Anterior Minimally Invasive Surgery

What makes this minimally invasive?

This is the only approach around the hip joint that is truly muscle-sparing. We utilize a natural muscle plane at the front of the hip and get excellent access to the hip joint without cutting or detaching any muscles! This results in is less bleeding, less pain and a much faster recovery for patients.

Is there any evidence that this is a better approach?

Yes. As a relatively new technique to North America, the clinical data is still in its infancy. This technique has been successfully performed in Europe since the 1930’s. We have a couple of studies which contain objective data that this Anterior Muscle Sparing Approach is better than conventional hip approaches. Gait studies using fancy “pedometers” collect data on patients after hip replacement surgery. We look at the numbers to determine when normal gait patterns return after hip replacement. AMIS patients have significantly shortened recovery and much faster return to normal function compared to traditional approaches. Another study looked at MRI’s done on patients 1 year after total hip replacement in patients with AMIS approaches and conventional THA approaches (posterior and lateral). The MRI study showed permanent damage to the muscles around the hip joint in the conventional approaches (scarring, fibrosis and fatty infiltration of muscles), however the AMIS patients looked great. There was no detectable muscle or tendon damage detected again proving that this is a tissue friendly, less invasive procedure. We tell our patients it’s like having a “natural zipper” at the front of your hip joint.

How long have you been doing this and how many have you done?

We started doing Anterior Minimally Invasive Hip replacements in 2003. We initially learned the technique from Dr. Joel Matta in Los Angeles and received additional training from Dr. Frederick Laude in Paris, France. When we first learned the technique, we were 2 of a handful of surgeons in the USA performing the Anterior Approach. Every primary hip replacement we have done in the past 8 years has been through this anterior approach and collectively we have done over 2000 hip procedures to date. We travel around the world teaching and lecturing other surgeons on this surgical technique. This AMIS technique is used around the world and gaining popularity in the US because of it’s many advantages.

Is this more dangerous? Is there a higher complication rate?

Absolutely not. In fact, we have a lower complication rate than traditional hip replacement surgery. The reasons for this fact are as follows: In conventional THA, dislocation and leg length inequality are two of the more common complications that surgeons can experience. The surgeons will often template pre-operatively to determine appropriate sizes for prosthetic implants and adjust the surgical technique (i.e.. bone cuts) bases on this static image. The procedure is then done with the patient in the “lateral decubitus” position (turned on your side) and often without any x-ray or imaging guidance. Surgeons will implant the prosthesis and adjust length based on soft tissue tension and the stability of the new hip replacement. Occasionally, this will result in over-lengthening of the leg to gain stability by tightening up the hip joint. Dislocations are most often the result of poorly placed components (i.e.. the cup is too tilted in the pelvis) combined with weakened and detached muscles and tendons.

In contrast, with the AMIS approach, our patients are flat on their back (“supine position”) and we have the ability to use x-rays for each and every step of the procedure to ensure proper positioning, sizing and placement of the implants. This allows for perfect hip replacements every time. As a result, we have not had any hip dislocations in years and our leg lengths are extremely close to perfect every time we do a hip replacement. This is despite us not having ANY hip precautions after AMIS hip surgery. We DO NOT impose bending, reaching or hip flexion precautions because the implants are well placed and the AMIS approach is considered a more stable approach. This is in part because we do not detach muscle and tendon from bone, and the posterior soft tissue sleeve remains intact.

Why doesn’t every surgeon perform AMIS?

This technique is not for every surgeon. Surgeons who specialize in joint replacement are the most likely candidates. Practice makes perfect and having a sufficient volume of cases to perfect the technique is preferable. The learning curve for this approach can be troublesome for surgeons new to this technique. Early in the learning curve, surgeons can have increased complications because of the technical difficulty of the procedure. Without proper training, many early adopters of this approach abandoned the technique because of higher than expected complication rates. There are many tricks that we have learned over the years to produce a very safe and reliable operation. As outlined above, we have an extremely low complication rate because of our expertise and experience with the approach. We take great pride in this and enjoy teaching fellow surgeons who are new to AMIS. Our goal is to assist our colleagues through the learning curve by taking them through a multi-step training pathway including lectures, cadaver labs and direct hands-on proctoring in the OR during initial cases. We have had great success and good feedback from those surgeons whom we have trained.

