1 Introduction

An aneurysmal bone cyst (ABC) is a benign, osteolytic, sprawling, and bleeding lesion at 0.14 per 100,000 of the population per year.¹ There is an insignificant female dominance. ABC is encountered at all ages, but most patients are in their second decade and 75%–90% of cases appear earlier than the age of 20. Recently, genetic research has shed some light on the etiology of ABC, leading to the conclusion that ABC is a true neoplasm rather than a reactive process, caused by the rearrangement of the USP6 oncogene, on chromosome 17.² ABC is usually more eccentrically located on the metaphysis than Unicameral Bone Cyst (UBC), with a more destructive performance, with walls more bulging and thinner. ABC's are locally aggressive causing bone resorption, leading to fragile bones and subsequent fractures. ABCs display a variety of clinical appearances, from the concealed to the energetic form and finally the rare aggressive alternate.³ The bones most commonly affected are the distal femur, the tibia, the humerus, and the fibula. ABC is often revealed by pain, sometimes by swelling, and more rarely by fracture. If ABC has a large dimension, it tends to deform the affected limb. In the proximal humerus, the growth plate can be affected. Angular deformity and/or shortening can occur due to growth plate destruction. On plain X-ray, ABC shows as an eccentric, osteolytic, expansive lesion containing fine-walled cystic cavities. MRI is the examination of choice to complement X-ray. The typical finding is an expansive lesion. Multiple fluid levels may be detected on T2-weighted axial sequences.

The combined bone cyst curettage and bone grafting is the most appropriate surgical method, especially for aneurysmal cysts with great potential for expansion.⁴ In contrast, in anatomical areas that are challenging to approach for curettage or resection and grafting, such as vertebral bodies and pelvic bones, the appropriate method is embolization.^5,6 De Cristofaro et al. reported that in 24 patients who undertook embolization the relapse rate was 8%.⁷ In cases of aneurysmal bone cysts that are difficult to approach or have recurred after their surgical treatment, radiotherapy could then be used. In cases where radiotherapy is used as a sole treatment, the reported relapse rate varies between 8% and 25%.⁸ There are other additional therapeutic means that have been used from time to time. These therapeutic means include Argon beam coagulation,⁹ cryotherapy,¹⁰ phenol,¹¹ polidocanol,¹² alcohol,¹³ and heat cauterization with polymethylmethacrylate polymerization.¹⁴ In the literature, the recurrence rate from the use of these different therapeutic agents differs significantly. The relapse rate varies 5%–40%.¹⁵ Presently, curettage and bone grafting, with or without the use of additional therapeutic means is the most acceptable method of treating ABCs. Recurrence is mostly due to residual tissue left after curettage^16,17. In children, the recurrence rate is greater than in adults. Open growth plates and the proximity of the bone cyst to the epiphysis have been regarded as threat factors for local relapse.¹¹ This is due to the fact that the most aggressive forms prevail in children. In addition, due to the proximity of the bone cyst to the epiphyseal plate and the risk of its injury throughout the curettage, the likelihood of inadequate excision and persistence of pathological tissue near the growth plate is significant. Despite the fact that relapses commonly occur within the first two years, a long-term follow-up of their course is necessary. Gibbs et al. proposed a method of complete excision of pathological tissue using high-speed burrs under the guidance of endoscopy¹⁷. Using this method, he reported a decrease in the relapse rate to 12%. Also, Wang et al. concluded that the combination of curettage with high-speed burrs and bone grafting is a fairly justified method as it reported only one relapse in a total of 31 cases.¹⁸

2 Case report

A nine-year-old boy was referred to our institution with a six-month history of painful and swollen lateral malleolus of the left ankle.

3 Clinical findings

Clinical examination of the left ankle revealed a significant degree of swelling of the lateral malleolus and a moderate degree of sensitivity to its palpation.

4 Diagnostic assessment

On plain radiographs of the left ankle, a well-defined osteolytic lesion, with a balloon-like appearance, extending up to the growth plate of the lateral malleolus was a distinctive feature (Fig. 1a and b). On MRI images of the left ankle, a well-defined low signal in T1 weighted images (Fig. 1c, d) and a well-defined high signal in T2 weighted images was evident. Additionally, multiple fluid levels were detected on T2-weighted axial sequences, characteristic features of an aneurysmal bone cyst (Fig. 1e and f).

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Fig. 1 Aneurysmal bone cyst (ABC) of the lateral malleolus in a 9 years old boy. Radiographic and MRI assessment. a, b; anteroposterior and lateral X-Rays of the ankle; a well-defined osteolytic lesion, with a balloon-like appearance, extending up to the growth plate of the lateral malleolus. c; frontal view MRI image of the ankle; a well-defined low signal in T1 weighted image of the lateral malleolus. d; sagittal view MRI image of the ankle; a well-defined low signal in T1 weighted image of the lateral malleolus. e; sagittal view MRI image of the ankle; a well-defined high signal in T2 weighted image of the lateral malleolus; a positive fluid-to-fluid sign is obvious, indicating aneurysmal bone cyst (ABC) of the lateral malleolus. f; transverse view MRI image of the ankle; a well-defined high signal in T2 weighted image of the lateral malleolus; a positive fluid-to-fluid sign.

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5 Therapeutic intervention

Complete excision of pathological tissue by the use of a high-speed arthroscopic burr was performed a few days later. Additionally, percutaneous bone grafting of the cyst was performed by harvesting iliac crest autologous cancellous bone, the crushed cancellous bone allograft and DBM putty allograft (SteriGraft).

