Blood in the Joint: Effects of Hemarthrosis on Meniscus Health and Repair Techniques

SUMMARY

Injury to the meniscus is common and frequently leads to the development of post-traumatic osteoarthritis (PTOA). Many times meniscus injuries occur coincident with anterior cruciate ligament (ACL) injuries and lead to a bloody joint effusion. Hemarthrosis, or bleeding into the joint, has been implicated in degeneration of joint tissues. The goal of this review paper is to understand the pathophysiology of blood-induced joint damage, the possible effects of blood on meniscus tissue, and the implications for current meniscus repair techniques that involve the introduction of blood-derived products into the joint. In this review, we illustrate the similarities in the pathophysiology of joint damage due to hemophilic arthropathy (HA) and osteoarthritis (OA). Although numerous studies have revealed the harmful effects of blood on cartilage and synovium, there is currently a gap in knowledge regarding the effects of hemarthrosis on meniscus tissue homeostasis, healing, and the development of PTOA following meniscus injury. Given that many meniscus repair techniques utilize blood-derived and marrow-derived products, it is essential to understand the effects of these factors on meniscus tissue and the whole joint organ to develop improved strategies to promote meniscus tissue repair and prevent PTOA development.

INTRODUCTION

Menisci are fibrocartilaginous structures that play an essential role in the proper biomechanics and function of the knee^1-3. Meniscus tears are common due to both sports-related injuries and age-related degeneration of the tissue. Meniscus injuries are associated with injury to the anterior cruciate ligament (ACL) in up to 70% of cases^4,5. Traumatic injuries to the meniscus and ACL cause acute pain, bloody joint effusion, joint immobility, and frequently lead to post-traumatic osteoarthritis (PTOA)^1,6,7. The exact mechanism(s) by which PTOA develops following these traumatic injuries is unknown. However, mechanical alterations, biological changes, and hemarthrosis, or bleeding into the intra-articular joint space, all likely play a role in the pathophysiology of PTOA development following joint injury.

The purpose of this review is to highlight the role hemarthrosis plays in joint degeneration and reveal potential implications on meniscus homeostasis and repair. Numerous studies have investigated blood-induced joint damage in the context of patients with hemophilic arthropathy (HA)^8-12. As well, several studies have discussed the harmful effects of blood on cartilage^13-18 and synovium^19,20. These studies have illustrated the following similarities in the pathophysiology of HA and osteoarthritis (OA): migration of macrophages and other immune cells^8,9,21-25, increases in pro-inflammatory cytokines^24,26,27, hypertrophic and hyperplastic synovium^22,23, and eventual cartilage damage^14-16,28. However, there is a lack of information in the literature regarding the direct effects of blood or blood components on meniscus tissue. Given that meniscus injuries are commonly present with hemarthrosis and a variety of blood-derived products are utilized as potential therapeutic tools to enhance meniscus repair, future studies investigating the direct effects of blood and its constituents on meniscus tissue are warranted to determine the potential beneficial or negative impacts blood has on meniscus repair.

CAUSES OF HEMARTHROSIS

Traumatic knee injuries, including ligamentous injury, intra-articular fracture, patellar dislocation, and meniscus injury, can result in hemarthrosis²⁹. In 132 patients with acute hemarthrosis of the knee, 77% of cases were caused by an ACL tear, while peripheral meniscus tears and osteochondral fractures accounted for 13% and 11% of cases respectively³⁰. Furthermore, acute trauma resulting in hemarthrosis within 4 hours of injury correlated with significant injury to meniscal, ligamentous, and osseous structures³¹. Moreover, even a single intra-articular bleed can cause irreversible cartilage and joint damage^16,17,19,28. Joints that suffer these traumatic injuries have a 50% increased risk of developing PTOA within 10-15 years of injury⁶. However, the specific factors that lead to PTOA following joint injury remain unclear and the contribution of hemarthrosis to PTOA development is understudied.

