Assessing the Stiffness of Spinal Fusion in Animal Models


The clinical goal of spinal fusion is to reduce motion and the associated pain. Therefore, measuring motion under loading is critical. The purpose of this study was to validate four-point bending as a means to mechanically evaluate simulated fusions in dog and rabbit spines. We hypothesized that this method would be more sensitive than manual palpation and would be able to distinguish unilateral vs bilateral fusion. Spines from four mixed breed dogs and four New Zealand white rabbits were used to simulate posterolateral fusion with polymethyl methacrylate as the fusion mass. We performed manual palpation and nondestructive mechanical testing in four-point bending in four planes of motion: flexion, extension, and right and left bending. This testing protocol was used for each specimen in three fusion modes: intact, unilateral, and bilateral fusion. Under manual palpation, all intact spines were rated as not fused, and all unilateral and bilateral simulated fusions were rated as fused. In four-point bending, dog spines were significantly stiffer after unilateral fusion compared with intact in all directions. Additionally, rabbit spines were stiffer in flexion and left bending after unilateral fusion. All specimens exhibited significant differences between intact and bilateral fusion except the rabbit in extension. For unilateral vs bilateral fusion, significant differences were present for right bending in the dog model and for flexion in the rabbit. Unilateral fusion can provide enough stability to constitute a fused grade by manual palpation but may not provide structural stiffness comparable to bilateral fusion.

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  1. 1.

    SJ Lipson (2004) ArticleTitleSpinal-fusion surgery—advances and concerns N Engl J Med 350 643–644 Occurrence Handle10.1056/NEJMp038162 Occurrence Handle1:CAS:528:DC%2BD2cXhtFCrtr4%3D Occurrence Handle14960739

    Article  CAS  PubMed  Google Scholar 

  2. 2.

    JN Katz (1995) ArticleTitleLumbar spinal fusion. Surgical rates, costs, and complications Spine 20 78S–83S Occurrence Handle1:STN:280:BymA28zmt1Q%3D Occurrence Handle8747260

    CAS  PubMed  Google Scholar 

  3. 3.

    C Lee J Dorcil TE Radomisli (2004) ArticleTitleNonunion of the spine: a review Clin Orthop Relat Res 419 71–75 Occurrence Handle15021134

    PubMed  Google Scholar 

  4. 4.

    JS Fischgrund M Mackay HN Herkowitz et al. (1997) ArticleTitle1997 Volvo Award winner in clinical studies. Degenerative lumbar spondylolisthesis with spinal stenosis: a prospective, randomized study comparing decompressive laminectomy and arthrodesis with and without spinal instrumentation Spine 22 2807–2812 Occurrence Handle10.1097/00007632-199712150-00003 Occurrence Handle1:STN:280:DyaK1c%2FptlKrtg%3D%3D Occurrence Handle9431616

    Article  CAS  PubMed  Google Scholar 

  5. 5.

    JC Steinmann HN Herkowitz (1992) ArticleTitlePseudarthrosis of the spine Clin Orthop Relat Res 284 80–90 Occurrence Handle1395317

    PubMed  Google Scholar 

  6. 6.

    AF DePalma RH Rothman (1968) ArticleTitleThe nature of pseudarthrosis Clin Orthop Relat Res 59 113–118 Occurrence Handle1:STN:280:CCeA2MjgtVM%3D Occurrence Handle5665452

    CAS  PubMed  Google Scholar 

  7. 7.

    MB Kornblum JS Fischgrund HN Herkowitz et al. (2004) ArticleTitleDegenerative lumbar spondylolisthesis with spinal stenosis: a prospective long-term study comparing fusion and pseudarthrosis Spine 29 726–733 Occurrence Handle10.1097/01.BRS.0000119398.22620.92 Occurrence Handle15087793

    Article  PubMed  Google Scholar 

  8. 8.

    DK Sengupta HN Herkowitz (2005) ArticleTitleDegenerative spondylolisthesis: review of current trends and controversies Spine 30 S71–S81 Occurrence Handle15767890

    PubMed  Google Scholar 

  9. 9.

    C Hidaka K Goshi B Rawlins et al. (2003) ArticleTitleEnhancement of spine fusion using combined gene therapy and tissue engineering BMP-7-expressing bone marrow cells and allograft bone Spine 28 2049–2057 Occurrence Handle14501913

    PubMed  Google Scholar 

  10. 10.

    GF Muschler S Negami A Hyodo et al. (1996) ArticleTitleEvaluation of collagen ceramic composite graft materials in a spinal fusion model Clin Orthop Relat Res 328 250–260 Occurrence Handle8653966

    PubMed  Google Scholar 

  11. 11.

    Y Kotani BW Cunningham A Cappuccino et al. (1998) ArticleTitleThe effects of spinal fixation and destabilization on the biomechanical and histologic properties of spinal ligaments. An in vivo study Spine 23 672–682 Occurrence Handle1:STN:280:DyaK1c3htV2mug%3D%3D Occurrence Handle9549789

    CAS  PubMed  Google Scholar 

  12. 12.

