Fracture incidence and characteristics in young adults age 18 to 49 years: A population-based study

Abstract

While fractures in both the pediatric and, especially, the elderly populations have been extensively investigated, comparatively little attention has been given to the age-group in between. Thus, we used the comprehensive (inpatient and outpatient) data resources of the Rochester Epidemiology Project to determine incidence rates for all fractures among young adult (age range, 18 to 49 years) residents of Olmsted County, Minnesota in 2009–11, and compared the distribution of fracture sites and causes in this young adult cohort with those for older residents age 50 years or over. During the 3-year study period, 2,482 Olmsted County residents age 18–49 years experienced one or more fractures. There were 1,730 fractures among 1,447 men compared to 1,164 among 1,035 women, and the age-adjusted incidence of all fractures was 66% greater among the men (1,882 [95% CI, 1,793–1,971] vs. 1,135 [95% CI, 1,069–1,201] per 100,000 p-y; p<0.001). Of all fractures, 80% resulted from severe trauma (e.g., motor vehicle accidents),compared to 33% in Olmsted County residents age ≥ 50 years who sustained a fracture in 2009–11. Younger residents (ages 18–49 years), when compared to older residents (age ≥ 50 years), had a greater proportion of fractures of the hands and feet (40% vs. 18%) with relatively few fractures observed at traditional osteoporotic fracture sites (14% vs. 43%).. Vertebral fractures were still more likely to be due to moderate trauma than at other sites, especially in younger women. In conclusion, whereas pediatric and elderly populations often fracture from no more than moderate trauma, young adults, and more commonly men, suffer fractures primarily at non-osteoporotic sites due to more significant trauma.

Introduction

It has long been known that fracture incidence in the population is bimodal, with elevated rates among children and adolescents,^(1–3) which then decline only to rise again with advancing age.⁽⁴⁾ It is also evident from previous work that some types of fractures (e.g., hands and feet) typically occur early in life, more often in men and linked with recreational and occupational injuries;^(5–12) other types (e.g., hip fractures) are seen disproportionately among the elderly, more often in women and associated with bone loss and frailty-related falls.⁽¹³⁾ While fractures in the pediatric and, especially, the elderly populations have been extensively investigated, relatively little attention has been given to the age-group in between. The purpose of this report is to update the incidence rates for limb fractures among young adults in this community, which was last determined for the years 1969–71,⁽⁵⁾ and to compare the distribution of fracture sites and causes in 2009–11 for 18–49 year-old Olmsted County, Minnesota, residents with those for residents 50 years old and over.⁽¹⁴⁾ This updated information is needed since low trauma fractures that occur prior to age 50 years are potentially relevant to risk assessment models like the World Health Organization’s fracture prediction algorithm, FRAX^®.⁽¹⁵⁾

Methods

Study population

Population-based epidemiologic research can be conducted in Olmsted County because medical care is virtually self-contained within the community, and there are relatively few providers.⁽¹⁶⁾ Most orthopedic care, for example, is provided by the Mayo Clinic, which has maintained a common medical record system with its two large hospitals (Saint Marys and Rochester Methodist) for over 100 years. Mayo Clinic records thus contain both inpatient and outpatient data. The diagnoses and surgical procedures recorded in these records are indexed, including the diagnoses made for outpatients seen in office or clinic consultations, emergency room visits or nursing home care, as well as diagnoses recorded for hospital inpatients, at autopsy examination or on death certificates. Medical records of the other health care providers who serve the local population, most notably the Olmsted Medical Center, are also indexed and retrievable. Thus, the details of virtually all of the medical care provided to the residents of Olmsted County are available for study through the Rochester Epidemiology Project, as has been previously comprehensively described.⁽¹⁷⁾ The study was conducted following approval of the respective Mayo Clinic and Olmsted Medical Center Institutional Review Boards.

