Image Interpretation II: Congenital Anomalies, Variants, Scoliosis and Trauma

Half of what you see on a spine or extremity film is not disease at all: it is anatomy that formed a little differently, or a growth center that has not yet fused. The board tests whether you can tell a harmless variant from a fracture, a congenital block from a fused infected segment, and a stable injury from one that needs the emergency department today. This chapter teaches the radiographic hallmarks of congenital anomalies, variants, scoliosis and trauma, and the exact recommendation you would write in the report for each.

Section 1Spinal congenital anomalies and variants

Why anomalies matter on the board and in the office

An anomaly matters clinically when it does one of three things: it makes a segment unstable (os odontoideum, some occipitalizations), it overloads the neighbors (block vertebra, Klippel Feil fusion, transitional segment), or it signals other anomalies elsewhere in the body (hemivertebra with renal and cardiac defects, Klippel Feil with Sprengel deformity and renal agenesis). On the exam, an item showing an upper cervical anomaly almost always asks one of two things: what is it, or what you should do before adjusting. The answer to the second is usually flexion and extension views to assess stability, and modification or avoidance of high velocity rotary thrust at that level.

Anomalies arise from failure of formation (part of a vertebra missing: hemivertebra, butterfly vertebra, pedicle agenesis) or failure of segmentation (two vertebrae fused: block vertebra, occipitalization, unsegmented bar), the same scheme used for congenital scoliosis.

Upper cervical anomalies

Occipitalization (atlanto occipital assimilation) is fusion of the atlas to the occiput. On the lateral film the posterior arch of C1 merges with the occiput with no joint space. The C1 to C2 joint must then provide all upper cervical motion, which promotes late atlantoaxial instability; it is also associated with basilar impression and Chiari malformation. Recommend flexion and extension views, and MRI if there are long tract signs or headache with Valsalva.

Os odontoideum is a smooth, round ossicle above a hypoplastic dens, separated from C2 by a wide gap (most likely an old ununited childhood fracture). The ossicle moves with C1, so the transverse ligament no longer controls C1 on C2. It has smooth, corticated margins and the anterior arch of C1 is often enlarged and rounded from chronic stress. A type II odontoid fracture has a thin, irregular, noncorticated line and a normal sized arch; an ossiculum terminale is a small, stable unfused center at the dens tip. Os odontoideum contraindicates upper cervical thrust; recommend flexion and extension views and surgical referral, urgently if there is myelopathy.

Agenesis of the posterior arch of atlas ranges from a trivial midline cleft to complete absence; the smooth margins and an enlarged C2 spinous process separate it from fracture. Ponticulus posticus (arcuate foramen) is ossification of the oblique atlantooccipital ligament forming a bony ring over the vertebral artery groove of C1, seen in roughly 10 to 15 percent of people; it is a variant, but many clinicians modify upper cervical rotary thrust in its presence, so mention it in the report. Basilar impression (dens tip more than about 7 mm above McGregor line) may accompany occipitalization, Paget disease and rheumatoid arthritis.

Block vertebra and Klippel Feil syndrome

A congenital block vertebra is failure of segmentation of two or more vertebral bodies, most often at C2 to C3 and C5 to C6 in the neck and in the lumbar spine less often. The hallmarks on the lateral film are: a narrow waist at the level of the absent disc (the wasp waist sign), a rudimentary disc space that may persist as a thin sclerotic line, a combined height equal to two vertebrae plus the missing disc, fused facets and often fused spinous processes, and an enlarged intervertebral foramen that is shaped like a keyhole or oval. The segments above and below are hypermobile and degenerate early.

Acquired fusion lacks the waist: surgical fusion shows hardware or graft, ankylosing spondylitis shows syndesmophytes, juvenile idiopathic arthritis fuses facets with small bodies, and healed infection leaves an irregular sclerotic, often kyphotic fusion.

