Accuracy in tunnel placement for ACL reconstruction. Comparison of traditional arthroscopic and computer-assisted navigation techniques

Clinical Paper

Accuracy in Tunnel Placement for ACLReconstruction. Comparison of Traditional

Arthroscopic and Computer-AssistedNavigation Techniques

F. Picard, M.D., A.M. DiGioia, M.D., J. Moody, M.S., V. Martinek, M.D., F.H. Fu, M.D.,M. Rytel, M.D., C. Nikou, M.S., R.S. LaBarca, M.S., and B. Jaramaz, Ph.D.

Center for Orthopaedic Research, Shadyside Hospital (F.P., A.M.D., J.M., M.R., C.N., B.J.);Center for Medical Robotics and Computer-Assisted Surgery, Robotics Institute, Carnegie Mellon

University (A.M.D., R.S.L., B.J.); and Department of Orthopaedic Surgery, University of Pittsburgh(V.M., F.H.F.), Pittsburgh, Pennsylvania

ABSTRACT The purpose of this randomized, prospective study was to compare accuracy in tunnelplacement as performed with a traditional arthroscopic anterior cruciate ligament (ACL) reconstruc-tion technique and with KneeNav™ ACL, a computer-assisted surgical navigation technique. Twosurgeons experienced in ACL reconstruction, but inexperienced in computer-assisted surgical nav-igation technique, each randomly used traditional arthroscopic guides or KneeNav™ ACL to drill atunnel in twenty identical foam knees. Placement of the resulting tibial and femoral tunnels wasmeasured with a computer-assisted digitizing method and compared to traditional biplanar radio-graphs. Statistical analysis with Student’s t-test was used to compare the distance from the idealtunnel placement to the femoral and tibial tunnels. Accuracy of tunnel placement with KneeNav™ACL was significantly better than that obtained with the traditional arthroscopic technique. Dis-tances from the ideal tunnel placement to the femoral and tibial tunnels were 4.2 � 1.8 mm (mean �SD) and 4.9 � 2.3 mm, respectively, for the traditional arthroscopic technique, and 2.7 � 1.9 mm(femur) and 3.4 � 2.3 mm (tibia) for KneeNav™ ACL. These differences were statistically different.Tunnel placement for ACL reconstruction with KneeNav™ ACL, an image-based, computer-assisted surgical navigation device with a simple and intuitive interface, was more accurate than withthe traditional arthroscopic technique. Comp Aid Surg 6:279–289 (2001). ©2002 Wiley-Liss, Inc.

Key words: computer-assisted surgery, navigation system, ACL reconstruction

OBJECTIVEOutcome following anterior cruciate ligament (ACL)reconstruction is highly dependent on surgical tech-nique, specifically tunnel placement in the tibia and

femur. Proper tunnel placement is important in thepositioning of the ACL graft and, along with propergraft tension, results in improved long-term out-

Received November 27, 2000; accepted July 9, 2001.

Address correspondence/reprint requests to: Anthony M. DiGioia, M.D., Institute for Computer Assisted Orthopaedic Surgery(ICAOS), The Western Pennsylvania Hospital, 4815 Liberty Avenue, Mellon Pavilion, Suite 242, Pittsburgh, PA 15224;Telephone: 412-578-2267; Fax: 412-605-6376; E-mail: [email protected].

Computer Aided Surgery 6:279–289 (2001)DOI 10.1002/igs.10014

©2002 Wiley-Liss, Inc.

come.1,2 It also minimizes complications such as graftfailure and diminished range of knee motion.3,4

Unfortunately, proper placement of tunnelsremains difficult with current arthroscopic tech-niques. This was evidenced in a prior study by thegreat variability in tunnel placement, even whenperformed by surgeons experienced in ACL recon-struction.5 Even though ACL reconstruction is acommon procedure, the incidence of misplacedtunnels may be as high as 40%.6,7

The aim of this study was to compare tunnelplacement performed with the traditional arthro-scopic technique using standard guide-pin tools andwith an image-based, computer-assisted surgicalnavigation device. Accuracy was measured withrespect to the placement of the tunnel at a prede-termined position.

