- Email: [email protected]
Autograft containment in posterolateral spine fusion
The Spine Journal 8 (2008) 563–569
Clinical Studies
Autograft containment in posterolateral spine fusion Raj D. Rao, MDa,*, Vaibhav Bagaria, MDa, Krishnaj Gourab, MDa, Steven T. Haworth, PhDb, Vinod B. Shidham, MDc, Brian C. Cooley, PhDa a
Department of Orthopaedic Surgery, Medical College of Wisconsin, 9200 W. Wisconsin Avenue, Milwaukee, WI 53226-0099, USA b Department of Internal Medicine, Medical College of Wisconsin, 9200 W. Wisconsin Avenue, Milwaukee, WI 53226-0099, USA c Department of Pathology, Medical College of Wisconsin, 9200 W. Wisconsin Avenue, Milwaukee, WI 53226-0099, USA Received 12 December 2006; accepted 30 April 2007
Abstract
BACKGROUND CONTEXT: Pseudoarthrosis rates in lumbar intertransverse fusion remain high. Compression and displacement of the developing fusion mass by the paraspinal musculature may be a contributory factor. Biocontainment devices have been clinically used in the skull and mandible to guide bone regeneration. The role of a mechanical device in containing graft material in the developing posterolateral lumbar spine fusion is unclear. PURPOSE: To determine the benefits of using a bioabsorbable graft-containment device for lumbar intertransverse fusion, and to evaluate the biocompatibility of this implant by histological analysis of the host tissue reaction. STUDY DESIGN: A rabbit intertransverse spine fusion model was used to evaluate a bioabsorbable graft-containment implant. Study and control groups were compared with regard to the rate, volume, and quality of fusion, as well as host tissue reaction to the graft and implant. METHODS: Fourteen adult male New Zealand White rabbits underwent bilateral posterolateral intertransverse spine arthrodesis at L3–L4. The control group (n57) received autograft alone, and the study group received autografts placed in open meshed hemicylinders fashioned from LactoSorb sheets (LactoSorb; Biomet Orthopedics Inc., Warsaw, IN). Spines were harvested at 6 weeks and imaged. Radiographs and computed tomography (CT) images were used to calculate the rate, area, and volume of fusion mass. Sections were fixed and stained with hematoxylin-eosin and Mallory trichrome for histological analysis of fusion and host tissue response. The Mann-Whitney nonparametric statistical test was used for the radiographic and CT qualitative assessments. The CT volume quantitation was analyzed using the Student t test. A p value of !.05 was used to assign statistical significance. RESULTS: The fusion rates on radiographs and CT imaging did not show a significant difference (pO.05) between the biocontainment and control groups. The volume of fusion revealed a significant increase with biocontainment (mean6standard error; total leftþright fusion sides52.8860.30 cc) compared with controls (2.1260.15 cc) (p!.05). Histology revealed no difference in the maturity or the quality of the fusion mass between the two groups. Inflammatory response around the developing fusion mass and muscle necrosis were slightly increased in the study group. The LactoSorb biocontainment material led to variable inflammatory reaction, with some areas showing little or no response and other showing an inflammatory response with fibrous connective tissue, lymphocyte infiltration, and focal foreign body giant cell reaction. CONCLUSIONS: The incidence of fusion was similar with or without a containment device for onlay bone graft. A significant increase in the volume of the fusion suggests that a biocontainment device does play a role in protecting the developing fusion mass from the mechanical effects of the paraspinal musculature. The clinical use of this device cannot be justified at this time, and further studies will determine whether this increase in fusion volume will translate
IRB Approval: N/A. * Corresponding author. Department of Orthopaedic Surgery, Medical College of Wisconsin, 9200 W. Wisconsin Avenue, Milwaukee, WI 532260099, USA. Tel.: (414) 805-7425; fax: (414) 805-7499. 1529-9430/08/$ – see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.spinee.2007.04.017
E-mail address: [email protected] (R.D. Rao)
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into a better incidence and volume of fusion in primate and human models. Ó 2008 Elsevier Inc. All rights reserved. Keywords:
Bioabsorbable implant; Graft containment; Intertransverse lumbar spine fusion
Introduction
Materials and methods
Pseudoarthrosis rates after noninstrumented lumbar intertransverse fusion remain high even in recently published studies using current surgical technique [1,2]. Multiple factors contribute to a low rate of union after lumbar intertransverse fusion including the tensile forces on graft tissue, less bone surface available for fusion, and mechanical obstruction of, or direct interposition within, the fusion process by adjacent paraspinal musculature. Prior studies on bone regeneration in craniofacial defects have shown better regeneration of bone within these defects when the process is protected from the mechanical effects of the surrounding muscles [3]. Another potential impediment to bone formation at the fusion site is compression by paraspinal musculature on the developing fusion mass, and Martin et al. [4] reported decreased spine fusion rates using bone morphogenetic protein (BMP)-soaked collagen grafts resulting from mechanical ‘‘squeeze’’ from the paraspinal muscles. Kim et al. reported further indirect evidence of this compressive effect of the surrounding muscles with a nearly 50% decrease in the volume of fusion bone 18 months postoperatively, even after successful posterolateral lumbar spine fusion [5]. The use of a biocontainment device around the graft material and fusion site may help protect against unwanted muscle encroachment and compression of graft tissue. Poynton et al. previously reported on the effect of a bioresorbable material to provide graft containment in a rabbit posterolateral spine fusion model [6]. They reported an increase in the radiographic quality of fusion bone and computed tomography (CT)–measured fusion volumes with the use of this device. Despite a higher quality of fusion bone, the incidence of fusion was similar in both groups. The aim of our study was to evaluate the effect of a biocontainment mesh device on the volume and quality of intertransverse fusion in autografted rabbit spines. Histological analysis of the fusion mass was carried out to study the quality of the fusion mass with and without the biocontainment device, investigate the role of muscle fiber interposition within the fusion process, and determine the biocompatibility of this device. The biocontainment device used in our study was a hemicylinder fashioned from LactoSorb sheets (LactoSorb; Biomet Orthopedics Inc., Warsaw, IN), a polyester derivative of L-lactic and glycolic acids (82% polylactic acid, 18% polyglycolic acid). This bioresorbable device is porous and radiolucent, and retains its formed shape for at least 6 to 8 weeks after implantation.
