The BIRMINGHAM HIP◊ Resurfacing System now has over eighteen years of clinical history and utilizes an as-cast cobalt chrome metal on metal bearing with a highly polished finish. As Cast CoCr components have shown superior wear resistance compared to other forms of the alloy. This is because the As Cast process maintains the block carbides integrated throughout the metal structure. These carbides are harder than the metal substrate and reduce wear, especially at startup.
The BHR is produced using the investment casting process from high carbon cobalt chrome in the As Cast micro-structural condition. 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. 1, 2, 3
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. 4, 5, 6
The importance of carbide structure has been demonstrated in independent testing with other devices. A 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. 7
First generation Metal-on-Metal implant retrieved after 26 years.
Diagram A: Micro-abrasive Wear of Cobalt Chrome Alloys. 6.
Diagram B: Linear Wear of As Cast device compared to HIP & HT device. 7
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.
Clearance is the term used to describe the effective gap between the femoral head and acetabular cup in a Metal-on-Metal bearing. It is calculated by subtracting the radius of the femoral head from the radius of the acetabular cup. This difference in radii is used to describe the gap at the equatorial position on the bearing when the femoral head is in contact with the acetabular cup in a polar orientation. Polar bearings operate with a large apparent contact surface area. However the real contact surface area is very small. It is at this point where the articular surfaces interact creating friction and wear.
What is clearance?
Generation of fluid film
A fluid film is present when the two articulating surfaces are separated by the lubricant. It is the clearance (entrainment) angle and motion which generates the fluid film.
Factors such as bone density, implant position and post- surgery may all affect the ability of the bearing to generate a fluid film. As well as a value of the difference between head and cup radii, clearance can be expressed as a ratio to head diameter. There is an optimal clearance associated with each head diameter. Although low clearances work well in laboratory conditions, there may be an issue in the clinical environment. Factors such as bone density, implant position and post surgery may all effect the ability of the bearing to generate a fluid film. With low clearances, there is reduced tolerance for correct function in less than perfect implantation or patient conditions.
As a Metal-on-Metal bearing is not in continuous motion, it operates in a mixed lubrication regime and its longevity is linked to its ability to generate and sustain a fluid film. Laboratory evidence confirms the BHR generates fluid film lubrication. Small clearances increase friction and may cause micro motion in the cup. This may hamper bony ingrowth resulting in impaired fixation. 8
The Stribeck Curve is a graphical representation of the measured frictional forces occurring in a bearing. From the shape of the curve, deductions can be made concerning the lubrication operating conditions of the bearing.
Results of friction testing of the BHR are shown below in Graph A. The friction tests suggest boundary lubrication pre-testing but at 1 million cycles, a mixed lubrication regime was evident. By 2 million cycles, the classical Stribeck curve had formed indicating a considerable contribution from fluid film, which continued to be evident at 3 million cycles. 9
Stribeck Curve Graph A
Changes in Friction and Lubrication during a 3 Million-cycle weat test on a CoCrMo/CoCrMo Hip Resurfacing Device.
Unsworth, K Vassiliou, APD Elfick, SC Scholes Centre for Biomedical Engineering, University of Durham, England.
It was clear that some of the early McKee/Farrar failures were due to poor manufacturing. In the modern era of metal on metal joints the highest possible technology is employed to achieve near perfect bearings.
Design and Technology: References
1. Ahier S, Ginsburg K. Influence of carbide distribution on the wear and friction of Vitallium. Poc Inst Mech Eng 1966; 181:127-9.
2. Clemow AJT, Daniell BL. The influence of microstructure on the adhesive wear resistance of a Co-Cr-Mo alloy.Wear 1980; 61:219-31.
3. Wang KK, Wang A, Gustavson LJ. Metal-on-Metal wear testing of chrome cobalt alloys. In: Digesi JA, Kennedy RL, Pillar, eds. Cobalt-based alloys for bio-medical applications, ASTM STP 1365: Wear Characterization. West Conshohocken, PA 1999; 135-44.
4. Que L. Effect of heat treatment on the microstructure, hardness and wear resistance of the as-cast and forged Cobalt-chromium implant alloys. Presented at the Symposium on cobalt-based alloys for biomedical application. Nov 3-4, 1998, Norfolk, Virginia, USA.
5. Varano R, Bobyn JD, Medley JB, Yue S. Does alloy heat treatment influence metal-on-metal wear? Poster #1399 presented at the49th Annual meeting of the Orthopaedic Research Society. New Orleans, Los Angeles, USA.
6. J. Cawley, J.E.P Metcalf, A.H. Jones, T.J. Band, A. Skupien, A Tribological Study of Cobalt Chromium Molybdenum Alloys Used in Metal-on-Metal Resurfacing Hip Arthroplasty. Wear, 255 (2003) pp. 999-1006.
7. Nelson K., Dyson J., 'Wear Simulation of a Metal-on-Metal Resurfacing Prosthesis.' AEA Technology Group, Harwell, UK. 1997.
8. McMinn BHR lecture, BOA Manchester 2004.
9. The Effect of “Running-in” on the Tribology and Surface Morphology of Metal-on-Metal hip Resurfacing Device (BHR) in Simulator Studies. (Submitted for publication) JEIM Part H 2 Unsworth et al.