The relationship between temperature and hip and wrist fracture incidence

Abstract

Introduction

Predicting when fracture incidence will rise assists in healthcare planning and delivery of preventative strategies. The aim of this study was to investigate the relationship between temperature and the incidence of hip and wrist fractures.

Methods

Data for adults presenting to our unit with a hip or wrist fracture over a seven and eight-year period respectively were analysed. Incidence rates were calculated and compared with meteorological records. A Poisson regression model was used to quantify the relationship between temperature and fracture rate.

Results

During the respective study periods, 8,380 patients presented with wrist fractures and 5,279 patients were admitted with hip fractures. All women (≥50 years: p<0.001; <50 years: p<0.001) and men aged ≥50 years (p=0.046) demonstrated an increased wrist fracture rate with reduced temperature. Men aged <50 years also had an increased wrist fracture rate with increased temperature (p<0.001).

The hip fracture rate was highest in women aged ≥50 years but was not associated with temperature (p=0.22). In men aged ≥50 years, there was a significant relationship between reduced temperature and increased fracture rate (p<0.001).

Conclusions

Fragility fracture of the wrist is associated with temperature. Compared with an average summer, an additional 840 procedures are performed for wrist fractures during an average winter in our trust with an additional 798 bed days taken up at a cost of £3.2 million. The winter increase seen in male hip fracture incidence requires approximately 888 surgical procedures, with 18,026 bed days, and costs £7.1 million. Hip fracture incidence in older women is not related to temperature.

Introduction

With an ageing population, fragility fractures pose an increasing challenge to healthcare services and resources. Fifty per cent of women and twenty per cent of men aged over 50 years will suffer a fracture.¹ Approximately 300,000 fragility fractures are seen in the UK each year.² The greatest economic burden is due to hip fractures, with nearly 85,000 UK hospital admissions per year accounting for over 1.8 million hospital bed days and an estimated cost of hospital care alone of over £1.9 billion.³ Incidence is rising and projections suggest the number of hip fracture hospital admissions could rise to 140,000 per year by 2036 if effective preventative strategies are not introduced.⁴

The cost of treating other fragility fractures is lower but still significant at over £200 million per year.⁴ Distal radius fracture is the most common adult fracture.⁵ In young patients, injury may occur owing to high energy trauma but it more commonly occurs in older patients with osteoporosis after a fall on to the outstretched hand. The direct medical cost alone for treatment of fractures of the forearm and wrist in the UK has been predicted to total over £35 million by 2020.⁶

The aim of this study was to investigate the relationship between temperature and the incidence of hip and distal radius fractures. Understanding how temperature affects fracture incidence has several advantages. Temperature is easy to measure and precise forecasts are readily available at a local level. The ability to accurately predict when fracture incidence is likely to rise assists with healthcare planning and delivery by allowing estimation of the number of patients expected to present. This helps improve planning of acute services, osteoporosis management, and care and rehabilitation services as well as predicting the need for surgical intervention. Identifying which patients are at high risk of fracture during high or low temperature will help target preventative strategies towards these patients.

Methods

Our trust provides care to a population of just over one million people living in a large city and the surrounding rural counties. All patients sustaining an orthopaedic injury within these areas are referred for review and follow-up at this trust, allowing comprehensive capture of all such presentations among the resident population.

Details of all adult patients presenting to our unit with a radiographically confirmed fracture of the distal radius between 1 January 2007 and 31 December 2014 were identified from the emergency department database. Similar data were reviewed for all patients presenting with a hip fracture for the period between 15 February 2008 and 31 December 2014. These data were obtained from the local prospective hip fracture database, which began data collection in February 2008.

Historical weather variation data were collected from the UK Meteorological Office and the Centre for Environmental Data Analysis.^7,8 Record was made of the minimum daily temperatures for every day during the study period (from the historical data of the closest Meteorological Office station, approximately 15 miles from the emergency department).