In addition to traumatic joint injury, hemarthrosis can also occur spontaneously in congenital bleeding disorders, such as hemophilia. Following a traumatic event that results in hemarthrosis, synoviocytes are extremely active in the uptake of foreign substances to evacuate the joint capsule of cell debris and materials³². However, with recurrent or significant joint bleeds, the ability of the synovium to evacuate the joint is impaired and inflammation of the synovium, or synovitis, worsens and becomes chronic⁹. This condition, known as hemophilic synovitis, then develops into HA, a chronic degenerative disease characterized by the degradation of articular cartilage and subchondral bone, which leads to significant joint impairment^9,12. Interestingly, there are many similarities in the pathophysiology of HA and other degenerative joint diseases, such as OA and rheumatoid arthritis (RA)¹¹.

PATHOPHYSIOLOGY OF BLOOD-INDUCED JOINT DAMAGE

Although the exact mechanisms by which blood in the joint space elicits joint pathology have yet to be elucidated, iron, angiogenic growth factors, and cytokines seem to play critical roles in the pathophysiology^10-12. As well, potential mediators of joint damage include both the direct effects of blood and blood components on the synovium and articular cartilage, as well as indirect inflammatory mediators and enzymes that are released by the inflamed synovium and inflammatory infiltrates in response to the blood (Figure 1)^12,24,33.

Figure 1:

Schematic representation of the pathophysiology of blood-induced joint damage. Synovial and cartilage degeneration are caused by both direct and indirect effects of blood in the joint space. Pro-inflammatory cytokines stimulate chondrocyte production of hydrogen peroxide (H₂O₂). The catalytic activation of H₂O₂ in the presence of iron from the lysed RBCs produces potent oxidant hydroxyl radicals, which can induce chondrocyte apoptosis. Although this diagram is specific to hemophilic arthropathy (HA), it highlights many changes that occur in osteoarthritis (OA), including synovial hyperplasia, synovial neovascularization, and production of pro-inflammatory cytokines and matrix metalloproteinases (MMPs). Reprinted from Pharmacological Research, Vol 115, Astrid E. Pulles, Simon C. Mastbergen, Roger E.G. Schutgens, Floris P.J.G. Lafeber, Lize F.D. van Vulpen, Pathophysiology of hemophilic arthropathy and potential targets for therapy, Pages 192-199, Copyright 2017, with permission from Elsevier ³³.

Hemosiderotic Synovium

Following a significant joint bleed, there is a deposition of iron as hemosiderin, a breakdown product of hemoglobin, into the superficial and subsynovial layers of the synovium resulting in hemosiderotic synovium^19,20. In particular, intracellular and extracellular highly iron saturated ferritin have been found in the superficial synovial layers of hemophilic patients following a joint bleed²⁰. In vitro, highly iron saturated ferritin increases peroxidation of lipid membranes³⁴ and causes the release of hydrolytic enzymes, such as cathepsin D³⁵, which are associated with tissue damage²⁰. Additionally, hemoglobin increases the production and secretion of plasminogen activators matrix metalloproteinase (MMP)-2 and −9³⁶ and increases expression of disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS)-5 and −9 in synoviocytes²⁸. These proteases are known mediators of cartilage degeneration. Furthermore, hemosiderin plays a pivotal role in the development of synovial hyperplasia and hypertrophy, fibrosis, neovascularization, and increases production of the pro-inflammatory cytokines interleukin (IL)-1, IL-6, and tumor necrosis factor alpha (TNF-α)^9,16,19.

