A novel method to obtain modulus image of soft tissues using ultrasound water jet indentation: a phantom study

Detecting early osteoarthritis through changes in biomechanical properties - A review of recent advances in indentation technologies in a clinical arthroscopic setup

Osteoarthritis (OA) is a degenerative joint disease currently affecting half of all women and one-third of all men aged over 65 and it is predicted to even increase in the next decades. In the variety of causes leading to OA, the first common denominator are changes in the extracellular matrix of the cartilage. In later stages, OA affects the whole joint spreading to higher levels of tissue architecture causing irreversible functional and structural damage. To date, the diagnosis of OA is only formulated in the late stages of the disease. This is also, where most present therapies apply. Since a precise diagnosis is a prerequisite for targeted therapy, tools to diagnose early OA, monitor its progression, and accurately stage the disease are wanted. This review article focuses on recent advances in indentation technologies to diagnose early OA through describing biomechanical cartilage characteristics. We provide an overview of microindentation instruments, indentation-type Atomic Force Microscopy, ultrasound, and water-jet ultrasound indentation, Optical Coherence Tomography-based air-jet indentation, as well as fiber Bragg grating.

Miniaturized Water-Jet Ultrasound Indentation System for Quantitative Assessment of Articular Cartilage Degeneration: A Validation Study

Osteoarthritis is a common joint disease affecting a large population especially the elderly where cartilage degeneration is one of its hallmark symptoms. There is a need to develop new devices and instruments for the early detection and treatment of cartilage degeneration. In this study, we describe the development of a miniaturized water-jet ultrasound indentation probe for this purpose. To evaluate the system, we applied it to characterize the degeneration of articular cartilage with the measurement of its morphologic, acoustic, and mechanical properties, using the enzymatic digestions of cartilage as a model of OA. Fifty cartilage samples were tested with 10 of them used for the reproducibility study and the other 40 for collagenase and trypsin digestions. Thickness, integrated reflection coefficient (IRC), effective stiffness, and energy dissipation ratio (EDR) were used to quantify the change of articular cartilage before and after degeneration. The measurement reproducibility as represented by the standardized coefficient of variation (SCV) was 2.6%, 10.2%, 11.5%, and 12.8% for thickness, IRC, stiffness, and EDR, respectively. A significant change of IRC, stiffness, and EDR was detected after degeneration by the designed probe (p < 0.05). There was also a significant difference of IRC, stiffness, and EDR between trypsin and collagenase digestions (p < 0.001). In conclusion, a miniaturized water-jet ultrasound indentation probe has been designed, which has been successfully used to detect and differentiate cartilage degeneration simulated by enzymatic digestions. This probe, with future development, can be potentially suitable for quantitative assessment of cartilage degeneration with an arthroscopic operation.

Assessment of Corneal Biomechanical Properties with Inflation Test Using Optical Coherence Tomography

Biomechanical properties are important for the cornea to maintain its normal shape and function. There is still a need to develop better methods for accurate measurement of corneal mechanical properties. In this study, we propose to introduce the optical coherence tomography (OCT) in inflation test for the imaging of corneal deformation in order to measure its biomechanical properties. Ten cornea-mimicking silicone phantoms with different stiffness and five fresh porcine corneas were tested using the proposed method. Intra-ocular pressure was changed from 10 to 90 mmHg using two different loading rates to observe the pressure-apex displacement relationship and calculate the apparent stiffness of the corneas. Stiffness of the corneal phantoms obtained by the inflation test ranged from 0.2 to 1 MPa, which was highly consistent with the results from the mechanical tensile test (y = 0.70x, p < 0.001). The porcine corneas showed highly viscoelastic behavior with obvious hysteresis in inflation. The apparent stiffness of the porcine corneas was 0.63 ± 0.07 and 1.05 ± 0.08 MPa with loading rates of 3.3 and 33 mmHg/min, respectively. Mapping of corneal surface displacement was also generated for both the phantom and porcine corneas. This study showed that it is feasible to incorporate the high resolution OCT imaging in inflation test to measure the biomechanical properties of the cornea.

