ARTHRITIS & RHEUMATISMVol. 64, No. 2, February 2012, pp 423–428DOI 10.1002/art.33367© 2012, American College of Rheumatology

The Incidence of Knee Arthroplasty for Primary OsteoarthritisGrows Rapidly Among Baby Boomers

A Population-Based Study in Finland

Jarkko Leskinen,1 Antti Eskelinen,2 Heini Huhtala,3 Pekka Paavolainen,4 and Ville Remes1

Objective. Total knee arthroplasty (TKA) is acommonly used treatment for severe primary kneeosteoarthritis (OA) that is refractory to conservativetreatment. Despite the presumed increase in the use ofthis treatment modality in younger patients, there arefew published data concerning this. The aim of ourstudy was to examine the changes in the age- andsex-standardized incidence of TKA and unicondylarknee arthroplasty (UKA) in Finland during 1980–2006and to identify factors that might affect the incidenceduring this period.

Methods. We obtained data on UKAs and TKAsfrom the Finnish Arthroplasty Registry and populationdata from Statistics Finland to analyze the incidence ofUKAs and TKAs in Finland for the period 1980–2006.The effects of sex, age group, and hospital volume on theincidence of these procedures were also evaluated.

Results. The annual cumulative incidence ofUKAs and TKAs has increased rapidly from 1980 to2006 among 30–59-year-old inhabitants of Finland. ForUKAs, the incidence increased from 0.2 per 100,000inhabitants to 10 per 100,000, and for TKAs, the inci-dence increased from 0.5 per 100,000 to 65 per 100,000.The incidence remained higher among women duringthe entire study period. Most of the increase occurredamong patients ages 50–59 years. The incidence grew

more rapidly in low-volume and intermediate-volumehospitals.

Conclusion. We demonstrated a rapid increase inthe incidence of arthroplasty among patients with pri-mary knee OA in Finland, especially in those ages 50–59years. There was no single explanatory factor behindthis finding, although some of the growth might be dueto the increased incidence noted in low- andintermediate-volume hospitals.

Total knee arthroplasty (TKA) is a treatmentmethod commonly used in patients with severe osteoar-thritis (OA) that is refractory to conservative treatment.Good long-term results with cemented condylar prosthe-ses have been described (1–3). The majority of patientsin these studies, however, have been older than 60 years.Meanwhile, the so-called baby-boom generation (thegeneration born after World War II) is growing older,placing increasing demands on the health care system(4).

The estimated demand for primary TKA hasbeen predicted to grow by 673% to 3.48 million proce-dures in the US by the year 2030. Similar projectionshave also been proposed in other Western countries(5–9). Wells et al (8) reported an increase in theincidence of TKA in younger patients (45–64 years ofage) with primary OA in Australia between 1988 and1998. Those investigators also showed a geographicvariation in the incidence of TKA between the Austra-lian states and territories. Jain et al (5) reported that theincidence of TKA had increased 54% in the group ages50–59 years and 95% in the group ages 40–49 years inthe US between the 1990–1993 cohort and the 1998–2000 cohort. They also noted that the increase in inci-dence was more rapid in high-volume centers (�400total knee replacements per year) than in low-volume

1Jarkko Leskinen, MD, Ville Remes, MD, PhD: Peijas Hos-pital, Vantaa, Finland, and Helsinki University Central Hospital,Helsinki, Finland; 2Antti Eskelinen, MD, PhD: COXA Hospital forJoint Replacement, Tampere, Finland; 3Heini Huhtala, MA: Univer-sity of Tampere, Tampere, Finland; 4Pekka Paavolainen, MD, PhD:ORTON Orthopaedic Hospital of the Invalid Foundation, Helsinki,Finland.

Address correspondence to Jarkko Leskinen, MD, PeijasHospital, Sairaalakatu 1, 00029 HUS, Finland. E-mail: jarkko.leskinen@hus.fi.

Submitted for publication April 14, 2010; accepted in revisedform September 20, 2011.

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centers. Similar data on the incidence of unicondylarknee arthroplasty (UKA) in young patients have notbeen published.

In the present study, we analyzed changes in theage group– and sex-standardized incidence of UKA andTKA in Finland between the years 1980 and 2006. Inaddition, we sought to identify the factors that mighthave effects on the incidence during the study period.

