An underlying diagnosis of osteonecrosis of bone is associated with worse outcomes than osteoarthritis after total hip arthroplasty

We performed a cohort study of patients who had undergone an elective unilateral primary THA using the Kaiser Permanente (KP) Total Joint Replacement Registry (TJRR). We identified a cohort of patients who underwent elective unilateral primary THA between 04/01/2001 and 12/31/2012. The KP is a integrated healthcare system that covers over 9.5 million members in 7 US geographical areas. We have previously reported details on the KP TJRR data collection procedures, structure, and participation [13, 14]. The KP TJRR uses paper and electronic data capture tools to collect information by surgeons and other healthcare providers at the point of care, as well as data from other databases (including the electronic medical records) using electronic screening algorithms. The information from different data sources of the registry is linked using the integrated healthcare system’s unique patient identifiers. The data repository for the registry is a SQL database. A quarterly quality control check of these data ensures high accuracy. Specifically, the KP TJRR prospectively collects outcomes (i.e. readmissions, revision, SSI, venous thromboembolism (VTE)). Several outcomes including revision, SSI and VTE are adjudicated by review of charts by trained research associates [13, 14].

In a chart review of random sample of osteonecrosis and OA cases (n = 60; 30 cases each of osteonecrosis and OA) performed by an abstractor blinded to the database diagnosis, we found that the true positive rate of diagnoses (OA vs. osteonecrosis) was 88%. All registered cases in the KP TJRR are tracked until outcome and/or end of their lives. The voluntary participation of the KP TJRR in 2011 was 95% [14].

This study included all unilateral primary elective THA procedures, in patients aged 18 years old or older, with surgery indication for OA and/or osteonecrosis registered during the study period (N = 47,523). Patients who had a diagnosis of both OA and osteonecrosis (N = 563) were not included in the study. Patients who underwent revision THA, partial THA, or conversion procedures, as the index procedure, were not included in the study. This was done for two reasons: (1) the majority of THA are elective primary THA, our condition of interest; and (2) to keep the population homogeneous and allow easy interpretation of results. Cases from the geographical regions where the organization owns the hospitals (Southern California, Northern California, and Hawaii), covering 92% of the KP TJRR registered cases, were included. The final sample had cases from 50 hospitals and 362 surgeons.

The outcomes of interest in this study were mortality within 90 days, SSI (30 days for superficial and 1 year for deep infections), VTE within 90 days, unplanned readmissions within 90 days of the index primary elective THA, and revision (ever and for any reason). All outcome measures were obtained from the KP TJRR using 2001–12 data; timing of outcome assessment was chosen based on clinical relevance, based on feedback from an expert surgeon (R.N.). SSIs, which can be either deep or superficial were defined according to the Centers for Disease Control and Prevention/National Healthcare Safety Network criteria [15]. Superficial and deep SSI were also combined into one measure in this study since we anticipated a small number of events for these two components individually. The VTE measure, which included symptomatic deep vein thrombosis (DVT) and pulmonary embolism (PE) in this study due to the small number of events, were identified using the Agency for Healthcare Research and Quality patient safety indicators technical specifications [16] and further confirmed by chart review validation by trained clinical research associates. Readmissions are captured by the KP TJRR using quarterly extracts from the institutional electronic medical records of all hospital readmissions post discharge from the original joint arthroplasty procedure, available from 2009. Only unplanned readmissions, which were determined based on the 2014 Centers for Medicare and Medicaid Services Procedure-Specific Readmission Measures Updates and Specification Report for Elective Primary THA [17] were included in the analysis. Planned admissions such as those likely for subsequent hip arthroplasty procedures (e.g. International Classifications of Disease, 9th Revision, code (ICD-9) 715.34, Osteoarthrosis, localized, pelvic region and thigh and ICD-9: 733.42, aseptic necrosis of head and neck of femur) were not included in the reported readmissions. Revisions were defined as any re-operation of the index procedure where a component was replaced.

An underlying diagnosis of osteonecrosis was the exposure of interest, compared with a diagnosis of OA. This information was obtained from the KP TJRR intra-operative forms completed by the surgeon at the time of surgery and does not rely on administrative data coding schemes. We also explored the associations by the cause/type of osteonecrosis, categorized as idiopathic vs. non-idiopathic, using the 2009–2012 data, the year of the beginning of prospective collection of intra-operative diagnosis and readmission data. Additional file 1 shows the sub-categories of non-idiopathic category glucocorticoid-induced vs. not glucocorticoid-induced (including hip fracture, alcohol abuse, sickle cell disease, lupus, pancreatitis, HIV, vasculitis and other autoimmune conditions).

Age (<65 and ≥65 years old, based on Medicare-eligible age of 65 years and higher), gender; race (White and Others), body mass index (BMI, continuous in 5 unit increment); American Society of Anesthesiologists (ASA) class (<3 vs. ≥3), and medical comorbidities using the Elixhauser co-morbidity measure [18] were evaluated as possible confounders/covariates.

We calculated summary statistics to describe the study sample by whether the primary diagnosis was osteonecrosis vs. OA. We compared categorical variables between the two groups using 2-sided chi-square or Fisher’s exact tests where appropriate. A logistic regression was used to model the relationship between osteonecrosis (OA being the reference category) and mortality, VTE, SSIs, and readmissions while accounting for the nesting of observations within the surgeon variable. We created Cox proportional hazard models to evaluate the association of osteonecrosis compared to OA and the time to revision surgery. Covariates tested as confounders, were included in the final adjusted model if they changed estimates by >10% and p < 0.10 or deemed clinically important for the relationship studied. Odds ratios (OR), hazard ratios (HR), 95% confidence internals (CI), and p-values are reported. A sensitivity analysis was conducted to evaluate the robustness of estimations accounting for missing data. We also performed sensitivity analyses that adjusted for the competing risk of death. No formal sample size calculations were done for this observational cohort study. Data were analyzed using SAS (Version 9.2, SAS Institute, Cary, NC, USA) and p-value <0.05 was considered statistical significant.

