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A Porcine Early-onset Scoliosis Model Created Using a Posterior Mini-invasive Method A Pilot Study Xin Zheng, PhD, MD, Xu Sun, MD, Yong Qiu, MD, Ze-zhang Zhu, MD, Wang Bin, MD, Yi-tao Ding, MD, and Bang-ping Qian, MD, PhD
Study Design: An animal study. Objective: To create a reliable porcine scoliosis model representative of early-onset scoliosis (EOS) without violation of the vertebral elements along the curve. Summary of the Background Data: To develop new nonfusion techniques for the treatment of EOS, a reliable large animal model with remarkable growth potential is required. However, a long tethering period which consumed the majority of the rapid growth phase or violation of the vertebral elements was thought to be essential in most of the previous models. Therefore, these models may be suboptimal for mimics of human EOS which was usually idiopathic type without vertebral anomalies. Materials and Methods: This study included 12 female Yorkshire pigs (aged, 5–6 wk; weight, 5–7 kg) in which scoliosis was created by posterior asymmetric tethering from T5 to L3. At the index surgery, 3 separate incisions were preformed, and ipsilateral rib tethering from the 10th to the 13th rib was performed while maintaining the vertebral elements along the maximal curve in a pristine state. Progressive deformity was documented with monthly radiographs. Frontal and sagittal profiles were assessed using the Cobb method. After an 8-week tethering period, the whole instrumentations were removed, and the pigs were observed for an additional 8-week period with serial radiographs to document the progression of the deformity.
fection, and the other experienced prolonged postoperative weakness). Of the 10 available for analysis, all pigs developed rapidly progressive, structurally 3-dimensional, idiopathic-type curves with convex to the right in the lower thoracic spine. The mean coronal Cobb angle was 29 degrees immediately postoperatively and progressed to 65 degrees after the 8-week tethering period. Eight weeks after removal of the tether, the scoliosis continued to progress and averaged 68 degrees (range, 58–78 degrees). On the sagittal plane, a mean lordosis of 32 degrees at the thoracic spine and a thoracolumbar kyphosis of 63 degrees were observed at study completion. Conclusions: A 3-dimensional rapidly progressive scoliosis model, that is closely approximate to human EOS, can be successfully created in pigs by unilaterally tethering the thoracolumbar spine and the ribcage. This model provides an equivalent EOS-like deformity and leaves adequate skeletal growth potential for biomechanical research as well as validation of fusionless scoliosis correction systems. Key Words: scoliosis model, early-onset scoliosis, nonfusion (J Spinal Disord Tech 2014;27:E294–E300)
T
Received for publication January 6, 2014; accepted May 9, 2014. From the Department of Spine Surgery, the Affiliated Drum Tower Hospital of Nanjing University Medical School, Nanjing, China. X.Z. and X.S. contributed equally. Supported by National Natural Science Foundation of China (81372010), Medical Science and Technology Development Foundation, Nanjing Department of Health (201108016), China Postdoctoral Science Foundation funded project (2013M541654), and Jiangsu Provincial Natural Science Foundation (BK20130090). X.Z., X.S.: designed the study and drafted the manuscript; X.Z., B.Q., X.S.: performed the experimental work and the statistical analysis. All authors have read and approved the final manuscript. The authors declare no conflict of interest. Reprints: Bang-ping Qian, MD, PhD, Department of Spine Surgery, the Affiliated Drum Tower Hospital of Nanjing University Medical School, Zhongshan Road 321, Nanjing 210008, China (e-mail: [email protected]). Copyright r 2014 by Lippincott Williams & Wilkins
reatment of the growing spine afflicted with progressive early-onset scoliosis (EOS) remains challenging for a long time.1 Both controlling the progressing deformity and maximizing spinal growth are the ultimate goals of treatment. Many established methods that are successful in adolescent patients, such as orthosis or spinal fusion, have very limited effectiveness in children with EOS. Recently, various types of distraction and growth-guiding techniques have been developed to minimize the adverse impact on spinal growth, including the use of growing rods2,3 and vertically expandable prosthetic titanium ribs,4 etc. However, like all other procedures for EOS, growing rods and vertically expandable prosthetic titanium ribs have been shown to have a high risk of complication.2–4 The high rates of complication may be the result of both multiple surgeries and the presence of critical health issues in these patients. For these concerns, further study of novel techniques engaged in the treatment of EOS are anticipated. Ideally, the study of growth modulation and the development of new nonfusion techniques involve the use of an
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J Spinal Disord Tech
Results: Of the 12 pigs enrolled in this study, 2 encountered substantial complications (1 developed a postoperative in-
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experimental scoliosis model. Although several scoliosis models have been established in a variety of large animals, very few are suitable for subsequent fusionless scoliosis studies. This limitation arises because the animals used for the models had already passed the peak height velocity when structural scoliosis was induced,5–8 with little residual growth potential available for further fusionless scoliosis investigations.9 In a recent publication, Odent et al10 established an idiopathic-type scoliotic deformity in immature pigs. On the basis of the size of the immature pigs, they thought that their model was similar to human EOS. Despite the rapid progression of scoliosis during their 8-week tethering period, the scoliosis in the pigs was suboptimal because of the small curve magnitude. To achieve the greatest similarity to human EOS, scoliosis created over a shorter tethering period with sufficient growth potential is required. In addition, less violation to the spine is also necessary for the subsequent validation of fusionless scoliosis implants. The aim of this study is to create an EOS model which is characterized by rapidly progressive, structurally 3-dimensional, and idiopathic type in the immature pigs with a mini-invasive method.