Spine fusion model Adult male New Zealand White rabbits (3–4 kg) were anesthetized with intramuscular ketamine (44 mg/kg) and meditomidine (0.5 mg/kg). Using sterile technique, a bilateral posterolateral intertransverse spine arthrodesis was carried out at L3–L4, as described by Poynton et al. [6]. An electric burr was used to decorticate the transverse processes. Autograft bone was harvested from the iliac crest and morselized. The fusion technique was carried out identically in two groups (n57 per group): one group (control) received autograft bone placed in standard fashion over the spine fusion site, and the second group (experimental) received the same amount of autograft bone plus a biocontainment device placed in an open hemicylinder (1.2-cm diameter; 2.5-cm length) over the transverse processes, holding the autografted bone in place and separating it from the surrounding muscle (Fig. 1). The amount of autograft bone was the same for both groups (1.5 g, divided between each side of the spine; 0.75 g/side on average). The dorsal lumbar fascia was approximated using absorbable 3-0 absorbable suture, and the skin closed with 4-0 nylon suture. All procedures were approved by the Institutional Animal Care and Use Committee and followed National Institutes of Health guidelines.
Fig. 1. Biocontainment hemicylinder device fashioned from LactoSorb sheets (LactoSorb; Biomet Orthopedics Inc., Warsaw, IN).
R.D. Rao et al. / The Spine Journal 8 (2008) 563–569 Table 1 Radiographic assessment of fusion
565
Assessment of fusion
Points
in ethylenediaminetetraacetic acid and processed for paraffin embedding after fixation in 10% formalin. The sections were stained with hematoxylin-eosin and Mallory trichrome.
Definite fusion Probable fusion Definite nonunion
2 1 0
Statistical analysis
Each fusion side was scored separately. Definite fusion was determined by continuous and dense bony trabeculae extending from L3 to L4. Probable fusion was determined by some trabecular continuity and intertransverse opacity on radiographs with small areas of intervening lucency. The score for each fusion side was taken as an independent variable for statistical analysis.
The radiographic and CT qualitative assessments were compared using the Mann-Whitney nonparametric statistical test. The CT volume quantitation was analyzed by the Student t test. A p value of .05 was used to assign statistical differences.
Evaluation Spines were harvested at 6 weeks and fixed in 4% formaldehyde in phosphate-buffered saline. Radiographs were evaluated by two blinded observers. CT images were obtained with a Fein-Focus FXE-100.50 X-ray tube (Ro¨ntgenSysteme, Garbsen, Germany) using a 3-mm focal spot, a North American Imaging AI-5830-HP image intensifier (North American Imaging, Camarillo, CA, USA) set at either the 17.8- or 23-cm aperture, and an SMD 1M15 charge-coupled device camera (Silicon Mountain Design, Colorado Springs, CO). Spines were placed within a cylinder positioned approximately 12 cm from the X-ray source. The source to image intensifier distance was approximately 81 cm. The 5 or 10 consecutive frames comprising each image set were averaged to produce a stored image for each 1 of rotation. Reconstructed volumes were rendered using commercially available software (Analyze 5.0; AnalyzeDirect, Lenexa, KS). The projection data were transferred via network to a Dell 340 workstation running Linux Red Hat, using a preprocessing program to compensate for field distortions and nonuniformities introduced by the imaging chain; isotropic reconstructions were obtained through an implementation of the Feldkamp cone-beam algorithm [7]. A threshold surface shaded rendering [8,9] was generated for each imaged spine (58-mm pixel size). The completeness of the fusion across the fusion site was assessed, using a semiquantitative scale (Table 1). CT slice data were used to measure the area of fusion mass, and a three-dimensional volumetric reconstruction of the fusion volume was calculated. After radiographic analysis, spines were decalcified
Results Fusion rates as determined by both X-ray evaluation and CT imaging showed no differences between the biocontainment and control groups (Fig. 2). Interobserver variability was less than 3% difference in scores, indicating high consistency for these measures. Radiographs obtained in the anterior-posterior plane were more likely to have hidden areas of poor fusion (Fig. 3), whereas CT images provided a more accurate assessment of fusion because of the threedimensional representation (Fig. 4). The volume of fusion as determined by CT image analysis revealed a statistically significant increase in fusion volume with biocontainment (mean6standard error; total leftþright fusion sides5 2.8860.30 cc) over controls (2.1260.15 cc) (p!.05) (Fig. 2). Histologic evaluation of the fusion mass revealed new bone formation with predominantly enchodral-type ossification both in the control and biocontainment groups. No difference in the maturity (qualitative assessment of the relative ratios of osseous to cartilaginous areas) or the quality (qualitative assessment of the amount of trabeculae in the intertransverse area) of the fusion masses was apparent between the control and study groups. In rabbits from either group where fusion did not take place, masses of fibrous and cartilaginous cells were evident in the central intertransverse areas with variable fibrosis and ossification. A mixed inflammatory response with acute and chronic inflammatory cells was observed around the developing fusion mass in adjacent muscle tissue with variable necrosis and interstitial fibrosis in both the control and study
Fig. 2. Bar graph of relative fusion rates and volume assessed on radiographs and computed tomography (CT) images. No significant differences were found in the rate of fusion between the control and study groups. The biocontainment group showed a statistically significant higher volume than the control group (p!.05) (gray bars5control group; white bars5biocontainment group).
