Ultrasound indentation of bovine knee articular cartilage in situ

We have earlier developed a handheld ultrasound indentation instrument for the diagnosis of articular cartilage degeneration. In ultrasound indentation, cartilage is compressed with the ultrasound transducer. Tissue thickness and deformation are calculated from the A-mode ultrasound signal and the stress applied is registered with the strain gauges. In this study, the applicability of the ultrasound indentation instrument to quantify site-dependent variation in the mechano-acoustic properties of bovine knee cartilage was investigated. Osteochondral blocks (n=6 per site) were prepared from the femoral medial condyle (FMC), the lateral facet of the patello-femoral groove (LPG) and the medial tibial plateau (MTP). Cartilage stiffness (dynamic modulus, Edyn), as obtained with the ultrasound indentation instrument in situ, correlated highly linearly (r=0.913, p<0.01) with the values obtained using the reference material-testing device in vitro. Reproducibility (standardized coefficient of variation) of the ultrasound indentation measurements was 5.2%, 1.7% and 3.1% for Edyn, ultrasound reflection coefficient of articular surface (R) and thickness, respectively. Edyn and R were site dependent (p<0.05, Kruskall–Wallis H test). Edyn was significantly higher (p<0.05, Kruskall–Wallis Post Hoc test) in LPG (mean±SD: 10.1±3.1 MPa) than in MTP (2.9±1.4 MPa). In FMC, Edyn was 4.6±1.3 MPa. R was significantly (p<0.05) lower at MTP (2.0±0.7%) than at other sites (FMC: 4.2±0.9%; LPG: 4.4±0.8%). Cartilage glycosaminoglycan concentration, as quantified with the digital densitometry, correlated positively with Edyn (r=0.678, p<0.01) and especially with the equilibrium Youngʹs modulus (reference device, r=0.874, p<0.01) but it was not associated with R (r=0.294, p=0.24). We conclude that manual measurements are reproducible and the instrument may be used for detection of cartilage quality in situ. Especially, combined measurement of thickness, Edyn and R provides valuable diagnostic information on cartilage status.