Porcine Early-onset Scoliosis Model
(Ethicon Inc., New Brunswick, NJ) from the 10th to the 13th rib was performed subsequently (Fig. 1). A 2.5-mm flexible stainless steel cable (Medtronic Spinal and Biologics, Memphis, TN) was secured to the proximal pedicle screws using connectors and then passed
MATERIALS AND METHODS This study was approved by Institutional Animal Care and Use Committee. A total of 12 female Yorkshire pigs (aged, 5–6 wk; weight, 5–7 kg) were enrolled in this study. Before the initial tethering surgery, confirmed by xray films, all of the pigs’ spines were normally aligned in the coronal, sagittal, and axial planes. To evaluate the spinal growth, a preliminary growth control study was undertaken in 5 female Yorkshire pigs over a period of 6 months. Body weight and length (the distance from the midpoint of the ears to the proximal end of the tail) were measured monthly under sedation. The body length of the pigs increased by 10.1 cm/mo on average, from 45.2 cm at 1 month to 95.8 cm at 6 months. The body weight of the pigs increased by 11.4 kg/mo on average, from 5.8 kg to 62.9 kg after 5 months. On the basis of these results, it was indicated that the 6-monthold pigs were still in the rapid growth phase and they still had considerable growth potential.
Surgical Procedure After a 1-week acclimatization period, the pigs were induced with ketamine (20 mg/kg), intubated, and then anesthetized with sodium pentobarbital. The pigs were placed in a prone position on a grounding pad. Two separate midline posterior skin incisions from T5 to T6 and from L2 to L3 were used to expose the tips of the left transverse processes. Four pedicle screws (4.5 mm in diameter and 25–30 mm in length; Medtronic Sofamor Danek, Memphis, TN) were then inserted bicortically into the pedicles of the left side at T5–T6 and L2–L3 level. Afterwards, at 5 cm lateral from the midline, a left-sided paramedian incision from T10 to T13 was made to expose the ribs. The rib periosteum was carefully exposed without damage to the pleura, and ligamentous tethering r
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FIGURE 1. Definitive offset design used with the pedicle screws, the cable with the connectors. www.jspinaldisorders.com |
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FIGURE 2. A and B, Before the surgery, the porcine spine was normally aligned in the coronal and sagittal planes. C, Immediately after the tethering surgery, a mild scoliosis of 27 degrees was created in pig 7. D, Lateral radiograph showed the kyphosis was 55 degrees on the sagittal plane. E, Eight weeks after tether release, a significant scoliosis (72 degrees) was observed. F, The thoracic lordosis was 53 degrees and the thoracolumbar kyphosis was 75 degrees on the sagittal plane. G, Three-dimensional reconstruction computed tomography (CT) showed a typical scoliosis. H, Axial CT scan showed apical vertebral rotation of 38 degrees.
intramuscularly and caudally to connect with the distal pedicle screws (Fig. 2C). On the basis of the flexibility of the porcine spine, the cable length was set 3–4 cm shorter than the distance from T5 to L3 to create a unilateral tethering. A mild deformity with convex to the right side was induced once the tether was fastened and locked to the connectors. Wound closure was then performed using multilayer absorbable sutures.
Observation and Radiography Dorsoventral and lateral x-ray radiographs as well as computed tomography (CT) scans were taken before surgery, immediately after surgery, and at 4-week intervals to assess curve progression and instrumentation positioning. Pigs were excluded if screws pull-out or implant breakage was observed in the radiographs. The pigs were sedated before radiographic acquisition to ensure standardized films. Body weight, body length, and the Cobb angle were measured at the time of each radiograph. The apical vertebra wedging was also measured from the dorsoventral x-ray films. Axial rotation was measured during the 8-week
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tethering period using the CT scans.11 The rib hump was evaluated from the CT images using a method proposed by Thulborne and Gillespie12 and Lykissas et al.13 A rib hump elevation on the side of the convexity was considered positive. After 8-week tethering period, the stainless steel cable and pedicle screws were removed in all pigs. The pigs were observed for an additional 8-week period, and serial radiographs were taken at 4-week intervals. At study completion, the animals were euthanized, and their spines were harvested and stored for further histologic analyses.
Statistical Analysis Statistical analysis was performed using the SPSS (version 13.0; SPSS Inc., Chicago, IL) software package. Radiographic data were compared using repeated-measures analysis of variance between radiographic acquisition points with post hoc tests to see if any overall significance came about differing from one another. The level of statistical significance was set at P < 0.05. r
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TABLE 1. General Data of the Pigs Cobb Angle Pig No. 1 2 3 4 5 6 7 8 9 10
Age (wk)
Weight (kg)
Immediate After Tethering (deg.)
5 5 6 5 6 5 6 5 6 6
5.0 5.5 7.0 6.0 6.5 5.5 6.0 6.0 7.0 6.5
25 22 27 30 32 25 27 39 32 30
RESULTS Of the 12 animals enrolled in this study, 2 pigs were excluded from the analysis (1 developed a postoperative infection, and the other experienced prolonged postoperative weakness). Thus, our data analyses were based on the remaining 10 pigs. Over the 8-week tethering period, all of the 10 pigs developed a severe structural scoliosis associated with a chest wall deformity. The mean rib hump elevation was positive, demonstrating a right rib prominence and a left depressed thoracic cage. All the progressive curves had identical radiographic features of right thoracic scoliosis, including apical vertebral rotation and sagittal deformity, showing thoracic lordosis and thoracolumbar kyphosis. There was a mean of 11.0 ± 0.7 vertebrae within the instrumented levels. The curve patterns of all the pigs were similar, with the apex approximately at T11–T13. Table 1 summarized the evolution of scoliosis over the tethering period and the subsequent untethering period in each pig. The scoliosis immediately after the tethering procedure averaged 28.9 degrees (range, 22–39 degrees) and progressed to 64.7 degrees on average (range, 54–77 degrees) over the 8-week tethering period. The average progression of 35.8 degrees (range, 29–43 degrees) was significant (P < 0.001) (Fig. 3). On the sagittal plane, the average kyphosis after the tethering procedure was 38.7 degrees (range, 32–47 degrees) and the sagittal profiles showed 32.0 degrees (range, 21–42 degrees) lordosis in the thoracic spine and 63.0 degrees (range, 40–72 degrees) kyphosis in the thoracolumbar spine after 8-week tethering period. The mean rotation at the apex increased from 12.1 degrees (range, 6–22 degrees) immediately after the tethering procedure to 38.8 degrees (range, 29–46 degrees) after 8-week tethering period. After removal of the tether, the average scoliosis slightly decreased during the first 4 weeks, but progressed to 67.5 degrees (range, 58–78 degrees) over the next 4 weeks. The average progression of 2.8 degrees during this 8-week untethering period was statistically significant (P < 0.05; Fig. 3). On the sagittal plane, 8 weeks after removal of the tether, the average lordosis in the thoracic spine was 31.0 degrees, whereas in the thoracolumbar spine, the average kyphosis was 60.3 degrees. Decreases of both the thoracic lordosis (P = 0.918) and thoracor
2014 Lippincott Williams & Wilkins
8-wk Postoperative (deg.)