PATIENTS AND METHODS

The Finnish population and health care system. Dur-ing the study period, the total population of Finland increasedfrom 4,787,778 inhabitants in 1980 to 5,276,955 inhabitants in2008. The Finnish health care system is funded throughgeneral taxation resources and is provided by the municipali-ties. There is considerable emphasis on primary care, preven-tive services, and documentation (10).

The Finnish Arthroplasty Register. Since 1980, theFinnish Arthroplasty Register has been collecting informationon total knee replacements. Consequently, health care author-ities, institutions, and orthopedic units are obliged by law toprovide the information that is essential for the maintenance oftheir registry. The coverage of the Finnish Arthroplasty Reg-ister was estimated for the period 1994–1995 by comparing itsdata with those of the discharge registries of the participatinghospitals, and was found to cover 90% of implantations. Since1995, the data from the registry have been compared withthose from hospital discharge registries every few years. Cur-rently, 98% of implantations are recorded (refs. 11–13, and M.Peltola: personal communication [National Research and De-velopment Centre for Welfare and Health in Finland; July 27,2008]).

Description of the population data. Every inhabitant ofFinland has a unique social security number. This numberindicates that its holder is entitled to the public health careservices of Finland. Statistics Finland keeps an accurate,up-to-date civil registry of the inhabitants of Finland. Inpractice, there are no known significant numbers of illegalimmigrants in Finland. Therefore, data derived from such asource are unlikely to be a confounding or biasing factor in ourstudy.

Inclusion criteria. All TKAs and UKAs performed onpatients under the age of 60 years because of primary knee OAthat had been entered in the Finnish Arthroplasty Registrybetween 1980 and 2006 were included in the present study. Allother diagnoses were excluded. Population data for Finlandduring the period 1980–2006 were obtained from StatisticsFinland. All of the population data, including the populationdata for each municipality and each county, were divided intocohorts by age (30–39 years, 40–49 years, and 50–59 years) andwere subgrouped according to sex.

Analysis of the incidence of knee arthroplasty. Annualincidence rates for TKAs and UKAs were calculated. Bilateralarthroplasties were handled as 2 separate operations in eachanalysis. The general incidence (in subjects ranging in age from30 years to 59 years), the incidence by sex, and the incidence byage cohort for the knee arthroplasties were calculated from the

number of operations performed in the entire study populationfor both sexes and for each of the 3 age cohorts. The generalincidence was presented as the number of operations per-formed per 100,000 inhabitants ages 30–59 years. Incidence bysex was presented as the number of operations performed per100,000 men or women ages 30–59 years. The incidence by agegroups was presented as the number of operations performedper 100,000 inhabitants ages 30–39 years, 40–49 years, and50–59 years, respectively.

We also calculated the incidence for each publichospital. All inhabitants of Finland have to be registered as aresident of some county or municipality. Each public hospitalhas its own catchment area (consisting of 1 or more countiesand/or municipalities), and all patients from those countiesand/or municipalities are primarily referred to the specifichospital in whose administrative area they reside. In addition,there are 2 major foundation-based private hospitals that treatpublic patients in Finland. In order to apportion the correctincidence in the respective catchment area for each communalhospital, TKAs and UKAs performed in the private institu-tions were added to the patients’ “own” communal hospitalsoperation numbers, based on the municipality or county inwhich they lived. Thus, the incidence of TKAs and UKAs ineach catchment area was calculated according to the numberof arthroplasties performed in both the respective communityhospitals and the 2 foundation-based hospitals for each partic-ular catchment area.

Analysis of the effect of hospital volume on incidence.For this analysis, the study period was restricted to the years2000 through 2006 because only in the twenty-first century didthe hospitals start to demonstrate definite differences betweenannual numbers of knee arthroplasties. Hospitals were dividedinto low-, intermediate-, and high-volume centers according tothe number of TKAs performed among all hospitals in Finlandduring 2006. All knee arthroplasties, regardless of the agegroup or the indication for the operation, were taken intoaccount when dividing the hospitals into these 3 categories.Hospitals that performed �100 knee replacements, 100–400knee replacements, and �400 knee replacements during 2006,regardless of the ages of the patients, were categorized as low-,intermediate-, and high-volume centers, respectively. All hos-pitals belonged to the same volume category throughout theentire study period as was assigned according to their volumein 2006. All university hospitals except 1 belonged to thehigh-volume center category, and almost every central hospitalbelonged to the intermediate-volume center category. Thelow-volume center category mainly consisted of smaller ruralhospitals.

