1. Introduction
Knee arthroplasty as a surgical intervention is considered an efficient treatment for osteoarthritis. It is a standard procedure in almost all hospitals in Europe and the United States of America (USA). The statistical data collected in the USA report about 1 million surgical interventions performed yearly [
1]. In 2010 there were reported 719,000 total knee arthroplasties, of which 10–15% were secondary revision operations. These types of interventions are performed after implant failure. It is expected that in the year 2030, the number of primary TKA will increase, with a percentage of 673%, equal to about 3.48 million procedures.
Regarding the secondary surgical interventions, it is estimated that an increase of 401% between 2022 and 2030 will occur [
2]. The main causes of the TKA procedures are the average life expectancy of the population and the high level of physical activity. Unfortunately, these surgical interventions are performed at a younger age than in the past. About 85% of patients reported an excellent post-operator course after a primary TKA [
3,
4]. Approximately 20% of patients suffered from long-time chronic pain, and supplementary measures and painkillers had to be taken into account [
5,
6].
The typical knee prosthesis failures are abnormal joint movements, aseptic loosening, infection, periprosthetic fracture, ligament/flexion instability, arthrofibrosis, and material particles’ side effects. In practice, it was concluded that a knee implant works improperly due to multiple causes mentioned above. The revision surgery is a complex procedure, and it is associated with a high risk of infection, complications, prolonged hospitalization time, and a short life expectancy for the secondary prosthesis [
7,
8].
The main functions of orthopedic knee implants include articular function restauration and pain relief [
9,
10]. The implant life expectancy depends on the device design and the used biomaterials [
11,
12]. All the materials must be Food and Drug Administration (FDA) approved or carry the Conformitè Europëenne certificate [
13,
14].
Usually, the knee implant femoral component, placed at the distal part of the femoral bone, is made from a binary Co-Cr or a ternary Co-Cr-Mo alloy [
15]. Oxidized zirconium (OXINIUM) was introduced by Smith + Nephew as an alternative to the Co-based alloys to reduce the polyethylene particle quantity and limit the number of failed prostheses. This material offers a superior advantage because it combines metallic alloy durability with ceramic material biocompatibility, exhibiting a high corrosion resistance [
16,
17]. The implants made from OXINIUM are characterized by a 15 years life expectancy and a very low revision intervention rate [
18,
19]. In the case of patients that exhibit nickel allergies, the femoral component can be manufactured from ceramic materials such as alumina (Al
2O
3) [
20,
21], yttria-stabilized zirconia (Y-TZP) [
22,
23], and zirconia toughened alumina (ZTA) [
24]. The ceramic components’ main disadvantage is implant squeezing and reduced mechanical characteristics [
25]. Recently, femoral components made from polyetheretherketone (PEEK) received FDA approval due to the good mechanical properties of the material. PEEK has a value of Young’s modulus close to the one of the human bone, so the stress-shielding effect is prohibited [
26,
27]. By using polymeric materials, the complications of the metallic particles’ emission are avoided. The principal drawback of PEEK is that an important bioinertia characterizes this material. In order to increase its biocompatibility, different solutions, such as surface treatments, are searched [
28,
29].
The tibial component positioned at the tibial bone proximal part is manufactured from a titanium alloy (Ti6Al4V) [
30,
31]. This ternary alloy exhibits adequate mechanical properties and can sustain the mechanical stress that appears during the patient gait cycle. Unfortunately, metallic ion emission can be observed in some cases, and allergic, teratogenic, toxic, and carcinogenic effects are evidenced during secondary surgical interventions. Zimmer—Biomet has developed tibial components made from porous tantalum (Ta), denoted as Trabecular Metal
TM, with excellent mechanical properties, no stress shielding effects, and high biocompatibility [
32,
33]. Recent studies present alternative materials such as reinforced PEEK with carbon fiber, the so-called PEEK-OPTIMA, and Zeniva ZA-600 CF [
34] and concluded that polymeric-based materials could be considered an alternative for titanium alloy in some cases. In the case of old age or overweight patients, UHMWPE tibial components are proposed, and good outcomes are reported because these two categories of people have a limited range of motion and reduced physical outdoor activities [
35]. Finally, the tibial component of the BPK-S Integration prosthesis developed by Peter Brehm GmbH, Weisendorf, Germany, was made from BIOLOX
® ceramic material [
36].
