The human body possesses a tremendous healing potential. However, despite its innate restorative capacity, in many instances the body’s ability to heal is limited. Musculoskeletal tissues such as tendon, ligament, and cartilage present challenges to clinicians because these tissues tend to heal slowly as a result of their limited blood supply and slow cell turnover. Furthermore, conservative management or surgical intervention alone may not reliably recapitulate the normal architecture and function of the injured tissue. Therein lies the potential benefit of biologic therapy, in which the addition of growth factors and reparative cells may not only augment the normal body healing process but also restore normal form and function.
The use of biologic therapy has grown exponentially in the field of sports medicine in recent years. Although these emerging therapies may be based on solid preclinical evidence, they still need to be viewed in the same light as novel surgical devices and undergo judicious evaluation prior to widespread clinical use. This chapter reviews the basic principles and best available clinical evidence on two growing categories of biologic therapy in sports medicine: platelet-rich plasma (PRP) and stem cell therapy.
Platelet-Rich Plasma and Sports Medicine
The use of human blood concentrates to treat various tendon, ligament, and cartilage disorders and to augment surgical repairs has been growing within sports medicine. The availability of PRP and its autologous nature allow for easy clinical application without the risk associated with allogenic products. As a result, PRP has been used in several clinical studies for the management of pathologic conditions of ligament, tendon, bone, and cartilage ( Fig. 5-1 ). Although an extensive body of literature on PRP exists, a clear consensus for or against the use of PRP in the treatment of musculoskeletal diseases still eludes the medical community. What is clear is that not all PRP preparations are created equal, and our understanding regarding the various components of PRP along with its optimal composition, timing, and delivery, continues to be refined.
Definitions and Properties of Platelet-Rich Plasma
PRP is an autologous concentration of human platelets in a small volume of plasma produced from a patient’s own centrifuged blood. The concentrated platelets contain increased amounts of growth and differentiation factors, which can then be delivered to an injury site to augment the body’s natural healing process. The normal human platelet count ranges anywhere from 150,000 to 350,000 per µL. Improvements in bone and soft tissue healing properties have been demonstrated with concentrated platelets of 1,000,000 per µL, and thus it is this concentration of platelets in a 5-mL volume of plasma that has been suggested as one working definition of PRP. A resultant three- to fivefold increase in growth and differentiation factors can be expected with PRP compared with normal nonconcentrated whole blood.
Preparation and Composition
Currently more than 16 commercial PRP systems are available on the market, and hence quite a bit of variation exists in the PRP collection and preparation protocol depending on the commercial system being used ( Table 5-1 ). Each commercial system has a different platelet capture efficiency that results in different whole-blood volume requirements to achieve the necessary final platelet concentration for PRP. The commercial systems may also differ in their isolation method (one- or two-step centrifugation), the speed of centrifugation, and the type of collection tube system and operation. In general, whole blood is usually collected and mixed with an anticoagulant factor, such as acid-citrate-dextrose, sodium citrate, or ethylene diamine tetraacetic acid. Centrifugation is then required to separate the whole blood into a red blood cell fraction and a plasma fraction, which contains platelets, white blood cells (WBCs), and clotting factors ( Fig. 5-2 ). The plasma can then be further divided into platelet-poor plasma and PRP. Most systems then require the platelet to be “activated” via the addition of either calcium chloride or thrombin, which then causes the platelets to degranulate and release the growth and differentiation factors. Approximately 70% of the stored growth factors are released within the first 10 minutes of activation, and nearly 100% of the growth factors are released within 1 hour of activation. Small amounts of growth factors may continue to be produced by the platelet during the remainder of its life span (8 to 10 days).
|Separating System||Whole Blood Volume (mL)||Anticoagulant||Steps||Centrifuge Time (Min)||Final Volume of PRP||PRP Leukocyte Content Relative to Whole Blood|
|Autologous Conditioned Plasma (ACP) (Arthrex)||16||ACD-A, 1.5 mL||Single spin||5||3||Reduced|
|Cascade, Musculoskeletal Transplant Foundation (MTF)||9-18||Sodium citrate, 2 mL||Single spin for PRP, double spin for PRFM||6 (PRP) or 6+15 (PRFM)||4-9||Reduced|
|Gravitational Platelet Separation System (GPS III) (Biomet)||54||ACD-A, 6 mL||Single spin||15||3-12||Concentrated|
|Magellan (Medtronic)||20-60||ACD-A, 4 to 8 mL||Double spin||7||6||Concentrated|
|SmartPrep 2 (Harvest Technologies)||20-120||ACD-A, 8 mL||Double spin||15||3-20||Concentrated|
Platelets contain a milieu of growth factors and mediators in their alpha granules ( Table 5-2 ). The specific composition of PRP, however, likely varies not only from person to person but also when the isolation process is repeated in the same individual. Several elements are known to influence the specific makeup of PRP, which includes patient-specific factors and different commercial system preparation methods. The variability in the cellular composition of PRP creates challenges in interpretation of the literature regarding the clinical efficacy of PRP.
