Surgeons offer insight into the pros and cons of emerging porous metal technologies: Part 2

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. Part 2 of this two-part Orthopedics Today Round Table continues the dialogue particular to the re-emergence of cementless tibial fixation employing these 3-D, highly porous matrices. They are an attempt to counter the fact that cementless tibial fixation, when first introduced in the 1970s, met with both loosening and bone loss, contributing to its abandonment. Particular emphasis in this current dialogue addresses questions and provides insights into the use of these materials as a substitute for structural allografts for treating bone deficiency in both hip and knee revision situations. Part 1 appeared in the December issue.

A. Seth Greenwald, DPhil (Oxon)
Moderator

A. Seth Greenwald, DPhil (Oxon): Cementless tibial fixation is again receiving increased interest, but component loosening as a result of bone loss and subsidence has discouraged this application. Will these new surfaces address these problems and how may they influence overall design geometries?

Paul F. Lachiewicz, MD: I do not use cementless tibial fixation in primary total knee arthroplasty in patients of any age.

Allan E. Gross, MD, FRCS(C): It is possible that Trabecular Metal trays with a conventional stem may offer ingrowth and stabilization better than the conventional porous metals. The issue here, however, is that cement fixation of tibial components in total knee arthroplasty (TKA) have provided an excellent reconstruction and the data shows that they last up to 15 years plus. There are, however, situations where we are dealing with obese patients or young patients where a total knee replacement is the only option and here I would concede that cementless fixation is of benefit. There are cementless tibial components available on the market today, but their use and the results have been limited.

Getting bony ingrowth into a conventional porous surface on the tibial side is difficult because of micromotion related to the tibial trays. In that sense, I think that Trabecular Metal (Zimmer Inc.; Warsaw, Ind.) would offer a better press fit, less micromotion and a better opportunity for bone ingrowth. Another question that arises is, “What you do about the stem?” Even in primary knee replacement there is a short stem and, if it is made out of Trabecular Metal, it may be difficult to remove at the time of revision arthroplasty. Having ingrowth into a Trabecular Metal stem would be optimal, but then again it may not be necessary. It may be better if the stem was cemented and the tibial tray was made of Trabecular Metal.

Adolph V. Lombardi, Jr., MD, FACS: While cementless fixation has been widely embraced in total hip arthroplasty (THA), its use in TKA is limited. The Achilles’ heel of cementless fixation in TKA has been the tibial component. Therefore, several manufacturers have introduced tibial components with ultraporous metals at the interface. Initial reports are extremely promising; however, long-term follow-up is required.

Michael E. Berend, MD: There is opportunity to improve fixation through implant geometries and improvement in bearing locking mechanisms and even a return to one-piece implants. Long-term data now exists on non-modular designs with durable molded polyethylene.

David G. Lewallen, MD: We are only now approaching a decade post-surgery with our earliest cases employing Trabecular Metal uncemented tibial components. Initial experience at minimum 5 years has been encouraging, but it is safe to assume that these implants will be of greatest benefit in select patients with better bone quality who are higher demand and perhaps of younger age. In patients who are older, with poor quality bone and where the failure of cemented implants such as cemented all polyethylene tibial components is extremely low, these implants will be perceived to have less added value. Additional data is necessary though at 10 years and beyond to validate this technology choice for the young, active patient population.

One additional advantage to selecting porous tantalum implants is the ability to use monoblock devices, which eliminate backside wear as a source of particulative debris and osteolysis. The combination of improved polyethylene technology and a monoblock highly porous ingrowth surfaces would be appealing in the young high-demand population in the years ahead.

Leo A. Whiteside, MD: New porous devices make the difference for the tibia, but the new surfaces add a little extra. Porous stems, wedges, cones and blocks make it feasible to construct an implant suitable for osteointegration is the toughest of circumstances of bone loss and deformity. Certainly, these devices will eliminate the concern about subsidence of the tibial component. Already there are many reports in the literature of highly successful fixation of the tibial component using osteointegration, and these new devices and surfaces will make this procedure accessible to a wider population of orthopedic surgeons.

