Theoretical MTBF estimates are confirmed by performing Highly Accelerated Life Tests (HALT). These are stress testing methodologies that are used to improve the reliability of a product and greatly reduce probability of in-service failures.
This figure below shows an example of a HALT on a spinal brace. The fixture was put together very quickly using off-the-shelf parts. It includes a motor drive, sensors and a computer. It automatically tightens and releases the brace under real load conditions while the sensors track any brace failures. Five years of patient use of the brace are tested in a short time frame to provide reliability data.