Abstract
Introduction:
Current pelvic plane software is not accurate and do not address normal acetabular center axis that has minimum impingement and maximum stability relation with femur in navigated THA
Methods:
A retrospective study of 137 consecutive THA, comparing ACA and APP registration data with postoperative pelvic CT, independently measuring acetabular and cup inclination and version. APP registration points were anterior superior iliac spines and symphysis pubis. Computer identifies preoperative ACA as ‘zero’ after averaging superior, anterior and posterior rim points of registration of acetabulum. Reaming and cup orientation is kept within 4 mm of ACA or within 5 degrees of inclination and anteversion of APP software to avoid impingement.
Results:
Of 134/137 eligible subjects, mean CT acetabular version = 19°(S.D. ± 5.9), ACA = 19° (S.D. ± 7.6), APP = 14.5°(S.D. ± 6.9), and CT Cup version = 22.9°(S.D. ± 9.4), ACA = 23.0°(S.D. ± 8.4) (p = 0.89), and APP = 12.7°(S.D. ± 12.1) (p = 0.0002 denoting APP inferiority to CT). However, CT inclination angles of APP and ACA showed no statistical difference). Reamer and Cup Center was maintained within 2 mm of ACA in 127(95%) cases (mean Harris score (H.S.) at 6 months = 96.14, range 91.2–100). 7 cases were more than 2 mm off center of the acetabulum of which 4 were more than 4 mm (3% with mean HS = 75.98, range 48–85).
Conclusion:
Unlike software that relies on points outside acetabulum, ACA provides both accurate cup orientation and identifying cup center in relation to acetabular center of rotation minimizing impingement in THA. Cups with good inclination and version angles may continue to impinge with lower Harris score because their orientation was more than 2 mm away from the center of acetabular axis. ACA concept will accurately add to our knowledge of the ideal acetabular cup position, helping surgeons to decrease the risk of impingement that results in dislocation, and unexplained pain.
