Come to think of it – The perfect fit

Delta, August 2005

‘The surgeon has to drill a hole through a metal cylinder in the mould that fits the shoulder perfectly. That way the shoulder prostheses will also fit perfectly.’ 

Fitting patients with shoulder, hip or knee prostheses is no picnic. With limited views and space to move around in between joints, it’s difficult for surgeons to drill, saw and screw in exactly the right spot and thus perfectly fit the prostheses.

Dr Edward Valstar of the biomechanics and imaging group (3mE) developed a computer programme in 2005 that allows surgeons to make perfectly fitting moulds using a 3D printer and X-ray data. Guidance channels or holes in these moulds help the surgeons perform their duties.

Cartilage however remains difficult to picture. “So the moulds didn’t always fit very well,” says Dr Valstar, who also works at Leiden University Medical Centre’s orthopaedics department and now wants to develop a mould that is adaptive, allowing small changes to be made to the mould once it’s in place. “You know that toy consisting of hundreds of metal spines that you push against your hand or face and it takes over the exact shape? Maybe we can use that same principle to make a patient-specific chirurgical instrument.”

Dr Valstar recently obtained a grant from the Netherlands Organisation for Scientific Research (NWO) to work on a technique that will enable surgeons to repair old hip implants much more efficiently. Over time, an inflammatory process is triggered in many patients by the prostheses’ worn off pieces of plastic or metal, causing, for instance, hip implants to loosen in the bone. Between the implant and the bone a liquid accumulates that must be removed.

“Surgeons usually completely open up the bone to scrape this sludge away,” Dr Valstar says. “We’re trying to figure out if we can’t clean up the implants differently by using ‘cutting water’, for instance, or ultra sound or laser.”

Developing a technique for monitoring implants is another of Dr Valstar’s occupations: “We add grains of the metal tantalum to the bone, which function as markers. In combination with a special calibration technique, these grains allow us to see, with an accuracy down to one-tenth of a millimetre, if any change in position of the implants has occurred over time.” This work recently earned Dr Valstar the Anna Prize, an award given by the Dutch Orthopaedic Research and Education Fund.

Wind force nine and a virtually flat sea: just the combination needed for some record breaking. These peculiar oceanic circumstances regularly occur in a bay in Namibia. Every year in October or November dozens of kite surfers head out to this bay.

In 2008, kite surfer Rolf van der Vlugt was one of them. Reaching a speed of 88 km/h, he became the new Dutch record holder. But that is not enough for him: he wants to beat all world records.

“In the 2008 race in Namibia, a French kite surfer became the world’s fastest sailor with an average speed of 93.7 km/h over a distance of 500 meters. The previous record (90.9 km/h) was held by a windsurfer. But now the record is held by a large hydrofoil trimaran, which in September 2009 sailed at a speed of 95 km/h. But I think kite surfers can go faster than that. Every possible perfection has been squeezed out of these trimarans, whereas the development of fast kite surfing equipment is just getting started.”
Van der Vlugt is not a professional sailor, but apart from that, one might say that he is doing everything in his power to break records. As a research assistant at the aerospace engineering group Asset (Aerospace for Sustainable Engineering and Technology), he can amass plenty of expertise, since one of Asset’s main activities is the development of kites that generate electricity by harvesting wind energy. Making the kites more aerodynamic is one aspect of this research.

On top of this, Van der Vlugt did his MSc research on the ‘Aero- and Hydrodynamic Performance Analysis of a Speed Kiteboarder’. ‘Breaking the World Speed Sailing Record’ was the subtitle of his thesis.
For his MSc degree, Van der Vlugt developed a model that describes all the forces that act on the surfer, the board and the kite. “With GPS receivers strapped to my body and to the kite and devices that measure the tension in the lines between me and the kite, I gathered data. It turned out that the drag that the body of a kite surfer produces is much more important than people imagined. Because the wind at low altitudes is much less than at the higher altitude of the kite, when you speed up you’re body quickly experiences head wind.”

Special strips on the body that create small vortices which reduce the wake, just like speed skaters use, might be the solution to this, Van der Vlugt believes: “But it’s difficult to figure out where you should put them, because the wind and your position on the board changes all the time.”
Still, it’s something he is keeping in mind for the next race in Namibia later this year. Are there any other adjustments needed? “I think I’ll also develop a suspension system on the board. Kite surfing is like doing a Formula 1 race on a bumpy road. With a good suspension system I should be able to have better control over the kite. I think I will also make the board a little bit longer, so that I can reduce the angle between the board and the water and reduce the spray of water behind the board.”

But is it wise to reveal all this in a newspaper? “Perhaps I shouldn’t tell everything, but I also like to share information. It’s a funny situation there in Namibia. We all want to break the world record, but we also have a common goal, which is that it will again be a kite surfer who beats all other sailors.”