Greenwald: With the growing number of TKAs being performed in the United States in a younger patient population and assuming that these porous metal technologies will be used more often as well, what potential complications are anticipated?

Gross: Once again, the potential complication is the difficulty of revision and the sacrificing of more bone stock if these implants have to be removed. Obviously, if ingrowth does not occur, then revision would be no more difficult than a standard revision. But if these implants are solidly ingrown, then revision might require excision of a Trabecular Metal implant with a lot of associated bone.

Lombardi: Indeed, there are a growing number of arthroplasties being performed in increasingly younger patients. Ultraporous metals provide an alternative fixation modality. However, fixation has not been our most significant problem in TKA. For example, infection is a significant cause of failure. With cement fixation we can use antibiotic impregnated cement, which has been demonstrated in registry data to decrease the incidence of postoperative infection. At this point, antibiotic delivery is not possible with ultraporous technology. Having said this, there is current research into the development of antimicrobial coatings for these ultraporous metals.

Berend: Most complications with shifts to uncemented implants will follow what we have learned from THAs, notably increased fracture rates, wear and osteolysis as unintended consequences, with improved fixation interface durability.

Lewallen: The only special issues or complications that might differ from standard knee component usage relates to removal of well-fixed highly porous metal implants used in an uncemented fashion. On the tibial side, this is relatively easy using a special method, as the pegs on the Trabecular Metal tibial components can be divided with a broad osteotome after a pencil tip or a thin blade saw is used to divide the bone beneath the flat portion of the tray. It is best not to use a saw or burr to go through the pegs as this generates particulative debris and they are easily transected with a broad osteotome.

Whiteside: Younger populations are being treated with TKA. Osteointegration techniques and devices are especially suited for these patients. The downside of this lies mostly with engineering. New devices always come with unexpected engineering problems. Experienced development teams are absolutely essential as we proceed.

Greenwald: Will these fixation surfaces have any influence on the bearing surfaces of both hip and knee implant systems?

Lachiewicz: My bearing surface in all primary and revision THA has been metal-on-highly cross-linked polyethylene since 2000. The results have been so good that I never considered metal-on-metal or ceramic-on-ceramic bearings. For TKA, we are studying the use of highly cross-linked polyethylene in primary knee arthroplasty. In revision TKA, many of the liners are constrained condylar, and standard polyethylene is used. So, the use of enhanced porous metals is independent of bearing surface for me.

Gross: I do not feel that there would be any influence in that we are always searching for a safer and longer lasting bearing surface. It is possible that this new porous metal technology, since the fixation is longer-lasting, will increase the efforts to search for a bearing surface that is even longer-lasting than the ones we have now. I think, however, that the new bearing surfaces including, crosslinked polyethylene and ceramics, have changed the game related to the problems of bearing surfaces.

Lombardi: Fixation and bearing surfaces are two separate issues in both hip and knee arthroplasty. However, it is well known, especially in TKA, that direct compression molded polyethylene has demonstrated minimal wear. These ultraporous metals allow for direct compression molding of polyethylene into the metal substrate itself, and therefore, can eliminate backside wear in knee arthroplasty. Furthermore, they allow for a viable option for patellar resurfacing.

Historically, metal-backed patellae were fraught with complications of polyethylene wear and dissociation. This dissociation is virtually eliminated with direct compression molding of the polyethylene into the substrate of ultraporous metal. Likewise, on the acetabular side, it is attractive to construct acetabular components with direct compression molding into the ultraporous metals. One could also consider a ceramic-on-ceramic option for hip resurfacing if these metals can be attached to a ceramic or ceramicized bearing.

Berend: A shift to one-piece implants will help improve the problems associated with modularity and uncemented implants.

Lewallen: The only influence highly porous metal surfaces may have on bearing surfaces in the hip and knee would be to help extend and improve their performance by reducing loosening and in the case of monoblock designs redirection of the generation of particulatve debris that occurs from modular connections to inserts or flat trays in modular designs.

Whiteside: New surfaces are already being applied to both hip and knee implants, including primary and revision components. These improved technologies will replace the old ones.

