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Study Design: An in vitro laboratory study., Objective: (i) To evaluate the effect of osteoporotic degree in determining the strength of sacral screw fixation and (ii) to compare the strength of unaugmented bicortical pedicle screw and polymethylmethacrylate (PMMA) augmented unicortical pedicle screw in sacral fixation., Summary of Background Data: Screw loosening is a clinical problem in lumbosacral fusions, especially in osteoporotic patients. To improve the screw anchoring strength of sacrum, bicortical and PMMA augmented sacral pedicle screw fixation techniques are widely used in clinical practice. However, the biomechanical strength of the bicortical and PMMA augmented sacral screw fixations remains undetermined in different degrees of osteoporosis., Methods: Twenty-five fresh osteoporotic cadavers were used in this study. According to the value of lumbar bone mineral density (BMD) assessed by DEXA, specimens were divided into 3 groups: group A (N=9): BMD=0.7 to 0.8 g/cm, group B (N=8): BMD=0.6 to 0.7 g/cm, and group C (N=8): BMD<0.6 g/cm. In each specimen, S1 pedicle screw was inserted bicortically on the left side, and S1 pedicle screw with PMMA augmentation was inserted unicortically on the right side of the sacrum. Following a dynamic cyclic loading from 30 to 250 N on the screw head for 2000 cycles, the subsidence displacement and axial pull-out strength of each screw were measured., Results: No anchoring failure (defined as the subsidence displacement exceeding 2 mm within 2000 loading cycles) occurred in group A and B. However, in group C, 6 cases (75%) in bicortical fixation and 5 cases (63%) in PMMA augmented fixation failed during cyclic loading. In group A, no significant difference between the bicortical and PMMA augmented fixations was detected in terms of the subsidence and maximal pull-out strength. In group B, significantly less subsidence and higher maximal pull-out strength were demonstrated in the PMMA augmented technique than that in the bicortical fixation. Both techniques exhibited lower subsidence of the screw in group A than in group B. The bicortical technique exhibited higher maximum pull-out strength in group A than that in group B. However, statistical difference in terms of PMMA augmentation was not detected between group A and B., Conclusion: For BMD value more than 0.70 g/cm, bicortical sacral pedicle screw fixation could obtain sufficient anchoring strength comparable with the PMMA augmented technique. When BMD value is within 0.6 to 0.7 g/cm, the PMMA augmented technique would be more beneficial in improving the fixation strength than the bicortical fixation. For BMD values less than 0.6 g/cm, early screw loosening may occur in both bicortical and PMMA augmented fixations.

Study Design: An in vitro biomechanical cadaver study., Objectives: To compare the subsidence displacement after cyclic loading among 4 sacral pedicle screw fixations of bicortical, tricortical, standard polymethylmethacrylate (PMMA) augmentation, and sub-endplate PMMA augmentation in osteoporotic condition., Summary of Background Data: Implant failure caused by screw loosening is a clinical problem for lumbosacral fusions, especially in osteoporotic patients. To improve sacral screw anchoring strength, the main fixation techniques need to be evaluated biomechanically., Methods: For this study, 11 fresh osteoporotic cadaver sacra were harvested and bone mineral density was measured with dual-energy radiograph absorptiometry. A 7 mm diameter monoaxial pedicle screw (S1) was randomly assigned by side (left vs. right) and placed bicortically or tricortically. The 2 screws, followed 2000 cyclic compression loading of 30 to 250 N, were removed. The screw tracts were filled up with PMMA, then, screws 5 mm shorter than the bicortical or tricortical fixation were reinserted (defined as standard and sub-endplate PMMA augmented sacral screw fixations, respectively). The PMMA augmented screws were then retested as before. Screw subsidence displacement after 2000 cyclic loading was measured and compared., Results: The average bone mineral density of 11 specimens was 0.71 g/cm, ranged from 0.65 to 0.78 g/cm. No significant difference of subsidence displacement was detected between tricortical and standard PMMA augmented screws (P>0.05), however, the 2 fixations exhibited markedly less subsidence than bicortical screw (P<0.05). Sub-endplate PMMA augmented screw showed the least subsidence among all the screws (P<0.05)., Conclusions: PMMA augmentation can increase the bonding strength of sacral screw-bone interface and the sub-endplate PMMA augmented sacral screw could obtain the highest stability among the 4 fixation techniques in osteoporotic condition.

