Pearls: Mind the Gap! The Art of the Static Antibiotic-laden Cement Spacer

As the number of arthroplasties performed around the world continues its year-over-year increase, the number of patients who develop prosthetic joint infection (PJI) will increase in proportion. Two-stage revision is the most-commonly performed approach for patients with PJI in many parts of the world, including the United States. Given the large number of patients who are likely to develop PJI in the years to come, it seems unlikely that all patients who experience this potentially limb- and life-threatening complications will be treated by subspecialists. That being so, it is important to develop approaches to the management of this problem that are easier to use. I believe that nonarticulating (static) spacers can be one such approach.

To take a step back, antibiotic-laden cement spacers are categorized as static or articulating. Articulating spacers may improve function while in situ and result in slightly improved ROM after reimplantation. However, articulating spacers are costly, can dislocate, and have not shown to be superior to static spacers with respect to infection eradication, patient-reported outcomes, or spacer-related complications [1, 2].

Alternatively, static spacers are easier to fashion and insert, making them perhaps more practical for surgeons who do not treat a high volume of patients with PJI. But static spacers are associated with some complications, including arthrofibrosis and spacer migration, which can result in bone loss or soft-tissue injury. Thus, it is imperative to “mind the gap” and be strategic about how the spacer is placed in the gap remaining after explantation of the implant and infected bone and soft tissues.

I believe it is possible to minimize these risks with just a few specific surgical steps. First, after removing the implant, cement, and infected tissue, I perform a thorough irrigation and débridement including the femoral and tibial intramedullary canals. Drapes, gowns, and gloves should be changed at this point to avoid recontamination. I add heat-stable antibiotic powder (vancomycin + aminoglycoside) to cement. Generally, I use a dose of 2 g of vancomycin and 2.4 g of tobramycin for every 40 g of cement with the exception of patients with renal failure or allergies to these medications who may require reduced dosing or alternative agents, respectively. I then add one to two drops of methylene blue to the cement to help distinguish spacer from bone at the time of the imminent replant.

Next, I make a cement dowel by placing one to two drops of mineral oil in the cement gun and then allow the cement to cure in the nozzle in the shape of the long cylindrical nozzle with a small “skirt” at the end where the nozzle flares. I then slide the dowel out of the cement gun, and place one in each of the reamed femoral and tibial canals (Fig. 1A).

Fig. 1A-B.

(A) A lateral view of spacer is shown. Cement dowels (denoted with *) are placed in the intra-medullary canals to enhance antibiotic contact in the tibia and femur. The tibiofemoral spacer fills the bone defect and space (hatched lines). The patellofemoral cement spacer prevents adherence of the extensor mechanism to the underlying femur thus facilitating exposure at the time of second stage revision (dotted line). (B) An AP view shows the patellofemoral spacer (dotted line) and the tibiofemoral spacer (hatched lines). (Illustration by Nicole Wolf MS, ©2020. Published with permission).

At this point, I make a tibiofemoral spacer first by mixing two or more packages of cement, (depending on the size of the gap) by hand with spatula in a bowl without vacuum, to increase porosity. The antibiotic is added during the dough phase to enhance antibiotic elution [3]. I mold the cement to fit between the tibia and femur. I ask an assistant to hold the lower extremity in neutral alignment, maintaining the correct limb length while the cement cures. It is crucial to contour the shape of the cement spacer to fit into the irregularly shaped bony defects (Fig. 1A-B). The surgeon should avoid a smooth “hockey puck” shaped spacer, which is potentially hazardous because it can gradually extrude through and damage the extensor mechanism (Fig. 2A-B). Reconstructions for this complication are complex and difficult, and few work well; techniques including allograft, synthetic grafts, or rotational flaps all have been used, but all can be associated with persistent functional impairment including chronic weakness and an extensor lag.

Fig. 2A-B.

(A) A symmetric hockey puck-shaped spacer is shown. (B) The hockey puck-shaped spacer can extrude catastrophically through the extensor mechanism. (Illustration by Nicole Wolf MS, ©2020. Published with permission).

Prior to closure, I take the time to protect the extensor mechanism by creating a patellofemoral spacer. I do this by creating a flat cement spacer to reside between the extensor mechanism and the femur. This will prevent adhesions between the extensor and the femur. In the absence of this interpositional patellofemoral component, the extensor mechanism will adhere to the underlying femur and interfere with adequate exposure at the time of the revision. The quadriceps and patellar tendons can be so scarred to the underlying bone that there is risk of catastrophic avulsion of the extensor mechanism when one attempts to evert the patella at the time of the second-stage reimplantation surgery. To minimize the risk of extensor mechanism disruption, one may need to may resort to extensile techniques of exposure such as quads snip or tibial tubercle osteotomy in order to gain access to the knee, each of which have associated pitfalls including persistent extensor lag or tibial tubercle nonunion. In order to avoid adhesions in the patellofemoral space and the risk of calling on these extensile exposures later, I simply place interpositional cement in this area.

A static spacer remains a viable option for the first stage of a two-stage revision as long as the surgeon takes care to “mind the gap” and respects the soft tissues.