Platelet-rich Plasma: The Path Forward

Proof for effective, targeted use of blood-derived biologics still trails their popularity, but knowledge is accumulating

For just about every orthopaedic condition, there is a platelet-rich plasma (PRP) treatment that purports to remedy it. Yet evidence of efficacy for PRP and other blood-derived formulations remains elusive for many applications. During the AAOS 2018 Annual Meeting symposium titled “Use and Misuse of Biologics,” Scott A. Rodeo, MD, of Hospital for Special Surgery, surveyed the current landscape of “indiscriminate and sometimes inappropriate use of biologics” and provided an overview of what is known about these agents and the questions that remain to be answered.

Dr. Rodeo cited six factors driving the popularity of PRP and its biologic cousins:

  1. consumer demand, with patients frequently asking about and requesting these approaches
  2. aggressive marketing, contributing to the demand
  3. a low regulatory bar for many of these agents
  4. the autologous nature that makes many approaches largely safe
  5. some early, positive data demonstrating symptom modification (but few data for structure modification)
  6. a lack of effective therapeutic alternatives for many soft-tissue injuries

“There is little regulation and some early positive data, and we don’t have great solutions as clinicians for some of our patients’ problems. This creates a perfect storm that leads patients to seek these various treatments,” Dr. Rodeo said.

In sorting through the possible uses of PRP, “the primary issue is the significant variability in outcomes, which relates to the variability in different PRP formulations,” Dr. Rodeo said. Among the formulations under the PRP umbrella are pure-platelet-rich plasma, platelet-leukocyte-rich plasma, pure-platelet-rich fibrin, and leukocyte- and platelet-rich fibrin. Other blood derivatives, including autologous conditioned serum and autologous protein solution (APS), have therapeutic potential as well.

The critical factors to be considered in PRP, or any blood-derived product, are leukocyte content, whether platelets are activated, fibrin content, and the “hundreds of proteins contained in plasma outside of those in platelets,” Dr. Rodeo said.

He explained that PRP works by activating cellular pathways in fibroblasts. Pathway analysis has shown that the most highly induced signaling pathways in PRP-treated tendon fibroblasts are TNFα and NRκB, “two fundamental inflammatory pathways,” Dr. Rodeo said. Researchers have found that more than 3,000 genes are related to these and other pathways, suggesting that PRP probably acts by inducing a transitory inflammatory event, which then triggers tissue regeneration.

Taking aim

To determine how to use PRP, “we need to identify biologic targets. What are we treating? What is the goal?” Dr. Rodeo asked. Among the therapeutic goals to be considered (or not) are to increase vascularity, stimulate cell migration, decrease inflammation, accelerate matrix remodeling, attract local or distant progenitor cells, decrease proteolytic degradation of provisional fibrin clot scaffold, and produce nociceptive mediators (reduce pain).

“All of these are potential goals, but they are all very different,” he said. “Identifying the ideal biologic target will ultimately allow matching of the specific PRP formulation to that target,” Dr. Rodeo said.

This objective requires identification of the underlying cellular and molecular mechanisms of tissue degeneration and healing. Dr. Rodeo noted that these will differ among conditions, such as chronic tendonosis versus acute tendonitis and arthritis.

“These are all different biologic milieux,” he said. “As we consider them, we need to rigorously define the composition and biological activity of various PRP formulations. Only then can we match the PRP type with the desired biologic target. Clearly, one size does not fit all. The same PRP preparation would not be expected to have the same [positive] effect on diverse types of tissues, an acute versus chronic process, in male versus female, and in younger versus older patients.”


At the AAOS 2018 Annual Meeting, Scott A. Rodeo, MD, said the goal is to identify “the ideal biologic target” to allow matching of the specific PRP formulation to that target.

To “identify what we are putting into the patient,” Dr. Rodeo said, “we need to identify sentinel markers to characterize a dose of PRP or other biologics, including stem cells. We need to standardize assays and methods used to quantify the composition and biologic activity of these agents.”

“A quantifiable potency test” also is needed. This might involve gauging a secretion of a defined factor or a specific set of genes, microRNAs, or proteins. In addition, he said, “We need quality-control criteria that are accepted across disciplines and across international boundaries. If you plan to incorporate biologic approaches in your practice, I would encourage you to save a small aliquot of the material given to the patient for later analysis.”

The goal is to have reliable predictors of outcome; that is, do the composition and biologic activity of the material implanted in the patient predict the clinical/imaging outcomes?

What do we know?