Fracture ascertainment

Using this unique medical records linkage system, we identified all reported fractures available within the medical records that occurred among Olmsted County residents 18 to 49 years old during the 3-year period, 2009–2011. Only a minority of fracture patients is hospitalized, but it was possible in our data system to identify those treated solely on an outpatient basis. Complete (inpatient and outpatient) community medical records were reviewed by trained nurse abstractors for all residents with any diagnosis attributable to rubrics 800 through 829 in the 9^th International Classification of Diseases.⁽¹⁸⁾ Of 3,311 potential subjects, 829 were excluded either because no fracture or no eligible fracture was found (n = 429; by our standard convention, amputation and avulsion injuries are excluded), the fracture occurred before 2009 or after 2011 (n = 211), the patient was not actually an Olmsted County resident at the time of fracture (n = 123) or for some other reason (n = 27); only 39 patients had to be excluded because they had not provided an authorization for review of their medical records for research in accordance with Minnesota privacy law.⁽¹⁷⁾ All fractures were radiographically confirmed, but the original X-rays were not reviewed. Thus, the diagnosis of an incident vertebral fracture, whether symptomatic or not, was accepted on the basis of a radiologist’s report of compression or collapse of one more thoracic or lumbar vertebrae and through review of medical documentation available to make a determination that it was new. The indexing system is very comprehensive, and we searched for fracture diagnoses made by any provider in any setting (i.e., emergency room, hospital, or follow-up outpatient care) from 1 January 2009 through 31 December 2011. Fracture ascertainment is believed to be complete⁽¹⁷⁾ except for vertebral and rib fractures, some of which are never recognized clinically. Fractures were classified by etiology according to information about each event that was recorded in the medical record: those caused by a specific pathological process (e.g., metastatic malignancy) as determined by the attending physicians, those resulting from severe trauma (e.g., motor vehicle accidents or falls from greater than standing height) and those due to no more than moderate trauma (by convention, equivalent to a fall from standing height or less).

Statistical analysis

In calculating incidence rates, the entire population of Olmsted County in the specific age groups was considered to be at risk. Denominator age- and sex-specific person-years (p-y) were estimated from an ongoing enumeration of Olmsted County residents by the Rochester Epidemiology Project, which varies by each calendar year.⁽¹⁹⁾ Incidence rates were summarized by 5-year age categories, but total incidence rates were calculated using 1-year age intervals. To obtain some sense of variability, it was assumed that, given a fixed number of p-y, the number of fracture cases follows a Poisson distribution; this allowed for the estimation of standard errors and the calculation of 95% confidence intervals (95% CI) for the incidence rates. Incidence rates, including those for Rochester (the central city of Olmsted County) in 1969–71⁽²⁰⁾ as well as rates from studies in other communities, were directly age- and/or age- and sex-adjusted to the population distribution of United States whites in 2010. The standard errors and confidence intervals for the adjusted rates were based on the same assumptions as above. Generalized linear models were used to test for differences in incidences between the sexes and as well as over age, assuming a Poisson error structure⁽²⁰⁾ and correcting for overdispersion when necessary. Predictions from generalized additive models, fit assuming a Poisson error structure, were used to graphically display the incidence rates over age, using a log scale. The χ² test was used to compare fracture cause and site between the older and younger residents. Fracture ascertainment for the older residents (age ≥ 50 years) in the community was identical to what has been described.⁽¹⁴⁾

Results

Over the 3-year study period, 2009–11, 2,482 Olmsted County residents age 18–49 years experienced one or more fractures, for an overall age- and sex-adjusted incidence of any fracture of 1,291 per 100,000 p-y (95% CI, 1,240–1,342). There were 1,447 men and 1,035 women, and 91% were white by self-report, in keeping with the racial composition of the community in this age-group (84% white in 2010). The age-adjusted annual incidence for women was 1,007 per 100,000 (95% CI, 945–1,069) compared to 1,567 per 100,000 (95% CI, 1,486–1,648) for men, for a male:female ratio of age-adjusted incidence rates of 1.6:1. Altogether, 2,200 subjects (87% of the men and 91% of the women) experienced a single fracture, whereas 215 had two fractures, 37 had three, and 30 had four or more fractures each (10%, 1% and 2% in men and 7%, 2% and <1% in women, respectively). Of the 282 people who had more than one fracture during the three-year time period, 154 people had multiple fractures on the same date and 150 people had fractures that occurred on different dates; there were 22 people who had both multiple fractures on the same date in addition to having fractures that occurred on different dates.

