William Kurtz, M.D. - Joint Replacement Surgeon
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Hip Resurfacing

Introduction

Hip ResurfacingA hip resurfacing involves replacing the just the top of the femoral head and the socket of the pelvis bone with metal parts that then act as a new hip joint. The acetabular component is "press fit" to your pelvic bone and the femoral component is cemented to your femoral bone. The parts are made of cobalt chrome. Multiple orthopedic companies have hip resurfacing  implants ((Birmingham / Smith & Nephew), (Cormet / Stryker), (Conserve / Wright), (ASR / Depuy), (Durom / Zimmer), (Recap / Biomet)), but currently only the Birmingham and Cormet are approved by the FDA.

Hip resurfacing is a good option for some patients, but not all patients.  The Australian joint replacement registry has the largest collection of hip resurfacing patients and their data suggest that hip resurfacing in younger men is as safe as a traditional hip replacement. Hip resurfacing in older women is significantly riskier than a traditional hip replacement. These facts are not debatable, but what is debatable is the indications for hip resurfacing for patients in-between younger men and older women.

First, we will discuss the good. My hip resurfacing patients are some of my happiness patients. They feel like their hip is normal, and they can do remarkable activities. However, these younger patients would also have done exceptionally well with a traditional hip replacement in my mind.  The greatest advantage of hip resurfacing is the preservation of the proximal femoral bone. Also, hip resurfacing patient may have a more nature feel to their hip because the patient's body weight is transmitted through the native femoral neck instead of through the metal component into the femoral shaft.

Next, we will discuss the bad. Hip resurfacing is not new. It was tried in the 70's and 80's and was generally regarded as a failure and abandoned.  The hip resurfacing of today is now a metal on metal bearing surface, and not the metal on polyethylene that was tried in the 80's. However, some of the biomechanical issues remain. Hip resurfacing has additional risk over a traditional hip replacement including femoral neck fracture and a higher rate of femoral component loosening.

The purpose of the following information is to explain to patients the major aspects of hip resurfacing surgery, inform them of its major risks, and hopefully help them make a well informed decision about their joint disease.  If after reading this discussion, you would like more information or would like to be evaluated for a hip resurfacing surgery, please a schedule an appointment.

History of Hip Resurfacing

Hip resurfacing was developed in the late 1970s. Many articles in the 1980's showed poor outcomes with hip resurfacing and most surgeons abandoned the procedure. The first generation hip resurfacing procedures failed largely because of the large metal femoral head rubbed on the plastic (polyethylene) socket and worn it out. The components loosened and femoral neck fractures occurred.  Advocates of today's hip resurfacing state that these problems are fixed with improved metal on metal surfaces.

Biomechanical concerns of Hip Resurfacing

Hip resurfacing has a serious biomechanical disadvantage, namely a small head-neck ratio. Orthopedic surgeons have realized for years that increasing the head to neck ratio improves the range of motion (ROM) of a total hip replacement. Not as much attention has been given to the head neck ratio in hip resurfacing though. The typical response from a hip resurfacing advocate is that the resurfacing just replaces exactly what it takes off, but for reasons discussed below, I do not think that is accurate.

Unfortunately, understanding my concern with the head-neck ratio in hip resurfacing requires a little background information and a technical discussion. The typical head size in a total hip replacement is anywhere from a 28 to a 36 mm for traditional metal on poly and can be as big as 50 mm with large metal on metal heads. The neck size for most femoral implants is between 12 and 14 mm. The head-neck ratio is therefore between 2 (28/14) and 4 (48/12) with a hip replacement. All hip resurfacing femoral components on the market have a difference between the head diameter and neck opening diameter of 6 to 9 mm.  The larger sizes typically have a 9 mm difference and the smaller sizes have a 6 mm difference. If a small lady has a 46 mm head diameter, her hip resurfacing would likely have a 46 mm head diameter, a 40 mm neck diameter, and a 1.15 (46/40) head-neck ratio. If a large man has a 58 head diameter, his resurfacing head would likely have a 58 mm head diameter, a 49 mm neck diameter, and a 1.18 head-neck ratio. If the surgeon selects a femoral component slightly larger than the native femoral head, then the neck opening diameter would be larger than with the actual femoral neck diameter. In other words, there would be a step off where the femoral component stopped and the femoral neck started, and the smaller femoral bone diameter would improve the head neck ratio. Our femoral bone has to have a larger diameter to support our body weight than a metal prosthesis. Because the femoral bone is larger, it gets in the way of hip motion.

But what does this mean for the range of motion before the components and bones start to hit one another? The equation for determining the functional range of motion of the hip joint is easy to determine for known geometrical shapes like in hip resurfacing and hip replacement. Dr. Yoshimine published this equation in Journal of Biomechanics in 2002.

