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Minimum cement volume required in vertebral body augmentation—A biomechanical study comparing the permanent SpineJack device and balloon kyphoplasty in traumatic fracture
JCLB-03964; No of Pages 6 Clinical Biomechanics xxx (2015) xxx–xxx
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Minimum cement volume required in vertebral body augmentation—A biomechanical study comparing the permanent SpineJack device and balloon kyphoplasty in traumatic fracture Robert Rotter a,⁎, Lena Schmitt b, Philip Gierer a, Klaus-Peter Schmitz b, David Noriega c, Thomas Mittlmeier a, Peter-J. Meeder d, Heiner Martin b a
Department of Trauma, Hand and Reconstructive Surgery, University of Rostock, Germany Institute for Biomedical Engineering, University of Rostock, Germany c Spine-Unit, University Hospital Valladolid, Royal Academy of Medicine and Surgery, Spain d University of Heidelberg, Germany b
a r t i c l e
i n f o
Article history: Received 15 July 2014 Accepted 28 April 2015 Keywords: Kyphoplasty Cement Spine Volume SpineJack Fracture
a b s t r a c t Background: Minimally invasive treatment of vertebral fractures is basically characterized by cement augmentation. Using the combination of a permanent implant plus cement, it is now conceivable that the amount of cement can be reduced and so this augmentation could be an attractive opportunity for use in traumatic fractures in young and middle-aged patients. The objective of this study was to determine the smallest volume of cement necessary to stabilize fractured vertebrae comparing the SpineJack system to the gold standard, balloon kyphoplasty. Methods: 36 fresh frozen human cadaveric vertebral bodies (T11-L3) were utilized. After creating typical compression wedge fractures (AO A1.2.1), the vertebral bodies were reduced by SpineJack (n = 18) or kyphoplasty (n = 18) under preload (100 N). Subsequently, different amounts of bone cement (10%, 16% or 30% of the vertebral body volume) were inserted. Finally, static and dynamic biomechanical tests were performed. Findings: Following augmentation and fatigue tests, vertebrae treated with SpineJack did not show any significant loss of intraoperative height gain, in contrast to kyphoplasty. In the 10% and 16%-group the height restoration expressed as a percentage of the initial height was significantly increased with the SpineJack (N 300%). Intraoperative SpineJack could preserve the maximum height gain (mean 1% height loss) better than kyphoplasty (mean 16% height loss). Interpretation: In traumatic wedge fractures it is possible to reduce the amount of cement to 10% of the vertebral body volume when SpineJack is used without compromising the reposition height after reduction, in contrast to kyphoplasty that needs a 30% cement volume. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Vertebroplasty and kyphoplasty are the standard methods for minimally invasive treatment of vertebral compression fractures. A key feature of these methods is stabilizing the fracture by cement augmentation. Disadvantages of this technique are changes in the mechanical properties of cancellous bone and an increased risk of complications due to leakage (Hulme et al., 2006; Taylor et al., 2006, 2007). Positive side effects of kyphoplasty were an increased restoration of vertebral height and filling of the cavity created with high viscosity cement compared to vertebroplasty. However, in order to stabilize the ⁎ Corresponding author at: Department of Trauma, Hand and Reconstructive Surgery, University of Rostock, Schillingallee 35, 18057 Rostock, Germany. Tel.: + 49 381 494 6050; fax: +49 381 494 6052. E-mail address: [email protected] (R. Rotter).
achieved height reduction, the entire cavity has to be filled with cement. Another problem with kyphoplasty, the loss of vertebral height following fracture reduction with a balloon tamp and its subsequent removal (Voggenreiter, 2005) has been resolved by developing newer alternatives (Furderer et al., 2002). This “third” generation of augmentation system uses specific implants that remain within the vertebra (Rotter et al., 2010). However, these implants also rely on cement injection to stabilize the vertebra permanently (Kruger et al., 2013). Using the combination of a permanent implant plus cement, it is now conceivable that the amount of cement can be reduced and so this augmentation could be an attractive opportunity for use in traumatic fractures in young and middle-aged patients (Wikipedia, 2014). One ex vivo study following percutaneous vertebroplasty showed that for restoration of vertebral body strength and stiffness, vertebral body cement filling degrees of 16% and 30%, respectively, are required (Molloy et al., 2003). A clinical study on the relationship between the
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Please cite this article as: Rotter, R., et al., Minimum cement volume required in vertebral body augmentation—A biomechanical study comparing the permanent SpineJack device and..., Clin. Biomech. (2015), http://dx.doi.org/10.1016/j.clinbiomech.2015.04.015
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volumetric analysis of cement in vertebroplasty with clinical outcome and complications showed that a volume larger than 11.65% leads to a significantly increased incidence of leakage and adjacent fractures (Jin et al., 2011). As mentioned above, to the best of our knowledge there is currently no effective information that allows the surgeon to reduce the amount of cement in the third generation kyphoplasty technique without decreasing vertebral strength and stiffness. It was the aim of this study to identify the minimum quantity of cement necessary while still sufficiently stabilizing the restored fractured vertebra in the SpineJack (SJ) system (Fig. 1) compared to the gold standard balloon kyphoplasty (BKP). 2. Methods 2.1. Specimens and experimental groups For equal group sample size 36 vertebrae from eight intact fresh human male cadaveric spines (T11-L3) were used in this study. The average age of the donors was 62 years (51–69 years). Each specimen was screened by computed tomography (CT) scan (Aquilion, Toshiba, Tokyo, Japan) for bone mineral density (BMD) measurements including calculation of vertebra body volume (Aquarius INtuition version 4.6.85.2800, TeraCcon, Frankfurt, Germany). All vertebrae were pooled to create two different groups consisting of each augmentation systems (SJ vs. BKP) and subdivided into three cement groups (10%, 16% and 30% cement filling of the vertebral body volume, n = 6). The SJ system (Vexim SA, Balma, France) consists of an expandable metal implant (titanium alloy) mounted on an expander, of which two are inserted bilaterally into the vertebral body and simultaneously expanded. The unexpanded implant (Ø 5 mm) is delivered in a prefolded state, and is gradually expanded to its final configuration until fracture reduction is satisfactory and/or the maximum expanded
height of 17 mm is reached. After the implant expansion both implants stay in place to maintain the restored height. PMMA cement is then injected through the central part of the implant (Fig. 1). 2.2. Fracture generation and instrumentation The vertebrae were isolated and all soft tissue was removed. The caudal endplates of the vertebrae were embedded into radiolucent PMMA (Beracryl; Troller Kunststoffe, Fulenbach, Switzerland). Vertebral compression fractures (AO A1.2.1) (Magerl et al., 1994) were performed using a universal material testing machine (MTS; Eden Prairie, MN, USA; 15 kN load cell, measurement error: 0.3%). Load was transferred by the pivot-mounted pressure plate in orthograde projection to the superior vertebral endplate to allow a wedge compression of the anterior wall until a compression of the anterior vertebral edge of more than 40% was reached (2 mm/min; 5 Hz) (Fig. 2). After generating the vertebral compression fractures, the vertebral body height was redetermined by CT scan including calculation of the vertebral body volume (Table 1). For augmentation the vertebrae were mounted into a custom made testing device performing reduction tests under a constant axial preload of 100 N. The specific working cannulae of both systems were placed bipedicular under the fluoroscopic imaging guidance of a C-arm (Ziehm Vario 3D, Ziehm imaging, Nuremberg). Either SJ (Ø 5 mm [diameter] × 25 mm [length]) or an inflatable kyphoplasty bone tamp/balloon (KyphX Xpander® 20/3, Kyphon Europe Zaventem, Belgium) was inserted simultaneously on both sides and expanded by two surgeons simultaneously. The re-alignment was continued until the vertebral body height was restored. Next, PMMA cement (Cohesion® Bone Cement CM0300, VEXIM SA, Balma, France) was injected synchronously on both sides using filling cannulae by Vexim or Kyphon, respectively, to comply with the manufacturers' recommendations. Each step was performed under
Fig. 1. A; Illustration of the SpineJack implant in the pre-folded and expanded states. B; SJ-representative lateral X-ray image after cement augmentation.