A total of 2,894 different fractures were observed in this age-group during the study period, and the incidence of all fractures, as opposed to unique individuals, was 1,513 per 100,000 p-y (95% CI, 1,458–1,569). There were 1,730 fractures among Olmsted County men compared to 1,164 among the women, and the age-adjusted incidence of all fractures was 66% greater among the men (1,882 [95% CI, 1,793–1,971] vs. 1,135 [95% CI, 1,069–1,201] per 100,000 p-y; p<0.001). Between the ages of 18 and 49 years, fracture incidence declined somewhat in men (p=0.229) and rose (p<0.001) among the women (Figure 1).

Figure 1.

Incidence of all fractures, plotted on a log scale, among Olmsted County, Minnesota, residents by sex and as a function of age.

As delineated in Tables 1 and 2, 87% of all fractures in men and 71% of all fractures in women resulted from severe trauma. Figures 2 and 3 show the incidence as a function of age for moderate trauma and severe trauma fractures, respectively, in both sexes. The severe trauma fractures included 319 and 115 motor vehicle accidents in 196 men and 76 women, respectively, 193 and 159 falls from greater than standing height in 156 men and 152 women, 465 and 207 recreational mishaps in 441 men and 196 women, and 523 and 346 occupational and other injuries in 498 men and 339 women. The age-adjusted incidence of fractures due to severe trauma was greater among men compared to the women (1,624 [95% CI, 1,541–1,707] per 100,000 p-y vs. 793 [95% CI, 738–847] per 100,000 p-y; p<0.001), whereas the incidence of pathologic fractures was somewhat greater among the women (6.2 [95% CI, 1.2–11.3] per 100,000 p-y vs. 1.8 [95% CI, 0.0–4.4] per 100,000 p-y; p=0.06). However, only 8 fractures were due to a specific local pathological process (2 bone cyst, 2 multiple myeloma, 1 Ewing sarcoma, 1 giant cell tumor, 1 endometrial cancer, and 1 squamous cell carcinoma). In the women and men combined, of the 513 fractures attributed to moderate trauma, no specific precipitating event was recognized in 93, i.e., fractures that occurred in the course of daily activities (“spontaneous”). These included 56 fractures of the thoracic or lumbar spine, 2 of the cervical spine, 16 of the feet/toes, 10 of the ribs, 4 of the pelvis, 3 of the tibia/fibula, 1 of the shaft/proximal radius and 1 of the proximal femur. Of those with a spontaneous cause, 23 were found incidentally in the course of evaluating another clinical problem. Altogether, 49 fractures were considered over-use/stress injuries by the attending physicians (17 spontaneous, 28 recreational, and 4 other). However, the single largest cause of fracture was a fall from standing height or less with 420 fractures in 408 individuals (165 fractures in 158 men, 255 fractures in 250 women). The incidence of fractures due to no more than moderate trauma was much greater among the women compared to the men (336 [95% CI, 299–372] per 100,000 p-y vs. 256 [95% CI, 223–290] per 100,000 p-y; p=0.005).

Table 1.