MATERIALS AND METHODS

Equipment and Material RequirementsThe main requirements for the study were as fol-lows:

1. Two surgeons experienced in ACL recon-struction, with no previous exposure to com-puter-assisted surgical navigation systems.

2. Twenty-one foam knee bones (Sawbones, Pa-cific Research Laboratories, Inc., Vashon,WA) per surgeon. These were custom kneemodels complete with meniscus, collateralligaments, and PCL. All knees were fullyenclosed in a simulated capsule of light-im-pervious elastic fabric (Fig. 1).

3. A traditional ACL tool set (Paramax™ ACLguide system, Linvatec, Largo, FL).

4. A computer-assisted navigation system(KneeNav™, CASurgica, Inc., Pittsburgh,PA) including trackers to be attached to thefemur, tibia, and the Paramax™ cruciateguide assembly (guide pin) (Fig. 2). It shouldbe noted that the FDA has not cleared theKneeNav™ device. However, UPMC IRBapproval has been obtained for clinical trialpurposes.

5. Traditional tools, including guide-pin guideand drill (Fig. 3).

6. An arthroscopic station suitable for ACL re-construction surgery, including a 30° arthro-scope, a camera, a central processing unit, alight cable, a monitor, and a VCR to recordall experimental data (Fig. 4).

Design and Execution of the ExperimentGeneralDuring the experiment, two surgeons indepen-dently prepared ACL tunnels on sets of Sawbonesknee models. Each surgeon used both traditionaland computer-guided methods in attempts to placethe femoral and tibial tunnels in a predeterminedorientation and location. Postoperatively, the tunnelpositions and orientations were measured using tra-ditional planar radiographic methods and in threedimensions using computer measurement tech-niques.

Foam knee bones were used in the experi-

Fig. 1. Knee model enclosed in simulated capsule. [Colorfigure can be viewed in the online issue, which is availableat www.interscience.wiley.com]

Fig. 2. The Linvatec Paramax™ Cruciate Guide Systemas fitted with trackers. [Color figure can be viewed in theonline issue, which is available at www.interscience.wiley.com]

280 Picard et al.: Computer Assisted Navigation in ACL Reconstruction

ment because they were anatomically accurate andphysically identical (to within manufacturing toler-ances). Thus, meaningful results can be obtained bycomparing follow-up measurements from all Saw-bones test specimens. Although not truly represen-tative of an actual surgical environment, carefultest-site preparation and the use of Sawbones kneemodels provided sufficiently realistic OR condi-tions for the purposes of this study.

PreparationSeveral days before the experimental sessions, eachsurgeon was given a “planning” Sawbones kneemodel from which the simulated capsule was re-

moved. On this model, the surgeon indicated theexact locations of the desired entry and exit holesfor ACL tunnels. This reference knee then became,in effect, that surgeon’s preoperative plan for thesurgical experiment. The participating surgeonsused their specific reference model throughout theexperiment. This approach gave each surgeon theadvantage of working within his personal frame ofreference.

Each “planning” reference knee’s tunnel lo-cations were marked with radiopaque fiducials(Fig. 5). The model was CT scanned using thefollowing protocol. All slices were 1 mm thick,with interslice density varying depending on thelocation: 1-mm spacing was used in critical areas,for example, areas used for registration and thosecontaining fiducials; 3-mm spacing was used inbetween high-density areas, and in other areas ofanatomic interest; 10-mm spacing was used in allother areas (the most proximal and most distalones).

Three-dimensional (3D) surface models weregenerated from the CT images. These models wereused intraoperatively to provide navigation, as wellas in the postoperative analysis for 3D measure-ments of alignment accuracy.

Randomized ProcedureThe study was a randomized prospective parallelinvestigation comparing traditional and image-guided navigation ACL reconstruction. Two groupsof twenty identical knee models were selected.Knees were numbered from 1 to 20 for the firstsurgeon and from 21 to 40 for the second surgeon.

