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ACL Replacement Using Gore-Tex Ligament Prosthesis

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Page 1: ACL Replacement Using Gore-Tex Ligament Prosthesis

SIMULATION-BASED DESIGN OF A POINTER FOR ACCURATEDETERMINATION OF ANATOMICAL LANDMARKS

A. Erdemir1 and S. J. Piazza1,2

The Center for Locomotion Studies and Departments of 1Kinesiology and 2Mechanical &Nuclear Engineering, Penn State University, University Park, PA

Email: [email protected] Web: www.celos.psu.edu

INTRODUCTION

The locations of anatomical landmarks areoften used during video-based motion analy-sis to establish local reference frames thatare aligned with anatomical axes. The coor-dinates of such points may be determined byplacing a marker on the skin above a bonyprominence, but this technique is subject toerrors due to skin movement (Lucchetti etal., 1998). An alternative technique wasproposed by Cappozzo et al. (1995) who lo-cated anatomical landmarks using a pointerto which reflective markers were fastened.The tip of this pointer was placed at the de-sired point, and marker locations were thenrecorded using a motion analysis system.The tip point coordinates were determinedfrom the known geometry of the pointer andthe marker coordinates. While avoidingerrors due to skin movement, the pointer de-scribed by Cappozzo et al (1995) is never-theless subject to error due to inaccuracyinherent in the motion analysis system.Small errors in marker positions maycombine to produce amplified error inlocation of the pointer tip (Figure 1).

Figure 1: Pointer for locating anatomicallandmarks. Errors in marker locations maycause larger errors at the tip.

The purposes of the present study were (i) touse a computer simulation to predict tip

errors in hypothetical designs and (ii) toconstruct and test pointers suggested by thesimulation results in order to compare actualtip errors.

COMPUTER SIMULATION

Computer simulation was used to predict tiplocation errors for various pointer designs.These errors were computed by perturbingmarker coordinates and calculating theresulting change in tip location. Mean errorswere computed from 1000 perturbationtrials. The magnitudes of marker errorswere assumed to be normally distributed(mean = 0 mm; SD = 1 mm), and thedirections of marker errors were assumed tobe randomly distributed in three-dimensions.

Errors associated with two pointer designswere simulated. The first design, proposedby Cappozzo et al. (1995) consisted of atleast two markers arranged linearly; lineardesigns incorporating up to 10 markers werealso considered. Tip locations weredetermined by fitting a line to the markercoordinates in three dimensions. Increasingthe number of markers from 2 to 10produced only a small decrease (~ 0.2 mm)in mean tip error. The second pointer designincorporated five markers arranged on asphere whose center was the pointer tip.The tip location was determined by fitting asphere of known radius to the measuredmarker locations. A t-test confirmed that themean simulated error computed for thespherical design was significantly smaller (p< 0.0001) than that computed for the linearpointer (Table 1).

Page 2: ACL Replacement Using Gore-Tex Ligament Prosthesis

EXPERIMENTAL ANALYSIS

Physical prototypes of each pointer (Figure2) were constructed to test for actualdifferences in tip location error.

Figure 2: Linear and spherical pointerprototypes.

Four indentations were placed at knownlocations on a steel plate. A local coordinatesystem was established using markersattached to three of these points. For each of35 trials performed using each pointer, thecoordinates of the fourth point weredetermined using a six-camera Vicon 370motion analysis system (Oxford Metrics,U.K.) to record marker locations for 3seconds at 60 Hz. The measured globalcoordinates were averaged over each trialand then transformed into the localcoordinate system. Tip errors were found bycomparing the measured and knownlocations of this point (Table 1). As in thecomputer simulation results, a significantdifference (p < 0.0001) was found betweenthe mean errors for the two designs.

DISCUSSION

Consistent interpretation of data collectedduring motion analysis is dependent uponaccurate measurement of anatomicallandmarks. The computer simulation resultsof the present study suggested that a novelpointer in which markers are distributed onthe surface of a sphere would exhibit less tip

location error than that of a previouslyproposed linear design. Experimentsconducted using physical prototypessupported this conclusion.

Error magnitudes predicted using the simu-lation were different from those actuallymeasured. We attribute these differences to:simulated error that does not reflect actualerror in the motion analysis system; failureto account for machining errors in thesimulation; and experimental error in estab-lishing the local coordinate system.

This study demonstrates the utility of com-puter simulation for evaluating differentpointer designs used for locating anatomicallandmarks. This and previous studies (Luand O’Connor, 1999) have shown thatsimulation of error that affects marker loca-tion is useful for improving the techniquesand tools used to perform motion analysis.

REFERENCES

Cappozzo, A. et al. (1995). Clin. Biomech.,10, 171-8.

Lu, T.-W. and O’Connor, J.J. (1999). J.Biomech., 32, 129-34.

Lucchetti et al. (1998). J. Biomech., 31, 977-984.

ACKNOWLEDGMENTS

The authors wish to thank Peter Cavanagh,Nori Okita, and Doug Tubbs.

Table 1: Pointer tip errors (in mm) for thelinear and spherical designs, both computedfrom 1000 simulated trials and measured in35 experimental trials.

simulation experimentlinear spherical linear spherical

mean 1.54 0.93 3.15 2.06SD 0.99 0.54 0.74 0.50max 6.62 3.05 4.53 3.30min 0.08 0.05 1.70 1.25