Tim Frank

Measurements and modelling of the compliance of human and porcine organs

Stress-strain data obtained from animal and human tissue have several applications including medical diagnosis, assisting in surgical instrument design and the production of realistic computer-based simulators for training in minimal access surgery. Such data may also be useful for corroborating mathematical models of tissue response. This paper presents data obtained from ex-vivo and in-vivo tissue indentation tests using a small indentor that is similar to instruments used in minimal access surgery. In addition, uniform stress tests provide basic material property data, via an exponential stress-strain law, to allow a finite element method to be used to predict the response for the non-uniform stresses produced by the small indentor. Data are obtained from harvested pig liver and spleen using a static compliance probe. Data for human liver are obtained from volunteer patients, undergoing minor open surgery, using a sterile hand-held compliance probe. All the results demonstrate highly non-linear stress-strain behaviour. Pig spleen is shown to be much more compliant than pig liver with mean elastic moduli of 0.11 and 4.0 MPa respectively. The right lobe of human liver had a mean elastic modulus of about 0.27 MPa. However, a single case of a diseased liver had a mean modulus of 0.74 MPa--nearly three times the stiffness. It was found that an exponential stress-strain law could accurately fit uniform stress test data and that subsequent finite element modelling for non-uniform stress around a small indentor matched measured force characteristics.

Real-time thermography during energized vessel sealing and dissection

AbstractBackground: Energized dissection systems facilitate laparoscopic dissection and hemostasis and reduce instrument traffic. However, they can introduce undesirable thermal collateral/proximity damage to adjacent structures mainly by heat conduction, although other mechanisms may be involved. The latest generation devices have the potential to reduce the incidence of such problems through use of active feedback control over the power output. This effectively regulates the delivery of energy to the target tissue with minimal thermal collateral damage. In addition, innovative heat-sink engineering of the device head ensures that the surface of the instrument tip remains cool (<45°C). In this study, we evaluated the performance of this technology (LigaSure) by using dynamic infrared thermography. The thermal imaging measurements were then correlated with histopathologic studies. The overall value of in situ thermography as an adjunct to energized surgical dissection systems was also assessed. Methods: Eight anesthetized pigs underwent open surgery to mobilize eight target vessels/organs in a randomized fashion. The LigaSure vessel sealing system with Instant Response Technology was used with three different interchangeable heads. In situ dynamic thermography was undertaken with a thermal imaging camera operating in the mid-infrared (3–5 µm) waveband and with each fully digitized 12-bit thermographic frame acquired at a rate of 60 Hz. Following sacrifice at the end of the dissection, tissue from the dissected regions was harvested for histology by an independent pathologist who was blinded to the thermographic data. Results: Seals made with both the LS1000 5-mm laparoscopic head (predominantly to the small bowel and colon) and the LS1100 10-mm (Atlas) device (on the liver and short gastric tissues) were outwardly satisfactory. The average thermal spread () with the LS1000 was = 4.4 mm, and the exposed surface of the instrument tip developed a temperature of approximately 100°C. This instrument thus has the potential, albeit small, for heat-related proximity iatrogenic injury. The more technologically advanced LS1100 10-mm laparoscopic instrument exhibited a superior performance, with = 1.8 mm, and with a maximal temperature on the exposed surface of the jaws well within tolerable limits (approximately 35°C) for use during surgery (laparoscopic or open). This was confirmed by histological studies that demonstrated negligible evidence of thermal damage. Conclusions: In situ thermal imaging represents a powerful modality for the monitoring of energized dissection/coagulation during surgery. The LigaSure system used with the LS1100 head constitutes a very safe option for energized dissection and hemostasis of vessels with a diameter of up to approximately 7 mm.