Data were analysed in four categories grouped by sex and age (women aged <50 years, women aged ≥50 years, men <50 years, men aged ≥50 years). This age stratification was chosen as bone mineral density remains stable up to 50 years of age, enabling differentiation between fractures in younger patients with good bone quality and fragility fractures in older patients.⁹

The population at risk for each of the four patient groups was identified using data from the Office for National Statistics.¹⁰ Daily incidence rates for hip and wrist fracture for each day in the study periods were calculated for the four groups as well as all for patients combined. Rates were calculated per million patients at risk. This denominator was chosen as the emergency department serves a population of just over one million, meaning it would allow easy identification of an expected change in the number of patients presenting to the unit and enabling simple calculation and adjustment for units of other sizes.

Poisson regression modelling was used to investigate the relationship between daily minimum temperature and daily fracture incidence. A model was constructed to assess minimum temperature against fracture incidence rate, offset by the population at risk. The Poisson regression was performed separately for each sex and age category as well as all patients combined for both hip and wrist fractures. Robust standard errors were calculated using the sandwich method to account for overdispersion. Data analysis was performed with R statistical software (R Foundation for Statistical Computing, Vienna, Austria; www.r-project.org).

Results

During the 2,922 days studied for wrist fractures, 8,380 patients presented to the emergency department with such a fracture. The mean fracture rate was 2.9 fractures per day. The overall mean age for these patients was 56.4 years (range: 18–104 years).

A total of 5,279 patients presented to the emergency department and were admitted to the trauma unit with a hip fracture during the 2,513 days studied. The mean rate of hip fracture was 2.1 per day. The overall mean age for these patients was 81.2 years (range: 21–106 years). A breakdown of the number of patients with hip and wrist fractures grouped by sex and age is given in Table 1.

Table 1.

Number of patients presenting with a hip or wrist fracture during the study period

Age	Wrist fracture		Hip fracture
	Men	Women	Men	Women
18–49 years	1,761	1,349	12	5
≥50 years	917	4,353	2,513	3,778

Wrist fracture rate and temperature

All age groups demonstrated a significant change in fracture rate with a change in temperature. A negative log change in rate with increasing temperature indicates a fall in fracture rate. All groups except men aged <50 years had an increased fracture rate with lower temperatures (Fig 1, Table 2).

Figure 1.

Relationship between temperature and wrist fracture rates per million of population at risk for the four patient groups

Table 2.

Logged values of change in wrist fracture rate per million population with a 1°C increase in temperature

Group	Log change in rate with temperature increase (per °C)	95% confidence interval	Robust standard error	p-value
All	-0.024	-0.032 to -0.017	0.0038	<0.001
Women <50 years	-0.030	-0.043 to -0.016	0.0069	<0.001
Women ≥50 years	-0.032	-0.042 to -0.023	0.0047	<0.001
Men <50 years	0.009	0.0001 to 0.020	0.0049	<0.001
Men ≥50 years	-0.040	-0.057 to -0.024	0.0082	0.046

Hip fracture rate and temperature

The hip fracture rate was highest in women aged ≥50 years but this was not associated with temperature. For men in both age groups, there was a significant relationship between reduced temperature and increased fracture rate. There was also a trend for women aged <50 years to have an increase in fracture rate with a reduction in temperature but this did not quite reach statistical significance (Fig 2, Table 3). The number of hip fractures in the younger age groups, however, was small (12 men, 5 women) so the results are likely to be unreliable and not useful clinically.

Figure 2.

Relationship between temperature and hip fracture rates per million of population at risk for the four patient groups

Table 3.

