Issue: January 2012
January 01, 2012
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Femoral stem modularity: Parts, pieces and connectors — Part 2

Issue: January 2012
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Introduction

The evolving use of component modularity in femoral stem design has found principal application in revision hip procedures. Mid-stem and distal neck interconnections have been successfully employed for a variety of patient skeletal pathology. But coincident with their advantages, concerns relative to design structural integrity, stability, debris generation and tissue response have been cited. The debris burden arising from microdisplacements at these tapered connections may also contribute to the tissue reactions attributed to metal-metal articulations suggesting a further source of foreign body intrusion into the surrounding tissue.

Part two of this Orthopedics Today Round Table continues the discussion on femoral stem modularity with our panel of experienced hip arthroplasty surgeons. The remaining questions focus on observed adverse local tissue reactions, the metal debris burden arising from modular connections, the difficulty of modular hip revision and, ultimately, their cost benefit.

A. Seth Greenwald, DPhil(Oxon)
Moderator

Round Table Participants

Moderator

A. Seth Greenwald, DPhil(Oxon)A. Seth Greenwald, DPhil(Oxon)
Orthopaedic Research Laboratories Cleveland

J. David Blaha, MDJ. David Blaha, MD
University of Michigan Ann Arbor, Mich.

Allan E. Gross, MD, FRCSCAllan E. Gross, MD, FRCSC
University of Toronto Toronto

Michael J. Dunbar, MD, FRCS(C), PhDMichael J. Dunbar, MD, FRCS(C), PhD
Dalhousie University Halifax, Nova Scotia, Canada

Joshua J. Jacobs, MDJoshua J. Jacobs, MD
Rush University Medical Center Chicago

A. Seth Greenwald, DPhil(Oxon): What complications have you encountered inclusive of component or bone fracture, loosening, infection and the presence of adverse local tissue reactions (ALTRs) when metal-on-metal interconnections are involved?

Joshua J. Jacobs, MD: All modular connections are subject to mechanically assisted crevice corrosion (MACC). While most modular reconstructions seem to perform well, there have been retrieval studies that have documented the presence of extensive damage on some components. In a few instances, MACC at neck-body modular junctions have been associated with ALTRs similar to that seen in metal-on-metal bearings. These ALTRs have necessitated revision surgery. MACC at modular junctions can also predispose to implant fracture, which we also have observed.

J. David Blaha, MD: We have encountered fracture of the modular neck in some heavy, active, male patients with long neck titanium implants. We also have found corrosion at the modular neck to femoral stem junction. We have reported several cases of “hydrogen pneumarthrosis,” a condition in which hydrogen gas, which is a product of the oxidation of titanium, is trapped below the modular neck and is suddenly released into the joint. This condition mimics an acute infection with a gas-forming organism but is, self-limited although it is a harbinger of significant corrosion of the taper junction.

Michael J. Dunbar, MD, FRCS(C), PhD: Long titanium femoral necks fractured in high body mass index patients at an alarming rate — approximately 2 years postoperatively. Loosening, infection and ALTR have not been observed.

Allan E. Gross, MD, FRCS(C): I do not use metal-on-metal articulations so I have not seen any ALTRs. We have not seen an increased incidence of infection or bone fracture or loosening using modular femoral stems. The main complication that we have encountered, and this problem has been remedied, is fractures at the junction of these modular stems.

We first started using the system that we employ when we inserted them into high demand patients based on their size, weight, occupation and lifestyle. If we used pure distal fixation, then they were at approximately a 1% risk for stem fracture at the junction of the body in the stem. This has since been remedied by using an extra strength junction when we use pure distal fixation in this demanding group of patients. The other complication we have seen is subsidence when we used modular stems in patients where distal fixation is just not feasible because the bone loss is already down into the metaphyseal flare of the femur distally.

Greenwald: It has been reported that microdisplacements at metallic, taper junctions result in fretting and also, fretting corrosion when the passivated surfaces are interrupted. In your practice, have you observed these phenomena and is their causality a major reason for in-situ failure?

Jacobs: Our implant retrieval laboratory at Rush has been tracking this phenomenon and has reported that fretting corrosion damage at modular junctions is relatively common in devices retrieved at revision surgery. This damage has been observed at a variety of modular interfaces including the head/neck junction, the neck/body junction and the body/stem junction. Actual clinical failure from MACC has been observed in my practice, but it is a small number of cases. My suspicion is that this clinical phenomenon is underdiagnosed as a consequence of the fact that it is often not considered. When faced with a symptomatic total hip replacement reconstruction when there is no other explanation for the symptoms, serum metal ion analysis has been helpful in identifying patients with significant MACC, regardless of the bearing surface combination.