8-wk After Removal of Tethering (deg.)
54 57 62 65 68 55 70 77 67 72
58 62 65 65 70 60 72 78 70 75
lumbar kyphosis (P = 0.774) were not significant. On the axial plane, the average axial rotation measured by CT scan was 28.8 degrees (range, 22–42 degrees). At study completion, CT scan analysis revealed apical vertebrae rotation in the axial plane and vertebral wedging mainly in the frontal plane. This wedging was more pronounced at the apical vertebrae, averaging 9.9 degrees (Fig. 5).
DISCUSSION Goals of treatment for EOS patients include controlling the progressive deformity as well as allowing for the growth of spine. For EOS patients, fusionless correction surgery has received increasing concerns over the past decades.3–5,7,14–17 There is a myriad of treatment options that are accompanied by complications, thus necessitating a new generation of fusionless correction systems. First and foremost, a proper animal model that allows for preclinical testing of fusionless scoliosis implants needs to be developed.
FIGURE 3. Evolution of scoliosis in pigs during the 16-week period. The scoliosis aggravated during the 8-week tethering period. Four weeks after the tether release, a small decrease of the Cobb angle was observed. However, the scoliosis progressed over the next 12th–16th weeks. www.jspinaldisorders.com |
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FIGURE 4. A, Before the tethering surgery, the porcine spine of pig 7 was well aligned in the coronal plane. Two posterior skin incisions from T5 to T6 and L1 to L3 levels were made for pedicle screws implantation. A left-sided paramedian incision from T10 to T13 levels was also made for tethering of the left ribs. B, Immediately after surgery, the local curvature of the thoracic spine showed a mild spinal deformity. C, Eight weeks after the tethering surgery, a typical, idiopathic-type scoliosis involving a right rib prominence was presented.
Two prerequisites are involved in setting up an ideal EOS model: it should provide a comparative size to human and experience rapid progression of scoliosis. Having a more rounded thorax and a comparable vertebral size, pigs are the most representative of humans in terms of spine shape.18–20 Therefore, in the present study, we used Yorkshire pigs to get close approximation of human scoliosis. Although many authors have created scoliosis models with remarkable curves,6,8,10 several shortcomings related to their study designs are evident. Firstly, the animals used by Schwab et al8 and Braun et al6 had passed the peak height velocity when scoliosis was induced.9 Thus, there were less growth available for subsequent fusionless scoliosis correction investigations. Secondly, while contralateral rib resections are performed, the chest wall is severely destroyed. This destruction can result in diminished pulmonary function and can be fatal in very young animals. It may also violate the integrity of the thoracic cage, which may cause the model to differ from human idiopathic scoliosis. Thirdly, to gain access for concave rib tethering, a huge incision is required, and the paraspinal muscle is violated, which might increase the damage to the immature animals. Finally, rough disturbance of the paraspinal muscle may eventually increase the risk of autofusion and thus limit fusionless correction investigations.
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To create an experimental scoliosis model in a large animal that not only approximates EOS but also results in minimal violation of the spinal elements along the curve, a novel method was developed in the present study. Three separate incisions were used: 2 for the insertion of the pedicle screws and 1 for rib tethering, which also served as an intermediary for the cable to connect to the 4 pedicle screws (Fig. 4A). Our scoliosis model was successfully established with the spinal elements intact despite some mild violation of the pedicles at the cephaled and caudal portions. The osseous structures, disks, and ligamentous attachments along the curve remained undisturbed. Compared with previous models, our model produces ideal experimental scoliosis for the subsequent study of treatments aimed at testing fusionless corrective techniques for EOS. Schwab et al8 demonstrated that the greater the initial deformity, the greater the rate of progression, which was consistent with the Hueter-Volkmann principle. Compared with the Odent et al’s study10 in which no scoliosis was initially presented, our results showed larger curves that were more representative of EOS patients for whom nonfusion treatment would be indicated. Moreover, using immature pigs, our model generated rapid scoliosis progression based on the exponential growth during the 8-week tethering period. Our scoliosis model can also permit r
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(increased from 60.1 to 67.5 degrees), which might be caused by the continued asymmetric vertebral growth. Although asymmetric mechanical loading originating from the tether diminished after removal of the spinal tether, vertebral growth continued at a slightly slower rate on the concavity, which caused scoliosis aggravation. Some caution, however, should be taken when comparing human scoliosis with experimental scoliosis in an animal model. The forces acting on the spine of bipedal humans differ from those on a quadruped animal. However, the marked curves present in our porcine scoliosis model are more representative of human EOS than previous models.
