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Fixation Factors in Cemented Total Knee Arthroplasty
Presented by Dr. Zachary D. Post
Cemented knee replacements have been, and will continue to be, very successful. According to data from the Australian Orthopaedic Association National Joint Replacement Registry, the success rate for cemented total knee arthroplasty is very high — 91.4% at 18 years.1 Data from private studies also underscore the long-term success of cemented TKA.2-5 In 2016, Ritter and colleagues published a review of 5,649 primary TKAs. The overall prosthesis survival rate was 94.2% at 25 years and 92.4% at 30 years.6 The ATTUNE® Knee System (DePuy Synthes), while relatively new to the market, has shown similar excellent survival in unbiased registry data with 97.3% survivorship at 5 years.7
Although cemented TKAs are successful, there is most certainly room for improvement, specifically with aseptic loosening. In the Ritter study, the top two mechanisms of failure were aseptic loosening and instability.6 When Sharkey and colleagues evaluated 781 revision TKAs from 2002 to 2012, they also found that loosening was the most common cause of failure (39.9%) followed by infection (27.4%) and instability (7.5%).8 The Sharkey study further emphasized that while other causes of TKA failure, including polyethylene wear, seemed to be diminishing over time, aseptic loosening continues to be a problem in TKA.
Registry data show that aseptic loosening is different from other causes of failure. Whereas TKA failure from infection and instability seem to plateau around 4 to 5 years after surgery, aseptic loosening continues to climb.1 This is an issue for younger patients who will need their knees to last not just 10 to 15 years, but likely 20 years or more. This growing discrepancy, greater patients demands and increased loosening failures over time presents a challenge for knee surgeons and the entire TKA implant industry.
How do we improve the longevity of TKA? Factors to consider
The ways to improve cemented knees can be divided into three categories: patient, operative and implant factors.
Although the age of patients undergoing TKA is decreasing, the number of patients with an elevated BMI undergoing joint replacement is increasing. It is predicted that by the year 2030, 50% of adults in the United States will be obese.9 Certainly many of these adults will require TKA. However, patients with a BMI greater than 35 have been shown to be at increased risk for failure.10-12 In my own experience, smaller female patients with a high BMI seem to be at greatest risk of failure. For these challenging patients, a traditional cemented TKA may not be the best choice. The addition of a stemmed tibial component may protect heavier patients from varus collapse and early failure. In my practice, we do not hesitate to use the additional fixation a stemmed tibia gives to patients who need it.
Operative factors: Cementing technique
Traditional cementing technique focused on mixing well and preventing movement of the implant before the cement hardened. We now recognize that there are many other important factors to maximizing the fixation of a cemented TKA. Billi and colleagues produced one particularly well-designed study that demonstrates the effect of variations in cement technique and its impact on the strength of the bond between the cement and the implant.14 For their study they used two cements (Simplex [Stryker] and Palacos [Heraeus Medical]) to cement tibial trays into acrylic holders. First, they varied the time from mixing to implantation. The cementing was performed early, normal (per manufacturer recommendations) or late. Next, they evaluated the effect of placing cement on the tibial plateau only (and not on the tibial implant) vs. placing cement on both the tibial plateau and separately on the tibial implant (“precoating” the implant). Finally, they studied the effect of marrow fat contamination at the metal-cement and cement-cement interfaces. What Billi and colleagues found is both interesting and instructive for surgeons looking to maximize cement bond strength. First, cementing early increased the mean strength of the interface by 48% for Simplex and 72% for Palacos. Late cementing, on the other hand, decreased strength 47% for Simplex and 73% for Palacos. Second, when cement was placed on the implant in addition to the tibial plateau, mean strength increased by 153% for Simplex and 147% for Palacos. Finally, and in my opinion, most importantly, when fat contaminated the metal-cement interface, mean strength decreased by 99% for Simplex and by 94% for Palacos. Interestingly this same effect was not found when lipid contamination occurred between two layers of cement.13
The knowledge I gained from the study by Billi and colleagues and others like it have changed how I practice. I still follow the traditional teaching to ensure the bone is clean and dry before cementing. We are meticulous with this step. In my operating room we cement soon after mixing of the cement is complete. This often results in using “wet” cement, which I now prefer. We also “precoat” the implants with cement and then separately press cement into the bone until we can see the trabecular pattern of the bone through the cement. This ensures cement penetration into the bone as well as maximizes the cement contact on the implant. Knowing that fat contamination reduces strength, I have incorporated several techniques to avoid it. One basic step is taking precautions to ensure that once the implant comes out of the box, no one touches it until cement is placed on the back. And then finally, once the knee is cemented in place, it is critical that the leg not move until the cement is cured. This prevents propagation of lipid under the tibial tray.14 These steps may sound simple, but I think it is important that we address every detail that could affect outcomes.
Implant factors: Macrolock and Microblast—ATTUNE S+™ Technology designed to enhance implant fixation
The adjustments I make in both patient and operative factors can be aided by my implant selection. The tibial base with ATTUNE S+™ Technology has two features to improve tibial fixation: Macrolock and Microblast. The Macrolock feature refers to the macro-lock at the cement-implant interface achieved by four cement pockets with 45-degree undercuts. The cement interdigitates in these pockets, inhibiting lipid transfer across the interface, maximizing cement/interface contact and strengthening the bond. Microblast refers to the relative roughness of the tibia’s undersurface. This roughness is intended to inhibit lipid infiltration because lipid flows more easily over a smooth surface.
Source: DePuy Synthes
Early data suggest that these implant changes may lead to improved longevity of cemented TKA. The Australian registry data report 7,386 total primary TKAs performed using ATTUNE S+™ Technology with 3,341 reported at 1-year follow-up. This tibial base is showing performance in line with class of all TKAs with a cumulative percent revision rate of 1.5% (1.1, 2.1) at 2 years compared with 2.0% (2.0, 2.0) for the class. Most notably, when looking at the reasons for revisions, failures due to loosening only make up 2.9% of the total revisions of the ATTUNE S+™ Technology tibias compared with 20.3% for the class of TKA.15*
In conclusion, cemented fixation has been, and likely will continue to be, the gold standard for TKA implantation. Data over the last 30 years have demonstrated excellent success. However, loosening, instability and infection continue to occur. It is my strong feeling that we can improve on the previous generation of TKA success with patient-specific implant constructs, improved operative technique, especially cement handling, and advances in the implant itself such as the ATTUNE S+™ Technology with Macrolock and Microblast features.
*Disclaimer: AOANJRR is confident in the accuracy of the data included in this report, at the time it was provided. However, it was generated using an automated reporting system and has not been reviewed by the AOANJRR personnel.
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