Background
The number of total knee arthroplasty (TKA) procedures is growing worldwide, with an expected future increase of 143% by 2050 [
1]. Good outcomes are described in the literature, with a survivorship of primary TKA ranging between 90% and 95% at 15-year follow-up [
2]. Considering these data, and the increased number of patients at higher risk of TKA failure (i.e., younger patients) [
3], the concomitant increase in the incidence of revision TKA (rTKA) procedures is not surprising. Some authors estimated that the number of rTKA will increase by 600% by 2030 [
4]. Unfortunately, the survivorship of rTKA is inferior compared to primary TKA, ranging from 71 to 86% at 10-year follow-up [
2,
5]. Adequate implant fixation accounting for bone loss amount and bone quality is paramount to improve implant survivorship. Morgan Jones et al. described “zonal” fixation in rTKA, considering three zones: epiphysis, metaphysis, and diaphysis. The authors concluded that good fixation should be achieved in at least 2 zones in rTKA [
6]. Bone loss in rTKA has been historically classified according to the Anderson Orthopaedics Research Institute (AORI) classification, which considers the location of bone loss and defect size [
7]. Different authors described the available options to treat bone losses in rTKA which include cement, impaction bone grafting, traditional metal augments, structural allograft, metal cones, or sleeves [
8,
9]. However, some authors reported high mid-term failure rates using cement, morselized, or structural bone allograft, probably due to poor bone quality in the metaphysis [
10,
11]. In these cases, Trabecular Metal
TM tantalum cones (Zimmer, Warsaw, IN, USA) can be used to reconstruct metaphyseal bone defects (2A or greater) and to improve implant fixation [
12‐
14]. Furthermore, in most rTKA (particularly after septic loosening), the bone quality is very poor and sclerotic, and it may be useful to add metaphyseal fixation with these cones to obtain more solid implant fixation.
The aim of this study was to evaluate the outcomes of a consecutive series of rTKA in which implant fixation has been obtained in at least two zones [
6]. Furthermore, rTKA and bone loss evaluation was performed according to a modified bone loss classification system, based on the one described by Engh [
7], but taking into consideration also the bone quality evaluation together. Lastly, possible risk factors for the development of radiolucent lines, including the presence of additional fixation in the metaphyseal zone, will be evaluated.
Discussion
This is a prospective study including 53 rTKA performed in 51 patients by the same surgeon at an average follow-up of 56.6 months.
The first finding of the study was that the most frequent cause of TKA failure in this series was aseptic loosening (41.5%), followed by septic loosening (30.2%). This is similar to other published reports [
29,
30]. The second finding of this study was that rTKA is a complex surgery, with a relatively high complication rate (6%), similarly to the results of other case series [
31].
However, if the revision is performed following a step-wise approach, such as the three-step technique [
23] and the level of constraint is accurately chosen based on bone loss and ligamentous insufficiency [
32] good mid-term clinical and radiological outcomes may be obtained [
21,
33]. A mid-term cumulative survivorship of 92.1% was reported in this series at the last follow-up. It can be considered a good mid-term survivorship, also compared to other studies in literature [
33,
34].
One of the main problems challenging the surgeon in rTKA is the evaluation and treatment of bone loss. The most used system to evaluate bone loss is the AORI classification by Engh et al. [
7], which considers the amount and location of bone loss. However, this classification does not account for bone quality. Different authors described the risk of developing radiolucent lines and bone resorption with full cementation in total knee arthroplasty [
35]. Other authors described an increased risk of radiolucent lines and aseptic loosening with metal augments in revision TKA, especially in the presence of sclerotic bone [
36‐
38]. For these reasons, a new classification for bone loss, which considers also bone quality, as the one proposed by the authors (Table
1), should be validated and introduced. Bone loss may be treated with different options, depending on the severity of the defect and the quality of bone losses, including cement, impaction bone graft, traditional metal augments, structural allograft, tantalum cones, or sleeves [
8,
9]. Particularly, tantalum cones have been relatively recently introduced to treat major bone loss, with good outcomes [
13,
39,
40]. Different authors described the biomechanical properties of tantalum, including high biocompatibility, high density, and possibility of porous structure with increased osteoconductive properties [
41]. For all these reasons and because of their osteoconductive and positive biological properties, tantalum cones may be also useful to achieve a good metaphyseal fixation in presence of poor bone quality one, allowing for a stable “zonal” fixation as previously described by Morgan [
6,
42]. Furthermore, different authors confirmed that radiolucent lines development may be correlated to instability, micromotion, inadequate load distribution, and different of elasticity between bone and metal and that they may decrease using some material more similar to bone properties, such as tantalum [
43].
All these aspects, including the amount of bone loss and bone quality, should be considered during rTKA. For this reason, when approaching to bone loss, the algorithm of treatment should consider location and amount of bone losses, bone quality, and need for solid “zonal” fixation, as the one proposed in this manuscript (Table
2). In this algorithm, tantalum cones may be also used to enhance implant fixation in the metaphyseal zone in the presence of sclerotic bone, to reduce the risk of aseptic loosening due to insufficient epiphyseal fixation, particularly at the tibial baseplate. The biological and mechanical properties of tantalum cones are well known [
14]. Considering the osteoconductive properties of tantalum, it is reasonable to think that it may allow for a stronger fixation in the metaphyseal zone, reducing the forces on the epiphyseal zone and, consequently, the risk for aseptic loosening [
12,
39,
44]. However, surgeons have to be careful because the internal diameter of the cone may limit the diameter of the stem, or the use of an offset, possibly resulting in suboptimal Canal Fill Ratio (CFR). However, some studies demonstrated that the cone is not an obstacle to obtain a good stem alignment and CFR [
45].
In this case series, only one failure was due to implant loosening (only femoral component) associated with hinge breakage was reported. In this case, no metaphyseal fixation was used during the first revision. Despite the small numbers of cones, mostly due to the recent introduction of tantalum cones, overall good results in terms of implant fixation were obtained in patients with sclerotic bone. However, logistic regression analysis found no association between the examined variables and the development of radiolucent lines, including poor bone quality. This is probably due to the small number of patients included in the case series, so further studies are necessary to confirm the role on metaphyseal fixation systems, such as tantalum cones, in enhancing implant fixation in the presence of sclerotic bone quality.
This study has several limitations. Firstly, it is a small series (53 cases) with a medium-term follow-up (56 months, minimum follow-up of 24 months). However, the case series cannot be enlarged so far in order to guarantee a minimum acceptable follow-up. Furthermore, the proposed classification has not been validated with testing for inter-and intra-observer reliability. Further studies are necessary to validate this classification, including inter and intra-observer reliability compared to other available classification, so that it can be applicable for every surgeon. The authors were not able to find any variable associated with radiolucent line development, including poor bone quality or number of augments or any variable related to insufficient epiphyseal fixation and need for metaphyseal fixation. This may be due to the small number of patients with poor bone quality (sclerotic or severe osteoporotic) and the small number of tantalum cones implanted. Another limitation is the lack of a matched control group, so a comparison between patients with improved metaphyseal fixation or cannot be compared. Lastly, this case series included only one type of tantalum cone, but there are different solutions available, such as sleeves, which may be useful to treat bone loss and to improve implant fixation in the presence of poor bone quality and which have not been considered in this case series. Furthermore, in the presence of massive bone losses, also customized implant may be considered, and they are not included in this case series.
However, also considering these limitations, this new proposed classification may be a valuable instrument for the surgeon to evaluate not only bone loss but also bone quality and to choose the right method of fixation required to obtain adequate bone loss treatment and implant fixation.
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