Knee Arthritis: Diagnostic Imaging Approaches II | El Paso, TX.

Sagittal Fluid Sensitivity

• Sagittal Fluid Sensitive MR slice showing large synovial popliteal (Baker’s) cyst (above top image) and large synovial effusion (above bottom image)
• Note multiple patchy dark signal areas on both images, representing fibrinoid inflammatory depostits aka “rice bodies” a characteristic MRI feature of RA

Management Rheumatological Referral & DMARM

• Conservative management followed by operative care in complicated cases of tendon ruptures and joints dislocations
• Supplemental reading:
• Diagnosis and Management of Rheumatoid Arthritis – AAFP
• www.aafp.org/afp/2011/1201/p1245.html

Septic Arthritis (SA)

• Septic arthritis – d/t bacterial or fungal contamination of the joint. SA may cause rapid joint destruction and requires prompt Dx and antibiotic administration
• Joints affected: large joints with rich blood supply (knee 50%>Routes of Joint Contamination
  • 1. Hematogenous (M/C)
  • 2. Spread from the adjacent site
  • 3. Direct inoculation
  • M/C organism-Staph aureus
  • N.B Gonococcal infection may be a top differential in some cases
  • IV drug users: pseudomonas, candida
  • Sickle cell: Salmonella
  • Animal (cats/dogs) bites: Pasteurella
  • Occaisonally fungal contamination may occur

  Radiography

  • Initially non-specific ST/joint effusion, obscuration/distortion of fat planes. Because it takes 30% of compact and 50-75% trabecula bone to be destroyed before seen on x-rays, radiography is insensitive to some of the early changes. MR imaging is the preferred modality
  • If MRI is not available or contraindicated. Bone scintigraphy with Tc-99 MDT can help
  • In children US preferred to avoid ionizing radiation. In children US can be more sensitive than in adults due to lack of bone maturation

  Radiographic Dx

  • Early findings are unrewarding. Early features may include: joint widening d/t effusion. Soft tissue swelling and obscuration/displacement of fat planes
  • 1-2 weeks: periarticular and adjacent osseous changes manifesting as patchy demineralization, moth-eaten, permeating bone destruction, loss and indistinctness of the epiphyseal “white cortical line” with increase in soft tissue swelling. MRI may be helpful with early Dx.
  • Late features: complete joint destruction and ankyloses
  • N.B. Septic arthritis may progress rapidly within days and requires early I.V. antibiotic to prevent major joint destruction

  T1 & T2 Knee MRI

  • T1 (above left) and T2 fat-sat sagittal knee MRI slices reveal loss of normal marrow signal on T1 and increase on T2 due to septic edema. Bone sequestrum d/t osteomyelitis progressing into septic arthritis is noted. Marked joint effusion with adjacent soft tissue edema are seen. Dx: OSM and septic arthritis
  • Imaging may help the Dx of septic joint. However, the final Dx is based on Hx, physical examination, blood tests and most importantly synovial aspiration (arthrosentisis)
  • Synovial fluid should be sent for Gram staining, culture, glucose testing, leukocyte count, and differential determination
  • ESR/CRP may be elevated
  • Synovial fluid: WBC can be 50,000-60,000/ul, with 80% neutrophils with depleted glucose levels Gram stain: in 75% gram-positive cocci. Gram staining is less sensitive in gonococcal infection with only 25% of cultures +
  • In 9% of cases, blood cultures are the only source of pathogen identification and should be obtained before antibiotic treatment
  • Articles: www.aafp.org/afp/2011/0915/p653.html  
  • www.aafp.org/afp/2016/1115/p810.html

  Crystal-Induced Knee Arthritis

  • Crystalline arthritis: a group of arthropathies resulting from crystal deposition in and around the joint.
  • 2-m/c: Monosodium urate crystals (MSU)  and Calcium Pyrophosphate Dehydrate crystals (CPPD) arthropathy 
  • Gout: MSU deposition in and around joints and soft tissues. Elevated levels of seurm uric acid (UA) (>7mg/dL) causd by overproduction or under-excretion of uric acid
  • Once UA reached/exceeded 7mg/dL it will deposit in the peripheral tissues. Primary gout: disturbed metabolism of nucleic acids and purines breakdown. Secondary gout: increased cell turnover: Psoriasis, leukemia, multiple myeloma, hemolysis, chemotherapy etc.
  • Gout presents with 5-characteristic stages:
  • 1)asymptomatic hyperuricemia (years/decades)
  • acute attacks of gouty arthritis (waxes and wanes and lasts for several years)
  • Interval phase between attacks
  • Chronic tophacious gout
  • Gouty nephropathy

  Clinical Presentation

  • Depends  on stages
  • Acute attacks: acute joint pain “first and the worst” even painful to light touch
  • DDx: septic joint (both may co-exist) bursitis etc.
  • Gouty arthritis typically presents as monoarthropathy
  • Chronic tophacious stage: deposits in joints, ear pinna, ocular structures and other regions. Nephrolithiasis etc. Men>women. Obesity, diet, and age >50-60.
  • Radiography: early attacks are unremarkable and may present as non-specific joint effusion
  • Chronic tophacious gout radiography: punched out peri-articular, para-aticular and/or intraosseous erosions with overhanging edges. Characteristic rim of sclerosis and internal calcification, soft tissue tophi. Target sites: lower extremity m/c
  • Rx: allopurinol, colchicin (esp. preventing acute episodes and maintenance)

  Synovial Aspiration

  • Synovial aspiration with polarized microscopy reveal negatively birefringent needle-shaped MSU crystals with large inflammatory PMN presence. DDx: positively birefringent rhomboid-shaped CPPD crystals (above bottom right) seen in Pseudogout and CPPD

  Large S.T.

  • Density and joint effusion puched out osseous erosion with overhanging margins, overall perservation of bone density, internal calcifications Dx: chronic tophacious gout

  MRI Gout Features

  • Erosions with overhanging margins, low signal on T1 and high on T2 and fat suppressed images. Peripheral contrast enhancement of tophacious deposits d/t granulation tissue
  • Dx: final Dx; synovial aspiration and polarized microscopy

  Additional Articles

  • 10-facts about gout www.pharmacytimes.com/publications/issue/2013/july2013/a-bout-of-gout-a-region-of-pain
  • www.aafp.org/afp/1999/0215/p925.html

  Knee Arthritis

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