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Sports Medicine and Rehabilitation

Platelet Rich Plasma: Postprocedural Considerations for the Sports Medicine Professional

Morey J. Kolber, PT, PhD, CSCS*D, Joseph Purita, MD,2 Christian Paulus, Dr. med,3 Jeremy A. Carreno, and William J. Hanney, DPT, PhD, ATC, CSCS Department of Physical Therapy, Nova Southeastern University, Fort Lauderdale, Florida; Institute of Regenerative Medicine, Boca Raton, Florida; American Academy of Regenerative Medicine, Lakewood, Colorado; and Doctor of Physical Therapy Program, Department of Health Professions, University of Central Florida, Orlando, Florida

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ABSTRACT


Owing to a growing interest in treatments that use the body’s innate healing mechanisms, sports medicine professionals are likely to encounter individuals with musculoskeletal injuries who received platelet-rich plasma (prp). this column presents strategies that foster recovery and harness the regenerative potential of prp. evidence underpinning the impact of loading biological tissues is presented to guide safe and efficacious exercise prescription. a companion article in this issue discusses the science and evidence surrounding prp.

INTRODUCTION

P latelet-rich plasma (PRP) is an autologous biological treatment that involves processing whole blood to obtain concentrated cells (primarily platelets) that are then injected directly into or in the proximity of a degenerated or injured anatomical structure (e.g., tendon, joint, muscle, and ligament) (64). Owing to an excellent safety profile and a growing body of scientific literature, it is easy to understand the widespread interest in PRP among practitioners in the sports medicine community (64,86). Enthusiasm for biological treatments that harness the body’s own healing mechanisms is not limited to the medical community, as media coverage of high-profile athletes who have received PRP for sports injuries has led to a growing public interest (5,35,64). Although information is available to the public, the accuracy and readability of such information may be limited, particularly regarding the internet (35). Despite a growing body of favorable outcomes–based research, a paucity of evidence exists to identify a consensus approach for the management of patients who have undergone a PRP injection (61,70,80). Nevertheless, professionals charged with managing patients or clients following a procedure should be familiar with relative precautions and understand the need for appropriately incorporating both rest and progressive physical activity into the postprocedural phases of healing. Sports medicine professionals, armed with an understanding of both musculoskeletal disorders and the regulatory elements of tissue loading, are in an ideal position to provide targeted exercise programming that augments the efficacy of PRP. The purpose of this column is to present considerations that may guide sports medicine and rehabilitation professionals working with patients or clients, hereafter referred to as “individuals,” who have undergone a PRP injection. The term “considerations” is used in lieu of guidelines when discussing management strategies, as conclusive clinical practice guidelines (from an authoritative body) and evidence to steer postprocedural management are currently in the infancy stage (80). This column, grounded in the available albeit limited evidence, covers the range of postprocedural considerations including use of medications, movement or exercise, and appropriate rest. Given the heterogeneity of musculoskeletal injuries, information presented follows general guidelines gleaned from individual research studies as well as the authors’ personal experience treating patients following PRP procedures. Although the content is written for the broad readership, scope of practice should dictate boundaries with these recommendations. Clearance from the physician who performed the PRP procedure is an absolute requirement before initiating any type of intervention or exercise programming. It should be recognized that although there are more common diagnoses being treated with PRP, each patient is unique and only those with appropriate licensing to provide such services should perform specific programming.

