Coefficients of friction, lubricin, and cartilage damage in the anterior cruciate ligament-deficient guinea pig knee

The Role of Exerkines in the Treatment of Knee Osteoarthritis: From Mechanisms to Exercise Strategies

With the increasing prevalence of knee osteoarthritis (KOA), the limitations of traditional treatments, such as their limited efficacy in halting disease progression and their potential side effects, are becoming more evident. This situation has prompted scientists to seek more effective strategies. In recent years, exercise therapy has gained prominence in KOA treatment due to its safety, efficacy, and cost-effectiveness, which are underpinned by the molecular actions of exerkines. Unlike conventional therapies, exerkines offer specific advantages by targeting inflammatory responses, enhancing chondrocyte proliferation, and slowing cartilage degradation at the molecular level. This review explores the potential mechanisms involved in and application prospects of exerkines in KOA treatment and provides a comprehensive analysis of their role. Studies show that appropriate exercise not only promotes overall health, but also positively impacts KOA by stimulating exerkine production. The effectiveness of exerkines, however, is influenced by exercise modality, intensity, and duration of exercise, making the development of personalized exercise plans crucial for KOA patients. Based on these insights, this paper proposes targeted exercise strategies designed to maximize exerkine benefits, aiming to provide novel perspectives for KOA prevention and treatment.

Intra-Articular Administration of Ganglioside Sugars Protects Cartilage from Progressive Degeneration in an Instability OA Rabbit Model

OBJECTIVE

Osteoarthritis (OA) is a degenerative joint disease that has no cure, and current therapies are intended to minimize pain. There is, therefore, a need for effective pharmacologic agents that reverse or slow the progression of joint damage. We report herein on an investigation of the effects of intra-articular injections of ganglioside sugars on the progression of OA in an experimental rabbit model.

DESIGN

Knee OA was induced Japanese in White rabbits by anterior cruciate ligament transection (ACLT). Ganglioside sugars at concentrations of 0.1, 0.3, and 0.9 mg/ml were then intra-articularly injected as a possible treatment for OA. Controls received intra-articular injections of saline. Knees were assessed macroscopically, histologically, and mechanically at 13 weeks after ACLT induction.

RESULTS

Macroscopically, knees of the groups that received ganglioside sugars at concentrations of 0.3 and 0.9 mg/ml exhibited milder cartilage degradation compared to the controls. Consistent with these results, histological scores for these knees were significantly higher than the corresponding values for the control knees. Lectin histochemistry staining revealed that the treatment with ganglioside sugars at concentrations of 0.3 and 0.9 mg/ml was associated with a remarkable increase in the levels of GalNAc-positive chondrocytes in cartilage. Coefficient of friction testing also demonstrated that cartilages treated with ganglioside sugars had a lower coefficient of frictions than the values for the control group.

CONCLUSIONS

Intra-articular injections of ganglioside sugars prevented cartilage degeneration in an OA-instability model. These results highlight the promising therapeutic potential for using ganglioside sugars in the treatment of progressive OA.

Cartilage Integrity: A Review of Mechanical and Frictional Properties and Repair Approaches in Osteoarthritis

Osteoarthritis (OA) is one of the most common causes of disability around the globe, especially in aging populations. The main symptoms of OA are pain and loss of motion and function of the affected joint. Hyaline cartilage has limited ability for regeneration due to its avascularity, lack of nerve endings, and very slow metabolism. Total joint replacement (TJR) has to date been used as the treatment of end-stage disease. Various joint-sparing alternatives, including conservative and surgical treatment, have been proposed in the literature; however, no treatment to date has been fully successful in restoring hyaline cartilage. The mechanical and frictional properties of the cartilage are of paramount importance in terms of cartilage resistance to continuous loading. OA causes numerous changes in the macro- and microstructure of cartilage, affecting its mechanical properties. Increased friction and reduced load-bearing capability of the cartilage accelerate further degradation of tissue by exerting increased loads on the healthy surrounding tissues. Cartilage repair techniques aim to restore function and reduce pain in the affected joint. Numerous studies have investigated the biological aspects of OA progression and cartilage repair techniques. However, the mechanical properties of cartilage repair techniques are of vital importance and must be addressed too. This review, therefore, addresses the mechanical and frictional properties of articular cartilage and its changes during OA, and it summarizes the mechanical outcomes of cartilage repair techniques.

Independent and combined effects of obesity and traumatic joint injury to the structure and composition of rat knee cartilage

Osteoarthritis (OA) is a multifactorial joint disease characterized by articular cartilage degradation. Risk factors for OA include joint trauma, obesity, and inflammation, each of which can affect joint health independently, but their interaction and the associated consequences of such interaction were largely unexplored. Here, we studied compositional and structural alterations in knee joint cartilages of Sprague-Dawley rats exposed to two OA risk factors: joint injury and diet-induced obesity. Joint injury was imposed by surgical transection of anterior cruciate ligaments (ACLx), and obesity was induced by a high fat/high sucrose diet. Depth-dependent proteoglycan (PG) content and collagen structural network of cartilage were measured from histological sections collected previously in Collins et al.. (2015). We found that ACLx primarily affected the superficial cartilages. Compositionally, ACLx led to reduced PG content in lean animals, but increased PG content in obese rats. Structurally, ACLx caused disorganization of collagenous network in both lean and obese animals through increased collagen orientation in the superficial tissues and a change in the degree of fibrous alignment. However, the cartilage degradation attributed to joint injury and obesity was not necessarily additive when the two risk factors were present simultaneously, particularly for PG content and collagen orientation in the superficial tissues. Interestingly, sham surgeries caused a through-thickness disorganization of collagen network in lean and obese animals. We conclude that the interactions of multiple OA risk factors are complex and their combined effects cannot be understood by superposition principle. Further research is required to elucidate the interactive mechanism between OA subtypes.

Small laboratory animal models of anterior cruciate ligament reconstruction

Anterior cruciate ligament (ACL) injuries are common knee ligament injuries. While generally successful, ACL reconstruction that uses a tendon graft to stabilize the knee is still associated with a notable percentage of failures and long-term morbidities. Preclinical research that uses small laboratory species (i.e., mice, rats, and rabbits) to model ACL reconstruction are important to evaluate factors that can impact graft incorporation or posttraumatic osteoarthritis after ACL reconstruction. Small animal ACL reconstruction models are also used for proof-of-concept studies for the development of emerging biological strategies aimed at improving ACL reconstruction healing. The objective of this review is to provide an overview on the use of common small animal laboratory species to model ACL reconstruction. The review includes a discussion on comparative knee anatomy, technical considerations including types of tendon grafts employed amongst the small laboratory species (i.e., mice, rats, and rabbits), and common laboratory evaluative methods used to study healing and outcomes after ACL reconstruction in small laboratory animals. The review will also highlight common research questions addressed with small animal models of ACL reconstruction.

Regenerative Effect of Low-Intensity Pulsed Ultrasound and Platelet-Rich Plasma on the Joint Friction and Biomechanical Properties of Cartilage: A Non-traumatic Osteoarthritis Model in the Guinea Pig

This study was aimed at investigating the effects of platelet-rich plasma (PRP) and low-intensity pulsed ultrasound (LIPUS) on the joint friction parameters and biomechanical properties of articular cartilage in a non-traumatic knee osteoarthritis (OA) model. Fifty adult male Dunkin Hartley guinea pigs were randomly divided into five groups: control, OA60, OA + US, OA + PRP and OA + US + PRP). Non-traumatic knee OA was induced with a single dose of 3 mg of mono-iodoacetate (MIA) by intra-articular injection. Intra-articular PRP was injected twice in the OA + PRP and OA + US + PRP groups. LIPUS was delivered in 10 sessions in the OA + US and OA + US + PRP groups. By use of the pendulum free oscillation test, joint friction (coefficient of friction) was measured. In addition, the instantaneous elastic modulus and aggregate modulus were measured using the stress-relaxation test. MIA injection decreased cartilage thickness, instantaneous elastic modulus and aggregate modulus, and increased joint friction. The friction coefficients in the OA + US and OA + US + PRP groups reached near-normal values, and there was no significant difference compared with the control group (p = 0.232 and p = 0.459, respectively). The instantaneous elastic modulus and aggregate modulus in the OA + US group increased significantly compared with the OA + PRP group (p < 0.05). It seems that both LIPUS and PRP injection effectively improved joint lubrication, but LIPUS was superior to PRP in improving the mechanical properties of the articular cartilage.

The synthetic artificial stem cell (SASC): Shifting the paradigm of cell therapy in regenerative engineering

This paper presents the synthetic artificial stem cell (SASC) system: a versatile therapy which provides the ability to tailor paracrine responses of different cells and provide a more potent regenerative effect for targeted tissues. Upon challenging the SASC system against an osteoarthritis model, we demonstrate that the factors combined tailored for chondrogenesis have a potent antiinflammatory and chondroprotective effect. This paper also demonstrates the in vivo capacity of SASC to attenuate proteoglycan depletion in the cartilage extracellular matrix while also improving biomechanical properties of the resulting cartilage. We report the first of many applications of the SASC system, which provides a promising step toward clinical translation of a minimally immunogenic stem cell with many commercial advantages over its biological counterpart.