Please cite this article as: Rotter, R., et al., Minimum cement volume required in vertebral body augmentation—A biomechanical study comparing the permanent SpineJack device and..., Clin. Biomech. (2015), http://dx.doi.org/10.1016/j.clinbiomech.2015.04.015
R. Rotter et al. / Clinical Biomechanics xxx (2015) xxx–xxx
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Finally, all specimens were axially loaded in controlleddisplacement mode (2 mm/min) until macroscopic failure of the vertebra occurred. The failure load was defined manually at the first significant decrease of slope of the load displacement diagram. For all biomechanical tests, the same pivot loading system was used (fracture generation, augmentation, cyclic loading, failure test). The loading point was always restored using special adjustment screws of the fixation device. 2.4. Statistical analysis All data were expressed as mean ± SD. After proving the assumption of normality and equal variance across groups, differences between groups were assessed using ANOVA followed by the appropriate post hoc comparison test (Holm-Sidak method (HS) or Kruskal–Wallis one-way analysis of variance on ranks (KW)). Statistical significance was set at P b 0.05. Statistics were performed using the SigmaStat software package (Jandel Corporation, San Rafael, CA, USA). Fig. 2. Illustration of vertebra during fracture test in the material testing machine. The vertebral body with the cement embedding is placed within the holding device. Load is transferred by the pivot and the pressure plate onto the upper vertebral body end plate. The holding device and the pivot are held by the hydraulic clamps of the material testing machine.
fluoroscopic control. Lateral fluoroscopic views were taken to determine the deformity corrections at the following time points: 1) after fracture generation and before reduction, 2) after maximum (balloon) inflation/ expansion of the repositioning tool 3) after balloon deflation and withdrawal/retraction of the repositioning tool, and 4) after injection and hardening of the cement. To minimize possible magnification effects and inter-radiographic precision errors a calibrated standard (20 mm ball mounted at the test device) was visible in all radiographs. Anterior vertebral body height, kyphotic angle and Beck Index (BI) were analyzed by two surgeons from these radiographs using special software (Media Viewer 1.0; Ziehm Imaging, Nuremberg, Germany). The measurements were triplicated and the mean value was calculated in each case. In accordance with McKiernan, vertebral height restoration was estimated as the absolute restoration in millimeters (restored vertebral height (c) − initial fracture height (b)) and percentage of restoration relative to initial fracture height ((c − b) / b ∗ 100) (McKiernan et al., 2003). Specimens were then re-evaluated by CT. 2.3. Mechanical testing Two different biomechanical tests were performed with the material testing machine (MTS). The augmented vertebrae were loaded by cyclic sinusoidal dynamic compression to simulate daily loads acting on the vertebra due to patient's activity. For this purpose, 10,000 cycles were performed between 200 N and 600 N with a frequency of 1 Hz (Kruger et al., 2013). Following the cyclical testing all vertebrae were examined by CT for the fourth time. The analysis of the height changes was made the same as for the C-arm radiograph results (see above).
3. Results All spines presented normal bone density values according to the WHO definition according to addressed middle-aged group of patients. After distribution of the vertebrae into the groups, no significant difference was seen between the BMDs of the examined groups (P = 0.987) (Table 1). 3.1. Results of fracture generation It was possible to generate a wedge compression fracture in all specimens. The resulting vertebral failure load values of the pooled vertebrae did not show any significant differences between the groups (Table 1; failure load P = 0.757). 3.2. Results of instrumentation The approach and placement of the SJ or bone tamps in all vertebrae were carried out without any complications. Vertebral body height restorations were achieved while maintaining full balloon inflation up to 94% for SJ and up to 100% for BKP (Fig. 3). However, there was a significant loss of reduction after balloon deflation in BKP compared to implant holder release in SJ, and a significant total height gain by SJ (Fig. 3). Anterior height loss after deflation in relation to preoperative height was significantly higher (16%, P b 0.001; KW) in BKP compared to SJ (1%, n.s.). Even more pronounced was the relation of anterior height loss after deflation to re-alignment height. Overall, 72% of the restored height was lost in BKP. SJ showed a significantly lower restored height loss of 10% (P b 0.001; KW). Correspondingly, there was an increase in total anterior height gain by SJ (11%) compared to BKP (7%) (P = 0.275) (Fig. 3). The mean changes in the kyphotic angles and BI corresponded to the above-mentioned results (data not shown).
