Distribution Characteristics of New AO Classification in Different Age Groups of Patients with Femoral Neck Fracture


 Objective: Femoral neck fractures(FNFs) patients from 2018 to 2020 were classified according to the new AO classification. To observe the proportion of new AO classification and provide reference for improving the upper age limit of internal fixation(IF) treatmentMethods: In this study, patients with femoral neck fracture hospitalized in Xi 'an Honghui Hospital from January 2018 to January 2021 were divided into four groups according to age: the young group (≤44 years old), the middle-aged group (45-59 years old),the young elderly group (60-74 years old), and the elderly group (≥75 years old) to analyze age,sex,side,injury mechanism,new AO classification and other features . A total of 2071 patients were included for analysis.Results: There were 742 males (35.8%) and 1,329 females (64.2%).There were 1,106 patient s(53.4%) on the left side and 965 on the right side (46.6%).There were 1781 patients of low energy injury (86%) and 290 patients of high energy injury (14%), and the main injury mechanism was falling.The number of patients with femoral neck fracture in each year was 719 patients in 2018, 694 patients in 2019, and 661 patients in 2020, respectively. The number of patients showed a decreasing trend year by year, and the gender ratio and injury mechanism showed no significant trend of change. In the age group, the proportion of the young group was increasing year by year.Among the new AO classification, there were 1023 patients of B1.3, accounting for the highest proportion of 49.4%.The proportion of B2.2 was the highest in the young group (32.4%) .B1.3 and B2.2 were the main types in the middle-aged group, accounting for 31.7% and 32.0, respectively.B1.3 was dominant in the young elderly and elderly group (47.1% and 63.4%, respectively).Conclusion: The new AO classification is more detailed and practical. 1/3 of the fracture types of middle aged and young elderly people may achieve satisfactory results through IF.Level of evidence: III.


Introduction
FNF is a common clinical proximal femoral fracture with high morbidity and mortality, especially in the elderly with osteoporosis (1).Three types of classi cation are commonly used in hospital: the Pauwels class cation according to the Angle of the distal fracture line, the Garden classi cation according to displacement type of the fracture and the anatomical classi cation according to the location of the fracture line. The AO classi cation has been rarely used due to its complexity and low consistency among doctors (2,3).
In 2018, AO proposed a new classi cation of FNFs (4), which combined the three commonly used classi cation.The new AO classi cation is easy to remember and distinguish fracture types more comprehensively.
So that doctors have a better classi cation method for FNFs. It will be of great help to guide clinical treatment.However, the treatment methods for FNF does not depend solely on fracture classi cation. Age is an important factor in this fracture to select treatment methods (5). Therefore, the purpose of this study was to explore the distribution characteristics of new AO types in different age groups.
In this study, all imaging data were classi ed by 3 attending orthopedic surgeons and 2 radiologists.Before the start, the doctors participating in the lm reading should be trained to ensure that they can master the classi cation standards. If a data has two or more results, the chief physician and 5 physicians discuss together to determine the nal classi cation.

Number of patients
A total of 2,071 patients were registered.The mean age of the patients was 68.31±15.85 years old (range, 16-99 years old). There were 742 males (35.8%) and 1329 females (64.2%), and the ratio of male to female was 1:1.79. The proportion of female patients was signi cantly higher than that of male patients. By comparing the number of patients in different age groups, the elderly group had the highest proportion (43.5%), and the young group had the lowest proportion (8.9%).
By comparing the sex ratio of male and female in different age groups, the proportion of male in the young group was much higher than female, while the proportion of female patients was higher than male in other age groups, among which the proportion of female patients in the elderly group was the largest (70.6%). Among the fracture sides, there were 1106 cases on the left side and 965 cases on the right side, and the left side accounted for more (53.4%). In all age groups, the left side was more than the right side. According to the statistics of medical records, the injury mechanism were divided into low-energy injury (fall, sports injury, ght) and high-energy injury (tra c accident, fall from high, heavy pound injury, machine injury). Among them, there were 1,781 cases of low-energy injury and 290 cases of high-energy injury. The most common mechanism of injury was falling (1764 cases). The proportion of low-energy injury was higher than that of high-energy injury in all age groups, and the proportion of low-energy injury gradually increased with the increase of age ( Table 2). Age classi cation standard(WHO,2020 ): the young group (< 44 years old), the middle-aged group (45-59 years old), the young old group (60-74 years old), an old). 3.2 Incidence Trend gender ratio and injury mechanism showed no obvious trend of change. In age groups, the proportion of the young group increased year by year, accounting for 8.66% in 2018, 8.93% in 2019 and 9.23% in 2020, the aging trend of this fracture may be alleviated (Figure 1).

