Radiostereometric analysis of newer cementless total knee arthroplasty designs

Optimizing total knee arthroplasty (TKA): the role of RSA measurements

The resurgence of cementless total knee arthroplasty

With the global aging population, the demand for TKA is steadily increasing [1]. The fixation of a knee prosthesis can be cemented, cementless, or hybrid. Cemented fixation is still widely regarded as the reference standard for TKA, with excellent clinical outcomes and implant longevity of up to 20 years [2–4]. Cemented implants rely on primary bone fixation achieved through cement interdigitation into trabecular bone. However, there are concerns regarding long-term failure of cemented TKA due to loss of cement-bone interlock and debonding at the cement-implant interface, particularly in younger, obese, and active patients [5, 6]. The most common cause of late TKA failure is aseptic loosening due to cumulative mechanical stress, abrasion from cement debris, or trabecular resorption [3].

With a growing population of younger, more physically active patients seeking TKA [7], the possibility of long-lasting biological fixation and prolonged implant survival has reignited interest in cementless TKA. Cementless TKA, as opposed to cemented TKA, relies on osseointegration for fixation [8]. This approach aims to provide a more physiological bond between bone and implant, resulting in improved stability and preservation of bone stock for future revisions [4, 5].

First-generation cementless TKA implants faced multiple design-related complications, particularly early aseptic loosening of the tibial component. This loosening was primarily due to factors such as osteolysis, inadequate bone ingrowth into the tibial tray, and micromotion at the bone-implant interface, leading to subsidence—the sinking of the prosthesis into its host bone [9]. However, new designs and, more recently, the integration of 3-dimensional (3D) printing have vastly improved their performance [4]. As introduced in Part 2, modern cementless implants rely on highly porous or roughened surfaces to facilitate bone formation and employ optimized component design to provide a better mechanical interlock, reducing initial micromotion.

Several different modern implants are now available for use in TKA. The latest 3D-printed components are designed to improve the bone-to-metal connection and feature an enhanced porous structure to promote better bone ingrowth, along with peripheral pegs to increase the contact surface area and reduce early micromotion. Some designs exhibit biomechanical properties, such as high compressive strength and elastic modulus, that are very close to those of trabecular bone [10]. These features are thought to facilitate fixation to the host bone, thereby promoting bone ingrowth and increase implant longevity [9–12].

Radiostereometric analysis as a surrogate marker for long-term implant survival

The latest cementless TKA implants have shown promising outcomes in RSA studies. Radiostereometric analysis is a highly accurate imaging technique used to predict implant stability. During the surgical procedure, small radiopaque tantalum markers are implanted in both the bone and the implants. Following surgery, stereoscopic x-rays are taken using a calibration cage with established reference points (Figure 1). These images reveal the positions of the tantalum beads, and when analyzed with an RSA software, they allow surgeons to accurately calculate micromotion between the implant and the bone in three dimensions. By conducting repeated measurements at different time points, RSA allows quantitative analysis of implant migration over time [13]. Given the documented correlation between implant micromotion and the risk of long-term aseptic loosening [14], RSA has emerged as the method of choice for assessing the performance of TKA implants.

Maximum total point motion migration

In TKA, the migration of the tibial component is quantified using maximum total point motion (MTPM), which estimates the length of the translational vector of the marker exhibiting the greatest migration [16]. In the first postoperative year, MTPM measurements provide surgeons with an instrument to detect early signs of implant loosening or instability prior to the manifestation of clinical symptoms, and they are routinely used for comparing different implant designs, fixation methods, and surgical techniques [17]. Thanks to the high accuracy and precision of RSA, late loosening of new implants can be predicted based on the 2-year RSA results, even with limited number of patients [14]. In a 2018 systematic review and metaanalysis aimed at evaluating migration patterns of tibial components in TKA, Pijls and colleagues [18] plotted RSA values in percentile from multiple studies, including two historical "disaster" implants known for their failure: the Boneloc cement and the Freeman Samuelson AP uncoated. Their analysis demonstrated that examining the MTPM value from early RSA data could have enabled the anticipation of impending failure as early as 6 months after surgery [18] (Figure 2).

Currently, irrespective of the fixation technique employed, excessive early migration is defined as MTPM migration exceeding 1.6 mm within the first postoperative year and a continuous migration of more than 0.2 mm of MTMP between the first and second year. These parameters have been identified as predictive indicators of long-term aseptic loosening. Conversely, MTPM migration of less than 0.5 mm at 1 year and less than 0.3 mm of MTPM between 2 and 5 years postoperatively are indicative of a stable implant [10, 11, 19]. Given that the majority of the early migration occurs in the first 6 postoperative months, the MTPM value at 6 months postoperative can serve as a suitable alternative to the 1-year mark [18].

Radiostereometric analysis outcomes

Cemented and cementless implants exhibit distinct RSA migration patterns

Multiple RSA studies have indicated that both cemented and cementless tibial components exhibit migration during the first year after surgery, primarily due to bone remodeling [11]. The dominant directions of migrations are subsidence and posterior rotation in the tibial host bone [19] (Figure 3). Cemented implants display very little early migration, likely because they rely on primary bone fixation through cement interdigitation. In contrast, cementless implants tend to exhibit a higher initial migration due to implant settling, followed by a migration plateau once biological fixation is achieved through bone ingrowth. In Pijls et al’s systematic review and metanalysis of RSA migration in TKA, the average 12-month MTPM of cemented TKA was lower compared to uncemented TKA: 0.44 (CI 0.38–0.50) versus 1.09 (CI 0.91–1.28) (Figure 4). However, following this initial “unstable” phase, typically lasting 3–6 months after surgery, stabilization is usually achieved for both cemented and cementless implants, and this stability remains evident for many years [18].

