7.4 Medical products: catheters
It is true to say medical technology has developed at a breathtaking pace with the introduction of new NDT examination methods for exploration of the inner workings of the human body, such as NMR, X-ray tomography and advanced ultrasonic scanning. While vital for diagnosis of disease, it has also stimulated the development of new surgical methods of intervention.
Doctors treating heart disease have many options for treatment. Angioplasty, for example, involves threading a catheter upon which a plastic balloon has been wound, into an artery. When the catheter has reached a congested part of the artery, where fatty deposits are restricting the blood supply, the balloon is inflated and the deposits flattened. Such a procedure has been further developed by enclosing the balloon in a small metal wire cage, called a stent, so that when the balloon is inflated, the stent deforms plastically and remains in place to support the artery permanently. A key part of the procedure is being able to observe the exact point of placement using X-rays or ultrasound.
Developments in medical technology have not been without problems however. Product failures have been investigated, and some of the failures have been widely publicised within the medical profession, which has led to improved designs. Other failures, such as with breast implants and hip joint replacements, have received widespread public attention, either because of litigation or because action has been taken by official bodies (such as the Medical Devices Agency, the government body that monitors failures of such products). Many new materials, as well as conventional materials, have been introduced, with mixed results, as the next case study shows.
7.4.1 New material for catheters
Catheters are small bore tubes by which fluids such as serum, blood, saline solutions, or drugs are supplied to a patient before, during, or after surgery or other medical treatment. A variety of polymers have been used in the past, such as plasticised PVC, but such materials can present the danger of plasticiser leaching into the human body.
Thermoplastic elastomers (TPE) offer a range of properties without the risk of leaching, so they have been adopted by many suppliers to hospitals. The case concerns a new type of TPE introduced in the 1980s, where the elastomeric properties were provided by short-chain poly-oxides, and the stability provided by physical cross-links of nylon segments scattered along the main chains. They co-crystallised into so-called hard blocks and so provided anchor points of stability for the material.
The TPE offers benefits for catheters, because it can be processed easily using conventional plastics processing methods, yet behaves like a vulcanised rubber. The stiffness can be modified simply by varying the hard-block content, producing at one end of the scale, a soft material, and at the other end a hard material, which are both tough and stable in use. Such a range of properties is not available either in conventional plastics or vulcanised rubbers.
7.4.2 Failed catheter
The product in this study is a catheter, about 1 metre long and with an outside diameter of 1 mm. One end of it had been sealed and provided with three small holes for the infusion of drugs into the patient – that is called the distal end – as shown in Figure 97. The other end – called the proximal end – was fitted with a socket into which a syringe could be inserted by the anesthetist to provide the appropriate drug.
The catheter-plus-syringe is used in childbirth, where the mother requests an epidural anaesthetic, which effectively eliminates any pain from the lower half of her body. The injection into the spinal column is carried out by an anesthetist by inserting a heavy duty hollow needle – a Tuohy needle – through which the distal end of the catheter is fed. The drug can then be drip-fed through the catheter into her spinal fluid. The whole device is provided in a sterile, hermetically-sealed pack ready for use by staff when needed.
In one such operation, on 18 August 1990, all seemed to go well, with the delivery of a healthy boy to a Mrs K. The catheter was found to have broken at the proximal end, but was mended satisfactorily, according to a witness statement from an attending nurse. Removal of the catheter at the end of childbirth occurs by withdrawal of the needle, and gently pulling the catheter until it slides out neatly. It should not be withdrawn through the needle, because the sharp edge of the hollow tip could cut the catheter and leave a chunk in the patient.
But something went wrong with the removal from Mrs K: the distal end sheared off at the proximal side hole and a small fragment was left in her spinal fluid. After consultation, it was decided there was more risk in attempting to remove the fragment than leaving the piece where it was. The fragment was small, sterile and apparently presented no further risk to Mrs K.
