Subsidence is a Predictor for Stem Revision due to loosening or osteolysis

Abstract

Subsidence is a Predictor for Stem Revision due to loosening or osteolysis 247 primary cemented stems with rough surface followed for 2-17 years with Radiostereometric Analysis Master thesis in Medicine By Martin Antonsson MStat, MScCSE Supervisor Johan Kärrholm MD, PhD Co-supervisor MaziarMohaddes MD, PhD The cemented Spectron EF stem, frequently used in Sweden between 1990 and 2010, underwent minor design changes in 1994 and was renamed Spectron EF Primary and smaller sizes were introduced. The length of the stem, previously 15 cm for all sizes, was reduced by steps for each reduction in size. According to data from the Swedish Hip Arthroplasty Register, these design changes were associated with a 3-4 times increased risk of revision due to loosening or osteolysis after 6 years. From 1995 to 2005 the Spectron EF Primary stem was used in several radiostereometric(RSA) studies to evaluate new types of cement and polyethylene. This study uses data from several of studies and aims to evaluate if stem subsidence measured with RSA could be used to predict later revision due to loosening or osteolysis. Ethical approval was received from the regional ethics review board in Gothenburg. Patients and Methods In total 279 hips operated with a Spectron EF Primary stem between 1996 and 2005 were identified. All had previously been included in RSA studies. The stems were supplied with small titanium towers press-fitted into the stem at the shoulder, at the collar and at the tip of the stem, each containing tantalum beads. For measuring purposes, the femoral head center was also included in the stem segment. RSA examinations were performed within one week after operation and further studies were scheduled after 6 months, 1 year, 2, 5, 7, 10, 13, 15 and 17 years. Conventional radiographic examinations were performed at each follow-up occasion past the 6-month follow-up. The inclusion criteria were existing migration data up to at least 2 years.This resulted in 247 hips (161 females, 86 males) available for further analysis. Median age at operation was 62 years (range 29-80) and median follow-up time was 13.9 years (range 2.6- 18.2). Case records and the Swedish Hip Arthroplasty Register database were used to obtain information about revision procedures. At the last follow-up 27 stem had been revised due to mechanical failures related to stem loosening with or without osteolysis. Another 5 were classified as radiographic failures. Results The 10-year stem survival was 93.8 percent (95% CI 89.7% - 96.3%), slightly below 95 percent which would be considered good for cemented stems. 15-year survival decreased to 86.3 percent (95% CI 81.3% – 91.3%). Wilcoxon signed-rank tests show that the stems are continually subsiding even after 12-13 years at a significant rate (p < 0.0005). Each mm of subsidence at 2 years increases the risk of later loosening 5.7 times (p <0.001). Subsidence exceeding the riskvalue 0.15mm at two years increased the risk of later loosening 5.2 times (p<0.001). Small stem size (size 1) increased the risk of later failure 7.8 times compared to stem size 2. For stem sizes 3 or bigger the risk decreased compared to size 2 (HR 0.06, 95% CI 0.004 – 0.80). High offset increased the risk of later loosening 3.2 times (95 % CI 1.5-7.0). Male gender was associated with increased risk of later loosening (HR 6.5 95% CI 3.0 -16). Discussion and Conclusion The reason why seemingly minor design changes of the Spectron stem resulted in substantially higher revision rates due to loosening and osteolysis is not clear but the introduction of smaller sizes seem to explain some of it. According to our study a rather minor increase of the stem subsidence and rotation into retroversion within 2 years increased the risk of revision. Poorer results with the smaller sizes associated with accelerating subsidence past 2 years suggest that debonding of the stem from the cement mantle due to repetitive load occurring more easily after the design changes were made. Thus, it seems probable that a major drawback with the altered design was reduction of the load bearing surface area of the stem facing the cement mantle. Our study indicates that RSA is of value in predicting failure of a cemented stem with rough surface.