BIRMINGHAM HIP^◊

The BIRMINGHAM HIP Resurfacing System now has over ten 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.

1. Metallurgy

2. Clearance

3. Optimal Clearance

4. Surface Finish

Metallurgy

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. ^{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.

Clearance

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.

Optimal Clearance

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 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.

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 millioncycles.

Changes in Friction and Lubrication during a 3 Million-cycle weat test on a CoCrMo/CoCrMo Hip Re-surfacing Device. Unsworth, K Vassiliou, APD Elfick, SC Scholes Centre for Biomedical Engineering, University of Durham, England.

Surface Finish

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.

The typical patient will be physically active, under 60 years of age, and suffering from hip arthritis, hip dysplasia or avascular necrosis of the hip. The implant can be used in patients over 60 whose bone quality is strong enough to support the implant. The BIRMINGHAM HIP Resurfacing System (BHR) is a single use device intended for hybrid fixation: cemented femoral head component and cementless acetabular component. The BHR system is intended for use in patients requiring primary hip resurfacing arthroplasty due to:

• Non-inflammatory arthritis (degenerative joint disease) such as osteoarthritis, traumatic arthritis, avascular necrosis, or dysplasia/DDH

• Inflammatory arthritis such as rheumatoid arthritis.

Contraindications
• Patients with infection or sepsis
• Patients who are skeletally immature
• Patients with any vascular insufficiency, muscular atrophy, or neuromuscular disease severe enough to compromise implant stability or postoperative recovery
• Patients with bone stock inadequate to support the device including:

•Patients with severe osteopenia or with a family history of severe osteoporosis or severe osteopenia
•Patients with osteonecrosis or avascular necrosis (AVN) with >50% involvement of the femoral head (regardless of FICAT Grade)
•Patients with multiple cysts of the femoral head (>1cm)

Note: In cases of questionable bone stock, a DEXA scan may be necessary to assess inadequate bone stock• Females of child-bearing age due to unknown effect on the fetus of metal ion release
• Patients with known moderate to severe renal insufficiency
• Patients who are immunosuppressed with diseases such as AIDS or persons receiving high doses of corticosteroids
• Patients who are severely overweight
• Patients with known or suspected metal sensitivity (e.g., jewelry)

For more information, don't forget to download the BHR Surgical Technique .

1. Is the BIRMINGHAM HIP Resurfacing implant clinically proven?

The BIRMINGHAM HIP  Resurfacing implant is not a new implant or technique. It has been in use worldwide since 1997, and the US Food and Drug Administration reviewed a tremendous amount of resulting clinical data before approving it for use in this country.

2. Who is a candidate for the BIRMINGHAM HIP Resurfacing System?

The typical patient will be physically active, under 60 years of age, and suffering from hip arthritis, hip dysplasia or avascular necrosis of the hip. The implant can be used in patients over 60 whose bone quality is strong enough to support the implant.

3. How long will the BIRMINGHAM HIP Resurfacing implant last?

It is impossible to say how long the implant will last because so many factors play into the lifespan of an implant. In the case of resurfacing, for instance, the metal-on-metal bearing surfaces of the new joint may extend its life longer than that of a traditional total hip replacement, but failure to comply with the physical rehabilitation regime may cause the implant to fail within months. A clinical study showed the BIRMINGHAM HIP ^◊ Resurfacing implant had a survivorship of 95.4% at the 10 year mark, which is comparable with the survivorship of a traditional total hip replacement in the under-60 age group, and 98.6% are pleased to extremely satisfied with BHR. ¹

1.  Minimum 10 Year Outcome of Birmingham Hip Resurfacing (BHR)

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