The tibial insert represents the vulnerable part of a knee implant because this component is usually the first that fails since it is made from polymeric materials [
37]. The most used material is ultra-high molecular weight polyethylene (UHMWPE), whose properties were improved over time to decrease wear and oxidation [
38,
39]. Different variants such as highly cross-linked polyethylene (HXLPE) or HXLPE combined with a synthetic analog of vitamin E (alpha-tocopherol) were proposed to prevent the oxidative degradation effect and to reduce the number of failed knees prosthesis due to osteolysis and aseptic loosening [
37,
40]. UHMWPE is a particular type of polyethylene, being a linear homopolymer, half crystalline with high molecular mass. The material that is used in the orthopedic field has a molecular weight between (3.5–6) × 10
6 g/mol and a crystallinity grade of about 50–55% [
37]. According to the American standard ASTM, UHMWPE must have a molecular weight higher than 3.1 × 10
6 a.m.u. (atomic mass units), since the standard ISO 11542 (ISO, 2001) recommends a molecular weight of at least 1 × 10
6 a.m.u [
38]. The ultra-high molecular weight polyethylene exhibits three phases. A crystalline lamellar phase, in which the polymeric chains are organized in an orthorhombic crystalline matrix, and an amorphous phase, in which the macromolecules form random wires or act as molecule ties that go out from a crystalline lamella and enter another one. A third phase partial order, which intercalates macromolecules with tight or loose chains, is present in the material. It can be noticed that the crystalline phase has an important contribution to increased mechanical rigidity, while the amorphous phase offers ductility and resilience [
39]. When the polyethylene quality is improper, mechanical stresses can result in polymeric particles that induce osteolysis, an acute inflammatory process. It was observed that this phenomenon occurs due to an inadequate sterilization process with gamma radiations. The standards recommend that the sterilization process of the material be carried out in an inert ethylene oxide atmosphere or gas plasma. The cross-linked effect is induced through irradiation, increasing the material wear resistance by a high amount [
37]. The most used types of UHMWPE are manufactured by Ticona (Summit, NJ) and are denominated GUR 1020 and GUR 1050 [
41]. The GUR 1020 material has an average molecular weight of 3.5 × 10
6 g/mol, a density of 0.93 g/mL, yield stress higher than 17 MPa, impact strength higher than 210 kJ/m
2, and an average particle size of 140 μm. Regarding the GUR 1030 type, the material molecular weight is comprised of between (5.5–6) × 10
6 g/mol. It has a density of 0.93 g/mL, a tensile modulus of 680 MPa, and an impact strength higher than 130 kJ/m
2 [
41]. The tibial insert can also be manufactured from PEEK, showing increased resistance to degradative processes reported in previous cases and good mechanical properties [
35].
In some clinical cases, replacing the native human patella with a dedicated part is necessary. There were proposed different solutions, such as a combination between a metallic base and a thin polyethylene surface, but major failure reports were found in the literature. Today, the worldwide accepted solution uses a UHMWPE component [
42].
Osteolysis due to metallic, ceramic, or polymeric particles represents one of the leading causes of aseptic loosening. After the knee prosthesis is introduced into the human body, it becomes a particle source due to wear and corrosion. The resulting particles infiltrate the patient’s circulatory system, or they can agglomerate in neighboring tissues such as bone or bone marrow. They have a dangerous potential to interact with osteoblasts, preosteoblasts, mesenchymal stem cells (MSCs), macrophages, osteoclasts, and fibroblasts [
43].
Figure 1 presents the adverse effect of the particles against the functions of the cells placed in the implant vicinity. The pro-inflammatory effect of the macrophages and osteoblasts has an important contribution to a chronic inflammatory environment near the implant and osteoclastogenesis [
44,
45]. In addition, a negative influence of the osteogenic differentiation and osteoblasts or MSCs mineralization was put in evidence in the literature [
46].
