July 2012 - Donna Magid, M.D., M.Ed.

Nutshell Pelvis

POSITION: As in most plain films CANNOT interpret until ADEQUACY OF POSITIONING AND TECHNIQUE ascertained: is film well-positioned, both for over-all pelvis and for proximal femoral necks (often mis-called ‘hips’)?   If not—“Minimally limited exam due to rotation/underexposure/nonvisualization of…”

*Symmetry of OBTURATOR FORAMINA, pelvic inlet easier to see than subtle asymmetry of iliac wings. Obesity, scoliosis, pain/guarding, trauma dressings, lower extremity injuries, can all compromise positioning. CHECKLISTS less reliable if ROTATED.

*PROX.FEMURS: require slight internal rotation of feet to see true AP profile of neck, cleared of trochanteric superimposition, with ‘waist’ or slight biconcave medial/lateral neck outlines. EXT. Rotation common with pain, superimposes trochanter over distal neck LIMITS EXAM.

Bilateral Symmetry (obturator foramina, pelvic inlet, sacrum symmetry = best landmarks) in well-positioned pt. allows assessment of trabecular patterns, trabecular changes (destruction, permeation, thickening as in Paget Disease, sclerosis/blastic, neurological asymmetry leading to asymmetry of bone, muscle, stance, developmental issues such as cerebral palsy or polio, etc. Back up and look for Big Picture; let the bones ‘talk’!

Obturator foramina should be absolutely symmetric in size, shape; assesses positioning/rotation of pelvis.

CHECK old reports and old films (or document ‘none available”)

CHECK “Normal Variants Which Can Simulate Disease” text (T.Keats).

Correlate with physical findings and reported mechanism of injury, and level of clinical concern.


  • AP = standard view; no true lateral possible
  • Lateral hip: = lateral ONLY of proximal femur, NOT acetabulum
  • Frog-leg (abduction) and operating-room laterals (drop or raise nl. Leg out of way)
  • True laterals: atrocious dose and usually suboptimal
  • Inlet and Outlet: need not move patient; beam angles cephalad/caudal. Safer for trauma, easily replicated.
  • (Judet) obliques: 30-45 degree axial rotation to L/R; BUT pt. must move, therefore risk/discomfort

Have low threshold to get CT for trauma, pelvic ring issues, or bony/calcified abnormalities.

SACRUM, SI JOINTS: symmetry, width of SI joint (2-4 mm, ‘meanders’ AP and MedLat so can look like ‘double railroad tracks” distally. True SI joint is distal ¼, more proximal ¾= are strongest ligaments in body for stability in gait and position. Sacrum angles posteriorly ~30-45 degrees; that plus air/feces/osteopeni all contribute to difficulty visualizing.

Neural foramina roofs S1-2-3 look like arcuate pairs of “eyebrows’, smooth, flowing, symmetric

Normal Variants L5, S1: segmentation anomalies VERY common, check normal variant book!!

Pseudarthrosis if L5 transverse processes expanded, trying to articulate w/ superior margin S1 fine sclerotic line or ‘pseudo’ joint—can cause low back pain (LBP); S1-2 pseudarthrosis less common.

SYMPHYSIS: smooth, 3-4 mm space (up to 6 mm in kids, cartilaginous)

Congruous across superior margin; no step-off superiorly (exc. Post-partum)

Osteitis symphysis: Sclerotic, irregular margins, step-off, wear-and-tear appearance common post partum (vaginal deliveries); goes at end of exam (i.e. is a ‘don’t worry’). Check old films (and pt. gender!).

Obturator foramina margins follow around 360 degrees for symmetry, continuity, fine cortical margin.


  • Ilio-ischial: follow fine cortical line from distal lateral SI joint through lateral margin obturator foramin.
  • Ilio-pubic: same starting point, around pelvic inlet to superior symphysis
  • Teardrop: on well-positioned film, “J” or looped line medial inferior acetab. Coming off ilio-ischial line. Conceptually difficult - line is AP ‘summation’ of pelvic sidewall, or quadrilateral plate surface (QLPS).

ACETABULUM: 2/3 to 3/4 of nearly-spherical concavity covering nearly-spherical femoral head.

  • Sits in inverted ‘Y’: Ischium supports POSTERIOR,and superior pubic ramus ANTERIOR, acetababulum
  • Roof: ‘from 10 AM to 2 PM’ superior acetabulum, 2 mm radiodense line reflecting Wolffe’s Law (transmission of weight-bearing forces, entire body, through two small surfaces)
  • Dome: inverted-wine-glass or arch of finely-etched trabeculae sitting on roof, tracing the path of that weight-bearing transfer
  • Joint space ~ 3 mm and symmetric; widens and is more ill-defined medially at fossa
  • Anterior and Posterior acetabular rims cross the femoral head obliquely, anterior more medially
  • Head should be covered by and congruent with roof from ‘10AM-2 PM’

PROXIMAL FEMUR: (is NOT synonymous with ‘the hip,’ which =s prox. femur plus acetabulum).

POSITION - must see trochanters rotated OFF neck, revealing biconcave or ‘peanut’ shaped neck ie true AP neck. If not - “LIMITED EXAM L FEMORAL NECK” should lead off report.

Sphericity: 2/3 near-perfect sphere; motion, pressure and Wolffe’s Law induce matching acetabulum.

Articular surface: smooth, lateral head-neck junction to medial head-neck junction, exc. fovea centralis.

  • Tiny head-neck ‘bumps’ may signal early femoral-acetabular impingement (FAI).

Trabeculae: Compressive = direct vertical weight-bearing forces, headneckmedial cortex femur.

  • Tensile = smooth arcuate flow transmitting/lateralizing force to lateral cortex femur
  • Ward’s Triangle: relatively radiolucent trabecular-free triangle base femoral neck, can look lytic in osteopenic pt: check bilateral symmetry and continuity of trabecular etching.
  • Trabeculae should always look like fine flowing hand-drawn etching with sharp instrument.

Head-neck osteophytes, synovial herniation pits (oval 3-5 mm lucencies w/ fine sclerotic rim) femoral neck = early femoral-acetabular impingement i.e. early arthritis/altered mechanism joint.

SOFT TISSUE: bladder, pelvic side wall muscles, psoas margins, muscle masses, fibroids.

  • Assess obesity; pannus can limit detail. Know varied appearances of bowel air/feces.
  • See if ‘feces’ has moved relative to bone since last exam (subtle permeative bone lesions mimic feces). If air, feces, osteopenia obscure posterior ring, sacrum—say so!
  • Fascial planes subtle, symmetric relative lucencies; separate sidewall (obturator internus) muscle from   bladder (which may be indented by feces or uterus but should be midline, smooth)
  • Bladder midline whether full/empty; mass, hemorrhage, can displace, elevate or indent (feces, gravid or enlarged uterus, can also create smooth extrinsic indentations).
  • Normal calcifications: fibroids (‘popcorn’ calcifications), phleboliths (can recannulate and look like Cheerios), ligation materials, surgical material.
  • Injection site granulomata, bone islands, calcified enthesopathies (‘whiskers’ at tendonous attachments), all ‘Don’t-Worry’ and/or on old films.

Clinical/radiographic/historical doubt: GET CT!

July 2012 - Donna Magid, M.D., M.Ed.