The loads acting in joint prostheses and other orthopaedic implants is still partially unknown. The acting loads are required for different purposes.
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- The safety and stability of new and improved implants has to be analyzed and tested under realistic conditions before they are used in patients.
- Strength and wear of new implants, as well as their fixation in bone, can only be optimized if the real mechanical loads are known.
- After joint replacement or fractures patients need advise, which activities are permitted at which postoperative time.
- Activities which cause extremely high implant loads should also be avoided by people suffering from arthrosis.
- Physiotherapists need information about the best treatment and about exercises which may overload the implants during the first postoperative time.
- Surgeons and thus the patients profit from new insight into the influence of implant positions on the load magnitudes.
- Despite the improvements of computer technology it is still difficult to calculate the loads acting in the human body. Precise in vivo data will help to make such computer simulations more realistic.
History of Research Projects
In the early 1980th Georg Bergmann, Josef Siraky und Friedmar Graichen from the Biomechanics Laboratory of the Free University of Berlin, located at the Oskar-Helene-Heim (Director: Univ. Prof. Dr. med. Ulrich Weber), started a research project with the goal of directly measuring the forces acting in hip implants. The micro-electronics developed for this purpose allows to measure the contact forces acting in the joint for an unlimited time. The loads are transmitted by a telemetry at radio frequency from inside the body. First measurements were taken in 1988. Since then the measuring technique has been permanently improved.
- 20 internal fixators for the spine; this study is terminated
- 19 hip implants in 17 patients
- 8 shoulder implants
- 9 knee implants
- 5 vertebral body replacements
- Additionally to the acting forces we investigated in 5 patients whether hip implants heat up after longer periods of walking to an extend which may contribute to implant loosening.
Basis of the instrumented implants are clinically proven models. This guarantees the long-term success of the implantations. Inside the implants sensors are arranged which allow to measure the magnitude and direction of the acting forces and moments. A telemetry transmitter, only 9.5 x 6 mm in size, contains a specially developed chip and transmits the data wireless to outside the body.
To guarantee the safety of the patients, the following requirements must be achieved:
- With regard to their endurance and other features the instrumented implants must comply with the clinically proven models.
- The load signals can only be transmitted telemetrically from inside the body.
- The electronics inside the implants must be hermetically and permanently sealed.
- The mechanical stability of the implants must be warranted. To achieve this, the inner stresses are simulated in a computer. From the colours (picture) the areas of highest stresses are determined. These are then enforced until any risk of failure is eliminated.
- The mechanical stability is practically tested by applying 10 million loading cycle with extremely high force magnitudes.
During the measurements the patient wears a power coil and an antenna close to the implant. The magnetic field of this coil powers the measuring electronics inside the implant. Without this magnetic field the electronics inside the implant are passive and the implant behaves exactly like the standard model. In contrast to an energy supply by batteries this concept enables unlimited measuring times.
The transmitted load signals are received by the antenna. They are processed in a computer and directly displayed on a monitor. Herewith it can systematically be searched for activities which cause outstandingly high implant loads. An additional video documentation of all investigated activities provides a complete documentation of the measurements.
The recorded load signals and the belonging videos of the patient activities are analyzed together. The forces are displayed in a special manner. In the left picture the forces in different joints are displayed in load-dependent colours. Additionally the forces and moments are shown in time diagrams and are also available as numerical data (right picture).
Synchronously the videos are shown which allows, for example, to identify the point of time when the loads are highest.
The combined measuring results can be downloaded from the free public data base OrthoLoad. This allows not only scientists and implant designers to inform themselves about the loading conditions in several implants during various activities, bus also patients, surgeons and physiotherapists (Video 918kB).