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Fig. 3. Representative anterior-posterior radiographs of rabbit spine showing incomplete or poor fusion in the control (Top, left panel) and study (Top, right panel) groups, and good fusion in the control (Bottom, left panel) and study (Bottom, right panel) groups.
(Fig. 5) groups. This inflammatory response and necrosis was qualitatively more pronounced in the study group. Inflammatory response to the LactoSorb biocontainment material was mixed, with some areas showing little or no response and others showing an inflammatory response with fibrous connective tissue, lymphocyte infiltration (Fig. 5, middle), and focal foreign body giant cell reaction. The biocontainment material was birefringent under a polarized microscope (Fig. 5, bottom). The intensity of the inflammatory reaction (infiltration by the lymphocytes) and
necrosis was qualitatively slightly increased in the study group. Discussion Guided bone regeneration under protective sheets is a recognized surgical technique for the treatment of craniofacial skeletal defects. Cornwall et al. demonstrated better regeneration of iliac crest defects after bone graft harvest, when the defect was protected by a bioabsorbable sheet.
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Fig. 4. Representative computed tomography images of rabbit spine from controls (Left, two panels) and biocontainment-treated rabbits (Right, two panels). The top panels show incomplete fusion, with apparent gaps in the fusion bar. The bottom panels show good fusions for each group.
The percentage of the defect area filled with bone (25.9% vs. 10.7%) and the area of the newly formed bone (64.8 vs. 16.3 mm2) were higher in the protected versus the unprotected group, respectively [10]. Bone regeneration has also been demonstrated in long bone defects bridged with a resorbable poly-L-lactide chamber [11] and in deficient alveolar ridges before endosseous implant placement [12– 15]. Our study demonstrated a greater volume of fusion mass with the use of a molded biocontainment device, but found no apparent difference in the histological quality of fusion. LactoSorb plates, mesh, and screws have been used in fracture fixation and craniofacial reconstructive surgeries, especially in the pediatric age group. Bioabsorbable plate and screw fixation systems have several advantages, chief among them being better load sharing, ease of radiographic assessment of fusion, and decreased likelihood of revision surgery resulting from hardware-related complications. Complications are infrequent [16–20] and more frequent with implants containing greater amounts of polyglycolic acid [17,20]. This may relate to the higher in vivo resorption rate of polyglygolic acid in comparison to that of polylactic acid [21]. Bioabsorbable cages, screws, plates, and meshes have been used in the cervical spine [22–26]. Using structural allograft, demineralized bone matrix, and
polylactide copolymer (polylactide-co D,L lactide) anterior cervical plate and screws, Vaccaro et al. reported radiographic union in seven of nine patients (77%) at a mean follow-up of 7 months, with all patients having an excellent or good outcome as per Odom’s criteria [22]. Cadaveric cervical constructs using 70/30 polylactic acid polymer plates have a stiffness comparable to that of similar constructs with conventional metallic screws and plates [27]. Histologic evaluation of the biocontainment devices used in our study showed a slight increase in inflammatory and fibrous reaction at the site of and surrounding the implant. The lack of serious adverse histologic or clinical process within the duration of follow-up confirms the favorable biocompatible profile previously reported for these implants. In a previous study, Fuchs et al. reported on the histological appearance of the tissues surrounding bioabsorbable screws in a minipig lumbar spine model. At 6 weeks, the screws were surrounded by a sparse fibrillar capsule 10 to 25 mm in thickness. At 26 weeks, the screws had integrated completely into the bone with a rarefaction of the surrounding fibrous capsule [28]. The authors noted that the inflammatory response of the bone to these polymers was ‘‘rather weak,’’ with the presence of inflammatory cells being negligible at different stages throughout their study period. Our finding of an apparently increased
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inflammatory response with the biocontainment implant suggests that more study of the use of such devices is indicated, to assure that the level of inflammation is not detrimental to spine fusion outcome. Previous animal studies have found that resorbable biocontainment has a modest influence on spine intertransverse fusion [6,29]. Poynton et al. found a statistically significant increase in the volume of the fusion mass and a higher radiographic fusion score when autograft was used with a biocontainment device as opposed to autograft alone [6], similar to the outcome of our study. Bawa et al. reported higher fusion rates using porous biocontainment sheets (100%) than with nonporous biocontainment sheets (62%) (p5.056), with histological evidence of vascular ingrowth from overlying paraspinal muscles through the porous sheet into the developing fusion mass [29]. The mesh design of the biocontainment device used in our study theoretically allows for ingrowth of both blood vessels and migration of mesenchymal cells from the surrounding tissues into the nascent fusion mass in response to the osteoinductive proteins present in the grafted bone. Our study showed that whereas fusion rates were similar with or without a biocontainment device, the volume of fusion bone was higher after the use of a meshed interposition layer between the fusion site and the surrounding musculature. This suggests that the effect of biocontainment may be primarily mechanical, that is, preventing the interposition of the muscle fibers between the autografted bone pieces and decreasing the mechanical squeeze of the paraspinal muscles on the developing fusion mass. The increased volume of the fusion mass may conceivably translate into a greater rate of fusion over a longer postsurgical period. However, the potentially modest gains with regard to fusion and the possibility of an increased inflammatory reaction suggest that further animal studies are warranted to clarify the risk-benefit ratio with the use of these devices for posterolateral spine fusion.
