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abstractFull article available online at Healio.com/Orthopedics. Search: 20120919-21

The purpose of this study was to determine the change of fixation strength after adjust-ing the height of polymethylmethacrylate (PMMA)-augmented pedicle screws.

Cement-augmented cannulated pedicle screws with or without PMMA augmenta-tion with a radial hole in the distal third of the screw thread were inserted into syn-thetic bone blocks used to model osteoporosis. Screws were left unchanged (in situ), screwed in 3 threads, or screwed out 3 threads. The change in screw height was made 24 hours after cement placement. Radiographs of the samples were taken before and after screw adjustment, and pullout strength testing was performed. In the cement group, a radiolucent cavity was present after screwing in due to the screw–cement complex migrating downward, whereas no obvious change in the bone−cement com-plex existed after screwing out. Mean pullout strength was significantly higher in the groups with cement as compared to those without cement. However, in the cement groups, the screw-in group had the lowest mean pullout strength among 3 groups, and the mean pullout strength in the screw-out group was also significantly lower than that in the in situ group (P,.05).

Adjustment of pedicle screw height after cement augmentation in a severely osteopo-rotic spine can significantly reduce the pullout strength of the screw.

Drs Ying, Yu, Wang, and Liu are from the Department of Orthopaedics, Taipei Veterans General Hospital, and the Department of Surgery, School of Medicine, National Yang-Ming University; Dr Kao is from the Orthopaedic Device Research Center, National Yang-Ming University; and Dr Chang is from the Department of Orthopaedics, Taipei Veterans General Hospital, and the Institute of Anatomy and Cell Biology, School of Medicine, National Yang-Ming University, Taipei City, Taiwan.

Drs Ying, Kao, Chang, Yu, Wang, and Liu have no relevant financial relationships to disclose.The authors thank Li-Chern Pan, PhD, and Cherng-Horng Yang.Correspondence should be addressed to: Ming-Chau Chang, MD, Department of Orthopaedics,

Taipei Veterans General Hospital, No. 201, Sec 2, Shipai Rd, Beitou District, Taipei City, Taiwan 11217 (mcchang@vghtpe.gov.tw).

doi: 10.3928/01477447-20120919-21

Fixation Strength of PMMA-augmented Pedicle Screws After Depth Adjustment in a Synthetic Bone Model of OsteoporosisSzu-Han Ying, MD; Hung-CHan Kao, PHD; Ming-CHau CHang, MD; Wing-Kuang Yu, MD; SHiH-Tien Wang, MD; CHien-Lin Liu, MD

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Figure: Diagram (A) and photograph (B) of the can-nulated pedicle screw. The cement-augmented can-nulated pedicle screw is conical in shape with a side hole in the distal third of the screw threads (arrow).

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ORTHOPEDICS | Healio.com/Orthopedics

n Feature Article

Osteoporosis is common in the ag-ing population, and once spine surgery is indicated due to spon-

dylolisthesis, infection, trauma, or ma-lignancy in the elderly, instrumentation is inevitable. It is challenging for a spine surgeon to place instruments into a spine with severe osteoporosis. Many studies have revealed that complications such as loosening, backout, or migration occur frequently after instrumentation in pa-tients with severe osteoporosis, resulting in a poor clinical outcome.1,2

Pedicle screws are a popular implant for posterior instrumentation, and many methods have been used to increase the fixation strength of pedicle screws in os-teoporotic bone, such as hook augmenta-tion, wiring, cement augmentation, and bicortical purchase.1,3,4 Many studies on cement augmentation of pedicle screws have shown that fixation strength and clinical outcomes are better when cement-augmented cannulated pedicle screws are used in osteoporotic bone than with other methods.5-11

When spinal instrumentation is per-formed in a deformed spine, the height of the pedicle screws must usually be adjust-ed intraoperatively to set up the system of screws and rods. However, no studies have been performed to determine the effects of screwing in or out with respect to pullout strength when the pedicle screws are aug-mented with cement or how this may affect the integrity of the screw–cement–bone interface.