Fig. 3. Traditional tools for pin insertion. [Color figurecan be viewed in the online issue, which is available atwww.interscience.wiley.com]

Fig. 4. Arthroscopic station setup. [Color figure can beviewed in the online issue, which is available at www.interscience.wiley.com]

Fig. 5. Knee tunnel locations marked with radiopaquefiducials. [Color figure can be viewed in the online issue,which is available at www.interscience.wiley.com]

Picard et al.: Computer Assisted Navigation in ACL Reconstruction 281

Each surgeon was given a set of 20 envelopesindicating whether the “traditional technique” or“navigation technique” was to be performed. The20 envelopes for each surgeon were thoroughlymixed, thus randomizing the subsequent selectionorder.

During the experimental sessions, the kneemodels were processed in numerical order. Oncethe model was set up, an envelope was selected andthe indicated technique was used to perform theACL tunnel placement on that model.

Surgical ProcedureA single approach surgery was performed on eachindividual Sawbones knee. Each model was placedin a natural posture and secured in a fixture de-signed to permit the surgeon to use their standardsurgical approach. All the models had fully intactcapsules so as to minimize any external visual cuesfor tunnel placement (Fig. 6).

In the traditional approach, the tibial guidepin was first positioned arthroscopically and drilledinto the bone. Afterward, using a 10-mm reamer, atibial tunnel was overdrilled along the pin orienta-tion (Fig. 7).

The second step consisted of fitting a femurguide-pin through the tibial tunnel. The pin wasdrilled through the femur in order to make a tunnel(Fig. 8).

For the navigation technique, the followingsteps were performed prior to the normal surgicalflow: calibration of the guides, registration of the

bone to the CT scan, and verification. Then, usingthe calibrated guide-pin, the surgeon placed thetibial and femoral tunnels as above, with the assis-tance of the fully 3D anatomy. It should be notedthat the arthroscope was not used during the navi-gation-assisted test (Fig. 9).

Green dots were used on the 3D images tomark the “planned” ACL sites (surgical goal), anda red line was used to represent the guide-pinalignment (Fig. 9e). Three different views of theknee were provided (tibial superior view, femoralinferior view, and tibio-femoral sagittal view) andcontinuously displayed on the monitor. The sur-geon could also select the view using a foot-pedal

Fig. 6. The capsule surrounding the knee model mini-mizes external visual cues. [Color figure can be viewed inthe online issue, which is available at www.interscience.wiley.com]

Fig. 7. Drilling of tibial tunnel. [Color figure can beviewed in the online issue, which is available at www.interscience.wiley.com]

Fig. 8. Drilling of femoral tunnel. [Color figure can beviewed in the online issue, which is available at www.interscience.wiley.com]


Fig. 9. Navigation-assisted test: (a) calibration of guides; (b,c) registration of bone to CT; (d) verification of registration;(b,c) (e) placement of tunnels. Red line in (e) represents guide-pin alignment. [Color figure can be viewed in the online issue,which is available at www.interscience.wiley.com]


control. The surgery then continued in a traditionalmanner.

Evaluation ProtocolTwo assessment techniques were used for measur-ing the tunnel positions: a traditional radiographicmethod and a 3D computer-assistance measure-ment.

X-Ray AssessmentThe first method used to assess alignment was thetraditional radiological measurement of the femoraland tibial hole positioning. The following parame-ters were measured from X-rays for each of thefoambone models: femoral hole sagittal positioning(FSP); femoral hole coronal positioning (FCP); tib-ial hole sagittal positioning (TSP); and tibial holecoronal positioning (TCP) (Fig. 10).

Each model was filmed in the AP and lateralaspects (Figs. 11 and 12). Radiographic images

were obtained with Oralix 70 (Philips, The Neth-erlands) using the following parameters: distance36 inches (91.37 cm), 70 kV, 0.25 mA. Prior toradiography, the simulated soft tissue (includingthe whole capsule) was removed from the kneemodel to obtain separate tibia and femur images. A10-mm tunnel obturator was placed in the tibialtunnels as a radiographic marker. This device en-hanced contrast and facilitated measurement of thetunnel positions. A single pin was secured in thefemur at an isometric location. To standardize the

Fig. 11. AP radiograph of knee model. Fig. 12. Lateral radiograph of knee model.

