Introduction
Deficiency in the alveolar ridge may occur from tooth loss, trauma, pathology, or congenital defects. This deficiency can compromise ideal implant placement and long-term treatment outcomes.1 The ridge deficiency can occur horizontally, vertically, or in combination.2 Hence, different therapeutic strategies have been developed to regenerate alveolar bone defects, including guided bone regeneration (GBR), block graft, ridge splitting, or distraction osteogenesis.3–8 GBR is a method that provides clinicians with an adequate amount of bone for implant site development and has been used with success in dental practice.9 However, reconstruction of vertical defects may be more challenging than reconstruction of horizontal defects.10–13 A major challenge in vertical regeneration is the eventual collapse of space resulting from excessive masticatory forces over the membrane and graft materials.14
To overcome some shortcomings of GBR for reconstruction of vertical bony defects, the use of a tent pole or tenting screws has been proposed. The rationale of the “tent-pole” grafting technique is that using screws will help prevent collapse of the graft no further than the level of the screw heads.15 In the tent-pole grafting technique, the membrane and supporting screws are the main components supporting the reconstructed area to prevent collapse of graft materials. Stability of the membrane and tent pole screws are important factors in ensuring safe dissipation of masticatory forces over the grafted area.16,17
Non-resorbable barrier membranes have been used to maintain the space for alveolar ridge augmentation in multiple clinical studies.18–20 Previously, studies reported the use of an expanded polytetrafluoroethylene (e-PTFE) membrane on vertical augmentation.16,19 However, a high incidence of infection after membrane exposure has seen its use fall out of favor.21 Dense-PTFE (d-PTFE) membrane has been used to achieve optimal bone width and height in GBR procedures, carrying less risk of infection after membrane exposure compared with e-PTFE.12,20,22–24 In addition, d-PTFE membranes are biocompatible, sufficiently occlusive, clinically manageable, and have superior mechanical stability and stiffness compared with resorbable membranes.24 Recently, Urban et al18 reported a prospective series of patients with advanced vertical defects who were treated with the tent-pole screws technique in addition to d-PTFE membranes. This approach was shown to be effective at achieving a significant amount of bone height.18
Masticatory forces result in oblique and vertical loads, inducing pressure on and bending screws, making make them more susceptible to fracture or failure to maintain the space.25,26 Nevertheless, there are ethical issues to performing in vivo studies to identify the forces exerted on tenting screws used for vertical bone augmentation.27 To date, however, there have been no clinical or experimental studies investigating the effect of occlusal forces on tenting screws, membrane, and the surrounding bone. An alternative to performing in vivo studies to predict biomechanical performance of dental structures and biomaterial designs in dentistry is a virtual test known as the “finite element analysis” (FEA) means of predicting.27 FEA has previously been used to investigate the stress distribution around implants and surrounding bone.28
The purpose of this study was to analyze stresses induced in tenting screws by occlusal forces in a vertically augmented ridge through use of 3-dimensional FEA.
Methods
Finite element model
The geometry and associated mesh of the posterior mandibular bone with vertical defect was reconstructed with 3-dimensional FEA code (ANSYS Workbench 11.0; ANSYS Inc, Canonsburg, Penn) and engineering design software (SolidWorks Professional 2013; SolidWorks Corp, Concord, Mass) to develop a digital study model. The reconstructed model consisted of two tenting screws placed and stabilized in the mandibular bone, then covered by d-PTFE membrane (Osteogenics Biomedical, Lubbock, Tex) for vertical ridge augmentation. A linear screw configuration was used that included two screws above the alveolar ridge in a straight line, a configuration applied earlier by several authors.15,19,29 Figure 1 shows the screw configuration as well as the mandibular bone digital study model when axial forces were applied. The tenting screw (Osteogenics Biomedical) used in this study was a 9mm self-drilling screw made of polished neck (5mm) for supporting membrane and space maintenance, and threaded portion (4mm) for stabilizing the screw into the bone. The width of the tenting screws was 1.5mm with broad 3.0mm head. Two tenting screws were buried in a simulated posterior mandibular bone model that included a cancellous core surrounded by a thick cortical layer (1mm thickness). The tenting screws were rigidly anchored in the bone model along the entire interface.The overall dimensions of this model were 10mm in buccolingual width, 17mm in mesiodistal length, and 15mm in height.
