What is spontaneous osteonecrosis of the knee? Spontaneous osteonecrosis of the knee is defined as osteonecrosis of the bone tissue of the knee that is caused by an idiopathic cause (i.e., no specific cause is known). It is usually common in middle-aged women, most often in the medial femoral condyle, but also in the lateral femoral condyle and tibial plateau, and can progress rapidly to the osteoarthritic stage of the medial femoral condyle, often with incomplete fractures. In the past, many patients have required total knee replacement surgery within months to years of onset. Causes of spontaneous osteonecrosis of the knee? We now know that almost all spontaneous osteonecrosis of the knee is due to a previously undiagnosed tear of the root of the medial meniscus. In these cases, often due to squatting positions, such as when cleaning floors, doing gardening work, or during deep squats for sports activities, such as skiing, the meniscal root can tear away from its attachment at the back of the tibial plateau at the back of the knee. In many patients, the meniscus root tear then slips outside the knee, which we call a circumferential protrusion, thus removing the main cushion against osteoarthritis on the inside of the knee. Biomechanical studies have shown that when a person tears the root of the meniscus, it is basically the complete removal of your medial meniscus. How do I treat spontaneous osteonecrosis of the knee? In patients with spontaneous osteonecrosis of the knee with an essentially normal cartilage surface, it is best to have the meniscal root tear repaired if possible. In these cases, restoring weight-bearing cushioning to the knee joint to promote healing of the meniscal root tear and weight-free crutching for a period of time while the meniscal root tear heals will always promote repair and healing of spontaneous osteonecrosis of the knee, even in the presence of an incomplete fracture. In this way most spontaneous osteonecrosis of the knee that is diagnosed early enough is able to heal. No more partial or total knee replacements due to unclear diagnosis and progression to advanced osteonecrosis regional collapse and advanced osteoarthritis. In most patients with a medial meniscal root tear with circumferential dislocation, early diagnosis and treatment will delay progression to the collapsed osteonecrotic area and advanced osteoarthritis stage in these patients. Frequently asked questions about osteonecrotic lesions of the knee Spontaneous osteonecrosis, called SONK, used to be thought to occur “spontaneously” as a result of inadequate response of the medial femoral condyle to loading stresses, eventually progressing to incomplete fracture and osteoarthritis of the medial compartment. It is common in middle-aged women and the exact cause is unknown. What is known is that most cases of spontaneous osteonecrosis of the knee are due to meniscal root tears that have not been diagnosed in the past. 1. What is spontaneous osteonecrosis of the knee? Spontaneous osteonecrosis of the knee is caused by overloading of the medial compartment, resulting in bone marrow edema that can be demonstrated on MRI, as well as overloading of the articular cartilage surface. These overloads can lead to incomplete fractures as well as rapidly progressive articular cartilage wear. Because professionals are well educated in meniscal root tears, we now recognize that most cases of spontaneous osteonecrosis of the knee are due to type II root tears of the posterior horn of the medial meniscus. 2. What is the presentation of spontaneous osteonecrosis of the knee on MRI? Spontaneous osteonecrosis of the knee is manifested on MRI as a diffusely increased signal on the fat-suppressed image of the medial femoral condyle. Incomplete fractures near the subchondral bone and articular cartilage surfaces are not uncommon. This is accompanied by a root tear of the posterior horn of the medial meniscus or a radial root tear. 3. What are the symptoms of spontaneous osteonecrosis of the knee? Most patients with spontaneous osteonecrosis of the knee initially experience pain in the back of the knee and a popping sensation when gardening, laying carpet, scrubbing floors, or doing deep knee flexion activities. Then, after a few weeks, they begin to experience significant pain on the medial side of the knee due to overloading of the medial femoral condyle bone and articular surface cartilage after the meniscus root has lost function. When a meniscal root tear is present, it often results in protrusion toward the outer edge of the joint and can lead to direct contact between the medial femoral condyle and the articular cartilage surfaces of the medial tibial plateau. In some patients, this can lead to a rapid progression of arthritis. 4. What is the treatment for spontaneous osteonecrosis of the knee? The first step in the treatment of spontaneous osteonecrosis of the knee is to determine its cause. If it is a meniscal root tear and the articular surface cartilage is still fairly intact, even if there is an incomplete fracture, repairing the meniscal root tear and then allowing the knee to avoid weight bearing for 6 weeks after surgery will often cure spontaneous osteonecrosis of the knee. In patients with pre-existing osteoarthritis, the use of crutches and a knee brace that allows for reduced weight bearing on the medial compartment allows for repair and healing of the osteonecrotic area, after which the need for a partial or total knee replacement is evaluated. 5. Why does spontaneous osteonecrosis occur after knee arthroscopy? Spontaneous osteonecrosis of the knee is thought to be caused by overloading of the medial compartment femoral condyle bone and articular surface cartilage. If there has been an excisional revision of the meniscal root or a radiographic tear that can lead to meniscal failure, these overloads due to the loss of shock absorption of the meniscus can lead to the development of spontaneous osteonecrosis of the knee. This is thought to be the main cause of spontaneous osteonecrosis of the knee after knee arthroscopy.T 6. Is the use of a bone scan useful in diagnosing spontaneous osteonecrosis of the knee? Although bone scans can show bone turnover and enhanced bone metabolism levels in the medial femoral condyle in patients with osteonecrosis of the knee, MRI scans are a more definitive and useful study for spontaneous osteonecrosis of the knee because they can show the underlying cause of SONK, which in approximately 80% of patients is due to a meniscal root tear or a radiographic root tear. 7. Is there a relationship between spontaneous osteonecrosis of the knee and osteoporosis? The majority of meniscal root tears occur in middle-aged female patients. These middle-aged female patients have an underlying bone loss or osteoporosis. Also, there is a relative decrease in bone mass due to decreased activity due to medial interval knee pain. In this way, when some patients have osteoporosis, it is important to recognize that the majority of patients with osteonecrosis of the knee are due to a meniscal root tear rather than underlying osteoporosis. 8. Is there a relationship between medial meniscus peripheral dislocation and spontaneous osteonecrosis of the knee? The majority of spontaneous osteonecrosis of the knee is due to a type II root tear of the posterior horn of the medial meniscus. Other types of meniscal dislocation include complete radial tears resulting in loss of meniscal function, or patients with severe osteoarthritis resulting in meniscal dislocation toward the periphery. The most common cause of spontaneous osteonecrosis of the knee is a meniscal root tear, which often causes the meniscus to dislocate toward the peripheral edge of the joint, which can lead to overloading, which can produce spontaneous osteonecrosis of the knee. In this way most cases of spontaneous osteonecrosis of the knee will have some degree of dislocation of the medial meniscus periphery present.