Iliosacral screw fixation has become a common method for surgical stabilization of acute disruptions of the pelvic ring. Placement of iliosacral screws into the first sacral (S1) body is the preferred method of fixation, but size limitations and sacral dysmorphism may preclude S1 fixation. In these clinical situations, fixation into the second sacral (S2) body has been recommended. The objective of this study was to evaluate the bone quality of the S1 compared to S2 in the described “safe zone” of iliosacral screw fixation in trauma patients.
Materials and methods
The pelvic computed tomography scans of 25 consecutive trauma patients, ages 18–49, at a level 1 trauma center were prospectively analyzed. Hounsfield units, a standardized computed tomography attenuation coefficient, was utilized to measure regional cancellous bone mineral density of the S1 and S2. No change in the clinical protocol or treatment occurred as a consequence of inclusion in this study.
Results
A statically significant difference in bone quality was found when comparing the first and second sacral segment (p = 0.0001). Age, gender, or smoking status did not independently affect bone quality.
Conclusion
In relatively young, otherwise healthy trauma patients there is a statistically significant difference in the bone density of the first sacral segment compared to the second sacral segment. This study highlights the need for future biomechanical studies to investigate whether this difference is clinically relevant. Due to the relative osteopenia in the second sacral segment, which may impact the quality of fixation, we feel this technique should be used with caution.
Level of evidence
III
Introduction
Iliosacral screw fixation has become a common method for surgical stabilization of acute disruptions of the pelvic ring [1‐4]. Iliosacral screw placement can be accomplished percutaneously in conjunction with closed reduction or after open reduction; providing stability, minimizing deformity, facilitating mobilization and improving outcomes in patients with posterior pelvic ring injuries [2, 3, 5]. However, loss of fixation, loss of function, neurovascular injury and malunion have all been reported as serious complications following unstable posterior pelvic ring injuries treated using this method [1, 4‐8]. Placement of iliosacral screws into the S1 body is the preferred method of fixation, but size limitations and sacral dysmorphism may preclude S1 fixation [4, 9]. In these clinical situations, fixation into the second sacral body (S2) has been recommended [3, 10, 11].
Although safe zones for screw fixation in both normal and dysmorphic second sacral segments have been established, challenges exist in achieving proper fixation into the S2 body because of its smaller size and decreased tolerance of variant screw trajectories [3, 11, 12]. In spite of multiple studies on surgical techniques for optimal placement of fixation, there is little mention of the quality of the surrounding bone in the S2 body. The purpose of this study is to investigate the bone density of the first and second sacral segments using Hounsfield units, a standardized computed tomography attenuation coefficient. We hypothesize that S2 bone density is inferior to that of S1, increasing the chances of screw loosening and fixation failure despite screw placement consistent with accepted methods in the literature.
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Materials and methods
The study was approved by our institutional review board and carried out in the radiology suite of a level 1 trauma center emergency department. Pelvic computed tomography (CT) scans obtained as part of the routine trauma workup of 25 consecutive trauma patients meeting the inclusion criteria were obtained between July 2008 and January 2011. All subjects were between the ages of 18–50 years of age to limit the effects of age-related bone loss or skeletal immaturity. Subjects were excluded from this study for the following reasons: previous documented sacral trauma, presence of a zone 3 sacral fracture, neoplasm of the pelvic girdle, documented history of rheumatoid arthritis, documented history of seronegative arthropathies, documented history of osteoporosis or osteopenia, history of paraplegia, non-ambulatory/wheelchair bound, an inadequate scan technique that would limit density determination, including, but not limited to, motion artifact, streak artifact from internal hardware or external metallic devices, beam-hardening artifact, or photon deprivation in the extremely obese patient, known use of bisphosphonates, steroids and/or hormone medications, or evident malnutrition. The patient’s age, gender and smoking history were recorded from the electronic medical record. No change in protocol or treatment occurred as a consequence of inclusion in this study.
