Biomechanical Effects of Implant-Root Fragment Contact in the Socket Shield Technique: A Finite Element Analysis

Background: The socket shield technique (SST) is widely applied in implant treatments in aesthetic zones, demonstrating significant advantages in preserving hard and soft tissues. Although SST was developed to maintain alveolar bone and gingival architecture by retaining a buccal root fragment, its biomechanical implications - particularly under direct implant-fragment contact - and the optimal geometric parameters of the shield remain poorly defined. Objective: This study utilized finite element analysis to evaluate the biomechanical behaviour of direct implant-root fragment contact in SST. Methods: Three-dimensional finite element models of a maxillary anterior socket shield implant were constructed from cone beam computed tomography data. The residual root fragment was modelled with a uniform thickness of 1.5 mm, a length of 6 mm, and labial arc angles of 60°, 90°, 120°, 150°, and 180° at the alveolar crest level. An insertion torque of 30 N·cm and an occlusal load of 100 N were applied to simulate clinical loading conditions. Stress distribution, peak stress, and displacement in the residual root, periodontal ligament (PDL), implant, and surrounding bone interface were analysed by Abaqus software. Results: Under insertion torque of 30 N·cm, the peak stress and displacement of the fragment and PDL decreased as the arc angle of the tooth fragment increased. The peak stress for a 60° fragment was 101.9 MPa, which decreased significantly to 75.48 MPa for a 90° fragment, and further to 69.62 MPa, 67.14 MPa, and 42.43 MPa for 120°, 150°, and 180° fragments, respectively. The stress concentration was located at the fragment area corresponding to the implant neck's maximum diameter. The PDL's peak stress decreased from 4.073 to 1.067 MPa, and fragment displacement decreased from 0.01651 to 0.007814 mm. Under a 100 N occlusal force, as the arc angle increased, the peak stress in the PDL and cortical bone increased, while the peak stress in the fragment and implant decreased. The PDL's peak stress was lowest (1.143 MPa) for a 60° fragment and highest (1.996 MPa) for a 180° fragment, both below the natural tooth's PDL peak stress of 2.472 MPa. Cortical bone stress peaked at 49.74 MPa for a 60° fragment, decreased for 90° and 120° fragments (47.77 and 47.62 MPa, respectively), and then increased significantly for 150° and 180° fragments (50.83 and 64.51 MPa). Conclusion: Within the limitations of this static finite element model, direct contact between the implant and a shield fragment with an arc angle of 120° to 150° appears to provide a favourable biomechanical environment. However, in vivo loading is cyclic and may result in long-term fatigue effects on living tissues and biomaterials that were not investigated in this study. Clinical studies are needed to validate these findings.