In Vivo Study of Osteochondral Defect Regeneration Using Innovative Composite Calcium Phosphate Biocement in a Sheep Model

This study aimed to clarify the therapeutic effect and regenerative potential of the novel, amino acids-enriched acellular biocement (CAL) based on calcium phosphate on osteochondral defects in sheep. Eighteen sheep were divided into three groups, the treated group (osteochondral defects filled with a CAL biomaterial), the treated group with a biocement without amino acids (C cement), and the untreated group (spontaneous healing). Cartilages of all three groups were compared with natural cartilage (negative control). After six months, sheep were evaluated by gross appearance, histological staining, immunohistochemical staining, histological scores, X-ray, micro-CT, and MRI. Treatment of osteochondral defects by CAL resulted in efficient articular cartilage regeneration, with a predominant structural and histological characteristic of hyaline cartilage, contrary to fibrocartilage, fibrous tissue or disordered mixed tissue on untreated defect (p < 0.001, modified O’Driscoll score). MRI results of treated defects showed well-integrated and regenerated cartilage with similar signal intensity, regularity of the articular surface, and cartilage thickness with respect to adjacent native cartilage. We have demonstrated that the use of new biocement represents an effective solution for the successful treatment of osteochondral defects in a sheep animal model, can induce an endogenous regeneration of cartilage in situ, and provides several benefits for the design of future therapies supporting osteochondral defect healing.     

Articular cartilage regeneration

The main functions of articular cartilages are load-bearing and reducing friction of the articular surfaces. Because the capacity of articular cartilage to repair is limited, many attempts have been made to repair articular cartilage defects, which include transplantations of various tissues or cells. Within them, autologous cultured chondrocyte or autologous bone marrow mesenchymal cell transplantations are reported to be useful methods to repair articular cartilage defects. Here, we introduce these methods of tissue engineering and gene therapy, which are expected to become new treatments of articular cartilage defects.     

Treatment of joint cartilage lesions with cell therapy

Articular cartilage lesions which do not affect the integrity of subchondral bone, they are not able to repair it expontaneously. The asymptomatic nature of these lesions induces articular cartilage degeneration and development of an arthrosic process. To avoid the necessity to receive joint replacement surgery, it has been developed different treatments of cellular therapy which are focused to create new tissues whose structure, biochemistry composition and function will be the same than native articular cartilage. Approaches used to access the stream produce a fibrocartilaginose tissue which is not an articular cartilage. Implantation of autologous chondrocytes and autologous mosaicplasties induces a quality better articular cartilage. Furthermore both techniques involve damage in the sane cartilage; because of trying to get a big amount of chondrocytes or because of extraction osteochondral cylinder which will be implanted in the injured joint. The stem cells are a promising toll to repair articular cartilage, however they are in a previous experimentation step yet. Although the present studies using cellular therapy improves clinically and functionally, it is not able to regenerate an articular cartilage which offer resistance the degeneration process.     

Cartilage damage in the haemophilic joints: pathophysiology, diagnosis and management

Intra-articular bleeding affects the metabolism and repair of articular cartilage. Biomechanical data have shown that blood causes harmful effects on overall cartilage function under loading conditions. Therefore, haemophilic patients suffering a haemarthrosis should be subjected to blood aspiration (arthrocentesis) to prevent cartilage damage. MRI and ultrasonography have shown themselves to be excellent noninvasive tools for the evaluation of early cartilage damage that remains undetectable by conventional radiography in haemophilic patients. Prophylaxis with the deficient factor can prevent cartilage deterioration and reduce the incidence of joint haemorrhage in children with haemophilia. Radiosynovectomy has proved to be a highly effective procedure to decrease both the frequency and the severity of recurrent intra-articular bleeding episodes. Nowadays, the most usual surgical procedures for treating articular cartilage defects (cartilage repair) include abrasion chondroplasty, microfracture, mosaicplasty, autologous chondrocyte implantation (ACI), and matrix-induced ACI. In small defects (<2-4 cm(2)), ostechondral autograft or microfracture are the recommended options. In large defects (>2-4 cm(2)), ACI or osteochondral allograft are indicated. However, these techniques have not been applied in haemophilic patients because inflammatory conditions and advanced degenerative change (>50% joint space narrowing) are contraindications for cartilage repair. Thus, prevention of cartilage damage is paramount in haemophilia. The definitive remedy for advanced cartilage damage is either (knee or hip) replacement or (ankle) arthrodesis. Primary prophylaxis and radiosynovectomy are the best alternatives at our disposal to protect our patients against cartilage damage and arthropathy in haemophilic joints.     