Stem cells are of great interest in tissue regeneration due to their ability to modulate the local microenvironment by secreting bioactive factors (collectively, secretome). However, secretome delivery through conditioned media still requires time-consuming cell isolation and maintenance and also may contain factors antagonistic to targeted tissue regeneration. We have therefore engineered a synthetic artificial stem cell (SASC) system which mimics the paracrine effect of the stem cell secretome and provides tailorability of the composition for targeted tissue regeneration. We report the first of many applications of the SASC system we have formulated to treat osteoarthritis (OA). Choosing growth factors important to chondrogenesis and encapsulating respective recombinant proteins in poly (lactic-coglycolic acid) 85:15 (PLGA) we fabricated the SASC system. We compared the antiinflammatory and chondroprotective effects of SASC to that of adipose-derived stem cells (ADSCs) using in vitro interleukin 1B-induced and in vivo collagenase-induced osteoarthritis rodent models. We have designed SASC as an injectable therapy with controlled release of the formulated secretome. In vitro, SASC showed significant antiinflammatory and chondroprotective effects as seen by the up-regulation of SOX9 and reduction of nitric oxide, ADAMTS5, and PRG4 genes compared to ADSCs. In vivo, treatment with SASC and ADSCs significantly attenuated cartilage degeneration and improved the biomechanical properties of the articular cartilage in comparison to OA control. This SASC system demonstrates the feasibility of developing a completely synthetic, tailorable stem cell secretome which reinforces the possibility of developing a new therapeutic strategy that provides better control over targeted tissue engineering applications.

Raman spectroscopic insight into osteoarthritic cartilage regeneration by mRNA therapeutics encoding cartilage-anabolic transcription factor Runx1

While joint arthroplasty remains nowadays the most popular option available to repair chronically degenerated osteoarthritic joints, possibilities are recently emerging for regeneration of damaged cartilage rather than its replacement with artificial biomaterials. This latter strategy could allow avoiding the quite intrusive surgical procedures associated with total joint replacement. Building upon this notion, we first apply Raman spectroscopy to characterize diseased cartilage in a mice model of instability-induced knee osteoarthritis (OA) upon medial collateral ligament (MCL) and medial meniscus (MM) transections. Then, we examine the same OA model after cartilage regeneration by means of messenger RNA (mRNA) delivery of a cartilage-anabolic runt-related transcription factor 1 (RUNX1). Raman spectroscopy is shown to substantiate at the molecular scale the therapeutic effect of the Runx1 mRNA cartilage regeneration approach. This study demonstrates how the Raman spectroscopic method could support and accelerate the development of new therapies for cartilage diseases.

Intra-Articular Adeno-Associated Virus-Mediated Proteoglycan 4 Gene Therapy for Preventing Post-Traumatic Osteoarthritis

OBJECTIVE

Lubricin, a glycoprotein encoded by the proteoglycan 4 (PRG4) gene, is an essential boundary lubricant that reduces friction between articular cartilage surfaces. The loss of lubricin subsequent to joint injury plays a role in the pathogenesis of post-traumatic osteoarthritis (PTOA). Here we describe the development and evaluation of an adeno-associated virus (AAV)-based PRG4 gene therapy intended to restore lubricin in injured joints. The green fluorescent protein (GFP) gene was inserted the PRG4 gene to facilitate tracing the distribution of the transgene product (AAV-PRG4-GFP) in vivo.

METHODS

Transduction efficiency of AAV-PRG4-GFP was evaluated in joint cells, and the conditioned medium containing secreted PRG4-GFP was used for shear loading/friction and viability tests. In vivo transduction of joint tissues following intra-articular injection of AAV-PRG4-GFP was confirmed in the mouse stifle joint in a surgical model of destabilization of the medial meniscus (DMM), and chondroprotective activity was tested in a rabbit anterior cruciate ligament transection (ACLT) model.

RESULTS

In vitro studies showed that PRG4-GFP has lubricin-like cartilage binding and anti-friction properties. Significant cytoprotective effects were seen when cartilage was soaked in PRG4-GFP prior to cyclic shear loading (n = 3). Polymerase chain reaction and confocal microscopy confirmed the presence of PRG4-GFP DNA and protein, respectively, in a mouse DMM (n = 3 per group). In the rabbit ACLT model, AAV-PRG4-GFP gene therapy enhanced lubricin expression (p = 0.001 versus AAV-GFP: n = 7-14) and protected the cartilage from degeneration (p = 0.014 versus AAV-GFP: n = 9-10) when treatments were administered immediately post-operation, but efficacy was lost when treatment was delayed for 2 weeks.

CONCLUSION

AAV-PRG4-GFP gene therapy protected cartilage from degeneration in a rabbit ACLT model; however, data from the ACLT model suggest that early intervention is essential for efficacy.

Intra-Articular Administration of a Synthetic Lubricin in Canine Stifles

OBJECTIVE

 The aim of this study was to evaluate the functional, systemic, synovial and articular changes after intra-articular administration of a synthetic lubricin within healthy canine stifles.

STUDY DESIGN

 A prospective randomized blinded placebo-controlled study composed of 10 dogs equally divided into either a treatment group (intra-articular synthetic lubricin injection, n = 5) or control group (saline, n = 5). Clinical (orthopaedic examination, gait observation, gait analysis), biochemical (complete blood count and biochemistry profile) and local tissue outcomes (joint fluid analysis, joint capsule and articular cartilage histopathology) were evaluated over a time period of 3 months.

RESULTS

 No significant differences between the treatment group and control group were identified with regard to baseline patient parameters. No clinically significant orthopaedic examination abnormalities, gait abnormalities, biochemical alterations, joint fluid alterations or histopathological alterations were identified over the course of the study.

CONCLUSION

 The synthetic lubricin studied herein is both biocompatible and safe for a single administration within the canine stifle joint. Further research is necessary to evaluate the clinical efficacy of the synthetic lubricin in canine osteoarthritic joints.

Long-term outcomes of anterior cruciate ligament reconstruction surgery: 2020 OREF clinical research award paper

ACL injuries place the knee at risk for post-traumatic osteoarthritis (PTOA) despite surgical anterior cruciate ligament (ACL) reconstruction. One parameter thought to affect PTOA risk is the initial graft tension. This randomized controlled trial (RCT) was designed to compare outcomes between two graft tensioning protocols that bracket the range commonly used. At 7 years postsurgery, we determined that most outcomes between the two tension groups were not significantly different, that they were inferior to an uninjured matched control group, and that PTOA was progressing in both groups relative to controls. The trial database was also leveraged to gain insight into mechanisms of PTOA following ACL injury. We determined that the inflammatory response at the time of injury undermines one of the joint's lubricating mechanisms. We learned that patients continue to protect their surgical knee 5 years postinjury compared to controls during a jump-pivot activity. We also established that presurgical knee function and mental health were correlated with symptomatic PTOA at 7 years, that there were specific anatomical factors associated with poor outcomes, and that there were no changes in outcomes due to tunnel widening in patients receiving hamstring tendon autografts. We also validated a magnetic resonance imaging technique to noninvasively assess graft strength. In conclusion, the RCT determined that initial graft tensioning does not have a major influence on 7-year outcomes. Therefore, surgeons can reconstruct the ACL using a graft tensioning protocol that is within the window of the two graft tensioning techniques evaluated in this RCT.

Resilience to height loss of articular cartilage of osteoarthritic stifle joints of old pigs, compared with healthy cartilage from young pigs in a tribological pin-on-plate exposure, revealing similar friction forces

Introduction

We saw a lack of data on the biomechanical behavior of degenerated articular cartilage (OA) compared with that of healthy cartilage, even though the susceptibility to wear and tear of articular cartilage plays a key role in the progression of osteoarthritis (OA). Therefore, we performed a comparison between naturally occurring OA and healthy cartilage from pigs, before and after tribological stress.

Aim

The aim of the study was to compare OA-cartilage with healthy cartilage and to analyze the resilience to tribological shear stress, which will be measured as height loss (HL), and to friction forces of the cartilage layers. The findings will be substantiated in macro- and microscopical evaluations before and after tribological exposure.

Methods

We assessed stifle joints of fifteen old and sixteen young pigs from the local abattoir radiologically, macroscopically and histologically to determine possible OA alterations. We put pins from the femoral part of the joints and plates from the corresponding tibial plateaus in a pin-on-plate tribometer under stress for about two hours with about 1108 reciprocating cycles under a pressure of approximately 1 MPa. As a surrogate criterion of wear and tear, the HL was recorded in the tribometer. The heights of the cartilage layers measured before and after the tribological exposure were compared histologically. The condition of the cartilage before and after the tribological exposure was analyzed both macroscopically with an adapted ICRS score and microscopically according to Little et al. (2010). We assessed the friction forces acting between the surfaces of the cartilage pair–specimens.

Results

Articular cartilage taken from old pigs showed significant degenerative changes compared to that taken from the young animals. The macroscopic and microscopic scores showed strong alterations of the cartilage after the tribological exposure. There was a noticeable HL of the cartilage specimens after the first 100 to 300 cycles. The HL after tribological exposure was lower in the group of the old animals with 0.52 mm ± 0.23 mm than in the group of the young animals with 0.86 mm ± 0.26 mm (p < 0.0001). The data for the HL was validated by the histological height measurements with 0.50 mm ± 0.82 mm for the old and 0.79 mm ±0.53 mm for the young animals (p = 0.133). The friction forces measured at the cartilage of the old animals were 2.25 N ± 1.15 N and 1.89 N ± 1.45 N of the young animals (p = 0.3225).

Conclusion

Unlike articular cartilage from young pigs, articular cartilage from old pigs showed OA alterations. Tribological shear stress exposure revealed that OA cartilage showed less HL than healthy articular cartilage. Tribological stress exposure in a pin–on–plate tribometer seemed to be an appropriate way to analyze the mechanical stability of articular cartilage, and the applied protocol could reveal weaknesses of the assessed cartilage tissue. Friction and HL seemed to be independent parameters when degenerated and healthy articular cartilage were assessed under tribological exposure in a pin–on- plate tribometer.

