A good page on SR from ActiveJoints.com. Here's a recent article on microfracture as a cartilage regrowth procedure.
A Bit of history
I see no prospect in the next few years for recovering a normal hip using pure biological healing strategies (gene therapy, cartilage transplantation...). I'm resigned to the fact that I need hip replacement surgery. Early on, I realized that standard hip replacement (with stemmed femoral component which occupies the femoral canal) was a poor option, especially at my current tender age of 52 (2005). There are many reasons...(1) abnormal bone growth due to stress shielding, (2) the loss of a significant amount of mechanical compliance by the loss of the femoral neck (3) the difficulty in doing satisfactory revision surgery if / when it fails (4) the stresses on normal impact act as a wedge to split your lower femur, so that athletic activities (running, jumping, backpacking.... anything which puts significant force on the hip) must be curtailed. (5) the documented short lifetime of these devices; 15-20 years at the very best. Until 2001, I thought I was stuck. But in 2001 I leared about a far better and more logical approach - just replace the articulating surface of the hip joint, not the entire area. "Surface replacement" (SR) was pioneered in the 1980's by Harlan Amstutz, Derrek McMinn, and a few others. Early designs had problems. Foremost was that the the cup and cap were made with exactly the same radii, leading to lock-up. Modern designs now have a cap with a slightly smaller radius. Another worry was that the metal/metal wear debris would lead to increased cancer risk. Clinical followup has not shown any increased cancer risk so far, although cancer has a notoriously long gestation, and so it is something which should continue to be followed. Metal ion content in the urine and serum of SR patients (and metal/metal THR patients) is several times higher than normal, but the body apparently has a capacity to shuttle excess metal ions out of the body (within reason, no doubt). Metal debris production is usually highest in the first year, and then drops after "wear in" gives a self-polished and mated interface. I would speculate that part of the reason for the higher metal ion debris rate in the first year is that it takes roughly 6 months for the join capsule to re-form completely and seal off the joint from blood. Blood is not nearly as good a lubricant as synovial fluid, which will fill the intact joint capsule. Here is an excellent paper on the subject of metal ion risks and what hip patients can do about it.
Surface Replacement Surgery Complications
Today, surface replacement surgery is offered by a number of surgeons around the U.S. and around the world. However, as of July 2005 no device has yet been FDA approved for general use. Biomet has their "ReCap" system approved for use in hemiarthroplasty (replacing just one of the articulating surfaces), but it is not approved for both to be implanted at the same time(!). Yes, that's our FDA tax dollars hard at work for us... All devices in this category are being implanted as part of long term studies, with the goal of proving the idea for general use. In Europe SR has been used quite successfully for many years. The published study with the longest term data for a modern device is that of McMinn and covers the first 8 years of the BHR device. In my conversation with Dr. Thomas Gross, his experience is similar to McMinn's. The results so far show that that there is a small risk of failure in the first year after implant. The level is 1-2% but varies a bit from surgeon to surgeon - (surgeon skill is the single most important variable in getting a good long term result!). The early failures are due to (1) infection. Internal infections can migrate to an artificial hip and grow there more easily. Especially true during the first 6 months, before the joint capsule regenerates and seals off the joint from the blood system. Typically, the implant has to be surgically removed. (2) fracture of the femoral neck. This may be due to "notching" of the neck accidentally during surgery, or due to poor bone quality and too high stress levels being put on the leg in the first months after surgery. (3) Avascular necrosis under the femoral cap. For reasons not well characterized yet, the bone under the cap, where it meets the bone cement, can die. This leads to a loose cap and further damage to the bone, requiring surgery and revision to a THR. Current devices use bone cement to attach the metal cap to the bone. If one can get past the first year without problems, SR is apparently subject to only a very low on-going failure rate at least through 8 years follow-up. Dr. Gross speculates that the AVN may be induced by the heated bone cement.
