Intra-articular injection of synovial mesenchymal stem cells improves cartilage repair in a mouse injury model.

Controversy remains whether articular cartilage has an endogenous stem/progenitor cell population, since its poor healing capacity after injury can lead to diseases such as osteoarthritis. In the joint environment there are mesenchymal stem/progenitor cells (MSCs) in the synovial membrane and synovial fluid that can differentiate into cartilage, but it is still under debate if these cells contribute to cartilage repair in vivo. In this study, we isolated a Sca-1 positive, chondrogenesis capable population of mouse synovial MSCs from C57BL6 and MRL/MpJ "super-healer" strains. Intra-articular injection of Sca-1 + GFP + synovial cells from C57BL6 or MRL/MpJ into C57BL6 mice following cartilage injury led to increased cartilage repair by 4 weeks after injury. GFP expression was detected in the injury site at 2 weeks, but not 4 weeks after injury. These results suggest that synovial stem/progenitor cells, regardless of strain background, have beneficial effects when injected into an injured joint. MSCs derived from MRL/MpJ mice did not promote an increased repair capacity compared to MSCs derived from non-healing C57BL6 controls; however, MRL/MpJ MSCs were observed within the defect area at the time points examined, while C57BL6 MSCs were not.

Embryonic stem cell therapy improves bone quality in a model of impaired fracture healing in the mouse; tracked temporally using in vivo micro-CT.

In the current study, we used an estrogen-deficient mouse model of osteoporosis to test the efficacy of a cell-generated bone tissue construct for bone augmentation of an impaired healing fracture. A reduction in new bone formation at the defect site was observed in ovariectomized fractures compared to the control group using repeated measures in vivo micro-computed tomography (µCT) imaging over 4 weeks. A significant increase in the bone mineral density (BMD), trabecular bone volume ratio, and trabecular number, thickness and connectivity were associated with fracture repair in the control group, whereas the fractured bones of the ovariectomized mice exhibited a loss in all of these parameters (p<0.001). In a separate group, ovariectomized fractures were treated with murine embryonic stem (ES) cell-derived osteoblasts loaded in a three-dimensional collagen I gel and recovery of the bone at the defect site was observed. A significant increase in the trabecular bone volume ratio (p<0.001) and trabecular number (p<0.01) was observed by 4 weeks in the fractures treated with cell-loaded collagen matrix compared to those treated with collagen I alone. The stem cell-derived osteoblasts were identified at the fracture site at 4 weeks post-implantation through in situ hybridization histochemistry. Although this cell tracking method was effective, the formation of an ectopic cellular nodule adjacent to the knee joints of two mice suggested that alternative in vivo cell tracking methods should be employed in order to definitively assess migration of the implanted cells. To our knowledge, this study is the first of its kind to examine the efficacy of stem cell therapy for fracture repair in an osteoporosis-related fracture model in vivo. The findings presented provide novel insight into the use of stem cell therapies for bone injuries.

Intra-articular anaesthesia mitigates established pain in experimental osteoarthritis: a preliminary study of gait impulse redistribution as a biomarker of analgesia pharmacodynamics.

OBJECTIVE: Develop a sensitive, functional biomarker of persistent joint pain in a large animal model of experimental osteoarthritis. Evaluate Impulse Ratio as a measure of weight distribution among supporting limbs throughout the early natural history of osteoarthritis and with local anaesthesia and analgesia. DESIGN: The distribution of weight bearing in the trot of 11 skeletally-mature dogs was analyzed before and after unilateral surgical intervention (cranial cruciate transection or distal femoral focal impact). The short-term effects of two analgesic treatments (intra-articular lidocaine and intra-dermal meloxicam) were then evaluated as an index of pain relief based on the redistribution of weight-bearing impulse between normal and injured limbs. RESULTS: Impulse Ratio was able to resolve weight redistribution between limbs in both long-term (weekly for over 400 days) and short-term (15 min intervals) joint evaluations. Joint pain relief from lidocaine administration could be reliably tracked over its brief acting time course. Meloxicam administration resulted in ambiguous results, where average weight bearing in the injured limb did not increase, but the variability of limb use changed transiently and reversibly. CONCLUSION: Joint function and the role of persistent joint pain in the development of osteoarthritis can be investigated effectively and efficiently in a large animal model through the use of Impulse Ratio. Impulse Ratio can be a functionally relevant and sensitive biomarker of locomotion-related joint pain.