The Elasticity Coefficients Measurement of Human Dentin Based on RUS

This paper proposed to take advantages of resonant ultrasound spectroscopy (RUS) to measure the mechanical properties of human dentin specimen. The resonant spectroscopy of the dentin specimen was obtained between the frequency bands 155 and 575 kHz, and resonant frequencies were extracted by linear predictive filter and then by Levenberg-Marquardt method. By inverse problem approach, 13 experimental resonant frequencies progressively matched to the first 30 orders of theoretical resonant frequencies calculated by Lagrangian variational method. The full second-order elastic tensor of dentin specimen was adjusted. The whole set of human dentin engineering moduli, including Young's moduli (E₁₁ = 22.641 GPa, E₃₃ = 13.637 GPa), shear moduli (G₁₂ = 10.608 GPa, G₂₃ = 7.742 Gpa), and Poisson's ratios (ν₁₂ = 0.067, ν₃₁ = 0.378), were finally calculated. This study demonstrates that RUS can be successfully adapted to measure the mechanical properties of low quality factor biomaterials.

Semianalytical Solution for the Deformation of an Elastic Layer under an Axisymmetrically Distributed Power-Form Load: Application to Fluid-Jet-Induced Indentation of Biological Soft Tissues

Fluid-jet-based indentation is used as a noncontact excitation technique by systems measuring the mechanical properties of soft tissues. However, the application of these devices has been hindered by the lack of theoretical solutions. This study developed a mathematical model for testing the indentation induced by a fluid jet and determined a semianalytical solution. The soft tissue was modeled as an elastic layer bonded to a rigid base. The pressure of the fluid jet impinging on the soft tissue was assumed to have a power-form function. The semianalytical solution was verified in detail using finite-element modeling, with excellent agreement being achieved. The effects of several parameters on the solution behaviors are reported, and a method for applying the solution to determine the mechanical properties of soft tissues is suggested.

Development of an arthroscopic ultrasound probe for assessment of articular cartilage degeneration

Quantitative assessment of articular cartilage is important for the early diagnosis of osteoarthritis, intra-operation joint tissue evaluation and judgment of repaired cartilage quality. This technique is also applicable to the cartilage if arthroscopic instrument embedding this technique can be developed. In this study, an arthroscopic water-jet ultrasound indentation probe was developed with the help of a small profile intra-articular ultrasound imaging (IAUS) catheter for the intra-articular measurement of cartilage condition. The probe can provide measurement of morphological, acoustical and mechanical properties of articular cartilage. Preliminary tests were conducted on 10 intact porcine knees with the guide of arthroscopy for the evaluation of cartilage degeneration, which was induced by trypsin digestion. Results showed the cartilage stiffness decreased significantly after the digestion (p < 0.001) with the measurement conducted by the developed probe. In summary, an arthroscopic ultrasound probe has been successfully developed and its utility in detecting the cartilage degeneration was demonstrated in this study. Future work includes the improvement of the probe design and studies on measurement of animal or human samples in vivo.

An ultrasound biomicroscopic and water jet ultrasound indentation method for detecting the degenerative changes of articular cartilage in a rabbit model of progressive osteoarthritis

It is important to assess the early degeneration of articular cartilage associated with osteoarthritis (OA) for early intervention and treatment planning. Previously, we have developed a high frequency ultrasound and water jet indentation method for the morphologic, acoustic and mechanical assessment of articular cartilage, using the enzymatic digestion as a model of osteoarthritic degeneration. No naturally degenerated articular cartilage has been tested with the developed method. In this study, we aimed to determine the usefulness of the developed method for detecting the natural degeneration of articular cartilage in a standard surgical model of OA in rabbits. Forty adult New Zealand white female rabbits were used in this study, which included 30 experimental rabbits undergoing the right anterior cruciate ligament transection surgery and 10 control rabbits. At the 3rd, 6th, and 9th week post-surgery, 10 experimental rabbits were sacrificed, respectively, for assessment of the knee cartilage quality. The cartilage at the medial and lateral femoral condyles and tibial plateaus (four points) was measured by the high frequency ultrasound biomicroscopy, the water jet ultrasound indentation and a contact mechanical indentation test before a histopathologic analysis for grading of degeneration severity. Measured parameters were compared among different groups classified either by post-surgery time or by histopathologic grade. The results showed a general trend of increase for ultrasound roughness index and a general trend of decrease for integrated reflection coefficient, stiffness coefficient from water-jet indentation and Young's modulus (E) from the mechanical indentation with the increase of post-surgery time. Comparisons among groups with different histopathologic grades showed similar trend with the increase of degeneration severity. The water jet ultrasound indentation method was demonstrated to be an effective method to measure the mechanical properties of the articular cartilage and with further development of arthroscopic ultrasound probe; it has the ability to assess the early degeneration of articular cartilage with measurement of morphologic, acoustic and mechanical properties of the cartilage in vivo.

