(G-277) Shear Wave Ultrasound Elastography for Estimating Cartilage Stiffness and Investigating the Impact of Subchondral Bone: Implications for Early Detection of Osteoarthritis. In-Vitro Study

Introduction:Osteoarthritis (OA) is a chronic degenerative joint disorder associated with debilitating pain that affects over 33.6% of the elderly population (>65yrs old) in United States, resulting in $186B spent annually. OA is characterized by several hallmarks, including degeneration of cartilage, subchondral bone architectural abnormalities, synovial inflammation, and diverse pain characteristics. Structural dystrophies of the knee joint are the major microscopic and macroscopic indicators of OA progression. Thus, it is important to study cartilage mechanical properties and investigate the effect of subchondral bone on overall cartilage stiffness. Currently, the major diagnostic methods for OA are joint aspiration, X-ray, MRI, and invasive scopes, which are cost ineffective and can be harmful to patients due to their invasiveness and exposure to radiation[1]–[3]. Shear wave ultrasound elastography (SWUE) is a high-resolution acoustic technique that has the potential to image knee joints noninvasively for precise diagnose of OA progression. Due to sensitivity to tissue stiffness and surface characteristics, SWUE is capable of differentiating between normal and damaged tissues. Therefore, the objective of this study is to assess the ability of SWUE to detect changes in cartilage stiffness, while also investigating the impact of subchondral bone abnormalities on cartilage stiffness in-vitro, to aid in early OA diagnosis.
Materials and Methods: The degeneration of the articular cartilage was achieved by utilizing the chemical action of the trypsin enzyme. Trypsin has been shown to digest proteoglycans with a slight effect on the collagen network, which can mimic cartilage degeneration in osteoarthritis [4]. Bovine subchondral units were immersed into a 0.05% trypsin-ethylenediaminetretra-acetic acid solution at 37C for 24 hr. Control samples were immersed in Dulbecco's Modified Eagle Medium for the same time period. After 24hr incubation, samples were washed three times with PBS before testing. Another group was employed to mechanical method by exposing the cartilage along sandpaper in both x and y direction using varying grades (320, 220, 150 Grit) to stimulate different grades of OA [4]. To assess the impact of subchondral bone defects on cartilage stiffness, we injected different concentrations of formic acid (1%, 3%, or 7%) into the subchondral bone. Due to the inherent geometric challenges associated with cartilage samples, cartilage samples were incubated into a gelatin-phantom prior to testing. This approach helped mitigate any potential interference caused by the geometry of the cartilage samples. To induce tissue deformation, a single pulse of 2 kHz and 500 mV was generated through function generator and amplified by power amplifier to derive the mechanical shaker. This shaker was connected to a metal rod that made gentle contact with the sample. The imaging process was facilitated using a programmable ultrasound research system, with a 15 MHz array probe employed for detecting the shear waves that propagate laterally along the sample. Compounded plane wave imaging with 3 angles and 13 KHz PRF was implemented. The system collected an in-phase/quadrature (IQ) dataset consisting of 2000 frames to obtain spatiotemporal maps with lateral distance versus propagation time. This data was then used to estimate the shear wave speed (SWS), with higher SWS indicating a stiffer sample.
Results and discussion: SWUE was successfully conducted to all groups, and SWS were estimated. The cartilage was detected through B-mode image, and displacement were only obtained to cartilage region. Three samples at least were tested each group. Our results indicate that Trypsin were significantly decrease cartilage stiffness, where the SWS of the sample group treated with trypsin were approximately 30% lower than control group (figure 3a). Using mechanical method, The average SWS of 320, 220, and 150 grit were 26.38±3.9,22.28±0.77 ,16.32±0.07 m/s, respectively. For different FA group, the results shown significant increase in SWS when FA increases. The average SWS of 1, 3, and 7% FA were 20.47±1.74,25.4±1.66 ,and 38.8±1.19 /s, respectively. In current state, there was no significant difference between control and 1% FA group. Our observations have revealed that the presence of surface defects not only affects the stiffness of cartilage, as measured by SWS, but also the signal-to-noise ratio (SNR). The cartilage layer was 1.5±0.5 mm in thickness.Thus, average of at least 30 layers were used to estimate the SWS of the cartilage. The present state of the study has demonstrated that SWUE is capable in differentiating various grade of OA based on the estimated SWS. Since higher OA grade led to lower the SWS and increase SNR, these observations were in favor of previous study that found higher OA grade led to decrease the uniaxial elastic modulus of the cartilage layers[4]. These finding may have implication for the early diagnose of OA. Our undergoing study has certain limitations that should be acknowledged. For instance, some samples had to be excluded due to a high SNR. By addressing the limitations and obtaining a comprehensive understanding of the relationship between SWS and OA grades, we remain optimistic that the results of this study could potentially lead to more accurate and earlier diagnoses of OA.
Reference:
[1] D. Burstein, A. Bashir, and M. L. Gray, “MRI Techniques in Early Stages of Cartilage Disease:,” Invest. Radiol., vol. 35, no. 10, pp. 622–638, Oct. 2000, doi: 10.1097/00004424-200010000-00008.
[2] T. Lang, P. Augat, S. Majumdar, X. Ouyang, and H. K. Genant, “Noninvasive Assessment of Bone Density and Structure Using Computed Tomography and Magnetic Resonance 11This manuscript was partially adapted from Genant, H., Engelke, K., Fuerst, T. Gluer, C. C., Grampp, S., Harris, S. T., Jergas, M., Lang, T., Lu, Y., and Majumdar S. Non-invasive assessment of bone mineral and structure: State of the art. J Bone Miner Res 11:707–729; 1996.,” Bone, vol. 22, no. 5, pp. 149S-153S, May 1998, doi: 10.1016/S8756-3282(98)00005-2.
[3] D. T. Ginat, G. Hung, T. R. Gardner, and E. E. Konofagou, “High-Resolution Ultrasound Elastography of Articular Cartilage in Vitro,” in 2006 International Conference of the IEEE Engineering in Medicine and Biology Society, New York, NY: IEEE, 2006, pp. 6644–6647. doi: 10.1109/IEMBS.2006.260910.
[4] S. Z. Wang, Y. P. Huang, S. Saarakkala and Y. P. Zheng, "Quantitative Assessment of Articular Cartilage with Morphologic Acoustic and Mechanical Properties Obtained Using High-Frequency Ultrasound", Ultrasound Med. Biol., vol. 36, no. 3, pp. 512-527, 2010.
[5] H. Niu et al., “Relationship between triphasic mechanical properties of articular cartilage and osteoarthritic grade,” Sci. China Life Sci., vol. 55, no. 5, pp. 444–451, May 2012, doi: 10.1007/s11427-012-4326-7.

[6] Niu et al. BMC Musculoskeletal Disorders. 2012.