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Differential Diagnosis of Musculoskeletal Disorders

Snapping Hip Syndrome!!navigator!!

  1. INTRAARTICULAR
    1. Osteocartilaginous bodies
  2. EXTRAARTICULAR = tendon slippage
    1. Fascia lata / gluteus maximus over greater trochanter
    2. Iliopsoas tendon over iliopectineal eminence
    3. Long head of biceps femoris over ischial tuberosity
    4. Iliofemoral ligament over anterior portion of hip capsule

Increase in Teardrop Width!!navigator!!

  • increase in distance between teardrop + femoral head
    Cause: hip joint effusion
  • increase in mediolateral size of teardrop
    Cause: hip dysplasia, chronic hip joint effusion during skeletal maturation

Protrusio Acetabuli!!navigator!!

= acetabular floor bulging into pelvis

  • center-edge angle of Wiberg of >40°
  • medial wall of acetabulum projecting medially to ilioischial line by >3 mm (in males) / >6 mm (in females)
  • crossing of medial + lateral components of pelvic “teardrop” (U-shaped radiodense area medial to hip joint)
  • Anatomy:
    1. lateral aspect = articular surface of acetabular fossa
    2. medial aspect = anteroinferior margin of quadrilateral surface of ilium)
  • obscured “teardrop” sign = pelvic teardrop obscured by femoral head
  1. UNILATERAL
    1. Tuberculous arthritis
    2. Trauma
    3. Fibrous dysplasia
  2. BILATERAL
    1. Rheumatoid arthritis
    2. Paget disease
    3. Osteomalacia

mnemonic: PROT

  • Paget disease
  • Rheumatoid arthritis
  • Osteomalacia (HPT)
  • Trauma

Pain with / after Hip Prosthesis!!navigator!!

= pain in groin / thigh after hip arthroplasty

Prevalence of pain: 40%; ~ 120,000 hip arthroplasties per year in USA

  1. Postoperative hematoma
    Incidence: 1.7% (within first 2 weeks)
    Cx: wound dehiscence, infection
  2. Heterotopic ossification
    Incidence: 50–60% (within 8 weeks after surgery)
    Risk factors: male gender, DISH, history of heterotopic ossification, osteoarthritis with preexisting heterotopic bone, ankylosing spondylitis
    • loss of motion ← ossifications bridging the joint
  3. Trochanteric bursitis
  4. Prosthetic / periprosthetic / cement fracture
    • audible crack during tapping of stem (intraoperative!)

    Incidence: in up to 18%
    Predisposition: osteoporosis, osteolysis, stress shielding (= bone resorption due to decreased stress to bone) typically at base of greater trochanter and calcar
  5. Dislocation
    Risk factors: component malposition, imbalance of tissue tension, implant design, surgical approach, extent of surgical soft-tissue dissection, small femoral head, failure of abductor mechanism
    1. posterior joint instability disruption of posterior joint capsule + short external rotator muscles / muscle atrophy
    2. anterior joint instability excessive acetabular cup anteversion >30° → impingement of femoral neck onto posterior rim of acetabular component → creation of posterior lever mechanism forcing femoral head out anteriorly
  6. Synovitis
    1. nonspecific mechanical irritation
      • small amount of joint fluid without debris
      • thin synovial lining
    2. polyethylene wear-induced = polymeric debris
      • slowly progressive typically bulky osteolysis
      • expansion of hip pseudocapsule by thick + particulate-appearing synovitis
    3. adverse local tissue reaction (metal hypersensitivity, metallosis)
      • synovial thickness >7 mm
    4. infection (see below)
  7. Iliopsoas impingement syndrome & tendinopathy
    Incidence: 4.3%
    Cause:
    1. idiopathic
    2. prominent oversized / malpositioned acetabular component
    3. retained cement
    4. excessively long iliac screws
    5. femoral head larger than native head
  8. gluteus medius and minimus tendon thickening / tear
  9. Aseptic loosening
    = complete loss of implant fixation
    Frequency: 50% of prostheses after 10 years; 30% require revision
    Cause:
    1. mechanical wear + tear of components
    2. small-particle disease
      • Path: particulate debris incites inflammatory / immune reaction → unsuccessful enzymatic destruction of debris → cytokines and proteolytic enzymes damage bone and cartilage → osteolysis
      • Histo: synovium-like pseudomembrane of histiocytes (95% of specimens), giant cells (80%), lymphocytes and plasma cells (25%), neutrophils (<10%)
    • thin and enlarging >2 mm radiolucent area around component / between cement mantle + bone
    • NEW radiolucent area <2 mm
    • increasing osteolysis (due to particulate debris with foreign body granuloma)
    • increasingly wide / asymmetric periprosthetic radiolucency
    • endosteal scalloping around femoral stem
    • pedestal formation = bone sclerosis distal to prosthetic tip in medullary canal ← micromotion
    • bead shedding = punctate pieces of metal around in-growth component
    • fractured cement mantle
    • fractured acetabular cup screw
    • newly tilted / migrated acetabular cup
    • rotated / migrated / toggled femoral stem:
      • subsidence (= distal migration) of prosthesis (up to 5 mm is normal for noncemented femoral component in first few months)
      • “sinking” of femoral flange into lesser trochanter
    • contrast medium between points of fixation
    • motion of components on stress views / fluoroscopy
    • inflammatory benign solid soft-tissue mass / pseudotumor around metal-on-metal implants

