Resumption of School After Lockdown in Covid-19 Epidemic: Occult Fracture on the Same Site of the Same Grade Adolescents


 Background：As the capital city of Hunan Province next to Hubei Province very closely, Changsha started Level 1 response of major public health emergency on January 23. After 75 days of lockdown due to Covid-19 epidemic, students in the 9th grade of junior high school started hard training for physical entrance examination. Case Presentation：We report three cases of occult fracture on the same site in adolescents of the same grade since resumption of school after lockdown in covid-19 epidemic. Three students in the 9th grade of junior high school who were facing the physical examination in two weeks are diagnosed the occult fracture of the distal femur. Conclusion：It is recommended that the students, parents, education providers and policy makers should all pay attention to the physical exercise of students that should be from a small amount to a medium amount and then to examination training. The sudden increase in the amount of exercise may cause hidden fracture of young people.


Introduction
Occult fracture usually refers to the fracture that can not be shown on X-ray, but can be con rmed by MRI or CT [1]. Occult fracture includes complete fracture and incomplete fracture. In clinical practice, occult complete fracture often leads to fracture displacement, while occult incomplete fracture rarely occurs fracture displacement, which is mainly manifested as local pain and limited function [2]. In this study, 3 cases of adolescent occult fracture were reported as follows: Case Report Case 1, male, 14 years old, was admitted in the outpatient department because of "pain of the right knee joint with limited activity for 3 days after 1000 meter long-distance running training". The patient endures the pain and nished the long-distance running. Physical examination: height 167 cm, weight 53 kg, BMI index: 19.0, slight swelling of the right knee joint, local tenderness at the posterior and lateral side of distal femur, slight limitation of the right knee joint activity, range of activity in the extension of 10 Treatment: the right lower limb was xed with plaster and free of weight bearing. At the same time, high calcium and protein diet was given. At 8 weeks, X-ray shows good fracture healing ( Fig. 1 e,f) and the patient was admitted to running training. Case 3, female, 13 years old, was admitted in the outpatient department because of "pain on the right knee joint after 1000 meter running training for one day". The patient continued training and come to hospital one day later. Physical examination: height 164 cm, weight 49 kg, BMI index: 18.2, slight swelling of the right knee joint, local tenderness in the distal part of the right femur, unrestricted movement of the left knee joint, the range of motion was from 0 degree to 120 degree, oating patella test (-),McMurray test (-), drawer test (-). Laboratory examination: blood regular test (-), CRP (-), ESR (-), serum calcium (-), 1,25-(OH)2-VitD3 test (-), rheumatism factors test (-), imaging examination: right knee joint positive and lateral radiograph(-) (Fig. 3 a,b), BMD (-), MRI of right knee joint showed incomplete fracture of right distal femur (Fig. 3 c,d). Diagnosis: occult incomplete fracture of the distal right femur. Treatment: the right lower limb was xed with plaster and the right lower limb was free of weight bearing. At the same time, high calcium and protein diet was given. In 8 weeks, X-ray shows good fracture healing (Fig. 3 e,f) and the patient was admitted to running training.

Discussion
China plays an important role in prevention and control of epidemic of Covid-19, and China is the rst country to resume school after long term lockdown in covid-19. Due to the P.E entrance test to senior high school, the sudden increase in the amount of exercise brings the increase of weight-bearing and muscle contraction of the lower extremities, the strength exerted on the lower extremities was also signi cantly increased, and the risk of fracture was greatly increased.
The three cases we reported are students in 9th grade of junior high school. In this year, they are about to take entrance physical examination test for senior high school in one month after returning to school. Therefore, after the beginning of school, they immediately carry out the long-distance or short-distance running training. The amount of exercise suddenly increases, and the pain symptoms comes out in the process of exercise, which indicates that excessive exercise is the cause of their fracture. In the subsequent X-ray, MRI, BMI, serum calcium, 1,25-(OH)2-VitD3, rheumatism factors and other laboratory tests, the diagnosis of pathological fracture was excluded. It is recommended that the diagnosis of fracture should be considered when there is a history of sudden increase in the amount of exercise, the pain located at the same area in different physical examination and con rmed by further imaging examination.
The main pathological changes of occult fracture were trabecular fracture, no cortical disruption, and local intra cancellous edema and hemorrhage [3,4]. There is no cortical interruption, which is not enough to cause the obvious change of X-ray attenuation coe cient, so the X-ray plain lm has no abnormal signs [5]. On MRI, local hyperemia of bone marrow and transitional perfusion of capillary bed lead to edema of bone marrow. Free water increases during edema. A series of changes in local chemical composition of bone tissue lead to changes in MRI relaxation time and abnormal signal changes [6][7][8]. Therefore, for the three children in this study, no obvious fracture line and surrounding soft tissue hematoma were found in the rst X-ray examination. However, the local pain of the child has not been improved signi cantly after rest. In combination with the patient's history of signi cantly increased activity in recent period, it should be highly suspected whether there is hidden fracture at this time. It is recommended to perform MRI examination. If the MRI examination cannot be performed temporarily, it is recommended to give the fracture xation according in the pain area rst, and then deal with it after the MRI examination is completed or review the X-ray after 3-4 weeks to see if there is callus formation, which helps diagnose.
The bone tissue of adolescents is in the stage of high-speed growth, so the metaphyseal bone is relatively weak, and it is easy to be fracture under external force [9,10]. In this study, 2 of the 3 cases were male and case 1 was female, aged 13-14years, which matches the principles.
Occult fracture is often incomplete fracture, so the probability of displacement is relatively small. Under the protection of plaster and other external xation, it can generally get a relatively smooth bone healing [11][12][13]. In our three cases, callus formation can be seen in the X-ray lm after 8 weeks follow up. The function of the patients restored.

Conclusion
Adolescents were isolated at home for a long time due to the epidemic situation. After the school starts, the students, parents, education providers and policy makers should all pay attention to the physical exercise of students that should be from a small amount to a medium amount and then to examination training. The sudden increase in the amount of exercise may cause hidden fracture of young people.
For the children who have been isolated at home for a long time during the epidemic, the amount of exercise suddenly increases after returning to school, the local pain has not been improved after rest, and the X-ray has shown no abnormal change, the occult fracture should be considered, and further examination such as MRI should be performed for diagnosis. Consent for publication: All authors approved the nal manuscript as submitted and agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.
Availability of supporting data All data are fully available without restriction.
Competing interests: The authors declare that they have no competing nancial interests.
Funding: Natural science foundation of Hunan province China (2019JJ50324).

Authors' contributions:
Jie Wen and Jianzhou Wang share co-rst author. Jie Wen, Jianzhou Wang and Djandan Tadum Arthur v collected data, carried out the initial analyses, Feng Xiang and Zhongwen Tang reviewed and revised the manuscript, Hong Liu and Sheng Xiao conceptualized and designed the study, drafted the initial manuscript, and reviewed and revised the manuscript.