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Study finds wide variation in modular femoral heads

Different degrees of roundness and surface finish seen between components of various materials and manufacturers.

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BIRMINGHAM, U.K. — An engineering study has found significant variability between modular femoral prosthetic heads made of different materials and by various device manufacturers.

Lee A. David, FRCS(Orth), of the Royal National Orthopaedic Hospital in Stanmore, England, presented the data to the 2005 British Orthopaedic Association’s Annual Congress, here. The investigation evaluated more than 100 brand-new heads using established quality control equipment. The heads were made of ceramic, cobalt-chrome and stainless steel.

“There is wide variability between heads of different materials and manufacturers, which may have important clinical and cost implications,” David told Orthopedics Today.

The differences seen were noted between “materials, between manufacturers of the same material and between heads of the same material and the same manufacturer within the same batch,” he said.

Quality control testing

The experiments were conducted at the Centre for Biomedical Engineering, University College London at the Royal National Orthopaedic Hospital. The femoral heads were supplied by the manufacturers free of charge. Each company provided six heads of each material, all of which were 28 mm in diameter, David said.

The researchers performed tests using custom made jigs and Telerond and Telesurf machines, which utilize a touch-sensitive probe under clean room conditions. Data were collected using computerized programs already in use for quality control testing.

“Mean roundness was calculated from three random measurements of each head through the equator and the apex,” David said. The equator was deemed the widest part of the head and the apex the proximal pole, he noted.

The researchers similarly calculated smoothness and expressed the results in terms of Ra (the arithmetic mean of absolute departures of the roughness profile from a mean line), Rz (the maximum deviation of the profile) and Rsm (the mean spacing between departures).

“Overall there were more than 7500 measurements, which took a long time,” he said. “We found variability not just between materials, but also within materials. Rather worryingly, there was variability even within batches.”

Overall, ceramic heads were superior in terms of both roundness and surface finish. And cobalt-chrome components were superior to stainless steel in all parameters, David said.

“There were no correlations between the cost of the heads and their quality in the parameters that we measured,” he noted. “This wide variability between heads may have important clinical and cost implications.”

For your information:

• David LA, Hilton AI, Back DL, et al. An analysis of the roundness and surface finish of modular prosthetic heads. Presented at the 2005 Annual Congress of the British Orthopaedic Association. Sept. 22, 2005. Birmingham, U.K.