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Surgeons offer insight into the pros and cons of emerging porous metal technologies

Issue: December 2012

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The evolving development of titanium- and tantalum-based porous metals represents a next step in achieving cementless component fixation and treating significant bone deficiencies in revision hip and knee arthroplasty reconstructions. Their claimed advantages is in the presentation of 3-D porous matrices with pore volumes approaching 70% of the available ingrowth surface. Increased coefficients of friction significantly reduce the prospect of micromotion to ensure osseous ingrowth, while their elastic moduli approach that of cancellous bone. Despite these advantages, limited clinical experience has been offered in the peer-reviewed literature.

Most of the major orthopedic implant manufacturers have developed porous metal technologies whose applications include direct implant-bone fixation as well as augmentation for bone deficiency (see Table). The figure on page 20 demonstrates the use of cementless fixation for primary and revision hip and knee arthroplasty in the United States.

In this first part of a two-part Orthopedics Today Round Table discussion, we bring together experienced orthopedic surgeons whose collective background is intended to provide readers insight into the pros and cons of these emerging porous metal technologies and their adaptability for use in primary and revision hip and knee arthroplasty reconstructions.

A. Seth Greenwald, DPhil (Oxon)
Moderator

A. Seth Greenwald, DPhil (Oxon): What percentage of your practice is cementless fixation for primary and revision hip and knee arthroplasty?

Paul F. Lachiewicz, MD: For primary total hip arthroplasty (THA), 100% of my acetabular components are cementless, usually titanium fiber-metal mesh coated, and routinely fixed with two screws. Our survivorship at 15 years with these components has been excellent, with no loosening. Thus, for routine primary hip arthroplasty, I see no reason to use the enhanced porous metal components except in unusual circumstances. Almost all of my primary femoral components are also cementless, and again, I use standard porous titanium tapered or cylindrical components.

Revision THA is a completely different situation, as we are faced with damaged or deficient bone structure. Based on our clinical studies, all of our cementless revision acetabular components are fabricated of or coated with enhanced porous metals. We use multiple screws for fixation as well. A variety of cementless femoral components are used for revision, including beaded full-coat prostheses and titanium fluted tapered prostheses.

I have a different philosophy for primary and revision total knee arthroplasty (TKA), and use cemented components in all cases. However, when there are major metaphyseal deficiencies (Engh 2B or 3 defects) of the distal femur or proximal tibia, the enhanced porous surface cones or sleeves (trabecular metal tantalum specifically) are routinely implanted to reconstruct the deficiency first.

Allan E. Gross, MD, FRCS(C): All of my primary knees are cemented and pretty well all of my primary hips are cementless. I do not do revision knees, but in our hospital I know revision knees are only partially cemented when trabecular metal cones are used. If the trabecular metal cones are not used, then the revision knees are primarily cemented. With regards to revision hips, reconstructions are primarily cementless. I do, however, cement polyethylene cups into cages and cup cage constructs. The only other time we would use cement in a revision is if we were doing a proximal femoral allograft in which case the femoral stem is cemented into the allograft but not into the host. Once again, the contact with host is cementless.

Adolph V. Lombardi, Jr., MD, FACS: During the past 25 years, I have used cement fixation for nearly 100% of my primary TKAs. With respect to revision knee arthroplasty, I have used hybrid fixation; that is, cemented fixation of the metaphysis with press-fit of the stems. With respect to hip arthroplasty, I have evolved from approximately 50% hybrid fixation, that is, cement fixation of the femoral component with cementless acetabular reconstruction, to nearly 100% cementless fixation of both components. Similarly, in revision hip arthroplasty I have evolved to 100% cementless fixation.

Michael E. Berend, MD: In 98% of primary THAs, I use cementless designs for both femoral and acetabular fixation, and in 100% of revision THAs. In 2% of primary TKAs, I use cementless designs for both femoral and acetabular fixation with a cemented all polyethylene patellar component. I have not used completely cementless revision TKAs for metaphyseal fixation. Depending on metadiaphyseal cancellous bone quality, I will either cement a shorter stem (~80 mm) or implant an uncemented diaphyseal engaging stem.

David G. Lewallen, MD: For primary THA, 95% are cementless acetabulum and femur, 5% hybrid fixation cementless socket cemented stem, and less than 1% cemented acetabulum and femur. For primary TKA, 80% are cemented, 20% reverse hybrid with cemented femoral and cementless tibial fixation. For revision THA, 100% are cementless acetabular fixation, 95% cementless femoral fixation, and 3% to 5% cemented femoral fixation. For revision TKA, 100% are cemented but with adjunctive femoral or tibial cones for supplemental cementless fixation within the metaphysis in 30% to 40%.

