how simple is too simple? an open-source approach to...

How Simple is Too Simple?An Open-Source Approach to Simplifying

Femoral Neck Axis Drilling

Jeremy Kooyman80109119

University of British Columbia,2329 West Mall Vancouver, [email protected]

http://www.ubc.ca

Abstract. Femoral head resurfacing is a favoured option for youngerpatients who are not suitable candidates for total hip replacement. Thisprocedure is more technically demanding than a total hip replacementand relies on mechanical alignment jigs that are time consuming to use.This study explores the effects that eliminating these jigs would have onthe accuracy of targeting and drilling the femoral neck axis. This studydescribes a procedure, using a porcine model and open-source software,to target the femoral neck axis and assess the accuracy of the drilled holecompared to a preoperative plan. This study shows that alignment jigsare an essential component of established surgical protocols and theirelimination produces unacceptably inaccurate results.

Keywords: Femoral Head Resurfacing, Femoral Neck Axis, 3D Slicer,ITKSnap

1 Introduction

While suitable for older patients, total hip arthroplasty (THA) has been shownto produce high rates of articular bearing wear and component loosening inyounger patients[1]. Instead, surgeons have begun to resurface the femoral head,a procedure that when compared to THA preserves more bone, maintains normalhip kinematics/joint stability, prevents stress shielding of the femur, and reducesthe risk of leg length discrepancies[2]. In order to minimize the risk of postop-erative complications, surgeons typically use a manual jig that is difficult andawkward to align, consuming expensive operating room time[3]. Correct place-ment of the femoral component in resurfacing hip arthroplasty is an importantfactor in long-term implant survival[4].

In order to extend research in this area, current surgical approaches need tobe analysed to create a holistic understanding of the procedure and associatedchallenges.

It has been suggested that there often exists less complex and technical so-lution to many biomedical engineering problems. With the accuracy of manual,

2 EECE 542 Final Report

and computer-assisted surgical guidance and navigation tools established andquantified, this project sought to examine the effects that removal of the manualguidance jig would have on femoral neck axis drilling.

2 Materials

2.1 Selection of an Animal Model

While human cadavers would be the ideal source for proximal femur specimens,there was insufficient time available for an ethics review meaning that a suitablealternative needed to be identified. Bones available from Sawbones Inc. werebriefly investigated but due to construction materials, they lacked the densityneeded for C-arm imaging, and due to their hollow nature it was hypothesizedthat they would fail to provide a sufficient framework for segmenting the post-operative drill trajectory. Further Sawbone complications included a manufac-turing hole approximately where the drill would enter the femur, as shown inFig. 1, which completely excluded Sawbones from use in this study. It was briefly

Fig. 1. ITKSnap segmentation of a Left Human Femur from Sawbones Inc. showinga manufacturing hole aligned approximately with the femoral neck axis, complicatingdrilling tasks.

thought that a solid plaster model of the Sawbone femur could be created, allow-ing identical copies of consistent density to be repeatedly produced. A negativemold was started but after a catastrophic failure of the whole mold during thefirst femur casting attempt, the idea was abandoned.

Literature reported frequent use of dog and sheep models, with the latterbeing preferred because of lower loosening rates[5]. With canine femurs subject

EECE 542 Final Report 3

to similar ethics restraints as human specimens, and a general lack of sheepfemurs in local butcher shops, neither option could be pursued. Discussions withsubject experts revealed a frequent and favourable use of porcine models becauseof their ease of availability. An abundant supply of porcine femurs were locatedat a local pet food store.

Fig. 2. Smoked pig femurs purchased from a local Kitsilano pet store.

While raw femurs were available, smoked porcine femurs were selected be-cause of their ease of handling, maintenance, and cleaning. A total of 6 porcinefemurs were purchased for this study, three of which are shown in Fig. 2.

3 Methods

3.1 Femur Preparation

To aid with imaging segmentation and fiducial attachment, the soft tissue sur-rounding the femoral neck was removed, with a cleaned example shown in Fig. 3.Three femurs were used for manual drilling, and thus did not need fiducials at-tached. The remaining three femurs had four fiducials attached at approximately90 degree intervals around the femoral neck, as shown in Fig. 4. The fiducials,used for targeting and registration in another project (and later abandoned),were robertson head screws approximately 2cm in length. Since all samples hadbeen smoked, they were stored at room temperature, similar to the environmentat the pet store where they were obtained.


Fig. 3. Porcine femoral neck after soft tissue removal.

