Spontaneous Osteonecrosis of the Knee

Abstract

Background Spontaneous osteonecrosis of the knee (SONK), first described in 1968 (1), is an ailment that typically affects the medial femoral condyle (94%) (3,7), although it also has been described to affect the lateral femoral condyle, tibia, and patella (4,8,11). Pathophysiology Likely due to subchondral insufficiency fractures in osteopenic bone without evidence of necrosis (2,16). Edema from insufficiency fracture can lead to secondary focal ischemia and necrosis. Typical Clinical Presentation (7,12,15) Women who are 50 to 60 years or older Typically active, exercises, and unilateral History of osteoporosis/osteopenia Acute onset medial-sided knee pain without precipitating trauma Exacerbated with weight bearing The most common physical examination finding is severe pain with palpation over the medial femoral condyle (10). Important Historical Information History of surgery and/or trauma to the affected knee Disorders of bone Prior meniscal injury Pain usually worse at night and with weight bearing Prior injectable and/or oral steroid usage Imaging Four view radiographs (anterior-posterior, tunnel view, sunrise, and lateral) should be obtained. There will be flattening of the respective condyle and/or radiolucencies if SONK is in the late stages; otherwise, radiographs will be normal in the early stages. Magnetic resonance imaging (MRI) is the imaging modality of choice if SONK is suspected (Fig. 1) (9). Bone scans also may show SONK (Fig. 2) but are not as sensitive as MRI (7).Figure 1: T2 Weight Coronal MRI. Notice the bone marrow edema isolated at the medial femoral condyle.Figure 2: Bone Scan. Notice the signal enhancement isolated to the left medial femoral condyle.Characteristics of SONK-MRI Bone marrow edema out of proportion to osteoarthritic changes (Fig. 1) Subchondral crescent linear focus on T1 and potentially T2 sequences Focal epiphyseal contour depression Subchondral low signal (5,9) Association with meniscal tears, specifically radial and root tears (14) Differential Diagnosis SONK (subchondral insufficiency fracture) Bone contusion Transient osteoporosis of bone Osteochondritis dissecans Exacerbation of osteoarthritis Treatment Treatment and prognosis is dependent on the extent of T2 subchondral signal and size of subchondral lesion (7,9,17). If detected early and the subchondral lesion is small (one algorithm proposes less than 3.5 cm2) (7), nonsurgical management is appropriate. Nonsurgical management includes non-weight bearing versus protected weight bearing with a medial off-loader knee brace, analgesics, nonsteroidal anti-inflammatory drugs as needed, and potentially bisphosphonates (6,17). Weight bearing status is dependent upon symptoms. If the lesion is larger (>50% of the femoral condyle or lesion size is greater than 5 cm), patients do not improve nonoperatively, or radiographically after 3 months, there is an increased risk of collapse and surgical referral is indicated (7,10,13). Pearl If there is pain out of proportion to the examination over the medial femoral condyle without trauma and normal radiographs (except for osteoarthritic changes), one should have a low threshold to order an MRI to detect early onset of SONK.