Ten polyethylene inlays were investigated using scanning electron microscopy. All inlays were of the same type and size. The model used was an 11 mm thick constrained inlay of the Genesis II total knee (Genesis II constraint, Smith & Nephew, Schenefeld, Germany). The size is called "5–6" which is identical for sizes 5 and 6 of the tibial component. The inlays consist exclusively of ultrahigh-molecular weight polyethylene (UHMWPE; ASTM F 648) without any metal reinforcement and were formed by milling to its final shape. All inlays were sterilized by gas sterilization using ethylene oxide.
Three polyethylene inlays were unused and acted as controls. The samples were subsequently prepared according to a standardized preparation protocol, mentioned below.
Seven inlays had been retrieved from patients, four men and three women, with a constrained prosthesis during revision surgery. The mean age of the patients was 66.4 years (min 48.6 years, max 80.0 years). The patients had a body mass index at the time of surgery of 31 kg/cm2 (min 24.2 kg/cm2, max 38.6 kg/cm2).
All UHMWPE inlays were retrieved during second revision TKA. The first revision was carried out for aseptic loosening in four cases, septic loosening in one case, and mediolateral instability in two cases. In this first revision, a condylar constrained implant was used for reconstruction. After a period of in average 25.4 months after the first revision (min 1.1 months, max 50.2 months), a second revision was necessary. The reasons for the second revision were aseptic loosening in three cases, deep infection in two cases and in two cases a painful combined medial and lateral instability. Preoperatively, the patients showed no significant deviation of the mechanical axis in coronal plane and had no trauma in their history.
The retrieved implants were cleaned with sterile water and afterwards prepared according to the same preparation protocol applied to the other inlays.
The polyethylene inlays, which have an overall size of about 75 mm mediolateral, 50 mm posteroanterior and a height of 36 mm (Fig. 1a), had to be reduced in size for the scanning electron microscopic investigation. To fulfill the instrumental requirements the outer regions of the UHMWPE prostheses were sawed off very carefully using a special saw with fine saw teeth. The final size of the prostheses after sawing amounts to approximately 26 mm × 26 mm × 20 mm (Fig. 1b). The prostheses were cleaned twice in 96% ethanol applying ultrasound each time for 5 minutes. Subsequently, the cleaned prostheses were mounted on aluminum specimen stubs with electrically conductive carbon (PLANO, Wetzlar, Germany) and sputter coated with gold using argon gas as the ionizing plasma. The average thickness of the gold film applied to the prostheses was approximately 15 nm.
Imaging was performed on a scanning electron microscope (SEM) S-450 (Hitachi Ltd., Japan) with secondary electrons (SE) at 20 keV and at room temperature [11, 12]. The primary magnifications were in the range of 50- to 3,000-times depending on whether an overview or details should be monitored. A very careful scanning electron microscopic screening of the surface structure was performed in the region where the post merges into the inlay plateau. To better compare the results obtained from different prostheses, we selected twelve specific locations (Fig. 1c) defined by their geometric positions. Micrographs were recorded from a high-resolution cathode ray tube using negative film (Agfapan, APX100). For the final demonstration of the experimental data, however, we used a total of eight different locations only, which correspond to the corner of the tibial post.