Musculoskeletal differential diagnosis
Bone and soft-tissue differentials depend on age, location within the bone, margin and matrix. Each row records the feature that separated the alternative in the original case.
| Alternative diagnosis | Feature that argues against it | Case |
|---|---|---|
| Aneurysmal bone cyst | An aneurysmal bone cyst occurs in a younger patient, is metaphyseal and contains multiple fluid-fluid levels without solid enhancing tissue, whereas a giant cell tumour is epiphyseal and solid. | Giant cell tumour |
| Angiosarcoma | Angiosarcoma is more aggressive, with ill-defined margins, haemorrhage and necrosis, and it does not typically occur as a multifocal skin and muscle lesion in an HIV-positive young man. | Kaposi sarcoma with skin and muscle involvement |
| Avulsion fracture | An avulsion fracture shows a donor site and a displaced bony fragment with marrow oedema after trauma, and it does not have the chronic, corticated appearance of a carpal boss. | Carpal boss |
| Bacillary angiomatosis | Bacillary angiomatosis occurs in HIV patients and produces similar vascular lesions, but it responds rapidly to antibiotics and is not associated with the mucocutaneous distribution of Kaposi sarcoma. | Kaposi sarcoma with skin and muscle involvement |
| Bifid median nerve with persistent median artery | A persistent median artery lies between the two nerve bundles and shows a Doppler signal, whereas in this case the duplication is present without a central artery. | Duplicated median nerve |
| Brodie abscess | A Brodie abscess has a central lytic area with a thick sclerotic rim and adjacent soft-tissue inflammation, and the patient usually has a history of infection and positive cultures. | Intra-articular osteoid osteoma of the proximal phalanx |
| Calcific tendinitis | Calcific tendinitis produces amorphous calcification within the rotator cuff tendon, not multiple well-defined intra-articular chondral bodies. | Primary synovial chondromatosis |
| Calcium pyrophosphate deposition disease | CPPD produces punctate or linear chondrocalcinosis in the menisci and hyaline cartilage, not large amorphous periarticular calcific masses. | Hydroxyapatite deposition disease (HADD), intra-articular |
| Degenerative joint disease | Osteoarthrosis shows joint space narrowing, subchondral sclerosis and marginal osteophytes, and the calcified bodies are not multiple loose bodies within the joint recesses. | Primary synovial chondromatosis |
| Degenerative sacroiliitis | Degenerative disease produces osteophytes, subchondral sclerosis and vacuum phenomenon with a preserved joint space in older patients, without erosions, fever or a soft-tissue abscess. | Infective sacroiliitis complicated by a pelvic muscle abscess |
| Enchondroma | An enchondroma is centrally located, contains chondroid ring-and-arc calcification and does not extend to the subarticular surface or enhance avidly. | Giant cell tumour |
| Fibroma | A fibroma is a solid, low-T2-signal mass that enhances homogeneously and does not show the fluid-like, diffusion-restricted keratin content of an epidermoid cyst. | Subungual epidermoid cyst |
| Flexor tenosynovitis | Tenosynovitis produces fluid and synovial thickening around the flexor tendons, compressing a single median nerve rather than creating a duplicated nerve. | Duplicated median nerve |
| Ganglion cyst | A ganglion cyst is a fluid-filled lesion that follows water signal on all sequences, does not contain bone marrow or a bony protuberance and may communicate with the joint. | Carpal boss |
| Glomus tumour | A glomus tumour is a small, intensely enhancing, hypervascular lesion in the subungual region that is exquisitely painful and cold-sensitive, without cystic keratin signal. | Subungual epidermoid cyst |
| Gout | Gout shows punched-out juxta-articular erosions with overhanging edges and tophi of intermediate signal, and the calcification is not typically cloud-like and intra-articular. | Hydroxyapatite deposition disease (HADD), intra-articular |
| Infective sacroiliitis without abscess | Early infection may show only joint effusion and erosions, but the presence of a rim-enhancing pelvic muscle collection indicates complicated disease beyond the joint. | Infective sacroiliitis complicated by a pelvic muscle abscess |
| Inflammatory sacroiliitis | Inflammatory sacroiliitis is usually bilateral and symmetric in a young patient with inflammatory back pain and HLA-B27 association, without a fluid collection or systemic infection. | Infective sacroiliitis complicated by a pelvic muscle abscess |
| Intramuscular haematoma | A haematoma shows blood signal and a history of trauma, does not enhance internally and resolves on follow-up imaging. | Kaposi sarcoma with skin and muscle involvement |
