Have the annual trends of total knee arthroplasty in ankylosing spondylitis patients decreased?

backgroundAnkylosing spondylitis (AS) is characterized by osteoproliferation-induced structural damage and spinal inflammation, which lead to spinal deformity and functional disability. Though AS commonly affects the axial skeleton and sacroiliac joints, up to 70% of patients have involvement of the knees and other joints. Despite pharmacological efforts, advancing joint involvement may ultimately require surgical intervention. Total knee arthroplasty (TKA) is effective in managing patients with AS, yet it remains unclear whether or not the annual rates of TKA have been affected. Therefore, the purpose of this study was to evaluate the annual trends of AS patients who underwent TKA. Specifically, we evaluated: (I) the annual trends of TKAs due to AS in the United States population; (II) the annual trends in the proportion of TKAs due to AS in the United States.

methodsThe Nationwide Inpatient Sample (NIS) was used to identify all patients who underwent TKA between 2002 and 2013 (n=6,492,873). Then, a subsequent query was performed to identify TKA patients who had a diagnosis of AS, defined by the International Classification of Disease 9th revision diagnosis code 720.0. The incidence of TKAs with a diagnosis of AS in the United States was calculated using the United States population as the denominator. Regression models were used to analyze the annual trends of AS in patients who underwent TKA.

resultsDuring the study period, 2,986 patients who had AS who underwent TKA were identified. The annual number of TKAs with a diagnosis of AS increased by 168% from 125 to 335. After normalizing to the US population, the incidence of TKAs with AS increased from 0.58 to 1.38 TKAs per million US adults [IRR =1.08 (95% CI: 1.07-1.09), P<0.001]. Out of the 350,122 TKAs in 2002, 125 (0.04%) were due to AS, whereas, out of the 640,695 TKAs in 2013, 335 (0.05%) were due to AS. The prevalence of AS in those who underwent TKA increased from 2002 to 2013 [coefficient =0.002 (95% CI: 0.001-0.003), P=0.004].

conclusionsThe annual trends of AS patients undergoing TKA significantly increased during the study period. To the authors' best knowledge, this is the first study to evaluate TKA trends in the AS population. The literature has reported on the ability of non-steroidal anti-inflammatory drugs (NSAIDs) and the potential of tumor necrosis factor-alpha (TNFα) inhibitors to hinder bone disease progression in AS, however, this was not shown to support to the significant changes found in TKA trends during the study period.