Progressive chondrocyte death after impact injury indicates a need for chondroprotective therapy

Abstract

Background: Impact injury to articular cartilage can lead to posttraumatic osteoarthritis.

Hypotheses: This study tests the hypotheses that (1) chondrocyte injury occurs after impact at energies insufficient to fracture the cartilage surface, and that (2) cartilage injury patterns vary with impact energy, time after injury, and cartilage thickness.

Study design: Controlled laboratory study.

Methods: Fresh bovine osteochondral cores were randomly divided into 5 groups: (1) control, (2) 0.35 J, (3) 0.71 J, (4) 1.07 J, and (5) 1.43 J impact energies. Cores were subjected to computer-controlled impact loading and full-thickness sections were then prepared and incubated in Dulbecco's Modified Eagle's Medium/F12 at 37 degrees C. Adjacent sections were harvested 1 and 4 days after impact for viability staining and fluorescent imaging. The area of dead and living chondrocytes was quantified using custom image analysis software and reported as a percentage of total cartilage area.

Results: The highest impact energy fractured the cartilage in all cores (1.43 J, n = 17). Seventy-three percent and 64% of the osteochondral cores remained intact after lower energy impacts of 0.71 J and 1.07 J, respectively. At lower energy levels, fractured cores were thinner (P <.01) than those remaining intact. In cores remaining intact after impact injury, chondrocyte death increased with increasing impact energy (P <.05) and with greater time after impact (P <.05). A progressive increase in dead cells near the bone/cartilage interface and at the articular surface was observed.

Conclusion: These data showing progressive chondrocyte death after impact injury at energies insufficient to fracture the cartilage surface demonstrate a potential need for early chondroprotective therapy.

Clinical relevance: These data show that efforts to reduce chondrocyte morbidity after joint injury may be a useful strategy to delay or prevent the onset of posttraumatic osteoarthritis.

Figure 1

Representative images of cores subjected to 1.43 J (A) and cores subjected to lower-impact energies (B). All cores subjected to high-energy impact (1.43 J, n = 17) were destroyed. Cores subjected to lower energy impact and controls usually remained intact.

Figure 2

Chondrocyte viability after impact. The percentage area of cell death is greater as impact energy increases and progresses with time. Intermediate- and high-energy impacts have significantly more death than low-energy and nonimpacted samples at both day 1 and day 4. Additionally, at day 4, there is an increase in cell death at low-energy impact when compared with control. Values reported are an average ± standard deviation. (P < .001***, P < .01**, P <.05* vs indicated group.)

Figure 3

Representative confocal images of core sections stained for living cells (green) and dead cells (red). Nonimpacted control (A), core impacted with energy of 0.71 J (B); a characteristic chondrocyte death pattern is visible at the site of impact (on the surface) and in the deep basal zone of the cartilage (at the subchondral bone/cartilage interface).

Figure 4

Chondrocyte viability after impact on day 1 (A) and day 4 (B) relative to core region. Values reported are averages ± standard deviations.