Effect of impact load on articular cartilage: cell metabolism and viability, and matrix water content.

Significant evidence exists that trauma to a joint produced by a single impact load below that which causes subchondral bone fracture can result in permanent damage to the cartilage matrix, including surface fissures, loss of proteoglycan, and cell death. Limited information exists, however, on the effect of a varying impact stress on chondrocyte biophysiology and matrix integrity. Based on our previous work, we hypothesized that a stress-dependent response exists for both the chondrocyte's metabolic activity and viability and the matrix's hydration. This hypothesis was tested by impacting bovine cartilage explants with nominal stresses ranging from 0.5 to 65 MPa and measuring proteoglycan biosynthesis, cell viability, and water content immediately after impaction and 24 hours later. We found that proteoglycan biosynthesis decreased and water content increased with increasing impact stress. However, there appeared to be a critical threshold stress (15-20 MPa) that caused cell death and apparent rupture of the collagen fiber matrix at the time of impaction. We concluded that the cell death and collagen rupture are responsible for the observed alterations in the tissue's metabolism and water content, respectively, although the exact mechanism causing this damage could not be determined.

Continuous cyclic load reduces proteoglycan release from articular cartilage.

OBJECTIVE: To study the effect of a continuous cyclic mechanical load on the release of newly synthesized proteoglycans (PGs) from mature bovine articular cartilage. METHODS: Viable cartilage explants were continuously loaded with 1 MPa cyclic stress at 1 Hz frequency for 24 h, and the release of labeled (35SO4) PGs measured before, during and after application of the compressive load. To separate the effect of active chondrocyte catabolism from that of passive PG release, PG release in live explants, with and without protease inhibitors to inhibit PG breakdown, was compared to PG release in explants whose chondrocytes were killed prior to loading. RESULTS: In live explants, a continuous cyclic load significantly reduced PG release by as much as 50% compared to unloaded explants. In killed explants which were unloaded, the PG release increased five to 10 times, while a cyclic load reduced PG release to that found in viable, loaded explants. Twenty-four hours after load removal PG release in all loaded explants returned (increased) to that of the unloaded explants. CONCLUSIONS: These results indicate that PG release from the cartilage matrix is inhibited by continuous cyclic mechanical loading, independent of cellular metabolism, and suggest that a primary mechanism for reducing PG release is by decreasing the interstitial porosity through which the PGs can escape.

Effect of impact load on articular cartilage: development of an intra-articular fracture model.

OBJECTIVES: To investigate the biological and mechanical effects of a single-impact load on articular cartilage. DESIGN: An in vitro laboratory study was performed using mature bovine cartilage and bone, and isolated cartilage explants. Each specimen was impacted with a single load applied with a specially designed impactor and materials test machine. Chondrocyte metabolic activity and cartilage structural integrity was investigated using force displacement curves, radionuclide labeling, histology, and changes in water content. SETTING: Laboratory for Soft Tissue Research, New York, New York, U.S.A. SPECIMENS: Viable mature bovine cartilage and cartilage and bone explants. MAIN OUTCOME MEASUREMENTS: Mechanical failure, proteoglycan synthesis, water content, histology, radiography, and scanning electron microscopy changes occurring during the twenty-four-hour period immediately following impact. RESULTS: Force/displacement curves for the cartilage and bone explants demonstrated two failure-stress peaks, the first at fifty megapascals, representing cartilage failure, and a second peak at seventy-five megapascals, representing bone failure. Fine grain radiographs, histology, and scanning electron microscopy all confirmed the destruction of the cartilage in the area of direct impact (zone I) and subchondral bone failure and the detachment of the cartilage within the lesser impacted area (zone II). Proteoglycan synthesis was reduced significantly (p < 0.05) in the areas of direct impact (zone I) compared with areas with less or no impact (zones II and III, respectively). Significantly greater water content (p < 0.05) was found within the cartilage of zone I compared with zones II and III. CONCLUSIONS: Significant and possibly irreversible articular cartilage damage occurs after a single high-energy impact load.

Characterization of cartilage metabolic response to static and dynamic stress using a mechanical explant test system.

A new mechanical explant test system was used to study the metabolic response (via proteoglycan biosynthesis) of mature, weight-bearing canine articular cartilage subjected to static and dynamic compressive stresses. Stresses ranging from 0.5 to 24 MPa were applied sinusoidally at 1 Hz for intervals of 2-24 h. The explants were loaded in unconfined compression and compared to age-matched unloaded explants. Both static and dynamic compressive stress significantly decreased proteoglycan biosynthesis (range 25-85%) for all loading time intervals. The inhibition was proportional to the applied stress but was independent of loading time. After rehydration upon load removal, the measured water content of the loaded explants was not different from the unloaded explants for all test variables. Autoradiographic and electron microscopic analysis of loaded explants showed viable chondrocytes throughout the matrix. Our results suggest that the decreased metabolic response of cyclically loaded explants may be dominated by the static component (RMS) of the dynamic load. Furthermore, the observed decreased metabolism may be more representative of the in situ tissue response than that of unloaded explants, in which we found an increasing rate of metabolism for up to 6 days after explant removal.

Effects of misoprostol and prostaglandin E2 on proteoglycan biosynthesis and loss in unloaded and loaded articular cartilage explants.

The effects of misoprostol, a prostaglandin E1 analog, and prostaglandin E2 on proteoglycan biosynthesis and loss were studied in unloaded and mechanically loaded mature bovine articular cartilage explants. The prostaglandins were administered daily at dosages of 0, 10, 100 and 1000 eta g/ml for up to seven days, and proteoglycan biosynthesis determined by measurement of radiolabelled sulfate incorporation. The presence of misoprostol lead to a significant (p < 0.001) dose-dependent inhibition (30%-50%) in proteoglycan biosynthesis which was also dependent on exposure time (p < 0.05). A significant decrease in biosynthesis (34%) was also found for prostaglandin E2, but only at the highest dose (1000 eta g/ml). Proteoglycan catabolism rates were not affected by either substance as assessed by loss of newly synthesized proteoglycan. The application of a continuous cyclic mechanical compressive load (stress of 1.0 MPa at 1 hertz for 24 hours) resulted in a significant inhibition of proteoglycan biosynthesis (up to 50%) as compared to unloaded explants. However, there was no additive effect when mechanical load and misoprostol or prostaglandin E2 were combined. These results suggest that prostaglandins may have a role in the degenerative and repair process in various forms of arthritis where elevated intra-articular levels of prostaglandin E2 are present.

A feasibility study for removing tissue contamination from porous implants.

Porous implants that are unpackaged in the operating room but not implanted are discarded because they must be considered potentially contaminated with tissue. To reduce this waste, a method was developed to decontaminate these implants so that they can be resterilized and implanted. This method consists of ultrasonic scrubbing, sequentially, in aqueous solutions of dishwashing detergent, 7% nitric acid, and 5.25% sodium hypochlorite. The effectiveness of the method was tested by contaminating samples of porous implants with tissue, subjecting them to the decontamination method, and then using the following techniques to determine whether any tissue remained. The weights of samples after decontamination were compared with their weights before contamination. The rate of removal of labeled protein contamination from samples was measured. The capacity of decontaminated samples to activate immune system cells was assayed. Bioburden evaluations were performed on decontaminated samples. Within the measurement capabilities of each technique, no tissue was detected in any sample after decontamination.