The trochanteric double contour is a valuable landmark for assessing femoral offset underestimation on standard radiographs: a retrospective study

Abstract

Background Inaccurate projection on standard pelvic radiographs leads to the underestimation of femoral offset—a critical determinant of postoperative hip function—during total hip arthroplasty (THA) templating. We noted that the posteromedial facet of the greater trochanter and piriformis fossa form a double contour on radiographs, which may be valuable in determining the risk of underestimating femoral offset. We evaluate whether projection errors can be predicted based on the double contour width. Methods Plain anteroposterior (AP) pelvic radiographs and magnetic resonance images (MRIs) of 64 adult hips were evaluated retrospectively. Apparent femoral offset, apparent femoral head diameter and double contour widths were evaluated from the radiographs. X-ray projection errors were estimated by comparison to the true neck length measured on MRIs after calibration to the femoral heads. Multivariate analysis with backward elimination was used to detect associations between the double contour width and radiographic projection errors. Femoral offset underestimation below 10% was considered acceptable for templating. Results The narrowest width of the double line between the femoral neck and piriformis fossa is significantly associated with projection error. When double line widths exceed 5 mm, the risk of projection error greater than 10% is significantly increased compared to narrower double lines, and the acceptability rate for templating drops below 80% (p = 0.02). Conclusion The double contour width is a potential landmark for excluding pelvic AP radiographs unsuitable for THA templating due to inaccurate femoral rotation.

A new understanding of radiographic landmarks of the greater trochanter that indicate correct femoral rotation for measurement of femoral offset

“…In a cadaveric study performed by Weber et al, 35% of radiographic femoral offset measurements exceeded a 5 mm difference from CT measurements, while leg length and global offset were more reliably within this range, at 1% and 15% respectively [ 15 ]. Blumel et al reported findings that showed a projection error greater than 10% when the distance between radiographic double lines exceeded 5 mm [ 7 ]. This consistent underestimation of femoral offset can exert significant effects on patient outcomes following THA, as decreased postoperative femoral offset negatively affects muscle tensioning, joint stability, implant wear, and leg length perception [ 3 – 6 , 16 ].…”

“…Preoperative templating via plain radiography is a standard practice to determine prosthetic size and position based on a patient’s native femoral offset, which is defined as the perpendicular distance from the center of rotation of the femoral head to the long axis of the proximal femoral shaft [ 2 ]. Subsequent prosthetic reproduction of femoral offset in THA is important for achieving maximum abductor force and active range of motion and preventing impingement and dislocation [ 3 – 6 ], and the need to accurately measure femoral offset on plain radiograph has become increasingly recognized in recent years [ 7 ].…”

“…Anteroposterior radiographs of the pelvis may underestimate femoral offset by as much as 13%, yet this view remains standard when templating for THA [ 2 ]. Despite the long history and high frequency of THA, and the associated millions of radiographic measurements, there is plenty of qualitative guidance to internally rotate the extremity and to minimize the appearance of the lesser trochanter on the image exists, but very limited quantitative guidance is available [ 7 , 8 ].

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Contribution of the BioballTM head–neck adapter to the restoration of femoral offset in hip revision arthroplasty with retention of a well-fixed cup and stem

“…All preoperative digital radiographs of the pelvis were taken in an anterior–posterior orientation. Since external rotation of the femur usually leads to underestimation of femoral offset [ 14 ], patients were positioned supine with both hip joints rotated inward by approximately 10–15°. A dual calibration marker ball system (KingMark TM ) was routinely used as a reference for determining the individual magnification factor [ 15 ].…”