Abstract
Purpose of the study: Correct positioning of the prosthetic cup requires reliable anatomic landmarks, particularly for navigation systems. Referring uniquely to the three dimensions fails to recognize interindividual differences in pelvic position. The anterior plane of the pelvis is a good indicator of the pelvic position which can be determined from radiographic measurements. Standard values are poorly known (age, gender, weight). The purpose of this study was to measure the APP radiographically in the upright and reclining positions before and after total hip arthroplasty and to correlate the observed values with those obtained with navigation and ultrasound.
Material and methods: Strictly standardized x-rays of the pelvis in the upright and standing position were obtained in 110 patients (40 men, 70 women, mean age 65 years). Films which did not meet strict standard criteria were removed from the analysis which thus included upright views in 57 patients, reclining view in 36, and upright and reclining views in 28. Navigation measurements were made in 20 patients and ultrasound measurements in 10.
Results: Before arthroplasty, anteversion was 6.42±6.9° in the reclining position, 0.29±7.39° in the upright position (significant difference). After arthroplasty, anteversion was 6.9±5.3° in the upright position and 0.28±5.03 in the reclining position (significant difference). The values ranged from −15° to +18° (three patients without change, four with anteversion). There was no significant difference by gender. There was no clear correlation between the navigation values and those measured on the standard x-rays. The navigation and ultrasound values appeared to be correlated.
Discussion: The anterior pelvic plane can be easily measured on standard x-rays (upright and reclining position). Its landmarks can be easily accessed by navigation enabling the constitution of a reference plane. Several authors have demonstrated the influence of pelvic tilt on the position of the prosthetic cup. Posterior tile produces acetabular anteversion and inversely. The difference between the reclining and upright position is to the order of 6°. There are however variants up to 20° observed in certain patients and which might explain malpositions or instabilities. A cup with correct anteversion in the reclining position may be malpositioned on the upright film because of pelvic tilt.
Conclusion: Pelvic tilt should be taken into consideration when positioning the cup. The anterior pelvic plane can be correctly measured on standard x-rays and used to evaluate this tilt then serve as a reference for navigation. It should be proposed in all patients to search for extreme values.
Correspondence should be addressed to SOFCOT, 56 rue Boissonade, 75014 Paris, France.