Fig. 10. Parameters measured for knee models: FSP � femoral hole sagittal positioning; FCP � femoral hole coronalpositioning; TSP � tibial hole sagittal positioning; TCP � tibial hole coronal positioning.


X-ray measurements, a casting was prepared tosecure the identical Sawbones models in exactly thesame orientation relative to the film. The jig wascomposed of malleable urethane foam mounted onplywood (both materials are essentially radiographi-cally transparent). A reference Sawbones tibia andfemur were placed on the jig, positioned to perfectalignment, and pressed into the foam to create aunique “footprint.” Subsequent test models wereplaced in the same footprint, thereby providing iden-tical alignment for all radiographs. Then, each bonewas X-rayed in two planes: AP and lateral.

For the AP view, the tibia and femur wereplaced in a flat position, with the posterior femoraland tibial condyles in contact with the undersur-face, and then aligned. For lateral views, the abovetechnique was repeated to generate a standardizedlateral image of each model.

Two observers measured the X-rays.

Computer-Assisted AssessmentTo precisely measure the overall 3D alignment of thetunnel, each model was carefully reregistered and thefinal position of the femoral and tibial holes recorded.For the follow-up measurements, the knee model wasequipped with rigid bodies (one each for the tibia andfemur) then registered in the normal fashion, andregistration was verified visually. A centering rod wasinserted through the tibial tunnel so that each end ofthe rod could be easily accessed, as illustrated inFigure 13. The ball probe tip was inserted into thedistal detent, and the position was measured withrespect to the tibia (Fig. 13a). The ball probe wasmoved to the proximal detent and a second measure-ment was recorded. For femoral alignment measure-ments, the ball probe was placed directly on each endof the femoral guide-pin tunnel (Fig. 13b). The pointswere collected with respect to the femoral tracker.The ball probe self-centered to the tunnel ends formost knee models, although care had to be taken tocenter the probe tip on models with oblique femoraltunnels. Postprocessing software computed the inter-section of the tunnel axes with the bone surfacemodel.

RESULTS

X-Ray EvaluationThere were no statistical differences between Ob-server 1 and Observer 2 for radiological measure-ments (Table 1). Although results for tunnel sagittalalignment (TSP) by Surgeon 1 revealed a trendshowing KneeNav™ to be more accurate than thetraditional technique, no clear statistical differences

were noticed between the two surgical techniquesusing planar radiographs as the basis for accuracycomparisons (Table 2).

Fig. 13. Measurement of 3D tunnel alignment in (a) tibiaand (b) femur. [Color figure can be viewed in the onlineissue, which is available at www.interscience.wiley.com]


Computer-Assisted MeasurementUsing the computer-assisted system as a 3D mea-surement tool, the “surgical error” was defined bythe distance between the preoperatively markedideal location and the actual femoral and tibialholes. Surgical error was 3.1 mm (� 2.14 mm) forGroup 1 (KneeNav™) and 4.6 mm (� 2.12 mm)for Group 2 (scope), and was statistically different(Table 3 and Fig. 14).

The surgeons had no prior experience withKneeNav™ ACL, yet after using it four times theyreported being comfortable with it. Subsequently,the tunnel placements tended to be more tightlyclustered around the planned locations (Fig. 14).

DISCUSSIONUse of KneeNav™ ACL, a computer-assisted surgi-cal navigation technique, resulted in more accuratetunnel placements in the femur and tibia than with thetraditional arthroscopic technique. This surgical nav-igation system represents the second generation ofcomputer-assisted surgical systems developed by theCenter for Orthopaedic Research at UPMC Shadysidein collaboration with the Center for Medical Robotics

and Computer Assisted Surgery (MRCAS) at Carne-gie Mellon University. Building on principles andresults obtained with the HipNav™ system,8 theKneeNav™ ACL image-guided system has been de-veloped as a broad-based platform to improve manytypes of knee reconstruction surgery. The initial em-phasis was on ACL reconstruction.

