Axial Perspective To Find The Largest Intraosseous Space Available For Anterior Column Screw Of The Acetabulum And Define The Safe Zone For Antegrade Posterior Column Screw Of The Acetabulum

BackgroundAcetabular fractures are usually caused by high energy and occur mainly in young adults. With the rapidly develop of the economy, the industrial and traffic accidents are increasing gradually, which lead to large numbers of patients with acetabular fractures. And acetabular fracture has been one of the hotspots in recent years in the field of orthopaedic research. In another way, because of its deep location, complex anatomy, reduction and fixation difficulties and postoperative complications, acetabular fracture is also a difficult problem in orthopaedic research.Plate osteosynthesis and lag screw fixation are two common fixation methods for acetabular fractures. Compared with plate osteosynthesis, lag screw fixation has been proven to achieve anatomical reduction and inter-fragmentary compression with good clinical outcomes. Additionally, biomechanical tests have shown that lag screw fixation of transverse or T-shaped fractures was at least as stiff as plate fixation. Currently, antegrade or retrograde lag screw fixation along the long axis of the posterior column is widely used in clinical practice for treatment of transverse and T type fractures. However, this procedure is technically demanding due to the unique and complex 3D anatomy of the anterior and posterior column. A misdirected or misplaced screw during the sugery may penetrate the hip joint or damage the rich neurovascular structures in the vicinity of the anterior and posterior column.To insert a long screw through a safe osseous path without penetrating the hip joint or bone cortex, familiarity with the osseous anatomy of the anterior and posterior column is essential. Because the optimal entry point and direction are determined by the largest screw path found during the research, the current research hotspot is focused on looking for the largest anterior and posterior column screw path. Cadaveric specimens and virtual 3D models of adult pelvis are used to study the the largest anterior and posterior column screw path. Ebraheim et al. were the first to study the anterior column of acetabulum on cadavers. They sectioned the anterior column perpendicular to its axis and then determined the diameter of the smallest cross section as the largest screw diameter. As we can imagine, the diameter of cross section is actually determined by the axis he chose. As we know, the anatomy of the anterior column is complex; it is very hard to make sure which its axis is. Reproducibility of his study can be affected by the arbitrary position of this axis. Attias et al used 3D computer models to evaluate the intra-osseous space for screw fixation of acetabular fractures. In his study, virtual cylindrical implants were superimposed upon the posterior column and then the position of the virtual bolt was adjusted until it was totally enveloped by the posterior column. However, the 3D adjustments have to be balanced gradually and carefully because they are hampered one another. Satisfactory adjustment of the virtual bolt in one orientation can happen while there is violation of the outer contour of the posterior column in another orientation. Their methods cannot compare or assess the size of osseous path where the virtual implant goes between different placements. Hence, it is considerably hard to make sure whether the implant is situated at the optimal position that can yield the largest space available to accommodate a long lag screw.To ensure safe insertion of a long screw into the anterior column, the screwing procedure has to be monitored radiographically to avoid iatrogenic neurovascular injury. Conventional 2-dimensional fluoroscopy remains a routine means for many surgeons to monitor the placement of implants in various trajectories for pelvic and acetabular fixation. In the percutaneous screw fixation of the anterior column fractures, traditional radiographic images are obtained at outlet view, inlet view, and obturator-outlet view. Each image plays a very important role in the percutaneous screwing procedure that surgeons have to adjusting the positions of a C-arm machine repeatedly to obtain those images intraoperative. Therefore, more operation time, more blood loss, and more radiation exposure are required. To overcome the shortcomings of the traditional radiographic images, a new x-ray projection has to be found to simplify the surgery operation.In the treatment of the posterior column fracture, antegrade lag screw fixation is used commonly. However, the method used by previous studies cannot find the largest screw path, which makes the entry point and direction less useful. Moreover, a safe zone on the inner surface of the ilium has not been previously defined for antegrade lag screw fixation of posterior column fracture.Objectives1. Using a novel metnod axial perspective to find the largest screw path of the anterior column;2. Test the existence and clinical feasibility of the anterior column axial view projection in cadaveric specimens and patients;3. Using a novel metnod axial perspective to find