Chapter 4 Hip Resurfacing
The concept of hip resurfacing is not new. In the 1970s, Calandruccio had experience with total hip articular replacement by concentric shells (THARIES) that consisted of an all-polyethylene acetabular component and a metal femoral cap. Both components were cemented, but because the femoral component was as large as a normal femoral head, acetabular bone had to be sacrificed. At the same time, the all-polyethylene cup design was very thin and failures were primarily on the acetabular side of the hip. The THARIES was abandoned by the mid-1980s in favor of total hip replacement with smaller femoral heads and thicker acetabular components. Despite these early failures with hip resurfacing, the appeal of a hip more appropriate for younger, more active patients persisted. Also appealing was the concept of bone preservation in the proximal femur that would reconstruct a painful joint and at the same time preserve a patient’s own natural anatomy and biomechanics. Leg length and offset are not altered with resurfacing, and the patient’s own proximal femoral bone is preserved. Additionally, with resurfacing, if a revision, especially of the femoral component, should become necessary, it would be like doing a primary total hip replacement.
Metal-on-metal total hip replacements had been used in Europe for many years, but they were not commonly accepted in the United States. The McKee-Farrar and Ring total hips, with metal-on-metal articulations, were observed for many years in Birmingham, England, by McMinn and Treacy. McMinn combined the theory of hip resurfacing with the technology of metal-on-metal implants to reintroduce the modern hip resurfacing in the 1990s.
Since the problems with early resurfacings were thin polyethylene acetabular components and loss of acetabular bone stock, these problems were eliminated by solid metal ingrowth type acetabular components. To achieve acceptably low rates of wear, attention to maintaining high levels of diamond-like molecules of carbide in the metal was necessary. This requires an exact manufacturing technique that calls for the beads on the back of the acetabular component to be cast into the metal rather than sintered in place as is the usual technique of manufacturing. In addition, wear is lessened by manufacturing tolerances within 2 microns and substantial implant stiffness to prevent deformation of the components during implantation.
The ideal patient for a hip resurfacing is still a matter of debate, but currently we limit hip resurfacing mainly to active men younger than the age of 60 years with osteoarthritis or posttraumatic arthritis. Nearly normal proximal femoral anatomy is needed to provide a satisfactory bony substrate for implant fixation. Hip resurfacing may be more beneficial for male manual laborers who are required to squat, which generally is not allowed after conventional total hip replacement. Also, the well-informed patient whose recreational activities, such as running, make a conventional hip replacement an unacceptable choice may be a good candidate for a hip resurfacing procedure.
We rarely consider resurfacing in women because femoral neck fracture is a greater risk in women, both in the short term after surgery as well as in the long term years later. Smaller femoral head sizes (<50 mm) are associated with elevated serum metal ions, and women may have a higher incidence of metal allergy than men.
Patients with osteonecrosis or large femoral head cysts may or may not be candidates for a resurfacing based on how much intact viable femoral head remains after contouring the head for the resurfacing component. As a rule, three fourths of the femoral head should be intact after contouring the head. There is a greater incidence of failure secondary to femoral component loosening in patients with osteonecrosis.
Because femoral neck fracture has been reported in 1% of the Australian Registry, bone quality should be considered before doing a hip resurfacing; patients with osteoporosis should not be considered candidates for this type of procedure.
In 2010, the results of 12,093 hip resurfacings followed in the Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR) were published. It was reported that at 8 years the cumulative revision rate for all patients and implants was 5.3% compared with 4.0% for total hip replacements. However, in patients with femoral head sizes of 50 mm or more, age younger than 55 years, and osteoarthritis, the revision rate was 3.0%. Patients with hip dysplasia and hip designs other than the Birmingham Hip also had increased rates of revision. In our series of over 150 Birmingham Hips we have had no revisions to date (6-year maximum follow-up).
The issue of metal hypersensitivity and the development of local tissue reactions have discouraged many from pursuing hip resurfacings. To date, we have not seen these reactions in the resurfaced hips in our practice, but we remain watchful for its development. It appears that these reactions may be associated with certain implant designs with which we have no experience. They are more likely to occur with metal-on-metal total hip replacements than resurfacings. With metal-on-metal hip replacements, increased corrosion is seen at the head-to-stem Morris taper than at the inner hip-bearing surface.
