Optical coherence tomography detection of subclinical traumatic cartilage injury

Abstract

Objectives: Posttraumatic arthritis is a major cause of disability. Current clinical imaging modalities are unable to reliably evaluate articular cartilage damage before surface breakdown, when potentially reversible changes are occurring. Optical coherence tomography (OCT) is a nondestructive imaging technology that can detect degenerative changes in articular cartilage with an intact surface. This study tests the hypothesis that OCT detects acute articular cartilage injury after impact at energy levels resulting in chondrocyte death and microstructural changes, but insufficient to produce macroscopic surface damage.

Methods: Bovine osteochondral cores underwent OCT imaging and were divided into a control with no impact or were subjected to low (0.175 J) or moderate (0.35 J) energy impact. Cores were reimaged with OCT after impact and the OCT signal intensity quantified. A ratio of the superficial to deep layer intensities was calculated and compared before and after impact. Chondrocyte viability was determined 1 day after impact followed by histology and polarized microscopy.

Results: Macroscopic changes to the articular surface were not observed after low and moderate impact. The OCT signal intensity ratio demonstrated a 27% increase (P = 0.006) after low impact and a 38% increase (P = 0.001) after moderate impact. Cell death increased by 150% (P < 0.001) and 200% (P < 0.001) after low and moderate energy impacts, respectively. When compared with unimpacted controls, both Mankin histology and David-Vaudey polarized microscopy scores increased (P = 0.036 and P = 0.002, respectively) after moderate energy impact.

Conclusions: This study shows that OCT detects acute cartilage changes after impact injury at levels insufficient to cause visible damage to the articular surface but sufficient to cause chondrocyte death and microscopic matrix damage. This finding supports the use of OCT to detect microstructural subsurface cartilage damage that is poorly visualized with conventional imaging.

Figure 1

OCT scanner generates a 2D cross-sectional image (6.5mm x 2mm) along the scan line depicted by the laser aiming light (dashed line). The OCT scan line defines the cartilage section in the mid-sagittal plane running from the 12 o'clock mark to the 6 o'clock position and all cores were scanned in this position in order to preserve the orientation of the acquired OCT images both before and after impact.

Figure 2

(A) OCT image of normal articular cartilage with corresponding superficial (1) and deep (2) analysis regions. (B) A significant alteration in OCT signal was seen after impact injury. Scale bar = 2mm.

Figure 3

The superficial to deep OCT signal intensity ratio demonstrated a significant 27% increase following low impact (*p=0.006) and a 38% increase following moderate impact injury (**p=0.001). Values reported are an average ± SD.

Figure 4

Representative fluorescence images of osteochondral core sections stained for live (green) and dead (red) cells. Few dead cells are present in the unimpacted control core (A), compared to increasing amounts in cores subjected to low (B), and moderate (C) energy impact. Scale bar = 2mm.

Figure 5

Linear regression analysis demonstrated a significant correlation between the percent cell viability and the OCT signal intensity ratio (p<0.001).

Figure 6

PLM microscopy images of (A) non-impacted and (B) moderate impacted articular cartilage. A) In the unimpacted specimen, polarized microscopy shows a dense layer of collagen fibrils in the superficial zone running parallel to the articular surface. The collagen fibrils in the transitional zone are regular and closely spaced. B) In the impacted specimen, the collagen fibers of the superficial zone are disrupted and attenuated. The collagen network in the transitional zone appears porous with irregular gaps between fibers. Scale bar = 125 microns.