The Role of Iron in Synovial Hyperplasia

Hyperplastic and neovascular synovial changes present in HA are similar to phenotypic changes in malignant tissues, including invasive and destructive behaviors^8,22. Normal human synovial fibroblasts that are exposed to ferric citrate in vitro have a 2-fold increase in cell proliferation, as compared to cells exposed to sodium citrate²². Similarly, ferric citrate increases DNA synthesis in synovial cells isolated from RA and OA patients³⁷. Iron increases the expression of the proto-oncogene cellular-myelocytomatosis (c-myc) in normal human synovial fibroblasts²² and c-myc expression has been associated with the degree of hyperplasia and lymphocytic infiltration in HA, OA, and RA^22,23. In addition, iron increases the expression of the p53 binding protein, murine double minute 2 (mdm2), in normal human synovial fibroblasts²¹. These iron-induced molecular changes may drive the dysregulation of synoviocyte proliferation, leading to synovial hyperplasia and pannus growth^11,21,22. Iron-induced hyperplasia results in the thickening of the synovial intima, which develops villous projections that create deep clefts in the subsynovial tissue⁸. This process allows for inflammatory cell infiltration and eventual fibrosis^8,21. In a mouse model of HA, as early as 2 days post-hemarthrosis, there is significant infiltration of mononuclear cells and neutrophils in the subsynovial tissue²¹. Together, these findings suggest that even acute blood exposure can result in hyperplastic and neovascular changes in the joint.

Hemarthrosis-induced Synovial Vascularization and Vascular Endothelial Growth Factor

Hemarthrosis-induced synovial hyperplasia and pannus growth results in an increased demand for oxygen and subsequent synovial hypoxia³⁸. This hypoxic environment upregulates the transcription factor hypoxia-inducible factor-1α (HIF-1α), which in turn increases vascular endothelial growth factor (VEGF) production to promote angiogenesis³⁹. Increased vascularity is observed in hemophilic synovitis, which is part of the HA phenotype¹². Patients with hemophilic synovitis have both upregulated VEGF in the synovium and peripheral blood, along with other pro-angiogenic mediators, such as MMP-9, stromal cell-derived factor-1 (SDF-1), and HIF-1α²⁵. Moreover, VEGF has been localized to CD68 expressing cells (monocytes/macrophages). Therefore, pro-angiogenic factors may contribute to the recruitment and infiltration of monocytes/macrophages, cells that have been implicated in OA⁴⁰, into the synovium²⁵. Interestingly, increased disease activity in patients with OA and RA has been directly correlated with increased serum levels of VEGF^41,42, further highlighting the similarities in the pathogenesis between hemarthrosis-induced joint damage and OA.

Hemarthrosis-Induced Cytokines

Hemosiderotic synovium produces increased levels of the inflammatory cytokines IL-1β, IL-6, and TNF-α in vitro, as compared to normal synovial tissue¹⁹. The concentrations of these inflammatory mediators due to hemarthrosis are comparable to concentrations produced by RA synovial tissue¹⁹. Furthermore, hemophilic mice with induced hemarthrosis have increased levels of IL-1β, IL-6, keratinocyte-derived chemokine (KC, murine homologue of IL-8)⁴³, and monocyte chemoattractant protein-1 (MCP-1)²⁴. In hemophilic synovitis, MCP-1 plays a vital role in recruiting mononuclear cells into the synovium and subsynovial tissue^9,24. The recruited monocytes/macrophages are activated by phagocytosis of red blood cells (RBCs) and secrete IL-1β, elevating concentrations of this inflammatory cytokine in the synovial fluid⁴⁴. The increased IL-1β subsequently stimulates reactive oxygen species production, such as superoxide and hydrogen peroxide (H₂O₂), by chondrocytes and monocytes/macrophages. The catalytic activation of H₂O₂ in the presence of iron from lysed RBCs produces potent oxidant hydroxyl radicals¹⁵, which can induce chondrocyte apoptosis⁴⁴. These findings are further supported by in vitro studies of cartilage co-cultured with RBCs (iron source) and mononuclear cells (secrete IL-1β) that resulted in prolonged inhibition of sulfated glycosaminoglycan (sGAG) synthesis, which was attenuated by dimethylsulphoxide (DMSO), an agent known to sequester hydroxyl radicals¹⁵. Similar to joints exposed to blood, increased levels of inflammatory cytokines have been well documented in the synovial fluid of injured and degenerative joints^24,26,27. In particular, a cross-sectional analysis performed by Sward et al. revealed significant elevation of synovial fluid concentrations of IL-1β, IL-6, IL-8, and TNF-α in acutely injured knees with hemarthrosis, as compared to healthy knees²⁷.