Blaha: As I have mentioned, we have seen hydrogen gas released into the joint with resultant pain that has required operative intervention because of the confusion of this condition with infection. In the fractures of the modular neck, when we have been able to disassemble the broken fragment from the stem, we have found significant corrosion. In theory, the corrosion could cause surface irregularities that could act as a “notch” in the titanium, and could be the start of a fatigue failure. In addition, the hydrogen gas could diffuse into the titanium metal and cause “hydrogen embrittlement” of the metal.

Dunbar: It is likely that the fracture I have reported on with scanning electron micrographs post-retrieval was related to pitting corrosion leading to failure under tension. On revision cases for infection in distal neck modularity, I have not noticed any pitting corrosion with titanium necks in titanium bodies. I have not noticed any corrosion in mid-stem modularity.

Gross: I personally have not observed this as I have not examined the fractures closely. Dror Lakstein and colleagues published a study in the Journal of Bone & Joint Surgery earlier this year, of which I am a co-author. This publication is an analysis of a group of fractured modular stems that closely examines these junctions. These are not my observations at surgery, but these fractures were examined more closely by this group of orthopedic surgeons and mainly engineers.

Greenwald: Specific to tapered interconnections, which can also include the femoral head-neck junction for both modular and monoblock stems, do you believe they play a role in the observed tissue manifestations that are associated with metal-metal articulations? Does this imply an increased metallic debris and ion burden into the surrounding tissues?

Jacobs: We have observed multiple cases in which MACC at modular junctions has led to an ALTR. In all but one case, serum metal ion levels, particularly cobalt levels, have been elevated implying that metal debris is etiopathologic.

Blaha: It would appear that the amount of corrosion seen at the modular neck junction would produce metal in sufficient quantity to contribute to the “ion burden” in the surrounding tissue. In my experience, however, only one case had the tissue surrounding a corroded modular neck that one would expect from reaction to metal debris and ions. I have observed corrosion at some of the head-to-taper connections but, until recently, had not considered it as a source of metal ions. With the experience with the modular necks, however, I believe that this taper junction could also contribute to the “ion burden.”

Dunbar: Likely it does. The additional taper increases the volume/risk for metallic debris. I think we will be seeing more on this in the future.

Gross: Personally I have not observed this. However, I have heard recently at meetings that there is some concern, particularly with femoral heads of 36 mm and larger, that there may be a metallic debris problem because of the taper. Certainly with the large head metal-on-metal, particularly when a sleeve was used on the neck, there is definitely a problem with metallic debris and ion release. This work was published by the University of British Columbia in an article with the main author being Don Garbuz, which won a Hip Society award. I think it is something that we have to keep a close eye on with regards to any tapers, but particularly with femoral heads that are 36 mm in diameter or larger. In my own practice, if it is a straightforward primary hip replacement, I do not go above 32 mm because I have not seen any problems with that. If it is a revision, however, then I would tend to go to 36 mm or 40 mm because of the increased risk of dislocation. This is something we are going to have to keep an eye on.

Greenwald: When revision is necessary, do modular systems present more of a technical challenge than monoblock designs? In both system options, when faced with a well-fixed stem but apparent damage at the head-neck junction, what is the clinical rationale for head replacement or complete stem removal?

Jacobs: Revision surgery in the face of MACC at modular junctions can pose technical challenges. In particular, the decision to retain or revise a well-fixed femoral component is complex, with no published data to guide the surgeon. When we have seen ALTRs in the setting of head-neck MACC, we have been retaining the well-fixed femoral stem while replacing the co-alloy femoral head with an alumina ceramic head containing a titanium alloy sleeve. In general, more than 2 years, this approach has been satisfactory, although longer term follow-up is needed.

Blaha: Both modular systems and monoblock stems present a significant technical challenge for revision. If the modular system could be disassembled and reassembled then, in theory, the revision of the modular system would be simpler. However, I have considerable concern about reassembling a taper that has been set once, cyclically loaded and then disassembled. The considerable damage to the femur that is necessary to completely remove a stem must be weighed against the worry of reassembling a taper junction. Up to now, I have replaced the head and left the well-fixed stem apparently with no ill effects. I may be reconsidering this as we learn more about the effects of the metal ions shed from the modular junction at the head and the fate of reassembled head junctions.

Dunbar: Distal neck modular stems may present as more of a technical challenge. In my experience, there are two potential pitfalls when revising these cases. Firstly, a retroverted femoral neck is difficult to dislocate from a direct lateral approach. Also, in some cases, distal modular titanium necks cold fuse to the titanium femoral stem and cannot be dissociated. A complete femoral revision is required in this case. Mid-stem modularity allows for dissociation of the bulky proximal portion that gives better access to medial bone and hence facilitates the removal of well-fixed stems. Revision of a well-fixed stem for a damaged head-neck junction should be decided based on patient factors, such as ability to tolerate the additional surgery. In general, I would be inclined to remove the stem.

