Incidence and excess mortality of hip fracture in young adults: a nationwide population-based cohort study

Hip fractures in young adults are uncommon and often caused by high-energy trauma, whereas hip fractures in elderly adults are more common and are generally sustained in falls [ 1– 11]. Previous studies estimated that less than 10 % of total hip fractures occurred among subjects aged under 50 to 60 years in North America [ 12, 13]. Studies likewise revealed that the one-year mortality after hip fractures among the elderly population was as high as 20 % to 30 % [ 7, 10, 12– 18]. Several studies have reported mortality rates in hip fracture patients aged under 50 to 65 years old [ 1, 5, 7, 10, 11, 18]. However, there are relatively few studies on mortality rates after hip fractures among young adults aged under 40 years [ 1– 11]. Studies from individual institutes generally did not use a large sample size and thus it was not possible to precisely assess the incidence of short-term and long-term mortalities after hip fractures. At present, no population studies have explored the incidence of hip fracture and mortality rates in a young Asian population. Therefore, this study assessed the incidence of hip fracture and excess post-hip fracture mortality using a sample of inpatients aged 20 to 40 years from a nationwide population database in Taiwan.

Between 2001 and 2008, 4,523 subjects were admitted for the first time with a primary diagnosis of hip fracture and then treated with surgery. Among these patients, 3439 (76.03 %) were male, 1084 (24.97 %) were female, 1931 (42.69 %) had trochanteric fracture, 2592 (57.31 %) had cervical fracture, 4363 (96.46 %) received internal fixation, and 160 (3.54 %) received hemiarthroplasty (Table  1). For the period 2001 to 2008, the annual incidence rates of hip fractures changed from 7.68 to 7.23 ( P =0.6519), from 11.09 to 11.29 ( P =0.9885), and from 4.15 to 3.07 ( P =0.5415) per 100,000 for the overall population, males, and females, respectively (Table  2).

For the same period, the annual mortality rates of hip fractures decreased significantly from 1.37 % to 0.94 % ( P =0.0464) and from 1.86 % to 0.95 % ( P =0.0094) for the overall population and males, respectively (Table  2). The values in females were unstable due to the small number of newly diagnosed hip fracture cases which resulted in fewer death cases within one year after fracture. We used the annual SMR to indirectly compare the annual mortality of young adults after hip fracture with that of the general population in Taiwan. The overall annual SMR was 9.06 in 2001 which decreased to 6.71 in 2008 ( P =0.0781) (Table  2).

For the entire cohort, the 1-year, 2-year, 3-year, 5-year, and 10-year follow-up mortality rates were 1.28 %, 2.44 %, 3.54 %, 5.32 %, and 10.50 %, respectively (Table  3). Males had higher mortalities than those of females within ten years after the occurrence of fracture (Table  3 and Fig.  1). The gender- and age-stratified follow-up mortalities indicated that all male age groups had higher death rates than those of females within ten years after the occurrence of fracture, except the 20–24 years group (Table  3). For the entire cohort, the 1-year, 2-year, 3-year, 5-year, and 10-year SMRs were 8.33, 7.59, 7.28, 6.39, and 5.82, respectively. Females had higher SMRs than those of males within ten years after the occurrence of fracture (Table  3). The gender- and age-stratified SMRs indicated that all female age groups had higher SMRs than those of males within ten years after the occurrence of fracture (Table  3). We compared the effects of the possible risk factors of mortality using a log-rank test (Fig.  1). The statistically significant risk factors for overall death were male gender, trochanteric fracture, hemiarthroplasty, and larger CCI scores.

The present investigation is the first population study to evaluate long-term excess mortality in young adults after hip fracture in Taiwan. The annual mortality and SMR of young adults after hip fracture increased gradually during 2001–2003 and decreased slightly after 2005. The launch of the national health insurance program in 1995 and the nationwide implementation of a case payment system in 2002–2003 provided better funding for hip fracture patients and allowed for the provision of more complete care. The decreasing trend of annual SMR may indicate that following the implementation of the NHI there was a decrease in mortality rates for young adults in the general population and also a decrease in mortality rates for young adults with hip fracture, but there was higher decreases in the mortality rate of young adult hip fracture patients compared with that of the general population of young adults. The general improvement in healthcare, as well as the year-on-year improvement in surgical techniques, the increased penality for drunk driving, and decreasing traffic accident death rates may explain, at least in part, the decrease in peri-operative mortality and short-term post-operative mortality after 2003.

The estimated 1-month, 3-month, 6-month, 1-year, 2-year, 3-year, 5-year, and 10-year follow-up mortality rates were 0.13 %, 0.48 %, 0.81 %, 1.28 %, 2.44 %, 3.54 %, 5.32 %, and 10.50 %, respectively (Table  3). Karantana et al. revealed that the cumulative mortality rates were 3.5 % and 13.3 % for 30 days and 1 year after hip fracture among women under 65 years [ 10]. The differences observed might be attributed to the selection of a greater number of subjects aged over 40 years in the Karantana et al. report [ 10]. The overall follow-up 1-year, 2-year, 5-year, and 10-year SMRs were 8.33, 7.59, 7.28, 6.39, and 5.82, respectively. Several studies demonstrated that hip fractures continuously affected the long-term mortality even one year after the occurrence of fracture among elderly adults [ 20– 22]. Our results support the premise that hip fractures affected both short-term and long-term mortality rates. To our best knowledge, the present investigation is the first study of long-term excess mortality among young adults. Furthermore, we analyzed the causes of death stratified by year of death for up to ten years following the index day (Additional file 1: Table S1). We found that accident injury, cancer, and suicide were the major causes of death, each of which is highly correlated with the mortality rate of the young adult population. Though the proportion of each of these causes of death fluctuated slightly from year to year, overall the contribution of each major cause to the death rate remained stable.

