The Use Of A Digital Model To Study The Percutaneous Retrograde Screw Fixation Of Acetabular Fractures

BackgroundSince the early 1960's,after the pioneering work of Judet and Letournel,there has been a gradual evolution in pelvic and acetabular surgery.Open reduction and internal fixation has become the standard method of treating acetabular fractures. Over the last 20 years,percutaneous fixation were proposed to the fracture of acetabular without displacement or with displacement of less than 2 mm,and also used in the acute acetabular fractures and nonunion of acetabular fractures.The outcomes were very well,which is biomechanically stable with lower rates of bleeding and infection,earlier ambulant and discharge.The percutaneous retrograde screws fixation of anterior and posterior acetabular fracture had become a most important part of minimally invasive surgery.But the application of this technique is limited by a lot of technical problem because of the anatomy problem and the difficulty of reduction,especially without the computer navigation.Therefore,the identification of the insert point,direction and diameter of the screws take an important part of the successful operation.Some researchers made a preliminary study of the percutaneous retrograde screw fixation of acetabular fracture in specimen.The diameter and direction of the screws were determined by the diameter and axis of the cross-section of anterior and posterior acetabular.But the results of study were variance and different from clinical application because of the technology deficient and limited account of specimen. Presently,there are not agreement on the insert point,direction and diameter of the screws of percutaneous retrograde fixation of acetabuluar fractures.Furthermore,we believe that the differences between sub-populations,such as men and women,should be investigated because of the different physiology structure of pelvis between them.The appearance of digital orthopaedic has created a new era for the basic application research of orthopaedic and traumatology.The elevation of scan precision of image equipment and the promotion of calculation of 3-demensinal model made the use of 3D reconstruction to application study the complicated structure to come truth,such as pelvis and acetabulum.Objective1.To create a new study method for the application research of percutaneous retrograde screws fixation of anterior and posterior column fracture of acetabulum.2.To provide an accurate and full-scale applied anatomical basis for the placement of percutaneous retrograde lag screw in the anterior and posterior column fractures of acetabulum through compare our method with that of other researchers.3.To provide empirical method and evidence for the personal preoperative prepare of percutanous retrograde screws fixation of acetabular.Methods1.Collection of imagesObtained 82 adult patien randomly,who took complete CT scan of pelvis in our hospital between April 2006 and March 2007 without any bony and anatomy abnormality.Forty males,forty two females,age range 18～80 years,164 cadaveric hemipelves were collected.Scan condition:tube tension 120kV,tube current 200～300mA,slice increment 1.25 mm,matrix 512×512.All the images were saved as DICOM format and imported to the personal computer.2.Reconstruct the 3-dimensional model of pelvisAll the sequent tomoscan images of pelvis were imported into the Materialise's Interactive Medical Image Control System(Mimics) in personal computer as DICOM format.The 3-dimensonal models were reconstructed by the same calculated parameter.3.Simulated the screws fixation of anterior and posterior column of acetabularThe 3D reconstruction model of pelvis was rotated to opposite side by 45 degrees(obturator oblique view) and transparencied.The virtual cylindrical implantⅠwas placed intraosseously in the anterior column.The diameter of the virtual cylindrical implantⅠwas progressively increased,and the outlet and inlet views of pelvis were obtained to make sure that the virtual cylindrical implant did not penetrate the cortical margin of superior ramus of pubis or into the acetabulum. Review the 2-dimensional images of the transverse plane;coronal plane and sagittal plane to avoid the virtual cylindrical implant penetrate the cortical margin of superior ramus of pubis or into the acetabulum.The maximum diameter of the virtual implantⅠwas defined as the largest diameter that did not penetrate the cortex of the column regardless of its orientation.Then the 3D reconstruction model of pelvis was rotated to homonymy by 45 degrees(obturator oblique view),and the diameter of virtual cylindrical implantⅡof posterior column were determined by the same way.4.Measured the insert point,direction and length of the screws The distance between the insert point and out point of virtual cylindrical implantⅠ,Ⅱwere measured directly by the tool of 3D measurement.Then the length of screwsⅠ,Ⅱwere defined.The direction 