The purpose of this study was to deter-mine the change of fixation strength, the degree of loosening, and the morphologic changes after adjusting the height of poly-methylmethacrylate (PMMA)-augmented pedicle screws.

Materials and MethodsSpecimen Preparation and Experimental Procedure

Commercially available synthetic bone (Sawbones; Pacific Research Laboratory, Inc, Vashon Island, Washington) with 2

different densities was used to simulate a spinal bone with extreme osteoporosis. The synthetic bone is rigid foam with an open-celled structure resembling that of human cancellous bone. The cell structure is more than 95% open, and the cell size is 1.5 to 2.5 mm. This product has been used for biomechanical studies of cement-aug-mented cannulated pedicle screw model-ing in osteoporotic cancellous bone.12 The lower-density product (.09 g/cc; model #1522-505) has a compressive strength of .11 MPa and a compressive modulus of 6.2 MPa, whereas the higher-density product (.12 g/cc; model #1522-507) has a compressive strength of .28 MPa and a compressive modulus of 18.6 MPa. The standard block size of the synthetic bone is 1331834 cm.

For the experiments, the materials were cut into 636.534-cm pieces. Each density of the synthetic bone was divided into 6 groups, with 10 samples in each group. In the 6 low-density groups (.09 g/cc), the first group (L1) was not augmented with bone cement after placing the pedicle screw; the second group (L2) was prepared in the same way as L1, but the depth of the screw was adjusted 3 threads inward; and the third group (L3) was also prepared in the same way as L1, but the depth of the screw was adjusted 3 threads outward. Group 4 (L4) was prepared in the same way as L1, but the screw was augmented cement; group 5 (L5) was prepared in the same way as L4, but the depth of the screw was adjusted 3 threads inward after 24 hours, the average time it takes for the cement to completely solidify. Group 6 (L6) was prepared in the same way as L4, but the depth of the screw was adjusted 3 threads outward after 24 hours. The high-density samples (.12 g/cc) were also divided into 6 groups (H1-H6) and were prepared as corresponding low-density samples.

The screws used were 6.2340-mm injectable cannulated pedicle screws with a 2.5-mm-diameter screw tip (OPS Pedicle Screw; Wellong Instruments Co Ltd, Taipei City, Taiwan) and a radial

hole in the distal third of the screw thread (Figure 1). After a 40-mm-deep pilot hole was made using a 3-mm drill, the pedicle screws were placed at the same depth until all of the threads were sunk into the syn-thetic bone. Polymethylmethacrylate bone cement (Surgical Simplex P; Howmedica International S. de R.L., Limerick, Ireland) was mixed at room tempera-ture. After mixing the liquid and powder, 3 mL of the bone cement, which was in the dough phase and looked like tooth-paste, was injected into the samples de-scribed previously.

The height of the pedicle screw above the synthetic bone, measured from the proximal end of the pedicle screw to the surface of the synthetic bone, was docu-mented before and after the screw was ad-justed. Radiographs were taken 1 day after cement injection, and then again after fur-ther adjustment of the screws. SmartIris version 1.2.0.14 medical image software (Taiwan Electronic Data Processing Corporation; New Taipei City, Taiwan) was used to measure the screw height on radiographs; a horizontal line was drawn

Figure 1: Diagram (A) and photograph (B) of the cannulated pedicle screw. The cement-augmented cannulated pedicle screw is conical in shape with a side hole in the distal third of the screw threads (arrow).

1A 1B

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from the surface of the synthetic bone that was perpendicular to the axis of the screw. The height of the screw was defined as the distance between the tail end of the screw to the horizontal line. The measurement was then repeated after adjustment. After taking radiographs of all samples, pullout testing was performed to record the maxi-mum pullout strength.

Test ApparatusEach specimen was tested for fail-

ure in axial pullout using the Bionix Servohydraulic Test System (MTS Systems Corporation, Eden Prairie, Minnesota). The test block, with a screw inserted, was placed on a specially de-signed universal fixture, and the fixture was clamped on the lower side of the test-ing machine. The screw heads were fixed in a 10-mm cylindrical rod with an inner thread that matched the outer thread of the screw head. The cylindrical rod was then attached to the testing machine with a stranded stainless steel wire (3 mm in diameter) that coincided with the screw axis (Figure 2). After the specimens were mounted, pullout force was applied at a constant crosshead rate of 5 mm/min. The relationship between force and displace-ment was recorded. The ultimate pullout force was defined as the maximum load sustained before failure.

