Brim Sign

1. High-Yield Overview & Pathognomonic Essence

• Radiographic Hallmark: Marked cortical thickening, dense sclerosis, and outward osseous expansion along the pelvic brim (the iliopectineal / arcuate line), transforming a normally slender 1 to 2 mm cortical line into a prominent, thickened, radio-opaque rim on anteroposterior (AP) pelvic radiographs.
• Primary Diagnostic Association: Pathognomonic radiographic sign of Paget’s disease of bone (osteitis deformans) involving the pelvic ring, the single most common skeletal site affected by Paget disease (documented in 70% to 75% of patients).
• Critical Diagnostic Utility: Serves as the quintessential radiographic differentiator separating pelvic Paget’s disease from osteoblastic metastases (e.g., metastatic prostate adenocarcinoma or breast carcinoma). Paget disease causes true bone enlargement, cortical expansion, and coarse trabecular remodeling, whereas osteoblastic metastases cause internal medullary sclerosis and trabecular obliteration without osseous expansion or cortical brim thickening.

2. Diagnostic Sign Summary Matrix

3. Clinical Case Imaging & Standardized Figures

4. Pathophysiology & Biomechanical Mechanism

The development of the pelvic brim sign in Paget’s disease of bone (osteitis deformans) reflects a profound disturbance of basic multicellular unit (BMU) remodeling coupled with the focal mechanical stress environment of the pelvic girdle. The sign emerges through distinct biological and biomechanical stages:

• Osteoclast and Osteoblast Remodeling Uncoupling: Paget’s disease is initiated by hyperactive, giant multinucleated osteoclasts containing up to 100 nuclei per cell. These abnormal osteoclasts, driven by mutations in the RANK-NF-kappa-B signaling cascade (frequently involving the SQSTM1/p62 gene), execute uncontrolled, aggressive bone resorption during the initial osteolytic phase. In response, osteoblasts are recruited in massive numbers during the second (mixed osteolytic-osteoblastic) phase. However, osteoblastic matrix production occurs at an accelerated, dysregulated rate without physiological lamellar organization. The resulting bone is structurally chaotic, highly vascular, and composed of woven bone intermixed with lamellar fragments. Over time, in the late inactive or sclerotic phase, osteoclastic resorption diminishes while disorganized osteoblastic bone deposition yields dense, mineralized bone characterized histologically by a pathognomonic “mosaic” or “jigsaw-puzzle” pattern of prominent, scalloped cement lines.
• Appositional Deposition and Osseous Expansion: Unlike secondary neoplastic processes (such as metastatic prostate or breast cancer), which deposit osteoid within the confines of pre-existing trabecular spaces, Pagetic remodeling involves both extensive endosteal remodeling and true subperiosteal appositional bone formation. This eccentric and concentric apposition causes true osseous expansion (bone enlargement). In tubular and flat bones alike, external dimensions expand outward while the cortex widens substantially.
• Mechanical Load Transmission Across the Arcuate Line: The pelvic brim represents the anatomical boundary between the pelvis major (false pelvis) and pelvis minor (true pelvis). Anatomically, it is formed by the arcuate line of the ilium and the pectineal line of the superior pubic ramus, extending from the sacroiliac joint to the pubic tubercle. Biomechanically, the iliopectineal line serves as the primary structural tension-compression strut transmitting weight-bearing forces between the axial vertebral column and the appendicular femoral head. In response to these persistent physiological mechanical load vectors, hyperdynamic Pagetic remodeling induces pronounced cortical thickening and trabecular strut reinforcement directly along this arcuate pathway. The normal delicate 1 to 2 mm cortical line progressively thickens into an expanded, dense sclerotic band measuring 8 to 15 mm or more in width, creating the classic brim sign.
• Hypervascular Marrow Transformation: During active remodeling, the medullary cavity undergoes extensive fibrovascular replacement. Capillary sinusoids dilate and form microscopic arteriovenous communications, resulting in local hyperemia, increased bone temperature, and elevated systemic cardiac output in severe polyostotic disease. This hyperdynamic circulation drives further subperiosteal osteoid deposition along lines of mechanical stress.

