Antibiotic Spacer Techniques in Hip and Knee

Introduction

Prosthetic joint infection (PJI) remains an uncommon but devastating complication following total hip arthroplasty (THA) and total knee arthroplasty (TKA), with a relatively low incidence of ∼1% to 2% of all primary TKAs and THAs but a variably higher incidence following revision surgery. The gold standard and most widely employed treatment for chronic PJI in the hip and knee is a two-stage exchange revision with the main goals of infection eradication and preservation of prosthetic joint function. Typically, the first stage comprises explantation of all total hip and knee components along with irrigation, infected tissue debridement and placement of a high-dose antibiotic-loaded bone cement (ALBC) spacer. This is followed by an extended course of antibiotics and a delayed second-stage procedure entailing antibiotic spacer removal and reimplantation of revision TKA or THA components once the joint is declared free of infection. The success of a two-stage exchange revision in eradicating hip and knee PJI is reportedly as high as 80% to 90% but is highly predicated on several factors, including the causative organism, extent of surgical debridement, and medical status of the host.

Antibiotic-Loaded Bone Cement Spacers in the Hip and Knee

The goals of ALBC spacers are local antibiotic delivery, provision of joint stability, alignment, and prevention of severe soft-tissue contracture before the second-stage reimplantation. They are broadly classified as articulating or static ( Fig. 30.1 ). ^, Articulating spacers are designed to allow for hip and knee range of motion in the interval between the two revision stages with the advantages of potentially increased range of motion after second-stage revision, less soft-tissue contracture and easier exposure, and increased patient function and satisfaction. These can be created using a variety of techniques, including hand molding, commercially available prefabricated spacers, sterile molds for intraoperative preparation ( Fig. 30.2 ), and metal with all-polyethylene components ( Fig. 30.3 ). Static spacers function as a temporary knee arthrodesis, providing more rigid joint stability than articulated spacers, which protect the collateral ligaments and the extensor mechanism, creating a more favorable environment for soft-tissue and wound healing. They are typically prepared with a manually placed antibiotic cement block, filling the extension gap along with intramedullary femoral and tibial antibiotic cement dowels with or without the use of Steinmann pins or small-diameter intramedullary rods for stability reinforcement ( Fig. 30.4 ). Static spacers of the hip are reserved for massive acetabular or femoral bone loss in which an articulating spacer cannot be placed but do not function as an arthrodesis of the hip. They can be handmade with ALBC hand-packed around rush rods or Kirschner wires (K-wires) or can be fashioned as a dowel from a cement gun nozzle (see Fig. 30.4 ).

There is controversy over which type of spacer overall provides the more superior treatment of PJI following TKA or THA. However, orthopaedic surgeons typically decide on which technique to use depending on multiple patient factors and the degree of bone and soft-tissue compromise. There is evidence suggesting equal infection eradication rates between the two methods but improved function and knee range of motion following revision with an articulated knee spacer. ^, In the hip, there is evidence suggesting equal infection eradication rates and functional outcomes when comparing static versus articulating spacers. Both articulating and static spacers carry the risk of complications, which may vary depending on technique. Nonetheless, these complications need to be recognized, avoided if possible, and, in the case of occurrence, must be managed appropriately by the treating surgeon.

Medical Complications of Hip and Knee Spacers

Mortality

The mortality rates surrounding ALBC spacer treatment of hip and knee PJI are an important consideration. In a retrospective review including 326 two-stage exchange treatments of knee PJI with cement spacer implantation, the 2-year mortality rate was approximately 12%. The mortality rate was even higher, at 18% at a mean of 3.7 years, in a different retrospective study of 134 patients who underwent the first of a two-stage treatment of knee PJI with antibiotic spacer placement. The literature on mortality during two-stage exchange of the hip is very similar to the knee. One retrospective study of 202 patients who underwent two-stage exchange for THA PJI found that 14 patients (7%) died before reimplantation, with a 90-day mortality rate of 4% after the first stage. With a minimum of 2-year follow-up, their overall mortality rate during their study period of 13 years was 48%. These data highlight the significant risk associated with two-stage treatment of chronic knee and hip PJI and the importance of medically optimizing patients undergoing this staged treatment.