|Factor||Target Cell and Tissue||Function|
To date, the optimal composition of PRP and the relative importance of each of its individual components remain unclear. The recent literature has focused on the proinflammatory component of PRP, which appears to be linked to its leukocyte content. High-concentration PRP has an elevated leukocyte content, which is associated with elevated catabolic cytokines, such as interleukin-1β, tumor necrosis factor-α, and metalloproteinases. When placed in vivo, preparation systems that yield leukocyte-rich PRP resulted in a greater short-term soft tissue inflammatory response compared with leukocyte-poor PRP. The clinical ramifications of these different PRP preparations, including leukocyte content, remain unclear given that most clinical studies have not critically evaluated the specific cellular components of the PRP and have not correlated outcome measures to the specific composition of PRP.
Platelet-Rich Plasma in Tendon- and Ligament-Related Disorders and Repair
Tendon and Ligament Injuries
The area of orthopaedic surgery in which the efficacy of PRP has been most actively evaluated is tendon and ligament injuries. Tendon and ligaments heal through a dynamic process with stages of inflammation, cellular proliferation, and subsequent tissue remodeling. Many of the cytokines found in PRP are involved in the signaling pathways that occur during this restorative process. PRP may also promote neovascularization, which may not only increase the blood supply and nutrients needed for cells to regenerate the injured tissue but may also bring new cells and remove debris from damaged tissue. Both these mechanisms of action are particularly attractive in chronic tendinopathic conditions in which the biologic milieu may be unfavorable for tissue healing.
Clinical studies on the use of PRP for tendon and ligament injuries have mainly focused on the areas of the elbow, knee, and ankle. Studies on lateral epicondylitis suggest that PRP may be effective and a reasonable option for patients who have failed to respond to a therapy regimen. In a prospective cohort study, Mishra et al. evaluated 230 patients who failed to respond to at least 3 months of conservative treatment for lateral epicondylitis. Patients were followed up for 24 weeks. At 12 weeks, the patients receiving PRP reported a 55.1% improvement in their pain scores compared with a 47.4% improvement in the active control group ( P = .094). At 24 weeks, the patients who received PRP reported a 71.5% improvement in their pain scores compared with a 56.1% improvement in the control group ( P = .027). The percentage of patients reporting significant residual elbow tenderness at 12 weeks was 37.4% in the PRP group versus 48.4% in the active control group ( P = .036). At 24 weeks, 29.1% of the patients receiving PRP reported significant elbow tenderness compared with 54.0% in the control group ( P < .001).
In terms of the sustainability of treatment effect, PRP may provide longer continuous relief of symptoms for lateral epicondylitis than do corticosteroids. Peerbooms et al. evaluated the efficacy of PRP versus corticosteroids in 100 patients who had a minimum 6-month history of recalcitrant chronic epicondylitis and had failed to respond to conservative management. Treatment success within this study was defined as, at minimum, a 25% reduction in the visual analog scale (VAS) score or Disability of Arm, Shoulder, and Hand score without a repeat intervention after 1 year. Although both groups improved in VAS scores from baseline, 73% (37 of 51 patients) in the PRP group versus 49% (24 of 49 patients) in the corticosteroid group were considered to have a successful response at 1 year ( P < .001). Furthermore, 73% (37 of 51 patients) in the PRP group versus 51% (25 of 49 patients) in the corticosteroid group noted improved Disability of Arm, Shoulder, and Hand scores at 1 year ( P = .005). Patients who received PRP also continued to report symptom relief 1 year after receiving the injection. In contrast, the short-term benefits of corticosteroids began to wane after 12 weeks. In a separate report, the improvement within this group of patients who received PRP continued to be noted 2 years after the PRP injection.
