Proc. of the Computer Assisted Orthopaedic Surgery Symposium,
`Bern, Switzerland,November, 1995
`
`HipNav: Pre-operative Planning and Intra-operative
`Navigational Guidance for Acetabular Implant Placement
`in Total Hip Replacement Surgery
`
`A.M. DiGioia. M.D.,‘-2, D.A. Simon”. B. Jaratnazm, M. Blackwellz, F. Morganz,
`RV. o"roo1e3, B. Colgan], E. Kischellz
`
`’Center for Orthopaedic Research
`Shadyside Hospital
`Pittsburgh, PA 15232
`
`2Robotics Institute
`Carnegie Mellon University
`Pittsburgh, PA 15213
`
`3Harvard Medical School
`25 Shattuck St.
`Boston MA 02 115
`
`Abstract
`
`The Hip Navigation or HipNav system allows a surgeon to determine optimal, patient-speczfc acetabular
`implantplacement, and accurately achieve the desired acetabular implantplacement during surgery. H1'p~
`Nav includes three components.‘ apre-operativeplanner, a range ofmotion simulator, and an intra-oper~
`ative tracking and guidance system. The goals of the currentHzpNav system are to: I ) reduce dislocations
`following total hip replacement due to acetabular malposition; 2) determine andpotentially increase the
`“safe ”range of motion: 3) reduce wear debris resultingfrom impingement of the implantsfemoral neck
`with the acetabular rim; and 4) track in real time theposition of thepelvis and acetab ulum during surgery.
`This information will help the surgeon achieve more reliable and accurate positioning of the acetabular
`cup and take into account specific anatomyfor individualpatients. The Hz_'pNav systemprovidesfor a new
`class ofresearch tools that can be used intra-operatively topermit surgeons to re-examine commonly held
`assumptions conceming bone and implant motion, range of motion testing, and the “optimal" alignment
`ofacetab ular cups.
`
`Keywords: computer—assisted surgery, total hip replacement, navigational guidance.
`
`1
`
`Introduction
`
`The incidence of dislocation following primary total hip replacement (THR) surgery is between 2-6% and
`even higher following revisions [5] {4]. It is, therefore, one of the most commonly occurring complications
`following hip replacement surgery. Dislocation of a total hip replacement causes significant distress to the
`patient and physician and is associated with significant additional costs in order to relocate the hip. Anoth-
`er complication of THR surgery is impingement between the neck of the femoral implant and the rim of
`the acetabular component, as shown in Figure l. Impingement can lead to advanced wear ofthe acetabular
`rim resulting in polyethylene wear debris shown to accelerate loosening of implant bone interfaces. The
`position at which impingement occurs is determined by the design and geometry of the implants (such as
`the size of the femoral head, the width of the neck, and the design of the acetabular liner), and more im-
`portantly by the relative position of the femoral and acetabular implants. In certain cases, impingement
`may result in dislocation, as seen in the X—Ray of Figure 2. The causes of dislocation following total hip
`replacement are multi-factorial and include not only malposition of the implants causing impingement,but
`also soft tissue and bone impingement, and soft tissue laxity [5]. However, the most common cause of both
`impingement and dislocation is malposition of the acetabular component [5].
`
`National Science Foundation - Award ECS—9422734. 53
`
`This work is supported in part by 5 National Challenge grant from the
`
`§2{D
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`Figure 1: Implant impingement.
`
`
`
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`Figure 2: X-Ray showing pelvic dislocation.
`
`A system has been developed to permit accurate placement of the acetabular component during surgery.
`As shown in Figure 3, the Hip Navigation or HipNav system includes three components: a pre-operative
`planner, a range of motion simulator, and an intra-operative tracking and guidance system. The pre—oper-
`ative planner allows the surgeon to manually specify the position of the acetabular component within the
`pelvis based upon pre-operative CT images. The range ofmotion simulatorestimates femoral range of mo-
`tion based upon the implant placement parameters provided by the pre-operative planner. The feedback
`provided by the simulator can aid the surgeon in determining optimal, patient specific acetabular implant
`placement. The intra-operative tracking and guidance system is used to accurately place the implant in the
`predetermined optimal position regardless of the position of the patient on the operating room table.
`
`By accurately placing the acetabular component in an optimally selected position, the HipNav system has
`the potential to reduce the risk of dislocations and the generation of wear debris caused by impingement
`resulting from malpositioned components and increase the “safe” range of motion.
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`Pre-operative
`Planner
`
`Simulator
`
`Range of Motion
`
`Intra-operative
`Tracking &
`Guidance
`
`Figure 3: HipNav system overview.
`
`2 Current Practice
`
`Current planning for acetabular implant placement and size selection is performed using acetate templates
`and a single anterior-posterior X-Ray of the pelvis. Acetabular templating is most commonly performed
`to determine the approximate size of the acetabular component, but there is little effort to accurately de-
`termine the ideal position of the implant.
