Osteoarthritis (OA) is a degenerative joint disease [1] and is the leading cause of long-term disability [2] [3] [4] resulting in progressive disorder of joint function. It is also recognized as the leading cause of disability and poor-quality life among elderly populations within developed countries [1] .

Knee OA is prevalent particularly within East Asian countries [5] . with its occurrence increasing with age, especially among women. In adults over the age of 45, 6% to 13% of men are affected, whereas 7% to 19% of women are affected, demonstrating a 45% lower risk of incidence for men [6] [7] . The prevalence of OA is markedly higher among women than men and increases noticeably with age [8] [9] . Knee OA occurs in 12% of American adults 65 years old or elder, and in 13% of women and 10% of men 60 years old or older in the United States [10] [11] [12] . The prevalence of OA is 15% in women and 5.6% in men 60 years old or older in Beijing, China [13] . The current occurrence of OA among the elderly population in Taiwan region is about 37% of individuals over 50 years old. The demographic crisis of aging is spreading worldwide, including in China but a full view of the epidemiology has not been obtained. In China, few medical reports have investigated the trends in the prevalence of primary TKRs. We sought to estimate the incidence of TKR from 2010 to 2019 by demographics, incidence rates, lengths of stay, and medical expenses.

Total knee replacement (TKR) is a general surgical operation with a high success rate that improves the function and quality of life in patients with a disorder in the knee joint [14] [15] . The rate of TKR has been steadily increasing over the last 2 decades [16] [17] . It has consistently been demonstrated to provide long-lasting pain relief and substantial and sustained improvement in joint functioning and health-related quality of life [14] [18] - [23] . TKR does not typically reduce mortality, but the procedure results in marked improvements in health-related quality of life and functional status and is highly cost-effective [24] [25] .

We obtained hospitalization records from the Joint Surgery Department at our hospital and utilized the data to identify all individuals who underwent primary total knee replacement (TKR) between 2010 and 2019. This study only included elective surgeries for individuals above 17 years of age. Procedures performed for the treatment of chronic or complicated diseases, unicompartmental arthroplasty, bilateral TKRs, revision TKR, medical complications requiring extended hospitalization, traffic accidents, and complex cases requiring additional procedures were excluded from the study. Cases where patients were accommodated in VIP wards that incurred additional hospital charges were also excluded. After analyzing detailed records of all hospital admissions, a total of 6770 TKR procedures were eligible for inclusion in the study.

The research variables included in this study comprised sex (male and female), and age distribution of TKR recipients with changes categorized into five groups: <50 years old, 50 - 59 years old, 60 - 69 years old, 70 - 79 years old, and ≥80 years old; primary diagnoses classified into three groups: osteoarthritis (OA), rheumatoid arthritis (RA), and Others (GA, OD, HA, etc); the average length of stay (ALOS); and unit cost associated with this operation.

We examined the demographic characteristics and prevalence of key co-morbid conditions for patients who underwent TKR between 2010 and 2019. For simplicity, data are presented separately for each year. We examined changes in the mean number during each 10 years. All analyses were performed for primary TKR.

TKA has a remarkable history of clinical success in relieving pain and improving function for patients with arthritic knees [26] . TKA substantially improves quality-adjusted life expectancy for patients with arthritic knees, and TKAs are cost-effective in the United States across all risk groups [24] [27] . Available data suggest that approximately 600,000 TKA procedures are performed annually in the United States at a cost of approximately $15,000 per procedure [24] [26] [28] .

Several studies have shown that the incidence of TKR has increased annually. Our study found that the rate of TKR robustly increased over the years 2010 to 2019, with a much larger increase observed among women compared to men. In 2010, only 172 individuals underwent primary TKR, which gradually increased to 885 in 2019. This represents an increase of 514.53% over 10 years. This upward trend is consistent with findings from studies conducted by Tien et al. and Kumar et al. [29] [30] . According to Tien’s study, the prevalence of TKR was 22.86 per 100,000 persons in 1996 and it increased to 54.95 per 100,000 persons in 2004-a rate increase of 140.38%. Kumar’s study showed that the prevalence rate of TKR was 28.5 per 100,000 persons in 1998 and increased to 56. 8 per 100,000 persons in 2009 an increase in the rate of 99.1% [29] [30] . The rise in TKA can be seen as an indication of the success of this procedure in safely reducing pain and improving functional status for an aging population [31] [32] . However, the surge in TKA can also be viewed as yet another source of strain on government, insurers, individuals, and businesses struggling with unremitting growth in healthcare spending [33] [34] [35] .