Biomechanical

The A variable is the hemispherical nature of the opening which is 180° for a hemispherical acetabular component in hip replacement.  Hip resurfacing components are less than a hemisphere and the A variable is simply 360° minus the acetabular sector angle. The acetabular sector angle is 159° for the Birmingham, 165° for the ASR and Durom and 168° for the conserve. The A variable for hip resurfacing is 192° to 201° for resurfacing components. The important understanding of this equation is that as the head-neck ratio approaches one (n=r), then the right side of the equations approaches 180° and the functional range of motion, θ, approaches zero for a hemispherical cup and 12-21° for a resurfacing cup.

Functional ROM in total hip replacement/resurfacing

Using the above formulas (anyone can put the numbers into a calculator), the functional ROM for any given head and neck sizes are listed below.

Components Head Size Neck Size Head-Neck Ratio A variable Functional ROM
Hip resurfacing 50 mm 44 mm 1.14 192° 69°
Typical total hip 28 mm 12 mm 2.33 180° 128°
Large MoM total hip 50 mm 12 mm 4.16 192° 164°

A picture is typically more informative than numbers and words.

Metal THA Hip Resurfacing
Large Metal on Metal Traditional THA with 28 head Hip Resurfacing

This decreased motion before impingement is more evident when the picture includes the femoral bone.

Hip Resurfacing Hip Resurfacing Hip Resurfacing
Degrees of flexion/extension
in hip resurfacing
Degrees of abduction/
adduction in hip resurfacing
Degrees of internal
and external Rotation in
hip resurfacing

One additional factor influences the head neck ratio in hip resurfacing, the femoral component height. Sometimes, a surgeon will attempt to lengthen a patient's leg during hip resurfacing by removing less bone off the top of the femur and placing the femoral component higher or more proud. When the surgeon raises the femoral component, he/she inadvertently raises the femoral head/neck junction. The femoral neck diameter increases as you move up the femoral neck until it fads into the femoral head. Therefore, if a surgeon raises the femoral component, he/she is ensuring an increased neck diameter and  a sub-optimal head/neck ratio.

Femoral Femoral
Raised femoral component with a larger
neck diameter and decreased head/neck ratio
Lowered femoral component with a smaller
neck diameter and increased head/neck ratio

The above information originally seemed to exaggerated to believe. After all, is not a hip resurfacing just replacing what is already there?  Yes and no. To answer this question, you have to understand what happens in normal hip flexion. The normal hip does have a poor head-neck ratio, but there are two things in its favor. First, the normal hip has excellent stability; the large femoral head and the conformity of the socket prevents dislocation.  Therefore, when the anterior femoral neck hits the acetabular rim with hip flexion, the hip does not dislocate, but instead the pelvis flexes and allows the leg to continue to flex. This femoral neck impinging on the acetabular rim occurs at 100° in normal hips and 90°in femoral acetabular impingement. Obviously, additional leg flexion is possible when the pelvis also flexes. In other words, impingement happens all the time in the native hip, but the hip joint is well accustomed to handle the impingement (i.e. the soft labrum cushions the blow)  Second, the acetabular bone has a particular shape to the anterior wall that allows more motion before the femoral neck hits the acetabular rim.

So, is a hip resurfacing just like a native hip? I think there are many similarities and some differences. First, both the native hip and the hip resurfacing have excellent stability, so when impingement occurs with hip resurfacing, the pelvis flexes and the leg continues to flex. However, a main difference between the native hip and a hip resurfacing is this impingement. In the native hip, a soft labrum cushions the femoral neck as it impinges of the relatively soft acetabular rim. In a hip resurfacing, the femoral neck impinges of the metal rim of the acetabular component. I feel that the repetitive hard impact is the main cause of femoral neck fractures. Currently, most surgeons feel that "notching" the femoral neck weakens the femoral bone and causes of femoral neck fracture. On the other hand, "notching" the femoral neck effectively diminishes the difference between the head and neck diameter, lowers the already poor head-neck ratio further, and increases the occurrence of this impingement. Retained bone cement at the head neck junction can also effective diminish the head/neck ratio and cause impingement and possibly femoral component loosening.

Interestingly, patients with a native hip joint and femoral acetabular impingement (i.e. alpha angle of Notzli > 50°) had a head-neck ratio of <1.15 in their diseased femoral bone. (Beaule PE, et al. JBJS 2007 Jan, 89B(1)) In other words, hip resurfacing has a similar head-neck ratio as femoral acetabular impingement. For these reasons, I feel that along the anterior femoral neck, the surgeon must ensure that there is some additional room or decrease in neck diameter where the femoral component stops and the neck starts; in other words, the surgeon must oversize the femoral component to improve the head neck ratio as much as possible.

The other main difference between the native hip and a hip resurfacing is the shape of the acetabular rim. The hip resurfacing does not have the anterior rim contour of the native acetabular rim and therefore impingement occurs sooner with flexion.