Bone Cement Issues
In the early days of hip replacements, bone cement was used to attach all components to underlying bone. However, it was found that bone cement is mechanically poor in tension and in shear (not unlike the situation with concrete; which is fabulous in compression, and almost useless in tension and shear [that's what re-bar is for!]). Within a few years, the bone cement was failing, leading to particle debris which would then induce osteolysis and separation of the device from the bone. This happened in the femoral stem, and in the acetabular cap both. For many years now, cemented acetabular caps have been replaced with bone in-growth cups. The result is that failures here are much rarer, especially if the bone quality is good. Bone in-growth devices have been studied and fairly well perfected. Small beads are sintered together and treated with titanium plasma and then with hydroxyapatite. The size of the beads, the preparation of the surface, and the other parameters have all been fine tuned and shown to provide a solid and stable fixation. Bone will grow into the interstices and make a firm yet flexible lock. Bone in-growth femoral stems in THR devices have also been in use for some years in many places. There is probably a reason why bone in-growth femoral caps were not introduced and approved at the same time as for surface replacement but I don't know what that reason is. Maybe it has to do with the fact that no SR devices were yet FDA approved, and getting approval is first on the device manufacturer's priority list so that some financial return can be gotten before further R&D is financed. And, because cemented caps have so far been successful, so why rock the boat?. McMinn's data show that the femoral caps have not yet failed due to bone cement issues. However, bone cement does appear to cause femoral neck narrowing when compared to uncemented (here's the paper).
Here, in 2011 - we have a couple of updates to share from Dr. Gross on the results of follow up studies of hip prostheses.
What is the Best Bone Fixation? (1/11/11)
What is the Best Bone Fixation Type? (1/11/11)
The Biomet ReCap Device
The Biomet device is the one I believe will be best. It was designed in close collaboration with Dr. Thomas Gross - from my homework, the best surgeon in the hip replacement business. It preserves more of the native femoral bone, and its geometry and surgical instrumentation reduce the risk of notching of the femoral neck, and it has already been granted FDA approval for general use in cases of AVN when one side of the hip still has healthy cartilage. While there is no surface replacement device which is designed specifically as a replacement in revision surgery when the femoral cap fails, it seems to me that the Biomet device would be the easiest to revise with a new SR device, since it preserves a hemispherical bone surface and more of the native bone than e.g. the BHR or the Conserve Plus. Dr. Gross now uses the Biomet devices and this is why I scheduled surgery with him in June of 2005. A .pdf file describing this system, and showing a finite element analysis under load, is here.
My June 2005 Surgical Adventure in South Carolina
I learned only after arriving in South Carolina at Dr. Gross's office that there is a bone in-growth version of the Biomet femoral cap. It is already being manufactured, it is cataloged by Biomet, and it is just awaiting "final approval of the paperwork"(?) before surgeries can begin. (so why did a google search on this subject turn up virtually nothing??). When and where, at this moment, is uncertain as I revise this in August '05. I thought long, hard, and carefully about whether to go through with my scheduled surgery using the cemented cap, or to wait. In fact, I spent most of the pre-surgery weekend in my motel room in Columbia, South Carolina talking with a mechanically inclined cohort (I too have a master's in mechanical engineering) and future SR patient in Ohio (and on the internet in the public library). We discussed and made sure we understood the results of the finite element analysis of the mechanical stresses on the hip, bone, and device, and the implications for us. I'm pretty healthy; I've lived in a clean, small town, been very athletic, eat an organic diet, have low blood pressure, healthy heart, no health problems (except my left hip, which was caused by AVN from a doctor's cortisone shot for poison oak; not some genetic or lifestyle or other fundamental health flaw)... and so I expect to live well into my 80's or perhaps 90's. That's up to 40 years more that I need to get out of my hip. I already know how it feels to make a decision which one regrets for the rest of one's life. I do not want to make any more. I don't want the prospect of being in a wheelchair in my late 80's with a bankrupt state school system pension...(I could go on, but you get the picture). And I want to keep my healthy, active lifestyle. That means running, cycling, rock climbing, kayaking, surfing.... and not worrying that every activity is taking weeks, months, or years off the end of my mobile life. Or that I'm one little slip away from destroying my hip.