Table 1 Basic data of tested vertebrae. System
SJ
Cement percentage [%]
10
3
BMD [g/cm ] Vertebra volume before fracture [cm3] Relative volume decrease due to fracture [%] Total cement filling volume [ml] Preop. failure load [N] Postop. failure load [N]
BKP 16
30
Total
10
16
30
Total
Mean
SD
Mean
SD
Mean
SD
Mean
SD
Mean
SD
Mean
SD
Mean
SD
Mean
SD
129 39.2 25 2.93 3201 4464
27 7.7 7 0.50 906 721
146 37.2 24 4.55 3510 4785
61 8.6 9 1.16 1551 1128
149 35.4 24 7.99 4075 5088
59 6.4 6 1.04 1260 1400
141 37.3 24 5.16 3596 4779
49 7.3 7 2.35 1247 1068
146 37.1 24 2.81 3191 4181
61 8.6 9 0.69 614 1411
147 34.0 24 4.13 4190 4241
60 5.2 9 0.85 719 1687
143 34.1 22 8.08 3753 5703
62 5.7 9 2.02 1274 2580
145 35.1 23 5.01 3711 4709
57 6.4 8 2.45 958 1953
Please cite this article as: Rotter, R., et al., Minimum cement volume required in vertebral body augmentation—A biomechanical study comparing the permanent SpineJack device and..., Clin. Biomech. (2015), http://dx.doi.org/10.1016/j.clinbiomech.2015.04.015
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Fig. 3. Values of the relative changes in height in % with SJ (filled circles) compared with BKP (open triangles) i) before and ii) after fracture generation, iii) after reduction, iv) after deflation and v) the resulting height gain after completion of reduction. Values are given as mean ± SD; ANOVA, post hoc comparison; *P b 0.05 BKP.
Fig. 5. Values of the Beck index by SJ (filled squares) compared with BKP (open squares) according to CT image evaluation. Values are given as mean ± SD; ANOVA, post hoc comparison; *P b 0.05 BKP.
3.3. Results of post-operative static and dynamic tests
failure load ratio improved by 30% in SJ compared to BKP (P = 0.212) (Fig. 6, Table 1).
After 10,000 cycles of axial compression, significant differences were found between the groups in the subsidence behavior (plastic deformation) of the vertebrae. Unlike BKP, in SJ differences between the cement groups were smaller than 2%. However, in the 10%-group the height restoration expressed as a percentage of the initial height was significantly increased with the SJ (P = 0.016; HS). Also, in the 16%-group the height restoration was significantly increased (P = 0.017; HS). Only for the 30%-group, no difference between the two systems could be seen (P = 0.690) (Fig. 4). Analyzing the BI, SJ showed better restoration values than BKP including a significant difference following cyclic loading (P = 0.010; HS) (Fig. 5). Postoperatively, SJ and BKP showed a substantial tendency to increasing failure loads after vertebral augmentation. SJ presented a non-significant increased failure load compared to BKP in the 10 and 16% groups, respectively. In the 16%-group the post- to preoperative
The latest developments in kyphoplasty include different implants that remain within the vertebra and restore the achieved reduction height temporarily until the cement is injected and hardened (Kruger et al., 2013; Rotter et al., 2010). However, none of these new procedures works without the additive use of cement (Kruger et al., 2013). Therefore, the problem of cement-associated complications is not yet solved (Hulme et al., 2006; Taylor et al., 2006, 2007). The current study provides information about the minimum required cement volume, height restoration and static behavior of vertebral compression fractures following augmentation by such a new technique (SpineJack) compared to BKP. The most important finding of this study was that a quantity of cement equivalent to 10% of the vertebral volume was sufficient to
Fig. 4. Global height restoration after cyclic loading according to CT image evaluation with SJ (filled bars) compared with BKP (blank bars) depending on the degree of cement filling. Values are expressed as percentages of the preoperative heights. Values are given as mean ± SD; ANOVA, post hoc comparison; *P b 0.05 BKP.
Fig. 6. Postoperative failure loads of SJ (filled bars) and BKP (blank bars) expressed as a percentage of the preoperative failure loads (the preoperative failure load was normalized as 100%). Values are given as mean ± SD.
4. Discussion
Please cite this article as: Rotter, R., et al., Minimum cement volume required in vertebral body augmentation—A biomechanical study comparing the permanent SpineJack device and..., Clin. Biomech. (2015), http://dx.doi.org/10.1016/j.clinbiomech.2015.04.015
R. Rotter et al. / Clinical Biomechanics xxx (2015) xxx–xxx
stabilize the fractured vertebra when using SJ but not BKP. A clinically adequate reduction in the fractured vertebral body height was achieved with both systems. However, with BKP, the absence of load-bearing within the created cavity after balloon deflation and before cement injection translated into a significant loss in the height gained initially compared to SJ. From the biomechanical point of view, Molloy et al. was able to show in vitro that strength and stiffness will be restored with 16 and 30% of vertebral cement filling, respectively, following vertebroplasty (Molloy et al., 2003). For this reason these quantities of cement were used for the current study. In contrast to Molloy, we used the post-fracture vertebra volume as a starting point to better simulate the clinical situation. In clinical practice, usually only post-fracture imaging is available. In our study the volume differences (pre- and post-fracture) were approximately 24%. Rotter et al. were able to demonstrate an increase in strength due to the combination of intervertebral implant plus cement (Rotter et al., 2010). Therefore, it could be assumed that with the SJ implant a reduced cement volume would be required for equivalent stability. Nevertheless, a study by Krueger et al. had previously shown the SJ implant alone does not achieve sufficient stability following a cyclic loading (Kruger et al., 2013). Accordingly, the 10%-group was introduced for this study, which appears clinically and biomechanically reasonable. In accordance with the implant design and the intra-operative load-bearing capability, there was a significantly higher preservation of initially gained height with SJ compared to BKP. The intra-operative reduction in restored height in BKP was due to the absence of a means of load-bearing within the created cavity after balloon deflation but before cement injection and hardening. This phenomenon has already been shown in other studies, in part with other intra-vertebral implants (Kruger et al., 2013; Rotter et al., 2010; Voggenreiter, 2005). How relevant this improved re-alignment is clinically has not yet been proven. However, there was a biomechanical study indicating that the anterior shift of the upper body is the dominant factor in adjacent vertebral fractures, so achieving the maximum possible fracture reduction could eventually decrease this complication (Baumbach et al., 2012; Ferraris et al., 2012; Rohlmann et al., 2006). The cyclic loading tests were performed to simulate the short-term in vivo performance of the materials (Kruger et al., 2013). Wilke et al. state that the load between 100 and 600 N is a compromise for moderate loading between lying (100 N, 180 N) and standing or walking (500 N, 900 N) (Wilke et al., 1999, 2006). We found a significant loss of height in the 10%-group and a nonsignificant trend at 16% with BKP. This may be due to a mismatch in the cavity created by the maximally inflated balloon but followed by incomplete cement filling. As a consequence vacuum phenomena, which are detectable by CT, caused weakness and sintering behavior. In contrast, the SJ implant does not create a cavity during the expansion procedure. The sintering in SJ is only achieved in part through cancellous displacement superior or inferior of the implant. The use of a smaller cement volume compared to the balloon volume during BKP is not recommended by the manufacturer. From our results it is obvious that it is not useful to reduce the cement filling to 10% by volume in BKP. However, from the biomechanical point of view this technique is possible with SJ. The examinations of static testing were carried out to correspond to the criteria of Belkoff et al. and Heini et al. (Belkoff et al., 2001; Heini et al., 2001). In concordance with these authors, we found a significant difference in maximum failure load between augmented and nonaugmented vertebrae but no significant differences between SJ and BKP. However, there was a tendency toward an increased relative failure load by SJ in the 10 and 16%-groups. Contrary to Molloy et al., strength restoration was already achieved with both SJ and BKP in the 10%-group and did not increase with further increases in the amount of cement filling (Molloy et al., 2003). One possible explanation might
