gms | German Medical Science

Objectives: In total hip arthroplasty the restoration of hip offset (HO) determines abductor muscle function and is considered essential in order to prevent adverse events such as impingement, dislocation and wear. The objectives of the present study were to (1) evaluate the accuracy and reliability of native acetabular offset measurements performed on conventional supine anterior-posterior (AP) pelvis radiographs with reference to computed tomography (CT) in patients with end-stage hip osteoarthritis (OA); (2) determine the minimum and maximum amount of medialisation of the centre of rotation (COR) simulating different reaming techniques for acetabular preparation; and (3) identify patients at increased risk of excessive medialisation of the COR.

Methods: A consecutive series 131 CT scans and radiographs of patients with end-stage primary hip osteoarthritis was evaluated using validated software for 3D measurements. Acetabular and femoral parameters were evaluated. Bland-Altman plots were used to assess the accuracy of HO measurements on pelvic radiographs compared to CT. We simulated the implantation of a hemispherical press-fit cup comparing anatomical and conventional reaming techniques and evaluated corresponding changes in acetabular offset. A medialization greater than 8mm was considered as clinically relevant. The proportion of patients at risk for under-reconstruction of HO was evaluated with odds ratios and Venn diagrams.

Results and conclusion: Standardised AP pelvis radiographs allowed for an accurate and reliable assessment of acetabular offset compared to CT based measurements. Cup placement in the most lateral position (anatomical reaming technique) resulted in a mean implant-related medialization of 5.9 mm. On average, the most medial aspect of the cup remained 0.1mm lateral to the acetabular floor ("true floor"). However, placing the cup in the most lateral position did not require reaming to the acetabular floor in 64 hips (49%). There was no difference in the number of patients with high FO and Dorr type A femora at high risk for excessive reaming between anatomical reaming and reaming to the outer lamina (n=4). Comparing the anatomical reaming (n=4) to the reaming to the inner lamina (n=15) a significantly higher proportion of patients were at high risk for hip offset underreconstrution (OR 4.1, p=0.01).

The present study quantifies changes of the native COR following cup implantation and highlights the variability of acetabular anatomy in patients with primary hip OA. Acetabular offset is independent of femoral shape or geometry and can be accurately and reliably determined on conventional AP pelvis radiographs. Depending on the preferred reaming technique a substantial number of patients appear at risk for excessive cup medialisation. The present findings support the use of pre-operative assessment and templating in order to improve individual restoration of hip offset in patients with primary hip OA.