Inflammatory and Noninflammatory Synovial Fluids Exhibit New and Distinct Tribological Endotypes

Inferior synovial lubrication is a hallmark of osteoarthritis (OA), and synovial fluid (SF) lubrication and composition are variable among OA patients. Hyaluronic acid (HA) viscosupplementation is a widely used therapy for improving SF viscoelasticity and lubrication, but it is unclear how the effectiveness of HA viscosupplements varies with arthritic endotype. The objective of this study was to investigate the effects of the HA viscosupplement, Hymovis®, on the lubricating properties of diseased SF from patients with noninflammatory OA and inflammatory arthritis (IA). The composition (cytokine, HA, and lubricin concentrations) of the SF was measured as well as the mechanical properties (rheology, tribology) of the SF alone and in a 1:1 mixture with the HA viscosupplement. Using rotational rheometry, no difference in SF viscosity was detected between disease types, and the addition of HA significantly increased all fluids' viscosities. In noninflammatory OA SF, friction coefficients followed a typical Stribeck pattern, and their magnitude was decreased by the addition of HA. While some of the IA SF also showed typical Stribeck behavior, a subset showed more erratic behavior with highly variable and larger friction coefficients. Interestingly, this aberrant behavior was not eliminated by the addition of HA, and it was associated with low concentrations of lubricin. Aberrant SF exhibited significantly lower effective viscosities compared to noninflammatory OA and IA SF with typical tribological behavior. Collectively, these results suggest that different endotypes of arthritis exist with respect to lubrication, which may impact the effectiveness of HA viscosupplements in reducing friction.

Synovial fluid lubricin increases in spontaneous canine cruciate ligament rupture

Lubricin is an important boundary lubricant and chondroprotective glycoprotein in synovial fluid. Both increased and decreased synovial fluid lubricin concentrations have been reported in experimental post-traumatic osteoarthritis (PTOA) animal models and in naturally occurring joint injuries in humans and animals, with no consensus about how lubricin is altered in different species or injury types. Increased synovial fluid lubricin has been observed following intra-articular fracture in humans and horses and in human late-stage osteoarthritis; however, it is unknown how synovial lubricin is affected by knee-destabilizing injuries in large animals. Spontaneous rupture of cranial cruciate ligament (RCCL), the anterior cruciate ligament equivalent in quadrupeds, is a common injury in dogs often accompanied by OA. Here, clinical records, radiographs, and synovial fluid samples from 30 dogs that sustained RCCL and 9 clinically healthy dogs were analyzed. Synovial fluid lubricin concentrations were nearly 16-fold greater in RCCL joints as compared to control joints, while IL-2, IL-6, IL-8, and TNF-α concentrations did not differ between groups. Synovial fluid lubricin concentrations were correlated with the presence of radiographic OA and were elevated in three animals sustaining RCCL injury prior to the radiographic manifestation of OA, indicating that lubricin may be a potential biomarker for early joint injury.

Lubricin in experimental and naturally occurring osteoarthritis: a systematic review

OBJECTIVE

Lubricin is increasingly being evaluated as an outcome measure in studies investigating post-traumatic and naturally occurring osteoarthritis. However, there are discrepancies in results, making it unclear as to whether lubricin is increased, decreased or unchanged in osteoarthritis. The purpose of this study was to review all papers that measured lubricin in joint injury or osteoarthritis in order to draw conclusions about lubricin regulation in joint disease.

DESIGN

A systematic search of the Pubmed, Web of Knowledge, and EBSCOhost databases for papers was performed. Inclusion criteria were in vivo studies that measured lubricin in humans or animals with joint injury, that investigated lubricin supplementation in osteoarthritic joints, or that described the phenotype of a lubricin knock-out model. A methodological assessment was performed.

RESULTS

Sixty-two studies were included, of which thirty-eight measured endogenous lubricin in joint injury or osteoarthritis. Nineteen papers found an increase or no change in lubricin and nineteen reported a decrease. Papers that reported a decrease in lubricin were cited four times more often than those that reported an increase. Fifteen papers described lubricin supplementation, and all reported a beneficial effect. Eleven papers described lubricin knock-out models.

CONCLUSIONS

The human literature reveals similar distributions of papers reporting increased lubricin as compared to decreased lubricin in osteoarthritis. The animal literature is dominated by reports of decreased lubricin in the rat anterior cruciate ligament transection model, whereas studies in large animal models report increased lubricin. Intra-articular lubricin supplementation may be beneficial regardless of whether lubricin increases or decreases in OA.

Synovial fluid lubricin and hyaluronan are altered in equine osteochondral fragmentation, cartilage impact injury, and full-thickness cartilage defect models

The objectives of this study were to evaluate temporal changes in lubricin, hyaluronan (HA), and HA molecular weight (MW) distributions in three distinct models of equine joint injury affecting the carpal (wrist), tarsal (ankle), and femoropatellar (knee) joints. To establish ranges for lubricin, HA, and HA MW distributions across multiple joints, we first evaluated clinically healthy, high-motion equine joints. Synovial fluid was collected from high-motion joints in horses without clinical signs of joint disease (n = 11 horses, 102 joints) and from research horses undergoing carpal osteochondral fragmentation (n = 8), talar cartilage impact injury (n = 7), and femoral trochlear ridge full-thickness cartilage injury (n = 22) prior to and following arthroscopically induced joint injury. Lubricin and HA concentrations were measured via enzyme-linked immunosorbent assays, and gel electrophoresis was performed to evaluate HA MW distributions. Synovial fluid parameters were analyzed via linear regression models, revealing that lubricin and HA concentrations were conserved across healthy, high-motion joints. Lubricin concentrations increased post-injury in all osteoarthritis models (carpal fragmentation P = .001; talar impact P < .001; femoral trochlear ridge cartilage defect P = .03). Sustained loss of HA was noted post-arthroscopy following carpal osteochondral fragmentation (P < .0001) and talar impact injury (P < .001). Lubricin may be elevated to compensate for the loss of HA and to protect cartilage post-injury. Further investigation into the mechanisms regulating lubricin and HA following joint injury and their effects on joint homeostasis is warranted, including whether lubricin has value as a biomarker for post-traumatic osteoarthritis.

Biotribology of Synovial Cartilage: A New Method for Visualization of Lubricating Film and Simultaneous Measurement of the Friction Coefficient

A healthy natural synovial joint is very important for painless active movement of the natural musculoskeletal system. The right functioning of natural synovial joints ensures well lubricated contact surfaces with a very low friction coefficient and wear of cartilage tissue. The present paper deals with a new method for visualization of lubricating film with simultaneous measurements of the friction coefficient. This can contribute to better understanding of lubricating film formation in a natural synovial joint. A newly developed device, a reciprocating tribometer, is used to allow for simultaneous measurement of friction forces with contact visualization by fluorescence microscopy. The software allowing for snaps processing and subsequent evaluation of fluorescence records is developed. The evaluation software and the follow-up evaluation procedure are also described. The experiments with cartilage samples and model synovial fluid are carried out, and the new software is applied to provide their evaluation. The primary results explaining a connection between lubrication and friction are presented. The results show a more significant impact of albumin proteins on the lubrication process, whereas its clusters create a more stable lubrication layer. A decreasing trend of protein cluster count, which corresponds to a decrease in the thickness of the lubrication film, is found in all experiments. The results highlight a deeper connection between the cartilage friction and the lubrication film formation, which allows for better understanding of the cartilage lubrication mechanism.

Effect of Lubricin Mimetics on the Inhibition of Osteoarthritis in a Rat Anterior Cruciate Ligament Transection Model

BACKGROUND

Lubricin, a mucinous glycoprotein, plays a chondroprotective role as a constituent of synovial fluid. Structural analogs have been synthesized to mimic the structure and function of native lubricin in an effort to recapitulate this effect with the goal of delaying progression of osteoarthritis (OA).

PURPOSE

To investigate the efficacy of intra-articular injections of lubricin mimetics in slowing or preventing the progression of posttraumatic OA by using a rat anterior cruciate ligament transection model.

STUDY DESIGN

Controlled laboratory design.

METHODS

Four lubricin mimetics were investigated, differing from one another in their binding orientations and steric interactions. Eighty skeletally mature Sprague-Dawley rats underwent bilateral anterior cruciate ligament transections and were randomly allocated to receive intra-articular injections (50 µL/injection) of 1 of the 4 mimetics in the right knee and equal volumes of saline injection in the contralateral knee (control). All rats were euthanized 8 weeks postoperatively and assessed via biomechanical analysis, which evaluated comparative friction coefficients across the 4 groups, and histological evaluation of articular cartilage, osteophytes, and synovitis. The Osteoarthritis Research Society International (OARSI) histopathological assessment system was used to evaluate the degree of articular cartilage degeneration and osteophytes, while synovitis was assessed through a semiquantitative scoring system. Binding efficacy of the 4 mimetics was assessed in vitro and in vivo through the immunohistochemical localization of polyethylene glycol. Articular cartilage degeneration and synovitis scoring data analyses were performed with generalized estimating equation modeling.

RESULTS

Injection of the group 3 mimetic (random 24 + 400 + 30) directly correlated with improved OARSI scores for femoral articular cartilage degeneration when compared with saline-injected contralateral control knees (P = .0410). No lubricin mimetic group demonstrated statistically significant differences in OARSI scores for tibial articular cartilage degeneration. Injection of the group 4 mimetic (AB 24 + 400 + 30) led to a statistically significant difference in osteophyte OARSI score (P = .0019). None of the 4 lubricin mimetics injections incited an additive synovial inflammatory response. Immunohistochemical staining substantiated the binding capacity of all 4 mimetics, while in vivo experimentation revealed that the group 1 and 3 mimetics were still retained within the joint 4 weeks after injection. There were no differences in friction coefficients between any pair of groups and no significant trends based on lubricin mimetic structure.

CONCLUSION

We demonstrated that the tribosupplementation of a traumatically injured knee with a specific lubricin structural analog may attenuate the natural progression of OA.

CLINICAL RELEVANCE

The current lack of efficacious clinical options to counter the onset and subsequent development of OA suggests that further investigation into the synthesis and behavior of lubricin analogs could yield novel translational applications.