Statistical analysis. We first described the data usingdescriptive statistics. We assessed trends in the general inci-dence of TKAs and UKAs between the years 1980 and 2006.Then, we examined the incidence of TKAs and UKAs sepa-rately for sex, hospital volume, and age groups. Finally, webuilt multivariable Poisson regression models to assess sex,hospital volume, and age groups with regard to the incidenceof TKAs and UKAs. The results are expressed as incidencerate ratios (IRRs) with 95% confidence intervals (95% CIs). Pvalues less than 0.05 were considered statistically significant.The statistical analyses were conducted with the SPSS 16.0software and Stata 8.2 software (StataCorp).

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RESULTS

Patient characteristics. Between the years 1980and 2006, a total of 95,577 primary knee arthroplastieswere performed in Finland, and primary OA was anindication in 74,715 of these operations (78.2%). Amongthese operations, 7,829 TKAs and 1,132 UKAs (total of8,961 knee arthroplasties) were performed for primaryOA in patients under the age of 60 years. Femalepatients represented 5,797 of these cases (64.5%). Themean age of the patients was 54.9 years (range 30–59years) in the TKA group and 53.9 years (range 30–59years) in the UKA group (Table 1).

General incidence of knee arthroplasty. Therewas a 130-fold increase in the incidence of TKAs overthe entire study period, from 0.5 operations per 100,000inhabitants (ages 30–59 years) to 65 per 100,000 (Figure1). The incidence increased very rapidly between 2001and 2006, from 18 per 100,000 to 65 per 100,000. The

IRR for the annual increase in the general incidence ofTKAs was 1.19 (95% CI 1.19–1.20; P � 0.001).

The incidence of UKAs increased from 0.2 oper-ations per 100,000 inhabitants to 10 per 100,000 (Figure1). The increase in the number of UKAs was less thanthat observed for the number of TKAs. There was,however, a period of rather fast growth toward the endof the study period (from 2000 to 2006) (Figure 1). TheIRR for the annual increase in the general incidence ofUKAs was 1.26 (95% CI 1.24–1.28; P � 0.001). Theincidence of TKAs was higher than the incidence ofUKAs, with an IRR of 6.92 (95% CI 6.50–7.36; P �0.001).

Incidence by sex. TKAs were performed morefrequently in women than in men (Figure 2). Theincidence of TKAs increased among both sexes, but itremained consistently higher among women throughoutthe study period, and there was a significant interactionbetween sex and time with regard to TKAs (P � 0.001).During the last 10 years of the study period, the inci-dence was 1.6–2.4-fold higher in women than in men.

UKAs were also performed more often in womenthan in men (Figure 2). Until year 1999, there were nomajor differences between the sexes. After 2000, how-ever, there was a larger increase in the incidence ofUKAs among women than among men, but there wasstill no significant interaction between sex and time withregard to UKAs (P � 0.036).

Incidence by age group. In patients ages 30–39years, there was no major increase in the incidence ofTKAs over the study period, with an IRR of 1.09 (95%CI 1.04–1.14; P � 0.001) (Figure 3). In patients ages

Table 1. Knee arthroplasties performed in Finland during the studyperiod*

Total number of primary knee arthroplasties 95,577No. (%) with indication of primary OA 74,715 (78.2)No. (%) of patients �60 years of age 8,961 (9.4)

Total knee arthroplasty 7,829 (8.2)Unicondylar knee arthroplasty 1,132 (1.2)

No. femaleTotal knee arthroplasty 5,047Unicondylar knee arthroplasty 750

Mean age of the patients (range), yearsTotal knee arthroplasty 54.9 (30–59)Unicondylar knee arthroplasty 53.9 (30–59)

* OA � osteoarthritis.

Figure 1. General incidence of knee arthroplasty for primary osteo-arthritis in Finland for the years 1980 through 2006. Shown are thenumber of total knee arthroplasties and unicondylar knee arthroplas-ties per 100,000 inhabitants ages 30–59 years.

Figure 2. Incidence of total knee arthroplasty (TKA) and unicondylarknee arthroplasty (UKA) for primary osteoarthritis by sex. Shown arethe number of TKAs and UKAs performed in male and femalepatients ages 30–59 years per 100,000 inhabitants.