The patient was disturbed by the decision to leave the fragment in her spine, and decided to bring proceedings against the hospital. The first action of her solicitor was to engage an expert to examine the remains of the catheter, which had eventually been passed on to the manufacturer of the device.
It was clear the distal end was the key to determining the cause of failure, and the expert duly examined it in the offices of the solicitor representing the manufacturer. However, he was not allowed to remove the sample for microscopic examination, and had only a short period to examine it with a hand-held lens. In his opinion the failed distal end of the catheter showed traces of knife or score marks running across the failed surface. It had probably been withdrawn incorrectly by the anesthetist.
7.4.3 Microscopy of the failed distal end
The matter rested there for some time before the hospital engaged its own expert. In the interim, an expert had also been hired by the manufacturer, who was also a party to the dispute (in the event that the catheter itself proved faulty). The three experts needed to re-examine the failed end to provide firmer evidence of the failure mode. The failed device was provided to the experts, and given the 1 mm diameter size of the failed end, they decided to use a scanning electron microscope to provide clear images.
Rather than coat the end, it was agreed to examine it using the then relatively new technique of environmental SEM (ESEM), so that this critical piece of evidence would be preserved intact (see Box 23). The method produced an image that was capable of detailed analysis (Figure 98). The interpretation of the failed end by the expert acting on behalf of the hospital is shown in Figure 99.
Box 23 Environmental SEM
The main problem with conventional SEM is the need to coat the sample surface of non-conductors with carbon or gold so as to conduct away the incoming electrons. If this is not done, they build up on the sample, and inhibit image formation. A very high vacuum is needed in conventional SEM because air molecules scatter and absorb the electron beam. A recent innovation, however, allows a small bleed of gas to pass over the sample being examined without entering the main column of the instrument (Figure 100).
As the primary electron beam hits the surface, any electrons that stay on the surface are neutralised rapidly by reaction with positive ions formed by interaction of the primary beam with the gas molecules bled into the microscope. This enables the electrons to be carried away from the surface of the sample, so preventing the unwanted build-up.
There are several versions of the new method available. In the version used for the failed end of the catheter – so-called low-vacuum SEM, or LVSEM – the pressure is kept rather low at about 30 Pa. But higher pressures are available in some instruments, rising up to about 2000 Pa.
The prime interest in these instruments is for observing living things, or materials that would deteriorate rapidly at low pressures. Water absorbent fibres and wood lose water rapidly at low vacuum, so suffer damage. Care is still needed for all polymers however, because the highly energetic electron beam – accelerated through 20 000 volts typically – can itself cause direct damage to samples by chemical reaction with the polymer chains.
Examine the failure surface map (Figure 99) produced by one of the experts. Summarise what the map shows in terms of the behaviour of the normally tough TPE material. Are there any traces on the failure surface that indicate the catheter was cut by the edge of the Tuohy needle?
Indicate the stress concentrations you would expect from the distal end of such a catheter if it were strained in tension along its major axis. Attempt to provide a quantitative estimate of the net stress-raising effect. The expert has marked apossible origin on one exterior surface of the sample. Is this a credible interpretation, given the presence of several severe stress concentrations?
The failed distal end of the catheter shown in Figure 98 exhibits both brittle and ductile behaviour. The brittle nature of the areas either side of the hole in the side wall is shown by the large flat zones, which also show contamination from dust and other debris on these relatively featureless zones.
There is, however, clear evidence of ductility in the remaining parts of the failure surface. In Figure 98, the quadrant between 7 o'clock and 11 o'clock shows two tear zones leading to a ductile tip. Moreover, there are traces of fibres pulled from the surface at 3 o'clock, adjacent to the edge of the side hole. The overall surface appears unusual for a normally tough and ductile material.
When a tough polymer is cut by a knife in a single event, tiny imperfections on the blade usually produce lines running straight across the surface. Also, a ductile tip is often created where the blade edge leaves the surface. There appear to be no parallel markings on the fracture surface. Alternatively, it might be argued that a brand new needle would have no such imperfections, and that the absence of such marks could not be taken as evidence that the sample had not been cut. On the contrary, the large flat zones would be what one would expect from a single cut.