Another factor in surgeons converting to the AMIS approach relates to how things change in medicine in general. New techniques are constantly evolving and often under great scrutiny. It takes years for new techniques to gain popularity among the masses. This is still a new technique for total hips in North America, despite the fact that it has been done in Europe since the 30’s. Many of our senior colleagues use traditional hip approaches (posterior and lateral) and have done so for years. They have historically had tremendous success and in general joint replacement is one of the greatest advances in modern orthopedics. Many of our distinguished teachers and professors have been doing joint replacements through the same approach for decades and it is less likely that they would be willing to change techniques at this point in their careers. In addition, to date, there have been only a few publications that evaluate AMIS techniques. As more surgeons around the world adopt this approach, we will continue to collaborate and publish studies supporting the use of AMIS in THA.

What prosthesis do you use and why?

Our preference is to use an un-cemented (press-fit) hip replacement whenever possible. The idea here is that after we implant the stem and cup, the bone grows onto the prosthesis over the first few months. The bone creates an extremely strong bond that will last for years. Bone is a living tissue that responds to it's environment and the stresses around it. We feel that this type of implant will function well for years and has the potential to adapt to changes in lifestyle and load. On rare occasion, a patients bone quality may be too poor to risk hammering in a press-fit prosthesis. Instead of risking a fracture, we will cement in a prosthetic stem.

There are many choices in terms of bearing surfaces. We have had tremendous experience with a variety of bearings and implants. In general, we want to choose a bearing surface that will last a long time (good wear rates) while not increasing the risk of complications as a result of the implant itself. Metal on metal (i.e.. head and cup/liner) and ceramic on ceramic are considered hard on hard bearing surfaces. Conceptually, these are extremely durable and have extremely low wear rates because of the nature of the materials. Unfortunately, many of these “alternative bearing” surfaces have been the source of many problems including recalls and revisions. Metal on metal hips (i.e.. Zimmer Durom and Depuy ASR) have caused an unacceptably high failure rate requiring revisions in many patients. There are many theories on the potential mechanisms of failure, however, it may be due to a tissue reaction to the metallic debris and wear particles that build up in the joint combined with implant stiffness and poor design features. Ceramic on ceramic hips, while durable, have had issues with breakage of the heads and squeeking hips. In short, we have moved away from these alternative bearings until we are confident in the technology. Over the last few years, there have been tremendous advances in the way we manufacture the plastic (polyethylene) that serves as the new bearing (cartilage) surface of the joint. New plastics are more resilient and have much longer lifespans with excellent wear rates. Ceramic on cross-linked plastic (polyethylene) has excellent wear rates when we review the literature. A ceramic head with a cross-linked plastic (polyethylene) liner is currently our ideal bearing choices.

Dual mobility cups are a relatively new concept (figure 1). Conventional fixed-bearing cups have a plastic liner that locks into the metal cup implanted into the pelvis. The ceramic head rotates freely within this liner, which is in turn locked into the cup/pelvis. Traditionally, ceramic head sizes are in the range of 28 to 36 mm in diameter. For this “fixed-bearing” hip, the distance the head must travel to dislocate is related to the radius of the head (i.e.. 14 to 18 mm travel). In general, the bigger the head, the better the stability and range of motion. In the dual mobility cup, a ceramic head is inserted into a freely mobile plastic liner. The liner also rotates freely within the smooth polished inner surface of the cup implanted into the pelvis. At final implantation, the ceramic head is popped into the liner trapping it within the plastic liner but allowing it to rotate freely as a second bearing surface. The advantage of this design are as follows: 1) the two arcs of motion (i.e.. plastic rotating in metal cup and ceramic head rotating in plastic liner) increase both stability and range of motion. The stability is increased because once the final ceramic head is locked into the rotating liner; it will not decouple creating an effective head diameter that is much larger than traditional heads. This increased head diameter creates an incredibly stable hip with excellent range of motion. 2) Studies show that the wear rates are better with the two arcs of motion in the dual mobility design. This is likely because the stress across the hip joint is dispersed over a greater surface area thereby protecting the plastic from early wear.

In short, we feel that the dual mobility cup is a great choice for our patients providing excellent range of motion, stability and outstanding wear potential. We have had tremendous success with this implant over the last three years, which is in stark contrast to some of the failures we have seen previously with the alternative bearings discussed above.

Is hip resurfacing better?

If we thought resurfacing was a better option we would still be performing this procedure. This is a question that frequently comes up and unfortunately requires a fairly lengthy answer. If you have not considered a resurfacing hip procedure then skip to the next question. If not, then read on:

We both trained in Canada and performed the Birmingham (BHR) resurfacing replacement in hips. A well-done BHR that avoids complications and does not develop any of the problems associated with a metal on metal bearing (as above) will do very well. The problem is that there is increasing evidence that the failure rate of this technique is increasing. We feel that there are a number of technical factors that contribute to these failures, which we will outline below:

BHR is marketed as an alternative to THA in younger active patients. Manufacturers claim advantages of increased range of motion, better wear rates and improved stability. Additional claims include less bone removal because of the preserved bone in the femoral head and neck region (Figure 2).

