The future of orthopaedic discovery will depend on advances in musculoskeletal basic science. While our surgical techniques and implant designs have seen considerable improvements over the last few years, the common sentiment is that we are reaching a plateau in implant development and related clinical outcomes. Therefore, it will come as no surprise that the developmental pipeline is focused on a new target: biologics and the translation of biomolecular discoveries from musculoskeletal basic science. In this review, we highlight the most recent discoveries in orthopaedic basic science related to aging; muscle regeneration, homeostasis, and bone-muscle crosstalk; tendon and bone regeneration; and musculoskeletal infection. We anticipate that these breakthroughs will influence orthopaedic care over the next decade. Other topic areas, including osteoarthritis and spine, will be covered in next year’s review. In addition, basic science breakthroughs in musculoskeletal oncology are not covered herein, as an update that is focused on musculoskeletal tumor surgery will be published in the upcoming issue of JBJS.
Aging has a dramatic effect on the regeneration of all tissues, bone included. When a young adult sustains a fracture, stem cells residing within the periosteum, endosteum, and bone marrow proliferate and differentiate into bone matrix, producing osteoblasts. This process results in the regeneration of the skeletal element, with complete restoration of its biomechanical properties. In the elderly, however, this process is often inadequate, which will result in delayed or unsuccessful fracture-healing. Chronic inflammation triggered and sustained by pro-inflammatory macrophages has recently been linked to a decline in the regenerative potential in aged skin, skeletal muscle, and the nervous system. Macrophages, derived from circulating or resident monocytes, undergo polarization into either pro-inflammatory (M1) or anti-inflammatory (M2) macrophages, depending on local cytokine cues. Recent focus has been on understanding the role of macrophage polarization and its resultant secretome on stem cell function during regeneration. Our current knowledge suggests that both M1 and M2 macrophages play crucial and necessary roles during fracture-healing. M1 macrophages regulate and sustain the early inflammatory phase, while M2 macrophages initiate the resolution of the inflammatory phase and, in doing so, initiate osteogenic differentiation and matrix deposition by osteoblasts. Either sustained M1-mediated inflammation or insufficient M2 activation and, thus, termination of the acute inflammatory response will lead to impaired bone regeneration. Aging has been associated with a delayed M1-M2 phenotype switch, and this dysregulation of macrophage activation may underlie the often-observed diminished and delayed healing response in the elderly, as discussed in a recent review by Pajarinen et al.1. In another noteworthy report, Vi et al. convincingly demonstrated that macrophages from young mice produce cytokines that promote bone formation, while macrophages from aged mice secrete factors that slow bone-healing in young mice. Using proteomic analysis of the secretome of young and aged macrophages, the authors identified low-density lipoprotein receptor-related protein 1 (LRP1) as a factor that is necessary for successful fracture-healing in young mice and, when administered as recombinant protein to aged mice, could rejuvenate fracture-healing2. This report and other studies further establish the importance of the local immune environment during fracture-healing. Immune cells serve as “first responders” after an injury, where they are responsible for orchestrating the complex logistics of stem cell recruitment and the initiation of angiogenesis, phagocytosis, and lineage commitment of skeletal stem cells.
Muscle Regeneration, Homeostasis, and Bone-Muscle Crosstalk
Nearly every traumatic orthopaedic condition is associated with a muscle injury, and muscle injuries are associated with considerable disability and time-consuming rehabilitation. While most muscle injuries heal without substantial sequelae, consequences of treatment failure may be dramatic, postponing the return to near-normal function for weeks or months. While surgical treatment of muscle injuries has been, and will likely remain, inconsequential, other treatment options, including cell therapy and the use of biologics, will possibly gain importance over the next few years. In order to advance these treatment avenues, we have to first better understand the underlying basics of skeletal muscle regeneration and homeostasis.
