P. V. Pistecky

Mechanical properties of porcine and human arteries: implications for coronary anastomotic connectors.

BACKGROUND To determine whether the pig is an appropriate experimental animal for studies on distal anastomotic connectors in coronary artery bypass surgery, the mechanical properties of young porcine and old human coronary and internal mammary arteries were compared within and beyond the physiologic range of strains. METHODS Coronary arteries from 6 humans and 8 pigs were studied as well as internal mammary arteries of 3 humans and 6 pigs (human, aged 61 to 85 years; pig, 78.7 +/- 5.8 kg [mean +/- SD]). Pressure-diameter, pressure-axial force, circumferential and axial stress-strain relations, and dimensions were measured. RESULTS The dimensions of the porcine and human coronary and internal mammary artery were generally similar but wall thickness was smaller in the porcine internal mammary artery (0.35 +/- 0.07 mm versus 0.71 +/- 0.06 mm, respectively, p = 0.002). The porcine internal mammary artery wall was less elastic than the coronary artery wall, whereas in humans both arteries displayed similar elasticity. Overall the porcine arteries were far more elastic in both circumferential and axial direction compared with the human arteries. Consequently the porcine arteries could be safely stretched by 60% to 70% compared with about 20% for the human arteries before reaching their maximum circumferential strain. CONCLUSIONS The three times greater elasticity of porcine compared with human coronary and internal mammary artery walls may result in underestimation of wall stress and the risk of wall injury when coronary connectors that involve overstretching of the wall are evaluated in the pig.

Assessment of patented coronary end-to-side anastomotic devices using micromechanical bonding

BACKGROUND Despite multiple patented ideas for vascular end-to-side anastomotic devices, and the growing need for them in minimally invasive coronary bypass procedures, no device has been evaluated clinically yet. This study assessed patents of micromechanical end-to-side anastomotic devices with respect to application in coronary artery bypass grafting. METHODS Patents were categorized with respect to their micromechanical bonding principle. Calculated values for the wall strain during the construction of an anastomosis were compared with the allowable strain for human coronary arteries. RESULTS From 51 patents describing vascular anastomotic devices, 11 ideas, categorized into four groups (staples, clips, mounting systems, and intraluminal stent structures), are serious candidates for coronary end-to-side anastomoses. Most ideas use an anvil for proper application of the bonding elements. For small (1.5 mm) coronary arteries, the calculated wall strain was 0.87, exceeding the breaking strain (0.45) in 60- to 79-year-old patients. CONCLUSIONS In a coronary anastomotic device, the concept of using an anvil for the application of micromechanical bonding elements is not attractive, because excessive wall strain is likely to occur.

Behavior of arthroscopic irrigation systems

In the literature, no consensus exists about optimal irrigation of joints during arthroscopic operations. The goal of this paper is to study the behavior of irrigation systems resulting in the proposal of guidelines for optimal irrigation. To this end, optimal irrigation is defined as the steady state of irrigation of a joint in which a sufficient positive intra-articular pressure and a sufficient flow are maintained. A model of the complete irrigation system was created to schematically elucidate the behavior of pump systems. Additionally, clinical experiments were performed during arthroscopic knee operations in which the pressure at different locations and the irrigation flow were measured. The combination of model prediction and clinical results could well be used to derive guidelines, since the clinical results, which showed considerable variation, were used to verify the model, and the model could be used to explain the typical trends. The main findings are twofold the set pressure is always higher than the intra-articular pressure, and the scope–sheath combination has a significant influence on irrigation control, because of its large restriction. Based on the results, we advice to increase the set pressure during active suction, and to include the sheath–scope combination in the control loop.

Measuring alignment of the hindfoot.

In subtalar arthrodesis operations, correction of the hindfoot alignment is performed in about half of the cases. To improve the quality of the operation, a measurement system was developed which reliably measures the hindfoot angle pre-, per-, and postoperatively. This device was evaluated by measuring subjects in standing weightbearing position and in prone nonweightbearing position. The results were compared with hindfoot angles constructed on posterior photographic images. The results are similar to other studies (all maximum values): intratester accuracy 1.4 degrees, intertester accuracy 2.2 degrees, intratester reliability 0.9, and intertester reliability 0.74. The proposed device will improve the quality of correction, because it enables peroperative measurement of hindfoot alignment.

