Author:Ji Xiaoxi & Wang Fang & Wang Dongmei & Li Fan &Li Xiaoqin & Su Yunlong & Zhang Jie & Wang Qiugen

Introduction

Comparedto along bone, the shape of the pelvis is morecomplex and irregular. Thus, plate shaping of the pelvis during surgeryis not perfect. In fact, repeatedly shaping the reconstructionplate during the operation is more likely toresult in prolonged operation time, breakage of the inter-nal fixator, and loss of stage I reduction due to the mismatch of the shape of implantandpelvis. All of the problems mentioned above increase the risk of the operation and theincidence of complications . For rotationally unstablepelvic fractures and peri-anterior column fractures of the acetabulum (including fractures of the anterior wall, anterior column, transverse fractures,T-shaped fractures, and frac- tures of the anterior columnor anterior wall associated with a hemi-transverse fracture), the incidence is relativelyhigh (52.4 % and 47.5 %, respectively) . The effects of thereduction and fixation of the fractures directly determine the occurrence of postoperative complications and the patient’s quality of life . The two groups of fractures usuallyresult in ruptures of the pelvic anterior ring, especially of the arcuate line. Therefore, to fix the aforementioned fractures,the inferior border of the iliac fossa and its forwardexten- sion are commonly chosenas areas to place the reconstruc- tionplate . This region is considered an ideal implant location, because it provides clear surgical exposure, aflatbone surface, and relativelythick bone tissue. According to the specificfracture type, the reconstruc- tion plate can extend along the pubic tubercle,pubis pecten, iliopubic eminence, arcuate line, or it may ap- proach the sacroiliacjoint. Letournel et al. reported that fixing the anterior wall, anterior column, or a fracture that extended outof thehip joint, couldbe accom- plished by positioning a long reconstruction plate along the pubis pectento thearcuateline .Jakob et al. treatedanterior column fractures of the acetabulum and pelvic ring fracturesby placing the plate along the superior ramus ofthepubis or the iliopubic eminence to the arcuate line. By doing this, the authors achieved satisfactory anatomic reduction . At present, due to thecomplex and irregular shape of the pelvis compared to a long bone, and the multi-plane and multi-radian fixation route in3D, no anatomical data for the fixation route along the supe- rior border of the arcuate line is currentlyavailable,whichresults in an inefficient and ineffective plate shaping proce- dure. Digital three-dimensional (3D) measurements, based onCT reconstruction, have the same accuracy and reliability astraditional measurements, and this 3D technology has beenconsidered a more efficient method for orthopaedic anatomic studies, design, and optimization ofimplants. Thus, the first hypothesiswasraised to obtain an optimized plateshape of pelvic fixator by digital 3D anatomical measure- ments, which eliminates the prolonged operation time, break-ageof the fixator and loss of stage I reduction.

Unlike other humanbones, the pelvic shape appears markedly different between males and females. For the female pelvis alone, there are four differentshapes, which are termed gynecoid, android, platypelloid and anthropoid. Although finding a perfect match for each pelvic shape can offer the maximalanatomical matches between the internal fixator and the pelvis, it is relatively unrealisticand costly for industrial manufac-ture.Differences among pelvic shapes are related to thechanges of thepelvic inlet shape; in particular, tothe ratio of the posteriorsagittal diameter to the anteriorsagittal diameter (DSp/DSa) andtheratioof thetrans- verse diameter to thesagittaldiameter (DT/DS). With the increaseof DSp/DSa  and DT/DS, the pelvic inletshape transitions fromthe heart-shapeof the android type to the approximately round, elliptical shape of thegynecoid and platypelloid types . Hence, the second hypothesis was raised to find the relationship betweenDSp/DSa, DT/DS   and the shape of the superior border of the arcuate line, which could used as a base on which to estimate the general shape of the internal fixator by pelvic type.

  A retrospective observational study by digital recon- structionand 3D measurements of a large sample of normal adult pelvic CT scans was conducted by the authors. Theaimof thisstudy wasto obtaina series of parameters describing theshape andbone thicknessof the fixation route along the superiorborder of thearcuate line to the pubic tubercle, and find the relation between DSp/DSa, DT/DS and the shape of the superior borderof the arcuate line; and finally, to provide a good reference for pelvic and acetabular surgeries and the design of pelvic anatomical internal fixators at the su- perior border of the arcuate line.

