Background: Most of studies suggest the cup inclination angle is controversial. All of the studies previous have not control the prosthesis type and head diameter, it is difficult to determine relative or synergistic effects of cup angle.
Methods: We retrospectively reviewed 54 patients (61 hips) with primary total hip arthroplasties which cup inclination angle bigger than 500 after a mean 11.8 years follow-up. All the prosthesis are noncemented cup with a 28 mm metal head(Trilogy Acetabular Shell, Zimmer). The cup inclination was divided into three groups, 50-550 in 26 hips, 55-600 in 21 hips, and bigger than 600 in 14 hips. An immediate postoperative radiograph was compared with a follow-up radiograph. The polyethylene wear rates and abduction of the acetabular cups was measured in all of the patients.
Results: The preoperative mean Harris hip score improved from 47.36 to 94.3 points at 10 years. The survivorship of the cup was 100% at 10 years. The mean rate of liner wear was 0.144 ± 0.031 mm/y (0.105-0.178 mm/y) in cup inclination angle between 500-550, and 0.260 ± 0.043 mm/y (0.215-0.394 mm/y) in angle between 550-600, 0. 403 ± 0.016 mm/y (0.378-0.423 mm/y) in angle bigger than 600. The different cup inclination groups are different with liner and volumetric wear.
Conclusion: For the metal-on-polyethylene prostheses, the liner wear obvious correlate with cup inclination after angle bigger than 500. It can be concluded that the ideal abduction angle for metal-on-polyethylene prostheses should be less than 55° in hip total replacement.
Keywords: Cementless; Cup inclination; Hip arthroplasty; Linear wear; Metal-on-polyethylene.