OVERVIEW OF BASIC SCIENCE AND EVIDENCE

Although the basic science, biological and clinical evidence, as well as procedures are discussed in a companion article in this issue, a brief overview is necessary to establish content fluency. The natural healing process of soft tissue entails 3 overlapping stages referred to as the acute, proliferative, and remodeling phases. After injury, blood cells arrive at the affected area and trigger numerous responses. Blood cell actions range from inflammation and clotting to the release of signaling molecules that foster repair (5,34). Although the phases of healing ultimately result in repair or regeneration under the desirable microenvironment, one must recognize that different injuries or circumstances may lead to an inadequate, altered, or failed response. A PRP procedure involves processing one’s own blood (autologous) to produce supraphysiologic concentrations of growth factors, cytokines, and chemokines (5,34). The concentrations produced with PRP are greater than those present through the normal healing process. Specifically, the blood is processed predominantly to isolate platelets, which contain essential growth factors including but not limited to platelet-derived growth factor that works to establish a blood supply through neogenesis (new vessels) and insulin-like growth factor (IGF) that promotes the synthesis, proliferation, and differentiation of cells. The purported benefit from PRP extends beyond growth factors, as tissue healing may be attributed to a myriad of signaling molecules (chemokines and cytokines) that attract stem cells to the injured area and serve to mitigate inflammation (4,5). After processing, the final PRP product is then injected into the injured region or structure with the goal of resolving the inflammatory response and promoting tissue regeneration (64). The in vitro evidence for PRP is based on coculturing cells in a PRP solution. A clear body of evidence has shown that tenocytes, myoblasts, chondrocytes, and the less differentiated fibroblasts and adult stem cells respond favorably to PRP coculturing (6,25,52,62,66,88). A systematic review of 8 in vitro studies identified increased cell proliferation and growth factor expression of tenocytes cultured in PRP (6). Furthermore, an in vitro investigation of myoblasts cultured in PRP resulted in increased cell proliferation and differentiation (52,62). Regarding cartilage, PRP has been shown to induce chondrogenesis of mesenchymal stem cells, and studies of osteoarthritic cartilage have indicated that PRP exposure leads to reduced apoptosis and increased proliferation of chondrocytes (65,66). The clinical evidence shows a relatively good safety profile with swelling, erythema, and pain as the primary adverse effects of PRP (16,84). PRP has shown comparable or superior long-term effects to corticosteroid injections and viscosupplementation for improving function and decreasing pain among individuals with various musculoskeletal disorders (osteoarthritis [OA] and tendinopathy) (7,24,31,33,36,75,78,85,87). Much of the evidence in favor of structural healing or repair comes from case series investigations as opposed to larger comparative trials. Given the lack of a comparison group with case series investigations, causation is difficult to conclude. Nevertheless, current research on PRP is conclusive of a noninferiority outcome and a greater safety profile than some of the more common interventions (e.g., corticosteroid injection or opiates). Aside from the cost of PRP, which has been estimated to range from $500 to 1,500 per injection (cost to patient) (86), there seems to be little downside to PRP. A more detailed overview of the evidence underpinning the use of PRP may be found in a companion article in this issue of the Strength and Conditioning Journal.

POSTPROCEDURAL CONSIDERATIONS

Individuals who have received a PRP injection require special consideration with respect to the underlying injury, premorbid activity level, and time interval since the injection. Sports medicine professionals who possess an understanding of the basic and clinical sciences of PRP and musculoskeletal injuries are in an ideal position to guide individuals through the recovery process and return to premorbid activities. Given that individuals may seek the services of a sports medicine or rehabilitation professional at various stages after a PRP injection, an awareness of the evidence underpinning activity return, use of nonsteroidal anti-inflammatory drugs (NSAIDs), and specific loading strategies is critical for the pursuit of favorable outcomes. After a PRP injection, return to activity considerations should commensurate with the standard phases of healing. Using a progression from the acute to proliferative and remodeling phases as a guide is a reasonable approach given the known biological effects of PRP and expected clinical symptom sequelae. These stages, as well as the underlying injury and severity (e.g., tendinopathy versus tear), should dictate the nature of progression and expectations. This section presents an overview of progression from the acute to remodeling phase (Tables 1 and 2). Although evidence-based, variable protocols exist for managing individuals with tendinopathy following PRP. Unfortunately, there is an absence of literature regarding optimal programming for degenerative joint disease (e.g., OA) and discogenic pathology. Given the paucity of conclusive evidence for managing individuals after a PRP injection, information presented in this article is derived from previously published protocols (14,21,27,36,51,73,75), biological plausibility (based on known cellular responses to loading), as well as the authors’ clinical experience.

