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© 1996 The Journal of Bone and Joint Surgery, Inc. Solitary Bone Cyst with Epiphyseal Involvement: Confirmation with Magnetic Resonance Imaging. A Case Report and Review of the Literature*  , CINCINNATI, OHIOInvestigation performed at Children's Hospital Medical Center, Cincinnati
Documented epiphyseal extension of a solitary unicameral bone cyst in the presence of an open physis is extremely rare3,15. Epiphyseal extension by such cysts has been noted on plain radiographs3, on computed tomography15, and intraoperatively10. We report an unusual case of a skeletally immature child in whom magnetic resonance imaging clearly demonstrated extension of a solitary bone cyst into the proximal humeral epiphysis. The portions of the cyst in the epiphysis and metaphysis communicated through a defect in the physis that was probably due to erosion by the cyst. Progressive limb-shortening developed. The purpose of the current report is to describe the natural history and pathogenesis of physeal involvement by a unicameral bone cyst.
A fourteen-year-old boy was seen in the emergency room of the Children's Hospital Medical Center in Cincinnati with a history of a twisting injury of the right arm. He had been managed for a pathological fracture of the humerus on the same side when he was nine years old. Roentgenograms made at that time had revealed a fracture through a unicameral bone cyst (Figs. 1-A and 1-B). He had had no specific problems since that time except that the right arm was noticeably shorter than the left.
The findings on the general physical examination when the patient was fourteen were unremarkable. The findings on the local examination of the right arm were clinically consistent with a fracture of the proximal part of the humerus. The results of the neurovascular examination of the limb were normal. Roentgenograms of the humerus showed the fracture to be through a loculated radiolucent lesion with a thinned cortex in the proximal metaphyseal and diaphyseal region. Proximally, the lesion abutted the physis and had extended into the epiphysis. The growth plate appeared to be open. The fracture did not involve the physis (Fig. 1-C). The fracture was treated with an arm sling. Magnetic resonance imaging was done to evaluate the character and extent of the lesion further.
Spin-density and T2-weighted magnetic resonance images showed a large lesion in the proximal part of the humerus; the maximum proximal-distal diameter measured ten centimeters (Figs. 1-D and 1-E). The lesion began in the mid-portion of the diaphysis and extended proximally through the physeal plate into the adjacent epiphysis. The physeal plate was seen clearly except in the area of the defect. There was no subchondral collapse around the cystic area, and the adjacent soft tissues were apparently normal on the T2-weighted images (Fig. 1-E). The lesion was characterized by septa in a high-signal-intensity matrix. There was extensive thinning of the cortex in the mid-portion, and a cortical fracture was seen in the distal third of the lesion. The signal intensity of the intralesional matrix on the spin-density images indicated proteinaceous fluid (Fig. 1-D).
The patient was managed with aspiration, irrigation with saline solution, and injection of methylprednisolone three times at six-week intervals, according to the standard protocol for the treatment of unicameral bone cysts at our center8. Serosanguineous fluid was aspirated each time, and histological examination of the clotted aspirate did not reveal any tumor cells. Progressive healing was noted after each injection. One year after the most recent injection, the patient had a full, free range of motion of the shoulder and elbow. The right arm had shortening of 4.5 centimeters, compared with 3.8 centimeters at the time of the fracture. Roentgenograms showed a well healed cyst. The growth plate appeared to be almost closed (Fig. 1-F).
Fifty per cent of solitary bone cysts occur in the proximal metaphyseal region of the humerus12,23. Cysts in close proximity to the physis have been classified as active and those away from the physis, as latent12. The apparent migration of the cyst toward the diaphysis is a result of the cyst remaining in the same location as the bone lengthens and the distance between the wall of the cyst and the physeal line increases. Occasionally, persistent juxtaphyseal cysts may extend to involve the epiphyseal region after skeletal maturity2,12. Epiphyseal involvement in the presence of an open physis is unusual3,15. The etiology of epiphyseal involvement by unicameral bone cysts is not clear. In a skeletally immature patient, the growth plate appears to act as a barrier to proximal expansion. However, theoretically there is a risk that a cyst abutting the physis will cause local erosion of the physeal plate and lead to limb-length discrepancy by means of growth retardation3,6 or stimulation5. Most authors have attributed physeal involvement and subsequent growth arrest to traumatic damage5,14,19. Recent reports have suggested erosion by the cyst as a possible cause of physeal damage and growth arrest3,25. Malawer and Markle reported on a patient who had involvement of the proximal femoral epiphysis and suggested a vascular etiology because of increased measurements of pulsatile pressure. Those authors also analyzed seven previously reported cases of epiphyseal involvement1,2,7,10,12,19,23. Of four lesions found in children with open physes, two were in the proximal part of the tibia and two were in the proximal part of the femur7,10,15,23. Two lesions in the proximal part of the humerus and one in the distal part of the femur were detected after closure of the physes1,19. Shortening of the limb was observed in only one patient and was attributed to previous trauma19. A vascular etiology was also considered by Clark, who thought that hyperemia after an injury could be responsible for erosion and damage to the physis. Capanna et al. reviewed the roentgenograms of 607 patients who had a unicameral bone cyst and identified twelve who had demonstrable epiphyseal or apophyseal involvement through an open physis. Four of the involved limbs had shortening of more than 2.5 centimeters. According to those authors, the growth-plate