July 2012 - Donna Magid, M.D., M.Ed.
Nutshell Developmental Hip Dysplasia
No longer called “congenital DH’ (or CDH) since many, esp. Cerebral palsy patients (CP), born with apparently normal hips which become increasingly dysplastic with on-going modified weight bearing and long-tem muscle imbalances (Wolff’s Law...again). Early diagnosis is key; the more time spent with the proximal femur and acetabulum correctly aligned, the less the long-term deformity, dysplagia, and disability. The first few years are the most ‘plastic’ in terms of reshaping a potentially dysplastic hip.
1 to 1.5/1000 live births, Left>Right (fetal lie across sacrum), 80% female
- Pre-cartilaginous anlage cleaves 7-11 wks, forming hip joint.
- Head grows faster than acetabulum, free ROM in utero induces sphericity of both.
- Laxity and uncovered head maximizes ROM for safer vaginal delivery.
- Acetabular growth spurt first few weeks.
- Action of head against acetabulum INDUCES spherical ‘ball’ in spherical ‘socket’
- Neonate pelvis mostly cartilage; fetal hip flexion→ limited XR landmarks, measurements
- Some genetic/ethnic influence/tendency (Native American, Lapp, etc.)
- Limited intrauterine space: first pregnancy, multiple fetuses, disproportionate paternal size, oligohydramnios
- Breech birth (25-40% of DDH?), over-swaddling
- Prenatal conditions: arthrogryposis, neurologic disease, Down and certain syndromes
- CP or neurologic imbalances: Wollf’s law
CAPITAL EPIPHYSIS (femoral head ossification center) does not ossify until 6-9 months. Once visible should be symmetric in size, shape, position, radiodensity. Successful treatment only with early/immediate diagnosis - clinical suspicion/experience primary to diagnosis. Many neonatal hips indeterminate or subluxable but rapidly ‘tighten’; physical exam highly operator-dependent
RADIOGRAPHIC LANDMARKS AND SIGNS (see drawing)
Asymmetry of capital epiphysis ossification, size, shape, and/or radiodensity. Describe.
HILGENREINER’S LINE (A): horizontal baseline across superior bony margin of triradiate cartilage lucency (i.e. superior aspect of the normal radiolucency mid-acetabulum).
PERKIN’S LINE (B): from superior lateral ossified acetabulum dropping perpendicularly to Hilgenreiner’s line. Ossification center should be in medial inferior quadrant formed by 2 lines’ junction
ACETABULAR ANGLE (D): angle formed by Hilgenreiner’s line and line connecting most inferior to most superior ossified acetabular landmarks. Normal ~28 degrees and symmetric; then decreases with age as acetabulum deepens/ossifies. In DDH: increased because acetabular growth/induction delayed→ shallow, vertical, acetabulum.
SHENTON’S LINE (C): reflects lateral subluxation of proximal femur rather than dysplasia, but persisting subluxation will produce or precedes dysplasia. Normal→ smooth arc or curve from inferior margin superior pubic ramus to medial femoral neck. In DDH this radius flattens and widens or elongates to more oval or egg-shaped. Caveat: Obturator foramina must look nearly symmetric to confirm pt. not rotated side-to-side. Lateral rotation throws Shenton—and other observations—off.
“ASSESS FOR DDH”: imaging options (Check ACR Appropriateness Criteria!)
US best first few months (operator-dependent); and no ionizing radiation. Also allows dynamic imaging (hip maneuvered into different position). As capital epiphyses begin ossifying 6-9 months, increasingly limited but worth a try to limit dose.
XR: requires meticulous positioning of pelvis and hips; less useful in infants until heads ossifying. Uses bony landmarks and bilateral symmetry.
CT: only if low-mAs, and VERY limited number of thin slices (2-5), in precisely positioned infant; 3D NOT INDICATED (too many slices, too much dose, for too little additional information!). Most useful for post-op in abduction spica cast: confirms head remains concentrically reduced into acetabulum and symmetric with contralateral side.
MR: pre- and post-op uses; evaluates cartilage, labrum, ligamentum teres, soft tissue abnormalities such as thickened ligamentum teres, hypertrophic pulvinar (fibrofatty debris preventing reduction of head into acetabulum) , or folded/abnormal labrum (ditto); and detects avascular necrosis femoral head (AVN or osteonecrosis, ON, is more often secondary to treatment than primary). BUT MR usually needs sedation and is expensive.
July 2012 - Donna Magid, M.D., M.Ed.