Bespoke stents

A new method of individually tailoring heart valves could lead to more treatments without the need for invasive surgery.

UCL biomedical engineer Silvia Schievano has developed a way to ensure the stents used in the procedure are designed perfectly for the unique shape and size of a patient’s heart. Schievano said a tailor-made stent was recently used in the surgery of a patient at the UCLH Heart Hospital in London.

With high-resolution CT images, Schievano demonstrated that a patient’s heart could be modelled using finite element analysis software and simulated surgeries could be performed on it to show the performance of a stent.

Following this, Schievano demonstrated that a rapid prototype of a patient’s faulty heart valve could be printed through stereolithography using a solid-state laser machine, meaning surgeons can now practise the procedure on a transparent model of the valve before beginning actual surgery.

The method for replacing a heart valve without invasive surgery was developed in 2000 by UCL cardiologist Philipp Bonhoeffer. The procedure sends a replacement valve surrounded by a compressed stent to the heart through a catheter inserted in a patient’s femoral vein, a large blood vessel in the groin.

The procedure has been used to treat more than 800 patients worldwide. Catheter aortic valve insertion followed in 2002 and has been used on thousands of patients. However, the shape of the faulty outflow tract that the replacement valve has to fit into is different for each patient.

Schievano worked with Bonhoeffer to find a more flexible alternative to the current stents made of platinium rhodium. The team decided on a new stent made of shape-memory alloy that can be fabricated in a variety of diameters and lengths.

The shape-memory material, Schievano said, makes the stent easier to implant. Currently the valve and metal stent are compressed and delivered to the implantation site with a balloon catheter. The balloon is inflated at the site of implantation to unfold the valve and expand the stent.

Schievano, an RAEng/EPSRC Research Fellow at the UCL Institute of Child Health and Great Ormond Street Hospital, said the new stent is self-expanding. It is compressed, she added, with a protective sheath on the catheter. When the catheter tip reaches the implantation site, the sheath is retracted and the stent is released. It then quickly recovers to its original shape.

The lack of balloon, Schievano said, makes the entire implantation device more compact and easier to manoeuvre through the body. The stent’s flexible material, she added, also makes it easier to slide around tight curvatures and the arterial wall of the heart.

The new stents were fabricated in the US by Medtronic. According to Schievano, they are made of zigzag-shaped nitinol wires sutured onto a fabric graft. Their shape resembles an hourglass: the extremities anchor the device to the implantation site and the central portion holds the valve, which is cut and shaped from porcine pericardium and sutured inside the stent.

The procedure is reportedly much less traumatic to a patient than open heart surgery, she said, and many patients will be able to walk without any problems in 24 hours. Schievano hopes the new specially designed stents will make non-invasive valve replacement surgeries available to a wider variety of patients, especially children.

The stents have undergone tests for durability and biocompatibility, and animal testing has also been performed. The stent has been used on one patient at the Heart Hospital in London, but mass clinical trials will not begin until January 2010.

Dr Schievano received recognition for her work earlier in the year when she was awarded the 2009 Sir George MacFarlane Medal from the Royal Academy of Engineering.

Siobhan Wagner