What is a Unicameral Bone Cyst?

Introduction

Endochondral ossification is one of the primary ways in which bone is formed during embryonic mesenchyme development. Bone tissue is formed through the replacement of hyaline cartilage, which forms the primary centre of ossification. Chondrocyte cells in this region begin to undergo heterotrophy and form the bone marrow cavity. Expansion of the bone marrow cavity goes on to form the bone shaft or diaphysis, and this connects with the epiphysis located at the proximal and distal ends of the bone via an interface known as the metaphysis. At the interface between the metaphysis and epiphysis is the epiphyseal growth plate; it is within this space that continued cartilage proliferation and maturation causes the elongation of bone tissue.¹ When growth ceases in adults, cartilage proliferation stops, and the physis are replaced by bone tissue that fuses the epiphysis and metaphysis.²

Unicameral bone cysts (UBCs) are fluid-filled lesions that occur most commonly in the metaphysis of the long bones of children and young adults. Although generally considered benign, as they tend to resolve on their own as the skeletal bone matures, they are prone to induce fractures and can have a significant impact on fracture healing and inherent tissue function. Some bone cysts, however, can cause complications in growth and run the risk of becoming necrotic if a fracture is experienced.³  

Epidemiology

Approximately 85% of UBCs occur in children and young adults. Given the physics of the femur and humerus are two of the most rapidly growing bones in the body, it is intuitive that nearly 90% of reported cases occur in the long bones.^{4, 5} UBCs tend to form primarily in the growth region of the humerus and femur, which then recedes after puberty. Unsurprisingly, the age range of patients with UBC is between three and fourteen, with the average patient age being 9 years.⁴ While 95% of reported UBCs involve long bones, it has been diagnosed in almost every human bone, including flat bones, which account for some 10% of cases.⁴ Additionally, approximately half of UBCs diagnosed in the proximal femur can be attributed to patients within a 17 to 54-year age bracket.⁵

Clinical features

UBCs, when they develop, are usually singular solitary cysts. However, during the healing process, multiple sections or septations can develop, turning a singular cyst into several cavities. The cyst as a whole is typically enclosed within a fibrous membrane that appears as either reddish brown or a grey-white. The membrane also tends to show bony ridges along its periphery.

When the cyst suffers a break or fracture, the infiltration of blood and other extra membranous components can cause complications to its inherent structure that can make treatment challenging. Fractures cause the cyst walls to thicken as fibroblasts, osteoclasts and hemosiderin, which subsequently cause calcification, further hardening of the membrane.⁴ Trauma to the cyst is also known to introduce foamy histiocytes, granulation tissue and cholesterol clefts.⁴

Diagnosis

Unicameral bone cysts are typically diagnosed and assessed using available cross-sectional imaging techniques such as X-ray, CT and MRI images. Unicameral bone cysts share pathophysiologic features with conditions such as aneurysmal bone cysts, enchondroma, eosinophilic granuloma and intraosseous ganglia.6 Therefore, it is important to carry out a differential diagnosis to confirm unicameral bone cysts. Careful assessment of images was obtained using the aforementioned methods, as each has specific pathological features such as signature fluid density and differentiating structural features such as the presence and number of septations. 

X-ray photography

Since simple bone cysts are nominally located in the diaphysis, X-rays are the most common option used by clinicians to diagnose UBCs. UBCs are routinely diagnosed with high accuracy using this method as a pathognomic ‘rising bubble sign’, which is considered specific for a unicameral cyst.⁷

Computerised tomography scans 

Computerised tomography (CT) imaging is used to diagnose UBCs that occur in areas that can not be readily ascertained using X-ray imaging. It is also used to evaluate cysts located in areas such as the pelvis or spine.⁸ CT scans are also adept at assessing fractured cysts and differentiating cysts from similar conditions, such as lipomas, for example.⁸

Magnetic resonance imaging       

When making a diagnosis, magnetic resonance imaging (MRI) is used together with either X-rays or CT scans to give more detail on the location and physiology of the cyst. This is particularly useful for assessing fractured cysts that have undergone structural changes. An MRI would be able to determine characteristics like the presence and viscosity of the fluid in the cyst, the thickness of the membrane wall, and if there is any significant physical alteration that may impede simple treatment procedures.⁴

Treatment

UBCs are generally benign, and most resolve on their own as bone growth matures and ceases. For bone cysts that do require treatment, approximately 70% of cysts heal completely.⁹ Of all cases, approximately 41% of UBCs are the cause of a pathological bone fracture, with a low proportion of these being prone to refracturing (4.1%).⁹

A number of treatment options are available for UBCs and these can be used singularly or in combination with one another.

Intralesional injections        

This process is the introduction of a drug directly into a lesion, or in this case, the cyst. The direct delivery of the drug to the target site allows for the use of a high concentration to obtain the maximum desired effect while simultaneously keeping any side effects that may be experienced by the body to an acceptable minimum. While there is currently no consensus on the ideal injectable to use for treating UBCs, steroids, bone marrow, demineralised bone, and synthetic bone substitutes such as calcium phosphate have been used.³ The implications of using these materials are unclear as there is little available data as to their safety and efficacy in the long term.¹⁰

Bone grafting

As bone tissue has superb regenerative capacities, surgically grafting a UBC with bone material obtained from an alternate site in the patient (autologous) is the optimal option for treating UBCs. Nonetheless, autologous bone grafting presents its own limitations: the procedure is invasive, and the creation of a secondary injury site rather than alternative means to obtain grafting materials remains a challenge. Bone substitutes, such as calcium phosphate or calcium sulphate, are good candidates as alternate graft materials because they are osteo-integrative and biodegradable.³ Bone substitute materials, however, possess very different mechanical and resorption characteristics from native bone, and these are reflected in their shorter durability and the potential to cause adverse reactions when in contact with other tissue types in the body.³

As a compromise, composite materials (a mixture of both natural and synthetic materials) have been fabricated. Composite materials allow for a middle ground to be reached by reducing the volume demand of autologous material and improving the resorption and mechanical strength of bone substitutes.¹¹

Other options for treating UBCs is to cause damage to the cyst’s lining to facilitate self-healing and ensure  structural stabilisation of cysts that have undergone a fracture.

Summary

The majority of UBCs develop in children and young adults. Although children and young adults, due to having actively growing long bones, are the main demographic, a proportion of cases have been documented in adults. UBCs can also develop in flat bones, usually due to the individual having suffered trauma to the bone, and as such, UBCs are known to be capable of occurring in almost every bone in the body.

These fluid filled lesions normally heal on their own during the growth process without the need for intervention. As UBCs are prone to fractures, complications can arise, bringing about the need for clinical intervention. Treatment approaches are varied but usually involve the use of bone or synthetic bone substitute materials introduced to the pathology site either via injection or surgical grafting.

Research is ongoing to understand the limitations of available treatment approaches and to ascertain how to fabricate new materials and methods that have the best possible outcome for the patient.