In-Vivo Wear Scar vs In-Vitro Wear Scar

There are a few distinct differences between wear scars found on a retrieved hip joint and wear scars found on a simulator component. These become obvious, when measured with a high-resolution coordinate measuring machine.

To illustrate these differences, a femoral head and an acetabular cup from a simulator test have been measured and are compared to a retrieved head and cup. Both sets of components have been measured on an OrthoLux 4AB:

Let us first look at a joint from the simulator:

Worn femoral head after a simulator test and worn acetabular cup after a simulator test.

A few points should be noted here:

• The head is worn centrally at the pole. This is due to the specific wear simulator that was used during the wear test. Some simulators are capable of loading slightly off-centre to represent in-vivo conditions better.
• The wear scar on the cup is near-perfectly round. This is because the gait profile applied by the hip simulator is very controlled and repeats precisely with every cycle of motion.

And in comparison, a retrieval:

Worn femoral head after a simulator test and worn acetabular cup after a simulator test.

In this case, the following differences are apparent:

• The wear scar on the head is located away from the pole (the deepest point of the wear scar lies 35° from the pole) instead of on the pole.

• The wear scars on the head and cup are elongated and the transition between worn and unworn regions is smoother as a result of the more random nature of human gait.

Additional information on hip joint wear analysis can be found here:

How to Analyse Wear Geometrically in 4 Simple Steps.

The optical CMM used for these measurements is described here.