CONCLUSIONS Using an approach involving 3 separate incisions, a 3dimensional, rapidly progressive scoliosis was created in immature pigs with reasonable similarity to human EOS. This model may allow for validation of new fusionless corrective techniques because the animals are still in the rapid growth phase when remarkable scoliosis is achieved. FIGURE 5. Eight weeks after removal of the tethering, coronal reformatted image of computed tomography showed wedging of the apical vertebrae of pig 7.
validation of fusionless corrective techniques due to the remarkable remaining growth. Similarly, as Schwab et al8 reported, we also found that the rib resection of the convex side was unnecessary for the progression of deformity. In the present study, ribs on the concave side were tethered to give an initial direction for the scoliosis to progress; this procedure might be less invasive. Although the curves in the present study were not idiopathic, our experimental scoliosis possessed 3-dimensional features, including lateral translation of the apical vertebra from the midline, thoracic lordosis measuring 31.0 degrees on average, and axial rotation with vertebral body rotation into the curve convexity. Radiographic analyses of the CT scans showed significant apical vertebral wedging and ribcage asymmetry. Similar to human EOS, in our model, it was observed that the wedging of vertebrae was greatest at the apex (9.9 degrees at 8 wk after the removal of the tether; Fig. 5). Despite the morphologic similarities to previously reported scoliosis models,5–8,10 our model shows an iatrogenic sagittal deformity, including thoracic lordosis and thoracolumbar kyphosis. The thoracolumbar kyphosis may be caused by the location of the rib tethering, which was more laterally and more ventrally located than the position of Schwab et al’s study.8 Fastening of the rib tethering might produce an initial kyphosis on the thoracolumbar spine, which is likely to aggravate over time according to the “vicious cycle hypothesis.”21 A small decrease of the Cobb angle was observed 4 weeks after tether release. This phenomenon might result from the viscoelasticity of the disks. However, scoliosis progression over the next 12–16 weeks was evident r
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ACKNOWLEDGMENTS The authors acknowledge Dr Mingyan Liu and Dr Jeffery Zhang for their assistance during the experiment design. REFERENCES 1. Akbarnia BA. Management themes in early onset scoliosis. J Bone Joint Surg Am. 2007;89(suppl 1):42–54. 2. Blakemore LC, Scoles PV, Poe-Kochert C, et al. Submuscular Isola rod with or without limited apical fusion in the management of severe spinal deformities in young children: preliminary report. Spine (Phila Pa 1976). 2001;26:2044–2048. 3. Thompson GH, Akbarnia BA, Kostial P, et al. Comparison of single and dual growing rod techniques followed through definitive surgery: a preliminary study. Spine (Phila Pa 1976). 2005;30: 2039–2044. 4. Campbell RM Jr, Smith MD, Hell-Vocke AK. Expansion thoracoplasty: the surgical technique of opening-wedge thoracostomy. Surgical technique. J Bone Joint Surg Am. 2004;86-A(suppl 1): 51–64. 5. Braun JT, Ogilvie JW, Akyuz E, et al. Experimental scoliosis in an immature goat model: a method that creates idiopathic-type deformity with minimal violation of the spinal elements along the curve. Spine (Phila Pa 1976). 2003;28:2198–2203. 6. Braun JT, Ogilvie JW, Akyuz E, et al. Creation of an experimental idiopathic-type scoliosis in an immature goat model using a flexible posterior asymmetric tether. Spine (Phila Pa 1976). 2006;31: 1410–1414. 7. Zhang YG, Zheng GQ, Zhang XS, et al. Scoliosis model created by pedicle screw tethering in immature goats: the feasibility, reliability, and complications. Spine (Phila Pa 1976). 2009;34:2305–2310. 8. Schwab F, Patel A, Lafage V, et al. A porcine model for progressive thoracic scoliosis. Spine (Phila Pa 1976). 2009;34:E397–E404. 9. Braun JT, Akyuz E, Ogilvie JW. The use of animal models in fusionless scoliosis investigations. Spine (Phila Pa 1976). 2005;30: S35–S45. 10. Odent T, Cachon T, Peultier B, et al. Porcine model of early onset scoliosis based on animal growth created with posterior miniinvasive spinal offset tethering: a preliminary report. Eur Spine J. 2011;20:1869–1876. 11. Aaro S, Dahlborn M, Svensson L. Estimation of vertebral rotation in structural scoliosis by computer tomography. Acta Radiol Diagn (Stockh). 1978;19:990–992.
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12. Thulbourne T, Gillespie R. The rib hump in idiopathic scoliosis. Measurement, analysis and response to treatment. J Bone Joint Surg Br. 1976;58:64–71. 13. Lykissas MG, Sharma V, Crawford AH. Assessment of rib hump deformity correction in adolescent idiopathic scoliosis with or without costoplasty using the double rib contour sign. J Spinal Disord Tech. 2012. [Epub ahead of print]. 14. Akbarnia BA, Emans JB. Complications of growth-sparing surgery in early onset scoliosis. Spine (Phila Pa 1976). 2010;35: 2193–2204. 15. Akbarnia BA, Marks DS, Boachie-Adjei O, et al. Dual growing rod technique for the treatment of progressive early-onset scoliosis: a multicenter study. Spine (Phila Pa 1976). 2005;30:S46–S57. 16. Cunningham ME, Frelinghuysen PH, Roh JS, et al. Fusionless scoliosis surgery. Curr Opin Pediatr. 2005;17:48–53.
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17. Newton PO, Fricka KB, Lee SS, et al. Asymmetrical flexible tethering of spine growth in an immature bovine model. Spine (Phila Pa 1976). 2002;27:689–693. 18. Bozkus H, Crawford NR, Chamberlain RH, et al. Comparative anatomy of the porcine and human thoracic spines with reference to thoracoscopic surgical techniques. Surg Endosc. 2005;19:1652–1665. 19. Busscher I, Ploegmakers JJ, Verkerke GJ, et al. Comparative anatomical dimensions of the complete human and porcine spine. Eur Spine J. 2010;19:1104–1114. 20. Sheng SR, Wang XY, Xu HZ, et al. Anatomy of large animal spines and its comparison to the human spine: a systematic review. Eur Spine J. 2010;19:46–56. 21. Stokes IA, Spence H, Aronsson DD, et al. Mechanical modulation of vertebral body growth. Implications for scoliosis progression. Spine (Phila Pa 1976). 1996;21:1162–1167.