Statistical AnalysisData were presented as mean6SD.

Three states were tested (in situ, screw-in, screw-out) under 4 conditions (.09 g/cc, no cement, control; .12 g/cc, no cement, control; .09 g/cc plus cement; .12 g/cc plus cement). Differences in mean pullout strength among the 3 states were com-pared using a paired t test for each of the 4 conditions. For a given state, the mean pullout strength among 4 conditions was also compared using 1-way analysis of variance with a post-hoc pairwise com-parison Bonferroni test. A P value less than .05 was considered statistically sig-nificant; an adjusted significance level of

.01 was used for the pairwise compari-sons. Statistical analysis was performed using PASW version 18.0 statistical soft-ware (SPSS, Inc, Chicago, Illinois).

resultsRadiographic Changes

In the screw-in group, all changes ap-peared at the interface between the bone cement and the synthetic bone (Figures 3A, B). An obvious radiolucent cavity re-sulting from a space left after movement of the initial cement position was found in 13 screws with remarkable depth change, whereas no remarkable change between the interface of the pedicle screw and the bone cement was noted. Only a minor radiolucent space was found around the bone cement in the screws with less depth change (Figures 3C, D). In the screw-out group, the change was mainly between the pedicle screw and bone cement (Figures 3C, D). No displacement of the cement was found in 19 samples in this group; the screw was only moved out from the cement. No radiolucent space existed be-tween the synthetic bone and bone cement, but a conical radiolucent space was present in the cement (Figure 3D), which was the original space occupied by the screw tip.

Pullout Strength TestingMean pullout strengths of the 3 states

(in situ, screw-in, and screw-out) tested in each of the 4 conditions are shown in Figure 4. In the .09 g/cc control group, mean pullout strength was not different be-tween the states (all, P..05). In the .12 g/cc control group, mean pullout strength was higher in the screw-in group than in the in situ group (P,.001) and screw-out group (P5.037). In the .09 g/cc plus cement and .12 g/cc plus cement groups, mean pull-out strengths were significantly lower in the screw-in groups than in the in situ or screw-out groups (all, P,.01). In addition, in each state, mean pullout strength was significantly higher in the groups with ce-ment compared with those without cement (all, P,.001) (Figure 4).

discussionIn this study, the pullout strength of

cement-augmented cannulated pedicle screws, either in situ, screwed in, or screwed out, was significantly higher than that of screws without bone cement augmentation. However, this study also showed that after depth adjustment of the cement-augmented cannulated screws, ei-ther screwed in or screwed out, the pullout strength of the screw is decreased because the primary stable structure formed by the screw–cement–bone complex is damaged.

These results are consistent with many studies that have shown that PMMA ce-ment augmentation of pedicle screws increases fixation strength in a severely osteoporotic bone.12-16 Burval et al5 re-ported that the use of PMMA can increase the fixation strength of pedicle screws by 2- to 3-fold in osteoporotic vertebrae. However, when pedicle screws are placed

Figure 2: Photograph of the materials testing ma-chine. The test block with a screw inserted was placed on a specially designed universal fixture (white arrow). The head of the screw was fixed in a cylindrical rod (large black arrow) with an inner thread that matched the outer thread of the screw head. The cylindrical rod was then attached to the testing machine with a stranded stainless steel wire (small black arrow) that coincided with the screw axis.

