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Tracking joint particles

BioPhotonics
Dec 2008
Anne L. Fischer, anne.fischer@laurin.com

Replacement joints are tricky. The joints’ constant movement can cause particles to wear off and to begin a reaction within the body that can lead to loosening and to yet another installation. Because this is not a fun scenario, researchers have sought ways to determine which particles cause problems and which materials work best.

A group at the Institute of Medical and Biological Engineering at the University of Leeds in the UK is using the Nanoparticle Tracking Analysis (NTA) system from NanoSight Ltd. of Salisbury, also in the UK, to learn more about the type and size of debris from implants. Led by Dr. Joanne L. Tipper, the investigators are studying the debris produced in in vitro joint simulators and in vivo from tissue surrounding failed hip replacements.

Tipper explained that the particles cause a “foreign-body-type response that results in the loss of bone [resorption].” As the particles slough off, she explained, they are attacked as foreign invaders by macrophages, resulting in bone loss and causing the replaced joint to loosen, to become painful and to require replacement.

“Particles are bad news for patients,” she said.

The researchers have worked on developing methodologies for isolating and characterizing particles to determine the extent of the problem with specific materials. They have examined the debris from hip replacements, although particles result from all joint replacements. Hip debris has been especially conducive to their study because the particles are smaller than those from a knee joint, for example, and they can cause even more trouble.

Motion tracking

The NanoSight system is based on a conventional optical microscope and uses a Class 1 laser device (Firefly LM10) that outputs 30 mW at 640 nm. Particles from 10 to 1000 nm are introduced to a liquid in the viewing unit and are visualized by the light they scatter from the laser source. The particles are filmed with a CCD monochrome digital video camera – Marlin F-033B – from Allied Vision Technologies of Stadtroda, Germany, which comes with the microscopy tool. By tracking the motion of each particle from frame to frame, the software relates it to a sphere-equivalent hydrodynamic radius calculated through the Stokes-Einstein equation.

The investigators previously used a field-emission gun scanning electron microscope to take conventional images and employed traditional image analysis tools to characterize particles, but as Tipper pointed out, that was “long-winded and tedious.” The new method is quick and easy, but one challenge lies in correctly sizing polyethylene materials because the materials tend to be “sticky … and the particle size can be overestimated.” The technique is most accurate with metals and ceramics suspended in water.

Once the images are captured, sized and counted, the results are displayed as a frequency size distribution graph and are output to a spreadsheet. “This enables us to compare the particle distributions from different materials produced using different size analysis techniques,” Tipper said. Manufacturers use the data to help understand the extent and characterization of wear debris.

The researchers plan to work on standardizing the sizing of ceramic and cobalt chrome wear particles to determine the size range of particles from other devices, such as surface replacement metal-on-metal implants. They also will be sizing particles before culture with various cell types, including fibroblasts and macrophages.


GLOSSARY
resorption
The absorption of a material by a medium or system that has formerly been released from absorption by that same medium or system.
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