r
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RIGINAL
ARTICLE
A Porcine Early-onset Scoliosis Model Created Using a Posterior Mini-invasive Method A Pilot Study Xin Zheng, PhD, MD, Xu Sun, MD, Yong Qiu, MD, Ze-zhang Zhu, MD, Wang Bin, MD, Yi-tao Ding, MD, and Bang-ping Qian, MD, PhD
Study Design: An animal study. Objective: To create a reliable porcine scoliosis model representative of early-onset scoliosis (EOS) without violation of the vertebral elements along the curve. Summary of the Background Data: To develop new nonfusion techniques for the treatment of EOS, a reliable large animal model with remarkable growth potential is required. However, a long tethering period which consumed the majority of the rapid growth phase or violation of the vertebral elements was thought to be essential in most of the previous models. Therefore, these models may be suboptimal for mimics of human EOS which was usually idiopathic type without vertebral anomalies. Materials and Methods: This study included 12 female Yorkshire pigs (aged, 5–6 wk; weight, 5–7 kg) in which scoliosis was created by posterior asymmetric tethering from T5 to L3. At the index surgery, 3 separate incisions were preformed, and ipsilateral rib tethering from the 10th to the 13th rib was performed while maintaining the vertebral elements along the maximal curve in a pristine state. Progressive deformity was documented with monthly radiographs. Frontal and sagittal profiles were assessed using the Cobb method. After an 8-week tethering period, the whole instrumentations were removed, and the pigs were observed for an additional 8-week period with serial radiographs to document the progression of the deformity.
fection, and the other experienced prolonged postoperative weakness). Of the 10 available for analysis, all pigs developed rapidly progressive, structurally 3-dimensional, idiopathic-type curves with convex to the right in the lower thoracic spine. The mean coronal Cobb angle was 29 degrees immediately postoperatively and progressed to 65 degrees after the 8-week tethering period. Eight weeks after removal of the tether, the scoliosis continued to progress and averaged 68 degrees (range, 58–78 degrees). On the sagittal plane, a mean lordosis of 32 degrees at the thoracic spine and a thoracolumbar kyphosis of 63 degrees were observed at study completion. Conclusions: A 3-dimensional rapidly progressive scoliosis model, that is closely approximate to human EOS, can be successfully created in pigs by unilaterally tethering the thoracolumbar spine and the ribcage. This model provides an equivalent EOS-like deformity and leaves adequate skeletal growth potential for biomechanical research as well as validation of fusionless scoliosis correction systems. Key Words: scoliosis model, early-onset scoliosis, nonfusion (J Spinal Disord Tech 2014;27:E294–E300)
T
Received for publication January 6, 2014; accepted May 9, 2014. From the Department of Spine Surgery, the Affiliated Drum Tower Hospital of Nanjing University Medical School, Nanjing, China. X.Z. and X.S. contributed equally. Supported by National Natural Science Foundation of China (81372010), Medical Science and Technology Development Foundation, Nanjing Department of Health (201108016), China Postdoctoral Science Foundation funded project (2013M541654), and Jiangsu Provincial Natural Science Foundation (BK20130090). X.Z., X.S.: designed the study and drafted the manuscript; X.Z., B.Q., X.S.: performed the experimental work and the statistical analysis. All authors have read and approved the final manuscript. The authors declare no conflict of interest. Reprints: Bang-ping Qian, MD, PhD, Department of Spine Surgery, the Affiliated Drum Tower Hospital of Nanjing University Medical School, Zhongshan Road 321, Nanjing 210008, China (e-mail: [email protected]). Copyright r 2014 by Lippincott Williams & Wilkins
reatment of the growing spine afflicted with progressive early-onset scoliosis (EOS) remains challenging for a long time.1 Both controlling the progressing deformity and maximizing spinal growth are the ultimate goals of treatment. Many established methods that are successful in adolescent patients, such as orthosis or spinal fusion, have very limited effectiveness in children with EOS. Recently, various types of distraction and growth-guiding techniques have been developed to minimize the adverse impact on spinal growth, including the use of growing rods2,3 and vertically expandable prosthetic titanium ribs,4 etc. However, like all other procedures for EOS, growing rods and vertically expandable prosthetic titanium ribs have been shown to have a high risk of complication.2–4 The high rates of complication may be the result of both multiple surgeries and the presence of critical health issues in these patients. For these concerns, further study of novel techniques engaged in the treatment of EOS are anticipated. Ideally, the study of growth modulation and the development of new nonfusion techniques involve the use of an
E294 | www.jspinaldisorders.com
J Spinal Disord Tech
Results: Of the 12 pigs enrolled in this study, 2 encountered substantial complications (1 developed a postoperative in-
Volume 27, Number 8, December 2014
J Spinal Disord Tech
Volume 27, Number 8, December 2014
experimental scoliosis model. Although several scoliosis models have been established in a variety of large animals, very few are suitable for subsequent fusionless scoliosis studies. This limitation arises because the animals used for the models had already passed the peak height velocity when structural scoliosis was induced,5–8 with little residual growth potential available for further fusionless scoliosis investigations.9 In a recent publication, Odent et al10 established an idiopathic-type scoliotic deformity in immature pigs. On the basis of the size of the immature pigs, they thought that their model was similar to human EOS. Despite the rapid progression of scoliosis during their 8-week tethering period, the scoliosis in the pigs was suboptimal because of the small curve magnitude. To achieve the greatest similarity to human EOS, scoliosis created over a shorter tethering period with sufficient growth potential is required. In addition, less violation to the spine is also necessary for the subsequent validation of fusionless scoliosis implants. The aim of this study is to create an EOS model which is characterized by rapidly progressive, structurally 3-dimensional, and idiopathic type in the immature pigs with a mini-invasive method.
