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Fractures of the spinal column are a common musculoskeletal injury, occurring nearly 150,000 times annually.1 Of these, the majority are in the thoracic or lumbar spine, and 75 to 90% are within the thoracolumbar junction.^1–3 Because of the frequency and potential severity of these injuries, a reliable and reproducible classification scheme that is both descriptive and helpful in determining the appropriate treatment is imperative. Several classification schemes have been developed for these injuries.^4–8 However, these schemes have poor reproducibility, little prognostic value, and a high level of complexity; for these reasons, and despite numerous attempts, none of these schemes has gained widespread acceptance.^3,9,10

The historical classification schemes can largely be grouped into either mechanistic or morphological classifications. Of the morphological schemes, the Magerl⁵ comprehensive classification, based on the AO classification for extremity fractures, is the most descriptive. Fractures are grouped into three mechanistic categories: compression, distraction, and translation. Each category is divided into three groups that are further subdivided to produce the final injury type. Although this classification is universally considered highly detailed, and it provides the most information regarding fracture anatomy, it has been criticized for being cumbersome and not appropriate for practical use.

More recently, the Thoracolumbar Injury Classification and Severity Score (TLICS), originally described in 2005, was developed to address the shortcomings of prior classification schemes, simplify the description of spine fractures, and guide treatment decisions.^9,10 This classification system categorizes spine fractures based on objective fracture morphology, the existence of a neurologic injury, and the presence of injury to the posterior ligamentous complex (PLC). Each of these categories is given a point value, the points are totaled, and the determination of operative versus nonoperative treatment is based on the final TLICS.¹⁰ Despite its recent success and widespread use, the TLICS is not without shortcomings, such as its inability to provide definitive treatment recommendations for various burst fracture configurations with possible disruption of the PLC.

The AOSpine Thoracolumbar Classification represents a hybrid of the TLICS and Magerl classifications that combines the strengths of each scheme.^11,12 Primarily, it provides a detailed yet simplified description of the fracture morphology, roughly based on what was previously described by Magerl. Additionally, it incorporates the neurologic status of the patient and the integrity of the PLC into the overall rating scheme, similar to the TLICS system and critical for clinical decision making. This chapter describes the derivation of this classification and discusses the methodology for grading injuries with this classification.

Unlike many of the historical classification schemes, which were based largely on individual surgeon experience, the AOSpine classification was developed by a number of academic spine surgeons who composed the AOSpine Classification Group (AOSCG). This working group used the Magerl classification as a starting point and systematically reviewed 750 cases from the AOSpine database to revise the classification.11 The AOSCG held seven meetings and five evaluation sessions, and produced a fracture morphology classification. The reliability of the new classification was determined using 40 randomly chosen trauma cases; nine fellowship-trained spine surgeons graded these 40 cases according to the new classification on two separate occasions 1 month apart. In Vaccaro et al’s¹¹ 2013 report on the new classification, the interobserver reliability, by kappa coefficient, for grading the fracture type and the overall classification was 0.72 and 0.64 respectively, indicating a substantial level of reliability. Intraobserver reliability had an average kappa coefficient of 0.77, again indicating a substantial level of reliability.

Classification

The AOSpine classification assesses three patient- or injury-specific variables: (1) the morphological classification of the injury, (2) the neurologic status, and (3) significant clinical patient-specific modifiers and comorbidities. The system uses the in formation provided by the three main injury categories from the original Magerl AO concept: A, compression injuries; B, tension-band injuries; and C, displacement-type injuries. Type A injuries were divided into subtypes A0 to A4; type B injuries were divided into subtypes B1 to B3; and type C injuries remained undivided.^11,12 Similar to the Magerl classification, a higher grade on the AOSpine classification scale denotes a higher severity of injury (i.e., B fractures are more severe than A fractures).

A more detailed description of each type and subtype is presented below.

Morphological Characteristics (Table 1.1)

Type A: Compression

Type A injuries, compression injuries, consist of fractures resulting from axial compression, with the anterior elements failing and the posterior elements remaining intact. This category represents fractures of the anterior elements, vertebral body, or disk, as well as fractures of the transverse or spinous processes.^11,12 There is no injury to the PLC, and there is no discernible displacement or dislocation. Type A compression injuries are further divided into five subtypes, as described in the following subsections.

Table 1.1 Fracture Morphology

A0: Minor Injuries

This subtype, minor injuries, includes fractures involving either the transverse process or spinous process without extension into the vertebral body. Because mechanical instability and neurologic deficit are not of concern in this type of injury, these injuries are often considered to be clinically insignificant.11

A1: Wedge Compression

The wedge compression subtype involves fracture of a single vertebral body end plate that results from compression or impaction of the cancellous bone in the vertebral body beneath the end plate. Fractures may involve either the upper or lower end plate without involvement of the vertebral body’s posterior wall (Fig. 1.1).

A2: Split or Pincer Type

This subtype involves injuries that have a fracture line running through both end plates in a split-type or pincer-type modality; however, it excludes involvement of the vertebral body’s posterior wall (Fig. 1.2).

A3: Incomplete Burst

Incomplete burst fractures involve one vertebral end plate and extend into the posterior vertebral wall with the potential for retropulsion of fragments into the spinal canal. This subtype of fracture affects a single end plate. The posterior tension band integrity is preserved, and vertebral translation is absent. Vertical lamina fractures are commonly found with this fracture morphology, as the compressive forces increase the distance between pedicles and produce stress across the posterior elements, leading to lamina fractures. The fracture pattern is associated with varying degrees of bony comminution and loss of vertebral body height depending on the quality of the bone and the energy of the injury. If the posterior tension band has been disrupted, the injury should be classified as a type B2 tension-band injury. Furthermore, A3 injuries that demonstrate a horizontal fracture in the axial plane through the posterior elements may be indicative of a distraction injury and should be classified as a type B injury as described below.

Fig. 1.1 Type A1: wedge compression fracture of the cephalad vertebral body end plate without involvement of the posterior vertebral wall.

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