Klippel Feil syndrome is congenital fusion of multiple cervical segments; the classic triad (short neck, low hairline, restricted motion) is present in under half. It is associated with Sprengel deformity (high scapula, sometimes with an omovertebral bone), scoliosis, renal anomalies, heart disease and hearing loss, so renal ultrasound is advised. Never manipulate the fused block and treat the hypermobile adjacent levels gently.

Hemivertebra and butterfly vertebra

A hemivertebra results from failure of formation of one half of the vertebral body. On the AP film it appears as a wedge shaped body with a single pedicle, producing an abrupt lateral angulation, a short sharp scoliosis. A lateral hemivertebra causes scoliosis; a dorsal (posterior) hemivertebra, in which the anterior body fails to form, causes a sharp kyphosis (gibbus) with risk of cord compression. Hemivertebrae may be fully segmented (discs above and below, greatest growth potential), semisegmented, or incarcerated (tucked into the curve, least deforming). The extra or missing pedicle and rib count help confirm the diagnosis.

A butterfly vertebra (sagittal cleft vertebra) results from failure of fusion of the two lateral chondral centers. On the AP film the body is narrowed centrally with two triangular halves pointing toward the midline, and the adjacent end plates of the vertebrae above and below are reciprocally shaped to fill the cleft. On the lateral film it can look like a compression fracture; the AP appearance and the reciprocal deformity of the neighbors settle the question. Butterfly vertebrae are associated with Alagille syndrome and other anomalies but are usually isolated and asymptomatic.

Spina bifida and other posterior element variants

Spina bifida occulta is a midline lucent cleft where the laminae fail to unite, most often at L5 and S1, and is usually insignificant. A hair tuft, dimple or lipoma over it in a child with gait change, foot deformity or bladder symptoms suggests tethered cord and calls for MRI. Spina bifida vera (meningocele, myelomeningocele) is evident at birth with widened interpedicular distances. A knife clasp deformity is an elongated L5 spinous process that drops into an S1 cleft in extension, causing extension pain.

Agenesis of a pedicle must be distinguished from the destroyed pedicle of metastasis (Chapter 14): congenital absence shows a hypertrophied, sclerotic contralateral pedicle. A limbus bone is a corticated triangular ossicle at the anterosuperior corner of a lumbar body, formed when disc material herniates through the ring apophysis before it fuses; it is often mistaken for a teardrop fracture. Unfused apophyses at transverse, spinous or articular process tips also mimic fractures but are smoothly corticated.

Transitional segments

A lumbosacral transitional segment occurs in roughly 10 to 15 percent of people. Sacralization means L5 takes on sacral features (an enlarged transverse process forming a joint with or fusing to the sacral ala); lumbarization means S1 behaves like a lumbar vertebra with a well formed disc below it. The Castellvi classification grades the anomaly: type I is a dysplastic, enlarged transverse process (at least 19 mm tall); type II is a pseudarthrosis (a diarthrodial joint) between the transverse process and the sacrum; type III is complete bony fusion; type IV is type II on one side and type III on the other. Each type may be unilateral (a) or bilateral (b).

A unilateral type II pseudarthrosis can itself cause pain (Bertolotti syndrome), and the disc above a transitional segment degenerates early. The practical point is level counting: state how you counted (from C2 or from the lowest rib bearing vertebra) so advanced imaging and procedures use the same numbering.

Cervical rib

A cervical rib arises from the costal element of C7 (about 0.5 to 1 percent of people, often bilateral). The C7 transverse process points downward, whereas T1 transverse processes point upward and carry the first rib. A complete rib or an invisible fibrous band from an incomplete one can compress the lower trunk of the brachial plexus or the subclavian artery (thoracic outlet syndrome). Correlate with C8 to T1 findings, and refer promptly for vascular studies if there are signs of arterial compromise.

Section 2Extraspinal anomalies and variants

The corticated margin rule

The single most useful principle in this section is simple: a developmental ossicle or unfused apophysis has a smooth, rounded, completely corticated margin on all sides, and the parent bone next to it is also corticated and often shaped to receive it. An acute fracture fragment has a sharp, irregular, noncorticated edge on the fracture side, it fits like a jigsaw piece into a defect in the parent bone, and there is usually soft tissue swelling and focal tenderness. Bilateral symmetry also favors a variant; when in doubt, a comparison view of the other side is a cheap and classic answer. An old ununited fracture will eventually corticate, so history matters as well.