5. Modality-Specific Imaging Findings

Plain Radiography

Conventional plain radiography remains the definitive first-line diagnostic investigation. Anteroposterior (AP) pelvis and 45-degree Judet oblique views (iliac and obturator obliques) provide optimal visualization of the pelvic anatomy:

• The Pelvic Brim Sign: Marked thickening and dense sclerosis along the iliopectineal (arcuate) line. In contrast to the contralateral normal hemipelvis, which exhibits a delicate, sharply demarcated pencil-thin cortex, the affected brim shows prominent widening, dense radiopacity, and outward osseous expansion.
• Pelvic Teardrop Thickening: Concomitant cortical thickening of the pelvic teardrop (ilioischial line and quadrilateral plate), forming another reliable secondary sign of pelvic Paget disease.
• Coarsened Trabecular Architecture: The normal fine, cancellous trabecular meshwork is replaced by thickened, wavy, prominent trabecular struts oriented along primary stress vectors. This creates a striated, “honeycomb”, or “pumice-stone” appearance. Secondary, non-weight-bearing trabeculae are resorbed, enhancing the visual contrast of primary struts.
• True Bone Enlargement: The overall dimensions of the affected hemipelvis, iliac crest, ischial tuberosity, and pubic rami are increased compared to the uninvolved side, an essential discriminator that never occurs in metastatic disease.
• Acetabular Remodeling & Protrusio Acetabuli: Progressive weakening and remodeling of the acetabular floor under chronic femoral head loading leads to medial displacement of the femoral head beyond Kohler’s line (ilioischial line), producing protrusio acetabuli (Otto pelvis) and secondary concentric or medial hip joint space narrowing.

Computed Tomography (CT)

Multi-detector CT with thin-slice axial acquisitions and multiplanar reformations (sagittal, coronal, and 3D volume-rendered reconstructions) using high-resolution bone algorithms (window width ~2000 to 2500 HU, window level ~400 to 500 HU) provides precise architectural detail:

• Cross-Sectional Brim Anatomy: CT demonstrates concentric expansion of the anterior column, quadrilateral plate, and arcuate line with marked cortical hypertrophy and thickened, disorganized endosteal struts.
• Differentiation of Marrow Architecture: CT confirms the preservation of medullary fatty elements intermixed with coarse sclerotic trabeculae, verifying the absence of aggressive medullary replacement or permeative bone destruction.
• Exclusion of Cortical Breach: High-resolution CT reliably confirms the integrity of the expanded cortex, excluding focal lytic cortical destruction or breakthrough that would indicate secondary malignant transformation (e.g., secondary osteosarcoma).
• Preoperative Structural Assessment: In patients undergoing total hip arthroplasty, CT precisely maps the depth of medial protrusio, the thickness and density of the anterior and posterior columns, and the presence of subchondral acetabular cysts.

Magnetic Resonance Imaging & Nuclear Medicine

Magnetic resonance imaging and scintigraphic investigations offer critical metabolic and tissue-characterization capabilities:

• MRI Signal in Inactive (Sclerotic) Disease: Demonstrates thickened cortical boundaries with profound signal voids on both T1-weighted and T2-weighted sequences. The central medullary space maintains normal hyperintense T1 signal characteristic of unreplaced yellow marrow, providing robust proof of a benign process.
• MRI Signal in Active (Mixed/Lytic) Disease: Shows heterogeneous low-to-intermediate signal on T1-weighted images and elevated signal on T2-weighted and STIR sequences, reflecting vascular fibrovascular marrow stroma. Prominent signal voids representing hyperdynamic arterial flow vessels can be identified within the marrow cavity.
• Malignant Transformation Surveillance: Occurs in less than 1% of patients with Paget disease. Development of an invasive soft-tissue mass, focal destruction of the thickened cortex, or confluent homogeneous replacement of marrow fat on T1-weighted images strongly alerts the clinician to secondary osteosarcoma, fibrosarcoma, or undifferentiated pleomorphic sarcoma.
• Technetium-99m MDP Bone Scintigraphy: Demonstrates intense, uniform, avid radiotracer uptake (“super-uptake”) tracking the entire anatomical contour of the affected hemipelvis and pelvic brim. Scintigraphy is highly sensitive (>95%) for staging the complete skeletal distribution of disease, identifying clinically occult monostotic or polyostotic involvement.
• Fluorine-18 NaF PET/CT: Demonstrates intense fluoride uptake reflecting accelerated regional bone turnover, offering superior spatial resolution compared with planar scintigraphy.