6 Surgical technique

Under fluoroscopic guidance a pin was inserted into the cyst, 20mm proximal to the growth plate of the lateral malleolus, as a guide pin for the insertion of the 2.9 mm arthroscope. A blood-tinged fluid was aspirated from the cyst and it was sent for laboratory analysis. A portal of 4mm was created for the insertion of the 2.9 mm arthroscope. Under fluoroscopic guidance a second pin was inserted into the cyst, 20mm proximal to the first one, as a guide pin for the insertion of a 3mm arthroscopic round burr. A second portal of 4mm was created for the insertion of a 3mm arthroscopic round burr. A 2.9 mm arthroscope, with a 30° oblique view, was used for the endoscopy (Fig. 2a). A pathological tissue sample from the cyst was taken with biopsy forceps for histopathological examination. The histopathological results were consistent with the ABC lesion. For a successful ESC, fluoroscopy was periodically used to avoid growth plate damage (Fig. 2b). Meticulous ESC with the arthroscopic 3mm round burr was performed (Fig. 2c, d, e). During this procedure, the arthroscope and arthroscopic burr exchanged portals to ensure full observation of the cavity and full aggressive high-speed resection of the tumor tissue of the cyst. The cystic lesion was thoroughly removed until a normal bone was seen in the medullary cavity. Finally, the percutaneous filling of the cyst cavity by the harvesting iliac crest autologous cancellous bone, the crushed cancellous bone allograft and the DBM putty allograft (SteriGraft) was performed (Fig. 3a).

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Fig. 2 Intraoperative photos. a; a 2.9 mm arthroscope, with a 30° oblique view, into the aneurysmal bone cyst of the lateral malleolus for the endoscopy; 3mm arthroscopic round burr in place for the cyst's curettage. b; Radiographic image. c, d, e; endoscopic photo images of the aneurysmal bone cyst of the lateral malleolus; endoscopic high-speed curettage; the cystic lesion is thoroughly resected until the normal bone can be seen in the medullary cavity.

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Fig. 3 Postoperative radiographs of the ankle. a; anteroposterior and lateral X-Rays of the ankle immediately after the percutaneous filling of the cyst cavity of the lateral malleolus with the harvesting iliac crest autologous cancellous bone, the crush cancellous bone allograft, and the DBM putty allograft. b, c, d; anteroposterior, oblique, and lateral X-Rays of the ankle 6 months postoperatively; good consolidation of the aneurysmal bone cyst.

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7 Follow up and outcomes

Postoperatively, instructions were given to use a below-knee scotch cast ankle splint for three weeks. Immediately after its removal, instructions were given for the beginning of active and passive exercises of the ankle and the progressive increase of the weight-bearing of the leg to its full weight-bearing at the end of the 6th post-operative week.

The consolidation of the cyst at the final evaluation was assessed according to the Modified Neer's Classification Scale (Table 1). Complete healing of the ABC of the lateral malleolus, according to the Modified Neer's Classification Scale was achieved 6 months postoperatively (Fig. 3b, c, d). The Modified Musculoskeletal Tumor Society Score (MSTS) (30/30 points) (Table 2), for functional evaluation, was used preoperatively and postoperatively at the final evaluation, to evaluate functional recovery. The Musculoskeletal Tumor Society Score was increased from 23 points preoperatively (Table 2) to 30 points (100°) postoperatively, at his final evaluation, 5 years postoperatively. No recurrence was observed in the last patient reassessment.

8 Discussion

The management of ABCs has continued to evolve with time. Endoscopic curettage (ESC) of ABCs is a much less invasive method than open access.¹⁹ Furthermore, because ESC requires small portals for the endoscope and high-speed burrs, the whole process is aesthetically better than that of open access. Endoscopy has the advantage of the immediate visualization of the pathological tissue so that it is possible to completely remove it.^20,21 Under endoscopic guidance, it is possible to immediately access the degree and extent of the curettage, until the appearance of normal bone in the medullary canal. Therefore, extreme curettage of the bone is avoided, which could lead to its fragility and bone fracture.²¹ Finally, ESC with high-speed burrs, due to the direct stimulation of the cysts, favors the formation of new bone. Limitations of this technique are often thick septa that can be seen more clearly on CT scans. Septa can act as an obstacle and consequently could prevent sufficient curettage and padding of the cyst. If a septum is thin or weak, its resection is recommended. Another limitation is the maneuverability of the arthroscopic instruments within the bone, which is not a problem for small cysts as in this case. For large cysts, multiple portals may be needed.

9 Conclusion

ESC with high-speed burrs and bone grafting represents an innovative approach to the surgical treatment of ABCs. Endoscopy offers the advantage of precisely evaluating tumor resection through direct inspection of the bone cavity.

Funding/sponsorship

This research did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors.

Informed consent

Informed consent was obtained from the individual participant included in this study. The patient and his parents were informed that data on their child's case would be submitted for publication and they agreed.

Institutional Ethical Committee approval

We assure you that all procedures performed in this study involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. We assure you that our study was approved by the Institutional Ethical Committee.

Authors contribution

I assure you that the co-authors involved in this study contributed to the.

Conceptualization; Data curation; Formal analysis; Investigation; Methodology; Project administration; Resources; Software; Supervision; Validation; Visualization; Roles/Writing – original draft; Writing – review & editing.