Bone cement has shown no problems so far in the femoral cap. Reasoned speculation is that this is because the cement is in compression here. However, finite element analysis (see the Biomet literature) shows that a significant wedge of cap is in tension and shear. And this can perhaps be understood as due to the fact that bone is much more flexible than is the stainless steel cap. The force on the cap is down, giving compression at the apex, but spreading out to produce shear farther away and tension near the edges of the cap near the femoral neck junction. Rythmic loading of a cemented cap as in walking or running may produce the same cement failures as seen in the femoral stems of THR's, or in the old acetabular cups. Both of these had bone cement in mostly pure shear. The incidence of these failures typically accelerated after 10 years and beyond. While there is no clinical evidence of this yet in surface replacement, the data only goes out 8 years (McMinn) so far, with spotty anecdotal evidence beyond. It's possible that in the next few years we'll begin to see some cement failures in the femoral caps. ~10 years.... that's pretty short compared to the 40 years I need. And if I get a THR after ~10~15 years, how long will IT last? With what quality of life, living in fear of the next failure? Once one has a hip replacement, there's no hope of anything along the lines of cartilage regrowth or bone healing or stem cells. You're future is with an artificial hip. Period. Another concern is that cement debris will cause lysis of the bone in the same way that polyethylene debris from standard THR's has done for decades. Clinical studies show that it is not polyethylene per se, but the size of the particles which causes the lysis. And, bone cement will never "heal", but live bone can...
Another point for a bone in-growth design - shock resistance. In fact, numerous studies show that it is impact loading which stimulates the osteoblasts to grow bone. Not swimming or cycling, but impact. Precisely the kind of loading which hip replacement patients are advised against because the implant may come loose. I'm thinking of how bone cement locks with the existing bone. Small trabeculations of exposed debrided bone are smothered in the cement, which then hardens. The bone cement is initially hot (which Dr. Gross speculates is why AVN is sometimes caused to the bone in a small fraction of cases. Perhaps I'm at increased risk, since early AVN is my diagnosis of the precipitating cause of my hip disease). Whether AVN is induced or not, the bone / cement interface is only as strong as those trabeculations at the time of surgery. But my bone (and most patient's bone) is underdense because it is under-impacted due to lack of physical activity due to chronic pain. The bone is likely osteoporotic to some extent. And when the bone strengthens later with use, it will not affect the bits of smothered bone impregnated inside the cement. And if that interface breaks with activity, there's no interstitial space for bone to grow back in to or re-attach to. A bone in-growth interface would seem to have at least the possibility of healing if it were to break (from e.g. a bad fall or a bicycling crash). After all, bone cysts are common in AVN and OA patients getting SR surgery. And these cysts can be filled in with bone grafts. A bone graft is basically ground up bone together with spun-down blood platelets and growth factors poured into the space. So, it seems reasonable that trabeculae broken off at the cap/bone interface might re-weld themselves together given the right mechanical stress and biochemical environment. A femoral prosthesis with the capacity to heal if over-stressed - this would seem to be an ideal hip replacement.
While some might balk at being one of the first guinea pigs for a new device, in fact the technology is already well proven. Medical engineers have already converged on the optimum parameters - bead size, sintering process, coating, geometry of the interstitial regions. These parameters are proven winners in the acetabular cap. I can't find a good reason to think that the same bone wouldn't be just as happy to integrate just as well into a femoral cap designed this way. The more I thought about it, the more different facets I was able to hold into my mind at the same time, and guage my final feeling about what to do - and it was fairly easy to decide - wait for the bone in-growth caps.
Questions and Concerns..
