STUDY OF THE HYDROXYAPATITE (HA)-COATED ACETABULAR CUPS RETRIEVED AT REVISION SURGERY — RELATION OF THE RESIDUAL HA TO THE FAILURE ODES

Abstract

Fifty-three retrieved hydroxyapatite (HA)-coated acetabular cups were studied with scanning electron microscope (SEM) for identification of the residual HA. Image processing technique was applied for quantification of the residual HA as percentage of the cup surface. The study included 28 Landos Atoll, 21 Osteonic Omnifit and 4 Howmedica ABG cups. These cups were retrieved at an average of 59.1 (13–107) months after implantation due to aseptic loosening in 24, septic loosening in 14, infection without loosening in 7, polyethylene wear in 7 and osteolysis in 1 cup. The relationships of residual HA to original coating thickness, failure mode, duration in the host and other factors were studied with Pearson's correlation, linear regression and ANOVA tests for its statistical significance.The residual HA is negatively correlated with the duration in the host (r = -0.375, p < 0.01 for all cups, r = -0.732, p < 0.001 for loose Atoll cups and r = -0.592, p < 0.01 for the loose Omnifit cups). The thicker coating (155 um, Atoll) had more residual HA than the thinner (50 um, Omnifit) coating (p < 0.001). The loose cups had less residual HA than the stable cups (p < 0.001). The loss of HA coating during the first 2 years averages 52% and 22% for the loose and stable Atoll cups, respectively. Thereafter, HA coating disappears at a rate of 7.4 ± 2.6% and 3.8 ± 1.8% per year. And the loss of HA surface from the Omnifit and ABG cups during the first 2 years averages 69% and 42% for the loose and stable cups, respectively. Then the following loss rates are 10.1 ± 3.8% and 7.1 ± 2.9% of surface per year, respectively.The HA coating is resorbed in the host. The resorption rate is higher on the loose cups than on the stable cups. A hemispherical smooth surfaced cup depends solely on screws for resistance of rotational torque after HA resorption. Clinical loosening and migration of the cup then follow screw loosening or failure. The design of HA-coated acetabular cup should avoid smooth surfaced hemispheric metal shell without a more dependable fixation than screws.