Regeneration of articular cartilage of the knee

Cartilage therapy for focal articular lesions of the knee has been implemented for more than a decade, and it is becoming increasingly available. What do we know on the healing response of cartilage lesions? What do we know on the treatment of focal cartilage lesions of the knee and the prognostic factors involved? PubMed articles related to articular cartilage regeneration of the knee in clinical studies were searched from January 2006 to November 2012, using the following key words: articular cartilage, regeneration, clinical studies, and knee. A total of 44 reports were found. They showed the following possibilities for the treatment of focal lesions of the articular cartilage of the knee: cartilage regeneration and repair including cartilage reparation with gene-activated matrices, autologous chondrocyte implantation (ACI) and matrix-induced ACI (MACI), microfracture, osteochondral autograft transfer (mosaicplasty), biological approaches (scaffolds, mesenchymal stem cells—MSCs, platelet-rich plasma, growing factors—GF, bone morphogenetic proteins—BMPs, magnetically labeled synovium-derived cells—M-SDCs, and elastic-like polypeptide gels), osteotomies, stem-cell-coated titanium implants, and chondroprotection with pulsed electromagnetic fields. Untreated cartilage lesions on the femoral condyles had a superior healing response compared to those on the tibial plateaus, and in the patellofemoral joint. Clinical outcome regarding the treatment of medial defects is better than that of the lateral defects. Improvement from baseline was better for patients < or = 30 years compared with patients > or = 30 years. ACI, MACI, and mosaicplasty have shown similar results. The results of comparative clinical studies using ACI have shown some superiority over conventional microfracturing in medium or large defects and in long-term durability. Some biological methods such as scaffolds, MSCs, GF, M-SDCs, BMPs, and elastic-like polypeptide gels still need more research.     

Surgical approaches to the repair of cartilage defects

Traumatic defects in articular cartilage are common in the young athletic population. They may occur in association with other injuries to the knee, particularly anterior cruciate ligament disruption. We understand that both theoretically and clinically, these defects may lead to an increased incidence of ongoing painful symptoms in the short term and degenerative joint disease in the long term. The orthopedic surgeon has several options to deal with theses injuries. The choice is dependent on the size and position of the lesion in the knee and any associated pathology. The most commonly practised methods aim to evoke a ‘repair’ response of fibrocartilage formation. This often helps in the short term, but the fibrocartilage degenerates over time, resulting in the return of symptoms. These ‘repair’ methods can be classified as: 1. Marrow stimulation techniques– these include abrasion chondroplasty, subchondral drilling and microfracture techniques. 2. Scaffold induced repair techniques. More recently, attempts to ‘re‐generate’ normal articular cartilage have been introduced to clinical practice. These are: 1. Mosaicplasty– autogenous osteochondral plug transplantation. 2. Autologous chondrocyte implantation (ACI)– this involves the harvest and culturing of autologous chondrocytes and then re‐implantation via injection under a periosteal or porcine collagen patch. The newest technology allow implantation of the collagen patch which has had chondrocytes cultured within its structure. The results of these autologous chondrocyte implantation show encouraging short to medium term results clinically. 3. Some orthopaedic centres have recently been advocating ‘articular cartilage paste grafting’– this is a simple technique based upon the theory that a paste of autogenous articular cartilage and subchondral bone should contain chondrocytes, stem cells and growth factors in its own scaffold. Orthopedic surgeons await with interest the outcome of pre‐clinical work in examining the use of growth factors, scaffold technology and gene therapy in helping with the clinical problem of articular cartilage defects.     