Fracture of the femoral neck is still going to be a risk to some small extent. How much of this might be due to the guide stem hole and stem which are part of all the SR designs? This stem goes all the way through the femoral ball and deep into the femoral neck. It is there as a guide for the instruments which machine the femoral ball and is not structural. Then why must it be so deep? What worries me is that this stem hole is filled with the stainless steel stem of the femoral cap. This metal will have a much different mechanical compliance than the underlying bone. Anyone who's looked at fresh chicken knows how flexible and resilient live bone is. The angled femoral neck is one of the areas which flexes to take up impact loading when running. If this flexes and deforms, but the metal stem inside doesn't, would this not stress the neck from the inside? Even without fracturing, a femoral neck which gets micro stress fractures can compromise the blood supply, which comes into the femoral neck from below the hip socket capsule and up through the neck to the ball, leading to avascular necrosis (this long pathway is why the femoral head is the most sensitive place in the body for AVN to develop). Dr. Gross' office could only tell me that they knew of no one who was worrying about that. I would hope that the designers of this device worried about it! It may be that the thin-ness of the stem combined with the thickness of the femoral neck and the fact that most of the stress and deformation in any cylindrical solid under bending load is taken up at the surface...means that this concern is not a problem. But I wish I could hear it from the designers, to be sure.
I'm scheduled for cementless hip replacement surgery using the new Biomet device in June '07. Wish me luck! It's been a long saga...
May '07 New Problem - Low Bone Mineral Density
July '07 - Posterior Approach Hip Surgery and the Blood Supply to the Femoral Head
Here's the On-Going Account of my surgery, recovery, and personel experience with a new Biomet hip.
Jan '08 - A letter from Dr. Smith in Canada: Results on Success of Cementless Surface Replacement (Cormet's Design)
"Thank you for your inquiry. In answer to your questions:
To date I have had no failures due to cementless heads. Two ladies have cracked their femoral necks after inappropriate activity level within 2-3 weeks of the operation. Out of 230 resurfacing hips done that is a low percentage. Both were salvaged with stems and heads the same size as the resurfacing so they are very stable.
Our CT program enables us to look inside the metal cap and we can see excellent bone response to theHA coating inside the head. Bone growth is not an issue.
Most weight bear as tolerated immediately post op. Exceptions are those with cysts in the head that require grafting. Cementless allows grafting, cement kills graft. V ast majority of patients resume normal activities by a year. I do require a bone density study if there is any question of osteopaenia before deciding that this is an appropriate procedure.
I hope that answers your questions."
Frank C Smith MB,ChB, FRCSC
Jan '08 - A Letter from Dr. Thomas Gross in South Carolina Responding to Questions Regarding Surface Replacement Hip Options
Thomas P. Gross M.D. 1/17/2008
1. Why do you choose to use the posterior approach over other approaches?
TG: I estimate that most total hip resurfacing done worldwide is done through a posterior approach ( > 90 %). There was an interesting report recently at the Annapolis MD HSR Meeting from Mr. Ronan Treacy indicating no difference in results between posterior and anterior-lateral approaches in a direct comparison looking at the rates of limp and other complications. Therefore, I do not think anyone has shown that it really matters which approach is used. My current personal preference is posterior minimally invasive (see my video) vascular sparing (as described by Dr. Koen DeSmet). I believe, that with this approach, I can accomplish the operation in virtually all case that are candidates ( only 2 of 1300 converted to THR intraoperatively), return patients back to activity as rapidly as possible, avoid a permanent limp, and place components in ideal position. This is personal preference, not science. Every surgeon should use the approach that he/she is most comfortable with.
2. How long do you feel it takes for the bone to be fully healed, grow
into the prosthesis?
TG: It is a gradual process that proceeds over 6-12 months. Dog studies have shown that the process has already begun by 6 weeks postop. At 6 months, I estimate that 90% of bone ingrowth has occurred. After 12 months bone remodeling around an implant porous surface should be back to the normal baseline with the implant well incorporated. Some impact early on will likely stimulate bone ingrowth, while too much may cause implant – bone micromotion and result in failure of ingrowth. How much then is too much?
3. What is the recommended time you tell your patients before they
can start to run again/do impact sports?
TG: Six months. This is not only because of bone ingrowth issues, but also because of the femoral neck fracture risk which is present for 6 months after surgery in my experience. Although it is clear that M/M HSR is much more durable than most THRs, exactly how much impact activities can be safely tolerated over the long term is unclear. Therefore, patients should exercise caution.
