Wavelength-dependent penetration depth of near infrared radiation into cartilage

Articular cartilage is a hyaline cartilage that lines the subchondral bone in the diarthrodial joints. Near infrared (NIR) spectroscopy is emerging as a nondestructive modality for the evaluation of cartilage pathology; however, studies regarding the depth of penetration of NIR radiation into cartilage are lacking. The average thickness of human cartilage is about 1-3 mm, and it becomes even thinner as OA progresses. To ensure that spectral data collected is restricted to the tissue of interest, i.e. cartilage in this case, and not from the underlying subchondral bone, it is necessary to determine the depth of penetration of NIR radiation in different wavelength (frequency) regions. In the current study, we establish how the depth of penetration varies throughout the NIR frequency range (4000-10 000 cm −1 ). NIR spectra were collected from cartilage samples of different thicknesses (0.5 mm to 5 mm) with and without polystyrene placed underneath. A separate NIR spectrum of polystyrene was collected as a reference. It was found that the depth of penetration varied from ∼1 mm to 2 mm in the 4000-5100 cm −1 range, ∼3 mm in the 5100-7000 cm −1 range, and ∼5 mm in the 7000-9000 cm −1 frequency range. These findings suggest that the best NIR region to evaluate cartilage with no subchondral bone contribution is in the range of 4000-7000 cm −1 . In this study, we established how the depth of penetration into cartilage varies throughout the NIR frequency range (4000-10 000 cm −1 ).