Hip pain is diagnostically heterogeneous because the hip is a deep, weight-bearing joint surrounded by complex osseous, cartilaginous, synovial, muscular and ligamentous structures. Conventional radiography remains an important initial investigation for many presentations of hip pain, but it may be insensitive to early marrow disease, early osteonecrosis, synovitis and several soft-tissue abnormalities. Current appropriateness guidance places radiography as the usual initial imaging examination for chronic hip pain, while MRI is particularly useful when radiographs are negative or nondiagnostic and a specific soft-tissue, marrow, inflammatory or intra-articular abnormality is suspected.¹ MRI is particularly valuable in femoral-head osteonecrosis because it can demonstrate pre-radiographic marrow abnormalities, delineate lesion extent and assist in staging. A systematic review and meta-analysis reported pooled MRI sensitivity and specificity of approximately 93% and 91%, respectively, for early osteonecrosis of the femoral head.² Earlier diagnostic studies also demonstrated substantially greater sensitivity of MRI than conventional radiography for osteonecrosis.³⁻⁵ Infective hip disease presents a different diagnostic challenge. MRI can demonstrate joint effusion, synovial thickening, bone-marrow edema, erosions and adjacent soft-tissue abnormalities; however, these findings may overlap between tuberculous and pyogenic infection. Aspiration, microbiology and/or histopathology therefore remain essential when infection is suspected.⁶˒¹⁶ MRI also contributes to characterization of pediatric hip disorders, including developmental dysplasia and Perthes disease, and to the detection and staging of osseous metastatic disease.¹⁴˒¹⁵˒¹⁷ The present study was undertaken to describe the diagnostic spectrum and MRI features in patients with painful hip presenting to a tertiary-care setting, with particular emphasis on avascular necrosis, septic arthritis, degenerative disease, pediatric/developmental disorders, metastatic disease and transient disorders. The study also examines characteristic MRI patterns that may assist clinical diagnosis, staging and differential diagnosis.⁶⁻⁸˒¹²˒¹⁸˒²⁰ Aim and Objectives Aim: To evaluate the diagnostic value of MRI in the assessment of painful hip joint. • To identify the spectrum of MRI findings in different conditions causing hip pain. • To assess the severity and extent of lesions using MRI. • To establish a differential diagnosis of painful hip disorders based on MRI findings.

Study design and setting: This was a hospital-based observational imaging study conducted over approximately 1.6 years in patients referred for MRI of the hip because of acute or chronic hip pain. The source document does not provide exact calendar dates. Participants: Fifty patients were included. Patients of all age groups and both sexes with acute or chronic unilateral or bilateral hip pain were eligible. Inclusion criteria: Acute or chronic unilateral or bilateral hip pain in patients referred for MRI. Exclusion criteria: Previous hip trauma; refusal to provide consent; contraindication to MRI; and patients receiving life-support systems. MRI protocol: Imaging was performed on a Siemens 1.5-T MAGNETOM Symphony system with the patient supine and both hips examined simultaneously using a body coil. Coronal T1-weighted, proton-density fat-suppressed, T2-weighted and STIR sequences were obtained. Sagittal and axial proton-density fat-suppressed/T2-weighted images were subsequently acquired. Axial gradient-echo imaging was obtained when required. For patients with clinically suspected infective pathology, renal function testing was performed before contrast administration; patients with normal renal function underwent contrast-enhanced MRI after fasting, with gadopentetate dimeglumine 0.1 mmol/kg and post-contrast T1-fat-suppressed imaging in axial, coronal and sagittal planes. Image interpretation and data collection: Relevant clinical and laboratory information was recorded on a prestructured proforma. MRI examinations were interpreted by an experienced radiologist and the findings were tabulated. Reference standard for infection: In the septic-arthritis subgroup, histopathology was used as the reference standard for the supplied diagnostic 2×2 tables. The source document does not describe a uniform reference standard for all other diagnoses; therefore, overall MRI sensitivity/specificity was not calculated. Statistical analysis: Descriptive statistics were used. Categorical variables were summarized as frequencies and percentages. The supplied 2×2 tables for tuberculous and pyogenic arthritis were independently recalculated using standard definitions of sensitivity, specificity and accuracy. No inferential analysis was performed for the overall diagnostic spectrum.