Greenwald: These porous metals have also contributed in a major way to the decreasing use of structural allograft materials in situations of bone deficiency. In this context, what are the advantages and disadvantages for both hip and knee revision reconstructions?

Lachiewicz: The enhanced porous metal acetabular augments have eliminated the use of bulk acetabular allografts in my practice, although, when needed, femoral or tibial allograft struts are used on the femoral side. With the use of Trabecular Metal cones for large tibial and femoral defects, a bulk allograft has not been necessary in revision TKA (except for extensor mechanism allograft) in more than 8 years.

Gross: The advantages of using the new porous metals for segmental defects when doing a revision hip or a revision knee is that they are off the shelf, easy to use, will not resorb and have less of a tendency to fracture. There is also still the threat of disease transmission with allografts; however, that threat has decreased significantly with all of the new testing procedures that the tissue banks use. The main disadvantages are that the porous metals may make the re-revisions more difficult in that they not only do not restore bone stock but as part of the revision the porous metal has to be removed, then bone stock may be destroyed even further. It has been well documented that even in failed allografts there is some restoration of bone stock, which makes the revisions easier.

Lombardi: While initially successful, structural allografts do not withstand the test of time. Initially, healing occurs at the allograft-host junction followed by creeping substitution. However, resorption occurs at a more rapid rate, which, therefore, leads to the loss of integrity of the structural allograft. With porous augments there is rapid bone ingrowth with stabilization of the device. Since it is a metallic device, there is no compromise in its mechanical properties with time. These augments are now provided in a variety of shapes and sizes, which allow for intraoperative customization. Therefore, they have taken over the role of structural allografts.

Berend: Once fixed to the bone with osseous integration, a lasting interface may be achieved. This may reduce the second decade graft resorption issues leading to implant loosening.

Lewallen: Our institutional experience with structural allograft has revealed a troubling incidence of failure at intermediate-term follow-up. In the knee for example, structural allografts failed approximately one in four cases at around 8 years related to resorption, implant loosening or infection. Our initial experience with both acetabular augments and cone-type augments in the knee suggests a lower intermediate-term mechanical failure rate for these so-called, “prosthetic structural allograft replacements.”

Reliable fixation appears to be achieved as well when the construct as a whole is stable initially, and well-fixed during the initial few months. Thus, for both the acetabular reconstructions and for reconstructions of bone defects in the distal femur and proximal tibia, Trabecular Metal augments have improved our ability to achieve and maintain durable long-term fixation out to 10 years. The disadvantages of these implants as previously discussed relates to the relatively rare difficult problem of late hematogenous infection in a well-fixed construct. This requires accessing the implant bone interface and dividing it with cutting tools, such as a pencil tip burr, and in the case of the proximal tibia, may require tibial tubercle osteotomy in order to open the bone and access the implant for removal purposes similar to extended osteotomies of the femur for removal of extensively ingrown femoral components.

Whiteside: Augments have certainly decreased the use of allografts and have also decreased the use of custom components. This will probably save money, and almost certainly will decrease the failure rate and complications associated with mechanical failure of block allograft.

Greenwald: How universal are these augments or are they system-specific, and does this suggest a downside of their application?

Lachiewicz: The acetabular augments of one manufacturer can be used with porous-coated acetabular hemispheres of a different manufacturer. However, the tibial and femoral enhanced metal cones and sleeves work best with the total knee implants of the same manufacturer.

Gross: I have had no experience with any of the other systems, as I have only used Trabecular Metal. I think, however, that if the other porous metals are made out of a material that is compatible with the other implants that are part of the reconstruction, then doing crossovers should not be a serious problem. The systems however have specific designs that make the diameter of the porous metal augments, etc. compatible with the components that you are putting in. Therefore, it makes them technically much easier to use. If, however, a surgeons finds himself in a situation where he has to do a crossover technique between companies and as long as the two metals are compatible, I do not think that there would be a problem. In a situation like that, a thin layer of cement might obviate a potential problem.

