Won C. Bae

Depth- and strain-dependent mechanical and electromechanical properties of full-thickness bovine articular cartilage in confined compression

Compression tests have often been performed to assess the biomechanical properties of full-thickness articular cartilage. We tested whether the apparent homogeneous strain-dependent properties, deduced from such tests, reflect both strain- and depth-dependent material properties. Full-thickness bovine articular cartilage was tested by oscillatory confined compression superimposed on a static offset up to 45%. and the data fit to estimate modulus, permeability, and electrokinetic coefficient assuming homogeneity. Additional tests on partial-thickness cartilage were then performed to assess depth- and strain-dependent properties in an inhomogeneous model, assuming three discrete layers (i = 1 starting from the articular surface, to i = 3 up to the subchondral bone). Estimates of the zero-strain equilibrium confined compression modulus (H(A0)), the zero-strain permeability (kp0) and deformation dependence constant (M), and the deformation-dependent electrokinetic coefficient (ke) differed among individual layers of cartilage and full-thickness cartilage. HiA0 increased from layer 1 to 3 (0.27 to 0.71 MPa), and bracketed the apparent homogeneous value (0.47 MPa). ki(p0) decreased from layer 1 to 3 (4.6 x 10(-15) to 0.50 x 10(-15) m2/Pa s) and was less than the homogeneous value (7.3 x 10(-15) m2/Pa s), while Mi increased from layer 1 to 3 (5.5 to 7.4) and became similar to the homogeneous value (8.4). The amplitude of ki(e) increased markedly with compressive strain, as did the homogeneous value: at low strain, it was lowest near the articular surface and increased to a peak in the middle-deep region. These results help to interpret the biomechanical assessment of full-thickness articular cartilage.

Follow-up of Osteochondral Plug Transfers in a Goat Model A 6-Month Study

Background Osteochondral transfer procedures are increasingly used to resurface full-thickness articular cartilage defects. There has not been long-term assessment/description of autogenous donor and recipient sites. Hypothesis The healing process occurs at the donor/host cartilage and bone interfaces. Study Design Histologic, biochemical, and biomechanical changes were assessed 6 months after an osteochondral transfer in a goat model. Methods Eight adult goats were studied. In the 6 osteochondral transfer goats, 2 autogenous plugs were transferred from the femoral trochlea to defects in the weightbearing portion of the medial femoral condyle. The goats were allowed free range for 6 months. Randomly assigned plugs were assessed. Results Knees of the sacrificed animals had preservation of the joint space with mild chondromalacic changes in both transfer and contralateral control groups. Histologically, no evidence of cartilage (host/donor) healing was seen. Subchondral bone of the plug was contiguous with the surrounding recipient bone. Cellular viability in the autogenous osteochondral plug was seen, and 35SO4 uptake of the articular cartilage was not statistically different from the contralateral control condyle. The indentation stiffness of the transfer plug (mosaicplasty) and the contralateral donor site were similar—much stiffer than normal cartilage including surrounding condylar cartilage. Large structural stiffness of transferred cores and donor sites appeared to be related to their thinner cartilage layer. Conclusions At 6-month follow-up, a cleft between host and transferred articular regions remained, with no integration between the two. Clinical Relevance Autogenous transplantation of osteochondral plugs is possible with integration of subchondral bone and preservation of chondral viability.

A morphologic, biochemical, and biomechanical assessment of short-term effects of osteochondral autograft plug transfer in an animal model.

