1 Introduction

Hip fractures are common injuries of the elderly¹ and already related to a majority of public health costs caused by fractures in people over 50 years.² Moreover, hip fractures of the elderly are suggested to become a growing public health issue in future because of aging society.³ Especially older people are at higher risk due to an uncertain gait, weakness in legs and often in combination with reduced bone mineral density.⁴ Therefore, patients who sustained a hip fracture in the past are at higher risk (5–10%) of sustaining a contralateral hip fracture within the next five years.⁵ Different screening strategies have been established to prevent hip fractures in elderly.⁶ However, the effectiveness of hip protectors are discussed controversially.⁷ Understanding of biomechanics of hip fractures and parameters that predict fracture probability could improve the effectiveness of such hip protectors. Previous studies suggest that geometry of the proximal femur could play an important role for fracture probability and fracture type.⁸ Especially the caput collum diaphyseal (CCD) angle seems to be an important factor influencing the fracture type in proximal femur fractures⁹ and could be a predisposing factor for fracture symmetry. Aim of this study was to elucidate the influence of CCD angle on the fracture type and symmetry of the proximal femur.

Therefore all patients suffering from a hip near fracture between January 2014 and December 2018 and being treated in a Level 1 Trauma Center in western Germany where analyzed.

2 Material and Methods

2.1

2.1 Data acquisition & measurement

This study includes a retrospective experimental setup. Being approved by the local ethics committee data of patients who underwent surgical treatment of an acute proximal femur fracture in a Level 1 trauma centre (in western Germany) between January 2014 and December 2018 were examined. Exclusion criteria were pathological hip fractures, radiotherapy, chemotherapy, active tuberculosis and metastasis in the femur or pelvic bone. According to the documented icd10 diagnosis, patients were divided into two groups: I) intra capsular femur fracture (femoral neck fractures) and II) extra capsular femur fracture (inter- and subtrochanteric femur fractures). Occurrence of a contralateral fractures, age, gender, weight, height and CCD angle of each group were further investigated. All patients had pre- and postoperative digital x-rays. All digital x-rays were analyzed by one board certified radiologist with seven years of experience in musculoskeletal radiology, who was blinded to the diagnosis. Only x-rays that ensured an adequate quality without rotation error were taken into further account. Exclusion criteria were an overlapping of the greater trochanter and the femoral neck as well as an incomplete pelvis overview or the availability of only one (pre- or postoperative) pelvis overview. In a standardized procedure,¹⁰ CCD angle was determined on the healthy contralateral joint pre- and postoperative using SECTRA IDS7 (Sectra AB, Linköping, Schweden) as illustrated in Fig. 1. To minimize the influence of rotation error the mean was taken into further analysis.

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Fig. 1 Example of standardized measurement of the CCD angle on the contralateral joint.¹⁰

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3 Results

Data of 448 patients, who underwent surgical treatment of a proximal femur fracture between 2014 and 2018, were included. Fig. 2 shows a comparison of age, weight, height, gender and CCD angle between intra capsular and extra capsular femur fractures.

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Fig. 2 Comparison of age, weight, height, gender and CCD angle between group (I) intra capsular femur fractures and group (II) extra capsular femur fractures.

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In group (I) 0 patients had a CCD angle <120° (varus), 214 patients (86%) 120°–140° and 36 patients (14%) angle>140° (valgus). In contrast, in group (II) 21 patients (10%) had a CCD angle <120° (varus), 170 patients (86%) 120°–140° and 7 patients (4%) >140° (valgus, Fig. 2).

3.1

3.1 CCD angle and fracture type

The ANOVA revealed a significant higher mean CCD angle for patients with intra capsular femur fractures (133.9 ± 6.6°) compared to patients who sustained an extra capsular femur fracture (127.6 ± 6.1°, F(1, 446) = 106.5, p = 0.00, Fig. 3).

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Fig. 3 Mean CCD angles of patients with intra capsular and extra capsular femur fractures. Error bars indicate standard deviation of the mean. Two ** indicate p-values<0.01.

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Moreover, the ANOVA revealed a significant higher age (F(1, 446) = 29.7, p = 0.00), a significant lower height F(1, 446) = 30.2, p = 0.00) and a significant lower weight F(1, 451) = 106.5, p = 0.01) for patients with intra capsular femur fracture compared to patients who sustained an extra capsular femur fracture (Fig. 2).