References
Fig. 5. Low-power view showing residual void of absorbed biocontainment device (v), with fibrous tissue (y) and bone formation (arrows) at the fusion site and adjacent muscle (z) (hematoxylin-eosin, 2) (Top). InsetdLymphocytic infiltration with fibrosis and necrosis of muscle seen focally around trapped residue of biocontainment material (Middle). InsetdPolarized microscopy shows birefringence of residual biocontainment material (Bottom).
[1] Fischgrund JS, Mackay M, Herkowitz HN, Brower R, Montgomery DM, Kurz LT. 1997 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 1997;22(24):2807–12. [2] Kornblum MB, Fischgrund JS, Herkowitz HN, Abraham DA, Berkower DL, Ditkoff JS. Degenerative lumbar spondylolisthesis with spinal stenosis: a prospective long-term study comparing fusion and pseudarthrosis. Spine 2004;29(7):726–33; discussion 33–4. [3] Lundgren D, Nyman S, Mathisen T, Isaksson S, Klinge B. Guided bone regeneration of cranial defects, using biodegradable barriers: an experimental pilot study in the rabbit. J Craniomaxillofac Surg 1992;20(6):257–60. [4] Martin GJ Jr, Boden SD, Marone MA, Marone MA, Moskovitz PA. Posterolateral intertransverse process spinal arthrodesis with rhBMP-2 in a nonhuman primate: important lessons learned regarding dose, carrier, and safety. J Spinal Disord 1999;12(3):179–86.
R.D. Rao et al. / The Spine Journal 8 (2008) 563–569 [5] Kim KW, Ha KY, Moon MS, Kim YS, Kwon SY, Woo YK. Volumetric change of the graft bone after intertransverse fusion. Spine 1999;24(5):428–33. [6] Poynton AR, Zheng F, Tomin E, Lane JM, Cornwall GB. Resorbable posterolateral graft containment in a rabbit spinal fusion model. J Neurosurg 2002;97(4 Suppl):460–3. [7] Feldkamp LA, Davis LC, Kress JW. Practical cone-beam algorithm. J Opt Soc Am 1984;1:612–9. [8] Robb RA, Hanson DP, Karwoski RA, Larson AG, Workman FL, Stacy MC. Analyze: a comprehensive, operator-interactive software package for multidimensional medical image display and analysis. Comput Med Imaging Graph 1989;13:433–54. [9] Robb R. The Biomedical Imaging Resource at Mayo Clinic. [guest editorial]. IEEE Trans Med Imaging 2001;20:854–67. [10] Cornwall GB, Wheeler DL, Thomas KA, Taylor WR, Turner AS. Use of a resorbable sheet in iliac crest reconstruction in a sheep model. J Neurosurg 2002;97(4 Suppl)):456–9. [11] Giardino R, Fini M, Aldini NN, et al. A reabsorbable tubular chamber for the treatment of large diaphyseal bone defects. Experimental study in rabbits. Int J Artif Organs 1998;21(8):473–82. [12] Leghissa GC, Zaffe D, Assenza B, Botticelli AR. Guided bone regeneration using titanium grids: report of 10 cases. Clin Oral Implants Res 1999;10(1):62–8. [13] Lorenzoni M, Pertl C, Polansky R, Wegscheider W. Guided bone regeneration with barrier membranesda clinical and radiographic follow-up study after 24 months. Clin Oral Implants Res 1999;10(1):16–23. [14] Friedmann A, Strietzel FP, Maretzki B, Pitaru S, Bernimoulin J-P. Histological assessment of augmented jaw bone utilizing a new collagen barrier membrane compared to a standard barrier membrane to protect a granular bone substitute material. Clin Oral Implants Res 2002;13(6):587–94. [15] Fugazzotto PA, Shanaman R, Manos T, Shectman R. Guided bone regeneration around titanium implants: report of the treatment of 1,503 sites with clinical reentries. Int J Periodontics Restorative Dent 1997;17(3):292–9. [16] Eppley BL. Zygomaticomaxillary fracture repair with resorbable plates and screws. J Craniofac Surg 2000;11(4):377–85. [17] Bostman OM, Pihlajamaki HK. Adverse tissue reactions to bioabsorbable fixation devices. Clin Orthop Relat Res 2000;371:216–27.