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be that the strength was measured following the cyclic loading protocol by the final static test. However, the maximum reduction of height of the augmented vertebra was already reached during the cyclic loading and so the failure load was postoperatively increased due to the impacted cancellous bone that was not filled by the cement. This biomechanical study was limited by the fracture model used and the experimental situation deviated from clinical reality. We are aware that kyphoplasty is a domain of osteoporotic fracture augmentation. Nevertheless, previous studies have already reported the use of kyphoplasty in younger patients (Gumpert et al., 2014; Schmelzer-Schmied et al., 2009). For our group the opportunity of a permanent implant with a minimum cement volume appears ideal for fracture stabilization in the middle-aged and young patient groups. That is the reason why we did not use osteoporotic vertebrae. Due to biological reasons there are not enough young specimens available for biomechanical studies. Therefore, vertebrae with average age of 62 (51–69 years) were used in this study. Inter- and intra-individual differences between the specimens were reduced by matching the vertebrae to the groups. To generate a typical wedge fracture, prevent the posterior wall to become fractured as well and to investigate the cancellous/ endplate interface the cranial endplate was loaded by a textured planar metal surface and was not embedded. Mirzaei et al. have indicated that the exact parallel embedding of the endplates as well as the exact vertical alignment with the vertical axis of the vertebra is not possible. As a consequence, unwanted bending moments would result (Mirzaei et al., 2009). Disadvantage not to embed the endplate is the occurrence of local crushing at the endplate which biases the stiffness measurement. The moderate transfer of force (2 mm/min) produces wedge fractures instead of shock loads generating burst fractures. However, the used force flow is not comparable to a low-energy trauma. Nevertheless, in literature there is no reference regarding different transfer of forces (1–10 mm/min) and the biomechanical quality of generated fractures. The instrumentation and augmentation of a single fractured vertebra, isolated from all soft tissue, is not similar to the clinical situation. However, the preload of 100 N used is comparable with treatment in prone position (Belkoff et al., 2001; Sato et al., 1999; Tohmeh et al., 1999). Using the same specimen for the final static test following the cyclic loading is a potential limitation. However, cyclic loading test were performed to simulate short in vivo performance (about 3 months until bony fusion) of the bone cement implant combination. We are aware that one million cycles represent about one year in a patient's life (Wilke et al., 2006). Simulating 3 months, 250,000 cycles should have been performed, however this is hardly practicable because of autolysis affecting the biomechanical behavior of the specimens. In line with the literature 10,000 cycles were performed and it revealed that about 80% of sintering occurred after one third of cycles (Kruger et al., 2013; Rotter et al., 2010). The vertebral height measurements were performed identically in all groups using CT and fluoroscopy. According to the literature a precise measurement by CT of 1.2% (coefficients of variation) and fluoroscopy of 2.2% can be assumed (Frobin et al., 1997; Tan et al., 2013). However, this error is unavoidable and is leveled down due to the group distribution.
5. Conclusions This in-vitro study demonstrated that it is possible to reduce the amount of cement to 10% of the vertebral body volume when using SJ following vertebral body augmentation due to traumatic wedge fracture. Furthermore, it was impossible to reduce the amount of cement in BKP without increased height loss. For clinical use these results implicate that the leakage rate could be reduced and consequently the risk of symptomatic complications. However, the true validity of this conclusion must be shown in clinical trials in comparison with BKP conducted in a proficient manner as the gold standard.
Please cite this article as: Rotter, R., et al., Minimum cement volume required in vertebral body augmentation—A biomechanical study comparing the permanent SpineJack device and..., Clin. Biomech. (2015), http://dx.doi.org/10.1016/j.clinbiomech.2015.04.015
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Acknowledgments The authors cordially thank the engineers of Vexim SA (Balma, France) for their generous assistance. This study was supported in part by Vexim SA, Balma, France. References Baumbach, S.F., Böcker, W., Mutschler, W., Schieker, M., 2012. Why do osteoporotic vertebral fractures cluster in the mid-thoracic and thoracolumbar region? Osteologie 21, 143–150. Belkoff, S.M., Mathis, J.M., Fenton, D.C., Scribner, R.M., Reiley, M.E., Talmadge, K., 2001. An ex vivo biomechanical evaluation of an inflatable bone tamp used in the treatment of compression fracture. Spine (Phila Pa 1976) 26, 151–156. Ferraris, L., Koller, H., Meier, O., Hempfing, A., 2012. The relevance of the sagittal balance in spine surgery. OUP 1, 502–508. Frobin, W., Brinckmann, P., Biggemann, M., Tillotson, M., Burton, K., 1997. Precision measurement of disc height, vertebral height and sagittal plane displacement from lateral radiographic views of the lumbar spine. Clin. Biomech. 12 (Suppl. 1), S1–S63. Furderer, S., Anders, M., Schwindling, B., Salick, M., Duber, C., Wenda, K., et al., 2002. Vertebral body stenting. A method for repositioning and augmenting vertebral compression fractures. Orthopade 31, 356–361. Gumpert, R., Bodo, K., Spuller, E., Poglitsch, T., Bindl, R., Ignatius, A., et al., 2014. Demineralization after balloon kyphoplasty with calcium phosphate cement: a histological evaluation in ten patients. Eur. Spine J. 23, 1361–1368. Heini, P.F., Berlemann, U., Kaufmann, M., Lippuner, K., Fankhauser, C., van Landuyt, P., 2001. Augmentation of mechanical properties in osteoporotic vertebral bones—a biomechanical investigation of vertebroplasty efficacy with different bone cements. Eur. Spine J. 10, 164–171. Hulme, P.A., Krebs, J., Ferguson, S.J., Berlemann, U., 2006. Vertebroplasty and kyphoplasty: a systematic review of 69 clinical studies. Spine (Phila Pa 1976) 31, 1983–2001. Jin, Y.J., Yoon, S.H., Park, K.W., Chung, S.K., Kim, K.J., Yeom, J.S., et al., 2011. The volumetric analysis of cement in vertebroplasty: relationship with clinical outcome and complications. Spine (Phila Pa 1976) 36, E761–E772. Kruger, A., Baroud, G., Noriega, D., Figiel, J., Dorschel, C., Ruchholtz, S., et al., 2013. Height restoration and maintenance after treating unstable osteoporotic vertebral compression fractures by cement augmentation is dependent on the cement volume used. Clin. Biomech. 28, 725–730. Magerl, F., Aebi, M., Gertzbein, S.D., Harms, J., Nazarian, S., 1994. A comprehensive classification of thoracic and lumbar injuries. Eur. Spine J. 3, 184–201.
