On late complications with HA coated hip arthroplasties

Abstract: 100 hips in 86 patients were operated with a total hip arthroplasty entirely coated with a 155±35 µm thick HA coating, and included in a clinical and radiological follow-up study. The female/male ratio was 75/25, and the mean age was 56 years (32-73). Early clinical results were excellent, especially pain relief. No thigh pain was found. Radiologically, bone formation adjacent to the components as well as periprosthetic bone remodelling developed in all hips. Radiologically defined direct bone-implant contact averaged 94% or more in all components at five years. Proximal lines developed along 82 of 93 stems at seven to nine years. Five radiologically well-integrated and painless cups in five females loosened with symptoms starting 3.8 to 5.5 years postoperatively. Macroscopically, almost no HA was left on the removed components. Accelerating polyethylene wear was found, necessitating reoperations of 18 hips after 5.2 to 8.3 years, whereas six hips were pending for revision. Radiological osteolysis was found in 66 hips. In seven of 17 hips reoperated for polyethylene wear without wear-through of the inlay, metalosis of the soft tissue was found. Matting of the stainless steel heads was seen. All reoperations revealed disappearance of HA coating from the surfaces of the stems that were uncovered with bone. Twenty hips with the same stem were reoperated after 2.0 to 7.5 (mean 5.5) years because of polyethylene wear (10), acetabular loosening (7), instability (2), and infection (1). Biopsies including bone and soft tissue were obtained from the proximal femur adjacent to the medial or anterior aspect of the stem. Ten well fixed HA coated cups were removed with adjacent tissue at reoperations after 0.3 to 5.8 (mean 3.3) years because of infection (5), polyethylene wear (3), and instability (2). Histological examination demonstrated that fixation of the implants took place by bone formation directly on the HA coating, coalescing with original bone. The threads as well as the corresponding outfolded mirror images contained about 50% bone. The metal interface of the cups and the calcar biopsies contained just more than 50% HA. In the cups, the resulting metal-tissue interface included significantly more soft tissue than bone, and the total implant-tissue interface was nearly equally shared between bone and soft tissue. Great variations were seen. The mean thicknesses of the remaining HA coating in the calcar biopsies, an unused cup, and the retrieved cups were 148 mm, 112 µm, and 97 µm, respectively. HA degradation was seen in all hips, typically resulting in the disintegration of complete segments of the coating, apparently being replaced by soft tissue and bone. Multinucleated giant cells and a great number of macrophages were seen, the latter often containing HA remnants. In the calcar biopsies, osteoid-like tissue, bone and soft tissue were observed between the surface and the HA coating. Bone resorption associated with the presence of metal and polyethylene particles undermined the HA coat, resulting in the liberation of large flakes of HA with free access to the joint. The study indicated that HA particles released from coating adjacent to the joint caused third body wear in the articulation.

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