2

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into an osteoporotic spine, an increased risk exists of screw loosening, pullout, and fixation failure; and osteoporosis has been considered a contraindication for pedicle screw fixa-tion. Many methods have been examined for improving fixa-tion strength in os-teoporotic bone, in-cluding expandable pedicle screws,17-21 various screw and rod configura-tions,12,22 and screws with different thread designs.12,23-25

Various stud-ies using different methods have exam-ined the biomechan-ics of pedicle screw

fixation. One study reported that cyclic fatigue loading results in a 20% decrease in pullout load in healthy, nonosteoporotic vertebrae, whereas the decrease is 33% in those with osteoporosis.5 In a study of short and long iliac screws by Zheng et al,26 the authors reported a markedly lower maximum pullout strength of short screws after fatigue conditioning; how-ever, no difference was noted when short screw fixation was done with cement aug-mentation. Chapman et al27 reported that the pullout screw of pedicle screws in-serted into cancellous bone was related to the major diameter of the screw, the length of engagement of the thread, the shear strength of the material the screw is inserted in, and a thread shape factor that is related to screw depth and pitch. The re-sults showed that decreasing thread pitch or increasing thread depth increases screw purchase strength. They also reported that tapping porous material decreased the pullout force.27 Zhang et al28 used finite el-ement modeling to examine the effects of bone materials on screw pullout strength in the spine and found that the effects of purchase length on the pullout strength were different for different bone material, indicating that the bone material has a sig-nificant effect on the stability of the screw. Polikeit et al29 also used an finite element model to study the effects of cement injec-tion into vertebral bodies to treat osteopo-rotic compression factors and found that although the cement restores the strength of the treated vertebrae, it leads to an in-creased endplate bulge and altered load transfer in the adjacent vertebra.

In the groups with cement augmen-tation in the current study, the pullout strength of the screw-in group was signifi-cantly decreased compared with that of the screws in situ, whereas the difference be-tween the screw-out group and in situ was much smaller. This finding is compatible with the changes seen on the radiographs, which showed that screwing in resulted in remarkable destruction to the bone–ce-ment interface, which is the main source

Figure 3: Radiographs with contrast and brightness adjusted to highlight the screw–cement interface. Sample before screwing in (A). After screwing in, a radiolucent cavity was present after the screw–cement complex migrated down-ward (white arrows) (B). Sample before screwing out (C). After screwing out, the screw migrated upward, a radiolucent space was present in the cement complex (white arrows), and no obvious change existed in the bone–cement interface (D).

3A 3B

3C 3D

Figure 4: Graph showing mean pullout strength among different conditions (n510 for each condition). Data are presented as mean6SD. †‡P,.001: indicates significant difference compared with .09 g/cc control and .12 g/cc control.

4

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of screw stability. In contrast, in the screw-out group, no obvious damage existed to the bone–cement interface. This finding also suggests that removing a cement-aug-mented cannulated pedicle screw in severe osteoporotic bone is possible and may not cause significant destruction to the bone, which is consistent with a previous study.30 Cho et al31 also examined backing out ped-icle screws augmented with PMMA in a cadaveric study and reported no pedicle or lamina fractures, but the study did not ex-amine the bone–cement–screw interface.

The results of the current study suggest that height adjustment of a cement-aug-mented cannulated pedicle screw should not be performed because it can signifi-cantly influence the stability of the screw. To ensure the stability of a pedicle screw, the ideal method is to pilot assemble the whole structure of the screw-rod system and adjust the screw height if necessary before cement injection. Although this method may increase the operative time, it can prevent the immediate loosening of the pedicle screw in severely osteoporotic bone. When screw adjustment is unavoid-able, screw-out is preferable to screw-in with respect to the influence on screw stability. Another alternative is to use a side loading system, which has an opened proximal screw end. Cement augmenta-tion can then be performed after assem-bling the entire screw-rod system.

The results of the current study also demonstrated that pullout strength was not statistically different between the 2 groups with different simulated bone density, ei-ther in situ, screw-in, or screw-out. This suggests that the difference in the synthetic bone density (.09 vs .12 g/cc) is too minor to have a significant mechanical difference in fixation strength of the screw.

