The influence of hip rotation on femoral offset in plain radiographs

Abstract

Background and purposeAdequate restoration of femoral offset (FO) is critical for successful outcome after hip arthroplasty or fixation of hip fracture. Previous studies have identified that hip rotation influences the projected femoral offset (FOP) on plain anteroposterior (AP) radiographs, but the precise effect of rotation is unknown.Patients and methodsWe developed a novel method of assessing rotation-corrected femoral offset (FORC), tested its clinical application in 222 AP hip radiographs following proximal femoral nailing, and validated it in 25 cases with corresponding computed tomography (CT) scans.ResultsThe mean FORC was 57 (29–93) mm, which differed significantly (p < 0.001) from the mean FOP 49 (22–65) mm and from the mean femoral offset determined by the standard method: 49 (23–66) mm. FORC correlated closely with femoral offset assessed by CT (FOCT); the Spearman correlation coefficient was 0.94 (95% CI: 0.88–0.97). The intraclass correlation coefficient for the assessment of FORC by AP hip radiographs correlating the repeated measurements of 1 observer and of 2 independent blinded observers was 1.0 and 1.0, respectively.InterpretationHip rotation affects the FOP on plain AP radiographs of the hip in a predictable way and should be adequately accounted for.

Evaluation of femoral neck shaft angle on plain radiographs and its clinical implications

“…5 The rotations and version of the femoral neck to the femoral condyles and its projection on the anterior posterior radiograph influences the implant choice during total hip arthroplasty and hence the rotational influences has to be considered using the biplanar radiographs. 6,7 The implants which are used to treat the proximal femur fractures include the Sliding Hip screws, The proximal femoral nail, cannulated cancellous screws, angle blade plates and even hemi replacement with bipolar or unipolar prosthesis etc. Majority of these implants come with different angular options and are manufactured considering the biomechanics and anthropological data and body morphology according to the western world.…”

The Influence of Hip Rotation on Femoral Offset Following Short Stem Total Hip Arthroplasty

“…[6] In reverse, the correction of FO by the correction factor is based on the same formulae and therefore possible imprecision of the measurements is neutralized.…”

“…The necessity to identify hip rotation and correct the FO has been shown in previous publications. [6,8] However, this method has not been applied to ssTHA before. For the correct interpretation of FO in postoperative radiographs, it is necessary to address the effect of hip rotation.…”

“…While the projected femoral offset (FO P ) can be directly measured in calibrated plain radiographs of the hip or pelvis, its projection can be significantly shortened due to hip rotation. [4,6,7] According to Lechler using standard femoral components, the effect of hip rotation on the projected femoral offset and rotation-corrected femoral offset has not been studied following ssTHA. [8,9] Due to the particular design of ssTHA, the long axis of the stem does not necessarily follow the long axis of the femoral shaft resulting in a wide range of acceptable varus or valgus positions.…”

“…Following Lechler et al, [6] the hip rotation can be calculated with the knowledge of the true and the projected NSA (NSA T , NSA P ) of the implant: Hip rotation (°) = arcos (tan (180-NSA P ) / tan (180-NSA T )).…”