Class II malocclusion is among the most prevalent orthodontic problems encountered in clinical practice. It may result from a prognathic maxilla, a retrognathic mandible, or a combination of both. In growing patients, functional and orthopedic appliances are commonly used to modify skeletal discrepancies.1,2 In contrast, treatment in non-growing patients is typically limited to camouflage approaches, including extraction of maxillary premolars or distalization of the maxillary dentition.3-5 The introduction of temporary anchorage devices (TADs) has significantly expanded the scope of nonextraction treatment. Placement of TADs in the infrazygomatic crest (IZC) region has gained popularity for en-masse distalization of the maxillary arch.6,7 Unlike inter-radicular implants, IZC screws are positioned extra-radicularly, thereby minimizing root interference and eliminating the need for repositioning during tooth movement. This allows efficient distalization of the entire arch while maintaining anchorage control and potentially reducing treatment duration.8,9 Despite their clinical advantages, there is no consensus regarding the optimal placement site and length of IZC screws. Placement has been advocated in both the IZC 6 region, near the mesiobuccal root of the first molar, and the IZC 7 region, between the first and second molars. Similarly, while shorter screws (8–12 mm) have been reported to provide adequate cortical engagement, longer screws (12–14 mm) are often recommended to compensate for increased soft tissue thickness in the buccal vestibule. These variations may influence stress distribution in the maxillary bone and surrounding structures during distalization.10-12 The biomechanical response generated during maxillary distalization is influenced by several factors, including the point of force application, direction of force, quality of the supporting bone, and the design and dimensions of the anchorage device. Even subtle variations in miniscrew length or insertion site may alter stress distribution within the surrounding bone and affect the pattern of tooth movement. A thorough understanding of these biomechanical interactions is essential for optimizing anchorage stability, minimizing undesirable side effects, and achieving predictable treatment outcomes during en-masse distalization. Finite element analysis (FEA) provides a reliable method to evaluate biomechanical responses, including stress distribution and displacement patterns, under simulated clinical conditions. However, limited data are available comparing the effects of different IZC screw lengths and placement sites on maxillary biomechanics. Therefore, the aim of this study was to evaluate the stress distribution and displacement patterns in the maxilla and associated structures during distalization using IZC screws of varying lengths and placement sites, employing a three-dimensional finite element model derived from computed tomography data.

A computed tomography (CT) scan of an adult patient with skeletal Class II malocclusion indicated for maxillary arch distalization was obtained from the Department of Oral Medicine and Diagnosis, Postgraduate Institute of Dental Sciences, Rohtak. The scan parameters included a slice thickness of 1 mm and a pixel size of 0.42 mm. The CT data were used to construct a three-dimensional finite element model of the craniofacial skeleton. The Digital Imaging and Communications in Medicine (DICOM) data were processed using Mimics software (version 9.0; Materialise, Leuven, Belgium) to generate geometric models through reverse engineering techniques. The surface models were further refined using Rapidform software to obtain accurate anatomical structures. These geometric models, consisting primarily of surface data, were subsequently imported into meshing software (Hypermesh; Altair Engineering, Troy, Mich) for discretization. In Hypermesh, the anatomical components, including cortical bone, trabecular bone, sutures, teeth, and periodontal ligament, were discretized into finite elements and assembled. Orthodontic components, including brackets, archwires, and infrazygomatic crest (IZC) screws composed of stainless steel, were modeled separately using reverse engineering techniques and positioned appropriately on the maxillary arch. The resulting finite element model consisted of nodes and elements representing the physical structure. Material properties, including Young’s modulus and Poisson’s ratio, were assigned to each component based on values reported in the literature. All materials were assumed to be homogeneous, isotropic, and linearly elastic. The interface between the mini-implant and bone was considered to be rigidly bonded. Boundary conditions were applied to simulate physiological constraints, and linear static analysis was performed using ANSYS software (version 12.1; ANSYS Inc., Canonsburg, Pa). Each model was solved, and the results were stored in database (.db) format. Infrazygomatic crest screws of lengths 8 mm, 12 mm, and 14 mm were evaluated under a traction force of 350 g applied for distalization of the maxillary arch. Two insertion sites were considered: the IZC 6 region, corresponding to placement in line with the mesiobuccal root of the maxillary first molar, and the IZC 7 region, corresponding to a more superior and lateral position between the first and second molars. The force was applied perpendicular to the long axis of the screw to simulate clinical loading conditions.(Figure 1). Stress distribution and displacement were evaluated in the cortical bone, trabecular bone, mini-implant, and periodontal ligament. Stress analysis was performed both qualitatively and quantitatively. Color-coded contour maps were used to visualize stress distribution, with red indicating maximum stress and blue indicating minimum stress. Intermediate values were represented along a continuous spectrum. Numerical values of von Mises stress were recorded, and deformation patterns were standardized to allow comparison among different models.

