Radiological Anatomy And Biomechanical Studies On The Lengthened Sacroiliac Screw Fixation

Objective:To provide the anatomical basis for the feasibility and clinical practice of lengthened sacroiliac screw fixation, by means of measuring various related indices of the safe insertion regions of S1&S2lengthened sacroiliac screws.Methods:66healthy pelvises of adults (33women and33men) were scaned by64-slice spiral CT. The length, width, height and the iliac length of the safe insertion regions for S1&S2lengthened sacroiliac screws were measured, and the screw entrance points were located with a quantitative method. The indicators were recorded by descriptive statistics, and the statistics of the left and right sides, the segments of S1and S2, and the different layers of S1&S2were compared, respectively. The superoinferior region of the screw placement was compared with the anteroposterior region, and the longitudinal region of the screw placement in sagittal plane was compared with the laterigrade region. The correlations between the screw placement entrance, the length of the screw and the anatomical indices were analyzed with correlation analysis.Results:There is no significant difference between the values obtained from the simulation of the screw placement from the left side and that from the right side; The lengths of ilium within the safe insertion region for the lengthened screw are more than16mm; With the descending of the screw entrance, the length of screw placement region is smaller and smaller. The length reference range of the lengthened sacroiliac screw in the safest placement region is:S1:136.90mm～174.34mm S2:120.50mm～149.90mm. The width and height of the safe insertion region of S1&S2are almost all more than7.3mm, generally, the width and height of S1are larger than those of S2; the reference range of the best entrance of lengthened sacroiliac screw is that:S1:42.21mm～63.69mm in front of posterior superior iliac spine,32.77mm～53.75mm above the highest point of the greater sciatic notch; S2:22.68mm～54.28mm in front of posterior superior iliac spine,14.06mm-33.70mm above the highest point of the greater sciatic notch. The superoinferior region of the screw placement is larger than the anteroposterior region and the longitudinal region of the screw placement in sagittal plane is larger than the laterigrade region;There is no clear correlations between the quantitative indices of the best entrance point, the length of the safest screw placement region and the length of lateral view of ilium.Conclusions:1. There is anatomical feasibility for the placement of S1&S2lengthened sacroiliac screw.2. There is no difference between various indicators of placement of lengthened sacroiliac screw from the left and right side in the same segment.3. φ7.3mm partial thread cannulated screw (thread length:16mm) and φ6.5mm partial thread cancellous screw (thread length:16mm) can be used as lengthened sacroiliac lag screw.4. The screw thread capacity of iliac bone in the widest screw implanting regions of S1is superior to that of S2.5. With the descending of the screw entrance, the length of screw needed is smaller and smaller.6. Theoretically, there is safe space for placement of at least one piece of lengthened sacroiliac screw in both S1and S2.7. The safe insertion space of S1is larger than that of S2.8. The best entrance location of S1and S2can be approximately ascertained with anatomical landmarks.9. Implanting the lengthened sacroiliac screw in the longitudinal direction of scrum is safer than in the laterigrade direction in sagittal plane and superoinferior implanting is safer than anteroposterior implanting.10. The length and the entrance point of lengthened sacroiliac screw cannot be determined by means of the length of the ligature between the anterior superior iliac spine and the posterior superior iliac spine in lateral view. Objective:To compare various biomechanical indices of the lengthened sacroiliac screw and sacroiliac screw for the treatment of bilateral vertical sacral fractures to provide biomechanics basis for the effectiveness and safety of lengthened sacroiliac screw and to provide biomechanics support for realizing the optimization of the fixation effect.Methods:A finite element model of Tile C pelvic ring injury (bilateral type Denis Ⅱ fracture of sacrum)was produced. The bilateral sacal fractures were fixed with a lengthened sacroiliac screw and a sacroiliac screw in seven types of models respectively. The translation and angle displacement of the superior surface of sacrum and the angle displacement of the hipbone in the case of standing on both feet were measured and compared. The maximum stress of screws and screw-bone interfaces, and the stress distribution of screws and pelvises were analyzed and compared.Results:The stability of one lengthened sacroiliac screw fixation in the S1or S2segment is superior to that of two bidirectional sacroiliac screws in the same sacral segment; the stability of one lengthened sacroiliac screw fixation in S1and S2segments, respectively, is superior to that of two bidirectional sacroiliac screw fixation in S1and S2segments, respectively; the stability of one lengthened sacroiliac screw fixation in S1and S2segments, respectively, is superior to that of one lengthened sacroiliac screw fixation in the S1or S2segment; the stability of two bidirectional sacroiliac screw fixation in S1and S2segments, respectively, is markedly superior to that of two bidirectional sacroiliac screw fixation in the S1or S2segment and is also markedly superior to that of one sacroiliac screw fixation in the S1segment and one sacroiliac screw fixation in the S2segment; the vertical and everted stability of the lengthened sacroiliac screw or the sacroiliac screw fixation in S2is superior to that of S1. The rotational stability of the lengthened sacroiliac screw or sacroiliac screw fixation in S1is superior to that of S2; The maximum stress of the bilateral symmetrical sacroiliac screw