Pericollapse Stage of Osteonecrosis of the Femoral Head: A Last Chance for Joint Preservation

Zhang, Qing-Yu; Li, Zi-Rong; Gao, Fu-Qiang; Sun, Wei

A subchondral fracture exhibits a high T2-weighted signal on MRI but is often obscured by bone marrow edema (BME) and joint effusion. BME is an ill-defined, diffuse abnormality in the femoral head and neck presenting as decreased signal intensity on T1-weighted images. Accordingly, increased signal intensity is observed on T2-weighted images, manifesting elevated free-water content or bleeding in the bone marrow. In contrast to the recognition that BME is the early change in ONFH, BME accompanies disease progression and the onset of collapse, even before subchondral fracture becomes apparent on imaging studies. [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] More cases of subchondral fractures could be detected on additional CT and axial reconstruction is clearer than coronal reconstruction is. [20, 21] Meier et al. analyzed 37 symptomatic hips in 27 consecutive adult patients with both ONFH and associated BME. Although only 19 cases showed fracture lines on MRI, subchondral fractures were observed in all patients on CT, which recategorized the remaining 18 cases to ARCO Stage III/IV diseases. [20] Studies in pediatric and adolescent patients also demonstrated that the presence of BME on MRI in symptomatic ONFH patients correlated with the occurrence of early subchondral fractures. [18] Delineation of the necrotic area using the linear low-density band on T1-weighted images and the double-line sign on T2-weighted images on MRI with no collapse identified on anteroposterior and frog-leg lateral radiographs are widely used criteria that constitute the diagnosis of ARCO Stage I/II ONFH, which may result in the underdiagnosis of subchondral fractures. [3, 28, 29] A crescent sign or interruption of the outline of the femoral head appearing on the X-ray film also denotes intra-articular fracture and subchondral collapse, indicating Stage III of the Steinberg classification system. [30] Nevertheless, crescent signs may not be present or only appear in specific planes; under normal conditions, by the time, X-ray radiographs reveal macroscopic epiphyseal fractures, joint motion impairment has become apparent, and core decompression (CD) is less successful. [15] Clinical manifestations after occurrence of subchondral fractures include sudden pain in the groin or hips, painful limping, and aggravated pain with strong internal rotation; most patients are first referred to orthopedists due to these symptoms and signs. [21, 26] It should be noticed that although aggravated pain, the presence of BME on short-tau inversion recovery (STIR), and accompanied joint effusion on MRI may indicate a high possibility of the appearance of a subchondral fracture, the gold standard technique for diagnosing the pericollapse stage remains CT. These imaging and clinical findings testify extensive trabecular fractures, accompanied structural instability, and further collapse [ Figure 1 ]. [10, 31] PeRIcollaPse stage May foRecast stRuctuRal InstabIlIty of the feMoRal head Close relationships exist among subchondral fractures, ONFH-associated BME of the proximal femur, worsening of clinical symptoms and signs, hip joint effusion, and the ultimate collapse of the femoral head [ Figure 2 ]. [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [32] [33] [34] [35] In fact, the latter four phenomena may occur secondary to subchondral fractures, which forecast the progression of structural instability of the femoral head.