Age-related changes in the cartilage of the temporomandibular joint

Osteoarthritis (OA) of the knee is closely associated with aging; however, little is known about the age-related degeneration in the mandibular condylar cartilage (MCC) of the TMJ. Our objective was to examine whether a correlation exists between aging and degeneration of the MCC of the TMJ. Thirty-two male C57BL/6J wild-type mice were aged to 2, 12, 18, and 25 months old. The mice were euthanized by CO₂ inhalation and were dissected and examined by micro-CT and histology. Sagittal sections of the condyles were stained for tartrate-resistant alkaline phosphatase, alkaline phosphatase, safranin O, picrosirius red, and toluidine blue. In addition, immunostaining for BMP2, BMP4, BMP7, PRG4, and MMP13 was performed. Bone volume fraction and tissue density significantly increased with the age of the animals. There was a significant increase in the Osteoarthritis Research Society International histopathological score and mineralization of the noncalcified cartilage in the aged animals. There was a decrease in cartilage thickness, proteoglycan distribution, and cellularity in the aged animals. Additionally, we noted increased picrosirius red staining with the increase in the age of the animals. Our protein expression showed increased BMP2, BMP4, BMP7, and MMP13, whereas there was a decrease in PRG4 expression in the aged animals. As the animal ages, there is decreased proteoglycan secretion, decreased cellularity, decreased cartilage thickness, increased fibrillation, and increased proteolytic activity. A better understanding of the basic mechanisms underlying the degeneration of the MCC in the older animals could provide novel ways to slow the development of OA.

Influence of reduction accuracy in lateral tibial plateau fractures on intra-articular friction - a biomechanical study

BACKGROUND

Lateral tibial split fractures (LTSF) usually require surgical therapy with screw or plate osteosynthesis. Excellent anatomical reduction of the fracture is thereby essential to avoid post-traumatic osteoarthritis. In clinical practice, a gap and step of 2 mm have been propagated as maximum tolerable limit. To date, biomechanical studies regarding tibial fractures have been limited to pressure measurement, but the relationship between dissipated energy (DE) as a friction parameter and reduction accuracy in LTSF has not been investigated. In past experiments, we developed a new method to measure DE in ovine knee joints. To determine weather non-anatomical fracture reduction with lateral gap or vertical step condition leads to relevant changes in DE in the human knee joint, we tested the applicability of the new method on human LTSFs and investigated whether the current limit of 2 mm gap and step is durable from a biomechanical point of view.

METHODS

Seven right human, native knee joint specimens were cyclically moved under 400 N axial load using a robotic system. During the cyclic motion, the flexion angle and the respective torque were recorded and the DE was calculated. First, DE was measured after an anterolateral approach had been performed (condition "native"). Then a LTSF was set with a chisel, reduced anatomically, fixed with two set screws and DE was measured ("even"). DE of further reductions was then measured with gaps of 1 mm and 2 mm, and a 2 mm step down or a 2 mm step up was measured.

RESULTS

We successfully established a measurement protocol for DE in human knee joints with LTSF. While gaps led to small though statistically significant increase (1 mm gap:ΔDE compared with native = 0.030 J/cycle, (+ 21%), p = 0.02; 2 mm gap:ΔDE = 0.032 J/cycle, (+ 22%), p = 0.009), this increase almost doubled when reducing in a step-down condition (ΔDE = 0.058 J/cycle, (+ 56%), p = 0.042) and even tripled in the step-up condition (ΔDE = 0.097 J/cycle, (+ 94%), p = 0.004).

CONCLUSIONS

Based on our biomechanical findings, we suggest avoiding step conditions in the daily work in the operating theatre. Gap conditions can be handled a bit more generously.

Influence of Block Length on Articular Cartilage Lubrication with a Diblock Bottle-Brush Copolymer

We report how the tribological properties of a class of diblock copolymers with architecture and function inspired by the lubricating glycoprotein lubricin correlate to chemical composition. This class of diblock copolymers, consisting of a cationic cartilage-binding block and a brush-lubricating block, demonstrates that boundary lubrication of articular cartilage more strongly depends on the cartilage-binding block than the lubrication block. Specifically, the cartilage-binding functional groups (tertiary or quaternary amines) and cartilage-binding block length significantly influence the degree of lubrication under boundary mode experimental conditions. An optimal number (∼24 in this case) of cartilage-binding groups led to the lowest coefficient of friction, and an increase or decrease in the number of cations in the binding block led to partial (>24, and between 12 and 24) or complete (=12) loss of lubricating ability. The length of the lubricating block (DP = 200 or 400) chosen in this study had no effect on the degree of lubrication. These results are put into context in terms of binding affinity to the cartilage and the spatial packing density of the polymer on the cartilage surface and can serve as a useful guide for future designs of synthetic lubricants that rival the efficacy of natural lubricants.

The Roles of Indian Hedgehog Signaling in TMJ Formation

The temporomandibular joint (TMJ) is an intricate structure composed of the mandibular condyle, articular disc, and glenoid fossa in the temporal bone. Apical condylar cartilage is classified as a secondary cartilage, is fibrocartilaginous in nature, and is structurally distinct from growth plate and articular cartilage in long bones. Condylar cartilage is organized in distinct cellular layers that include a superficial layer that produces lubricants, a polymorphic/progenitor layer that contains stem/progenitor cells, and underlying layers of flattened and hypertrophic chondrocytes. Uniquely, progenitor cells reside near the articular surface, proliferate, undergo chondrogenesis, and mature into hypertrophic chondrocytes. During the past decades, there has been a growing interest in the molecular mechanisms by which the TMJ develops and acquires its unique structural and functional features. Indian hedgehog (Ihh), which regulates skeletal development including synovial joint formation, also plays pivotal roles in TMJ development and postnatal maintenance. This review provides a description of the many important recent advances in Hedgehog (Hh) signaling in TMJ biology. These include studies that used conventional approaches and those that analyzed the phenotype of tissue-specific mouse mutants lacking Ihh or associated molecules. The recent advances in understanding the molecular mechanism regulating TMJ development are impressive and these findings will have major implications for future translational medicine tools to repair and regenerate TMJ congenital anomalies and acquired diseases, such as degenerative damage in TMJ osteoarthritic conditions.

Lubricin expression in the lumbar endplate and its association with Modic changes

Objective

To explore the expression of lubricin in the lumbar endplate and its association with Modic changes (MCs).

Methods

Human endplate specimens harvested from patients undergoing surgery for thoracolumbar spine fractures ​or lumbar interbody fusion were divided into two groups: MCs group and normal group. Lubricin expression was examined by immunohistochemistry, and differences between the groups were analysed using quantitative polymerase chain reaction (qPCR). Lubricin expression and differences between endplates with MCs and normal endplates were confirmed using a rabbit model. In a final experiment, rabbit endplate chondrocytes were cocultured with Propionibacteria acnes (P. acnes) supernatant, and the expression of lubricin and endplate degeneration related genes were evaluated. In addition, the expression of matrix metalloproteinase 1(MMP-1), A disintegrin-like and metalloproteinase with thrombospondin type 5 motif (ADAMTS5) and inflammatory factors (Interleukin- 1β (IL-1β) and Interleukin-6 (IL-6)) were evaluated after lubricin overexpression.

Results

Lubricin was found in human lumbar endplates and its expression was lower in the MCs group compared to the normal group. In the rabbit model, lubricin was also found in the endplate. In rabbits injected with P. acnes (the MCs group), lubricin expression of endplate decreased compared to the normal group. In the culture of rabbit endplate chondrocytes with P. acnes supernatant, the expression of lubricin, aggrecan, sox9 and collagen type-II decreased significantly, while that of MMP-1 and ADAMTS5 increased significantly. Moreover, lubricin overexpression could downregulate the expression of MMP-1, ADAMTS5 and inflammatory factors (IL-1β and IL-6) compared to negative control.

Conclusion

Lubricin is present in the lumbar endplate where it may have an anti-inflammatory role. P. acnes infection inhibits lubricin expression by cartilage endplate cells and this may facilitate the progression of MCs and endplate degeneration.

The translational potential of this article

Lubricin may have an anti-inflammatory role. P. acnes infection inhibits lubricin expression by cartilage endplate cells and this may facilitate the progression of MCs and endplate degeneration.

Temporal changes in synovial fluid composition and elastoviscous lubrication in the equine carpal fracture model

The objective of this study was to examine temporal variations in synovial fluid composition and lubrication following articular fracture. Post-traumatic osteoarthritis (PTOA) was induced by creating an osteochondral fracture in the middle carpal joint of four horses while the contralateral limb served as a sham-operated control. Horses were exercised on a high-speed treadmill, and synovial fluid was collected pre-operatively and at serial timepoints until 75 days post-operatively. Lubricin and hyaluronic acid (HA) concentrations were measured using sandwich ELISAs, and the molecular weight distribution of HA was analyzed via gel electrophoresis. Synovial fluid viscosity and cartilage friction coefficients across all modes of lubrication were measured on days 0, 19, 33, and 61 using a commercial rheometer and a custom tribometer, respectively. HA concentrations were significantly decreased post-operatively, and high molecular weight HA (>6.1MDa) did not recover to pre-operative values by the study termination at day 75. Lubricin concentrations increased after surgery to a greater extent in the OA as compared to sham-operated limbs. Viscosity was significantly reduced after surgery. While boundary and elastoviscous mode friction coefficients did not vary, the transition number, representing the shift between these modes, was lower. Although more pronounced in the OA limbs, similar derangements in HA, HA molecular weight distribution, viscosity, and transition number were observed in the sham-operated limbs, which may be explained by synovial fluid washout during arthroscopy. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

A computational algorithm to simulate disorganization of collagen network in injured articular cartilage

Cartilage defects are a known risk factor for osteoarthritis. Estimation of structural changes in these defects could help us to identify high risk defects and thus to identify patients that are susceptible for the onset and progression of osteoarthritis. Here, we present an algorithm combined with computational modeling to simulate the disorganization of collagen fibril network in injured cartilage. Several potential triggers for collagen disorganization were tested in the algorithm following the assumption that disorganization is dependent on the mechanical stimulus of the tissue. We found that tensile tissue stimulus alone was unable to preserve collagen architecture in intact cartilage as collagen network reoriented throughout the cartilage thickness. However, when collagen reorientation was based on both tensile tissue stimulus and tensile collagen fibril strains or stresses, the collagen network architecture was preserved in intact cartilage. Using the same approach, substantial collagen reorientation was predicted locally near the cartilage defect and particularly at the cartilage-bone interface. The developed algorithm was able to predict similar structural findings reported in the literature that are associated with experimentally observed remodeling in articular cartilage. The proposed algorithm, if further validated, could help to predict structural changes in articular cartilage following post-traumatic injury potentially advancing to impaired cartilage function.

