Joint efforts - How to simulate a shoulder

By Katharina Kramer

With a click and a sound, the machine starts moving. A light is glowing on one of the modules, signaling that it is ready to operate. Slowly it turns to the front, twists a bit and then moves backwards again. The movements remind of those of our arms, swinging back and forth, changing only by a few little jabs on the control stick. What sounds like a futuristic robot is actually a machine built for the research group of Daniel Smolen to test artificial shoulder joints in various projects of the shoulder team.

Max Warendorf designed, constructed and built the machine as part of his Master thesis project with the TU Vienna and the Ludwig Boltzmann Institute for Experimental and Clinical Traumatology. “The machine is a biomechanics simulator”, Max explains. “It helps test motion sequences, strength and agility of artificial shoulder joints”, he continues. The aim is to develop and test an artificial joint that is better than the ones that already exist in order to enhance movement ability and the quality of life of patients that need new shoulder joints.

New and artificial shoulder joints are needed when the old joint is damaged to the point where it causes the affected person a lot of pain and limits their mobility. Damages arise mainly in elderly people due to chronic diseases such as arthrosis. With arthrosis, the cartilage that covers both the socket and the joint head wears off as the surfaces of the two parts chafe against each other.

The machine is a biomechanics simulator. It helps test motion sequences, strength and agility of artificial shoulder joints.

Problematic thereby is that the cartilage does not regenerate itself so that the damage worsens over time until the joint has to be replaced. In Austria, almost 30 per cent of the population that is over 60 years old suffers from arthrosis as a health survey from 2014 by the Austrian Ministry of Health shows. To help these people, good and reliable artificial joints need to be developed and tested.

The construction process: from idea to implementation

Building such a machine that simulates processes of the human body is very complex and takes a lot of expertise. Three different main steps are involved in completing the machine: the construction, the electronics and the programming. “You start out with a sketch and think about how the machine could look like or what kind of movements it needs to be able to do”, Max says. Afterwards, a lot of calculations are done to determine the proportions as well as the strain and forces that act upon the machine. “When all the calculations are finished, I can begin with the construction on the computer and then start building and assembling the different parts”, Max explains. To bring the construct to life, electronics such as microcontroller that make the different parts move need to be connected and subsequently programmed.

“All these steps are done more or less simultaneously to ensure their compatibility”, Max relates. 

Holding one part of the machine with his hand, Max loosens one of the screws on the main module to adjust the movable arm. He secures the socket of the artificial joint to the fixated part of the machine and carefully attaches the joint head to its designated position. With one look at the control board and a few clicks that adjust the strain put on the joint, the testing can start. A soft whirr can be heard as the machine begins to move. This time programmed to simulate the motion of our arms when we stretch, the joint head is moved upwards and twisted a bit to the side. After the testing is done, Max takes one last look at the control board and turns off the machine. With another sound the machine goes back into its initial position and the lights flicker out.