On the Necessity of a Customized Knee Spacer in Peri-Prosthetic Joint Infection Treatment: 3D Numerical Simulation Results

Abstract

Peri-prosthetic joint infections (PJIs) dramatically affect human health, as they are associated with high morbidity and mortality rates. Two-stage revision arthroplasty is currently the gold standard treatment for PJI and consists of infected implant removal, an accurate debridement, and placement of antimicrobial impregnated poly-methyl-metha-acrylate (PMMA) spacer. The use of antibiotic-loaded PMMA (ALPMMA) spacers have showed a success rate that ranges from 85% to 100%. ALPMMA spacers, currently available on the market, demonstrate a series of disadvantages, closely linked to a low propensity to customize, seen as the ability to adapt to the patients' anatomical characteristics, with consequential increase of surgical complexity, surgery duration, and post-operative complications. Conventionally, ALPMMA spacers are available only in three or four standard sizes, with the impossibility of guaranteeing the perfect matching of ALPMMA spacers with residual bone (no further bone loss) and gap filling. In this paper, a 3D model of an ALPMMA spacer is introduced to evaluate the cause- effect link between the geometric characteristics and the correlated clinical improvements. The result is a multivariable-oriented design able to effectively manage the size, alignment, stability, and the patients' anatomical matching. The preliminary numerical results, obtained by using an "ad hoc" 3D virtual planning simulator, clearly point out that to restore the joint line, the mechanical and rotational alignment and the surgeon's control on the thicknesses (distal and posterior thicknesses) of the ALPMMA spacer is mandatory. The numerical simulations campaign involved nineteen patients grouped in three different scenarios (Case N° 1, Case N° 2 and Case N° 3) whose 3D bone models were obtained through an appropriate data management strategy. Each scenario is characterized by a different incidence rate. In particular, the observed rates of occurrence are, respectively, equal to 17% (Case N° 1), 74% (Case N° 2), and 10% (Case N° 3).

Keywords: custom made knee spacer; peri-prosthetic infection; two stage knee revision; virtual surgery.

Figure 1

3D model of the ALPMMA spacer: (a) 3D view of the proposed ALPMMA femoral component; (b) frontal view of the proposed ALPMMA femoral component; (c) top view of the proposed ALPMMA femoral component; and (d) lateral view of the proposed ALPMMA femoral component.

Figure 2

3D view of the proposed augmented ALPMMA femoral component.

Figure 3

3D view of the proposed augmented ALPMMA femoral component.

Figure 4

Frontal (a) and lateral (b) view of mechanical (red) and anatomical (blue) axis drawing.

Figure 5

Frontal view of a virtually reconstructed femur, with a representation of the mechanical (red line) and anatomical (blue line) axis.

Figure 6

Virtual planning results (Case N° 1): (a) and (b) lateral view of the right knee with the evidence of bone defect of about 3 mm involving medial and lateral condyle; (c) frontal view of the right knee; and (d) customization result with an increase of medial and lateral distal thickness.

Figure 7

Virtual planning results (Case N° 2): (a) and (b), lateral view of the left femur with an evidence of bone loss exclusively on medial condyle (b); (c) correct position of the spacer in the frontal plane; and (d) customization result with an increase of medial distal thickness.

Figure 8

Virtual planning results (Case N° 3): (a) posterior view with an evidence of severe bone loss exclusively on posterior-lateral condyle; (b) customization result with an increase of posterior-lateral thickness.