A pilot study to measure vascular compliance changes during fistula maturation using open-source software.

PURPOSE:: Autogenous arteriovenous fistulas are the preferred access for hemodialysis. Yet when created, fistulas often fail to mature, requiring surgical or radiologic interventions before their use. This pilot study measures the vascular wall elasticity and flow gradient using an open-source ultrasound software program designed to aid in assessing fistula maturation. METHODS:: A total of seven end-stage renal disease patients were enrolled for our study after providing informed consent. Ultrasound scanning was performed for the inflow artery, post-arterial anastomotic segment, and outflow vein at initial and follow-up evaluation. Conventional digital imaging and communications in medicine data were collected from the ultrasound machine. The vessel diameter and distensibility of artery, post-arterial anastomotic segment, and vein were computed from the digital imaging and communications in medicine data using an open-source ultrasound software program. RESULTS:: The vessel diameter of artery and vein increased from 4.6 ± 1.1 mm to 6.0 ± 1.1 mm and from 5.8 ± 0.7 mm to 7.5 ± 0.9 mm from 1 to 6 weeks post-operation, respectively. Conversely, the vessel diameter of post-arterial anastomotic segment decreased from 4.2 ± 1.0 mm to 3.5 ± 0.9 mm from 1 to 6 weeks post-operation. The distensibility of artery and post-arterial anastomotic segment increased from 3.4% ± 0% to 5.9% ± 1.1% and 3.7% ± 1.2% to 4.9% ± 1.4%, respectively, while the distensibility of vein decreased from 5.0% ± 1.3% to 2.6% ± 0.4% from 1 to 6 weeks post-operation. CONCLUSION:: This study demonstrates that the change in vessel diameter and distensibility related to the healthy remodeling as the vein dilates during maturation.

Continuous Inferior Vena Cava Diameter Tracking through an Iterative Kanade-Lucas-Tomasi-Based Algorithm.

Ultrasound assessment of the respiratory-induced change in size of the inferior vena cava is a useful technique in the evaluation and management of critically ill patients. We have developed an automated technique based on the Kanade-Lucas-Tomasi feature tracker and pyramidal segmentation to continuously track the diameter of the inferior vena cava during ultrasound. To test the accuracy of this automated process, the inferior vena cava of 47 spontaneously breathing patients were measured by trained ultrasound physicians and compared against the results obtained via the automated tracking. Good agreement between the techniques was found, with intra-class correlation coefficients for maximum vessel diameter, minimum diameter and caval index of 0.897, 0.967 and 0.975, respectively. More than 95% of the difference between physicians and automated measurements agreed to within 10% of the inferior vena cava collapse. Furthermore a phenomenon of cardiac collapsibility index variability was observed and reported. The accuracy and precision of this algorithmic technique provide a foundation for future automated measures for critical care ultrasound.

An Open-Source Ultrasound Software for Diagnosis of Fistula Maturation.

Vascular access is essential for hemodialysis patients. The mature native arteriovenous fistula has been the preferred vascular access for hemodialysis, because it has greater longevity than synthetic grafts. However, once surgically created, fistulas often fail to develop (mature) into viable points of vascular access, requiring surgical or radiologic interventions before their use. Because maturation depends on vascular mechanics (e.g., distensibility and wall shear), we developed open-source ultrasound software to investigate these metrics clinically. We demonstrated in a single patient the ability of the software for consistent measurements from various locations within a cardiac cycle and between different cardiac cycles. We further assessed the ability of the software to identify changes in distensibility of a patient's fistula from 1 to 6 weeks postoperation. The routine frame rates of clinical machines demonstrated high fidelity tracking within cardiac cycles (coefficient of variation [CV] = 2.4% ± 0.011) and between cardiac cycles (CV = 2.4% ± 0.004). The distensibility of the patient's fistula from 1 to 6 weeks postoperation increased from 4% to 7% in the arterial inflow and from 3% to 4% in the postarterial anastomotic segment (PAAS). In contrast, the distensibility of the outflow vein decreased from 4% to 2%. These results corroborate that in addition to diameter changes, the mechanical properties of the vascular segments changed during fistula maturation. This demonstrates that our software-based approach may allow ultrasound-based mechanical measurements to become more accessible for wider clinical research.

Notched K-wire for low thermal damage bone drilling.

