Metallurgy

The First Generation Metal-on-Metal bearings manufactured in the 1950s and 1960s were produced by the investment casting process (Ring and McKee Farrar prostheses). From these devices we have recorded the longest benign clinical history of cobalt chrome alloys with extremely low linear wear rates.

Forensic studies of these successful first generation Metal-on-Metal bearings were conducted to determine the material chemistry, micro-structural condition, bearing clearance, and evidence of the wear mechanism. These implants were typically produced from the investment casting process from high carbon Cobalt Chrome in the As Cast condition. The material contained large block carbides.

The BHR^* is produced using the investment casting process from high carbon cobalt chrome in the As Cast micro-structural condition.

Wear studies have shown that Cobalt Chrome in its As Cast form has superior wear resistance to other forms of the alloy. ^{10, 11, 12}

Heat treating, which includes hot isostatic pressing (HIP), solution heat treatment (HT), wrought forging or sintering modifies the microstructure, reducing the block carbides in both quantity and quality. This directly affects the wear resistance of the metal, as shown in diagram A. ^{13, 14, 15}  

The importance of carbide structure has been demonstrated in independent testing with other devices. A recent publication highlighted the difference in the wear rates of heat treated and As Cast products. The cumulative linear wear rate data showed substantially more wear with the heat treated metallurgy when compared to the As Cast devices. ¹⁶

First generation Metal-on-Metal implant retrieved after 26 years

Diagram A:

Micro-abrasive Wear of Cobalt Chrome Alloys. Reference 15.

Diagram B:

Linear Wear of As Cast device compared to HIP & HT device. Reference 16

Typical Microstructures of First Generation Metal-on-Metal.

This image shows a cross-section micrograph through the articulating surface and shows the coarse primary, block carbide in the Cobalt Chromium matrix.

The BHR^* has a hemispherical cup design with a cast-in porous ingrowth surface called Porocast^*. This ingrowth surface does not require a heat treatment to attach the beads and therefore preserves the carbide structure.