Investigation of mechanisms leading to early aseptic loosening of hip prostheses. A retrieval study of 27 failed cases of cemented Spectron EF stem in combination with Reflection acetabular cup
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Background: More than 1 million people with arthroplasties undergo total hip arthroplasty (THA) surgery every year worldwide. In Norway more than 10 % of the THA cases needed revision operations due to aseptic loosening. The Spectron EF femoral stem was reported by the Norwegian Arthroplasty Register to have 3.8 times higher relative risk of revision than the Charnley total hip replacement. The aim of this research project was to determine the failure mechanisms responsible for the unexpectedly high rate of aseptic loosening of the Spectron EF hip stem in combination with the Reflection acetabular cup. Methods: In this study 27 cases of failed hip prostheses were collected as part of an on-going retrieval program. The appearance of osteolytic lesions, the alignment of the components and linear penetration in the cups were measured from patient radiographs. The wear of components collected from revision surgery was graded. Tissue biopsies for adverse local chronic inflammation reactions were analyzed histologically. Additionally, wear particle exposure was quantified using different microscopy techniques and energy-dispersive X-ray analysis. Results: The mean wear rate of the Reflection acetabular cup was 0.21 mm/year. The total linear wear was positively correlated with: 1. in vivo time of prosthesis; 2. the number of PE particles in tissue samples; 3. osteolysis area and percentage behind the cup. PE particle count was also positively correlated with osteolysis area. Most of the cups were severely worn (grade 20 to 30) with 6 different wear modes: pitting, scratching, burnishing, abrasion, permanent deformation, embedded particles. Scratching, abrasion and burnishing were observed on the stems (20% to 55%) due to stem-cement micro and macro-motion. Posterior medial and anterior lateral were the most worn parts. Cases with stem loosening accompanied by relative movement within the cement shell had higher total particle count. Tissue samples near the stem also had higher total particle count. Discussion: Mechanical load on the prosthesis during physical activity caused PE particles from wear of cups, cement and metal particles from wear in stem-cement interfaces. Particles migrating along joint space caused third body wear of the implants, which accelerated particle release. Mechanical loosening of the stem led to further release of cement and metal particles. Conventional UHMWPE with low wear resistance was used as the cup material; proximal roughened Spectron EF" was used as the stem. This combination was shown to be prone to causing increased number of different wear particles. Conclusion: In this study of 27 failed cases of cemented Spectron EF stem in combination with Reflection acetabular cup, the loosening of cup was due to particle induced osteolysis in the acetabulum. The predominant cause of this osteolysis was PE particles. Loosening of stem was mainly mechanical, commonly occurring between the cement and stem. Under subsequent loading this led to release of wear particles and osteolysis.