`
`The intra—operative positioning devices currently used by surgeons attempt to align the acetabular compo-
`nent with respect to the sagittal and coronal planes of the patient [6]. These devices assume that the pa-
`tient’s pelvis and trunk are aligned in a known orientation, and do not take into account individual
`variations in a patient’s anatomy or pelvic position on the operating room table. Use of this type of posi-
`tioner can lead to a wide discrepancy between the desired and actual implant placement, possibly resulting
`in reduced range of motion, impingement and subsequent dislocation.
`
`3 System Description
`
`The first step in using the HipNav system is the pre-operative CT scan which is used to determine the pa-
`tient’s specific bony geometry. The CT images are used in the pre-operativeplanner which allows the sur-
`geon to determine appropriate implant size and placement. In the current version of the planner, the
`surgeon can position cross sections of the acetabular implant upon orthogonal views of the pelvis, as seen
`in Figure 4. We are investigating other methods of presenting CT data to the surgeon, including an ap-
`proach which displays implant placement on multiple CT cross sections, each of which passes through the
`acetabulum’ s central axis (the axis which passes through the center of pelvic rotation and which is perpen-
`dicular to the plane of the acetabular rim).
`
`Once the surgeon has selected the position of the acetabular implant, the range of motion simulator is used
`to determine the femoral positions (in terms of extension/flexion, abduction/adduction, and internal/exten
`nal rotation) at which impingement would occur for that specific implant design and position. Based upon
`this range of motion information, the surgeon may choose to modify the selected position in an attempt to
`achieve the “optimal” cup position for the specific patient. The range of motion simulator performs a ki-
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`Figure 4: Pre-operative planner.
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`nematic analysis which determines an “envelope” of the safe range of motion, as seen in Figure 5. A more
`detailed description of the range of motion simulator appears in [3].
`
`The optimal patient specific plan is used by the HipNav System in the operating room on the day of sur-
`gery. HipNav permits the surgeon to determine where the pelvis and acetabulum are in “operating room
`coordinates” at all times during surgery. Knowing the position of the pelvis during all phases of surgery,
`and especially during preparation and implantation of the acetabular implant, permits the surgeon to accu-
`rately and precisely position the cup according to the pre—operative plan. Alternately, using HipNav the
`surgeon can align the component to an accepted standard such as “true” 45 degrees of abduction and 20
`degrees of anteversion.
`
`There are several devices that are used intra—operativelyto allow the surgeon to accurately execute the pre-
`operative plan, as seen in Figure 6. One device is an “Optotrak” optical tracking camera (Northern Digital
`Inc., Ontario, Canada) which is capable of tracking the position of special light emitting diodes or “LEDS”.
`These LEDs can be attached to bones, tools, or other pieces of operating room equipment to allow highly
`reliable tracking. Optotrak can achieve accuracies of roughly 0.1mm at speeds of 100 measurements per
`second or higher.
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`
`Cup orientation: 45” Lateral Opening
`15”Antcvcrsion
`
`impingement
`circle \
`
`Impingenzenr.
`
`
`
`
`I \ External
`I-
`Rotation
`
`
`/I
`Impingement
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`Figure 5: Kinematic simulations: Left - implant geometry. Right - motion envelope.
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`Figure 6: Intra-operative execution.
`
`In order to determine the location of the pelvis and the acetabular implant during surgery, Optotrak targets
`are attached to several conventional surgical tools, as seen in Figure 7. The pelvis is tracked by attaching
`a target to the pelvic portion of a Harris leg length caliper (Zimmer, Inc., Warsaw, IN), and inserting this
`device into the wing of the ilium. The acetabular implant is tracked by attachinga second target to the han-
`dle of an HGP II acetabular cup holder and positioner (Zimmer, Inc., Warsaw, IN). A third Optotrak target
`is required by the HipNav system to determine operating room coordinates (i.e., left, right, up and down
`with respect to the surgeon).
`
`Several key steps are necessary to use the HipNav intra-operative guidance system. One of the most im-
`portant is the registration of pre—operative information (i.e., the CT scan and pre—operative plan) to the po-
`sition of the patient on the operating room table. One limitation of current registration systems used in
`Orthopaedics is the need for pins to be surgically implanted into bone before pre—operative images are ac-
`quired (e. g. [9]). An alternative technique being investigated within our group uses surface geometry to
`perform registration [8] [7]. In this approach, the surfaces of a bone (such as the pelvis or acetabulum) can
`be used to accurately align the intra-operative position of the patient to the pre—operative plan without the
`use of pins or other invasive procedures. Using this technique, it is necessary to sense multiple points on
`the surface of the bone with a digitizing probe during surgery. These “intra-operative data points” are then
`
`
`
`Figure 7: Standard surgical tools instrumented with optical tracking targets.
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`Figure 8: Surface-based registration.
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`matched to a geometric description of the bony surface of the patient derived from the CT images used to
`plan the surgery.