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[18] Westermark A. LactoSorb resorbable osteosynthesis after sagittal split osteotomy of the mandible: a 2-year follow-up. J Craniofac Surg 1999;10(6):519–22. [19] Edwards RC, Kiely KD, Eppley BL. The fate of resorbable polyL-lactic/polyglycolic acid (LactoSorb) bone fixation devices in orthognathic surgery. J Oral Maxillofac Surg 2001;59(1):19–25. [20] Laine P, Kontio R, Lindqvist C, Suuronen R. Are there any complications with bioabsorbable fixation devices?: a 10 year review in orthognathic surgery. Int J Oral Maxillofac Surg 2004;33(3):240–4. [21] Vaccaro A. Resorbable polymersdapplication in spine surgery. Paper presented at: Proceedings of the Annual Meeting of the AAOS; March 22–26, 2006; Chicago, IL; Spine symposium. [22] Vaccaro AR, Venger BH, Kelleher PM, et al. Use of a bioabsorbable anterior cervical plate in the treatment of cervical degenerative and traumatic disk disruption. Orthopedics 2002;25(10 Suppl):s1191–9; discussion s9. [23] Vaccaro AR, Madigan L. Spinal applications of bioabsorbable implants. Orthopedics 2002;25(10 Suppl):s1115–20. [24] Ames CP, Acosta FL Jr, Chamberlain RH, Larios AE, Crawford NR. Biomechanical analysis of a newly designed bioabsorbable anterior cervical plate. Invited submission from the joint section meeting on disorders of the spine and peripheral nerves, March 2005. J Neurosurg Spine 2005;3(6):465–70. [25] .Ames CP, Crawford NR, Chamberlain RH, Deshmukh V, Sadikovic B, Sonntag VKH. Biomechanical evaluation of a bioresorbable odontoid screw. J Neurosurg Spine 2005;2(2):182–7. [26] Ames CP, Crawford NR, Chamberlain RH, Cornwall GB, Nottmeier E, Sonntag VKH. Feasibility of a resorbable anterior cervical graft containment plate. Orthopedics 2002;25(10 Suppl): s1149–55. [27] Ames CP, Crawford NR, Chamberlain RH, Deshmukh V, Sadikovic B, Sonntag VKH. Biomechanical analysis of a resorbable anterior cervical graft containment plate. Spine 2005;30(9): 1031–8. [28] Fuchs M, Koster G, Krause T, Merten HA, Schmid A. Degradation of and intraosseous reactions to biodegradable poly-L-lactide screws: a study in minipigs. Arch Orthop Trauma Surg 1998;118(3):140–4. [29] Bawa M, Schimizzi AL, Leek B, et al. Paraspinal muscle vasculature contributes to posterolateral spinal fusion. Spine 2006;31(8):891–6.
Clinical Studies
Autograft containment in posterolateral spine fusion Raj D. Rao, MDa,*, Vaibhav Bagaria, MDa, Krishnaj Gourab, MDa, Steven T. Haworth, PhDb, Vinod B. Shidham, MDc, Brian C. Cooley, PhDa a
Department of Orthopaedic Surgery, Medical College of Wisconsin, 9200 W. Wisconsin Avenue, Milwaukee, WI 53226-0099, USA b Department of Internal Medicine, Medical College of Wisconsin, 9200 W. Wisconsin Avenue, Milwaukee, WI 53226-0099, USA c Department of Pathology, Medical College of Wisconsin, 9200 W. Wisconsin Avenue, Milwaukee, WI 53226-0099, USA Received 12 December 2006; accepted 30 April 2007
Abstract
BACKGROUND CONTEXT: Pseudoarthrosis rates in lumbar intertransverse fusion remain high. Compression and displacement of the developing fusion mass by the paraspinal musculature may be a contributory factor. Biocontainment devices have been clinically used in the skull and mandible to guide bone regeneration. The role of a mechanical device in containing graft material in the developing posterolateral lumbar spine fusion is unclear. PURPOSE: To determine the benefits of using a bioabsorbable graft-containment device for lumbar intertransverse fusion, and to evaluate the biocompatibility of this implant by histological analysis of the host tissue reaction. STUDY DESIGN: A rabbit intertransverse spine fusion model was used to evaluate a bioabsorbable graft-containment implant. Study and control groups were compared with regard to the rate, volume, and quality of fusion, as well as host tissue reaction to the graft and implant. METHODS: Fourteen adult male New Zealand White rabbits underwent bilateral posterolateral intertransverse spine arthrodesis at L3–L4. The control group (n57) received autograft alone, and the study group received autografts placed in open meshed hemicylinders fashioned from LactoSorb sheets (LactoSorb; Biomet Orthopedics Inc., Warsaw, IN). Spines were harvested at 6 weeks and imaged. Radiographs and computed tomography (CT) images were used to calculate the rate, area, and volume of fusion mass. Sections were fixed and stained with hematoxylin-eosin and Mallory trichrome for histological analysis of fusion and host tissue response. The Mann-Whitney nonparametric statistical test was used for the radiographic and CT qualitative assessments. The CT volume quantitation was analyzed using the Student t test. A p value of !.05 was used to assign statistical significance. RESULTS: The fusion rates on radiographs and CT imaging did not show a significant difference (pO.05) between the biocontainment and control groups. The volume of fusion revealed a significant increase with biocontainment (mean6standard error; total leftþright fusion sides52.8860.30 cc) compared with controls (2.1260.15 cc) (p!.05). Histology revealed no difference in the maturity or the quality of the fusion mass between the two groups. Inflammatory response around the developing fusion mass and muscle necrosis were slightly increased in the study group. The LactoSorb biocontainment material led to variable inflammatory reaction, with some areas showing little or no response and other showing an inflammatory response with fibrous connective tissue, lymphocyte infiltration, and focal foreign body giant cell reaction. CONCLUSIONS: The incidence of fusion was similar with or without a containment device for onlay bone graft. A significant increase in the volume of the fusion suggests that a biocontainment device does play a role in protecting the developing fusion mass from the mechanical effects of the paraspinal musculature. The clinical use of this device cannot be justified at this time, and further studies will determine whether this increase in fusion volume will translate
