Osteonecrosis of the knee (ONK) is a serious condition involving the death of bone tissue due to a lack of blood supply. Severity varies greatly, ranging from cases that resolve without aggressive intervention to those that lead to joint destruction and the need for major surgery. A timely diagnosis is the most important factor influencing the outcome, as the stage of progression dictates treatment options and the likelihood of preserving the natural knee joint. Delaying treatment significantly increases the risk of bone collapse and the rapid onset of debilitating osteoarthritis.
Understanding Osteonecrosis of the Knee
The underlying mechanism of ONK is ischemia, where blood flow is compromised to the subchondral bone, the dense layer just beneath the joint cartilage. This lack of oxygen and nutrients causes the bone cells to die, weakening the internal architecture of the bone. Over time, the dead bone area cannot withstand normal mechanical stress, leading to structural failure and collapse of the joint surface.
ONK is categorized into spontaneous (SONK), secondary, and post-arthroscopic types. Secondary osteonecrosis is linked to systemic factors that compromise the vascular system and often affects younger patients in multiple joints. Primary risk factors include chronic, high-dose corticosteroid use, excessive alcohol intake, and certain underlying systemic diseases.
Conditions like lupus, sickle cell anemia, and Gaucher’s disease increase the risk of developing ONK. Trauma, such as a severe knee injury, or the use of certain medications for HIV care can also predispose an individual to the condition. Women over the age of 60 are disproportionately affected by the spontaneous type of ONK, which primarily occurs on the inner side of the knee on the medial femoral condyle.
Stages of Progression and Severity
The severity of osteonecrosis is directly related to the stage of bone damage, which progresses predictably over time. In the earliest stages, a patient may experience sudden, intense pain, but standard X-rays often appear normal. Magnetic Resonance Imaging (MRI) is necessary at this point to detect the characteristic bone marrow edema and the crescent-shaped lesion of dead bone.
The next phase involves detectable changes in the bone structure, but without the joint surface collapsing, meaning the overlying cartilage is still intact. This is a transition period where the area of dead bone is structurally weak and the overlying joint surface risks flattening. Timely intervention in these early stages has the best chance of halting the disease and preventing further damage.
The disease becomes more severe when subchondral collapse occurs, marked by a visible flattening or depression of the femoral condyle on X-ray. This collapse creates a mechanical irregularity in the joint, leading to severe, chronic pain and the rapid breakdown of the articular cartilage. This structural failure marks the end-stage of the disease.
In the final, advanced stage, the joint space begins to narrow, and widespread degenerative changes like bone spurs develop. The joint surface is destroyed, and the knee becomes functionally compromised, leading to difficulty with standing and bearing weight. Joint-preserving measures are no longer effective at this stage.
Non-Surgical and Early Interventions
Management of ONK in the early stages focuses on joint preservation by reducing stress and promoting healing within the bone. For small lesions without any sign of collapse, conservative measures are often the first line of treatment. This typically involves protected weight-bearing using crutches or a cane for several weeks to months to reduce mechanical load on the weakened area.
Medications like non-steroidal anti-inflammatory drugs (NSAIDs) are used to manage pain and swelling in the joint. Physicians may also prescribe bisphosphonates, which help slow bone breakdown, or statins, which can alter fat metabolism that may contribute to the disease. These pharmacological treatments aim to create a more favorable environment for the bone to repair itself before structural failure occurs.
When conservative treatment is insufficient or the lesion is larger, minimally invasive surgical options are considered to prevent collapse. Core decompression is a common early surgical technique where small channels are drilled into the dead bone area. This procedure relieves internal pressure within the bone and encourages new blood vessels to grow, stimulating a healing response.
Core decompression can be combined with the use of bone grafts or the introduction of cellular therapies. These additions fill the void created by the drilling and supply regenerative cells to the necrotic area. These joint-sparing procedures are generally performed before the joint surface collapses, as their effectiveness drops dramatically afterward.
Surgical Management for Advanced Disease
When ONK progresses to the point of joint surface collapse, a major reconstructive surgical approach becomes necessary. The damage at this late stage is too extensive for joint-preserving techniques to be successful. The primary goal of these advanced surgical interventions is to restore joint function and eliminate pain caused by the structural deformity and severe arthritis.
One option is an osteotomy, which involves surgically cutting and realigning the bone to shift the weight-bearing axis away from the damaged area of the knee. This procedure is often utilized in younger, more active patients to delay the need for a full joint replacement. Recovery from an osteotomy is significant, but it can successfully restore a more natural mechanical environment to the joint.
For most patients with end-stage ONK, a knee replacement is the definitive treatment. A partial knee replacement, or unicompartmental arthroplasty, may be an option if the damage is confined to one side of the joint, typically the medial condyle. If the disease is extensive or has led to widespread arthritis, a total knee replacement (TKA) is required to resurface the entire joint.
Both types of arthroplasty involve removing the damaged bone and cartilage and replacing them with metal and plastic implants. These procedures offer reliable pain relief and restoration of mobility, but they represent the highest level of intervention for the disease. Recovery is a structured process involving physical therapy to regain strength and full range of motion in the artificial joint.