There were 47,523 patients included in the study from 2001 to 12. Of these, 2,271 (4.8%) had an underlying diagnosis of osteonecrosis. Demographic and clinical features of patients with osteonecrosis are shown in Table 1. Compared to the cohort of patients with OA, the osteonecrosis group was younger (median (interquartile range (IQR)): 55 (46–64) vs. 67 (59–75)) years old, had more males (1,306 (57.5%) vs. 18,875 (41.7%)), and had less White (1,358 (59.8%) vs. 35,009 (77.4%)) patients. Patients with osteonecrosis had lower BMI (<30 kg/m²: 1,505 (66.3%) vs. 26,358 (58.3%)), slightly higher ASA scores (ASA ≥ 3: 897 (39.5%) vs. 15,827 (35.0%)), and were less likely to have diabetes (451 (19.9%) vs. 10,042 (22.2%)). Additional file 2 shows that several comorbidities were more prevalent in patients with osteonecrosis compared with those with osteoarthritis.

Between 2009 and 2012 (period of prospective collection of intra-operative diagnosis and readmission data), of a total of 670 patients, 507 (76%) had idiopathic osteonecrosis and ​163 (24%) had non-idiopathic osteonecrosis. Among the non-idiopathic category, 40 (6%) patients had glucocorticoid-induced osteonecrosis and 123 (18%) patients had causes other than glucocorticoid-induced osteonecrosis. Additional file 3 shows that patient characteristics were similar between idiopathic vs. non-idiopathic etiology; similarly Additional file 4 shows that characteristics were similar between glucocorticoid-induced vs. not glucocorticoid-induced osteonecrosis.

Compared to patients with OA, patients with osteonecrosis had higher incidence of the following complications post-THA: 90-day mortality (0.7% vs. 0.3%), SSI (combined: 1.2% vs. 0.8%; 90-day deep infection: 0.6% vs. 0.5%; 30-day superficial infection: 0.6% vs. 0.3%), 90-day VTE (1.4% vs. 1.2%, DVT: 0.8% vs. 0.7%, PE: 0.6% vs. 0.5%), 90-day unplanned readmission (9.6% vs. 5.2%) and revision at any time during the follow-up (3.1% vs. 2.4%; Table 2). The median follow up time for revision outcome was 3.2 years (Interquartile range, 1.4–5.8). Unadjusted revision-free survival showed a trend favoring osteoarthritis compared to osteonecrosis (Fig. 1; p = 0.108).

Additional files 5 and 6 shows that the unadjusted rates of outcomes were similar by underlying cause of osteonecrosis except the following: higher 90-day readmission (14.7% vs. 7.7%) and revision rates (3.1% vs. 1.2%) in non-idiopathic group vs. idiopathic group (see Additional file 5); and higher 90-day readmission rate (17.5% vs. 13.8%) in glucocorticoid-induced vs. not glucocorticoid-induced osteonecrosis (see Additional file 6),

After adjusting for age, race, BMI, and ASA class, the risk of 90-day mortality was significantly higher for patients with osteonecrosis than OA for the overall sample (OR: 2.48; 95% CI, 1.31–4.72) (Table 3). After adjusting for BMI, the odds of SSI (deep and superficial) in patients with osteonecrosis were 1.67 (95% CI: 1.11–2.51) times higher than patients with OA (Table 3). In the age-adjusted model, the risk of VTE (DVT and PE) was not statistically significantly different between two groups (OR: 1.37; 95% CI, 0.92–2.03) (Table 3). After adjusting for age, 90-day unplanned readmission was statistically significantly higher in patients with osteonecrosis than those with OA (OR: 2.20; 95% CI, 1.67–2.91) (Table 3).

A time interaction was observed for revision after adjusting for age. While the risk estimates for revision surgery were not statistically significant overall, the direction of the risk estimates changed for patients within one year of the surgery vs. longer time, for OA vs. osteonecrosis. Specifically, compared to patients with OA, patients with osteonecrosis did not have any significantly different hazard of revision (HR = 0.92, 95% CI: 0.63–1.33) within one year of the surgery, but after one year, patients with osteonecrosis seemed to have a 1.32-times higher (95% CI: 0.94–1.84) hazard of revision surgery (Table 3).

Exploring differences by the underlying cause of osteonecrosis, the age-adjusted 90-day readmission was significantly higher in non-idiopathic vs. idiopathic group, with adjusted odds ratio of 2.13 (95% CI: 1.23–3.68). Additional file 7 shows that the revision rate seemed higher, but did not meet statistical significance. Additional file 8 shows that there was no significant difference in 90-day readmission was noted in glucocorticoid-induced vs. not glucocorticoid-induced osteonecrosis.

The five most common principal discharge ICD-9 codes for the unplanned 90-day readmissions in patients with osteonecrosis were as follows: 996.42 (Dislocation of prosthetic joint), 038.9 (Unspecified septicemia), 486 (Pneumonia), 282.62 (Hemoglobin SS disease, sickle cell disease with crisis), and 493.22 (Chronic obstructive lung disease/asthma with exacerbation). In comparison, the five most common principal discharge ICD 9 codes for 90-day readmissions in patients with OA were as follows: 996.42 (dislocation of prosthetic joint), 996.66 (infection & inflammation reaction due to internal joint prosthesis), 998.59 (other postoperative infection), 38.9 (unspecified septicemia), and 996.44 (periprosthetic fracture around prosthetic joint). Additional file 9 shows the top 15 causes for readmission.