Patient-Specific Restoration of Biomechanics in Total Hip Arthroplasty.

Restoration of biomechanics is crucial in total hip arthroplasty, in order to ensure a free postoperative range of motion for activities of daily life. Failure may result in impingement and thus instability, limping and finally patient dissatisfaction. In daily practice, preoperative templating is usually performed on magnification-corrected, plain radiographs of the hip. Due to the divergence of the X-ray beams and the position of the pelvis in relation to the film, measurements on plain radiographs are susceptible to error. Whereas leg length measurements are highly accurate, femoral offset is frequently underestimated on plain radiographs. The benchmark for acceptable postoperative leg length and offset differences is still under debate in the literature. However, discrepancies of 1 cm in relation to the non-affected, contralateral side should not be exceeded. Gait analysis is able to detect even differences in leg length and offset above 5 mm due to altered gait kinematics. For intraoperative control of leg length and offset, a variety of different methods have been described in the literature. Beside alignment guides, rulers and other tools, intraoperative fluoroscopy and computer-guided navigation allow the surgeon to intraoperatively analyse leg length and offset changes. Both techniques show high accuracy, although the precision is higher for navigation with a reduced number of outliers. For measurement of stem torsion, computed tomography is the gold standard. However, stem torsion can also be evaluated on plain hip radiographs by measuring the projected prosthetic neck-stem angle. Native femoral anteversion can be assessed by the size of the lesser trochanter. Since the version of the femoral stem can highly vary from the native femoral anteversion, the Isthmus Ratio (iRatio) - the relationship between canal isthmus and calcar isthmus in the anteroposterior and Lauenstein view - helps to predict the torsion of the final intraoperative stem.