Estimation of the Young's modulus of articular cartilage using an arthroscopic indentation instrument and ultrasonic measurement of tissue thickness

We evaluated whether the use of cartilage thickness measurement would impro ve the ability of the arthroscopic indentation technique to estimate the in trinsic stiffness of articular cartilage. First, cartilage thickness and ul trasound reflection from the surface of bovine humeral head were registered in situ using a high-frequency ultrasound probe. Subsequently, cartilage w as indented in situ at the sites of the ultrasound measurements using arthr oscopic instruments with plane-ended and spherical-ended indenters. Finally , full-thickness cartilage disks (n = 30) were extracted from the indented sites (thickness = 799-1654 mum) and the equilibrium Young's modulus was de termined with a material testing device in unconfined compression geometry. We applied analytical and numerical indentation models for the theoretical correction of experimental indentation measurements. An aspect-ratio (the ratio of indenter radius to cartilage thickness) correction improved the co rrelation of the indenter force with the equilibrium Young's modulus from r (2) = 0.488 to r(2) = 0.642-0.648 (n = 30) for the plane-ended indenter (di ameter = 1.000mm, height = 0.300 mm) and from r(2) = 0.654 to r(2) = 0.684- 0.692 (n = 30) for the spherical-ended indenter (diameter = 0.500 mm, heigh t = 0.100 mm), depending on the indentation model used for the correction. The linear correlation between the ultrasound reflection and the Young's mo dulus was r(2) = 0.400 (n = 30). These results suggest th;it with large ind enters, knowledge of the cartilage thickness improves the reliability of th e indentation measurements, especially in pathological situations where car tilage thickness may be significantly lower than normal. Ultrasound measure ments also provide diagnostically important information about cartilage thi ckness as well as knowledge of the integrity of the superficial zone of car tilage. (C) 2001 Elsevier Science Ltd. All rights reserved.