Surg Radiol Anat (2010) 32:455–462 DOI 10.1007/s00276-009-0589-5
ORIGINAL ARTICLE
Morphometry of iliac anchorage for transiliac screws: a cadaver and CT study of the Eastern population Xiguang Tian • Jiazhen Li • Weichao Sheng Dongbin Qu • Jun Ouyang • Dachuan Xu • Shenghua Chen • Zihai Ding
•
Received: 9 November 2008 / Accepted: 23 October 2009 / Published online: 20 November 2009 Ó Springer-Verlag 2009
Abstract Objectives To describe the morphometry of iliac columns for transiliac screw and to testify the conformity among the anatomic measurement, two-dimensional (2D) and threedimensional (3D) computed tomography. Methods We evaluated the length, inner width, and angle of three screw trajectories starting at the iliac tubercle, posterior superior iliac spine, and posterior inferior iliac spine toward the anterior inferior iliac spine. Measurements were made on specimen, two- and 3D computed tomography using 18 embalmed cadaveric pelves. Results There was no significant difference among three measure methods. The path between the posterior superior iliac spine and anterior inferior iliac spine had the largest iliac column length, with 135 mm in male and 110 mm in female. The canal allowed placement of 8-mm screw in male and 6.5 mm in female with the angle of 25° laterally directed from the midsagittal plane. The line between the posterior inferior iliac spine and anterior inferior iliac spine
was below or just located at the top of greater sciatic notch in the majority measurements. The safe section for transiliac screw approximately located above the greater sciatic notch and could be divided into anterior and posterior parts. Conclusion The measurements among anatomic measurement, 2D and 3D computed tomography are consistent. The screw path from the posterior superior iliac spine toward anterior inferior iliac spine provided the longest anchor site. At the same time, the line between the posterior inferior iliac spine and anterior inferior iliac spine is not available for transiliac screw insertion of eastern population. The posterior of the safe section also can be regarded as another ilium anchorage area for transiliac screws. Keywords Three-dimensional reconstruction Ilium screw Pelvic morphometry Spinal-pelvic instrumentation Anatomy
Introduction Our study complies with the current laws of our country and regulations of the university. At the same time, all process is permitted by local ethics committee. X. Tian W. Sheng J. Ouyang D. Xu S. Chen Z. Ding (&) Department of Anatomy, Southern Medical University, 510515 Guangzhou, China e-mail:
[email protected] J. Li Department of Orthopaedics, The First Affiliated Hospital of Zhengzhou University, 450052 Zhengzhou, China D. Qu Department of Orthopaedics, Nanfang Hospital, Southern Medical University, 510515 Guangzhou, China
Since the transiliac technique was done by Letournel [1], the supra acetabular, transiliac implantation have promoted the management of spine deformity [2–5], postradical sacrectomy [6, 7], revision spondylolisthesis surgery [8, 9], unstable H- or U-shaped sacral fractures [10, 11], and fractures and dislocations of the pelvic ring [1, 12–14], especially Type 2 fractures according to the description of Day et al. [15]. In clinical management, repeated attempts to optimize the transiliac screw position will undoubtedly decrease the biomechanical results. Moreover, the vicinal vessels, nerves and organs will be damaged once the screw punctures the cortical table. Therefore, accurate iliac screw placement with maximum diameter and length for greatest
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pullout strength requires familiarity with the anatomy and morphometry of the ilium. Furthermore, it is essential to obtain more than one screw anchor for stable lumbopelvic fixation in managing the severe posterior pelvic ring fracture and revision surgery with diminished bone stock at the posterior superior iliac spine (PSIS) after bone grafting, sacroiliac joint tuberculosis or tumor, which require at least two different starting points and screw paths or use uncommon trajectory. Miller [16] identified the area just above the sciatic notch as the narrowest isthmus with substantial cortical bone and recommended limiting anchor length to 90 mm. Then Berry [17], based on external measurement of desiccated human pelvic specimens, further concluded that a path from the PSIS to the anterior inferior iliac spine (AIIS) provided a longer and larger, and potentially safer anchor site for performing the Galveston technique. Measurement of outer cortical diameters, however, does not provide direct information on inner cortical thickness of the ilium, because cortical thickness is related to age, osteoporosis, and other factors [18]. Moreover, the medullary width may be more important, because screw sizes are more limited by the inner diameters than the outer widths. Afterward, Starr [19] surveyed the iliac crosssectional slices and regarded from the posterior inferior iliac spine (PIIS) to AIIS as the optimize path. But it is cumbersome, time-consuming, and specimen-demolishing process while making slices. In addition, the boundary between cortical and cancellated bone is difficult to be distinguished by naked eye. It has been well documented that characteristic differences of the pelvis exist among different races [20]. However, the anatomy and morphometry of the ilium mentioned above come only from white pelvis. To our knowledge, detailed anatomic research on ilium anchorage for transiliac screws of the eastern population is rare. The practices of medicine always have relied on images to detect and treat disease that disturb or threaten normal life processes. The revolutionary capabilities of new three-dimensional (3D) reconstruction along with computer reconstruction and rendering of multidimensional medical and histologic volume image data obviate the need for physical dissection or abstract assembly of anatomy and provide powerful new opportunities for medical diagnosis and treatment. Generally speaking, these image data come from either computed tomography (CT) [21–23], or histological sections [24], or magnetic resonance imaging [25]. Nevertheless, the research on the conformity among anatomic measurement, two-dimensional (2D) CT image and 3D-image is rarely seen in the medical literature [21–23]. In our study, we aim to testify the conformity among anatomic measurement, 2D- and 3D-image. Then, we also make some morphologic measurements on ilium anchorage
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for transiliac screws of the eastern population using 2Dand 3D-imagine to determine the length, diameter, and angle of supra sciatic transiliac implant anchor sites, to define maximum safe section of the anchor with purpose that the results obtained from this research could give a light on the surgical procedures and hardware manufacture.