Distribution of all fractures by skeletal site and cause among Olmsted County, Minnesota, male residents 18–49 years of age, 2009–11

	Fracture cause
	Severe trauma		Moderate trauma		Pathological		Uncertain		All causes
Fracture site	n	%a	n	%a	n	%a	n	%a	n	%b
Skull/face	180	94.2%	10	5.2%	0	0.0%	1	0.5%	191	11.0%
Hands/fingers	403	91.2%	35	7.9%	0	0.0%	4	0.9%	442	25.5%
Distal forearm	87	80.6%	20	18.5%	1	0.9%	0	0.0%	108	6.2%
Proximal humerus	19	82.6%	3	13.0%	0	0.0%	1	4.3%	23	1.3%
Other arm	80	84.2%	15	15.8%	0	0.0%	0	0.0%	95	5.5%
Clavicle/scapula/sternum	104	91.2%	9	7.9%	0	0.0%	1	0.9%	114	6.6%
Ribs	120	80.5%	24	16.1%	0	0.0%	5	3.4%	149	8.6%
Thoracic/lumbar vertebrae	50	56.8%	28	31.8%	0	0.0%	10	11.4%	88	5.1%
Cervical vertebrae	42	85.7%	6	12.2%	1	2.0%	0	0.0%	49	2.8%
Pelvis	19	90.5%	2	9.5%	0	0.0%	0	0.0%	21	1.2%
Proximal femur	8	88.9%	1	11.1%	0	0.0%	0	0.0%	9	0.5%
Other leg	180	84.5%	31	14.6%	0	0.0%	2	0.9%	213	12.3%
Feet/toes	208	91.2%	18	7.9%	0	0.0%	2	0.9%	228	13.2%
All sites	1500	86.7%	202	11.7%	2	0.1%	26	1.5%	1730	100%

^aPercentage (%) by cause of each type of fracture

^bPercentage (%) by site of total

Table 2.

Distribution of all fractures by skeletal site and cause among Olmsted County, Minnesota, female residents 18–49 years of age, 2009–11

	Fracture cause
	Severe trauma		Moderate trauma		Pathological		Uncertain		All causes
Fracture site	n	%a	n	%a	n	%a	n	%a	n	%b
Skull/face	60	76.9%	17	21.8%	1	1.3%	0	0.0%	78	6.7%
Hands/fingers	160	83.8%	30	15.7%	0	0.0%	1	0.5%	191	16.4%
Distal forearm	56	57.1%	41	41.8%	1	1.0%	0	0.0%	98	8.4%
Proximal humerus	11	52.4%	10	47.6%	0	0.0%	0	0.0%	21	1.8%
Other arm	35	49.3%	35	49.3%	0	0.0%	1	1.4%	71	6.1%
Clavicle/scapula/sternum	23	92.0%	1	4.0%	1	4.0%	0	0.0%	25	2.1%
Ribs	42	59.2%	24	33.8%	1	1.4%	4	5.6%	71	6.1%
Thoracic/lumbar vertebrae	14	26.4%	38	71.7%	0	0.0%	1	1.9%	53	4.6%
Cervical vertebrae	23	88.5%	1	3.8%	1	3.8%	1	3.8%	26	2.2%
Pelvis	16	72.7%	5	22.7%	1	4.5%	0	0.0%	22	1.9%
Proximal femur	5	71.4%	2	28.6%	0	0.0%	0	0.0%	7	0.6%
Other leg	131	65.8%	65	32.7%	0	0.0%	3	1.5%	199	17.1%
Feet/toes	251	83.1%	42	13.9%	0	0.0%	9	3.0%	302	25.9%
All sites	827	71.0%	311	26.7%	6	0.5%	20	1.7%	1164	100%

^aPercentage (%) by cause of each type of fracture

^bPercentage (%) by site of total

Figure 2.

Incidence of mild/moderate trauma fractures, plotted on a log scale, among Olmsted County, Minnesota, residents by sex and as a function of age.

Figure 3.

Incidence of severe trauma fractures, plotted on a log scale, among Olmsted County, Minnesota, residents by sex and as a function of age.

