To the Editor:

We thank Dr. Leidinger for his comments regarding our article. Dr. Leidinger is correct that the periosteum was not removed at the time of resection of the simple cyst from the proximal fibula. We agree that a resection that does not include a cuff of normal tissue around the lesion does not meet the definition of a “wide” or “en bloc” resection. Such a resection has never, to our knowledge, been a recommended treatment for simple bone cyst. The purpose of the report was to demonstrate that with a subperiosteal en bloc resection of a simple cyst, recurrence was possible. This would seem to indicate that the factors responsible lie somewhere outside of the resected bone, possibly in the periosteum.

It is possible that “tumor cells” mentioned by Dr. Leidinger could play a role. To our knowledge, however, no such cells have been identified. A thin layer of tissue containing flattened, mesothelial-like cells lines the inner surface of simple bone cysts. Their presence has not been described on the outer cortex of these lesions, and it is unclear what role if any these cells play any role in the development of the cyst. Without knowing the etiologic cells in the unicameral bone cyst, it is difficult to determine the margin of resection.

We are grateful to Dr. Leidinger’s comments regarding our manuscript and hope that the further discussion clarifies the issues.

To the Editor:

We read with interest the article, "Recurrence of a Unicameral Bone Cyst in the Proximal Part of the Fibula After en Bloc Resection. A Case Report." (2004; 86:154- 58), by Bowen et al(1). This interesting case again shows the anything but simple behaviour of bone cysts. Generally, recurrence after surgical treatment of UBC has been frequently (30-50%) reported regardless of the chosen therapy(2,3). Neither the suggested relationship of the periosteum nor any extrinsic factor or disordered endochondral ossification explains the recurrence of the cyst. Although the authors excised the proximal fibula with the fractured cyst, by leaving the periosteum intact the resection margin was not wide according to the criteria of Enneking(4). With an active UBC, this might be the reason for some tumor cell remnants in the periosteal tube.

In active benign stage 3 lesions, recurrence is less likely only after wide en bloc resection but such an aggressive surgical treatment for a benign tumor-like-lesion with its associated morbidity is indicated only when other options have been tried, especially in an expendable bone like the fibula. Our recommendation is to first proceed with a minimally invasive procedure such as a steroid injection as the authors did with proven success.

References

1. Bowen RE and Morrissy RT. Recurrence of a Unicameral Bone Cyst in the Proximal Part of the Fibula After en Bloc Resection. A Case Report. J Bone Joint Surg [Am] 2004;86:154-58.

2. Enneking WF, Spanier SS, Goodman MA. A system for the surgical staging of musculoskeletal sarcoma. Clin Orthop. 1980;153:106-20.

3. Inoue O, Ibaraki K, Shimabukuro H, Shingaki Y. Packing with high- porosity hydroxyapatite cubes alone for the treatment of simple bone cyst. Clin Orthop. 1993;293:287-92.

4. Wilkins RM. Unicameral bone cysts. J Am Acad Orthop Surg 2000;8:217-24.

To the Editor:

This interesting case report raises more questions than it answers. However<I think it is fair to say that this case is not entirely novel as a recurrence rate of up to 10% after diaphysectomy was reported en passant by Kaelin and MacEwen (1) in describing their cyst index to assess risk of fracture.

It seems agreed that bone cysts originate at growth plates and can become distanced from the physis with growth. If latency is a true phenomenon in terms of response to treatment, then the periosteum can only have a secondary influence on cyst behaviour.

The apparent centrifugal pattern of cyst development is also difficult to reconcile with periosteal induction. We have seen cyst-healing reverse completely in cysts some distance from the physis of origin but find this is unusual.

Clearly bone cyst behaviour is anything but simple.

1. Kaelin AJ,MacEwen GD. Unicameral bone cysts. Natural history and the risk of fracture. Int Orthop.1989;13:275-282.