최소직경의 임플란트와 다양한 직경의 미니임플란트간의 압축강도 비교

Abstract

[한글]

임플란트(endoosseous implant)가 다양해지면서 직경이 3.0mm이하의 임플란트를 “미니임플란트”로 분류하고 있다. 초기의 미니임플란트는 임플란트를 저작압으로부터 보호하여 골유착이 성공적으로 일어나게 3-6개월동안 임시적으로 사용되어져왔다. 하지만, 최근 미니임플란트도 임플란트와 유사한 골유착을 보인다고 보고되고, 골폭이 좁은 치아상실부에서 영구적 목적으로 사용된 임상증례들이 발표되면서 이에 대한 관심이 높아지고 있다.

본 연구의 목적은 최소직경의 임플란트와 다양한 직경의 미니임플란트간의 압축강도의 비교실험과 30°, 60°, 90°의 다른 하중방향에 따른 압축강도의 비교실험을 통해 미니임플란트가 영구적 목적으로 이용가능한지 알아보고, 이의 사용시 고려해야 할 사항을 제시하고자 함이다.

본 연구의 실험결과 3.0mm이하의 모든 직경의 미니임플란트에서 3.3mm 직경의 임플란트보다 낮은 압축강도가 나타났다. 이는 연조직의 공간을 확보하고 보철의 용이성을 위해 좁혀진 임플란트-지대주 연결부가 원인으로 사료된다. 또한, 미니임플란트에 30°이상의 각도에서 하중을 가하였을 때에는 평균 저작력보다 낮은 하중에서 변형이 일어났다.

미니임플란트의 식립시에는 최소직경의 임플란트보다도 강도가 낮은 것을 고려하여 구치부에서 사용하는 것보다는 저작력이 낮은 전치부에서의 사용이 추천되며, .가능한 저작력의 방향에 일치하게 미니임플란트를 식립하여야 한다.

[영문]

Introduction

When MI was introduced, it was used to protect the conventional implant from loading during the healing period. The MI provided fixed temporary prosthesis for the patient. It was called as transitional implant because it is removed after 3 - 6 months. Nowadays, the paradigm of MI is being shifted from the transitional purpose to long-term usage. MI has been used in partially edentulous area with narrow mesio-distal space where conventional implant cannot be placed. Some authors reported that bone-to-implant contact ratio of MI is similar to the conventional implant and MI has high survival rate in the mandibular anterior area. Unfortunately, the report about the mechanical strength MI is very rare. Only one report compared the mechanical properties of two prosthetic mini-implants with 1.8mm diameter. Current MIs has diameter 2mm, or greater to support biting force for long term usages. We compared the ultimate strength of conventional narrow diameter implant(NI) and mini-implants with various diameters. The ultimate strength of 2.5mm diameter mini-implant was compared at different loading angles.

Materials & Methods

- Comparison of ultimate strength of NI and MI

We compared the ultimate strength of conventional narrow implant(NI) and mini-implants with various diameters using cantilever Beam Method. NI (USII, 13 X 3.3mm, Osstem Implant, Korea, Seoul) is made of Cp-Ti type IV. MI (EZI, Warantec Co., Korea, Seoul) is made of Ti-alloy. Each MI is 13mm in length. MI is 2.0, 2.5, 3.0, 3.5mm diameter and has the solid body. We tested 7 NIs and 7 MIs for each diameter to measure the ultimate strength. Prefabricated cement-type abutment was connected to the NI with prosthetic screw. Friction-type abutment was connected to the MI by malleting. Each specimen was horizontally fixed to a holder 11 mm from the tip and a perpendicular load was added 1 mm from the tip.

- Comparison of ultimate strength of NI at various loading directions

Twenty one 2.5mm diameter MI(EZI, Warantec Co., Korea, Seoul) were tested for comparing the ultimate strength at different loading directions. Seven abutment-to-implant experiment samples were fixated to the testing instrument (4201, Instron, USA) at 30, 60, 90 degrees from the vertical direction. Each testing samples had a force applied at 11mm from the fixation point, at speed of 5.0mm/min, until the permanent deformation occurred.