Biomechanics of cartilage articulation: effects of lubrication and degeneration on shear deformation

OBJECTIVE: To characterize cartilage shear strain during articulation, and the effects of lubrication and degeneration. METHODS: Human osteochondral cores from lateral femoral condyles, characterized as normal or mildly degenerated based on surface structure, were selected. Under video microscopy, pairs of osteochondral blocks from each core were apposed, compressed 15%, and subjected to relative lateral motion with synovial fluid (SF) or phosphate buffered saline (PBS) as lubricant. When cartilage surfaces began to slide steadily, shear strain (Exz) and modulus (G) overall in the full tissue thickness and also as a function of depth from the surface were determined. RESULTS: In normal tissue with SF as lubricant, Exz was highest (0.056) near the articular surface and diminished monotonically with depth, with an overall average Exz of 0.028. In degenerated cartilage with SF as lubricant, Exz near the surface (0.28) was 5-fold that of normal cartilage and localized there, with an overall E(xz) of 0.041. With PBS as lubricant, Exz values near the articular surface were approximately 50% higher than those observed with SF, and overall Exz was 0.045 and 0.062 in normal and degenerated tissue, respectively. Near the articular surface, G was lower with degeneration (0.06 MPa, versus 0.18 MPa in normal cartilage). In both normal and degenerated cartilage, G increased with tissue depth to 3-4 MPa, with an overall G of 0.26-0.32 MPa. CONCLUSION: During articulation, peak cartilage shear is highest near the articular surface and decreases markedly with depth. With degeneration and diminished lubrication, the markedly increased cartilage shear near the articular surface may contribute to progressive cartilage deterioration and osteoarthritis.     

Bioinspired nanofibers support chondrogenesis for articular cartilage repair

Articular cartilage repair remains a significant and growing clinical challenge with the aging population. The native extracellular matrix (ECM) of articular cartilage is a 3D structure composed of proteinaceous fibers and a hydrogel ground substance that together provide the physical and biological cues to instruct cell behavior. Here we present fibrous scaffolds composed of poly(vinyl alcohol) and the biological cue chondroitin sulfate with fiber dimensions on the nanoscale for application to articular cartilage repair. The unique, low-density nature of the described nanofiber scaffolds allows for immediate cell infiltration for optimal tissue repair. The capacity for the scaffolds to facilitate cartilage-like tissue formation was evaluated in vitro. Compared with pellet cultures, the nanofiber scaffolds enhance chondrogenic differentiation of mesenchymal stems cells as indicated by increased ECM production and cartilage specific gene expression while also permitting cell proliferation. When implanted into rat osteochondral defects, acellular nanofiber scaffolds supported enhanced chondrogenesis marked by proteoglycan production minimally apparent in defects left empty. Furthermore, inclusion of chondroitin sulfate into the fibers enhanced cartilage-specific type II collagen synthesis in vitro and in vivo. By mimicking physical and biological cues of native ECM, the nanofiber scaffolds enhanced cartilaginous tissue formation, suggesting their potential utility for articular cartilage repair.     

Biology of articular cartilage repair--present status and prospects

The cartilage response to injury differs from the classic wound healing because of two important features of the cartilage structure; lack of vascular system and rich matrix preventing chondrocyte from migration to the injury site. Three distinct types of cartilage injuries can be identified based on the depth of injury and the repair response: microdamage, chondral injury, and osteochondral injury. There have been numerous attempts to repair cartilage injury, however their results have reinforced that articular cartilage has limited potential to repair itself. However, current research is helping to clarify the mechanism of cartilage injury and repair based on its gene expression. Biology, gene therapy, and tissue engineering may provide a breakthrough to treat cartilage injury.     

Osteochondral repair in synovial joints

PURPOSE OF REVIEW: One of the major challenges in rheumatology remains the induction of osteochondral repair in synovial joints. Remarkable progress has been made in controlling the inflammatory pathways of chronic synovitis and tissue damage in rheumatoid arthritis and spondyloarthropathy. Here, we provide an overview of the current knowledge on the mechanisms involved in osteochondral repair in degenerative joint diseases, as well as in immune mediated inflammatory arthritides, with special emphasis on tumor necrosis factor alpha and IL-1. RECENT FINDINGS: Homeostasis of articular cartilage and subchondral bone are essential for maintaining the integrity of osteochondral structures within synovial joints. This is achieved by the regulation of a delicate balance between anabolic and catabolic signals. In articular cartilage one cell type, the chondrocyte, is responsible for regulation of homeostasis. In bone, however, two distinct cell types, osteoblasts and osteoclasts, are responsible for anabolic and catabolic pathways, respectively. In inflammatory joint disorders, this tight regulation is profoundly dysregulated, with tumor necrosis factor alpha acting as an important catalyst of a disturbed homeostasis, together with IL-1. Targeting these cytokines may restore the intrinsic repair capacity of osteochondral structures. SUMMARY: To restore catabolic cytokine balances appears to be a suitable strategy to promote osteochondral repair.     