PURPOSE The objective of this study was to assess the short-term changes that occur after an osteochondral autograft plug transfer from the femoral trochlea to the medial femoral condyle in a goat model. TYPE OF STUDY Articular cartilage repair animal study. METHODS Six adult male goats were used in this study. Two 4.5-mm osteochondral plugs were transferred from the superolateral femoral trochlea to 2 recipient sites in the central portion of the medial femoral condyle for a survival period of 12 weeks. Postmortem, the global effects of the procedure were assessed by gross morphologic inspection and by analyzing the synovial DNA for inflammatory response. The recipient sites were also evaluated histologically and biomechanically. Metabolic activity was determined by (35)SO(4) uptake, and viability was assessed using a live/dead stain and by confocal laser microscopy. RESULTS There was no evidence of significant gross morphologic or histologic changes in the operative knee as a result of the osteochondral donor or recipient sites. The patella, tibial plateau, and medial meniscus did not show any increased degenerative changes as a result of articulating against the donor or recipient sites of the osteochondral autografts. Analysis of synovial DNA revealed no inflammatory response. Biomechanically, 6- to 7-fold greater stiffness was noted in the cartilage of the transferred plugs compared with the control medial femoral condyle. Furthermore, on histologic examination, the healing subchondral bone interface at the recipient site had increased density. Glycosaminoglycan synthesis as determined by (35)SO(4) uptake was upregulated in the transplanted cartilage plug relative to the contralateral control, showing a repair response at the site of implantation. And finally, confocal microscopy showed 95% viability of the transferred plugs in the medial femoral condyle region. CONCLUSIONS Our findings demonstrate the ability to successfully transfer an osteochondral autograft plug with maintenance of chondrocyte cellular viability. The transferred cartilage is stiffer than the control medial femoral condyle cartilage, and there is concern regarding the increased trabecular mass in the healing subchondral plate, but these do not result in increased degenerative changes of the opposing articular surfaces in the short term.

Quantitative ultrashort echo time (UTE) MRI of human cortical bone: Correlation with porosity and biomechanical properties

In this study we describe the use of ultrashort echo time (UTE) magnetic resonance imaging (MRI) to evaluate short and long T2* components as well as the water content of cortical bone. Fourteen human cadaveric distal femur and proximal tibia were sectioned to produce 44 rectangular slabs of cortical bone for quantitative UTE MR imaging, microcomputed tomography (µCT), and biomechanical testing. A two‐dimensional (2D) UTE pulse sequence with a minimal nominal TE of 8 µseconds was used together with bicomponent analysis to quantify the bound and free water in cortical bone using a clinical 3T scanner. Total water concentration was measured using a 3D UTE sequence together with a reference water phantom. UTE MR measures of water content (total, free, and bound), T2* (short and long), and short and long T2* fractions were compared with porosity assessed with µCT, as well as elastic (modulus, yield stress, and strain) and failure (ultimate stress, failure strain, and energy) properties, using Pearson correlation. Porosity significantly correlated positively with total (R2 = 0.23; p < 0.01) and free (R2 = 0.31; p < 0.001) water content as well as long T2* fraction (R2 = 0.25; p < 0.001), and negatively with short T2* fraction and short T2* (R2 = 0.24; p < 0.01). Failure strain significantly correlated positively with short T2* (R2 = 0.29; p < 0.001), ultimate stress significantly correlated negatively with total (R2 = 0.25; p < 0.001) and bound (R2 = 0.22; p < 0.01) water content, and failure energy significantly correlated positively with both short (R2 = 0 30; p < 0.001) and long (R2 = 0.17; p < 0.01) T2* values. These results suggest that UTE MR measures are sensitive to the structure and failure properties of human cortical bone, and may provide a novel way of evaluating cortical bone quality.

Chondrocyte Viability is Higher after Prolonged Storage at 37°C than at 4 C for Osteochondral Grafts

Background Osteochondral allografts are currently stored at 4°C for 2 to 6 weeks before implantation. At 4°C, chondrocyte viability, especially in the superficial zone, deteriorates starting at 2 weeks. Alternative storage conditions could maintain chondrocyte viability beyond 2 weeks, and thereby facilitate increased graft availability and enhanced graft quality. Purpose The objective of the study was to determine the effects of prolonged 37°C storage compared with traditional 4°C storage on chondrocyte viability and cartilage matrix content. Study Design Controlled laboratory study. Methods Osteochondral samples from humeral heads of adult goats were analyzed (i) fresh, or after storage in medium for (ii) 14 days at 4°C including 10% fetal bovine serum, (iii) 28 days at 4°C including 10% fetal bovine serum, (iv) 28 days at 37°C without fetal bovine serum, (v) 28 days at 37°C including 2% fetal bovine serum, or (vi) 28 days at 37°C including 10% fetal bovine serum. Portions of samples were analyzed by microscopy after LIVE/DEAD staining to determine chondrocyte viability and density, both en face (to visualize the articular surface) and vertically (overall and in superficial, middle, and deep zones). The remaining cartilage was analyzed for sulfated glycosaminoglycan and collagen. Results The 37°C storage maintained high chondrocyte viability compared with 4°C storage. Viability of samples after 28 days at 37°C was ˜80% at the cartilage surface en face, ˜65% in the superficial zone, and ˜70% in the middle zone, which was much higher than ˜45%, ˜20%, and ˜35%, respectively, in 4°C samples after 28 days, and slightly decreased from ˜100%, ˜85%, and ˜95%, respectively, in fresh controls. Cartilage thickness, glycosaminoglycan content, and collagen content were maintained for 37°C and 4°C samples compared with fresh controls. Conclusion The 37°C storage of osteochondral grafts supports long-term chondrocyte viability, especially at the vulnerable surface and superficial zone of cartilage. Clinical Relevance Storage of allografts at a physiologic temperature of 37°C may prolong storage duration, improve graft availability, and improve treatment outcomes.

