Brunner, P.

Bühler, D.W.

Nolte, L.-P

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Design and Evaluation of a Device for Measuring 3D-Micromotions ofPress-Fit Femoral Stern ProsthesesBühler D.W., Brunner R, Nolte L.-P.M.E. Müller Institute for Biomechanics, University of Bern, 3010 Bern, SwitzerlandINTRODUCT1ONPrimary stability of uncemented total hip replacements isregarded äs a major factor necessary for good qualitybony ongrowth to the prosthetic femoral stem andtherefore for a successrul long-term outcome [1,2]. Theremodelling of the endothelial bone in contact with theimplant is affected by the relative micromotions of thebone-implant interface and the stress state in the femoralbone. As a result, for optimal biological fixation,translational movements (i.e. primary stability) are ofgreatest interest. These interface-micromotions generallyoccur in all three spatial directions. Under physiologicloading, they consist of a cyclic amplitude and change inthe mean, which in the cranio-caudal direction representssubsidence of the prosthesis.Existing measurement strategies which are based on dialgauges, extensometers, LVDTs, hall effect transducers orstrain gauge techniques provide information about onlyone component of the general three-dimensionalmicromovement [3,4,5]. Furthermore, fixation of sensorsaway from the point of interface motion measurement, ässeen in various studies [4,5], may result in the registrationof motions due to elastic deformation of the femoral orprosthetic shaft in addition to the interface motion.The objective of this study was to develop, validate, andapply a new technique which allows the precisemeasurement of the isolated 3D interface motion at threedifferent points along the femoral shaft.Local Sensor COSThermistor,Global Prostheticx· cos RodLinear SpringCustom Ball and Socket JointAdapter UnrtZero Adjustment ScrewsMeasuring PointsFigure 1: Conceptual Design ofthe SensorMATERIALS AND METHODSThe present measuring technique is based on a sensorcombining optoelectronic with precision mechanicalcomponents. Spherical measuring tips of three sensorswere placed on the prosthesis at predefined locations: thedistal tip of the prosthesis and at the middle and proximalthird of the femoral stem. The sensors were mounted onan adapter unit which was anchored in a transverse 9 mmhole drilled in the cortex. Precision, play-free, ballbearing mechanisms allowed transition of the detectedmotion to a infrared emitting diode on the opposite end ofthe sensor (Fig. 1). Photons emitted by this LED wereregistered by a two dimensional silicon position sensitivedetector (PSD). The AT-movements of the LED weremeasured directiy by the PSD. Micromovement of theprosthesis in the Z-axis of the transducer brings the LEDcloser or farther to the photodiode, thereby increasing ordecreasing the intensity of the light beam on the PSD.Since the two-dimensional position sensitive detectorgenerates photocurrents, which sum is proportional to theintensity, the highly non-linear correlation between LED-PSD distance and light intensity was used to acquire thethird dimension at the same time äs measuring the XY-motions. Due to the triermal response of the PSD, a ntc-resistor was also integrated to exclude thermodrifts duringlong-tenm testing.Static and dynamic validation indicated that the maximumSystem error was ±4.9 microns within a measuring rängeof ±0.75 mm in each spatial orientation.To demonstrate the Utility of the sensors in interfacemotion measurement, an in-vitro experiment wasperformed. Seven paired fresh cadaveric femora wereused for testing two different types of uncementedfemoral stems: CLS stem (Spotorno) and Cone prosthesis(Wagner) both from Protek AG, Switzerland. Using acustom loading rig the femora were subjected tosinusoidal cyclic loading in a bi-axial materials testingmachine. A cranio-caudal force FCC with a frequency of lHz was combined with a 0.5 Hz antero-posterior forceFap having a magnitude of 10% of FCC. Loading Steps ofl-, 2- and 3-times body weight (BW) were applied.Custom Software allowed for real-time graphical displayof the applied forces and the detected movements.RESULTSThe postprocessing Software allowed the analysis of: (A)Three-dimensional motion displayed in cylindricalcoordinates (Fig. 2); (B) Motion amplitude (MA), whichis the difference of the measured minimum and maximum394 Biomedizinische Technik · Band 41 · Ergänzungsband 1 · 1996value for one loading cycle and (C) Total motion (TM),which is the time-based average micromotion of oneloading cycle (Fig. 3).