The major stress raisers exposed in the failed surface of the catheter are:
the large hole in the side wall of the catheter;
two inner corners to the hole.
The inner corners were probably created when the hole was made during manufacture, probably by a hot needle. The standard effect of a hole is to produce a Kt value of about 3, although in a sample like this, the effect may be larger because the size of the hole is comparable with the inner bore. The effect of the corners can be estimated using Figure 40, Box 14. The relevant dimensions can be estimated directly from the failure surface map or photograph. Taking r to be about 2 mm, and d to be about 24 mm gives
r/d = 0.08
So, reading from Figure 40 gives
So the net stress concentration is at least 3 × 3 = 9.
The problem is that there appears to be no clear origin either at or near the edge of the hole, which would be expected from this calculation. On the contrary, the only possible origin shown on the surface appears to be some distance away, on the outer edge of the tube, and adjacent to a large flat zone. The failure surface shows apparently contradictory features; it appears not to offer an easy explanation.
The failed surface was clearly rather complex, showing areas of both brittle and ductile behaviour in a nominally tough material. The large flat areas could be interpreted as cut areas, with a ductile tip representing the point at which the sharp edge of the needle left the tube. This would fit with a supposed entry point on the opposite side of the catheter, near the proximal side hole. On the other hand, there were no traces of parallel markings from the minute defects in a knife edge. Further investigation was clearly needed, but what was the next step? Comparison of the failure surface with other parts of the failed catheter, such as the failed proximal end, and a deliberately cut catheter could be useful.
In an effort to produce further evidence of the state of the tube, conventional SEM was used to examine the state of the catheter elsewhere. Several other areas of the intact catheter showed traces of brittleness (Figure 101). Taken from a broken edge well away from the failure surface, the conventional SEM micrograph shows a completely brittle fracture surface, with hackles and sharp edges characteristic of completely brittle behaviour – just as you would expect from, say, broken glass. In addition, a sample of tough new catheter was deliberately cut with a Tuohy needle, producing the failure surface shown in Figure 102. The cut surface shows some features that are common to the failed catheter end, such as two areas of ductility and a flat area. However, there are parallel marks showing defects from the edge of the needle.
7.4.4 Material and mechanical analysis
If a product shows traces of brittle cracking when it is normally tough and ductile, it is essential to check the material has not degraded in any way. So samples of new and failed catheter were subjected to DSC, FTIR, and even NMR in an effort to provide some evidence of its quality. No degradation was found at first, although the exercise was useful in determining the composition of the block copolymer. As was found in the case of the cracked radiator seals described in Section 5, minute traces of degradation are often easy to miss using spectroscopic methods. Attack is concentrated at the tip of a crack or cracks, so the spectra are swamped by normal polymer.
However, there was one piece of evidence that was obtained using FTIR microscopy. This is a method where a narrow infrared beam is passed through areas of the sample selected while in an optical microscope, so that very small parts of a sample can be analysed. The result of one such experiment from the expert representing the manufacturers is shown in Figure 103, where the failed catheter is shown by the lower curve, and a good catheter in the upper curve. Quantitative analysis from the base lines shown indicated the presence of significant amounts of esters and other products in the material.
An inspection of the scientific literature on the worldwide web produced an important article by a group of French chemists, the abstract of which is shown in Figure 104. Their work showed photo-oxidation by ultraviolet rays attacks the amorphous or elastomeric parts of the molecules, producing esters – among other degradation products. As UV attack cuts the main chains, the strength can drop rapidly where exposure has occurred.
Tensile testing of the material also showed the strength of new catheters, but several entirely brittle failures were also obtained from the length of failed catheter still available. As before, the experiments were conducted with all the experts present, so that later, when the case came to trial, there could be no argument among the experts about the validity of the experiments – arguments are a possible problem where experts perform their own research in isolation. Figure 105 compares the load-elongation curves from a good and the failed catheter.