Many of these inflammatory cytokines regulate the haptoglobin-CD163-hemoxygenase (HO)-1 pathway, which is the first line of defense against hemoglobin/heme-induced toxicity during hemolysis⁴⁵. In this pathway, hemoglobin binds to haptoglobin and CD163 mediates the uptake of this complex into macrophages. Lysosomal breakdown of the complex occurs and HO-1 catalyzes the conversion of heme into carbon monoxide, biliverdin, and ferrous iron. While haptoglobin is the rate-limiting factor in this pathway, both IL-1 and IL-6 upregulate haptoglobin. Furthermore, IL-6 increases expression of CD163; in contrast to interferon gamma (IFN-γ) and TNF-α that decrease CD163. Interestingly, CD163 can also facilitate uptake of free hemoglobin, but this induces IL-6 and IL-10 secretion. Finally, HO-1 is highly-inducible by IL-1α, TNF-α, and nitric oxide (NO) and is expressed by many cell types, including macrophages⁴⁵, chondrocytes⁴⁶, and meniscus cells⁴⁷. Knock-out of the HO-1 transcriptional repressor Bach-1 decreases the severity of age- and injury-induced OA in a mouse model⁴⁶. Bach-1 deficiency causes high constitutive expression of HO-1 and protects against meniscus degradation in the aging mouse model^46,47. Meniscus degeneration mediated by oxidative stress via IL-1β or cadmium chloride treatment is reduced in Bach-1 deficient mice, suggesting an important role of HO-1 as an anti-oxidant mediator⁴⁷.

It is well established that IL-1 and TNF-α lead to increased NO production, inhibition of proteoglycan synthesis, and upregulation of MMPs, resulting in degeneration of articular cartilage and meniscus^11,26,48,49. IL-1 and TNF-α also reduce the integrative shear strength of meniscus repair^48-53. Furthermore, MCP-1, which is elevated in the synovial fluid of meniscus-injured knees⁵⁴ and is released by inflamed synovium, correlates with increased levels of IL-1β, IL-8, and IL-6 in the synovium of patients with RA, OA, and PTOA⁵⁵. These findings highlight the similarities between the underlying pathogenesis of HA and OA and the connection of these inflammatory cytokines to intra-articular bleeding and tissue degeneration.

Hemarthrosis-Induced Cartilage Damage

Several in vitro studies have illustrated the harmful effects of whole blood on cartilage^13-18. Short-term exposure (4 days) of combined mononuclear cells and RBCs, as seen in whole blood, increased and prolonged cartilage matrix turnover^13,14,18, inhibited GAG synthesis^14,15,17,18, increased protein levels of IL-1β and TNF-α^15,16, and increased chondrocyte apoptosis^14,15,17, all of which are implicated in the disruption of cartilage matrix integrity. Prior work has demonstrated that blood inhibits chondrocyte GAG synthesis but that N-acetylcysteine, which can increase levels of the anti-oxidant glutathione, mitigates this inhibition¹⁶. This suggests that oxidative stress is a key player in the inhibition of GAG synthesis. Together these results indicate that short-term, acute exposure to whole blood following joint injury or joint bleeding contributes to in vivo cartilage damage^28,44 and activation of pathways that have been implicated in the development of PTOA. It is common practice among orthopedic surgeons to aspirate fluid from the knee joint following a bloody effusion. However, there may be additional ways to mitigate the negative effects of hemarthrosis, such as the introduction of iron-chelation agents or anti-oxidant administration.