Images were viewed using the bone algorithm default windows on picture archiving and communication system (PACS) viewing software. Using axial images, the mid-body location of S1 and S2 was determined for each subject and confirmed by cross-referencing position with coronal and sagittal reconstructions (Fig. 1). To standardize measurement while accounting for normal anatomic variation and optimal iliosacral screw trajectory as described in the literature, four standardized circular voxel regions of interests (ROIs) were drawn at determined mid-body S1 and S2 levels of each subject (Fig. 2). These standardized circular ROIs were drawn with areas ranging from 23.2 to 26.2 mm2. This range was chosen after pilot testing to maximize the area of trabecular bone tested in line with the potential screw trajectory, while limiting overlap of adjacent ROIs. When placing ROIs, one horizontal reference line was drawn tangential to the most anterior points of both sacral foramina (Fig. 2a, e). One transecting vertical reference line was then drawn from the tip of the spinous process through the midpoint of the anterior cortex of the vertebral body (Fig. 2b, f). ROIs were then drawn with their center corresponding to 25 and 75 % of the distance from the anterior cortex to the horizontal reference line. A vertical reference line was then drawn tangential to the most medial point of the sacral foramina (Fig. 2c, g). An ROI was then drawn with the center of the ROI at 50 % of the distance between the anterior cortex and horizontal reference line drawn previously. This method was then repeated on the adjacent side. Figure 2d and h demonstrates the placement of ROIs. Hounsfield unit (HU) density values for each ROI were then collected and averaged to yield the mean value for each segment.
Fig. 1
CT images depicting the cross-referencing technique used for localizing the mid body point of S1 (a) and S2 (b) for measurement in axial, sagittal and coronal reconstructions
Fig. 2
Axial CT sections demonstrating the technique for ROI placement as described in the methods section. a Horizontal reference lines at S1 (blue). b Vertical midline reference line at S1 (green dashed). c Vertical reference line tangential to the medial border of the sacral foramina at S1 (red dashed). d Placement of ROIs at 25 and 75 % of the vertical midline distance and 50 % of the vertical lateral distance at S1 (white). e–h The same technique at S2 (color figure online)
×
×
Prospective power analysis was conducted and revealed that a sample size of 25 patients was necessary to detect a difference in bone density of SI compared to S2 at the 0.05 alpha level with 80 % power. Statistical analysis of the data was performed on the mean values for each segment in the four ROIs examined using paired Student’s t tests with statistical significance being set at p < 0.05.
Results
Twenty-five patients, with a mean age of 35.2 years, were studied (ages 18–49 years). Thirteen patients had a positive smoking history. Nineteen patients were male and 6 female. The difference between the average Hounsfield unit (HU) of the first and second sacral segment was 89.9 (p = 0.0001). Comparisons of the mean bone density of the first and second sacral segments are presented in Fig. 3.
Fig. 3
Comparison of mean aggregate bone density measurement of S1 vs. S2 in each subject, as measured in Hounsfield Units (HU)
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The 13 patients with a positive smoking history had a mean HU of 93.7 compared to the mean HU value of non-smokers, 85.8 (p = 0.66). The average HU difference when comparing males, 87.2, versus females, 98.6, was 11.4 (p = 0.58). Age had no significant effect on HU difference (p = 0.53).
The values of bone mineral density in Hounsfield units for each of the tested points are detailed in Table 1 for each of the patients. All four points were found to have a statistical difference between S1 and S2 (anterior p = 0.0011, posterior p < 0.0001, right p < 0.0001, left p < 0.0001). The percentage difference of mean density measured with Hounsfield units between S1 and S2 is presented in Table 2.
Table 1
Values of bone mineral density in Hounsfield units of analogous ROI locations at S1 and S2 with differences
Patient no.
