Abstract
Objective: Contrast-enhanced computed tomography (CECT) has been introduced for the evaluation of cartilage integrity. Furthermore, CECT enables imaging of the structure and density of subchondral bone. In this laboratory study, we investigate the potential of microCECT to simultaneously image cartilage and subchondral bone for the evaluation of tissue healing. Design: Osteochondral lesions (Ø = 6 mm) were surgically created in equine intercarpal joints (n = 7). After spontaneous healing for 12 months, the horses were sacrificed and osteochondral plugs (Ø = 14 mm),including the repair cartilage and adjacent intact tissue, were harvested. The nonfibrillar and fibrillar moduli and the permeability of cartilage were determined using indentation testing. Contrast agent diffusion into the samples was imaged for 36 hours using high-resolution CT. Results from CECT, mechanical testing, and microscopic analyses were compared and correlated. Results: The contrast agent diffusion coefficient showed a significant (P < 0.05) difference between the repair and adjacent intact tissue. MicroCECT revealed altered (P < 0.05) bone volume fraction, mineral density, and microstructure of subchondral bone at the repair site. The contrast agent diffusion coefficient correlated with the moduli of the nonfibrillar matrix (R = -0.662, P = 0.010), collagen fibril parallelism index (R = -0.588, P = 0.035), and glycosaminoglycan content (R = -0.503, P = 0.067). The repair cartilage was mechanically and structurally different from adjacent intact tissue (P < 0.05). Conclusions: MicroCECT enabled simultaneous quantitative evaluation of subchondral bone and monitoring of cartilage repair, distinguishing quantitatively the repair site from the adjacent intact tissue. As the only technique able to simultaneously image cartilage and determine subchondral bone mineral density and microstructure, CECT has potential clinical value.
| Original language | English |
|---|---|
| Pages (from-to) | 235-244 |
| Number of pages | 10 |
| Journal | Cartilage |
| Volume | 3 |
| Issue number | 3 |
| DOIs | |
| Publication status | Published - 1 Jul 2012 |
Funding
In this study, for the first time to our knowledge, the ability of microCECT to distinguish between spontaneously healed cartilage defects and intact cartilage was investigated ex vivo in an animal model. Furthermore, the biomechanical properties of cartilage were determined using the creep indentation protocol and FEA. Finally, the subchondral bone microstructure was assessed from the microCT images, and the organization of the collagen fibril network and GAG distribution were determined histologically. Based on microCECT imaging, spontaneously healed tissue could be quantitatively discerned from the adjacent intact tissue. Importantly, the technique allowed simultaneous evaluation of contrast agent diffusion dynamics, cartilage thickness, and subchondral bone microstructure and mineral density. The contrast agent diffusion coefficient showed a significant difference between the repair and adjacent intact tissue. It was higher in the repair tissue than in the adjacent intact tissue, indicating that the diffusion equilibrium was reached faster in the repair tissue. This may be due to the lower GAG content in the repair tissue. 28 , 42 However, the structure and orientation of the collagen network have also been suggested to affect the diffusion of solutes within the cartilage matrix. 43 , 44 In the present work, the structure of the collagen network of the repair cartilage was found to be abnormal: based on the present PLM findings, the collagen fibrils were not tangentially oriented at the superficial layer of the repair tissue. This could partially explain the present findings since the integrity of the superficial cartilage has been speculated to significantly affect the contrast agent diffusion dynamics. 26 - The subchondral bone under the repair site included both repair tissue and remodeled bone underneath. However, the region of interest extended only to the repair area. Lower BMD in the subchondral bone underneath the repair cartilage may partly be due to the new mechanical environment of the bone, which affects the healing and remodeling response of bone after an osteochondral injury. 45 Furthermore, reduction in the bone volume fraction and BS/BV are signs of the subchondral reaction, also visually detectable in the microCT images. These findings are in line with earlier studies in goats and pigs. 46 48 X-ray attenuation coefficients determined for the repair cartilage and adjacent intact tissue did not show significant differences at any time point. This was surprising since previous studies 17 , 19 , 49 imply that contrast agent intake should be higher in tissue with lower GAG content. This apparent contradiction could be explained by the possibility that other constituents in addition to the GAGs (e.g., collagen) affect the diffusion and the distribution of contrast agent in the cartilage. 