Etiology and classification of the unstable total knee arthroplasty
Total knee arthroplasty (TKA) is a common and successful surgery for the treatment of osteoarthritis of the knee with good outcomes in pain, function, and quality of life. However, in some patients instability occurs after the primary arthroplasty leading to the need for a revision TKA. The causes of instability are manifold and the types of instability occurring are unique, each requiring a specific treatment strategy. In this article, Dario E Garin, Hospital Ángeles Tijuana, Tijuana, Mexico, takes a closer look at instability after TKA, starting with an examination of the etiology and classification of the unstable TKA.
Dario E Garin
The number of primary and revision total knee arthroplasties is increasing on a global scale
Total knee arthroplasty is a well-established procedure with high success rates in the treatment of patients with osteoarthritis of the knee ; patients undergoing TKA have reported improvements in pain and function as well as in quality of life . In 2007, Kurtz et al  predicted that the demand for TKA would increase by 291% from 2005 to 2020 in the US alone. Over the last years, data from the US has shown a steady increase in the prevalence of primary TKA and revision TKA [3, 4], and this trend is supported by the yearly data from different national joint replacement registries [4-6], with an 8–9% increase in number of primary TKAs [5, 6] and ~ 7% increase in revision TKAs from 2020 to 2021 [5, 6].
This global trend towards an increased volume of primary and revision TKAs is set to continue [1, 7] and adding to this burden is the expanded demographic of the patients to include those who are young and active . As Dario E Garin notes, the number of TKAs has increased worldwide due to early osteoarthritis secondary to obesity, sports-related injuries, and patients with a more active lifestyle.
Instability is an important major indication for revision
In 2015, McNabb et al  summarized the causes of primary TKA failure as infection, aseptic loosening, instability, stiffness, and polyethylene wear. As depicted in Figure 1, which shows the reasons for primary TKA revisions across different registries, there are manifold reasons for revision. Although there are differences observed in the diagnostic codes used across countries, what is consistent is that infection is the top reason for revision [4–6, 8]. In the US, periprosthetic joint infection (PJI) and aseptic loosening are the main reasons for nearly half of all revision TKAs  and in Germany, for example, PJI is a primary reason driving the projected increase in number of revision TKAs over the next decades . In contrast, aseptic loosening or lysis is the primary reason for revision in the UK .
Aside from PJI or aseptic loosening, instability is one of the main causes of revision TKA (Figure 1) [5, 6, 8–10]. Data show that the number of revision TKAs associated with instability appears to be increasing [9, 11]. In an analysis by Upfill-Brown et al , a significant increase (P < .001) in the proportion of procedures associated with instability was observed from 2012 to 2019 (9–12.8%). Furthermore, in Australia there was a consistent increase in revision TKAs due to instability seen from 6% in 2003 to 18% in 2019 . The reason for this increase is poorly understood, but it has been suggested to be related to an increased awareness of different forms of instability, such as midflexion instability, by Lewis et al .
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- Increasing number of primary and revision total knee arthroplasties on a global scale
- Instability is an important major indication for revision
- The etiology and symptoms of instability after total knee arthroplasty
- Risk factors for instability in total knee arthroplasty
- Classification of the unstable total knee arthroplasty
- Extension instability
- Flexion instability
- Midflexion instability
- Genu recurvatum
- Global multiplanar instability
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Dario E Garin
Clínica El Rosario
AO Recon Joint Preservation Knee Curriculum Taskforce
University College Hospital London
This issue was written by Lyndsey Kostadinov, AO Innovation Translation Center, Clinical Science, Switzerland.
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