Recent investigations have shed light on the cellular key players of muscle regeneration, and these findings will hopefully allow further development of clinical treatment options for patients with muscle sprains, tears, contusions, and loss. Wosczyna et al. recently established the critical role of fibroadipogenic progenitors (FAPs), also called mesenchymal stromal cells, in muscle repair and maintenance. Using genetic mouse models, they were able to demonstrate that FAPs are necessary for muscle regeneration by supporting the muscle stem cell (satellite cell)3. Besides their role in muscle regeneration and homeostasis, FAPs have been linked to the process of fatty degeneration of the muscle4. While healthy young muscle does not support adipogenic differentiation of FAPs during regeneration, aged and diseased muscle often fails to regenerate and leads to adipocytic infiltration. Further research into this field of muscle regeneration and homeostasis and the role of FAPs during these processes will provide valuable insights that may lead to the development of therapeutics to prevent or delay fatty infiltration in conditions such as rotator cuff or hip abductor tendon tear.
Muscle fibrosis after injury, similar to fatty degeneration, results in compromised tissue architecture with subsequent impaired regeneration and function. Murray et al. identified a cell population within muscle that is platelet-derived growth factor receptor beta (PDGFRβ)-positive and responds to injury with excessive collagen deposition and thus fibrosis5. This fibrosis is αv integrin-dependent, and when αv integrin was depleted in these cells, the muscle was protected from posttraumatic fibrosis. Discoveries such as this one will allow targeting of specific cell populations within muscle that will then abolish unwanted side effects such as fibrosis and fatty degeneration, while still allowing for successful muscle tissue regeneration.
Over the past few years, important advances have been made in the understanding of the connection between exercise and physiologic well-being. Most strikingly has been the discovery that both skeletal muscle and bone, the 2 organs responsible for locomotion, have been functioning as an endocrine organ that regulates exercise tolerance. Bone secretes osteocalcin, and muscle provides interleukin (IL)-6, and both of these proteins promote adaptation to exercise in the young and old. In the elderly, however, this crosstalk between bone and muscle is diminished, resulting in decreased muscle mass, function, and exercise capacity. Recent studies have shown that osteocalcin administration can restore this bone-muscle crosstalk and thus improve exercise capacity in the elderly6. Besides this effect of osteocalcin on the musculoskeletal system, osteocalcin has been shown to have a far wider function throughout the body. As reviewed by Obri et al., the bone-derived hormone osteocalcin is necessary for optimal brain development and function. While being able to regulate anxiety and cognition in adult mice, its declining systemic levels with age are thought to be responsible for the cognitive decline observed in aging7. If decreased systemic levels of osteocalcin are the root cause for cognitive deterioration, then one could consider exogenous treatment with this bone-related protein in an attempt to restore brain function. A step toward this therapeutic direction was taken by Villeda et al., who presented compelling evidence that it was, in fact, osteocalcin present in serum from young mice that was responsible for the restoration of cognitive function in aged mice treated with serum from young mice8. These data further support the notion that the musculoskeletal system serves many other functions besides allowing locomotion and storing calcium.
Almost every orthopaedic surgeon will diagnose and treat tendon injuries. Many of these injuries can be treated nonoperatively with physical therapy and anti-inflammatory modalities, resulting in an acceptable clinical outcome. Substantial gaps in knowledge regarding the identity and origin of tendon stem cells have resulted in a dearth of successful biologic treatments for tendon injuries. Tendon cells express the transcription factor Scleraxis, which has allowed lineage tracing and subsequently the identification of the tendon stem progenitor cell (TSPC) that can be found in the tendon fascicle, epitenon, and tendon-surrounding vasculature. Future regenerative approaches aimed at tendon-healing will likely consist of 3-dimensional scaffolds containing TSPCs and the appropriate growth factors to induce tenogenic differentiation, as discussed in a recent report by Walia and Huang9. Using Scleraxis as a lineage tracer, Best and Loiselle identified a Scleraxis-positive cell population in the bridging scar tissue after tendon injury10. While the Scleraxis-positive cells formed a highly aligned tissue resembling tendon tissue, S100 calcium-binding protein A4 (S100A4)-positive fibroblasts generated a disorganized scar tissue filling the gap. These findings suggest that TSPCs are present in the adult tendon and actively contribute to the healing response; however, their small number does not result in a successful regeneration of the tendon but rather scar formation with interspersed tendon tissue. Future research will aim at harnessing the regenerative potential of the TSPC to restore tendon form and function10.