the largest screw path of the posterior column and define the safe zone for antegrade lag screw fixation of posterior column fracture.Part I:Axial perspective to find the largest intraosseous space available for percutaneous screw fixation of fractures of the acetabular anterior columnMaterials and methodsData CollectionFifty-eight human subjects (32 men,26 women with the mean age of 50.28 years. range:18-89 years) admitted to our institution from February 2013 to December 2013 without pelvic and acetabular injury or lesions were recruited in this study. All patients underwent a sixteen-line pelvic helical computed tomography scan (GE, US) with 1.0 mm slices at 0.1-s intervals for imaging of the acetabulum. The raw data obtained were stored in Dicom format.Model ReconstructionThe raw data sets were reconstructed into 3D models using MIMICS 10.01. The left hemipelvis was exported in STL format and then imported into the image-processing software Geomagic Studio 12.0. Next, the inner triangular patches which represented the contents of the marrow cavity were deleted to make the marrow cavity hollow in the 3D models. After processed in Geomagic Studio 12.0, the images were exported in STL format and imported again into MIMICS where all simulations and measurements were carried out.Largest screw path analysisTo distinguish the screw path in the 3D models at one same perspective, we downgraded the transparency of 3D models and turned the 3D models at the axial perspective, a view perpendicular to the cross section of the anterior column axis. As a result, a translucent area with a darker outline was seen clearly. The translucent area represented the marrow cavity of the anterior column and the outline showed the cortical bone.First, we adjusted the positions of 3D model in the axial perspective to find the likely largest triangle-like translucent area by visual observation. Then a virtual computer-aided design screw was placed perpendicular to the screen at the centre of the translucent area. We increased the screw diameter progressively to accurately determine the maximum implant diameter the triangle-like transparent area could accommodate. The maximum implant diameter was defined as the largest diameter of the screw that did not penetrate the outline of the translucent area. After the above steps were repeated in three likely largest triangle-like transparent areas by three observers, we obtained three accurate maximum implant diameters. Comparison was conducted to finalize the accurate maximum implant diameter of the screw the specific 3D model could accommodate. The maximum screw diameters, the directions of the screw to the transverse, coronal and sagittal planes were measured respectively.Moreover, adjustment of the positions of 3D model in the axial perspective might result in ellipse-like translucent areas which seem suitable for placement of two virtual screws rather than one largest implant. Likewise, The maximum screw diameters were measured.Statistic analysisAll the experimental data were presented as the mean and SD or median and range. Independent-samples t-test was used to compare the data between male and female. Statistical significant was accepted at p< 0.05. The SPSS statistical software package for windows (version 19.0) was used for statistical analysis.ResultsThe mean maximum diameters of Screw I and Screw II were 11.20±1.73 mm (7.80-14.60 mm) and 8.71±0.91mm (6.60-10.60 mm) respectively. The differences regarding the mean maximum diameters of the two different screws (Screw I and Screw II) between male and female were of statistical significance (p< 0.05). The diameters of Screw I in all the 58 cases were bigger than 7.3 mm. For Screw II,2 cases were smaller than 7.3 mm (>6.5mm) and both female. The diameters of Screw II in all males were bigger than 7.3 mm.The angles of Screw I to the transverse, coronal and sagittal planes were 41.16° ± 4.59°,18.18° ± 1.15°,44.33°±4.31° respectively. The differences of the angles to the coronal plane between the male and female were of statistical significance.ConclusionsThe acetabular anterior column could safely accommodate not only one 7.3 mm screw but also two 6.5 mm screws. The anterior column axial projection may be clinically feasible.Part II:Exploration research of the anterior column axial view projectionMaterials and methods1. Test the existence and clinical feasibility of the anterior column axial view projection in pelvic cadaveric specimenFive cadaveric specimens of hemipelvis without fracture, tumor, or deformity were used in our test. First, the cadaveric specimen of hemipelvis was fixed on the operating table. And then, we positioned the C-arm along the axis of the anterior column and adjusted the positions of the C-arm until a likely largest translucent area was visualized clearly. After the angle of the C-arm was stabilized, we located the entry point of a guide pin at the center of the translucent area and determined the inserting direction when the projection of the guide pin became a point inside the translucent area. After the entry point and direction of the guide pin were determined, the guide pin was inserted using battery-powered equipment. The cadaveric specimens were