| Intraosseous ganglion | An intraosseous ganglion is a small, well-defined subarticular cyst with fluid signal and a sclerotic rim, without the solid enhancing tissue of a giant cell tumour. | Giant cell tumour |
| Keratoacanthoma | A keratoacanthoma is a rapidly growing solid epidermal tumour with a central keratin plug that enhances and may invade the nail bed, unlike a simple subungual cyst. | Subungual epidermoid cyst |
| Lymphoma | Lymphoma produces a bulky, homogeneous mass with restricted diffusion and less avid enhancement, without the vascular, skin-based and fascial involvement seen here. | Kaposi sarcoma with skin and muscle involvement |
| Median nerve schwannoma | A schwannoma is a focal enhancing mass that displaces the nerve and does not show two discrete fascicular bundles with a normal nerve texture. | Duplicated median nerve |
| Monoarticular arthropathy | Inflammatory or degenerative monoarthritis shows uniform joint space narrowing, erosions and synovial proliferation without a discrete nidus on CT. | Intra-articular osteoid osteoma of the proximal phalanx |
| Nodular fasciitis | Nodular fasciitis is a small, rapidly growing nodule with a fascial tail and homogeneous enhancement, rather than a large mass with central ischaemia. | Fasciitis with ischaemic features |
| Normal variant median nerve | A normal median nerve is a single bundle without a longitudinal septum or two separate epineurial sheaths, as seen here. | Duplicated median nerve |
| Osteoarthrosis | Osteoarthrosis produces joint space narrowing, subchondral sclerosis and osteophytes around the affected joint, but it does not explain a discrete dorsal bony boss with overlying bursitis. | Carpal boss |
| Osteochondritis dissecans | Osteochondritis dissecans shows a donor site in the articular surface with a detached osteochondral fragment, rather than diffuse synovial chondromatosis. | Primary synovial chondromatosis |
| Osteophyte | A simple osteophyte is a marginal bony outgrowth of a degenerated joint without a separate ossicle or the characteristic carpometacarpal boss configuration. | Carpal boss |
| Postoperative seroma or haematoma | A seroma or haematoma is a fluid collection at the surgical bed with a thin enhancing rim and no solid enhancing component. | Fasciitis with ischaemic features |
| Proliferative fasciitis | Proliferative fasciitis contains ganglion-like cells and is a histological variant, and it shows similar fascial oedema without the central ischaemic necrosis seen here. | Fasciitis with ischaemic features |
| Rheumatoid arthritis | Rheumatoid arthritis causes marginal erosions, uniform joint space loss and pannus enhancement, without multiple calcified loose bodies. | Primary synovial chondromatosis |
| Sacral insufficiency fracture | An insufficiency fracture shows a sclerotic band or fracture line parallel to the sacroiliac joint without erosions, joint widening or an abscess. | Infective sacroiliitis complicated by a pelvic muscle abscess |
| Septic arthritis | Septic arthritis produces a hot, swollen joint with fever and rapid cartilage destruction, and it shows a joint effusion with synovial enhancement but no calcified nidus on CT. | Intra-articular osteoid osteoma of the proximal phalanx |
| Simple bone cyst | A simple bone cyst is central, fluid-filled and typically occurs in the proximal humerus or femur of a child, with no solid enhancing component. | Giant cell tumour |
| Soft-tissue sarcoma | A sarcoma is usually larger, with nodular enhancement, necrosis and invasion of adjacent structures, and it does not follow a fascial plane with the diffuse oedema typical of fasciitis. | Fasciitis with ischaemic features |
| Squamous cell carcinoma | Squamous cell carcinoma shows an irregular solid mass with bone destruction and marrow invasion, and it does not have the well-defined cystic appearance with peripheral enhancement. | Subungual epidermoid cyst |
| Stress fracture | A stress fracture shows a linear fracture line with periosteal reaction and marrow oedema, without a rounded nidus containing calcification. | Intra-articular osteoid osteoma of the proximal phalanx |
| Synovial chondromatosis | Synovial chondromatosis produces multiple well-defined osteochondral bodies with ring-and-arc calcification and marrow signal, rather than amorphous hydroxyapatite deposits. | Hydroxyapatite deposition disease (HADD), intra-articular |
| Synovial sarcoma | Synovial sarcoma is a soft-tissue mass that enhances and may calcify, but it arises adjacent to the joint rather than forming multiple intra-articular calcific bodies within synovium. | Hydroxyapatite deposition disease (HADD), intra-articular |
The confirmed diagnosis of each case is stated on the case page and is not repeated here. Sorted alphabetically; the order says nothing about probability.