Lombardi: Ultraporous bone augments are completely universal. They are not system specific. While overall similar in shape, each manufacturer has slightly different sizes and shapes, which maybe optimal in certain revision scenarios. Therefore, it is not inappropriate to use augments from one manufacturer and an implant system from a different manufacturer. This is applicable in both the hip and the knee.

Berend: Many are now not system-specific, but rather, shape-specific. They may well enlarge and bridge an important gap between standard implants and custom implants based on preoperative CTs.

Lewallen: One of the advantages of the initial generation of both acetabular augments for the hip and femoral and tibial augments for the knee, is they can be used interchangeably with a variety of different components and implant systems, and thus once reconstruction of the bone occurs and a foundation is established for the revision implant, the surgeon may use whatever device suits the patient’s clinical needs or the surgeon’s preference. These implants allow us to achieve good support for an off the shelf revision system into an area of bone deficiency that otherwise would not have allowed the use of a standard revision implant. The main downside of these implants, which have limited their use, remains their expense.

Whiteside: Many of the augments are not system-specific, and this is probably an advantage for the implant companies as well as for the patients and surgeons. However, some components cannot be interchanged with ease, and new systems that employ exclusive technology are welcome.

Greenwald: What role does bone preparation play in achieving apposite fit and in this context, are they used in conjunction with acrylic bone cement, bone graft, bone graft substitutes and adjunctive screws?

Lachiewicz: The use of enhanced metal acetabular augments and tibial and femoral cones do require specific bone preparation, usually with a high-speed burr (such as MidasRex or BlackMax). The recent addition of tibial rasps for the Trabecular Metal tibial cones has somewhat simplified the technique and ease of implantation.

The acetabular augments are always fixed with one or two screws, then a thin layer of bone cement “unitizes” the augment with the enhanced porous metal hemisphere, which also is fixed with multiple screws.

For the tibial and femoral cones, these are press-fit into the respective metaphyseal defect, and bone graft seals the periphery and any gaps between the enhanced porous metal and host bone. The tibial and femoral components are then cemented into the inner surface of the cone along with a stem extension of appropriate length.

Gross: The bone has to be prepared not only to fix the augment, but it also has to bleed so that ingrowth and eventual stabilization occurs. Fixing a porous metal augment on an avascular surface will not lead to ingrowth and stabilization. Bone cement is not used to fix the augment. It is, however, useful in situations where the bleeding host bone surface is some distance from the rest of the reconstruction. For example, we have had situations lately where we could not use a cup cage because there was insufficient bleeding host bone in the acetabulum, or we could just not get a press fit with a Trabecular Metal cup. In this situation, we used a conventional cage. In addition to the conventional cage however, we managed to secure an augment to bleeding host bone a centimeter or two away from the cage. The cage reconstruction was then cemented to the augment so that the entire reconstruction then had at least one part in contact with bleeding host bone for eventual permanent ingrowth and stabilization. The theory is that this would cut down on the micromotion of the conventional cage and eventual failure. We have done this approximately half a dozen times with good preliminary results. I think that morsellized bone grafting is of some advantage to restoring bone stock, and we place graft in the windows of the porous metal augment or adjacent to them. Screw fixation is absolutely necessary to obtain rigid fixation allowing ingrowth.

Lombardi: Bony preparation is integral to the success of any cementless device. The bone bed upon which the augment will be placed should be meticulously prepared to accept the augment. Hand tools, such as rongeurs, curettes and osteotomes, used in conjunction with high-speed burrs and reamers, are required to prepare the bone to accept these metallic augments. In acetabular reconstruction, augments utilized in uncontained deficits are fixed to the host with multiple 6.5-mm screws. In contained acetabular reconstruction and in the knee where most of augments are conical in shape, press-fit fixation of the augments to the host bone is adequate. Once the augments are secure, the implants are placed and the interface between the definitive implant and the augment should be cemented or unitized.

Berend: Bone preparation is more of an art with porous metal augments about the hip and in the methaphyses of the knee. Few reproducible instrument systems make this easier as each defect pattern is unique.

