Conventional fluoroscopic views are inadequate to predict the Tip Apex Distance and articular clearance of sliding screw devices: A computed tomography-based analysis

Abstract

Tip-apex-distance (TAD) has been widely advocated as the index to predict the risk of screw cut out in hip fractures treated with sliding hip screw devices. The fluoroscopic extents of the femoral head can change with the slightest change in the limb position which includes internal and external rotations, adduction and abduction. These changes can affect the visible TAD and articular-clearance of the screw-tip. The purpose of this Computed-tomography(CT) based analysis is to analyze the variations of the radiographically visible articular-clearance and TAD measurements with limb positioning and to determine the appropriate fluoroscopic projections for different screw-positions within the femoral head. We retrospectively analysed CT studies of twenty healthy proximal femora. Spatial markers simulating screw tips were placed in the different combinations of anteroposterior(AP) and cephalocaudal(CC) positions. Software-based AP and lateral radiographs were developed for each screw position. Additional AP radiographs with femur in internal and external rotation, and lateral radiographs with the femur in adduction and abduction were developed. The variation of TAD and articular-clearance of the screw was measured among these radiographs for individual screw tip positions. Screw tip placed centrally in AP and CC planes position didn't show any significant variation in TAD and articular-clearance with modified AP and lateral radiographs. Significant differences were observed in TAD and articular-clearance values for other screw tip positions. Anteriorly placed screw tips had higher TAD and articular-clearance values with external rotation and similar changes were observed with posteriorly placed tips in internal rotation. Inferiorly placed tips had higher articular-clearance and TAD with limb abduction and similar changes were observed for superiorly placed tips with limb adduction. For the sliding screws placed in non-central locations, the clearance of the screw tip from the articular margins can not be appropriately estimated with conventional AP and lateral views. Additional views with the limb in internal rotation and external rotation in AP view, and adduction-abduction in lateral view are required to safely place the sliding screw in the femoral head. The limb should be brought to a neutral alignment for the accurate estimation of TAD.