Hemarthrosis Effects on Meniscus Tissue

Although there have been several studies on the effects of blood-induced cartilage damage, there is a lack of knowledge about the direct effects of blood on meniscus tissue. Peripheral tears in the red zone of the meniscus near the meniscocapsular junction most commonly lead to hemarthrosis due to an isolated meniscus tear³⁰. Additionally, meniscus tears are often associated with other traumatic knee injuries, including ACL ruptures that frequently result in hemarthrosis. Recently hemarthrosis was induced by femoral drill holes in a rabbit model that simulates intra-articular bleeding that occurs with ACL reconstruction (ACLR)⁵⁶. This injury resulted in upregulation of inflammatory and catabolic gene expression in the rabbit meniscus tissue, which was mitigated by intra-articular injection of dexamethasone. However in the presence of blood, the dexamethasone treatment reduced medial meniscus cell viability⁵⁶. This study provides valuable information regarding changes in meniscus cell gene expression to a drill hole injury but questions still remain regarding the biochemical, biomechanical, and histological changes to the meniscus tissue, effects on meniscus tissue repair, and the effects of blood on these outcomes without the introduction of the drill hole injury. Therefore, future studies investigating the direct effects of blood or blood components on meniscus tissue are warranted to elucidate the impact this environment has on meniscus tissue and its ability to heal.

MENISCUS REPAIR AND ACL RECONSRUCTION

It is important to note that meniscus injuries frequently occur coincident with injury to other joint structures. In particular, meniscus tears are highly correlated with ACL injuries^29,30. In a study of 1145 patients with knee trauma resulting in ACL injury, 88% had other structural knee injuries, and meniscus tears were the most common concurrent injury occurring in 41% of cases⁵⁷. Other studies have reported that up to 70% of patients with an ACL rupture have a concurrent meniscus tear^4,5.

Interestingly, the healing rates of meniscus tears are greater when tears are repaired concurrently with ACLR as opposed to when they are repaired in isolation^4,5,58,59. These observations may be explained by the re-establishment of proper knee kinematics, longer rehabilitation associated with ACLR, and/or the introduction of “marrow elements,” which occurs during ACLR with the creation of bone tunnels for graft placement⁶⁰. However, the exact mechanism(s) by which this occurs or if there are confounding variables that may explain these observations has yet to be determined. For example, 2 years after surgery, patients with an assumed stable knee and meniscus repair had a failure rate of 16.7%, while age and sex matched patients with a combined meniscus repair and ACLR had a meniscal reoperation rate of 9.7% (p<0.001)⁵⁸. This difference in failure rates could be explained by several factors, including potential knee instability in assumed stable knees⁵⁸. Alternatively, isolated meniscus tears commonly occur in joints with age-related changes and degenerative menisci, likely reducing the success of repair, as compared to a healthy meniscus with a traumatic tear that commonly occurs concurrently with an ACL rupture⁶¹. In support of this theory, there is a low failure rate of isolated meniscus repair (8.7%) in young athletes due to traumatic injury 12-33 months post-surgery⁶². Furthermore, at 9 months follow-up, patients who underwent a combined ACLR and meniscus repair had a failure rate of 14%, while patients who had meniscus repair alone had a failure rate of 16% (p=0.2)⁵⁹, and others observed similar failure rates at 1-year follow-up⁴. On the other hand, patients with meniscus repair and delayed ACLR (after 6 weeks of injury and after initial meniscus repair) had a significantly higher failure rate of 27%, compared to a 14% failure rate when meniscus repair and ACLR were performed concurrently or within 6 weeks of injury⁵⁹. Similar failure rates of meniscus repair concurrent with ACLR of 14% have been reported at 6 years follow-up⁵. However, it is notable that 27% of failed meniscus repairs were associated with ACLR graft failure⁵, suggesting that knee stability and restoration of knee kinematics is essential for proper meniscal healing.

