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]}