Continuous decompression treatment of unicameral bone cyst in a small‐breed dog: A case report

Abstract

Objective

To report the long‐term outcomes of continuous decompression for repairing a unicameral bone cyst in a dog.

Study design

Case report.

Animal

A 1‐year‐old Maltese‐Poodle mixed breed dog presented with left forelimb lameness.

Methods

A well‐defined cyst in the left humeral condyle was detected on radiography and computed tomography. A lateral surgical approach revealed the cyst in the distal metaphysis of the left humerus. The attained cyst fluid and incisional biopsy led to the diagnosis of unicameral bone cyst. To prevent cyst expansion and recurrence, aspiration, open curettage, ethanol cauterization, and continuous decompression using introducer needles were performed.

Results

The dog recovered from surgery, and a Robert Jones bandage was retained for 2 weeks. The follow‐up at 1 month showed resolution of lameness. At 12 months post‐surgery, radiographs revealed increased cortical thickness and no recurrence of the cyst or lameness.

Conclusion

Continuous decompression using introducer needles could be used to repair unicameral bone cysts. Small diameter introducer needles were especially useful, as the lesion was located at the joint. The procedures described in this report could be applied before other salvage procedures to promote healing and prevent recurrence.

1. INTRODUCTION

A bone cyst is an expansive lesion within the bone marrow cavity that gradually weakens the bone, predisposing it to fracture. ¹ Bone cysts are categorized as unicameral (cystic, simple), aneurysmal, or subchondral. ² , ³ Unicameral bone cysts are benign intraosseous fluid‐filled cavities lined with fibrous connective tissue. ⁴ While the overall prevalence of canine bone cysts is not well‐documented, they are rarely reported in dogs. ⁵ , ⁶ They are most often diagnosed in young dogs, usually 5–15 months old, with a higher incidence in medium‐to‐large breeds. ⁷

Unicameral bone cysts often develop in the proximal or distal metaphysis of long tubular bones adjacent to the growth plate and migrate to the diaphysis as the bone grows. ⁴ , ⁵ Although the exact pathophysiology of unicameral bone cysts remain unclear, metaphyseal venous obstruction with secondary osteolysis has been proposed as a contributing factor. ⁸ , ⁹ , ¹⁰

Treatment of unicameral bone cysts is crucial to the prevention of recurrent fractures, prolonged restriction of activities, and deformities or shortening of long bones. ¹¹ , ¹² Surgical treatment options range from simple curettage with bone grafting to subperiosteal resection. ¹³ In human medicine, minimally invasive procedures targeting all possible causes of the lesion are used to improve long‐term outcomes by promoting better healing and preserving function. ¹⁴ One such technique involves inserting a cannulated screw to provide continuous decompression. ¹⁵ However, for small dogs, utilizing a cannulated screw may be unsuitable owing to screw diameter and cortical thinning, necessitating an alternative device for this method. This case report reviews the application of introducer needles in the treatment of unicameral bone cysts to prevent pathological fractures and maintain elbow joint function.

To the authors' knowledge, this is the first report that describes successful surgical management of the unicameral bone cyst using continuous decompression, wherein diagnosis, treatment, and follow‐up are described in detail.

2. CASE DESCRIPTION

A 1‐year‐old neutered male Maltese–Poodle mix weighing 5 kg presented with left forelimb lameness. The dog exhibited non‐weight‐bearing in his left forelimb when standing or sitting after slipping while jumping onto a table the day before presentation. In the hospital, the dog showed grade 3 of 5 lameness during a trot, with obvious head bobbing. Physical examination revealed no pain response, swelling, or warmth in either the elbow or carpal joints. Forelimb radiography revealed a round‐to‐oval‐shaped radiolucent lesion on the left humeral condyle (Figure 1), with no sign of fracture. To prevent progression to a pathologic fracture, a modified Robert Jones bandage was applied to the left forelimb, and activity was restricted for 1 week until a computed tomography (CT) scan was performed.

FIGURE 1.

(A) Craniocaudal and (B) mediolateral radiographs of antebrachium at presentation. A round to oval‐shaped radiolucent lesion (arrow) is located on the left humeral condyle, with cortical thinning. No periosteal reaction or soft tissue swelling is observed.

