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Research Papers

# Validation of Cartilage Thickness Calculations Using Indentation Analysis

[+] Author and Article Information
Matthew F. Koff1

Department of Radiology and Imaging, Hospital for Special Surgery, New York, NY 10021koffm@hss.edu

Le Roy Chong

Department of Diagnostic Radiology, Changi General Hospital, Singapore 529889, Singapore

Patrick Virtue

GE Healthcare, Waukesha, WI 53188

Dan Chen, Xioanan Wang, Timothy Wright

Department of Biomechanics, Hospital for Special Surgery, New York, NY 10021

Hollis G. Potter

Department of Radiology and Imaging, Hospital for Special Surgery, New York, NY 10021

1

Corresponding author.

J Biomech Eng 132(4), 041007 (Mar 18, 2010) (6 pages) doi:10.1115/1.4000989 History: Received November 12, 2009; Revised January 11, 2010; Posted January 12, 2010; Published March 18, 2010; Online March 18, 2010

## Abstract

Different methods have been used to cross-validate cartilage thickness measurements from magnetic resonance images (MRIs); however, a majority of these methods rely on interpolated data points, regional mean and/or maximal thickness, or surface mean thickness for data analysis. Furthermore, the accuracy of MRI cartilage thickness measurements from commercially available software packages has not necessarily been validated and may lead to an under- or overestimation of cartilage thickness. The goal of this study was to perform a matching point-to-point validation of indirect cartilage thickness calculations using a magnetic resonance (MR) image data set with direct cartilage thickness measurements using biomechanical indentation testing at the same anatomical locations. Seven bovine distal femoral condyles were prepared and a novel phantom filled with dilute gadolinium solution was rigidly attached to each specimen. High resolution MR images were acquired, and thickness indentation analysis of the cartilage was performed immediately after scanning. Segmentation of the MR data and cartilage thickness calculation was performed using semi-automated software. Registration of MR and indentation data was performed using the fluid filled phantom. The inter- and intra-examiner differences of the measurements were also determined. A total of 105 paired MRI-indentation thickness data points were analyzed, and a significant correlation between them was found ($r=0.88$, $p<0.0001$). The mean difference (±std. dev.) between measurement techniques was $0.00±0.23 mm$, with Bland–Altman limits of agreement of 0.45 mm and −0.46 mm. The intra- and inter-examiner measurement differences were $0.03±0.22 mm$ and $0.05±0.24 mm$, respectively. This study validated cartilage thickness measurements from MR images with thickness measurements from indentation by using a novel phantom to register the image-based and laboratory-based data sets. The accuracy of the measurements was comparable to previous cartilage thickness validation studies in literature. The results of this study will aid in validating a tool for clinical evaluation of in-vivo cartilage thickness.

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## Figures

Figure 1

Prepared bovine femoral condyle with L-phantom rigidly attached (arrow) using nylon screws

Figure 2

Needle indentation testing to measure cartilage thickness. (a) Specimen mounted in the material testing system for indentation testing. A 3D digitizing stylus was used to record phantom location and orientation and site of indentation in the LCS. (b) Close-up of indentation test using needle probe oriented perpendicular to the articular surface. (c) Representative force-displacement curve from indentation at one testing location. The circle indicates cartilage thickness.

Figure 3

((a) and (b)) Reformatted images used to display, localize, and define the location of the L-phantom (arrows) in the MR coordinate system. Image slice showing user set threshold (c), semi-automated segmentation (d), and final segmentation (e) of the femoral condyle.

Figure 4

Correlation plot of MR thickness measurements versus indentation thickness measurements. A significant correlation between the paired measurements was found.

Figure 5

Bland–Altman analysis of paired MR and indentation thickness measurements displaying mean difference (solid line) and limits of agreement (dashed lines)

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