Patients and methods

Samples and equipment

We collected the computed tomography scans from out- patients with varicoseveins of the lower limb, who needed enhanced computed tomography scan (CT scan) from pelvis to feet. Each patientwith pelvic deformity,trauma, tumor and other diseases on CT scans  and intheir history was eliminated.From December 2009 to November2010,175 complete pelvic computed tomography scans of unrelated ethnic HanChinese adults (meas age 61.0years;105 men,70 women) were collectedfrom the medical image database of the department of radiology of a largecomprehensive hospital.The age distribution of all 175 patients is seen in Table 1. During CT scanning, patients kept the standard anatomical horizontal position with lower limbs unbent. All the CT scanswere performedat 12 kV and 300 mA with a slice thickness of.75mmby a 64- channelCT, Light Speed VCT XTe (GE Healthcare; Milwaukee, WI, USA), and  the  scanning time of each slice was 20ms. There were approximately380DICOMformat CT images for the pelvic part of eachpatient. For the data that was collected from outpatients,the height and weight of the patient were not recorded inthis study. The study was performed with the help of the following software: the interactivemedical image control system MIMICS 13., the reverseengineering software Geomagics 10, and the mechanical design soft-ware Unigraphics NX. The study was approved by the Review Board of Shanghai First People’s Hospital.

              Table1 Age distribution of all 175patients

Measurement of parameters

Each volunteer’s DICOM format CT images were imported to Mimics 13.. After removing the soft tissue by thresholding segmentation, region growing, and a subsequent smoothing process, an entire 3D digital pelvic model was established andsaved in Stereo Lithography (STL) format.

The pelvic model was importedto the Geomagics soft- ware as a mesh model in the STL format. The horizontal, coronal and sagittal plane were determined in the first place.According to the standard anatomical position, the pubictubercle and anterior superior iliac spineare in the same coronal plane. The plane passing the midpointof the pubic symphysis, the midpoint of the anterior border of the sacral promontory and the coccyxtip was identified the sagittal plane.The plane constructed perpendicularly to the sagittaland the coronal plane was the horizontal plane. Along with the route from the pubictubercle, pubis pecten, iliopubiceminence, arcuate line, and sacroiliac joint, several pointswere picked at the cortical surface, 10 mm lateralto the pelvic brim in the superior border of the arcuateline, to draw

the space curve of the bone surface. By intersecting extru- sion,the required measurement model and the anatomicalcurve were extracted.

The measurement model and the anatomical curve wereimported to UG in the STL and IGES format. Thus, the total length of the curve (L) can be measured. Bydividing the curve into elevenequal parts, the radius of curvature at eachdecile point, r1, r2, …, r1, and the bone thickness of eachdecile point, d1, d2, …, d10, were achieved from front toback. The curve section was divided into front-acetabular,acetabular, and post-acetabular areas by the locationof the acetabulum, and the thinnest bone regions of thethree parts, D1, D2, D3, were measured (Fig.1).

Fig.1  By dividing the curve into ten equal parts,theradius of curvature at each decile point，r₁，r₂，…，r₁₀，and

the bone thickness of each decilepoint，d₁，d₂，…，d₁₀，were achieved from front to back.The thinnest bone regionsof the three parts，D1，D2，D3，were  measured

The pelvis wasimported to the Imageware 12.0 software as a mesh model in the STLformat. After being rotated to the position of the pelvic inlet, and compressingthe pelvis to the inlet plane as with X-ray films, the transverse diameter of thepelvic inlet DT(the widest distance), the sagittal diameter DS (the distancefrom the anterior border of the sacral promontory to the posterior border of thepubic symphysis), the anterior sagittal diameter DSa, and the posterior sagittal diameter DSp could all be measured (Fig.2). Finally, the ratio of the posterior sagittal diameter to the anterior sagittal diameter (DSp/DSa) and the ratio of the transverse diameter to the sagittal diameter (DT/DS)were calculated.