Viscoelasticity of the articular cartilage surface in early osteoarthritis.

OBJECTIVE: Structural and biochemical changes in articular cartilage occur throughout the pathogenesis of osteoarthritis (OA). Early changes include proteoglycan loss and collagen network disorganization at or near the articular surface. These changes accompany reductions in mechanical properties of cartilage, yet the relationships between mechanics and structure in early OA are poorly defined. Thus, the overall goal of this work was to measure changes in the microscale mechanics and structure of the articular surface in an in vivo model of OA to better understand the early pathogenesis of cartilage degeneration in this disease. DESIGN: A canine cranial cruciate ligament transection (CCL(x)) model was used. The contralateral joint served as an internal control (Ctl). The frequency dependence of the dynamic indentation modulus (E(∗)) was evaluated, and creep behavior was measured to estimate the instantaneous (E(i,inst)) and equilibrium (E(i,eq)) indentation moduli and longest creep time-constant (τ). These functional parameters were related to microscopic metrics of cartilage structure and biochemistry, measured by polarized light microscopy and digital densitometry of proteoglycan staining by safranin-O. RESULTS: CCL(x) and Ctl cartilage exhibited frequency sensitivity. E(i,inst), E(i,eq), and τ were lower in CCL(x) vs Ctl cartilage. These mechanical changes were accompanied by a reduction in superficial zone thickness and changes in superficial zone collagen organization, as well as a non-significant reduction in superficial zone proteoglycan staining. CONCLUSIONS: Changes in the microscale viscoelastic behavior of the cartilage surface are a functional hallmark of early OA that accompany significant changes to the microstructural organization of the collagenous extracellular matrix.

New understanding of the complex structure of knee menisci: implications for injury risk and repair potential for athletes.

Menisci help maintain the structural integrity of the knee. However, the poor healing potential of the meniscus following a knee injury can not only end a career in sports but lead to osteoarthritis later in life. Complete understanding of meniscal structure is essential for evaluating its risk for injury and subsequent successful repair. This study used novel approaches to elucidate meniscal architecture. The radial and circumferential collagen fibrils in the meniscus were investigated using novel tissue-preparative techniques for light and electron microscopic studies. The results demonstrate a unique architecture based on differences in the packaging of the fundamental collagen fibrils. For radial arrays, the collagen fibrils are arranged in parallel into ∼10 µm bundles, which associate laterally to form flat sheets of varying dimensions that bifurcate and come together to form a honeycomb network within the body of the meniscus. In contrast, the circumferential arrays display a complex network of collagen fibrils arranged into ∼5 µm bundles. Interestingly, both types of architectural organization of collagen fibrils in meniscus are conserved across mammalian species and are age and sex independent. These findings imply that disruptions in meniscal architecture following an injury contribute to poor prognosis for functional repair.

Non-uniform strain distribution within rat cartilaginous growth plate under uniaxial compression.

Growth plates are highly inhomogeneous in morphology and composition. Mechanical loading can modulate longitudinal bone growth, though the mechanisms underlying this mechanobiology are poorly understood. The proximal tibial growth plates of six rats were tested in vitro under uniaxial compression to 5% strain, and confocal microscopy was used to track and capture images of fluorescently labeled cell nuclei with increasing applied strains. The local strain patterns through the growth plate thickness were quantified using texture correlation analysis. The technique of texture correlation analysis was first validated by comparing theoretical simulated strain maps generated from numerically distorted images. The texture correlation algorithm was sensitive to the grid size superimposed on the original image, but remained insensitive to parameters related to the size of the final image mask, which was searched by the correlation algorithm for each grid point of the original image. Within the growth plate, experimental strain distributions were non-uniform in all six specimens. Growth plates were mostly under compression strains. The strain distributions differed among the histomorphological zones of the growth plate, which was most obvious in specimens with regular growth plate shape: higher compressive strains (4-8 times higher than the applied 5% strain) were located mainly in regions overlapping the reserve and hypertrophic zones with lower compressive strains in the proliferative zone. This study documents the non-uniform mechanical behavior of growth plate across its three histological zones when exposed to compression. Further investigation is required to establish the significance of non-uniform strain fields during growth in vivo.