This study demonstrated that traditional sur-gical tools could be easily adapted for use in with acomputer-assisted surgical navigation technique. Italso demonstrated that the image-guided systemenabled the surgeon to accurately place the tibialand femoral tunnels without visualization.

Each surgeon placed tunnels in accordancewith a specific preoperative plan, and they differedappreciably. The femoral and tibial tunnel goalswere different for each surgeon in the coronal andsagittal plane. This occurred despite extensive priorstudy of proper tunnel placement for ACL recon-struction.1,2 Previous studies found similar variabil-ity in tunnel placement, even when performed bysurgeons experienced in ACL reconstruction.5,9,10

This indicated the importance of allowing eachsurgeon to select individual, ideal tunnel place-

Table 1. Comparison between the Two Observers

Tunnelposition

Surgeon 1 Surgeon 2Scope KneeNav Scope KneeNav

Obs. 1 Obs. 2 Obs. 1 Obs. 2 Obs. 1 Obs. 2 Obs. 1 Obs. 2FCP (%) 75.7 � 6.7 75.9 � 5.23 70.7 � 8.9 71.2 � 9.7 79.8 � 8.7 78.2 � 4.9 99.1 � 4.9 99.4 � 3.4

p � 0.99 p � 0.52 p � 0.96 p � 0.90

FSP (%) 67.9 � 3.3 66.7 � 3.1 68 � 7.1 68.2 � 6.5 66.9 � 4.4 70.8 � 5.7 82.5 � 8.4 85.5 � 5.9p � 0.98 p � 0.98 p � 0.85 p � 0.91

TCP (%) 47.5 � 1.5 50 � 3.5 50.3 � 1.4 52.4 � 2.7 48 � 2.0 49 � 2.6 51.9 � 1.9 49.3 � 1.6p � 0.79 p � 0.74 p � 0.35 p � 0.84

TSP (%) 41.1 � 2.0 41.2 � 3.2 35.1 � 4.5 36.8 � 2.0 33.5 � 4.2 33.8 � 2.9 35.5 � 3.0 37.9 � 3.5p � 0.94 p � 0.64 p � 0.85 p � 0.91

FSP � Femoral hole sagittal position; FCP � femoral hole frontal position; TSP � tibial hole sagittal position; TCP � tibial hole frontal position.

Table 2. Measurements of the Bone Tunnels PositioningHoleposition

Surgeon 1 Surgeon 2Scope KneeNav Scope KneeNav

FCP (%) 75.7 � 6.7 70.7 � 8.9 79.8 � 8.7 99.1 � 4.9Goal � 70.1 (p � 0.08) Goal � 100 (p � 0.01)

FSP (%) 67.9 � 3.37 68 � 7.13 66.9 � 4.41 82.5 � 8.4Goal � 72 (p � 0.94) Goal � 83.4 (p � 0.71)

TCP (%) 47.5 � 1.52 50.3 � 1.48 48 � 2.03 51.9 � 1.93Goal � 52.2 (p � 0.98) Goal � 50.5 (p � 0.86)

TSP (%) 41.1 � 2.04 35.1 � 4.5 33.5 � 4.21 35.5 � 3.04Goal � 39.1 (p � 0.02) Goal � 36 (p � 0.08)

FSP � Femoral hole sagittal position; FCP � femoral hole frontal position; TSP � tibial hole sagittal position; TCP � tibial hole frontal position.


ments, and also suggested a requirement for anysurgical navigation system to support customizableplanning parameters.11 Moreover, the surgeon mustbe given the option to verify and/or change his planintraoperatively. Such capability is not currentlyavailable in robotic-based surgical systems.12

With radiographic assessment, there was not anundeniable statistically significant difference in tunnelplacement between the two techniques. The initialconclusion might be that the KneeNav™ ACL tech-nique had comparable accuracy to the traditional ar-throscopic technique, and even worse for the TSPparameter (Surgeon 1). However, the computer nav-igation measurements demonstrated a difference inaccuracy between the two techniques, suggesting in-stead that the radiographic technique was inadequatefor the measurement of tunnel placement. This wasalso found in a previous study of component positionassessment after ACL reconstruction, as discussed byKlos in his thesis (Chapter 5),11 and total joint re-placement.13,14 A reason for this finding is that radio-graphic views are not able to establish the alignmentin three dimensions.

