How The TAD Will Affect The Stabilization Of Intertrochanteric Fractures Fixed By The DHS: A Biomechanical Study

Objective: DHS is widely used in intertrochanteric fractures, and is still classical surgical methods.Postoperative complications such as coxa vara,hip screws loose,and cut out which can affect the fracture healing or abnormal healing,will lead to changes in hip joint mechanics,causing pain,limited mobility,and so on. In order to avoid the occurrence of complications, numerous scholars have been doing continuous efforts.In 1995, Baumgaertner proposed the concept that the tip-apex distance (TAD), he noted that the tip-apex distance is the sum of the distance from the tip of the lag screw to the apex of the femoral head on an anteroposterior radiograph and this distance on a lateral radiograph, after controlling for magnification. When the TAD is less than 25mm the failure rate can significantly be reduced. In our article, A Biomechanical Study was made in order to test the validity of this theory,when the bone quality, fracture type,the extent of fracture reduction is controlled.Method: The study was performed with 4 matched pairs ofembalmed human cadaveric proximal femurs obtained from 3 males and 1 females aged 60–75 (67) years who had no known history of malignant tumors.A hip disarticulation was made in order to get the femur, and then get rid of the soft tissues.To make sure The length of the femur was 30 mm. All of these specimens were visually examined and then X-rayed, CT examination to exclude the diseases such as obvious tuberculosis, tumor and other pathologic abnormality.With the help of the image intensifier,we put the guide-pin in the femoral head and make it centrally in the lateral view and central or inferior in the Ap view before the screw has been drilled and inserted.Good anteropostenior radiograph are required.The length of the guide-pin in the proximal femur is measured. The model of intertrochanteric fracture (Evans I) was established, with the fracture line perpendicular to the long axis of Femoral neck.Reduction the frcrure and using 135°- dynamic hip screw to fix the fracture.Make sure that the length of the screw is appropriate and pointing to the apex of the femoral head.4 holes lateral plate was used.The study has used one-leg stance model and applied axial loading through the femoral head to simulate weight bearing. The femurs were loaded vertically in the CSS-44020 vitodynamics machine and the displacements of femoral head will be measured. At the first time we inster the screw 75mm long in the femur neak .The pitch of the hip screw is 3mm, so we inster the screw 3mm more each time.So the screw will be inster 75mm, 78mm, 81mm, 84mm ... ... until the tip of the screw is very close to surface of articular.Each time the pressure will be start at 0N gradually increasing at a rate of 20N/S. At the pressure of 600N,the displacement was recorded.During the testing procedure, the specimens were kept moist with saline solution to decrease the organism denaturation owing to the test, so that the experimental results were precise and commeasurable. The dynamic hip screw should be adjusted before each measurement in order to ensure that the femul completely reset.After each test,let the specimen revert to the initial state by taking a 5 minutes rest in order to eliminate the effect because of the creep. To input the data and carry on statistics analysis with spss13.0 (one way ANOVA).Results:There was a direct relationship between an decreased tip-apex distance and an decreased distance of the fracture.The beast TAD is 28-23mm.Compared with those the TAD is more than 28mm the difference is significant.Compared with those the TAD is less than 23mm there is no ddifference.Conclusion: When the TAD became smaller, the implants give more stability to the fracture.when theTAD less than a certain value, in other words, when all the hip screw threads beyond the ward triangle,the displacement of fracture is the smallest.Instering the screw deeper,the displacement is no longer decreases, and when the hip screw is located under the articular surface of the femoral head,it will increase the risk of the screw cutting out of the femoral head.