What Are the Considerations for Static vs. Articulating Antibiotic Spacers in Revision THA?
Dr. Scott Sporer answers ICJR’s questions about the types of antibiotic spacers available to surgeons performing revision total hip arthroplasty, the pros and cons of the different types of spacers, and his recipe for antibiotic cement.
ICJR: What are the options for the antibiotic spacer used in a revision procedure for an infected total hip arthroplasty (THA)?
Scott M. Sporer, MD: Antibiotic-eluting hip spacers can be categorized into either static or articulating designs:
Static antibiotic spacers provide no formal connection between the femur and the acetabulum. They can be constructed with multiple antibiotic beads placed over suture material or wire or with a femoral dowel (with or without metal reinforcement) and an associated antibiotic puck to fill the acetabular void.
Articulating antibiotic spacers allow motion to occur between the antibiotic-eluting spacer and the acetabulum. In general, they should be used in patients with minor cavitary defects of the acetabulum and proximal femur and avoided in patients with severe acetabular and/or femoral bone loss or with abductor deficiency.
Many methods are available for constructing an articulating antibiotic spacer:
- Self-constructed articulating antibiotic spacers are generally assembled intraoperatively and resemble a polymethylmethacrylate (PMMA) hemiarthroplasty. These are often constructed by placing PMMA (with or without the addition of antibiotics) over some sort of metal “backbone” (eg, Rush rod, Ender’s Nail, small femoral hip stem), along with a femoral head portion. The femoral head can be constructed of PMMA (handmade or formed using a bulb syringe as a mold) or, if a hip stem is chosen as the metal backbone, a prosthetic femoral head can be used.
- A hip mold system can be used to construct articulating spacers intraoperatively. Two general designs are available. The Prosthesis with Antibiotic Loaded Acrylic Cement (PROSTALAC) is the most well-known intraoperative mold. It uses a femoral stem core over which antibiotic cement is applied and subsequently clamped together by the mold. A femoral head is then attached to the trunnion of the core, along with an all polyethylene constrained liner cemented to the acetabulum. The alternative intraoperative mold system uses a silicon reverse mold into which PMMA is injected.
- Prefabricated spacers offer the advantage of an off-the-shelf prosthesis with antibacterial characteristics. These spacers have been approved by the US Food and Drug Administration for 2-stage treatment of infected hip prostheses.
ICJR: What does the literature indicate are the pros and cons of these options?
Dr. Sporer: The potential advantages of a static spacer include the increased surface area for elution of antibiotics and the minimization of stress directly onto the surrounding bone. Many surgeons recommend this type of spacer when the patient has severe acetabular or proximal femoral bone loss.
The use of a static spacers, however, results in the overall shortening of the leg between the first and second stages of the revision procedure, and also frequently results in soft tissue contracture. Patients with a static spacer are required to be non-weight-bearing until the second stage of reimplantation. In addition, theoretical concerns with static spacers include disuse osteopenia due to the lack of stress applied to the proximal femur and the acetabulum and the higher risk of postoperative instability following the second stage of reconstruction.
Advantages of self-constructed articulating antibiotic spacers include the relatively low cost and the ability to adjust the length of the stem and the diameter of the femoral head to match an individual patient’s anatomy. The surgeon can also easily adjust the type and dosage of antibiotics contained in the spacer.
Making a self-constructed antibiotic spacer is time-intensive and requires personnel in the operating room who are familiar with the technique. Dislocation and fracture have been observed with this type of spacer and have been attributed to the large head/neck ratio and failure to adequately secure the spacer to the proximal femur.
Both the PROSTALAC and the silicon hip mold systems allow the surgeon to alter the antibiotics and dose included in the spacer. The surgeon can also choose the femoral stem length and the femoral head size independent of each other for a fit that most closely resembles the host anatomy. However, these mold systems are more costly than self-constructed antibiotic spacers.
Prefabricated antibiotic spacers have traditionally contained a relatively low-dose aminoglycoside. Newer-generation designs have included vancomycin and have allowed an all-PMMA articulation along with a PMMA acetabular component.
Despite the lower total antibiotic dose, many prefabricated spacers have been shown to provide higher local antibiotic concentrations due to their dimpled surface finish and resulting increased surface area. Additional advantages to prefabricated spacers include surgical efficiency and the ability to avoid the surface irregularities of a self-constructed spacer.
Similar to a mold system, prefabricated spacers are more costly than self-constructed spacers. They are also available in limited sizes.
ICJR: Do you have a preferred type of articulating antibiotic spacer when you’re revising an infected THA?
Dr. Sporer: Few studies have compared clinical outcomes with the multiple antibiotic hip spacers that are available. This is not surprising: Attempting to compare treatment outcomes is difficult given the various spacer sizes/geometries, antibiotic dosing, antibiotic concentration, infecting organism, and definitions of “cure.”
Despite the paucity of randomized controlled trials, available data support the use of articulating antibiotic spacers whenever possible, including a strong recommendation from the recent International Consensus Meeting on Musculoskeltal Infection. It was noted at the Consensus Meeting that patients with a periprosthetic joint infection had improved range of motion and fewer functional limitations with this type of spacer. Major femoral/acetabular bone loss or loss of the abductor mechanism were felt to be the only reasons to consider a static spacer.
I personally use a prefabricated articulating antibiotic spacer for the majority of my patients with an infected THA. When using a prefabricated spacer, I will also secure the femoral spacer (and acetabulum, depending on the vendor) with PMMA containing high-dose antibiotics, including an organism-specific antimicrobial agent.
ICJR: What is your “recipe” for antibiotic cement?
Dr. Sporer: The ultimate goal when using any antibiotic-impregnated spacers is to provide local delivery of a high concentration of antibiotics and promote the eradication of infection. Antibiotic elution from a PMMA spacer depends on the antibiotic dose, the antibiotic chosen, the type of PMMA cement, and the geometry of the spacer.
Adding more antibiotics to the cement is an “off-label” use. Having said that, vancomycin and tobramycin are frequently mixed together and have been shown to provide a high level of infection control. Both antibiotics remain heat stable and have been shown to increase the overall elution of vancomycin when mixed together – a “passive opportunism.”
The most common “recipe” I use for articulating and static spacers is 3 grams of vancomycin and 2.4 grams of tobramycin per 40-gram batch of high-viscosity cement. The antibiotic dosing is doubled when low-viscosity antibiotic cement is used. In addition, I add an antifungal agent to the cement when a patient has a deep fungal infection.
It is important to note that renal function should be monitored postoperatively, especially in patients with underlying renal insufficiency.
Scott M. Sporer, MD, is Professor of Orthopedic Surgery at Rush University Medical Center, Chicago, Illinois, and a hip and knee reconstruction and replacement orthopedic surgeon at Midwest Orthopaedics at Rush, Chicago, Illinois.
Disclosure: Dr. Sporer has disclosed that he receives royalties from and is a paid consultant for OsteoRemedies.