Also associated with early failure is acetabular dysplasia. This may well be because the acetabular component is fixed in a more “open” or abducted position in these patients. That position of the acetabulum is known to increase wear on the edge of the acetabular component against the metal head of the femoral component. For this reason, patients with acetabular deficiencies or dysplasia should have resurfacing with a specific “dysplasia” cup placed in a more horizontal position and use of screws (described later) to supplement fixation.
Patients who have had their hips resurfaced rather than replaced have a much more rapid recovery to a much more normal level of activity than patients with traditional hip replacements in our practice. Because the risk of dislocation is negligible (0.3% in the Australian Registry), no abduction pillow is used, and patients do not have to follow strict hip range of motion precautions. The patient is mobilized to low impact activity very rapidly. Most patients return to a higher level of activity by 2 to 3 weeks.
For the first 6 months after surgery, the patient is encouraged to participate in low-impact activities such as walking, swimming, and cycling. This gives the acetabular bone a good chance for fully stable ingrowth and allows the femoral neck to remodel after the trauma of the surgery. After this, patients are released to full activity, including running, soccer, and skiing. They should be informed of course that these activities do carry increased risk of fracture (acute or stress fractures); the risk of injury should be balanced by the reward of activity.
As with all arthroplasty hip surgery, preoperative templating, radiographic evaluation, and planning are critical to success. If a patient’s anatomy is distorted by previous surgery, injury, or deformity, he may not be a candidate for hip resurfacing. The femoral head and neck bone quality should be normal, and there should not be more than 25% to 30% of the head involved with avascular necrosis or cyst formation as seen on radiographs. If the femoral neck is enlarged by remodeling, there may not be a clear delineation between the head and neck, with the head being larger than the neck (Fig. 4-1). If the neck and head are of the same width, especially along the superior neck as seen on an anteroposterior radiograph, then removing bone from the head will risk notching the femoral neck and thus risk neck fracture.
The first step in templating is to measure the size of the femoral component. A template is laid over a radiograph of the proximal femur. The width of the opening of the femoral component should be wider than the femoral neck by 2 to 4 mm total. If not, the next larger template should be used. Then, the center post of the implant is aligned over the center of the femoral neck on radiograph. The line from the top of the greater trochanter to where the line on the template intersects the lateral cortex is measured and documented (Fig. 4-2A). This distance will be used when measuring the valgus angle of the implant intraoperatively (Fig. 4-2B).
Position the patient in the lateral position with the affected hip up. Stabilize the pelvis with a pelvic clamp or pegboard, with the pelvis oriented straight up and down. If the pelvis is leaning forward, the acetabular component may be placed in retroversion; and if it is leaning backward, the acetabular component may be placed in excessive anteversion.
To resurface the hip, extensive exposure is necessary to allow the acetabulum to be visible and later on in the procedure to keep the femoral head visible over its entire surface. Therefore, steps must be taken to achieve exposure not commonly used in total hip replacement surgery. Obviously, the femoral head is removed during a total hip replacement, which greatly aids in exposure.
Make a curved skin incision over the greater trochanter, angling the proximal portion posteriorly, pointing toward the posterior superior iliac spine (Fig. 4-3A). Carry the incision over the center of the greater trochanter and then distally over the shaft of the femur to end over the attachment of the gluteus maximus on the linea aspera.
Divide the subcutaneous tissue in a single plane over the fascia of the gluteus maximus proximally and the fascia of the iliotibial band distally. Make a longitudinal incision over the middle to posterior third of the fascia over the greater trochanter and extend it distally over the femoral shaft. Extend the proximal end of the incision through the thin fascia over the gluteus maximus in the same direction as the skin incision. Bluntly split the fibers of the gluteus maximus muscle, taking care to find and cauterize any bleeding.
Release the tendinous attachment of the gluteus maximus from the linea aspera to maximally internally rotate the femur to provide satisfactory exposure of the proximal femur and femoral head. If the gluteus maximus is not released, the sciatic nerve may be at risk of compression at the time of preparation of the femoral head. Place a hemostat under the gluteus maximus tendon as the tendon is divided to avoid injuring branches of the medial femoral circumflex artery and the first perforating artery. Leave a centimeter of tendon attached to the linea aspera and femoral shaft for later repair.
