Higher Risk of Loosening for a Four-Pegged TKA Tibial Baseplate Than for a Stemmed One: A Register-based Study
Authors: Robertsson Otto , Sundberg Martin , Sezgin Erdem Aras , Lidgren Lars , W-Dahl Annette 
Published: May 2019

Abstract

Background
Modern modular implants allow surgeons to mix different combinations of components within the same brand. From 1999 to 2012, the NexGen®-CR Option femoral component, together with a NexGen® Option Stemmed tibial plate (stemmed baseplate), which uses a short central stem, was the most-frequently used NexGen® combination in the Swedish Knee Arthroplasty Register. However, from 1999 to 2012, the same femoral component was also used along with the NexGen® Precoat four-pegged tibial baseplate (pegged baseplate). Considering the difference in the fixation concepts for these two tibial baseplates, we wanted to study whether their revision rates differed.

Questions/purposes
To investigate the difference in (1) all-cause revision and (2) the risk of revision for aseptic loosening between the NexGen® pegged and stemmed baseplates when used with the NG-CR Option femoral component and the same two types of inserts.

Methods
The Swedish Knee Arthroplasty Register provided data. The register, which was started in 1975, has since 1999 registered part numbers for individual implant components, allowing it to assess the combinations of components used in each patient. It has been shown to have high completeness (97%) and validity [12, 15]. The inclusion period was 1999 to 2012; during that time, 137,143 primary knee arthroplasties were registered, of which 125,094 were TKAs. Only TKAs performed for osteoarthritis and without patellar resurfacing were included, since not resurfacing the patella is the standard procedure in Sweden. This left 15,287 knees with the stemmed baseplate and 2479 with the pegged baseplate, or 12% and 2% of the total number of TKAs, respectively. Two general hospitals used the pegged baseplate exclusively during that period. Thus, specific patients were not selected for having the pegged plate. The mean age, mortality, and length of followup were similar for the two groups.
We used the Kaplan-Meier statistics to calculate the cumulative revision rate (CRR) and Cox regression to compare risk ratios after adjusting for age and sex. The end point was a knee revision for respective all causes or aseptic loosening. The study ended on December 31, 2016. Due to the free healthcare system in Sweden it is highly unusual for patients to seek elective revision abroad, and by use of the extensive Swedish census register, we estimate the level of followup approximately 97%.

Results
Knees with the pegged baseplate had a higher risk for all-cause revision than did those with the stemmed baseplate (5.8% [95% confidence interval {CI}, 4–8.3] and 3% [95% CI, 2.6–3.5] at 15 years; p = 0.003). After controlling for age and sex, the aseptic loosening risk in the pegged baseplate group was still higher than that in the stemmed group (relative risk, 5.40; 95% CI, 3.64–8.02; p < 0.001).

Conclusions
In this Swedish registry study, we observed a higher loosening risk with the pegged baseplate than the stemmed one, even after controlling for age and sex. Because this was only a comparison of implants from one vendor, and because there may have been other between-group differences for which we could not fully control, this concerning finding should be explored using data from other registries.

Level of Evidence
Level III, therapeutic study.