The CT scan was performed to comprehensively examine the presence of lesions in other limbs and cortical thickness at the lesion site. The results confirmed a well‐defined, round‐to‐oval‐shaped, non‐contrast‐enhancing, fluid‐attenuating lesion with a distinct transitional zone at the level of the left humeral condyle (Figure 2). Additionally, the lesion was accompanied by cortical thinning in adjacent areas. No other lesions were identified, and no periosteal reactions or changes in the adjacent soft tissue were observed. While the lesion was suggestive of a bone cyst, the differential diagnoses included giant cell tumor, fibrous dysplasia, and atypical bone neoplasia. Therefore, biopsy was necessary for a definitive diagnosis. Surgical treatment was planned to prevent pathologic fractures, prolonged activity restriction, and abnormal range of motion in the elbow joint, considering the lesion's site.

FIGURE 2.

Both three‐dimensional bone reconstruction and computed tomography (CT) are illustrated. The CT showed well‐defined, non‐contrast enhancing, fluid‐attenuating lesion (arrow) at the left humeral condyle, with cortical thinning. The thickness of the thinnest cortex is measured as 2.17 mm.

The patient underwent sedation with butorphanol (0.2 mg/kg IV), anesthesia induction with propofol (6 mg/kg IV), and maintenance with isoflurane in oxygen. Perioperative antibiotics were provided by cefazolin (30 mg/kg IV). During surgery, the dog was placed in the right lateral recumbent position, and the left forelimb and left lateral thorax were clipped and prepared aseptically. A lateral approach to the humeral condyle and epicondyle was selected to preserve the blood vessels and nerves surrounding the elbow joint. The incision made extended over the extensor muscle, starting distally in the intermuscular septum between the extensor carpi radialis and common digital extensor muscles, continuing proximally into the periosteal origin of the distal half of the extensor carpi radialis muscle on the lateral epicondylar crest of the humerus, the extensor carpi radialis muscle was elevated from the bone and underlying joint capsule.

Upon exposure of the supratrochlear foramen, a membrane‐like elastic tissue was observed (Figure 3A). Using a 24‐gauge catheter, the membrane was punctured, and intraluminal fluid was aspirated, revealing a clear‐to‐yellowish serous fluid, which was sampled for bacterial and fungal culture. The cranial part of the cyst covering the supratrochlear foramen was excised and sent for histopathological examination (Figure 3B). Palpation of the lumen confirmed a solid cortex on the medial and lateral surfaces, while an elastic membrane was adhered to the caudal surface. Curettage was performed to remove the membrane lining of the cyst and a 95% ethanol solution was injected to prevent recurrence due to the residual active lining of the membrane, inducing chemical cauterization. ¹⁴ , ¹⁶ Drilling was performed with a 0.7 drill bit from the distal diaphysis into the cyst cavity, followed by the installation of two 16‐gauge introducer needles (Figure 4A). Care was taken to avoid affecting the range of motion of the anconeal process in the elbow joint with the needles. The lateral ends of the needles were bent to ensure firm fixation and prevent skin puncture (Figure 4B), and patency was confirmed with saline (Figure 4C). The dissected muscles, subcutaneous tissue, and skin were routinely closed.

FIGURE 3.

(A) Lateral approach to the left humeral condyle and epicondyle. When exposing the supracondylar foramen, a membrane‐like elastic tissue (arrow) is visualized. (B) The cranial part of the cyst is excised (arrowhead).

FIGURE 4.

Installation of two introducer needles. Drilling with 0.7 drill bit (arrow) as a guide is followed by insertion of 16‐gauge introducer needle. (A) The location of introducer needle's end (arrowhead) is confirmed in the cyst lumen. (B) Two introducer needles (arrows) are installed on the distal humerus without limiting the range of motion of the anconeal process. The lateral ends of the needles are curved to confirm fixation. (C) The patency of the installed introducer needle is confirmed by inserting the 26‐gauge introducer needle into the opening of the needle (arrow) and injecting 0.9% normal saline.

A modified Robert Jones bandage was applied to prevent edema and external impact. The dog was hospitalized for 7 days, receiving analgesia (meloxicam 0.1 mg/kg SC, every 24 h) and antibiotics (cefazolin 30 mg/kg IV, every 12 h) because of the limitations in antimicrobial penetration in the intra‐articular environment and the risk of implant‐associated infection. Mobility restriction was maintained for 2 weeks for stabilization, and laser therapy was employed to promote bone healing.

Postoperative radiographs confirmed the placement of the introducer needles within the bone cyst (Figure 5). Cultures of the fluid and biopsy of the cyst tissue showed no infectious agents, and the tissue was characterized as fibrovascular tissue with scant chondrous differentiation and foci of chronic and active inflammation (Figure 6). No evidence of overt neoplastic processes was found, confirming unicameral bone cyst diagnosis.

FIGURE 5.

Immediate postoperative (A) craniocaudal and (B) mediolateral radiographs showing the installation of two introducer needles positioned within the cyst (arrow).

FIGURE 6.

