From images to data to theory

Quantitative relationship of thigh adipose tissue with pain, radiographic status, and progression of knee osteoarthritis: longitudinal findings from the osteoarthritis initiative

OBJECTIVE

The aim of this study was to explore the relationship of thigh subcutaneous fat (SCF) and intermuscular fat (IMF) content with knee osteoarthritis (KOA) cross-sectionally and longitudinally, using quantitative magnetic resonance imaging. Specifically, we examined relationships with frequent knee pain, various radiographic strata, and structural progression status of KOA.

MATERIALS AND METHODS

Subjects who showed between-knee discordance of frequent versus no pain (n = 48), osteophyte versus no radiographic KOA (n = 55), and radiographic joint space narrowing versus no joint space narrowing (n = 44) were selected from the Osteoarthritis Initiative. Knees with structural progression of KOA (n = 23) were compared with knees without progression. Axial thigh magnetic resonance images were used to determine IMF and SCF. Differences between knees and between baseline and 2-year follow-up were examined using paired t tests.

RESULTS

Women displayed similar amounts of IMF, but twice the SCF of men. Limbs of women with frequently painful knees had statistically significant greater IMF areas (+4.2%; P = 0.05) than contralateral pain-free limbs did. No significant cross-sectional differences were observed between other strata. Men with structural progression appeared to have a greater longitudinal increase in SCF (+13.2%; P < 0.05) than did men without progression (-1.9%), and women with progression appeared to have a greater increase in IMF (+11.6%) than did those without progression (+1.5%).

CONCLUSION

In women, painful knees display greater IMF content than do contralateral pain-free knees. Other between-knee comparisons did not reveal a regional association between radiographic KOA and thigh adipose tissue status. Structural progression of KOA may be associated with greater longitudinal increases in SCF in men and greater increases of IMF in women, compared with nonprogressive controls.

In vivo optical imaging of early osteoarthritis using an antibody specific to damaged arthritic cartilage

Background

The lack of specific and sensitive serum and radiographic biomarkers for early diagnosis of osteoarthritis (OA) as well as for monitoring subtle changes in disease activity in clinical trials has hampered the development of treatments for OA. We previously showed that 1-11E, a human single chain fragment variable (scFv) specific to collagen type II that has been post-translationally modified by reactive oxidants (ROS-CII), binds exclusively to arthritic cartilage. Here we test the validity of 1-11E as a radiographic biomarker for early disease in experimental OA.

Methods

Murine OA was induced by destabilisation of the medial meniscus (DMM) in adult male mice. Immunohistochemistry of destabilised or sham-operated knees was performed from 2 to 8 weeks post-surgery with Cy5.5-labelled 1-11E and negative control scFv, C7. Prospective in vivo optical images were taken 4 and 8 weeks post-DMM following intra-articular injection of Cy5.5-labelled scFvs, or intravenous injection of Cy5.5-labelled full length monoclonal antibodies (mAbs).

Results

Specific cartilage staining with 1-11E was apparent as early as 4 weeks post-DMM at the time of earlier cartilage degradation assessed by histology. Prospective in vivo optical images taken 4 and 8 weeks post-DMM following local intra-articular injection of Cy5.5-labelled scFv (n = 7) showed specific in vivo retention of Cys5.5-1-11E scFv following local administration into the knee joint (tissue half-life >78 hours, n = 7, signal to noise ratio (SNR) > 2.1). Specific localization of Cys-5.5-1-11E-mAb to DMM knees (SNR >1.65) was also observed (p < 0.01, n = 8, SNR >1.65). In both cases the SNR increased with time post-DMM.

Conclusions

1-11E binds specifically to early osteoarthritic cartilage and can be used as a radiographic biomarker following local or systemic delivery to facilitate early diagnosis and monitor disease progression in OA.

Contralateral knee effect on self-reported knee-specific function and global functional assessment: data from the Osteoarthritis Initiative

OBJECTIVE

To analyze the effect of contralateral knee pain on sensitivity of patient-reported outcomes and objectively measured functional performance tests in subjects with knee osteoarthritis (OA).

METHODS

Subjects with discordant knee pain status (i.e., 1 knee being painful [≥4 on a numeric pain rating scale (NPRS)], with the contralateral knee being pain free [NPRS 0]) were selected from the Osteoarthritis Initiative and matched to subjects with bilaterally pain-free and painful knees by age, sex, body mass index, and radiographic knee OA. The Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) physical function score, the global Physical Activity Scale for the Elderly (PASE), and objective functional performance tests were cross-sectionally compared in a matched case-control design.

RESULTS

A total of 378 subjects with discordant knee pain status were matched to 359 controls with bilaterally pain-free knees and to 323 controls with bilaterally painful knees. WOMAC scores in pain-free knees of discordant knee pain cases significantly differed compared to scores of bilaterally pain-free knees (P = 0.003). Likewise, scores in painful knees of discordant knee pain cases significantly differed compared to scores of bilaterally painful knees (P < 0.001). PASE levels between these groups were not significantly different (P > 0.68). Functional performance tests differed in subjects with discordant knee pain compared to subjects with bilaterally pain-free knees and when compared to subjects with bilaterally painful knees, with the chair stand test showing the strongest effect size (standardized response mean 0.28 and 0.33, respectively).

CONCLUSION

The WOMAC physical function score, although knee specific, is impacted by the contralateral knee pain status. The repeated chair stand test appears to be the most sensitive assessment in differentiation between groups with different status of knee pain.

Sex-differences of the healthy infra-patellar (Hoffa) fat pad in relation to intermuscular and subcutaneous fat content--data from the Osteoarthritis Initiative

The infra-patellar fat pad (IPFP) is composed of intra-articular adipose tissue; it represents a potential source of pro-inflammatory cytokines and has been associated with osteoarthritis of the knee. Yet, to what extent the size of the IPFP differs between healthy men and women, and how sex differences compare to those in inter-muscular and subcutaneous fat tissue content is unknown. We studied healthy reference subjects from the Osteoarthritis Initiative, without knee pain, without radiographic signs or without risk factors of femorotibial osteoarthritis. Sagittal magnetic resonance images (MRIs) of 99 right knees were used to segment the IPFP; in a subset, axial images of the thigh were available to segment inter-muscular and subcutaneous fat. Healthy men (n=40) displayed a 41% greater (p<0.001) IPFP volume and a 9% greater (p<0.01) ratio of IPFP volume/body weight than women (n=59). Men (n=13) displayed 15% greater intermuscular fat content (not significant), and a 50% lesser (p<0.01) subcutaneous fat content than women (n=12); when related to total thigh cross-sectional areas, these sex differences were +2% (not significant) and -53% (p<0.001). This is the first study to explore quantitative measures of the IPFP in healthy men and women, and to relate these to sex differences of inter-muscular and subcutaneous fat tissue content. Men displayed a significantly greater ratio of IPFP volume/body weight than women, similar amounts of inter-muscular fat, and strikingly less subcutaneous fat. These data provide a basis for further systematic studies of the variability of the IPFP with the body mass index and its role in knee osteoarthritis.