Automated MicroCT-based bone and articular cartilage analysis using iterative shape averaging and atlas-based registration

Molecular mapping of articular cartilage CXCR4 expression after ACL injury via a novel small molecule-based probe

PURPOSE

Molecular imaging is a powerful modality to spatially resolve molecular changes across tissues, but application to articular cartilage remains limited. CXCR4 is an established marker of chondrocyte hypertrophy and potential therapeutic target for osteoarthritis. The purpose of this study was to develop and apply a CXCR4-targeted, near-infrared fluorescent (NIR) probe to a rat model of post-traumatic osteoarthritis (PTOA).

METHODS

A CXCR4-targeted, small molecule-based NIR probe ("Cy7-AMD") was synthesized. Sensitivity and specificity of Cy7-AMD to CXCR4 was validated in vitro (HUVECs), and ex vivo (rat osteochondral explants). To induce PTOA, female Lewis rats underwent noninvasive anterior cruciate ligament (ACL) rupture. At 7- and 28-days post-injury, injured/contralateral femora and tibiae were dissected, incubated in Cy7-AMD vs a non-targeting control, and imaged via NIR imaging, as well as conventional and contrast-enhanced micro-computed tomography. Imaging datasets were co-registered, cartilage tissue volumes were segmented, and paired cartilage thickness and NIR signal maps were generated and analyzed for PTOA-relevant changes.

RESULTS

Compared to a non-targeting control probe, in vitro and ex vivo assays confirm sensitivity and specificity of Cy7-AMD to CXCR4. Flow cytometry confirmed high correspondence between Cy7-AMD- and antibody-based measurement of CXCR4 expression. Cy7-AMD rapidly equilibrated within cartilage, and fluorescent histology confirmed full-thickness penetration. Injured femoral cartilage exhibited heterogeneous CXCR4 expression, with increased signal deviation compared to contralateral femora. Spatial CXCR4 expression patterns correlated to cartilage thickness patterns; high CXCR4 expression at boundaries of low-thickness lesions suggests an association between CXCR4 expression and cartilage loss.

CONCLUSIONS

Small molecule-based probes are advantageous for mapping spatial patterns of molecular expression in rodent articular cartilage, deepening our understanding of PTOA progression.

Quantitative Skeletal Imaging and Image-Based Modeling in Pediatric Orthopaedics

PURPOSE OF REVIEW

Musculoskeletal imaging serves a critical role in clinical care and orthopaedic research. Image-based modeling is also gaining traction as a useful tool in understanding skeletal morphology and mechanics. However, there are fewer studies on advanced imaging and modeling in pediatric populations. The purpose of this review is to provide an overview of recent literature on skeletal imaging modalities and modeling techniques with a special emphasis on current and future uses in pediatric research and clinical care.

RECENT FINDINGS

While many principles of imaging and 3D modeling are relevant across the lifespan, there are special considerations for pediatric musculoskeletal imaging and fewer studies of 3D skeletal modeling in pediatric populations. Improved understanding of bone morphology and growth during childhood in healthy and pathologic patients may provide new insight into the pathophysiology of pediatric-onset skeletal diseases and the biomechanics of bone development. Clinical translation of 3D modeling tools developed in orthopaedic research is limited by the requirement for manual image segmentation and the resources needed for segmentation, modeling, and analysis. This paper highlights the current and future uses of common musculoskeletal imaging modalities and 3D modeling techniques in pediatric orthopaedic clinical care and research.

Early patellofemoral cartilage and bone pathology in a rat model of noninvasive anterior cruciate ligament rupture

OBJECTIVE

Anterior cruciate ligament rupture (ACLR) is a risk factor for the development of post-traumatic osteoarthritis (PTOA). While PTOA in the tibiofemoral joint compartment is well-characterized, very little is known about pathology in the patellofemoral compartment after ACL injury. Here, we evaluated the extent to which ACLR induces early patellofemoral joint damage in a rat model.

METHODS

Adult female Lewis rats were randomized to noninvasive ACLR or Sham. Two weeks post-injury, contrast-enhanced micro-computed tomography (µCT) of femoral and patellar cartilage was performed using 20% v/v ioxaglate. Morphometric parameters of femoral trochlear and patellar cartilage, subchondral bone, and trabecular bone were derived from µCT. Sagittal Safranin-O/Fast-Green-stained histologic sections were graded using the OARSI score in a blinded fashion.