Novel role of CCN3 that maintains the differentiated phenotype of articular cartilage

Knowledge of the microenvironment of articular cartilage in health and disease is the key to accomplishing fundamental disease-modifying treatments for osteoarthritis. The proteins comprising the CCN Family are matricellular proteins with a remarkable relevance within the context of cartilage metabolism. CCN2 displays a great capability for regenerating articular cartilage, and CCN3 has been shown to activate the expression of genes related to articular chondrocytes and to repress genes related to endochondral ossification in epiphyseal chondrocytes. Moreover, mice lacking CCN3 protein have been shown to display ostearthritic changes in their knee articular cartilage. In this study, we employed a monoiodoacetic acid (MIA)-induced osteoarthritic model to investigate whether osteoarthritic changes in the cartilage are reciprocally accompanied by CCN3 down-regulation and an inducible overexpression system to evaluate the effects of CCN3 on articular chondrocytes in vitro. Finally, we also investigated the effects of exogenous CCN3 in vivo during the early stages of MIA-induced osteoarthritis. We discovered that CCN3 is expressed by articular chondrocytes in normal rat knees, whereas it is rapidly down-regulated in osteoarthritic knees. In vitro, we also discovered that CCN3 increases the proteoglycan accumulation, the gene expression of type II collagen, tenascin-C and lubricin, as well as the protein production of tenascin-C and lubricin in articular chondrocytes. In vivo, it was discovered that exogenous CCN3 increased tidemark integrity and produced an increased production of lubricin protein. The potential utility of CCN3 as a future therapeutic agent and possible strategies to improve its therapeutic functions are also discussed.

Early genetic restoration of lubricin expression in transgenic mice mitigates chondrocyte peroxynitrite release and caspase-3 activation

OBJECTIVE

This study investigated the ability of endogenous lubricin secretion to restore joint health following a brief <21 day, postnatal lubricin-null state, in a C57BL/6J Prg4 gene trap (GT) mouse under the control of cre-recombinase. Previously we showed that re-expression of lubricin at 21 days was partly restorative of joint lubrication.

DESIGN

The tibio-femoral joints of adult C57BL/6J mice containing lubricin, lacking lubricin, and postnatally lacking lubricin until restoration of lubricin expression at 7 days or 14 days of age were evaluated ex vivo. At 8-weeks of age, whole joint coefficient of friction (COF), and caspase-3 activation were measured and the tibial-femoral joints histologically analyzed for degenerative changes, following progressive cyclic loading. The peroxynitrite content of femoral head cartilage from these mice prior to cyclic loading was measured.

RESULTS

Mice that underwent gene recombination at 7 and 14 days of age did not reestablish low COF as joint cycling time increased and were histopathologically indistinguishable from the joints of lubricin-null littermates. However, cartilage from tibio-femoral joints that underwent recombination at 7 and 14 days of age had significantly fewer caspase-3 positive cells and significantly reduced peroxynitrite content compared to lubricin-null littermates.

CONCLUSIONS

The biological effects of lubricin, which include limiting inflammation via peroxynitrite production and caspase-3 activation, may be achieved without completely restituting low COF. However, fully recapitulating low COF may require undamaged cartilage surfaces or absence of biofouling, which may interfere with the activity of lubricin.

Cartilage matrix remodelling differs by disease state and joint type

Dramatic alterations in mechanical properties have been documented for osteoarthritic (OA) cartilage. However, the matrix composition underlying these changes has not been mapped and their aetiology is not entirely understood. We hypothesised that an understanding of the cartilage matrix heterogeneity could provide insights into the origin of these OA-related alterations. We generated serial transverse cryo sections for 7 different cartilage conditions: 2 joint sites (knee and hip), 2 disease states (healthy and OA) and 3 tissue depths (superficial, middle and deep). By laser capture microscopy, we acquired ~200 cartilage matrix specimens from territorial (T) and interterritorial (IT) regions for all 7 conditions. A standardised matrix area was collected for each condition for a total of 0.02 ± 0.001 mm3 (corresponding to 20 µg of tissue) from a total of 4800 specimens. Extracted proteins were analysed for abundance by targeted proteomics. For most proteins, a lower IT/T ratio was observed for the OA disease state and knee joint type. A major cause of the altered IT/T ratios was the decreased protein abundance in IT regions. The collagenase-derived type III collagen neo-epitope, indicative of collagen proteolysis, was significantly more abundant in OA cartilage. In addition, it was enriched on average of 1.45-fold in IT relative to T matrix. These results were consistent with an elevated proteolysis in IT regions of OA cartilage, due to degenerative influences originating from synovial tissue and/or produced locally by chondrocytes. In addition, they offered direct evidence for dynamic remodelling of cartilage and provided a cogent biochemical template for understanding the alterations of matrix mechanical properties.

Effect of bone-soft tissue friction on ultrasound axial shear strain elastography

Bone-soft tissue friction is an important factor affecting several musculoskeletal disorders, frictional syndromes and the ability of a bone fracture to heal. However, this parameter is difficult to determine using non-invasive imaging modalities, especially in clinical settings. Ultrasound axial shear strain elastography is a non-invasive imaging modality that has been used in the recent past to estimate the bonding between different tissue layers. As most elastography methods, axial shear strain elastography is primarily used in soft tissues. More recently, this technique has been proposed to assess the bone-soft tissue interface. In this paper, we investigate the effect of a variation in bone-soft tissue friction coefficient in the resulting axial shear strain elastograms. Finite element poroelastic models of bone specimens exhibiting different bone-soft tissue friction coefficients were created and mechanically analyzed. These models were then imported to an ultrasound elastography simulation module to assess the presence of axial shear strain patterns. In vitro experiments were performed to corroborate selected simulation results. The results of this study show that the normalized axial shear strain estimated at the bone-soft tissue interface is statistically correlated to the bone-soft tissue coefficient of friction. This information may prove useful to better interpret ultrasound elastography results obtained in bone-related applications and, possibly, monitor bone healing.

Tailoring hydrogel surface properties to modulate cellular response to shear loading

Biological tissues at articulating surfaces, such as articular cartilage, typically have remarkable low-friction properties that limit tissue shear during movement. However, these frictional properties change with trauma, aging, and disease, resulting in an altered mechanical state within the tissues. Yet, it remains unclear how these surface changes affect the behaviour of embedded cells when the tissue is mechanically loaded. Here, we developed a cytocompatible, bilayered hydrogel system that permits control of surface frictional properties without affecting other bulk physicochemical characteristics such as compressive modulus, mass swelling ratio, and water content. This hydrogel system was applied to investigate the effect of variations in surface friction on the biological response of human articular chondrocytes to shear loading. Shear strain in these hydrogels during dynamic shear loading was significantly higher in high-friction hydrogels than in low-friction hydrogels. Chondrogenesis was promoted following dynamic shear stimulation in chondrocyte-encapsulated low-friction hydrogel constructs, whereas matrix synthesis was impaired in high-friction constructs, which instead exhibited increased catabolism. Our findings demonstrate that the surface friction of tissue-engineered cartilage may act as a potent regulator of cellular homeostasis by governing the magnitude of shear deformation during mechanical loading, suggesting a similar relationship may also exist for native articular cartilage.

STATEMENT OF SIGNIFICANCE

Excessive mechanical loading is believed to be a major risk factor inducing pathogenesis of articular cartilage and other load-bearing tissues. Yet, the mechanisms leading to increased transmission of mechanical stimuli to cells embedded in the tissue remain largely unexplored. Here, we demonstrate that the tribological properties of loadbearing tissues regulate cellular behaviour by governing the magnitude of mechanical deformation arising from physiological tissue function. Based on these findings, we propose that changes to articular surface friction as they occur with trauma, aging, or disease, may initiate tissue pathology by increasing the magnitude of mechanical stress on embedded cells beyond a physiological level.

Reduction of friction by recombinant human proteoglycan 4 in IL-1α stimulated bovine cartilage explants

A boundary lubricant attaches and protects sliding bearing surfaces by preventing interlocking asperity-asperity contact. Proteoglycan-4 (PRG4) is a boundary lubricant found in the synovial fluid that provides chondroprotection to articular surfaces. Inflammation of the diarthrodial joint modulates local PRG4 concentration. Thus, we measured the effects of inflammation, with Interleukin-1α (IL-1α) incubation, upon boundary lubrication and PRG4 expression in bovine cartilage explants. We further aimed to determine whether the addition of exogenous human recombinant PRG4 (rhPRG4) could mitigate the effects of inflammation on boundary lubrication and PRG4 expression in vitro. Cartilage explants, following a 7 day incubation with IL-1α, were tested in a disc-on-disc configuration using either rhPRG4 or saline (PBS control) as a lubricant. Following mechanical testing, explants were studied immunohistochemically or underwent RNA extraction for real-time polymerase chain reaction (RT-PCR). We found that static coefficient of friction (COF) significantly decreased to 0.14 ± 0.065 from 0.21 ± 0.059 (p = 0.014) in IL-1α stimulated explants lubricated with rhPRG4, as compared to PBS. PRG4 expression was significantly up regulated from 30.8 ± 19 copies in control explants lubricated with PBS to 3330 ± 1760 copies in control explants lubricated with rhPRG4 (p < 0.001). Explants stimulated with IL-1α displayed no increase in PRG4 expression upon lubrication with rhPRG4, but with PBS as the lubricant, IL-1α stimulation significantly increased PRG4 expression compared to the control condition from 30.8 ± 19 copies to 401 ± 340 copies (p = 0.015). Overall, these data suggest that exogenous rhPRG4 may provide a therapeutic option for reducing friction in transient inflammatory conditions and increasing PRG4 expression. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:580-589, 2017.

