Femoral stem modularity: Parts, pieces and connectors — Part 1

Introduction

The evolving use of component modularity in femoral stem design has found principal application in revision hip procedures. The advantages of these systems include off-the-shelf flexibility for customizing proximal and distal canal filling, preservation of soft-tissue structures, biomechanical restoration of offset, version and leg length as well as accommodating difficult situations of femoral deformity and bone loss. 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 continuously cited since their introduction.

According to the Millennium Research Group, almost 460,000 primary and revision total hip arthroplasties were performed in the United States in 2010. Of these, 7% employed modular femoral stem designs. The graph on page 34 suggests their increasing use, although still a relatively small percentage of the total.

In part one of a two-part Orthopedics Today Round Table discussion, we bring together experienced hip arthroplasty surgeons whose collective background in the use of modular femoral stems is intended to provide readers insight into the pros and cons of these stems.

— A. Seth Greenwald, DPhil(Oxon)
Moderator

Round Table Participants Moderator A. Seth Greenwald, DPhil(Oxon) Orthopaedic Research Laboratories Cleveland J. David Blaha, MD University of Michigan Ann Arbor, Mich. Allan E. Gross, MD, FRCSC University of Toronto Toronto Michael J. Dunbar, MD, FRCS(C), PhD Dalhousie University Halifax, Nova Scotia, Canada Joshua J. Jacobs, MD Rush University Medical Center Chicago

A. Seth Greenwald, DPhil(Oxon): What is the extent of femoral stem modularity use in your practice? Is it confined to revision or are there situations where you have employed these designs in primary reconstruction?

Joshua J. Jacobs, MD: While there are indications for the use of femoral stem modularity in the revision situation with a bone-deficient femur, the indications in primary total hip arthroplasty are uncommon – limited to severe proximal femoral deformity.

J. David Blaha, MD: I use femoral stem modularity as minimally as possible. Virtually all heads are modular, so this use of modularity is unavoidable. In primary replacement, I have used the titanium modular neck in a titanium femoral stem extensively. A review of my experience showed that I used the same stem-neck combination in 85% of cases. It can be argued in these cases that the modularity was not necessary. Modular neck fractures and corrosion have made me wary of the use of the titanium alloy modular neck. My previous experience with the Wagner implant and recent experience with an updated version of that implant convinces me that modularity of the body stem of a revision hip implant is not necessary.

Michael J. Dunbar MD, FRCS(C), PhD: Femoral stem modularity is basically limited to revisions in my practice. In rare occasions, with particularly dysplastic and anteverted hips, I will consider a modular primary component. The routine use of cemented femoral stems allows me flexibility in component positioning that would otherwise require modularity. I use femoral stem modularity (mid-stem) in almost all revisions.

Allan E. Gross, MD, FRCSC: Femoral stem modularity is used mainly in hip revision procedures. In my practice, which deals with a lot of complex hip revisions, we use modular stems in approximately 90% of the cases, and we use monoblock full porous coated stems in the other 10%. We also use modular stems in some difficult primary cases where we have to perform a femoral osteotomy for deformity or for subtrochanteric shortening for high riding CDH, and on occasion, in very young patients where we want to get good proximal fixation with ingrowth but because it is a difficult primary we want to get some initial stability with our stem. In this latter situation, we use a porous body with a smooth press-fit stem. This allows us to use the diaphysis for later revision procedures. This is very similar to the S-ROM reconstruction. We, therefore, use femoral stem modularity in complex femoral revisions, which would be about 90% of the practice.

Greenwald: Do you employ both mid-stem and distal neck modularity options? In this context, what do you consider the advantages of these systems in comparison to their monoblock counterparts?

Jacobs: Beyond head-neck modularity, femoral component modularity is generally avoided unless there is a severely bone-deficient and/or deformed proximal femur.

Blaha: As I said in my previous answer, I use modularity sparingly. In the past year, I have not used a mid-stem or distal neck prosthesis for revision surgery.

Dunbar: I only use mid-stem modularity. The main advantages are ease of use and ability for the distal stem to find the best position in the femoral canal, without compromising the proximal version. This is particularly true in the varus-remodeled femur with a significant anterior bow. I do not see any advantages for distal neck modularity over a cemented stem.