Retract the gluteus medius muscle and tendon anteriorly. A hooked instrument such as a Hibbs retractor is useful. Under the gluteus medius is the piriformis, which is exposed. Tag the piriformis tendon with suture, and then release it from the femur. Under and anterior to the piriformis tendon are the muscle fibers of the gluteus minimus. With an elevator, raise the gluteus medius off the capsule of the hip completely. The entire capsule of the hip should be exposed superiorly. Use of a narrow cobra retractor is helpful to see this area when it is placed under the gluteus minimus and medius.
Expose the plane distally between the capsule and the short external rotator muscles. Release the short external rotator muscles off the femur including the quadratus femoris distally. Coagulate the vessels in this area.
The capsule of the hip is now completely exposed posteriorly, superiorly, and inferiorly. The lesser trochanter also is visible. With the hip in internal rotation, make an incision in the capsule circumferentially, leaving at least a centimeter of capsule still attached to the femoral neck. This centimeter of capsule is later used to repair the capsule back as well as to provide protection to the intraosseous vessels needed to maintain vascularity of the femoral neck.
Dislocate the femoral head and perform a complete anterior capsulotomy with sharp scissors. The inferior portion of the capsule is seen by extending and internally rotating the femur. The psoas tendon is exposed at the lesser trochanter, and the capsule is isolated just in front of the psoas tendon. While maintaining the scissors just posterior to the psoas tendon, incise the capsule from inferior to superior (Fig. 4-3C). Maintain the femur in internal rotation and apply anterior traction with a bone hook on the lesser trochanter.
Perform the proximal end of the capsulotomy by flexing the femur 90 degrees and maintaining a narrow cobra retractor under the gluteus muscles. Incise the capsule with sharp scissors while internally rotating the femur to beyond 100 degrees. If a complete capsulotomy is not performed, exposure of the femur is compromised.
Measure the femoral neck from superior to inferior, its longest dimension (Fig. 4-3D). The Birmingham Hip comes with heads in 2-mm increments. The measurement tool should loosely fit over the femoral neck to avoid undersizing the femoral component, which could cause notching of the femoral neck. Femoral neck notches may weaken the neck and predispose it to early postoperative fracture. If there is any doubt, choose the next larger size of the femoral head component.
Once the size of the femoral component is known, the acetabular component size also is known because the acetabular component is matched with components either 6 or 8 mm larger than the femoral component. So, if the femoral head measures 52 mm, the acetabular component will need to be either 58 or 60 mm. That means (in this case) the acetabulum will need to be reamed to 57 or 59 mm, respectively.
The key to exposure of the acetabulum is to dislocate the femoral head out of the way anteriorly and superiorly. Create an anterosuperior pouch large enough for the femoral head under the gluteus muscles and above the ilium. This is done by sharply dissecting the soft tissues off the bone of the ilium, including the capsule and tendons of the rectus femoris from the superior acetabular lip and the anterior inferior iliac spine.
Once the pouch has been created, dislocate the femoral head into the pouch under the gluteus muscles and retract it with a sharp, narrow Hohmann retractor driven into the ilium superior to the acetabulum and resting on the femoral neck (Fig. 4-3E). Additional pins may be driven into the ilium and ischium to help with the acetabular exposure. A retractor also is placed inferiorly to expose the transverse acetabular ligament. Sharply excise the labrum.
Ream the acetabulum medially through the cotyloid notch of the acetabulum to the medial wall. Take care not to ream through the medial wall. Once medialized, the reamers are used to increase the bony acetabulum to the desired size. The acetabulum usually is underreamed by 1 mm from the desired component size. Use an acetabular trial to assess the potential component’s stability. The trial components in the Birmingham Hip Resurfacing System are 1 mm smaller than their stated size to provide for tighter fitting of the actual component. Impact the trial into the acetabulum with a mallet, and excise osteophytes for unobstructed cup insertion (Fig. 4-3F and G