Arthroscopy offers qualitative means to evaluate the surface of articular cartilage. However, possible degeneration of the deep cartilage and subchondral bone remains undetected. High frequency ultrasound imaging is an advanced cartilage evaluation method which is conceivable to arthroscopic use and brings diagnostic information also from deeper cartilage and subchondral bone.
In this study, we characterized spontaneous repair of porcine cartilage in situ with quantitative 2D-ultrasound imaging. At the age of 7-8 months, a cartilage lesion (diameter 6mm, not penetrating into subchondral bone) was created on the lateral facet of the right femoral trochlea (n=8). The animals were sacrificed 3 months after the surgery. The lesion site, adjacent cartilage and the corresponding control area at the contralateral (left) knee were imaged in situ with 20 MHz ultrasound. Ultrasound reflection coefficients were determined from the cartilage surface (R) and from the cartilage-bone interface (R(bone)). Microtopography of the articular surface was quantified by calculating ultrasound roughness index (URI) parameter from the ultrasonically determined surface profile.
Lesion site was spontaneously filled with visually cartilage-like soft tissue with smooth surface. However, ultrasonic images and histological analyses revealed erosion of subchondral bone under the lesion site. Ultrasound reflection (R) at the surface of the spontaneously repaired tissue was significantly lower (-73.5+/-7.6%, P<0.05) than at the surface of intact cartilage. R(bone) was lowest at the lesion site. The surface roughness of spontaneously repaired cartilage was significantly higher than that of the intact tissue (44.0+/-26.0 microm vs 7.5+/-2.3 microm, P<0.05).
Quantitative ultrasound parameters offered diagnostic information revealing impaired structural integrity of the spontaneously repaired porcine cartilage and subchondral bone. These changes are not detectable by traditional arthroscopic means.