Next-Generation Heart Valve Stent Implants for Children

Using the latest advances in cardiac imaging and computer modeling, researchers at UCL Institute of Child Health (ICH) and Great Ormond Street Hospital for Children (GOSH) in London, UK, are developing a new generation of implants for children with faulty heart valves, potentially removing the need for open-heart surgery for thousands of patients every year.

Figure 1. Examples of different patients’ implantation site anatomies.

The procedure is based on an innovative concept developed by Professor Philipp Bonhoeffer in the late 1990s. A valve from a bovine jugular vein can be sewn inside an expandable stent and mounted on a balloon catheter for delivery. The catheter is simply inserted into a vein in the leg and advanced, through the vascular pathways, into the chambers of the heart. Once at the desired implantation site, inflation of the balloon deploys the valved stent and anchors it within the old, dysfunctional valve. This technique drastically decreases the risk of death and stroke associated with open-heart surgery and increases patient comfort, reducing the hospital stay to less than 24 hours.

This minimally invasive procedure has formed the basis for a successful clinical program, which to date has treated hundreds of patients. However, the non-surgical procedure is currently available for only a small portion of cases; less than 15% of the patients who need valve replacements. The reason is that everyone’s heart is unique. The shape and size are different from patient to patient (Figure 1), especially in children who are born with cardiac problems and have already undergone multiple surgeries throughout their life. The current device (Melody™, Medtronic Inc., USA), though lifesaving, was never designed to be a one-size-fits-all solution.

Simulation for Virtual Testing

The challenge for the research team is to find a way to develop implants for a wider range of patients, while ensuring optimal safety and reliability. Animal experiments in this area are of limited value because they are not representative of human anatomy. Bench tests are also of limited use, because of the difficulties in reproducing the in-vivo conditions with an experimental apparatus.

The researchers at ICH/GOSH, in collaboration with Medtronic Inc., plan to use the latest engineering technologies to create a virtual, realistic environment in which to test patient-specific implants, without the patient having to enter an operating theater.

Magnetic resonance (MR) and computerized tomography (CT) images of the patient’s heart are the input data to study cardiac structures because they provide a reliable representation of the patient anatomy and dynamics. This information can be translated in computer models (finite element method) that allow for a virtual simulation of the procedure (Figure 2). The results of the computer analysis give indications on the implant performances across a variety of anatomical settings, and guide the optimization process towards a robust final device design.

Unlike animal experiments and real life, computer simulations can be repeated a large number of times, in a relatively short time, and at low cost. The design of the device can be modified and tested using finite element analyses until the optimal implant for the patient is achieved.

Figure 2. Abaqus FEA simulation results after device expansion in a patient-specific implantation site.

For example, a common complication of the current device is fracture, which can lead to major clinical events. Abaqus FEA software from SIMULIA (Dassault Systèmes’ brand for realistic simulation) was used to understand the mechanical properties of the current device and to compare different innovative solutions. This led to the development of a “stent-in-stent” concept, where a first stent is placed before the valved stent. This appears to be an effective solution to reduce the rate of device fracture and increase the success of the procedure, without compromising its technical ease.

Rapid Prototyping a Heart “Copy”

Another engineering technology introduced in the study of valve implants by the research group at ICH/GOSH is rapid prototyping, commonly used in the manufacturing industry. A rapid prototyping system works as a printer in 3D: the machine uses a polymer to build a physical object, layer by layer. Using a patient’s MR and CT images as input, rapid prototyping creates a detailed 3D photocopy of the heart vessels (Figure 3).

Figure 3. (top) A patient’s right heart (pink) and spine (blue) reconstructed from MR/CT data; and (bottom) the correspondent rapid prototyping model.

Establishing the precise morphology of the patient’s valve implantation site with conventional techniques (echocardiography, MR, and CT imaging) is extremely valuable, but the 3D heart is still presented on a 2D computer screen. The use of rapid prototyping models lets the cardiologist and/or surgeon hold a 3D physical object representative of the patient’s anatomy in their hands. These models enable them to physically examine the structure of the patient’s heart prior to an intervention or surgery, and if necessary, trial the implantation of a device, testing its correct and safe placement, therefore enabling better decisions about treatment.

Engineering methodologies that create both physical (rapid prototyping) and virtual (finite element) models are instrumental to design optimal devices and progress the field of heart valve implantations. This may lead to significant reductions in manufactured prototypes and animal experiments, shorter learning curves for doctors, and lower numbers of device failures, thus increasing patient safety and enabling a larger number of patients to avoid multiple open-heart surgeries. The ultimate aim of this pioneering work is to develop methods that will enable the rapid translation of device development, via accurate models of the heart, into safe patient treatment options.

This article was written by Claudio Capelli, Ph.D. student; Philipp Bonhoeffer, Professor; and Silvia Schievano, Research Fellow, at the University College of London. For more information, Click Here