Computer Assisted Orthpaedic Surgery in Adult
Reconstructive Surgery: Basic Concepts
James B. Stiehl, MD
Columbia St Mary’s Hospital
Milwaukee, Wisconsin
1. Historical Landmarks
a. Bargar and Paul, 1987, first robotic application
i. IBM collaborative project
ii. Developed digital “pixel” accuracy, 20-30 microns
iii. Robodoc required preoperative CT, and fiducial placement
b. DiGioia, Jarmaz
i. Developed first CT total hip application in 1996
ii. Required CT segmentation protocol
iii. Allowed intraoperative fiducial placement
iv. Claimed accuracy of 1/1 mm
c. Krakow; Saragaglia, 1997,1998
i. Separately developed total knee imageless referencing protocols
ii. Required the development of hip/ankle kinematic referencing method
iii. Validated precision on the order of 0.5 to 1 (Saragaglia,2007)
2. Components of Contemporary Computer Navigation Systems
a. Computer platform
i. Labtop or tower requires powerful processors and memory
ii. Unix and Linnix codes preferred for stability with high digital
processing, but Windows XP works if set up correctly
iii. Open platform: allows use with any total knee prosthesis
iv. Proprietary platform: limited by CAD models to one particular
prosthesis
v. Typical “capital” system: computer tower, CCD optical camera, LCD
monitor, mouse, foot pedal activator.
vi. Labtop System: Similar but with ‘suitcase’ portability.
b. Tracking Technologies
i. Optical: Requires Dynamic Reference Base with reflective balls
(minimum 3; preferably 4-5) attached specifically to an array that is
attached to the “target”. Target may be bone, instrument, prosthesis.
Requires 2 or 3 optical cameras, that view passive or active (LED)
arrays from distance of 6-8 feet. Must have unobstructed “line of
sight”. Accurate to 0.25 mm.
ii. Electromagnetic: Trackers emit electromagnetic impulse that is
tracked by an electromagnetic coil, target must be within a box of 2030 centimeters. Does not require line of sight. Must have software
algorithm that controls for ferrous, magnetic, or electrical interference
to the electromagnetic impulse from the tracker. This is the current
limiting factor for this technology! Accurate to 0.5 mm.
iii. Ultrasound: Uses 2D, 2.5D or 3D probes to identify the position of a
target landmark, bone, or object. Accurate to 0.25 mm.
iv. Video Monitoring: Allows for realtime viewing of DRB movement.
c. Referencing Methods: Means of Registering or Describing the “Target”
i. Computed Tomography: Used initially for high 3D digital portrayal of
the target object. First application was acetabular total hip placement.
Requires segmentation of the image and standardization to a known
reference frame, ie the anterior pelvic plane of the pelvis in THA.
Highly reproducible and accurate to 1/1mm. Most systems utilize a
‘best fit’ validation scheme in the software. Currently requires a preop
CT, but future systems with intraoperative CT may allow for
‘automatic’ registration.
ii. Fluoroscopic Radiography: Two dimensional registration, that allows
for biplanar images of the target object portrayed on screen, on which
virtual movement of target objects such as drill or prosthesis may be
observed by the surgeon. Requires registration of the images by direct
‘touch pointing’ of anatomical features that can be recognized on the
images in real time. Similar accuracy as CT but relies on grid to
correct for earth’s gravitational forces, and anatomical features that are
specific enough to be precisely identified both on the patient and on
the image.
iii. Imageless: Requires direct ‘touch point’ matching of specifically
predefined landmarks that are recognized by the computer software
protocol. Example is the femoral center point in total knee
application. One system defines this point as the center of the roof of
the intercondylar notch which also is on Whiteside’s line, and the
transepicondylar axis. Total knee is one of the applications that allows
ready access to landmarks than can be referenced under direct vision.
This method is accurate to 0.25 mm.
iv. Kinematic: Novel method described for total knee where the femur is
marked with a DRB, and then passed in a circular motion to define a
cone shaped movement whose center is the center of the femoral head.
The computer then identifies the finite projection of the cone which is
the center of the femoral head. Similar method defines the
transmalleolar axis of the ankle joint.
v. Ultrasound: Uses 2D, 2.5D, 3D imaging of ultrasound to define points
in space, to a submillimeter accuracy. Problem to date has been the
inability to accurately describe broad anatomical surfaces such as the
ASIS which lead to errors of 2-5 mm.
vi. Bone Morphing: Secondary referencing method where surfaces of the
target bone are described and overlayed onto a ‘baseline’ structure
such as the distal femur.
3. Validation of Computer Systems:
a. Descriptive terms of Standard Metrology
i. Unit of Measurement: Value of measurable quantity
ii. True Value: Value determined by perfect measurement, considered
infinite, as infinite number of measurements needed to determine
iii. Conventional True Value: Best guess measure, also ‘ground truth’ or
reference value
iv. Measurand: Quantity subject to measurement
v. Influence Quantity: Sum of slight variance that may occur in
measurement, such as identifying the exact edge of acetabular
component
vi. Accuracy of Measurement: Qualitative assessment of measured value
in relation to the true value
vii. Precision of Measurement: Closeness of agreement of independent test
results obtained under stipulated conditions, such as repeatability or
reproducibility, and usually defined as standard deviation.
viii. Repeatability: Measurement under identical prescribed conditions
ix. Reproducibility: Measurement when one condition is altered in
measurement, such as having different observers.
x. Error of Measurement: Result of measured value minus the true value
of the measurand.
xi. Random Error: Measurement of measurand minus the mean after
infinite number of measurements.
xii. Systematic Error: Mean measurement of measurand minus the true
value after an infinite number of measurements.
xiii. Random Error equals Error minus Systematic Error
xiv. Systematic Error equals Error minus Random Error
xv. Correction: Amount added to measured value to correct for systematic
error.
xvi. Type A Error: Deals with errors created by uncertainty
xvii. Type B Error: Error other than statistical error.
b. Descriptive Statistics:
i. Mean
ii. Standard Deviation
iii. Experimental Standard Deviation
4. Process Capability Analysis
a. After manufacturing process is brought under control assessment of precision
or process capbability is done by following formula: Cp
Cp 
(USL - LSL )
6
b. Where USL and USL is upper and lower specification limits beyond which an
outlier is determined.  is standard deviation.
c. Typical Cp for process capable function is 1.3
d. Where there is offset of measurand from true value, offset capability index is
determined: Cpk
 (USL - x ) ( x - LSL ) 
Cpk  min 
,

3
3


e. For this measurement, the minimum of either the first or second value is
determined, with a goal of at least 2.0
f. This analysis basically determines the 6 or the bell shaped curve for a
process, and defines outliers as anything below or above the bell shaped
curve. General Electric states that an outlier, beyond the specification limits
will occur in 3.4 of 1,000,000 trials. The advantage of this method is that it
allows simple calculations that can be applied across a variety of technologies.
g. The most difficult aspect of this method is to establish the upper or lower
specification limits. For example, we would recommend that the specification
limits for mechanical axis alignment in total knee arthroplasty would be 5.
Our reasoning, normal knees have been shown to be up to 5 from the
mechanical axis. Also Ritter,et.al. has shown that tibial components with 5
of varus will fail. Another biomechanical study has shown that 5 of variation
from the mechanical axis creates unacceptable forces in the knee joint.
Examples:
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