Heterogeneity in patellofemoral cartilage adaptation to anterior cruciate ligament transection; chondrocyte shape and deformation with compression.

OBJECTIVE: The purpose of this study was to determine if the opposing cartilages of the feline patellofemoral joint adapted differently to short-term anterior cruciate ligament transection (ACL-T) and if the magnitude of chondrocyte deformation upon tissue loading was altered under ACL-T conditions compared to contralateral controls. In situ static compression of physiological magnitude was applied to the feline patellofemoral cartilage 16 weeks post-ACL-T and cartilage and chondrocyte deformation were evaluated by histomorphometry. DESIGN: Six adult cats were euthanized 16 weeks after unilateral ACL-T. A peak surface pressure of 9 MPa was applied to the fully intact patella and femoral groove cartilages. After in situ fixation under compression, sections from the centre of the indent and from an adjacent unloaded area of the cartilages were analysed. Chondrocyte shape, size, clustering and volumetric fraction were quantified. RESULTS: Experimental patellar articular cartilage was thicker, contained larger chondrocytes that were more frequently arranged in clusters and had, on average, a larger chondrocyte volumetric fraction compared to contralateral controls. In contrast, the experimental femoral groove cartilage demonstrated little adaptation to ACL-T. CONCLUSIONS: The patellar articular cartilage adapts to short-term ACL-T to a greater extent than femoral groove cartilage. We speculate that differences in the histological parameters of control tissues, such as cartilage thickness and the magnitude and depth distribution of chondrocyte shape, size and volumetric fraction may contribute to predisposing patellar cartilage, and not femoral groove cartilage, to adaptation after ACL-T.

Opposing cartilages in the patellofemoral joint adapt differently to long-term cruciate deficiency: chondrocyte deformation and reorientation with compression.

OBJECTIVE: The purposes of this study were to quantify patellofemoral histology in the feline knee 67 months post-anterior cruciate ligament transection (ACL-T) and to apply an in situ static load of physiological magnitude to the articular cartilage and evaluate the resulting cartilage and chondrocyte deformation. DESIGN: Six cats were sacrificed 67+/-6 months post-unilateral ACL-T. Static compression was applied to the cartilage surfaces of the patellofemoral joint using a cylindrical metal indentor. After fixation, full thickness osteochondral blocks were harvested and sections cut from not-indented and indented areas. Chondrocyte shape, orientation and volumetric fraction as well as cartilage thickness were evaluated. RESULTS: Experimental and contralateral patellae were histologically different compared to normal with thickened cartilage, rounded superficial chondrocytes, and uneven proteoglycan staining throughout. In contrast, no differences were apparent in 10 of the 12 femoral groove samples. The structural reorganisation of the experimental patellae cartilage that occurred with load was also different compared to normal. Specifically, the indentation shape was deeper and had steeper sides and the realignment of deep zone cells at angles of 45 degrees and 135 degrees observed in normal cartilage was no longer apparent in the experimental tissue. CONCLUSIONS: Two directly articulating cartilage surfaces of the feline patellofemoral joint have completely contrasting responses to long-term ACL-T. We speculate that this could be a result of the different nature of the loads experienced by the two surfaces (intermittent vs constant) and/or the differences in the histology and material properties of the two tissues in their normal state, and/or an inherent difference in the biological response capabilities of the articular cartilages.

Static compressive loading reduces the mRNA expression of type II and X collagen in rat growth-plate chondrocytes during postnatal growth.