EARLY ACUTE STAGE (0–3 DAYS)

Immediately following a procedure, it is common to experience swelling, erythema, and increased pain (16). Furthermore, many individuals who receive a PRP injection have had clinical signs and symptoms for a variable period before the injection. Moreover, most individuals have been recalcitrant to previous conservative treatments. Thus, the patient will not be pain-free after the injection and may have increased pain and swelling until the potential PRP-induced inflammatory symptoms resolve. At the early postprocedural stage, protection of the treated area, active rest, and management of the acute symptoms are the goal (41,61). Patients are encouraged to resume use of previously prescribed protective measures such as bracing or ambulatory aides. The decision to immobilize a joint is individualized and based on injury severity and location, as well as the patient’s typical activities of daily living. In the early acute stage, the physician administering the PRP injection will provide specific instructions for immediate and early care. Given the absence of specific guidelines from the literature, material presented in this section will be largely drawn from the authors’ experience, individual outcome–based studies, biological evidence, and previously published narrative discussion articles (3,21,28,41,51,75,80). Nevertheless, the physician who provided the injection should be consulted before engaging in any activities at this stage. Generally, in the early acute stage, the joint is protected for up to 3 days. Protection may be in the form of partial weight-bearing, use of sling or motionrestricted walking boot, or specific instructions to avoid use. Complete rest is discouraged, and active movements of the affected body part should be performed a few times a day to avoid the deleterious cellular effects of immobilization. The decision to limit weight-bearing is based on the patient’s presentation before the injection and the diagnosis. For example, a PRP injection to the plantar fascia may interfere with pain-free ambulation for a few days leading to the use of a restricted motion walking boot or partial weight-bearing. After the third day, a gradual increase in activity is encouraged, provided movements are tolerable (pain during movement that ceases after completion). Individuals with restricted weight-bearing are encouraged to progressively increase walking (duration and distance), and a progressive loading program is initiated. Failure to move the involved body part or weight-bear through the extremity may result in impairments that perpetuate or compound the pathology being treated. Similarly, overzealous activity will be counterproductive and may prolong the acute inflammatory phase. During the early and late acute stages, patients may be provided with medications to reduce pain and interfere with inflammation. Although there is no consensus on the use of medication, approximately 42% of physicians limit the use of NSAIDs in some manner, despite limited evidence of these medications interfering with PRP outcomes (61,80). Alternative medications that may be used include narcotics (e.g., Percocet) and acetaminophen (e.g., Tylenol) (80). Corticosteroids are strictly discouraged because they are associated with reduced chondrocyte and tenocyte proliferation as well as cellular apoptosis (11,17,68). Furthermore, corticosteroids are likely to interfere with the inflammatory phase, which may preclude regeneration. Moreover, in vitro evidence suggests that adding a corticosteroid to a PRP preparation may reduce cellular proliferation (17). Regardless of the evidence, patients or clients should consult their physician before taking any medications. Cryotherapy (e.g., ice) may be used to reduce pain and inflammation; however, a concern is that ice may reduce platelet activation and interfere with the intended inflammatory cascade after PRP (61,80). Regarding ice, evidence suggests that hypothermia offers a protective effect of cartilage after trauma (76) and may reduce subacromial bursal thickness (72). Moreover, there is no published evidence that implicates ice as a factor that may hinder PRP outcomes. Given that no study has demonstrated an inferior result from ice and studies have used ice after a PRP injection with positive outcomes (51,75), there is no conclusive evidence to preclude use.

