Approach Considerations
The decision to pursue surgical intervention in patients with unicameral (simple) bone cysts (UBCs) is a highly individualized one. An asymptomatic lesion with satisfactory maintenance of cortical thickness may require only observation. A lesion with precarious cortical thinning (with or without insufficient pain) may demand surgical intervention.
In addition, factors such as an upper-extremity (lower stress) vs a lower-extremity location (higher stress) and younger children (more amenable to cast immobilization) vs older adolescents (less amenable to cast immobilization) may strongly influence surgical decisions. Simple treatment of the pathologic fracture may result in cyst resolution in as many as 25% of cases. [28]
Some authors have suggested the use of a cyst index aimed at predicting the future risk of a pathologic fracture. Kaelin and MacEwen discussed this concept and defined their cyst index as the area of the UBC measured via its widest dimensions divided by the diameter of the diaphysis of the same bone. [53] On the basis of a statistical analysis of 57 patients with UBCs, they recommended mainly observation for humeral cysts with an index below 4 and femoral cysts with an index below 3.5. When either threshold was exceeded, stronger consideration of surgical intervention was deemed appropriate.
Surgical therapy for a UBC may be divided into open and percutaneous procedures. The success rates of such procedures have varied considerably, and the very definition of success has also varied from author to author. [19, 20, 54, 55, 33, 56, 57, 58, 59]
The main contraindication for surgical treatment of a UBC is a patient who otherwise meets indications for surgery but is unable to tolerate anesthesia. Another relative contraindication for surgery is a patient with a small asymptomatic latent cyst with a low likelihood of a pathologic fracture.
A survey of 444 members of the European Pediatric Orthopedic Society (EPOS) and the Pediatric Orthopedic Society of North America (POSNA) documented great variation with regard to the treatment of UBCs in the humerus. [60] For painless UBCs, 67% of the respondents preferred no treatment at all except with cases involving a high fracture risk, in which case 53% recommended surgery; 71% of the respondents would treat painful UBCs surgically. The most commonly employed surgical techniques were as follows:
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Curettage (45%)
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Artificial bone substitutes (37%)
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Corticosteroid injection (29%)
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Intramedullary stabilization (eg, rodding; 24%)
A large majority of the respondents in this survey preferred nonoperative treatment for fractured nondisplaced and mildly displaced proximal humerus UBCs and mildly displaced pathologic humerus shaft fractures (94%, 91%, and 83%, respectively). [60] Severely displaced pathologic proximal humerus fractures were more likely to be treated surgically (40%) than conservatively (36%), and surgical treatment (63%) by intramedullary stabilization (60%) was preferred for severely displaced humerus shaft fractures. There is a need for prospective randomized controlled studies to evaluate the outcomes of the different surgical approaches versus those of nonoperative strategies.
Medical Therapy
Nonoperative treatment of a UBC usually amounts to closed fracture care following pathologic fracture through the lesion. It has been suggested that in as many as 25% of cases, spontaneous healing of the cyst may occur after such pathologic fractures. [28] However, not all authors have reported such a high frequency of spontaneous healing. Thus, watchful waiting and routine fracture care only are not a universally accepted treatment option. [19, 20, 13, 54, 55, 33, 56, 61, 62, 57, 58]
Open Surgical Procedures
In a study of 144 UBCs treated by means of curettage and packing with freeze-dried crushed cortical-bone allograft, Spence et al demonstrated higher recurrence rates in patients younger than 10 years, female patients, those with active cysts, and those with incompletely packed cysts. [63] Of the completely packed cysts, 88% healed. The authors suggested that freeze-dried allogeneic crushed cortical bone yielded results superior to those of similarly processed cancellous bone but comparable to those of fresh autogenous cancellous bone.
Other studies have also demonstrated multiloculated cysts and fracture immobilization as a primary treatment as associated with cyst recurrence or persistence.
Neer et al stressed that reported recurrence rates in UBC surgery could be quite misleading if complete cyst obliteration was the criterion for success. [64] They believed that true recurrences were characterized by the cyst cavity reappearing and enlarging, causing expansion and thinning of the cortex and the threat of fracture.