Gross: The modular designs are somewhat easier to revise than monoblock. If you have a modular femoral component that is fixed distally, then the body can be disassembled from the stem and the stem can be removed using trephines. It is a time-consuming arduous procedure, but it is possible. Usually under those circumstances, the body is not ingrown and the disassembly is relatively easy.

With a monoblock that is distally fixed and needs removal, the upper part of the stem where you have the flair, neck, etc., has to be cut off and then trephines can be used. Using a modular system the body, head and neck can be replaced with the distal portion of the stem, which is well fixed, left inside providing it is not for infection. There is, in fact, a couple of papers out recently that the surgeons elected to leave well-fixed stems in when doing a revision for infection, but occurred after ingrowth of the stem had taken place. There are only a few papers in the literature that will back this up, but it is reasonable in high-risk patients or in patients that have late hematogenous infections, you would think that the already ingrown stem has provided a barrier against infection going down into that interface.

Greenwald: At the end of the day, modular femoral stems, on balance, are more costly than their monoblock alternatives. Do their advantages outweigh the added cost in contributing to an optimal outcome?

Jacobs: Revision surgery in the setting of a severely bone-deficient femur and primary surgery in the setting of a complex proximal femoral deformity are situations where the benefits of modular femoral stems may outweigh the risks and added expense. In routine primary total hip replacement, the benefits of femoral stem modularity are far less clear.

Blaha: I judge that the modular systems do not have advantage enough in terms of optimal outcome to outweigh the cost and, more importantly, the potential detrimental effect of the modular junction itself.

Dunbar: Not in distal-neck modular options. The efficiency and ease of use of mid-stem modularity in the revision scenario is worth the premium, in my opinion.

Gross: I think that for the difficult femoral revision there is no question that the advantages outweigh the added cost, and I would predict that their use will increase with time.

A note from the editors

Read part one of this Round Table discussion in the December issue.

References:
  • Garbuz DS, Tanzer M, Greidanus NV, et al. The John Charnley Award: Metal-on-metal hip resurfacing versus large-diameter head metal-on-metal total hip arthroplasty: A randomized clinical trial. Clin Orthop Relat Res. 2010;468(2):318-325.
  • Lakstein D, Eliaz N, Levi O, et al. Fracture of cementless femoral stems at the mid-stem junction in modular revision hip arthroplasty systems. J Bone Joint Surg Am. 2011;93:57-65.
  • J. David Blaha, MD, can be reached at 1500 E. Medical Center Dr., Taubman 2916A, Ann Arbor, MI 48109; 734-647-9961; fax: 734-647-3125; email: jdblaha@med.umich.edu.
  • Michael J. Dunbar, MD, FRCS(C), PhD, can be reached at QEII Health Sciences Center, New Halifax Infirmary, Suite 4822, 1796 Summer St., Halifax NS B3H 3A7, Canada; 902-473-7337; fax: 902-473-7370; email: michael.dunbar@dal.ca.
  • A. Seth Greenwald, DPhil(Oxon), can be reached at Orthopaedic Research Laboratories, 2310 Superior Avenue East, Cleveland, OH 44114; 216-523-7004; fax: 216-523-7005; email: seth@orl-inc.com.
  • Allan E. Gross, MD, FRCS(C), can be reached at Mount Sinai Hospital, 600 University Ave., Suite 476(A), Toronto, ON, M5G lX5 Canada; 416-586-4800; fax: 416-586-8673; email: agross@mtsinai.on.ca.
  • Joshua J. Jacobs, MD, can be reached 1611 W. Harrison St., Suite 300, Chicago, IL 60612; 312-432-2344; fax: 708-492-5344; email: jjacobs@rushortho.com.
  • Disclosures: Blaha is a consultant to Wright Medical Technology and receives royalties from IP regarding knee replacement technology; Dunbar is a consultant to and receives royalties from Stryker, receives research support from the Canadian Institute of Health Research, Atlantic Innovation Fund, Natural Sciences and Engineering Research Council of Canada, Stryker, Wright Medical,Depuy, Smith & Nephew and Zimmer; Greenwald has no relevant financial disclosures; Gross is a consultant for Zimmer and has worked on the ZMR modular component; Jacobs is a paid consultant for Johnson & Johnson, Medtronic Sofamor Danek, Smith & Nephew, Spinal Motion and Zimmer, is an unpaid consultant for Implant Protection, and receives research support from Medtronic Sofamor Danek, Spinal Motion and Zimmer.