Retrieval analysis of well-functioning and failed implants is of great importance in the orthopedic biomaterials field because surgical technique, implant design, and material quality can be improved in order to repair joints affected by diseases or trauma [
14,
47]. The standard ASTM F561-97 recognizes the importance of this analysis and concludes that two factors are met to obtain a successful implant integration [
48]. The first one is related to the material, and the second one is biologically dependent [
49]. Some authors identified the existence of a dynamic interface placed at the implant-tissue boundary [
50]. Osseointegration was defined in [
51] as a direct structural and functional link between the load stress implant and vital bone. The main types of cells that act in bone healing and formation are the osteoblasts, that form bone at a rate of 0.17 mm
3/day; osteocytes, which are considered an important stress detector for bone; and osteoclasts, that resorb bone at a rate of 100 μm/day [
50]. Three different modalities of bone response were identified. The first one is called fibrointegration, which appears in a case of an extent trauma and consists of fibrous tissue apparition around the prosthesis. Then if new blood vessels are not formed, dead bone can be seen in the implant vicinity, and if all the conditions are met, normal bone can be observed in the region of the prosthesis. This last response is called osseointegration. Three types of osseointegration are present: biointegration, mechanical integration, and chemical integration. Biointegration is possible when active biomaterials that link collagen fibers to the bone, such as calcium phosphate coatings, are used. Mechanical integration is considered when the bone is connected to the implant with the help of screws, vents, or cement. Chemical integration is obtained when chemical or physical bonds such as covalent bonds, hydrogen bonds, or van der Waals forces appear. The main factors identified in the literature [
52] that influence the osteointegration process are surgical technique, the biocompatibility of the materials, implant design and surface, quality of the patient bone, and loading conditions of the prosthesis. The materials can be classified according to their biocompatibility as biotolerant, bioinert, and bioactive. Regarding the stress conditions, it was noticed that implant design and geometry have a strong influence on the loads applied and can be a critical factor in the retrieval analysis. Additionally, aspects linked to the surgical technique, such as implant alignment and surgical insert point of the prosthesis, are important in stress management. The surface state influences the bonds between the bone and the implant, and critical factors such as coatings or roughness must be considered for good osseointegration.
There are many studies regarding the retrieval analysis of UHMWPE tibial inserts, and the main failure reasons are due to oxidation and wear phenomena. Liza et al. [
53] investigated the failure analysis of a UHMWPE tibial insert from Apollo
® Total Knee System that was removed after 10 years of service. Using different testing methods such as scanning electron microscopy (SEM), infinite focus microscope (IFM) coupled with energy disperse spectroscopy (EDS), Fourier-transform spectroscopy (FTIR), and gel permeation chromatography (GPC), they concluded that the UHMWPE material exhibited high oxidation and wear degradation. Moreover, delamination, folding, pitting, and scratching were predominant. The retrieved sample had a reduced molecular weight contributing to the implant failure. Kurz et al. [
54,
55] and Edidin et al. [
56,
57] presented a mechanical test strategy adapted to UHMWPE tibial inserts called the small punch test. They have prepared small, thin disks indented by a hemispherical punch, determining a biaxial tension state. Mechanical quantities such as Young’s modulus, ultimate load, ultimate displacement, and work to failure were measured and correlated with the in vivo service. Cho et al. [
58] showed that microscopic surface asperities on the knee prosthesis’s metallic femoral components have an important role in increasing the wear process of UHMWPE tibial inserts. Medel et al. [
59] collected 119 tibial inserts, from which 29 were gamma sterilized in normal atmospheric conditions and 90 were conventional gamma sterilized using nitrogen atmosphere. They have found a direct link between in vivo oxidation and fatigue deterioration. It was concluded that oxidation strongly influences delamination since pitting damage is not increased by this phenomenon. Berry et al. [
60] made a similar study on 132 gamma-air and 174 gamma-inert-sterilized UHMWPE tibial inserts. They noticed that gamma-inert-sterilized tibial inserts reached the critical oxidation index value at an average of 13 years after the primary surgery. Cerquiglini et al. [
61] studied the new antioxidant polyethylene retrieval analysis. They used 24 Press Fit Condylar (PFC) Sigma (DePuy) and 17 Attune Knee System (DePuy) with fixed bearing and rotating platform design. A higher value of the Hood score was found in the case of Attune tibial inserts on the backside surface. Material properties’ modifications were reported due to implant design and potentially reduced implant performance was foreseen. Raman spectroscopy was used by Tone et al. [
62] to analyze the wear rate and creep of e-beam-sterilized conventional UHMWPE tibial insert. The main conclusion is that the body mass index (BMI) greatly influences implant failure, and a separation of the creep and wear components of thickness reduction was made. There are few recent papers regarding failure analysis of primary cemented Zimmer Biomet knee implants. Our study fulfills this research and sustains the conclusions of Granquist et al. [
63], that found a minimum 5-year survival time for total hip or knee arthroplasty. Additionally, the paper brings new information about the potential of microscopy techniques in failure explant analysis.
In the paper, we investigated four UHMWPE tibial inserts retrieved from patients from Clinical Hospital Colentina, Bucharest, Romania. We performed macrophotography, stereomicroscopy, and scanning electron microscopy investigations to establish the samples’ wear grade and to correlate it with the Hood index computation. We checked the chemical composition, crystallinity grade, and oxidation index through Fourier-transform infrared spectroscopy (FTIR). We analyzed the material’s mechanical behavior based on the small punch test.