in the 1980s , the prevalence of rickets in extremely low birth weight ( elbw ) infants was around 50% and was 17% for fractures ( 1 , 2 ) . after introduction of a high mineral containing diet in the late 1980s , the prevalence decreased to 30% or lower ( 3 , 4 ) . there is little information , however , beyond 1985 , concerning the incidence of rickets in extreme prematurity and factors that relate to its occurrence . in addition to inadequate mineral intake as a major contributing factor ( 5 ) , chronic co - morbidities , like chronic lung disease , short bowel syndrome , and parenteral nutrition associated cholestasis ( pnac ) could also be related factors ( 6 ) . medications used in the neonatal intensive care unit ( nicu ) , such as steroids , methylxanthines , and diuretics could theoretically increase the risk of inadequate bone mineralization ( 7 - 9 ) . other risk factors , such as immobilization from sepsis , cerebral pathology , muscular disorders and paralysis are associated with bone disease of prematurity ( 10 ) . since mineral enriched formula for the preterms began 30 yr ago , it is the standard practice to provide minerals in an attempt to reach in utero mineral accretion rates . in theory , under ideal conditions , the adventage of high mineral intake formulas should be associated with a marked reduction of rickets . however , it is often practically difficult to achieve ideal nutrition support in sick elbw infants , the current incidence of rickets in elbw infants is unknown , and the relationship of the severity of overall illness and degree of prematurity in this population is unclear . the aim of this study was to characterize the risk factors for development of rickets in elbw infants fed with high mineral containing preterm formula . as a retrospective case - control study , medical records of 55 elbw infants admitted into the severance children 's hospital nicu from 2004 to 2008 that survived until 1 month of age were reviewed . all infants had their serum alkaline phosphatase ( alp ) , calcium ( ca ) and phosphate ( p ) levels reported weekly and bone screen radiography at 1 month of age . for nutritional care , fortified human milk ( ca 131 mg/100 kcal , p 71 mg/100 kcal ) or a mineral fortified preterm formula ( ca 112 - 141 mg / kcal , p 52 - 63 mg / kcal ) was used . during parenteral nutrition , ca , p and multivitamins with trace minerals were added to parenteral nutrition , appropriately . grade 1 was defined as a loss of dense white line , increased lucency of bone shaft , and thinning of cortex ; grade 2 showed irregularity and fraying of a metaphysis with splaying and cupping ; and grade 3 presented evidence of fractures . as co - morbidities , hyaline membrane disease ( hmd ) , patent ductus arteriosus ( pda ) , necrotizing enterocolitis ( nec ; using the modified bell 's staging criteria ) , parenteral nutrition associated cholestasis ( pnac , defined as direct bilirubin [ db ] greater than 2.0 mg / dl ) , bronchopulmonary dysplasia ( bpd ; using nih classification ) , retinopathy of prematurity ( rop , included above stage 2 ) , and intraventricular hemorrhage ( ivh , above grade 3 ) were analyzed . severe pnac was defined as elevation of db greater than 4.0 mg / dl for more than 1 month and aspartate aminotransferase ( ast ) and alanine aminotransterase ( alt ) greater than 60 moderate / severe bpd was defined in infants requiring either 28 days of supplemental oxygen therapy with oxygen , nasal continuous positive airway pressure ( cpap ) or positive pressure ventilation at 36 weeks postmenstrual age or discharge , whichever came first , whose gestational age was younger than 32 weeks , or at 56 days postnatal age or discharge , whichever came first , in infants whose gestational age was older than 32 weeks . statistics were calculated with the student t - test and chi - square test for means and frequencies . numeric data are presented as mean and standard deviation because the data were normally distributed . calculations were performed with spss software version 17.0 ( spss , chicago , il , usa ) . the present retrospective review of database in this study was approved by the institutional review board of the yonsei university health system , severance hospital ( approval no . , 4 - 2012 - 0157 ) . the present retrospective review of database in this study was approved by the institutional review board of the yonsei university health system , severance hospital ( approval no . , 4 - 2012 - 0157 ) . twenty - four elbw infants ( 26.3  1.9 weeks , 792  99 g ) were diagnosed as rickets of prematurity , and 31 infants without rickets ( 27.2  2.3 weeks , 871  93 g ) were the control group . rickets of prematurity was diagnosed in 43.9% of elbw infants and fracture in 13% . for the rickets group , 7 infants ( 29% ) showed g1 , 14 infants ( 58% ) g2 and 3 infants ( 13% ) g3 . rickets of prematurity was diagnosed at 48.2  16.1 days of life and improved by 85.3  25.3 days of life . in patient characteristics such as mean gestational ages , birth weight , gender , and frequency of intrauterine growth retardation ( iugr ) the cumulative amounts ( either enteral or parenteral ) of ca and p administration during hospitalization were not different between the groups . for the rickets group in comparison with the control group , mean days on oxygen and ventilator use were significantly longer ( p < 0.05 ) . the duration of hospitalization and parenteral nutrition were significantly longer in the rickets