4 Discussion

Hip fractures are a danger to the elderly. Probably 18% of women and 6% of men all over the world are at danger in sustaining a hip fracture once in their life.¹¹ Medical treatment of hip fractures can be subdivided into emergency treatment, surgery, after care and prevention. To minimize the socio-economic costs and the growing burden for the future society, all aspects of these medical treatment should be addressed with special regard for easy approaches of a better trauma prevention.^12–14 In the past, different strategies have been suggested to prevent hip fractures in older people [6] but the effectiveness of most approaches, like hip protectors, is still discussed controversially [7]. Previous studies suggest, that patients who sustained a hip fracture are at higher risk of approximately 5–10% of sustaining a contralateral hip fracture within the next five years.⁵ Our data suggest that in 69% of all cases, the second hip fracture will be of the same fracture type. Thus, understanding of biomechanics of hip fractures and easy assessable parameters that could predict fracture type and probability, could improve the effectiveness of such hip protectors and prevention strategies.

Typical geometrical parameters of the hip that can be easily assessed using preoperative x-rays are the femoral shaft width, femoral neck width, femoral head diameter, femoral neck axis and hip axis length. A meta analysis revealed, that for femoral neck fractures only CCD angle, femoral neck width and hip axis length had a significant influence.^15,16 In this study, we concentrated our efforts on the CCD angle and its significance on fracture location and fracture type. Data analysis revealed a higher CCD angle in patient with intra articular femur fractures compared to patients with extra articular femur fractures (Fig. 2). Our results are in line with previous studies.¹⁷ There is evidence that ethnic differences in bone geometry with special regard to the CCD angle do exist.¹⁸ Besides biomechanical analysis, our data give further insights into biometrical data of Caucasian patients who underwent treatment after a hip fracture (Fig. 2). Our results revealed a significant higher age, a significant lower height and a significant lower weight for patients with extra capsular femur fracture compared to patients who sustained an intra capsular femur fracture. Patients who sustained an intra capsular femur fracture are approximately 5 kg heavier and 5 cm taller compared to patients with an extra capsular femur fracture. These results are in line with the study of Armstrong et al.¹⁹ However, a previous study suggest that, despite lower bone mass, Japanese women are at lower risk suffering an intra capsular femur fracture compared to American women because of a lower CCD angle.²⁰ This underlines the CCD angle as one of the crucial parameters when evaluating and predicting the fracture type probability. Our data show that intra capsular femur fractures are correlated with an indifferent (120–140°) or valgus (>140°) where as extra capsular femur fractures are correlated with an indifferent (120–140°) or varian (<120°) femoral neck configuration. Our results as well as previous studies suggest that a higher CCD angle is correlated with a higher risk of intra capsular femur fractures and a lower CCD angle is correlated with a higher risk of extra capsular femur fracture. A possible explanation could be, that a higher CCD angle is related to a higher compressive force component relative to the bending force component.²¹

Prediction of the fracture type and the fracture probability of the contralateral side could play an important role for the effectiveness of hip protector pads. It is still discussed controversially whether hip protectors offer sufficient biomechanical protection during a fall.²² Taking our results into account, a CCD-angle-adjusted biomechanical design for hip protectors could be a decisive approach. Moreover, the poor acceptance of bulky hip protectors by senior citizens could be reduced by a well-appointed geometric and biomechanical more efficacious design of the hip pads.²³ However follow up studies are needed to test this hypothesis.

Additionally, besides the shown influence of the anatomical configuration of the proximal femur, the patient cohort will also be analyzed on for further patient based influences to the fracture morphology.

5 Conclusion

Our data suggest that the CCD angle can be used as an easy available predictor of extra or intra capsular femur fractures. Elderly, who already had a hip fracture in the past, are at higher risk for the same fracture type on the healthy contralateral side. This fact should be explained during doctor-patient consultation. Moreover, present results may benefit prosthetists in developing new and more efficient hip protectors, which could lead to a higher acceptance and by modifying the impact after falling on the hip reduce the number of patients with intra capsular femoral neck fractures.