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Computed Tomography Methods for Assessing Total

Knee Replacement Mechanics

Karen C.T. Ho

University of Calgary, Canada Centre for Bioengineering Research & Education

Imaging and Measurements in Biomedical Engineering Paris, France

October 2-3, 2008

Outline

Total Knee Arthroplasty Project Motivation and Rationale Postoperative Knee Mechanics

Protocol Development and Validation

Preliminary Results Future Work & Significance

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Total Knee Arthroplasty (TKA)   Treatment for severe knee osteoarthritis   Goals: Relieve pain, restore joint alignment   Over 500,000 TKA procedures per yr in North America1

  Over 80% of patients satisfied with overall results and improved quality of life (15 year follow-up study)2

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Tibia

Femur Patella

Femoral Component

Tibial Component

1. Kozak 2006. 2. Loughead 2008.

Project Motivation

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Knee Pain • 10-25% report postoperative anterior knee pain (AKP)1

•  Patellar tracking is a likely cause of AKP but not proven •  Does patellar tracking differ for patients with & w/o AKP?

Gender Differences •  Tracking differences exist2 •  Gender-specific (GS) implants are designed to suit female femurs (ML/AP ratio) •  Does GS implant design affect patellar tracking? Pain? Zimmer Gender Solutions Knee

[www.genderknee.com] 1. Helmy N, 2008. 2. Anglin C, Ho KCT et al. 2008.

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Project Rationale

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Postoperative AKP

Patellar Tracking

Gender (ML/AP Ratio)

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Post-TKA Mechanics

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X-ray & Fluoroscopy1

  2D   poor accuracy   single flexion angle (X-ray)   sagittal view (fluoro)

Computed Tomography (CT)   3D   Better cortical bone

definition   Higher flexion angles   Faster acquisition time   Less noise discomfort

Limitations   Metal Artifact   Radiation

Magnetic Resonance Imaging (MRI)2,3

  3D   limited accuracy (low res.)   accuracy not validated for

patellar kinematics   no contact area   metal artifact

1. Stiehl JB, 2001. 2. von Eisenhart-Rothe R, 2007. 3. Lee KY, 2005.

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Objectives

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Development Develop an accurate protocol to measure in vivo 3D knee arthroplasty mechanics and joint contact areas using computed tomography

Validation Validate the accuracy and the intra- and inter-observer repeatability of the CT protocol

Implementation Evaluate the effects of gender-specific implant design on knee mechanics

CT Protocol Development

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Image Acquisition

Segmentation

Registration

Joint Mechanics & Contact Areas

CT scans at multiple flexion angles Partial weight-bearing knee rig1

Metal artifact reduction Components matched to 3D TKA models Segment 3 bones + 3 components

Each flexion angle registered to full extension position

6-DOF patellofemoral (PF) mechanics 6-DOF tibiofemoral (TF) mechanics PF & TF contact areas Patellar tracking relative to femoral groove

1. Connelly K, MSc Thesis, University of Calgary, 2006

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Defining Patellofemoral Mechanics

Patellar Maltracking   defined as shift > 5 mm or tilt > 5˚

  could lead to clinical differences1

9 1. Shih H, 2004. 2. Katchburian M, 2003.

Clinical Standard: 30˚ or 45˚ radiographs

Biomechanics Reference: Mechanical Axis vs.

Prosthetic Femoral groove2

CT Protocol Validation

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Accuracy

Repeatability

Expected Outcomes

At least 3 cadaveric knee specimens Compare CT method against Optotrak system using bone-mounted markers

Intra-observer repeatability perform protocol at least 3 times

Inter-observer repeatability 2 different observers perform protocol

Accuracy of at least an order of magnitude smaller than clinically relevant differences in mechanics: 5 mm or 5º (translations and rotations)1

1. Shih H, 2004.

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Preliminary CT Scan

TKA components on Sawbone CT scanner: Siemens Sensation 64 Image analysis software: Amira 4.1.2 Scan Parameters:   120 kVp, 108 mAs   FOV: 150mm   Matrix: 512 × 512   Slice Thickness: 2 mm   Number of slices: 38

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Segmentation

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Goal Achieve adequate segmentation such that key

distinctive features of each knee component are accurate for 3D TKA model registration

Patellar Button Pins

Tibial Plateau Flange / Holes

Femoral Component Pins

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Segmentation Stages (1)

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Original Image Resampled data

0.8 mm slice resolution

Rough Threshold Sawbone [-650 to -300] Metal [+2000 to +3071]

Segmentation Stages (2)

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Mask Artifact Tibial Component Threshold

[+2000 to +3071]

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Segmentation Stages (3)

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Femoral Component Morphological Filter: Closing Operation

Island Removal Dilation

Erosion

Segmentation Stages (4)

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Patellar Component Mask patellar region

Threshold to identify pins [-200 to 3071]

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Segmentation Results

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Surface generation using unconstrained 3D smoothing

•  After segmenting implants and masking the artifact, the remainder of the image is bone segment bones •  If preop CT available, register preop to postop •  Assign coordinate systems to bones and components

CAD Model Registration

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Metal Artifact

  Photon starvation (not enough photons hitting the detectors) producing incomplete data

  Beam hardening (increase in effective energy as lower energy photons are attenuated)

  Streak artifact is a combined effect

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Metal Artifact Reduction (MAR)

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Before scanning Material selection (no control for our study)

During scanning Patient positioning Standard MAR algorithm available on scanner Scanning parameters (e.g. adjust kVp, mAs)

After scanning Use iterative forward & backward projections to correct and reduce metal artifact given known implant geometry (future research)

Register to preop CT, when available

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Patient Positioning Artifact band appears in region of greatest material thickness

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Full Extension 30˚ Flexion 60˚ Flexion

Foot on CT Table

Foot Raised

Raising the patient’s leg may move the patella out of the artifact and decrease the width of the artifact band

Full Extension 30˚ Flexion 60˚ Flexion

Radiation

  Radiation dose limits number of flexion angles & number of patients

  Ethically reasonable because in extremity and in older (non child-bearing) individuals

  Will optimize acquisition parameters to reduce radiation dose

  Aiming to combine with fluoroscopic imaging so only need one CT image (future work)

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Conclusion Anticipated Outcome   Accurate, validated in vivo 3D CT method able to detect

clinically relevant differences in patellofemoral & tibiofemoral mechanics

  Patellar mechanics reported relative to femoral groove, to improve clinical relevance

Potential Applications   Gold standard for testing other techniques   Testing clinical hypotheses (gender-specific vs. standard

components, pain vs. no pain)   Use results to improve implant design, surgical technique,

preop screening, postop diagnosis, prehab & rehab

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Acknowledgements

Research Team Dr. Carolyn Anglin – Supervisor

Dr. Carol Hutchison – Orthopaedic Surgeon Jack Fu – MSc Student

Jeff Wai – Research Assistant

Funding CAOS-International Travel Fellowship

NSERC Alberta Ingenuity Fund

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