Study On Influencing Factors Of Screw Stability

Fracture is one of the most common acute diseases in bone surgery,which always caused by car accident,movement impact and high altitude fall.Once the fracture occurs,effective treatment must be done immediately.Internal fixation is the main method of the treatment of fracture.Bone screw plays an important role in internal fixation surgery,the stability of screw at early and late stage after implantation will affect the clinical therapeutic effect.Despite surgical technological advances,screws failure is not uncommen,which will do harm to the patient,also it brings much mental pressure to the surgeon.Therefore,it is necessary to understand and analysis the factors affecting screw stability,which can provide an effective method to reduce the risk of screw failure,also it is of great value for improving the people's health and quality of life.The hold stability of the screw is influenced by multiple factors,this study concentrate on the most important factors which has a strong influence to the stability of bone screw,the bone mineral density,and the surgeon ' s insertion technique such inserting angle,polit-hole size and screw geometry.Simulation experiment,simulation and 3D printing technology were used in the research.Medical twist drill,medical thin tooth deep thread tap and medical pendulum saw were used in the operation.Screw stability test uses stainless steel deep thread screw with diameter 4.0mm.The test materials involved artificial bone materials,animal bones,and human bones.Six-axis manipulator assisted platforms were used to simulate clinical procedures including bone drilling,tapping,and placement of bone screws.Observing the quality of the bone thread after tapping by the bone drilling machine,measure the depth and cross section of the bone thread,and propose a new evaluation index of the screw stability;observing the failure mode of the screw extraction process,including the bone hole extrusion injury and the inner wall of the bone hole The tape of crushing failure and fracture type of thescrew holes was studied.The standard pullout strength test of the screw was used to analyze the final comprehensive evaluation of the screw's immediate stability.Integrate the above research content and comprehensively analyze the key factors affecting the stability of the thread.The main conclusion of this paper is as follows:(1)The trend of the pullout strength curve of low bone density polyurethane materials is very different from that of medium density polyurethane,artificial bone and human bones.The experimental results obtained with low density polyurethane as the experimental material have little significance.The trend of the medium-density polyurethane pullout curve is similar to that of the corpse,which can be used as an effective bone substitute experimental material.The destructive form of the bovine bone material screw is similar to human bone when it is pulled out.It can be used as a substitute material for ideal pullout force test.The artificial bone material maintains good consistency with the human bone from the shape to the mechanical properties and is the most ideal bone substitute material.(2)Bone density is an important factor affecting the stability of the screw.As the minera density increases,the pullout strength increases gradually.Low bone density increases the risk of screw loosening;Cortical bone provides the main mechanical load for the screw.When the screws are arranged in the harder bony layer with thicker average thickness,it can obtain high pullout strength and improve the fastening effect of the screw steel plate system.(3)The ideal screw mechanics load model is the vertical placement of the screw.When the screw is placed at an angle,it is recommended to use a smaller insertion angle as much as possible,at the expense of a small amount of pullout force to reduce the damage caused by screw loosening.Placing screw at large angles will increase the risk of secondary fractures at the bone interface which caused by compression failure of the bone.(4)As the diameter of the pilot hole decreases,the screw pullout strength increases,but as the diameter of the pilot hole decreases,it cause more damage to the bone during the insertion process,which results in the phenomenon of "extrusion" of pull-out force,that is,bone structure failure of the secondary fracture occurs at the bone interface before the screw achieved the maximum pullout strength.Measurement of the three-dimensional shape of the bone thread can be recommended as a new method for evaluating the quality of the bone thread.(5)The pullout strength is affected by the TSF factor of the screw and the shape of the tooth.With the increase of the TSF factor,the pullout strength increases obviously.The HA light-screw screw can provide the maximum pull-out force in the system screw,and the HB deep-thread screw has the highest risk of fracture failure under the same load.The area of thread stress concentration of the HD asymmetrical screw is the smallest,and the stress distribution of the thread is the most uniform.when selecting the screw tooth profile,HA light thread screws are the best recommended.(6)3D printing technology can provide a quick solution for optimization of bone screw structure selection.Selection of medical 3D printing insertion materials for screw stability test can predict the stability of different structural screws and can reduce cost for customized screws...