Restoration of the hip center during THA performed for protrusio acetabuli is associated with better implant survival

Abstract

Background: Acetabular protrusio is an uncommon finding in hip arthritis. Several reconstructive approaches have been used; however the best approach remains undefined.

Questions/purposes: Our purposes in this study were to (1) describe the THA survivorship for protrusio as a function of the acetabular component, (2) evaluate survivorship of the cup as a function of restoration of radiographic hip mechanics and offset, and (3) report the long-term clinical results.

Methods: One hundred twenty-seven patients (162 hips) undergoing primary THA with acetabular protrusio were retrospectively reviewed. The mean age of the patients at surgery was 66±13 years, and the mean followup was 10±6 years (range, 2-25 years).The cup fixation was uncemented in 107 (83 with bone graft) and cemented in 55 hips (14 with bone graft). Preoperative and postoperative radiographs were reviewed for restoration of hip mechanics and offset.

Results: The THA survival from aseptic cup revision at 15 years was 89% (95% CI, 75%-96%) for uncemented compared with 85% (95% CI, 68%-94%) for cemented cups. The risk of aseptic cup revision significantly increased by 24% (hazards ratio, 1.24; 95% CI, 1.02-1.5) for every 1 mm medial or lateral distance away from the native hip center of rotation to the prosthetic head center. Harris hip scores were improved by mean of 27±20 points (n=123) with a higher postoperative score for uncemented bone grafted compared with solely cemented cups (81±16 versus 71±20 points).

Conclusions: Restoring hip center of rotation using an uncemented cup with or without bone graft was associated with increased durability in our series. There was a 24% increase in the risk of aseptic cup revision for every 1 mm medial or lateral distance away from the native hip center to the prosthetic head center.

Level of evidence: Level III, therapeutic study. See Guidelines for Authors for a complete description of levels of evidence.

Fig. 1A–B

(A) Preoperative and (B) postoperative pelvic AP radiographs are shown for a patient with bilateral idiopathic protrusio acetabuli. The patient had a right THA with bone autograft for the acetabular reconstruction. The two horizontal lines shown in Illustration A mark the height of the pelvis. One-fifth of this distance determines the approximate height of the acetabulum (AB line). On the right side in Illustration A, point A is located 5 mm lateral to the intersection of Kohler’s line (KL) and Shenton’s line (SL). On the left side a similar point is marked A. The perpendicular line is drawn through point A, and point B is marked at one-fifth the height of the pelvis. The second perpendicular line then is drawn laterally from point B to a distance equal to AB. This marks point C. By joining points A and C, the isosceles triangle is completed. The midpoint of the AC line is the native hip center of rotation or the approximate femur head center which represents the reference point (0, 0). The femur offset is represented with a perpendicular line on the femur axis (FA) to the center of rotation (femur head center preoperatively and prosthetic femur head center postoperatively).

Fig. 2A–B

(A) A Kaplan-Meier survival curve for 162 THAs using aseptic cup revision for any reason as an end point is shown. The dashed lines represent the 95% CI. (B) A Kaplan-Meier survival curve for THA using aseptic cup revision for any reason as an end point based on etiology of protrusio, cup fixation, and uncemented cup design generation is shown.

Fig. 3A–B

(A) Preoperative femur head center of rotation and (B) postoperative prosthetic femur head center of rotation are shown relative to the native hip center of rotation (0, 0) based on the Ranawat triangle method. *Preoperative and postoperative centers of rotation for the revised cups.

Fig. 4A–C

(A) AP and (B) oblique view radiographs of the left hip of a 65-year old woman with idiopathic protrusio acetabuli are shown. The patient underwent a left THA using uncemented cup fixation with bone autograft for the acetabular reconstruction. (C) She had satisfactory radiographic 10-year followup.