Figure 1. Finite element analysis (FEA) model of bone and tenting screws covered with membrane when axial forces were applied. Figure 2. Von Misses stress results in bone when vertical loading was applied. Figure 3. Von Misses stress in the tenting screw when vertical loading was applied. Figure 4. FEA model of bone and tenting screws covered with membrane when off-axial forces were applied. Figure 5. Von Misses stress results in bone when off-axial loading (45 degree) was applied. Figure 6. Von Misses stress in the tenting screw when off-axial loading (45 degree) was applied.
Figure 1. Finite element analysis (FEA) model of bone and tenting screws covered with membrane when axial forces were applied. Figure 2. Von Misses stress results in bone when vertical loading was applied. Figure 3. Von Misses stress in the tenting screw when vertical loading was applied. Figure 4. FEA model of bone and tenting screws covered with membrane when off-axial forces were applied. Figure 5. Von Misses stress results in bone when off-axial loading (45 degree) was applied. Figure 6. Von Misses stress in the tenting screw when off-axial loading (45 degree) was applied.
The mechanical properties of the simulated model were considered to be isotropic, homogenous, and linearly elastic.30 It was assumed that tenting screws engaged completely with the bone and were subjected to shear stress. Young's modulus and Poisson's ratio of all simulated structures were given by the manufacturer (Table 1). In total, the 3-dimensional simulated model consisted of 91 093 nodes and 60 985 elements. Schneider et al31 showed that the maximum stress the elastic properties of a fresh human mandible can tolerate is 85 MPa. Selecting adequate boundary conditions prevents any torsion of the mandibular bone. Tensile stresses in the bone and screws were then evaluated applying 3-dimensional FEA.
Loading protocols
Since the pattern of stress distribution was significantly affected by location and size of force induction, the masticatory force in the lower first molar area was simulated. An axial occlusal forces of 100 N and an off-axial of 30 N (45 degrees of inclination in relation to the long axis of the tenting screws) was applied to simulate the mean value of the posterior bite force in humans.32,33
Data analysis
The strength estimation of the 3-dimensional screw system was calculated using von Mises and Henckey's “change of shape” energy hypothesis.34–36 All the data needed to contour patterns and develop stress values in and around tenting screws and a 3-dimensional model of the mandible were meshed and analyzed by ANSYS Workbench 11.0 (ANSYS). Von Mises stresses were used to present the stress values. A stress map was gecnerated that showed stress distribution on the bone and screws. If the maximum tensile stress for either the tenting pole screws or the bone is exceeded, the structure may fail. The results of the FEAs do not have a variance; therefore, there was no need to perform statistical analyses.
Results
The von Mises stress pattern on tenting screws and the surrounding bone with a masticatory force of 100 N in vertical and 25 N in oblique directions was evaluated by FEA (Figures 1 through 6).
When vertical force was applied, the maximum von Mises stress on the surrounding bone was located coronally (Figures 1 and 2). A magnification of the tensile stress distribution after application of the vertical load on tenting pole screws is shown in Figure 3. The maximum and minimum stresses are located at the top and end of the screws, respectively.
Application of the oblique occlusal load resulted in high stress values at the coronal and middle portions of the bone and screws, respectively, as illustrated in Figures 4 through 6.
Table 2 summarizes the maximum stress values in screws and the surrounding bone when axial and off-axial loading forces were applied. Distribution of stress in both tenting screws was comparable, as was the pattern of stress distribution on the bone surrounding the two screws. Furthermore, stresses as a result of axial and off-axial forces did not cause mechanical instability.
Discussion
Space maintenance is the main parameter of successful bone regeneration over the residual bone defect. It is noteworthy that complete reconstruction of vertical bone defects may not feasible using barrier membranes in all cases and may result in membrane collapse at some locations. These collapsed points are known as “leaky spots” and may be the result of the membrane folding due to lack of adaptation to a 3-dimensional structure or insufficient support and stability against masticatory forces.16,19 Application of tenting screws allows for effective space maintenance by creating a support for the barrier membrane against external forces, thereby helping the membrane hold its volume and shape during the healing period.37 Tenting screws have been used successfully for bone augmentation with a low risk of complications.19,38 However, successful bone regeneration may be compromised by mechanical instability of the tenting screws.39 To the best of our knowledge, this is the first study that assesses the stability of tenting screws against external forces as well as illustrating the pattern of stress distribution on the surrounding bone. Based on the findings from this study, different stress patterns resulting from the application of axial and off-axial forces do not cause mechanical instability in tenting screws.