IRB Approval: N/A. * Corresponding author. Department of Orthopaedic Surgery, Medical College of Wisconsin, 9200 W. Wisconsin Avenue, Milwaukee, WI 532260099, USA. Tel.: (414) 805-7425; fax: (414) 805-7499. 1529-9430/08/$ – see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.spinee.2007.04.017
E-mail address: [email protected] (R.D. Rao)
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into a better incidence and volume of fusion in primate and human models. Ó 2008 Elsevier Inc. All rights reserved. Keywords:
Bioabsorbable implant; Graft containment; Intertransverse lumbar spine fusion
Introduction
Materials and methods
Pseudoarthrosis rates after noninstrumented lumbar intertransverse fusion remain high even in recently published studies using current surgical technique [1,2]. Multiple factors contribute to a low rate of union after lumbar intertransverse fusion including the tensile forces on graft tissue, less bone surface available for fusion, and mechanical obstruction of, or direct interposition within, the fusion process by adjacent paraspinal musculature. Prior studies on bone regeneration in craniofacial defects have shown better regeneration of bone within these defects when the process is protected from the mechanical effects of the surrounding muscles [3]. Another potential impediment to bone formation at the fusion site is compression by paraspinal musculature on the developing fusion mass, and Martin et al. [4] reported decreased spine fusion rates using bone morphogenetic protein (BMP)-soaked collagen grafts resulting from mechanical ‘‘squeeze’’ from the paraspinal muscles. Kim et al. reported further indirect evidence of this compressive effect of the surrounding muscles with a nearly 50% decrease in the volume of fusion bone 18 months postoperatively, even after successful posterolateral lumbar spine fusion [5]. The use of a biocontainment device around the graft material and fusion site may help protect against unwanted muscle encroachment and compression of graft tissue. Poynton et al. previously reported on the effect of a bioresorbable material to provide graft containment in a rabbit posterolateral spine fusion model [6]. They reported an increase in the radiographic quality of fusion bone and computed tomography (CT)–measured fusion volumes with the use of this device. Despite a higher quality of fusion bone, the incidence of fusion was similar in both groups. The aim of our study was to evaluate the effect of a biocontainment mesh device on the volume and quality of intertransverse fusion in autografted rabbit spines. Histological analysis of the fusion mass was carried out to study the quality of the fusion mass with and without the biocontainment device, investigate the role of muscle fiber interposition within the fusion process, and determine the biocompatibility of this device. The biocontainment device used in our study was a hemicylinder fashioned from LactoSorb sheets (LactoSorb; Biomet Orthopedics Inc., Warsaw, IN), a polyester derivative of L-lactic and glycolic acids (82% polylactic acid, 18% polyglycolic acid). This bioresorbable device is porous and radiolucent, and retains its formed shape for at least 6 to 8 weeks after implantation.
Spine fusion model Adult male New Zealand White rabbits (3–4 kg) were anesthetized with intramuscular ketamine (44 mg/kg) and meditomidine (0.5 mg/kg). Using sterile technique, a bilateral posterolateral intertransverse spine arthrodesis was carried out at L3–L4, as described by Poynton et al. [6]. An electric burr was used to decorticate the transverse processes. Autograft bone was harvested from the iliac crest and morselized. The fusion technique was carried out identically in two groups (n57 per group): one group (control) received autograft bone placed in standard fashion over the spine fusion site, and the second group (experimental) received the same amount of autograft bone plus a biocontainment device placed in an open hemicylinder (1.2-cm diameter; 2.5-cm length) over the transverse processes, holding the autografted bone in place and separating it from the surrounding muscle (Fig. 1). The amount of autograft bone was the same for both groups (1.5 g, divided between each side of the spine; 0.75 g/side on average). The dorsal lumbar fascia was approximated using absorbable 3-0 absorbable suture, and the skin closed with 4-0 nylon suture. All procedures were approved by the Institutional Animal Care and Use Committee and followed National Institutes of Health guidelines.