Clinically-driven approach to improve arthroscopic techniques.

In this paper, a clinically-driven approach is introduced as the starting point for the improvement of arthroscopic techniques. The approach was divided into two phases. Phase 1 consisted of a combination of observations and discussions that resulted in the definition of clinically-relevant research topics. Phase 2 consisted of an interview which aimed at analysis of the general opinion on arthroscopy, and which assigned a priority ranking to the topics. Six research topics were defined. Based on the information collected, the conclusion is that the surgeons are satisfied with current arthroscopic techniques. A majority gives priority to the optimization of cartilage treatment and to the design of a steerable arthroscopic cutter. A minority gives priority to the expansion of arthroscopic techniques. The two-phase approach was fruitful in terms of establishing clinical problem areas, and of involving surgeons in the technical improvement of arthroscopic techniques.

Mechanical properties of coronary arteries and internal mammary arteries beyond physiological deformations

Passive circumferential and axial mechanical properties of porcine coronary arteries and internal mammary arteries (IMA) were measured and compared. The cylindrical specimens were subjected to axial stretch and internal pressures up to 300 mmHg. Stress-strain relations of the arteries were calculated from the measured data and compared. It was found that the stresses in the IMA were much higher and that the IMA is stiffer than the coronary artery, especially in the circumferential direction. The axial stress in the coronary artery increased substantially after axial stretching, this in contrast to the axial stress in the IMA.

A Compliant Instrument for Arthroscopic Joint Fusion

Due to its complex shape and its tightness, it is difficult to prepare the subtalar joint for fusion by means of a minimally invasive approach and conventional straight instruments. The preparation implies the establishment of bleeding contact surfaces. It is desirable to preserve the joint’s shape to keep congruent and smooth contact surfaces for optimal fusion. A compliant instrument was designed to facilitate the preparation by starting the design process from a clinically driven approach. The concept consists of a drill/mill unit at the tip that is steered through the joint by means of a passive automatic control. Since the joint will be fused, the cartilage and the subchondral bone layers should intentionally be damaged. This allows the use of the subtalar joint surfaces themselves as a guiding tunnel to preserve the joint’s shape. Thereto, a guidance frame was placed at the tip. The instrument is equipped with a special shaft that is compliant in one direction (perpendicular to the joint surfaces), and stiff in the two other directions to resist and transmit machining forces. The dimensioning of the instrument was performed by taking into account the specific shape of the subtalar joint. A prototype of the instrument was manufactured. The prototype is powered by a commonly used shaver system. The compliant instrument was tested in cadaver material, and gives promising results. In practice, the difference of the stiffness in y- and z-direction is at least a factor 100.Copyright

Permanent wall stretching in porcine coronary and internal mammary arteries

BACKGROUND Anastomotic connectors may induce substantial arterial wall deformation and, hence, wall injury. We studied arterial wall damage and repair after sustained large longitudinal elongation in the porcine coronary and internal mammary arteries in vivo. METHODS A stretch device that elongates a part of the artery by 80% was implanted in 8 pigs. Elongated coronary arteries (n = 14) and internal mammary arteries (n = 15) were examined histologically at either 2 days (4 pigs) or 5 weeks of follow-up (4 pigs). RESULTS No mural thrombus was observed at the elongated site. In the coronary artery at 2 days, few and only minor histologic changes were found. At 5 weeks, in two of seven coronary segments, a thin rim of intimal hyperplasia was found, in one case with a maximum thickness of 76 micro m. The internal mammary artery hardly showed any changes. CONCLUSIONS Permanent longitudinal elongation by 80% caused little structural changes in the porcine coronary and internal mammary artery wall. Anastomotic connectors that impose relatively large deformations can be safely evaluated in the pig.