S1 anterior
S2 anterior
Difference
S1 posterior
S2 posterior
Difference 2
S1 right
S2 right
Difference 3
S1 left
S2 left
Difference 4
1
295.9
226.9
69
261.5
205
56.5
321.2
158.9
162.3
288.1
156.3
131.8
2
240.9
140.9
100
170.3
98.9
71.4
270.2
106.1
164.1
259.7
92.3
167.4
3
1313
1272.4
40.6
1358.7
1298.6
60.1
1369
1294.5
74.5
1310
1288.7
21.3
4
280.9
236.2
44.7
211.8
200
11.8
296.7
186.8
109.9
294.8
174.3
120.5
5
229.5
239.3
−9.8
263
164.9
98.1
263
218.1
44.9
256.8
244.3
12.5
6
230.1
236.3
−6.2
256.7
134.9
121.8
313.5
177.5
136
323.8
180.7
143.1
7
1341.3
1279.4
61.9
1348.8
1262.9
85.9
1335.9
1306.1
29.8
1355.6
1277.3
78.3
8
1342.1
1279.2
62.9
1292.7
1265.5
27.2
1263.3
1237.2
26.1
1258.9
1222.2
36.7
9
1292.3
1237.3
55
1263.8
1146.6
117.2
1148.1
1103.6
44.5
1154.5
1142.2
12.3
10
306.7
268.6
38.1
331.4
200.1
131.3
266.5
222.4
44.1
307
224.7
82.3
11
361.4
373.9
−12.5
547.3
391.2
156.1
447.3
344.5
102.8
411.9
393.5
18.4
12
1392.7
1230.8
161.9
1389.2
1234.3
154.9
1398.8
1191.3
207.5
1417.7
1240.4
177.3
13
293.4
233.4
60
393.3
80.3
313
283.4
160.8
122.6
267.9
153.4
114.5
14
139.5
97
42.5
138.5
41.9
96.6
190
38.7
151.3
216.6
43
173.6
15
287.1
408.7
−121.6
341.9
290.6
51.3
379
256.7
122.3
309.1
296.6
12.5
16
247.8
223
24.8
212.3
151.7
60.6
305.2
136
169.2
254.1
163.2
90.9
17
212.2
167.7
44.5
188.2
106.2
82
150.2
152.6
−2.4
146.1
124.3
21.8
18
248.7
277.9
−29.2
261.7
158
103.7
388.4
197.2
191.2
328.5
173.2
155.3
19
223.4
196.3
27.1
248.9
215.2
33.7
268.2
139.7
128.5
275.3
220.4
54.9
20
271.7
279.3
−7.6
292.4
189.4
103
305.2
217.8
87.4
317.7
176.8
140.9
21
207.9
77.9
130
235
58
177
128.1
39.3
88.8
167.1
44.5
122.6
22
407.5
207.7
199.8
401.9
202.4
199.5
311.9
191.3
120.6
365.2
237.7
127.5
23
155.5
129.4
26.1
162.1
166.2
−4.1
254.9
97.6
157.3
203.3
119.1
84.2
24
367.5
181.4
186.1
172.5
66.4
106.1
232.7
143.9
88.8
258.8
137.9
120.9
25
422.2
306.8
115.4
370.9
223.9
147
384.3
196.2
188.1
367.5
224
143.5
52.14 (p = 0.0011)
102.47 (p < 0.0001)
110.41 (p < 0.0001)
94.6 (p < 0.0001)
All values listed are in Hounsfield units
Table 2
Values of bone mineral density in Hounsfield units of analogous ROI locations at S1 and S2 with differences
Patient no.
Difference anterior ROI
% Difference S2 vs. S1 anterior ROI
Difference posterior ROI
% Difference S2 vs. S1 anterior ROI
Difference right ROI
% Difference S2 vs. S1 anterior ROI
Difference left ROI
% Difference S2 vs. S1 anterior ROI
1
69
77
57
78
162
49
132
54
2
100
58
71
58
164
39
167
36
3
41
97
60
96
75
95
21
98
4
45
84
12
94
110
63
121
59
5
−10
104
98
63
45
83
13
95
6
−6
103
122
53
136
57
143
56
7
62
95
86
94
30
98
78
94
8
63
95
27
98
26
98
37
97
9
55
96
117
91
45
96
12
99
10
38
88
131
60
44
83
82
73
11
−13
103
156
71
103
77
18
96
12
162
88
155
89
208
85
177
87
13
60
80
313
20
123
57
115
57
14
43
70
97
30
151
20
174
20
15
−122
142
51
85
122
68
13
96
16
25
90
61
71
169
45
91
64
17
45
79
82
56
−2
102
22
85
18
−29
112
104
60
191
51
155
53
19
27
88
34
86
129
52
55
80
20
−8
103
103
65
87
71
141
56
21
130
37
177
25
89
31
123
27
22
200
51
200
50
121
61
128
65
23
26
83
−4
103
157
38
84
59
24
186
49
106
38
89
62
121
53
25
115
73
147
60
188
51
144
61
Mean difference (p value)
52 (p = 0.0011)
102 (p < 0.0001)
110 (p < 0.0001)
95 (p < 0.0001)
Percent difference of S2 compared to S1
86
68
65
69
Average global density of S2 compared to S1
71.9
Discussion
Iliosacral screw fixation has emerged as the treatment of choice for unstable injuries involving the posterior pelvic ring. However, the posterior pelvic anatomy is complex and variable, and thus placement of fixation can be technically challenging. A 44 % incidence of sacral dysmorphism has been reported; therefore, a thorough understanding of the typical as well as atypical individual anatomy is critical for reliably placing safe iliosacral screws [3, 11]. In dysmorphic sacra, the first sacral safe zone was 36 % smaller compared to the normal counterparts, and with more oblique orientation from caudal to cranial and posterior to anterior [11]. In the second segment safe zone, the cross-sectional area was more than twice as large in the dysmorphic sacra compared to normal [11]. Additionally, it was found that a transverse screw could be safely placed at the S2 level in 95 % of dysmorphic sacra but only in 50 % of normal sacra [11].
