Solitary or unicameral bone cysts are benign bone lesions in children and adolescents that are commonly observed in the humerus as well as the femur and the calcaneus [1] . Most patients present with pain due to pathologic or impending fractures or they can be found incidentally [2] . Solitary bone cysts in adults are rare, but are occasionally seen in elderly persons [3] . Treatment options for solitary bone cysts include careful observation, open curettage and bone grafting, steroid or autologous bone marrow injections, various methods of cyst decompression using cannulated screws or pins, and replacement of the cyst cavity using a bone substitute, such as calcium phosphate cement [4] [5] [6] . The treatment of solitary bone cysts has evolved from open curettage and bone grafting to less invasive surgery. The recent development of arthroscopic technology has allowed operators to perform curettage of the cyst wall safely and with greater precision under direct visualization [2] [7] [8] [9] [10] [11] . Using such a minimally invasive surgery for the calcaneus bone is well reported [2] [7] [9] [10] , but has rarely been reported for use in treating the humerus [11] . We present an adult case of a solitary bone cyst of the proximal humerus with concomitant refractory stiff shoulder treated with arthroscopic capsular release and arthroscope-assisted resection of the bone cyst.

A 73-year-old woman was referred to our clinic with a complaint of persistent left shoulder pain and a limited range of motion despite 3 months of conservative treatment. She had been treated with nonsteroidal anti-inflammatory medication and injection of hyaluronan, but the pain continued to worsen. From her examination, both active and passive shoulder motions were restricted at 120˚ in flexion, 90˚ in abduction, 20˚ in external rotation, and at the 5th lumber vertebra in internal rotation. Tenderness of the rotator interval was observed, but not in the lateral aspect of the humerus. There was no history of previous shoulder pain or trauma. There was no abnormality in the plain radiographs (Figure 1), but magnetic resonance imaging showed an approximately 4.5-cm long diameter cystic bone lesion without a septum from the humeral neck to the shaft with T1 low and T2 high signal intensities (Figure 2). Effusion of the glenohumeral joint and thickening of the rotator interval were also observed. Thinning of the cortex was not apparent by computed tomography (Figure 3). Laboratory findings were all in the normal range, including a white blood cell count (5400/μl), erythrocyte sedimentation rate (11 mm/hour), and C-reactive protein (0.11 mg/dl). Culture of the effusion of the glenohumeral joint was negative. Steroid injection into the glenohumeral joint was effective, but limited.

Diagnosis at that time was a refractory stiff shoulder with an incidentally observed solitary bone cyst of the proximal humerus. Surgery was performed in a beach-chair position under general anesthesia. Passive shoulder motion was also restricted under general anesthesia. At first, arthroscopic synovectomy and

pan-capsular release were performed using anterior and posterior portals. Active synovitis and stiffness of the capsule were observed (Figure 4(a)). After the procedure, normal range of motion was obtained. In the next step, two 1-cm skin incisions and 8-mm drill holes into the lateral wall of the cyst with fluoroscopic guidance were created with meticulous care not to damage the axillary nerve

(Figure 5). After insertion of the arthroscope through the holes, the white membranous cyst wall was observed and resected with a shaver until bone marrow was observed (Figure 4(b)). An arthroscope was introduced to the other drill hole, resection was similarly performed, and sufficient curettage of the inner surface of the cyst was confirmed endoscopically. Finally, an 8-mm cannulated hydroxyapatite pin (Japan Medical Next, Osaka, Japan) was inserted for continuous cannulation (Figure 6(a)). Histologic analysis showed a fibrous tissue membrane with occasional giant cells, which was coincident with a finding of the solitary bone cyst. After surgery, no remarkable complications were observed, and passive shoulder exercise was allowed the following day. At 1-year after the operation, pain was relieved, and motion was improved to 170˚ in flexion and abduction, 60˚ in external rotation, and to the 10th thoracic vertebra in internal rotation. Ossification of the cyst was insufficient, but enlargement of the cyst was not observed (Figures 6(b)-(d)). Written informed consent was obtained from the patient for publication of this case report and accompanying images.