Compared to Olmsted County residents age 50 years or over with a fracture in 2009–2011, the younger residents had a much greater proportion of fractures of the hands and feet (39% vs. 16% in men [Table 3] and 42% vs. 19% in women [Table 4]). A corresponding dearth of fractures was observed at the traditional osteoporotic fracture sites, including the distal forearm, proximal humerus, thoracic/lumbar vertebra or proximal femur (13% vs. 39% in men and 15% vs. 45% in women). Likewise, 87% and 71% of the fractures in men and women, respectively, among the 18–49 year-olds resulted from severe trauma (particularly motor vehicle accidents, recreation and occupational injuries) compared to only 45% and 27% of the fractures in men and women, respectively, among older residents, the majority of whose fractures resulted from no more than moderate trauma, particularly falls from a standing height. That said, of all fractures, vertebral fractures in young women were more likely to occur as a result of no more than moderate trauma than due to severe trauma (72% vs. 26%, Table 2), while in young men the proportion of vertebral fractures due to mild/moderate vs. severe trauma was greater than at other fracture sites (32% with mild/moderate trauma vs 57% with severe trauma, Table 1).

Table 3.

Comparison by skeletal site and cause of all fractures in 2009–11 among Olmsted County, Minnesota, male residents 18–49 years of age versus those 50 years old or older.

	Age-group
	18–49 years		≥ 50 years
Variable	n	%	n	%
Fracture site
Skull/face	191	11.0%	91	5.1%
Hands/fingers	442	25.5%	164	9.2%
Distal forearm	108	6.2%	89	5.0%
Proximal humerus	23	1.3%	54	3.0%
Other arm	95	5.5%	57	3.2%
Clavicle/scapula/sternum	114	6.6%	91	5.1%
Ribs	149	8.6%	269	15.1%
Thoracic/lumbar vertebrae	88	5.1%	428	24.1%
Cervical vertebrae	49	2.8%	64	3.6%
Pelvis	21	1.2%	56	3.1%
Proximal femur	9	0.5%	117	6.6%
Other leg	213	12.3%	172	9.7%
Feet/toes	228	13.2%	127	7.1%
Fracture cause
Severe trauma	1500	86.7%	802	45.1%
Motor vehicle accident	319	18.4%	180	10.1%
Fall from height	193	11.1%	236	13.3%
Recreational	465	26.9%	133	7.5%
Occupational and other	523	30.2%	253	14.2%
Moderate trauma	202	11.7%	868	48.8%
Fall from standing height	165	9.5%	547	30.7%
“Spontaneous” (incidentally diagnosed)	10	0.6%	96	5.4%
“Spontaneous” (non-incidental)	27	1.6%	225	12.6%
Pathological	2	0.1%	51	2.9%
Uncertain	26	1.5%	58	3.3%

Statistically significant differences are bolded.

Table 4.

Comparison by skeletal site and cause of all fractures in 2009–11 among Olmsted County, Minnesota, female residents 18–49 years of age versus those 50 years old or older.

	Age-group
	18–49 years		≥ 50 years
Variable	n	%	n	%
Fracture site
Skull/face	78	6.7%	87	2.5%
Hands/fingers	191	16.4%	203	5.8%
Distal forearm	98	8.4%	345	9.9%
Proximal humerus	21	1.8%	155	4.5%
Other arm	71	6.1%	122	3.5%
Clavicle/scapula/sternum	25	2.1%	71	2.0%
Ribs	71	6.1%	271	7.8%
Thoracic/lumbar vertebrae	53	4.6%	809	23.3%
Cervical vertebrae	26	2.2%	72	2.1%
Pelvis	22	1.9%	217	6.2%
Proximal femur	7	0.6%	253	7.3%
Other leg	199	17.1%	426	12.3%
Feet/toes	302	25.9%	442	12.7%
Fracture cause
Severe trauma	827	71.0%	953	27.4%
Motor vehicle accident	115	9.9%	128	3.7%
Fall from height	159	13.7%	350	10.1%
Recreational	207	17.8%	97	2.8%
Occupational and other	346	29.7%	378	10.9%
Moderate trauma	311	26.7%	2294	66.1%
Fall from standing height	255	21.9%	1535	44.2%
“Spontaneous” (incidentally diagnosed)	13	1.1%	210	6.0%
“Spontaneous” (non-incidental)	43	3.7%	549	15.8%
Pathological	6	0.5%	59	1.7%
Uncertain	20	1.7%	167	4.8%

Statistically significant differences are bolded.