Accessory ossicles of the foot and ankle

• Os trigonum: unfused lateral tubercle of the posterior talar process; causes posterior ankle impingement in dancers.
• Accessory navicular: medial to the navicular at the posterior tibial tendon insertion; the most often symptomatic foot ossicle, especially with flatfoot.
• Os peroneum: in the peroneus longus tendon near the cuboid; proximal migration on serial films suggests tendon rupture.
• Os vesalianum: at the base of the fifth metatarsal, rare, with smooth margins. The normal fifth metatarsal apophysis in a child runs parallel to the shaft; a fracture line runs transverse (perpendicular) to the shaft. This is a heavily tested distinction.
• Bipartite medial hallux sesamoid: more common medially and often bilateral; it is smooth, and the two parts together are larger than the opposite sesamoid. A fractured sesamoid has sharp edges and the pieces add up to a normal size.

Other common variants

• Bipartite patella: an unfused ossification center at the superolateral pole, usually bilateral, with smooth sclerotic margins. A transverse patellar fracture is mid patella; a fragment at the inferior pole after trauma is a sleeve fracture or fracture, not a variant.
• Fabella: sesamoid in the lateral gastrocnemius head behind the lateral femoral condyle, not a loose body.
• Os acromiale: unfused acromial apophysis after about age 25, best on the axillary view, linked to impingement.
• Nutrient foramen: a thin oblique lucent line through one cortex of a long bone, angled away from the growing end (toward the elbow in the upper limb, away from the knee in the lower limb). A fracture line usually crosses both cortices.
• Mach band: a perceptual dark line where two densities overlap, classically across the base of the dens from the overlying posterior arch of C1 or the teeth. It extends beyond the bone margins, which a true fracture line does not.

Developmental dysplasia of the hip

Developmental dysplasia of the hip (DDH) ranges from a shallow acetabulum to frank dislocation. Risk factors are female sex, breech presentation, first born, family history and tight swaddling; the left hip is involved more often. In the newborn, Barlow (adduction with posterior pressure) dislocates an unstable hip and Ortolani (abduction with anterior lift) reduces it with a clunk. After about 3 months these become negative, and the signs are limited abduction, a positive Galeazzi sign and asymmetric thigh folds; a walking child limps. Use ultrasound before the femoral head ossifies (about 4 to 6 months), then an AP pelvis:

Imaging depends on age. Before the femoral head ossifies (about 4 to 6 months), ultrasound is the study of choice. After that, an AP pelvis is used with these lines:

• Hilgenreiner line: a horizontal line through both triradiate cartilages.
• Perkins line: a vertical line through the lateral edge of the acetabular roof, perpendicular to Hilgenreiner line. Together they form quadrants; the femoral head ossification center (or the medial metaphyseal beak if not yet ossified) should lie in the lower inner quadrant. A head in an upper or outer quadrant is subluxated or dislocated.
• Acetabular index: the angle between Hilgenreiner line and a line along the acetabular roof. Normal is about 30 degrees or less at birth, decreasing toward about 20 to 22 degrees by age 2. A value over 30 degrees suggests dysplasia.
• Shenton line: a smooth arc from the inferior border of the superior pubic ramus along the medial femoral neck. A break indicates superior displacement of the femur.
• A smaller, delayed femoral head ossification center on the affected side is another sign.

Residual adult dysplasia shows a shallow acetabulum with a lateral center edge angle under about 20 to 25 degrees and leads to early osteoarthritis. Refer any suspected DDH to pediatric orthopedics promptly; a Pavlik harness works well in the first months.

