Abstract: BACKGROUND: During the restoration of ultrashort implants, different restorative methods and occlusal forces in different directions affect the internal structure of the implant and the distribution of stresses at the implant-bone interface, which determine the longevity of the implant and the stability of the surrounding bone level.
OBJECTIVE: To analyze the effects of different restorative methods on the implant and surrounding bone tissue after implantation of ultrashort implants in the first and second molar areas with severe alveolar bone resorption (insufficient distance from the mandibular nerve).  
METHODS: Cone beam CT images of a patient with implant restoration in the first and second mandibular molar areas were selected from Department of Stomatology, Second Affiliated Hospital of Xinjiang Medical University. This patient met the Class II bone quality classification of the jaw. Two Φ5 mm×5 mm ultra-short implants were used. Based on cone beam CT imaging data, a three-dimensional model of the mandible was established. A joint-crown restoration model or two single-crown restoration models for the implants in the lower first and second molar areas were established. A force perpendicular to the long axis of the dentition with the long axis of the dentition was applied in the first and second molar areas at 45° obliquely (the points of action were located on the fossa of the dentition and buccal occlusal half surface, respectively) with a loading force of 150 N on the points of the first and second molars respectively. The stress distribution in the internal structures and bone tissues of the implant was analyzed using finite element software. 
RESULTS AND CONCLUSION: (1) When the vertical load was applied, using single-crown restoration, the stress of the crown was concentrated at the connection between the inner wall of the crown and the abutment, and the peak stress of each component was concentrated on the central screw. When using the joint-crown restoration, the peak stress of the crown was concentrated near the gingival margin of the joint-crown connector, and the peak stress of each component was concentrated on the joint crown. The peak stress of each component of single-crown restoration was higher than that of joint-crown restoration, and the peak stress of bone tissue was also significantly higher than that of joint-crown restoration. (2) When the oblique load was applied, using single-crown restoration, the stress concentration area of the crown was located at the connection between the inner wall of the crown and the abutment, and the peak stress of each component was concentrated on the central screw. When using the joint-crown restoration, the peak stress of the crown was concentrated near the gingival margin of the joint-crown connector, and the stress distribution of the crown, central screw, abutment, and ultrashort implant was relatively concentrated. The peak stress of a single-crown prosthesis and its components was significantly lower than that of the joint-crown prosthesis, and the peak stress of the central screw, ultrashort implant and abutment was larger when the joint-crown prosthesis was performed. (3) There was no significant difference in stress distribution between the two kinds of loads when the single-crown restoration was used. When using the joint-crown restoration, the stress of the implant prosthesis was mainly distributed on the connector in the middle of the joint crown after applying two kinds of loads, and the peak stress of each component was small and uniform when applying the vertical load. (4) If only the bone stress distribution of the ultrashort implant is considered, two different repair methods can be selected. If the long-term retention rate of the ultrashort implant is considered, the single-crown restoration can be selected to avoid mechanical complications.

Key words: oral implant, ultrashort implant, bone insufficiency, single-crown restoration, joint-crown restoration, stress, biomechanics, three-dimensional finite element