Materials and methods In accordance with the rules and regulations of the university, we used 18 embalmed cadaveric pelves (9 male, 9 female) without any gross structural abnormality. The following specimen details were recorded: age, gender, body weight and stature. The femurs, lumbar vertebrae, and all muscles were removed. Anatomic measurements included the length of three screw trajectories on bilateral ilium of each skeleton with a calliper (±0.1 mm) (Fig. 1). LIT extended from the iliac tubercle (IT) to AIIS. LPSIS extended from the PSIS to AIIS. LPIIS extended from the PIIS to AIIS. Markers were placed at these four landmarks which were used both for the anatomic and CT measurements. At the same time, the drilling angle of LPSIS in the horizontal plane was determined with a goniometer. The remaining studies were performed on a 64-row CT scanner (Brilliance 64; Philips Medical Systems, Cleveland, USA) using 0.9 mm slice thickness, pitch of 0.45 mm, 300 mAs/slice, 120 kV. Each pelvis specimen was placed into the CT scanner in a supine position with the three lines in bilateral ilium mentioned above oriented vertically. An almost transverse plane image was thus obtained, showing the shape and orientation of three iliac
Fig. 1 A desiccated illium demonstrating three screw trajectories: LIT extending from the IT to AIIS, LPSIS extending from the PSIS and AIIS, LPIIS extending from the PIIS to AIIS
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columns, respectively. The digitized scans were imported into Mxliteview DICOM Viewer software (Philips Medical Systems, Cleveland, USA). CT measurements included the length, smallest medullary width, and angle of three columns in the horizontal plane. Window/Level adjustment was an essential tool to represent the boundary between cortical and cancellated bone. Subsequently, all specimens underwent spiral CT scanning again in the supine position. Correct stance was achieved by alignment of the anterior superior iliac spine and pubic symphysis in a coronal plane, and the ischial tuberosities in a horizontal plane. The scanning range was from the top of the iliac crests to the bottom of ischial tuberosity. The digitized scans were imported into MIMICS 11.0 image analysis software (MIMICS software by Materialise N.V., Haasrode, Belgium). For an accurate measurement of the cortical bone geometric parameters, an edge recognition application was performed based on gray level thresholds to extract the cortical surfaces of the pelvis. In this study, a lower threshold of 84 Hounsfield units (HU) and an upper threshold of 1273 HU were used. The length of three columns and angle of LPSIS were measured on 3D-image. The measurements were performed by a superior surgeon and a superior radiologist consults together. At last, the length and angle of LPSIS obtained from three measure methods were analyzed to testify the conformity among these measure techniques. Continually, the 3D-image was reconstructed once again with the same gray level threshold. In this stage, we deleted the iliac area in which the medullary width is less than 6.5 mm and the area anterior to the constriction in which the smallest medullary width is less than 6.5 mm on each 2D-image. With the same proceeding, the 3D-images were obtained which demonstrated the maximum safe section for transiliac screws on bilateral ilium. For statistical analysis, we used the repeated measures analysis of variance and paired t test. SPSS 13.0 statistical software (SPSS, Chicago, USA) was used to analyse the data: P value below 0.05 was considered to be significant.
