union. This is crucially important as it could facilitate earlier identification and targeted treatment of high-risk patients, along with improving our understanding on pathophysiology of fracture non-union. Since this is an update on our previous systematic review, we searched the literature indexed in PubMed Medline; Ovid Medline; Embase; Scopus; Google Scholar; and the Cochrane Library using Medical Subject Heading (MeSH) or Title/Abstract words (non-union(s), non-union(s), human, tissue, bone morphogenic protein(s) (BMPs) and MSCs) from August 2014 (date of our previous publication) to 2 October 2021 for non-union tissue studies, whereas no date restrictions imposed on non-union-related tissue studies. Inclusion criteria of this systematic review are human studies investigating the characteristics and properties of non-union tissue and non-union-related tissues, available in full-text English language. Limitations of this systematic review are exclusion of animal studies, the heterogeneity in the definition of non-union and timing of tissue harvest seen in the included studies, and the search term MSC which may result in the exclusion of studies using historical terms such as 'osteoprogenitors' and 'skeletal stem cells'. A total of 24 studies (non-union tissue: n = 10; non-union-related tissues: n = 14) met the inclusion criteria. Soft tissue interposition, bony sclerosis of fracture ends and complete obliteration of medullary canal are commonest macroscopic appearances of non-unions. Non-union tissue colour and surrounding fluid are two important characteristics that could be used clinically to distinguish between septic and aseptic non-unions. Atrophic non-unions had a predominance of endochondral bone formation and lower cellular density, when compared against hypertrophic non-unions. Vascular tissues were present in both atrophic and hypertrophic non-unions, with no difference in vessel density between the two. Studies have found non-union tissue to contain biologically active MSCs with potential for osteoblastic, chondrogenic and adipogenic differentiation. Proliferative capacity of non-union tissue MSCs was comparable to that of bone marrow MSCs. Rates of cell senescence of non-union tissue remain inconclusive and require further investigation. There was a lower BMP expression in non-union site and absent in the extracellular matrix, with no difference observed between atrophic and hypertrophic non-unions. The reduced BMP-7 gene expression and elevated levels of its inhibitors (Chordin, Noggin and Gremlin) could potentially explain impaired bone healing observed in non-union MSCs. Expression of Dkk-1 in osteogenic medium was higher in non-union MSCs. Numerous genetic polymorphisms associated with fracture non-union have been identified, with some involving the BMP and MMP pathways. Further research is required on determining the sensitivity and specificity of molecular and genetic profiling of relevant tissues as a potential screening biomarker for fracture non-unions.