    T Steffen D Marchesi M Aebi (2000) ArticleTitlePosterolateral and anterior interbody spinal fusion models in the sheep Clin Orthop Relat Res 371 28–37 Occurrence Handle10693547

    PubMed  Google Scholar 

  13. 13.

    JN Grauer DA Bomback R Lugo et al. (2004) ArticleTitlePosterolateral lumbar fusions in athymic rats: characterization of a model Spine J 4 281–286 Occurrence Handle10.1016/j.spinee.2003.10.001 Occurrence Handle15125850

    Article  PubMed  Google Scholar 

  14. 14.

    SD Boden JH Schimandle WC Hutton (1995) ArticleTitleAn experimental lumbar intertransverse process spinal fusion model. Radiographic, histologic, and biomechanical healing characteristics Spine 20 412–420 Occurrence Handle1:STN:280:ByqB28ngtlI%3D Occurrence Handle7747224

    CAS  PubMed  Google Scholar 

  15. 15.

    AA Sama SN Khan ER Myers et al. (2004) ArticleTitleHigh-dose alendronate uncouples osteoclast and osteoblast function: a study in a rat spine pseudarthrosis model Clin Orthop Relat Res 425 135–142 Occurrence Handle15292798

    PubMed  Google Scholar 

  16. 16.

    JN Grauer AR Vaccaro M Kato et al. (2004) ArticleTitleDevelopment of a New Zealand white rabbit model of spinal pseudarthrosis repair and evaluation of the potential role of OP-1 to overcome pseudarthrosis Spine 29 1405–1412 Occurrence Handle10.1097/01.BRS.0000115137.11276.0E Occurrence Handle15223930

    Article  PubMed  Google Scholar 

  17. 17.

    HS Kim M Viggeswarapu SD Boden et al. (2003) ArticleTitleOvercoming the immune response to permit ex vivo gene therapy for spine fusion with human type 5 adenoviral delivery of the LIM mineralization protein-1 cDNA Spine 28 219–226 Occurrence Handle12567021

    PubMed  Google Scholar 

  18. 18.

    H Suzuki K Takahashi M Yamagata et al. (2001) ArticleTitleSpatial and temporal collagen gene expression in lumbar intertransverse fusion in the rabbit J Bone Joint Surg Br 83 760–766 Occurrence Handle10.1302/0301-620X.83B5.10792 Occurrence Handle1:STN:280:DC%2BD3MvivFShtQ%3D%3D Occurrence Handle11476319

    Article  CAS  PubMed  Google Scholar 

  19. 19.

    SS Liao K Guan FZ Cui et al. (2003) ArticleTitleLumbar spinal fusion with a mineralized collagen matrix and rhBMP-2 in a rabbit model Spine 28 1954–1960 Occurrence Handle1:STN:280:DC%2BD3svjt1KjsQ%3D%3D Occurrence Handle12973141

    CAS  PubMed  Google Scholar 

  20. 20.

    AJ Yee HW Bae D Friess et al. (2004) ArticleTitleAccuracy and interobserver agreement for determinations of rabbit posterolateral spinal fusion Spine 29 1308–1313 Occurrence Handle15187630

    PubMed  Google Scholar 

  21. 21.

    GF Muschler B Huber T Ullman et al. (1993) ArticleTitleEvaluation of bone-grafting materials in a new canine segmental spinal fusion model J Orthop Res 11 514–524 Occurrence Handle10.1002/jor.1100110406 Occurrence Handle1:STN:280:ByyA2c%2FoslU%3D Occurrence Handle8340824

    Article  CAS  PubMed  Google Scholar 

  22. 22.

    JA Szivek RF Roberto JM Slack et al. (2002) ArticleTitleAn implantable strain measurement system designed to detect spine fusion: preliminary results from a biomechanical in vivo study Spine 27 487–497 Occurrence Handle10.1097/00007632-200203010-00009 Occurrence Handle11880834

    Article  PubMed  Google Scholar 

  23. 23.

    F Beer E Johnston (1992) Mechanics of Materials McGraw-Hill New York

    Google Scholar 

  24. 24.

    B Tabacknick L Fidell (1989) Using Multivariate Statistics Harper and Row New York 63–65

    Google Scholar 

  25. 25.

    JN Grauer JS Erulkar TC Patel et al. (2000) ArticleTitleBiomechanical evaluation of the New Zealand white rabbit lumbar spine: a physiologic characterization Eur Spine J 9 250–255 Occurrence Handle10.1007/s005860000141 Occurrence Handle1:STN:280:DC%2BD3M%2FgvVGgug%3D%3D Occurrence Handle10905445

    Article  CAS  PubMed  Google Scholar 

  26. 26.

    SL Blumenthal K Gill (1993) ArticleTitleCan lumbar spine radiographs accurately determine fusion in postoperative patients? Correlation of routine radiographs with a second surgical look at lumbar fusions Spine 18 1186–1189 Occurrence Handle1:STN:280:ByyA28bnt1U%3D Occurrence Handle8362324

    CAS  PubMed  Google Scholar 

  27. 27.

