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.
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.
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.
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
Conventional total hip replacement has failed the young active adult. The Birmingham Hip Resurfacing addresses this difficult patient group.
We have shown that the femoral head remains viable after hip resurfacing.
Femoral neck fracture is a rare occurrence and in our experience has only occurred in the presence of poor bone quality.
Femoral head fixation with cement has been shown by us and others to be extremely durable.
We have seen early and extensive ingrowth into the hydroxyapatite coated Porocast™ socket.
It has been shown that thick film lubrication is possible in these large headed metal on metal hip resurfacing arthroplasty bearings.
We have observed no wear on retrieved Birmingham Hip Resurfacing bearings.
We have measured low blood metal ion levels in a highly active group of patients fitted with the Birmingham Hip Resurfacing.
We have measured low frictional torque in these Birmingham Hip Resurfacing bearings on a realistic loading pendulum apparatus.
In 1,839 metal metal hip resurfacings performed between Feb 1991 and Dec 1999 no dislocations have occurred.
We have treated a young active patient group with arthritic hips when they needed their treatment most, allowing them to return to work.
Patients have achieved excellent functional results after this procedure with many participating in recreational sport allowing the health gain associated with activity.
In this young and active population we have seen in our 1,720 hybrid hip resurfacings in the Birmingham series an overall 99% success rate and in our longest followups we have a 98% survivorship at 5 – 6 years.
The BIRMINGHAM HIP™ Resurfacing (BHR™) has demonstrated exceptional clinical results worldwide. High survival rates of 98% or better were achieved in clinical centres around the world. 1,2,3,4,5 In addition, the recently published Australian Orthopaedic Association National Joint Replacement Registry reports BHR as having the lowest revisions per 100 observed ‘component’ years when comparing all resurfacing implants used in the country. 6 Other clinical studies have focused on predictive measurements to project long-term survivorship of the resurfaced femoral head. Researchers at the University of Oxford, England used roentgen stereophotogrammetric analysis (RSA) to measure the stability of the femoral head. At 24 months, the total three-dimensional migration of the head was not statistically significant at 0.2mm.Previous studies have shown that implants that loosen quickly have rapid early migration. According to the authors, these results suggest the BHR femoral component is an inherently stable device predicting a good long-term performance. 7,8 It is widely accepted that the Bone Mineral Density (BMD) of the proximal femur generally decreases after cementless THA using standard designs of femoral components. However, BMD studies conducted at Osaka University, Japan reported the post-operative BMD in the proximal femur was significantly greater in patients treated with the BHR system compared to the conventional system. The patients treated with the BHR system demonstrated preservation of the BMD in Gruen zone 1 and an increase in zone 7. These results suggest that transfer of load to the proximal femur was more normal after surface replacement with the BHR system. These findings also show the BHR system preserves the bone stock of the proximal femur after surgery. 9
Survivorship: Results Comparable Globally
Follow Up (months)
Shimmin et al 1
Ebied et al 2
De Smet et al 3
6 – 42
Treacey et al 4
McMinn et al 5
Australian Joint Registry. Published Figures
Australian Nation Joint Registry Annual Report 2006 (Clinical Results)6 : Resurfacing Hip systems requiring revision.
Observed Component Years
Revisions per 100 Observed Component Years
Additional Radiographic Studies
Glyn-Jones et al 7
Femoral migration < 0.2mm at 2 Years
Itayem et al 8
Vertical migration femoral component < 0.01mm at 5 Years Vertical migration acetabular component < 0.03mm at 5 Years
Kishida et al 9
BMD preserved / improved in proximal femur at 2 years