It was also noted that the surgical systemcould provide navigation that could not be fullyexploited due to limitations in the traditional instru-mentation. For example, in our study, the trackedtibial guide-pin could be oriented such that thetibial tunnel, when overreamed, would provide op-timal access to the intercondylar notch for the sub-sequent femoral tunnel placement. However, thisoptimal orientation could not always be maintainedduring tibial guide pin drilling because the obliqueangle between the ACL guide cannula and the tibiaprevented a stable purchase of the guide. A lesserangle had to be used to ensure stable guide posi-tioning during pin drilling. Thus, although tradi-tional tools can easily be incorporated into comput-er-assisted navigation systems, instrumentationmay have to be redesigned before surgeons cantake full advantage of a computer-assisted system’scapabilities. The second limitation of the computer-assisted navigation system is the current imagingprocess. CT scan is certainly not the ideal medicalimaging modality for ACL reconstruction because

it is not routinely performed, and CTs provide poorcartilage and ligament visibility. Current work on3D MRI reconstruction is expected to provide anaccurate, safe, and traditional examination for CASsystems. A final limitation of the study was the useof foambone knees, but identical bones wereneeded to provide a common baseline for follow-upcomparison.

In a future study, an improved KneeNav™ACL technique will permit individual, ideal tunnelplacements to be selected and trialed intraopera-tively. Also, a virtual ACL graft will be createdwith a diameter that can be modified. Intraopera-

Fig. 14. Comparison between traditional and KneeNavtechniques for femoral (a) and tibial (b) tunnel placement. Ineach case, the target is in red, placements with computer-assisted technique are in green, and placements with tradi-tional technique are in yellow. [Color figure can be viewedin the online issue, which is available at www.interscience.wiley.com]

Table 3. Comparison between Traditionaland KneeNav TechniquesTechnique Arthroscope KneeNavLocation Femur Tibia Femur Tibiadistances(mm) 4.2 � 1.8 4.9 � 2.3 2.7 � 1.9 3.4 � 2.3t-test p � 0.01 p � 0.04


tively determined cut planes would be defined sothat the virtual ACL graft and anatomical structuressuch as the femoral notch can be depicted simulta-neously. With a passive range of knee motion, thesurgeon will be able to change the position of thevirtual ACL graft if there is impingement or exces-sive graft lengthening (Figs. 15 and 16). Eventhough the software program is already running forthis feature, virtual ACL graft evaluation must stillbe clinically tested. This computer-assisted proce-dure enables the surgeon to follow the traditionalapproach using augmented assembly. A standardmechanical guide-pin equipped with a tracker en-ables the surgeon to visualize his own mark and thepin orientation. Four consecutive views can be dis-played on the monitor screen by pressing the foot-pedal control. Hence, the surgeon can verify inter-actively the tunnel orientations relative to theintraoperative planning before doing any drilling.

The surgeons were exposed to new technicalconcepts related to CAS systems. Tracking issues,tool calibration, model registration, and operationof the user interface were quickly grasped andposed no problem in the execution of the experi-ment.15 Computer-assisted navigation systems willprovide viable alternatives to current surgical tools.Regardless of the complexity of the underlyingtechnology, a truly practical system must provide asimple and intuitive interface.

ACKNOWLEDGMENTThe authors thank Chuck Cheetham from Linvatec,Bob Brown from Zimmer Randall Associates, Inc.,and Patrick McMahon (Department of Orthopaedic

Surgery, University of Pittsburgh) for his valuableassistance in the drafting of this document.

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Accuracy in tunnel placement for ACL reconstruction. Comparison of traditional arthroscopic and computer-assisted navigation techniques