Fifty patients were evaluated. There were 35 males (70%) and 15 females (30%), giving a male-to-female ratio of 2.3:1. The age range was 2–74 years. Nineteen patients (38%) were 21–40 years old, 14 (28%) were 41–60 years, 11 (22%) were 0–20 years and 6 (12%) were 61–80 years. Sex n % Male 35 70 Female 15 30 Age group (years) n % 0–20 11 22 21–40 19 38 41–60 14 28 61–80 6 12 Eighteen patients (36%) had bilateral hip involvement and 32 (64%) had unilateral involvement. The diagnostic distribution is shown below. Diagnosis Patients (n=50) % Avascular necrosis 18 36 Septic arthritis 12 24 Osteoarthritis 8 16 Metastasis 3 6 Developmental dysplasia of hip 3 6 Perthes disease 2 4 Transient synovitis 2 4 Transient osteoporosis of hip 2 4 Age-wise distribution of pathology Age group AVN Septic arthritis OA Metastasis DDH Perthes Transient synovitis Transient osteoporosis 0–20 1 4 0 0 3 2 1 0 21–40 4 6 2 0 0 0 0 0 41–60 12 1 4 2 0 0 1 0 61–80 1 1 2 1 0 0 0 2 AVN and osteoarthritis were most frequently observed in the 41–60-year group. DDH and Perthes disease were confined to the 0–20-year group, while transient osteoporosis was most frequent in the 61–80-year group. Avascular necrosis AVN was identified in 18 patients (36%). Ten patients (55.5%) had bilateral disease and eight (44.4%) unilateral disease, corresponding to 28 affected hips. MRI detected all 28 involved hips. Idiopathic AVN was the most frequent reported risk category (10/18, 55.5%), followed by alcohol (4/18, 22.2%), smoking (2/18, 11.1%) and steroid exposure (2/18, 11.1%). Risk factor n % Idiopathic 10 55.5 Alcohol 4 22.2 Smoking 2 11.1 Steroids 2 11.1 ARCO stage Affected hips % ARCO I 0 0 ARCO II 3 10.7 ARCO III 11 39.2 ARCO IV 14 50.0 MRI finding Affected hips (n=28) % Subchondral signal abnormality 27 96.4 Femoral head fragmentation with collapse 16 57.1 Bone marrow edema 13 46.4 Double-line sign 10 35.7 Septic arthritis Twelve patients (24%) were classified as having septic arthritis. MRI showed synovial thickening and bone marrow edema in 11/12 patients (91.6% each), joint effusion and adjacent soft-tissue T2 hyperintensity in 10/12 (83.3% each), bone erosion in 9/12 (75%), and joint-space reduction in 3/12 (25%). MRI finding n (12) % Synovial thickening 11 91.6 Bone marrow edema 11 91.6 Joint effusion 10 83.3 Soft-tissue T2 hyperintensity 10 83.3 Bone erosion 9 75.0 Joint-space reduction 3 25.0 Histopathology-based diagnostic analysis of septic arthritis The source supplied 2×2 tables for tuberculous and pyogenic arthritis. Recalculation is necessary because the percentages printed in the source are mathematically inconsistent with the cell counts. For tuberculosis, the supplied matrix was TP=2, FP=3, FN=0, TN=10; therefore sensitivity was 100%, specificity 76.9%, and accuracy 80%. For pyogenic arthritis, the supplied matrix was TP=7, FP=0, FN=3, TN=2; therefore sensitivity was 70%, specificity 100%, and accuracy 75%. These calculations should be checked against the original patient-level dataset before submission. Condition TP FP FN TN Sensitivity Specificity Accuracy Tuberculous arthritis 2 3 0 10 100% 76.9% 80% Pyogenic arthritis 7 0 3 2 70% 100% 75% Osteoarthritis Eight patients (16%) had osteoarthritis; three had bilateral involvement and five unilateral involvement, producing 11 affected hips. MRI detected all eight patients. Among the 11 involved hips, marginal osteophytes, joint effusion and bone marrow edema were each present in 11 (100%); joint-space narrowing in 9 (81.8%), subchondral cysts in 7 (63.6%), and adjacent soft-tissue edema in 3 (27.2%). MRI finding Affected hips % Marginal osteophytes 11 100 Joint effusion 11 100 Bone marrow edema 11 100 Joint-space narrowing 9 81.8 Subchondral cysts 7 63.6 Adjacent soft-tissue edema 3 27.2 Metastasis Three patients (6%) had metastatic disease. The source described one 50-year-old woman with breast malignancy and two men with prostate carcinoma. MRI demonstrated T2/STIR hyperintensity in all three cases, T1 hypointensity in two, altered femoral contour in one and adjacent soft-tissue T2 hyperintensity in one. MRI finding n (3) % T2/STIR hyperintensity 3 100 T1 hypointensity 2 66.7 Altered femoral contour 1 33.3 Soft-tissue T2 hyperintensity 1 33.3 Developmental dysplasia of the hip Three patients (6%), all younger than 5 years, had DDH. Displaced