At the 2022 Equine Regenerative Medicine and Orthobiologics Summit (ERMOS), many experts in the field presented information on pathophysiology of orthobiologic processes, and the benefits to healing.
Lisa Fortier, DVM, PhD, DACVS, discussed how the inflammatory process can work for or against wound resolution. Wound healing takes the form of three phases, she explained. Inflammation of an injury elicits cell proliferation. Then the matrix remodels, leading to cell differentiation and resolution through anti-inflammatory cytokines, T cells, and growth factors, all of which are important for functional tissue regeneration. Fortier remarked that “just because the wound doesn’t look nasty or hot doesn’t mean there isn’t still on-going inflammation.”
However, with prolonged inflammation from extensive tissue damage or a failure of resolution, the wound undergoes scarring, fibrosis, and dysfunctional repair. To achieve resolution, she stressed the necessity of a balance of regulatory T-cells and macrophages along with pro-inflammatory macrophages. She explained the varied responses of interleukins within wounded tissue:
- IL-1 doesn’t tend to stick around.
- IL-6 tends to stick around and stimulates macrophages and pro-inflammatory cytokines. IL-6 is a biomarker for persistent inflammation in synovial fluid and is less likely to respond to treatment and time.
- IL-10 is secreted from M2 macrophages to cause non-functional tissue healing.
- IL-4 elicits metalloproteinases.
- IL-17 causes tissue damage.
Orthobiologics to Treat Osteoarthritis
Using osteoarthritis (OA) as an example, she stressed that OA is not actually a wear-and-tear disease. Instead, there is usually immune-mediated inflammation based on the presence of cytokines. A preceding traumatic incident might lead to post-traumatic OA (PTOA), which has the same pathogenesis as age-acquired OA.
Normally, T-regulatory cells are in balance with the secretion of IL-10. But with PTOA, increases in IL-17 levels exceed IL-10 levels. Cells undergo a phenotypic switch that secretes both IL-17 and IL-10. This creates a target in the joint capsule and synovium to switch them back to secreting only IL-10. A potential treatment target is neutrophil activation from IL-17. Fortier remarked that if you can balance the immune system within a joint, then it is possible to mitigate “wear-and-tear” destruction.
Corticosteroids are potent anti-inflammatory drugs but do not provide growth factors or anabolic reparation, whereas orthobiologic approaches do. Orthobiologic materials affect chondrocytes, synovial fibroblasts, osteoblastic macrophages, T-cells, and neutrophils.
PRP (Platelet-Rich Plasma)
Goodrich noted that platelet-rich plasma (PRP) is not entirely without negative effects due to the presence of pro-inflammatory cytokines. However, PRP provides positive growth factors. Platelet concentration is positively correlated with repair. In contrast, white blood cell concentration is negatively correlated with repair due to release of metalloproteinases and inflammatory cytokines from neutrophils. Ideally, treatment is best accomplished with leucocyte-poor, platelet-rich plasma.
Jose Garcia Lopez, VMD, DACVS, DACVSMR, Associate Profession of Large Animal Surgery at the University of Pennsylvania veterinary school, presented information about actions by blood-derived orthobiologics. He noted that studies with PRP often lack strong clinical evidence due to bias and variability. While 2.5-6 times platelet concentration is ideal, a concentration of 10 times might lessen healing. In one study, leucocytes did not significantly affect the clinical outcome. Similar improvement in 192 cases of human knee osteoarthritis after 12 months was achieved by giving 5 mg PRP weekly for three treatments. Several injections gave better benefits than a single treatment.
Another study looked at PRP versus steroid treatment (three injections weekly) after 12 months. There was a high variability in the processing method for PRP. However, PRP achieved better results than corticosteroids, and the leucocyte-rich platelets enhanced improvement compared to platelets with low leucocyte concentrations. This contrasts with other studies regarding the benefit or not of leucocytes in the PRP preparation.
Comparing the use of PRP to hyaluronic acid, Garcia Lopez reported that PRP improves the clinical outcome. In this case, less leucocyte concentration did better than abundant leucocytes.
Efficacy still needs to be proven in treating joint disease with PRP. By 24 hours post-injection, growth factors and cytokines return to pre-injection levels. Thus, it is better for a patient to receive multiple injections such as one treatment weekly for three weeks rather than a single treatment.
IRAP (Interleukin-1 Receptor Antagonist Protein)
IRAP (interleukin-1 receptor antagonist protein or autologous conditioned serum) blocks IL-1 from attaching in the joint to significantly decrease inflammation (prostaglandin E2). Immunoglobulins normalize in the joint to a more anabolic state. A metastudy demonstrated that IRAP can decrease pain and improve function in human knee OA.
An equine study of 40 osteoarthritic horses treated with autologous protein solution (APS or Pro-Stride) achieved significant improvements at 12 and 52 weeks in a subjective analysis. APS treatment was compared to hyaluronic acid and triamcinolone in a canine study of the coxofemoral OA in 23 dogs over six months. There was no difference between groups although both improved. If treatment is started early in the disease, APS is associated with significant pain improvements.
Mesenchymal Stem Cells (MSC)
Mesenchymal stem cells secrete extracellular vesicles, chemokines, cytokines and growth factors. They are able to turn pro-inflammatory monocytes (M1) into M2 reparative monocytes.
Bone-marrow-derived MSCs significantly decrease prostaglandin E2 compared to adipose-derived MSCs.
Many studies have investigated the use of intra-articular MSCs for managing microfractures and chondral defects. Garcia-Lopez reported some results:
- There was no clinical significance with injected MSCs but arthroscopically, tissue appeared firmer.
- MSCs yield a positive effect on wear lines in joint treatment.
- Positive effects are seen with MSCs at 6 and 18 weeks post-injury.
- Treatment of stifle meniscal injury in a group of 33 horses: Those that had surgery and MSC fared better than surgery alone.
- Another stifle meniscal study of 76 sport horses showed no association of long-term outcome with use of orthobiologics. Return to use was achieved in 86%; return to previous level in 40%; and to a lower performance level in 46%.
Adding MSC to PRP can generate a fibrin scaffold in chondral defects due to increased GAGS and collagen-2 concentrations. Combining this with rehabilitation protocols further improves the outcome.
The Bottom Line for Orthobiologics
In general, Garcia-Lopez noted that orthobiologics might have a positive role in enhancing surgical repair or might serve as a scaffold for full-thickness cartilage and chondral bone lesions. He quoted a concept from the American Medical Society for Sports Medicine that is worth considering: “Responsible use of orthobiologics and other novel regenerative treatments requires multiple considerations before translation into routine clinical practice.” He concluded, “Uniform clinical evidence regarding the effectiveness of orthobiologics in osteoarthritis treatment is lacking.” More research might yield answers going forward.