The mechanisms by which chondrocytes modulate longitudinal bone growth are not well understood. This in vitro study investigated the effects of loading on the mRNA expression pattern of key molecular components of the growth-plate related to the extracellular matrix (type II and type X collagen) and the PTH-PTHrP feedback loop. Short-term static compressive loading was applied to rat proximal tibial growth-plate explants. Four age groups at specific developmental stages were investigated. The spatial variation in the mRNA expression was compared among loaded explants, their contralateral sham controls, and uncultured growth plates from normal animals. Basic cell metabolism (18S rRNA) was unaffected by load. Results indicated a narrower spatial distribution of mRNA expression of type II collagen throughout the growth plate; similarly, a narrowed distribution of expression of type X collagen was noted in the lower hypertrophic zone of the growth-plate. This suggests that mechanical compression influences chondrocytes of the hypertrophic zone to alter their expression of specific genes encoding proteins of the extracellular matrix, while PTH-PTHrP receptor mRNA, a regulatory protein, remained unaffected by loading. The effects of compression were similar at the different stages of growth, suggesting that additional factors may be involved in the clinical progression of skeletal deformities observed during growth spurts. Although this study was done in vitro and limited to static loading, it furthers our understanding of growth-plate mechanobiology as a first step toward providing a scientific rationale for treating progressive musculoskeletal deformities.

Detecting structural changes in early experimental osteoarthritis of tibial cartilage by microscopic magnetic resonance imaging and polarised light microscopy.

OBJECTIVES: To detect changes in the collagen fibril network in articular cartilage in a canine experimental model of early osteoarthritis (OA) using microscopic magnetic resonance imaging (microMRI) and polarised light microscopy (PLM). METHODS: Eighteen specimens from three pairs of the medial tibia of an anterior cruciate ligament transection canine model were subjected to microMRI and PLM study 12 weeks after surgery. For each specimen, the following experiments were carried out: (a) two dimensional microMRI images of T(2) relaxation at four orientations; (b) the tangent Young's modulus; and (c) two dimensional PLM images of optical retardance and fibril angle. Disease induced changes in tissue were examined across the depth of the cartilage at a microMRI resolution of 13.7-23.1 microm. RESULTS: Several distinct changes from T(2) weighted images of cartilage in OA tibia were seen. For the specimens that were covered at least in part by the meniscus, the significant changes in microMRI included a clear shift in the depth of maximum T(2) (21-36%), a decrease in the superficial zone thickness (37-38%), and an increase in cartilage total thickness (15-27%). These microMRI changes varied topographically in the tibia surface because they were not significant in completely exposed locations in medial tibia. The microMRI results were confirmed by the PLM measurements and correlated well with the mechanical measurements. CONCLUSION: Both microMRI and PLM can detect quantitatively changes in collagen fibre architecture in early OA and resolve topographical variations in cartilage microstructure of canine tibia.

Comparison of microcomputed tomographic and microradiographic measurements of cortical bone porosity.

Cortical bone is perforated by a network of canals that have a significant impact upon its material properties. Microcomputed tomography offers the possibility of noninvasively visualizing and quantifying cortical pores in both two and three dimensions. Establishing how two-dimensional (2D) microcomputed tomographic (microCT) analysis compares with conventional methods for analyzing cortical porosity is an important prerequisite for the wider adoption of this technique and the development of three-dimensional (3D) analysis. Therefore, we compared porosity-related parameters from 2D microcomputed tomographic images with those from matching microradiographic sections. Samples from five human femora were scanned at a 10-microm resolution and then sequentially sectioned and microradiographed. An average of eight image pairs were produced from each femur (total, n = 41). The repeatability and comparability of the two techniques was assessed for three parameters; cortical porosity (%), mean pore area (microm(2)), and pore density (pores/mm(2)). For repeatability, no significant difference ( P > 0.05) was found between the two methods for cortical porosity and mean pore area; however, pore density differed significantly ( P < 0.001). For comparability, the bias (+/- error) between the methods was found to be 0.51% (+/-0.31%) for cortical porosity and -155 microm(2) (+/-293 microm(2)) for mean pore area. The bias for pore density was dependent upon measurement size with microcomputed tomographic images having 14% (+/-9.3%) fewer pores per millimeter squared. The qualitative and quantitative similarities between the two techniques demonstrated the utility of 2D microcomputed tomographic for cortical porosity analysis. However, the relatively poor results for pore density revealed that a higher resolution (<10 microm) is needed to consistently visualize all cortical pores in human bone.

The notochordal cell in the nucleus pulposus: a review in the context of tissue engineering.