4. What is your take on cementless devices?
Derek McMinn's comments on cementless HSR: At the beginning of my experience, all my resurfacings were cementless. The results were not good for cementless femoral components, but cementless acetabular cups were excellent. Of course I have occasional patients with a great result following a cementless femoral component 16+ years post-op. For the total group of patients, however, cementless femoral components were not successful. In 1994 I started with hybrid fixation using a cemented femoral component and a cementless cup. In my BHR series, i.e. commencing 1997, I continued with hybrid fixation and I have had no loose cups and no loose femoral components. It would be hard to do better than have a zero loosening rate in this large series of BHRs in patients with varying bone quality. Thankfully patients who need resurfacing today need not be the Guinea pigs for a new experiment. Hard information does exist on this subject and should be used by surgeons and patients alike. Those are the short answers to your questions, the long answers are in a multi author book called Modern Hip Resurfacing which I have edited and which will be published by Springer early next year. When it is published, I will send you a signed copy. Best Regards, Derek "
TG: "Uncemented femoral resurfacing components are a new development. The acetabular components used routinely are uncemented, while the femoral components routinely have been cemented to bone. Recently, I have developed a fully porous coated femoral component with Biomet Inc. (Uncemented ReCap). This component is FDA approved in the US for femoral resurfacing. I have been using this together with the uncemented Magnum acetabular component for uncemented total Hip Surface Replacement since April 2007. No other surgeons are yet using this device. Corin also has a device available outside the US for uncemented femoral resurfacing; it is partially porous coated plus HA coated (Corin Cormet 2000 uncemented femoral component [version 2]). Several early reports on the Corin device out of England are promising. Mr. Richard Villar has compared the uncemented Corin Cormet 2000 version I (no porous surface, only HA coating) to a cemented femoral component. This study showed less bone remodeling (neck narrowing) with the uncemented component. The significance of this is completely unknown. We will watch these developments with interest. I also have a favorable experience (fewer patients but 8year follow up) with this original Corin version I device which I presented at the first Comprehensive US hip Resurfacing Course in Annapolis MD in November 2007. I am currently working on a manuscript for publication. Mr. Derek McMinn has recently added some insight into his past experience with uncemented femoral components based on the Corin McMinn component, which was made of cobalt chrome with no bone ingrowth or ongrowth surface whatsoever. Mr. McMinn tried this about 15 years ago and it predictably failed with this surface. At that point (early 1990's) we already knew (from joint implant work done in the 1950's) that an uncemented uncoated press-fit implant of any sort would not work reliably more than 1 – 2 years in most patients. At this point uncemented fixation of the femoral component in total hip resurfacing is an interesting concept. My hypothesis is that an implant that has fixation to bone by an appropriate bone ingrowth surface may result in less early complications because the thermal necrosis of bone with curing of cement is avoided. Also it may be longer lasting, if cement is subject to late fatigue failure in HSR as it is in most other joint replacement applications. But, again, I stress that this is a hypothesis which can only be validated or refuted by clinical data (That means 2+ year results in large numbers of patients).On the negative side, we need to discover how reliable the bone ingrowth process is (with these new implants and instruments) in the femoral head. Mr. McMinn's statement that he has already tried uncemented femoral fixation and that this did not work is not completely accurate. There is no doubt that he showed that "a femoral component placed without cement and without a bone ingrowth (or ongrowth) surface will not give reliable results." He has not published any results on a femoral component with a bone ingrowth coating, and to my knowledge, his company (MMT, which makes the Birmingham hip) has not marketed any such device. The reference article is: Derek McMinn; Ronan Treacy; Kyaw Lin; Paul Pynsnnt, Metal on Metal Surface Replacement of the Hip. Experience of the McMinn Prosthesis CLINICAL ORTHOPAEDICS AND RELATED RESEARCH, Number 329S, 1996,pp S89- S98 There is very little published data on the use of an uncemented femoral component. I will need to gather data for at least three to four years before I will be able to provide any useful data on this subject. Fixation of total joint implants to bone can be accomplished by cement or by porous ingrowth technology. Cement fixation is immediate, while permanent fixation with uncemented components requires six to twelve months of bone ingrowth before it is considered well fixed. Some total joint replacement components have had more success with cement fixation, while others have done better with uncemented technology. In hip surface replacement at eight years, approximately 95% of implants are still working in young patients with cemented femoral fixation. Only rarely can these 5% failures be blamed directly on the cement fixation itself."