Tenting screws have been used successfully to vertically augment the alveolar ridge by 3–5mm by supporting and maintaining the geometry of the membrane.18,19,38,40,41 Several studies have demonstrated that after use of tenting screws, the bone height appeared stable and the particulate bone graft was not resorbed farther than the level of the screw heads.15,19 The size of the vertical defects may affect the success of the procedure, and it has been suggested that vertically augmenting less than 5–6mm may reduce the risk of complications.15
The size of the tenting screws used for vertical augmentation is also critical in the evaluation of their mechanical stability when occlusal forces are applied during the healing period. The size of the tenting screws employed in this present investigation is comparable to those used in the previous studies.18,19,38,40,41 Moreover, it has been suggested that at least 3mm of vertical bone height is needed to achieve tenting screw primary stability.37 Based on the results of the current study, primary stability of the screws was obtained by placing the screws 4mm inside the bone.
Non-resorbable barrier membranes have been used in multiple clinical studies to maintain the space for alveolar ridge augmentation.18–20 Since e-PTFE membrane has been removed from the market, the d-PTFE membrane has become a more commonly used biomaterial for GBR procedures.42,43 However, a recent systematic review suggested that further research is needed to evaluate the success of vertical ridge augmentation using d-PTFE membranes.44 The use of non-resorbable barrier has not been evaluated using FEA in any previous studies.
Biomechanical characteristics of bone grafting procedures can also be explored by FEA.45 Finite element analysis is a mathematical method designed to simulate and evaluate different shapes, including bone and tenting screws used in this study, as well as the mechanical properties of these structures. A strong correlation between the amount of force applied and the stress distribution on implants and the surrounding bone has been reported using FEA.46,47 The experimental model in this study evaluated axial and off-axial loading forces over a simulated vertical ridge defect that was augmented by tenting screws and a d-PTFE membrane. As illustrated in the present study, the point of load transfer significantly affects the amount and pattern of stress on screws and the surrounding bone. In comparison with axial loading, off-axial loading forces caused more stress at the coronal portion of the bone-to-screw interface, and the pattern of stress shifted from the coronal to the middle portion of the screws. Axial loading induced higher von Mises stress values at the screws (146.08 MPa) than did oblique loading (34.36 MPa) and similar stress values within the bone with axial loading (25.1 MPa) and off-axial (24.68 MPa). The masticatory force used in the present study—100 N for axial and 25 N for off-axial—is within the range of masticatory force generated in the molar region (75 to 300 N).42,45 The results of the present study reveal that the largest amount of stress to bone when vertical and oblique forces are applied occurs in the cortical bone adjacent to the coronal part of the bone-to-screw interface, which is consistent with the findings of previous studies.48–50 In addition, the von Mises stress pattern showed an uneven stress distribution in the bone.
It is important to note that the model in the present study was homogenous, isotropic, and linearly elastic, which are conditions that do not necessarily exist in clinical practice.29 For that reason, the mechanical trends and the trends of changes in the stress observed in the study should be considered rather than the absolute values. In addition, the materials used and physical characteristics of each layer of the model affect the stress distribution patterns.
To perform the FEA, the clinical condition for vertical bone augmentation was simplified. Note that certain differences are present in the strength and quality of the bone, as well as the strength of the bone graft, tenting screw, and barrier membrane. However, the mechanical properties of different types of these bone grafts, tenting screws, and barrier membranes have not been well established. Further, different locations, angulation and size of forces, temperature, type of diet, opposing dentition, and use of fixture screws to stabilize the barrier membrane are also clinical factors that can affect stress generated around the tenting screws and barrier membrane during vertical bone augmentation. Thus, inherent limitations in this study should be considered. By considering the limitations of FEA, the clinician will be better equipped to interpret the outcomes and extrapolate these findings to clinical practice.
Conclusion
Within the limits of this study, all tenting screws tested were sufficiently stable to maintain membrane volume against vertical and oblique forces. Further animal and clinical studies are necessary to confirm that tenting screws can support forces as depicted by the models used in the present study.
Abbreviations
Acknowledgments
The authors are grateful to Professor Yu Zhang, Department of Biomaterials & Biomimetics, for his advice during the study, and Kira Nightingale, MS, CCRC, Bluestone Center for Clinical Research, New York University College of Dentistry, New York, for her help with the revision of the English text of the manuscript.
Note
The authors claim no financial interest, either directly or indirectly, in the products or information listed in this paper. There was no external source of funding for the present study.