Binding and lubrication of biomimetic boundary lubricants on articular cartilage

The glycoprotein, lubricin, is the primary boundary lubricant of articular cartilage and has been shown to prevent cartilage damage after joint injury. In this study, a library of eight bottle-brush copolymers were synthesized to mimic the structure and function of lubricin. Polyethylene glycol (PEG) grafted onto a polyacrylic acid (pAA) core mimicked the hydrophilic mucin-like domain of lubricin, and a thiol terminus anchored the polymers to cartilage surfaces much like lubricin's C-terminus. These copolymers, abbreviated as pAA-g-PEG, rapidly bound to cartilage surfaces with binding time constants ranging from 20 to 39 min, and affected lubrication under boundary mode conditions with coefficients of friction ranging from 0.140 ± 0.024 to 0.248 ± 0.030. Binding and lubrication were highly correlated (r²  = 0.89-0.99), showing that boundary lubrication in this case strongly depends on the binding of the lubricant to the surface. Along with time-dependent and dose-dependent behavior, lubrication and binding of the lubricin-mimetics also depended on copolymer structural parameters including pAA backbone length, PEG side chain length, and PEG:AA brush density. Polymers with larger backbone sizes, brush sizes, or brush densities took longer to bind (p < 0.05). Six of the eight polymers reduced friction relative to denuded cartilage plugs (p < 0.05), suggesting their potential to lubricate and protect cartilage in vivo. In copolymers with shorter pAA backbones, increasing hydrodynamic size inhibited lubrication (p < 0.08), while the opposite was observed in copolymers with longer backbones (p < 0.05). These polymers show similar in vitro lubricating efficacy as recombinant lubricins and as such have potential for in vivo treatment of post-traumatic osteoarthritis. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:548-557, 2017.

Use of pre-clinical surgically induced models to understand biomechanical and biological consequences of PTOA development

Post-traumatic osteoarthritis (PTOA) development is often observed following traumatic knee injuries involving key stabilising structures such as the cruciate ligaments or the menisci. Both biomechanical and biological alterations that follow knee injuries have been implicated in PTOA development, although it has not been possible to differentiate clearly between the two causal factors. This review critically examines the outcomes from pre-clinical lapine and ovine injury models arising in the authors' laboratories and differing in severity of PTOA development and progression. Specifically, we focus on how varying severity of knee injuries influence the subsequent alterations in kinematics, kinetics, and biological outcomes. The immediate impact of injury on the lubrication capacity of the joint is examined in the context of its influence on biomechanical alterations, thus linking the biological changes to abnormal kinematics, leading to a focus on the potential areas for interventions to inhibit or prevent development of the disease. We believe that PTOA results from altered cartilage surface interactions where biological and biomechanical factors intersect, and mitigating acute joint inflammation may be critical to prolonging PTOA development. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:454-465, 2017.

Implications of trauma and subsequent articulation on the release of Proteoglycan-4 and tissue response in adult human ankle cartilage

The purpose of this study was to investigate the effects of trauma and subsequent articulation on adult human ankle cartilage subjected to an injurious impact. Trauma was initiated through impaction on talar cartilage explants. Articulation and loading were applied in a joint bioreactor over 5 consecutive days. The early (24 h) effects of impaction included a reduced chondrocytes viability (51% vs. 81% for non-impacted; p = 0.03), increased levels of apoptosis (43% vs. 27%; p = 0.03), and an increase in the histopathology score (4.4 vs. 1.7; p = 0.02) as compared to non-impacted cartilage explants. One of the key findings was that damage also stimulated the PRG4 release (2.2 vs. 1.5 μg/ml). Subsequent articulation for 5 days did not lead to further changes in tissue histopathology and cell viability, neither for injured nor non-injured samples. However, articulation led to an increased apoptosis in the injured samples (p = 0.03 for the interaction term). Articulation also caused a significant increase of PG/GAG release into the culture medium (p = 0.04) for both injured and non-injured samples; however, the synthesis of PG was not affected by articulation (p = 0.45) though the PG synthesis was higher in injured samples (p < 0.01). With regard to the PRG4 release, impacted samples continued to show higher amounts (p = 0.01), adding articulation led to a reduction (p = 0.02). The current study demonstrated that adult human talar cartilage increases both the PRG4 release and biosynthetic activity as an immediate cellular response to injury. Articulation played a less contributing role to biosynthesis and remodeling, behaving mostly neutral, in that no further damage emerged. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:667-676, 2017.

Lubricin/proteoglycan 4 increases in both experimental and naturally occurring equine osteoarthritis

OBJECTIVE

The goals of this study were (1) to quantify proteoglycan 4 (PRG4) gene expression; (2) to assess lubricin immunostaining; and (3) to measure synovial fluid lubricin concentrations in clinical and experimental models of equine carpal osteoarthritis (OA).

DESIGN

Lubricin synovial fluid concentrations and cartilage and synovial membrane PRG4 expression were analyzed in research horses undergoing experimental OA induction (n = 8) and in equine clinical patients with carpal OA (n = 58). Lubricin concentrations were measured using a custom sandwich enzyme-linked immunosorbent assay, and PRG4 expression was quantified using qRT-PCR. Lubricin immunostaining was assessed in synovial membrane and osteochondral sections in the experimental model.

RESULTS

Lubricin concentrations increased in synovial fluid following induction of OA, peaking at 21 days post-operatively in OA joints vs sham-operated controls (331 ± 69 μg/mL vs 110 ± 19 μg/mL, P = 0.001). Lubricin concentrations also increased in horses with naturally occurring OA as compared to control joints (152 ± 32 μg/mL vs 68 ± 4 μg/mL, P = 0.003). Synovial membrane PRG4 expression increased nearly 2-fold in naturally occurring OA (P = 0.003), whereas cartilage PRG4 expression decreased 2.5-fold (P = 0.025). Lubricin immunostaining was more pronounced in synovial membrane from OA joints as compared to controls, with intense lubricin localization to sites of cartilage damage.

CONCLUSIONS

Although PRG4 gene expression decreases in OA cartilage, synovial membrane PRG4 expression, synovial fluid lubricin concentrations and lubricin immunostaining all increase in an equine OA model. Lubricin may be elevated to protect joints from post-traumatic OA.

Lubricin in synovial fluid of mild and severe temporomandibular joint internal derangements

BACKGROUND

To understand the molecular basis of temporomandibular joint (TMJ) pathologies, we aimed to investigate the lubricin levels in the TMJ synovial fluid (SF) of patients with mild to severe internal derangements (IDs).

MATERIAL AND METHODS

A total, 34 joints were the study group. Only patients, with a Wilkes stage of III, IV and V were included, in this sample. Control group consisted of SF from eight joints, from patients undergoing to orthognatic surgery. Concentrations of lubricin in the SF from both samples were measured using ELISA system.

RESULTS

The mean lubricin concentration was 7.029 ± 0.21 µg/mL in stage III patients; 5.64 ± 0.10 µg/mL in stage IV patients, and 4.78 ± 0.11 µg/mL in stage V patients. The lubricin levels from stage IV and stage V patients differed significantly (P ≤ 0.001) from those of control subjects. Lubricin levels were inversely correlated with age and to VAS score.

CONCLUSIONS

The results of this cross-sectional study highlight the relationship between disease severity and the levels of lubricin in TMJ SF. Our findings suggest that novel biotherapeutic approaches, including the administration of recombinant lubricin in the joint cavity, for the treatment of TMJ diseases can be developed.

Galectin-3 Binds to Lubricin and Reinforces the Lubricating Boundary Layer of Articular Cartilage

Lubricin is a mucinous, synovial fluid glycoprotein that enables near frictionless joint motion via adsorption to the surface of articular cartilage and its lubricating properties in solution. Extensive O-linked glycosylation within lubricin’s mucin-rich domain is critical for its boundary lubricating function; however, it is unknown exactly how glycosylation facilitates cartilage lubrication. Here, we find that the lubricin glycome is enriched with terminal β-galactosides, known binding partners for a family of multivalent lectins called galectins. Of the galectin family members present in synovial fluid, we find that galectin-3 is a specific, high-affinity binding partner for lubricin. Considering the known ability of galectin-3 to crosslink glycoproteins, we hypothesized that galectins could augment lubrication via biomechanical stabilization of the lubricin boundary layer. We find that competitive inhibition of galectin binding results in lubricin loss from the cartilage surface, and addition of multimeric galectin-3 enhances cartilage lubrication. We also find that galectin-3 has low affinity for the surface layer of osteoarthritic cartilage and has reduced affinity for sialylated O-glycans, a glycophenotype associated with inflammatory conditions. Together, our results suggest that galectin-3 reinforces the lubricin boundary layer; which, in turn, enhances cartilage lubrication and may delay the onset and progression of arthritis.