Study Design: Comparison of the biomechanical fixation strengths offered by 3 iliac screw fixation techniques: short screw, short screw augmented with cement, and long screw., Objective: Evaluate the effect of screw length and bone cement augmentation on the fixation strength of iliac screw upon fatigue loading., Summary of Background Data: Iliac screws have been used in treating spinal disorders such as spinal deformity, spondylolisthesis, and sacral tumor. In clinical practices, both short screws and long screws are being used. It has been reported that short iliac screws have a higher rate of loosening. Therefore, short iliac screws are being used with bone cement augmentation to improve fixation. To date, no biomechanical study has compared the strengths of these 3 different iliac screw fixation techniques., Method: Fresh, frozen human cadaveric pelvis specimens (n = 18, 12 males, 6 females, average age 61 y) were used. Bone density was measured to characterize bone quality. The specimens were randomly divided into 2 groups. In group 1 (n = 8), short screws of 7.0-mm diameter and 70 + or - 4 mm length (as the length of exceeding over ischial notch) and long screw of 7.0-mm diameter and 120 + or - 4 mm length were placed on either side of the pelvis (left and right). In group 2 (n = 10), short iliac screws were placed after augmentation with polymethyl methacrylate bone cement on 1 side of the pelvis and long iliac screw were placed on the other side (left and right). Cyclic loading ranging from 20 to 200 N was applied to each screw at a frequency of 2 Hz up to 5000 cycles. Pullout tests were then conducted at the rate of 5 mm/min after the fatigue test, and the maximum pullout strength for each screw was recorded and analyzed., Results: The maximum pullout strength of the long screw and short screw groups after fatigue conditioning were 2386 + or - 1470 and 833 + or - 681 N respectively. Significant difference was found between the 2 groups (P < 0.05). The short iliac screw had a higher loosening rate. The pullout force of the short screw fixation with augmentation and the long screw fixation after cyclic loading were 2436 + or - 915 and 2529 + or - 1055 N, respectively. No significant difference was found between the 2 groups (P > 0.05)., Conclusions: Short iliac screws are susceptible to loosening after cyclic loading. Bone cement augmentation of short screws has demonstrated a significant increase in the fixation strength of short screws to an extent similar to that of long iliac screws. Thus, short iliac screw fixation after augmentation with bone cement will be a viable clinical option for spino-pelvic reconstruction.

Study Design: Comparison of feasibility and safety of the placement of short and long iliac screws by anatomic and biomechanical evaluations as they apply to lumbo-iliac fixation construct., Objective: To compare the stability of the short and long iliac screw fixations for lumbo-iliac reconstruction by anatomic and biomechanical evaluations., Summary of Background Data: Spinopelvic reconstruction remains a challenge to spine surgeons. Despite the advent of many fixation methods, the use of iliac screws seems most favorable so far. Various lengths of iliac screws are applied in surgical treatments; however, no biomechanical comparison has been reported based on the screw length., Methods: For anatomic observation, CT scan data of 60 Chinese adults were used to measure the details of the iliac spine structures. For biomechanical evaluation, 7 adult human cadavers (L3-pelvis) were observed. L4-S1 pedicle screw fixation was performed with posterior spinal fixation system. On the basis of the lengths of iliac screws, 2 groups were tested (short screw group using 70 mm screws and long screw group using 138 mm screws). In this study, short and long iliac screws were placed in the same specimen. Biomechanical testing was performed on a material testing machine under 800 N compression and 7 Nm torsion loading modes for stiffness evaluations. Finally, pullout testing was performed for all the iliac screws to measure the maximum pullout force., Results: The length of the line between posterior superior iliac spine and anterior inferior iliac spine was 140.6 +/- 1.1 mm, and the distance between this line and the greater sciatic notch was 18.3 +/- 0.8 mm. The length of the line between posterior superior iliac spine and the second narrowest point was 67.1 +/- 0.62 mm in men and 70.1 +/- 1.4 mm in women. Insertion lengths of the short and long iliac screws were 70 +/- 2 mm and 138 +/- 4 mm, respectively. The lumbo-pelvic reconstruction using short and long iliac screws restored 53.3% +/- 13.6% and 57.6% +/- 16.2% of the initial stiffness in compression testing respectively. In torsion testing, the use of short and long iliac screws harvested 55.1% +/- 11.9% and 62.5% +/- 9.2% of the initial stiffness, respectively. No significant difference was detected between the 2 reconstructions in terms of compressive and torsional stiffness (P > 0.05). However, the maximum pullout strength of long iliac screw group was significantly higher than the short screw group (P < 0.05)., Conclusion: The local stability is rather difficult to be restored to the original levels regardless the length of iliac screws. Obviously, long iliac screws resisted significantly greater axial pullout force. However, under physiologic, torsional, and compressive loading conditions, the mechanical stability of lumbo-pelvic fixation construct with short iliac screws was comparable with that of the long ones. Therefore, the use of short iliac screws, which are only about half the length of the long iliac screws, could reduce the implantation risk without significantly compromising on the stability of the construct.

