gms | German Medical Science

Objectives: Longevity of hip prostheses is essential for total hip arthroplasty (THA). In the 1990s, the main reason for long-term failure of THA was polyethylene wear-related complication. At end of the last century (i.e., cross-linked polyethylene was not yet on clinical use), to solve the problem, hard on hard bearing couples, such as metal on metal (MoM) or ceramic on ceramic (CoC), was improved with an expectation to reduce polyethylene wear-related complications, also to improve the longevity. At same time, the use of MoM bearing surface was started with Bicon-Plus cup and SL-Plus stem in our institute. Time passed, many studies of low-carbide (LC) MoM have been reported its long-term results were pessimistic due to adverse reaction of metal debris (ARMD) and osteolysis. The aim of this study is to evaluate the long-term result of MoM bearing couple comparing LC and high-carbide (HC) metal alloy in dysplastic hips.

Methods: In all hips, all components are common (Bicon-Plus Cup and SL-PLUS Stem) except the metal composition of bearing. Thus the difference of the result may show the difference of the metal composition. These THAs are composed of two groups; 392 THAs with LC (320 patients, M:F=32:288), operated since April 1997 till September 2003, and 74 HC THAs (64 patients, M:F=5:59), operated October 2003 to November 2006. The diameter of the femoral head was 28 mm in all hips (in those days only 28mm small heads were available in Japan).

The mean follow-up period was 14.5(0.1-21.0) years in LC, 10.9(0.2-15.0) years in HC, and the mean age at primary surgery was 52.2 (28-75) in LC, 54.5 (40-69) in HC, respectively. Modified Watson-Jones approach was used in all case.

Results and conclusion: In LC, 47 hips in 43 patients were revised at average 10.6 (1.1-19.5) years after the initial surgery. The reasons for revision were osteolysis in 20 hips, ARMD in 16 hips, recurrent dislocation in 5, infection in 3, periprosthetic fracture in 1, breakage of the acetabular shell in 1 and thigh pain due to excessive leg-length extension in 1, respectively. In contrast only one HC hip needed to be revised 6.5 years after initial operation due to the postoperative thigh pain following excessive leg-length correction. The survival rates with removal of any component for any reason at the endpoint were 79.1% at 21 years (95% confidence interval [CI], 71% to 87%) in LC, 98.4% at 15 years (95% CI, 95% to 100%) in HC, respectively(p=0.046).

Our study showed disappointing result in LC similar to other articles. While good survivorship in HC, even in young and active dysplastic hips, we abandoned the use of the threaded cup and MoM bearing surface at the end of 2006. Failures are still at risk even in LC well-functioning in situ. Thus careful and frequent follow-up is essential. Furthermore, prophylactic conversion of the bearing couple to CoC when the implants remain stable may be a good option to prevent progression of ARMD or osteolysis.