Construction of a static cement spacer provides good stability to the knee between stages
Infection following total knee arthroplasty remains a rare, but potentially devastating complication, with a reported incidence of approximately 1% to 2%. Traditionally, chronic infections have been treated with staged revision arthroplasty procedures. These procedures include an initial removal of components, followed by placement of a temporary antibiotic-impregnated cement spacer with concomitant intravenous antibiotics (typically 6 weeks) and ultimately a reimplantation of components after the infection has been eradicated.
Surgical options for spacers include static and articulating designs, with each having advantages and disadvantages. Advantages of static spacers include low-cost, ease of implantation and immobility of the wound, which may be important for soft tissue healing. While the literature shows comparative results between both static and articulating spacers for the eradication of infection, static spacers may be preferred in cases of severe bone loss, a compromised soft tissue envelope that require immobility or an incompetent extensor mechanism where the risk of an articulating spacer dislocating is high.
Rachel M. Frank
Construction and implantation
Our preference for a static spacer includes the use of two intramedullary dowels fashioned over a threaded Steinmann pin. The Steinmannn pin adds strength to the construct (similar to rebar) and the use of a threaded, as opposed to a smooth Steinmannn pin, is recommended to prevent delamination of the cement off of the Steinmann pin during extraction. The Steinmann pin used is typically 4.8 mm in diameter and 23 cm in length. Although the amount of antibiotics to be used in spacers is controversial, our preference is 3 g of vancomycin powder and 1.2 g of tobramycin powder per package of high viscosity cement. High viscosity cement has been shown to elute antibiotics more efficiently than lower viscosity cement, and a combination of antibiotics (as opposed to the use of one agent) further improves elution and expands bacterial coverage; higher doses of antibiotics may be safe to use if a low viscosity cement is selected, however patients should be monitored closely for complications such as renal failure if higher doses are utilized.
The antibiotic powder is delivered onto the field and crushed using a metallic strainer (Figure 1) and then is mixed with the cement powder. The monomer is added to the powder and mixed by hand and then divided into thirds; each aliquot is then rolled by hand to create a dowel approximately the same length as the Steinmann pin (Figure 2A) followed by embedding the Steinmann pin into the cement (Figure 2B) and then rolling the dowel to completely cover the Steinmann pin with cement (Figure 2C). The dowels are now allowed to completely harden on the back table. One package of cement is typically adequate to construct three dowels, which can be made of slightly varying diameters (10 mm to 14 mm) to give the surgeon a choice of sizes to use. Keep in mind that the tendency is to make the dowels too large in terms of diameter, and these will not fit into the medullary canal.
While the cement dowels are hardening, an anterior synovectomy is performed followed by removal of the implanted components and cement (if present) followed by a posterior synovectomy and a meticulous debridement of any remaining cement, infected appearing bone or soft tissue. The bony surfaces can be further debrided and cut flat with an oscillating saw. The tibial and femoral canals are then opened with a drill and cleared of loose cancellous bone using a crochet hook (Figure 3) followed by cleansing the entire wound with pulsatile lavage. Subsequently, the wound is soaked with dilute betadine for 3 minutes, followed by additional pulsatile lavage of both the wound and the canals.
Once a complete debridement of all infected appearing tissue and bone has been confirmed, the previously constructed dowels (which are now completely hardened) are placed into the intramedullary canals; one dowel goes down the tibial canal and one dowel goes up the femoral canal (Figure 4A), which allows them to overlap in the middle (Figure 4B). We then mix two additional packets of cement with 3 grams of vancomycin and 1.2 grams of tobramycin per package of cement, as previously described, to create the static spacer by gently distracting the joint and placing the additional cement around the dowels and filling up the space within the joint including the placement of some additional cement for a short distance into the metaphysis of the femur and tibia to augment stability. Finally, a thin patty of cement (approximately 2 cm wide and 10 cm long) is placed into the patellofemoral joint to prevent the extensor mechanism from scarring down to the distal femur (Figure 4C).
Postoperative care and removal
Postoperatively, the patient is maintained in a knee immobilizer and the wound is examined on postoperative day 3. If the wound is dry, a long leg cast including the foot is typically applied taking care to place sufficient padding at the heel to prevent skin irritation or breakdown; a knee immobilizer can also be used as an alternative based on physician preference. Patients are allowed to ambulate with touchdown weight-bearing restrictions until the time of revision arthroplasty (Figure 5). The patient is then seen at 3 weeks postoperatively, and the cast is removed and the wound is checked. If staples or sutures were used, they are removed and a new cast or knee immobilizer is applied until the time of attempted reimplantation.
At the time of reoperation, our preference is to use a medial-parapatellar approach. Proximally, as described earlier, the thin patty of cement placed into the patellofemoral articulation and suprapatellar pouch facilitates exposure as the extensor mechanism does not scar down to the distal femur. Following an anterior synovectomy, the cement spacer is broken with osteotomes until the Steinmannn pins are identified and subsequently cut with a metal-cutting burr. The dowels are then easily removed from the canal.
Surgical options for spacers are variable, and include static and articulating designs. We have described a reliable, method for construction of a static antibiotic-loaded cement spacer that provides good stability to the knee between stages. The use of more limited static spacers (such as a “hockey puck” of cement) that do not utilize the intramedullary canals for stability is discouraged as these can lead to bone loss and potentially damage to the soft tissue envelope and extensor mechanism (Figure 6).
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Disclosures: Frank has no relevant financial disclosures. Della Valle is a consultant for Biomet, Convatec, DePuy and Smith & Nephew. He receives research support from Biomet, CD Diagnostics, Smith & Nephew and Stryker. He has stock options and is on the scientific advisory board for CD Diagnostics.