Section 3Scoliosis

Definitions and classification

Scoliosis is a lateral curvature of the spine of 10 degrees or more measured by the Cobb method on an upright AP or PA film. It is a three dimensional deformity: the vertebrae also rotate, with the bodies turning toward the convexity and the spinous processes toward the concavity, which is why the ribs form a posterior hump on the convex side during the Adams forward bend test. Curves are named for the side of the convexity (a right thoracic curve is convex to the right) and by the apex: cervical (apex C1 to C6), cervicothoracic (C7 to T1), thoracic (T2 to T11), thoracolumbar (T12 to L1), lumbar (L2 to L4) and lumbosacral (L5 to S1).

A structural curve has fixed rotation and does not correct on side bending; a nonstructural (functional) curve is flexible and has a cause outside the spine, such as a leg length inequality, muscle spasm or a painful lesion. A functional curve from leg length difference disappears when the patient sits or when the short leg is lifted. Curves are also called major (largest, most structural) and minor or compensatory.

Features that make a curve atypical and call for MRI of the neural axis: a left thoracic curve, a short sharp curve, rapid progression, onset under age 10, pain (especially at night), abnormal abdominal reflexes, cavus foot or other neurological signs (syrinx, Chiari, tethered cord, tumor). Painful scoliosis with night pain relieved by NSAIDs suggests osteoid osteoma on the concave side near the apex (Chapter 14).

In congenital scoliosis the worst prognosis is a unilateral unsegmented bar with a contralateral hemivertebra. Because the somites form alongside the kidneys and heart, renal (about 20 to 30 percent), cardiac and intraspinal anomalies are common and must be screened.

Measuring the curve: the Cobb method

The Cobb method is the standard measurement. First identify the end vertebrae: the most tilted vertebrae at the top and bottom of the curve, those whose end plates tilt most toward the concavity. Draw a line along the superior end plate of the upper end vertebra and along the inferior end plate of the lower end vertebra. If the end plates are hard to see, the tops or bottoms of the pedicles may be used. Then draw perpendiculars to those two lines; the angle where the perpendiculars intersect equals the Cobb angle (with large curves you can measure the angle between the end plate lines directly). The measurement error between observers is about 3 to 5 degrees, so a change must exceed about 5 degrees to count as true progression. The apical vertebra is the most horizontal and most laterally displaced vertebra (or disc) in the curve, and it shows the most rotation.

Rotation is graded by the Nash Moe method, by how far the convex side pedicle migrates toward the midline.

Skeletal maturity: the Risser sign

The Risser sign grades ossification of the iliac crest apophysis, which advances from the ASIS posteriorly: 0 none, 1 to 4 by quarters, 5 fused to the ilium (skeletal maturity). Peak growth occurs before Risser 1, near triradiate closure and before menarche.

Predicting progression

Progression risk rises with larger curves, Risser 0 to 1, premenarchal status, female sex and thoracic or double curves: a 20 to 29 degree curve progresses in about two thirds at Risser 0 to 1 versus about one quarter at Risser 2 to 4. After maturity, thoracic curves over about 50 degrees progress about 1 degree a year.

Bracing (worn roughly 13 or more hours a day) reduces progression to surgical size but does not permanently correct a curve. Chiropractic care may help pain and function but has not been shown to halt progression, so a progressing curve in a growing child calls for referral for bracing assessment. Use PA projection and minimal views to limit radiation in serial films.

Section 4Skeletal trauma

Who needs films after neck trauma

Two validated rules decide whether an alert, stable adult needs cervical imaging. NEXUS clears the neck if there is no posterior midline tenderness, no focal deficit, normal alertness, no intoxication and no distracting injury. The Canadian C spine rule mandates imaging for age 65 or older, a dangerous mechanism (fall from 3 feet or 5 stairs, axial load, high speed or rollover crash, ejection, bicycle collision) or extremity paresthesias, and otherwise requires that the patient can actively rotate 45 degrees each way. In hospitals, CT has replaced plain films for significant trauma; in the office, recognize instability and transfer rather than image and treat.