The age-adjusted total knee replacement (TKR) rate in Wisconsin, United States increased by 81.5% from 1990 to 2000, rising from 162 to 294 per 100,000 [16] . In Southern California, the primary TKR rate also saw a significant increase, rising from 6.3 per 10,000 in 1995 to 11.0 per 10,000 in 2004, representing a growth of 74.60% [36] . Ravi’s study utilized databases from the Healthcare Cost and Utilization Project and the Institute for Clinical Evaluative Sciences to estimate the prevalence of total joint arthroplasty (TJA) for hip and knee in the United States and Ontario, Canada. The rates of total knee arthroplasty (TKA) increased by approximately 59% in the United States and 73% in Ontario between 2001 and 2007 [37] . The annual primary TKR volume increased 161.5% from 93,230 to 226,177 between 1991 and 2010 [38] .

Furthermore, the annual volume of primary TKA surged by 161.5%, increasing from 93,230 to 226177 between 1991 and 2010. In Australia, an increase was observed as well with primary TKA for osteoarthritis jumping from 10132 in 1994 to 14472 in 1998, a rise of 42.8% [39] . Meanwhile, Korea experienced an astounding 407% increase in primary TKA rates from 2001 to 2010, based on data from the Health Insurance Review and Assessment Service of Korea [40] . Another prior study conducted in Taiwan region revealed an even more dramatic surge, in which the annual incidence rate skyrocketed by 202.59% during the time frame of 1996 to 2010. Policies addressing an aging population should include considerations for healthcare utilization of TKR and prevention of osteoarthritis [41] .

Gender differences were evident when it came to TKR rates. Women exhibited higher rates due to a greater prevalence of osteoarthritis-related disability compared to me [42] . Our study found that women had a threefold higher rate of TKR than men, a finding consistent with previous studies [30] [31] . Hawker et al. also reported similar results, stating that the women’s potential need for arthroplasty was three times greater than that of men [43] . Furthermore, TKR rates showed an upward trend with increasing patient age. Similar findings regarding gender-specific TKR studies have been reported in other countries. Many studies have indicated that knee osteoarthritis is more prevalent among women than men [44] [45] [46] [47] . In Sweden, the total knee replacement (TKR) rate is 1.93 times higher in women than in men. A similar gender gap in TKR rates has also been observed in the United States [16] [48] . This indicates that arthritis is a significant health concern for women, who are more likely than men to experience disability due to arthritis [43] . Numerous studies have demonstrated that joint arthroplasty is an effective and cost-efficient treatment for advanced arthritis of the hip and knee, providing pain relief and reducing functional disability [49] [50] [51] [52] .

The results of this study revealed higher TKR rates among individuals aged 60 to 69, which contrasts with findings from other Western and Asian countries where TKR rates were higher among those aged 70 to 79. This trend can be attributed to the increase in symptomatic osteoarthritis (OA) with advancing age [1] [30] [41] [53] . The average age of patients undergoing TKR has increased over time. Advances in the medical management of OA may lead patients to postpone joint replacement surgery, resulting in a gradual rise in the average age of TKR recipients [54] . Notably, there has been a significant increase in the number of procedures performed on younger patients (<50) in this study. This shift towards younger recipients is concerning as most evidence regarding outcomes following total knee arthroplasty (TKA) comes from studies focused on elderly patients. There are several potential explanations for the rising TKA rates among younger patients; our findings may indicate a genuine increase in the number of young individuals diagnosed with end-stage arthritis due to either increased incidence or improved surveillance efforts.

In the United States, the main diagnosis of TKR patients is OA (approximately 86% - 87%) [17] [29] . A study by Tien et al. in Taiwan Region noted that about 94% of TKR patients had OA from 2002 to 2004 [29] . Our study revealed that the majority of patients who underwent TKR procedures were diagnosed with OA, with 90.84% of them having OA in 2019. The close association between an aging society and the risk of OA has not been thoroughly investigated in epidemiologic studies on TKR utilization in the Chinese population over a long period [41] .