Some have proposed that the improved success rate of meniscus repair when performed concurrently with ACLR is due to biological factors released by intra-articular bleeding⁵⁹ and exposure of “marrow elements” during bone tunnel drilling and microfracture of the intercondylar notch during the ACLR⁶⁰. For example, the bleeding from bone tunnels during ACLR in rabbits increased extrinsic cell infiltration into an in situ frozen-thawed ACL at 6 weeks post-surgery⁶³. However, this study did not identify the cell infiltrates in the ACL graft. Therefore, it is unclear if cells were reparative cells or immune cell infiltrates, which have been identified in other tissues when exposed to intra-articular bleeding^8,21. Others have investigated marrow venting procedures of the intercondylar notch during isolated meniscus repair and demonstrated no significant differences in the failure rates of meniscal repair with marrow venting and meniscal repair with concurrent ACLR⁶⁴. The authors suggest that a reduction in meniscus repair failure rates, as compared to other studies, is attributable to the introduction of marrow elements. However, this study did not look at the direct comparison of an isolated meniscus repair without the introduction of marrow elements. On the other hand, in New Zealand white rabbits an isolated intra-articular bone injury via drill holes to simulate bone tunnel drilling in the femoral-notch revealed significant increases in cartilage TGF-β and MMP-13 mRNA levels at both 72 hours and 3 weeks post-operatively⁶⁵. In addition, this model showed short-term increases in synovial inflammatory marker expression, resulting in chronic synovial changes and progressive cartilage damage consistent with PTOA without disrupting the mechanical integrity of the joint⁶⁵. In a subsequent study utilizing this drill hole injury model, Heard and colleagues found upregulation of inflammatory and catabolic gene expression in the meniscus tissue⁵⁶. However, this study did not evaluate the effects of this injury on meniscus tissue repair; therefore, future studies are needed to investigate the direct effects of these blood and marrow elements on meniscus repair.

Many studies have looked at the effects of fibrin clots^66,67, fibrin glue^3,68,69, and constituents of bone marrow, including bone marrow-derived mesenchymal stem cells (MSC)^68,70 on meniscus healing. However, there has been a lack of investigation on the direct effects of intra-articular blood and bone marrow, which are frequently present following joint injury and during ACLR, on meniscus tissue repair. Future research in this area is necessary to either mitigate or maximize the potential of intra-articular blood and/or bone marrow elements on meniscus repair.

STRATEGIES TO PROMOTE MENISCUS HEALING

Even though total or partial meniscectomy is strongly correlated with the development of PTOA^7,71, partial meniscectomy remains one of the most common orthopaedic procedures^1,72. Meniscus repair procedures are performed to preserve native meniscus tissue but these repair procedures typically focus on the peripheral, vascularized region of the meniscus, rather than the inner, avascular region of the tissue that has a lower healing capacity³. Therefore, research is needed on biologic strategies to promote meniscus healing. Many current meniscus repair techniques involve exposure to blood- or marrow-derived components.

Bone Marrow-derived Mesenchymal Stem Cells

Much research has focused on the use of bone marrow-derived mesenchymal stem cells (MSCs) that have the ability to differentiate into various cell types and secrete bioactive factors that can aid in repair⁷³ and meniscal regeneration. A number of preclinical studies have shown augmented meniscal repair with the use of MSCs in various meniscus repair models^74-78. In a clinical study, intra-articular injection of allogeneic MSCs following partial meniscectomy increased meniscus volume (>15% volume increase determined by quantitative MRI), as compared to a vehicle control at 12 months post-meniscectomy⁷⁹. MSC-seeded scaffolds also show improved clinical outcomes at 12⁸⁰ and 24 months⁸¹. In these studies, lack of long-term follow-up and the relatively low number of subjects in clinical trials limit the conclusions regarding the ability of MSCs to enhance meniscus repair and prevent PTOA development. Furthermore, difficult bone marrow and MSC isolation procedures, time and cost associated with expanding MSCs, and the unclear repair mechanism(s), have limited the widespread clinical use of MSCs for meniscus repair⁷⁴.