epiphyses, dysplastic acetabulum and dysplastic femoral head were each present in all three patients; hyperintense epiphyseal signal and marrow edema were present in two. MRI finding n (3) % Displaced epiphysis 3 100 Dysplastic acetabulum 3 100 Dysplastic femoral head 3 100 Hyperintense epiphyseal signal 2 66.7 Bone marrow edema 2 66.7 Perthes disease Two patients (4%), both boys, were diagnosed with Perthes disease. Bone marrow edema was present in both patients, while epiphyseal T1 hypointensity was recorded in one patient in the table supplied with the source document. MRI finding n (2) % Bone marrow edema 2 100 Epiphyseal T1 hypointensity 1 50 Transient synovitis Two patients (4%) had transient synovitis. Joint effusion and synovial thickening were present in both patients. MRI finding n (2) % Joint effusion 2 100 Synovial thickening 2 100 Transient osteoporosis of the hip Two patients (4%) had transient osteoporosis. MRI demonstrated marrow edema of the femoral head and neck and joint effusion in both patients, with sparing/absence of subchondral lesions and edema. In one case, follow-up MRI at three months was reported as normal. MRI finding n (2) % Bone marrow edema in femoral head/neck 2 100 Joint effusion 2 100 Absence of subchondral lesions/edema 2 100

The present study demonstrates a broad diagnostic spectrum among patients referred for MRI because of hip pain and illustrates the ability of MRI to characterize osseous, marrow, synovial and periarticular abnormalities. The cohort was predominantly male (70%), and the largest age group was 21–40 years. Similar variation in demographic and diagnostic distributions has been described in institutional MRI series of painful or non-traumatic hip presentations, reflecting differences in referral patterns, age composition and inclusion criteria.⁶⁻⁹ AVN was the most frequent diagnosis in the present cohort (36%). This finding is consistent with published Indian MRI series in which osteonecrosis represents an important cause of painful-hip referrals.⁶⁻⁸ MRI is particularly valuable in osteonecrosis because marrow abnormalities can precede definitive radiographic changes and MRI can demonstrate the extent of disease and structural collapse.²⁻⁵˒²⁰ Meta-analytic evidence has reported pooled MRI sensitivity and specificity of approximately 93% and 91% for early femoral-head osteonecrosis.² In the present study, 50% of involved hips were ARCO stage IV and 39.2% were stage III, indicating that a substantial proportion of patients in this tertiary-care cohort had advanced structural disease at imaging. The dominant AVN feature was focal subchondral signal abnormality, present in 96.4% of involved hips, followed by femoral-head fragmentation with collapse (57.1%), bone-marrow edema (46.4%) and the double-line sign (35.7%). These findings are recognized MRI manifestations of osteonecrosis and reflect the spectrum from subchondral structural abnormality to collapse and reactive marrow change.³⁻⁵˒²⁰ The double-line sign, in particular, represents the interface between necrotic and reactive bone and is a characteristic MRI finding in established osteonecrosis.⁵˒²⁰ Septic arthritis represented the second-largest diagnostic group. Synovial thickening and bone-marrow edema were each present in 91.6% of cases, while joint effusion and adjacent soft-tissue T2 hyperintensity were present in 83.3%. These findings are consistent with the recognized MRI appearance of infective arthritis, although no single MRI feature is pathognomonic.⁶˒¹⁶ Differentiation between tuberculous and pyogenic arthritis may remain difficult on imaging alone, and microbiological sampling, aspiration and/or tissue diagnosis remain important for definitive classification.⁶˒¹⁶ The present source provided histopathology-based 2×2 tables; recalculation produced 100% sensitivity, 76.9% specificity and 80% accuracy for tuberculous arthritis, and 70% sensitivity, 100% specificity and 75% accuracy for pyogenic arthritis. These values should be verified against the original patient-level dataset before submission. Osteoarthritis was diagnosed in 16% of the cohort and involved 11 hips. Marginal osteophytes, joint effusion and bone-marrow edema were the most frequent MRI findings, followed by joint-space narrowing and subchondral cysts. MRI can demonstrate both structural and inflammatory components of degenerative hip disease, although imaging abnormalities should be interpreted in conjunction with symptoms and clinical examination because degenerative findings may not always be the sole source of pain.