Histochemical investigation of adjuvant-induced arthritis

Rats with adjuvant-induced arthritis were observed to have increased alkaline phosphatase, acid phosphatase (two isozymes), and ATPase activity in the radial zone of articular cartilage, at the osteochondral junction, and in the bone marrow elements. A qualitative and quantitative reduction of azure A, PAS, colloidal iron, alcian blue critical electrolyte concentration staining (0.4 and 0.9 M (mg Cl₂) was also observed in corresponding areas. These findings suggest the degradation of the articular cartilage matrix with possible simultaneous or resultant calcification.     

Stage oriented surgical cartilage therapy. Current situation

Chondral or osteochondral lesions are typical injuries in orthopaedics and traumatology. Since there is no regeneration of damaged articular cartilage, these lesions can lead to premature osteoarthritis. Therefore, an adequate therapy for these injuries is an important goal. Nowadays, common methods in cartilage therapy are procedures for the recruitment of mesenchymal stem cells: autologous osteochondral transplantation and autologous chondrocyte transplantation. Currently, autologous osteochondral transplantation is the only procedure that allows the replacement of the defect with hyaline cartilage. However, this procedure has the problem of donor-site morbidity and limited availability of transplants. Stem cell recruiting procedures and autologous chondrocyte transplantation normally achieve a regeneration of the defect with only fibrocartilage tissue, but both can achieve good medium-term clinical results. Each of these therapeutic principles has certain major indications. In order to select an adequate therapy, the classification of chondral or osteochondral lesion is needed. From a multiplicity of classification systems, those of the ICRS are of particular clinical relevance.     

Tissue-engineered composites for the repair of large osteochondral defects

OBJECTIVE: To test the hypothesis that engineered cartilage can provide a mechanically functional template capable of undergoing orderly remodeling during the repair of large osteochondral defects in adult rabbits, as assessed by quantitative structural and functional methods. METHODS: Engineered cartilage generated in vitro from chondrocytes cultured on a biodegradable scaffold was sutured to a subchondral support and the resulting composite press-fitted into a 7-mm long, 5-mm wide, 5-mm deep osteochondral defect in a rabbit knee joint. Defects left empty (group 1) or treated with cell-free composites (group 2) served as controls for defects treated with composites of engineered cartilage and the support, without or with adsorbed bone marrow (groups 3 and 4, respectively). RESULTS: Engineered cartilage withstood physiologic loading and remodeled over 6 months into osteochondral tissue with characteristic architectural features and physiologic Young's moduli. Composites integrated well with host bone in 90% of cases but did not integrate well with host cartilage. Structurally, 6-month repairs in groups 3 and 4 were superior to those in group 2 with respect to histologic score, cartilage thickness, and thickness uniformity, but were inferior to those in unoperated control tissue. At 6 months, Young's moduli in groups 2, 3, and 4 (0.68, 0.80, and 0.79 MPa, respectively) approached that in unoperated control tissue (0.84 MPa), whereas the corresponding modulus in group 1 (0.37 MPa) was significantly lower. CONCLUSION: Composites of tissue-engineered cartilage and a subchondral support promote the orderly remodeling of large osteochondral defects in adult rabbits.     

Muscle derived,cell based ex vivo gene therapy for treatment of full thickness articular cartilage defects

OBJECTIVE: To evaluate the effectiveness of transplanted allogeneic muscle derived cells (MDC) embedded in collagen gels for the treatment of full thickness articular cartilage defects, to compare the results to those from chondrocyte transplantation, and to evaluate the feasibility of MDC based ex vivo gene therapy for cartilage repair. METHODS: Rabbit MDC and chondrocytes were transduced with a retrovirus encoding for the beta-galactosidase gene (LacZ). The cells were embedded in type I collagen gels, and the cell proliferation and transgene expression were investigated in vitro. In vivo, collagen gels containing transduced cells were grafted to the experimental full thickness osteochondral defects. The repaired tissues were evaluated histologically and histochemically, and collagen typing of the tissue was performed. RESULTS: The MDC and chondrocyte cell numbers at 4 weeks of culture were 305 +/- 25% and 199 +/- 25% of the initial cell number, respectively. The initial percentages of LacZ positive cells in the MDC and chondrocyte groups were 95.4 +/- 1.9% and 93.4 +/- 3.4%, and after 4 weeks of culture they were 84.2 +/- 3.9% and 76.9 +/- 4.3%, respectively. In vivo, although grafted cells were found in the defects only up to 4 weeks after transplantation, the repaired tissues in the MDC and chondrocyte groups were similarly better histologically than control groups. Repaired tissues in the MDC group were mainly composed of type II collagen, as in the chondrocyte group. CONCLUSION: Allogeneic MDC could be used for full thickness articular cartilage repair as both a gene delivery vehicle and a cell source for tissue repair.     