This study had some limitations. The synthetic bone block used in this study has no cortical bone structure. In addition, the open cell form tested is weaker than human osteoporotic bone, and dry testing conditions without the influence of bodily fluids may not model the physiologic state

accurately.32 The amount of cement used in this study was 3 mL, and a different amount may influence screw stability in a different manner. Although axial pull-out tests are a commonly used method for evaluating the fixation strength of spinal screws, clinical screw failures may re-sult from bending and shear loads on the screws; straight pullout force rarely oc-curs in the clinical setting.

conclusionThe current study revealed that screw

height adjustment after cement augmenta-tion, either screw-in or screw-out, will de-crease screw stability, particularly when screwed in. Screw-in results in damage to the bone–cement complex, whereas screw-out leaves the bone–cement com-plex intact. The results suggest that height adjustment of pedicle screws after cement hardening should not be performed.

references 1. Ponnusamy KE, Iyer S, Gupta G, Khanna AJ.

Instrumentation of the osteoporotic spine: biomechanical and clinical considerations. Spine J. 2011; 11(1):54-63.

2. Cook SD, Salkeld SL, Stanley T, Faciane A, Miller SD. Biomechanical study of pedicle screw fixation in severely osteoporotic bone. Spine J. 2004; 4(4):402-408.

3. Cook SD, Salkeld SL, Whitecloud TS III, Barbera J. Biomechanical evaluation and pre-liminary clinical experience with an expan-sive pedicle screw design. J Spinal Disord. 2000; 13(3):230-236.

4. Santoni BG, Hynes RA, McGilvray KC, et al. Cortical bone trajectory for lumbar pedi-cle screws. Spine J. 2009; 9(5):366-373.

5. Burval DJ, McLain RF, Milks R, Inceoglu S. Primary pedicle screw augmentation in os-teoporotic lumbar vertebrae: biomechanical analysis of pedicle fixation strength. Spine (Phila Pa 1976). 2007; 32(10):1077-1083.

6. Chang MC, Liu CL, Chen TH. Polymethylmethacrylate augmentation of pedicle screw for osteoporotic spinal surgery: a novel technique. Spine (Phila Pa 1976). 2008; 33(10):E317-E324.

7. Becker S, Chavanne A, Spitaler R, et al. Assessment of different screw augmentation techniques and screw designs in osteoporot-ic spines. Eur Spine J. 2008; 17(11):1462-1469.

8. Evans SL, Hunt CM, Ahuja S. Bone cement

or bone substitute augmentation of pedicle screws improves pullout strength in poste-rior spinal fixation. J Mater Sci Mater. 2002; 13(12):1143-1145.

9. Paré PE, Chappuis JL, Rampersaud R, et al. Biomechanical evaluation of a novel fenes-trated pedicle screw augmented with bone cement in osteoporotic spines. Spine (Phila Pa 1976). 2011; 36(18):E1210-E1214.

10. Waits C, Burton D, McIff T. Cement aug-mentation of pedicle screw fixation us-ing novel cannulated cement insertion device. Spine (Phila Pa 1976). 2009; 34(14):E478-E483.

11. Frankel BM, D’Agostino S, Wang C. A bio-mechanical cadaveric analysis of polymeth-ylmethacrylate-augmented pedicle screw fixation. J Neurosurg Spine. 2007; 7(1):47-53.

12. Chen LH, Tai CL, Lee DM, et al. Pullout strength of pedicle screws with cement aug-mentation in severe osteoporosis: a compara-tive study between cannulated screws with cement injection and solid screws with ce-ment pre-filling. BMC Musculoskelet Disord. 2011; 12:33.

13. Boger A, Bisig A, Bohner M, Heini P, Schneider E. Variation of the mechanical properties of PMMA to suit osteoporotic can-cellous bone. J Biomater Sci Polym Ed. 2008; 19(9):1125-1142.

14. Boger A, Bohner M, Heini P, Verrier S, Schneider E. Properties of an injectable low modulus PMMA bone cement for osteo-porotic bone. J Biomed Mater Res B Appl Biomater. 2008; 86(2):474-482.

15. Kayanja M, Evans K, Milks R, Lieberman IH. The mechanics of polymethylmethacry-late augmentation. Clin Orthop Relat Res. 2006; (443):124-130.