7.4.5 Degradation hypothesis
At this stage, the experts still disagreed about the cause of failure, but the possibility of degradation had been strengthened by the observation of traces of foreign ester groups in FTIR microscopy. Discovery from the makers of the catheter produced information about extrusion and sterilisation, enabling a traceability diagram to be constructed (Figure 106). The diagram shows at left the unknown dates of the various events in the history of the failed catheter, while the right-hand column shows the various unknown conditions of the various manufacturing processes used.
Exposure to sunlight is unknown at all stages, but could have been considerable. No quality control procedures had been revealed that could have caught any degraded material. Excessive heat during extrusion could also have damaged the material. Most significant of all in the sequence is the gamma radiation exposure step, the final step where the product was sterilized. The problem arises of further degradation here if earlier exposures to sunlight had occurred. Gamma radiation is powerful, and would have exacerbated any damage to the polymer chains. But no other failures of such catheters had occurred, so what was unique about this catheter?
Consider the traceability diagram of Figure 106, and determine any stages where photo-oxidation could have occurred. Supposing you were the expert acting for the manufacturers, what action would you take to check the degradation hypothesis? What quality records are likely to have been made at the time? What counterarguments would you use to challenge the degradation hypothesis?
The traceability diagram of Figure 106 shows several stages when photo-degradation could have occurred. Attack at the granule stage is unlikely, and the degradation would be distributed uniformly through the product by melting and mixing in the injection-moulding machine. Degradation is more likely on extruded tube, and should include any stage when the product could be exposed to sunlight. Degradation could have happened in any of four ways – when it was:
stored after extrusion;
cut into lengths and heat processed;
taken from the package early at the hospital;
taken from the package prior to the childbirth.
Acting on behalf of the manufacturers, the expert would check the internal quality records, which were never made available in discovery. The tubing was actually extruded in Ireland, so the search for records would start there. Quality records would include extrusion machine printouts that recorded temperatures and pressures for the specific batches in question (FLA 234/5). Although an extrusion problem is unlikely, it cannot be excluded. The product is made in batches, and there would be a period when non-specification tubing would be produced at the start-up of the machine – analogous to the problem in injection moulding that probably created the faulty radiator reservoir.
The most important quality records needed for examination would be the testing of small samples of tubing taken at random during production of a batch. The expert would be looking for any drop in mechanical strength – if such a test had been employed. There would also be, no doubt, an internal standard that specifies the frequency of testing and the kind of testing to be used, and at what stage the sample would be taken. There would probably be a random check after packaging, and possibly after the critical radiation sterilisation stage.
The expert would probably start by suggesting any exposure was caused after the supply of the catheter pack by the manufacturers, perhaps by a nurse opening the pack some time before it was needed. It might then have been left on a window ledge exposed to sunshine. Brittleness was confined to specific zones, and most of the catheter was tough – otherwise it could not have been used at all. It suggests accidental exposure by the hospital itself rather than a manufacturing fault.
But is pre-use degradation the answer? The theory is open to attack from several angles. Firstly, the catheter had probably suffered degradation in the period since the accident, because little attempt was made to protect the device from sunlight – although this point goes against the manufacturers, because they kept the device for several years following the accident. Secondly, could the catheter have been exposed to solvents, causing ESC or SCC? Hospitals use many organic solvents that might have attacked the material at isolated zones along its length, if solvent had been spilled. The expert would ask for further details from the attending nurses, and new witness statements to clarify the matter. No doubt, they would also ask the manufacturers for details of any known cracking or crazing agents, and then cross-check with the new witness statements. There is clearly some scope to attack the pre-use degradation theory.
