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Articular surface irregularities are often observed in collapsed femoral heads with osteonecrosis, while the effects of the degree of collapse on the articular surface are poorly understood. We first macroscopically assessed the articular surface irregularities on 2‐mm coronal slices obtained using high‐resolution microcomputed tomography of 76 surgically resected femoral heads with osteonecrosis. These irregularities were observed in 68/76 femoral heads, mainly at the lateral boundary of the necrotic region. The mean degree of collapse was significantly larger for femoral heads with articular surface irregularities than for those without (p < 0.0001). Receiver operating characteristic analysis showed that the cutoff value for the degree of collapse in femoral heads with articular surface irregularities at the lateral boundary was 1.1 mm. Next, for femoral heads with <3‐mm collapse (n = 28), articular surface irregularities were quantitatively assessed based on the number of automatically counted negative curvature points. Quantitative evaluation showed that the degree of collapse was positively correlated with the presence of articular surface irregularities (r = 0.95, p < 0.0001). Histological examination of articular cartilage above the necrotic region (n = 8) revealed cell necrosis in the calcified layer and abnormal cellular arrangement in the deep and middle layers. In conclusion, articular surface irregularities of the necrotic femoral head depended on the degree of collapse, and articular cartilage was already altered even in the absence of macroscopically determined gross irregularities. [ABSTRACT FROM AUTHOR]

Stress distribution remains unclear in early‐stage osteonecrosis of the femoral head (ONFH). To clarify this issue, we generated patient‐specific finite element models (FEMs) from 51 patients with ONFH. Patients' hips were classified into three groups: ONFH without a sclerotic boundary (Stage 1, n = 6), ONFH with a sclerotic boundary (Stage 2, n = 10), and ONFH with both a sclerotic boundary and <2 mm collapse (Stage 3, n = 35). Four hips without ONFH were used as controls. Stress distribution in each FEM was compared with magnetic resonance imaging (MRI) and computed tomography (CT) results. Fifteen wholly resected femoral heads in Stage 3 hips were assessed by micro‐CT. Furthermore, we histologically examined three Stage 2 femoral heads that subsequently developed subchondral fractures after FEM analyses. In all FEMs of both control and Stage 1 hip, stress was equally distributed on the femoral head surface. However, in all FEMs of both Stages 2 and 3 hips, stress was concentrated at the lateral boundary of the femoral head surface, corresponding to both a low‐intensity band on T1‐weighted MRI images and sclerotic changes on CT. On micro‐CT, subchondral fractures consistently began at the lateral boundary with sclerotic changes, in which bone volume fraction was increased. Histology showed breakage of subchondral plates at the junction between necrotic and reparative zones. In early‐stage ONFH, sclerotic changes caused stress concentration, which can trigger subchondral fractures at the lateral boundary. Clinical Significance: Our results will clarify the pathogenic mechanism of collapse in ONFH. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:3169–3177, 2018. [ABSTRACT FROM AUTHOR]

Objectives To quantify the volume of bone-resorptive lesions in post-collapse osteonecrosis of the femoral head (ONFH) using micro-computed tomography (micro-CT) and assess their characteristics in post-collapse ONFH. Methods We investigated 35 femoral heads resected from 35 patients with ONFH (20 men and 15 women; mean age, 47.2 years). On each of seven coronal high-resolution micro-CT slices of the femoral head, the bone-resorptive areas were extracted using bone microstructure measurement software. Next, the total bone-resorptive volume ratio, defined as the ratio of all bone-resorptive cross-sectional areas to all femoral head cross-sectional areas in all seven slices, was calculated. Associations between total bone-resorptive volume ratio and sex, age, ONFH-associated factors, patient workload levels, ONFH stage, ONFH type, necrotic volume on magnetic resonance imaging, and duration from the onset of pain to surgery were analyzed. Lesion location and the association between bone-resorptive lesion and collapse were also evaluated. Results The mean total bone-resorptive volume ratio was 7.0±6.0%, which varied significantly by ONFH stage (ARCO collapse quantitation 3A, 3.5±2.1%; 3B, 6.8±3.0%; and 3C, 13.6±8.8%). ONFH stage was independently associated with total bone-resorptive volume ratio (P<0.05). Furthermore, high bone-resorptive volume ratios were found in the anterior femoral head and were associated with collapse. Conclusions This study demonstrated that bone-resorptive volume in post-collapse ONFH was significantly associated with the disease stage, which was more widespread in the anterior portion of the femoral head than in the posterior portion. [ABSTRACT FROM AUTHOR]