BHR is a technique that requires the surgeon to “mill” the bone of the femoral head (ball) and place a metal cap on the head. In addition, a cup is placed in the pelvis and the new hip is a large diameter metal on metal joint.

Range of motion and stability relates to the size of the head and the “head-neck” ratio. As discussed above, larger heads have better range of motion and improve stability. The problem with the resurfacing is that it employs a large diameter metal on metal articulation. There have been a tremendous number of early failures with these large diameter metal on metal bearing (i.e.. Durom and ASR). One primary concern is the possibility that this problem we have observed may extend to these resurfacing bearings.

Bone is preserved in the femoral neck, however, often times the surgeon must remove more bone from the socket to accommodate the size of the metal cap placed on the head. As a total joint surgeon, I am more concerned with preserving bone in the socket that a few centimeters of bone in the femoral neck. The other main issue with the resurfacing cap is that it requires precise placement on the head. As the surgeon mills the head and neck region on the femur, great care is take to not “notch” or hit the bone at the junction of the head and the neck. If this occurs, it can weaken the bone and create an area where the femoral neck may fracture. Also, the area in the femoral neck at the junction between the metal cap and the native hip bone is normally under a great deal of stress with normal human activity. Adding a metal cap which is much more stiff that the underlying bone creates a significant difference in “modulus of elasticity” (i.e.. bending term in engineering) at that critical junction. This has the potential for breaking through the neck of the femur below the metal cap during high stress/load activities. Younger, more active patients will put significantly higher stress across this region and as such we feel that the marketing of this implant is almost paradoxical. We would have significant worry about a neck fracture in younger more active patients with a resurfacing device. In various joint registries around the world (i.e.. Australia), we are seeing increased complications from BHR including fractures and early failures. It is for this reason that we have stopped doing these procedures. We feel that an AMIS THA with a dual mobility implant provides a minimally invasive approach with excellent range of motion, durability and stability without the fear of fracture and early complications secondary to metallosis (the generation of metallic wear particles in the hip joint).

What are the risks? What are the future risks and considerations?

As outlined above, we feel that many potential complications surrounding hip replacement are avoided with the AMIS approach. The AMIS approach, in our experience, results in a significantly reduced risk of leg length inequality, dislocation and intra-operative fractures associated with total hip replacement. There are general risks associated with any joint replacement, the most common of which include infection and blood clots (DVT and PE). National infection rates for total joints are approximately 1% for primary total joint replacement in healthy individuals. We take special precautions to avoid both infection and blood clots. To prevent infections we give patients antibiotics before and after surgery. In addition, we utilize dedicated teams and special operating rooms with modern technology (ie. laminar air flow) to reduce the risk of infection. We even use special skin cleansing solutions and anti-bacterial drains and bandages. Patients with total joints are at increased risk of infection for the rest of their life. This is in part because of the metal and plastic in their hip or knee. If a patient is exposed to a significant bacterial infection, it has the potential to seed the joint replacement. Unfortunately, this becomes an easy spot for the bacteria to grow because the immune cells in the body cannot penetrate the metal and the bacteria can create “slime” layers on the prosthesis which essentially overpower the normal human defense cells. These infections can create a very unfortunate situation where we may have to remove the knee joint entirely placing an antibiotic spacer in the knee while the patient receives intravenous antibiotics to eradicate the infection. This increased infection risk is why we recommend a single oral dose of prophylactic antibiotics before any invasive procedure after receiving a total joint replacement. The AAOS (American Academy of Orthopedic Surgeons) recommends this for two years after joint replacement before these invasive procedures. We consider it reasonable to take a dose before an invasive procedure for the rest of your life. There is minimal downside risk from taking these antibiotics. Simple dental cleanings do not require prophylaxis, however, anything more invasive does. Please refer to our website for a list of procedures.

How long is the hospital stay? Who will see me while in hospital?

A majority of our knee and hip replacements are done at outpatient surgery centers. Patients are going home the day as surgery or within 23 hours of their surgery. Patients with medical problems are best to have surgery at the hospital and discharge home the next day following surgery. While in the hospital, either the surgeon and/or the Physician Assistant from our team will see you every day. In addition, most patients are evaluated and closely followed by our internal medicine specialists while in hospital. The ensure proper functioning of your heart, lung, kidneys and vital body functions. We are also fortunate to have a very experienced Orthopedic nurse practitioner who is at the hospital every day to assist our patients.