16. Sawakami K, Yamazaki A, Ishikawa S, Ito T, Watanabe K, Endo N. Polymethylmethacrylate augmentation of pedicle screws increases the initial fixation in osteoporotic spine patients. J Spinal Disord Tech. 2012; 25(2):E28-E35.

17. Wan S, Lei W, Wu Z, Liu D, Gao M, Fu S. Biomechanical and histological evaluation of an expandable pedicle screw in osteo-porotic spine in sheep. Eur Spine J. 2010; 19(12):2122-2129.

18. Wan SY, Wu ZX, Zhang W, et al. Expandable pedicle screw trajectory in cadaveric lumbar vertebra: an evaluation using microcomputed tomography. J Spinal Disord Tech. 2010; 24(5):313-317.

19. Wu ZX, Cui G, Lei W, et al. Application of an expandable pedicle screw in the severe osteo-porotic spine: a preliminary study. Clin Invest Med. 2010; 33(6):E368-E374.

20. Wu ZX, Gao MX, Sang HX, et al. Surgical treatment of osteoporotic thoracolumbar compressive fractures with open vertebral cement augmentation of expandable pedicle screw fixation: a biomechanical study and a

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2-year follow-up of 20 patients. J Surg Res. 2010; 173(1):91-98.

21. Wu ZX, Gong FT, Liu L, et al. A compara-tive study on screw loosening in osteoporotic lumbar spine fusion between expandable and conventional pedicle screws. Arch Orthop Trauma Surg. 2012; 132(4):471-476.

22. Chen LH, Tai CL, Lai PL, et al. Pullout strength for cannulated pedicle screws with bone cement augmentation in severely osteo-porotic bone: influences of radial hole and pilot hole tapping. Clin Biomech (Bristol, Avon). 2009; 24(8):613-618.

23. Kim YY, Choi WS, Rhyu KW. Assessment of pedicle screw pullout strength based on vari-ous screw designs and bone densities—an ex vivo biomechanical study. Spine J. 2012; 12(2):164-168.

24. Ha KY, Hwang SC, Whang TH. Biomechanical stability according to differ-ent configurations of screws and rods [pub-lished online ahead of print November 19,

2011]. J Spinal Disord Tech.

25. Mehta H, Santos E, Ledonio C, et al. Biomechanical analysis of pedicle screw thread differential design in an osteoporotic cadaver model. Clin Biomech (Bristol, Avon). 2012; 27(3):234-240.

26. Zheng ZM, Zhang KB, Zhang JF, Yu BS, Liu H, Zhuang XM. The effect of screw length and bone cement augmentation on the fixa-tion strength of iliac screws: a biomechanical study. J Spinal Disord Tech. 2009; 22(8):545-550.

27. Chapman JR, Harrington RM, Lee KM, Anderson PA, Tencer AF, Kowalski D. Factors affecting the pullout strength of can-cellous bone screws. J Biomech Eng. 1996; 118(3):391-398.

28. Zhang QH, Tan SH, Chou SM. Effects of bone materials on the screw pull-out strength in human spine. Med Eng Phys. 2006; 28(8):795-801.

29. Polikeit A, Nolte LP, Ferguson SJ. The effect of cement augmentation on the load trans-fer in an osteoporotic functional spinal unit: finite-element analysis. Spine (Phila Pa 1976). 2003; 28(10):991-996.

30. Blattert TR, Glasmacher S, Riesner HJ, Josten C. Revision characteristics of cement-aug-mented, cannulated-fenestrated pedicle screws in the osteoporotic vertebral body: a biome-chanical in vitro investigation. Technical note. J Neurosurg Spine. 2009; 11(1):23-27.

31. Cho W, Wu C, Zheng X, et al. Is it safe to back out pedicle screws after augmentation with polymethyl methacrylate or calcium phosphate cement? A biomechanical study. Spinal Disord Tech. 2011; 24(4):276-279.

32. Patel PS, Shepherd DE, Hukins DW. Compressive properties of commercially available polyurethane foams as mechani-cal models for osteoporotic human cancel-lous bone. BMC Musculoskel Disord. 2008; 9:137.

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