The three column concept

The Denis three column model divides the thoracolumbar spine into the anterior column (anterior longitudinal ligament, anterior two thirds of the body and annulus), the middle column (posterior third of the body and annulus, posterior longitudinal ligament) and the posterior column (pedicles, facets, laminae, spinous processes and the posterior ligament complex). Injury to two or more columns, and in particular any injury involving the middle column, is considered unstable. Newer systems (TLICS and the AO classification) also weigh neurological status and the integrity of the posterior ligament complex, but the three column model remains the board standard.

Upper cervical fractures

• Jefferson fracture: a burst of the C1 ring from axial load (diving, a blow to the vertex). On the open mouth view the C1 lateral masses overhang C2; a combined overhang over about 7 mm (rule of Spence) implies transverse ligament rupture and instability. An isolated posterior arch fracture from hyperextension is stable but must be separated from a congenital cleft.
• Odontoid (dens) fractures, Anderson and D'Alonzo: type I an avulsion of the tip (rare, usually stable); type II through the base of the dens (the most common and the most prone to nonunion because of poor blood supply, especially in the elderly after a simple fall); type III extending into the body of C2 (better union). Clues on the lateral film include anterior or posterior tilt of the dens, disruption of the Harris ring (the ring of cortex projected over the body of C2, broken in type III) and prevertebral swelling.
• Hangman fracture (traumatic spondylolisthesis of the axis): bilateral fractures through the pars interarticularis of C2 from hyperextension with axial loading, typically a deceleration crash with the chin striking the dashboard. The lateral film shows lucent lines through the C2 pedicles, anterior slip of the C2 body and disruption of the spinolaminar line at C2 (the posterior cervical line of Swischuk helps separate it from pediatric pseudosubluxation). The canal widens, so the cord is often spared.

Lower cervical fractures and dislocations

A fragment from the anteroinferior corner of a body after trauma is a teardrop fracture until proven otherwise; a corticated triangle at the anterosuperior corner of a lumbar body is a limbus bone. A fused spine (ankylosing spondylitis, DISH) fractures like a long bone after minor trauma; new pain after a fall in such a patient needs CT.

Thoracic and lumbar fractures

• Compression (wedge) fracture: anterior column failure in flexion, with reduced anterior height, an intact posterior wall, a buckled anterior cortex and a dense zone of impaction below the end plate in recent fractures. Height loss over about 50 percent raises concern for posterior ligament injury.
• Burst fracture: anterior and middle column failure from axial load, usually T12 to L2, with posterior height loss, retropulsion and a widened interpedicular distance on AP. All need CT.
• Chance fracture (seatbelt, flexion distraction): a horizontal fracture through spinous process, laminae, pedicles and body at T12 to L2. Up to half have intra abdominal injuries, so send to the emergency department.
• Fracture dislocation: all three columns, translational displacement, highly unstable, high rate of paraplegia.

Osteoporotic versus pathological compression fracture

Favoring osteoporosis: diffuse osteopenia, an intact posterior wall and pedicles, and a level below T6. Favoring malignancy: a destroyed pedicle, a convex posterior wall, a soft tissue mass, a level above T5 and other lytic or blastic lesions (Chapter 14). Recommend MRI and labs when features are atypical.

Pars defects and spondylolisthesis

Spondylolysis is a pars interarticularis defect, usually a fatigue fracture, in about 5 to 6 percent of people and more in athletes who hyperextend (gymnasts, linemen, fast bowlers); L5 accounts for about 85 to 90 percent. On the oblique view the posterior elements form the Scotty dog (nose transverse process, eye pedicle, neck pars, ear superior facet, front leg inferior facet, body lamina); a defect is a collar, and a slip decapitates the dog. Early stress reactions need SPECT, CT or MRI.

Spondylolisthesis is forward slip of one vertebra on the one below. Wiltse types: I dysplastic, II isthmic (pars defect), III degenerative (facet arthrosis, typically L4 to L5 in women over 50, intact pars, rarely beyond grade I), IV traumatic, V pathological and VI iatrogenic. Meyerding grading uses quarters of the lower end plate: I up to 25 percent, II to 50, III to 75, IV to 100, and V (spondyloptosis) beyond, with the inverted Napoleon hat sign on the AP view. In isthmic slips the palpable step is at the spinous process above the slipped level (its posterior arch stays behind); in degenerative slips it is at the slipped level.

