The aim of this project is to develop a prototype of an instrumented hip endoprosthesis that can measure cup wear and femoral head decentration. With this system, a necessary revision of the HTEP should be identified more easily and quickly and more precise data on wear and decentration should be collected. This should ultimately improve the situation for patients. Severe pain does not have to occur before a revision is necessary. In this way, an early intervention can take place and costly revisions, in which not only the endoprosthesis head and the cup inlay, but also the endoprosthesis cup and possibly also the endoprosthesis stem have to be changed, can be avoided.
In contrast to the currently used procedure, the system should have the following characteristics:
- Higher accuracy
- Simple operation with a low level of knowledge
- High durability
- Widely applicable
- Better planning reliability
- No radiation exposure for the patient
- Low time expenditure
The wealth of experience gained in the project should be transferable to most endoprostheses with mechanical loading.
To achieve this requirement, the following main work objectives can be defined:
- Development of a novel sensory measuring device that can detect head decentration caused by abrasion on an endoprosthetic hip.
- Development of an energy supply and data transmission device as well as corresponding peripherals and a read-out unit with which measurement results can be transmitted in vivo.
- Creation of an installation space within the endoprosthesis that is adequate for the measurement system.
- The external dimensions and the familiar appearance are not changed.
- The mechanical properties are adequate for the typical loads of an endoprosthesis, despite the created installation space.
Furthermore, the following essential sub-work objectives can be defined:
- Development and construction of a validation test bench
- Creation of a functional model for the complete measurement process
- Creation of a peripheral unit with which the measurement can be carried out
- Power supply
- Processing of the measurement data
- Finding a suitable manufacturing process for an endoprosthesis with measuring equipment
- Creation of a functional model
In the following, the mode of operation of the intended measuring system will be described in its entirety. For better clarification, a measuring system ready for series production will be assumed in order to explain the potential.
The total hip endoprosthesis with measuring device for head decentration measurement is implanted in a patient like a conventional endoprosthesis. No changes occur for the surgeon. After the successful operation, the patient comes to the clinic or to his or her orthopaedist in private practice for routine check-ups at regular intervals as usual. The patient stands upright and moves his leg with the artificial hip joint in the sagittal plane and in the frontal plane while standing next to a measuring device or wearing a measuring belt containing the measuring electronics. This apparatus, which the examining orthopaedic surgeon uses, provides a contactless power supply and communication with the measuring electronics built into the implant. The electronics in the implant are put into operation by the now available power supply or the transmitted “wake-up” signal and send measurement data to the external apparatus.
By moving the leg, the ball head moves in the socket. Depending on the abrasion, the ball head moves along the abraded surfaces. By moving the ball head, the decentration and the abrasion can be measured. By comparing historical measurement data, a more accurate statement about the abrasion condition can be made. The access to the historical data runs according to the scheme shown in the figure:
The following properties and functionalities are to be achieved:
- Measurement of endoprosthesis inlay abrasion to ± 1 mm.
- Measurement of the decentration of the endoprosthesis head
Power supply and data transmission:
- Creation of a contactless power supply
- Creation of a contactless data transmission
- Evaluation software for the measurement data
- Medical safety
- Creation of installation space in an endoprosthesis.
- Consideration of data and energy transmission
- Consideration of the stability of the endoprosthesis while maintaining the same external size
- Hermetic sealing of the construction space
- Medical safety