Logged values of change in hip fracture rate per million population with a 1°C increase in temperature

Group	Log change in rate with temperature increase (per °C)	95% confidence interval	Robust standard error	p-value
All	-0.009	-0.014 to -0.003	0.0029	0.004
Women <50 years	-0.092	-0.184 to 0.001	0.0471	0.051
Women ≥50 years	-0.004	0.010 to 0.002	0.0032	0.22
Men <50 years	-0.034	-0.157 to 0.088	0.0627	<0.001
Men ≥50 years	-0.020	-0.031 to -0.008	0.0059	<0.001

Comparison between hip and wrist fracture rates and temperature in patients aged ≥50 years revealed that the wrist fracture rate in women was highest during colder weather but when the temperature reached 6°C, it dropped below the hip fracture rate for these women. Men aged ≥50 years had a lower rate than women in this age group for both hip and wrist fractures at all temperatures. The rates fell significantly for both types of fracture with increasing temperature (Fig 3).

Figure 3.

Relationship between temperature and hip and wrist fracture rates in patients aged ≥50 years

The Poisson regression model was used to predict the expected daily number of fractures for the groups studied. Using sex and age specific data for the total UK population, the expected number of fractures seen during a 90-day summer and winter was calculated at average UK temperatures for these seasons (0.9°C for winter, 10.2°C for summer). Approximately 4,667 extra wrist fractures would be treated during an average winter compared with an average summer and 1,277 more hip fractures.

Discussion

This study provides useful information about the relationship between hip and wrist fracture rate and temperature. Significantly higher wrist fracture incidence was seen with lower temperatures for all patients except men aged <50 years. Wrist fracture incidence was associated with higher temperatures in younger men. There was no association between hip fracture incidence and temperature in women aged ≥50 years.

Even relatively modest changes in fracture rate with temperature will have a large impact on health services. The UK DRAFFT (Distal Radius Acute Fracture Fixation Trial) study and economic analysis provide relevant and up-to-date information from which projections can be made for wrist fracture resource use.^11,12 An estimate of fracture fixation rate can be calculated from the DRAFFT screening data (Table 4). Our model predicts that an extra 4,667 fractures will be seen during an average winter compared with an average summer. Given the estimated fixation rate of 18%, this suggests there will be 840 additional surgical procedures for wrist fractures required during winter, utilising an extra 798 bed days (Table 5). Increased winter surgical fixation equates to an additional expenditure of £3,222,521 just for surgical costs alone.

Table 4.

Calculation of wrist fracture fixation rate from DRAFFT data

Eligible for trial and wanted surgery	617
Ineligible for trial but required surgery  Fracture required opening: 430  Open fracture (Gustilo grade >1): 43	473
Total fractures undergoing surgery	1,090
Total adult fractures within 3cm of radiocarpal joint	5,947
Fixation rate (1,090/5,947) x 100	18%

Table 5.

Estimation of additional cost and bed days taken up in our trust owing to wrist fractures in winter (assumes that half of the procedures would be K-wire fixation and half would be volar locking plate fixation)

Additional winter wrist fractures	4,667
Additional surgical procedures for wrist fractures 4,667 x 0.18 fixation rate	840
Mean length of hospital stay for wrist fracture surgery	0.95 days11
Additional winter bed days 840 x 0.95 days	798 days
K-wire fixation
Surgical time  420 x 53.57 minutes11	375 hours
Cost  420 x £3,384.7811	£1,421,607.60
Volar locking plate fixation
Surgical time  420 x 70.00 minutes11	490 hours
Cost  420 x £4,287.8911	£1,800,913.80
Total additional winter surgical cost	£3,222,521

The hip fracture rate for older men increases in winter. The model suggests that an extra 888 male hip fractures will be treated in UK hospitals during winter compared with summer. The national median total hospital cost of a primary hip fracture is £8,049¹³ and the average length of stay is 20.3 days.¹⁴ Consequently, winter hip fracture costs will increase by £7,147,512 (888 x £8,049) and an additional 18,026 bed days (888 x 20.3 days) will be taken up.

These predictions are made based on Meteorological Office data from 1981 to 2010 for average summer and winter temperatures in the UK. December 2010 was the coldest month for the last 100 years with a mean temperature of -4°C. In a similar exceptionally cold month, our model would predict 360 more wrist fractures and 35 more hip fractures than in an average single winter month, and 1,395 more wrist fractures and 157 more hip fractures than in an average summer month.