3D coordinate of the virtual cylindrical implantⅠandⅡwere obtained directly.Then the angles to transverse plane,coronal plane and sagittal plane ofⅠ,Ⅱwere calculated through pythagoras theorem and inverse trigonometric function.The direction of screws ofⅠ,Ⅱwere defined.The perpendicular distant from the insert point(A) of virtual cylindrical implantsⅠto the pubic symphyses(B) and the rim of superior ramus(C) were measured.The distant between the insert point of virtual cylindrical implantsⅡ(J) and the distal of the ischiadic tuberosity(K) was measured.5.Create the cross-section and measurementCross-sections of 1 cm of the anterior and posterior columns of the intact side of the virtual three-dimensional reconstruction pelvis were created and their largest dimensions were measured.The initial virtual cuts were made in the posterior margin of the acetabulum,perpendicular to the anterior surface of the anterior column.The other cuts were parallel to the initial cut as previously described by Ebraheim.Our technique differed only in that the cross-sectional measurements started at the pubic tubercle and ended at the anterior inferior iliac spine for the anterior column,and started at the ischial tuberosity and ended at the anterior inferior iliac spine for the posterior column.Line EF,connecting the(E) lateral bony acetabular margin and the(F) pelvis brim,was drawn perpendicular to the pelvic brim.A perpendicular line GH,was drawn on the midpoint of line EF.The length of line EF and GH were measured respectively.A circle was placed intraosseously in the minimal section of anterior column as Wang QX.Then the diameter of the circle was measured. 6.The experimental data was analyzed by the software of SPSS 12.0. Independent-samples t test and pair-sample t test were used,and an associated probability of 5%(P≤0.05) was considered significant.Results1.The anterior column of acetabular164(eighty males,eighty four females) hemipelves models were obtained.The mean maximum diameter and length of virtual cylindrical implant were 8.16±1.21 mm(5.60～10.80 mm) and 109.39±8.95 mm.The diameters of cylindrical implant in 19 cases were less than 6.5 mm,and they were all female(22.62%).And the angle to the horizontal plane,coronal plane and sagittal plane were 39.66±3.92°,20.81±4.58°and 42.66±3.23°respectively.The distant of AB and AC were 18.42±4.82 mm and 17.76±2.63 mm.The differences of the diameter,length,distance of AB of virtual cylindrical implantsⅠbetween the male and female were of statistical significance (P＜0.001).The mean smallest lateral medial diameter of the anterior column cross-section was 14.98±2.14 mm.The mean smallest superior inferior diameter of the anterior column cross-section was 13.92±1.81 mm.They were all larger than the maximum diameter of the equivalent virtual cylindrical implant obviously.The mean maximum diameter of anterior column screws according to the disc method was (11.80±1.53) mm(8.64～15.02 mm).It was larger than the maximum diameter of the equivalent virtual cylindrical implant,and the differences between them were of statistical significance(P＜0.0001).2.The posterior column of acetabular164(eighty males,eighty four females) hemipelves models were obtained.The mean maximum diameter and length of virtual cylindrical implant was(13.27±1.40) mm and 121.72±8.56 mm,and the angle to the coronal plane,sagittal plane and horizontal plane were 71.24±5.53°,15.93±7.22°and 8.79±3.83°respectively.The insert point was on the central line of the ischiadic tuberosity.The distant of JK was 15.80±2.28 mm.The differences of the diameter,length,angle to sagittal plane of virtual cylindrical implantsⅡbetween the male and female were of statistical significance(P＜0.001).The mean smallest lateral medial diameter of the posterior column cross-section was 20.34±2.18 mm.The mean smallest superior inferior diameter of the posterior column cross-section was 19.36±2.04 mm.They were all larger than the maximum diameter of the equivalent virtual cylindrical implant obviously.Conclusion1.The anterior column of male would accommodate with the virtual implant of 6.5 mm,but not to 22.62%of female.The insert points of screw are difference between male and female.It is at the medial inferior of pubic tubercle in male and lateral inferior of pubic tubercle in female.2.The posterior column would accommodate with the virtual implant of 7.3 mm in both male and female,but the insert direction and length of screws between the male and female are different.3.The size of the screw used for percutaneous fixation of anterior or posterior column fractures of acetabular should not be based solely on the measurement of cross-sectional diameter.4.The digital model of pelvis can not only provide a new method for the study of application basis research of percutaneous retrograde screws fixation of anterior and posterior column fracture,but also offer an ideal preoperative prepare for the the use of the technique.