An understanding of developmental biology can provide useful insights into how different tissue-engineered repairs might be designed. During embryogenesis of the intervertebral disk, the cells of the notochord play a critical role in initiating tissue formation, and may be responsible for development of the nucleus pulposus. In some species, including humans, these notochordal cells may eventually be lost, either through apoptosis or terminal differentiation, and are replaced by chondrocyte-like cells. However, there is some evidence that the notochordal cells may persist in at least some humans. This review discusses some of the potential applications of notochordal cells in tissue engineering of the nucleus pulposus.

Angiogenesis in the distal femoral chondroepiphysis of the rabbit during development of the secondary centre of ossification.

In the developing chondroepiphyses of long bones, the avascular cartilaginous anlage is invaded by numerous blood vessels, through the process of angiogenesis. The objective of this study was to investigate the chronology of this vascular invasion with the spontaneous calcification of the cartilaginous epiphysis during development of the secondary ossification centre in the rabbit distal femur. The time-course of chondroepiphyseal vascular invasion was determined histologically and standardized for eight gestational and four postnatal intervals by plotting kit body mass against crown-rump length. Similarly, microcomputed tomography (micro-CT) helped to visualize calcification at those same gestational and postnatal intervals. To confirm the angiogenic nature of the avascular chondroepiphysis, such samples were assayed on the chick chorio-allantoic membrane (CAM). Neovascular outgrowths from the CAM were apparent 48 h following introduction of an 18-day (gestational) chondroepiphyseal sample. Chondroepiphyseal samples were assayed for the potent developmental angiogenic factors bFGF and VEGF, with the mRNA expression for both these mediators being confirmed using RT-PCR. As angiogenesis and calcification during chondroepiphyseal development occur in a defined tissue environment initially devoid of blood vessels and mineral, those processes provided a unique opportunity to study their progression without complication of injury-related inflammation or extant vasculature and mineral. Furthermore, the discovery of angiogenic, angiostatic or mineral-regulating mediators specific to developing connective tissue may prove useful for analysing the regulation of vascular and mineral pathogenesis in articular tissues.

In situ chondrocyte deformation with physiological compression of the feline patellofemoral joint.

The mechanical environment is an important factor affecting the maintenance and adaptation of articular cartilage, and thus the function of the joint and the progression of joint degeneration. Recent evidence suggests that cartilage deformation caused by mechanical loading is directly associated with deformation and volume changes of chondrocytes. Furthermore, in vitro experiments have shown that these changes in the mechanical states of chondrocytes correlate with a change in the biosynthetic activity of cartilage cells. The purpose of this study was to apply our knowledge of contact forces within the feline patellofemoral joint to quantify chondrocyte deformation in situ under loads of physiological magnitude. A uniform, static load of physiological magnitude was applied to healthy articular cartilage still fully intact and attached to its native bone. The compressed cartilage was then chemically fixed to enable the evaluation of cartilage strain, chondrocyte deformation and chondrocyte volumetric fraction. Patella and femoral groove articular cartilages differ in thickness, chondrocyte aspect ratio, and chondrocyte volumetric fraction in both magnitude and depth distribution. Furthermore, when subjected to the same compressive loads, changes to all of these parameters differ in magnitude and depth distribution between patellar and femoral groove articular cartilage. This evidence suggests that significant chondrocyte deformation likely occurs during in vivo joint loading, and may influence chondrocyte biosynthetic activity. Furthermore, we hypothesise that the contrasts between patella and femoral groove cartilages may explain, in part, the site-specific progression of osteoarthritis in the patellofemoral joint of the feline anterior cruciate ligament transected knee.

The effects of mechanical loading on the mRNA expression of growth-plate cells.

Bone growth is a complex process involving proliferation, maturation and hypertrophy of chondrocytes in the growth plates. Mechanical forces applied to growing bones alter their longitudinal growth. However, the mechanisms by which chondrocytes modulate longitudinal bone growth are not well understood. This in vitro study investigated the effects of mechanical loading on the mRNA expression pattern of key molecular components of the growth-plate. Short-term static loading was applied to rat proximal tibial growth-plate explants. Various age groups at specific developmental stages were investigated. In situ hybridization was used to assess the mRNA expression of the cells in different zones of the growth-plate. Four key components were investigated: 18s (basic cell metabolism), type II collagen (major extracellular matrix component), type X collagen (matrix component in hypertrophic zone) and PTH-PTHrP receptors (pre-hypertrophic chondrocytes). The spatial variation in the mRNA expression between loaded explants and their contralateral controls was compared to establish: -the sensitivity of the different growth-plate zones to mechanical loading; -the sensitivity of the different developmental stages to loading. Preliminary results indicated that static loading on the growth plate of 80 d.o. rats affects type II and X collagen gene expressions while PTH-PTHrP remains insensitive to static loading. Improved understanding of growth-plate mechanics and the underlying biology is required to provide a scientific basis for the treatment of progressive deformities.