Results

- Comparison of ultimate strength of NI and MI

The mean maximum load to bend 3.3mm NI was 254.8±8.9N. The mean maximum load for 2.0mm, 2.5mm, 3.0mm, 3.5mm MI was 68.1±6.0N, 135.7±4.9N, 223.1±10.2N, 293.5±28.8N respectively. In case of NIs, all failures occurred at the prosthetic screw by fracture. In case of MIs, all failures occurred at the neck portion of the fixture by bending. Only for 3.5mm MIs, friction-type abutment was disconnected from the fixture.

- Comparison of ultimate strength of NI at various loading directions

The ultimate strength of MI at 30° was 532.2±51.7N. The ultimate strength of MI at 60° was 156.7±12.6, and at 90° was 117.3±6.1N. All friction-type abutments were not disconnected from the fixture. The ultimate strength of MI loading angle at 30° showed 4.5times higher than that at 90°.

Discussion

MI has been used to avoid the load to the conventional implant during osseointegration period for 3 - 6 months. It can provide the fixed provisional crown to the patient for the healing period. Also, it can protect the guided bone graft site and be used for radiographic stent. For those days, the diameter of MI was 1.8mm, and made of pure Cp-Ti. MI was one-body implant because it couldn't have the screw hole inside due to the narrow body.

Kanie et al compared the mechanical properties of 2 mini-implants using bending test. One mini-implant is MTI implant made of Cp-Ti and the other is the MDI implant made of Ti-alloy. The diameter of each was 1.8mm. They reported that MDI made of TI-alloy has the higher strength than the MTI made of Pure Ti and recommended MDI if priority is given to the intensity. Froum et al reported that histological data of bone-implant contact of immediately loaded mini-implant that have functioned for more than 6 months. A total of 49 mini-implants were studied. The average BIC rate is similar to that of conventional machined implant reported by other studies. Recently, they reported 1 - 3 years survival rates of implant and prosthesis when mini-implant was loaded immediately for overdenture. The survival rate is 94.1% for implant and 100% for prosthesis. Based on these studies, MI can be used for permanent purpose in the space narrower than 6.3mm where conventional implant cannot be placed. MI has evolved to withstand the biting force for long-term usages. The diameter has become larger from 1.8mm to 2.0 and 2.5mm. The surface has changed from the machined to the rough. Recently some of MI have abutment system to make the prosthetic procedure easier and staged-approach possible when the primary stability is inadequate. The MI used in this study has the abutment system that is connected to the fixture with friction force.

We measured the maximum load that broke the complex of fixture and abutment. The maximum load of 3.3mm NI complex was lower than that of 3.5mm MI but higher than 3.0mm, 2.5mm and 2.0mm MI. The portion of prosthetic screw failed in the complex of NI, however MI failed at the neck portion by bending without the abutment disconnection. It is speculated that the weakest portion of the 3.3mm NI is the prosthetic screw and that of the NI is the neck portion.

The maximum load to bend the 2.5mm NI at 30°angle is 532.2±51.7N and 3.5 and 4.5 times greater than at 60°, and at 90°.

Conclusion

The results of this study are that

1. Mini-implant having the diameter less than 3.0mm is weaker than conventional narrow implant when they are loaded connecting the abutment.

2. The weakest portion of the mini-implant is the neck and that of conventional narrow implant is the prosthetic screw.

3. The ultimate strength of 2.5mm MI at 30° loading angle is higher than that of 60° and 90°. But, it was lower than average posterior biting force of male.

On the base of this results, we concluded that mini-implants should be placed parallel to the occlusal force direction since they are weaker than the small diameter conventional implant and they should be indicated for anterior area rather than posterior area.