**1. Introduction**

Adjacent segment disease (ASD) is the symptomatic structural failure of the spine or sacrum adjacent to an area of previous operative intervention, most notably fusion [1]. Internationally, the numbers of spinal fusions being performed is increasing. Within the USA approximately 457,500 adult spinal fusions and 38,000 paediatric spinal deformity corrections are performed annually, with similar rates per capita reported worldwide [2]. Of concern is that more than 20% of patients undergoing lumbar deformity surgery will develop ASD within 8 years, most of which occurs early with 40% requiring revision within 6 months [3–9]. This has a significant clinical effect on patient outcomes, with pain, neurological, emotional, social and occupational concerns, but also carries a large financial burden, with an estimated cost in the USA for revision being \$77,432 USD per patient [10, 11]. This would suggest that over 500 million USD is spent annually on the surgical treatment of ASD. Yet, a complete understanding of the aetiology of this problem has not been compiled.

It is believed that the cause of ASD is multifactorial [12]. These causes can be separated into non-modifiable, potentially modifiable and modifiable risk factors.

Non-modifiable risk factors include patient age and expected baseline motion segment degeneration that cannot be modified with current known treatments [13, 14]. These factors are particularly pertinent in the adult population where their index procedure is often related to degeneration which itself renders patients at higher risk of degeneration at other levels.

Potentially modifiable risk factors include bone density, which may be amenable to medical treatment [15]. Others include fusion without instrumentation and limiting the fusion length, however the pathology often dictates the length of fusion and implant requirement [13, 16].

Modifiable risk factors include intraoperative surgical techniques, notably motion preservation to reduce the adjacent segment load, avoiding circumferential fusion of the most cranial segment that increases the stresses on the adjacent level, ensuring spinal balance, avoiding extensor musculature and ligamentous damage, protecting the adjacent facets, endplates and intervertebral disc (IVD) [13, 14].

While multiple classification systems and definitions have been proposed, none have attempted to group these into anatomical or pathological considerations. Our classification broadly categorises ASD into five groups according to the anatomical region of failure which can then determine the likely pathological cause and offer treatment direction.