6. Diagnostic Utility, Sensitivity & Specificity

The primary clinical dilemma in elderly patients presenting with pelvic sclerosis and elevated serum alkaline phosphatase is the differentiation between Paget’s disease and osteoblastic skeletal metastases, most notably from prostate adenocarcinoma in men and breast adenocarcinoma in women. The brim sign serves as the pivotal diagnostic discriminator:

• Osseous Expansion vs Invariant Bone Caliber: Paget disease causes true bone expansion (enlargement) of the hemipelvis and pelvic brim. Osteoblastic metastases stimulate osteoblasts to lay down woven bone within existing marrow spaces without altering the external cortical dimensions or expanding the pelvic contour.
• Cortical Thickening vs Preservation or Destruction: In Paget disease, the brim sign represents substantial cortical thickening along the iliopectineal line. In osteoblastic metastases, the cortex retains its normal thickness or becomes engulfed by amorphous medullary sclerosis without true cortical hypertrophy.
• Trabecular Coarsening vs Trabecular Obliteration: Paget disease reinforces primary stress trabeculae, yielding coarsened, striated struts. Metastases deposit dense, irregular osteoid that obliterates or “blots out” the underlying trabecular architecture, creating poorly defined, round or confluent “cotton-wool” sclerotic patches.
• Teardrop Involvement: In Paget disease, thickening of the pelvic teardrop accompanies the brim sign. In blastic metastases, the teardrop remains structurally normal or is nonspecifically obscured by diffuse radiodensity.
• Statistical Accuracy: Thickening of the pelvic brim is observed in approximately 85% to 90% of patients with pelvic Paget disease. When identified in conjunction with osseous enlargement and coarsened trabecular architecture, the radiographic specificity for Paget disease exceeds 95%, with a positive predictive value greater than 96%. This high diagnostic certainty frequently eliminates the need for invasive bone biopsy in elderly patients with elevated alkaline phosphatase.

7. Differential Diagnosis & Radiographic Mimics

8. Clinical Significance & Management Implications

Identification of the pelvic brim sign has direct therapeutic, reconstructive, and surgical ramifications for patient management:

• Hip Joint Biomechanics & Secondary Osteoarthritis: Involvement of the pelvic brim and periacetabular bone fundamentally alters the mechanical stiffness and load distribution of the acetabulum. As the disorganized woven bone deforms under physiological joint forces, patients develop premature secondary osteoarthritis characterized by progressive medial and axial joint space narrowing (distinct from idiopathic osteoarthritis, which predominantly causes superior/superolateral narrowing) and progressive protrusio acetabuli (Otto pelvis).
• Pelvic Insufficiency & Stress Fractures: Disorganized mosaic bone exhibits impaired tensile strength and increased brittleness. Micro-fissures and pseudofractures (Looser zones or banana-like insufficiency fractures) frequently occur along the convex tensile cortices of the pubic and ischial rami or along the thickened pelvic brim. Advanced imaging with CT or MRI is required when patients experience sudden, severe localized pelvic pain to rule out complete structural ring discontinuity.
• Targeted Medical Pharmacotherapy: Potent nitrogen-containing bisphosphonates represent the standard of care for active Paget disease. A single 5 mg intravenous infusion of zoledronic acid (Reclast / Aclasta) is the first-line therapeutic agent, achieving normal alkaline phosphatase levels in over 95% of patients with sustained remission lasting several years. Alternative regimens include oral risedronate (30 mg daily for 2 months). Medical suppression is mandatory for active disease involving high-risk weight-bearing sites (pelvic brim, acetabulum, femoral neck) to halt progressive deformity, relieve bone pain, and reduce intraosseous hypervascularity.
• Surgical Hemorrhage Mitigation in Total Hip Arthroplasty (THA): Active Pagetic pelvic bone is hypervascular, characterized by extensive arteriovenous communications within the marrow that can cause catastrophic intraoperative bleeding during acetabular reaming. Preoperative administration of intravenous zoledronic acid at least 6 to 12 weeks prior to elective THA suppresses osteoclast-driven hyperemia and substantially reduces surgical blood loss.
• Acetabular Reconstruction Nuances: THA in patients with pelvic Paget disease presents major technical challenges. Sclerotic bone can be exceedingly hard and brittle (“ivory bone”), predisposing to reamer deflection, inadequate cup seating, or periprosthetic fractures. Conversely, active lytic bone may be soft and structurally fragile. Protrusio acetabuli requires structural bone grafting (autograft or allograft morcellized bone) and multi-hole cementless cups with supplemental screw fixation to transfer loads to peripheral host bone. Modern cementless porous-coated or trabecular metal cups demonstrate superior long-term survival compared to cemented cups, which historically suffered from unacceptably high aseptic loosening rates.
• Heterotopic Ossification Prophylaxis: Patients with Paget disease undergoing hip reconstructive surgery carry an elevated risk of developing high-grade heterotopic ossification (Brooker class III-IV). Routine postoperative prophylaxis with oral non-steroidal anti-inflammatory drugs (indomethacin 75 mg daily for 3 to 6 weeks) or single-fraction targeted radiation therapy (700 to 800 cGy) is recommended.