Objective: To evaluate the efficacy of hBMP-4 gene modified tissue engineered bone graft in the enhancement of rabbit spinal fusion and find an ideal kind of substitute for the autograft bone., Methods: Rabbit BMSCs were cultured and transfected with AAV-hBMP-4 using different MOI value. The optimal MOI value were determined by observing cell's morphology change. BMSCs were then transfected with AAV-hBMP4 and AAV-EGFP respectively, following which the transfected cells were evenly suspended in a collagen sponge I, and implanted to either side of the L5,6 intertransverse spaces posterolateral in the New Zealand rabbits to induce spinal fusion. Fourteen rabbits were randomly divided into 2 groups. Group 1: AAV-hBMP-4 transfected BMSCs in the right side (hBMP-4 side) and autograft bone in the left side. Group 2: AAV-hBMP-4 transfected BMSCs in the right side (hBMP-4 side) and AAV-EGFP transfected BMSCs in the left side (EGFP side). Radiographs and three-dimensional CT of the spine, manual palpation, gross and histological examination of the fusion masses for all the animals were performed subsequent to animals having been sacrificed at 12 weeks after surgery., Results: Evaluation has been taken in 12 New Zealand rabbits delivered into 2 groups which meet the criterion after operation. Eleven in 12 implemented sides involved hBMP-4 achieved bony fusion, to which 5 in 6 autografted sides was similar. But only 2 in 6 sides in EGFP-group achieved bony fusion meanwhile. Three-dimensional CT scan and palpation also evidenced the results. Bone formation was observed obviously on specimen both in hBMP4 sides and autografted ones. EGFP-group also got bony integration, but the quantity was small., Conclusion: Tissue-engineered bone graft constructed from application of hBMP4 is a fine substitute for autograft. Effective enhancement of bony integration in spinal fusion surgery has been evidenced in vivo.

Background: Although both single and dual iliac screw techniques are used in spino-pelvic reconstruction following sacrectomy for treating sacral tumors, the basis for choosing between the two techniques for different instability types remains undetermined. The purpose of this study was to evaluate the effects of the extent of sacrectomy on the stability of the lumbo-iliac fixation construct using single and dual iliac screw techniques. Methods: Nine human L2-pelvic specimens were tested for their intact condition simulated by L3–L5 pedicle screw fixation. Sequential partial sacrectomies and L3-iliac fixation using bilateral single and dual iliac screws were conducted on the same specimens as follows: under-S1 sacrectomy+single screw, under-½S1 sacrectomy+single screw, one-side sacroiliac joint resection+single screw, total sacrectomy+single screw, and total sacrectomy+dual screw. Biomechanical testing was performed on a material testing machine for evaluating the stiffness of the L3-iliac fixation construct in compression and torsion. Findings: Single iliac screw technique was found to effectively restore the local stability in under-½S1 sacrectomy. However, it could not provide adequate stability for further resection of one-side sacroiliac joint in torsion and total sacrectomy in compression (P <0.05). On the other hand, dual iliac screw technique could restore the stability to the intact condition after total sacrectomy in both compression and torsion. Interpretation: The single iliac screw technique for L3-iliac fixation could effectively restore the local stability for under-½S1 sacrectomy. However, for instabilities of the under-½S1 sacrectomy with one-side sacroiliac joint resection or total sacrectomy, the dual iliac screw technique should be considered. [Copyright &y& Elsevier]