Nephrotoxicity

The primary function of an ALBC spacer is the local elution of high doses of antibiotics at the site of infection. However, the elution of certain antibiotics—particularly vancomycin and aminoglycosides (gentamicin, tobramycin)—with subsequent systemic absorption can result in nephrotoxicity. A prospective study of 21 patients (10 THA and 11 TKA) who underwent placement of an antibiotic cement spacer demonstrated persistently detectable serum levels of vancomycin, gentamicin, and tobramycin for 8 weeks, indicating prolonged systemic absorption of these antibiotics. Patient factors—including age, male gender, diabetes, and blood urea nitrogen levels—correlated with serum vancomycin or aminoglycoside levels. Another prospective study encompassing the same cohort as part of a 37-patient sample (17 THA and 20 TKA) monitored postoperative serum creatinine levels to assess the risk of nephrotoxicity (acute kidney injury [AKI]). A total of 10 patients experienced some degree of renal insufficiency (27%), while 2 subjects (5%) developed renal failure. No patient risk factors were identified as predictors of nephrotoxicity in this study; however, the sample size was limited. A different prospective study of 50 patients who received tobramycin (± gentamicin) ALBC spacers (THA and TKA) demonstrated a 20% incidence of AKI and identified intraoperative blood transfusions, the postoperative use of nonsteroidal antiinflammatories, and the use of gentamicin-added bone cement as risk factors for AKI. A higher incidence of AKI (26%) was found in a larger retrospective study of 247 patients (THA and TKA) who underwent two-stage exchange revision with placement of an ALBC spacer. Risk factors for AKI included high body mass index, lower baseline hemoglobin levels, and concurrent medical comorbidities.

The incidence of nephrotoxicity following ALBC spacer placement is not negligible and warrants careful consideration of antibiotic dosing when preparing ALBC spacers for revision TKA or THA. Timely recognition of nephrotoxicity following ALBC spacer placement is important and possible through close monitoring of serum creatinine levels postoperatively, particularly in patients potentially at risk based on the risk factors identified earlier. Appropriate management includes adequate hydration and the avoidance of nephrotoxic medications in the immediate postoperative period. Considering the balance between delivery of high doses of local antibiotics and the risk of nephrotoxicity, we routinely use 2 g of vancomycin and 2.4 g of tobramycin per 40-g bag of high-viscosity cement when making a cement spacer.

Surgical Complications of Hip and Knee Spacers

Wound Complications

The incidence of wound complications following antibiotic spacer placement is not uncommon given the frequency of a compromised soft-tissue envelope in patients undergoing two-stage treatment of knee PJI. The reported incidence is around 6% to 7.4% in institutional case series of patients undergoing two-stage exchange. ^, This predominantly includes poor healing, wound dehiscence, and superficial infection. Depending on severity, it may require a secondary procedure in the form of irrigation and debridement and possibly spacer exchange. In a single institution’s retrospective review of its complications of hip ALBC spacers as part of a two-stage exchange, 4 of 82 patients (4.9%) developed a draining sinus between stages that required repeat debridement. This lower rate of wound complications in the hip compared with the knee may be attributed to a larger soft-tissue envelope that may be protective from persistent drainage. Systemic host grade and local extremity grade are major factors associated with wound complications following spacer placement. Careful host and extremity selection for static versus articulated spacers may also play a role in the occurrence of wound complications. However, current wound complication rates are similar between static and articulated spacers. Given the potential for complexity of these wound complications in a compromised soft-tissue envelope, early consultation with plastic surgery may be appropriate for wound management.

Spacer Fracture-Dislocation

Another potentially catastrophic complication following the placement of a static or articulated ALBC spacer is dislocation or fracture of the spacer itself or the surrounding host bone ( Fig. 30.5 ). This may compromise the bone stock available for subsequent revision or may cause soft-tissue injury ( Fig. 30.6 ), including extensor mechanism disruption. The incidence of fracture around a static spacer was reported at 10.5% in a case series of 133 patients who underwent two-stage treatment for PJI with a static spacer. The majority of those were fissures on the tibial side. In a larger institutional database study that included 326 knee PJI cases treated with a two-stage exchange with articulated or static spacers, there were 5 cases of spacer dislocation (1.5%) and only 1 case of tibial fracture (0.3%). All 6 of these cases required spacer revision, adding to patient morbidity and cost of care. The incidence and risk of spacer fracture or dislocation increases with spacer retention for prolonged periods of time. A retrospective review including 34 retained knee spacers due to being medically unfit for reimplantation found a 21% spacer revision rate at 2 years, most frequently for supracondylar femur fractures and spacer dislocations.