Greenwald: During the last 2 decades, the clinical utility of porous surfaces and coatings used particularly in hip implant systems has established their efficacy as an alternative to cement fixation. What biological and mechanical advantages do these emerging porous metal technologies offer over their predecessors as a means of providing more optimal and enduring fixation?

Lachiewicz: These enhanced porous metal surfaces have greater surface friction than standard porous beads or mesh, and many have a modulus of elasticity closer to that of cancellous bone. There is also greater porosity than standard metal coatings. These factors may help explain the greater bone ingrowth seen with these enhanced porous metal components, and their greater success in revision arthroplasty.

Gross: I think the main difference between the new porous implants compared to the conventional porous uncemented implants is the fact that they are much more efficient in providing a press fit, and an environment for biological ingrowth and remodeling. In addition, their elasticity is similar to bone decreasing the chances of stress shielding. Comparing them to the conventional porous implants, I think there are situations where you can get a good press fit with a good chance of getting ingrowth. In the past, a conventional porous implant might not do the trick, and we would resort to cemented types of reconstructions including rings and cages.

In this latter type of reconstruction, cement actually is used to fix the poly to the cage or the ring, but the cement also goes through the cage or the ring and bonds to the underlying bone. These cages and rings do not provide a surface into which bone can grow into, and therefore, are similar to having plate fixation where permanent biological stabilization does not occur. These rings and cages are prone to fatigue and eventually fail. The new porous metals, therefore, give us the opportunity to stay cementless in situations where in the past we have had to resort to cemented reconstructions, including not only cemented cups and stems, but also cages and rings.

Lombardi: During the past 2 decades, the clinical use of porous surfaces and coatings used particularly in hip implant systems has established their efficacy as an alternative to cement fixation.

Modern day cementless fixation has its roots in the work of Albrektsson and Brånemark. These two individuals coined the term “osseointegration.” They recognized that bone would grow in intimate contact with unloaded grit-blasted titanium dental implants. This bone apposition provided stable implants upon which they could build their reconstructions. Their work was expanded upon by the work of Bobyn who recognized that bone could grow into 3-D structures. This led to an evolution of implant coatings which foster bone ingrowth. These included beaded surfaces, plasma-sprayed surfaces, structured cancellous titanium and fiber metal.

Initially, bone deficits were dealt with by using cement and sometimes metal reinforced cement. However, as these constructs began to fail, allografting techniques developed. With long-term follow-up, it was noted that allografts began to resorb, and therefore, other options were sought which led to the development of ultraporous metals. By virtue of their increased porosity, these metal augments and coatings have significantly enhanced potential for bone ingrowth. They have dramatically changed the paradigm by which acetabular revision arthroplasty is performed. While the exact percentage of intimate contact of the acetabular component with host bone which is necessary for bone ingrowth is not known, it is certainly far less than the historically stated 70% contact needed with traditional porous coatings. The metallic augments have essentially replaced allograft bone reconstruction. These devices have adequate mechanical strength and will maintain that strength. They also allow for vascularized bone ingrowth.

Berend: Cementless femoral fixation in THA has improved second decade performance with decreased loosening. On the acetabular side, cementless fixation is less technique sensitive compared to cemented cup insertion. It allows for adjustment of headsize and levels of constraint. In the absence of osteolysis, most porous implants when ingrown are unlikely to loosen.

Lewallen: Our experience has been primarily with trabecular metal (or highly porous tantalum) beginning with its introduction in the late 1990s. The advantages in primary arthroplasty are documented reduction in stress shielding around the acetabular components as demonstrated on quantitative CT studies and exceptional performance with regard to apparent bone ingrowth. Within our institutional database beginning with the first trabecular metal acetabular component we used in 1997 up through January 2011, there were no cases of aseptic loosening in 1,649 primary arthroplasties using trabecular metal sockets. In 1690, revision cases during the same time period on the acetabular side, we documented three cases of aseptic loosening, despite bias towards use of trabecular metal cups in the most challenging of cases performed at our institution during those years. The failures occurred in the setting of pelvic disassociation where nonunion had persisted, and in an instance of trabecular metal use against dead allograft with failure of bone ingrowth by the dead bone not surprisingly.

Greenwald: Are there any recognized or potential downsides to their use?

Lachiewicz: The downsides to the use of these enhanced porous metal components are: difficulty in removal, especially for infection or malposition; increased cost; and relative lack of published clinical results, except with the trabecular metal acetabular components and tibial cones. We have published our results with these at 5 years or more follow-up time and have seen no downside as yet.