Histopathologic examination of the biopsy tissue reveals fibrovascular tissue with chondrous differentiation and inflammation foci. No evidence of neoplastic processes or infectious agents is observed.

The dog was discharged to the care of his owner with gabapentin (10 mg/kg orally, every 12 h) and trazodone (3 mg/kg orally, every 12 h). The modified Robert Jones bandage remained in place for 7 days and was removed along with the sutures during a follow‐up visit. Subsequent follow‐ups at 1, 2, 4, 8, and 12 months post‐surgery revealed an increased cortical diameter at the supratrochlear foramen site and heightened opacity within the cyst on mediolateral radiographs (Figure 7). The dog's gait progressively improved, achieving full normalization without any signs of lameness at 1 month post‐surgery. No recurrence of lameness or bone cysts was observed during the 1 year of follow‐up, and an increase in cortical thickness was detected on radiographs, suggesting that the continuous decompression induced by the introducer needle was considerable.

FIGURE 7.

(A) Craniocaudal and (B) mediolateral radiographs of 1 (1), 2 (2), 4 (3), and 8 months (4), and 1 year (5) post‐surgery. Gradual increase of the cortical thickness of the humeral condyle and opacity (asterisks) are visible where the bone cyst is confirmed (arrows).

3. DISCUSSION

Unicameral bone cysts are benign cystic lesions that are commonly found in humans and horses but are rare in small animals. The prevalence of bone cysts in small animals is not well documented. ¹⁷ While they can manifest in any bone, they predominantly occur in the proximal or distal metaphysis of long bones, with over 80% reported in the proximal humerus and femur in humans. ¹⁸

Unicameral bone cysts are often asymptomatic and may be found incidentally. A complete body radiograph is recommended to detect multifocal lesions. CT is useful for identifying pathologic fractures that may not appear on radiographs. Definitive diagnosis requires histopathologic confirmation, typically obtained through fine‐needle aspiration or biopsy. ³

Although the unicameral bone cyst's pathogenesis is not well‐documented, previous studies found that the internal pressure of a unicameral bone cyst is higher than that of the contralateral limb's intraosseous pressure, and the external oxygen tension (PaO₂) of bone cyst's fluid is lower than that of intravascular and intra‐arterial PaO₂. ¹⁹ Consequently, interference with venous blood outflow prevents the circulation of intercellular fluids, forming a bone cyst. Additionally, the fluid content in unicameral bone cysts is suggested to be transudate, which supports the theory of vascular obstruction. ²⁰ Until recently, bone resorption by venous obstruction in cancellous bone and increased intraosseous pressure have been widely accepted as the pathogenesis of unicameral bone cysts.

Unicameral bone cysts can continue to increase in size. ²¹ Therefore, surgical treatment can be considered for cases presenting clinical signs, similar to this case report, to prevent pathologic fractures due to cortical thinning, deformity, or shortening of long bones. ¹¹ , ¹² The treatment method is determined based on lesion etiology and location. Several surgical treatments used include percutaneous aspiration and local steroid injection, open or percutaneous curettage and bone‐grafting, electrical or chemical cauterization, use of cannulated screws or flexible intramedullary nails for continuous decompression, and wide resection. ¹⁴ Typically, the initial surgical intervention has a favorable success rate; however, alternative surgical correction, other than wide resection, does not guarantee complete treatment and has a failure rate of 15%–30%. ² Because of the low prevalence in small dogs, information on treatment methods and success rates is limited.

In another veterinary study on the surgical management of unicameral bone cysts, bone grafting was used to prevent pathologic fractures during stabilization and to promote osteoconductive, osteoinductive, and osteogenic activity. ¹⁷ , ²¹ , ²² , ²³ , ²⁴ Notably, in this particular case, a bone graft was not performed because of concerns regarding the potential limitations in joint range of motion, potential interference with olecranon movement, and the risk of leakage outside the joint, which can lead to inflammation.

Ethanol cauterization is another surgical technique used in human medicine for the treatment of bone cysts. ¹⁴ , ²⁵ However, it can cause mild complications, such as local inflammatory reactions, skin discoloration at the injection site, and pain. ²⁵ Caution is needed for lesions near the growth plate, as complications such as growth disturbances or deformities might occur. ²⁵ In this case, ethanol was applied carefully to avoid leakage, followed by thorough irrigation, and no related complications were observed postoperatively.