Solitary bone cysts are benign bone lesions that usually occur in the humerus, calcaneus, and femur of children or adolescents. When present in adults, they are usually diagnosed as incidental findings [1] . The bone cyst observed in this case was found incidentally in relation to the concomitant stiff shoulder. It was successfully treated with arthroscope-assisted resection of the bone cyst and insertion of the hydroxyapatite pin for continuous cannulation. This simple technique allowed direct visualization and complete resection of the cyst wall with two small incisions. Such a technique may be superior to conventional open procedures or those under image guidance.

Treatment strategies for the solitary bone cyst are changing from conventional open curettage and autologous bone grafting to minimally invasive surgeries to decrease the rate of recurrence and to avoid disadvantages, such as donor-site morbidity, increased operative time, and intraoperative blood loss [9] . Dormans et al. described successful results of a minimally invasive technique that consists of percutaneous intramedullary decompression, curettage, and calcium sulfate pellets grafting for the solitary bone cysts of variable bones under image intensifier [4] . Alvarez et al. first reported a case of a calcaneal bone cyst treated with curettage and bone grafting under arthroscope assistance [7] . Thereafter, several authors reported successful cases of a calcaneal bone cyst with use of an arthroscope [2] [9] [10] . Meanwhile, there are few reports of a bone cyst on the humerus [11] [12] . Otsuka et al. reported a case series of arthroscope-assisted resection of the cyst of variable bones, which included a case involving the humerus; however, detailed descriptions about the surgical technique are lacking [11] . Saccomanni reported an atypical case of aneurysmal bone cyst of the humeral head treated with arthroscope-assisted resection of the bone cyst [12] . To our knowledge, this is the first case of treatment of a solitary bone cyst of the humerus that includes a detailed surgical technique under arthroscope assistance.

Optimal treatment after arthroscope-assisted resection of the cyst wall is still unclear. Recent reports have shown the efficacy of continuous decompression using a cannulated screw [5] [6] [13] [14] . This technique may be reasonable because a main hypothesized cause of a solitary bone cyst is accumulation of interstitial fluid within the bone due to venous obstruction [1] [13] . Shirai et al. reported good clinical results with this procedure, of which 38 out of 43 cases achieved bone healing [6] . In this case, we used a cannulated hydroxyapatite pin for continuous decompression of the cyst. Such a pin does not require removal and has osteoconductive properties that may stimulate bone healing [6] . However, there has been no evidence regarding the time required to achieve sufficient structural strength using this technique. In this case, ossification of the cyst was insufficient at one year after the operation. As another, less-invasive technique, injection of bone substitute, such as calcium phosphate cement, has been performed [2] [4] [9] [10] . Mainard et al. first reported a case of a solitary bone cyst on the calcaneus that was treated using arthroscope-assisted resection of the cyst and injection of calcium phosphate cement with a successful outcome [9] . Innami et al. reported superior results using a similar technique in 16 young athletes with symptomatic calcaneal bone cysts [2] . They found that optimal visualization, which allows complete curettage with minimum violation of the structural integrity, and percutaneous injection of bone substitute instead of autologous bone grafting permit an early return to sports activities in young athletes. Notably, while the maximum strength of the cement is obtained within 3 days, the process of integration and eventual degradation requires several years [9] [10] . Patient age, activity, and cyst site (i.e. upper or lower extremities) should be considered for suitable treatment.

We present an adult case of solitary bone cyst of the proximal humerus successfully treated with arthroscope-assisted resection of the bone cyst and insertion of the hydroxyapatite pin for continuous cannulation.

We would like to thank Editage (http://www.editage.jp/) for English language editing.

No conflicts declared.

Ando, A., Koide, M., Hagiwara, Y., Matsuda, M. and Itoi, E. (2018) Solitary Bone Cyst of the Proximal Humerus with a Concomitant Stiff Shoulder Treated with both Arthroscopic Capsular Release and Arthroscope-Assisted Resection of the Bone Cyst: A Case Report. Open Journal of Orthopedics, 8, 77-84. https://doi.org/10.4236/ojo.2018.83010