LATE ACUTE AND EARLY SUBACUTE PHASE (DAYS 4–14)

After the early protective stage, stretching and isometrics are introduced at the affected regions. Isometrics have been shown to beneficially affect tendon healing, and evidence suggests that unloading of muscle activity (through Botox) impairs the regenerative effects of platelets (44,83). In cases where contractile tissue is being treated (e.g., tendon and muscle), the isometric efforts are submaximal. If the treated pathology is noncontractile tissue such as ligament or capsule, then the isometric contractions may be maximal effort. Isometrics should be repeated multiple times per day with 10-second holds working toward longer durations of up to 2 minutes by the subacute phase if tolerated. Activities such as cycling, deep-water running or water walking, and upperbody ergometry are introduced and advanced based on tolerance. In cases where a PRP injection is administered to treat OA, progressive and repetitive loading of the joint is encouraged. Activities such as low-resistance cycling and high-repetition heel slides (Figure 1) should be performed multiple times a day. A progressive increase in overall activities is pursued until the conclusion of week 2, when more pronounced advancements could be made. Before progression into the next phase, symptom tolerance to exercises must be gauged. The presence of some discomfort or pain during exercises is generally acceptable, provided symptom resolution occurs on completion. Pain or discomfort that occurs during exercises and remains elevated on completion suggests the need to evaluate and potentially regress programming.

SUBACUTE PHASE (WEEK 3–6)

The subacute phase is a key milestone, as this is where the preinjection heterogeneity has a considerable influence. Specifically, the variability of clinical presentation before the PRP procedure will influence progression. Although those who were functioning at a high level (sport participation or weight lifting) before injection will be placed on a more progressive routine, others will follow a more conservative approach. In consideration of safety, a more conservative approach is presented. The objective at this phase is to ensure that the cointroduction of mechanical forces is a primary component of the management strategy. The introduction of new exercises (and advancement of previously prescribed activities) that have been associated with regenerative changes in pathological tissue, upregulation of growth factors, and stimulation of cellular tissues is the overarching goal. Tables 1 and 2 highlight the associated exercises and progressions. At week 3, isometric exercises are advanced to full effort for all conditions and increased hold times are recommended (full or reduced effort to failure if tolerated). Although isometrics may be considered easy for some, continuation is recommended, given the biological benefits (anabolic) previously discussed. Eccentric-focused muscle actions are first introduced within a limited tissue range and ultimately advanced to full range consistent with published protocols (27,51,73). Studies using eccentric loading after PRP range in performance frequency from 3 times a week to twice daily (2,19,21,26,51,73,79). The authors of this article generally base frequency on premorbid conditioning level with a frequency of 3 times a week up to once daily. Isotonic strengthening is introduced based on impairments, and joint loading is gradually increased from protected to full weight-bearing. An elliptical trainer may be used for lower extremity pathology, and squatting on an incline device to unload the joints is encouraged (e.g., Total Gym), particularly in cases of OA or cartilage pathology. In some published investigations, individuals with focal tendinopathy or muscle strain have been progressed in a more aggressive manner after a PRP injection. For example, a case-control study of professional football players with a grade 2 (average grade) hamstring strain reported initiating a stretching program immediately and commencement of resistance training within the first week (75). In another study, patients with patellar tendinosis began a progressive eccentric loading program performed 3 times a week at the onset of week 2 (51). In the study, eccentric loading of the patellar tendon was gradually progressed with knee flexion limited to 458 for 1 week, which was progressed to 608 until week 6. At week 6, flexion was progressed to 908 with a decline board initiated shortly afterward (Figure 2). In another study of patellar tendinopathy, athletic participants were allowed to resume premorbid activities within a week after their first PRP injection (32). Boesen et al. (14) initiated a twice-daily eccentric loading program and allowed 5 minutes of running within the first week after PRP injection for patients with Achilles tendinopathy. In contrast to the aforementioned studies, a case report of a patient with a partial distal triceps tear used 2 weeks of rest before any exercise-based intervention and initiated eccentric loading of the triceps at week 6 (21). The above studies highlight the variability of programs and the need to individualize progressions based on symptoms and underlying injury.