However, the risk of recurrence associated with curettage and bone grafting is 45%. Considering the putative pathophysiology of unicameral bone cysts, the continuous drainage method was introduced. ¹⁵ Initially, this involved creating multiple drill holes using Kirschner wires, which showed early positive results; however, these holes were closed within 4–6 months, resulting in cyst recurrence. ¹⁹ , ²⁶ , ²⁷ To address this limitation, a method employing cannulated screws was introduced for the treatment of humans, demonstrating a higher healing rate compared to curettage and bone grafting. ¹⁵ , ²⁸ Nonetheless, the choice of the screw must align with the limb's girth. ¹⁴ However, the use of conventional cannulation poses a challenge for small dogs, because the diameter is too large for small dogs' bone thickness. To overcome this, a new device with an appropriate diameter for small dogs was needed to ensure effective continuous decompression. Inspired by the use of a 13‐gauge, 10 cm bone biopsy needle in humans for methylprednisolone injection, ²⁹ the authors were motivated to employ an introducer needle. In this patient, a 16‐gauge introducer needle with an external diameter of 1.63 mm was successfully used, and sufficient patency was confirmed. As a clear serous fluid was confirmed through surgery, the risk of reobstruction by debris was considered low. An additional decompression may be expected by placing the opening of the needle in the direction of gravity, but in this case, it was difficult to apply, due to the presence of the articular surface of the elbow in the direction of gravity. The absence of recurrence in this case suggested that this positioning did not adversely affect healing. However, given the combination of multiple surgical interventions, it is difficult to state with certainty how much the needle placement contributed to the outcome.

Although specific reports on the preoperative application of a modified Robert Jones Bandage in canine bone cyst cases are lacking, its application has potential benefits. In this case, the bandage was applied preoperatively to prevent traumatic pathologic fracture caused by external forces. Given the cortical thinning observed at the lesion site, this stabilization was critical to maintain structural integrity until surgical intervention.

In this case, there may be a question as to whether the fluid within the bone cyst was continuously generated and drained outside the bone. However, other studies have concluded that disturbed venous blood outflow within the bone obstructs the circulation of intercellular fluids, ultimately leading to cyst formation. ¹⁹ , ²⁰ Thus, relieving the pressure within the cyst using an introducer needle addresses the impairment of venous blood outflow, which is considered a key aspect of unicameral bone cyst treatment. Effective continuous decompression likely prevented fluid reaccumulation. The needle primarily functioned to normalize elevated intraosseous pressure from venous outflow obstruction, which is central to cyst pathogenesis. Another question that may arise is whether effective drainage can occur even if the opening of the introducer needle is not oriented in the direction of gravity. Unlike sesamoid bones, the metaphysis contains a medullary cavity and is composed of porous cancellous bone, which suggests that sufficient drainage can be achieved without relying on additional gravitational decompression. ³⁰ Furthermore, in this particular case, the articular surface was situated in the gravity‐dependent direction, and positioning the needle distally posed a risk of iatrogenic fracture due to cortical thinning, limiting its application. Nevertheless, orienting the opening in the direction of gravity based on the lesion's location and morphology could potentially enhance drainage efficiency.

Although needle patency could not be directly confirmed postoperatively, the absence of recurrence suggests that continuous decompression via the needle may have contributed to cyst resolution by alleviating intraosseous hypertension. This aligns with previous findings using cannulated screws, where continuous decompression was linked to higher healing rates and lower recurrence, supporting the role of mechanical decompression in targeting the proposed pathophysiology of venous obstruction and elevated intraosseous pressure. ¹⁴ , ¹⁵ While the independent efficacy of continuous decompression remains unproven, the favorable outcome in this case underscores its potential clinical value and justifies further investigation.

In conclusion, although bone cysts are rare in small dogs, they should be considered in a differential diagnosis for radiolucent lesions lacking neoplastic features, such as fibrous dysplasia. Before considering salvage procedures, such as arthrodesis or amputation, attempting less aggressive treatment options may still result in favorable outcomes. This case illustrates that a combination of traditional treatments and continuous decompression can be a feasible treatment strategy, especially when using an introducer needle in small dogs where the application of cannulated screws may pose a challenge.

AUTHOR CONTRIBUTIONS

Lee ES: Case management, manuscript, figure image preparation, drafted, and revised the manuscript. Kim SY: Collected data and revised the manuscript. Kim JI: Assisted with data collection and manuscript revision. Lee SH: Histopathologic analysis and interpretation (investigation). Yoon HY: Critical revision and final approval of the manuscript.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflicts of interest related to this study.

Lee E‐S, Kim S‐Y, Kim J‐I, Lee S‐H, Yoon H‐Y. Continuous decompression treatment of unicameral bone cyst in a small‐breed dog: A case report. Veterinary Surgery. 2025;54(8):1680‐1687. doi: 10.1111/vsu.14315