In addition to the use of PRP to treat lateral epicondylitis, some encouraging results support the use of PRP to treat chronic refractory patellar tendinopathy. Gosens et al. evaluated the use of PRP in 36 patients with chronic patellar tendinopathy. Patients were subdivided into two groups: those who had undergone previous treatments for patellar tendinopathy (e.g., corticosteroids and surgery) and those who had not undergone any prior treatments. Although both groups of patients showed a statistically significant reduction in pain after PRP treatment, the group that had no prior treatments reported significantly better functional outcome relative to the group in whom previous treatments had failed (recalcitrant cases). Filardo et al. similarly reported the benefit of PRP injections for treatment of chronic refractory patellar tendinopathy. A statistically significant improvement in all scores (as measured by the Tegner score, EuroQol–VAS, and pain level) was observed at the end of the PRP injections, and further improvement was noted with the addition of physiotherapy at 6 months.
Compared with findings for lateral epicondylitis and patellar tendinopathy, the higher level data with regard to the use of PRP in persons with Achilles tendinosis have been less promising. In a prospective randomized trial, de Jonge et al. found no significant benefits with PRP versus a saline solution injection as an adjunct to eccentric exercises for mid Achilles tendinosis. The authors reported no significant differences in Achilles tendon structure, the degree of neovascularization, and clinical outcome compared with the saline solution group. In another randomized study, de Vos et al. similarly reported no significant benefit in terms of pain reduction, activity level, and tendon appearance on ultrasound at 1 year after injection of PRP for chronic Achilles tendinopathy.
Rotator Cuff Repair
Compared with studies relating to lateral epicondylitis, patellar tendinopathy, and Achilles tendinopathy, fewer high-quality studies have been performed on the use of PRP for tendon repair. Most studies in this area have focused on rotator cuff repairs. Some studies used a fibrin matrix as a carrier for the PRP, whereas others injected PRP directly into the repair site ( Fig. 5-3 ). Rodeo et al. reported their outcomes regarding the use of platelet-rich fibrin matrix (PRFM) in 79 patients undergoing rotator cuff repairs (small to large tears). The patients were randomized to receive either the PRFM at the rotator cuff tendon-bone interface (n = 40) or a standard repair without the matrix (n = 39). The authors reported no benefit with repairs augmented with PRFM in terms of tendon-bone healing rate (67% in the PRFM group and 81% in the control group, P = .2), rotator cuff strength, and the clinical outcome at 12 months. The patients’ preoperative platelet count, which may affect the quality of the PRFM, did not correlate with healing. Interestingly, additional regression analysis suggested that the PRFM might have a negative effect on healing (odds ratio: 5.8) in this study. Use of the fibrin matrix was a significant predictor for the presence of a rotator cuff repair defect at 12 weeks. In another randomized study, Castricini et al. similarly reported no significant benefit with PRFM for small (<1 cm) and medium-sized (1 to 3 cm) rotator cuff tear repairs. The authors found no significant differences between the PRFM and the control group in terms of the Constant score, tendon thickness, and repeat rupture rate. Finally, Randelli et al. evaluated use of PRP alone in a randomized control study of 53 single-tendon rotator cuff repairs. Patients in the treatment group (PRP, n = 27) reported short-term pain reduction and increased Simple Shoulder Test scores at early postoperative periods. In their subgroup analysis, patients who had undergone repairs of small-sized tears had lower repeat tear rates (9 of 16 patients or 40% vs. 12 of 19 patients or 52%) and increased external rotation strength at follow-up. Overall, several confounding factors may account for variability in the results between studies, including lack of standardization regarding nonsteroidal antiinflammatory drug use, repair techniques, and variation in the type of the PRP. A consensus regarding the use of platelet concentrates for rotator cuff repair is far from clear based on the available literature.
Achilles Tendon Repair
Data are limited on the use of PRP in the repair of acute Achilles tendon tears, and findings in the existing literature are conflicting. Sanchez et al. reported their experience in athletes undergoing an open suture repair of a complete Achilles tendon rupture. In six athletes, PRFM was added to the repair site. In this case control study, athletes who had undergone PRFM-augmented repairs recovered their range of motion earlier, took less time to take up gentle running (11 vs. 18 weeks, P = .042), and took less time to resume training activities (14 vs. 21 weeks, P = .004). Schepull et al. similarly evaluated the use of PRP in Achilles tendon repair in a randomized study (n = 30). Although no differences were reported with regard to tendon elasticity and heel raise index between the PRP and control groups, the authors did note a lower Achilles Tendon Total Rupture score among the PRP group, which suggests a detrimental effect of PRP on subjective outcome after repair.