Relationships between fracture rate and temperature are likely to be explained by bone fragility, location and mechanism of injury. Wrist fractures in older patients tend to occur owing to a trip or slip outdoors in those who are relatively active whereas hip fractures generally occur indoors in more frail, older patients.^15,16 Consequently, inclement weather is less likely to affect the hip fracture rate.

Jacobsen et al investigated weather and fracture rates in Minnesota.^17,18 Similar to our findings, they noted that wintery conditions were associated with the greatest increase in incidence of wrist fracture for women. There was minimal influence of weather on the risk of hip fracture in women aged ≥75 years. In our study, the hip fracture rate in men aged ≥50 years did increase with lower temperatures. This may represent fragility fractures in men who are remaining reasonably active for longer than women and mobilising outdoors. While women make up the majority of hip fracture patients, men are an important group who demonstrate higher mortality.

Younger men were the only group in our study population to see an increase in wrist fracture risk with increasing temperature. These injuries are likely to be high energy injuries in non-osteoporotic bone due to sports and other outdoor activities, which more commonly take place during warmer conditions.

Modern weather forecasting is accurate and easily available. It should not surprise healthcare providers that numbers of certain fractures will increase during cold weather. Other studies have demonstrated similar findings but not quantified the rates such as with our model.^19,20 These data can assist in the efficient use of resources and in planning workforce deployment. Most wrist fractures can be dealt with on an outpatient basis, and day case surgery is safe, effective and popular with patients. During cold periods, suitably trained staff could be allocated to emergency departments to assess and manipulate displaced fractures, apply casts and organise appropriate clinic follow-up review or surgery for wrist fracture patients. Trauma units should ensure suitable surgeons and theatre space are available. Hip fractures almost always require surgery with lengthy hospital admission.

Interestingly, the relationship between fracture rate and temperature was almost linear for all groups studied, without a large change in fracture rate at a certain threshold of temperature, even though a non-linear regression method was employed. This model can be beneficial for trauma units when planning expected admissions and number of hip fracture procedures, and it has identified important sex differences in fracture risk for different temperatures, which requires further investigation. This can help develop falls prevention interventions for those at risk.

Study limitations

While the overall population studied was large, the number of hip fracture patients aged <50 years was small. Nevertheless, the main purpose of this study was to look at the influence of temperature on fragility fractures.

Falls are usually due to multiple factors and there are likely to be confounding variables that were not investigated in this study. Despite extensive weather data being obtainable, our study used daily minimum air temperature for analysis. These measurements are widely available at a local level. Further subanalysis of fracture incidence associated with different meteorological data regarding wind, ice or snow may yield interesting results but is likely to lead to errors, and these measurements are not easily accessible by clinicians and healthcare planners. Average seasonal temperatures were used to investigate change in fracture numbers between winter and summer. This provides a crude estimate to demonstrate seasonal differences but more accurate calculations can be made with other information.

Only two fracture types were studied. Wrist fractures are the most common adult fracture⁶ and hip fractures represent the largest economic burden with high morbidity and mortality.³ The differences in the number of fractures treated between seasons may be offset by other injuries such as paediatric fractures, which more commonly occur in spring or summer.^21–24 However, paediatric fractures usually require simple interventions such as manipulations without internal fixation and a short length of stay.

Conclusions

This study demonstrates that fragility fracture of the distal radius is associated with lower temperature. Falls prevention strategies with particular reference to behaviour in cold weather should be directed towards individuals at risk. Emergency departments and trauma units must be aware that more patients will present at times of low temperature, and plans should be put in place to deal with the additional workload. Hip fracture incidence in older women does not appear to be related to temperature. Hip fracture is most common in this group so the need for specific hip fracture services is likely to remain relatively constant all year.