24 , 26 , 27 Thus, the present microCECT findings may partially be explained by differences in the structure of the collagen network in repair and intact tissues. This is consistent with a recent study 50 in which the influence of enzymatic degradation of collagen and proteoglycans was evaluated by contrast-enhanced MRI. Not only proteoglycan loss but also the alterations in the collagen network were reported to influence the accumulation of the contrast agent in cartilage. In this study, the abnormally organized collagen network could hinder the penetration of the contrast agent molecules into the repair tissue. Possibly for this reason, the contrast agent in the repair cartilage with lower GAG content showed similar x-ray attenuation values as in the adjacent intact tissue. The modulus of the nonfibrillar matrix was smaller in the repair tissue than in the adjacent intact cartilage. This finding is supported by the optical density measurements showing that GAG content, known to primarily control the modulus of the nonfibrillar matrix, 37 was lower in the repair cartilage than in the adjacent intact tissue. Furthermore, the modulus of the fibril network was smaller in the repair tissue. This may be explained by the abnormal orientation and organization of the collagen fibril network in spontaneously repaired cartilage. Despite the differences in GAG content, there was no difference in the hydraulic permeability between repair and intact tissue. This is interesting since a decrease in tissue GAG content is usually linked to an increase in permeability. 37 , 51 However, significant differences in the organization of the collagen network can have a significant independent effect on tissue hydraulic permeability. 37 , 52 It has been suggested that the permeability would be higher along the collagen fibril direction than in the direction perpendicular to the fibrils. 52 Since the fluid flows mainly in a horizontal direction in indentation testing, the reduction in permeability in the repair tissue due to the random organization of fibrils in the superficial tissue may compensate the other structural and compositional changes (GAG content) that result in an increase in permeability. Furthermore, permeability of cartilage has been suggested to be strain dependent 53 (i.e., to decrease as a function of compressive strain). Since the creep protocol created larger strains for the softer repair cartilage than for the intact tissue, it may have reduced the permeability of the repair tissue. Since cartilage and subchondral bone can be considered a functional unit, imaging of cartilage defects or cartilage repair should also focus on the subchondral bone. 34 , 54 Currently, MRI is considered the most sensitive imaging method for the evaluation of cartilage and subchondral bone. 34 Depending on the MRI technique in question, it is possible to detect edema or cysts in the subchondral bone, but direct evaluation of the microstructure or BMD is not possible. 34 In contrast, computed tomography provides direct information on subchondral BMD, and peripheral computed tomography devices, having spatial resolution up to 100 µm, enable imaging resolution sufficient for quantitative evaluation of bone microstructure. In conclusion, microCECT could distinguish quantitatively between repair tissue and adjacent intact tissue. If successfully developed for in vivo use, CECT with simultaneous acquisition of quantitative information on the properties of cartilage and subchondral bone could be clinically valuable. Currently, no clinical imaging technique simultaneously provides direct information on contrast agent diffusion dynamics in cartilage, cartilage thickness, and subchondral bone microstructure and mineral density. Acknowledgments and Funding We thank Mr. Alpi Linjama for conducting the biomechanical measurements. This work was supported by the Sigrid Jusélius Foundation, Finnish Cultural Foundation North Savo Regional Fund, Kuopio University Hospital (EVO 5041715), the strategic funding of the University of Eastern Finland, and Foundation for Advanced Technology of Eastern Finland. CSC–IT Center for Science is acknowledged for providing computing services. Declaration of Conflicting Interests The author(s) declared no potential conflicts of interests with respect to the authorship and/or publication of this article. Ethical Approval This study was approved by our institutional review board.
Keywords
- cartilage
- computed tomography
- contrast agent
- repair
- subchondral bone
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