Quantitative imaging of young's modulus of soft tissues from ultrasound water jet indentation: a finite element study

Indentation testing is a widely used approach to evaluate mechanical characteristics of soft tissues quantitatively. Young's modulus of soft tissue can be calculated from the force-deformation data with known tissue thickness and Poisson's ratio using Hayes' equation. Our group previously developed a noncontact indentation system using a water jet as a soft indenter as well as the coupling medium for the propagation of high-frequency ultrasound. The novel system has shown its ability to detect the early degeneration of articular cartilage. However, there is still lack of a quantitative method to extract the intrinsic mechanical properties of soft tissue from water jet indentation. The purpose of this study is to investigate the relationship between the loading-unloading curves and the mechanical properties of soft tissues to provide an imaging technique of tissue mechanical properties. A 3D finite element model of water jet indentation was developed with consideration of finite deformation effect. An improved Hayes' equation has been derived by introducing a new scaling factor which is dependent on Poisson's ratios v, aspect ratio a/h (the radius of the indenter/the thickness of the test tissue), and deformation ratio d/h. With this model, the Young's modulus of soft tissue can be quantitatively evaluated and imaged with the error no more than 2%.

Quantification of stiffness change in degenerated articular cartilage using optical coherence tomography-based air-jet indentation

Articular cartilage is a thin complex tissue that covers the bony ends of joints. Changes in the composition and structure of articular cartilage will cause degeneration, which may further lead to osteoarthritis. Decreased stiffness is one of the earliest symptoms of cartilage degeneration and also represents the imperfect quality of repaired cartilage. An optical coherence tomography (OCT)-based air-jet indentation system was recently developed in our group to measure the mechanical properties of soft tissues. In this study, this system was applied to quantify the change of mechanical properties of articular cartilage after degeneration induced by enzymatic digestions. Forty osteochondral disks (n = 20 × 2) were prepared from bovine patellae and treated with collagenase and trypsin digestions, respectively. The apparent stiffness of the cartilage was measured by the OCT-based air-jet indentation system before and after the degeneration. The results were also compared with those from a rigid contact mechanical indentation and an ultrasound water-jet indentation. Through the air-jet indentation, it was found that the articular cartilage stiffness dropped significantly by 84% (p < 0.001) and 63% (p < 0.001) on average after collagenase and trypsin digestions, respectively. The stiffness measured by the air-jet indentation system was highly correlated (R > 0.8, p < 0.001) with that from the other two indentation methods. This study demonstrated that the OCT-based air-jet indentation can be a useful tool to quantitatively assess the mechanical properties of articular cartilage, and this encourages us to further develop a miniaturized probe suitable for arthroscopic applications.