Finite element models were successfully constructed to evaluate the biomechanical effects of infrazygomatic crest (IZC) screw placement at IZC 6 and IZC 7 sites with screw lengths of 8 mm, 12 mm, and 14 mm. The 8-mm model consisted of 62,871 nodes and 301,954 elements, the 12-mm model consisted of 66,454 nodes and 323,836 elements, and the 14-mm model consisted of 67,266 nodes and 328,688 elements.(Table 1). Table 1. Finite Element Model Characteristics Model (IZC Screw Length) Number of Nodes Number of Elements 8 mm IZC screw 62,871 301,954 12 mm IZC screw 66,454 323,836 14 mm IZC screw 67,266 328,688 Displacement contours of the full model demonstrated similar distribution patterns for both insertion sites and all screw lengths, with maximum displacement localized at the region of screw insertion. (Figure 2) In cortical bone, maximum displacement at the screw region ranged from 0.175 mm to 0.176 mm for IZC 6 and was approximately 0.19 mm for IZC 7 across all screw lengths. In cancellous bone, displacement patterns were comparable between IZC 6 and IZC 7 models, with no appreciable variation in magnitude among the different screw lengths. Von Mises stress distribution in cortical bone was concentrated at the screw–bone interface and the point of force application in all models. (Figure 3, 4) The overall stress distribution pattern was similar across insertion sites; however, higher stress magnitudes were observed in the 8-mm screw models compared with the 12-mm and 14-mm models. (Table 2,3) In cancellous bone, stress levels were lower than those observed in cortical bone and followed a similar distribution pattern across all configurations. Table : 2 Comparison of Von Mises stress in maxilla ; screw positioned at IZC 6 with varying screw length Measured stress 8mm 12mm 14mm P value Post hoc comparison Von mises stress in maxilla cortical bone ( in MPa) 27.99 23.42 21.29 <0.0001* 8mm>12mm,14 mm Von mises stress in maxilla cancellous bone ( in MPa) 11.10 8.87 7.39 <0.0001* 8mm>12mm,14 mm Table : 3 Comparison of Von Mises stress in maxilla ; screw positioned at IZC 7 with varying screw length Measured stress 8mm 12mm 14mm P value Post hoc comparison Von mises stress in maxillary cortical bone ( in MPa) 31.76 24.20 24.14 <0.0001* 8mm>12mm,14 mm Von mises stress in maxillary cancellous bone ( in MPa) 9.80 6.00 5.03 <0.0001* 8mm>12mm,14 mm Distal movement of the maxillary dentition was observed in all models, with a maximum displacement of approximately 0.24 mm, irrespective of screw length or placement site. Intrusion was observed in both anterior and posterior teeth. In IZC 6 models, anterior intrusion was approximately 0.06 mm and posterior intrusion was approximately 0.09 mm. In IZC 7 models, anterior intrusion was approximately 0.04 mm, whereas posterior intrusion was approximately 0.10 mm. The periodontal ligament demonstrated similar stress and displacement patterns across all models, with no appreciable differences observed between screw lengths or insertion sites.