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Please cite this article as: Rotter, R., et al., Minimum cement volume required in vertebral body augmentation—A biomechanical study comparing the permanent SpineJack device and..., Clin. Biomech. (2015), http://dx.doi.org/10.1016/j.clinbiomech.2015.04.015
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Clinical Biomechanics journal homepage: www.elsevier.com/locate/clinbiomech
Minimum cement volume required in vertebral body augmentation—A biomechanical study comparing the permanent SpineJack device and balloon kyphoplasty in traumatic fracture Robert Rotter a,⁎, Lena Schmitt b, Philip Gierer a, Klaus-Peter Schmitz b, David Noriega c, Thomas Mittlmeier a, Peter-J. Meeder d, Heiner Martin b a
Department of Trauma, Hand and Reconstructive Surgery, University of Rostock, Germany Institute for Biomedical Engineering, University of Rostock, Germany c Spine-Unit, University Hospital Valladolid, Royal Academy of Medicine and Surgery, Spain d University of Heidelberg, Germany b
a r t i c l e
i n f o
Article history: Received 15 July 2014 Accepted 28 April 2015 Keywords: Kyphoplasty Cement Spine Volume SpineJack Fracture
a b s t r a c t Background: Minimally invasive treatment of vertebral fractures is basically characterized by cement augmentation. Using the combination of a permanent implant plus cement, it is now conceivable that the amount of cement can be reduced and so this augmentation could be an attractive opportunity for use in traumatic fractures in young and middle-aged patients. The objective of this study was to determine the smallest volume of cement necessary to stabilize fractured vertebrae comparing the SpineJack system to the gold standard, balloon kyphoplasty. Methods: 36 fresh frozen human cadaveric vertebral bodies (T11-L3) were utilized. After creating typical compression wedge fractures (AO A1.2.1), the vertebral bodies were reduced by SpineJack (n = 18) or kyphoplasty (n = 18) under preload (100 N). Subsequently, different amounts of bone cement (10%, 16% or 30% of the vertebral body volume) were inserted. Finally, static and dynamic biomechanical tests were performed. Findings: Following augmentation and fatigue tests, vertebrae treated with SpineJack did not show any significant loss of intraoperative height gain, in contrast to kyphoplasty. In the 10% and 16%-group the height restoration expressed as a percentage of the initial height was significantly increased with the SpineJack (N 300%). Intraoperative SpineJack could preserve the maximum height gain (mean 1% height loss) better than kyphoplasty (mean 16% height loss). Interpretation: In traumatic wedge fractures it is possible to reduce the amount of cement to 10% of the vertebral body volume when SpineJack is used without compromising the reposition height after reduction, in contrast to kyphoplasty that needs a 30% cement volume. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Vertebroplasty and kyphoplasty are the standard methods for minimally invasive treatment of vertebral compression fractures. A key feature of these methods is stabilizing the fracture by cement augmentation. Disadvantages of this technique are changes in the mechanical properties of cancellous bone and an increased risk of complications due to leakage (Hulme et al., 2006; Taylor et al., 2006, 2007). Positive side effects of kyphoplasty were an increased restoration of vertebral height and filling of the cavity created with high viscosity cement compared to vertebroplasty. However, in order to stabilize the ⁎ Corresponding author at: Department of Trauma, Hand and Reconstructive Surgery, University of Rostock, Schillingallee 35, 18057 Rostock, Germany. Tel.: + 49 381 494 6050; fax: +49 381 494 6052. E-mail address: [email protected] (R. Rotter).