Partial chondroprotective effect of zoledronate in a rabbit model of inflammatory arthritis.

OBJECTIVE: To test the hypothesis that bisphosphonate treatment has a chondroprotective effect in the carrageenan model of inflammatory arthritis (IA). METHODS: Experimental IA was induced in rabbits by intraarticular injections of carrageenan. One group also received subcutaneous injections of zoledronate, a new bisphosphonate. After 4 weeks, the joints were harvested. Articular cartilage degradation and the degree of synovitis were assessed by light microscope using qualitative grading scores. Articular cartilage and subchondral and cancellous bone were evaluated histomorphometrically. Bone microhardness was measured. RESULTS: Carrageenan injected knees showed changes of inflammatory arthritis with cartilage erosion. Zoledronate treatment partially protected the articular cartilage from degradation. This effect was unlikely due to an antiinflammatory effect of the drug as the carrageenan induced synovitis was unaffected by zoledronate treatment. The treatment preserved subchondral bone thickness and cancellous bone volume and prevented focal breaks in the osteochondral barrier. The subchondral bone hardness was also maintained. CONCLUSION: Zoledronate had a partial effect in a rabbit model of inflammatory arthritis. The chondroprotection may be due to the prevention of bone resorption. By maintaining an intact subchondral bone, normal joint loading may have been maintained and contact between the bone marrow and the articular cartilage averted.     

The effect of cartilage degeneration on ultrasound speed in human articular cartilage

Objectives: We investigated the effect of cartilage degeneration on ultrasound speed in human articular cartilage in vitro.

Methods: Ultrasound speed was calculated by the time-of-flight method for 22 femoral condyle osteochondral blocks obtained from osteoarthritis patients. In parallel, histological evaluation of specimens was performed using the modified Mankin and OARSI scores.

Results: The mean ultrasound speed was 1757?±?109 m/s. Ultrasound speed showed significant negative correlation with OARSI score, and a decreasing tendency with high Mankin scores. Good correlation was found between the optically measured and the calculated cartilage thickness.

Conclusion: Our results show that articular cartilage degeneration has relatively little influence on ultrasound speed. In addition, morphological evaluation of articular cartilage using a preset value of ultrasound speed seems to offer relatively accurate results.     

Accuracy of quantitative magnetic resonance imaging in the detection of ex vivo focal cartilage defects

BACKGROUND: No established, non-invasive diagnostic procedure for quantifying focal cartilage defects is currently available. OBJECTIVE: To test the accuracy of quantitative magnetic resonance imaging (qMRI) for reliable determination of cartilage defect size in various compartments of the human knee. METHODS: 24 tibial and patellar cartilage plates were harvested during knee arthroplasty. 74 cylindrical defects with diameters of 3, 5, and 8 mm were created with a punch. In 15 specimens (51 defects), the cartilage cylinders (inside the punch) were removed (approach 1), while in 9 specimens (23 defects) the surrounding tissue was removed mechanically and the cartilage cylinder was left in place (approach 2). All plates were imaged with a T(1) weighted water excitation gradient echo sequence at a resolution of 1.5 mm x 0.31 mm x 0.31 mm. The defect size was computed from the image data after interactive segmentation and compared with the known dimensions of the cylinders. RESULTS: Although there was a significant overestimation of the defect size by qMRI in 3 mm defects (mean (SD) +1.3 (0.58) mm = +/-42%; p<0.001), the overestimation was only +1.0 (0.57) mm (+/-21%; p<0.05) in 5 mm defects and +0.1 (0.39) mm (+/-4%; p = 0.31) in 8 mm defects (approach 1). Values were similar for approaches 1 and 2 and for patellar and tibial cartilage plates. CONCLUSIONS: These findings show that qMRI allows accurate quantification of focal cartilage defects. It may therefore represent a valuable tool in the diagnosis of traumatic cartilage lesions, osteochondrosis dissecans, and osteochondral fractures, and in monitoring their responsiveness to surgical or other treatments.     