REMODELING PHASE (WEEK 7+)

The remodeling phase begins approximately at week 6 or 7. It is at this time when more advanced loading is initiated. Eccentric overload exercises are progressed to full range and heavy slow concentric actions are emphasized, with efforts to promote further soft-tissue remodeling. Although some examples of eccentric loading are provided in this article, readers may consult previously published articles in the Strength and Conditioning Journal for additional examples (18,58,50). Depending on the underlying injury, running is generally introduced, and individuals who are more active may be progressed to plyometric training with efforts to resume premorbid activities. Although running may be considered a risk factor for musculoskeletal conditions, much of this is speculative because recreational running is not a risk factor for lower extremity OA, and there is a body of evidence that it may positively increase the composition (e.g., hydration, proteoglycan content, and fiber hypertrophy) of the intervertebral disc (12,56). Individuals with OA are progressed to an appropriate exercise routine that promotes full range of motion with variable loading patterns based on clinical presentation. It is important to note that many of the activities introduced in this phase may be commenced earlier in athletic individuals who were functioning at a high level before injection. In keeping with a “one size does not fit all” approach, individuals who were functioning at a much lower level (e.g., potential surgical candidate, use of assistive device for walking, or advanced degenerative disease) before injection may not reach more advanced performance milestones such as treadmill or plyometric training.

ADDITIONAL CONSIDERATIONS

The indications for a PRP injection are wide ranging and inclusive of OA, tendinopathy, cartilage defects, and muscle or tendon tears. Although each of these conditions (and the individual affected) have their own clinical presentation, degree of healing after the onset, and characteristic response to PRP, overarching considerations exist and should be used to narrate the postprocedural phases. Musculoskeletal injuries or disorders present a constellation of signs and symptoms, with no paucity of evidence for rehabilitation or postrehabilitation guidelines because resumption of premorbid function is the goal. After a PRP injection, these goals remain; however, there are additional considerations for which this column sought to address, namely, the protective phase where pain and inflammation are addressed, as well as the subacute and remodeling phases where exercise principles are used to augment the effects of PRP.

NONSTEROIDAL ANTIINFLAMMATORY DRUGS

In the early acute phase, evidence from numerous studies and narrative articles suggest a short period of pain and inflammation. Although there is a consensus for a few days of rest, variability exists regarding the use of NSAIDs. The concern for NSAID use after PRP is based on reduced platelet aggregation, impaired growth factor release, interruption of the intended inflammation, and deleterious cellular effects. Regarding cellular function, the evidence is divergent. In one study of individuals exposed to a prolonged bout of running, NSAID use abolished the adaptive rise in collagen synthesis seen in the subjects who took a placebo (22). In another study, NSAIDs were infused into the quadriceps of subjects before, during, and for 4.5 hours after an eccentric training bout (63). Results indicated that the NSAIDs prevented satellite cell activity in the infused quadriceps, whereas the control extremity showed a 96% increase in activity. In contrast to the aforementioned study, Mackey et al. (60) studied the effect of NSAID ingestion on satellite cell activity of the quadriceps muscle (through biopsy) before and after (up to 30 days) a laboratoryinduced muscle injury. In the study, NSAID consumption was associated with increased satellite cell activation compared with placebo, suggesting that NSAIDs may have a different effect on injured versus healthy muscle (60). Additional evidence on the effect of NSAIDs may come from clinical studies. In one study, individuals with chronic Achilles tendinopathy were randomized to 1 week of NSAIDs or placebo (42). Results from biopsy and ultrasound indicate that ibuprofen had no effect on collagen expression and growth factors. Dideriksen et al. (30) studied the effects of NSAID use on patellar tendons of elderly adults who were immobilized for 2 weeks. Although collagen synthesis was decreased in the immobilized limbs as expected, the NSAID group revealed no difference when compared with placebo. Although evidence supports the premise that nonselective NSAIDs (e.g., naproxen) may interfere with platelet aggregation (55), growth factor levels in PRP are not altered (59,82). In conclusion, NSAID use does not compromise growth factor release and is more likely to have a deleterious effect on healthy structures as opposed to pathological tissues. Moreover, other medication classes such as acetaminophen and select opioid drugs may negatively influence postexercise protein synthesis and produce in vitro chondrotoxicity, respectively (1,81). Given the potential for NSAIDs to reduce pain and inflammation and the potential of other medication classes to negatively influence cells, it seems there is little downside to intermittent, low-dose use in the early acute phase

