Editorial: Different, yet strong together: the Nordic Arthroplasty Register Association (NARA)

The Nordic Arthroplasty Register Association (NARA) was established in 2007 by hip and knee arthroplasty registry leaders from Denmark, Norway, and Sweden. Purportedly, the idea of combining national databases was presented and discussed while sitting in a bar. The 1st NARA manuscript by Havelin et al. (2009), published in Acta Orthopaedica, described differences in the demographics of patients receiving total hip arthroplasty (THA) among these 3 participating countries. The paper was well read in Finland, especially by author KM of this editorial, with disbelief and envy. It became obvious that the other Scandinavian registries were flourishing while the Finnish Arthroplasty Register (FAR) was suffering. The Finnish arthroplasty surgeons started to develop FAR in collaboration with the National Institute for Health and Welfare in Helsinki, Finland became a preliminary member at NARA meetings around 2010, and FAR data became electronic in 2014 (Finnish Arthroplasty Register 2021).

Simultaneously, the scientific work of NARA reached high standards, concerning both quality and quantity (Havelin et al. 2009, Dale et al. 2012, Mäkelä et al. 2014a, Lazarinis et al. 2017). Frequent face-to-face meetings enabled mutual confidence building between collaborators. The Scandinavians allowed NARA to be under Finnish leadership for the time period 2014–2020. “NordForsk,” a funding agency under the Nordic Council of Ministers, financially supported NARA from 2014 to 2016, and currently all participating registries support NARA financially and logistically, but there is no central funding.

All Nordic countries have similar state-funded public healthcare systems, but there are large dissimilarities between the participating countries when it comes to the practice of orthopedics, such as for example concerning fixation techniques and surgical approaches: the use of uncemented THA fixation is much more frequent in Denmark and Finland than in Norway and Sweden (Mäkelä 2014a), and the use of the posterior approach to the hip dominates in Denmark, whereas almost half of the Swedish exposures are by direct lateral approaches (Swedish Hip Arthroplasty Register 2019, Danish Hip Arthroplasty Register 2020). In total knee arthroplasty (TKA), the use of cemented fixation and of patellar resurfacing varies considerably between NARA countries (Irmola et al. 2021). These differences result in what is termed a “natural experiment” by epidemiologists, such that country-wise outcomes can be investigated (Mäkelä et al. 2014b).

Today, in June 2021, there are more than 50 scientific NARA publications, and many of them have influenced treatment practices, at least in the participating countries. For example, in 2014 a NARA paper concerning THA fixation methods in elderly patients showed that the proportion of THAs with uncemented implants had increased from 10% to 39% between 1995 and 2011, although survival for uncemented implants was much lower compared with cemented implants in patients aged ≥ 65 years (Mäkelä et al. 2014a). Gradually, cemented stems were favored in the elderly based on these and other similar data, especially in Denmark and Finland where uncemented fixation was common even in patients with hip fractures (Danish Hip Arthroplasty Register 2020, Finnish Arthroplasty Register 2021).

The role of hydroxyapatite coating in THA has been myth-busted based on NARA data. Hydroxyapatite-coated femoral stems and cups of different brands did not reduce the risk of revision due to aseptic loosening (Hailer et al. 2015, Lazarinis et al. 2017). These, together with other similar findings, suggested that hydroxyapatite coating does not render primary hip implants more stable.

Reducing dislocation rates is one of the major remaining challenges when developing modern THA further. Based on NARA data, Kreipke et al. (2019) showed that osteoarthritis patients operated on with a dual mobility cup (DMC) had a lower risk of revision due to dislocation, but a higher risk of revision caused by infection. Similarly, the use of a DMC in the treatment of patients with displaced femoral neck fractures was associated with a lower risk of revision, both for any reason and due to dislocation (Jobory et al. 2019). The total of 4,500 patients with DMC analyzed by use of the NARA database exceeded any cohort of DMC-treated hip fracture patients published previously.

NARA has increased focus towards research on TKA: In agreement with data from outside the Nordic countries (Nugent et al. 2019), Niemeläinen et al. (2020) found that both cemented and hybrid TKA had excellent 10-year survival rates in patients below 65 years, and that cemented TKA still deserves the status of gold standard in working-age patients. Another NARA paper indicated that totally uncemented TKA fixation was associated with an increased risk of revision when compared with cemented fixation in elderly TKA patients (Irmola et al. 2021).

After the success of NARA research on THA and TKA, the Nordic shoulder registries also joined the NARA collaboration. Using the NARA shoulder database, Lehtimäki et al. (2018) found that the overall mid-term risk of revision after reverse shoulder arthroplasty performed for rotator cuff tear arthropathy was low, and they identified patient factors associated with an increased risk of revision.