gms | German Medical Science

Objectives: Cartilage degeneration is an important cause of disability in humans. Early diagnosis and treatment are, therefore, clinically highly relevant. Early-stage pathological alterations include loss of surface integrity, which is currently hard to evaluate objectively. Arthroscopically principally available, Optical Coherence Tomography (OCT) is a light-based non-destructive technology allowing imaging at micrometer resolutions up to millimeter depths. As yet, OCT-based surface evaluation standards remain to be defined.

Methods: In total, 105 cartilage samples of different degenerative stages were obtained from Total Knee Arthroplasties (6 male, 14 female; mean age 71.9 years [range 48–83 years]), cut to standard size and imaged using a spectral-domain OCT device (Thorlabs, Germany). The central 2-D OCT scan was obtained and used for image processing (i.e. image adjustments, morphological filtering). Subsequent automated surface identification algorithms were used to obtain the primary profile, which was then filtered and processed using established algorithms employing ISO standards. The roughness profile thus obtained was used to calculate 11 2-D roughness parameters, i.e. average of ordinates [Ra, Rq], surface stratification [Rk, Rpk, Rvk], amplitude [Rz, Rp, Rv, Rt] and characteristic average [Rsk, Rku]. Subsequently, samples were correlated with routine histology according to a modified version of the Degenerative Joint Disease (DJD) grading system (a surface-focused subcategory of Mankin's). Correlations were statistically assessed using Spearman's rho, while Mann-Whitney or Kruskal-Wallis tests were used for inter-parameter and inter-group comparisons in Graphpad Prism Software (Version 5.0, US).

Results and Conclusion: The majority of roughness parameters, in particular average of ordinates, surface stratification and amplitude, revealed a close-to-linear correlation with only marginal differences within the entire spectrum of degeneration. Analysis of the characteristic average Rsk indicated a predominance of peaks (Rsk > 0), which significantly increased with increasing degeneration. Rku indicated a random surface (Rku = 3) in healthy cartilage (DJD 0) and an increasingly sharp and pointed surface (Rku > 3) with higher degeneration (DJD > 3). Differences between DJD grades were significant for all parameters but Rku. Likewise, a strong and highly significant correlations for all parameters was shown, which was weakest for Rku.

In conclusion all relevant 2-D roughness parameters were of distinct diagnostic value in the assessment of roughness, although fine degeneration grade-dependent differences were detectable. Therefore, surface integrity should be assessed by a combination of roughness parameters rather than by individual parameters to improve the diagnostic performance of OCT-based surface assessment.