Superficial zone cellularity is deficient in mice lacking lubricin: a stereoscopic analysis

Background

Lubricin, a mucinous glycoprotein secreted by synoviocytes and chondrocytes plays an important role in reducing the coefficient of friction in mammalian joints. Elevated cartilage surface friction is thought to cause chondrocyte loss; however, its quantification and methodological approaches have not been reported. We adapted a stereological method and incorporated vital cell staining to assess cellular loss in superficial and upper intermediate zones in lubricin deficient mouse cartilage.

Methods

The femoral condyle cartilage of the intact knees from lubricin wild type (Prg4^+/+), heterozygote (Prg4^+/-), and knockout (Prg4^-/-) mice was imaged using fluorescein diacetate (FDA), propidium iodide (PI), and Hoechst staining, and confocal microscopy. Three dimensional reconstructions of confocal images to a depth of 14 μm were analyzed using Matlab to determine the volume fraction occupied by chondrocytes in cartilage of both medial and lateral femoral condyles. Living chondrocyte volume fraction was defined as FDA stained chondrocyte volume/total volume of superficial + upper intermediate zone. Living and dead (total) chondrocyte volume fraction was defined as FDA + PI stained chondrocyte volume/total volume of superficial + upper intermediate zone. MicroCT provided an orthogonal measure of cartilage thickness. Immunohistology for activated caspase-3 and TUNEL staining were performed to evaluate the presence of apoptotic chondrocytes in Prg4 mutant mice.

Results

Living chondrocyte volume fraction of the medial femoral condyle was significantly lower in Prg4^-/- mice compared to Prg4^+/+ (p = 0.002) and Prg4^+/- (p = 0.002) littermates. There was no significant difference in medial condyle chondrocyte volume fraction between Prg4^+/+ and Prg4^+/- mice (p = 0.82). No significant differences were observed for the chondrocyte volume fraction for the lateral condyle (p > 0.26). Cartilage thickness increased in the medial condyle for Prg4^-/- mice compared to Prg4^+/+ (p = 0.02) and Prg4^+/- (p = 0.03) littermates, and the lateral condyle for Prg4^-/- mice compared to Prg4^+/+ (p < 0.0001) and Prg4^+/- (p < 0.0001) littermates, indicating that a multi-dimensional increase in cartilage volume did not artifactually lower the chondrocyte volume fraction in the medial condyle. Significantly higher number of caspase-3 positive cells were observed in the superficial and upper intermediate zone cartilage of the medial femoral condyle of Prg4^-/- mice compared to Prg4^+/+ (p = 0.01) and Prg4^+/- (p = 0.04) littermates, and the lateral femoral condyle of Prg4^-/- mice compared to Prg4^+/+ (p = 0.02) and Prg4^+/- (p = 0.02) littermates. There were no significant differences in TUNEL staining among different Prg4 genotypes in both condyles (p > 0.05 for all comparisons).

Conclusions

Increased Caspase-3 activation is observed in Prg4 deficient mice compared to Prg4 sufficient littermates. Absence of Prg4 induces loss of chondrocytes in the superficial and upper intermediate zone of mouse cartilage that is quantifiable by a novel image processing technique.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-016-0967-4) contains supplementary material, which is available to authorized users.

Cartilage Derived from Bone Marrow Mesenchymal Stem Cells Expresses Lubricin In Vitro and In Vivo

OBJECTIVE

Lubricin expression in the superficial cartilage will be a crucial factor in the success of cartilage regeneration. Mesenchymal stem cells (MSCs) are an attractive cell source and the use of aggregates of MSCs has some advantages in terms of chondrogenic potential and efficiency of cell adhesion. Lubricin expression in transplanted MSCs has not been fully elucidated so far. Our goals were to determine (1) whether cartilage pellets of human MSCs expressed lubricin in vitro chondrogenesis, (2) whether aggregates of human MSCs promoted lubricin expression, and (3) whether aggregates of MSCs expressed lubricin in the superficial cartilage after transplantation into osteochondral defects in rats.

METHODS

For in vitro analysis, human bone marrow (BM) MSCs were differentiated into cartilage by pellet culture, and also aggregated using the hanging drop technique. For an animal study, aggregates of BM MSCs derived from GFP transgenic rats were transplanted to the osteochondral defect in the trochlear groove of wild type rat knee joints. Lubricin expression was mainly evaluated in differentiated and regenerated cartilages.

RESULTS

In in vitro analysis, lubricin was detected in the superficial zone of the pellets and conditioned medium. mRNA expression of Proteoglycan4 (Prg4), which encodes lubricin, in pellets was significantly higher than that of undifferentiated MSCs. Aggregates showed different morphological features between the superficial and deep zone, and the Prg4 mRNA expression increased after aggregate formation. Lubricin was also found in the aggregate. In a rat study, articular cartilage regeneration was significantly better in the MSC group than in the control group as shown by macroscopical and histological analysis. The transmission electron microscope showed that morphology of the superficial cartilage in the MSC group was closer to that of the intact cartilage than in the control group. GFP positive cells remained in the repaired tissue and expressed lubricin in the superficial cartilage.

CONCLUSION

Cartilage derived from MSCs expressed lubricin protein both in vitro and in vivo. Aggregation promoted lubricin expression of MSCs in vitro and transplantation of aggregates of MSCs regenerated cartilage including the superficial zone in a rat osteochondral defect model. Our results indicate that aggregated MSCs could be clinically relevant for therapeutic approaches to articular cartilage regeneration with an appropriate superficial zone in the future.

Hyaluronan concentration and size distribution in human knee synovial fluid: variations with age and cartilage degeneration

Background

One potential mechanism for early superficial cartilage wear in normal joints is alteration of the lubricant content and quality of synovial fluid. The purpose of this study was to determine if the concentration and quality of the lubricant, hyaluronan, in synovial fluid: (1) was similar in left and right knees; (2) exhibited similar age-associated trends, whether collected postmortem or antemortem; and (3) varied with age and grade of joint degeneration.

Methods

Human synovial fluid of donors (23–91 years) without osteoarthritis was analyzed for the concentrations of protein, hyaluronan, and hyaluronan in the molecular weight ranges of 2.5–7 MDa, 1–2.5 MDa, 0.5–1 MDa, and 0.03–0.5 MDa. Similarity of data between left and right knees was assessed by reduced major axis regression, paired t-test, and Bland-Altman analysis. The effect of antemortem versus postmortem collection on biochemical properties was assessed for age-matched samples by unpaired t-test. The relationships between age, joint grade, and each biochemical component were assessed by regression analysis.

Results

Joint grade and the concentrations of protein, hyaluronan, and hyaluronan in the molecular weight ranges of 2.5–7 MDa, 1–2.5 MDa, and 0.5–1 MDa in human synovial fluid showed good agreement between left and right knees and were similar between age-matched patient and cadaver knee joints. There was an age-associated decrease in overall joint grade (–15 %/decade) and concentrations of hyaluronan (–10.5 %/decade), and hyaluronan in the molecular weight ranges of 2.5–7 MDa (–9.4 %/decade), 1–2.5 MDa (–11.3 %/decade), 0.5–1 MDa (–12.5 %/decade), and 0.03–0.5 MDa (–13.0 %/decade). Hyaluronan concentration and quality was more strongly associated with age than with joint grade.

Conclusions

The age-related increase in cartilage wear in non-osteoarthritic joints may be related to the altered hyaluronan content and quality of synovial fluid.

Non-invasive mouse models of post-traumatic osteoarthritis

Animal models of osteoarthritis (OA) are essential tools for investigating the development of the disease on a more rapid timeline than human OA. Mice are particularly useful due to the plethora of genetically modified or inbred mouse strains available. The majority of available mouse models of OA use a joint injury or other acute insult to initiate joint degeneration, representing post-traumatic osteoarthritis (PTOA). However, no consensus exists on which injury methods are most translatable to human OA. Currently, surgical injury methods are most commonly used for studies of OA in mice; however, these methods may have confounding effects due to the surgical/invasive injury procedure itself, rather than the targeted joint injury. Non-invasive injury methods avoid this complication by mechanically inducing a joint injury externally, without breaking the skin or disrupting the joint. In this regard, non-invasive injury models may be crucial for investigating early adaptive processes initiated at the time of injury, and may be more representative of human OA in which injury is induced mechanically. A small number of non-invasive mouse models of PTOA have been described within the last few years, including intra-articular fracture of tibial subchondral bone, cyclic tibial compression loading of articular cartilage, and anterior cruciate ligament (ACL) rupture via tibial compression overload. This review describes the methods used to induce joint injury in each of these non-invasive models, and presents the findings of studies utilizing these models. Altogether, these non-invasive mouse models represent a unique and important spectrum of animal models for studying different aspects of PTOA.

Does intra-articular fracture change the lubricant content of synovial fluid?

BACKGROUND

Lubrication function is impaired and the lubricant content of synovial fluid (SF) changes immediately after plateau tibia fractures. Here, we aimed to analyze the lubricant content of SF at chronic term following plateau tibia fracture.

METHODS

Forty-eight surgically treated patients without joint incongruency (<2 mm displacement) were included in the study. Joint aspiration had been possible in 16 of the participants. However, sampling could be made from healthy knees in only ten of these patients. Twenty-six SF samples (16 injured knees, 10 healthy knees) were analyzed for concentrations of hyaluronic acid (HA), proteoglycan-4 (PRG4), TNF-α, IL-1β, and IL-6.

RESULTS

The group of experimental samples were obtained at a mean of 31 (12-66) months after injury from patients with a mean age of 45.1 (32-57) years. There were no relationships detected between biochemical analysis results and patient ages, sexes, postoperative time, and fracture type. After excluding six patients for whom we could not sample from their healthy knee, ten patients' values were compared with paired Wilcoxon signed rank test and no significant differences detected between the healthy and injured knee in terms of the SF concentrations of HA and PRG4 (p = 0.225 and 0.893, respectively). Similarly, there were no statistically significant differences in SF sample concentrations of TNF-α, IL-1β, and IL-6 between healthy and injured knees.