Fig.2 In the position ofthe pelvic inlet，just like the inletX-ray plane，the transverse diameter of pelvicinlet D_T，the sagittal diameter D_S，the anterior sagittal diameter D_Sa，and the posrerior sagittal diameter D_Spwere measured

Statistical analysis

The data were expressed as mean ± standard deviation andanalyzed by using the descriptive methods with SPSS19.0. The data distributions were analyzed with normality tests.A p value less than 0.05 was considered to be statistically significant.

All the pelvis were categorized into seven groups by DSp/DSa   (≤0. 40, 0.41–0.5, 0.51–0.6, 0.61–0.7, 0.71–.8, .81–.9, ≥ .9), and the relationship between theradius of curvature and the decile point was analyzed. Each group was tested and showed normal distributed. For eachgroup, we compared the mean values of the radius of curva-turebetween two adjacentpoints from decile point 1 to 10 with the T test.

Results

The radius and the bone thickness of curvature at each decilepoint are seen in Table 2 and Table 3. The thicknessof the front-acetabular, acetabular, and post-acetabular areas were 11.67± 2.94, 13.97 ± 3.36, and 28.21 ± 3.46 mm, respectively (Table 4). The transverse diameter, the sagittaldiameter, the anterior sagittal diameter, andthe posterior sagittal diameter of the pelvic inlet were 124.14 ± 11.69, 107.38 ± 13.51,66.01 ± 9.15, and 41.36 ± 8.19 mm, respectively (Table 5).

Table 2 The radius of curvatureat each decile point

Table 3 The bone thickness of each decile point

Table 5 The parameters of the pelvic inlet

Fig.3 For any group divided by DSp/DSa, the decile points 1，3 ，and 10 have smaller radii of curvature than the beforeand after points，respectively

Fig.4 The radius of curvatureat decile point 3 showed an obvious rising trend with therising of DSp/DSa, while the radii of curvature at decile points 1 and 10 had no obvioustrend

The comparisons between the mean values of the radius of curvature betweentwo adjacent points from decilepoint 1 to 10 showed that the mean value of decilepoint 1 was smaller than point 2, which of point 3 was smaller than point 2 and 4, point 1 smaller than point 9. And there were significant differences between the mean values of the radi- usof curvature at decile points 1 and 2; 2 and 3; 3 and 4; and 9 and 10 (p < 0.05) (Table 6). Hence, for all groups divided by DSp/DSa, the decile points 1, 3, and 10 had smaller radii ofcurvature than the beforeand after points, respectively(Fig.3). The radius of curvature at decile point 3 showed anobvious rising trend with the rising of DSp/DSa.  However,theradii of curvature at decilepoints 1 and 1 had no obvious trend (Fig.4).

By dividingthe whole pelvis by DT/DS and repeating thework above, we tried to describe the general situationof the radius of curvature at each decile point for differentDT/DS groups. However, we did not identify any obvious trends.

Discussion

When dealing with peri-anterior column fractures of the acetabulum (including fractures of the anterior wall, anterior column, transverse fractures, T-shaped fractures, fractures of the anterior column or anterior wall associated with a hemi-transverse fracture, and fracturesof both columns) and rotationally unstable pelvicfractures,the superior border of the arcuate line,including its forwardextension,is commonly chosen as the region to place the reconstruction plate. According to the specific fracture type,the reconstruction plate can extend along the pubic tubercle,pubis pectin, iliopubic eminence, arcuate line, or it may approach the sacroiliac joint. This area is considered an ideal Implant location, because it provides clearsurgical exposure, flat bone surface,and relatively thick bone tissue .No anatomical data for the fixation route along the superior borderof the arcuate line is currently available, which is associated with prolonged operation times due to shaping the reconstructionplate during the surgery, breakageof the internal fixatordue to being repeatedly bent, and loss of stageI reduction due to the mismatchin the shape of implantand pelvis. All of these possibilities increase the risk of theoperation and the incidence of complications. The two hypotheses of this study areto beobtain an optimized plateshape of pelvic fixator by digital 3D anatomical measure- ments, which eliminates the prolonged operation time, breakage of the fixator and loss of stage I reduction, and to find the relation betweenDSp/DSa, DT/DS and the shapeof the superior border of the arcuate line, which could be usedas a base on which to estimate the generalshape of the internal fixator by pelvic type.