Extremity fractures and dislocations

The Ottawa ankle and knee rules limit unnecessary films (malleolar or midfoot bone tenderness at the specified sites, or inability to bear weight four steps). Every fracture needs two views at right angles, including the joints above and below for shaft injuries.

Pediatric fractures

Children's bones are more pliable, have a thick periosteum and contain open physes (growth plates), which are weaker than the ligaments. A force that would sprain an adult ligament fractures a child's physis instead. Patterns unique to children include the torus (buckle) fracture, a bulge of the cortex on the compression side of the distal radial metaphysis; the greenstick fracture, which breaks the tension side cortex and bends the compression side; and plastic bowing, a bend without a visible fracture line, most often in the forearm.

The Salter Harris classification describes physeal fractures. Type I runs straight through the physis (Straight or Slipped; often a normal film with tenderness over the plate). Type II passes through the physis and exits through the metaphysis (Above; the most common, about 75 percent; the metaphyseal fragment is the Thurston Holland fragment). Type III passes through the physis and exits through the epiphysis (Lower, below the plate; intra articular). Type IV crosses metaphysis, physis and epiphysis (Through). Type V is a crush injury of the physis (Rammed or Ruined), often recognized only later by growth arrest. The mnemonic is SALTR. Risk of growth disturbance rises from type I to V; types III and IV need anatomic reduction, often surgical.

Slipped capital femoral epiphysis (SCFE) affects overweight adolescents (about 10 to 16 years) and may present only as knee pain. On the AP view Klein line along the superior neck fails to cross the epiphysis and the physis is widened; the frog leg lateral shows the slip earliest. It is bilateral in up to a third. Make the patient non weight bearing and refer urgently; no hip manipulation.

Nonaccidental trauma is suggested by metaphyseal corner fractures, posterior rib fractures, fractures of different ages, and any long bone fracture in a nonwalking infant, especially when the history does not fit. Chiropractors are mandated reporters: report to child protective services and arrange medical evaluation with a skeletal survey.

Stress fractures

A fatigue fracture is normal bone under abnormal repetitive load; an insufficiency fracture is weakened bone (osteoporosis, osteomalacia, radiation, steroids) under normal load. Plain films are often normal for 2 to 3 weeks, then show faint periosteal reaction or a sclerotic band perpendicular to the trabeculae; MRI is the best early test. Common sites are the second and third metatarsals (march fracture), tibia, calcaneus, femoral neck, pubic rami and, in elderly women, the sacrum (the H shaped or Honda sign on bone scan). High risk sites that displace or fail to heal are the tension side (superolateral) femoral neck, anterior tibial cortex, navicular and proximal fifth metatarsal diaphysis; a superolateral femoral neck stress fracture needs same day referral.

Section 5Soft tissue trauma signs on film

Why soft tissues are read first

A fracture line may be invisible, but the bleeding and swelling it causes usually are not. Soft tissue signs are therefore the indirect evidence of injury, and they often decide whether a normal looking film is truly normal. The habit to build is a systematic soft tissue search on every trauma film: the prevertebral space on the lateral cervical view, the fat pads of the elbow, the fat stripes of the wrist, the suprapatellar recess of the knee, the paraspinal lines of the thoracic spine and the psoas shadows of the lumbar spine.

Prevertebral soft tissue swelling in the neck

On the neutral lateral film, measure the retropharyngeal space from the anteroinferior corner of C2 to the airway and the retrotracheal space at C6. Adult upper limits are 7 mm at C2 and 22 mm at C6 (children 7 and 14 mm). Swelling suggests hemorrhage from fracture or ligament tear, but also abscess or tumor, and crying infants can mimic it. A normal width does not exclude injury because swelling can take hours to develop.