Anterior Cruciate Ligament Surgery
The success of anterior cruciate ligament (ACL) surgery not only hinges on technical factors (e.g., graft tunnel placement and graft fixation) but also biologic healing of the ACL graft. Given its potential for improving tissue vascularity and ligament healing, PRP has been used to augment ACL graft maturation and graft–bone tunnel incorporation after reconstruction. Within the research literature, ACL graft maturation tends to be assessed with magnetic resonance imaging (MRI). The assumption is that a low homogenous intensity signal on T2- and proton density–weighted MRI is likely indicative of a healthy maturing ACL graft. In terms of the effect of PRP on ACL graft maturation, some studies have demonstrated improved graft maturation with PRP, whereas others reported a lack of significant differences. Authors of a recent systematic review of eight controlled trials, which included studies in which statistical significance was not reached, concluded that PRP likely improves ACL graft maturation by an average of 20% to 30%. The authors pointed to insufficient sample size as a potential rationale for lack of statistical significance despite a 20% to 30% improvement in some metrics measuring ACL graft maturation.
The other component to successful biologic healing of an ACL graft is graft–bone tunnel incorporation. The existing literature on the use of PRP to augment healing of the graft-bone interface is inconclusive at best. Vogrin et al. evaluated the effects of PRP gel treatment for hamstring autograft ACL reconstruction in a controlled double-blinded study. MRI was used after the operation to assess vascularization along the ACL graft-bone interface. The authors reported evidence of improved vascularization along the interface at 3 months with use of PRP, but the observed benefit dissipated by 6 months after the surgical procedure. Other studies have similarly reported limited to no evidence to support the use of PRP to augment ACL graft–bone tunnel incorporation.
One final point of consideration is whether any of the observed benefit of PRP on ACL graft maturation or graft-tunnel healing would translate into improved clinical results. The current best available evidence seems to suggest no significant benefit for functional outcome with use of PRP. Ventura et al. found no differences in Knee Injury and Osteoarthritis Outcome score (83 vs. 84 points), KT-1000 (0.8 mm vs. 1.2 mm), or Tegner scores (0.9 vs. 0.8 difference preoperative to postoperative) between the PRP-treated group and control subjects at 6 months despite reporting a significant difference in graft appearance. Orrego et al. similarly noted no significant benefit in both Lysholm and International Knee Documentation Committee scores at 6 months after the operation despite identifying a positive effect of PRP on graft maturation. In summary, evidence from the current literature suggests that PRP may improve the rate at which ACL grafts achieve a low signal on MRI T2-weighted imaging but have little to no effect on graft-tunnel incorporation. A demonstrable benefit in patient outcome after use of PRP in patients undergoing ACL surgery is also lacking.
Platelet-Rich Plasma in Cartilage Restoration
When considering biologic approaches to cartilage problems, it is important to understand that focal cartilage injuries differ from arthritis in terms of joint biology, homeostasis, and levels of metalloproteases and inflammatory cytokines. In this way, PRP application and clinical results may be quite different for either condition, and clinical reports are available on the use of PRP for treatment of both cartilage lesions and osteoarthritis. The idea of using PRP for cartilage repair is based on the expectation that growth factors released by the platelet alpha granules may promote cell survival and induce chondrocyte proliferation and subsequent matrix synthesis. Insulin growth factor (IGF), especially IGF-1, is considered one of the main anabolic growth factors for articular cartilage. IGF stimulates synthesis of integrins, type II collagen, and proteoglycans. IGF-1 also stimulates chondrocyte adhesion, improves tissue integration, and inhibits matrix degradation. Platelet-derived growth factor increases chondrocyte proliferation, but it seems to have more influence on meniscal cells than articular cartilage. Transforming growth factor (TGF)-αβ1 is one of the three isoforms of TGF-β and has a potent effect on chondrocytes and cartilage synthesis. On the other hand, the mechanism of action of TGF-β1 and its effects on synovial joint biology are not completely understood, because distinct differences exist between in vitro and in vivo behaviors. Also, in vivo, it is believed that TGF-β1 is released within the initial days after the injury, in contrast to delivery of IGF-1, which is long-lasting. Some in vitro studies demonstrate that TGF-β1 may antagonize IGF-1–induced glycosaminoglycan synthesis when applied concomitantly. PRP may also influence the conversion of human osteoarthritic chondrocytes by inhibiting the action of inflammatory cytokines such as IL-1 and nuclear factor–kB.