Fig. 4. Porcine femur with 4 fiducials attached at 90 degree intervals around the femoralneck.


3.2 Imaging and 3D Model Creation

All samples were imaged using a Siemens Arcadis Orbic 3D C-arm in an ap-proximately anatomic position. The C-arm collected 256 images over a revolu-tion of 180 degrees, consistent with the built-in capturing protocol for 3D modelgeneration. The captured DICOM was opened in 3D Slicer (version 3.6) andconverted to a .NRRD file in order to take advantage of the superior segmenta-tion capabilities of ITKSnap. Unlike 3D Slicer’s inferior threshold segmentation(ignoring the unstable alternative segmentation methods), ITKSnap employs asnake-based segmentation that first requires the application and tuning of anintensity region filter followed by the seeding of bubbles in the region of interest.After several hundred iterations of the segmentation algorithm, a 3D model sim-ilar to that shown in Fig. 5 is obtained and saved as a .VTK model for furtherprocessing.

Fig. 5. The results of a snake segmentation using the open-source program ITKSnap.Yellow regions are porcine femur, and red indicates the location of fiducials.

3.3 Femoral Neck Axis Targeting

The femoral neck axis was targeted using an adaptation of the protocol describedin the ReCap Femoral Head Resurfacing System Surgical Protocol from BiometOrthopedics Inc. where the axis is determined by quartering the femoral neck(in the protocol this is done with the cauterizing tool), and using the crossinglines to determine the neck axis. In this study, the .VTK model of the femur washalved using the clipping tool in Paraview (version 3.12.0 64-bit) and a line wasfit along the neck axis as shown in Fig. 6.


Fig. 6. The femoral neck axis is identified by a line of 8 blue balls, as determinedpreoperatively using Paraview.

3.4 Femoral Neck Axis Drilling

The femoral neck axis trajectory identified in Paraview was saved in .STL formatand converted to an IGSTK compatible .MSH format using the open-sourceGMSH (version 2.5.0) program. The three samples that were to be used as anon-tracked control were hand drilled using a visual inspection of where thefemoral neck axis appeared to be located. Unfortunately, due to irreconcilableincompatibilities with the IGSTK Navigator program, this portion of the projecthad to be abandoned.

Instead, the femoral neck axis drilling was done by continuing the surgicalprotocol from Biomet Orthopedics, using lines that bisected the femoral neck intotwo planes (anterior/posterior and medial/lateral). While the Biomet procedurethen offers a drill alignment jig, no such tool existed for porcine femurs so it wasintentionally omitted, a choice that served to create the hypothesis of this study.

3.5 Post-Drilling Accuracy Assessment

Once samples had been drilled, a piece of wire was placed in the drill hole to aidin identification of the drill trajectory. They were then imaged using the methodsdetailed in 3.2. The pre and post-drilling DICOM volumes were loaded into 3DSlicer and registered using the Expert Automated Registration module, set to usethe pipeline affine approach. The drilled volume was registered to the pre-drillingvolume, with a new .NRRD volume being created from the output. Using thesnake segmentation feature of ITKSnap, the drill trajectory was segmented andsaved as a new .VTK model. The two trajectories, pre-operatively determined


in Paraview and post-operatively identified with wire, could now be comparedin 3D Slicer, as shown in Fig. 7. The measurement and angle modules in 3D

Fig. 7. The pre-operative trajectory (grey balls) and actual trajectory (red cylinder)compared against each other with the two DICOM volumes registered.

slicer were used to determine the degree of retro/anteversion and the degree ofvarus/valgus, in a manner similar to the approach used by Hodgson et al. (2005).

4 Results

The results of the manual and Biomet targeted femoral neck axis drilling task areshown in Fig. 8. Drilling trajectories were plotted according to their deviationfrom the preoperative plan. The x-axis denotes the degree of varus or valgusof the trajectory, or the inward or outward rotation of the distal end of thefemur as a result of the hypothetical implant. The y-axis denotes the degree ofretroversion or anteversion, or the anterior or posterior rotation of the implant.The manually drilled holes are displayed as circles, and the Biomet targetedholes are displayed as triangles.

The manually drilled holes ranged from 11 degrees varus to 5 degrees valgus,and 4 degrees retroversion to 8 degrees anteversion. The Biomet targeted holesranged from 3 to 30 degrees valgus, and 5 to 29 degrees retroversion.