¹˒¹² Contemporary imaging guidance similarly supports radiography as the usual initial examination for many chronic hip-pain presentations, with MRI used selectively when the clinical question remains unresolved or marrow, soft-tissue or intra-articular pathology is suspected.¹ The pediatric component of the cohort included developmental dysplasia of the hip and Perthes disease. MRI demonstrated displaced epiphyses and dysplastic acetabular/femoral-head morphology in DDH, while Perthes disease was characterized by epiphyseal signal abnormality and bone-marrow edema. MRI is useful in pediatric hip disorders because it provides multiplanar assessment of the developing femoral head, cartilage, marrow and surrounding soft tissues.¹⁷ The two cases of transient synovitis showed joint effusion and synovial thickening. These findings may overlap with infection, and clinical assessment, inflammatory markers and follow-up remain important when septic arthritis is a consideration.⁶˒¹⁷ Metastatic disease was identified in three patients. MRI demonstrated marrow signal abnormalities and, in some cases, cortical contour alteration and adjacent soft-tissue extension. MRI is particularly useful in skeletal metastasis because it depicts medullary involvement and the relationship of disease to cortex and surrounding soft tissues, information that contributes to staging and treatment planning.¹⁴˒¹⁵ Transient osteoporosis was uncommon but diagnostically important. Both cases demonstrated diffuse marrow edema involving the femoral head and neck with relative subchondral sparing, and one case showed normalization on three-month follow-up MRI. This pattern has been described as characteristic of transient osteoporosis and may help distinguish it from osteonecrosis when interpreted alongside the clinical course and follow-up imaging.¹⁸˒²⁰ Several limitations should be acknowledged. First, this was a single-centre observational cohort of only 50 patients and therefore cannot provide population-level estimates of disease prevalence. Second, the source does not document a uniform independent reference standard for all MRI diagnoses; consequently, global sensitivity, specificity, predictive values and diagnostic accuracy should not be claimed. Third, some results were recorded at patient level while others were reported at hip level, which can create denominator differences in bilateral disease. Fourth, the source document contains a few internal inconsistencies in wording and numerical interpretation, including the printed infection sensitivity calculations; these have been transparently recalculated where possible. Finally, the source does not provide sufficient detail about blinding of the MRI reader, interobserver agreement, or formal sample-size calculation.

In this 50-patient hospital-based cohort, MRI demonstrated a wide spectrum of causes of painful hip, with AVN, septic arthritis and osteoarthritis accounting for the largest diagnostic groups. MRI characterized the location, extent and severity of disease, including ARCO staging in AVN and detailed marrow, synovial and soft-tissue abnormalities in infective, degenerative, developmental, neoplastic and transient disorders. The study supports the role of MRI as a problem-solving and staging modality in appropriately selected patients with hip pain. Diagnostic performance should be reported against a defined reference standard rather than inferred from MRI findings alone. Strengths and Limitations Strengths • Inclusion of patients across a broad age range. • Standardized 1.5-T MRI protocol with bilateral hip imaging. • Detailed characterization of several distinct hip pathologies. • Histopathology-based analysis available for the infective subgroup. Limitations • Single-centre, relatively small cohort. • No universal reference standard for all diagnoses. • Patient-level and hip-level denominators are mixed in some analyses. • Limited reporting of blinding and interobserver reliability. • Exact study dates and formal sample-size calculation are not provided in the source.

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