Potential pathogenic mechanism for stress fractures of the bowed femoral shaft in the elderly: Mechanical analysis by the CT-based finite element method
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.
References (25)
- et al.
Stress fracture of the bowed femoral shaft is another cause of atypical femoral fracture in elderly Japanese: a case series
J Orthop Sci
(2014) - et al.
Unilateral stress fracture of the femoral shaft combined with contralateral insufficiency fracture of the femoral shaft after bilateral total knee arthroplasty
J Orthop Sci
(2008) - et al.
Bilateral stress fracture of the femoral shaft after total knee arthroplasty
Knee
(2011) - et al.
A comparison of coronal plane axial femoral relationships in Asian patients with varus osteoarthritic knees and healthy knees
J Arthroplasty
(2009) - et al.
An emerging pattern of subtrochanteric stress fractures: a long-term complication of alendronate therapy?
Injury
(2008) - et al.
Evolution of the Koch model of the biomechanics of the hip: clinical perspective
J Orthop Sci
(2002) - et al.
Severely suppressed bone turnover: a potential complication of alendronate therapy
J Clin Endocrinol Metab
(2005) - et al.
Subtrochanteric insufficiency fractures in patients on alendronate therapy: a caution
J Bone Joint Surg Br
(2007) - et al.
Atypical fractures of the femoral diaphysis in postmenopausal women taking alendronate
N Engl J Med
(2008) - et al.
American Society for Bone and Mineral Research. Atypical subtrochanteric and diaphyseal femoral fractures: report of a task force of the American Society for Bone and Mineral Research
J Bone Miner Res
(2010)
Fatigue, insufficiency, and pathologic fractures
JAMA
Stress fractures of the femur
Cited by (59)
Three-dimensional morphologic features of Asian atypical femur and clinical implications of cephalomedullary nail fixation: Computational measurement at actual size
2022, InjuryCitation Excerpt :Based on clinical experience, authors of the present study have found that detailed 3D understandings of diaphyseal bowing are needed by using an atypical femur model at actual size and that their components and impact should be clearly identified. Concerning 3D measurement of diaphyseal bowing, a few authors [14,17] have conducted 3D modeling studies using a specialized software. However, to the best of our knowledge, clinical CT-based 3D modeling of Asian atypical femur divided into cortex and medullary canal at actual size has not been reported.
Biomechanical mechanisms of atypical femoral fracture
2021, Journal of the Mechanical Behavior of Biomedical MaterialsReduced cortical bone thickness increases stress and strain in the female femoral diaphysis analyzed by a CT-based finite element method: Implications for the anatomical background of fatigue fracture of the femur
2020, Bone ReportsCitation Excerpt :Keaveny previously reported that femoral strength calculated using a finite element analysis decreased in Caucasian women and men with age, and an additional increase was noted in the prevalence of low femoral strength in women than in men entering the seventh decade of life (Keaveny et al., 2010). This method also revealed that femoral geometries, such as the neck-shaft angle and femoral bowing, were related to tensile stress and strain and may be a risk marker for AFF (Oh et al., 2014b; Oh et al., 2017; Haider et al., 2018). Therefore, to investigate whether the risk of fracture may be predicted based on morphological parameters, the distribution of biomechanical parameters in the femoral diaphysis and their relationships with cortical bone thickness, cross-sectional area, and the cortical index were examined using a CT-based finite element method in the single-leg stance configuration.