Gross: The most serious potential downside is that these new porous metals do not restore bone stock for further revisions of the hip and the knee. For example, if the next revision has to be done and the porous metal implant has to be removed and if it is well ingrown, then the re-revision could involve removal of the porous metal plus a lot of the bone that has grown into it. In other words, instead of restoring bone stock for the next revision, the porous metal might make the next revision more difficult because of removal of bone along with the porous metal.

On the other hand, I have had two cases where we were able to use well ingrown trabecular metal augments as part of a rerevision on the acetabular side. In those two cases, the trabecular metal augment actually served like a structural allograft, in that it helped to stabilize the new cup without having to resort to bone grafting or another augment. There is, however, still the potential risk of having to remove these trabecular metal implants in order to do the next revision.

Lombardi: The only potential disadvantages or downsides to the use of ultraporous augments is the significant difficulty which is encountered if they are required to be removed in the case of a septic total joint arthroplasty. The classic example of this is the extreme difficulty encountered when attempting to remove a porous cone from an infected proximal tibia. This, without a doubt, makes for a long day in the operating room.

Berend: We have reported increased intraoperative femoral fractures with uncemented stems. Uncemented acetabular components have had higher rates of poly wear compared to all poly cups. This may have improved with highly crosslinked polyethylene and improved acetabular liner locking mechanisms. Long-term data will help clarify advantages and disadvantages.

Lewallen: The only potential downside to use of highly porous metal implants in our hands has been similar to those encountered with other well-fixed uncemented devices. Once bone ingrowth occurs, regardless of the porous surface, and late infection or component malposition require removal of the device, it is necessary to divide all bony connections to the implant along all interfaces. This can be most challenging in cases where there were large defects managed with trabecular metal implants and augments, but are much akin to the challenges encountered with well-fixed devices with other porous surface treatments or with well-ingrown allografts which succumb to late infection. Basically, if one begins with a large defect and subsequent infection occurs, then a large defect results once the implant and bone graft materials have been removed regardless of the technology employed.

Greenwald: What patient populations may be the beneficiaries of these advanced fixation technologies?

Lachiewicz: In addition to their routine use for acetabular revisions, enhanced porous metal components should be considered in those primary hip arthroplasty patients who have had previous pelvic irradiation or unusual bone disorders, such as sickle cell anemia, other bone marrow disorders and osteopetrosis.

Gross: I think that the patient population that benefits from a long-lasting reconstruction option would be the main beneficiaries of this technology. For example, in an elderly patient where you are looking for a solution that would last 10 years or less, then the use of cement is advantageous in that it allows immediate weight bearing, and you do not have to depend on a biological response by the host vis-a-vis bony ingrowth and remodeling. I think porous metals in the elderly population are perhaps not realistic in that ingrowth is unlikely to occur and cement provides an instant solution with easy rehabilitation. I think this is also true in the cancer population where the prognosis is guarded, and also where there has been radiation to the pelvis or to the proximal femur. In those situations, the reconstructive bed may be avascular and cement has to be used.

I recognize that there have been some series of patients that have had trabecular metal cups in post-radiation acetabuli with early promising results. Overall, however, I think that the elderly and the cancer population would still be best served with an instant solution like cement.

Lombardi: All patient populations are potential beneficiaries of ultra-porous metal technology. However, this technology has revolutionized the reconstruction of patients who require revision hip and knee arthroplasty compromised by significant bone loss.

Berend: Patients with compromised bone quality and quantity may benefit from advanced fixation technologies. Acetabular augments are a primary example that have facilitated more anatomic hip center restoration with superior acetabular deficiencies.

For TKAs, metaphyseal defects secondary to severe osteolysis may now be candidates for cone, augment and uncemented metaphyseal fixation. This may reduce the need for structural allograft for metaphyseal support.

Lewallen: One could argue that virtually all patients would benefit from improved highly porous bone ingrowth surfaces due to the reduced risks of aseptic loosening, if cost was not an issue. Unfortunately, cost is an issue, and this may require judicious use of these devices in clinical scenarios where they are of greatest benefit. The tendency at our institution has been to use these materials more frequently in revision cases, especially those with large defects or poor quality bone where the risk of failure of standard implants is greater, and to use these implants in primary arthroplasty in younger patients where long-term durability of fixation is of greatest concern and where the effects of stress shielding around the implant may be issues many years later should subsequent revision be necessary.

As an increasing number of highly porous metal systems become available, one would anticipate more competitive pricing and this might result in a broader use of the technology in more routine patient populations.