MECHANOBIOLOGY

Formal postprocedural management strategies have been documented in the literature after PRP injection; however, these studies are primarily limited to tendon pathology and muscle strains (14,21,27,36,51,73,75). Although variability exists with respect to exercise selection, progressions, and inclusion time, a majority of programming includes the introduction of mechanical forces designed to work in synergy with respect to PRP. Specifically, most programs focus on initiating early active movement to combat the deleterious effects of immobilization, followed by progressive mechanical loading of the tissues. Progressive mechanical loading (referred to as mechanotherapy) has been shown to produce a favorable anabolic environment and promote tissue regeneration. Understanding the mechanical stimuli to which musculoskeletal cells best respond and the mechanisms these cells use to convert mechanical signals into a cellular response is required to effectively introduce interventions that function synergistically with PRP. A brief, albeit important discussion of the concept of mechanotransduction is necessary to appreciate the benefits associated with mechanical loading. In general, when joints move, mechanical energy is added to an existent equilibrium of the body and stress is introduced to the load-bearing tissues (48). Each muscle, tendon, and bone is composed of cells that are linked together through an extracellular matrix. Cells adhere to these extracellular matrix scaffolds (composed of collagen, glycoproteins, and proteoglycans) through binding of specific receptors of the cell surface (48). Integrins span the cell surface membrane and communicate signals or stress from the external to the internal environment. Essentially, integrins serve as mechanoreceptors given they are the first molecules on the cell surface to sense a mechanical signal (e.g., compression, tension, etc.) and transmit it across the membrane. These signals can alter gene expression, protein synthesis, and metabolism in a manner similar to hormones, growth factors, and cytokines (48). Mechanotransduction is the process by which the tissues undergo an adaptive structural change in response to a mechanical load (53). With mechanotransduction, efforts aim to convert potentially destructive mechanical effects into constructive events that promote recovery through adaptation at the vicinity of the “stressed” cell or neighboring cells (47). Similar to bone getting stronger in response to weightbearing (another form of mechanotransduction), tendon or muscle would remodel in response to loading. Mechanotransduction follows a sequence of 3 steps, which include mechanocoupling, cell-to-cell communication (passing of loading message from one area of tendon to another), and a cellular response such as collagen synthesis or chondrogenesis (53). A necessary requisite for a cellular response is the presence of an appropriate load to the tissue of interest. The physical load that induces mechanotransduction is referred to as mechanocoupling (53). A key point with mechanocoupling is that the “overload” needs to be appropriate, progressive, and short of the point where injury risk presents itself. For example, eccentrically overloading (mechanocoupling) an involved tendon sets off the remaining physical events of mechanotransduction through stimulation of the tendon cell (tenocyte), which in turn leads to cellto-cell communication and a cellular response (collagen synthesis). A detailed discussion of mechanotransduction with illustrations of the cellular processes described can be found in the article by Khan and Scott (53). In summary, for mechanotransduction to occur, a load is applied to the region of interest. This load, through integrins, communicates the load’s message to the cell where the DNA and cytoplasmic elements are located. When the signal of a “load” is received at the cell, a message is communicated to the cell’s nucleus (DNA). Once the nucleus receives the signal, the messenger RNA transcribes the message and shuttles it to the cytoplasm where it is translated into a regenerative response such as collagen synthesis, which would be incorporated into the cellular matrix. The more common exercise principles used to promote mechanotransduction after PRP include active and passive movements, which primarily serve to prevent the deleterious effects of immobilization (30,69,77) and muscle performance activities inclusive of isometric contractions, eccentric overload, and heavy slow resistance training. Fortunately, the mechanical load on the tissues from both isometric and eccentric training induces other positive anabolic responses that further support the effects of PRP in promoting a regenerative environment.

