Introduction: One of the main causes for the success of the total hip arthroplasty (THA) is the use of the ultra high molecular weight polyethylene (UHMWPE) as acetabular component, due to the fact that this polymer is very wear resistant and able to counteract with the metallic femoral head[1]. In the contact of the tribological metal-UHMWPE pair, the roughness of both contact surfaces has a strong relationship with the wear mechanism. So the control of the surfaces roughness is very important to avoid accelerated polymeric wear rate.
For the acetabular liner the roughness parameter Ra should be determined with a cut-off of 0.08mm according ISO 7206-2[2], a standard specific for surgery implants, and the maximum Ra value allowed is 2.0 µm. However, a more general existing ISO standard, the ISO 4288[3], indicates the use of a cut-off of 0.8mm when the roughness measuring is realized for surfaces where Ra values in the range of 0.1 to 2.0 µm are expected[4]. It is worth to note that this is the range of Ra values generally found in the acetabular liner of the hip implants.
The aim of the present study is to evaluate the influence of the cut-off used to determine the Ra in the acetabular liner surface of THA implants of different world wide manufactures.
Materials and Methods: The acetabular liners analyzed have internal diameter of 28 mm and external diameter of 52 mm. The measurements were made with a probe of 2 µm, using a measuring speed of 0.5 mm/s and a measuring force of 1.5 mN. The implants were analyzed using a cut-off filter (Lc) of 0.08mm and of 0.8mm with a Gaussian wave filter. It was used a measured length of 0.48mm, and half the first and half of the last cut-off were discarded. Initially were made three cycles of measurement on position 1 as show in Figure 1, and on the four remaining positions were used one cycle of measurement. The calibration was made at 20 ºC and relative humidity at 50%. This procedure was done in four acetabular liners from five different manufacturers, totaling twenty specimens.
Figure 1. Roughness measurement positions.
Results and discussion: Values of Ra of the acetabular liners measured with cut-off 0.8mm and 0.08mm for each manufacturer are shown on Figure 2. For all acetabular liners tested the values of Ra found with cut-off 0.08mm are below the maximum allowed value established by standard. However, the proposed measurement using a cut-off of 0.8mm provided values of Ra well above to those found before. Hence, when using a greater measurement length, the acetabular liner from manufacturer B presented Ra values higher than that allowed by the ISO 7206-2[2].
Figure 2. Measurement of acetabular liner roughness with two different cut-off values.
Conclusion: It was noted that the cut-off 0.08mm might not be the most adequate to check the roughness of hip implants contact surfaces. Therefore, it is important to review the ISO 7206-2[2] to alter the indicated cut-off value from 0.08mm to 0.8mm when measuring acetabular polymeric liners.
References:
[1] A. Wang, A Essner, V. K. Polieni, C. Stark and J. H. Dumbleton, "Lubrification and war of high molecular weight polyethilene in total joint replacements".1998.
[2] ABNT NBR ISO 7206-2, "Implantes para cirurgia — Próteses parcial e total de articulação de quadril Parte 2: Superfícies de articulação feitas de materiais metálico, cerâmico e plástico. Fev. 2012.
[3] ABNT NBR ISO 4288, "Especificações geométricas do produto (GPS) – Rugosidade: Método do Perfil – Regras e procedimentos para avaliação de rugosidade". Dez. 2008.
[4] M. M. Souza, "Estudo da forma e da rugosidade em próteses ortopédicas de quadril". Jul. 2011.