Socioeconomic Status Is Associated with Risk of Above-knee Amputation After Periprosthetic Joint Infection of the Knee
Authors: Lieber Alexander M. , Kirchner Gregory J. , Kerbel Yehuda E. , Moretti Vincent M. , Vakil Jeffrey J. , Brahmabhatt Shyam 
Published: April 2019

Abstract

Background
Above-knee amputation (AKA) is a severe but rare complication of TKA. Recent evidence suggests there are sex and racial disparities with regard to AKA after TKA. However, whether lower socioeconomic status is associated with an increased risk of AKA after TKA has not been conclusively established.

Questions/purposes
(1) Is low socioeconomic status or use of public health insurance plans associated with an increased risk of AKA after periprosthetic joint infection (PJI) of the knee? (2) Is race or sex associated with an increased risk of AKA after PJI of the knee?

Methods
This cross-sectional study screened the National Inpatient Sample (NIS) between 2010 and 2014 using the International Classification of Diseases, 9th Revision, Clinical Modification (ICD-9-CM) procedure and diagnosis codes to identify 912 AKAs (ICD 84.17) among 32,907 PJIs of the knee. The NIS is a large national database of inpatient hospitalizations frequently used by researchers to study outcomes and trends in orthopaedic procedures. The NIS was selected over other databases with more complete followup data such as the American College of Surgeons National Surgical Quality Improvement Program (ACS NSQIP) due to its unique ability to examine income levels and insurance type. Cases were identified by taking all patients with an ICD diagnosis code related to PJI of the knee and limiting that cohort to patients with an ICD procedure code specific to TKA. A total of 912 AKAs after PJI were identified (912 of 32,907, [3%] of all PJIs of the knee) with males comprising 52% of the AKA sample (p = 0.196). Multivariate logistic regression was used to compare risk of AKA after PJI of the knee after controlling for patient demographics, hospital characteristics, and comorbidities.

Results
Compared with the wealthiest income quartile by ZIP code, patients in the lowest income quartile by ZIP code were more likely to sustain an AKA (OR = 1.58; 95% confidence interval [CI] 1.25–1.98; p < 0.001). Compared with patients with private insurance, patients with Medicare (OR = 1.94; 95% CI, 1.55–2.43; p < 0.001) and Medicaid (OR = 1.86; 95% CI, 1.37–2.53; p < 0.001) were at higher risk of AKA. There were no differences with regard to risk of AKA for white patients (670 of 24,004 [3%]; OR = 0.99; 95% CI, 0.77–1.26; p = 0.936) and black patients (95 of 3178 [3%], OR = 0.95; 95% CI, 0.69–1.30; p = 0.751) when compared with others (reference, 83 of 3159 [3%]). When compared with female patients, male patients did not have a greater risk of undergoing AKA (OR = 1.02; 95% CI, 0.88–1.29; p = 0.818).

Conclusions
This study did not observe any racial or sex disparities with regard to risk of AKA after PJI. However, there was a greater risk of AKA after PJI for poorer patients and patients participating in Medicare or Medicaid insurance plans. Surgeons should be cognizant when treating PJI in patients from lower income backgrounds as these patients may be at greater risk for AKA. Future research should explore the role of physician attitudes or preconceptions about predicted patient followup in treating PJI, as well as the effect of concurrent peripheral vascular disease on the risk of AKA after PJI.

Level of Evidence
Level III, therapeutic study.