Bone Marrow Aspirate Concentrate (BMAC)

In an attempt to circumvent some of these issues, autologous biologically active procedures using MSCs have been investigated. Bone marrow aspirate concentrate (BMAC) is typically harvested from the iliac crest and generated by centrifugation and filtering of bone marrow aspirate (BMA) cells. Hemodilution or peripheral blood contamination is common in large volumes of BMA and is mitigated by the collection of 2-10mL per aspirate site⁸². Once centrifuged, the BMA separates into three layers: plasma containing platelets and growth factors, buffy coat with mononuclear hematopoietic stem cells and MSCs, and the bottom fraction composed of RBCs. BMAC is primarily composed of the buffy coat layer but also contains a portion of both the plasma and RBC layers, areas rich in platelets and large sized MSCs and RBCs respectively⁸³. This mixture ultimately results in an autologous cell source that contains concentrated MSCs and growth factors present in the bone marrow.

The BMAC preparation is a “one-step process” that is more time and cost-effective than the isolation of MSCs, which requires plating and cell expansion after bone marrow harvest. While bone marrow has a low percentage of MSCs ranging from 0.01%-0.02%⁸⁴, BMAC contains 3 to 41-fold concentrated MSCs⁸⁵. In addition, the BMAC preparation concentrates growth factors found in the bone marrow, such as VEGF and PDGF, and platelets that may promote healing and angiogenesis in a meniscus injury^85,86. In a clinical study performed by Massey et al., the introduction of BMAC following meniscus repair improved patient reported pain at 6 weeks and 3 months post-surgery when compared to meniscal repair without the use of BMAC⁸⁷. However, at 1 year follow-up, there were no differences between the control and BMAC groups on visual analogue scale (VAS) reported pain, Lysholm knee score, and the International Knee Documentation Committee (IKDC) knee evaluation form⁸⁷.

As a result of the concentration process to generate BMAC, it contains 10 to 100-fold increased concentrations of pro-inflammatory cytokines, such as IL-1β, as compared to simple BMA⁸⁵. Since BMAC is prepared and concentrated from autologous bone marrow, there is not a controlled, characterized, and consistent product that can be used for treatment of meniscal injuries. However, what is consistent in each BMAC preparation is that all factors present in the bone marrow are relatively concentrated, including RBCs and pro-inflammatory cytokines that can have detrimental effects on joint tissues.

Platelet-Rich Plasma

Platelet-rich plasma (PRP) is another autologous source of concentrated growth factors. PRP is generated by the centrifugation of peripheral blood. After the removal of the RBC and leukocyte layers, the blood is centrifuged again, yielding a platelet-rich cell pellet. The majority of the plasma is discarded and the pellet is resuspended, resulting in PRP⁷⁷. PRP contains the majority of growth factors present in the peripheral blood, including high levels of TGF-β and PDGF⁷⁷, both of which have been shown to increase meniscal cell differentiation and proliferation⁸⁸. While some studies have demonstrated beneficial effects on the treatment of meniscal tears in the avascular regions with PRP^89,90, others have reported no significant benefit on meniscus repair outcomes⁹¹. These differences in clinical success may be due to the concentration of factors that inhibit healing during PRP processing. In particular, concentrated inflammatory and degradative mediators may contribute to the inconsistent outcomes observed with PRP treatment. In an experiment performed by O’Donnell et al., chondrocytes were treated with PRP collected from healthy donors or patients with OA (OA-PRP)⁹². The OA-PRP significantly decreased Col1a1, Col2a1, and Sox9 gene expression as compared to PRP from healthy donors. Interestingly, MMP-1,−3,−9, and −13 gene expression was upregulated in both healthy PRP and OA-PRP treated chondrocytes when compared to the negative control. Furthermore, OA-PRP significantly increased the gene expression of TNF-α, and MMP-9 in monocyte-derived macrophages⁹². Together these results highlight that different disease states influence the bioactivity of PRP, which may decrease cartilage and meniscus biosynthesis, increase tissue degeneration, and promote macrophage-mediated inflammation rather than enhancing meniscus healing.

