How to best measure femoral length and lateralisation after total hip arthroplasty on antero-posterior pelvic radiographs.

Abstract

Various methods exist for measuring limb length and lateralisation after total hip arthroplasty. Most of them utilise standard anteroposterior (AP) pelvic radiographs, but their results can be affected by patient position during imaging and thus the position of the lower limb on the coronal plane. The aim of this study is to evaluate how commonly used measuring methods of limb lengthening and femoral offset are affected by the position of the lower limb in the coronal plane. A standing pelvic AP radiograph post implantation of a right total hip prosthesis was digitised. The right femur and its femoral stem were digitally segmented, such that they could be positioned orthogonal to the pelvis horizontal reference, with 10° of adduction, and with 10° of abduction, with respect to the centre of rotation. Various limb lengths and implant lateralisation were also digitised. We obtained nine x-rays differing one to three variables. Twice four independent surgeons performed three femoral length measurement methods and femoral offset measurement methods. Intra and inter-observer error as well as the effect of the femoral position on the measurements were studied. With respect to length measurements, the distance between the centre of rotation (C) and the tip of the lesser trochanter (LT) increased by 3mm per cm of lateralisation. This measurement was not affected by the hip position in abduction or adduction. The distance between the tip of the lesser or greater trochanter (GT) and the horizontal passing through the centre of rotation was strongly affected by the hip position in abduction or adduction. With respect to offset, the distance between the centre of rotation and the greater trochanter (C-GT) was the most consistent and was not affected by variations in lengths or femoral axis. At the level of the lesser trochanter, the distance of the femoral anatomical axis and to Perkin's line was heavily influenced by the femoral position. The C-LT distance was consistent in measuring limb length and the C-GT distance was reliable in determining femoral offset regardless of the relative position of the femur.