    Rx: 30% require single-stage revision arthroplasty
  10. Infection of Hip Prosthesis (= septic loosening)
    • Frequency: 0.3–1.7–9.0%; <2% of primary arthroplasties; <5% of revisions
    • Organism: Staphylococcus epidermidis (31%), Staphylococcus aureus (20%), Streptococcus viridans (11%), Escherichia coli (11%), Enterococcus faecalis (8%), group B streptococcus (5%)
    • Time of onset: ¹/³ within 3 months, ¹/³ within 1 year, ¹/³ >1 year
    • Path: bacteria bind to implant
    • Histo: neutrophils present in large numbers
    • Rx: excisional arthroplasty + protracted course of antimicrobial therapy + revision arthroplasty
    • clinical signs of infection often absent

    Plain film:
    • “aggressive” osteolysis with ill-defined margins ← particulate debris with foreign body granuloma / abscess
    • periostitis = periosteal new bone (100% specific, 16% sensitive for infection)
    • periarticular fluid collection with irregular walls communicating with joint (CT arthrography) and sinus track to skin

    NUC (83% sensitive, 88% specific): (see below)
    PET:
    • NO advantage over bone marrow imaging as a combination of 111In-labeled leukocytes and 99mTc sulfur colloid

    Arthrography:
    • irregularity of joint pseudocapsule
    • filling of nonbursal spaces / sinus tracts / abscess cavities

    Aspiration of fluid under fluoroscopy (12–93% sensitive, 83–92% specific for infection):
    • joint aspiration: high number of FP + FN
    • injection of contrast material to confirm intraarticular location

Evaluation of Total Hip Arthroplasty!!navigator!!

MEASUREMENTS

Reference line: transischial tuberosity line (R)

  1. Leg length = vertical position of acetabular component
    = comparing level of greater / lesser tuberosity (T) with respect to line R
    High placement: shorter leg, less effective muscles crossing the hip joint
    Low placement: longer leg, muscles stretched to point of spasm with risk of dislocation
  2. Vertical center of rotation
    = distance from center of femoral head (C) to line R
  3. Horizontal center of rotation
    = distance from center of femoral head (C) to teardrop / other medial landmark
    Lateral position: iliopsoas tendon crosses medial to femoral head center of rotation increasing risk of dislocation
  4. Lateral acetabular inclination = horizontal version
    = angle of cup in reference to line R (40° ± 10° desirable)
    Less angulation: stable hip, limited abduction
    Greater angulation: risk of hip dislocation
  5. Acetabular anteversion (15° ± 10° desirable)
    = lateral radiograph of groin
    Retroversion: risk of hip dislocation
  6. Varus / neutral / valgus stem position
    Varus position: tip of stem rests against lateral endosteum, increased risk for loosening
    Valgus position: tip of stem rests against medial endosteum, not a significant problem
  7. Femoral neck anteversion works synergistically with acetabular anteversion, true angle assessed by CT
  8. Cup overhang >12 mm associated with iliopsoas impingement, assessed by CT
  9. Unfavorable position of screw tip eg, abutting the L5 nerve root in sciatic notch