Lewallen: The goal in these reconstructions is to maximize contact of host bone against the highly porous metal surface whether this is an acetabular component or an augment on the acetabular side, the distal femur or the proximal tibia. Bone preparation occurs with either a burr or, in the case of some of the smaller size augments, companion rasps for preparation of the cancellous bone within the metaphysis of the tibia or femur. Any residual gaps between the augments and host bone should be filled with either bone graft material, such as autograft or allograft cancellous chips, or with a variety of other bone graft substitutes such as one of the moldable putties.

This improves the potential for bone apposition, the potential ingrowth to the implants and can help prevent cement extravasation into the interface between the augment and the bone, which is undesirable. Adjunctive screw fixation has been used most often with acetabular augments depending on their position and can provide additional stability to the entire acetabular reconstruction when cement is used to unitize the augment to the acetabular component, which is our routine.

Whiteside: Bone preparation will always be among the most important technical issues in total joint arthroplasty. The best systems make bone preparation obvious and straight-forward with tools and alignment devices that are nearly foolproof. Polymethylmethacrylate cement is a great adjunct to construction of complex implants that are designed at the operative table for complex cases, but its use as a bone-implant grout will disappear. Bone graft already is almost exclusively limited to filler autograft and allograft, and will continue to play this type of role in complex joint reconstruction. Screws will always be an important part of revision surgery, but with good design and materials, will seldom be needed in simple primary cases.

Greenwald: What has been the learning curve of their employ? Do they require increased technical proficiency?

Gross: I do not think that they are technically difficult to use particularly in the hands of somebody who does this type of revision surgery. This technique is similar but somewhat easier than using a structural allograft. However if the surgeon is not particularly experienced in revision surgery, then hands-on-courses would be of some advantage.

Lombardi: Surgeons who are proficient in acetabular reconstruction and have used structural allografts to reconstruct the acetabulum will find that these augments are perhaps easier to use than structural allografts. Furthermore, ultraporous technology has demonstrated enhanced fixation, which in turn has led to increased use of jumbo cups in more difficult revision scenarios. This in of itself has decreased the complexity of acetabular revision.

Berend: Based upon their infrequent presentation, there is certainly a learning curve. I would suggest experience in number of cases is needed, rather than a unique technical proficiency. The main limiting issue in using these technologies is obtaining adequate exposure of the acetabulum or knee in order to safely and accurately implant many of these designs.

Lewallen: Major bone defects reconstructed with these high porous metal augments end up being exercises in custom carpentry. The learning curve is relatively short for surgeons with any prior allograft experience, but it requires some innovation on the part of the surgeon in optimizing the fit of the available shapes and sizes to the particular defect encountered. In many ways, this is similar to the technical exercise of fashioning allografts to fit acetabular, distal femoral or proximal tibial bone defects. The main difference is that we modify the host bone to fit the Trabecular Metal augments, rather than burring or modifying the surface of the augment which will be against the host as this otherwise would cause smearing and deleterious effects in the surface porosity of the material and this might impair bone ingrowth. It is therefore necessary to try and pick an augment that best fits the defect encountered and then rebuild the knee off of the new metaphyseal platform created.

Whiteside: The learning curve should be worked out before the new devices are released. Design surgeons and experienced engineers must work this out meticulously, then the devices should be used by general orthopedic surgeons under close scrutiny as the procedures, instruments and implants are finalized. Then a straight-forward and simple system should be released to the public. We have had enough heavy-duty marketing, early release and recalls.

Greenwald: How do the outcomes of your structural allograft experience compare with those where contemporary porous metal augments were used?

Gross: The early outcomes of porous metal augment are impressive. The results however are mainly early- to mid-term, and therefore cannot be compared to the results of allografts yet. Our experience with structural allografts is that even when they fail, as long as they fail after 5 years, there is restoration of bone stock. We have data on structural allografts with a follow-up of more than 15 years that says the re-revisions, in most cases, do not have to have any kind of augmentation with a porous metal or another structural graft.