The Kirschner wire (K-wire) is a common bone drilling tool in orthopedic surgery to affix fractured bone. Significant heat is produced due to both the cutting and the friction between the K-wire and the bone debris during drilling. Such heat can result in high temperatures, leading to osteonecrosis and other secondary injuries. To reduce thermal injury and other high-temperature associated complications, a new K-wire design with three notches along the three-plane trocar tip fabricated using a thin micro-saw tool is studied. These notches evacuate bone debris and reduce the clogging and heat generation during bone drilling. A set of four K-wires, one without notches and three notched, with depths of 0.5, 0.75, and 1mm, are evaluated. Bone drilling experiments conducted on bovine cortical bone show that notched K-wires could effectively decrease the temperature, thrust force, and torque during bone drilling. K-wires with notches 1mm deep reduced the thrust force and torque by approximately 30%, reduced peak temperatures by 43%, and eliminated blackened burn marks in bone. This study demonstrates that a simple modification of the tip of K-wires can effectively reduce bone temperatures during drilling.

Polyvinyl chloride as a multimodal tissue-mimicking material with tuned mechanical and medical imaging properties.

PURPOSE: The mechanical and imaging properties of polyvinyl chloride (PVC) can be adjusted to meet the needs of researchers as a tissue-mimicking material. For instance, the hardness can be adjusted by changing the ratio of softener to PVC polymer, mineral oil can be added for lubrication in needle insertion, and glass beads can be added to scatter acoustic energy similar to biological tissue. Through this research, the authors sought to develop a regression model to design formulations of PVC with targeted mechanical and multimodal medical imaging properties. METHODS: The design of experiment was conducted by varying three factors-(1) the ratio of softener to PVC polymer, (2) the mass fraction of mineral oil, and (3) the mass fraction of glass beads-and measuring the mechanical properties (elastic modulus, hardness, viscoelastic relaxation time constant, and needle insertion friction force) and the medical imaging properties [speed of sound, acoustic attenuation coefficient, magnetic resonance imaging time constants T1 and T2, and the transmittance of the visible light at wavelengths of 695 nm (Tλ695) and 532 nm (Tλ532)] on twelve soft PVC samples. A regression model was built to describe the relationship between the mechanical and medical imaging properties and the values of the three composition factors of PVC. The model was validated by testing the properties of a PVC sample with a formulation distinct from the twelve samples. RESULTS: The tested soft PVC had elastic moduli from 6 to 45 kPa, hardnesses from 5 to 50 Shore OOO-S, viscoelastic stress relaxation time constants from 114.1 to 191.9 s, friction forces of 18 gauge needle insertion from 0.005 to 0.086 N/mm, speeds of sound from 1393 to 1407 m/s, acoustic attenuation coefficients from 0.38 to 0.61 (dB/cm)/MHz, T1 relaxation times from 426.3 to 450.2 ms, T2 relaxation times from 21.5 to 28.4 ms, Tλ695 from 46.8% to 92.6%, and Tλ532 from 41.1% to 86.3%. Statistically significant factors of each property were identified. The regression model relating the mechanical and medical imaging properties and their corresponding significant factors had a good fit. The validation tests showed a small discrepancy between the model predicted values and experimental data (all less than 5% except the needle insertion friction force). CONCLUSIONS: The regression model developed in this paper can be used to design soft PVC with targeted mechanical and medical imaging properties.

Experimental investigation of the abrasive crown dynamics in orbital atherectomy.

Orbital atherectomy is a catheter-based minimally invasive procedure to modify the plaque within atherosclerotic arteries using a diamond abrasive crown. This study was designed to investigate the crown motion and its corresponding contact force with the vessel. To this end, a transparent arterial tissue-mimicking phantom made of polyvinyl chloride was developed, a high-speed camera and image processing technique were utilized to visualize and quantitatively analyze the crown motion in the vessel phantom, and a piezoelectric dynamometer measured the forces on the phantom during the procedure. Observed under typical orbital atherectomy rotational speeds of 60,000, 90,000, and 120,000rpm in a 4.8mm caliber vessel phantom, the crown motion was a combination of high-frequency rotation at 1000, 1500, and 1660.4-1866.1Hz and low-frequency orbiting at 18, 38, and 40Hz, respectively. The measured forces were also composed of these high and low frequencies, matching well with the rotation of the eccentric crown and the associated orbital motion. The average peak force ranged from 0.1 to 0.4N at different rotational speeds.