9. Master Class Pro-Tip

Master Class Pro-Tip: When evaluating dense hemipelvic sclerosis in an elderly patient with elevated alkaline phosphatase, resist the reflex to order invasive prostate or bone biopsies until you have carefully scrutinized the pelvic brim and teardrop on the AP pelvis radiograph. The diagnostic hallmark of Paget’s disease—true bone enlargement, the brim sign (thickening and sclerosis of the iliopectineal line), and coarsened trabecular architecture—readily separates benign osteitis deformans from blastic metastases, which blot out the marrow space without expanding the bone cortex. Furthermore, if elective total hip arthroplasty is indicated for secondary Pagetic coxopathy or protrusio acetabuli, always check baseline serum alkaline phosphatase and administer intravenous zoledronic acid (5 mg) at least 6 to 12 weeks preoperatively. This crucial pause allows metabolic suppression of hyperdynamic intraosseous arteriovenous shunts, dramatically curbing potentially catastrophic intraoperative hemorrhage during acetabular reaming.

Master Class Radiological Pearl: Clinical and Surgical Management of Pelvic Paget’s Disease

10. Evidence-Based Synthesis

11. References

1. Marshall TR, Ling JT. The brim sign. A new sign found in Paget’s disease (osteitis deformans) of the pelvis. Am J Roentgenol Radium Ther Nucl Med. 1963;90:1267-1270. PubMed [Level IV: Original Case Series & Sign Validation]
2. Smith SE, Murphey MD, Motamedi K, Mulligan ME, Resnik CS, Gannon FH. From the archives of the AFIP. Radiologic spectrum of Paget disease of bone and its complications with pathologic correlation. RadioGraphics. 2002;22(5):1191-1216. DOI [Level V: Comprehensive Radiologic-Pathologic Review]
3. Ralston SH, Corral-Gudino L, Cooper C, Francis RM, Fraser WD, Gennari L, Guañabens N, Javaid MK, Layfield R, O’Neill TW, Russell RGG, Stone MD, Simpson K, Wilkinson D, Wills R, Zillikens MC, Tuck SP. Diagnosis and Management of Paget’s Disease of Bone in Adults: A Clinical Guideline. J Bone Miner Res. 2019;34(4):579-604. DOI [Level I: Evidence-Based Clinical Practice Guideline]
4. Theodorou DJ, Theodorou SJ, Kakitsubata Y. Imaging of Paget Disease of Bone and Its Musculoskeletal Complications: Review. AJR Am J Roentgenol. 2011;196(6 Suppl):S64-75. DOI [Level V: Comprehensive Musculoskeletal Imaging Review]
5. Lombardi AF, Aihara AY, Fernandes ADRC. Imaging of Paget’s Disease of Bone. Radiol Clin North Am. 2022;60(4):665-677. DOI [Level V: Multi-Modality Imaging Review]
6. Winn N, Lalam R, Cassar-Pullicino V. Imaging of Paget’s disease of bone. Wien Med Wochenschr. 2017;167(1-2):9-17. DOI [Level V: Clinical Imaging Review]
7. Singer FR, Bone HG, Hosking DJ, Lyles KW, Murad MH, Reid IR, Siris ES. Paget’s Disease of Bone: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2014;99(12):4408-4422. DOI [Level I: Clinical Practice Guideline]
8. Hadjipavlou AG, Gaitanis IN, Kontakis GM. Paget’s disease of the bone and its management. J Bone Joint Surg Br. 2002;84(2):160-169. DOI [Level V: Surgical and Clinical Review]
9. Helliwell PS, Porter G. Controlled study of the prevalence of radiological osteoarthritis in clinically unrecognised juxta-articular Paget’s disease. Ann Rheum Dis. 1999;58(12):762-767. DOI [Level III: Controlled Comparative Imaging Study]