cartilage has only a partial inhibitory effect on extension of the tumor or pseudotumor, and growth disturbance ensues after the cyst has extended into the epiphysis3. A medial epiphyseal slip after a fracture of the proximal part of the humerus can cause further physeal damage and can lead to varus angulation. Recent electron microscopic studies of the growth plate in the area of the cyst also suggested destructive changes involving the hyaline cartilage and proliferative zones of the growth plate25. Chondrocytes exhibited morphological signs of impaired function, such as an increase in glycogen granules and lipid droplets and a reduction in organelles25. Impaired synthesis of the lipoproteins, disappearance of organelles, and fragmentation of cytoplasm were observed, suggesting a severe lesion of the cells and a major role of hydrolytic enzymes in the pathogenesis25. The role of lysosomal enzymes in the formation and persistence of a cyst is also well documented9. Increased concentrations of acid phosphatase16 and high levels of prostaglandins22 in the cyst fluid, consistent with osseous destruction in the cyst wall, have been reported. Jaffe11 and Neer et al. demonstrated histologically a spongy osseous interface between the cyst wall and the physis in a study of cysts abutting the physis. We believe that this minimum osseous interface is due to the constant osteogenesis at the physeal level, which probably surpasses the biodegradation within an active cyst and thus protects the physeal cartilage from direct exposure to the cyst fluid and its contents. When an active cyst is in an extreme stage of biological degradation, it expands because of the increased hydrostatic pressure4,6 and the lysosomal9 and other enzyme activity16,25 within it. This causes resorptive erosion and absorption of the interface between the cyst wall and the physis, along with cortical thinning. The attenuated cortex is probably more vulnerable to injury, and a pathological fracture occurs through the thinned cortex before extensive erosion of the growth plate can take place. Decompression of the cyst due to the fracture changes the dynamics of the cyst temporarily in favor of osteogenesis5,12, but as soon as the fracture has healed the dynamics recur. Cohen observed that increased collateral drainage or removal of the vascular blockage leads to healing of the cyst. This finding is supported by the fact that wider cortical decompression14 or continued drainage of the cyst4 results in good healing. In the absence of any osseous interface between the cyst wall and the physis, persistent exposure to the contents of the cyst as it expands could cause direct local erosion of the growth plate. The result may be, first, gradual distal erosion of the physis leading to growth inhibition. Complete growth arrest does not occur because the germinal and proliferative zones of the physis are not involved. This corresponds to the physeal involvement reported by Cohen and by Clark. Next, continued erosion of the cartilage may cause a perforation in the growth plate at its weakest point. The growth plate is relieved of the pressure as the metaphyseal portion of the cyst is decompressed and as the cyst expands into the epiphysis. This phenomenon was reported previously, in patients in whom no growth arrest was observed during a three-year follow-up period10. Such a defect may not inhibit growth until the peripheral portion of the growth plate has also been destroyed. Finally, as the cyst expands on both sides of the physis, growth may be inhibited because of the partial destruction of the physis3,25. Vascular compromise of the epiphysis has also been suggested as an etiological factor11. Since the growth potential of the physis depends mainly on the vascular supply on the epiphyseal side24, it is possible that the extensive expansion of the cyst may cause vascular disruption and affect the physis and subsequent growth. The combination of these factors was probably responsible for the damage to the physis in our patient. On the basis of our observations, we believe that growth arrest is related to the damage to the physis caused by the cyst and that it (rather than the previously postulated possibility of extrinsic traumatic damage to the physis6,17,19) may account for the shortening associated with these lesions. Lesions that abut the physis in young children therefore should be evaluated carefully. Plain roentgenograms alone may not reveal the exact state of the lesion3,6. Because of its multiplanar imaging and better contrast sensitivity, magnetic resonance imaging is useful for evaluating these benign lesions and for differentiating them from malignant lesions such as purely lytic osteosarcoma21. The high signal intensity of the intralesional matrix on the spin-density images of our patient suggested that the lesion probably was not fibrous, and the absence of a soft-tissue mass on the T2-weighted images suggested the diagnosis of a benign lesion with fluid, such as a unicameral or an aneurysmal bone cyst. Critical evaluation of epiphyseal lesions is also essential because a large cyst can cause subchondral collapse, joint incongruity7, and avascular necrosis13. Because of the potential for physeal erosion by an active cyst in a young patient, magnetic resonance imaging may be justified. If there is encroachment into the physis, early decompression and definitive treatment should be considered to prevent additional destruction. Recent reports have supported the intracystic injection of steroids for the treatment of unicameral bone cysts without any noticeable effect on the physis9,15,20,22. Since growth retardation had already begun in our patient long before the injections of steroids, it seems unlikely that the steroids had any important additional effect on the final limb-length discrepancy.
NOTE: The authors thank Dr. George Bisset, M.D., staff radiologist, Children's Hospital Medical Center, Cincinnati, for his valuable suggestions during the preparation of this manuscript. They also thank the World Health Organization, as the work was done during the tenure of one of us (A. K. G.) on a World Health Organization fellowship.
*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study.
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