Dynamic Limb Bioimpedance and Inferior Vena Cava Ultrasound in Patients Undergoing Hemodialysis.

Assessment of volume status in critically ill patients poses a challenge to clinicians. Measuring changes in the inferior vena cava (IVC) diameter using ultrasound is becoming a standard tool to assess volume status. Ultrasound requires physicians with significant training and specialized expensive equipment. It would be of significant value to be able to obtain this measurement continuously without physician presence. We hypothesize that dynamic changes in limb's bioimpedance in response to respiration could be used to predict changes in IVC. Forty-six subjects were tested a hemodialysis session. Impedance was measured via electrodes placed on the arm. Simultaneously, the IVC diameter was assessed by ultrasound. Subjects were asked to breathe spontaneously and perform respiratory maneuvers using a respiratory training device. Impedance (dz) was determined and compared with change in IVC diameter (dIVC; r = 0.76, p < 0.0001). There was significant relationship between dz and dIVC (p< 0.0001). Receiver-operator curves for dz at thresholds of dIVC (20% to70%) demonstrated high predictive power with areas under the curves (0.87-0.99, p < 0.0001). This evaluation suggests that real-time dynamic changes in limb impedance are capable of tracking a wide range of dynamic dIVC. This technique might be a suitable surrogate for monitoring real-time changes in dIVC to assess intravascular volume status.

Heat accumulation during sequential cortical bone drilling.

Significant research exists regarding heat production during single-hole bone drilling. No published data exist regarding repetitive sequential drilling. This study elucidates the phenomenon of heat accumulation for sequential drilling with both Kirschner wires (K wires) and standard two-flute twist drills. It was hypothesized that cumulative heat would result in a higher temperature with each subsequent drill pass. Nine holes in a 3 × 3 array were drilled sequentially on moistened cadaveric tibia bone kept at body temperature (about 37 °C). Four thermocouples were placed at the center of four adjacent holes and 2 mm below the surface. A battery-driven hand drill guided by a servo-controlled motion system was used. Six samples were drilled with each tool (2.0 mm K wire and 2.0 and 2.5 mm standard drills). K wire drilling increased temperature from 5 °C at the first hole to 20 °C at holes 6 through 9. A similar trend was found in standard drills with less significant increments. The maximum temperatures of both tools increased from <0.5 °C to nearly 13 °C. The difference between drill sizes was found to be insignificant (P > 0.05). In conclusion, heat accumulated during sequential drilling, with size difference being insignificant. K wire produced more heat than its twist-drill counterparts. This study has demonstrated the heat accumulation phenomenon and its significant effect on temperature. Maximizing the drilling field and reducing the number of drill passes may decrease bone injury.

Two-Finger Tightness: What Is It? Measuring Torque and Reproducibility in a Simulated Model.

OBJECTIVES: Residents in training are often directed to insert screws using "two-finger tightness" to impart adequate torque but minimize the chance of a screw stripping in bone. This study seeks to quantify and describe two-finger tightness and to assess the variability of its application by residents in training. METHODS: Cortical bone was simulated using a polyurethane foam block (30-pcf density) that was prepared with predrilled holes for tightening 3.5 × 14-mm long cortical screws and mounted to a custom-built apparatus on a load cell to capture torque data. Thirty-three residents in training, ranging from the first through fifth years of residency, along with 8 staff members, were directed to tighten 6 screws to two-finger tightness in the test block, and peak torque values were recorded. The participants were blinded to their torque values. RESULTS: Stripping torque (2.73 ± 0.56 N·m) was determined from 36 trials and served as a threshold for failed screw placement. The average torques varied substantially with regard to absolute torque values, thus poorly defining two-finger tightness. Junior residents less consistently reproduced torque compared with other groups (0.29 and 0.32, respectively). CONCLUSIONS: These data quantify absolute values of two-finger tightness but demonstrate considerable variability in absolute torque values, percentage of stripping torque, and ability to consistently reproduce given torque levels. Increased years in training are weakly correlated with reproducibility, but experience does not seem to affect absolute torque levels. These results question the usefulness of two-finger tightness as a teaching tool and highlight the need for improvement in resident motor skill training and development within a teaching curriculum. Torque measuring devices may be a useful simulation tools for this purpose.