Fig. 1. Biocontainment hemicylinder device fashioned from LactoSorb sheets (LactoSorb; Biomet Orthopedics Inc., Warsaw, IN).
R.D. Rao et al. / The Spine Journal 8 (2008) 563–569 Table 1 Radiographic assessment of fusion
565
Assessment of fusion
Points
in ethylenediaminetetraacetic acid and processed for paraffin embedding after fixation in 10% formalin. The sections were stained with hematoxylin-eosin and Mallory trichrome.
Definite fusion Probable fusion Definite nonunion
2 1 0
Statistical analysis
Each fusion side was scored separately. Definite fusion was determined by continuous and dense bony trabeculae extending from L3 to L4. Probable fusion was determined by some trabecular continuity and intertransverse opacity on radiographs with small areas of intervening lucency. The score for each fusion side was taken as an independent variable for statistical analysis.
The radiographic and CT qualitative assessments were compared using the Mann-Whitney nonparametric statistical test. The CT volume quantitation was analyzed by the Student t test. A p value of .05 was used to assign statistical differences.
Evaluation Spines were harvested at 6 weeks and fixed in 4% formaldehyde in phosphate-buffered saline. Radiographs were evaluated by two blinded observers. CT images were obtained with a Fein-Focus FXE-100.50 X-ray tube (Ro¨ntgenSysteme, Garbsen, Germany) using a 3-mm focal spot, a North American Imaging AI-5830-HP image intensifier (North American Imaging, Camarillo, CA, USA) set at either the 17.8- or 23-cm aperture, and an SMD 1M15 charge-coupled device camera (Silicon Mountain Design, Colorado Springs, CO). Spines were placed within a cylinder positioned approximately 12 cm from the X-ray source. The source to image intensifier distance was approximately 81 cm. The 5 or 10 consecutive frames comprising each image set were averaged to produce a stored image for each 1 of rotation. Reconstructed volumes were rendered using commercially available software (Analyze 5.0; AnalyzeDirect, Lenexa, KS). The projection data were transferred via network to a Dell 340 workstation running Linux Red Hat, using a preprocessing program to compensate for field distortions and nonuniformities introduced by the imaging chain; isotropic reconstructions were obtained through an implementation of the Feldkamp cone-beam algorithm [7]. A threshold surface shaded rendering [8,9] was generated for each imaged spine (58-mm pixel size). The completeness of the fusion across the fusion site was assessed, using a semiquantitative scale (Table 1). CT slice data were used to measure the area of fusion mass, and a three-dimensional volumetric reconstruction of the fusion volume was calculated. After radiographic analysis, spines were decalcified
Results Fusion rates as determined by both X-ray evaluation and CT imaging showed no differences between the biocontainment and control groups (Fig. 2). Interobserver variability was less than 3% difference in scores, indicating high consistency for these measures. Radiographs obtained in the anterior-posterior plane were more likely to have hidden areas of poor fusion (Fig. 3), whereas CT images provided a more accurate assessment of fusion because of the threedimensional representation (Fig. 4). The volume of fusion as determined by CT image analysis revealed a statistically significant increase in fusion volume with biocontainment (mean6standard error; total leftþright fusion sides5 2.8860.30 cc) over controls (2.1260.15 cc) (p!.05) (Fig. 2). Histologic evaluation of the fusion mass revealed new bone formation with predominantly enchodral-type ossification both in the control and biocontainment groups. No difference in the maturity (qualitative assessment of the relative ratios of osseous to cartilaginous areas) or the quality (qualitative assessment of the amount of trabeculae in the intertransverse area) of the fusion masses was apparent between the control and study groups. In rabbits from either group where fusion did not take place, masses of fibrous and cartilaginous cells were evident in the central intertransverse areas with variable fibrosis and ossification. A mixed inflammatory response with acute and chronic inflammatory cells was observed around the developing fusion mass in adjacent muscle tissue with variable necrosis and interstitial fibrosis in both the control and study
Fig. 2. Bar graph of relative fusion rates and volume assessed on radiographs and computed tomography (CT) images. No significant differences were found in the rate of fusion between the control and study groups. The biocontainment group showed a statistically significant higher volume than the control group (p!.05) (gray bars5control group; white bars5biocontainment group).
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Fig. 3. Representative anterior-posterior radiographs of rabbit spine showing incomplete or poor fusion in the control (Top, left panel) and study (Top, right panel) groups, and good fusion in the control (Bottom, left panel) and study (Bottom, right panel) groups.