One possibility that could have caused a rogue length of tubing could have been exposure of a coil of extruded tube to sunlight, shortly after manufacture in the Republic of Ireland (stage 2 of Figure 106). Exposure to sunlight could have damaged just a few outer layers of tube, just enough to initiate photo-oxidation and leave damaged chains. It is also important to appreciate that such a catheter would incorporate no additives to protect the material against such attack – something that is normal for other, non-medical products. The reason for using virgin material is to prevent leaching into the patient.
7.4.6 Expert meetings and joint reports
A final meeting of the three experts was fraught, because they could not agree over the balance of causes that led to final failure. The expert for the claimant still maintained that the anesthetist could have cut the catheter, although support for his case was weakened by detailed analysis of the fracture surfaces (which he recognised). The expert for the manufacturer was, however, resistant to any problems with the catheter as supplied to the hospital, and proposed a joint report be prepared for the court. This is an option courts do prefer, especially as it means the judge has less reading before the trial. The issues raised by expert evidence are frequently just as complex as shown here, and at the end of the day, the court has to rule on which opinion it prefers. With one agreed report, argument over causation is limited. This might be an easy task for just two experts, but becomes more difficult where three or more are involved. Two of the experts agreed to produce a joint report, while the third – acting on behalf of the hospital – produced an independent report. Alternatively, courts require an agreed statement of what is agreed and what is disagreed between the experts. It was not produced in this case, owing to a breakdown in relations between the experts.
7.4.7 How was the catheter stressed?
There remained one final issue, which puzzled the expert acting on behalf of the hospital. That was the problem of the failure itself. Some stress must have been put on the catheter during withdrawal, irrespective of the material properties. The catheter should slip out easily from the spine, so how was stress induced? The answer came by questioning a hospital consultant in the obstetrics department, and finally, by direct examination of an intact catheter from a recent epidural. The end of the catheter was deformed, mainly due to pressure from the surrounding tissues during withdrawal, and such permanent strain was commonly observed following delivery of the baby (Figure 97). It would clearly require considerable stress to remove the tip of the catheter, so the problem was resolved.
Construct a diagram showing the likely sequence of events, starting with the polymer granules and ending with the failure of the catheter. Indicate on the diagram the evidence on which each event is based. Indicate the main areas of uncertainty in the sequence, and what action could have been taken to reduce that uncertainty.
Based on the information supplied in the case study, Figure 107 indicates the main sequence of events leading to the final failure – some of which is inevitably conjecture. Down the right side of Figure 107, pieces of evidence suggest or corroborate the event in the box.
The main areas of uncertainty are below. In brackets, are some actions that would reduce the uncertainty.
Possible photo-oxidation after manufacture. There is no direct evidence of exposure to sunlight. (Take witness statements from staff in Ireland; plus do further FTIR microscopic examination of the failed catheter to improve the resolution of spectra.)
Possible further gamma radiation degradation of catheter. (Extend literature search for any information on gamma radiation effects on copolymers, nylons, or poly ethers.)
Interpretation of fracture surface features. (Clean sample and re-examine; plus do FTIR microscopy on failed end.)
The case was settled by mutual agreement between the three parties the weekend before the trial was due to start in the High Court, with substantial savings in costs. The hospital paid its own costs, and the manufacturer of the catheter paid damages to Mrs K.
The case highlights many of the problems faced by investigators not just in medical cases, but also more generally. Firstly, hasty attempts to study the surviving evidence can often lead to misleading conclusions. In the catheter case, the initial examination of the failed catheter tip with a hand lens had produced a mistaken inference that the device had failed by being cut, despite the evidence of brittle behaviour elsewhere on the tubing. It is perhaps a natural human response to a request for assistance to provide an instant explanation, but is no substitute for deeper, more penetrating analysis.
Secondly, the sample had not been preserved at all well in the period between the accident and the deeper investigation the device demanded. The failed catheter tip was badly contaminated with debris – presumably from a solicitor's office – and no attempt had been made to store the device under dark conditions so that no deterioration could occur. It is likely that further damage had been created by exposure to light in the four years between investigations. Part of the problem lies in the long time it can take to bring court actions, and the lack of expertise among those holding key specimens upon which a case might hinge.