achieved height reduction, the entire cavity has to be filled with cement. Another problem with kyphoplasty, the loss of vertebral height following fracture reduction with a balloon tamp and its subsequent removal (Voggenreiter, 2005) has been resolved by developing newer alternatives (Furderer et al., 2002). This “third” generation of augmentation system uses specific implants that remain within the vertebra (Rotter et al., 2010). However, these implants also rely on cement injection to stabilize the vertebra permanently (Kruger et al., 2013). Using the combination of a permanent implant plus cement, it is now conceivable that the amount of cement can be reduced and so this augmentation could be an attractive opportunity for use in traumatic fractures in young and middle-aged patients (Wikipedia, 2014). One ex vivo study following percutaneous vertebroplasty showed that for restoration of vertebral body strength and stiffness, vertebral body cement filling degrees of 16% and 30%, respectively, are required (Molloy et al., 2003). A clinical study on the relationship between the
http://dx.doi.org/10.1016/j.clinbiomech.2015.04.015 0268-0033/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article as: Rotter, R., et al., Minimum cement volume required in vertebral body augmentation—A biomechanical study comparing the permanent SpineJack device and..., Clin. Biomech. (2015), http://dx.doi.org/10.1016/j.clinbiomech.2015.04.015
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volumetric analysis of cement in vertebroplasty with clinical outcome and complications showed that a volume larger than 11.65% leads to a significantly increased incidence of leakage and adjacent fractures (Jin et al., 2011). As mentioned above, to the best of our knowledge there is currently no effective information that allows the surgeon to reduce the amount of cement in the third generation kyphoplasty technique without decreasing vertebral strength and stiffness. It was the aim of this study to identify the minimum quantity of cement necessary while still sufficiently stabilizing the restored fractured vertebra in the SpineJack (SJ) system (Fig. 1) compared to the gold standard balloon kyphoplasty (BKP). 2. Methods 2.1. Specimens and experimental groups For equal group sample size 36 vertebrae from eight intact fresh human male cadaveric spines (T11-L3) were used in this study. The average age of the donors was 62 years (51–69 years). Each specimen was screened by computed tomography (CT) scan (Aquilion, Toshiba, Tokyo, Japan) for bone mineral density (BMD) measurements including calculation of vertebra body volume (Aquarius INtuition version 4.6.85.2800, TeraCcon, Frankfurt, Germany). All vertebrae were pooled to create two different groups consisting of each augmentation systems (SJ vs. BKP) and subdivided into three cement groups (10%, 16% and 30% cement filling of the vertebral body volume, n = 6). The SJ system (Vexim SA, Balma, France) consists of an expandable metal implant (titanium alloy) mounted on an expander, of which two are inserted bilaterally into the vertebral body and simultaneously expanded. The unexpanded implant (Ø 5 mm) is delivered in a prefolded state, and is gradually expanded to its final configuration until fracture reduction is satisfactory and/or the maximum expanded
height of 17 mm is reached. After the implant expansion both implants stay in place to maintain the restored height. PMMA cement is then injected through the central part of the implant (Fig. 1). 2.2. Fracture generation and instrumentation The vertebrae were isolated and all soft tissue was removed. The caudal endplates of the vertebrae were embedded into radiolucent PMMA (Beracryl; Troller Kunststoffe, Fulenbach, Switzerland). Vertebral compression fractures (AO A1.2.1) (Magerl et al., 1994) were performed using a universal material testing machine (MTS; Eden Prairie, MN, USA; 15 kN load cell, measurement error: 0.3%). Load was transferred by the pivot-mounted pressure plate in orthograde projection to the superior vertebral endplate to allow a wedge compression of the anterior wall until a compression of the anterior vertebral edge of more than 40% was reached (2 mm/min; 5 Hz) (Fig. 2). After generating the vertebral compression fractures, the vertebral body height was redetermined by CT scan including calculation of the vertebral body volume (Table 1). For augmentation the vertebrae were mounted into a custom made testing device performing reduction tests under a constant axial preload of 100 N. The specific working cannulae of both systems were placed bipedicular under the fluoroscopic imaging guidance of a C-arm (Ziehm Vario 3D, Ziehm imaging, Nuremberg). Either SJ (Ø 5 mm [diameter] × 25 mm [length]) or an inflatable kyphoplasty bone tamp/balloon (KyphX Xpander® 20/3, Kyphon Europe Zaventem, Belgium) was inserted simultaneously on both sides and expanded by two surgeons simultaneously. The re-alignment was continued until the vertebral body height was restored. Next, PMMA cement (Cohesion® Bone Cement CM0300, VEXIM SA, Balma, France) was injected synchronously on both sides using filling cannulae by Vexim or Kyphon, respectively, to comply with the manufacturers' recommendations. Each step was performed under
Fig. 1. A; Illustration of the SpineJack implant in the pre-folded and expanded states. B; SJ-representative lateral X-ray image after cement augmentation.
Please cite this article as: Rotter, R., et al., Minimum cement volume required in vertebral body augmentation—A biomechanical study comparing the permanent SpineJack device and..., Clin. Biomech. (2015), http://dx.doi.org/10.1016/j.clinbiomech.2015.04.015
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Finally, all specimens were axially loaded in controlleddisplacement mode (2 mm/min) until macroscopic failure of the vertebra occurred. The failure load was defined manually at the first significant decrease of slope of the load displacement diagram. For all biomechanical tests, the same pivot loading system was used (fracture generation, augmentation, cyclic loading, failure test). The loading point was always restored using special adjustment screws of the fixation device. 2.4. Statistical analysis All data were expressed as mean ± SD. After proving the assumption of normality and equal variance across groups, differences between groups were assessed using ANOVA followed by the appropriate post hoc comparison test (Holm-Sidak method (HS) or Kruskal–Wallis one-way analysis of variance on ranks (KW)). Statistical significance was set at P b 0.05. Statistics were performed using the SigmaStat software package (Jandel Corporation, San Rafael, CA, USA). Fig. 2. Illustration of vertebra during fracture test in the material testing machine. The vertebral body with the cement embedding is placed within the holding device. Load is transferred by the pivot and the pressure plate onto the upper vertebral body end plate. The holding device and the pivot are held by the hydraulic clamps of the material testing machine.
fluoroscopic control. Lateral fluoroscopic views were taken to determine the deformity corrections at the following time points: 1) after fracture generation and before reduction, 2) after maximum (balloon) inflation/ expansion of the repositioning tool 3) after balloon deflation and withdrawal/retraction of the repositioning tool, and 4) after injection and hardening of the cement. To minimize possible magnification effects and inter-radiographic precision errors a calibrated standard (20 mm ball mounted at the test device) was visible in all radiographs. Anterior vertebral body height, kyphotic angle and Beck Index (BI) were analyzed by two surgeons from these radiographs using special software (Media Viewer 1.0; Ziehm Imaging, Nuremberg, Germany). The measurements were triplicated and the mean value was calculated in each case. In accordance with McKiernan, vertebral height restoration was estimated as the absolute restoration in millimeters (restored vertebral height (c) − initial fracture height (b)) and percentage of restoration relative to initial fracture height ((c − b) / b ∗ 100) (McKiernan et al., 2003). Specimens were then re-evaluated by CT. 2.3. Mechanical testing Two different biomechanical tests were performed with the material testing machine (MTS). The augmented vertebrae were loaded by cyclic sinusoidal dynamic compression to simulate daily loads acting on the vertebra due to patient's activity. For this purpose, 10,000 cycles were performed between 200 N and 600 N with a frequency of 1 Hz (Kruger et al., 2013). Following the cyclical testing all vertebrae were examined by CT for the fourth time. The analysis of the height changes was made the same as for the C-arm radiograph results (see above).