Age- and sex-specific incidence rates for each of the different fracture sites are shown in Table 5, with the same data but with 95% CI around rates provided in Supplementary Tables 1–3. Age-adjusted fracture rates were greater, often substantially so, among 18–49 year-old men than women at all fracture sites except the ankle and feet/toes. Interestingly, fracture incidence at all sites was highest among 18–24 year-old men relative to any other age-group. Further, among 25–49 year-old men, the age-related fracture incidence at many skeletal sites tended to be higher with older age groups, except, in particular, the skull/face and hands/fingers where the age-specific rates were usually lower. Similar to the men, the age-related fracture incidence among women also tended to be higher with older age groups. This was particularly evident at sites including the hands/fingers, proximal humerus, ribs, thoracic/lumbar vertebrae, ankle, and feet/toes.

Table 5.

Incidence^a of all fractures by skeletal site among Olmsted County, Minnesota, residents 18–49 years of age, 2009–11, by sex and age-group

Age- group	Skull/ face	Hands/ fingers	Distal forearm	Proximal humerus	Proximal forearm	Shaft/ distal humerus	Clavicle/ scapula/ sternum	Ribs	Thoracic/ lumbar Vertebrae	Cervical vertebrae	Pelvis	Proximal femur	Shaft/ distal femur	Patella	Tibia/ fibula	Ankle	Feet/ toes	All sites
Men
18–24	387	730	144	20	79	25	194	74	89	45	45	15	25	20	94	139	189	2313
25–29	274	566	75	25	75	12	81	75	68	44	6	0	6	19	50	106	224	1704
30–34	206	406	90	26	90	0	58	90	32	19	6	6	39	6	52	90	252	1470
35–39	84	352	84	15	99	8	84	184	130	61	15	15	8	0	115	99	245	1599
40–44	71	392	173	31	110	24	141	235	118	47	31	16	0	8	110	110	337	1954
45–49	112	297	132	33	86	13	158	356	145	106	26	7	13	13	86	158	264	2006
Subtotalb	199	473	119	25	89	14	126	169	99	54	24	10	15	12	86	120	248	1882
Women
18–24	83	167	117	13	63	8	13	21	29	33	29	0	4	0	33	75	192	879
25–29	115	186	55	5	50	10	40	60	25	10	25	20	10	0	25	85	266	989
30–34	65	183	47	6	47	18	18	41	35	24	12	0	0	0	47	195	330	1067
35–39	50	136	65	22	65	7	29	57	72	29	29	0	7	7	65	136	316	1092
40–44	58	197	102	22	36	15	22	109	80	29	7	7	0	22	80	160	372	1320
45–49	55	225	170	61	73	12	24	146	85	24	18	12	12	18	37	183	317	1474
Subtotalb	73	183	97	22	56	12	24	72	55	26	20	6	6	8	47	136	293	1135
Totalc	137	330	108	24	73	13	75	121	77	40	22	8	11	10	67	128	271	1513

^aIncidence per 100,000 person-years

^bIncidence per 100,000 person-years using one-year age intervals directly age-adjusted to the population structure of 2010 United States whites age 18–49 years

^cIncidence per 100,000 person-years using one-year age intervals directly age- and sex-adjusted to the population structure of 2010 United States whites, age 18–49 years