Articular cartilage defects of the knee: correlation between magnetic resonance imaging and gross pathology.

Magnetic resonance imaging (MRI) of the knee articular cartilage is possible owing to the contrast provided by different signal intensities of adjacent menisci and subchondral bone. The objective of this study was to determine the accuracy of MRI in quantitatively detecting thinning and focal defects of articular cartilage in vivo. High resolution MRI was performed followed by dissection of the knee within one hour of amputations above the knee of eight patients (62-89 years) with peripheral vascular disease. Articular cartilage was examined for erosions, surface irregularities, and appearance. Mean thicknesses of femoral and tibial articular cartilage sagittal sections from MRI were statistically indistinguishable from matched gross thicknesses. In those joints in which cartilage erosions, thinning, or irregularities were detected by MRI the same defects were apparent by gross examination. Cartilage that appeared normal by MRI had a normal gross appearance by gross examination. Thus high resolution MRI can accurately predict gross articular cartilage appearance and thickness, allowing an objective, quantitative, noninvasive assessment of eroded cartilage.     

Tonic activation of hypoxia-inducible factor 1alpha in avascular articular cartilage and implications for metabolic homeostasis

OBJECTIVE: To determine whether oxygen-dependent activation patterns of hypoxia-inducible factor 1alpha (HIF-1alpha) observed in vascularized tissues are conserved within avascular and hypoxic articular cartilage and whether HIF-1alpha affects cartilage matrix synthesis. METHODS: Explants of bovine articular cartilage and primary chondrocytes were exposed to normoxia (21% O2), hypoxia (2% O2), and simulated hypoxia (21% O2 plus CoCl2). Western blot and immunofluorescence analyses of HIF-1alpha were performed to determine HIF-1alpha activation patterns. To simulate cartilage loss from disease or injury, the top layers of cartilage were removed from osteochondral explants, and the residual cartilage was assessed for HIF-1alpha immunolocalization and proteoglycan synthesis. RESULTS: We demonstrated continuous nuclear translocation of HIF-1alpha in deeper layers of intact articular cartilage. HIF-1alpha was not completely degraded in chondrocytes exposed to normoxia, but rather, colocalized to the Golgi complex, a finding not previously reported for any cell type. Following alteration of the oxygen gradient by removal of the top layers of cartilage, predominantly perinuclear HIF-1alpha was found in the deeper layers. Restoration of intranuclear HIF-1alpha to these areas was achieved by hypoxia and simulated hypoxia. Under conditions in which HIF-1alpha was inactivated, matrix synthetic activity was altered (P < 0.0001) compared with control cartilage. CONCLUSION: These findings demonstrate that hypoxia-dependent activation of HIF-1alpha is highly conserved and that changes in oxygen tensions following cartilage loss from injury or disease alter cartilage metabolism in part by changing HIF-1alpha activity. The discovery of tonic activation of HIF-1alpha within intact articular cartilage underscores its potential importance to cartilage homeostasis.     

Neurovascular invasion at the osteochondral junction and in osteophytes in osteoarthritis

BACKGROUND: Normal adult articular cartilage is thought to be avascular and aneural. OBJECTIVE: To describe neurovascular structures at the osteochondral junction and in osteophytes in tibiofemoral osteoarthritis (OA) displaying a range of severity of cartilage changes. METHODS: Articular surfaces were obtained from 40 patients at total knee joint replacement surgery for tibiofemoral OA (TKR) and seven patients post mortem (PM). Antibodies directed against CD34 (vascular endothelium), protein gene product 9.5 (pan-neuronal marker), substance P and calcitonin gene-related peptide (sensory nerves) and C-flanking peptide of neuropeptide Y (sympathetic nerves) were used to localise blood vessels and nerves by immunohistochemistry. Severity of OA cartilage changes was graded histologically. RESULTS: TKR and PM samples displayed a range of OA cartilage changes including tidemark breaching by vascular channels. Sympathetic and sensory nerves were both present within vascular channels in the articular cartilage, in both mild and severe OA. Perivascular and free nerve fibres, and nerve trunks were observed within the subchondral bone marrow and within the marrow cavities of osteophytes. Sensory and sympathetic nerves displayed similar distributions in each region studied. CONCLUSION: Vascularisation and the associated innervation of articular cartilage may contribute to tibiofemoral pain in OA across a wide range of structural disease severity.