Osteotomies around the knee address lower limb deformities that affect the leg posture and especially the knee joint. Knee arthritis is a very common age-related joint disease that can be treated with osteotomy, as an alternative to knee replacement by implants. The preoperative planning process is demanding and significantly determines the surgical out-come. The choice of the osteotomy type and the determination of parameters, like the correction angle, are crucial for long-term success. We present a software tool for preoperative 3D planning of femoral and tibial opening, closing and derotation osteotomies, thus a particularly wide range of interventions. Our tool is based on patient specific 3D bone models and allows the user to position cutting planes and to simulate cut and alignment of the bone.
Lower limb osteotomies correct different forms of malalignment. A common disease, attended by varus (bowlegs) or valgus (knock-knees) deformity, is knee osteoarthritis. Those deformities are addressed by opening or closing osteotomy which try to establish a better alignment by passing the load bearing leg axis through hip, knee and ankle joint. Another type of deformity, the inner torsion of the bone, may result in disturbance in gait and can be addressed by derotation osteotomy (Fig. 1). In order to exactly define the characteristic of the osteotomy, angles between bone and limb axes are measured. Thus the deformity can be located and the anatomical parameter to be corrected can be determined. Following, the osteotomy mode, bone, level and orientation of the cut are determined. An important issue is the correction angle, ie. how many degrees to open, close or rotate the bone. These parameters are crucial for the good out-come of the surgery.
The femorotibial mechanical angle (FTMA), measured on a frontal standing
radiograph, characterizes the magnitude of varus/valgus and is important in
opening and closing osteotomy [
Brainlab developed VectorVision ⃝R osteotomy, an intra-operative planning
and navigation system for high tibial osteotomy [
We developed a preoperative osteotomy planning system for surgeons, based on
patient specific 3D models provided by SterEOS [
We implemented an algorithm to determine the correction angle given the
desired postoperative FTMA angle using trigonometrical laws [
In the opening and closing mode, the cutting plane(s) can be translated along the z axis, rotated around a hinge axis perpendicular to the frontal plane of the bone frame and it can be fitted to the bone amplitude at a certain level. This is useful to fit the hinge axis on the bone surface. In derotation mode, the cutting plane does not have a hinge axis and can be translated along the z axis.
The GUI consists of two parts (Fig. 3). The right part contains the 3D scene with bone and cutting plane objects. To enable optimal perception, a radiograph-like color scheme is used: the background color is dark and the bones are visualized in a lighter, natural color. The left part contains patient and surgery data as well as buttons for the osteotomy determination and simulation. Thus the effect of user inputs can be seen directly when changing parameter values. Expert talks along with demonstrations of our software helped us to confirm the correctness and appropriateness of the tool during the development.
A final test was made to evaluate the usability of OstEOS. Test persons were two osteotomy practicing surgeons. They were asked to plan and virtually execute three osteotomies using OstEOS, performing one opening, one closing and one derotation on pathologic bone models. Person 1, a surgeon interested in computer support, plans osteotomies manually on 2D radiographs. Person 2, a pediatric surgeon not familiar with software tools for osteotomy planning, does approximate planning and uses rarely planning sketches due to the fact that children’s growing is not stable.
The time taken for the planning of one surgery by person 1 was 3-4 minutes.
He commented that he was able to perform all necessary planning steps, that
he would like to show the tool to colleagues and that there were no barriers
using OstEOS. Wrong decisions could be easily undone with the help of reset
buttons. The approach to the osteotomy simulation and the steps to attain a
cut and axis alignment were clear. Person 1 was able to get the answers on his
questions; for example where to place the cutting plane, how many degrees to
open the bone and which size of wedge to use to fill the resulting gap. The 3D
bone representation helped to comprehend the shape of bone and and surgery
effects. Person 2 took 5-10 minutes and had been helped when she was stuck.
She found the tool very useful but in her opinion some functions were missing,
e.g. the possibility to locate the center of deformity (see [
The user test showed that all important basic functionalities exist, but that
further functionalities need to be implemented in order to enlarge the spectrum of
supported patient cases, and to allow the planning of more advanced osteotomies.
The efficiency is very satisfying since the planning can be done in three minutes,
whereas a traditional planning with manual calculations can take up to 15
minutes. The pre and postoperative parameters, updated and displayed in real time,
contributed a lot. Both test persons were interested in using a tool like OstEOS
in clinical routine work. OstEOS allows the planning of many types of osteotomy
and thus stands out from other 3D planning tools which are often limited to high
tibial osteotomy (see [
We conclude that the 3D aspect enriches the planning in comparison to 2D tools, but that it holds further challenges. Most notably is the interaction with the 3D scene that requires practice. Ideally, a planning tool should allow a realistic simulation of all manipulations that can be done during the surgery in the operating block. Future prospects are a better user-interaction with the 3D scene, the automation of the process and to take into account the knee joint force distribution.