Discussion

Based on complete ascertainment from all sources of care in the community, the annual incidence of fractures among Olmsted County residents age 18–49 years was 1513 per 100,000 p-y, or 1.5% of this population per year. When compared to population-based data for 25–54 year-olds from an English community in 1980–82,⁽⁶⁾ our overall incidence rate was 208% higher in 25–54 year-olds when both sets of rates were directly adjusted to the age and sex distribution of the United States white population in 2010 (1633 vs. 530 per 100,000 p-y). That discrepancy has been attributed to under-ascertainment of some fractures, particularly fractures of the spine and pelvis,⁽⁹⁾ in data from the Leicestershire Health Authority obtained from local hospitals and a fracture clinic. Indeed, the same authors reported a much higher overall fracture rate for 25–54 year-old English residents in 2002–2004 (2975 per 100,000 p-y adjusted to 2010 United States whites) based on self-report from a population-based survey sample.⁽¹²⁾ The overall incidence of fractures in Olmsted County was also 7% higher than comparably adjusted rates for 25–54 year-olds from Cardiff, Wales (1516 per 100,000 p-y) based on emergency department data for 1994–1995⁽⁷⁾, 45% higher than those for 20–49 year-old residents of Edinburgh, Scotland in 1992–1993 (1481 versus 1024 per 100,000 p-y) also using emergency department data,⁽⁸⁾ 31% greater than rates for 20–49 year-olds in Tasmania, Australia (1134 per 100,000 p-y) based on X-ray reports in 1997–1999⁽¹¹⁾ and 52% higher than data for 20–49 year-olds that were derived from the General Practice Research Database in the United Kingdom for 1988–1998.⁽¹⁰⁾ However, whereas we focused on all fracture events, van Staa and colleagues⁽¹⁰⁾ counted individuals with any fracture during the study period. Considering only unique individuals, the annual age- and sex-adjusted incidence of any fracture among Olmsted County residents age 20–49 years was 1267 per 100,000 p-y compared to 834 per 100,000 p-y in the comparably adjusted English data. Overall fracture incidence in Olmsted County residents age 18–49 years remained similarly higher even to more recent comparable data from the UK between 1988–2012.⁽²¹⁾ On the other hand, reported fracture incidence in ages 18–49 from Denmark in 2011 tended to be more similar to our findings.⁽²²⁾ Finally, the incidence of all arm and hand fractures among 18–49 year-olds in 2009–11 was 6% lower, and that of all leg and foot fractures 5% lower, compared to 1969–71 rates for those sites among 18–49 year-olds in Rochester⁽⁵⁾, the central city in Olmsted County that comprised 74% of the County population in 2010.

The etiology and presentation of limb fractures was described in the earlier Rochester study,⁽⁵⁾ but fractures at axial skeletal sites were not included, and the 18–49 year age-group was not separately distinguished. When all skeletal sites were included, we found, as reported by others,^{(5–8,10–12)} that rates were much greater among young men than young women. Much of the male excess was accounted for by fractures of the hands and feet, and it is noteworthy that these accounted for 40% of the total, compared to only 18% among residents 50 years of age or older.⁽¹⁴⁾ In their classic work,⁽⁴⁾ Buhr and Cooke attributed hand and foot fractures mainly to occupational injuries. While severe trauma did account for nearly 90% of the hand and foot fractures in our study, occupational injuries were only responsible for 8% of the total, possibly reflecting changes in occupations or occupational safety standards over time. Indeed, it should be noted that heavy industry is not prominent in Olmsted County, and more recent studies have attributed fractures of the hands/fingers to sports, falls and direct contact (e.g., punching or hitting walls and doors)⁽²³⁾ and fractures of the foot/toes to twisting and falling.⁽²⁴⁾ Consistent with these reports, hand fractures in our study were primarily attributed to recreational activities (26%) and other reasons (50%), while foot fractures were mainly the result of falls from greater than standing height (14%), recreational activities (18%), and other reasons (50%).

