CG is a term of histology, which is a special type of granulation tissue reaction. It is the result of the body's cellular response to cholesterol crystal foreign bodies produced by the decomposition of blood and local tissues. CG is often yellowish-brown sludge under naked eye observation. A large number of cholesterol crystals with diamond-shaped fissures could be seen under microscopy. There are foreign body multinucleated giant cells, macrophages, tissue cells and a large number of lymphocytes, fibrin and abundant blood vessels, and there are black particles of hemosiderin in between. Because this blood vessel is easy to rupture, fresh or old bleeding can be seen at the same time. Therefore, CG lesions were characterized by rhomboid or rhombic cholesterol crystals in chronic inflammatory granulation tissue, surrounded by foreign body giant cells, often fresh or old bleeding, accompanied by hemosiderin[17].
Since CG is a very rare disease, the current research reports are mostly case reports. Among them, the most common report of CG was related to petrous apex, so the hypothesis of its pathogenesis was also based on it. There were two main types: the classic obstruction-vacuum hypothesis and the exposed marrow hypothesis[18]. Zheng Wenkui et al. reported a case of CG of the humerus, and tried to give a preliminary description of its possible formation process[15]. Since both the humerus and femur belong to the upper bone in the skeletal structure of the limbs, they have certain similarities in structure and function. Therefore, we guess that the two may also have a certain similarity in the formation process of CG. Therefore, when we try to describe the CG formation process of the femur, refer to his description of the process of the humerus.
Our patient was an elderly woman, so her femoral bone marrow was mainly yellow bone marrow, and the main component of the marrow was fat tissue. On the one hand, the formation of CG could not be separated from the accumulation of cholesterol. The yellow bone marrow in the femur was rich in fat tissue that broken down to produce cholesterol. This undoubtedly provided a prerequisite for the formation of CG. On the other hand, the formation of CG was also closely related to blood, because blood could also provide a sufficient source of cholesterol. Generally speaking, when cholesterol crystals appeared in the femur as a foreign body, the body's immune response was that phagocytes engulfed it. Therefore, under normal circumstances, a small amount of cholesterol crystals were produced and CG was not formed. However, when the rate of production of cholesterol crystals increased and the rate of output exceeded the speed of the body's phagocytosis, this situation would inevitably lead to the accumulation of cholesterol crystals, resulting in a continuous increase in its volume. At this time, the body's response was to unite the phagocytic cells to form a new type of cell, that was, the foreign body giant cell. When an increasing number of these new cells slowly surrounded the cholesterol crystals, their phagocytosis became relatively weak, to some extent. It was worth noting that there was a self-promoting phenomenon in the formation of CG. It was because blood was an important source of cholesterol, but bleeding often occurred in CG. The formation of CG was often accompanied by blood vessel proliferation, and most of these blood vessels were pathological types of blood vessels, which were easy to rupture and bleeding; bleeding leaded to the continuous increase of cholesterol content, while red blood cells continued to degenerate and decompose to produce cholesterol crystals, which stimulated the foreign body, leading to further aggravation of bleeding and inflammation; eventually a hemorrhage-inflammation malignant circulatory system was formed[19]. Theoretically, patients with CG should have higher serum cholesterol levels than normal. However, by consulting the reported cases in the literature, the patient's serum cholesterol was in the normal range, and there was no systemic bleeding factor[20]. Our patient's serum cholesterol was also in the normal range. Therefore, we believed that there was a possibility that the key role in the formation of CG was the ability to produce cholesterol locally in the lesion, and it was not directly related to serum cholesterol content or coagulation disorders.
CG was not only a very rare disease; it was also a disease that was not easily detected because of its milder clinical symptoms and slow progress. Therefore, when CG occurred in the distal femur, the initial symptoms were mainly mild pain, which had similar symptoms to common knee arthritis and was easily misdiagnosed or missed. Our patient had a history of as long as 10 years. Because of the pain in the knee joint, he had been diagnosed with knee arthritis and chosen to use symptomatic analgesia and other treatments. At the same time, because the pain symptoms were not very serious and had not yet reached the level of affecting their lives, she only had received knee arthritis treatment instead of MRI examination. It also suggested that it was difficult to find the CG lesions of the femur in the early stage, and it was easy to be concealed by other diseases of adjacent tissues.
MRI had certain advantages in distinguishing normal tissues from diseased tissues. Therefore, it was a wise choice to perfect the relevant examination of MRI when the disease could not be clearly defined. CG had special performance on MRI images with diagnostic and differential diagnosis value, such as performance on T1 and T2 weighted images-high signal and no enhancement or only slight peripheral enhancement signal[21]. After we performed MRI and contrast enhanced MRI examinations on the patients, we also got similar results with diagnostic value. Because the CG of the femur developed slowly and was localized in the bone marrow cavity, the bone quality of the femur began to be slightly damaged only after a long period of the disease. Therefore, during X-ray examination, generally only local bone destruction could be found, and there was no special phenomenon other than that. This X-ray appearance was very similar to bone cysts, which needed to be differentiated. However, the transmittance of X-ray film of bone cyst was higher[15]. Generally speaking, pathological examination was the most important examination for diagnosing the CG of the femur, and a large amount of cholesterol crystals could be seen. We treated the disease for the first time via surgery and finally achieved satisfactory results. The reason for choosing surgery was because CG had already caused a certain amount of damage to the structure of the femur. Moreover, after 3 months of follow-up, the patient still maintained satisfactory feedback, indicating that the surgical treatment was correct. Therefore, we believed that for the treatment of CG of the femur, surgery should be the main treatment method when there was bone damage.
In short, we reported this case for two purposes. First, CG was a rare disease, and most of the literature reports were based on case reports. There has been no report on CG of the femur. Second, it was the first time that we had reported on the CG of the femur, so we hoped that our report could provide some references for scholars who were studying CG. Meanwhile, our research also had shortcomings, especially the few references and materials that could be sought, which inevitably leaded to shortcomings in the research results. If there were any deficiencies, scholars were also asked to correct me.