Acetabular component position is important in traditional hip replacement for instability reasons; however, acetabular component position is important in hip resurfacing for impingement reasons. Unfortunately, it is extremely difficult to tell the acetabular component position on a x-ray in a hip resurfacing because of the overlap of the large femoral head.

In short, impingement naturally occurs in the both the native hip and hip resurfacing. The femoral neck bone diameter must be big enough to support our body weight and therefore the head-neck ratio suffers in both the native hip and hip resurfacing. Both the native hip and hip resurfacing have excellent stability, so that when impingement occurs, the pelvis flexes instead of the hip dislocating. The recurrent impingement in hip resurfacing due to poorly positioned acetabular or femoral component, might cause femoral neck fractures and/or femoral component loosening.

It should be noted that many studies have shown good outcomes with hip resurfacing, and the clinical implications of the above biomechanical concerns is uncertain. It is possible that a patient's pelvis may flex and extend enough to allow the hip to move in all of the necessary directions and that the poor head neck ratio is unimportant in hip resurfacing.

Pros of Hip Resurfacing

  • Proximal femoral bone preserved
  • Any subsequent femoral revision would be easier
  • Large metal head offers excellent hip stability, but this is available in a traditional hip replacement also
  • Patients with large femoral offset can keep their femoral offset
  • Proprioceptive benefits - it feels more like a normal hip

Cons of Hip Resurfacing

  • Large soft tissue dissection - more muscles cut
  • More difficult to change leg length or offset
  • Increased impingement as described above
  • Possibility of femoral neck fracture
  • Acetabular component size linked to femoral neck size

Patient Selection

So who is a good candidate for a hip resurfacing? The best candidate is a large male with large femoral offset and health bone. The Surface Arthroplasty index was designed by Beaule to identify the patients that had the greatest risk of failure. Patients who should consider avoiding hip resurfacing are people smaller than 82 kg (they have a smaller femoral head), people with previous hip surgery, patients with femoral cyst, and people with activity levels greater 6 on the UCLA activity scale. Obviously, these criteria are conservative, and many patients outside of this criteria do very well with hip resurfacing.

Patient Selection

Beaule PE, Dorey FJ, LeDuff MJ, Gruen T, Amstutz HC. Risk factors affecting outcome of metal on metal surface arthroplasty of the hip. Clin Orthop 2004;418:87-93.

Surgical Approach

Hip resurfacing is performed through a lateral (trans-gluteal) or posterior (Kocher) approach, although other approaches are possible.

Posterior Approach

The posterior approach (Kocher-Langenbock) has the advantage of not injuring the abductor muscles and the dissection can be extended in case more access to the femur or pelvis is necessary. The posterior approach is probably the most popular approach for a total hip replacement used today.  The disadvantage of the posterior approach is that the posterior capsule and muscles are cut during the approach. They are typically repaired at the end of the case which helps prevent dislocations, but the posterior approach does have a higher dislocation rate than the other approaches. Most surgeons limit the patient’s motion after surgery with a posterior approach to prevent any compromising leg positions that might cause a hip dislocation. Because the abductor muscles are spared, most patients have historically had the lowest rate of limp with the posterior approach.

With regards to hip resurfacing, some physicians are concerned about damaging the blood supply to the remaining femoral head with the posterior approach because the major blood vessels (lateral epiphyseal branch of the medial femoral circumflex artery) to the femoral head are cut during a posterior approach.  Some physicians feel that the femoral component loosening is a result of the decreased blood supply. In addition, both the anterior and posterior capsule is often cut and removed during a hip resurfacing in order to get the femur out of the way to fix the socket. The gluteus maximum tendon is usually cut during this approach as well.   The typical skin incision is typically 2-3 times larger for a hip resurfacing (> 9 inches) than for a hip replacement (3 inches).

Bearing Surface options:

The bearing surfaces for hip resurfacing are only available in metal-on-metal. 

Metal

Polished metal bearing surfaces have the advantage of good wear properties and the ability to make thin implants.  Metal bearing surfaces therefore come in the largest head sizes. The use of larger head sizes increases the stability. Metal bearing surfaces increase the blood levels of metal ions and have been reported to cause a metal hypersensitivity reaction in select patients. Currently, women who are contemplating child birth and patients with renal failure should probably not have metal bearing surfaces. The increased metal ions in the blood are excreted in the urine and have not been shown to increase cancer risks over the past 30 years.

