PII: S0301-5629(02)00480-5 ● Original Contribution REAL-TIME ULTRASOUND ANALYSIS OF ARTICULAR CARTILAGE DEGRADATION IN VITRO HEIKKI J. NIEMINEN* † ,JUHA TO ¨ YRA ¨ S* ‡ ,JARNO RIEPPO † ,MIIKA T. NIEMINEN † , JANI HIRVONEN*, RAMI KORHONEN* † and JUKKA S. JURVELIN* ‡ *Department of Clinical Physiology and Nuclear Medicine, Kuopio University Hospital and University of Kuopio, Kuopio, Finland; Departments of † Anatomy and ‡ Applied Physics, University of Kuopio, Kuopio, Finland (Received 25 January 2001; in final form 17 December 2001) Abstract—The sensitivity of the reflection coefficient, attenuation and velocity to the enzymatic degradation of bovine patellar cartilage was evaluated in real-time with high-frequency ultrasound (US) (29.4 MHz). These parameters were estimated from the radiofrequency (RF) signal, which was recorded at 5-min intervals during the digestion of the tissue by collagenase or by trypsin. The coefficient of reflection at cartilage surface decreased by 78.5% and 10.5% (p < 0.05) after 6 h of exposure to collagenase and 4 h of exposure to trypsin, respectively. During the trypsin digestion, the attenuation in cartilage increased by 0.274 dB/mm (p < 0.05) and the velocity decreased by 7 m/s (p < 0.05). The coefficient of reflection at the cartilage surface was the most sensitive acoustic parameter to the enzymatic degradation of cartilage and may be the easiest to implement for clinical diagnosis of cartilage quality. US velocity was found to be insensitive to degradation. The small difference in mean velocity between the control and degraded cartilage suggests that a constant predefined US velocity value can be used to obtain diagnostically acceptable measurement of the cartilage thickness. (E-mail: juha.toyras@uku.fi) © 2002 World Federation for Ultrasound in Medicine & Biology. Key Words: Ultrasound, Articular cartilage, Enzymatic digestion, Osteoarthrosis. INTRODUCTION Articular cartilage is a connective tissue that protects articulating bones against friction and contact loads. It is generally described as a biphasic tissue constituted of a solid matrix and of an interstitial fluid representing 70% to 80% of the total volume (Muir 1980). The matrix consists of a fibrous network mainly of type II collagen, which traps electrolytes and macromolecules, proteogly- cans (PGs). These hydrophilic macromolecules bind large amounts of water in the tissue. Collagens and PGs are the most important structural components of the cartilage, and the interaction between these constituents is essential for the mechanical integrity of the tissue (Broom and Poole 1983). Osteoarthrosis (OA) is a severe musculoskeletal disease, commonly diagnosed among elderly people. For successful treatment of the disease, early detection of the pathologic changes in cartilage is essential. Current di- agnostic modalities of cartilage degradation include non- invasive imaging techniques such as X-ray and magnetic resonance imaging (MRI), and invasive techniques such as arthroscopy and mechanical indentation (Lyyra et al. 1995). However, X-ray techniques are insensitive for evaluation of pathologic changes because they cannot reveal structure or properties of soft tissues. The resolu- tion of clinical MRI techniques, on the other hand, is insufficient to reveal superficial tissue damage. Conven- tional arthroscopy relies on the qualitative, visual eval- uation and reveals only macroscopic tissue changes of progressed disease. The mechanical indentation tech- nique lacks information on the cartilage thickness, mak- ing determination of intrinsic tissue stiffness impossible (Hayes et al. 1972; Mak et al. 1987). High-frequency US may provide sensitive means for a quantitative evaluation of the structural and func- tional properties of articular cartilage (Adams and Wal- lace 1991; Agemura et al. 1990; Cherin et al. 1998; Kim et al. 1995; Lefebvre et al. 1998; Saied et al. 1997; Senzig et al. 1992; To ¨yra ¨s et al. 1999; Youn et al. 1999). Morphologic changes in cartilage and subchondral bone induced by experimental OA were detected using a pulse-echo technique at 50 MHz (Cherin et al. 1998; Address correspondence to: Juha To ¨yra ¨s, Department of Applied Physics, University of Kuopio, POB 1627, 70211 Kuopio, Finland. E-mail: juha.toyras@uku.fi Ultrasound in Med. & Biol., Vol. 28, No. 4, pp. 519 –525, 2002 Copyright © 2002 World Federation for Ultrasound in Medicine & Biology Printed in the USA. All rights reserved 0301-5629/02/$–see front matter 519