5 Discussion and Conclusion

The original goal of this project was to compare the accuracy of three differentsurgical techniques for drilling the femoral neck axis: Manual drilling, computer-assisted navigation with preoperative planning, and rapid-prototyped, custom


Fig. 8. The results of the manual and Biomet targeted femoral neck axis drilling taskare shown above. The completely manually drilled axes are shown as circles with theBiomet targeted axes shown as triangles. The accuracies are classified as the deviation(degrees) from the preoperative surgical plan. The x-axis denotes the degree of varus orvalgus of the trajectory, or the inward or outward rotation of the distal end of the femuras a result of the hypothetical implant. The y-axis denotes the degree of retroversionor anteversion, or the anterior or posterior rotation of the implant.

drilling templates[7]. After realizing the scope of the former proposal, combinedwith technical incompatibilities, two of the three approaches were abandoned.

The paper detailing the custom drilling templates proved to be too sparsein details to adequately reproduce their approach. Of particular difficulty wasthe lack of access to the Mimics (Materialise NV) software program, a powerfulsegmentation program designed to interface with engineering applications. At-tempts to open 3D models created with open-source alternatives in SolidWorksproved to be too computationally intensive to make any progress.

Computer-assisted navigation was to be done using the Navigator programincluded with IGSTK 4.4, however due to contrast issues with the DICOM vol-umes obtained from the C-arm, the image slices would not render properly uponopening and could not be used for navigation. The Navigator program wouldcrash whenever a drill trajectory was loaded as well, rendering it useless for thisstudy.

Finally, since pseudo-human femurs from Sawbones were wrought with con-trast issues, surgical tools and alignment jigs were also excluded from this study.

After all of the exclusions, it was decided that this study would explore overlysimple attempts at targeting and drilling the femoral neck axis by a non-medicalresearcher. A completely manual (eyeballed) drilling task served as the studycontrol, and was compared to the surgical procedure detailed in section 3.4. Itwas hypothesized that the elimination of the complex guidance jig would not


drastically impact the accuracy of the procedure (compared to literature values)and would be more accurate than a completely unguided approach.

As shown in Fig. 8, the governing hypothesis of the study was completelyincorrect, despite the small sample size. While data pertaining to the accuracyof the Biomet ReCap protocol used for this study was not available, a studyexamining the accuracy of the Durom system (Zimmer, Inc) reported meanaccuracies of 5 degrees in both retroversion and varus angles. If +/- 5 degrees istaken as acceptable (acceptable placement ranges have not been biomechanicallydetermined at the time of writing [8]) and deviations beyond this implicated inimplant failure and rejection, the completely unplanned, manual drilling controlwould appear to be more accurate than the Biomet targeted approach.

While this appears to be supportive of more simple orthopaedic surgicalsolutions, Shimmin et al. (2005) have reported that implants placed in varusincreases the risk of a postoperative femoral neck fracture. This position in-creases the tensile stresses on the superior cortex of the neck, raises the medialcompressive stresses, and allows sheer stresses to develop at the mouth of theprosthesis[8]. Indeed, surgical protocol for both femoral head resurfacing and to-tal hip arthroplasty typically dictates that the surgeon air on the side of valgusto reduce the risk of postoperative fracture.

A potential explanation of the catastrophic failure of the adapted Biomettargeting approach is the unskilled and untrained nature of the person doingthe drilling. In order to maximize the accuracy of the approach, the drill had tobe kept parallel to the lines on the medial/lateral and anterior/posterior planessimultaneously. This required constant checking and repositioning of the drillrelative to the femur, and introduced a hesitancy that was not present in thecontrol. This hesitation would manifest itself with a lower rotational speed ofthe bit, resulting in the bit wandering away from the trajectory before plunginginto the femoral head.

Further complications can be attributed to the anatomical differences infemoral neck lengths of humans and pigs. This made the visualization and identi-fication of the femoral neck difficult, even when being digitally analysed. WhileMuller et al. (2005) have presented methods for automatically detecting thefemoral neck axis, journal articles detailing their methods were unable to belocated. If automatic targeting of the femoral neck axis is able to be adapted forporcine model use, it will eliminate human factors as a source of error.

With both approaches producing surgically inadequate outcomes, this studyhas failed to show that there exists potential for further simplification of femoralhead resurfacing protocol through the elimination of alignment jigs. Further workwill revisit the abandoned IGSTK-based drill navigation, and the identificationof the femoral neck axis in a porcine model will be refined.

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