Materials
This in vitro study was approved by the Ethics Committee of Chinese PLA General Hospital. Due to the fact that the cadaver pelvises used in this study were provided by Anatomy Lab of Chinese Aerospace General Hospital, no informed consent was needed. For this study we used 5 pelvises, self-made pelvis upper and lower pads, a C-arm X-ray machine (SIEMENS ARCADIS ORBIC), minimally invasive percutaneous lag screw guiding apparatus, and protractor.
The design principle of minimally invasive percutaneous lag screw guiding apparatus
The principle of “Two Points Form a Line” helps determine the position of the two points, the entry and exit points. Based on the structure of the rectangle, which shows that the length of the corresponding two sides are equal, the minimally invasive percutaneous lag screw guide apparatus is developed and is expected to solve the difficult problem that the hollow screw cannot be placed successful in the acetabula anterior column (Fig. 1).
The usage and surgical indication of minimally invasive pecutaneous lag screw guide apparatus
This apparatus has two important parts, including two locators (A1 and A2), two side connection bars (B1 and B2) and one square connection bar (C), which ensure that the entire apparatus form a rectangle and fulfill the principle of “Two Points Form a Line”. (See Fig. 1) C arm navigation should be applied during the surgery. First, the entry position (E in Fig. 2) is determined by a locater (A1 in Figs. 1 and 2) and the exit position (D in Fig. 2) by another locater (A2 in Figs. 1 and 2). Then, B1 and B2 are connected by a square connection bar (C in Figs. 1 and 2). Lastly, an electric drill is used to drill from locater (A1) to locater (A2) or from locater (A2) to locater (A1). Finally, the guiding needle moves through entry and exit points (D and E in Fig. 2) successfully and the hollow screw is secured. (See Fig. 2) The surgical indication of this apparatus includes simple fracture of the anterior column of the acetabulum with minimal or no displacement after miniinvasive or closed anatomical reduction and the transverse fracture of the acetabulum with minimal or no displacement.
Operation mimesis in vitro
Place the pelvis specimens in the self-made pelvis lower pad, keep the plane of the pelvis at 60° to the horizontal plane and then cover the self-made pelvic upper pad (Fig. 3a). Let the pelvic completely be wrapped up by the pelvis pad to simulate a patient who is lying flat on the operation table. The C-arm is placed on the contralateral side. Conventional views obtained are frontal, inlet and outlet views of the pelvis, the obturator oblique view and iliac oblique view. Following the steps mentioned above, the guiding needle is successfully placed through the locater tube of the posterior column of the ilium (Fig. 3). Then, the guide apparatus is used and the hollow lag screw is attached along with the guiding needle.
Evaluation Index
Visual observation
When the operation was finished, the five pelvises were visually observed to find out whether the hollow lag screw punctured the cortex or entered into the hip joint
X-ray examination
When the operation was finished, the five pelvises were placed under the C-arm to obtain views from the front and back, as well as inlet and outlet view of pelvis to confirm whether the guiding needle is in the acetabular anterior column.
CT Scan and analysis
After the experiment, CT scan and 3-D reconstructions were obtained for the five pelvises to confirm the position of the screws. The CT data was entered into Mimics by .docm Format and a cross-section vertical to the screw by reslice function was obtained along the Screw (Fig. 4).
The authors reconstructed the cross-section of acetabular notch (Fig. 4a) and the two cross-sections of the screw at which the distance of the screw and iliopectineal line’s arc roof is at its shortest (Fig. 4b). Within the two cross-sections, the radius of the biggest circle containing R (mm) and the distance of the circle center to the screw center T (mm) are measured. Here we used a 5.0-mm diameter hollow screw (the radius is 2.5 mm).
Make scatter diagram to describe distribution
The scatter diagram was obtained with the measured data (R-r) as the transverse axis and T as the vertical y-axis to describe the distribution. When points (T, r) were under the line with an inclination of 1 (namely T = r-r), the screw was suggested to be within the cortex and not puncturing the acetabula anterior column or the hip joint.