Mechanical properties of the human abdominal wall measured in vivo during insufflation for laparoscopic surgery

BackgroundCarbon dioxide insufflation of the peritoneal cavity for laparoscopic surgery offers a unique opportunity to measure some mechanical properties of the human abdominal wall that hitherto have been difficult to obtain.MethodsThe movement and change of the abdominal wall during insufflation to a pressure of 12 mmHg was studied in 18 patients undergoing laparoscopic surgery using a remote motion analysis system that does not compromise the sterility of the operative filed. These data together with the known abdominal wall thickness of each patient (measured by preoperative ultrasound scanning) enabled estimates of mechanical stiffness.ResultsThe findings showed that the abdominal wall changes from a cylinder to a dome during inflation, and that its area is increased by 15%. A volume, averaging 1.27 × 10−3m3, results from expansion, reshaping of the abdominal wall, and displacement of the diaphragm. The abdominal wall is stiffer in the transverse plane than in the sagittal plane (Young’s modulus, 42.5 ± 9.0 kPa vs 22.5 ± 2.6 kPa; p = 0.03; paired t-test).ConclusionsMeasurements of mechanical properties of the abdominal wall in patients undergoing laparoscopic surgery were obtained using a remote motion analysis system.

Influence of handle design on the surgeon's upper limb movements, muscle recruitment, and fatigue during endoscopic suturing

Background: Thus far, little has been done to investigate the kinematics (motion analysis) and kinetics (muscle work, muscle fatigue, comfort) of surgeons during laparoscopic surgery. Therefore, we set out to study these ergonomic aspects of task performance in the dominant upper limb of surgeons during endoscopic suturing. Methods: Three different handles-conventional finger loop, rocker, and ball handle prototype-were compared in a study involving 10 surgeons suturing porcine enterotomies with each of the three instruments. The endpoints were performance parameters, motion analysis and muscle work, and fatigue of the surgeons dominant upper limb; subjective scores for comfort level and maneuverability were also elicited from the subjects. Results: Task quality and efficiency during endoscopic suturing, were significantly better with the ball and rocker handle needle drivers than with the finger loop instrument, with lower angular velocity at the elbow and shoulder joints, more pronation, and less supination. The integrated muscle work was much lower for both the rocker and the ball handles. Significant muscle fatigue, especially of the arm flexors and deltoid, was observed only with finger loop instruments. Comfort and maneuverability rating scores were higher with both handles than with the conventional finger loop. The ball handle was easier to maneuver, but it was somewhat less comfortable than the rocker system. Conclusion: A different pattern of joint movements, a reduction in muscle power exerted during endoscopic suturing, and hence an absence of muscle fatigue were documented with ergonomic needle drivers (rocker and ball) when compared to the conventional finger loop instruments. These differences translate to better and more efficient task performance with enhanced comfort.

Objective assessment of endoscopic knot quality

BACKGROUND Studies of the surgeons skill and the ergonomics of task performance in endoscopic surgery can be based on knot-tying tasks. The aim of this study was to establish an objective method for assessing the quality of surgical knots for use in such studies. METHODS In all, 2,700 surgeons endoscopic knots were studied. Each knot was distracted using a tensiometer, and a computerized system analyzed force-extension curves. The breaking force was taken as an index of knot strength while the force integrated over the slope of the curve reflected knot tightening. A knot quality score (KQS) was obtained from the product of the knot-breaking force and the integrated force expressed as a percentage of the product for the untied ligature. RESULTS The mean breaking force (24 Newton +/- 2.5) and integrated force (7.4 Newton +/- 2.8) for broken knots were 71% and 35%, respectively, of those for untied ligature. The integrated force yielded a narrower range of variability for untied ligature (SD 3.5% of mean) than for knots (SD 37% of mean). The KQS was higher for broken (25.3%+/-10.3%) than slipped knots (7.1%+/-5.1%). CONCLUSION The KQS provides a reliable assessment of knot security and reflects the strength and degree of tightening of the knot.

