Adjacent Segment Degeneration and Disease of the Cervical Spine

Terminology

Using terms such as “adjacent segment degeneration” and “adjacent segment disease” interchangeably can result in imprecise classifications and inadequate postoperative analysis. In 2012, AOSpine released a statement establishing simple, descriptive terminology to standardize the language used among physicians, patients, researchers, payers, and others. ASP was designated as an umbrella term encompassing two specific concepts: radiographic ASP (RASP or adjacent segment degeneration) and clinical ASP (CASP or adjacent segment disease). RASP refers to the development of new radiographic degenerative changes (i.e., intervertebral disc height loss, osteophyte formation, end plate sclerosis) at an adjacent level, regardless of type of imaging modality, without associated clinical symptoms. CASP, on the other hand, refers to new degenerative pathology at an adjacent level with associated symptoms (i.e., new-onset myelopathy, radiculopathy, persistent neck pain). ^,

Biomechanics of Adjacent Segment Pathology

The cervical spine is subject to a complex interplay of axial and rotatory forces, encouraging wide range of motion and distribution of weightbearing loads. The functional spinal unit (FSU, or motion segment) consists of the superior and inferior adjacent vertebrae/facet joints, as well as the intervening disc and surrounding soft tissue structures. Disruption of the FSU, whether it is iatrogenic or pathological, can result in biomechanical alterations at the index and adjacent segment levels.

Previous studies have linked the development of ASP with soft tissue disruption. A systematic review by Kim et al. reported a two to five times higher risk of adjacent level ossification when the plate-to-disc distance was less than 5 mm. Additionally, a study by Nassr et al. showed that incorrect needle localization at an adjacent level was associated with a three times higher likelihood of adjacent segment degeneration caused by inflammatory changes subsequent to needle puncture. These studies highlighted the importance of minimal soft tissue disruption near the adjacent levels to prevent degenerative changes, although neither author was able to correlate the development of these degenerative changes with clinical symptoms.

Furthermore, cadaveric models have elucidated the effects of anterior cervical fusion on adjacent spinal segments as a result of weightbearing forces. In one of the first studies of its kind, Fielding conducted a fluoroscopic study evaluating the motion of the cervical spine. His study found levels adjacent to the site of the fusion construct experienced a compensatory increase in motion and instability. Similarly, Eck et al. found segmental motion increased at both superior and inferior adjacent levels upon flexion and extension in a C5‒C6 ACDF model. In this same study, intradiscal pressures were increased by 73% at C4‒C5 and by 45% at C6‒C7 compared with controls. These findings were further supported by Park et al., who demonstrated a significant increase in intradiscal pressure at C4‒C5 in a C5‒C7 ACDF model. However, no study to date has been able to account for the substantial amount of motion between the occiput and C2.

Recent studies have examined and compared the kinematic effects of CDA versus ACDF. Dmitriev and colleagues showed that, among cadaveric models that underwent fusion vs. arthroplasty, intradiscal pressures under flexion-extension were higher in the cervical fusion cohorts compared with the arthroplasty cohort. Furthermore, CDA preserved adjacent-level kinematics relative to preoperative levels. In a prospective randomized trial, Park et al. corroborated results from previous studies, showing preserved kinematics in the CDA group and increased range of motion at the superior adjacent segment in the ACDF group. Finally, Ghandi et al. went one step further, evaluating a cadaveric model comparing cervical fusion with arthroplasty and hybrid (arthroplasty + fusion) constructs. Their results showed that cadavers that underwent a hybrid surgery had increased motion at all unfused segments, although to a lesser degree than the fusion cohort. Additionally, the force required to achieve preoperative range of motion was 35% and 50% higher (compared with the arthroplasty group) in the hybrid and fusion groups, respectively. Whereas the altered adjacent-level biomechanics of fusion constructs may result in the activation of inflammatory cascades and eventual exacerbation of degenerative processes, motion-preserving procedures may prove to be an alternative option to slow down or prevent adjacent level pathology.