(Fig. 5) groups. This inflammatory response and necrosis was qualitatively more pronounced in the study group. Inflammatory response to the LactoSorb biocontainment material was mixed, with some areas showing little or no response and others showing an inflammatory response with fibrous connective tissue, lymphocyte infiltration (Fig. 5, middle), and focal foreign body giant cell reaction. The biocontainment material was birefringent under a polarized microscope (Fig. 5, bottom). The intensity of the inflammatory reaction (infiltration by the lymphocytes) and
necrosis was qualitatively slightly increased in the study group. Discussion Guided bone regeneration under protective sheets is a recognized surgical technique for the treatment of craniofacial skeletal defects. Cornwall et al. demonstrated better regeneration of iliac crest defects after bone graft harvest, when the defect was protected by a bioabsorbable sheet.
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Fig. 4. Representative computed tomography images of rabbit spine from controls (Left, two panels) and biocontainment-treated rabbits (Right, two panels). The top panels show incomplete fusion, with apparent gaps in the fusion bar. The bottom panels show good fusions for each group.
The percentage of the defect area filled with bone (25.9% vs. 10.7%) and the area of the newly formed bone (64.8 vs. 16.3 mm2) were higher in the protected versus the unprotected group, respectively [10]. Bone regeneration has also been demonstrated in long bone defects bridged with a resorbable poly-L-lactide chamber [11] and in deficient alveolar ridges before endosseous implant placement [12– 15]. Our study demonstrated a greater volume of fusion mass with the use of a molded biocontainment device, but found no apparent difference in the histological quality of fusion. LactoSorb plates, mesh, and screws have been used in fracture fixation and craniofacial reconstructive surgeries, especially in the pediatric age group. Bioabsorbable plate and screw fixation systems have several advantages, chief among them being better load sharing, ease of radiographic assessment of fusion, and decreased likelihood of revision surgery resulting from hardware-related complications. Complications are infrequent [16–20] and more frequent with implants containing greater amounts of polyglycolic acid [17,20]. This may relate to the higher in vivo resorption rate of polyglygolic acid in comparison to that of polylactic acid [21]. Bioabsorbable cages, screws, plates, and meshes have been used in the cervical spine [22–26]. Using structural allograft, demineralized bone matrix, and
polylactide copolymer (polylactide-co D,L lactide) anterior cervical plate and screws, Vaccaro et al. reported radiographic union in seven of nine patients (77%) at a mean follow-up of 7 months, with all patients having an excellent or good outcome as per Odom’s criteria [22]. Cadaveric cervical constructs using 70/30 polylactic acid polymer plates have a stiffness comparable to that of similar constructs with conventional metallic screws and plates [27]. Histologic evaluation of the biocontainment devices used in our study showed a slight increase in inflammatory and fibrous reaction at the site of and surrounding the implant. The lack of serious adverse histologic or clinical process within the duration of follow-up confirms the favorable biocompatible profile previously reported for these implants. In a previous study, Fuchs et al. reported on the histological appearance of the tissues surrounding bioabsorbable screws in a minipig lumbar spine model. At 6 weeks, the screws were surrounded by a sparse fibrillar capsule 10 to 25 mm in thickness. At 26 weeks, the screws had integrated completely into the bone with a rarefaction of the surrounding fibrous capsule [28]. The authors noted that the inflammatory response of the bone to these polymers was ‘‘rather weak,’’ with the presence of inflammatory cells being negligible at different stages throughout their study period. Our finding of an apparently increased
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inflammatory response with the biocontainment implant suggests that more study of the use of such devices is indicated, to assure that the level of inflammation is not detrimental to spine fusion outcome. Previous animal studies have found that resorbable biocontainment has a modest influence on spine intertransverse fusion [6,29]. Poynton et al. found a statistically significant increase in the volume of the fusion mass and a higher radiographic fusion score when autograft was used with a biocontainment device as opposed to autograft alone [6], similar to the outcome of our study. Bawa et al. reported higher fusion rates using porous biocontainment sheets (100%) than with nonporous biocontainment sheets (62%) (p5.056), with histological evidence of vascular ingrowth from overlying paraspinal muscles through the porous sheet into the developing fusion mass [29]. The mesh design of the biocontainment device used in our study theoretically allows for ingrowth of both blood vessels and migration of mesenchymal cells from the surrounding tissues into the nascent fusion mass in response to the osteoinductive proteins present in the grafted bone. Our study showed that whereas fusion rates were similar with or without a biocontainment device, the volume of fusion bone was higher after the use of a meshed interposition layer between the fusion site and the surrounding musculature. This suggests that the effect of biocontainment may be primarily mechanical, that is, preventing the interposition of the muscle fibers between the autografted bone pieces and decreasing the mechanical squeeze of the paraspinal muscles on the developing fusion mass. The increased volume of the fusion mass may conceivably translate into a greater rate of fusion over a longer postsurgical period. However, the potentially modest gains with regard to fusion and the possibility of an increased inflammatory reaction suggest that further animal studies are warranted to clarify the risk-benefit ratio with the use of these devices for posterolateral spine fusion.
References
Fig. 5. Low-power view showing residual void of absorbed biocontainment device (v), with fibrous tissue (y) and bone formation (arrows) at the fusion site and adjacent muscle (z) (hematoxylin-eosin, 2) (Top). InsetdLymphocytic infiltration with fibrosis and necrosis of muscle seen focally around trapped residue of biocontainment material (Middle). InsetdPolarized microscopy shows birefringence of residual biocontainment material (Bottom).
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