Inflicted fractures and other non-accidental musculoskeletal injury — formative SAQs

SAQ 1 — The high-specificity fracture and the multi-domain assessment (10 marks)

A six-month-old girl who cannot yet crawl is brought with a swollen, irritable left thigh. The history is a "roll off the sofa." The radiograph shows a spiral femur fracture, and the skeletal survey additionally reveals a classic metaphyseal lesion at the distal femur and a posterior rib fracture. [1] [10]

Questions

1. Which fracture patterns are high-specificity for abuse, and why is specificity, not sensitivity, the governing principle? (3 marks) [1]

2. Explain the biomechanics of the classic metaphyseal lesion and the posterior rib fracture, and cite the evidence. (4 marks) [7]

3. Outline the imaging protocol, including the role of the follow-up skeletal survey, and the whole-child assessment this infant requires. (3 marks) [4] [14]

Model answer

High-specificity patterns (3). The classic metaphyseal lesion, the posterior or postero-lateral rib fracture, and fractures of the scapula, spinous process and sternum head the specificity ladder — Kemp's systematic review placed posterior rib fractures and the CML at the top. The governing principle is that no fracture is pathognomonic: specificity outranks sensitivity because the question is how strongly a fracture, in this child, points away from an accidental explanation, not whether it can occur in abuse. [1]

Biomechanics (4). The classic metaphyseal lesion 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. Tsai's finite-element model reproduced the lesion only with the forces of shaking, not simple falls — that is the evidence the CML is an inflicted-force lesion. 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. [7]

Imaging and whole-child assessment (3). A complete skeletal survey (frontal and oblique views of every bone) is the standard for any infant under 24 months with suspected inflicted injury, and the follow-up survey at about two weeks is mandatory because occult rib, CML and extremity fractures mature into visibility as they heal. Add a CT head and a dilated retinal examination to exclude abusive head trauma, and exclude the rickets and osteogenesis-imperfecta differentials with biochemistry and genetics where the picture fits. [4] [8] [14]

SAQ 2 — The femur fracture in the pre-mobile infant and the safeguarding loop (10 marks)

A four-month-old boy who does not yet roll is brought with a femur fracture. The parents describe a changing mechanism, and there is a four-hour delay in presentation. There is an infant sibling at home. [10]

Questions

1. Why does a femur fracture in a pre-mobile infant mandate an abuse evaluation, and what does the developmental stage tell you? (3 marks) [10]

2. How do you address the rickets and osteogenesis-imperfecta differentials without dismissing them or reflexively invoking them? (3 marks) [8]

3. Outline the definitive management, including the safeguarding loop, the sibling assessment, and the disposition error to avoid. (4 marks) [4] [14]

Model answer

Mandate and developmental stage (3). Wood's systematic review established that the abuse prevalence among young children with a femur fracture falls steeply with age, highest in non-ambulatory infants. A four-month-old who cannot roll cannot generate the force to spiral a femur; the developmental stage is itself a diagnostic test, and a spiral fracture in a pre-ambulatory child implies an external rotational force. The changing history and the delay compound the concern. This mandates a skeletal survey and a child-protection referral — the threshold is concern, not proof. [10]

Differentials (3). Osteogenesis imperfecta and rickets are addressed on evidence, not assertion. The candidate cites Perez-Rossello's finding that rachitic change was absent in infants with fatal abusive head trauma and classic metaphyseal lesions — which meets the reflexive "it is rickets" defence without dismissing genuine bone fragility where it exists. Run the bone-fragility and rickets biochemistry (calcium, phosphate, alkaline phosphatase, vitamin D) and an OI screen in parallel with the imaging, and document the differential reasoning explicitly. [8]

Safeguarding loop and sibling (4). Deliver the stepped pathway: treat the orthopaedic injury on its merits, complete the skeletal survey and book the follow-up survey, consult child protection early, and exclude abusive head trauma with CT and retinal examination. Because inflicted injury is a household phenomenon, assess and survey the infant sibling. Confirm a safe place of care before discharge, name the clinician who owns the plan, and document the return precautions. The disposition error to avoid is the open-loop referral — treating the fracture, noting a concern, and owning no follow-up — which leaves a vulnerable infant exposed to recurrence. [4] [14]