A third problem lies in the new materials and process methods that have been introduced so rapidly there has been no time to appreciate some of the subtleties of property variation with time, degradation, interaction with other substances, and so on. The catheter was made from a new kind of thermoplastic elastomer, where knowledge of its sensitivity to its environment was known and published – like the French paper in Figure 104. It is uncertain, however, whether or not that information was available to, or had been acted upon by the quality control staff at the device manufacturers.
The lack of communication of key information on material properties also, paradoxically, extends to materials that have been available for much longer periods. Silicone rubber has been used in many medical devices, but the design of some has led to serious and extensive failures, that of breast implants being a notable example (Box 24). This elastomer is notoriously sensitive to fatigue, and is in any case not a tough material, showing poor mechanical behaviour. Designers using the material for products that will experience stress in service, must recognize this fact and design in ample fail-safe features to withstand such stresses.
Box 24 Breast implant failures
Extensive reporting of breast implant failures started to occur in the USA in the late 1980s. Most of the failures have occurred from permanent implants made from silicone rubber filled with silicone gel. However, saline implants are often used after the mastectomy operations that are done to treat breast cancer. They are expanded incrementally by repeated additions of saline solution to a silicone pouch, so that the tissue slowly expands to give a space ready for a permanent implant. Such tissue expanders have also failed, especially towards the end of the filling stages, when the load is greatest.
One such failed tissue expander is shown in Figure 108. The fracture occurred at the joint between the catheter to the upper dome, used for injecting saline, and the bag itself.
The material of both catheter and bag is crosslinked silicone rubber, an inert material, so good for implants, but rather weak mechanically – so bad for stressed implants. The junction failed suddenly and caused considerable distress to the user. So what caused failure? Examination of the failed surface in the ESEM showed a crack had grown in one event from one side of the junction, where there was a sharp corner between the catheter and the bag. See the fracture surface map in (Figure 109).
There was a single origin at the junction, but there were also several areas of microcracks elsewhere in the junction. Presumably, the stress concentration at the sharp corner led to rapid growth of one of the cracks there. But what caused the microcracks to develop in the first place?
It was suggested that one possible cause might be the adhesive used to bond the two parts together, because if the density of crosslinks is too high, the adhesive will be both stiffer and less tough than normal. Normal stresses from body movement may therefore have caused minute cracks to develop early in its life, so that when the load from the bag was greatest, the largest one at the junction became critical.
As silicone rubber is known to be weak when tensioned or fatigued, care is needed in the design of critically stressed areas such as junctions. Any stress raisers should be ameliorated as far as is possible, and the device provided with substantial redundancy, or fail-safe features.
Two of the three cases studied in this section under the heading of piping problems were settled before trial, a common result for product liability claims. An approaching trial sharpens the issues, and expert meetings focus on the shrinking areas of disagreement between the experts. Further key tests are performed quickly, attempting to clarify and define those issues more closely.
One area of uncertainty in many medical cases is the question of stresses in the body. It goes beyond the problem of the storage tank in Section 6.2, where the contents were of known mass, and the wall loads could be calculated easily. True, the mass of a breast implant contents are fixed and known, but there are extra loads imposed by the body, which are not known in detail and can be difficult to determine exactly. A woman with an implant will impose extra loads when exercising, or when moving the upper part of her body, or even when just sleeping. For similar reasons, a designer should provide the greatest possible safety margin when specifying catheter diameters, and the amelioration of known stress raisers. Medical failures are a key part of the learning process, but should not displace well known engineering principles.