To get an idea of “where we are today,” Dr. Rodeo reviewed all the meta-analyses and systematic reviews covering PRP in major journals as they pertained to six musculoskeletal conditions. His “bottom lines” for each of these were:

  1. Tendinopathy: A positive effect is seen for leukocyte-rich PRP systems compared to leukocyte-poor PRP.
  2. Rotator cuff repair: A positive effect is seen in some studies using leukocyte-poor PRP; leukocyte-rich PRP and platelet-rich fibrin matrix are not effective.
  3. Anterior cruciate ligament surgery: PRP does not affect graft-tunnel healing; PRP may accelerate graft maturation; there is no effect on clinical outcomes.
  4. Knee osteoarthritis (Fig. 1): Leukocyte-poor PRP has a positive effect on symptoms, not structure.
  5. Meniscus (Fig. 2): Arthroscopic repair with platelet-rich fibrin matrix has no effect at two years; in open repair of horizontal tear, the PRP group was better for Knee Injury and Osteoarthritis Outcome Score (KOOS) subscales for pain and sports.
  6. Hamstring injury: One of three randomized clinical trials showed beneficial effect of PRP over control for full recovery significantly earlier than physiotherapy alone; no other study found any significant difference between PRP and autologous blood, placebo injection, or platelet-poor plasma.

Dr. Rodeo said that clinical data regarding the effects of PRP on muscle are inconclusive. One laboratory study found that removal of platelets stimulated myoblast differentiation, which is necessary for skeletal muscle regeneration. A rat study found no synergistic effect between daily exercise training, as in rehabilitation, and PRP therapy. “This may explain why PRP therapies have failed in clinical trials where athletes have adhered to post-injection early active mobilization protocols,” he said.


Fig. 1 A platelet-rich fibrin matrix is prepared for insertion into the knee through an arthroscopic cannula.
Courtesy of Scott A. Rodeo, MD

Addressing next-generation approaches for improving PRP, Dr. Rodeo said possible modifications include removing undesirable proteins and neutralizing undesirable proteins by combining PRP with other agents. To address the issue of variability, lyophilized PRP could provide a product with defined growth factor in an off-the-shelf formulation. Also, it would be helpful to identify factors that affect the “quality” of PRP, including time of day, recent meals, and recent activity.


Fig. 2 Arthroscopic view of placement of a platelet-rich fibrin matrix at the site of meniscus repair.
Courtesy of Scott A. Rodeo, MD

Research on next-generation preparations included one study based on the knowledge that myostatin and TGF-β1 are detrimental for muscle regeneration. In the laboratory, Jason Dragoo, MD, and associates at Stanford University removed TGF-β1 and myostatin from PRP using antibodies attached to sterile beads. “Although there was minimal effect on myoblast differentiation, this demonstrates ‘proof of principle’—of modifying PRP to take out the bad stuff,” Dr. Rodeo observed.

Johnny Huard, PhD, and colleagues at the University of Pittsburgh treated mice with PRP and losartan, which has an antifibrotic effect. They found that muscle regeneration and muscle function were significantly better in the combined PRP plus losartan treatment group, probably due to reduced tendency toward fibrosis, suggesting that blocking the expression of TGF-β1 with losartan improves the effect of PRP therapy on muscle healing after a contusion.

Another study examined lyophilized PRP powder that was prepared with 12 pooled platelet concentrates derived from different donors. The investigators detected elevated concentrations of growth factors VEGF, bFGF, PDGF-AB, and TGF-β1 compared to whole blood.

“Pooled PRP by means of lyophilization may allow physicians to apply a defined amount of growth factors by using a defined (off-the-shelf) amount of PRP powder,” Dr. Rodeo said.

In a study to address the possible effect of nonsteroidal medications on PRP, researchers assessed the effects of a COX-2 inhibitor on platelet activation and anabolic growth factor release from canine PRP. No statistically significant effects of COX-2 inhibitor on platelet activation or on concentrations of TGF-β1, PDGF-BB, or TXB2 were seen. “This suggests that there is no need to withhold a COX-2 inhibitor before PRP preparation, particularly if thrombin is going to be used to activate the PRP,” Dr. Rodeo said.

Finally, he discussed APS, which is approved for use in Europe and South Korea but not in the United States. Prepared through a process that isolates platelets and white blood cells and concentrates them along with plasma proteins, APS contains a high concentration of anti-inflammatory cytokines and anabolic growth factors. In a study of patients with knee arthritis, it yielded no benefits at six months, but at 12 months, the APS group had better improvement on Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) pain score and some improvement in bone marrow lesion size. “APS seems to have some promise,” Dr. Rodeo said, noting that this is in the investigational stage in the United States.

Summary

Dr. Rodeo closed with the following points:

  • PRP does have promise, but further refinement is required.
  • The biologic target needs to be identified. What are we trying to treat?
  • Standardized assays and methods to quantify the composition and biologic activity of these agents are needed.
  • A quantifiable potency test and quality-control criteria that are accepted across clinical disciplines and international boundaries also are needed.

Terry Stanton is the senior science writer for AAOS Now. He can be reached at tstanton@aaos.org.

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