CONCLUSIONS

Despite acute changes, the long-term concentrations of HA and PRG4 were similar after plateau tibial fracture. We could not detect any concentration level differences between healthy knees and injured knees regarding HA and PRG4 in the long-term follow-up.

Treatment with recombinant lubricin attenuates osteoarthritis by positive feedback loop between articular cartilage and subchondral bone in ovariectomized rats

Osteoarthritis (OA) is a most commonly multifactorial degenerative joint disease along with the aging population, particularly in postmenopausal women. During the onset of OA, articular cartilage and subchondral bone act in concert as a functional unit. This present study is to investigate the effects of early or late treatment with recombinant lubricin on the onset of osteoarthritis (OA) in ovariectomized (OVX) rats. We found that both early and late recombinant lubricin treatments attenuated the onset of OA by positive feedback loop between articular cartilage and subchondral bone, although late treatment contributed to a lesser effect compared with early treatment. Specifically, treatment with recombinant lubricin protected articular cartilage from degeneration, demonstrated by lower proteoglycan loss, lower OARSI scores, less calcification cartilage zone and reduced immunostaining for collagen X (Col X) and matrix metalloproteinase (MMP-13) but increased the expression of lubricin, in comparison with vehicle-treated OVX rat group. Further, chondroprotective effects of lubricin normalized bone remodeling in subchondral bone underneath. It's suggested that treatment with recombinant lubricin inhibited the elevation of TRAP and Osterix positive cells in OVX rats and led to the normalization of subchondral bone microarchitectures with the suppression of subsidence of bone volume ratio (BV/TV) and trabecular thickness (Tb.Th) and the increase of trabecular separation (Tb.Sp) in vehicle-treated OVX rats. What's more, the normalization of subchondral bone in turn attenuated the articular cartilage erosion by inhibiting vascular invasion from subchondral bone to calcified cartilage zone, exemplified by inhibiting the elevation of CD31 positive cells in calcified cartilage and angiography in subchondral bone. Together, these results shed light that both early and late recombinant lubricin treatments attenuate the onset of OA by balancing the interplay between articular cartilage and subchondral bone in OVX rats, while also providing a further rationale for its therapeutic targeting to postmenopausal OA and suggesting that treatment timing is a pivotal factor for better effect acquisition.

Cartilage boundary lubrication of ovine synovial fluid following anterior cruciate ligament transection: a longitudinal study

OBJECTIVE

To assess ovine synovial fluid (oSF) from different post-injury time points for (1) proteoglycan-4 (PRG4) and hyaluronan (HA) concentration, (2) HA molecular weight (MW) distribution, (3) cartilage boundary lubrication function, and (4) lubricant composition-function relationships. The association between cartilage boundary lubrication and gross cartilage changes after injury was also examined.

METHODS

oSF was collected 2, 4, 10, and 20 weeks post anterior cruciate ligament (ACL) transection in five skeletally mature sheep. PRG4 and HA concentrations were measured using sandwich enzyme-linked immunosorbent assay, and HA MW distribution by agarose gel electrophoresis. Cartilage boundary lubrication of oSF was assessed using a cartilage-cartilage friction test. Gross damage to articular cartilage was also quantified at 20 weeks using modified Drez scoring protocol.

RESULTS

Early (2-4 weeks) after ACL injury, PRG4 concentrations were significantly higher (P = 0.045, P = 0.037), and HA concentrations were substantially lower (P = 0.005, P = 0.005) compared to 20 weeks. The HA MW distribution also shifted towards lower ranges in the early post-injury stage. The kinetic friction coefficients were significantly higher 2-4 weeks post injury (P = 0.008 and P = 0.049) compared to 20 weeks. Poor cartilage boundary lubricating ability early after injury was associated with cartilage damage at 20 weeks.

CONCLUSION

Altered composition and diminished boundary lubrication of oSF early after ACL transection may pre-dispose the articular cartilage to degenerative changes and initiate osteoarthritis (OA). These observations also provide potential motivation for biotherapeutic interventions at earlier time points post injury.

Simulation of in vivo dynamics during robot assisted joint movement

Background

Robots are very useful tools in orthopedic research. They can provide force/torque controlled specimen motion with high repeatability and precision. A method to analyze dissipative energy outcome in an entire joint was developed in our group. In a previous study, a sheep knee was flexed while axial load remained constant during the measurement of dissipated energy. We intend to apply this method for the investigation of osteoarthritis. Additionally, the method should be improved by simulation of in vivo knee dynamics. Thus, a new biomechanical testing tool will be developed for analyzing in vitro joint properties after different treatments.

Methods

Discretization of passive knee flexion was used to construct a complex flexion movement by a robot and simulate altering axial load similar to in vivo sheep knee dynamics described in a previous experimental study.

Results

The robot applied an in vivo like axial force profile with high reproducibility during the corresponding knee flexion (total standard deviation of 0.025 body weight (BW)). A total residual error between the in vivo and simulated axial force was 0.16 BW. Posterior-anterior and medio-lateral forces were detected by the robot as a backlash of joint structures. Their curve forms were similar to curve forms of corresponding in vivo measured forces, but in contrast to the axial force, they showed higher total standard deviation of 0.118 and 0.203 BW and higher total residual error of 0.79 and 0.21 BW for posterior-anterior and medio-lateral forces respectively.

Conclusions

We developed and evaluated an algorithm for the robotic simulation of complex in vivo joint dynamics using a joint specimen. This should be a new biomechanical testing tool for analyzing joint properties after different treatments.

Gene expression of catabolic inflammatory cytokines peak before anabolic inflammatory cytokines after ACL injury in a preclinical model

Background

The response of the joint to anterior cruciate ligament (ACL) injury has not been fully characterized. In particular, the characterization of both catabolic factors, including interleukin-6 (IL-6), interleukin-8 (IL-8), and markers of ongoing tissue damage (CRP), and anabolic factors, including vascular endothelial growth factor (VEGF), transforming growth factor β-induced (TGFβI), and the presence of CD163+ macrophages, have not been well defined. In this study, we hypothesized ACL injury would catalyze both catabolic and anabolic processes and that these would have different temporal profiles of expression.

Methods

Adolescent Yucatan minipigs were subjected to ACL transection. Within the joint, gene expression levels of IL-6, IL-8, VEGF, and TGFβI were quantified in the synovium, ligament, and provisional scaffold located between the torn ligament ends at days 1, 5, 9, and 14 post-injury. Macrophage infiltration was also assessed in the joint tissues over the two week period. Serum C-reactive protein (CRP) levels were measured at multiple time points between 1 hour to 14 days after injury.

Results

Increases in IL-6 and IL-8 gene expression peaked at day 1 after injury in the synovium and ligament. CRP levels were significantly increased at day 3 before returning to pre-injury levels. VEGF and TGFβI gene expression did not significantly increase until day 9 in the synovium and were unchanged in the other tissues. CD163+ macrophages increased in the ligament and synovium until day 9.

Conclusion

Taken together, these results suggest that the response within the joint is primarily catabolic in the first three days after injury, switching to a more anabolic phase by nine days after injury. The effect of medications which alter these processes may thus depend on the timing of administration after injury.

Synovial Fluid Lubricant Properties are Transiently Deficient after Arthroscopic Articular Cartilage Defect Repair with Platelet-Enriched Fibrin Alone and with Mesenchymal Stem Cells

BACKGROUND

Following various types of naturally-occurring traumatic injury to an articular joint, the lubricating ability of synovial fluid is impaired, with a correlated alteration in the concentration and/or structure of lubricant molecules, hyaluronan and proteoglycan-4. However, the effect of arthroscopic cartilage repair surgery on synovial fluid lubricant function and composition is unknown.

HYPOTHESIS

Arthroscopic treatment of full-thickness chondral defects in horses with (1) platelet-enriched fibrin or (2) platelet-enriched fibrin+mesenchymal stem cells leads to equine synovial fluid with impaired lubricant function and hyaluronan and proteoglycan-4 composition.

STUDY DESIGN

Controlled Laboratory Study.

METHODS

Equine synovial fluid was aspirated from normal joints at a pre-injury state (0 days) and at 10 days and 3 months following fibrin or fibrin+mesenchymal stem cell repair of full thickness chondral defects. Equine synovial fluid samples were analyzed for friction-lowering boundary lubrication of normal articular cartilage (static and kinetic friction coefficients) and concentrations of hyaluronan and proteoglycan-4, as well as molecular weight distribution of hyaluronan. Experimental groups deficient in lubrication function were also tested for the ability of exogenous high-molecular weight hyaluronan to restore lubrication function.

RESULTS

Lubrication and biochemical data varied with time after surgery but generally not between repair groups. Relative to pre-injury, kinetic friction was higher (+94%) at 10 days but returned to baseline levels at 3 months while static friction was not altered. Correspondingly, hyaluronan concentration was transiently lower (-64%) and shifted towards lower molecular weight forms, while proteoglycan-4 concentration was increased (+210%) in 10-day samples relative to pre-injury levels. Regression analysis revealed that kinetic friction decreased with increasing total and high molecular weight hyaluronan. Addition of high molecular weight hyaluronan to bring 10-day hyaluronan levels to 2.0mg/ml restored kinetic friction to pre-injury levels.

CONCLUSION

Following arthroscopic surgery for cartilage defect repair, synovial fluid lubrication function is transiently impaired, in association with decreased hyaluronan concentration. This functional deficiency in synovial fluid lubrication can be counteracted in vitro by addition of high molecular weight hyaluronan.

CLINICAL RELEVANCE

Synovial fluid lubrication is deficient shortly following arthroscopic cartilage repair surgery, and supplementation with high molecular weight hyaluronan may be beneficial.