We observed a rise in the mean cost per patient and a notable decline in the ALOS for TKR, from 15 days to 5 days. This reduction may be attributed to advances in medical technology and sciences [55] . In the United States, LOS decreased from 7.9 days (95% CI: 7.8 – 7.9) during 1991 - 1994 to 3.5 days (95% CI: 3.5 – 3.5) during 2007 - 2010 [38] . The finding of a decreasing trend in ALOS was consistent with results from other studies conducted in Taiwan Region [29] [56] [57] [58] . However, the ALOS for TKR in China was longer than that in the United States, indicating potential for improvement in TKR treatment.

The median charge for TKR procedures at hospitals increased from $19,309 to $29,509 (a factor of 1.53) between 1997 and 2004 in the United States [36] which aligns with our study’s observation of rising costs as well - average hospital charges for TKR increased from ¥38,261 to ¥53,115.17 (an increase by ~39%). Previous studies have identified strong relationships between various clinical parameters and hospital charges [41] leading patients to seek hospitals with high surgical quantity and treatment from experienced surgeons when faced with increasing charges [59] [60] . Another possible reason for rising total medical expenditures is an aging population coupled with increased life expectancy [61] .

A recent survey report by the Ministry of Health of the People’s Republic of China showed the total number of joint arthroplasties performed annually is growing rapidly with a 19% increase per year reported between 2000 and 2006 [62] . Despite this dramatic increase, concerns exist as the high cost of surgical procedures and relatively low insurance reimbursement are significant barriers to gaining access to joint arthroplasty that likely contributed to some patients forging joint arthroplasty. This financial constraint may limit access to care and has potentially decreased the quality of life for patients with arthritis and their families, increased burden on society, increased societal costs to care for these individuals, and also led to a decrease in the productivity of the labor force in China. Furthermore, it may have also negatively impacted the advancement of medical services in China [63] .

There are several limitations to the present study. This study retrospectively analyzes the cost of a single center. It is important to consider that costs may vary significantly between institutions, especially across different countries, for institutional costs, patient costs, and third-party payer costs (whether government or private insurance companies) for the same procedure. Additionally, it is crucial to note that the data from the single institution examined in this study may not be indicative of charges at other institutions in China; however, we believe it is representative of potential charges at other institutions.

Our findings indicate a substantial increase in the utilization of primary knee replacement with the majority of TKR recipients being >60 years old and osteoarthritis being the main cause. Women are more susceptible to knee arthritis owing to a major health problem. Furthermore, the demographics of joint replacement recipients shifted to younger people which is concerning. We also observed a significant decline in hospital LOS and increases in hospital costs. Likewise, this study provides an analysis of TKR in China, and the results may be used as a reference for future planning of resources for TKR. Further study may investigate the specific rehabilitation interventions and components following lower extremity joint replacement so that we can better understand the effect of different treatment modalities.

The study protocol received approval from the Institutional Review Board (IRB) of West China Medical Center, Sichuan University. Formal consent was not required for this type of study. All data were obtained from the National Health Database of China, which is hosted on a server at our institution. Our center was responsible for managing the database and ensuring data entry standardization and accuracy. This study included consecutive patients undergoing primary unilateral total knee arthroplasty (TKA) in our department from January 2010 to December 2019.

This research was supported by the National Natural Science Foundation of China (No. 82172394 and U22A20280); the National Key Research and Development Program of China (No.2022YFC2503100 and2022YFC2503104); 1.3.5 project for disciplines of excellence, West China Hospital, Sichuan University (No. 2023HXFH012) and Tibet Autonomous Region Science and Technology Plant Joint Funding Projects (No. XZ202201ZY0041G). The funding sources had no role in manuscript conception, paper writing and design, data analysis interpretation, and publication.

The authors would like to express their appreciation to Mr. Hirdaya Ratna Shakya M.Sc. for his guidance and expert opinion regarding the improvement of this manuscript and Lang Li for his support.

*Hirose Shakya and Anjing Chen contributed equally to this study.

^#Corresponding author.