Type of Hip Prosthesis

  1. Hemiprosthesis
    • preservation of acetabular cartilage + subchondral bone plate
    • only femoral stem is fixed by
      1. bone cement (polymethylmethacrylate)
      2. press-fit / in-growth with textured surface facilitating in-growth of trabecular bone
    1. Unipolar head
      = 1 articulation between metal implant head + native acetabular cartilage
    2. Bipolar head
      = 2 articulations to improve range of motion:
      1. between inner head + liner of bipolar head
      2. between bipolar head (shell) + acetabulum
      • smooth outer surface
      • slightly greater than hemispheric shape
      • no screw holes
  2. Total prosthesis (mostly for treatment of arthritis)
    • acetabulum reamed
    1. Conventional
      • screw holes in acetabular cup
    2. Resurfacing
      • = replacing articulating surfaces of hip joint and removing very little bone
      • small acetabular cup in pelvic socket
      • femoral head component seated with a pegged stem inside a preserved femoral neck

Radiographic Findings in Total Hip Arthroplasty

  1. NORMAL
    • irregular cement-bone interface
      • = normal interdigitation of PMMA (polymethylmethacrylate) with adjacent bone remodeling providing a mechanical interlock
      • PMMA is not a glue!
    • thin lucent line along cement-bone interface
      • = 0.1–1.5-mm thin connective tissue membrane (“demarcation”) along cement-bone interface accompanied by thin line of bone sclerosis
  2. ABNORMAL
    • wide lucent zone at cement-bone interface
      • = ≥2-mm lucent line along bone-cement interface due to granulomatous membrane
      • Cause: component loosening ± reaction to particulate debris (eg, PMMA, polyethylene)
    • lucent zone at metal-cement interface along proximal lateral aspect of femoral stem
      • = suboptimal metal-cement contact at time of surgery / loosening
    • well-defined area of bone destruction (= histiocytic response, aggressive granulomatous disease)
      • Cause: granulomatous reaction as response to particulate debris / infection / tumor
    • asymmetric positioning of femoral head within acetabular component
      • Cause: acetabular wear / dislocation of femoral head / acetabular disruption / liner displacement / deformity
    • cement fracture
      • Cause: loosening

Scintigraphy for Prosthetic Failure!!navigator!!

  • increased uptake of bone agent, 67Gallium, 111Indium-labeled leukocytes, complementary technetium-labeled sulfur colloid + combinations
  1. Bone Scintigraphy (high NPV):
    • negative
      • periprosthetic uptake indistinguishable from surrounding nonarticular bone = no prosthetic abnormality (= high NPV)
    • positive for infection / loosening:
      • diffuse intense uptake around femoral component (= generalized osteolysis unreliable in separating infection from loosening)
        • diffuse periprosthetic uptake favors infection
        • focal uptake at distal tip of femoral component in >1 year old prosthesis = aseptic loosening
    • nonspecific:
      • periprosthetic activity generally decreases with time:
        • variable uptake patterns in 1st year after implantation
        • persistent uptake >1 year is frequent in cementless / porous-coated hip replacements
  2. Sequential bone/gallium scintigraphy (60–80% accurate, modest improvement over bone scintigraphy alone):
    • negative for infection:
      • gallium distribution normal regardless of findings on bone imaging
      • spatially congruent distribution of both radiotracers + gallium intensity less than bone tracer
    • inconclusive:
      • spatially congruent radiotracer distribution + similar uptake intensity for both radiotracers
    • positive for infection:
      • spatially incongruent distribution of the 2 radiotracers
      • gallium uptake intensity exceeds that of bone agent
  3. Labeled leukocyte scintigraphy:
    • positive for infection:
      • intensity exceeds that of a reference point
      • activity outside normal distribution
  4. Combined labeled leukocyte–marrow scintigraphy
    = WBC/sulfur colloid scintigraphy (study of choice):
    • Accuracy: 88–98%
    • Concept:99mTc-sulfur colloid maps aberrantly located normal bone marrow as a point of reference for leukocyte tracer
    • positive for infection:
      • labeled leukocyte activity without corresponding sulfur colloid activity ← osteomyelitis stimulates WBC uptake + depresses sulfur colloid uptake
    • negative for infection:
      • spatially congruent distribution of both radiotracers / any other pattern of uptake

Outline