Cathérine Ruther

A Novel Sensor Concept for Optimization of Loosening Diagnostics in Total Hip Replacement

The main reason for the revision of total hip replacements is aseptic loosening, caused by stress shielding and wear particle induced osteolysis. In order to detect an implant loosening early, the osseointegration of endoprosthetic implants must be measured exactly. Currently applied diagnostic methods, such as standard radiographs and clinical symptomatology, often result in an imprecise diagnosis. A novel radiation-free method to improve the diagnostic investigation of implant loosening is presented. The osseointegration of an implant can be identified using mechanical magnetic sensors (oscillators), which impinge on small membranes inside an implant component, e.g., the femoral hip stem. The maximum velocity after impingement of the oscillator depends on the osseointegration of the implant. Excitation of the oscillator is realized by a coil outside the human body. Another external coil is used to detect the velocity of the oscillator. To demonstrate the principle of the novel loosening sensor, an overdimensioned test device was designed to measure simulated loosening phases in the first experimental tests with different material layers. The overdimensioned test device of the loosening sensor showed significant differences in the various phases of fixation. Analysis of the membrane without any material layer in the case of advanced loosening resulted in a 23% higher maximum velocity compared to an attached artificial bone layer. Based on these preliminary results, the sensor system shows potential for the detection of implant loosening. Moreover, the proposed system could be used in experimental applications to determine the quality of bioactive coatings and new implant materials.

Investigation of a Passive Sensor Array for Diagnosis of Loosening of Endoprosthetic Implants

Currently, imaging methods are used to diagnose loosening of endoprosthetic implants, but fail to achieve 100% accuracy. In this study, a passive sensor array which is based on the interaction between magnetic oscillators inside the implant and an excitation coil outside the patient was investigated. The excited oscillators produce sound in the audible range, which varies according to the extent of loosening. By performing several experimental tests, the sensor array was optimized to guarantee reproducible and selective excitation of the sound emission. Variation in the distance between the oscillators demonstrated a definite influence on the quality of the generated sound signal. Furthermore, a numerical design analysis using the boundary element method was generated for consideration of the magnetic field and the selectivity of the oscillators during excitation. The numerical simulation of the coil showed the higher selectivity of a coil with a C-shape compared to a cylindrical coil. Based on these investigations, the passive sensor system reveals the potential for detection of implant loosening. Future aims include the further miniaturization of the oscillators and measurements to determine the sensitivity of the proposed sensor system.

Current Possibilities for Detection of Loosening of Total Hip Replacements and How Intelligent Implants Could Improve Diagnostic Accuracy

Where pain is experienced following a total hip replacement (THR), there needs to be clarification as to whether the cause is due to an infected or mechanically loose THR. The major complication after implantation of a THR is aseptic loosening, caused by stress shielding and wear-particle induced osteolysis with an incidence of 75 % (Malchau et al., 2002). A further prevalent reason for implant loosening is a sepsis due to infection of the periprosthetic membrane. The optimal management in case of hip pain is an often discussed controversy. Currently, several diagnostic methods are used to identify the loosening status of the THR and to establish a basis for revision management. All these techniques are based on imaging methods. An overview of the main imaging methods used is given in figure 1. Although the devices and technology are highly developed, a 100 % diagnostic accuracy is not available (Temmerman et al., 2005). Plain radiographs are mainly used to identify the loosening status of a THR and most decisions on how to treat disorders after THR can be made (Ostlere & Soin, 2003). The time period between e.g. the onset of an infection and the possibility to identify any changes within the THR can be very long (Itasaka et al., 2001). Hence, in early loosening diagnosis, identifying radiolucent lines or increased uptake in radionuclide scanning can be very complex owing to the difficulty with excluding loosening (Love et al., 2001; Udomkiat et al., 2001). Therefore, surgeons cannot verify the actual loosening status accurately until the point of surgical intervention. Thus, the surgeon carries the risk of revising a sufficiently integrated THR. A major clinical problem in diagnosing loosening of a THR is to identify the moment where revision surgery is required. Loosening of THR should be diagnosed precisely and early in order to avoid massive osteolysis of the femur.

In vivo monitoring of implant osseointegration in a rabbit model using acoustic sound analysis.

Implant osseointegration can currently only be assessed reliably post mortem. A novel method that relies on the principle of acoustic sound analysis was developed to enable examination of the longitudinal progress of osseointegration. The method is based on a magnetic sphere inside a hollow cylinder of the implant. By excitation using an external magnetic field, collision of the sphere inside the implant produces a sound signal. Custom‐made titanium implants equipped thusly were inserted in each lateral femoral epicondyle of 20 New Zealand White Rabbits. Two groups were investigated: Uncoated, machined surface versus antiadhesive surface; and calcium phosphate‐coated surface versus antiadhesive surface. The sound analysis was performed postoperatively and weekly. After 4 weeks, the animals were euthanized, and the axial pull‐out strengths of the implants were determined. A significant increase in the central frequency was observed for the loose implants (mean pull‐out strength 21.1 ± 16.9 N), up to 6.4 kHz over 4 weeks. In comparison, the central frequency of the osseointegrated implants (105.2 ± 25.3 N) dropped to its initial value. The presented method shows potential for monitoring the osseointegration of different implant surfaces and could considerably reduce the number of animals needed for experiments.

A novel in vivo sensor for loosening diagnostics in total hip replacement

Approximately 200,000 patients are treated with total hip replacements (THR) in the United States per annum, in European Countries over 500.000. The main reason for revision of total hip replacements (THR) is the aseptic or septic loosening. The Osseointegration of the uncemented hip stem in the femoral bone has to be detected exactly, in order to enable early state loosening detection. All present diagnostic methods, e.g. radiographs and arthroscopy, show insufficient sensitivities and specificities between 70% and 80%. Osseointegration can be identified in-vivo by use of ‘active’ acoustic methods. The acoustic waves can be generated, e.g. by a mechanical hammer placed on the inside of the femoral hip stem wall. The mechanical-acoustic properties of the bone-implant interface give information about the status of the loosening process. A functional in-vitro model of the measuring principle shows significant differences in varied phases of fixation. The new acoustic sensor system demonstrates its potential to detect aseptic loosening.

Localization of Uncemented Hip Stem Loosening with a Novel In-vivo Sensor System Based on Vibration Analysis

The major reason for revision surgery of load bearing implants is aseptic loosening, especially with regard to total hip arthroplasty. Osseous anchorage of loosened implants needs to be measured exactly in order to ensure the indication for revision surgery and additionally to enable early detection of the loosening process. However, current diagnostic methods show unsatisfying results concerning sensitivity and specificity. Therefore, a new sensor system is being developed based on an acousto-mechanical approach, which can be used non-invasively and radiation-free to measure the stability of endoprosthesis fixation on the bone stock. This paper intends to show initial results of functional models, which are used to prove the measuring principle.

A New Approach for Diagnostic Investigation of Total Hip Replacement Loosening

Diagnosis of total hip replacement (THR) loosening using imaging often fails to provide reliable results. New implants instrumented with sensors shall conform to this challenge. Therefore, a novel concept of an in vivo sensor was tested. This simple mechano-acoustical sensor is integrated inside the total hip stem and enables detection of osseous fixation. The sensor is excited by an external coil and impinges inside the THR. The spring-back of the sensor can be detected by an extracorporeal coil, while the structure-borne sound is measured with a vibration sensor placed at the patient’s leg. Experiments of a THR with one integrated sensor showed the possibility to differentiate between well fixed and loosened implants. The presented in vivo sensor system has a promising potential to detect loosening and to analyze osseointegration.