Mechano-acoustic diagnosis of cartilage degeneration and repair.

Background: The combined use of high-frequency ultrasound and mechanical indentation has been suggested for the evaluation of cartilage integrity. In this study, we investigated the usefulness of high-resolution B-mode ultrasound imaging and quantitative mechanical measurements for the diagnosis of cartilage degeneration and for monitoring tissue-healing after autologous chondrocyte transplantation.

Methods: In the first study, osteochondral samples (n = 32) were obtained from the lateral facet of a bovine patella, and the samples were visually classified as intact (n = 13) or degenerated (n = 19) and were graded with use of the Mankin scoring system. Samples were imaged with use of a 20-MHz ultrasound instrument, and the dynamic modulus (Edyn) of cartilage was determined in unconfined compression with use of a high-resolution materials tester. In the second study, cartilage chondrocytes were harvested from the low-weight-bearing area of six-month-old porcine knee joints and cultured. A month later, a cartilage lesion was created on the facet of the femoral trochlea and was repaired with use of the autologous chondrocyte transplantation technique (n = 10). Three months later, to estimate cartilage Edyn, the repair tissue, the adjacent cartilage, and the sham-operated contralateral joint cartilage (control) were analyzed in situ with an arthroscopic indentation instrument. Subsequently, the same sites were imaged with ultrasound.

Results: All visually degenerated bovine samples (mean Mankin score = 4) and five visually normal samples (Mankin score = 1) showed reduced Edyn (<2.1 MPa) as compared with histologically normal cartilage (Edyn = 13.8 +/- 3.2 MPa, Mankin score = 0). Cartilage stiffness, as shown by the indenter force, was lower (0.6 +/- 0.3 N, p < 0.05, Wilcoxon's signed-rank test) in the porcine tissue repaired with autologous chondrocyte transplantation than it was in the adjacent (1.6 +/- 0.1 N) or the control (1.9 +/- 0.4 N) tissue. The superficial and internal structure of the degenerated and repaired tissue, including the subchondral erosion at the repair site, was sensitively demonstrated by the ultrasound imaging.

Conclusions: Measurement of cartilage Edyn is an objective method with which to follow changes in the mechanical integrity of cartilage. B-mode ultrasound imaging offers detailed information on the structural properties of cartilage and subchondral bone.