The present finite element study evaluated the biomechanical behaviour of the maxilla during en-masse distalization using infrazygomatic crest (IZC) screws of varying lengths and insertion sites. The findings demonstrated that both screw length and placement site influence stress distribution and displacement patterns, although the overall biomechanical response remained consistent across models. Maximum stress concentration was observed at the screw–bone interface and at the point of force application, which is consistent with the fundamental principles of load transfer in skeletal anchorage systems.15 Cortical bone exhibited higher stress values than cancellous bone, reflecting its greater stiffness and load-bearing capacity.16 Importantly, the stress levels observed in all models remained within physiologic limits, suggesting that IZC screws provide a safe and stable anchorage system for maxillary distalization. A notable finding of this study was the relatively higher stress concentration associated with the 8-mm screw compared with the 12-mm and 14-mm screws. This may be attributed to reduced bone engagement and decreased surface area for stress distribution in shorter screws, leading to increased stress intensity at the cortical interface. In contrast, longer screws likely provide improved primary stability by engaging a greater volume of cortical and trabecular bone, thereby distributing forces more evenly. This observation supports the clinical preference for longer screws in regions with thicker soft tissue and variable bone density.15 With respect to insertion site, the IZC 7 region demonstrated slightly greater displacement in cortical bone compared with IZC 6. This may be related to differences in bone morphology and screw orientation, which can influence the direction and magnitude of force transmission.12,14 Additionally, the IZC 7 site showed reduced anterior intrusion and slightly increased posterior intrusion compared with IZC 6, indicating a more favourable vertical control pattern during distalization. These findings suggest that IZC 7 placement may offer biomechanical advantages for achieving controlled distalization with minimal undesirable tooth movement. Distal movement of the maxillary dentition was consistent across all models, with similar magnitudes irrespective of screw length or placement site. This indicates that IZC-based anchorage is effective in producing en-masse distalization without significant dependence on screw dimensions. However, the presence of simultaneous intrusion, particularly in posterior teeth, highlights the complex three-dimensional nature of force systems involved in IZC mechanics. Such vertical components may be advantageous in cases requiring bite control but should be carefully considered during treatment planning. The periodontal ligament (PDL) response was uniform across all models, indicating that variations in screw length and placement site did not significantly alter stress transmission to the supporting structures of teeth.16 This suggests that IZC mechanics primarily influence skeletal anchorage behaviour without inducing excessive stress within the PDL, thereby supporting their clinical safety. The findings of this study are in agreement with previous investigations that have highlighted the efficiency of extra-radicular anchorage systems in achieving full-arch distalization while minimizing root interference.6,7,13 The use of finite element analysis in the present study provides additional insight into the biomechanical mechanisms underlying these clinical outcomes by allowing precise evaluation of stress distribution and displacement patterns under controlled conditions. The findings of the present study are consistent with previous finite element investigations evaluating infrazygomatic crest anchorage. Khan et al. demonstrated that the IZC 7 insertion site produced more favourable distalization mechanics than IZC 6, particularly when the line of force approximated the centre of resistance of the maxillary dentition.17 Similarly, studies evaluating IZC biomechanics have reported that stress is concentrated primarily at the screw–cortical bone interface while remaining within physiologic limits, supporting the mechanical stability of extra-alveolar anchorage. Recent finite element analyses have further confirmed that variations in force application and anchorage position influence displacement patterns and vertical control during en-masse distalization.18,19 The observed stress distribution pattern within the maxillary complex further reflects the biomechanical characteristics of extra-alveolar anchorage. Higher stress concentrations were primarily localized around the infrazygomatic screw insertion site and the adjacent cortical bone, whereas stresses dissipated progressively through the surrounding maxillary structures. This pattern indicates efficient transfer of orthodontic forces from the temporary anchorage device to the maxillary dentition while minimizing excessive stress accumulation in distant regions. Furthermore, the relatively uniform displacement of the dentition across all models suggests that the applied force system produced controlled en-masse distalization without marked asymmetrical deformation of the maxillary complex. These findings reinforce the biomechanical stability of IZC anchorage and support its use for predictable distalization mechanics. Despite its strengths, this study has certain limitations inherent to finite element analysis. The assumption of homogeneous, isotropic, and linearly elastic material properties does not fully replicate the complex biological behaviour of bone and periodontal tissues. Additionally, the model represents a static loading condition and does not account for time-dependent factors such as bone remodeling and orthodontic tooth movement dynamics. Therefore, the results should be interpreted in conjunction with clinical findings. Within these limitations, the present study provides valuable biomechanical evidence supporting the use of IZC screws for maxillary distalization. Longer screws (12 mm and 14 mm) and placement in the IZC 7 region appear to offer more favourable stress distribution and displacement characteristics, which may translate into improved clinical outcomes. Clinically, these findings may aid in optimizing IZC screw selection and placement to improve efficiency and control during maxillary distalization.

Within the limitations of this finite element study, infrazygomatic crest (IZC) screws provided effective and stable anchorage for en-masse distalization of the maxillary arch. Stress distribution was primarily concentrated at the screw–bone interface and remained within physiologic limits in all models. Screw length influenced stress patterns, with shorter screws (8 mm) demonstrating relatively higher stress concentrations compared with longer screws (12 mm and 14 mm), suggesting improved load distribution with increased screw length. Insertion site also affected biomechanical behaviour. The IZC 7 region exhibited slightly greater displacement with more favourable vertical control, characterized by reduced anterior intrusion and controlled posterior intrusion compared with IZC 6. Distalization of the maxillary dentition was consistent across all models, irrespective of screw length or placement site, indicating the reliability of IZC-based anchorage systems. These findings support the clinical preference for IZC screw placement in the IZC 7 region using longer screws (12–14 mm) to achieve more favorable biomechanical outcomes during maxillary arch distalization.

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