The Ilizarov Method

Overview

The Ilizarov method is used to correct bone deformities by using an adjustable frame to simultaneously perform alignment and distraction of an open-wedge osteotomy. We seek to improve the accuracy of Ilizarov’s method by (a) planning the correction from a patient-specific CT scan, and (b) using image-guidance technology to apply the frame to a patient. We use many of the same ideas from our fixation-based method of distal radius osteotomy: both that work and the Ilizarov method employ fixation as an essential part of distraction and alignment of bone fragments.

The Ilizarov Method

The Ilizarov method is used to correct bone deformities by using an adjustable frame to simultaneously perform alignment and distraction of an open-wedge osteotomy. The Ilizarov method of osteogenesis by distraction and fixation is used to correct rotational and translation deformities in the three axes of motion. The Ilizarov method is done by performing osteotomies on the healthy portions of the deformed bone and distracting at a regulated rate to promote bone growth. The method eliminates the need to destroy tissue, insert permanent artificial screws or metals, inject compounds or immobilize bones for months in a cast, and has been found to be highly successful in the treatment of angular deformities, malunions, non-unions, pseudoarthroses, bone infections, open fractures, post-traumatic osteomyelitis, limb lengthening, bone gaps, poliomyelitis, club foot, congenital and acquired disorders of the limbs, dwarfism, skeletal defects, stump elongation and joint contractions.

The Taylor Spatial Frame is an external orthopedic fixator device used to implement the Ilizarov method. The device consists of two circular bases or rings, six telescopic linkage rods (also called struts), and twelve universal joints that connect the struts to the rings. The six measurements used to characterize deformities are re- lated to the displacements on the struts by inverse kinematics.

The Traditional Technique

The surgeon first planned the procedure by determining the nature of the deformity and the specific mechanical parameters of the Taylor frame. Determing the parameters is no trivial task. Indeed, planning errors are frequent in the traditional technique.

Computer-Assisted Planning

The preoperative planning stage can be achieved in seven steps:

1. Develop a surface model of the deformed bone from a CT scan.
2. Determine the anatomical frame of reference (usually distal to the deformity).
3. Perform “virtual surgery” by cutting the deformed model and moving one fragment into a corrected position and orientation.
4. Position a model of the fixation pins, for a neutral Taylor frame shape, through the fragments.
5. Calculate the rotational and translational components of the correction.
6. Calculate the initial Taylor frame shape. Calculate the initial positions of the fixation pins.

Computer Assisted Intraoperative Guidance

Intraoperatively, a tracked calibrated drill is used to drill the pilot holes under computer guidance according to the preoperative plan. The surgeon has the option of using images of the bone and drill on the screen, or using numerical feedback on the distance and angles to the center of a planned pilot hole. The planned osteotomy site was also marked using the tracked drill. The following picture shows the guidance screen for the osteotomy site and a pin location. The drill hole is located at the intersection of guidance planes. Note that the distances and angles of the tracked drill tip to the planes are presented in both graphical and numerical form.

This is the final step of the computer assistance for the technique. After drilling, the Taylor frame was attached to the bone as per the traditional technique.

References

Further information on our work can be found using the links below:

1. O. O. Iyun, D. P. Borschneck, and R. E. Ellis. Computer-assisted correction of bone deformities using a 6-DOF parallel spatial mechanism. In T. Dohi and R. Kikinis, editors, Medical Image Computing and Computer-Assisted Intervention - MICCAI 2002, pages 232-240. Springer Lecture Notes in Computer Science #LNCS 2488, 2002.
2. O. O. Iyun. Computer-assisted Planning and Guidance of the Ilizarov Method with the Taylor Spatial Frame. Master's thesis, Queen's University at Kingston, 2002.