3. Results All spines presented normal bone density values according to the WHO definition according to addressed middle-aged group of patients. After distribution of the vertebrae into the groups, no significant difference was seen between the BMDs of the examined groups (P = 0.987) (Table 1). 3.1. Results of fracture generation It was possible to generate a wedge compression fracture in all specimens. The resulting vertebral failure load values of the pooled vertebrae did not show any significant differences between the groups (Table 1; failure load P = 0.757). 3.2. Results of instrumentation The approach and placement of the SJ or bone tamps in all vertebrae were carried out without any complications. Vertebral body height restorations were achieved while maintaining full balloon inflation up to 94% for SJ and up to 100% for BKP (Fig. 3). However, there was a significant loss of reduction after balloon deflation in BKP compared to implant holder release in SJ, and a significant total height gain by SJ (Fig. 3). Anterior height loss after deflation in relation to preoperative height was significantly higher (16%, P b 0.001; KW) in BKP compared to SJ (1%, n.s.). Even more pronounced was the relation of anterior height loss after deflation to re-alignment height. Overall, 72% of the restored height was lost in BKP. SJ showed a significantly lower restored height loss of 10% (P b 0.001; KW). Correspondingly, there was an increase in total anterior height gain by SJ (11%) compared to BKP (7%) (P = 0.275) (Fig. 3). The mean changes in the kyphotic angles and BI corresponded to the above-mentioned results (data not shown).
Table 1 Basic data of tested vertebrae. System
SJ
Cement percentage [%]
10
3
BMD [g/cm ] Vertebra volume before fracture [cm3] Relative volume decrease due to fracture [%] Total cement filling volume [ml] Preop. failure load [N] Postop. failure load [N]
BKP 16
30
Total
10
16
30
Total
Mean
SD
Mean
SD
Mean
SD
Mean
SD
Mean
SD
Mean
SD
Mean
SD
Mean
SD
129 39.2 25 2.93 3201 4464
27 7.7 7 0.50 906 721
146 37.2 24 4.55 3510 4785
61 8.6 9 1.16 1551 1128
149 35.4 24 7.99 4075 5088
59 6.4 6 1.04 1260 1400
141 37.3 24 5.16 3596 4779
49 7.3 7 2.35 1247 1068
146 37.1 24 2.81 3191 4181
61 8.6 9 0.69 614 1411
147 34.0 24 4.13 4190 4241
60 5.2 9 0.85 719 1687
143 34.1 22 8.08 3753 5703
62 5.7 9 2.02 1274 2580
145 35.1 23 5.01 3711 4709
57 6.4 8 2.45 958 1953
Please cite this article as: Rotter, R., et al., Minimum cement volume required in vertebral body augmentation—A biomechanical study comparing the permanent SpineJack device and..., Clin. Biomech. (2015), http://dx.doi.org/10.1016/j.clinbiomech.2015.04.015
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Fig. 3. Values of the relative changes in height in % with SJ (filled circles) compared with BKP (open triangles) i) before and ii) after fracture generation, iii) after reduction, iv) after deflation and v) the resulting height gain after completion of reduction. Values are given as mean ± SD; ANOVA, post hoc comparison; *P b 0.05 BKP.
Fig. 5. Values of the Beck index by SJ (filled squares) compared with BKP (open squares) according to CT image evaluation. Values are given as mean ± SD; ANOVA, post hoc comparison; *P b 0.05 BKP.
3.3. Results of post-operative static and dynamic tests
failure load ratio improved by 30% in SJ compared to BKP (P = 0.212) (Fig. 6, Table 1).
After 10,000 cycles of axial compression, significant differences were found between the groups in the subsidence behavior (plastic deformation) of the vertebrae. Unlike BKP, in SJ differences between the cement groups were smaller than 2%. However, in the 10%-group the height restoration expressed as a percentage of the initial height was significantly increased with the SJ (P = 0.016; HS). Also, in the 16%-group the height restoration was significantly increased (P = 0.017; HS). Only for the 30%-group, no difference between the two systems could be seen (P = 0.690) (Fig. 4). Analyzing the BI, SJ showed better restoration values than BKP including a significant difference following cyclic loading (P = 0.010; HS) (Fig. 5). Postoperatively, SJ and BKP showed a substantial tendency to increasing failure loads after vertebral augmentation. SJ presented a non-significant increased failure load compared to BKP in the 10 and 16% groups, respectively. In the 16%-group the post- to preoperative
The latest developments in kyphoplasty include different implants that remain within the vertebra and restore the achieved reduction height temporarily until the cement is injected and hardened (Kruger et al., 2013; Rotter et al., 2010). However, none of these new procedures works without the additive use of cement (Kruger et al., 2013). Therefore, the problem of cement-associated complications is not yet solved (Hulme et al., 2006; Taylor et al., 2006, 2007). The current study provides information about the minimum required cement volume, height restoration and static behavior of vertebral compression fractures following augmentation by such a new technique (SpineJack) compared to BKP. The most important finding of this study was that a quantity of cement equivalent to 10% of the vertebral volume was sufficient to
Fig. 4. Global height restoration after cyclic loading according to CT image evaluation with SJ (filled bars) compared with BKP (blank bars) depending on the degree of cement filling. Values are expressed as percentages of the preoperative heights. Values are given as mean ± SD; ANOVA, post hoc comparison; *P b 0.05 BKP.
Fig. 6. Postoperative failure loads of SJ (filled bars) and BKP (blank bars) expressed as a percentage of the preoperative failure loads (the preoperative failure load was normalized as 100%). Values are given as mean ± SD.