(A) Three-Dimensional Motion: A typical 3Dinterface motion behavior for one loading cycle (loadingstep 3: Fcc=2.07 kN; Fap=207 N) is illustrated for bothtypes of prostheses in Fig. 2. The midshaft of bothprostheses was the point with the least motion in alldirections. At the distal tip the Cone prosthesis alsoshowed very little movement. A comparably large MAwas measured at the proximal position for bothprostheses. This motion loop was mainly oriented in theX'Z' -plane (=frontal, see figure 1).for all seven paired tested femora is presented in Fig. 3(for axes orientation refer to Fig. 1).Fo«e«Fecp<NJFigure 2: SD-Motion Analysis(B) Motion Amplitude: At higher forces the MAmeasured in a loading cycle increased for both prostheses.All MA values for the CLS prosthesis measured at themiddle third and proximal third of the femoral stem werelarger in every direction than the values for the Coneprosthesis (values for 3xBW: CLS frorn 20 to 87μηι,Cone from 12 to 35μιη). However, at the proximal end ofthe shaft the Cone prostheses allowed a larger MA (CLSfrom 26 to 78um, Cone from 50 to 120 um). In mostcases the largest single component of the MA was foundin the mediolateral direction.(C) Total Motion: At the end of the first andsecond loading step both prostheses showed relativelysmall TM in any of the motion directions at the variousmeasurement points. In the third loading step TM for bothprostheses became much larger with the main increaseduring the first 250 cycles. At the end of stage 3 the TMof the Cone was larger than the TM of the CLS prosthesisat all measuring positions. The caudal subsidence was thelargest contributor to the TM for both prostheses. In X*(M-L) and Y' (A-P) directions the Sterns appeared morestable. The average TM at the distal tip of the prosthesis1000Figure 3: Mean Total MotionDISCUSSIONThe developed method seems sensitive in reflecting theactual postoperative Situation of the implant. Our noveltechnique provided accurate and repeatable dataillustrating the 3D interface motion of uncementedfemoral stems. Furthermore, it is necessary to measure thestability of the prosthesis at several points in all threedirections of the coordinate System to account for the 3Dmotion behavior of the implant with respect to thesurrounding bone.The two prostheses investigated in this study showedconsiderable differences in both their motion amplitudeand total motion behavior. This reflects diiferences intheir anchoring design. Analysis of different concepts offemoral stem fixation with the presented standardizedapproach may further elucidate the role of primarystability in successful long-term clinical outcomes.In addition the investigation of biomechanical problemsoften makes it necessary to measure relative smalltranslations or micromotions in the three-dimensionalspace. By changing the design of the adapter this easy-to-handle optical measuring System could also be used in avariety of other tasks like detection of translations of bonefragments after a fracture or the measurement of relativemovements between adjacent spinal vertebrae underdifferent loads.REFERENCES[1] R.M. Pilliar et al.; Clin. Orthop. 208,108-113 (1986)[2] R.J. Haddad Jr. et al.; J. Bone Joint Surg. 69A, 1459-1466 (1987)[3] L.A. Whiteside et al.; J. Arthroplasty 8,147-155(1993)[4] D.W. Burke et al.; J. Bone Joint Surg. 73B, 33-37(1991)[5] P.S. Walker et al.; J. Biomech. 20(7), 693-702 (1987)ACKNOWLEDGEMENTSThis study was supported in part by Protek AG,M nsingen, SwitzerlandBiomedizjnische Technik · Band 41 · Erg nzungsband 1 · 1996 395

Design and Evaluation of a Device for Measuring 3D-Micromotions of Press-Fit Femoral Stem Prostheses

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