Successful healing and regeneration of orthopaedic tissues after injury requires highly orchestrated interactions between cell types responding to multiple signaling inputs. Bioactive factors will be defined here as signals that have the potential to help injuries heal and tissues regenerate. A bioactive factor may be a naturally occurring protein or may be an engineered moiety created to mimic the action of a naturally occurring molecule. The targets of bioactive factors are cells with the potential to engage in the healing process. These cells may be present at the injury site, recruited to the tissue during the repair process, or added from exogenous sources to enhance healing. Additionally, a bioactive factor might be secreted into the extracellular space by resident cells, or could be tethered to or embedded in native extracellular matrix (ECM), or might be provided exogenously in engineered biocompatible tissue-specific matrices. Figuring out the most effective combinations of bioactive factors, biomaterials, and tissue-specific target cells able to initiate and sustain repair until a tissue returns to homeostasis remains both the biggest challenge and best opportunity for advancing the regeneration of orthopaedic tissues. The molecules chosen for discussion¹,²,³,⁴,⁵,⁶,⁷,⁸,⁹,¹⁰,¹¹,¹²,¹³,¹⁴,¹⁵,¹⁶,¹⁷,¹⁸,¹⁹,²⁰,²¹,²²,²³,²⁴,²⁵,²⁶ (Table 1) meet an important set of criteria: each is a known component of a tissue repair cascade or mimetic of a naturally occurring repair component; each has been tested for the ability to enhance repair in relevant preclinical and/or clinical models; each can be produced using good manufacturing practices. What remains to be determined is how to best use each alone or in combination to achieve successful repair and regeneration in a tissue-specific manner.

Platelet-rich plasma (PRP) is the designation given to preparations of autologous human plasma with increased platelet concentration produced by centrifuging the patient’s blood to concentrate contents. As platelets contain many growth factors including transforming growth factor beta, platelet-derived growth factor (PDGF), basic fibroblast growth factor, vascular endothelial growth factor, epidermal growth factor, and insulinlike growth factor-1 (Figure 1), using PRP would deliver combinations of these bioactive proteins to the wound site in a greater than normal amount.²⁷,²⁸ The ability of PRP to provide anabolic stimulus for healing by exerting mitogenic, chemoattractant, and proliferative effects would depend on the presence of growth factors in PRP known to mediate these processes. Plasma processing can be tailored to remove specific blood cell components, which would then change the combination of bioactive factors present in the final PRP.²⁹

There is no general consensus in the orthopaedic community as to what constitutes optimal PRP and it is likely that the optimal PRP for individual tissue-specific healing indications would need to be determined based on information about the healing and/or regenerative process in that tissue. For example, knowledge of the strength and duration of the inflammatory environment immediately after tissue injury would be key data when designing PRP. As leukocyte-rich PRP is thought to be proinflammatory by containing elevated catabolic cytokines, Zitsch et al³⁰ conducted a prospective randomized double-blind clinical trial for leukocyte-reduced PRP for patients undergoing surgery for pilon fractures and found that a single intra-articular injection of leukocyte-reduced PRP significantly reduced proinflammatory and degradative biomarkers while increasing anabolic biomarkers.

Several confounding factors exist for proving the efficacy of PRP for enhancing tissue repair and regeneration. There is inherent variability in blood contents collected from patients based on age, sex, medication status, and general overall individual health. In fact, PRP collected from the same patient has been found to change throughout the day.³¹,³² Additionally, commercial concentrating systems differ on platelet capture efficiency, platelet isolation method, centrifugation speed, and collection tubes.²⁸ These variations led Obana in a 2021 review of the previous 11 years of PRP clinical data to conclude that there is a dearth of high-level evidence and methodologic standardization about PRP despite the trend upward in use of PRP.³³ Future developments to be considered for optimizing PRP use include: creating a test kit that would be available before PRP administration that can determine if the PRP meets specific quality standards; creating
a point-of-care blood test that can determine if a donor’s PRP will be of high quality; and combining PRP with tissue-compatible carriers specific to the repair site.

PDGFs are a family of dimeric isoforms: PDGF-AA, PDGF-BB, PDGF-C, and PDGF-D.³⁴ PDGF-BB is the isoform that binds both PDGF receptors, PDGF receptor alpha and PDGF beta, so it is considered the most universal of PDGF proteins.³⁵ PDGF was originally identified as a product made and stored in the alpha granules of platelets that is released into serum during blood clotting.³⁶ Although platelets are a major storage site for PDGF, PDGF is also produced by many other cell types, including macrophages, osteoblasts, and fibroblasts.³⁷ PDGF has many functions in target cells and has been shown to stimulate proliferation, control chemotaxis (directed cell movement), enhance cell survival, and induce differentiation, all functions that are integral components of the tissue repair process.³⁶,³⁷ It is not surprising then that PDGF also has many target cells including smooth muscle, pericytes, connective tissue fibroblasts, and mesenchymal stem cells (MSCs).³⁸ As such, successful use of PDGF as a bioactive factor is highly dependent on controlling where and when it acts during the healing cascade.

The current clinical product, recombinant human (rh) PDGF-BB, is made in yeast in a commercial process that first gained FDA approval for use in topical indications in soft-tissue wound healing (Regranex).³⁹ PDGF-BB has a short half-life when delivered without a carrier, but biologic activity can be extended by the use of a carrier such as tricalcium phosphate (TCP).⁴⁰ PDGF-BB interactions with specific carriers also prevent large amounts of growth factor from entering systemic circulation and initiating off-target effects. PDGF-BB has shown positive effects on healing in numerous animal studies including osteoporotic rat fracture healing, diabetic rat fracture healing, and rodent distraction osteogenesis.⁴¹ These preclinical successes led to multicenter trials to ascertain the safety and efficacy of using rhPDGF-BB in foot and ankle fusion surgery as an alternative to autograft. Based on clear efficacy, FDA approval of rhPDGF-BB applied using TCP as carrier for foot and ankle fusions was achieved in 2012. Currently, AUGMENT, the drug name for rhPDGF-BB, is available in two formulations: as an injectable containing rhPDGF-BB, beta-TCP particles, and collagen or as an implant consisting of PDGF-BB and beta-TCP.²¹ rhPDGF-BB has also demonstrated preliminary efficacy in clinical trials for periodontal repair.

There continue to be substantial efforts to expand the potential uses of rhPDGF-BB in tendon reattachment surgeries such as rotator cuff repair. However, successful enhancement of healing appears to be dependent on the specifics of the surgical model used in the study, the mode of rhPDGF-BB delivery, the concentration of PDGF-BB used, and the timing of factor application. For example, Condron et al⁸ concluded in 2021 that there was no difference in strength after treatment of animal/human rotator cuff with PDGF-BB. PDGF-BB has also been unsuccessful in driving fusion in the setting of spine surgery, although the reasons for this lack of success remain to be determined. Niemiec et al⁴² in a 2021 study noted that the presence of a polymorphism in the patient’s PDGF-BB gene was associated with effectiveness of PDGF-BB therapy, suggesting that gene sequencing before treatment may make a difference in the successful use of PDGF-BB for shoulder injuries. An additional concern for using PDGF-BB is the fact that overactivity of PDGF has been linked to certain malignancies and other disorders that involve an excess of cell proliferation, including atherosclerosis, and fibrotic conditions.⁴³