Porcine Early-onset Scoliosis Model
(Ethicon Inc., New Brunswick, NJ) from the 10th to the 13th rib was performed subsequently (Fig. 1). A 2.5-mm flexible stainless steel cable (Medtronic Spinal and Biologics, Memphis, TN) was secured to the proximal pedicle screws using connectors and then passed
MATERIALS AND METHODS This study was approved by Institutional Animal Care and Use Committee. A total of 12 female Yorkshire pigs (aged, 5–6 wk; weight, 5–7 kg) were enrolled in this study. Before the initial tethering surgery, confirmed by xray films, all of the pigs’ spines were normally aligned in the coronal, sagittal, and axial planes. To evaluate the spinal growth, a preliminary growth control study was undertaken in 5 female Yorkshire pigs over a period of 6 months. Body weight and length (the distance from the midpoint of the ears to the proximal end of the tail) were measured monthly under sedation. The body length of the pigs increased by 10.1 cm/mo on average, from 45.2 cm at 1 month to 95.8 cm at 6 months. The body weight of the pigs increased by 11.4 kg/mo on average, from 5.8 kg to 62.9 kg after 5 months. On the basis of these results, it was indicated that the 6-monthold pigs were still in the rapid growth phase and they still had considerable growth potential.
Surgical Procedure After a 1-week acclimatization period, the pigs were induced with ketamine (20 mg/kg), intubated, and then anesthetized with sodium pentobarbital. The pigs were placed in a prone position on a grounding pad. Two separate midline posterior skin incisions from T5 to T6 and from L2 to L3 were used to expose the tips of the left transverse processes. Four pedicle screws (4.5 mm in diameter and 25–30 mm in length; Medtronic Sofamor Danek, Memphis, TN) were then inserted bicortically into the pedicles of the left side at T5–T6 and L2–L3 level. Afterwards, at 5 cm lateral from the midline, a left-sided paramedian incision from T10 to T13 was made to expose the ribs. The rib periosteum was carefully exposed without damage to the pleura, and ligamentous tethering r
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FIGURE 1. Definitive offset design used with the pedicle screws, the cable with the connectors. www.jspinaldisorders.com |
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FIGURE 2. A and B, Before the surgery, the porcine spine was normally aligned in the coronal and sagittal planes. C, Immediately after the tethering surgery, a mild scoliosis of 27 degrees was created in pig 7. D, Lateral radiograph showed the kyphosis was 55 degrees on the sagittal plane. E, Eight weeks after tether release, a significant scoliosis (72 degrees) was observed. F, The thoracic lordosis was 53 degrees and the thoracolumbar kyphosis was 75 degrees on the sagittal plane. G, Three-dimensional reconstruction computed tomography (CT) showed a typical scoliosis. H, Axial CT scan showed apical vertebral rotation of 38 degrees.
intramuscularly and caudally to connect with the distal pedicle screws (Fig. 2C). On the basis of the flexibility of the porcine spine, the cable length was set 3–4 cm shorter than the distance from T5 to L3 to create a unilateral tethering. A mild deformity with convex to the right side was induced once the tether was fastened and locked to the connectors. Wound closure was then performed using multilayer absorbable sutures.
Observation and Radiography Dorsoventral and lateral x-ray radiographs as well as computed tomography (CT) scans were taken before surgery, immediately after surgery, and at 4-week intervals to assess curve progression and instrumentation positioning. Pigs were excluded if screws pull-out or implant breakage was observed in the radiographs. The pigs were sedated before radiographic acquisition to ensure standardized films. Body weight, body length, and the Cobb angle were measured at the time of each radiograph. The apical vertebra wedging was also measured from the dorsoventral x-ray films. Axial rotation was measured during the 8-week
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tethering period using the CT scans.11 The rib hump was evaluated from the CT images using a method proposed by Thulborne and Gillespie12 and Lykissas et al.13 A rib hump elevation on the side of the convexity was considered positive. After 8-week tethering period, the stainless steel cable and pedicle screws were removed in all pigs. The pigs were observed for an additional 8-week period, and serial radiographs were taken at 4-week intervals. At study completion, the animals were euthanized, and their spines were harvested and stored for further histologic analyses.
Statistical Analysis Statistical analysis was performed using the SPSS (version 13.0; SPSS Inc., Chicago, IL) software package. Radiographic data were compared using repeated-measures analysis of variance between radiographic acquisition points with post hoc tests to see if any overall significance came about differing from one another. The level of statistical significance was set at P < 0.05. r
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TABLE 1. General Data of the Pigs Cobb Angle Pig No. 1 2 3 4 5 6 7 8 9 10
Age (wk)
Weight (kg)
Immediate After Tethering (deg.)
5 5 6 5 6 5 6 5 6 6
5.0 5.5 7.0 6.0 6.5 5.5 6.0 6.0 7.0 6.5
25 22 27 30 32 25 27 39 32 30
RESULTS Of the 12 animals enrolled in this study, 2 pigs were excluded from the analysis (1 developed a postoperative infection, and the other experienced prolonged postoperative weakness). Thus, our data analyses were based on the remaining 10 pigs. Over the 8-week tethering period, all of the 10 pigs developed a severe structural scoliosis associated with a chest wall deformity. The mean rib hump elevation was positive, demonstrating a right rib prominence and a left depressed thoracic cage. All the progressive curves had identical radiographic features of right thoracic scoliosis, including apical vertebral rotation and sagittal deformity, showing thoracic lordosis and thoracolumbar kyphosis. There was a mean of 11.0 ± 0.7 vertebrae within the instrumented levels. The curve patterns of all the pigs were similar, with the apex approximately at T11–T13. Table 1 summarized the evolution of scoliosis over the tethering period and the subsequent untethering period in each pig. The scoliosis immediately after the tethering procedure averaged 28.9 degrees (range, 22–39 degrees) and progressed to 64.7 degrees on average (range, 54–77 degrees) over the 8-week tethering period. The average progression of 35.8 degrees (range, 29–43 degrees) was significant (P < 0.001) (Fig. 3). On the sagittal plane, the average kyphosis after the tethering procedure was 38.7 degrees (range, 32–47 degrees) and the sagittal profiles showed 32.0 degrees (range, 21–42 degrees) lordosis in the thoracic spine and 63.0 degrees (range, 40–72 degrees) kyphosis in the thoracolumbar spine after 8-week tethering period. The mean rotation at the apex increased from 12.1 degrees (range, 6–22 degrees) immediately after the tethering procedure to 38.8 degrees (range, 29–46 degrees) after 8-week tethering period. After removal of the tether, the average scoliosis slightly decreased during the first 4 weeks, but progressed to 67.5 degrees (range, 58–78 degrees) over the next 4 weeks. The average progression of 2.8 degrees during this 8-week untethering period was statistically significant (P < 0.05; Fig. 3). On the sagittal plane, 8 weeks after removal of the tether, the average lordosis in the thoracic spine was 31.0 degrees, whereas in the thoracolumbar spine, the average kyphosis was 60.3 degrees. Decreases of both the thoracic lordosis (P = 0.918) and thoracor
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8-wk Postoperative (deg.)
