Hip Resurfacing

Hip Resurfacing

A hip resurfacing procedure is a variation on the concept of the traditional hip replacement. It consists of two components: 1) The acetabular shell and 2) a metal cap placed over the ball (head of the femur). This cap includes a small peg that goes into the neck of the femur rather than the more traditional stem which extends further down into the marrow canal of the femoral shaft. This results in a large diameter ball made of a metal surface, which will rotate inside of a large diameter cup (acetabular component).

Numerous variations of this design have evolved over the last several decades. Currently, the technology allows metal on metal gliding surfaces, with dramatically decreased wear rates compared to previous designs. The original conception of a form of hip resurfacing extends back to the late 1940's. Numerous designs have come and gone involving materials such as Teflon, as well as polyethylene. Various types of fixation have also been tried including cementing of the acetabular component, as well as the bone ingrowth surface. Several designs on the cap shape, and fixation method have also been used. In previous designs polyethylene wear, and inflammatory debride was felt to be one of the contributors to the failure mechanics. This may now be much improved in view of the metal on metal design features. Previous limitations also involved materials and manufacturing with quality control, and understanding of bearing surface mechanics.

This procedure would be most applicable to the "younger" patient. One of the main advantages would be that a revision operation would be easier to do. The cap with a small peg is easier to revise than the shaft of the standard femoral component, which extends into the canal of the femur. In principle, there is less initial bone disruption, preserving the neck of the femur, as well as the marrow of the proximal shaft of the femur. This can allow for a more natural pattern of stress transfer to the bone of proximal femur.

Current materials are chrome cobalt cap that fits onto the reshaped ball of the proximal femur. It is secured with an acrylic cement grout process. On the socket side, the acetabular component has a textured bone ingrowth surface. The inner surface is also chrome cobalt to allow for the metal on metal articulation, which has been shown in laboratory analysis to have dramatically lower wear rates. Due to the larger diameter of surfaces in contact and undergoing rotation, this design does have a higher level of metal ions in the blood and urine than the smaller head articulations. As a result, there is some concern in using this implant in patients that could have any compromised kidney function.

The metal cap that fits onto the femoral head is of a larger diameter then that of a standard hip replacement. This affords advantages in allowing a greater range of motion with better stability of the hip joint with a lower risk for dislocation.

Certain patients will be at higher failure risk for this procedure, and these may be considered relative contraindications:

  1. Previous surgery involving internal fixation hardware or significant changes in the anatomy.
  2. Very heavy patients.
  3. Notable cystic changes within the femoral head or neck.
  4. Osteoporosis.
  5. Congenital deformities.
This procedure is continuing to undergo clinical trials at various centers around the country. There are several concerns with hip resurfacing compared to a standard hip replacement. The blood supply of the neck and ball of the femur, which is compromised to some degree during the normal procedure, can cause loss of support, resulting in loss of fixation of the metal cap. Due to stress accumulation or inherent weakness in the neck there can also be fractures of the femoral neck resulting in failure of this resurfacing replacement. The cap secured onto the reshaped ball of the femur is also cemented in place, which can undergo loosening process. Current design results are in the early term of five to ten years with some promising findings.

Personal opinion now in 2006:
Advantages:
A.) Advantages of the hip resurfacing procedure include a "time buying" procedure with promising results in short to mid-term data.
B.) An easier revision should this fail, to a total hip replacement when compared to revising another hip replacement.
C.) Preservation of more normal bone such of the femoral neck and avoiding violation of the proximal femoral marrow canal.
D.) Decreased dislocation rate and a more desirable mechanics of stress transfer to the proximal femur.

The above stated advantages have minor significance when considered in practical terms.
Disadvantages:
A.) Less predictable longevity of the implants.
B.) More likely need of revision for this implant, rather than well proven metal/metal "total hip".
C.) Location and importance of this extra bone being "saved" is relatively insignificant.
D.) Improved stress transfer biomechanics negligible in bone preservation.
E.) Dislocation risk marginally better compared to the 36 millimeter head size of the "total hip" design.

John G. Mayer, M.D.

-JGM