Results The characteristics of the specimen are presented in Table 1. The mean age was 65.6 for male (range from 49 to 79) and 59.6 for female (range from 37 to 79). Basing on the findings of Berry [17], which showed no clinically significant age-related morphologic differences from 20 to 79 years of age, we believe the study belong to analysis in adult.
457 Table 1 The characteristics of 18 adult human cadavers Age (years)
Gender
Body weight (kg)
Stature (cm)
1
46
Female
61
163
2
55
Female
55
158
3
71
Female
49
154
4
79
Female
47
161
5
63
Female
63
169
6
73
Female
54
160
7
37
Female
53
158
8
44
Female
62
170
9
68
Female
64
168
10
65
Male
59
169
11 12
79 78
Male Male
55 70
165 173
13
61
Male
76
175
14
58
Male
81
182
15
56
Male
68
172
16
73
Male
59
168
17
71
Male
61
170
18
49
Male
72
166
In the majority measurement of LPIIS (6 from male and 5 from female), we found the line was below or just located at the top of greater sciatic notch (Fig. 2). As a result, the length, smallest medullary width and angle of LPIIS were not recorded. The length and angle of LPSIS obtained from anatomic measurement, 2D- and 3D-image are shown in Table 2. There was no significant difference among three sort values. According to the measurements of the length, angle and smallest medullary width of LPSIS and LIT from 2D-CT, there was no significant difference between the right and left sides in both sexes pelvic (using paired t test), so those data were combined to take descriptive statistics. The mean, standard deviation and range of the measured parameters are given in Table 3. During the measurement on 2D-CT imagine, window/ level adjustment was an essential tool to represent the boundary between cortical and cancellated bone. After manipulation, the iliac columns were shown like ‘‘sandwich structure’’: two slice of cortical bone with hollow structure between. As a result, the measurement became convenient and accurate. All specimens had relatively straight and rectangular shaped iliac columns along LPSIS that would allow for implantation of the entire length of the column. On average, the typical female pelvis had iliac columns along LPSIS that were 125 mm in length, 10 mm in width, with the drilling angle in the horizontal plane of 25° laterally directed from the midsagittal plane. Generally, the typical male pelvis had iliac columns that were 135 mm
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typical male pelvis had iliac columns that were 126 mm in length, 10 mm in width, with the angle of 25°. Although variable in size, even the smallest pelvis was reliably available for instrumenting a length of 90 mm in both sexes. We observed the stereotypical narrow areas of the defined screw paths on 3D-image. The constriction was approximately at the narrowest part of the iliac wing, just anterior or posterior to the greater sciatic notch. The 6.5 and 8 mm diameter screws always pass the constriction safely along LPSIS trajectory in female and male pelvic, respectively. In many pelvises, 6.5-mm screws were too large for the inner diameter of the ilium at constriction along LIT trajectory for both sexes. Nevertheless, 5.5-mm screws should be acceptable considering the cortical canal diameters. The 3D-image was reconstructed with the smallest medullary width more than 6.5 mm. Therefore, the 3D-image demonstrated the maximum safe section of the anchor for transiliac screws on bilateral ilium (Fig. 3). Generally, the safe section mainly located above the greater sciatic notch without significant difference between bilateral ilium. The vertical dimension above the greater sciatic notch of the safe section was approximately 30–40 mm. Similarly, the vertical dimension between the top of the section and the superior rim of the acetabulum, just above and below the AIIS, was approximately 50–55 mm. The posterior of the area extended from the iliac crests to the PIIS with a distance 100 mm more or less. The horizontal range at the level of PSIS was approximately 65 mm.
Discussion
Fig. 2 a A embalmed cadaveric pelvis shows the LPIIS is below the top of greater sciatic notch (arrow). b An axial computed tomography displays the discontinuation of illium wing (arrow) along the LPIIS because of the line below the top of greater sciatic notch. c An axial computed tomography presents the LPIIS just located the top of sciatic notch. The constriction (arrow) is so narrow that it is impossible to implant screw into this trajectory
in length, 13 mm in width, with the angle of 26°. Considering the minimum intrailiac distances, screw lengths of 110 and 90 mm should always be possible for transiliac insertion into LPSIS of male and female, respectively. Relatively, the canal length along LIT was shorter than LPSIS in both sexes. The typical female pelvis had iliac columns that were 117 mm in length, 8.2 mm in width, with the drilling angle in the horizontal plane of 26° laterally directed from the midsagittal plane. Generally, the
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Generally, the 3D-reconstruction algorithm is divided into two methods of surface fitting and direct volume rendering. Different algorithm has different advantages and disadvantages. Therefore, the relative accuracy or contradiction is possibly appeared during the image processing. In the current study, we testified the conformity of distance and angle measure among the anatomic measurement, 2D- and 3D-image. There was no significant difference among these three measure methods. Therefore, it implies that the measurements obtained from these three techniques are consistent, effective and reliable. Furthermore, it also testifies the authenticity of our clinical anatomy research on ilium, which will be mentioned below. Based on measurement on CT imagines, Thomas [23] argued LPSIS was largest screw path and lengths of 120 and 90 mm should always be possible for transiliac insertion in the general population of male and female patients, respectively. In comparison with current study, we believe
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459
Table 2 The length and drilling angle of LPSIS from anatomic measurement Female (n = 9) Mean 1.
