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Summation of the distance between the end of the screw and the apex of the femoral head on AP and lateral radiographs
94%
3766/3992
Distance from the acetabular teardrop to the tip of the screw on an AP radiograph of the hip
2%
64/3992
Multiplication of the distance between the end of the screw and the apex of the femoral head on AP and lateral radiographs
1%
53/3992
Distance from the center of the lesser trochanter to the tip of the screw on an AP hip radiograph
37/3992
Summation of the distance between the tip of the greater trochanter and end of the screw on AP and lateral hip radiographs
48/3992
Select Answer to see Preferred Response
Tip-apex distance (TAD) as it relates to a lag screw in the femoral head is the summation of the distance between the end of the screw and the apex of the femoral head on AP and lateral radiographs, corrected for radiographic magnification of the image. This is shown in Illustration A. TAD is a useful intraoperative indicator of deep and central placement of the lag screw in the femoral head, regardless of whether a nail or a plate is chosen to fix a fracture. A TAD of <25mm has been shown to minimize the risk of fixation cut-out in stable and unstable intertrochanteric hip fractures. Baumgaertner et al. examined factors leading to the failure of sliding hip screws (SHS) in the treatment of 198 intertrochanteric fractures. They determined that the TAD is a reproducible, standard measurement to predict SHS failure. The average TAD for successful fractures was 24mm while the average TAD for failures was 38mm. No screw with a TAD <25mm failed. Geller et al. evaluated the TAD of intramedullary devices as an independent predictor of cut-out failure in intertrochanteric hip fractures. The authors found that there was a statistically significant increase in screw cut-out with TAD > 25mm as well as with increasing fracture severity. They concluded that TAD is a valuable consideration for intramedullary devices as well. Illustrations B and C show a lag screw with an excessive TAD, and subsequent failure of fixation. Incorrect Answers: Answers 2-5: TAD is defined as the sum of the distance between the end of the lag screw and the apex of the femoral head on the AP and lateral radiographs, corrected for radiographic magnification of the image.
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