    AE Brodsky ES Kovalsky MA Khalil (1991) ArticleTitleCorrelation of radiologic assessment of lumbar spine fusions with surgical exploration Spine 16 S261–S265 Occurrence Handle1:STN:280:By6A38%2Fitlw%3D Occurrence Handle1862422

    CAS  PubMed  Google Scholar 

  28. 28.

    A Minamide M Kawakami H Hashizume et al. (2001) ArticleTitleEvaluation of carriers of bone morphogenetic protein for spinal fusion Spine 26 933–939 Occurrence Handle10.1097/00007632-200104150-00017 Occurrence Handle1:STN:280:DC%2BD3MzhsVCqtQ%3D%3D Occurrence Handle11317116

    Article  CAS  PubMed  Google Scholar 

  29. 29.

    SD Boden GJ Martin SuffixJr M Morone et al. (1999) ArticleTitleThe use of coralline hydroxyapatite with bone marrow, autogenous bone graft, or osteoinductive bone protein extract for posterolateral lumbar spine fusion Spine 24 320–327 Occurrence Handle1:STN:280:DyaK1M7msV2jug%3D%3D Occurrence Handle10065514

    CAS  PubMed  Google Scholar 

  30. 30.

    JS Erulkar JN Grauer TC Patel et al. (2001) ArticleTitleFlexibility analysis of posterolateral fusions in a New Zealand white rabbit model Spine 26 1125–1130 Occurrence Handle10.1097/00007632-200105150-00006 Occurrence Handle1:STN:280:DC%2BD3MzkvVWhsg%3D%3D Occurrence Handle11413423

    Article  CAS  PubMed  Google Scholar 

  31. 31.

    RC Spiro AY Thompson JW Poser (2001) ArticleTitleSpinal fusion with recombinant human growth and differentiation factor-5 combined with a mineralized collagen matrix Anat Rec 263 388–395 Occurrence Handle10.1002/ar.1119 Occurrence Handle1:CAS:528:DC%2BD3MXmsVyksrY%3D Occurrence Handle11500816

    Article  CAS  PubMed  Google Scholar 

  32. 32.

    PA Glazer MR Heilmann JC Lotz et al. (1997) ArticleTitleUse of electromagnetic fields in a spinal fusion. A rabbit model Spine 22 2351–2356 Occurrence Handle1:STN:280:DyaK1c%2Fhs1ygtw%3D%3D Occurrence Handle9355215

    CAS  PubMed  Google Scholar 

  33. 33.

    BK Tay AX Le M Heilman et al. (1998) ArticleTitleUse of a collagen–hydroxyapatite matrix in spinal fusion. A rabbit model Spine 23 2276–2281 Occurrence Handle10.1097/00007632-199811010-00005 Occurrence Handle1:STN:280:DyaK1M%2FjvVentQ%3D%3D Occurrence Handle9820906

    Article  CAS  PubMed  Google Scholar 

  34. 34.

    GF Muschler H Nitto Y Matsukura et al. (2003) ArticleTitleSpine fusion using cell matrix composites enriched in bone marrow-derived cells Clin Orthop Relat Res 407 102–118 Occurrence Handle12567137

    PubMed  Google Scholar 

  35. 35.

    ASTM (1997) Standard test methods for flexural properties of unreinforced and reinforced plastics and electrical insulating materials. 790–797

  36. 36.

    DR Carter DM Spengler (1978) ArticleTitleMechanical properties and composition of cortical bone Clin Orthop Relat Res 135 192–217 Occurrence Handle361320

    PubMed  Google Scholar 

  37. 37.

    G Lewis (1997) ArticleTitleProperties of acrylic bone cement: state of the art review J Biomed Mater Res 38 155–182 Occurrence Handle1:CAS:528:DyaK2sXjsFCmur4%3D Occurrence Handle9178743

    CAS  PubMed  Google Scholar 

  38. 38.

    FP Cammisa SuffixJr G Lowery SR Garfin et al. (2004) ArticleTitleTwo-year fusion rate equivalency between Grafton DBM gel and autograft in posterolateral spine fusion: a prospective controlled trial employing a side-by-side comparison in the same patient Spine 29 660–666 Occurrence Handle10.1097/01.BRS.0000116588.17129.B9 Occurrence Handle15014276

    Article  PubMed  Google Scholar 

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We thank Kai Zhang, MD, for help with specimen collection and Alex Aguila, DVM, for assistance with specimens and manual palpation. We would also like to thank Dr. Timothy Wright for his help in reviewing the manuscript.

This work was supported by the College of Engineering, Cornell University, and an unrestricted educational gift from Synthes USA.

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Correspondence to Jocelyn M. Cottrell BS.

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Cottrell, J.M., van der Meulen, M.C.H., Lane, J.M. et al. Assessing the Stiffness of Spinal Fusion in Animal Models. HSS Jrnl 2, 12–18 (2006).

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Key words

  • lumbar spinal fusion
  • biomechanics
  • animal model
  • spine