observed for cortical penetration or joint violation.2. Anterior column axial view projection in patientData CollectionSix volunteers (3 men,3 women with mean age of 50.28 years and range of 48-52 years, without pelvic and acetabular deformity, injury or lesions) admitted to our institution were recruited in this study. All patients underwent a sixteen-line pelvic helical computed tomography scan with 1.0 mm slices at 0.1-s intervals for imaging of the pelvic. The raw data obtained were stored in Dicom format in a computer.Model ReconstructionThe raw data sets of each patient were reconstructed into 3D model using the software MIMICS 10.01. The 3D pelvic model was exported as an STL model and then imported into the image-processing software Geomagic Studiol2.0. Then, the inner triangular patches, representing the contents of the marrow cavity, were deleted to make the marrow cavity hollow in the 3D models. Following processing in Geomagic Studio 12.0, the images were exported to an STL model and imported again into MIMICS where all simulations were carried out.Anterior column axial view projection simulationTo determine the screw path in the 3D models from one single perspective, the transparency of the 3D pelvic model was downgraded and the 3D model was turned at a view perpendicular to the cross section of the anterior column axis. Thus, a translucent area with a darker outline was seen clearly. The translucent area represented the screw path of the anterior column (The projection of the anterior column screw path). And then the position of the 3D model was adjusted carefully to find the largest screw path (translucent area that could accommodate a screw with largest diameter). The image with the largest translucent area showing in the computer was the simulation of the anterior column axial view projection, which would be helpful in indentifying and confirming the anterior column axial view image obtained in patient intraoperative.Virtual screw positionAfter the largest translucent area was determined, a virtual computer-aided design screw was placed perpendicular to the screen at the centre of the translucent area to make sure the screw was placed in the largest screw path of the anterior column.3D reconstructed the skin model of each patient and turned the pelvic model opaque; we could get the relative position of the virtual screw and the body, which would be helpful to guide the initial C-arm position intraoperative.Anterior column axial view projection in patientsTo obtain the anterior column axial view fluoroscopic image, each patient was supine in a fully radiolucent operation table with the leg of abnormal side straight and the leg of the contralateral side flexion, abduction and external rotation. The C-arm machine was placed at the caudal end of the operation table with the C-arm fluoroscopic intensifier position at the pelvic lateral view. Guided by the relative position between the virtual screw and the body, the C-arm intensifier was first tilted approximately 30° toward the "abnormal side" of the patient and then rotated approximately 45° toward the caudal side of the operation table.With the patient and C-arm position above, we obtained a fluoroscopic image which was similar with the corresponding simulation image in each patient. Adjust the position of the C-arm fluoroscopic intensifier carefully until the largest translucent area that represents the screw path show up; the fluoroscopic image with the largest translucent area was defined as the ideal anterior column axial view projection. Each anterior column axial fluoroscopic image was compared with the images obtained in cadaveric pelvis specimens to analyze the difference between them. Each anterior column axial fluoroscopic image was compared with the corresponding simulation image to find and analyze potential anatomic landmarks that were helpful to indentify the translucent area in each patient.Results and ConclusionTranslucent areas were successfully observed in all the cadaveric hemi-pelves and guide pins were successful inserted in all the cadaveric hemi-pelves with the assistance of the anterior column axial view projection without cortex penetration or joint violation. The anterior column axial view images were successfully obtained in all six patients.To obtain the ideal anterior column axial view image, the position of the patient should be supine with the leg of abnormal side straight and the leg of the contralateral side flexion, abduction and external rotation; the C-arm machine should be placed at the caudal end of the operation table with the C-arm fluoroscopic intensifier first positioned at the pelvic lateral view, and then tilted approximately 30° toward the "abnormal side" of the patient and rotated approximately 45° toward the caudal side of the operation table.Unlike the anterior column axial view images obtained from cadaveric pelvic specimens that the translucent area and anatomy structures were so clear to indentify, the fluoroscopic images obtained from the living patients were quite ambiguous that it’s too hard to identify the corresponding anatomical structure to final determine the translucent area that represents the screw path. With the help of the