MOVEMENT AND MOBILIZATION

Active and passive motion should be introduced in the early acute phase to deter the negative effects of immobilization. Aside from mitigating movement impairments such as arthrofibrosis, repetitive movements have a favorable effect on cartilage and the intervertebral disc. Unfortunately, much of what we have gleaned in the area of articular cartilage is based on studies investigating the effects of continuous passive movement (CPM) versus immobilization. Nevertheless, these studies provide an understanding of the biophysiological effects that steer early movement and avoidance of prolonged immobilization. Specifically, the biological evidence has shown that integrins in diseased human chondrocytes are sensitive to mechanical stimulation and increase expression of type 2 collagen and proteoglycans (49). In support of these findings, an in vitro study of human arthritic chondrocytes determined that repeated compression influenced chondrogenesis and reduced catabolic events through downregulation of collagenase expression (29). A key point here is that arthritic chondrocytes are characterized by a catabolic phenotype and a repeated compression model downregulated this environment (29). Furthermore, a greater understanding may be gleaned from in vivo laboratory studies of nonhuman knees. In a controlled laboratory study of rabbit knees, the anterior cruciate ligament was resected and the knee joint lines were evaluated 4 weeks after surgery. During the 4 weeks, the rabbits were allocated to CPM, treadmill, or sedentary environments. After 4 weeks, the CPM group had normal articular cartilage, whereas the treadmill and sedentary groups experienced surface abrasion. In addition, the CPM group had lower levels of inflammatory cytokines and a normal level of chondrocytes without damaged collagen fibers when compared with the other groups that had chondrocyte apoptosis, reduced cartilage thickness, and collagen damage (20). Shimizu et al. (77) investigated the repair response of cartilage defects in rabbit knee models and found that joints exposed to CPM had higher chondrocyte numbers than immobilization groups. In addition, delaying CPM for 1 week significantly reduced the beneficial effects on cartilage. Although the aforementioned in vivo studies further highlight the effects of activity and inactivity, one must be cautious in the generalization of these findings to the human clinical environment. Regarding the lumbar intervertebral disc, a body of evidence has suggested that increased diffusion of water content into the disc is associated with pain reduction among individuals with low back pain (8–10). Studies have shown that, when appropriate, posterior to anterior joint mobilization (nonthrust) and prone extension exercises may increase water content within the disc and subsequently be associated with reduced pain (8,10). Assuming that these interventions are appropriate to the individual’s diagnosis, they may be considered as part of routine care.

ISOMETRIC TRAINING

Although the clinical research on isometric training is limited, evidence does suggest a beneficial response for acute tendinopathy (72). In particular, evidence from one investigation (3-arm [group] trial) has indicated that isometric strengthening produced significant improvements in pain and function as well as reduced tendon thickening (observed for 71% of participants albeit not statistically significant) after intervention (72). Unfortunately, the details of the isometric dosing were unclear, as the authors reported progressing from 3 to 5 times per day with the duration of contraction progressing from 10 to 20 seconds with no mention of repetitions performed with each session. Research on pain ratings during contraction and pressure pain thresholds (sensitivity to pressure) have been favorable regarding isometric training; however, the use of healthy adults in these studies certainly skews the clinical value (46,57). From a dosing perspective, evidence suggests that submaximal isometric contractions are best held until failure, whereas maximal effort contractions may be held for durations as short as 5 seconds (39,43,44,46,57). Regarding anabolic hormones, growth factors, and cytokines, isometric training has been shown to produce increases in growth hormone, testosterone, and IGF (IGF1Ea and MGF) and decreases in myostatin (37,39,43,44). In the study by Hakkinen et al. (39), repeated maximum effort 5-second isometric contractions were used and although favorable changes were noted in all subjects, younger men had a greater response than older men.