In vivo studies using cell therapy in combination with, or without, scaffolds have revealed promising results. Adipose-derived stromal cells11-14 and bone marrow stromal cells15,16 have been implanted in multiple tendon-regeneration models and have shown integration and often accelerated tendon-healing compared with control injuries.
Tendinopathy, symptomatic or asymptomatic, has been suggested to be a predictor of future tendon injury. Abraham et al. identified the upregulation of NF-κB (nuclear factor kappa-light-chain-enhancer of activated B cells) and its downstream targets in human rotator cuff tendons with tendinopathy17. Using a transgenic mouse model in which IKKβ (inhibitor of nuclear factor kappa-B kinase subunit beta), a key regulator of inflammation, was overexpressed, they demonstrated the development of tendinopathy in mouse rotator cuff tendons. Conversely, the deletion of IKKβ had a protective effect from chronic overuse and also resulted in improved outcome after surgical repair in this animal model. These data offer a potential therapeutic target in the prevention and treatment of human tendon disease17.
Successful bone-healing after fracture is highly dependent on the presence and timely activation of skeletal stem cells. While the importance of stem cells to successful bone regeneration has long been recognized, our field has struggled to define the skeletal stem cell itself by using reproducible molecular and cellular attributes. Only if we can reliably identify the stem cell itself can we begin to consider stem cell therapy as an adjunct to nonoperative or operative treatment of fractures or other orthopaedic conditions. Without the precise definition, any therapy using a heterogenous mixture of cells, such as bone marrow aspirate concentrate, can only be termed “cell therapy,” rather than “stem cell therapy.” Chan et al. precisely defined the human skeletal stem cell (hSSC) using a battery of cell surface markers18. Once hSSCs were identified, they demonstrated their role in human fracture repair and provided evidence that these cells generate a bone marrow-supportive niche. Most surprisingly, hSSCs do not differentiate into fat, but rather give rise to bone, cartilage, and stromal progenitor cells, which then give rise to osteoprogenitors and chondroprogenitors, before differentiating into bone, cartilage, and stroma18.
Besides bone marrow, the periosteum has been known to contain skeletal stem and progenitor cells and, in fact, the majority of cells within the soft callus formed after fracture are periosteum-derived cells. Duchamp de Lageneste et al. identified a population of skeletal stem cells residing within the periosteum of mice that exhibit greater clonogenicity, growth, and differentiation capacity than their counterparts found within the bone marrow cavity. In addition, they uncovered Periostin, a gene required for successful callus formation after fracture and necessary for maintenance of the periosteal stem cell (PSC) pool19. Similarly, Debnath et al. identified a PSC with clonal multipotency and self-renewal that sits at the apex of the differentiation hierarchy. While most PSCs contribute to fracture-healing by forming a cartilaginous soft callus first, the PSC described by Debnath et al. gives rise to intramembranous bone by direct differentiation into osteoblasts20. Finally, we now have at hand a means to characterize and name the distinct stages of lineage progression of the skeletal stem cells and their progeny. This knowledge will allow us to identify cellular deficiencies in acute and chronic orthopaedic injuries and will enable us to use cell-therapy approaches to directly address these maladies. For example, if we knew which exact cell type on the skeletal stem cell lineage-progression tree serves an immunomodulatory function when injected into an arthritic joint, then we could design cell-specific purification methods and enrich these cells, using technology that would make ex vivo expansion unnecessary. This would not only lead to faster and more efficient patient care but it would also be in line with the rules and guidelines set forth by the U.S. Food and Drug Administration (FDA).