Effect of impact on chondrocyte viability during insertion of human osteochondral grafts

BACKGROUND Osteochondral grafts, used to treat chondral and osteochondral defects, require high insertional forces that may affect the viability of chondrocytes in the graft. The objectives of this study were to (1) measure the loading impact during insertion of osteochondral grafts, (2) evaluate the effect of insertional loading on chondrocyte viability, and (3) assess this effect on chondrocyte apoptosis and activation of caspase-3. METHODS The distal parts of twelve fresh femora from six adult human cadavers were harvested within seventy-two hours after the death of the donor. From each femur, four 15-mm-diameter cylindrical osteochondral grafts were isolated; two of these grafts (a total of twenty-four grafts in the study) were transplanted with standard impact insertion into recipient sockets in the other condyle of the ipsilateral femur. The other two grafts served as unloaded controls. Loads were measured during the insertion of ten of the twenty-four transplanted grafts. Full-thickness cartilage disks were then removed from the grafts, incubated for up to forty-eight hours, and analyzed for cell viability, TUNEL (terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling)-positive reactivity, and caspase-3 activation, each as a function of the depth from the articular surface. RESULTS The insertion of an osteochondral graft was characterized, on the average (and standard deviation), by 10 +/- 4 impacts, each generating 2.4 +/- 0.9 kN of load and 13.3 +/- 4.9 MPa of stress for a duration of 0.57 +/- 0.13 ms with a 0.62 +/- 0.25 N.s impulse. Impact insertion increased cell death in the superficial 500 mum to 21% at one hour (p < 0.001) and 47% at forty-eight hours (p < 0.001) and also increased cell death in deeper layers at forty-eight hours. Some cell death was due to apoptosis, as indicated by an increase in caspase-3 activation at eight hours (p < 0.01) and TUNEL-positive cells at forty-eight hours (p < 0.05) in the superficial 500 mum of impacted cartilage. CONCLUSIONS Impact insertion of osteochondral grafts generates damaging loads that cause chondrocyte death, particularly in the superficial zone, mainly as a result of apoptosis mediated by the activation of caspases. CLINICAL RELEVANCE Chondrocyte death that occurs during impact insertion of osteochondral grafts may lead to compromised function. Understanding the mechanisms and consequences of such impact loading may provide insights into potential therapeutic interventions, or lead to changes in the insertion technique, to decrease the cell injury associated with impact loading.

Increased Hydraulic Conductance of Human Articular Cartilage and Subchondral Bone Plate With Progression of Osteoarthritis

OBJECTIVE Osteoarthritis (OA) is characterized by progressive degeneration of articular cartilage and remodeling of the subchondral bone plate, comprising calcified cartilage and underlying subchondral bone. Calcified cartilage remodeling due to upward invasion by vascular canals or to calcified cartilage erosion may contribute to biomechanical alteration of the osteochondral tissue and its subchondral bone plate component. The study hypothesis was that hydraulic conductance of osteochondral tissue and subchondral bone plate increases with structural changes indicative of increasing stages of OA. METHODS Osteochondral cores were harvested from the knees of cadaveric tissue donors and from discarded fragments from patients with OA undergoing knee surgery. The osteochondral cores from tissue donors were macroscopically normal, and the cores from patients with OA had partial-thickness or full-thickness erosion to bone. The cores were perfusion-tested to determine the hydraulic conductance, or ease of fluid flow, in their native state and after enzymatic removal of cartilage. Adjacent portions were analyzed by 3-dimensional histology for calcified cartilage, subchondral bone, and subchondral bone plate thickness and vascular canal density. RESULTS Hydraulic conductance of native osteochondral tissue and subchondral bone plate was higher (2,700-fold and 3-fold, respectively) in fully eroded samples than in normal samples. The calcified cartilage layer was thicker (1.5-fold) in partially eroded samples than in normal samples but thinner and incomplete in fully eroded samples. Subchondral bone plate vascularity was altered with increasing stages of OA. CONCLUSION During joint loading, increased hydraulic conductance of the osteochondral tissue and subchondral bone plate could have deleterious biomechanical consequences for cartilage. Increased fluid exudation from overlying and opposing cartilage, increased fluid depressurization, and increased cartilage tissue strains could lead to chondrocyte death and cartilage damage.