RESULTS

Cartilage and bone remodelling consistent with an early PTOA phenotype were observed in both femoral trochleas and patellae. ACLR caused osteophyte formation of the patella and pathology in the superficial zone of articular cartilage, including surface fibrillation, fissures, increased cellularity, and abnormal chondrocyte clustering. There were significant increases in thickness of patellar and trochlear cartilage. Loss of subchondral bone thickness, bone volume fraction, and tissue mineral density, as well as changes to patellar and trochlear trabecular microarchitecture, were indicative of catabolic bone remodelling. Several injury-induced changes, including increased cartilage thickness and trabecular spacing and decreased trabecular number were more severe in the patella compared to the trochlea.

CONCLUSION

The patellofemoral joint develops mild but evident pathology in the early period following ACL rupture, extending the utility of this model to the study of patellofemoral PTOA.

Open-source pipeline for automatic segmentation and microstructural analysis of murine knee subchondral bone

μCT images are commonly analysed to assess changes in bone density and microstructure in preclinical murine models. Several platforms provide automated analysis of bone microstructural parameters from volumetric regions of interest (ROI). However, segmentation of the regions of subchondral bone to create the volumetric ROIs remains a manual and time-consuming task. This study aimed to develop an automated end-to-end pipeline, combining segmentation and microstructural analysis, to evaluate subchondral bone in the mouse proximal knee.

METHODS

A segmented dataset of μCT scans from 62 knees (healthy and arthritic) from 10-week male C57BL/6 mice was used to train a U-Net type architecture to automate segmentation of the subchondral trabecular bone. These segmentations were used in tandem with the original scans as input for microstructural analysis along with thresholded trabecular bone. Manually and U-Net segmented ROIs were fed into two available pipelines for microstructural analysis: the ITKBoneMorphometry library and CTan (SKYSCAN). Outcome parameters were compared between pipelines, including: bone volume (BV), total volume (TV), BV/TV, trabecular number (TbN), trabecular thickness (TbTh), trabecular separation (TbSp), and bone surface density (BSBV).

RESULTS

There was good agreement for all bone measures comparing the manual and U-Net pipelines utilizing ITK (R = 0.88-0.98) and CTAn (R = 0.91-0.98). ITK and CTAn showed good agreement for BV, TV, BV/TV, TbTh and BSBV (R = 0.9-0.98). However, limited agreement was seen between TbN (R = 0.73) and TbSb (R = 0.59) due to methodological differences in how spacing is evaluated. Microstructural parameters generated from manual and automatic segmentations showed high correlation across all measures. Using the CTAn pipeline yielded strong R² values (0.83-0.96) and very strong agreement based on ICC (0.90-0.98). The ITK pipeline yielded similarly high R² values (0.91-0.96, except for TbN (0.77)), and ICC values (0.88-0.98). The automated segmentations yield lower average values for BV, TV and BV/TV (ranging from 14 % to 6.3 %), but differences were not found to be influenced by the mean ROI values.

CONCLUSIONS

This integrated pipeline seamlessly automated both segmentation and quantification of the proximal tibia subchondral bone microstructure. This automated pipeline allows the analysis of large volumes of data, and its open-source nature may enable the standardization of microstructural analysis of trabecular bone across different research groups.

Semi-automatic micro-CT segmentation of the midfoot using calibrated thresholds

PURPOSE

In the field of skeletal research, accurate and reliable segmentation methods are necessary for quantitative micro-CT analysis to assess bone quality. We propose a method of semi-automatic image segmentation of the midfoot, using the cuneiform bones as a model, based on thresholds set by phantom calibration that allows reproducible results in low cortical thickness bones.

METHODS

Manual and semi-automatic segmentation methods were compared in micro-CT scans of the medial and intermediate cuneiforms of 24 cadaveric specimens. The manual method used intensity thresholds, hole filling, and manual cleanup. The semi-automatic method utilized calibrated bone and soft tissue thresholds Boolean subtraction to cleanly identify edges before hole filling. Intra- and inter-rater reliability was tested for the semi-automatic method in all specimens. Mask volume and average bone mineral density (BMD) were measured for all masks, and the three-dimensional models were compared to the initial semi-automatic segmentation using an unsigned distance part comparison analysis. Segmentation methods were compared with paired t-tests with significance level 0.05, and reliability was analyzed by calculating intra-class correlation coefficients.

RESULTS

There were statistically significant differences in mask volume and BMD between the manual and semi-automatic segmentation methods in both bones. The intra- and inter-reliability was excellent for mask volume and bone density in both bones. Part comparisons showed a higher maximum distance between surfaces for the manual segmentation than the repeat semi-automatic segmentations.

CONCLUSION

We developed a semi-automatic micro-CT segmentation method based on calibrated thresholds. This method was designed specifically for use in bones with high rates of curvature and low cortical bone density, such as the cuneiforms, where traditional threshold-based segmentation is more challenging. Our method shows improvement over manual segmentation and was highly reliable, making it appropriate for use in quantitative micro-CT analysis.