After performing the direct anterior total hip procedure for over 5 years now, I have learned where some of the pitfalls are in this procedure. In my mind, the area of greatest potential concern is the degree of initial stability achieved between the femoral component and the femoral canal. This implant needs to have a very high degree of initial stability because I encourage patients to fully weight bear immediately after surgery and return to activities as quickly as they can. In my mind, the best strategy for prevention of blood clots is physical activity, since this allows the body own natural defense mechanisms against blood clot formation to work.
I use an uncemented femoral stem which means that the stem has to have a solid press-fit into the femoral canal and no bone cement is used. The femoral stem is coated with a special metal layer that mimics the micro-architecture of bone. As the bone remodels around the stem, it locks onto the minute variations in the surface of the metal and essentially makes it a permanent part of the femur. This process is called osteointegration, and it is absolutely a requirement for a successful long term implant. Because patients are allowed to weight bear immediately after surgery they will put a lot of stress and strain on the hip prosthesis before the bone has solidly grown into the micro-architecture of the femoral stem.
The biology of osteointegration is not completely understood and all of the major manufacturers of orthopedic implants have their own proprietary metal surfaces and coating that are designed to facilitate osteointegration. We speculate that some stress is essential for osteointegration to occur, but too much stress occurring before the femoral stem has completely bonded to the bone may cause micromotion at interface between the bone and metal which may inhibit the process of osteointegration. If this occurs, a layer of fibrous tissue may stabilize the stem, but in the worse case scenario, the stem is loose inside the femoral canal and becomes a source of pain for the patient. Therefore, we try to balance early activity with the benefits of decreased blood clots, faster rehabilitation, higher patient satisfaction, and possibly even enhanced ingrowth if the stresses are appropriate, with the theoretical concerns of too much stress early on may be detrimental to the biology of integration.
Choosing the correct stem for the correct patient is a little bit art and a little bit science, and it is also influenced heavily by the maxim that if you do the same thing over and over again, you tend to get better at it. Most surgeons in the US try to limit themselves to one or two stems for most of their index procedures. Currently, the design that I am using is the Biomet Taperloc stem. I believe that this stem does a good job of meeting the needs of a proximal fit and fill stem with good initial stability, ease of insertion for the direct anterior approach, has a good track record, and is manufactured by a solid company with a long history in orthopedics in the United States. Biomet orthopedics (www.biomet.com) is on record as saying that they have over 22 years of experience with this stem design, that it has been successfully implanted in a very large group of patients, and that it has functioned well in the young and the elderly. In my mind, the greatest advantage of this stem is the reduced diameter of the distal portion of the stem which allows for solid contact between the proximal metaphyseal area of the femur and the porous coating and decreases the incidence of post-operative thigh pain.