Assessment of cartilage thickness utilising reflectance spectroscopy

Abstract

A new principle for cartilage layer thickness assessments in joints is presented. It is based on the differences between the absorption spectra of cartilage and subchondral bone (containing blood). High-resolution ultrasound measurements of cartilage thickness were compared with reflection spectroscopy data from the same area of bovine hip joint condyles. A simple mathematical model allowed calculation of thickness and comparison with ultrasound data. The cartilage thickness was changed by being ground in short episodes. For thicker cartilage layers, a high degree of reflection in the 400-600 nm wavelength interval was seen. For thinner cartilage layers, the characteristics of the spectra of blood and bone dominated those of cartilage. The mean (+/- SD) thickness of intact cartilage was 1.21 +/- 0.30 mm (n = 30). In an exponential regression model, spectroscopic estimation of cartilage thickness showed a correlation coefficient of r = 0.69 (n = 182). For thinner cartilage layers (d < 0.5 mm), the mean model error was 0.19 +/- 0.17 mm. Results from a bi-layer Monte Carlo simulation supported the assumption of an exponential relationship between spectroscopy data and reference ultrasound data. The conclusion is that optical reflection spectroscopy can be used for cartilage layer thickness assessment.

Optical spectral imaging of degeneration of articular cartilage

“…11,13 The characteristic absorption of the hemoglobin around 550 nm and below 450 nm ͑Ref. 13͒ was evident for samples after 15-h collagenase digestion ͑Fig.…”

“…All the earlier spectroscopic assessments of the articular cartilage [11][12][13][14] have been performed by measuring only a small surface area at the time. Because the degenerative changes in cartilage are highly dependent on the location, the actual state of the cartilage might not be reliably recorded with measurements at a small area.…”

“…After the 15-h digestion, some gaps in spectrum appear at wavelengths below 450 and around 550 nm, i.e., at the same spectral bands to maximal absorption of the hemoglobin. 13 The slopes after 15-h collagenase degradation are significantly different from those after mechanical degradation. After the 6-h collagenase degradation the slopes are significantly different only from the femoral samples ͑Table 1͒.…”

“…11,12,14 A model for the reflectance spectrum as a function of the cartilage thickness has also been proposed. 13 Further optical properties, such as scattering and absorption coefficients and anisotropy, within the whole visible range of the spectrum have been reported for cartilage tissue. 15,16 Based on those studies, cartilage has been found to be highly forward-scattering tissue.…”

“…Optical spectral assessment was applied earlier for bovine hip joint, 13 rabbit cartilage plugs after osteochondral autografting, 11 rabbit cartilage after microfracture repair, 12 and human articular cartilage. 14 The studies have found color coordinate changes in the CIELAB colorimetric system when the cartilage integrity changes.…”