2.
3.
Male (n = 9)
SD
Range
Mean
SD
Range
Length of LPSIS from Anatomic measurement (mm) Right
125.4
15.6
97.6–144.4
135.5
14.2
117.8–159.3
Left
125.5
15.4
97.9–144.7
136.0
14.4
117.4–159.6
Length of LPSIS from 2D-CT (mm) Right
125.2
15.7
96.8–145.1
135.6
14.5
117.9–159.5
Left
125.3
15.6
96.8–145.0
135.6
14.4
117.6–159.5
Length of LPSIS from 3D-imagine (mm) Right 125.2
15.6
96.9–145.1
135.4
14.4
117.6–158.9
15.5
97.1–144.8
136.0
14.7
117.6–159.7
Left
125.1
Analysis among 1, 2 and 3
4.
5.
6.
Right
p = 0.884*
p = 0.688*
Left
p = 0.543*
p = 0.458*
Angle of LPSIS from anatomic measurement (°) Right
25.2
5.2
17.9–32.2
26.6
4.6
17.1–31.5
Left
25.3
5.4
17.2–31.9
26.4
4.6
17.0–31.5
Right
25.5
5.4
17.4–33.2
26.2
4.8
16.4–31.6
Left
25.5
5.4
17.1–32.4
26.3
4.6
17.0–31.8
Right
25.6
5.3
17.5–32.9
26.4
4.7
16.6–31.5
Left
25.6
5.3
17.3–32.3
26.4
4.7
16.7–32.0
Angle of LPSIS 2D-CT (°)
Angle of LPSIS from 3D-imagine (°)
Analysis among 4, 5 and 6 Right Left
p = 0.307*
p = 0.316*
p = 0.153*
p = 0.337*
2D- and 3D-image: female and male * 1. Repeated measures analysis of variance was used 2. There was no difference among three measurement methods in tests of within-subjects effects 3. There was no difference among three measurement methods in multiple comparisons Table 3 The length, drilling angle and smallest medullary width of LPSIS and LIT from 2D-CT: left and right combined Female (n = 9)
Male (n = 9)
Mean
SD
Range
Mean
SD
Range
1. Length of LPSIS from 2D-CT (mm)
125.3
15.2
96.8–145.1
135.6
14.0
117.6–159.5
2. Length of LIT from 2D-CT (mm)
117.1
14.1
93.7–132.7
126.9
15.5
98.9–150.8
3. Angle of LPSIS from 2D-CT (°)
25.5
5.2
17.1–33.2
26.3
4.5
16.4–31.8
4. Angle of LIT from 2D-CT (°)
26.5
5.4
19.1–37.5
25.8
4.5
17.4–31.9
5. Smallest medullary width of LPSIS from 2D-CT(mm)
10.8
2.5
6.6–14.4
13.0
2.2
8.4–16.2
6. Smallest medullary width of LIT from 2D-CT (mm)
8.2
2.1
5.6–11.4
10.1
2.4
5.9–14.1
the screw lengths of 110 and 90 mm should always be acceptable for transiliac insertion in the eastern population of male and female, respectively. As to the difference of the results, we attribute it to the racial difference. Berry [17], based on external measure of desiccated human pelvic specimens, described the LPSIS with the averaged thickness of 14.4 and 17.3 mm, length of 147 and 141 mm for male
and female pelvis, respectively. Relatively, our measurements were considerably smaller than the dimensions previously described by Berry. Measurement of outer cortical diameters, however, does not provide direct information on inner cortical thickness of the ilium, because cortical thickness is related to age, osteoporosis, and other factors [18]. Moreover, the medullary width may
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Fig. 3 The 3D-images demonstrating the location of constrictions of screw trajectories approximately anterior or posterior to the greater sciatic notch, and the maximum safe section of the anchor for transiliac screws on bilateral ilium on left view (a), front view (b), and back view (c)
be more important, because screw sizes are more limited by the inner diameters than by the outer widths. Donovan [26] reported on supra sciatic intrailiac passage anterior post diameters in fresh specimens, observing diameters ranging from 14.3 to 27.3 mm. The difference between fresh specimens and embalmed cadavers and the racial difference may explain the somewhat larger diameters of Donovan. Concerned with the measurement of the LPIIS, Schildhauer [23] argued the pathway was 86.3 ± 7.9 in male and 99.7 ± 29.4 in female using the CT data. Starr [19]
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surveyed the iliac cross-sectional slices and believed the LPIIS was wide and straight and allowed easy placement of screws. In contrast, the line was founded below or just located on the top of greater sciatic notch in the majority specimens in our study (Fig. 2). As results, it is impossible to implant screw into this trajectory. We tentatively believe racial difference is a substantial factor to account for the anatomic distinction. In clinical management, the posterior pelvic ring is exposed through a dorsal midline approach. Under this circumstance, it is difficult for surgeons to place the screw accurately along the pathway described above. Moreover, repeated attempts to optimize the screw position will undoubtedly decrease the biomechanical results. Consequently, it is necessary to demonstrate the maximum