Displacement of the prevertebral fat stripe and, in the thoracic spine, bulging of the paraspinal line are other signs of hematoma.

Cervical alignment lines and the atlantodental interval

Four lines should be smooth on the lateral film: the anterior vertebral, posterior vertebral (George line; a step over about 3 mm means translation), spinolaminar and posterior spinous lines. The atlantodental interval (ADI) should be no more than 3 mm in adults and 5 mm in children; more implies transverse ligament insufficiency (trauma, rheumatoid arthritis, Down syndrome). A sagittal canal under about 12 mm (Torg Pavlov ratio under 0.8) indicates developmental stenosis.

In young children, pseudosubluxation of C2 on C3 (up to about 3 to 4 mm, normal under about age 8, confirmed by the posterior cervical line of Swischuk) and the pseudo Jefferson overhang of C1 on the open mouth view are normal.

Fat pad and fat stripe signs in the extremities

Joint fluid displaces the lucent fat around a joint. On a true lateral elbow at 90 degrees, a thin anterior fat pad is normal, but when lifted it forms a triangle, the sail sign. The posterior fat pad hides in the olecranon fossa and is never visible normally. After trauma a visible posterior fat pad means an occult intra articular fracture: a radial head fracture in adults, a supracondylar fracture in children. Treat it as a fracture (splint, repeat films in 7 to 10 days or CT).

• Pronator quadratus fat line: a thin lucent stripe on the volar aspect of the distal forearm on the lateral wrist view; bowing or obliteration suggests a distal radius fracture.
• Scaphoid (navicular) fat stripe: a lucent line parallel to the radial side of the scaphoid on the PA view; displacement or loss suggests a scaphoid fracture.
• Knee effusion: on the lateral view, the suprapatellar recess between the quadriceps fat and the anterior femoral fat widens beyond about 5 to 10 mm.
• Lipohemarthrosis: on a horizontal beam (cross table) lateral of the knee, a fluid level separates lucent fat floating on top of denser blood, the FBI sign (fat blood interface). Fat comes from the marrow, so a lipohemarthrosis means an intra articular fracture, most often of the tibial plateau, even when the fracture is not seen. It also occurs in the shoulder and hip. A vertical beam film will not show it.
• Soft tissue gas: after a penetrating wound, gas tracking along tissue planes raises concern for gas forming infection (gas gangrene) or a foreign body track and needs emergency care.

Flexion and extension views for instability

Ligament injuries may leave a neutral film perfectly aligned, because muscle spasm holds the segments in place. Flexion and extension lateral views (dynamic or stress views) are used to uncover instability, particularly after whiplash with persistent pain, in rheumatoid arthritis, in Down syndrome, and in patients with degenerative spondylolisthesis being considered for surgery. Signs of cervical instability include:

• Horizontal translation of one vertebral body on the next of more than 3.5 mm (or more than 20 percent of the body width), measured at the posterior body line.
• Angulation between adjacent end plates that differs by more than 11 degrees from the adjacent segments (White and Panjabi criteria).
• Fanning, an abnormal widening of the interspinous distance at one level, indicating posterior ligament complex disruption.
• Loss of facet parallelism, perched facets, or a focal kyphotic angle; an ADI over 10 mm implies alar ligament disruption too.

In the lumbar spine, translation over about 4.5 mm (about 15 percent) is commonly considered abnormal.

Timing and safety matter as much as the measurements. In acute trauma, flexion and extension views are contraindicated until fracture and gross instability have been excluded by neutral films or, more often today, CT; a patient with a neurological deficit or high risk mechanism should never be asked to flex and extend. The movements must be active and patient controlled, never forced by the examiner. Early after injury, muscle guarding can prevent the motion needed to show instability, producing a false negative; so if pain persists, repeat dynamic views at about 7 to 14 days (some protocols say 2 to 3 weeks) when spasm has eased, or use MRI to look for ligament edema. Delayed instability is a classic cause of a missed injury after whiplash.

Words worth knowing