Measurement of Impact Loads Applied to an Implanted Drug Pump Connector in a Porcine Cadaver Specimen

Abstract

Anecdotal and documented reports from both patients and doctors have described unanticipated breaks in connections between implanted catheters and drug pumps. In extreme cases, such disconnections in patient-required therapies could result in either withdrawal symptoms or possible deaths. Patients typically attribute such device failures to falls or impacts associated with vigorous physical activity; subsequent failure analyses most often have indicated pump connector uncouplings. We fabricated a facsimile of the Medtronic®SynchroMed® II pump that included both an accelerometer and a force sensor. The force sensor measured forces imparted on the pump connector via the attached catheter and surrounding tissues. The test pump was implanted in the lower left abdominal areas of porcine cadavers in various orientations. Wire-reinforced catheters were tunneled for 20–25cm under the abdominal epidermis, anteriorly toward the head, and the non-connector pump ends were secured by sutures. Following each simulated implant, the cadaver specimens were loaded into a harness and hoisted to a height where either their buttocks or backs were 80–86cm above the floor, simulating a worst-case scenario in which a patient might have fallen down a flight of stairs or off a step stool. The cadavers were then quick released from the hoist attachment, while forces (X, Y, and Z) and accelerations (X, Y, and Z) versus time were simultaneously recorded. Six porcine cadaver specimens were utilized for a total of 72 trials. Subsequent Monte Carlo analyses allowed us to model the variation in stress imparted onto the pump connectors and the estimated variation of the pump connector strength, as a means of predicting required connector retention impact specification for future designs. The recorded forces applied onto the connectors, including data from all three connector axes (X, Y, and Z), were typically within the range of 4.5–9N. However, in several trials, applied forces ranged as high as 30–49N. Monte Carlo modeling provided a maximum resultant load specification of 100.4N for a 0.033msec duration. Based on this value, due to predicted impact events, subsequent failures of future designs would be estimated at 7ppm. Based on our data, a new design requirement has been generated to ensure that implantable drug pump connector assemblies will, in high probability, perform their intended functions.