reliable section for transiliac screws. By familiar with the anatomy of the section, surgeons will have precise conception of the available section of screw and some extra attempts to optimize the screw position may be canceled. Therefore, the destruction of cancellous bone owing to extra attempts to implant guide pin or screw will be avoided. As a result, the biomechanic effects of screw will be increased. To our knowledge, all current literatures focus on the research on the pathway of transiliac screws [17, 23]. There are few investigations related to the safe section for transiliac screws in English literature. Today, a 6.5 mm diameter spinal rod or transiliac screws is commonly used in clinical practices. In the design of current study, we set the smallest medullary width greater than 6.5 mm as the reliable section on 3D-image. While using a surface-rendering algorithm, the selection of the window thresholds is critical for making an accurate measurement. Setting a lower threshold too low includes the soft tissues in the ilium wing, resulting in an overestimation of the cortical bone. In contrast, setting too high the lower threshold excludes some of the cancellated bone, resulting in an overestimation of the medullary width. After 18 specimen research, we located the safe section approximately above the greater sciatic notch (Fig. 3). Generally, it could be divided into anterior and posterior parts. The anterior part was approximately same to the position previously described by Miller et al. [16]. The posterior part, extending from the top of iliac crests to PIIS, was mainly situated at the posterior pelvic ring. Similar clinical anatomy report is not seen in current English literature. It is important in management of the severe posterior pelvic ring fracture, revision surgery after bone grafting, sacroiliac joint tuberculosis or posterior pelvic ring tumors which require at least two different starting points and screw paths or use uncommon trajectory. According to the posterior part of the safe section, Korovessis et al. [27] inserted a multiaxial 7.5 9 55 mm screw
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on the ground of a notch made at the PSIS within the cortices of the iliac bone, and 1 or 2 additional 6.5 9 45 mm multiaxial screws on S1 pedicles. All screws were connected transversally with an appropriately contoured 120-mm long horizontal 5 mm Cotrel–Dubousset rod to reach firm spinal fixation. Thomas [28] implanted in two iliac screws directed above the anatomic canal, the upper of them was mainly located in the iliac crests, to gain solid bony purchase to manage gunshot wounds to the pelvis. As results, we believe the posterior area also could be determined as another ilium anchorage for transiliac screws. This study met with challenge that is often encountered when implanting transiliac screws in precise position. The sample size of 18 embalmed cadaveric pelves, however, remains prohibitively small from the standpoint of statistical power and analysis, and the ability to establish anatomic regulation. The issue can most likely be overcome by a structured multi-center collaborative effort or by using CT data of consecutive patients with pelvic CT examinations in clinical practice. With large-scale data, we can analysis the related differences in pelvic morphometry, such as age, gender, and race. In conclusion, the distance and angle measurements obtained from anatomic findings, 2D- and 3D-CT image are consistent. With regard to eastern population, the transiliac screw with the length of 110 and 90 mm and the diameter of 8 and 6.5 mm is acceptable between the PSIS and AIIS for male and female, respectively. By familiar with the anatomy finding, surgeons can implant screw with the larger length and diameter as far as possible. Importantly, this trajectory is useful for surgeons to accomplish percutaneous implantations successfully because there is no possibility for direct check and everything is done under intra operative radiography. Disparity in western population, the LPIIS is not available for transiliac screw insertion of the eastern population. The safe section for screw fixation, which can be divided into anterior and posterior parts, is approximately located above the greater sciatic notch. Furthermore, the posterior of the safe section also can be regarded as another ilium anchorage area for transiliac screws.
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