Thermographic assessment of tumor growth in mouse xenografts

In human breast tumors, a 1–2°C increase in skin surface temperature is usually observed at the periphery; it has been proposed that this change is due to the hypervascularity and increased blood flow resulting from tumor‐associated angiogenesis. Here we tested the hypothesis that thermal imaging might represent a useful adjunctive technique in monitoring the growth dynamics of human tumor xenografts. Xenografts were established in immunocomprised nude mice using MDA‐MB‐231 or MCF7 breast cancer cells. We exploited the inherent noncontact and noninvasive advantages of infrared thermography to detect skin surface temperature changes. Continuous thermographic investigation was performed to detect and monitor tumor growth in vivo and high resolution digital images were analyzed to measure the tumor temperature dynamics. In contrast to the skin temperature increases associated with human breast cancer, a consistent temperature decrease was found in the xenograft mice. In one case, a smaller secondary tumor, otherwise undetectable, was clearly evident by thermal imaging. The tumors were cooler than the surrounding tissue with a maximum temperature reduction of 1.5°C for MDA‐MB‐231 tumor and 3°C for MCF7 tumors observed on day 14. In addition, the temperature of the xenograft tumors decreased progressively as they grew throughout the observation period. It was demonstrated that thermographic imaging could detect temperature changes as small as 0.1°C on the skin surface at an early stage of tumor development. The findings of the study indicate that thermographic imaging might have considerable potential in monitoring human tumor xenografts and their response to anticancer drugs.

Instruments based on shape-memory alloy properties for minimal access surgery, interventional radiology and flexible endoscopy

Instrumental manipulations during minimal access surgery (MAS) are limited by the four degrees of freedom (d.o.f.) possible when long instruments are introduced into the peritoneal cavity through access ports. This is in sharp contrast to an equivalent instrument held in the hand by the surgeon during open conventional surgery, where six d.o.f. are available. Steerable motorised endo-effectors are one engineering option to overcome this intrinsic limitation of MAS. However, it is difficult to meet the torque requirements of such instruments using existing electric motor technology. We have sought a different, though not mutually-exclusive approach to the design and development of more effective instrumentation for MAS, based on shape-memory alloy (SMA). SMA formed from nickel-titanium (NiTi) has unique superelastic and shape-recovery properties, and are therefore an advantageous construction material for instrument components and devices within the limited size constraints imposed by the ports used in MAS. In general, novel MAS instrumentation based on SMA components falls into two categories, depending on whether superelastic, or shape recovery properties are utilised for a particular instrument design. This paper is based on research and development carried out in Dundee during the past 15 years on NiTi-SMA based instrumentation for MAS.

Shape Memory Alloy Fixator System for Suturing Tissue in Minimal Access Surgery

AbstractA new technique for suturing human tissue is described in which tissue closure is achieved by means of small fixators made from shape memory alloy. The aim of the development is to provide an alternative to thread suturing in minimal access surgery, which is quicker and requires less skill to achieve the required suturing quality. The design of the fixators is described in terms of the thermal shape recovery of shape memory alloy and a novel form of finite element analysis, which uses a nonlinear elastic element for the material property. Thermal analysis of the fixators and surrounding tissue is used to predict the temperature distribution during and after the application of electric current heating. This was checked in an in vitro experiment, which confirmed that deployment caused no detectable collateral damage to surrounding tissue. In vivo animal studies on the use of the shape memory alloy fixator for suturing tissue are ongoing to establish safety and healing effects.

Shape Memory Alloy Clip for Compression Colonic Anastomosis

This study was setup to investigate the design and performance of a shape memory alloy clip for colonic anastomosis. The thermo-mechanical properties of the shape memory alloy material were studied and the data were used to derive a nonlinear material model. This enabled the development of computer computer aided design models and finite element analysis of the clip and tissue compression. The maximum strain of the anastomosis clip was within the recoverable range, and it exerted parallel compression of the colonic walls with a uniform pressure distribution. The design of the anastomosis clip was optimized for safe, simple, and effective use in colon surgery.

Force‐sensitive tactile sensor for minimal access surgery

A new approach to detecting abnormalities in organ tissue, particularly in relation to minimal access surgery, is presented. Prototype sensors, based on piezoresistive material, were developed and assembled into a forceps for evaluation on simulated diseased tissue. Data on the resilience and location of phantom tumours were recorded and displayed visually for ease of interpretation. Both single sensor and multiple sensor arrays (one‐ and two‐dimensional) were manufactured and tested.