8-wk After Removal of Tethering (deg.)
54 57 62 65 68 55 70 77 67 72
58 62 65 65 70 60 72 78 70 75
lumbar kyphosis (P = 0.774) were not significant. On the axial plane, the average axial rotation measured by CT scan was 28.8 degrees (range, 22–42 degrees). At study completion, CT scan analysis revealed apical vertebrae rotation in the axial plane and vertebral wedging mainly in the frontal plane. This wedging was more pronounced at the apical vertebrae, averaging 9.9 degrees (Fig. 5).
DISCUSSION Goals of treatment for EOS patients include controlling the progressive deformity as well as allowing for the growth of spine. For EOS patients, fusionless correction surgery has received increasing concerns over the past decades.3–5,7,14–17 There is a myriad of treatment options that are accompanied by complications, thus necessitating a new generation of fusionless correction systems. First and foremost, a proper animal model that allows for preclinical testing of fusionless scoliosis implants needs to be developed.
FIGURE 3. Evolution of scoliosis in pigs during the 16-week period. The scoliosis aggravated during the 8-week tethering period. Four weeks after the tether release, a small decrease of the Cobb angle was observed. However, the scoliosis progressed over the next 12th–16th weeks. www.jspinaldisorders.com |
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FIGURE 4. A, Before the tethering surgery, the porcine spine of pig 7 was well aligned in the coronal plane. Two posterior skin incisions from T5 to T6 and L1 to L3 levels were made for pedicle screws implantation. A left-sided paramedian incision from T10 to T13 levels was also made for tethering of the left ribs. B, Immediately after surgery, the local curvature of the thoracic spine showed a mild spinal deformity. C, Eight weeks after the tethering surgery, a typical, idiopathic-type scoliosis involving a right rib prominence was presented.
Two prerequisites are involved in setting up an ideal EOS model: it should provide a comparative size to human and experience rapid progression of scoliosis. Having a more rounded thorax and a comparable vertebral size, pigs are the most representative of humans in terms of spine shape.18–20 Therefore, in the present study, we used Yorkshire pigs to get close approximation of human scoliosis. Although many authors have created scoliosis models with remarkable curves,6,8,10 several shortcomings related to their study designs are evident. Firstly, the animals used by Schwab et al8 and Braun et al6 had passed the peak height velocity when scoliosis was induced.9 Thus, there were less growth available for subsequent fusionless scoliosis correction investigations. Secondly, while contralateral rib resections are performed, the chest wall is severely destroyed. This destruction can result in diminished pulmonary function and can be fatal in very young animals. It may also violate the integrity of the thoracic cage, which may cause the model to differ from human idiopathic scoliosis. Thirdly, to gain access for concave rib tethering, a huge incision is required, and the paraspinal muscle is violated, which might increase the damage to the immature animals. Finally, rough disturbance of the paraspinal muscle may eventually increase the risk of autofusion and thus limit fusionless correction investigations.
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To create an experimental scoliosis model in a large animal that not only approximates EOS but also results in minimal violation of the spinal elements along the curve, a novel method was developed in the present study. Three separate incisions were used: 2 for the insertion of the pedicle screws and 1 for rib tethering, which also served as an intermediary for the cable to connect to the 4 pedicle screws (Fig. 4A). Our scoliosis model was successfully established with the spinal elements intact despite some mild violation of the pedicles at the cephaled and caudal portions. The osseous structures, disks, and ligamentous attachments along the curve remained undisturbed. Compared with previous models, our model produces ideal experimental scoliosis for the subsequent study of treatments aimed at testing fusionless corrective techniques for EOS. Schwab et al8 demonstrated that the greater the initial deformity, the greater the rate of progression, which was consistent with the Hueter-Volkmann principle. Compared with the Odent et al’s study10 in which no scoliosis was initially presented, our results showed larger curves that were more representative of EOS patients for whom nonfusion treatment would be indicated. Moreover, using immature pigs, our model generated rapid scoliosis progression based on the exponential growth during the 8-week tethering period. Our scoliosis model can also permit r
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(increased from 60.1 to 67.5 degrees), which might be caused by the continued asymmetric vertebral growth. Although asymmetric mechanical loading originating from the tether diminished after removal of the spinal tether, vertebral growth continued at a slightly slower rate on the concavity, which caused scoliosis aggravation. Some caution, however, should be taken when comparing human scoliosis with experimental scoliosis in an animal model. The forces acting on the spine of bipedal humans differ from those on a quadruped animal. However, the marked curves present in our porcine scoliosis model are more representative of human EOS than previous models.
