12.2 DXI Extraspinal Skeletal Variants

DXI Extraspinal Skeletal Variants

Quick Answer: Extraspinal congenital/skeletal variant items test recognition of accessory ossicles, bipartite patella, cervical ribs, and tarsal coalitions. The unifying skill is distinguishing a smoothly corticated, often bilateral developmental variant from an acute fracture or a clinically significant structural anomaly that changes management.

Beyond the spine, the DXI Congenital/Skeletal Variant category tests extremity and thoracic outlet anomalies that commonly appear as incidental findings on radiographs ordered for trauma, foot pain, or thoracic outlet symptoms. As with spinal variants, the exam consistently rewards the candidate who recognizes when a finding is a normal developmental variant that requires no further workup versus when it represents (or predisposes to) a genuine clinical problem.

Accessory Ossicles

Accessory ossicles are extra centers of ossification that fail to fuse with the adjacent parent bone, most often found around the foot and ankle. Because acute avulsion fractures can occur in the same regions, DXI items frequently pair an accessory ossicle image with an avulsion-fracture distractor.

Common Accessory Ossicles

The accessory navicular is further subdivided by the Lester and Mercer / Sella classification:

1. Type I — A small sesamoid-type ossicle embedded within the posterior tibial tendon, not in continuity with the navicular; rarely symptomatic
2. Type II — A larger ossicle joined to the navicular by a synchondrosis (fibrocartilage); the most common source of symptomatic accessory navicular syndrome due to traction stress at the synchondrosis
3. Type III — A bony (cornuate) prominence fused directly to the navicular, essentially an enlarged navicular tuberosity

Key distinguishing features from an avulsion fracture: accessory ossicles show smooth, rounded, fully corticated margins on all sides, a consistent characteristic location, and are frequently bilateral and symmetric. Acute avulsion fragments show a sharp, non-corticated margin that matches the contour of the donor bone, a clear soft-tissue injury history, and are typically unilateral.

Bipartite Patella

Bipartite patella results from failure of one of the patella's secondary ossification centers to fuse with the main patellar body, producing a separate bony fragment connected by a fibrocartilaginous synchondrosis.

• Most common location: Superolateral pole (accounts for roughly 75% of cases), described using the Saupe classification:
  • Type I — Inferior pole fragment (rare)
  • Type II — Lateral margin fragment
  • Type III — Superolateral fragment (most common)
• Margins: Smooth and corticated on both the fragment and the main patella at the synchondrosis
• Bilaterality: Present bilaterally in up to 50% of cases, which is a strong clue favoring a developmental variant
• Symptomatic bipartite patella: Repetitive stress (e.g., quadriceps traction in athletes) can cause pain and edema at the synchondrosis, sometimes mimicking an acute injury clinically, though the radiographic appearance remains that of a chronic, corticated fragment unless acutely disrupted

Bipartite patella vs. acute patellar fracture: an acute transverse or stellate patellar fracture shows a sharp, irregular, non-corticated fracture line, is typically unilateral, follows an acute trauma history, and may show an associated hemarthrosis or displaced fragment; bipartite patella shows a smooth, rounded, corticated superolateral fragment, is frequently bilateral, and lacks an acute trauma mechanism.

Cervical Ribs

A cervical rib is a supernumerary rib that arises from the seventh cervical vertebra (C7), representing persistence and ossification of the costal element that normally regresses during development.

• Prevalence: Roughly 0.5-1% of the population; bilateral in about 50% of affected individuals
• Appearance: A rib-like bony structure extending laterally and inferiorly from C7, which may be a short rudimentary process, a complete rib with a fibrous band to the first rib, or (rarely) a complete rib with a true costal cartilage and sternal articulation
• Clinical significance: The leading identifiable structural cause of thoracic outlet syndrome, because the anomalous rib or its associated fibrous band can compress the brachial plexus and/or subclavian vessels in the scalene triangle

Cervical rib vs. elongated C7 transverse process: the key differentiator is the direction and origin of the bony process. A cervical rib curves downward/inferiorly from its origin (mimicking the downward slope of a normal thoracic rib), while an elongated C7 transverse process typically points laterally and slightly upward, similar to a normal cervical transverse process, and does not have a separate costal element or synostosis pattern with the first rib.

Tarsal Coalitions

Tarsal coalition is an abnormal congenital union (fibrous, cartilaginous, or bony) between two or more tarsal bones, most commonly involving the calcaneonavicular and talocalcaneal (subtalar) joints.

Calcaneonavicular Coalition

• Best seen on: Lateral and oblique foot radiographs
• Classic sign: The "anteater sign" — an elongated anterior process of the calcaneus that projects toward and overlaps the navicular, resembling an anteater's snout on the lateral view
• Age of presentation: Often becomes symptomatic in adolescence (roughly ages 8-12) as the coalition ossifies

Talocalcaneal (Subtalar) Coalition

• Best seen on: Axial/Harris-Beath view and, most reliably, CT or MRI
• Classic sign: The "C-sign" — a continuous C-shaped line formed by the outline of the medial talar dome and the sustentaculum tali on a lateral radiograph, indicating abnormal bony bridging at the middle facet
• Secondary sign: Talar beaking — a dorsal osteophyte at the talonavicular joint from altered subtalar biomechanics; this can also be seen with calcaneonavicular coalitions

Both coalition types classically present with a rigid, painful peroneal spastic flatfoot in adolescents, with restricted subtalar inversion/eversion on exam, and DXI items may ask you to correlate the radiographic sign with the affected joint or to select the next best imaging step (typically CT) when plain films are inconclusive.