SIFFH was considered one kind of pathological fracture due to poor bone quality, which is a disease predominantly affecting elderly patients. Currently, labral tears [10–12], acetabular over-coverage [13–15], and deficient anterior coverage of the acetabulum [16, 17] were considered the risk factors of SIFFH. Uchida et al. observed that the acetabular labral of all 9 SIFFH patients were torn via hip arthroscopy [11]. Acetabular over-coverage is one of the manifestations of pincer impingement [18] and usually be thought to have high morbidity of FAI in youth [19]. However, Kimura et al. first reported a case of a 53-year-old woman with SIFFH due to acetabular over-coverage. The patient’s bone mineral density was relatively low (T-score: −1.3) [13]. The above previous studies were all based on insufficiency-type fractures, but seldom researches reported that hip geometry could also serve as an intrinsic factor related to fatigue-type one in young, especially in young military recruits.
In 2021, Kim et al. first found that NSA is the risk factor associated with SFFFH [7]. The NSA of SFFFH was higher than the NSA of the femoral neck stress fracture (FNSF), which means that a higher NSA might increase the morbidity and severity of SFFFH. On the other hand, Previous data by Kuhn et al. [20] confirmed that acetabular retroversion is associated with an increased risk of sustaining an FNSF. Franken et al. [21] also found that impingement-associated deformities of the hip may cause femoral neck stress fractures. Tokyay et al. [22] pointed out that acetabular morphology can predict the types of proximal femoral fractures among elderly patients. NSA of trochanteric fracture was found to less than the femoral neck fracture, but there was no statistically significant difference. In our study, the NSA of SFFFH was significantly higher than the NSA of ONFH. However, we can’t identify the clinical significance from the results of logistics regression. Above all, we believed that NSA might be related to SFFFH but NSA can’t cause SFFFH directly.
The acetabular morphology is believed to provide mechanical stability to the hip joint. The morphology of the acetabulum distinct differences to skeletal maturity. Albers et al. [23] found distinct differences of acetabular version related to skeletal maturity. Version, assessed by MRI, was decreased in hips with open triradiate cartilage complex and increased during skeletal maturation. By contrast, Monazzam et al. [24] found that there were no substantial differences of acetabular version in children younger than 11 years, but a progressive increase of anteversion was noted in patients older than 12 years of age. Hingsammer et al. [25] reported decreased anteversion in skeletally immature compared with skeletally mature subjects. However, it should be noted that data of both studies were derived from CT. CT measurements cannot measure the true dimensions of the acetabulum because MRI was suitable for accurately measuring the margins of the acetabular wall in both skeletally mature and immature subjects. In our study, clinical data from the new recruits were analyzed by MRI and the age of SFFFH was significantly younger than that of ONFH. Of the 98 patients of SFFFH, 3 patients age 17 years in skeletally immature subjects. However, we still can’t identify the significant differences in terms of age from the results of logistics regression.
We classified the acetabular morphology with a focus on acetabular coverage parameters such as the SASA, AASA, and PASA. None of the patients with SFFFH or ONFH had severe acetabular dysplasia (SASA < 110°). Therefore, the lateral coverage was normal in patients with SFFFH and ONFH. In addition, a PASA of less than 86° is associated with sports-related posterior hip dislocation [26]. In our study, none of the patients with SFFFH or ONFH was defined as a posterior wall defect. Although both the lateral coverage and posterior wall were normal, the AASA of SFFFH was smaller than the AASA of ONFH, which verified deficient anterior coverage of the acetabulum should be the risk factor for SFFFH. Deficient anterior coverage of the acetabulum causes SFFFH might attribute to the abnormal stress distributions on the articular weight-bearing area [27]. Hip dysplasia is characterized by shallow acetabulum leading to increased mechanical load on the femoral head and labrum [28]. Henak et al. indicated that the labrum under poor coverage supported 2–10% of the applied load more than normal labrum by performing subject-specific finite element modeling [27]. However, there is no further study about how poor coverage induces SFFFH. Combining activities beyond the physiologic tolerance level with abnormal mechanical properties of the hip joint, SFFFH might be triggered easily. In the future, more research should focus on the mechanism of deficient anterior coverage causing SFFFH.
MRI was the most essential imaging material for both patients with SFFFH and ONFH, so the comprehensiveness and accuracy of the retrospective study were guaranteed. We found some evidence about distinguishing SFFFH and ONFH mainly based on the MRI. Firstly, reactive interface (RI), surrounding the necrotic area, could be mainly seen in ONFH but not in SFFFH. RI, also named sclerosis rim in X-ray or CT, had a protective effect on femoral head collapse[29, 30]. Yu et al. found that there was a negative correlation between collapse rate and the proportion of proximal RI. When the proportion is > 30%, the collapse risk is low, whereas at < 30%, the risk is high[31]. Secondly, cystic lesion, often close to RI in viable areas of the femoral head, could be a common pathologic feature in ONFH but not in SFFFH. Gao et al. also observed that the predilection locations of cystic lesion were mainly close to RI in ONFH and the main factor inducing the formation of cystic lesions may be the peak stress at RI edge via an OA-like mechanism [32]. Thirdly, the SFFFH patients’ femoral head collapsed rapidly due to a large amount of joint fluid flowing into the cancellous bone, causing osteocyte death once the cartilage is destroyed. The above three points could be explained that collapse ratio of femoral head of SFFFH (69.4%) higher than that of ONFH (25%) in our study. Fourthly, SFFFH usually involves the unilateral femoral head, unlike ONFH.
For those SFFFH patients with the pre-collapse lesion, conservative treatment including non-weight bearing with crutches and non-steroidal anti-inflammatory agents should be performed [33]. The patients with collapse lesion can be divided into two groups: one is a non-progression of collapse; the other is a progressive collapse with the diversity of etiologies. It is difficult to highlight reasons for collapse of the femoral head on imaging in some patients. Besides, variation of recovery or worsening could be found even among similar etiological cohorts[34]. Once the femoral head progress to collapse following loss of the articular bony margin or arthritic change, their prognosis seemed to the radiological findings in rapidly destructive arthrosis of the hip joint (RDA) [6, 35, 36]. Kubo et al. found Tartrate-resistant acid phosphatase 5-b (TRACP-5b), a specific bone resorption marker associated with osteoclast cells, reflect the condition of progressive collapse in SIFFH as well as RDA[37]. Due to high activity level in SFFFH patients, hip preservation techniques, such as TARO, have been recommended by many previous studies[38–40] .