Inflicted fractures and other non-accidental musculoskeletal injury — branching viva

Stem

The examiner opens with a six-month-old, pre-mobile infant and a spiral femur fracture, then escalates through the specificity ladder, the biomechanics, the skeletal-survey protocol, the differentials, and a safeguarding conversion when an occult rib fracture emerges. [1] [10]

Branch 1 — The specificity ladder

Examiner: A spiral femur fracture in a six-month-old who cannot crawl. Which fracture patterns are high-specificity for abuse, and why specificity over sensitivity? [1]

Strong answer: The classic metaphyseal lesion, the posterior or postero-lateral rib fracture, and fractures of the scapula, spinous process and sternum top the specificity ladder — Kemp's systematic review placed posterior rib fractures and the CML highest. Specificity outranks sensitivity because the question is how strongly a fracture, in this child, points away from an accidental explanation. A linear parietal skull fracture is sensitive but low-specificity; a posterior rib fracture is uncommon in accidental trauma and therefore high-specificity. No fracture is pathognomonic — the balanced multi-domain assessment makes the call. [1]

Examiner: Is the spiral femur itself high-specificity? [10]

Strong answer: Not in isolation — the developmental stage is the discriminator. Wood's systematic review showed abuse prevalence among young children with a femur fracture falls steeply with age, highest in non-ambulatory infants. A spiral femur in a six-month-old who cannot crawl implies an external rotational force she cannot generate, which raises the pattern well above a transverse femur in a school-age child who fell from a bike. [10]

Branch 2 — Biomechanics

Examiner: Explain the classic metaphyseal lesion. [7]

Strong answer: The CML is a shearing injury through the zone of provisional calcification — the weak interface between the physis and the primary spongiosa — produced when the limb is grasped and accelerated or decelerated, as in shaking. It appears radiographically as a corner or bucket-handle fragment. Tsai's finite-element model reproduced the lesion only with the acceleration-deceleration forces of shaking, not with simple falls, which is the evidence the CML is an inflicted-force lesion rather than an accidental one. [7]

Examiner: And the posterior rib fracture — could CPR have done it? [6]

Strong answer: The posterior rib fracture is a lever injury: thoracic compression levers the rib over the transverse process at the rib neck, which is why the more posterior the fracture, the higher its specificity. Maguire's systematic review showed that cardiopulmonary resuscitation, even correctly and repeatedly performed, rarely causes rib fractures in children — so a posterior rib fracture in an infant without a credible high-force explanation remains highly concerning. [3] [6]

Branch 3 — Imaging protocol

Examiner: What imaging will you obtain? [4]

Strong answer: A complete skeletal survey — separate frontal and oblique views of every bone by an infant-experienced radiographer — for this child under 24 months, plus a CT head and a dilated retinal examination to exclude abusive head trauma. Critically, I book the follow-up skeletal survey at about two weeks, because occult rib, classic metaphyseal and extremity fractures mature into visibility as they heal and may reveal a differing-age pattern. [4]

Examiner: Why is the follow-up survey not optional? [14]

Strong answer: Harper's study confirmed child-abuse paediatricians widely use repeat surveys because they add diagnostic information, and Hansen showed even a limited-view follow-up survey detects fractures invisible on the initial films. A single initial survey under-counts injuries; the differing-age fractures that emerge on follow-up are themselves diagnostic of recurrent inflicted injury, so omitting it is a recognised system failure. [14]

Branch 4 — Differentials

Examiner: The family raises osteogenesis imperfecta and rickets. How do you respond? [8]

Strong answer: I address both on evidence, not assertion, and in parallel with the imaging. For rickets I cite Perez-Rossello's finding that rachitic change was absent in infants with fatal abusive head trauma and classic metaphyseal lesions — a discrete CML is not rickets, and I meet the defence with that evidence. For osteogenesis imperfecta I look for osteopenia, blue sclera, dentinogenesis imperfecta, the family history and the fracture distribution, and I arrange genetic testing where the picture fits; a high-specificity pattern in a well, non-osteopenic infant points away from OI. I run the biochemistry alongside the survey so a genuine diagnosis is neither missed nor used as a reflexive shield. [8]

Branch 5 — Safeguarding conversion

Examiner: The follow-up survey shows two healing rib fractures of differing age. What now? [14]

Strong answer: Differing-age fractures — a healing fracture alongside an acute one — are the radiographic signature of recurrent inflicted injury, not a single event, and they convert the plan firmly onto a safeguarding pathway. I treat the orthopaedic injuries on their merits, make a child-protection referral — the threshold is concern, not proof — confirm a safe place of care before discharge, assess and survey the infant sibling, name the clinician who owns the plan, and document the return precautions. I engage the family honestly without interrogating or coaching them; my role is to investigate and document, and child protection decides thresholds and disposition. The error to avoid is the open-loop referral — treating the fracture and owning no follow-up. [4] [14]

Examiner scoring cues

• Ranks the specificity ladder (CML, posterior rib, scapula, spinous, sternal) and defends specificity over sensitivity; states no fracture is pathognomonic. [1]
• Weighs the spiral femur against the developmental stage, citing Wood — highest abuse prevalence in non-ambulatory infants. [10]
• Explains the CML as shearing at the zone of provisional calcification, citing the Tsai finite-element model. [7]
• Explains the posterior rib as a lever mechanism and cites Maguire that CPR rarely fractures ribs. [3] [6]
• Describes the complete skeletal survey and defends the mandatory follow-up survey, citing Harper. [4] [14]
• Meets the rickets defence with Perez-Rossello evidence, and the OI differential on clinical and genetic grounds. [8]
• Converts to safeguarding on differing-age fractures, confirms safe disposition, assesses the sibling, and closes the loop. [14]