Our data supports that these structural allografts have, in fact, restored bone stock. We do not have that experience yet with the augments since they have not been around long enough. There is the potential that these augments may, in fact, provide a stable base for the next re-revision, but there is also the potential that they have to be removed and removal of them may require excision of host bone.

Lombardi: In short-term follow-up, structural allografts provide excellent bone reconstruction and fixation for complex revisions. However, in long-term follow-up, it is noted that these structural allografts are plagued by resorption, and therefore, compromise of fixation. Porous metal augments provide for immediate structural augmentation. Bone ingrowth occurs rapidly and predictably. They are relatively easy to use and do not carry with them the risk of disease transmission.

Berend: This is a tough question to answer as these cases are challenging and have a myriad of complicating factors such as, instability about the hip, pelvic discontinuity, instability about the knee, extensor mechanism problems and infection. Often, the complexity of the cases involves a complex multifactorial failure mechanism rather than problems directly related to the structural allograft resorbing or lack of fixation with a porous metal augment.

Lewallen: In addition to the above, the intermediate follow-up of porous metal augments are outperforming structural allograft. Longer term follow-up will be necessary to validate the appearance superiority of these augments at 10 years and beyond.

Whiteside: Outcomes are better with porous augments than allografts in the hands of an expert surgeon, and will probably be improved even more for the less expert operator. Allografts must be shaped and prepared before they make good implants, and their mechanical qualities are not uniform. These issues are addressed by engineering, design, instrumentation and procedures in a good system of implants and augments.

Greenwald: At the end of the day, can you prognosticate whether these advanced porous metal materials for both fixation and bone deficiency will further minimize cement and bone allograft usage in hip reconstructions, while increasing their application particular to TKA?

Gross: Whether they become a standard for a primary hip or knee replacement will depend on the need of revision. Our success with conventional hip and knee replacement has been so good, that there is no need to use porous metals in the primary situation in most cases. Also, they are considerably more expensive. The revision situation is different. On the acetabular side of hip revision, if these porous metals prove to be long lasting and if the re-revision is not only possible but enhanced by these augments, then they could become the standard and they could replace structural allograft. I think it is too early to say that. Yet, however, at our institution in the young patient who is being revised we still use structural allografts because we know they will restore bone stock for the next revision. In the revision situation, it is therefore too early to state that they will completely replace the use of structural allograft, although things might be headed in that direction.

Lombardi: Ultraporous technology has truly revolutionized the field of adult reconstruction of the hip and knee. To date, it has had a greater impact on the hip than the knee and more on the acetabulum than the femur. This technology will continue to be enhanced. Currently, there are several new formulations under investigation. With the concurrent increasing advances of 3-D reconstructive technology, patient-specific custom augments will become the norm. On the femoral side, the enhanced fixation afforded by these materials may allow for the introduction and use of shorter femoral components. As a result, enhanced soft tissue attachment sleeves and trochanteric attachment devices are being developed to facilitate a greater degree of soft-tissue attachment in large femoral reconstructions.

With respect to the knee, several cementless tibial components have been introduced with ultraporous technology and have gradually been gaining in popularity while the femoral and tibial coned augments have virtually eliminated the need for structural allograft. While cement seems to work well in TKA there is an increasing interest in cementless TKA. Perhaps, this ultraporous technology will allow for enhanced cementless fixation and ultimately the adoption of cementless fixation in TKA.

Berend: I believe porous augments will be a valuable tool to treat complex deformities reducing, but not eliminating, the important role cement and allograft play in the treatment of these conditions.

Lewallen: For major acetabular reconstruction and for the management of major defects in the knee at the time of revision surgery, we have essentially abandoned structural allograft and converted to the use of highly porous augments to fill the major defects in more than 90% of the cases that require these strategies. The rare exception might be bone defects in young patients where the operating surgeon wishes to restore bone stock for the future as opposed to filling the void with metal. However, even this indication appears to be decreasing as our favorable experience with highly porous augments increases.

Whiteside: Improved porous implant technology will continue to displace allograft and cement from arthroplasty surgery. Dead bone will disappear, and cement will remain as a good way to stick two dead things together.