On efficacy of implant thread design for bone stimulation

Abstract: Introduction – The mechanism and efficiency of force transfer by dental implants to surrounding biologic tissues are important determinants in the development of the implant-to-bone/tissue interface and implant longevity. Threads are used to improve the initial stability by maximizing bone contact through an enlarged implant surface area and thereby favor distribution of interfacial stresses. However, knowledge about optimal thread design for an enhanced implant integration in bone tissue is still lacking. Aim – The aim of this thesis was to evaluate the efficacy of implant micro thread design when combined with macro threads, for bone stimulation. The hypothesis is that the short threads will contribute with compression forces that may stimulate bone healing, while the larger threads will provide with primary stability necessary during the healing process. A further aim was to use an FEA model to describe the optimal thread form for reduced stress concentration immediately after implant insertion as well as after completed bone healing. Materials and methods – In study 1, Two-dimensional finite element models were made from 8 different thread designs. The crest module and apex of the implants were removed from the implant models, in order to enhance the effect of the thread designs only. Thus, the suprastructures and microstructures of the implants were not considered. All the eight implant models were assumed to be embedded in cortical bone. In addition, a 3D model was used to evaluate stress in the bone generated by 6 different thread designed implants when the implant models included the entire implant. In the In vivo studies 2 and 3, experimental turned implants with a diameter of 4mm and 8mm in length were prepared with micro threads in between macro threads along the body of the implants. These were used as test implants. Implants without micro-threads were used as controls. In study 4, similar implants were made but with alteration in depth of the macro-thread to improve the possibility for bone stimulation by compression during healing. Insertion and removal torque analysis along with histomophometric analyses were done to evaluate the bone response. Results – In study 1, stresses were calculated using von Mises stress analysis. The stress levels in the bone were in the range of 5-13 MPa in osseointegrated model and 14-107 MPa in immediate 2D models. 3D Analysis results showed the von Mises stress in the range of 4.8-30.9 MPa, when a load of 100N was applied vertically. In Study 4 FEA demonstrated stress levels in the range of 0.28 MPa to 62MPa for the control implant model designs, whereas the test implant models displayed a range of 0.28 MPa to 31Mpa. In study 2, the mean values of the ITQ for the control and test groups in the tibia were 15 and 20 Ncm respectively, and in the femur, the values were 11 and 12 Ncm, respectively. In study 4, the ITQ values were 11Ncm and 14 Ncm respectively in the tibia, and in the femur 13 Ncm and 19 Ncm respectively. The RTQ values for the control and test groups in tibia was 11Ncm and 17Ncm, respectively and in the femur, 13Ncm and 23Ncm, respectively. The histomorphometric analysis of study 3, showed the mean total bone area, BA% (SD) to be higher in the test implants, when compared to the control implants in both the tibia 24 (4), and 21(4), the femoral bone 29 (5), and 25 (7), respectively with no statistical significance. In study 4, the total bone area BA% was higher for the test implants with a mean value of 72% compared to 48% for the control group in tibial bone. In femural bone, the bone area was 63% for the test and 38 % for the control group implants with p value of 0.10 for both tibia and femur. Bone to implant contact showed significantly higher value for the test implants in the femur, p= 0.04. Conclusion – The impact of different thread designs, with respect to the magnitude of the transferred stress peak in the bone, was higher for the immediately inserted implants than for the osseointegrated implant model. The stress distribution was more effective in experimental micro-thread implant models, when compared to the non-micro thread models. The addition of pitch shortened threads in the test implant, did significantly improve the primary and secondary stability of the test implants, when mechanically evaluated with ITQ and RTQ analysis in corticular or trabecular bone rabbit bone. Histomophometrical analysis showed that the addition of the pitch shortened threads in between the macro threads did have a bone stimulatory effect in the femur of the rabbits.