fixations (including lengthened sacroiliac screw fixations) in double segments is less and the stress distribution of lengthened sacroiliac screws is more uniform than that of sacroiliac screws; The maximum stress of screw-bone interfaces of sacroiliac screws is larger than that of lengthened sacroiliac screws; The pelvic stress distribution of bilateral symmetrical fixations is relatively close to normal.Conclusions:In respect of the fixation stability, S1and S2lengthened sacroiliac screws should be used for the fixation in bilateral sacral fractures of Tile C pelvic ring injury as far as possible and the most stable fixation is the combination of the lengthened sacroiliac screws of S1and S2segments; Even if lengthened sacroiliac screws cannot be used due to limited conditions, two bidirectional sacroiliac screw fixation in S1and S2segments, respectively, is recommended; No matter which kind of sacroiliac screw is applied, the fixation combination of S1and S2segments is strongly recommended to maximise the stability of the pelvic posterior ring. With regards to the screw's safety (the maximum stress and stress distribution of screws), the combination of the lengthened sacroiliac screws of S1and S2segments is the best choice, which may minimize the risk of the screw fracture. Compared with the sacroiliac screw, the risk of the lengthened sacroiliac screw loosening is lower. The pelvic stress distribution of bilateral symmetrical fixations (including lengthened sacroiliac screw fixations) is relatively close to normal, which would contribute to the healing of sacral fractures. We hold the opinion that, compared with sacroiliac screws, lengthened sacroiliac screws have better and safer biomechanical performance and the combination of lengthened sacroiliac screws of double segments (S1&S2) would contribute to get the best curative effect. We suggest that a comprehensive evaluation should be made in clinical practice according to the actual conditions, the comparative results in this study and the surgical intended purpose so as to determine a final fixation mode reasonably. Objective:To compare various biomechanical indices of the lengthened sacroiliac screw and sacroiliac screw for the treatment of unilateral vertical sacral fractures to provide biomechanics basis for the effectiveness and safety of lengthened sacroiliac screw and to provide biomechanics support for realizing the optimization of the fixation effect.Methods:A finite element model of Tile C pelvic ring injury (unilateral type Denis Ⅱ fracture of sacrum)was produced. The unilateral sacal fractures were fixed with a lengthened sacroiliac screw and a sacroiliac screw in six types of models respectively. The translation and angle displacement of the central superior surface of sacrum in the case of standing on both feet were measured and compared. The maximum stress of screws and screw-bone interfaces and the stress distribution of screws and pelvises were analyzed and compared.Results:The stability of one lengthened sacroiliac screw fixation in the S1or S2segment is superior to that of one sacroiliac screw in the same sacral segment; the stability of one lengthened sacroiliac screw fixation in S1and S2segments, respectively, is superior to that of one sacroiliac screw fixation in S1and S2segments, respectively;the stability of one lengthened sacroiliac screw fixation in S1and S2segments, respectively, is superior to that of one lengthened sacroiliac screw fixation in the S1or S2segment; The stability of one sacroiliac screw fixation in S1and S2segments, respectively, is markedly superior to that of one sacroiliac screw fixation in the S1or S2segment; the vertical and rotational stabilty of the lengthened sacroiliac screw and sacroiliac screw fixation in S2is superior to that of S1; The maximum stress of the lengthened sacroiliac screw is less than that of the sacroiliac screw; The stress distribution of lengthened sacroiliac screws is more uniform than that of sacroiliac screws and the stress distribution of screws in double segments is more uniform than that of screws in single segment; The maximum stress of screw-bone interfaces of sacroiliac screws is larger than that of lengthened sacroiliac screws; The difference of pelvic stress distribution between the pelvises of screw fixations in double segments and the normal pelvise is less.Conclusions:In respect of the fixation stability, S1and S2lengthened sacroiliac screws should be used for the fixation in unilateral sacral fractures of Tile C pelvic ring injury as far as possible and the most stable fixation is the combination of the lengthened sacroiliac screws of S1and S2segments.Even if lengthened sacroiliac screws cannot be used due to limited conditions, one sacroiliac screw fixation in S1and S2segments, respectively, is recommended. No matter which kind of sacroiliac screw is applied, if only one screw is implanted, the fixation in S2segment is more recommended than that in S1. With regards to the screw safety (the maximum stress and stress distribution of screws), the lengthened sacroiliac screw fixation in double segments is the best choice which may minimize the risk of the screw fracture. Compared with the sacroiliac screw, the risk of the lengthened sacroiliac screw loosening is lower. The pelvic stress distribution of the lengthened sacroiliac screw and sacroiliac screw fixations in double segments is relatively close to normal, which would contribute to the healing of sacral fractures.We hold the opinion that, compared with the sacroiliac screw, the lengthened sacroiliac screw has better and safer biomechanical performance and the combination of lengthened sacroiliac screws in double segments (S1&S2) would contribute to get the best curative effect. We suggest that a comprehensive evaluation should be made in clinical practice according to the actual conditions, the comparative results in this study and the surgical intended purpose so as to determine a final fixation mode reasonably.