Osteoarthritis (OA) is a debilitating disease that affects millions of people. It is characterized by the degeneration of articular cartilage, narrowing of the joint cavity, damage to the subchondral bone, loss of synovial fluid, and osteophyte formation. These symptoms can cause muscle weakness, decrease in range of motion at the joint, and pain. Pain is the symptom that is most frequently treated. However, pinpointing the exact origin and location that is causative to the pain can be difficult. Patients of OA commonly have areas of subchondral edema identified on an MRI as marrow bone lesions (BML). On a CT image, these BML are found to be areas of bone resorption within the subchondral bone of the affected joint called subchondral bone cysts (SBC). These cysts are hypothesized to be a source of pain in OA as well as progression of the disease. The synovial intrusion theory for cyst formation states that there is a physical connection made between the joint cavity and the SBC through which synovial fluid travels. The bony contusion theory describes micro cracks developing below the articular cartilage within the subchondral bone causing bone necrosis and SBC formation. This study was undertaken to investigate the histological presentation of the contents of the SBC and the surrounding area.Seven human femoral head samples, ranging from age 49-72 years old, from patients who received total hip arthroplasty due to OA were examined. Three areas were sectioned from each femoral head including an area containing a cyst, an area of primary compressive bone, and an area at the medial side of the femoral head. These sections were then stained for bone, cartilage, and nerves and examined histologically. Sclerotic bone was shown to surround each cyst cavity, while cysts were composed of a mixed connective tissue infiltrated with multiple blood vessels and potential nerve fibers. With further investigation of these structures, the location of the nerve fibers within the SBC could be a possible source of pain in OA and a target for future treatments and therapies. Within the femoral head, cysts were found to be both shallow and deep to the articular cartilage. Shallow cysts may support the synovial intrusion theory for cyst formation while deep cysts could support the bony contusion theory. In relation to articular cartilage, cysts were not only found below a degraded articular cartilage surface as expected but also below intact, less degraded cartilage. This in depth look at the cells and tissues present within and surrounding the cyst provides information to better understand the pathology of OA and possibly an alternative method for treating the disease.

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