Gross: We use only mid-stem modularity. We do not feel it is necessary to use the distal neck modularity options because when we use a mid-stem modularity we can easily adjust the version at the body stem junction. Also modular systems give you increased offset so we can therefore adjust both version and offset at the junction of the body and stem, thus negating the need for distal neck modularity. The monoblock options do not offer these adjustments of version and offset, and once it is inserted, you are pretty well stuck with what you have.

With a modular stem in the system that we use, the stem is inserted first after preparation of the femur, and then we have trial body options that we attach to the stem that is already in place and allows us to decide on the length of body we are going to use, its offset and version. I think this is a big advantage over the monoblock.

Greenwald: Are there particular patient contraindications that emerge from your experience with the modular designs?

Jacobs: Femoral stem modularity should be used with particular caution in the presence of a metal-on-metal bearing. The amount of metal debris produced is at least additive, and there may be a synergistic effect leading to accelerated mechanically assisted crevice corrosion and/or accelerated bearing surface wear.

Blaha: In the past, I have used a “modular neck” both for primary and revision hips, and was happy with the results of the prosthesis because of the flexibility offered for stem fixation and femoral head placement. Fractures have occurred in titanium alloy modular necks. The vast majority of those fractures were in cases in which a long modular neck was used in a heavy male patient who used the hip in heavy labor. This would suggest that in heavy men who do heavy work or in any patient who will load the prosthesis heavily, the long titanium alloy neck would be contraindicated.

Dunbar: Younger, high body mass index patients with high offset are a contraindication for distal neck modularity in my region based on our experience with an alarming fracture rate in this population.

Gross: The only contraindication I can think of is if there is absolutely no diaphysis left so that distal fixation is impossible. In that situation, either a proximal femoral allograft or a tumor prosthesis with replacement of the knee is necessary.

Greenwald: What is the learning curve of their employ? Do they require increased technical proficiency compared to monoblock stems?

Blaha: As with any prosthesis, there is a learning curve. My use of mid-stem modularity and modular necks did not seem to have a long learning curve. As long as the limitations of the devices in question are respected – and this refers more to preoperative planning than to technical skill at the operating table – then the learning curve is neither long nor steep.

Dunbar: The learning curve for mid-stem modularity is perhaps less steep in the revision scenario than the use of monoblock stems. There are technical issues with engaging the taper, but they are not dissimilar to engaging the femoral head onto the taper.

Gross: I do not think there is a significant learning curve, but it is helpful to attend at least one operative session with a surgeon who has had experience with a modular system. They definitely do not require increased technical proficiency compared to monoblock stems because they are safer with regards to the iatrogenic fractures and getting the version offset and leg lengths correct. In case I forgot to mention it in my previous answers, another advantage of the modular stem over the monoblock is that you can adjust leg length, not only through the femoral head, but also through the length of the body you use.

Greenwald: In your practice, how do the clinical outcomes of modular femoral stem usage compare with monoblock alternatives?

Blaha: I do not have data from my personal series of mid-stem, modular neck and monoblock so this answer must be an impression. I do not think there is any advantage to the mid-stem modularity in terms of clinical outcome. Again, as an impression without data, I believe that placing the head of the implant in the best position compared with the shaft improves patient function. The modular neck facilitates placing the head in the best position, but it also is possible to accomplish this positioning with fixed neck stems.

Dunbar: We experienced an unexpected series of fractures of titanium distal modular necks, which clearly skewed the clinical outcomes. There is no obvious difference in the outcomes of mid-stem modular components vs. monoblocks in the revision scenario. Operating times are shorter, however.

Gross: They are clinically better because the monoblock alternatives are usually stiffer and made out of cobalt-chrome, which can cause thigh pain. There are some publications that support this.

A note from the editor

Look for part 2 of this Round Table discussion in the January issue of Orthopedics Today.

• J. David Blaha, MD, can be reached at 1500 E. Medical Center Dr., Taubman 2916A, Ann Arbor, MI 48109-5328; 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 Ste. 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, FRCSC, 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., Ste. 300, Chicago, IL 60612-3828; 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 and receives royalties from Stryker and 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.