ECCENTRIC LOADING

A compelling body of evidence exists to support the inclusion of both isometric and eccentric overload training. This evidence is in the form of isolated studies showing a regenerative effect from eccentric training on tendinopathy (23,71) as well as additional studies indicating a favorable anabolic response (hormones and growth factors) and reduced pain when compared with concentric-based training (43,44,54,74). Furthermore, many published PRP studies have incorporated both isometric and eccentric overload training with favorable outcomes (14,21,27,51). Heel drops are a frequently prescribed eccentric exercise for Achilles tendinopathy. A program of twice-daily eccentric overload exercises (Figure 3) resulted in cellular matrix remodeling and reversal of degenerative changes among individuals with Achilles tendinopathy (71). In another investigation of Achilles tendinopathy, the effect of adding PRP to a program of eccentric overload activity was evaluated (27). In the study, both groups showed considerable improvement; however, the addition of a PRP injection offered no additional benefit. In contrast to the aforementioned results, Boesen et al. (14) studied the effect of eccentric training alone versus eccentric with PRP to individuals with Achilles tendinopathy and found that PRP plus eccentric overload has superior outcomes regarding tendon structure and pain reduction when compared with eccentric alone. In another study, heavy slow resistance training offered a comparable benefit (tendon structure and clinical outcomes) to eccentric loading, thus may serve as a valuable alternative or be prescribed concurrently (13). In addition to the clinical benefits and regenerative effects of eccentric overload training on tendinopathy, a considerable body of evidence has identified favorable anabolic growth factor responses. Eccentric overload activity has been shown to induce autocrine and paracrine IGF responses (both IGF-1Ea and mechanogrowth factor [IGF-1Ec]) in tendon and muscle (38,40,43,44), which further supports and influences the desired matrix remodeling effect from PRP. Other benefits of eccentric overload include myostatin inhibition (44). Myostatin is a myokine (cytokine) found in muscle tissue that alters muscle cell differentiation in favor of scar-forming myofibroblasts and has an inhibitory effect on satellite cell activation, hypertrophy, and protein synthesis (15,38). Finally, evidence suggest that repeated bouts of eccentric overload exercise promote a reduction in the inflammatory cytokines (TNF-a and interleukin 8) and an increase in interleukin-10 (anti-inflammatory cytokine) (45). The most appropriate dosage (frequency and repetition range) for eccentric training has yet to be determined (67); however, the protocol that is often followed after a PRP injection was developed by Alfredson et al. (2,14,71). Eccentric overload exercises, using the Alfredson protocol, are performed for 3 sets of 15 repetitions, twice daily, 7 days per week, for 12–24 weeks (2). This protocol has resulted in increased peak torque, decreased pain, and a return to previous level of function in addition to imaging confirmed tendon regeneration (2,71). Recent investigations found that performance of such exercises once daily or 3 times a week may offer similar benefits to twice daily regarding tendinopathy (19,52). Moreover, Stevens et al. (79) found that with respect to clinical outcomes, a “do as tolerated” program revealed comparable clinical outcomes; however, structural changes were not assessed.

CONCLUSION

Regenerative medicine is a growing field of medicine with rapidly expanding indications and widespread use. Sports medicine and rehabilitation professionals will contend with individuals who have had these procedures. An understanding of the treating physician’s recommendations, basic science, clinical evidence, and postprocedural precautions is necessary for safely reintegrating exercise programming. Although PRP and targeted exercise strategies share common goals of promoting an anabolic or regenerative environment, individuals receiving these procedures are contending with an injury or disorder that requires safety considerations. Specifically, a short period of active rest with the use of ice and NSAIDs is not precluded during the acute postprocedural stage. After the acute period, an ideal microenvironment that promotes appropriately timed movements with progressive overload will seemingly enhance functional outcomes and augment the regenerative response from PRP. Research into postprocedural management is still in its infancy and there is a need for an improved understanding of how mechanical forces may be used to optimize outcomes following PRP procedures.

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