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Micro-CT-Based Measurement of Local Micromotion Around a Cement-less Straight Femoral Stem During Compressive and Torsional LoadingV. Malfroy Camine1, H. A. Rüdiger2,3, D. P. Pioletti1, A. Terrier1
1Laboratory of Biomechanical Orthopaedics, Ecole Polytechnique Fédéral de Lausanne, Switzerland, 2CHUV, Lausanne University Hospital, Switzerland, 3Schulthess, Klinik Zürich, Switzerland
lbo.epfl.ch
INTRODUCTIONPrimary stability of the stem is essential for the long-term success of cementless total hip-arthroplasty (Engh, 1992). Poor primary stability is characterized by excessive micromotion at the bone-implant interface, which leads to aseptic loosening of the prosthesis.
Some physical activities such as stair climbing or rais-ing from a chair induce high torsional loads, which are thought to endanger more the implant primary stability than compressive loads.
Current techniques available to measure experimentally micromotion allow only a limited number of simultane-ous measurement points at the bone-implant interface.
OBJECTIVESDevelop a micro-CT-based technique to provide a com-plete map of micromotion during both compressive and torsional loadings.
Compression Torsion
METHODS
Image processing allows to detect automatically all markers position in each scan.
Micromotion is the displacement of corresponding bone markers from the loaded scan to the unloaded refer-ence scan. Interpolation between measurement points provides the continuous map of micromotion on the stem surface. Method accuracy is determined by meas-uring micromotion between two successive unloaded scans.
Bone reaming Radiopaque markers on bone and stem
Implantation
The technique relies on small (< ø 800 µm) radiopaque markers placed on the endosteal surface of a cadaveric femur and on a straight cementless collared stem (Corail, DePuy-Synthes).
Implanted bones with markers are placed in loading de-vices designed to fit inside a micro-CT scanner.
Loaded Micro-CT Scan
Unloaded Reference Micro-CT Scan
Superimpose implant markers to compute micromotion
RESULTS
Over 300 measurement points were spread homogene-ously around the stem. The measurement accuracy was 4 µm and was uniformly distributed.
COMPRESSION
Anterior Lateral Posterior Medial
Micromotion Amplitude [µm]
0 12.5 25
TORSION
Anterior Lateral Posterior Medial
Micromotion Amplitude [µm]
0 12.5 25
Compression0
Torsion
5
10
15
20
25
Mea
n M
icro
mot
ion
[µm
]
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CONCLUSIONS
We obtained a complete map of micromotion around a cementless stem in compression and torsion. The ac-curacy of the present technique is sufficient to discrim-inate between stable and unstable implants, allowing the method to be used for implant design, pre-clinical testing or finite element (FE) models validation (Vice-conti, 2000).
Local variation of micromotion can be observed on the different parts of the stem and between loading types, underlining the need for local measurement of micro-motion. Higher micromotion on the distal part of the stem was already reported in the literature (Fottner, 2011), and is consistent with stems designed to achieve metaphyseal fixation.
Increased measured micromotion in torsion confirms that torsional loads may be more detrimental to the pri-mary stability of cementless stems than compressive loads.
FUTURE DIRECTIONS
Exhaustive validation of patient-specific preoperative planning FE models that predict implant primary sta-bility is key to the translation of such models to clinical practice. The present study proposes a new technique based on micro-CT imaging that provides a complete map of micromotion around a femoral stem, paving the way to validation of such FE models.
Valérie MALFROY CAMINEEPFL STI IBI LBOStation 19CH-1015 LausanneSwitzerland
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