The optimal fixation construct remains unclear; however, injuries with multiplanar instability have increased the rates of fixation failure [13]. Biomechanical studies have suggested improved stability using two points of posterior fixation for the treatment of unstable pelvic ring injuries [14, 15]. Therefore, the placement of two fixation screws has been recommended to aid with stability. Several clinical scenarios necessitate the placement of fixation into the second sacral segment.
Multiple cadaveric and in vivo studies have investigated proving the efficacy and safety of S2 screw fixation using both fluoroscopic and computer tomography-based multiplanar guidance systems to identify reliable and reproducible landmarks to establish a safe corridor [10, 12, 16‐19]. Several case series have established the placement of fixation into the second sacral segment as a dependable alternative or adjunct fixation method to the more common first sacral segment [3, 4, 13].
However there is a paucity of data examining the quality of bone of the second sacral segment compared to the first sacral segment. In one clinical series with 62 patients treated with closed reduction and placement of percutaneous iliosacral screws for unstable pelvic ring injuries, 2 patients were managed with 2 S1 screws, 3 with 2 screws in S2, 56 with 1 S1 and another in S2, and 1 patient with 2 screws in S1 and a 3rd in S2. Fixation failure occurred in 4 of 62 patients. Retrospectively, five patients were identified as being osteopenic, with two of these five patients having early fixation failure. This led the authors to conclude that S2 screws should be used with caution in patients with suspected pelvic and sacral osteopenia/osteoporosis [13]. Additionally, in a series of 49 patients all treated with S2 screws, 2 had postoperative loss of reduction requiring revision surgery, both with radiographic evidence of osteopenia. This led to the recommendation of finding alternative fixation methods in those patients with osteopenia and in patients with questionable intraoperative screw purchase during placement [3]. To our knowledge, our study is the first to specifically compare the bone densities of the first two sacral segments.
Multiple modalities of measuring bone density have been described and validated, including dual X-ray absorptiometry (DEXA), plain radiographs and quantitative computed tomography [20]. More recent studies have demonstrated that computed tomography examinations utilizing automatic exposure control are able to accurately measure regional cancellous bone mineral density [21]. In our study we utilized Hounsfield units, a standardized computed tomography attenuation coefficient, which has been shown to correlate with both the DEXA and compressive strengths of osseous models. We hypothesized that S2 bone density is inferior to that of S1, increasing the chances of screw loosening and fixation failure despite screw placement consistent with accepted methods in the literature.
We prospectively assessed the pelvic computed tomography scans of 25 consecutive trauma patients evaluated in the Emergency Department of a level 1 trauma center. We found a statistically significant difference in the bone density at all four points and the aggregate of S1 compared to S2. Smoking history, gender and age were not found to be independent factors in contributing to this difference.
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One of the limitations of our study is that Hounsfield units on computed tomography were used as a surrogate measurements of “bone density” or “bone quality.” This non-invasive method is well described in the literature [21] and has previously been utilized as a tool to draw conclusions about bone mineral density; however, it should be noted that it is a quantitative and not a qualitative measurement. To directly calculate bone quality and thus truly investigate the local trabecular microarchitecture of bone, would require a bone biopsy.
The optimal fixation for posterior pelvic ring injuries remains unclear. Our study demonstrates that in relatively young, otherwise healthy trauma patients there is a statistically significant difference in the bone density of the first sacral segment compared to the second sacral segment. This study highlights the need for future biomechanical studies to investigate whether this difference has a clinically relevant effect on the quality of fixation. Previous studies have highlighted clinical scenarios in which fixation in the second sacral segment is warranted and have proposed that this technique is safe and effective. However, given our findings of relative osteopenia in the second sacral segment, which may impact the quality of fixation, we feel this technique should be used with caution.
Conflict of interest
None.
Ethical standard
The study was authorized by the local ethical committee and was performed in accordance with the Ethical standards of the 1964 Declaration of Helsinki as revised in 2000.
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Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
Mit dem demographischen Wandel versorgt auch die Chirurgie immer mehr betagte Menschen. Von Entwicklungen wie Fast-Track können auch ältere Menschen profitieren und bei proximaler Humerusfraktur können selbst manche 100-Jährige noch sicher operiert werden.
Patienten, die von Ärztinnen behandelt werden, dürfen offenbar auf bessere Therapieergebnisse hoffen als Patienten von Ärzten. Besonders gilt das offenbar für weibliche Kranke, wie eine Studie zeigt.
Ob bei einer Notfalloperation nach Schenkelhalsfraktur eine Hemiarthroplastik oder eine totale Endoprothese (TEP) eingebaut wird, sollte nicht allein vom Alter der Patientinnen und Patienten abhängen. Auch über 90-Jährige können von der TEP profitieren.