Low intensity pulsed ultrasound increases the mechanical properties of the healing tissues at bone-tendon junction

The re-establishment of bone-tendon junction (BTJ) tissues is involved in many trauma and reconstructive surgeries. A direct BTJ repair requires a long period of immobilization which may be associated with a postoperative weak knee. In this study, we investigated if low-intensity pulsed ultrasound treatment increases the material properties of healing tissues at bone-tendon junction (BTJ) after partial patellectomy using rabbit models. Standard partial patellectomy was conducted on one knee of twenty four rabbits which were randomly divided into an ultrasound group and a control group. The bony changes of BTJ complexes around the BTJ healing interface were measured by anteroposterior x-ray radiographs; then the volumetric bone-mineral density (BMD) of the new bone was assessed using a peripheral computed tomography scanner (pQCT). The stiffness of patellar cartilage, fibrocartilage at the healing interface and the tendon were measured in situ using a novel noncontact ultrasound water jet indentation system. Not only significantly more newly formed bone at the BTJ healing interface but also increased stiffness of the junction tissues were found in the ultrasound group compared with the controls at week 18. In addition, the ultrasound group also showed significantly 44% higher BMD at week 6 than controls.

Quantitative assessment of articular cartilage with morphologic, acoustic and mechanical properties obtained using high-frequency ultrasound

Osteoarthritis (OA) is one of the most common joint diseases among adults, and its early detection is still not possible. In this study, high-frequency ultrasound and ultrasound-assisted mechanical testing systems were used to quantitatively measure the morphologic, acoustic and mechanical properties of normal and enzymatically degraded bovine articular cartilages in vitro. A total of 40 osteochondral cartilage plugs were prepared from 20 bovine patellae (n=20x2) and divided into two groups for collagenase and trypsin digestions, respectively. A high-frequency ultrasound system (center frequency: 40 MHz) was used to analyze the surface integrity (ultrasound roughness index, URI), thickness and acoustic properties of the articular cartilages before and after enzymatic degradations. Acoustic parameters included the integrated reflection coefficient (IRC) from the cartilage surface, reflection from the cartilage-bone interface (AIB(bone)), integrated attenuation (IA) and integrated backscatter (IBS) of the internal cartilage tissue. A newly developed ultrasound water jet indentation system was used to assess the mechanical properties of the cartilage samples. The results showed that the URI increased significantly (p<0.05) after collagenase digestion while no significant change (p>0.05) was found after trypsin digestion. With regard to acoustic parameters, the IRC decreased significantly (p<0.05) after collagenase digestion while no significant change (p>0.05) was found after trypsin digestion. The AIB(bone) demonstrated an insignificant change after collagenase digestion (p>0.05) but a significant decrease after trypsin digestion (p<0.05). Both enzymatic degradation groups showed insignificant differences (p>0.05) in the IA but a significant increase (p<0.05) in the IBS after both enzymatic degradations. The apparent stiffness measured by ultrasound water jet indentation suggested that articular cartilage from both groups became significantly softer (p<0.05) after the enzymatic degradations. A significant relationship was found to exist between the IRC and URI (p<0.05). This study showed that high-frequency ultrasound can be a comprehensive tool to quantitatively and systematically analyze the morphologic, acoustic and mechanical properties of articular cartilage in association with its degeneration.

Evaluation of bone-tendon junction healing using water jet ultrasound indentation method

The re-establishment of bone-tendon junction (BTJ) tissues with the junction, characterized as a unique transitional fibrocartilage zone, is involved in many trauma and reconstructive surgeries. Experimental and clinical findings have shown that a direct BTJ repair requires a long period of immobilization, which may be associated with a postoperative weak knee. Therefore, it is necessary to evaluate the morphologic and mechanical properties of BTJ tissues in situ to better understand the healing process for the purpose of reducing the adverse effects of immobilization. We previously reported a noncontact ultrasound water jet indentation system for measuring and mapping tissue mechanical properties. The key idea was to utilize a water jet as an indenter as well as the coupling medium for high-frequency ultrasound. In this article, we used ultrasound water jet indentation to evaluate the BTJ healing process. The system's capability of measuring the material elastic modulus was first validated using tissue-mimicking phantoms. Then it was employed to assess the healing of the BTJ tissues after partial patellectomy over time on twelve 18-week-old female New Zealand White rabbits. It was found that in comparison with the normal control samples, the elastic modulus of the fibrocartilage of the postoperative samples was significantly smaller, while its thickness increased significantly. Among the postoperative sample groups, the elastic modulus of the fibrocartilage of the samples harvested at week 18 was significantly higher than those harvested at week 6 and week 12, which was even comparable with the value of the control samples at the same sacrifice time. The results suggested that the noncontact ultrasound water jet indentation system provided a nondestructive way to evaluate the material properties of small animal tissues in situ and thus had the ability to evaluate the healing process of BTJ.