We also determined that the long-term mortalities and SMRs varied by gender and age in Taiwan, as indicated in the literature on elderly adults [ 23– 25]. We found that in the long-term mortalities stratified by gender and age, the young male group had a higher mortality than that of the young female group, while the result for SMRs was reversed within ten years after the occurrence of fracture (Table  3). One of the reasons for the higher risk of mortality in males might involve the greater prevalence of high-energy trauma with severe injury in males than in females, which might result from differences in behaviors such as vehicular speeding and lifestyle factors between the genders. Meanwhile, high-energy trauma causes the most hip fractures among young adults, and females might be less biomechanically resilient than male, resulting in worse outcomes following a traumatic impact for females when compared to those in their corresponding gender-specific subgroup in the general population. With increasing age, hip fractures tend to show a greater correlation with osteoporosis or osteopenia. Several studies reported that the prevalence of osteoporosis rapidly increased after the age of 40 to 45 years [ 26– 29]. Other studies revealed that the incidence of osteoporotic hip fractures increased after 40 to 50 years of age [ 10, 28, 30, 31]. We postulate that the prevalence of osteopenia begins to increase even before the age of 40, and contributes to the increased mortality after hip fractures.

We found that the annual incidences of hip fracture among young adults decreased from 7.68 to 7.23, 10.18 to 11.84, and 3.07 to 4.20 per 100,000 persons per year for overall, males, and females, respectively, from 1999 to 2008. These incidence rates of hip fractures among young adults were lower than those of elderly adults [ 18, 30, 32– 34]. Wang et al. reported that the incidence rates of hip fractures among elderly adults were 405 and 476 per 100,000 persons in 1998 and 2009, respectively [ 34]. Only a few studies have reported population incidence rates of hip fractures among young adults [ 5, 7, 18, 30, 32, 35]. Previous studies reported that the annual incidence rates of hip fracture varied from 1.9 to 16.3 per 100,000 persons per year [ 5, 7, 18, 30, 32, 35]. These incidence rates of hip fractures among young adults were close to those reported in Germany [ 36] and Japan [ 30], but higher than those in England [ 18] and Spain [ 35]. The incidence rates of hip fractures among young adults fluctuated in Taiwan, showing a similar pattern to that observed in England [ 18]. However, Icks et al. revealed that the incidence rates of hip fractures gradually decreased from 1995 to 2004 [ 32]. Possible factors that contributed to the differences among regions include different inclusion criteria, distributions of age and gender in the studied populations, trauma severity, and bone mineral density near middle age.

The statistically significant risk factors of overall death were male gender, older age, trochanteric fracture, hemiarthroplasty, and larger CCI scores. These risk factors for mortality after hip fractures among young adults in Taiwan were similar to those reported for the elderly population [ 10, 17, 34, 37]. Only a few studies analyzed the risk factors, including comorbidities and lifestyles, for the mortality among young adults [ 10, 11]. We found that among young adults, males had a higher risk for overall death than females. Moreover, males had a higher prevalence of multiple comorbidities, especially chronic pulmonary disease, than females, which might explain one of the reasons for the higher risk of mortality. Duckworth et al. revealed that excess alcohol consumption and pre-existing morbidities such as renal, liver, and respiratory diseases, were predictors of surgery failure among 122 adult fracture patients younger than 60 years [ 11]. Karantana et al. investigated 315 hip fractures among younger women aged less than 65 years, and determined that smoking and alcohol abuse were important risk factors for mortality [ 10]. In their meta-analysis, Hu et al. identified multiple comorbidities as risk factors of overall mortality among elderly adults [ 37]. However, the comorbidities used in the multivariable analysis varied among these studies. There were no clinical measurements of individuals in our database that could be used to sufficiently evaluate these risk factors. We used CCI score to represent the combined severity of multiple comorbidities, which was demonstrated to play a key role in the mortality rate after hip fractures [ 38, 39]. Hip fractures among young adults are typically treated with internal fixation. We found that hemiarthroplasty had a higher mortality rate compared with that of internal fixation. However, only 160 subjects (3.54 %) received hemiarthroplasty in our database; the sample size was thus much too small to obtain a meaningful conclusion of the effect of surgery type.

The overall annual incidence rates of hip fracture for young adults aged 20 to 40 years in Taiwan were 7.68 and 7.23 per 100,000 in 2001 and 2008, respectively. The cumulative mortality rate was as high as 10.5 % during the ten-year follow-up. Although the one- to ten-year follow-up SMRs decreased, all values remained five-fold greater or more, which indicated that the relative impact on the mortality rate was quite high in young adults with hip fracture compared with the mortality rate in the general young adult population. We recommend that young adults that have been treated for hip fracture should be closely followed up to reduce the risk of mortality.

CCI, Charlson comorbidity index; ICD-9-CM, international classification of disease, ninth revision, clinical modification; IRB, institutional review board; NHI, The National Health Insurance; NHIRD, The NHI Research Database; SMR, standardized mortality ratio