Absolute concentrations of mRNA for type I and type VI collagen in the canine meniscus in normal and ACL-deficient knee joints obtained by RNase protection assay.

Relatively little is known about the cellular and molecular responses of the knee joint meniscus to joint injury, despite the functional importance of the tissue. We investigated how meniscus cells respond to joint injury in the early stages of post-traumatic osteoarthritis by characterizing the changes in matrix gene expression in menisci at 3 and 12 weeks post-surgery in dogs in which the anterior cruciate ligament (ACL) in one joint was transected and the other unoperated joint served as a control. Changes in the total RNA and DNA concentrations of the menisci were determined. Absolute concentrations of the mRNA of the COL1A1 gene of type 1 collagen, the major fibrillar collagen of the meniscus, and the COL6A3 gene of type VI collagen, a major repair molecule, were determined by quantitative ribonuclease (RNase) protection assay. The concentration of total RNA in medial and lateral menisci increased from 40 to 60 microg RNA/g wet wt in unoperated, control joints to 200-350 microg RNA/g wet wt in ACL-deficient joints. No significant changes were detected in the concentration of DNA (900-1200 microg DNA/g wet wt). Low concentrations of COL1A1 (2-3 pmol mRNA/g DNA) and COL6A3 (0.3-0.6 pmol mRNA/g DNA) mRNA transcripts were measured in normal menisci. ACL-deficiency induced a 20-38 fold increase in COL1A1 and COL6A3 mRNA concentration at 3 weeks, and an 11-19 fold increase at 12 weeks post-surgery. In general, the increase in COL1A1 and COL6A3 mRNA concentrations was greater in medial menisci than in lateral menisci. These results demonstrate that the menisci initiate a vigorous biosynthetic response to transection of the ACL.

Evidence of chondrocyte repopulation in adult ovine articular cartilage following cryoinjury and long-term transplantation.

OBJECTIVE: To characterize the response of articular chondrocytes to a specific cryoinjury that leads to cluster formation following long-term transplantation. DESIGN: Osteochondral dowels from 20 adult sheep were cryopreserved to optimize the recovery of chondrocytes immediately after thawing. The dowels were transplanted as allografts and observed at 3 and 12 months. Chondrocyte distribution and viability was assessed using paravital dyes after transplantation. Chondrocyte phenotype was assessed by in situ hybridization and immunohistochemistry to detect type II collagen. An anticentrosome antibody was used to identify cells undergoing cell cycle progression towards mitosis. RESULTS: All cryopreserved grafts showed the presence of spheroidal clusters of chondrocytes 1 year after transplantation while the host cartilage adjacent to the graft appeared morphologically normal. The average size of the clusters increased from four cells at 3 months to 12 cells at 1 year. The chondrocytes in the clusters displayed newly formed type II collagen protein and mRNA. Some cells within clusters were observed with two centrosomes, indicative of cells progressing through the S phase of the cell cycle. CONCLUSION: Adult articular chondrocytes retain the ability to repopulate the matrix, an ability which is demonstrated with this specific cryoinjury. This may be an initial stage of cartilage regeneration.

Vascular alterations in the rabbit patellar tendon after surgical incision.