Biomechanical, structural, and biochemical indices of degenerative and osteoarthritic deterioration of adult human articular cartilage of the femoral condyle

OBJECTIVE To compare the tensile biomechanical properties of age-matched adult human knee articular cartilage exhibiting distinct stages of degenerative or osteoarthritic deterioration and to determine the relationships between tensile properties and biochemical and structural properties hypothesized to underlie functional biomechanical deterioration. METHODS Age-matched articular cartilage samples, obtained from the lateral and medial femoral condyles (LFC and MFC), exhibited (1) minimal fibrillation, characteristic of normal aging (NLA), (2) overt fibrillation associated with degeneration (DGN), or (3) overt fibrillation associated with osteoarthritis (OA). DGN samples were from knees that exhibited degeneration but not osteophytes while OA samples were from fragments removed during total knee arthroplasty. Cartilage samples were analyzed for tensile properties, cell and matrix composition, and histopathological structure. RESULTS Differences in tensile, compositional and surface structural properties were indicative of distinct stages of cartilage degeneration, early (OA) advanced (DGN) and late (OA) with early degenerative changes in NLA samples being more advanced in the MFC than the LFC, including higher surface fibrillation, lower intrinsic fluorescence, and lower mechanical integrity. The transition from early to advanced degeneration involved a diminution in mechanical function, surface integrity, and intrinsic fluorescence. The transition from advanced to late degeneration involved an increase in cartilage water content, an increase in degraded collagen, and loss of collagen. CONCLUSIONS These results provide evidence of coordinated mechanical dysfunction, collagen network remodeling, and surface fibrillation. Even in the cartilage of knees exhibiting overt fibrillation but not extensive erosions characteristic of clinical osteoarthritis, most features of advanced cartilage degeneration were present.

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.

Ultrashort Echo Time MR Imaging of Osteochondral Junction of the Knee at 3 T: Identification of Anatomic Structures Contributing to Signal Intensity

PURPOSE To image cartilage-bone interfaces in naturally occurring and experimentally prepared human cartilage-bone specimens at 3 T by using ultrashort echo time (TE) (UTE) and conventional pulse sequences to (a) determine the appearance of the signal intensity patterns and (b) identify the structures contributing to signal intensity on the UTE MR images. MATERIALS AND METHODS This study was exempted by the institutional review board, and informed consent was not required. Five cadaveric (mean age, 86 years +/- 4) patellae were imaged by using proton density-weighted fat-suppressed (repetition time msec/TE msec, 2300/34), T1-weighted (700/10), and UTE (300/0.008, 6.6, with or without dual-inversion preparations at inversion time 1 = 135 msec and inversion time 2 = 95 msec) sequences. The UTE images were compared with proton density-weighted fat-suppressed and T1-weighted images and were evaluated by two radiologists. To identify the sources of signal on the UTE images, samples including specific combinations of tissues (uncalcified cartilage [UCC] only, calcified cartilage [CC] and subchondral bone [bone] [CC/bone], bone only; and UCC, CC, and bone [UCC/CC/bone]) were prepared and imaged by using the UTE sequence. RESULTS On the UTE MR images, all patellar sections exhibited a high-intensity linear signal near the osteochondral junction, which was not visible on protein density-weighted fat-suppressed or T1-weighted images. In some sections, focal regions of thickened or diminished signal intensity were also found. In the prepared samples, UCC only, CC/bone, and UCC/CC/bone samples exhibited high signal intensity on the UTE images, whereas bone-only samples did not. CONCLUSION These results show that the high signal intensity on UTE images of human articular joints originates from the CC and the deepest layer of the UCC, without a definite contribution from subchondral bone. UTE sequences may provide a way of evaluating abnormalities at or near the osteochondral junction. (c) RSNA, 2010.