By contrast, only 14% of all fractures (43% in the age-group 40–49 years) occurred at fracture sites most commonly associated with osteoporosis (spine, hip, proximal humerus and distal forearm). Moreover, only 38% of the fractures at these sites were attributed to no more than moderate trauma, and therefore of potential relevance to FRAX. Instead, the epidemiology differed considerably from that seen in older community residents.⁽¹⁴⁾ For example, although vertebral fractures were uncommon, they were almost twice as frequent among men as compared to women, especially in the age-group 35–49 years. This has been observed previously and attributed to occupationally-induced excessive spinal loading in men.^(8,11,25,26) That said, vertebral fractures were more likely to be due moderate trauma than severe trauma in women and more often related to moderate trauma than any other fracture site in men. Hip fractures were also slightly more common in younger men than women but, in any event, were rare among the 18–49 year-olds, as seen elsewhere.^{(6–8,10,11,21,22,27,28)} Unlike older individuals,⁽¹⁴⁾ hip fractures in this cohort were mostly due to severe trauma (13 of 16 total), as were proximal humerus fractures (30 of 44 total). Finally, two-thirds of distal forearm fractures were attributed to severe trauma (143 of 206 total), the highest proportion (46%) of which occurred during recreational activities. Only 30% of forearm fractures resulted from a “simple” (standing height or less) fall as compared to 71% among Olmsted County residents age 50 years and over.⁽¹⁴⁾ Of note, however, incidence rates at each of these four fracture sites (spine, hip, proximal humerus and distal forearm) did begin to drift higher around age 55 years among the women, which is consistent with a perimenopausal increase in the likelihood of falling⁽²⁹⁾ and the accelerated phase of bone loss that accompanies the menopause.⁽³⁰⁾

A topic of particular interest in the young adult age-group has been the occurrence of over-use/stress fractures. Much of the literature on this topic has been devoted to the problem of stress fractures in military recruits undergoing strenuous basic training.⁽³¹⁾ Even among the civilians evaluated here, however, stress fractures were seen predominantly in those beginning or extending (e.g., training for a marathon) an exercise program, or among those engaged in long distance running or hiking, as observed previously.⁽³²⁾

Our study has a number of noteworthy strengths. Except for those which do not come to clinical attention (e.g., some fractures of the vertebrae and ribs), ascertainment of fractures should be complete.⁽¹⁴⁾ and the denominator population is well characterized.⁽¹⁹⁾ Moreover, our analysis did not require assumptions inherent in studies based on administrative data,⁽³³⁾ nor the limitations of self-report,⁽³⁴⁾ although comparisons of rates from other published studies had to be extrapolated from the data contained therein. However, the number of fractures observed at some skeletal sites was relatively small in this study, and those particular incidence estimates were limited as a result. In addition, given the demographic makeup of the community,⁽¹⁷⁾ there were only 210 (150 Black/African American, 10 American Indian/Alaskan Native, 47 Asian, 3 Native Hawaiian/Pacific Islander) non-white residents in our cohort. Fractures among young adults may⁽³⁵⁾ or may not⁽³⁶⁾ be more frequent in minority populations, but the majority of fractures nationally occur in the white population,⁽³⁷⁾ and Olmsted County rates for hip fractures are comparable to previous incidence estimates for the United States white population generally.⁽³⁸⁾

In conclusion, these Olmsted County data represent the only recent age- and sex-specific incidence rates for all fractures among adults age 18 to 49 years in the United States. In contrast to pediatric and elderly populations, who often fracture from no more than moderate trauma, young adults, and more commonly men, suffer fractures primarily at non-osteoporotic sites due to significant trauma. In light of our previous data demonstrating that children with mild, but not moderate, trauma fractures have significant skeletal deficits,⁽³⁹⁾ young adults who suffer mild trauma fractures may also have underlying skeletal deficits; they may therefore warrant closer evaluation and further counseling on measures to optimize skeletal health and prevent future fractures.