Discussion
In the past application of the classi cation of FNF, the position of fracture line is not distinguished, and the classi cation of FNF is often confused.
Studies on the classi cation consistency of FNFs in recent years indicate that the previous AO classi cation, Pauwels classi cation and Garden classi cation have interobserver variation, and the AO classi cation is the most obvious(2, 3, 6).This is mainly because the original AO classi cation was based on X-ray images and was self-concluded based on patient imaging data.The rules for fracture typing are not used frequently and are not well understood by clinicians.However, the new AO classi cation cleverly combines the three types of FNF classi cation commonly used by clinicians, so that the widely used classi cation has been standardized and improved, which will bring great help to the clinical classi cation of FNF.
In this class cation, 31B1, 31B2, 31B3 corresponds to subcapital ,transcervical and basicervical. 31B1 is divided into three subgroups, B1. The proportion of fracture types varied among age groups, B2.2 (60,32.4%) was predominant in the young group, B2.3 fractures with high shear angle had the largest proportion in the young group, 18 (9.7%). Young patients generally have better bone quality.High-energy mechanism can cause a FNF pattern that is vertically oriented with a shear component making it biomechanically more unstable. Most cause transcervical fractures. Young patients are the same with elderly people,big shear angle of the fracture non-union, implant failure,osteonecrosis rate is higher than other type in B2. But because of the active patients following high-speed trauma when a head-sparing technique is required, almost all young patients choose ORIF(9-11).
As  (12,13).After multiple considerations,we can devise a more rational protocol for these patients.
In previous studies on Garden classi cation of femoral neck fracture, the existence of Garden TYPE I has been controversial (14,15).In this study, a total of 6 incomplete fractures based on radiographic evaluation. By observing its CT scan, only 1 patient looks like incomplete fracture and the patient was only 22 years old, similar to greendstick fracture.
Femoral neck fractures are affected by a variety of factors and are generally considered to be closely associated with Femoral neck bone mineral density and osteoporosis. In our study, FNFs were more common in women, with an overall male to female ratio of 1:1.79. In the young group, The male/female ratio was 2.19:1. In the middle-aged group, the ratio of male to female was 1:1.18, the male/female ratio was almost equal. In the young old group and the elderly group, the ratio of male to female was seriously unbalanced, which was 1:2.46 and 1:2.24.
Women in both age groups are in postmenopause.The loss of estrogen in the body leads to osteoporosis which leads to more serious bone loss than that of men. Elderly women are more likely to have such fractures(16). However, there are also articles that suggest that femoral neck bone mineral density, physical function were not associated with FNFs (17).
In the study of FNF, the main injury mechanism of the elderly is fall with low energy. And high energy injury is the main cause of femoral neck fracture in young people (18,19).In our study, the main injury mechanism of all age groups is low energy injury dominated by fall, and high energy injury accounts for 42.7% of the young group and only 2.4% of the elderly group.A large sample survey of hip fracture in Japan found that fall was the injury mechanism of 80% of hip fracture patients.The number of patients is the highest in January of every year (19).In Catalonia, Spain and Czech Prague, the incidence rate of femoral neck fracture is related to seasonal changes, and more in winter (20,21).
In recent years, international studies on the incidence trend of femoral neck fracture show that the incidence of the elderly population has a decreasing trend year by year, which may be related to the safety awareness of the elderly and the formation of prevention of osteoporosis (20,22). In this study, there was no obvious trend in the proportion of the elderly over 60 years old, but the proportion of patients in the young group increased, which may mean the improvement of people's health awareness, indicating that the age of patients with femoral neck fracture is getting younger. In some European countries, the COVID-19 has great in uence on the incidence of FNF, compared to the past few years have a certain percentage of the drop.This is related to the decrease in people going out during the pandemic.In the UK, the number of femoral neck fractures decreased from 410 to 327 during the 12-week coronavirus lockdown period, a decrease of 20.2% compared to the same period in the previous year (23).In Italy(ASST Sette Laghi, Varese, Lombardy, Italy), the number of femur neck fractures fell from 54 last year to 46 in the two months of the pandemic emergency, a decrease of 14.8% (24).The Clinical Center of Nis (Nis, Serbia) decreased from 28 to 17, with a decrease rate of 39.3% (25).Xi 'an city of Shaanxi Province is a medium-low risk area. From January 23, 2020 when Wuhan was closed to April 8, 2020 when Wuhan was lifted, a total of 199 patients with femoral neck fracture were admitted to our hospital during the three months from January to April, 190 in the same period in 19 and 192 in the same period in 18. The number of patients with femoral neck fracture was not affected.

Conclusion
The new AO classi cation is easy to remember after combining the Garden Pauwels and anatomical classi cation. This classi cation can distinguish stable and unstable fractures and is more helpful to guide surgical treatment. In this study, stable fractures occupy a certain proportion in middle-aged and young old people.Garden I fracture exists, but it is rare. The proportion of young patients with femoral neck is increasing year by year, which may be related to the increase of private vehicles and the improvement of health awareness among the elderly. During the COVID-19 outbreak, the hospitalization of femoral neck fracture in the middle and low risk areas in China has not been affected.

Limitations And Future Areas Of Research
The limitations of this study due to the short study period analysed it doesn't do a good job of showing age trends and the sample size of the single center analysis was too small to be representative of the entire region.Future studies will improve the age of ORIF in patients with FNFs based on the new AO classi cation, delaying hip replacement and avoiding repeated revision.A multi-center, multi-year big data study was conducted on patients with femoral neck fracture to observe the age-changing trend.