The key aspects of the Neer rating system for the purposes of evaluating treatment response are as follows [55] :
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Excellent – Complete obliteration of the cyst
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Residual defect – One or more static cystlike areas with good reestablishment of bone strength
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Reoperation – Subsequent operation required because of recurrence
Open techniques that have been reported include subtotal resection with and without bone grafting. Different bone-grafting materials that have been used include autograft, allograft, demineralized bone matrix, high-porosity hydroxyapatite bone-grafting material, and plaster-of-paris pellets. Some evidence suggests that the use of synthetic bone grafts to treat UBCs may lead to reduced reoperation rates. [65]
In 1962, Fahey and O'Brien introduced a technique for UBC treatment that they referred to as subtotal resection and grafting. [66] This technique involved subperiosteal exposure of the cyst and a portion of the adjacent normal bone, followed by removal of two thirds to four fifths of the cyst. Cortical struts of bone graft harvested from the patient's iliac crest or tibia were then used to fill the defect. Freeze-dried allograft was also used at times.
At an average of almost 4 years' follow-up, Fahey and O'Brien reported a 95% (19/20) success rate with their technique (using the absence of a cyst as their criterion). [66] They considered the operation to be the procedure of choice for individuals with latent primary cysts and for persons with cysts that recurred after a conventional operation. One patient required repeat grafting to achieve healing, and no other specific complications were reported with the technique.
Fifteen years later, McKay et al reported on a similar subtotal resection approach to UBCs that did not include bone grafting. [67] Their technique yielded a 90% (19/21) success rate (also defined as complete cyst obliteration). No infectious or neurocirculatory complications were reported, but the authors did identify three patients who suffered humeral growth disturbances and seven patients whose bones fractured during the procedure (though this did not substantively affect their later outcome).
Subtotal resection and bone grafting thus remains an option for UBC treatment. The procedure certainly carries a higher level of surgical morbidity than other procedures do, and the surgeon's enthusiasm for the procedure may be directly proportional to the cyst's distance from the growth plate.
Curettage [68] and bone grafting procedures have been used extensively in the treatment of a UBC.
Percutaneous Procedures
In 1974, Scaglietti introduced a procedure by which steroids were percutaneously instilled within UBCs (as well as other types of bone lesions). [69] A minimum of 40 mg of methylprednisolone acetate was used for smaller cysts in young patients, and as much as 200 mg was used for larger cysts in older patients. The described technique included, on average, three or four injections in a period of 12-20 months, but as many as nine injections over 4 years were used.
Scaglietti et al reported complete healing of the bone cysts in 55% of his cases and some improvement (eg, cortical thickening within the area of the cyst or areas of new bone formation within the cyst) in 45%. [69] Whereas 24% of their patients required only one steroid injection, 76% required multiple injections. The image below shows the typical appearance of the fluid aspirated from UBCs.
Typical appearance of cyst fluid is depicted. Initial aspiration often yields thin, clear, yellow fluid that rapidly becomes blood-tinged.
Simple mechanical disruption of the cyst wall has also been investigated as a treatment option. Komiya et al called this trepanation, and they reported good results in 91% (10/11) of their patients. [70] The technique consisted of aspiration of the cyst until venous hemorrhage became visible, perfusion of the cyst with saline, and cyst wall as well as proximal and distal medullary bone drilling with a Kirschner wire (K-wire).
Chigira et al reported similar success with multiple drilling in 86% (6/7) of their patients. [10] Their technique included leaving the 2.0-mm K-wires in place in some instances to allow drainage of the fluid through the cyst wall.
Various other authors have reported their results and suggested refinements of the percutaneous steroid technique. [56, 61, 58, 71, 72, 73, 74, 75, 76] Rosenberg et al emphasized the importance of eliminating fibrous or osseous septa within UBCs to facilitate bathing the entire lesion with the injected steroids. [77]
Capanna et al also pointed out that contrast examination allows the surgeon to identify noncontiguous septated areas of UBCs (see the image below). [78] This is important if optimizing the treatment response is desired. [78]
Double-cannula technique demonstrates intraoperative use of contrast material for evaluation of cyst's interior. In this case, large partial septum remains along inferior portion of cyst.