Numerical evaluation of sequential bone drilling strategies based on thermal damage.

Sequentially drilling multiple holes in bone is used clinically for surface preparation to aid in fusion of a joint, typically under non-irrigated conditions. Drilling induces a significant amount of heat and accumulates after multiple passes, which can result in thermal osteonecrosis and various complications. To understand the heat propagation over time, a 3D finite element model was developed to simulate sequential bone drilling. By incorporating proper material properties and a modified bone necrosis criteria, this model can visualize the propagation of damaged areas. For this study, comparisons between a 2.0 mm Kirschner wire and 2.0 mm twist drill were conducted with their heat sources determined using an inverse method and experimentally measured bone temperatures. Three clinically viable solutions to reduce thermally-induced bone damage were evaluated using finite element analysis, including tool selection, time interval between passes, and different drilling sequences. Results show that the ideal solution would be using twist drills rather than Kirschner wires if the situation allows. A shorter time interval between passes was also found to be beneficial as it reduces the total heat exposure time. Lastly, optimizing the drilling sequence reduced the thermal damage of bone, but the effect may be limited. This study demonstrates the feasibility of using the proposed model to study clinical issues and find potential solutions prior to clinical trials.

Comparison of cortical bone drilling induced heat production among common drilling tools.

OBJECTIVES: Significant data exist regarding heat production of twist drills; however, there are little data regarding cannulated drills or Kirschner (K) wires. This study compared the heat produced during bone drilling with twist drills, K wires, and a cannulated drill. It was hypothesized that drilling temperature would increase with tool sizes used in orthopaedic surgery; with twist drills producing the least amount of heat followed by cannulated drills and K wires. METHODS: Twist drills (2.0, 2.5, and 3.5 mm), K wires (1.25, 1.6, and 2.0 mm), and a cannulated drill (2.7 mm) were driven into warmed human cadaveric tibia by a battery-powered hand drill. The drill was secured on a servo-controlled linear actuator to provide a constant advancing speed (1 mm/s) during drilling. Two thermocouples were embedded 2 mm from the surface at 0.5 and 1.5 mm from the drill hole margin. Eight tests were performed for each tool. RESULTS: Twist drills exhibited a positive trend between size and heat production. The size effect was less significant with K wires. K wires resulted in significantly (P = 0.008 at 0.5 mm) higher peak temperatures than twist drills of the same size. A 2.7-mm cannulated drill produced more than double the temperature rise of a 2.5-mm twist drill. CONCLUSIONS: Twist drills produced the smallest temperature rise among all bit types. Thermal effects should not be a reason for choosing K-wire size. The cannulated drill showed significantly higher temperatures when compared with standard drills, reaching maximal temperatures comparable with K wires.

An apparatus to quantify anteroposterior and mediolateral shear reduction in shoe insoles.

BACKGROUND: Many of the physiological changes that lead to diabetic foot ulceration, such as muscle atrophy and skin hardening, are manifested at the foot-ground interface via pressure and shear points. Novel shear-reducing insoles have been developed, but their magnitude of shear stiffness has not yet been compared with regular insoles. The aim of this study was to develop an apparatus that would apply shear force and displacement to an insole's forefoot region, reliably measure deformation, and calculate insole shear stiffness. METHODS: An apparatus consisting of suspended weights was designed to test the forefoot region of insoles. Three separate regions representing the hallux; the first and second metatarsals; and the third, fourth, and fifth metatarsals were sheared at 20 mm/min for displacements from 0.1 to 1.0 mm in both the anteroposterior and mediolateral directions for two types of insoles (regular and shear reducing). RESULTS: Shear reduction was found to be significant for the intervention insoles under all testing conditions. The ratio of a regular insole's effective stiffness and the experimental insole's effective stiffness across forefoot position versus shear direction, gait instance versus shear direction, and forefoot position versus gait instance was 270% ± 79%, 270% ± 96%, and 270% ± 86%, respectively. The apparatus was reliable with an average measured coefficient of variation of 0.034 and 0.069 for the regular and shear-reducing insole, respectively. CONCLUSION: An apparatus consisting of suspended weights resting atop three locations of interest sheared across an insole was demonstrated to be capable of measuring the insole shear stiffness accurately, thus quantifying shear-reducing effects of a new type of insole.