Intravascular Ultrasound (IVUS): A Potential Arthroscopic Tool for Quantitative Assessment of Articular Cartilage

Conventional ultrasound examination of the articular cartilage performed externally on the body surface around the joint has limited accuracy due to the inadequacy in frequency used. In contrast to this, minimally invasive arthroscopy-based ultrasound with adequately high frequency may be a better alternative to assess the cartilage. Up to date, no special ultrasound transducer for imaging the cartilage in arthroscopic use has been designed. In this study, we introduced the intravascular ultrasound (IVUS) for this purpose. An IVUS system with a catheter-based probe (Ø approximately 1mm) was used to measure the thickness and surface acoustical reflection of the bovine patellar articular cartilage in vitro before and after degeneration induced by enzyme treatments. Similar measurement was performed using another high frequency ultrasound system (Vevo) with a probe of much larger size and the results were compared between the two systems. The thickness measured using IVUS was highly correlated (r = 0.985, p < 0.001) with that obtained by Vevo. Thickness and surface reflection amplitude measured using IVUS on the enzymatically digested articular cartilage showed changes similar to those obtained by Vevo, which were expectedly consistent with previous investigations. IVUS can be potentially used for the quantitative assessment of articular cartilage, with its ready-to-use arthroscopic feature.

An optical coherence tomography (OCT)-based air jet indentation system for measuring the mechanical properties of soft tissues

A novel noncontact indentation system with the combination of an air jet and optical coherence tomography (OCT) was presented in this paper for the quantitative measurement of the mechanical properties of soft tissues. The key idea of this method is to use a pressure-controlled air jet as an indenter to compress the soft tissue in a noncontact way and utilize the OCT signals to extract the deformation induced. This indentation system provides measurement and mapping of tissue elasticity for small specimens with high scanning speed. Experiments were performed on 27 silicone tissue-mimicking phantoms with different Young's moduli, which were also measured by uniaxial compression tests. The regression coefficient of the indentation force to the indentation depth (N mm(-1)) was used as an indicator of the stiffness of tissue under air jet indentation. Results showed that the stiffness coefficients measured by the current system correlated well with the corresponding Young's moduli obtained by conventional mechanical testing (r = 0.89, p < 0.001). Preliminary in vivo tests also showed that the change of soft tissue stiffness with and without the contraction of the underlying muscles in the hand could be differentiated by the current measurement. This system may have broad applications in tissue assessment and characterization where alterations of mechanical properties are involved, in particular with the potential of noncontact micro-indentation for tissues.

Noncontact evaluation of articular cartilage degeneration using a novel ultrasound water jet indentation system

We previously reported a noncontact ultrasound water jet indentation system for measuring and mapping tissue mechanical properties. The key idea was to utilize a water jet as an indenter as well as the coupling medium for high-frequency ultrasound. In this paper, the system was employed to assess articular cartilage degeneration, using stiffness ratio as an indicator of the mechanical properties of samples. Both the mechanical and acoustical properties of intact and degenerated bovine patellar articular cartilage (n = 8) were obtained in situ. It was found that the stiffness ratio was reduced by 44 +/- 17% after the articular cartilage was treated by 0.25% trypsin at 37 degrees C for 4 h while no significant difference in thickness was observed between the intact and degenerated samples. A significant decrease of 36 +/- 20% in the peak-to-peak amplitude of ultrasound echoes reflected from the cartilage surface was also found for the cartilage samples treated by trypsin. The results also showed that the stiffness obtained with the new method highly correlated with that measured using a standard mechanical testing protocol. A good reproducibility of the measurements was demonstrated. The present results showed that the ultrasound water jet indentation system may provide a potential tool for the non-destructive evaluation of articular cartilage degeneration by simultaneously obtaining mechanical properties, acoustical properties, and thickness data.