SUPERIOR 10-YEAR RESULTS WITH THE ROMANUS CUP RINGLOC LINER VERSUS THE HEXLOC LINER IN PRIMARY TOTAL HIP ARTHROPLASTY

Abstract

Introduction: Uncemented total hip arthroplasty (THA) is gaining in popularity. Modern stem designs function well also in the long term perspective. However, on the acetabular side, results have been more discouraging, with excessive wear and focal osteolysis being two major problems, which, though often asymptomatic, are common reasons for revision. The quality of the polyethylene liner, geometry and locking mechanism are often discussed as possible causes.

Since 1990, an uncemented titanium screw-in cup with the same outer design, but with two different types of polyethylene liners, has been used at our department. The aim of this study is to detect any differences between the two types of liners in terms of wear behaviour, focal osteolysis and revisions. We present results after a minimum 10 years of follow-up.

Patients and Methods: All patients undergoing a primary uncemented THA at our institution since 1990, who received a Romanus screw-in cup with a hydroxyapatite and porous coated titanium alloy shell were included in the study. Between 1990 and 1994 a cylindrical Hexlocliner with a snap-fit locking mechanism was used (125 patients, 160 hips) and a hemispherical Ringloc-liner was used between 1995 and 1997 (94 patients, 114 hips). All liners articulated with a 28 mm Cobolt-Chrome head on a uncemented Bi-Metric femoral stem. Ein-Bild-Roentgen-Analyse (EBRA) was used to measure the linear wear rate at the 5 and 10 year postoperative follow-up. Osteolysis was assessed on plain radiographs and verified with computed tomography.

Results: Nine patients were lost to follow-up. There were 17 dislocations (8 Hexloc, 9 Ringloc), of which 5 required revision due to dislocation (2 Hexloc, 3 Ringloc). No sign of aseptic loosening in any of the cups or stems was seen. The 10-year survival rate, with revision for excessive wear and/or osteolysis as endpoint, was 88% for the Hexloc group and 98% for the Ringloc group. Patients undergoing cup revision were significantly younger (p=0.029). The mean linear wear rate was 0.21 mm/year in the Hexloc group and 0.10 mm/year in the Ringloc group (p=0.01). After 10 years, the frequency of osteolysis was significantly higher in the Hexloc group (p< 0.001). Risk factors increasing the risk for revison was age below 53 years at surgery, Charnley class A and a Body Mass Index (BMI) below 25.

Discussion: As has been shown in other studies, the Hexloc liner performed poorly with a high percentage of focal osteolysis, a high wear rate and a low 10-year survival. The Ringloc liner performed better in all these aspects. Possible explanations for this could be the better quality of the polyethylene and the hemispherical geometry of the Ringloc-liner. A more rigid attachment to the shell and thereby less fluid pressure changes may also minimize bone resorption.