Structural associations of symptomatic knee osteoarthritis

Abstract

Objective: Structural changes of osteoarthritis (OA) may occur in the absence of pain. In this study, we aimed to identify histopathologic features that are associated with symptomatic knee OA.

Methods: Medial tibial plateaus and synovium samples were obtained at the time of total knee replacement (TKR) surgery for OA (advanced OA group) or were obtained postmortem from subjects who had not sought medical attention for knee pain during the last year of life (non-OA control group). To identify features of OA, we compared the patients with advanced OA with the age-matched non-OA controls (n = 26 per group). To identify OA features associated with symptoms, we compared two additional groups of subjects who were matched for severity of chondropathy (n = 29 per group): patients undergoing TKR for symptomatic OA (symptomatic chondropathy group) and postmortem subjects with similar severity of chondropathy who were asymptomatic during the last year of life (asymptomatic chondropathy group). The histologic features of the samples were graded, and immunoreactivities for macrophages (CD68) and nerve growth factor (NGF) in the synovium were quantified. The cellular localization of synovial NGF was determined by double immunofluorescence analysis.

Results: Advanced OA cases displayed more severe changes in the synovium (synovitis, increased synovial NGF, and CD68-immunoreactive macrophages) and cartilage (loss of cartilage surface integrity, loss of proteoglycan, tidemark breaching, and alterations in chondrocyte morphology) than did the non-OA controls. Synovial NGF was localized predominantly to fibroblasts and to some macrophages. The symptomatic chondropathy group displayed greater levels of synovitis, synovial NGF, and loss of cartilage integrity, in addition to alterations in chondrocyte morphology, than did the asymptomatic chondropathy group (P < 0.05 for each comparison).

Conclusion: Synovitis, increased synovial NGF, alterations in chondrocyte morphology, and loss of cartilage integrity are features of knee OA that may be associated with symptoms.

© 2014 The Authors. Arthritis & Rheumatology is published by Wiley Periodicals, Inc. on behalf of the American College of Rheumatology.

Figures

Figure 1

Histopathologic features of non-osteoarthritic (non-OA) (A–D) and OA (E–J) knees. Non-OA knees typically displayed mild cartilage surface irregularities of the superficial zone, with normal proteoglycan staining (A), whereas in some patients with advanced OA of the knee, there were deep clefts in the articular cartilage surface and severe proteoglycan loss (E) (Safranin O–fast green stained). Channels can be seen in the calcified cartilage beneath an intact tidemark in a non-OA knee (B), whereas an OA knee shows a channel breaching the tidemark and entering the noncalcified cartilage (F) (hematoxylin and eosin [H&E] stained). Normal subchondral bone marrow spaces are filled with fatty tissue in a non-OA knee (C), whereas subchondral bone marrow is replaced by fibrovascular tissue in an OA knee (G) (H&E stained). Mild synovial lining hyperplasia is shown in a sample from a non-OA knee (D), whereas severe synovitis, characterized by hyperplasia and hypertrophy of the synovial lining and sublining hypercellularity, can be seen in an OA knee (H) (H&E stained). Macrophage immunoreactivity for CD68 (I) and immunoreactivity for nerve growth factor (J) in OA knees was localized to the synovial lining and sublining regions (staining [blue-black] developed with nickel-enhanced diaminobenzidine). Arrows indicate the synovial surface. Bars = 500 μm.

Figure 2

Cellular localization of nerve growth factor (NGF) immunoreactivity in human synovium. A, Hsp47 immunoreactivity localized to cells with fibroblast morphology in the lining and sublining regions of synovium from a patient with osteoarthritis (OA). Arrow indicates a fibroblast colocalized to NGF (same cell as indicated in B); arrowhead indicates a fibroblast not colocalized to NGF. B, NGF immunoreactivity colocalized with Hsp47 (arrow) and in a cell that is not immunolabeled for Hsp47 (arrowhead) in the section shown in A. C, CD68 immunoreactivity of macrophages in synovium from a patient with OA. Arrow indicates a macrophage colocalized to NGF (same cell as indicated in D); arrowhead indicates a macrophage not colocalized to NGF. D, NGF immunoreactivity colocalized with CD68 (arrow) and in a cell that is not immunolabeled for CD68 (arrowhead) in the section shown in C. Double immunohistochemistry was visualized by Texas Red (Hsp47 and CD68 [red]) and fluorescein (NGF [green]) staining. Dotted lines indicate the synovial surface. Bars = 50 μm.

Figure 3

Histopathologic features of osteoarthritis (OA). Scatterplots illustrate the differences between the non-OA control group and the advanced OA group for the scores on cartilage surface integrity (A), proteoglycan loss (B), chondrocyte morphology (C), and synovitis (D), and for the synovial fractional area positive for CD68-immunoreactive macrophages (E) and nerve growth factor (NGF) (F). Each symbol represents an individual subject; horizontal lines and error bars show the median and interquartile range. ∗ = P < 0.05; ∗∗∗ = P < 0.001 versus the non-OA control group.

Figure 4

Histologic associations of symptomatic osteoarthritis (OA). Scatterplots illustrate the differences between subjects with similar macroscopic chondropathy scores who did not (asymptomatic chondropathy) or did (symptomatic chondropathy) undergo total knee replacement surgery. The cartilage surface integrity score (A), chondrocyte morphology score (B), synovitis score (C), and synovial fractional area positive for nerve growth factor (NGF) (D) were greater in the symptomatic chondropathy group, but differences in the synovial fractional area positive for CD68-immunoreactive macrophages (E) did not reach statistical significance. Each symbol represents an individual subject; horizontal lines and error bars show the median and interquartile range. ∗ = P < 0.05; ∗∗ = P < 0.01 versus the asymptomatic chondropathy group.