CONCLUSIONS Using an approach involving 3 separate incisions, a 3dimensional, rapidly progressive scoliosis was created in immature pigs with reasonable similarity to human EOS. This model may allow for validation of new fusionless corrective techniques because the animals are still in the rapid growth phase when remarkable scoliosis is achieved. FIGURE 5. Eight weeks after removal of the tethering, coronal reformatted image of computed tomography showed wedging of the apical vertebrae of pig 7.
validation of fusionless corrective techniques due to the remarkable remaining growth. Similarly, as Schwab et al8 reported, we also found that the rib resection of the convex side was unnecessary for the progression of deformity. In the present study, ribs on the concave side were tethered to give an initial direction for the scoliosis to progress; this procedure might be less invasive. Although the curves in the present study were not idiopathic, our experimental scoliosis possessed 3-dimensional features, including lateral translation of the apical vertebra from the midline, thoracic lordosis measuring 31.0 degrees on average, and axial rotation with vertebral body rotation into the curve convexity. Radiographic analyses of the CT scans showed significant apical vertebral wedging and ribcage asymmetry. Similar to human EOS, in our model, it was observed that the wedging of vertebrae was greatest at the apex (9.9 degrees at 8 wk after the removal of the tether; Fig. 5). Despite the morphologic similarities to previously reported scoliosis models,5–8,10 our model shows an iatrogenic sagittal deformity, including thoracic lordosis and thoracolumbar kyphosis. The thoracolumbar kyphosis may be caused by the location of the rib tethering, which was more laterally and more ventrally located than the position of Schwab et al’s study.8 Fastening of the rib tethering might produce an initial kyphosis on the thoracolumbar spine, which is likely to aggravate over time according to the “vicious cycle hypothesis.”21 A small decrease of the Cobb angle was observed 4 weeks after tether release. This phenomenon might result from the viscoelasticity of the disks. However, scoliosis progression over the next 12–16 weeks was evident r
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ACKNOWLEDGMENTS The authors acknowledge Dr Mingyan Liu and Dr Jeffery Zhang for their assistance during the experiment design. REFERENCES 1. Akbarnia BA. Management themes in early onset scoliosis. J Bone Joint Surg Am. 2007;89(suppl 1):42–54. 2. Blakemore LC, Scoles PV, Poe-Kochert C, et al. Submuscular Isola rod with or without limited apical fusion in the management of severe spinal deformities in young children: preliminary report. Spine (Phila Pa 1976). 2001;26:2044–2048. 3. Thompson GH, Akbarnia BA, Kostial P, et al. Comparison of single and dual growing rod techniques followed through definitive surgery: a preliminary study. Spine (Phila Pa 1976). 2005;30: 2039–2044. 4. Campbell RM Jr, Smith MD, Hell-Vocke AK. Expansion thoracoplasty: the surgical technique of opening-wedge thoracostomy. Surgical technique. J Bone Joint Surg Am. 2004;86-A(suppl 1): 51–64. 5. Braun JT, Ogilvie JW, Akyuz E, et al. Experimental scoliosis in an immature goat model: a method that creates idiopathic-type deformity with minimal violation of the spinal elements along the curve. Spine (Phila Pa 1976). 2003;28:2198–2203. 6. Braun JT, Ogilvie JW, Akyuz E, et al. Creation of an experimental idiopathic-type scoliosis in an immature goat model using a flexible posterior asymmetric tether. Spine (Phila Pa 1976). 2006;31: 1410–1414. 7. Zhang YG, Zheng GQ, Zhang XS, et al. Scoliosis model created by pedicle screw tethering in immature goats: the feasibility, reliability, and complications. Spine (Phila Pa 1976). 2009;34:2305–2310. 8. Schwab F, Patel A, Lafage V, et al. A porcine model for progressive thoracic scoliosis. Spine (Phila Pa 1976). 2009;34:E397–E404. 9. Braun JT, Akyuz E, Ogilvie JW. The use of animal models in fusionless scoliosis investigations. Spine (Phila Pa 1976). 2005;30: S35–S45. 10. Odent T, Cachon T, Peultier B, et al. Porcine model of early onset scoliosis based on animal growth created with posterior miniinvasive spinal offset tethering: a preliminary report. Eur Spine J. 2011;20:1869–1876. 11. Aaro S, Dahlborn M, Svensson L. Estimation of vertebral rotation in structural scoliosis by computer tomography. Acta Radiol Diagn (Stockh). 1978;19:990–992.
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12. Thulbourne T, Gillespie R. The rib hump in idiopathic scoliosis. Measurement, analysis and response to treatment. J Bone Joint Surg Br. 1976;58:64–71. 13. Lykissas MG, Sharma V, Crawford AH. Assessment of rib hump deformity correction in adolescent idiopathic scoliosis with or without costoplasty using the double rib contour sign. J Spinal Disord Tech. 2012. [Epub ahead of print]. 14. Akbarnia BA, Emans JB. Complications of growth-sparing surgery in early onset scoliosis. Spine (Phila Pa 1976). 2010;35: 2193–2204. 15. Akbarnia BA, Marks DS, Boachie-Adjei O, et al. Dual growing rod technique for the treatment of progressive early-onset scoliosis: a multicenter study. Spine (Phila Pa 1976). 2005;30:S46–S57. 16. Cunningham ME, Frelinghuysen PH, Roh JS, et al. Fusionless scoliosis surgery. Curr Opin Pediatr. 2005;17:48–53.
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17. Newton PO, Fricka KB, Lee SS, et al. Asymmetrical flexible tethering of spine growth in an immature bovine model. Spine (Phila Pa 1976). 2002;27:689–693. 18. Bozkus H, Crawford NR, Chamberlain RH, et al. Comparative anatomy of the porcine and human thoracic spines with reference to thoracoscopic surgical techniques. Surg Endosc. 2005;19:1652–1665. 19. Busscher I, Ploegmakers JJ, Verkerke GJ, et al. Comparative anatomical dimensions of the complete human and porcine spine. Eur Spine J. 2010;19:1104–1114. 20. Sheng SR, Wang XY, Xu HZ, et al. Anatomy of large animal spines and its comparison to the human spine: a systematic review. Eur Spine J. 2010;19:46–56. 21. Stokes IA, Spence H, Aronsson DD, et al. Mechanical modulation of vertebral body growth. Implications for scoliosis progression. Spine (Phila Pa 1976). 1996;21:1162–1167.
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