Injectable materials other than steroids, such as alcohol-based fibrosing agents, have also been suggested as treatment options for benign bone cysts. [79, 80]
Killian et al also reported the use of demineralized bone matrix via a similar percutaneous technique. [81] No steroids were used. At 2-year follow-up, 82% (9/11) of their patients demonstrated completely healed cysts.
Several authors investigated the effectiveness of autologous bone-marrow injection as a treatment for UBCs. [19, 82, 83] Spurred on by their earlier published work, a group of Israeli researchers reported marked improvement in cortical thickness and cyst remodeling in 10 of their patients following a single bone-marrow injection. [84] Highlights of their technique include the following [85] :
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A single puncture into the cyst with a thin trocar or needle
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Aspiration of cyst fluid, which is sent for pathologic analysis
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Disruption of the lining and septations within the cyst using the trocar or needle
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Contrast media examination (may be used)
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Injection of autologous bone marrow aspirated from the iliac crest (average volume, 25 mL)
All cysts healed completely with one procedure within 6-12 months. [19]
Other authors demonstrated similar results with the percutaneous bone-marrow injection procedure. [82, 85] Yandow noted that 83% (10/12) of patients with UBCs responded satisfactorily to the procedure, [82] and Delloye et al had good results in approximately 88% (7/8) of their patients. [85] Gundle et al reported on injection of bone-marrow aspirate and demineralized bone matrix. [41]
Kose et al evaluated the outcome of the autologous bone-marrow procedure in 12 patients. [86] Only 42% (5/12) cases responded to the treatment, whereas 50% (6/12) recurred and 8% (1/12) exhibited no response at all. These authors concluded that the technique may be less effective in large cysts and in multiloculated cysts, and they recommended that the procedure be applied to selected patients only.
A 2017 Cochrane review concluded that the available evidence was not sufficient to allow a determination of the relative merits of bone-marrow injections and steroid injections for treatment of UBCs in the long bones of children. [87]
In a retrospective comparative study of 53 consecutive patients (37 females, 16 males; mean age, 10.6 ± 1.53 y) with UBCs, D'Amato et al reported good results from using a combination of intralesional autologous bone-marrow concentrate and equine-derived demineralized bone matrix injections, as compared with methylprednisolone acetate injections. [88]
Zhang et al compared three methods of treating UBCs—autologous bone-marrow injection, elastic stable intramedullary nailing (ESIN), and a combination of the two—and found that the combined approach was most effective and had the highest cure rate. [89]
In a small study (N = 12; 10 male, two female), Fillingham et al evaluated the results of debridement and one percutaneous injection of a bioceramic bone-graft substitute for treatment of UBC. [90] Functional outcome was excellent, and 75% of the patients experienced complete healing, with 25% having a residual cyst. The largest of the residual cysts was viewed as a local recurrence and was treated with repeat percutaneous bioceramic injection 1.5 years later; 4 years subsequently, the patient was still disease-free.
In another small study (N = 15; mean follow-up, 118 wk), Sivakumar et al assessed the results of injection of a bone substitute for treatment of UBCs. [91] They reported clinical resolution (absence of pain at latest follow-up) in 86.7% of patients and radiographic resolution in 80%.
Complications
Injury to the growth plate (physis) may occur secondary to direct cyst expansion, pathologic fracture, or unintended mechanical disturbance during surgical intervention.
Direct cyst expansion across the growth plate and into the epiphysis of the proximal humerus has been well documented by Gupta et al via magnetic resonance imaging (MRI). [92] Growth arrest has also been reported following treatment either with local steroid injection or with curettage and bone grafting. [6] Growth disturbance leading to angular deformity or disturbed longitudinal growth has been estimated to occur in approximately 14% of cases. [19, 20]
Steroid injection has been a successful treatment, even in the setting of cyst extension into the epiphysis. [93]
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Large proximal humeral unicameral bone cyst demonstrates early cortical healing following pathologic fracture.
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Large unicameral bone cyst of pelvis. Pathologic fracture is depicted. Note extension of cyst into region of proximal femoral physis.
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Typical appearance of cyst fluid is depicted. Initial aspiration often yields thin, clear, yellow fluid that rapidly becomes blood-tinged.
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Double-cannula technique demonstrates intraoperative use of contrast material for evaluation of cyst's interior. In this case, large partial septum remains along inferior portion of cyst.
