How to make better and long-lasting artificial hips.

The “unofficial title”: How to let grandma cha-cha again!

WHY?

Hip replacement surgery replaces a worn out or damaged hip joint with an artificial joint called a prosthesis or implant. This prosthesis or artificial hip need to be not only biocompatible but long lasting to avoid any revision surgeries.

Even after having tested numerous medical devices through the year, each new request is typically looking at different aspects of these products. For the artificial hip, the manufacturer must consider many different qualities of the ball / cup interface: biocompatibility, low wear, low friction and surface integrity.

Many models exist that use different sets of materials.

The product that we tested was the ball head side of the implant and was coated with diamond like carbon (DLC). The goal of the tests was to answer the questions from the manufacturer regarding wear, friction and surface integrity for this product and compare their older products.

Testing conducted

The ball of the implant was tested in different configurations with worst case scenarios stresses.

Different modules were used on the Rtec Instruments MFT 5000 to conduct all tests. The tribology part of the test used normal and lateral forces mimicking the loading of the implant for someone jumping.

Results

The head of the device was mounted directly in the instrument for testing. Indentation, wear and scratch testing were spaced enough to avoid the interaction of deformations.

Mechanical properties

The first thing is the understanding of the material properties that play a significant role in the strength of the implant. A series of 10 indents were made on top of the implant head to calculate the elastic modulus and hardness. Indents were made according to ISO 14577 (100 mN/30/10/30)

The new product exhibits higher modulus and hardness than the old ones. This confirms that the DLC formulation and deposition were improved on this new product.

Wear

The study of wear is critical to ensure the implant can withstand the general stresses seen in the hip joint without damaging the surface of the head. Wear debris could also create an inflammation response in the body (not good!)

The wear of the product was measured using a 4 mm sapphire ball in reciprocating motion. Testing followed ASTM G133 protocol. Each wear groove was measured using the confocal microscope to evaluate the volume of material lost.

The wear track volume showed an improvement of 37% on the new product compared to previously tested products. The combination of the coating material and the deposition technique improved the wear resistance of the new product significantly. Very few particles are seen at the end of the wear track. Their presence can also be explained by the more drastic contact (sapphire on DLC) than what the implant will see (UHMWPE on DLC).

Friction

To provide a good range of motion without any difficulty, the implant needs to have low friction.

The friction coefficient was measured during the 300 minutes linear reciprocating wear test. Both new and old products were compared to see the possible improvement.

The new product shows an improved behavior at two levels. The first improvement is a shorter and less dramatic run-in, while the second is the lower coefficient of friction.

Scratch resistance

Scratch testing is used here for two reasons: first, it is the most accurate way to quantify adhesion of thin coatings, second it illustrates the possible damage that the implant would see if scratched by a surgical tool or small foreign object.

Scratch tests were conducted with a 2 μm diamond tip to keep the stresses in the coating as long as possible.

Three main failures were observed: the first cracks in the coating (Lc1), the first chipping (Lc2) and finally the complete removal of the DLC coating (Lc3). Lc1 of the new product shows an improvement of 22% compared to previous products. Lc2 improved less than 10% for the new product while, Lc3 didn’t show statistically significant improvement.

Conclusions

It is always great to deliver the good news to a customer! This was the case with a new product outperforming their old ones and providing a longer lasting implant with better coatings and processes. The combination of tribology (wear, friction), indentation (modulus & hardness) and scratch testing on Rtec Instruments, provides a more complete analysis of the product surface.

These tests allowed the customer to clearly quantify improvements between old and new products along with measuring the impact of new production processes.

Grandma can be confident in her hip the next time she goes dancing!