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Hip resurfacing creates the most normal artificial hip joint possible.
This is most important for the athletic person who wants to return to impact sports or who wants to do heavy physical work. But even at fast walking speeds hip resurfacing gait is more normal than gait with a total hip replacement.
This is the real and over-arching reason to choose hip resurfacing instead of a standard stemmed total hip replacement (THR). The central problem in a hip with severe arthritis of any type is that the 3mm surface layer of hyaline cartilage on both sides of the joint is worn away. This is sometimes called a bone-on-bone joint. It is not yet possible to reestablish this natural hyaline cartilage.
Microfracture during hip arthroscopy can stimulate fibrocartilage to heal small defects. Sometimes this helps, if the defect is tiny, but in defects more than 1cm2 it rarely results in a happy hip. If the bone is not covered with hyaline cartilage, the pain receptors in the bone are stimulated, especially during weight-bearing and impact activities.
Stem cell injections are also not able to regrow this cartilage and are therefore ineffective in solving the problem in severe arthritis. Depending on the pain tolerance of the patient and their willingness to limit vigorous activities, sometimes medications and injections of various types can help them live an acceptable life for some time. But cartilage never regrows – the process of cartilage loss is typically progressive. Sometimes patients become debilitated within 6 months, but usually, the problem progresses for a few years before patients become convinced that an operation is their best option.
Joint replacement is the only adequate way to address severe cartilage loss. The basic principle is that the remaining cartilage and some bone on both sides of the joint are removed and replaced with an artificial surface. This surface must be solidly attached to the bone on either side of the joint. After this, when the joint moves, the artificial surfaces are rubbing on each other, and the bone behind them is again protected as in the original joint with hyaline cartilage.
But an artificial joint is never “normal”. Surgeons cannot create “normal”. Imperfect healing of the soft tissue envelope (muscles ligaments and tendons) that the surgeon must pass through as well as the biomechanics of the artificial joint itself determine residual symptoms. Many claims are made about the healing process for the method of “approaching” the joint, that cannot be verified. In recent times an “anterior approach” for THR has gained many surgeon proponents who claim that recovery is faster and better with this approach. This has not been verified with scientific data. In analyzing the large sum of published papers on this topic, it does seem likely that the dislocation rate using small bearing THR is lower from the anterior rather than the posterior approach, but the femoral fracture and infection rates seem to be higher. It is a wash. The surgical approach cannot be demonstrated to affect the final outcome of THR.
However, Hip Resurfacing Arthroplasty (HRA) has been demonstrated in numerous controlled studies to lead to a more normal hip than a THR. This is likely due to the fact that a more biomechanically normal hip is created with the resurfacing operation. The hip is reconstructed closer to its normal state. Only a thin layer of bone on either side of the joint is sacrificed, and the final bearing size is virtually the same as the original hip. About 4mm of bone on both sides of the joint is replaced with metal. The resulting artificial joint has near normal stability once the ligaments heal and load is transferred near normally to adjacent bone.
In a THR, on the other hand, a similar amount of bone is removed from the socket, but it is replaced with a much thicker 2-piece implant (metal shell and bearing liner) that results in a downsizing of the artificial bearing size compared with normal. This has two consequences. First, the head cannot be whittled down enough to place an artificial surface that will fit in this smaller bearing cup; it must therefore be completely removed and replaced with a smaller head sitting on a stem. The stem is a large piece of metal that completely fills the femoral canal for 4-10 inches. The second consequence is that the smaller bearing size is not as stable as the original natural joint. This will be discussed further below.
Although instability is a big problem for THR, the main problem that limits high activity levels after THR seems to be the stem. This mainly affects people who wish to participate in impact sports. In some ways, the stem provides a Safety Device for a THR, because it prevents patients from being able to participate in high-impact activities and thereby 'protects' the THR from failure to some degree. Some THR surgeons counsel their patients to resume any activity that they desire even though it is well-known that THR fails faster the younger or more active the patient is. Most surgeons do advise limitations to high impact. But even without these surgeon-imposed limitations, most patients are unable to perform high-impact activities.
Patients who desire to return to such activities are routinely described by their surgeons as having “unrealistic expectations”. But high impact is routinely possible with HRA and does not result in any higher failure rate with HRA. The reason THR patients cannot adequately perform impact activities is the stem. The HRA stem is tiny and is NOT load-bearing. The force applied to the HRA head is transferred directly to the surrounding remaining femoral head bone in a near-normal fashion.
The THR stem is large, canal filling, and transfers load to the femoral shaft. This is very unnatural and the body doesn’t like it. The result is thigh pain. Most THR patients are improved (although 20% have been shown to have at least moderate residual “unexplained” pain), but thigh pain of at least a slight degree occurs in 30% of patients even with light activities of daily living (ADL). 3-5% have moderate or worse thigh pain with ADL. In gait lab analysis THR and HRA patients alike walk normally at slow gait speeds. At a fast-paced gait, the THR patient unconsciously shortens his gait and shifts load to the other hip. The HRA doesn’t. This likely means the stem is bothering people to some degree even at fast walking speeds. Impact sports cannot be analyzed in a gait lab. Routine hip follow-up questionnaires that we use do not set a very high bar, therefore THR and HRA seem similar in many studies that use these methods. If standardized “sport activity” questionnaires are instead utilized, HRA patients are much more likely to enjoy impact sports.
A hip resurfacing looks much more like a normal hip and scientific evidence demonstrates that it functions more like one. This is obvious to most patients who understand how they differ. The question I have is why so many surgeons fail to see this and persist in giving young patients THR and then pronounce them “unrealistic” if they want to play sports but are unable to do so. On the other hand, THR is reliable and makes most people better. Old folks who think an “active lifestyle” is golf and walking on the beach are usually satisfied with THR. But younger patients who still want to participate in extreme motion or high-impact activities are much better off with an HRA. It is the closest thing to a normal hip that surgeons can create. Patients are almost never limited in any activity by their hip after 1 year.
The only “expectations” that I need to set for my patients is for long-distance running. I have found that this is the single most demanding activity for the hip. I have many patients who run distances regularly including triathlons, marathons, and even ultra-marathons. I allow this starting at 6 months. Even this extreme activity does not result in a higher failure rate. But it is not always possible. Some patients still have hip pain after 2-3 miles of running and are therefore disappointed. These patients can typically perform all other extreme sports (such as basketball, soccer, or singles tennis) or exercise programs (P90X) that they like, but are unable to distance run. The x-rays and ion levels are excellent. The cause of the pain is “unexplained”. We can speculate that stress transfer from implant to bone is not normal, tissue healing is not perfect, or even that the hip pain is “referred” from the back. Some patients can work through this with persistent progressive weekly running on a soft surface. Others have to shift to other exercises such as cycling.
Even though HRA is an imperfect copy of the original hip, it is today the closest a patient can come to a normal hip. I cannot promise a return to distance running; on the other hand, even distance running does not result in a higher rate of implant failure and I encourage it.