Cartilage degeneration is associated with complex changes in cartilage matrix composition that result in decreased collagen and proteoglycan content and increased water content. These changes in matrix composition correlate with decreased mechanical stiffness. As a result, four stages of cartilage lesions develop. The consensus conference of the ICRS I (international Cartilage Repair Society) suggested a score for cartilage lesions in 2002 : The grades were as follows: ICRS grade 0 (normal), ICRS grade 1 (nearly normal, superficial lesions with soft indentation and/or superficial fissures and cracks), ICRS grade 2 (abnormal, lesions extending down to <50% of cartilage depth), ICRS grade 3 (severely abnormal, cartilage defects extending down > 50% of cartilage depth) and ICRS grade 4 (severely abnormal, complete defect).
Clinical examination (crepitus, effusion and pain) is nonspecific. Radiological signs of osteoarthritis (joint space narrowing, subchondral sclerosis, osteophytes) are primarily associated with high-grade cartilage damage, and are therefore inadequate for diagnosing early disease stages.
The diagnosis of cartilage lesions can be made by MRI or arthroscopic evaluation. MRI is the most important non-invasive diagnostic method for assessing cartilage lesions. Many studies have demonstrated the high validity of MRI for evaluating chondral lesions in the knee as well as in other joints, but MRI has low accuracy for initial lesions. Only specific techniques like dGEMRIC (delayed Gadolinium Enhanced MRI of Cartilage) are sufficient for evaluating initial chondral lesions [2–4].
The arthroscopic diagnosis of chondral lesions depends on the surgeon's subjective rating alone. Diagnosis is made by visualizing the lesion on the video monitor and by probing with the arthroscopy hook.
Friemert et al.  found a good correlation between MRI and arthroscopic findings in the case of deep cartilage lesions. One criterion for the validity of arthroscopic diagnosis is the "inter-observer-validity". The diagnosis of deep cartilage defects has high validity but the diagnosis of low grade lesions is often inaccurate [6–11]. Detection of these early stages primarily involves palpation with the arthroscopic hook. This palpation depends on the power applied by the manual pressure of the surgeon and, as Li and Herzog demonstrated, on the geometry of the distal end of the hook .
Recent studies have used mechanical devices to attempt to measure cartilage softening objectively by evaluating the reduced stiffness of the cartilage [13–17]. Duda et al. created an instrument for stiffness measurements in a "low-contact-modus". The reduction of cartilage stiffness was measured as a function of surface deformation produced by a pulsed flow . However, the practical use of these instruments is limited by their dimensions. Furthermore, the necessity of positioning these instruments exactly vertical to the cartilage surface may limit their practical use in routine arthroscopy.
Early disease stages are characterized by a rise in the water content of the extracellular matrix and, as a result, cartilaginous swelling [19–23]. The water inflow strongly correlates with the loss of mechanical stiffness . In the last decade NIRS (near-infrared-spectroscopy) became an important method for analyzing materials with complex mixes of chemical substances , in particular for measuring the water content of the material In a recent ex vivo study Spahn et al.  demonstrated a decrease of NIR-absorption in low-grade degenerated cartilage. The decreased NIR absorption did correlate with an increase of water content in early degenerated cartilage.
This study introduces a novel NIRS-device that is capable of identifying low-grade cartilage lesions under arthroscopic control.