Subchondral fatigue fracture of the femoral head (SFFFH) mainly occurs in young military recruits and might be confused with osteonecrosis of the femoral head. However, less research focuses on the risk factor for SFFFH.

To evaluate the intrinsic risk factors for SFFFH in young military recruits.

X-ray and magnetic resonance imaging data were used for analysis. Acetabular anteversion of the superior acetabulum, acetabular anteversion of the center of the acetabulum (AVcen), anterior acetabular sector angle (AASA), posterior acetabular sector angle, superior acetabular sector angle, neck-shaft angle (NSA), inferior iliac angle (IIA), and ischiopubic angle were calculated. Then, logistic regression, receiver operating characteristic curve analysis, and independent samples t-test were performed to identify the risk factors for SFFFH.

Based on the results of logistic regression, age [odds ratio (OR): 1.33; 95% confidence interval (95%CI): 1.12-1.65; P = 0.0031] and treatment timing (OR: 0.86; 95%CI: 0.75-0.96; P = 0.015) could be considered as the indicators for SFFFH. AVcen (P = 0.0334), AASA (P = 0.0002), NSA (P = 0.0007), and IIA (P = 0.0316) were considered to have statistical significance. Further, AVcen (OR: 1.41; 95%CI: 1.04-1.95) and AASA (OR: 1.44; 95%CI: 1.21-1.77), especially AASA (area under curve: 66.6%), should be paid much more attention due to the higher OR than other indicators.

We have for the first time unveiled that AASA and age could be key risk factors for SFFFH, which further verifies that deficient anterior coverage of the acetabulum might be the main cause of SFFFH.

Subchondral fatigue fracture of the femoral head (SFFFH) is rare but occurs in people with vigorous exercise including soldiers and athletes[1,2]. Apart from subchondral insufficiency fracture of the femoral head (SIFFH) secondary to osteoporosis, SFFFH occurs in healthy adults with normal bone quality after repeated abnormal stress or sudden increases in activity[2-4]. SFFFH is characterized by hip pain, limited range of motion, and collapse of the femoral head, similar to the manifestation of osteonecrosis of the femoral head (ONFH)[2,5,6]. At the onset of hip pain, magnetic resonance imaging (MRI) but not X-ray should be performed to detect the lesion[7]. However, there is little research on risk factors related to the occurrence of SFFFH.

Some studies indicated that intrinsic factors are related to SFFFH. Kim DK and Kim TH[8] indicated that the femoral neck-shaft angle (NSA) is associated with the location of the femoral stress fracture after retrospectively studying 37 patients with femoral stress fractures. Iwasaki et al[9] found that both the acetabular head index and band length ratio are important factors in the progression of SIFFH after reviewing 29 patients with this condition. According to the previous observation, we believed that some hip geometry indicators play crucial roles in the development of SFFFH.

In this study, we reviewed 122 patients from the Chinese People's Liberation Army who complained of hip pain to find some characteristics of SFFFH. Furthermore, we aimed to discover some new risk factors for SFFFH by comparing SFFFH with ONFH patients.

SIFFH was considered a kind of pathological fracture due to poor bone quality, which is a disease predominantly affecting elderly patients. Currently, labral tears[12], acetabular over-coverage[13,14], and deficient anterior coverage of the acetabulum[15,16] were considered the risk factors for SIFFH. Uchida et al[12] observed that the acetabular labrum of all nine SIFFH patients were torn via hip arthroscopy. Acetabular over-coverage is one of the manifestations of pincer impingement[17] and usually thought to have high morbidity of femoral acetabular impingement in youth[18]. However, Kimura et al[13] 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 few studies reported that hip geometry could also serve as an intrinsic factor related to fatigue-type fracture in young people, especially in young military recruits.

