There are two basic types of computer assisted orthopedic systems:
1. Surgical freehand navigation systems which determine the spatial location that is held in surgeons hand and provide
a positional feedback to the computer in real time. This method in which a device is used as oreintation aid is not very different
to GPS satellite navigation system.
2. Robotic group: Medical robots autonomously carry out steps of a surgery without surgeon intervention. The basic principle
behind both the systems is similar although the task is carried out very differently in both.
Conceptual Structure: Three cpomponents that are interrelated form the heart of the entire sytem: The surgical site
or therapuetic object; the navigator and the virtual object. Each of these can be considered as non deformable rigid structure
with a 3 d cordinate value attached to it.
Therapeutic object: Usually refers to boney structures in the operative feild but may also be implants at the surgical
site. Pure soft tissue surgeries cannot be supported by these principles.
Navigator: The free hand technique uses optoelectronic tracking of all the surgical instruments used in the feild. The
spatial position of a rigid surgical instrument can be mapped by three non co linear points using triangulation technique.
These points are created using LEDS mounted on the instrument. Typically 4-6 LEDs are placed on one target marker
and lighted in sequence so that the position of each acn be determined seperately. A camera system that consists of two or
three CCDs (Charged couple device) record the signals emitted from these diodes. With use of large number of markers on the
instrument, an arbitary spatial location of the instrument can be defined.
An alternative to above optical loclaizer (LED) system is elctromagnetic tracking system that uses the elctromagnetic
feild generated by the emitting coils in instruments to determine the spatial location of the instrument.
Virtual object; A three dimensional preoperative Ct scan is used by computer to represent the image of the therapeutic
object. MRI can also be used but because of low bone soft tissue contrast they are less preferred. Digital X rays and scanned
conventional Xrays are two dimensional and hence cannot be used.
Preoperative imaging and the process of creation of virtual
object can alternatively be done intraoperatively using C arm images that can be used to recreate virtual objects intraoperatively,
however it suffers from the diadvantage of providing a two dimensional data. On the positive side it spares the additional
financial and logistic efforts involved in procuring the preoperative CT scan.
Registration and verification: This process matches the therapeutic object to the virtual object. It can be done
using some predefined anatomical landmarks or artificial surface markers like pins, spheres implanted under local anaesthesia
prior to planned intervention and CT scan.
The verification is done by touching the various palpable boney anatomical landmarks with a tracker and confirming wether
the corresponding position is displayed correctly by the navigation system.
While using C Arm for recreating virtual objects, the calibrationn can be made easy by attaching the markers to image
intensifier unit of a C Arm allowing its aptial position to be determined while acquiring the image.
A conventional approach that relies on surgeons judgment completly abandons any use of radiological images. The surgeon
uses the marked instruments to recreate a virtual model of the anatomy by digiting the specific points, lines, surfaces etc.
Tracking: the basic principle of any surgical navigation is the ability to determine accurately and precisely at every
step the location of your target bone and your surgical tools. The oreintation of tools relative to bone of interest
can then be calculated and compared to planned one.
Registration: Involves geometric transformation that would map the patients intraopertaive position to coincide with
the virtual object. The calculation of this position is called registration. It amy involve use of implanted physical markers
or fiducials or use of well defined surface landmarks.
Navigation: Once the tracking, verification and registration is done , the surgeon can now use his instruments to
know the position of his instruments relative to the target bone and how does it compare with the planned one. Simple interfaces
similar to those that are used by airplane pilot can guide the surgeon to desired tool alignment.
An example in hip nav surgery: One tracking marker is attached to the illiac wing of pelvis through a small incision,
other markers are attached to the cup placement tools and the pointing probe respectively. The verification and registration
is carried outby collecting 46 different cordinate points using the pointing probe and matching it with surface model
of the pelvis. Once the registration is accomplished the pelvis can be tracked in real time. The current oreintation of cup
can then be compared against the planned ones and displayed as a simple cross hair interface that allows the surgeon to interactively
allow the surgeon to interactively align the tool in accordance with preoperatively planned alignment.
Intraopertaive feedback: Continous updated images showing relative positions of tools and bones, including those modifies
as a result of bone cuts can be provided.