simulation image, we found that the translucent area could be quickly identified through three anatomic landmarks, including the greater sciatic notch, the true pelvis edge, and the acetabulum. The translucent area is just located in the area surrounded by the three anatomic landmarks. The boundary lines of translucent area were the projection of the anterior column superior cortex, the anterior column medial cortex, and the acetabulum.Part III:Definition of a safe zone for antegrade lag screw fixation of fracture of posterior column of the acetabulum by axial perspective Materials and MethodsData CollectionFifty-nine Chinese subjects without pelvic and acetabular injury or lesions were recruited between February 2013 and December 2013 in this study. There were 35 male and 24 female. Mean age was 50.28 (range 17-89). All subjects underwent a sixteen-line pelvic helical computed tomography scan (GE, US) with 1.0 mm slices at 0.1-s intervals for imaging of the acetabulum. The raw data obtained were stored in Dicom format in a computer.Model ReconstructionThe raw data sets were reconstructed into 3D models using the software MIMICS 10.01. The left hemipelvis was exported as an STL model and then imported into the image-processing software Geomagic Studio 12.0. Then, the inner triangular patches, representing the contents of the marrow cavity, were deleted to make the marrow cavity hollow in the 3D models. Following processing in Geomagic Studio 12.0, the images were exported to an STL model and imported again into MIMICS where all simulations and measurements were carried out.Largest screw path analysisTo determine the screw path in the 3D models from one single perspective, the transparency of the 3D models was downgraded and the 3D models were turned to the axial perspective, a view perpendicular to the cross section of the posterior column axis. Thus, a translucent area with a darker outline was seen clearly. The translucent area represented the screw path of the posterior column while the outline of this area represented the overlaying cortical bone.First, the position of 3D model was adjusted in the axial perspective to find the largest translucent area by visual observation. Then a CAD screw (7 mm in diameter) was placed perpendicular to the screen into the centre of the translucent area. The CAD screw diameter was increased progressively in order to accurately determine the maximum implant diameter that the translucent area could accommodate. The maximum implant diameter was defined as the largest diameter of the screw that did not penetrate the outline of the translucent area. After the above steps were repeated in three likely largest translucent areas by three observers, three accurate maximum implant diameters were obtained. Comparisons between them were conducted in order to finalize the accurate maximum implant diameter of the screw that the specific 3D model could accommodate. The largest diameter of the virtual screw and the angles of the screw to the coronal and sagittal planes were measured.Definition of the safe zoneAfter the largest translucent area was determined, the’Draw Freeform Curve’ function in mimics was used to draw the outline of the largest transparent area in the 3D model. The model was turned opaque when the drawing process was completed and a triangle-like area surrounded by the curve was found in the iliac fossa. This area was defined as the safe zone for antegrade lag screw fixation of posterior column fractures.To define the safe zone quantitatively, five points A, B, C, D, O were defined as follows. The triangle-like curve intersected with the upper edge of the pelvic brim at point A and C. The two ends of the base of the triangle-like curve were defined as point B and D respectively. The arcuate line end at the Sacroiliac joint was defined as point O. The safe zone was defined as ABDC. The line segments of OA, AB, OC, CD, the angles of OAB and OCD were measured respectively and precisely.Statistical analysisAll experimental data were presented as the mean and standard deviation or median and range. Independent-samples t-test was used to compare the data between males and females. The threshold for statistical significance was set at p smaller than 0.05. SPSS (version 19.0. for Windows) was used to analyze the data.ResultsThe mean maximum diameters of the virtual screw were 16.81 mm (14.20-20.40 mm), and the angles of the screw to the coronal sagittal planes were15.34 ± 5.81°and 9.85°±4.27°, respectively. We determined that the differences of the mean maximum diameters of the virtual screw and the angles of the screw to the sagittal plane between male and female were of statistical significance (p< 0.05).The line segments OA, AB, OC, CD, and the angles OAB and OCD were recorded in Table 2. The differences between male and female were of statistical significance (p< 0.05).ConclusionUsage of an axial perspective allows better virtual screw placement. It delineates a larger osseous space in the posterior column available for screw path and defines a safe zone for posterior column antegrade lag screw insertion. With careful preoperative planning, screw placement can be safely done within the safe zone as defined by our method.