# The Effect of Cement Aging on the Stability of a Cement-in-Cement Revision Construct
## 4 Discussion  
The arthroplasty registries show an increase in joint arthroplasties. Related to this, the incidences of revisions are increasing too, while the aseptic loosening of the endoprosthesis is still the main cause [1]. Despite the positive long-term prognosis of cemented endoprostheses, a revision is a challenging task. Due to the previous implant, in revisions, the loss of bone is already processed. To avoid or at least minimize possible complications, the cement-in-cement technique can be an alternative for hip or knee revision surgery. In our experimental in vitro study, the bone cement Palacos R+G showed no significant difference between the reference group and the revision group at any testing time. The mean value of the reference and revision groups with Palacos R+G bone cement also had a bending strength of more than 50 MPa, which is recommended by ISO 5833:2002-05 [17]. A revision with the cement-in-cement technique and the bone cement Palacos R+G appears to be comparable to a standard cement layer under standardized conditions without the contamination of blood and lipids. Figure 4: Images of the fractured surface with the scanning electron microscope (SEM); Palacos R+G revision at 4W + 1d (**a**), 4W + 3d (**c**), and 4W + 10d (**e**); (**b**) SmartSet MV revision at 4W + 1d (**b**), 4W + 3d (**d**), and 4W + 10d (**f**), red arrows show the line between the two layers. The SmartSet MV bone cement showed a different result between the reference and the revision groups. After only 4 weeks plus 10 days of aging, the bending strength did not differ significantly between the two groups. All aging points before showed that the revision group had a significantly decreased bending strength. Especially at the testing times of 4 weeks plus 1 day and 4 weeks plus 3 days, the mean value of the revision group was below the recommended 50 MPa. While comparing the two kinds of cement, the references of the Palacos R+G and the SmartSet MV had similar values of bending strength. Therefore, there were no significant differences between the two reference groups at any testing time. The SmartSet MV bone cement sample had a significantly lower performance than the Palacos R+G sample with the revision technique. The aging of a revision construct influenced the bending strength of the SmartSet MV more than Palacos R+G. [PERSON] et al. also showed that PMMA bone cements had a decrease in bending strength over the aging time [18]. If the bone cement gets in contact with blood or lipids, it can further decrease the bending strength [20; 21]. Further investigations have to be conducted. In recent years, the use of the cement-in-cement technique has increased more and more. It showed promising clinical results. [PERSON] et al. had an excellent outcome using the cement-in-cement technique [22]. Other studies also showed results with the new technique and a decrease in complications due to the revision [5; 13; 23; 24]. [PERSON] et al. also investigated the cement-in-cement technique in multiple revised total hip replacements. Even though the results were less promising than other studies investigating the cement-in-cement technique, the outcome was still acceptable, especially for patients with a background with more comorbidity [12]. A systematic review of [PERSON] et al. in 2020 showed that the cement-in-cement technique is a useful option to minimize intraoperative complications and immediate postoperative morbidity, and the rate of implant survival also remains low [10]. However, it is often unclear which cement was used in the primary arthroplasty, and therefore, different combinations of cements are possible. Due to the time interval between the primary replacement and the revision and the possible change of hospital, the cement types may differ from each other. It is often unclear which bone cement was used in the primary replacement [11]. A further problem could be the preparation of the old cement mantle. [PERSON] et al.