Risks of a Hip Resurfacing

  • Femoral Neck Fracture: For reasons discussed above, hip resurfacing replacements have a risk of a femoral neck fracture. This could occur because blood supply to the femoral bone is injured during surgery or because of the repetitive impact of the femoral neck bone against the metal acetabular component. If a femoral neck fracture occurred, then the femoral component would have to be replaced to a total hip femoral component. The published rates of femoral neck fracture are 1 to 4% over 5 years
     
  • Poor Bone Growth: Occasional the bone will not grow into the implanted components. The components may loosen and change position. The motion of the loose component may cause pain and require another surgery to revise the components. Earlier hip resurfacing implants had a relatively high incidence of acetabular and femoral loosen likely from the femoral component impacting the acetabular component and loosening it. The acetabular component can not receive any screws, so the initial press fit fixation is imperative
     
  • Blood Clots: Blood clots in your leg veins are possible after any surgery on the lower extremities. The occurrence of blood clots can be minimized with blood thinners, foot pumps, and early mobilization. The main danger of blood clots is if they dislodge and travel to your veins in your lungs. This phenomenon is called a pulmonary embolus and can result in respiratory difficulty, chest pain, or even death. Blood clots may or may not hurt or cause swelling in your leg and can occur anywhere in either leg. If you have unexplained pain or swelling in your legs, let Dr. Kurtz know as he may order a duplex ultrasound to look for a blood clot. If you feel chest pain or breathing difficulties, you should call 911 and then call Dr. Kurtz. The risk of these clots causing death has been drastically reduced, and is less than 0.1%. The treatment for a proven blood clot is additional blood thinners, and occasionally a filter in your veins
     
  • Implant Dislocation: The ball of the new hip joint may become dislodged from the socket. The risk of this occurring can be less with hip resurfacing but has occurred. If your hip resurfacing dislocates, Dr. Kurtz would manipulate your leg with sedation to place the ball back in the socket. An unstable hip resurfacing could need to be revised to correct this condition if it keeps occurring
     
  • Infections: Antibiotics are given before and after surgery to decrease the risk of infection, but an infection still can occur immediately or even years after the surgery. It is usually treated with another surgery to remove infected tissue and often the prosthesis as well. If the components are removed, a revision hip prosthesis can sometimes be inserted months later if the infection clears, but sometimes the patient is left without a hip joint. Although patients rarely have life threatening problems from their joint infection, an infection is a devastating complication
     
  • Leg Length Inequality: The length of your legs usually is within 1 centimeter after surgery, but it is difficult to adjust the leg length with a hip resurfacing. If the unequal lengths are bothersome, a lift can be built or inserted into the shoe of your shorter leg
     
  • Nerve Damage: Although extremely rare, nerves to your leg and feet, or the nerve to your thigh can be damaged by the surgery. These nerves may or may not recover by themselves. If they do not, you may need a brace for your ankle or for your knee to walk, and your walking ability could be limited
     
  • Blood Vessels Damage: Rarely, the blood vessels around the hip are damaged by the surgery and excessive bleeding occurs after or during the surgery, requiring either surgery or special procedures under X-ray control to correct the problem. Occasionally, blood gathers in the wound even if no major blood vessel is damaged and further surgery (or observation) is required to correct the problem
     
  • Limping: The limp that most people have before the surgery usually persists until the muscles become stronger after surgery. It sometimes never goes away, and sometimes the surgery creates a new limp. Most people, however, note that the way they walk is greatly improved by the surgery
     
  • Pelvic Fracture: The femur or pelvis can crack when preparing the bone for insertion of the components, actually inserting the components, or even years after the surgery. Fractures usually are treated with metal cables or a plate, and usually heal
     
  • Bearing Surface Complications: Each bearing surface has its unique risk. Polyethylene bearings carry the risk of wearing and causing osteolysis. Ceramic bearings can squeak and break. Metal bearing surfaces can cause a hyper-sensitivity allergic reaction and release metal ions into the blood and surrounding tissue
     
  • Implant Failure: Though uncommon, hip replacements occasionally fail sooner than expected. Some other problems can also make further surgery necessary, including: bone forming where it should not, breaking of the bone around the prosthesis (during or after surgery), and irritation of the soft tissues by wire or sutures
     
  • Death & Other Medical Complications: Though very rarely, patients have died following hip replacements. This can be due to underlying medical or heart problems that arise or worsen after the surgery. It can also be due to blood clots traveling to the lungs as mentioned above, or from the stress placed on the body by more than the usual amount of bleeding

This list is meant to cover only the major problems most frequently encountered. Just as everyone is unique, so are many problems. It is important to remind patients that though numerous complications have been reported in the literature, most are minor and rare

Measures that you can take to help prevent complications include:

  • Telling your doctor immediately of any possible infection anywhere on your body. Also let my office know
     
  • Receiving antibiotics before any dental, urinary, or rectal procedure for two years. You will require pre-procedure antibiotics for a longer period if you have a disease that compromises your immune system. (Call my office if there are any questions)
     
  • Always mention to any doctor performing an invasive procedure on you that you have a hip replacement
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