Elsevier

Injury

Volume 45, Issue 11, November 2014, Pages 1764-1771
Injury

Potential pathogenic mechanism for stress fractures of the bowed femoral shaft in the elderly: Mechanical analysis by the CT-based finite element method

https://doi.org/10.1016/j.injury.2014.08.037Get rights and content

Abstract

Introduction

Stress fractures of the bowed femoral shaft (SBFs) may be one of the causes of atypical femoral fractures (AFFs). The CT-based finite element method (CT/FEM) can be used to structurally evaluate bone morphology and bone density based on patient DICOM data, thereby quantitatively and macroscopically assessing bone strength. Here, we clarify the pathogenic mechanism of SBFs and demonstrate this new understanding of AFFs through mechanical analysis by CT/FEM.

Patients and methods

A prospective clinical study was performed from April 2012 to February 2014. We assembled two study groups, the bowed AFF group (n = 4 patients; mean age, 78.0 years) including those with a prior history of AFF associated with bowing deformity and the thigh pain group (n = 14 patients; mean age, 78.6 years) comprising outpatients with complaints of thigh pain and tenderness. Stress concentration in the femoral shaft was analysed by CT/FEM, and the visual findings and extracted data were assessed to determine the maximum principal stress (MPS) and tensile stress–strength ratio (TSSR). In addition, we assessed femoral bowing, bone density, and bone metabolic markers. Wilcoxon's rank sum test was used for statistical analysis.

Results

All patients in the bowed AFF group showed a marked concentration of diffuse stress on the anterolateral surface. Thirteen patients in the thigh pain group had no significant findings. However, the remaining 1 patient had a finding similar to that observed in the bowed AFF group, with radiographic evidence of bowing deformity and a focally thickened lateral cortex. Patients were reclassified as having SBF (n = 5) or non-SBF (n = 13). Statistical analysis revealed significant differences in MPS (p = 0.0031), TSSR (p = 0.0022), and femoral bowing (lateral, p = 0.0015; anterior, p = 0.0022) between the SBF and non-SBF groups, with no significant differences in bone density or bone metabolic markers.

Conclusions

Significant tensile stress due to bowing deformity can induce AFFs. SBFs should be considered a novel subtype of AFF, and patients with complaints of thigh pain and femoral shaft bowing deformity must be considered at high risk for AFFs.

This project (Ref: AOTAP 13-13) was supported by AOTrauma Asia Pacific.

Introduction

Severely suppressed bone turnover after prolonged bisphosphonates (BPs) therapy has been considered a cause of low-energy diaphyseal femoral fractures, commonly called atypical femoral fractures (AFFs) [1], [2], [3], [4]. We previously studied stress fractures of the bowed femoral shaft (SBFs) among elderly Japanese for over a decade. Stress fractures of the femoral shaft are well recognised as fatigue fractures among young athletes and military marchers, whereas SBFs are insufficiency fractures caused by the daily stress load [5], [6]. SBFs have been confused with AFFs caused by severely suppressed bone turnover, especially in Asians. Oh et al. [7] previously reported a case series of SBFs not associated with BPs use and advocated that SBFs should be recognised as one of the causes of AFFs. In most cases, SBFs occur bilaterally and in active elderly women. Some reports on stress fractures of the femoral shaft after total knee arthroplasty have also described involvement of femoral shaft bowing deformity and bilaterality [8], [9].

On the other hand, the case definition of AFFs was revised by the American Society for Bone and Mineral Research (ASBMR) Task Force 2013, and specific diseases and exposure to certain drugs were removed from the minor features [10]. Though lower limb geometry and Asian ethnicity are clearly stated as risk factors of AFFs, the mechanical basis for AFFs remains poorly understood [10], [11]. Elderly women with femoral shaft bowing deformity and complaints of thigh pain show a diffuse uptake pattern in the lateral femoral cortex on bone scintigraphy, suggesting a stress concentration in the femoral shaft, which is a pathogenic factor of SBFs [7].

The CT-based finite element method (CT/FEM) can be used to structurally evaluate bone morphology and bone density based on patient DICOM data, thereby quantitatively and macroscopically assessing bone strength. Here, we report a prospective clinical study designed to clarify the pathogenic mechanism of SBFs and to demonstrate this new understanding of AFFs through mechanical analysis by CT/FEM.

This study was approved by the institutional review board of the first author's institution. Consent was obtained from all patients, complying with the principles laid down in the Declaration of Helsinki. Our reporting of patients are completely anonymous, protecting their privacy and dignity.

Section snippets

Patients and methods

A prospective clinical study was performed in a rural hospital in Japan from April 2012 to February 2014. We assembled the following two study groups, the bowed AFF group (n = 4 patients; mean age, 78.0 years), which include those with a prior history of AFF associated with femoral shaft bowing deformity, and the thigh pain group (n = 14 patients; mean age, 78.6 years), which include outpatient subjects with complaints of thigh pain and tenderness. The case definition of AFF was based on the

Results

On the CT/FEM colour charts, all 4 patients in the bowed AFF group showed a marked diffuse stress concentration on the anterolateral surface throughout the length of the femoral shaft (Fig. 2). Of the 14 patients in the thigh pain group, 13 showed no significant visual findings (Fig. 3). However, the remaining 1 patient (case 13) in the thigh pain group had a visual finding similar to that of the bowed AFF group, with radiographic evidence of an apparent bowing deformity and a focally thickened

Discussion

AFFs have already been reported as a multifactorial disease since osteoporosis, stress fractures, comorbid conditions, and pharmaceutical agents are known to be involved in their pathogenesis [10], [14]. In addition, the fracture pattern in AFFs is similar to the typical development seen in stress fractures [10]. The lateral cortex of the femur is known to sustain high levels of tensile stress due to bending. Koh et al. [15] emphasised that periosteal reactions and cortical stress lesions occur

Conclusions

Mechanical analysis by CT/FEM demonstrated that significant tensile stress caused by bowing deformity can induce AFFs in the femoral shaft. SBFs should thus be considered a novel subtype of AFF. Patients with complaints of thigh pain and femoral shaft bowing deformity must be considered at high risk for AFFs.

Funding

This project (Ref: AOTAP 13-13) was supported by an unrestricted grant funding from AOTrauma Asia Pacific.

Ethics

This study was approved by the institutional review board of the first author's institution. We obtained consent from all patients, complying with the principles laid down in the Declaration of Helsinki. Our reporting of patients is completely anonymous, protecting the privacy and dignity of these patients.

Conflict of interest

The authors declare no conflicts of interest.

Acknowledgements

We would like to express our deepest gratitude to Dr. Sawaguchi (AOTrauma Japan chairperson) and Dr. Shindo (AOTrauma Japan vice-chairperson) for providing support and encouragement in the present study.

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