Open incision of the patellar tendon (PT) is thought to promote acute vascular responses which ultimately result in an enhanced degree of tendon repair. Such a clinical procedure is commonly applied to patients with refractory tendinitis. The objective of this study was to quantify the vascular adaptations (both anatomical and physiological) to longitudinal incision of the PT, and the resultant effects on tendon organisation. Fifty-four New Zealand White rabbits were separated into 3 experimental groups and 2 control groups. Experimental groups underwent surgical incision of the right PT, and were assessed 3 d, 10 d and 42 d following injury; normal unoperated controls were evaluated at time zero, and sham-operated controls were evaluated at 3 d to control for the effects of incising the overlying skin. Quantitative measures of PT blood supply (blood flow, microvascular volume) and geometric properties of PT substance were obtained for each PT. Histomorphology was assessed to evaluate vascular remodelling and matrix organisation in the healing PT. Longitudinal open incision surgery of the PT led to rapid increases in both blood flow and vascular volume. The incision of overlying tissues alone (sham-operated) contributed to this measurable increase, and accounted for 36% and 42% of the elevated blood flow and vascular volume respectively at the 3 d interval. In the incised PT, blood flow significantly increased by 3 d compared with both time zero and sham-operated controls, and remained significantly elevated at the 10 d interval. Similarly, vascular volume of the incised PT increased at 3 d compared both with time zero and sham-operated controls. At the 10 d interval, the increase in vascular volume was greatest in the central PT substance. By 42 d both blood flow and vascular volume of the incised tendon had diminished, with only blood flow remaining significantly different from controls. In the contralateral limb, a significant neurogenically mediated vasodilation was measured in the contralateral PTs at both early time intervals, but was not seen by the later 42 d interval. With respect to PT geometric properties in the experimental animals, a larger PT results as the tendon matrix and blood vessels remodel. PT cross-sectional area increased rapidly by 3 d to 1.3 times control values, and remained significantly elevated at 42 d postinjury. Morphological assessments demonstrated the disruption of matrix organisation by vascular and soft tissue components associated with the longitudinal incisions. Substantial changes in matrix organisation persisted at 42 d after surgery. These findings suggest that open longitudinal incision of the PT increases the vascular supply to deep tendon early after injury. These changes probably arise through both vasomotor and angiogenic activity in the tissue. Since PT blood flow and vascular volume return towards control levels after 6 wk but structural features remain disorganised, we propose that vascular remodelling is more rapid and complete than matrix remodelling after surgical incision of the PT.

Periarticular cancellous bone changes following anterior cruciate ligament injury.

To understand more fully the early bone changes in an experimental model of osteoarthrosis, we quantified periarticular bone mineral density and bone mechanical properties in anterior cruciate ligament transected (ACLX) knee joints (4, 10, 32, and 39 wk post-ACLX) compared with contralateral joints and unoperated normal joints of skeletally mature animals. Maximal stress and energy were significantly reduced in ACLX cancellous bone from the medial femoral condyles at 4 wk postinjury. All mechanical properties (e.g., yield stress and elastic modulus) declined after 4 wk and were significantly reduced at 10 wk. ACLX bone mineral density was significantly reduced at all measured time points. Ash content was significantly reduced at 10 and 32 wk. Changes in the lateral condyles were similar but less pronounced than in the medial condyles. These bony changes accompanied the earliest articular cartilage molecular changes and preceded changes in the articular cartilage gross morphology. We suggest that these early changes in bone mechanical behavior contribute to the progression of osteoarthrosis and pathogenic changes in the joint.

Early regional adaptation of periarticular bone mineral density after anterior cruciate ligament injury.

The present study measured early-stage adaptation of bone mineral (BMD) in the periarticular cancellous bone of the canine knee (stifle) joint after anterior cruciate ligament (ACL) transection (ACLX). Regional changes in BMD in the tibia and femur were analyzed by using quantitative computed tomography (qCT) at 3 wk and 12 wk after unilateral ACLX to determine whether there were focal points for BMD changes and whether these changes occurred early after the induced knee injury. BMD decreased rapidly after ACLX, and the more pronounced response was in the femur. In the 3-wk group, there were decreases in BMD in the tibia and the femur, and these changes were significant in the posterior-medial region of the femur, which showed a decrease of BMD in the ACLX limb (-0.048 +/- 0.011 g/cm(3)). In the 12-wk group, all regions in the tibia and femur exhibited significant decreases in BMD, and the average decrease was greatest in the posterior-medial region of the femur (-0.142 +/- 0.021 g/cm(3)). The regions of pronounced periarticular cancellous BMD adaptation corresponded to observed focal cartilage defects. Early decreases in BMD in the injured knee may be related to altered loading and kinematics in the knee and may be an important link in the pathogenesis of posttraumatic osteoarthritis.