INCIDENCE OF KNEE ARTHROPLASTY FOR PRIMARY OA IN BABY BOOMERS 425

40–49 years, the increase was also quite limited, rangingfrom 0 per 100,000 inhabitants to 18 per 100,000, and theIRR was 1.19 (95% CI 1.18–1.21; P � 0.001). In patientsages 50–59 years, however, there was a large increase inincidence during the study period, from 1.5 per 100,000in 1980 to 160 per 100,000 in 2006, and the IRR was 16.8(95% CI 12.2–23.2; P � 0.001). There was a significantinteraction between the age groups and time with regardto TKAs (P � 0.001).

In patients who underwent UKA, there was asimilar variation in incidence among the age groups aswas observed in those who underwent TKA (Figure 3).The incidence of UKAs remained low in patients ages30–39 years (varying from 0 per 100,000 to 0.3 per100,000), and the IRR was 1.30 (95% CI 1.06–1.60; P �0.001). In patients ages 40–49 years, the incidence grewslightly over the period from 2000 to 2006 (from 0 per100,000 to 5 per 100,000), with an IRR of 1.33 (95% CI1.28–1.40; P � 0.001). However, the incidence grewmost rapidly in the oldest age group. In this group, theincidence increased from 0.5 per 100,000 to 24 per100,000, and the majority of the growth occurred duringthe period 2000–2006. The IRR in this age group was1.23 (95% CI 1.21–1.24; P � 0.001). Comparing agegroup 40–49 years with age group 30–39 years, the IRRfor an increase in the incidence of UKA was 24.4 (95%CI 10.8–55.2; P � 0.001). Comparing age group 50–59years with age group 30–39 years, the IRR was 147.2(95% CI 66.0–328.5; P � 0.001). There was no signifi-cant interaction between the age groups and time withregard to UKAs (P � 0.822).

Incidence by hospital volume. The incidence ofTKA grew in every group during the period 2000 to2006. The highest increase was seen in the low-volumeand intermediate-volume hospitals. Between theintermediate- and high-volume centers, the IRR was1.23 (95% CI 1.16–1.31; P � 0.001), and between thelow-volume and high-volume centers, the IRR was 1.23(95% CI 1.13–1.34; P � 0.001) (Figure 4). There was nosignificant interaction between the hospital groups andtime with regard to TKAs (P � 0.071).

The incidence of UKA varied between 0 to 13operations per 100,000 during the study period. Theincrease was highest in the low-volume hospital groupfrom the year 2004 and there were significant differencesin the incidence between the hospital groups and timewith regard to UKAs (P � 0.001).

DISCUSSION

We found that the incidence of TKAs and UKAsin Finland increased very rapidly in patients with pri-mary knee OA who were under the age of 60 yearsduring the study period 1980–2006. In addition, theincidence of TKA grew faster than that of UKA. Re-gardless of the arthroplasty method, the incidence wasconsistently higher in women than in men throughoutthe entire study period. Most of the increase in incidenceoccurred in patients ages 50–59 years. The incidence ofTKAs increased more in the intermediate-volume and

Figure 3. Incidence of total knee arthroplasty (TKA) and unicondylarknee arthroplasty (UKA) for primary osteoarthritis by age group.Shown are the number of TKAs and UKAs performed in each of the3 age groups (30–39 years, 40–49 years, and 50–59 years) per 100,000inhabitants.

Figure 4. Incidence of total knee arthroplasty (TKA) and unicondylarknee arthroplasty (UKA) by hospital volume from the year 2000 to theyear 2006. Shown are the number of TKAs and UKAs performed ineach of the 3 hospital volume categories (high-volume, intermediate-volume, and low-volume centers) per 100,000 inhabitants ages 30–59years.

426 LESKINEN ET AL

low-volume centers than in the high-volume centersduring the 2000–2006 study period.

Registry-based studies have certain limitations. Ithas been shown that the Finnish Arthroplasty Registercovers 90% of hip implants for the year 1995, and afterthat, the coverage increased to 98% (13). This mightexplain a minor part of the increase shown in our study.Because of the accurate civil register of Statistics Fin-land and the low level of illegal immigration in Finland,the population data can be assumed to closely corre-spond to the actual population. However, divergencebetween international health care systems might con-found direct comparisons between reports from differ-ent countries. The boundaries between low-,intermediate-, and high-volume centers were arbitrary,but they adapt the previous joint registry–based studiesdone in Finland and in the US (5,14), and they formedwell-distinguished groups of hospitals performing ar-throplasties during the twenty-first century.

