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Technical Briefs

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

[+] Author and Article Information
Cathérine Ruther1

Department of Orthopedics,  University of Rostock, 18057 Rostock, Germanycatherine.ruther@med.uni-rostock.de

Hartmut Ewald

Institute of General Electrical Engineering,  University of Rostock, 18057 Rostock, Germany

Wolfram Mittelmeier, Andreas Fritsche, Rainer Bader, Daniel Kluess

Department of Orthopedics,  University of Rostock, 18057 Rostock, Germany

1

Corresponding author.

J Biomech Eng 133(10), 104503 (Nov 03, 2011) (5 pages) doi:10.1115/1.4005222 History: Received January 03, 2011; Accepted September 27, 2011; Revised September 27, 2011; Published November 03, 2011; Online November 03, 2011

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.

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

Principle of the new sensor system for diagnosing aseptic loosening, e.g., applied in the total hip stem. Left: Hip stem with integrated membranes. Middle: Sensor principle with close bone contact. Right: Sensor principle with loosening layer (fibrous layer).

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Figure 2

Overdimensioned experimental setup to validate the functionality of the sensor principle

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Figure 3

Trigger circuit to control the excitation (L1) and the detection coil (L2)

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Figure 4

Cross-section of the overdimensioned model with the material layers attached to investigate different loosening phases

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Figure 5

Material layers to be attached on the external side of the membrane. Left: Water-filled pad with artificial bone. Right: Simulated partial osteolysis.

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Figure 6

Characteristic velocity signal of the oscillator recorded by the detection coil. The first negative peak is the maximum velocity of the oscillator after impingement on the membrane (n = 20).

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Figure 7

Maximum velocity after impingement of the oscillator on the membrane for different attached material layers simulating various implant loosening progression stages (n = 20)

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Figure 8

Frequency spectrum of the oscillator signal with the eigenfrequency at 67 Hz and the first harmonic at 134 Hz

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Figure 9

The amplitude in the frequency spectrum when testing with different material layers (n = 20)

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