Dr Rogerson, Madison Wisconsin comments on cementless devices:" I think the presentation and the xrays shown by Dr. Papavasileou were very telling. Many of the xrays showed significant neck narrowing not seen on the cemented BHR xrays. I think this is because the ingrowth on the cementless heads is "spotty" and leads to variable stress shielding of the distal inferior neck and calcar bone. Some of the xrays were very ominous appearing and will likely result in delayed neck fracture. Therefore, I will stay with the thin cement mantle produced by the BHR technique to spread out the forces coming down the femoral neck and avoid the stress shielding."
-Dr Rogerson, Madison, WI
Dr. Thomas Gross response: "Dr. Rogerson is mistaken. I suggest that he reads Mr. Richard Villar's work. The reference is: FEMORAL NECK RESORPTION AFTER HIP RESURFACING ARTHROPLASTY – A COMPARISON OF CEMENTED AND UNCEMENTED PROSTHESES. P Katrana; JR Crawford; S Vowler; A Lilikakis; and RN Villar. Journal of Bone and Joint Surgery - British Volume, Vol 88-B, Issue SUPP_II, 234. Copyright © 2006 by British Editorial Society of Bone and Joint Surgery. Mr. Richard Villar has compared the uncemented Corin Cormet 2000 version I (no porous, only HA coating) to a cemented femoral component. This study showed less bone remodeling (less neck narrowing) with the uncemented component. The significance of this is completely unknown. We will watch these developments with interest. Overall the short term results are equivalent between these two types of femoral implants.
5. Recently, comments have been made re: FDA status of implants. Can
you clarify this issue?
TG: "Statements have been made that the only FDA approved hip resurfacing device is the BHR with S&N. Furthermore, if patients get any other one the insurance companies at any time can make them pay them back for non-FDA approved devices. Both of these statements are false. The Birmingham (Smith & Nephew, Richards) total HSR was the first to get FDA approval in the US based on an unprecedented FDA decision to approve this implant on the basis of single (developing) surgeon's foreign data. Cormet 2000 ( Corin Ltd., Stryker) total HSR was the first to be approved based on the usual mechanism of a US run Multi-center FDA study( approval 7/2007, I was the lead investigator). Therefore, there are now 2 implants available in the US that have an FDA indication for total HSR. The Recap/Magnum (Biomet), Conserve Plus (Wright Medical) and APR (Depuy) are all also FDA approved implants in the USA. They may be legally used by any surgeon for the purpose of total Hip Surface Replacement. (The FDA does not regulate how surgeons may use approved implants.) However, the implant companies may not promote them as total HSR devices. (The FDA does regulate how implant companies may market approved implants.) Approximately 20 different companies sell total HSR implants on the worldwide market, including all of those mentioned above. A specific use of an implant (or drug) that is approved by the FDA is termed an FDA approved indication. As soon as a company has received one indication for use of their implant (or drug), the implant (or drug) is FDA approved. The use of this same implant (or drug) for a purpose other than for the approved indication is termed an off – label use. Off-label use is legal. In fact, much of American medical practice involves off-label use of drugs and implants. It would be impossible for the FDA to regulate doctors' practice in every situation. Actually the Supreme Court has expressly stated that this is not the FDA role (see Rehnquist opinion). Many insurance companies do have contract clauses that deny payment for "experimental" treatment. How to define what is experimental is very controversial. However, 'experimental' is something altogether different than off-label use of implants and drugs. Therefore, an insurance company would be treading on thin ice if they were to use FDA indications as a basis to deny payment."
(posted on SurfaceHippy by Vicky; LBHR Dr. Bose Dec 01 2005)