Comparison of BMAC, PRP, and MSCs for Meniscus Repair

Recently both BMAC and PRP were evaluated in combination with suture for the treatment of a longitudinal meniscal tear in rabbits⁸⁶. At 12 weeks post-surgery, BMAC plus suture showed significant improvement in meniscus repair as compared to PRP plus suture or suture alone⁸⁶. These results suggest that the presence of MSCs and other mononuclear cells in BMAC may be beneficial for meniscus repair. In addition, others have shown that stimulating MSCs with PRP causes a significant increase in MSC proliferation and expression of the chondrogenic differentiation markers RUNX2, SOX9, and ACAN⁹³. These findings suggest that the combined use of MSCs and PRP may be beneficial in meniscal regeneration and repair. An in vitro study using MSCs seeded on hyaluronan-collagen composite scaffolds implanted into a meniscal punch defect showed that MSCs enhanced the generation and integration of meniscus-like repair tissue as compared to acellular scaffolds, BMA-seeded scaffolds, or PRP-seeded scaffolds^77,78. This study suggests that there may be factors present in BMAC and PRP that are detrimental to the regeneration and repair of the meniscus due to the concentration of harmful blood and marrow components. Therefore, additional studies are necessary to elucidate the long-term effects of these blood- and marrow-derived factors on meniscus repair and PTOA development.

Understanding the Effects of Blood versus Potential Therapeutics

As this review has highlighted, much research is still needed to understand the effects of blood and marrow components and potential therapeutics containing these components on meniscus tissue homeostasis and repair. However, several factors may be critical to the success of therapeutics. These include the following: (1) local versus global concentrations, (2) the magnitude, and (3) the duration of blood and blood components within the joint. For example, local delivery of a fibrin clot (composed of fibrin, platelets, and some RBCs) promotes meniscus healing⁶⁶ and has very different properties than whole blood in the joint. Some in vitro work has investigated the effects of concentration and duration of blood on cartilage. Human cartilage explants treated with increasing concentrations of blood results in a concentration-dependent reduction in proteoglycan synthesis and an increase in proteoglycan release and MMP activity¹⁸. With as little as 10% blood for 2 days, longer-term cartilage degeneration was detected following 12 days without blood treatment. On the other hand, only 1 day of exposure to 50% blood resulted in only transient cartilage catabolism. In the joint, blood takes at least 4 days to be cleared⁹⁴. These findings suggest that it is likely beneficial to aspirate blood following joint injury with a hemarthrosis and it may be important to determine the concentration of blood introduced and remaining in the joint due to bone tunnel drilling, microfracture, and/or marrow venting.

CONCLUSION

Overall, the studies of hemarthrosis have revealed that iron, angiogenic growth factors, pro-inflammatory cytokines, and inflammatory infiltrates contribute to synovial hyperplasia, pannus growth, and cartilage degeneration. However, there is a gap in knowledge regarding the effects of hemarthrosis on meniscus tissue homeostasis, meniscus repair, and PTOA development following meniscus injury. Given the many similarities in the pathophysiology of HA and OA, it is likely that hemarthrosis also contributes to meniscus degeneration. Furthermore, it is currently unclear how ACLR concurrent with meniscus repair influences the success of meniscus repair. Additional studies are needed to carefully interrogate the effects of the marrow elements released during ACLR on meniscus repair. Findings from these studies could inform biologic techniques to improve meniscus healing. With the evolution of meniscus repair techniques using blood components and marrow-derived products, it is essential to more fully understand how meniscus tissue homeostasis and repair are influenced by injurious and therapeutic introduction of blood and blood-derived products into the joint. Future research in these areas is critical to develop successful strategies to promote meniscus tissue repair and ultimately prevent PTOA development.