Introduction

Because osteoarthritis of the knee is one of the most common disease processes in humans, data concerning the incidence of knee arthroplasty are important to estimate the societal cost of treatment for this condition. While utilization in countries that sponsor joint registries is well delineated, few data are available in the United States due to the lack of a national implant registry. While the Medicare databases give some idea of utilization of knee arthroplasty, partial versus total cannot be differentiated, and only patients covered by Medicare (generally greater than 65 years of age) are captured. Utilization of partial knee replacement can be considered in several ways, including as a percentage of total knee arthroplasties as well as the percentage of patients who have isolated unicompartmental arthritis. There have been previous spikes in the utilization of partial knee replacement that usually corresponded to an increase in the number of surgeons performing partial knee replacement, usually followed by a pullback related to an increase in the failure rate.

While the number of patients with symptomatic osteoarthritis has continued to grow significantly, it appears that the percentage of patients in which the osteoarthritis is confined to predominantly one compartment (and thus amenable to partial knee replacement) has remained relatively constant, and has been estimated at 30%. This chapter reports the incidence of partial knee arthroplasty (PKA) in all countries that have an existing registry. For the United States, which does not have a registry, we have estimated the incidence of unicondylar arthroplasty by indirect methods, using implant sales data from the major manufacturers.¹ We have estimated market share by utilizing commercial databases that track market share.²

Growth of PKA in the United States

To estimate the number of PKAs performed each year, we utilized a retrospective cross-sectional design. We solicited sales data from the four major PKA orthopaedic manufacturers in the U.S. market, which included Biomet (Warsaw, IN), Zimmer (Warsaw, IN), Depuy Orthopaedics (Warsaw, IN), and Stryker Corporation (Mahwah, NJ). Stryker Corporation elected not to contribute sales figures. We multiplied each company’s market share based on determinations of a market analyst firm (DataMonitor, New York, NY) by the number of unicompartmental arthroplasty implants sold, and this allowed us to determine total units sold. Sales data and market share were used to estimate total unicondylar implants implanted for 2003 (83% market share), 2004 (74% market share), and 2005 (87% market share). We used regression analysis to estimate the sales numbers for the years 1998–2002. We then used the National Hospital Discharge Survey (NHDS) to provide data for the total amount of knee arthroplasties performed in the United States during the study years, and subtracted our estimates of unicondylar knee arthroplasties (UKAs) from those to arrive at an estimate of the number of total knee replacements (TKRs) performed each year. The estimated number of unicondylar arthroplasties increased from 6570 in 1998 to 44,990 in 2005, whereas the number of total knee arthroplasties increased from 259,000 in 1998 to 441,000 in 2004 (no NHDS data available for 2005). As a percentage of all knee arthroplasties, the number of unis increased from 2.5% in 1998 to 9.8% in 2005. Between 1998 and 2005, the incidence of PKA increased by an average of 30% a year.

Incidence of PKA in Scandinavia

The Swedish Knee Arthroplasty Register has made annual reports for the period 1998–2005.³ Between 2002 and 2005, UKAs represented between 9.4% and 11.7% of all arthroplasties. In 1998, 4400 TKRs were performed and approximately 1000 UKAs, and in 2006 the number of TKRs had grown to 9700, and the number of UKAs in relative terms declined to 900. During this period the register published several reports of a relatively high failure rate, and this may have affected the growth of UKA, because a different experience existed in Finland and Norway, and a higher rate of growth was seen. The Norwegian registry reported in their 2008 report that the number of UKAs increased from 87 in 1998 to 455 in 2005, an increase of 400%.⁴ During the same period the number of total knees implanted in Norway increased from 1320 in 1998, to 2800 in 2005. The Finnish joint registry does not report their finding on the Internet, but has some data in published papers. Between 1988 and 1995, 540 UKAs were performed as compared to 12,480 TKRs, and between 1996 and 2003 the number of UKAs increased to 1251 as compared to 34,132 TKRs.

Incidence of Partial Knee Replacement in Other Countries

The Australian Joint Registry reported that the annual use of unicondylar knee replacement increased four times from 6700 unis implanted in 2000 to 28,822 unis in 2008.⁵ During this same time period, the number of TKRs increased fivefold from 36,442 to 197,301. The percentage of unis relative to all knee arthroplasties went from 19% to 16% during the same period.

The New Zealand Arthroplasty Registry reported that unicondylar arthroplasty utilization doubled from 2000 to 2003, with a moderate decrease from 2004 to 2008.⁶ Nonetheless, in 2000, of the 3015 knee arthroplasty procedures, unicondylar arthroplasty represented 11% of all knee arthroplasty procedures, and in 2008 unicondylar procedures represented 10% of all knee arthroplasty procedures. The British arthroplasty register in 2009 reported that unicondylar arthroplasty represented 8% of all knee replacements done between 2000 and 2009.⁷ In 2008, there were 71,527 primary knee arthroplasties reported to the U.K. registry. Of these, 5573 were unicondylar arthroplasties (8% of the total amount of procedures), and 1030 were patellofemoral arthroplasties (1% of the total). Consequently, 9% of knee arthroplasties were partial knee replacements, and this percentage has remained stable since 2000.

The Canadian Arthroplasty Register reported that the total number of knee arthroplasty procedures per year increased from 16,709 in 1998 to 38,922 in 2008, a change of 125%.⁸ Of this figure, partial knee replacement represented about 8% of the TKR procedures in 2004, 9% in 2005, and 8% in 2007.

Conclusion

The use of partial knee replacement increased steadily during the 1990s, but has remained relatively stable in the first decade of the 21st century across the world. In round numbers this percentage approaches 10% of all knee arthroplasties. Those registries that have recorded a decline in the relative utilization of PKA have also reported a higher early failure rate of partial replacements, and this may have impacted surgeon utilization of PKA in these countries. The stability of incidence of PKA across the world indicates that it remains a viable option for patients with unicompartmental arthritis of the knee.