For focal cartilage lesions, Dhollander et al. reported a pilot case series of five patients surgically treated with autologous matrix-induced chondrogenesis combined with a PRP gel. The rationale for the PRP augmentation is to stimulate mesenchymal stem cell (MSC) migration to the microfracture site. The authors reported that although VAS scores for pain decreased, the lesion was not filled after 12 and 24 months based on MRI evaluations, and intralesional osteophytes developed in three of the five patients. Haleem et al. reported on five patients with femoral condyle full-thickness cartilage defects who were treated with culture-expanded autologous bone marrow–derived MSCs combined with platelet-rich fibrin glue as a biologic scaffold. The authors described complete cartilage filling on MRI in three patients and partial filling in the other two patients. Very few clinical reports and no clinical trials have evaluated the use of PRP for treatment of focal cartilage lesions.
Most clinical reports on the use of PRP for cartilage problems involve patients with degenerative arthritis. Kon et al. described improvement in functional scores and VAS pain scores after intraarticular PRP injections in 100 patients with degenerative articular cartilage lesions. The results were stable from the end of the three-injection cycle up to 6 months but worsened at 1-year and 2-year evaluations. The same group also reported a comparative study between PRP and hyaluronic acid that showed similar clinical results. The authors described a trend favorable to PRP in patients with low-grade articular degeneration (Kellgren-Lawrence score up to 2). Patel et al. performed a prospective randomized trial comparing single- or double-injection WBC-filtered PRP with saline solution in 78 patients with early osteoarthritis. They concluded that a single injection of PRP was as effective as a double injection. They also reported that the results deteriorated after 6 months. At present, the effectiveness of PRP in the treatment of generalized arthritis is still not well established, and the sustainability of any benefit is likely short term.
Platelet-Rich Plasma in Meniscal Repair
The idea of augmenting meniscus repair with growth factor is not new. In 1988, Arnoczky et al. proposed the use of an exogenous fibrin clot to stimulate a reparative response in the avascular portion of the meniscus. In 1990, Henning et al. reported that the failure rate for an isolated meniscus tear repair was 41% without a fibrin clot versus 8% with a fibrin clot. Bhargava et al. demonstrated that platelet-derived growth factor increases the number of cells and tissue formation in a meniscus defect explant model. Several other in vitro and in vivo studies have demonstrated the potential for cytokines found in PRP to improve meniscal cell growth and meniscus repair healing. Ishida et al. performed a combined in vivo and in vitro study demonstrating that PRP led to improved meniscal repair in full-thickness 1.5-mm diameter defects in the avascular zone of the rabbit meniscus.
Augmenting meniscal repair with PRP is logical because it may deliver growth factors to the healing tissue. Despite being an appealing approach, at present no clinical studies have evaluated the routine use of PRP during meniscus repair. However, tissue healing is a highly complex biologic process that involves precise coordination of numerous signaling molecules to recreate the complex structure and composition of the meniscus. For this reason, it is likely that application of a single exogenous factor may not mimic the highly coordinated spatial and temporal expression of all involved factors required to direct proper cell proliferation, matrix synthesis, and tissue remodeling.
Platelet-Rich Plasma for Muscle Injuries
The use of PRP in the treatment of muscle injuries has attracted a significant amount of interest in recent years. Similar to tendon healing, the steps in muscle healing involve the initial inflammatory response, which is then followed by cell proliferation, tissue remodeling, and regeneration of intramuscular nerve branches. Several animal studies have shown efficacy of the use of PRP for improved healing of muscle injuries. However, human studies evaluating the use of PRP in the treatment of such injuries remain limited at this time. In one retrospective case series, Wright-Carpenter et al. reported on the treatment of muscle injury in 18 professional athletes with autologous condition serum (ACS). Unlike PRP, ACS involves not only harvesting autologous blood but also physically and chemically stimulating the blood to increase growth factor concentration within the blood prior to injection. The authors found a significant decrease in injury recovery time in the ACS group (16.6 ± 0.9 days) compared with the control group, who were treated with Actovegin/Tramuell (22.3 ± 1.2) ( P = .001). MRI analysis supported the finding of improved recovery in the ACS treatment group. The authors concluded that ACS injection is a promising approach for the reduction of muscle injury recovery time. Of note, a randomized, single-blinded controlled clinical trial evaluating PRP in the treatment of grade II hamstring muscle injuries is currently underway.