`
`The registration process is illustrated in Figure 8. The pelvic surface model was constructed from CT data
`using techniques described in [1]. The discrete points were collected using a digitizing probe which was
`physically touched to the indicated points. The goal of the process is to determine a “registration transfor-
`mation” which best aligns the discrete points with the surface model. An initial estimate of this transfor-
`mation is first determined using manually specified anatomical landmarks to perform corresponding point
`registration [2]. Once this initial estimate is determined,the surface—based registration algorithm described
`in [8] uses the pre- and intra—operative data to refine the initial transformation estimate.
`
`Once the location of the pelvis is determined via registration, navigational feedback can be provided to the
`surgeon on a television monitor, as seen in Figure 9. This feedback is used by the surgeon to accurately
`position the acetabular implant within the acetabular cavity. To accurately align the cup within the acetab-
`ulum in the position determined by the pre-operative plan, the cross hairs representing the tip of the im-
`plant and the top of the handle must be aligned at the fixed cross hair in the center of the image. Once
`aligned, the implant is in the pre—operatively planned position and orientation.
`
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`Figure 9: Navigational feedback.
`
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`Figure 10: Real-time tracking of the pelvis.
`
`Registration also allows the position of the pelvis to be tracked during surgery using the Optotrak system,
`as demonstrated in Figure 10. This eliminates the need for rigid fixation of the pelvis. In addition, this
`tracking ability allows us to record the position of the pelvis during surgery, and especially at key times
`such as at the time of implantation of the acetabular component or during range of motion testing.
`
`4 Conclusions
`
`The goals of the HipNav system are to: l)reduce dislocations following total hip replacement due to ace-
`tabular malposition; 2) determine and potentially increase the “safe” range of motion; 3) reduce wear de-
`bris resulting from impingement of the implant’s femoral neck with the acetabular rim; and 4) track in real
`time the position of the pelvis and acetabulum during surgery. This information will help the surgeon
`achieve more reliable and accurate positioning of the acetabular cup and take into account specific anato-
`my for individual patients.
`
`HipNav will also provide clinicians and researchers with a new class of tools for critically examining com-
`mon assumptions concerning range of motion, bone motion, and “optimal,’ alignment. For example, the
`pelvis can be tracked during surgery to determine its position at key times such as: prior to dislocation,
`following dislocation, and during acetabular component implantation. Using these tools, we can evaluate
`the efficacy of the HipNav system in placing the acetabular implant compared to traditional techniques,
`and critically examine commonly help beliefs of optimal acetabularposition (i.e., 45 degrees of abduction,
`20 degrees of anteversion).
`
`The HipNav system holds the promise of reducing dislocation rates in primary and revision total hip re-
`placement by optimizing the relative position of the acetabular implants and minimizing impingement. In
`addition, it will provide a new category of “smart” tools that will be useful to study issues in total hip re-
`placement and ultimately other procedures.
`
`References
`
`[1 }
`
`B. Geiger. Three-dimensional modeling cf human organs and its application to diagnosis and surgical plan-
`ning. PhD thesis, Ecole des Mines de Paris, April 1993.
`
`[2]
`
`B. K. P. Horn. Closed-form solution of absolute orientation using unit quaternions. Journal ofthe Optical’ S0-
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`ciety ofAmerica A , 4(4):629-642, April 1987.
`
`[3]
`
`B. Jaramaz, S. M. Kladakis, A. M. Digioia, L. F. Kallivokas, and O.Ghattas. Simulation of implant impinge-
`ment and dislocation in total hip replacement. In ComputerAssisted Radiology, 10th International Sympo-
`sium and Exhibition, Paris, June 1996.
`
`[4] D. E. McCoIlum, MD. and W. J. Gray, M.D. Dislocation after total hip arthroplasty. Clinical Orthopaedics,
`(261): 159-170, 1990.
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`[5]
`
`[5]
`
`B. F. Morrey, editor. Reconstructive Surgery if the Joints, chapter 91- Dislocation, pages 1247-1260.
`Churchill Livingston, 1996.
`
`B. F. Morrey, editor. Reconstructive Surgery of the Joints, chapter Joint Replacement Arthroplasty,
`pages 605-608. Churchill Livingston, 1996.
`
`[7] D. A. Simon, M. Hebert, and T. Kanade. Real-time 3-d pose estimation using a high—speed range sensor. In
`Proceedings oyFIEEEIntemutional Conference on Robotics andAutomation, pages 2235-2241, San Diego,
`CA, May 1994.IEEE.
`
`[8] D. A. Simon, M. Hebert, and T. Kanade. Techniques for fast and accurate intra—surgical registration. Journal
`d Image Guided Surgery, 1(1): 17-29, April 1995.
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`[9]
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`R. H. Taylor, B. D. Mittelstadt, H. A. Paul, W. Hanson, P. Kazanzides, J. F. Zuhars, B. Williamson, B. L.
`Musits, E. Glassman, and W. L. Bargar. An image-directed robotic system for precise orthopaedic surgery.
`IEEE Transactions on Robotics andAutomation, 10(3):26l—275, June 1994.
`
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