This is the first study that analyzed data collected from the largest inpatient database in the US, and several observations warrant further discussion. An important finding was that the in-hospital mortality rate is low and not different between CPPD and non-CPPD patients; therefore, TKA remains a safe procedure. Although the total hospital charges were not different between the 2 groups, CPPD patients had a longer length of stay and were more likely to be discharged to a non-home setting.
TKA is one of the most common orthopedic procedures performed in the US, and in the current study, we found that CPPD accounts for approximately 0.2% of TKA performed on an annual basis in the US, comprising a small proportion of total TKA surgeries compared to all other causes. A retrospective study utilizing data obtained from a Medicare claims database reported that the most common diagnosis associated with TKA was osteoarthritis (94%) followed by rheumatoid arthritis (3.6%) and other causes in 2.4%, without commenting if CPPD codes were examined . Given the estimated prevalence of chondrocalcinosis in the general population is between 4%-7%, and the results of a published study demonstrating that 20% of randomly selected patients undergoing TKA found to have CPP crystals in their synovial fluid, it appears CPPD is an underdiagnosed condition and the percentage of TKA related to CPPD might be higher than those recorded [2, 8].
We also noted that more than two-thirds of patients had Medicare as their primary insurance payer, and approximately one every five CPPD patients were older than 80. This is not a surprising finding, given the strong association between aging and CPPD [1, 2]. A small cross-sectional study that evaluated the characteristics of 53 CPPD patients undergoing TKA and compared them with 48 osteoarthritis patients, revealed that CPPD patients were older with a mean age of 70.3 versus 67.5 (p=0.037) .
Several comorbidities were more frequently observed in TKA patients with CPPD, including the 2 most common inflammatory arthritides, rheumatoid arthritis and gout. Similarly, Kleiber et al. analyzed patients’ data obtained from the Veterans Affairs (VA) database and reported a positive association between CPPD and gout (OR 2.82), CPPD and RA (OR 1.88) (9). Furthermore, another study examined 4630 synovial fluid specimens for the presence of crystals, and approximately 5% of patients had mixed crystals, monosodium urate and CPP crystals, indicating the two conditions can coexist .
Similar associations between CPPD and other comorbidities such as chronic kidney disease, hypomagnesemia and hypophosphatasia, have been previously reported in studies investigating the relationship between CPPD and coexisting conditions [5, 9, 16, 18, 19]. Noteworthy, coronary artery disease, smoking and avascular necrosis were also more frequently observed in the CPPD population, and in particular, avascular necrosis is a known condition that leads to bone necrosis and joint destruction requiring joint replacement . Overall, CPPD patients had a greater comorbidity burden based on the Charlson comorbidity index score, and given the older age, these factors may increase the complexity of managing these patients.
Regarding the length of stay and post-discharge site of care, the CPPD patients were more likely to stay longer and be discharged to other care facilities compared to non-CPPD patients. Although there are no post-discharge resource utilization data in the NIS database, the disposition to a non-home setting is most likely associated with increased cost. Healthcare providers and payers should be aware of these findings in order to develop strategies for proper in-hospital rehabilitation to reduce the length of stay and the disposition to a non-home setting.
In the current study, we found that CPPD patients have a low in-hospital mortality rate and similar to the non-CPPD patients. This finding is in line with existing evidence reporting a downward trend in deaths following TKA [21, 22]. A study by Kirskey et al. analyzed the NIS database and noted that the in-hospital mortality decreased from 0.2% in 1998 to 0.09% in 2008 . A systematic review of 37 studies reported a decline in the mortality following TKA over a 10-year period, with an estimated 30-day mortality of 0.10% in 2015 . Although there are no studies examining the in-hospital mortality of TKA in CPPD patients, a retrospective study of TKA in gout patients, using data from NIS database, reported an estimated mortality of 0.1% .
In addition, we found that the risk for in-hospital complications such as myocardial infarction was higher among the CPPD patients. In general, TKA patients have a greater risk of myocardial infarction during the first postprocedural month, particularly patients older than the age of 80 and those with underlying coronary artery disease [24, 25]. This observation might explain the higher risk of myocardial infarction, given the older mean age in the CPPD population and the greater frequency of coronary artery disease, and highlights the importance for awareness and prevention of this devastating complication.
There are several limitations in the interpretation of the study’s results. First, there are no available longitudinal data, such as discharge complications, outpatient resource utilization, post-discharge mortality, and due to the design of the cohort study, there is a risk of confounding bias. Additionally, the database does not provide information about patient-reported outcomes, laboratory studies, and medications, variables that might influence the outcomes. Our study has several strengths. The nationwide study design is more representative of the population, and the large sample size provides more reliable and accurate results.