Surgeons who help with device development go to great effort to inform colleagues of their own failures, perhaps the best example being heart surgery. If mistakes happen, the life of the patient may be at stake, so there is a clear incentive to publicise failure in specialist learned journals, and learn the lessons for product design. For example, there have been many failures of stents during angioplasty operations in arteries, but there has been a feedback loop to stent designers to improve the materials of construction, and also to maximise the balloon strength. Indeed, heart surgeons have a special kit of tools designed specifically to retrieve the remains of stents should they fail mechanically in the body.
No doubt the importance of adequate quality control testing and sampling has been appreciated by the extrusion company that made the catheter, and the medical manufacturer who supplied it to the hospital. The requirements of regulatory bodies (such as the MDA in the UK) are stringent, and rogue or maverick products should not enter the supply chain. The catheter was on the edge of failure, but still possessed enough residual ductility to allow its use before the failure occurred. In the event, the medical product company was taken over by another during the time it took for the action to come to trial in 1995, due liability being assumed by the original insurers.
Describe the key issues where the experts in the catheter case agreed, and those where they disagreed. Draft a one page statement of areas of agreement and disagreement to inform the legal teams in the case, and the court. Assume the experts meet on the Friday before the start of the full trial the following Monday. Suggest why the case is settled over the weekend, just before trial.
The three experts agreed about the following matters.
The catheter cracked in the early stages of the childbirth, and failed only at the last stage of the procedure.
The distal tip of the catheter was left in the spinal column of Mrs K.
The catheter was made by extrusion in the Republic of Ireland and then modified by cutting to length, one end sealed and three holes cut near the tip for drug infusion.
The tip of the catheter failed across the width of the tube, across the proximal hole. The failed surface showed traces of ductility, together with flat zones.
Mechanical testing and comparison of new and failed catheter tubing showed that the new product was tough and ductile, but some parts of the failed catheter were brittle.
FTIR, DSC and NMR results generally showed the material of the failed polymer tubing to be normal, although one spectrum was abnormal.
The main areas where the experts disagreed were as follows.
The cause of failure of the catheter at the end of the birth. The claimant's expert maintained it had been cut by the anesthetist.
The degradation of the catheter during manufacture. The manufacturer's expert probably believed either that it may have degraded by exposure at the hospital, or alternatively, that it had been cut by the anesthetist.
The expert acting on behalf of the hospital believed that, although the direct evidence was thin (one FTIR trace), the admitted brittleness of the catheter during the epidural could be explained by degradation. In addition, the failure surface showed brittleness as well as ductility.
The joint statement (if it had been made) might have been constructed as follows, although there are many ways the statement could have been expressed. Needless to say, the exact form of expression and the slight variations in meaning are quite critical to any ensuing trial.
Statement of areas of agreement and disagreement
The experts agree the catheter failed when the tip was pulled from the spinal column of Mrs K, in the final stage of the birth. They agree the catheter probably showed brittle behaviour prior to the final failure. They disagree about the cause of the failure.
The experts agree the failure surface when examined using ESEM, showed traces of ductility and other flat zones. They disagree about the interpretation of the flat zones.
The experts agree that when other parts of the failed catheter were tested mechanically, several brittle fractures were observed. They disagree about the interpretation of the brittle state of the catheter.
The experts agree that chemical analysis of the polymer of which the catheter was composed showed, in one example, traces of abnormality. However, they disagree about the implications for the strength of the material.
The case was settled because the strength of the evidence for brittleness in the catheter before the tip finally broke during withdrawal was strong, and could be explained by some of the chemical evidence. That explanation was supported by independent evidence from the scientific literature – the French paper. In addition, the fracture surface itself did not show the parallel marks characteristic of cutting by the Tuohy needle. And it was clear from the direct evidence of medical staff, that the tip was stressed during the final stages of the operation. It was likely, on the balance of all the evidence, that the material had been degraded before the operation started. The chances of degradation during manufacture were greater than at the hospital. Liability therefore lay with the manufacturer rather than the hospital. A trial would have sharpened the arguments, and would have tested the witness evidence, but it is unlikely that a credible alternative explanation would have emerged.