Excellent RSA outcomes of new generation 3D cementless TKA

Hasan and coworkers [18] conducted a comparative study assessing the migration of a newly developed 3D-printed cementless TKA in comparison to a cemented TKA of similar design over a 2-year period. They used RSA from 36 cemented and 36 cementless TKAs to compare the mean MTPM between the two groups at 3, 12, and 24 months postoperatively. The mean MTPM for the cementless group was higher than that of the cemented group, with values of 0.52 mm, 0.62 mm, and 0.64 mm, respectively, compared to 0.33 mm, 0.42 mm, and 0.47 mm for the cemented group (P =.003). However, when using 3 months as the baseline, there was no difference in mean migration between the two groups (P =.497). They concluded that cementless TKA showed increased migration in the initial 3 months after surgery but stabilized thereafter [19]. Another group observed a similar trend studying the migration of a 3D-printed TKA in 29 patients. They reported the largest tibial migration within the first 6 weeks postoperative (0.68 mm MTPM), but no change in mean tibial migration after 6 months [11]. It is worth noting that the 12-month MTPM values for the new generation 3D-printed TKAs, reported as 0.62 by Hasan and colleagues and 0.68 by Sporer and colleagues, tend to be lower than the previously published mean MTPM for cementless TKA at 12 months, which was reported as 1.09 mm [18] (Figure 4). This may be related to the design and the material properties of the new cementless TKA that better match the elasticity and stiffness of the bone, which could result in less stress-shielding around the implant [19].

The new cementless 3D-printed TKA implants have demonstrated excellent outcomes in short- and mid-term follow-ups [8–11, 17, 19, 20]. However, only a limited number of studies have tracked the progression of migration beyond 2 years. Van der Lelij and colleagues [12] investigated the migration of cementless 3D-printed and cemented tibial components over a 5-year follow-up period. The MTPM was compared between the groups at 5 years and the progress in migration assessed between 2 and 5 years. No significant differences (P =.09) were found between mean MTPM at 5 years. For the cementless group, the mean MTPM was 0.66 mm (95% CI: 0.56–0.78 mm), and for the cemented group it was 0.53 mm (95% CI: 0.43–0.64 mm). In addition, while cemented implants displayed less migration during the initial phase, they showed a progression of migration beyond the first year, possibly due to continued bone resorption. In contrast, cementless implants after the initial phase remained stable over time [12]. Other studies observed a similar trend, with cementless components stabilizing after an initial phase of early migration while cemented implants exhibited lower migration initially, followed by progressive migration beyond 2 years [21, 22]. However, this trend is not always consistent, and other RSA studies have not reported continuous migration of cemented tibial implants beyond the 2-year mark [23]. While progressive migration does not necessarily indicate a clinically significant increase in aseptic loosening rates, further long-term studies for both cemented and cementless implants are warranted to better understand their performance over time.

Potential limiting factors for cementless implants

There are other factors at play that may impact the RSA results with cementless TKAs. Patient-specific factors as well as surgeon factors have been shown to impact outcomes and should be considered when selecting a specific prosthesis fixation method. For instance, in cementless TKA, osseointegration is crucial for component fixation and concerns arise regarding the bone quality necessary for successful fixation, particularly in patients with osteoporosis or osteopenia. Andersen et al [16] conducted a study aiming to assess the impact of tibial bone quality on uncemented tibial component fixation. Preoperative bone mineral density (BMD) was measured and RSA was performed at different timepoints postoperatively. The study revealed a correlation between low preoperative BMD in the tibia and high MTPM values. The authors concluded that patients with poor bone density may experience increased migration of uncemented tibial components [24]. 
Another aspect to consider is the accurate sizing and placement of tibial components in cementless TKA. In a study aiming to assess tibial component migration of highly porous 3D-printed TKA implants, RSA was performed at various intervals up to 24 months postoperatively. Results indicated that undersized cementless tibial components are at higher risk of poor fixation, leading to continuous migration. Consequently, a higher risk of aseptic loosening can be expected [17]. Surgeons should be aware of the fact that undersizing the cementless tibial component can increase the risk of poor fixation, causing continuous migration, increased subsidence, and posterior tilt.

Conclusion

In conclusion, RSA studies have provided a valuable insight into the performance and outcomes of both cemented and cementless TKA. These studies have highlighted the importance of factors such as early migration patterns, bone quality, implant design and materials, and surgical technique in determining the success of cementless implants. Furthermore, RSA studies can predict loosening of new implants with extreme precision prior to the onset of clinical symptoms.
The RSA data presented in this article suggest that newer generation cementless prostheses offer a compelling alternative to cemented TKA, with promising results in terms of stability and long-term fixation. As discussed in Part 2, this is particularly true in younger, obese, and more active patient populations. Ongoing research and longitudinal studies will further elucidate the factors that influence the success of cementless TKAs and optimize patient outcomes.