Orthopaedic infections, and in particular periprosthetic joint infections (PJIs), are among the most dreaded complications in our specialty. To address this worldwide problem, a 2‐year-long international consensus meeting (ICM) process produced 652 consensus questions, which were discussed and voted on by 658 of the total of 869 delegates from 92 countries who attended the meeting in person in Philadelphia, Pennsylvania, from July 25 to 27, 2018. The 2018 ICM also had a Research Workgroup, which established consensus on the current and projected incidence of infection and the costs per patient for all orthopaedic subspecialties, which range from 0.1% to 30%, and $17,000 to $150,000, respectively21. The Research Workgroup used Delphi methodology to define 23 high-priority research questions within 6 thematic categories: acute versus chronic infection, host immunity, antibiotics, diagnosis, research caveats, and modifiable factors21. Major recent advances in this field include the discovery of Staphylococcus aureus invasion and colonization of the osteocytic-canalicular network of live cortical bone22,23, which provides an explanation for why this most common bacterial infection of bone is, as yet, incurable.
Bacteria exhibit a high affinity for adhering to foreign materials, such as orthopaedic implants. Once colonized on the surface of the implant, bacteria will begin to secrete a matrix composed of extracellular polysaccharides (EPS), proteins, and extra-cellular DNA, which is called biofilm. The importance of this topic was also recognized by the 2018 ICM, which included a Biofilm Workgroup that addressed 13 high-priority issues in this field24. Over 50% to 60% of all orthopaedic infections involving a biofilm are composed of S. aureus and S. epidermidis, which are the most common biofilm-producing bacteria. Formation of a biofilm provides a physical barrier to antibiotic therapy, which can increase the minimum inhibitory concentration (MIC) of antibiotics by up to 1,000-fold24. Therefore, new approaches to treating PJI are highly sought after and 1 such approach may involve the use of bacteriophages, which are naturally occurring viruses against bacteria. Phages are highly specific toward certain bacterial species, which allows targeted therapy without affecting the human microbiota. Lytic phages detect specific bacterial surface proteins, and after adhering to the bacterium, they inject their genetic material into the cell. Using the bacterial replication and translation machinery, large numbers of new phages are generated, which can then infect other bacteria, as discussed in a recent review by Taha et al.25. Kumaran et al. investigated the in vitro efficacy of a combination treatment algorithm using the lytic S. aureus phage SATA-8505 and 5 antibiotics26. The data from this report and from a previous report by Chaudhry et al.27 suggested that phage treatment preceding antibiotic treatment resulted in synergistic effects leading to improved biofilm reduction and reduced MIC. In a staggered treatment scenario, the lytic phages will result in fragmentation of the biofilm, allowing deeper penetration of the antibiotics into the biofilm, which will result in a more efficient eradication of the bacterial load within the biofilm26,27. Recent clinical trials have shown the safety of phage therapy; however, no efficacy studies using phage therapy for PJI have yet to be reported, to our knowledge. It is important to note that phage therapy in the U.S. has not yet been approved by the FDA, and the 2018 ICM Biofilm Workgroup cited major obstacles to bacteriophage therapy, including the fact that bacteriophages are neutralized in serum, and relevant pathogens contain CRISPR-cas9 (clustered regularly interspaced short palindromic repeats [CRISPR]-associated protein 9) immunity against bacteriophages24. Additionally, phages are usually bacterial-strain-specific; thus, a cocktail of different bacteriophage lineages may be necessary to effectively treat biofilm‐mediated infections24.
Upcoming Meetings and Events Related to Orthopaedic Basic Science
- The Gordon Research Conference on Bones and Teeth will be held February 2-7, 2020, in Galveston, Texas.
- The 2020 Annual Meeting of the Orthopaedic Research Society (ORS) will be held February 8-11, 2020, in Phoenix, Arizona.
- The European Calcified Tissue Society will meet May 16-19, 2020, in Marseille, France.
- The Annual Meeting of the International Society for Stem Cell Research will take place June 24-27, 2020, in Boston, Massachusetts.
- The Gordon Research Conference on Musculoskeletal Biology and Bioengineering will be held August 9-14, 2020, in Andover, New Hampshire.
- The American Society for Bone and Mineral Research (ASBMR) 2020 Annual Meeting will be held September 11-14, 2020, in Seattle, Washington.
- The European Orthopaedic Research Society (EORS) Annual Meeting will be held September 16-19, 2020, in Izmir, Turkey.
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