In 2021, Kim DK and Kim TH[8] first found that NSA is a risk factor associated with SFFFH. The NSA of SFFFH was higher than that 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[19] confirmed that acetabular retroversion is associated with an increased risk of sustaining an FNSF. Franken et al[20] also found that impingement-associated deformities of the hip may cause FNSF. Tokyay et al[21] pointed out that acetabular morphology can predict the types of proximal femoral fractures among elderly patients. NSA of trochanteric fracture was found to be less than that of the femoral neck fracture, but there was no statistically significant difference between them. In our study, the NSA of SFFFH was significantly higher than that of ONFH. However, we cannot identify the clinical significance from the results of logistic regression. Above all, we believe that NSA might be related to SFFFH but NSA cannot cause SFFFH directly.

The acetabular morphology is believed to provide mechanical stability to the hip joint. The morphology of the acetabulum distinctly differs from skeletal maturity. Albers et al[22] found distinct differences of the 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[23] found that there were no substantial differences in the 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[24] reported decreased anteversion in skeletally immature compared with skeletally mature subjects. However, it should be noted that the data of both studies were derived from computed tomography (CT). CT cannot measure the true dimensions of the acetabulum, while MRI is 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 patients was significantly younger than that of ONFH ones. We identified that age was a risk factor for SFFFH based on logistic regression. Younger people are more susceptible to SFFFH. Besides, patients with SFFFH tend to ignore the disease, leading to delayed treatment. Therefore, encouraging high-risk groups to promptly seek medical attention upon the detection of symptoms has been shown to be conducive to a shortened duration of the disease.

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[25]. In our study, none of the patients with SFFFH or ONFH was found to have a posterior wall defect. Although both the lateral coverage and posterior wall were normal, the AASA of SFFFH patients was smaller than the AASA of ONFH patients, which verified that deficient anterior coverage of the acetabulum should be a risk factor for SFFFH. Deficient anterior coverage of the acetabulum causes SFFFH possibly due to the abnormal stress distributions on the articular weight-bearing area[26]. Hip dysplasia is characterized by shallow acetabulum leading to increased mechanical load on the femoral head and labrum[27]. Henak et al[26] indicated that the labrum under poor coverage supported 2%-10% of the applied load more than the normal labrum by performing subject-specific finite element modeling. 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 is the most essential imaging modality 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 MRI findings. First, 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[28,29]. Yu et al[30] found that there was a negative correlation between the 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. Second, cystic lesions, often close to RI in viable areas of the femoral head, could be a common pathologic feature in ONFH but not in SFFFH[31]. Gao et al[32] also observed that the predilection locations of cystic lesions 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 osteoarthritis-like mechanism. Third, the femoral head of SFFFH patients collapses 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 explain the observation that the collapse ratio of the femoral head of SFFFH (69.4%) was higher than that of ONFH (25%) in our study. Fourth, SFFFH, unlike ONFH, usually involves the unilateral femoral head.

For those SFFFH patients with pre-collapse lesions, conservative treatment including non-weight bearing with crutches and non-steroidal anti-inflammatory agents should be performed[33]. Patients with collapse lesions can be divided into two groups: Non-progression of collapse and progressive collapse with a 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 progressed to collapse following the loss of the articular bony margin or arthritic change, their prognosis seemed to be the radiological findings in rapidly destructive arthrosis of the hip joint (RDA)[7,35]. Kubo et al[36] found that tartrate-resistant acid phosphatase 5b, a specific bone resorption marker associated with osteoclast cells, reflects the condition of progressive collapse in SIFFH as well as RDA. Due to high activity levels in SFFFH patients, hip preservation techniques, such as transtrochanteric anterior rotational osteotomy, have been recommended by many previous studies[37,38].

Taken together, we have, for the first time, identified AASA as a risk factor for SFFFH, which means that deficient anterior coverage of the acetabulum could induce SFFFH directly. Besides, young people, especially those with a history of intense training, are at higher risk for SFFFH. We advocate for the dissemination of scientific knowledge amongst high-risk populations in order to promote timely intervention in cases of SFFFH. However, we only compared the indicators of SFFFH and ONFH, and healthy people need to be included to solidify the results.