4. Discussion
Please cite this article as: Rotter, R., et al., Minimum cement volume required in vertebral body augmentation—A biomechanical study comparing the permanent SpineJack device and..., Clin. Biomech. (2015), http://dx.doi.org/10.1016/j.clinbiomech.2015.04.015
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stabilize the fractured vertebra when using SJ but not BKP. A clinically adequate reduction in the fractured vertebral body height was achieved with both systems. However, with BKP, the absence of load-bearing within the created cavity after balloon deflation and before cement injection translated into a significant loss in the height gained initially compared to SJ. From the biomechanical point of view, Molloy et al. was able to show in vitro that strength and stiffness will be restored with 16 and 30% of vertebral cement filling, respectively, following vertebroplasty (Molloy et al., 2003). For this reason these quantities of cement were used for the current study. In contrast to Molloy, we used the post-fracture vertebra volume as a starting point to better simulate the clinical situation. In clinical practice, usually only post-fracture imaging is available. In our study the volume differences (pre- and post-fracture) were approximately 24%. Rotter et al. were able to demonstrate an increase in strength due to the combination of intervertebral implant plus cement (Rotter et al., 2010). Therefore, it could be assumed that with the SJ implant a reduced cement volume would be required for equivalent stability. Nevertheless, a study by Krueger et al. had previously shown the SJ implant alone does not achieve sufficient stability following a cyclic loading (Kruger et al., 2013). Accordingly, the 10%-group was introduced for this study, which appears clinically and biomechanically reasonable. In accordance with the implant design and the intra-operative load-bearing capability, there was a significantly higher preservation of initially gained height with SJ compared to BKP. The intra-operative reduction in restored height in BKP was due to the absence of a means of load-bearing within the created cavity after balloon deflation but before cement injection and hardening. This phenomenon has already been shown in other studies, in part with other intra-vertebral implants (Kruger et al., 2013; Rotter et al., 2010; Voggenreiter, 2005). How relevant this improved re-alignment is clinically has not yet been proven. However, there was a biomechanical study indicating that the anterior shift of the upper body is the dominant factor in adjacent vertebral fractures, so achieving the maximum possible fracture reduction could eventually decrease this complication (Baumbach et al., 2012; Ferraris et al., 2012; Rohlmann et al., 2006). The cyclic loading tests were performed to simulate the short-term in vivo performance of the materials (Kruger et al., 2013). Wilke et al. state that the load between 100 and 600 N is a compromise for moderate loading between lying (100 N, 180 N) and standing or walking (500 N, 900 N) (Wilke et al., 1999, 2006). We found a significant loss of height in the 10%-group and a nonsignificant trend at 16% with BKP. This may be due to a mismatch in the cavity created by the maximally inflated balloon but followed by incomplete cement filling. As a consequence vacuum phenomena, which are detectable by CT, caused weakness and sintering behavior. In contrast, the SJ implant does not create a cavity during the expansion procedure. The sintering in SJ is only achieved in part through cancellous displacement superior or inferior of the implant. The use of a smaller cement volume compared to the balloon volume during BKP is not recommended by the manufacturer. From our results it is obvious that it is not useful to reduce the cement filling to 10% by volume in BKP. However, from the biomechanical point of view this technique is possible with SJ. The examinations of static testing were carried out to correspond to the criteria of Belkoff et al. and Heini et al. (Belkoff et al., 2001; Heini et al., 2001). In concordance with these authors, we found a significant difference in maximum failure load between augmented and nonaugmented vertebrae but no significant differences between SJ and BKP. However, there was a tendency toward an increased relative failure load by SJ in the 10 and 16%-groups. Contrary to Molloy et al., strength restoration was already achieved with both SJ and BKP in the 10%-group and did not increase with further increases in the amount of cement filling (Molloy et al., 2003). One possible explanation might
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be that the strength was measured following the cyclic loading protocol by the final static test. However, the maximum reduction of height of the augmented vertebra was already reached during the cyclic loading and so the failure load was postoperatively increased due to the impacted cancellous bone that was not filled by the cement. This biomechanical study was limited by the fracture model used and the experimental situation deviated from clinical reality. We are aware that kyphoplasty is a domain of osteoporotic fracture augmentation. Nevertheless, previous studies have already reported the use of kyphoplasty in younger patients (Gumpert et al., 2014; Schmelzer-Schmied et al., 2009). For our group the opportunity of a permanent implant with a minimum cement volume appears ideal for fracture stabilization in the middle-aged and young patient groups. That is the reason why we did not use osteoporotic vertebrae. Due to biological reasons there are not enough young specimens available for biomechanical studies. Therefore, vertebrae with average age of 62 (51–69 years) were used in this study. Inter- and intra-individual differences between the specimens were reduced by matching the vertebrae to the groups. To generate a typical wedge fracture, prevent the posterior wall to become fractured as well and to investigate the cancellous/ endplate interface the cranial endplate was loaded by a textured planar metal surface and was not embedded. Mirzaei et al. have indicated that the exact parallel embedding of the endplates as well as the exact vertical alignment with the vertical axis of the vertebra is not possible. As a consequence, unwanted bending moments would result (Mirzaei et al., 2009). Disadvantage not to embed the endplate is the occurrence of local crushing at the endplate which biases the stiffness measurement. The moderate transfer of force (2 mm/min) produces wedge fractures instead of shock loads generating burst fractures. However, the used force flow is not comparable to a low-energy trauma. Nevertheless, in literature there is no reference regarding different transfer of forces (1–10 mm/min) and the biomechanical quality of generated fractures. The instrumentation and augmentation of a single fractured vertebra, isolated from all soft tissue, is not similar to the clinical situation. However, the preload of 100 N used is comparable with treatment in prone position (Belkoff et al., 2001; Sato et al., 1999; Tohmeh et al., 1999). Using the same specimen for the final static test following the cyclic loading is a potential limitation. However, cyclic loading test were performed to simulate short in vivo performance (about 3 months until bony fusion) of the bone cement implant combination. We are aware that one million cycles represent about one year in a patient's life (Wilke et al., 2006). Simulating 3 months, 250,000 cycles should have been performed, however this is hardly practicable because of autolysis affecting the biomechanical behavior of the specimens. In line with the literature 10,000 cycles were performed and it revealed that about 80% of sintering occurred after one third of cycles (Kruger et al., 2013; Rotter et al., 2010). The vertebral height measurements were performed identically in all groups using CT and fluoroscopy. According to the literature a precise measurement by CT of 1.2% (coefficients of variation) and fluoroscopy of 2.2% can be assumed (Frobin et al., 1997; Tan et al., 2013). However, this error is unavoidable and is leveled down due to the group distribution.
5. Conclusions This in-vitro study demonstrated that it is possible to reduce the amount of cement to 10% of the vertebral body volume when using SJ following vertebral body augmentation due to traumatic wedge fracture. Furthermore, it was impossible to reduce the amount of cement in BKP without increased height loss. For clinical use these results implicate that the leakage rate could be reduced and consequently the risk of symptomatic complications. However, the true validity of this conclusion must be shown in clinical trials in comparison with BKP conducted in a proficient manner as the gold standard.
Please cite this article as: Rotter, R., et al., Minimum cement volume required in vertebral body augmentation—A biomechanical study comparing the permanent SpineJack device and..., Clin. Biomech. (2015), http://dx.doi.org/10.1016/j.clinbiomech.2015.04.015
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Please cite this article as: Rotter, R., et al., Minimum cement volume required in vertebral body augmentation—A biomechanical study comparing the permanent SpineJack device and..., Clin. Biomech. (2015), http://dx.doi.org/10.1016/j.clinbiomech.2015.04.015