group versus the control group ( table 1 ) . in univariate analysis , elbw infants with rickets had a significantly higher occurrence of pda ( p = 0.025 ) , pnac ( p = 0.041 ) , severe pnac ( p = 0.013 ) , bpd ( p = 0.019 ) , and moderate / severe bpd ( p = 0.012 ) ( table 2 ) . common neonatal illnesses such as hmd , air leak , nec , rop , ivh and periventricular leucomalacia ( pvl ) were not different between the groups . in multiple regression analysis after adjustment of gestation and birth weight , rickets in elbw infants significantly correlated with severe pnac ( or 18.5 ; 95% ci , 1.1 - 285 ; p = 0.042 ) and moderate / severe bpd ( or 3.2 ; 95% ci , 1.2 - 26.5 ; p = 0.04 ) ( table 3 ) . the mean level of peak alp was significantly higher in the rickets group than the control group ( 952.2  413.8 vs 524.7  158.2 iu / l ) , while the mean level of the lowest ca and p levels were not different between groups ( table 4 ) . in the 1980s , fortified mineral formulas were introduced to reduce the rates of rickets and fractures in very low birth weight ( vlbw < 1,500 g ) infants ( 11 ) . reports at that time indicated that up to 30% of vlbws had rickets with fractures ( 12 ) . with advances in preterm nutrition , especially with the introduction of mineral fortified formulas , theoretically the incidence of rickets of prematurity should have dropped ( 3 , 4 ) . however , for increasing the survival of elbw infants , significant co - morbid conditions could affect actual mineral intake which might mitigate the effect of high mineral fortification ( 13 ) . this is the first study to focus on the incidence of rickets in elbw infants after 2 decades of fortified formula use in korea . it is also the first to characterize and compare risk factors and neonatal morbidities for rickets of prematurity in elbw infants . we found that the incidence of radiologic rickets in elbw infants was extremely high at 44% ( 24/55 ) due to increased survival , and the incidence of significantly increased 18 times and 3 times for severe pnac and moderate to severe bpd , respectively . factors commonly thought to relate to rickets of prematurity include mismatch of postnatal intake of minerals compared to intrauterine mineral transfer , low bone loading effects in the nicu , intrauterine growth restriction , chorioamnionitis , steroids , methylxanthines and diuretic usage and necrotizing enterocolitis ( 7 - 10 , 14 - 16 ) . inadequate mineral nutrient ( calcium and phosphorous ) intake appears to play a major role in causing rickets of prematurity ( 10 , 17 ) . prolonged parenteral nutrition ( pn ) and delayed enteral nutrition complicate delivery of adequate mineral intake and would be particularly acute in elbw infants ( 10 ) . it is standard practice in nicu to provide high mineral preterm formula and human milk fortifier as an attempt to reach in utero mineral accretion rate , including the use of minimal early enteral feeding , which appears to improve bone mass at term ( 3 , 17 - 19 ) . however , in elbw infants , co - morbid illnesses often preclude full enteral tolerance and the actual delivery of mineral nutrients ( 13 ) . further , when pn is needed , the provision of minerals in pn can be limited by solubility issues compounded by fluid restriction in sick infants ( 3 ) . specific chronic co - morbidities , particularly chronic lung disease and short bowel syndrome which is generally associated with prolonged parenteral nutrition and pnac , are potentially serious risk factors for poor bone mineralization ( 6 , 20 , 21 ) . thus our findings of markedly higher risk of rickets in elbw infants with pnac ( 18 times ) and bpd ( 3 times ) support these associations . the limitations of this study are a small sample size of a single institution and a retrospective design . in conclusion , there is a high incidence ( 44% ) of rickets in elbw infants , which shows an 18-fold increase in severe pnac and a 3-fold increase in moderate to severe bpd . we suggest that in elbw infants with severe pnac or moderate / severe bpd , aggressive prevention or treatment for rickets and fractures can be instituted earlier in the course of management .
risk factors for rickets of prematurity have not been re - examined since introduction of high mineral formula , particularly in elbw infants . 
 we analyzed the incidence and the risk factors of rickets in extremely low birth weight ( elbw ) infants . 
 as a retrospective case - control study from 2004 to 2008 , risk factors were analyzed in 24 patients with rickets versus 31 patients without . 
 the frequency of rickets in elbw infants was 24/55 ( 44% ) . 
 infants with rickets were diagnosed at 48.2  16.1 days of age , and improved by 85.3  25.3 days . by radiologic evaluation , 
 29% were grade 1 rickets , 58% grade 2 and 13% grade 3 . in univariate analysis , infants with rickets had significantly higher incidence of patent ductus arteriosus , parenteral nutrition associated cholestasis ( pnac ) , severe pnac and moderate / severe bronchopulmonary dysplasia ( bpd ) . 
 in multiple regression analysis , after adjustment for gestation and birth weight , rickets significantly correlated with severe pnac and with moderate / severe bpd . 
 serum peak alkaline phosphatase levels were significantly elevated in rickets ( p < 0.001 ) . in elbw infants , 
 the incidence of rickets of prematurity remains high and the incidence of severe pnac and moderate / severe bpd was significantly increased 18 and 3 times , respectively .