There have been several reports on the generalincidence of TKA in the restricted population, but mostof them focus on elderly patients with primary OA of theknee (5–8). In some of those studies, the incidence ofknee arthroplasty has been shown to have increased byup to 10% per year for the last 25 years (15).

In our study, a rapid increase in the incidence ofTKAs was determined among younger patients in Fin-land, especially over the last 10 years. Jain et al (5)demonstrated the same phenomenon, although theirstudy period ended in 2000. In an analysis from acommunity joint registry in St. Paul, Minnesota, Gioe etal (16) reported that over time, the total percentage ofyoung patients (under 55 years of age) in the overallregistry TKA population increased from 4.6% in 1991 to17.1% in 2005. Their study design was similar to ours. Inour study, growth was especially fast between 2000 and2006. It must also be noted that data collected fromnationwide registers describes the true incidence ofUKA and TKA in different age groups more accuratelythan do data from narrower regional studies.

In a recent population-based study, the incidenceof OA especially among women was found to havedecreased in Finland during the period 1980–2000 (17).Despite the decreasing incidence of knee OA in Finland,a reciprocal rapid increase in TKAs was found in thepresent study. Possible explanations for this phenome-non include the high functional and quality-of-life de-mands of younger patients under the age of 60 years.Another reason could be that the baby boomers may optto undergo elective operations at an earlier stage, whenthey have milder symptoms, than previous generations.

The baby-boom generation is well-educated and awareof the contemporary treatment options, which includeTKA and UKA. Nowadays, information about the dif-ferent treatment options is readily available via theinternet and other media. Good long-term results interms of implant survival in TKAs among older agegroups, the continuous development of anesthetic andsurgical techniques, in addition to the constant updatingand retraining of orthopedic surgeons may all have ledto the observed increase in the incidence of kneearthroplasty.

Simultaneous with the rapid increase in the inci-dence of knee arthroplasties, we witnessed a cleardecrease in the incidence of high tibial osteotomy inFinland (18). This may partly explain the growth inincidence in both arthroplasty groups.

In our study, there was a higher age group–standardized incidence of knee arthroplasties in womenthan in men. Similar findings have recently been pub-lished regarding TKAs (5–7). We found that the actualgrowth in the incidence of knee arthroplasties amongyounger patients took place in the subgroup of patientsages 50–59 years. There was, however, a minor increasein the incidence of UKAs among patients ages 40–49years. Nonetheless, the incidence of total knee arthro-plasty in the same group remained at the same low levelthroughout the study period. This is consistent with theresults reported recently by Jain et al (5). Our studypopulation consisted of only patients with primary kneeOA as the recorded indication for arthroplasty. If wehad included data for patients with secondary (posttrau-matic) OA, rheumatoid arthritis, and osteonecrosis, theincidence of knee arthroplasties would have been higherin the youngest age groups.

In our study, the most rapid growth in theincidence of knee arthroplasties occurred in the low-volume and intermediate-volume hospitals. In contrastto our findings, Jain et al (5) reported that the increasein incidence was fastest in the high-volume hospitals inthe US. One could reason that smaller hospitals with thecombination of fewer peer review indications for kneearthroplasty on the one hand and less limited localresources on the other would be expected to vary morethan larger-volume hospitals.

In conclusion, the age group– and sex-standardized incidence of TKAs and UKAs has grownrapidly in patients in Finland with primary OA who areyounger than 60 years, and most of the increase tookplace in the 50–59-year age group. This phenomenonhas been especially strong during the twenty-first cen-tury. There is no single explanatory factor for this

INCIDENCE OF KNEE ARTHROPLASTY FOR PRIMARY OA IN BABY BOOMERS 427

growth. Some of the increased incidence can be ex-plained by hospital volume. Long-term results in youngpatients may differ from those reported in older pa-tients, and the risk for revision surgery may be higher.More long-term data are warranted before the wide-spread use of TKA in younger patients can be justified.

AUTHOR CONTRIBUTIONS

All authors were involved in drafting the article or revising itcritically for important intellectual content, and all authors approvedthe final version to be published. Dr. Leskinen had full access to all ofthe data in the study and takes responsibility for the integrity of thedata and the accuracy of the data analysis.Study conception and design. Leskinen, Eskelinen, Huhtala, Paavol-ainen, Remes.Acquisition of data. Leskinen, Eskelinen, Huhtala, Paavolainen,Remes.Analysis and interpretation of data. Leskinen, Eskelinen, Huhtala,Paavolainen, Remes.

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