**1. Introduction**

Recent studies have focused on how sagittal malalignment of the cervical spine influences outcomes and promotes impairment of quality of life. In order to further understand cervical movement, compensatory mechanisms and pathologies, there are basic biomechanical properties parameters that should be considered. These include mass (m), force (F), standard gravity (g), moment arm (L), bending moments (M) and instantaneous axis of rotation (IAR). In the upright position the head creates a gravitational force on the cervical spine with a magnitude, of F = m x g. This gravitational force then creates a forward bending moment, M, around a fulcrum of rotation, also known as the IAR. The magnitude of the bending moment is calculated by M = F x L, in which L is the distance between the IAR and the center of gravity line.

Yogadanan et al. [1–6] showed that for cadaver studies conducted in the last five decades the center of gravity (COG) or center of mass (COM) of the head is located approximately 1.8 cm anterior and 6.0 cm superior to the occipital condyle. The numbers vary from one cadaver study to the next [6–16]. The head to total body mass (TBM) ratio was 7.37% + − 0.6%. The mean head mass was 4.770.3 kg [17].

In a normally aligned lordotic cervical spine, the posterior tension band and paraspinal muscles counteract the forward bending movement created by the

weight of the head, maintaining the natural cervical alignment. When cervical kyphotic deformity is present, the head COM moves anteriorly and the moment arm, L, increases relative to the IAR, creating a larger bending moment, M. This results in greater paraspinal muscle contraction to keep the head erect, ultimately followed by exertion and pain.

The weight-bearing features of the cervical spine have been grouped into an anterior column, including the vertebral bodies and intervertebral discs, and two posterior columns, consisting of the facet joints [6]. It has been estimated that the anterior column is responsible for bearing up to 82% of the weight of the head while the posterior column is responsible for up to 33% [18]. By creating a larger bending moment, M, the kyphotic cervical deformity shifts the axial load anteriorly, which probably accelerates cervical disc degeneration. Disc degeneration might cause further cervical kyphosis, leading to an apparent vicious cycle.

Likewise, junctional failures of fusion are clearly the result of an imbalance of anterior column compression forces and posterior column tension band strength [1]. Biomechanical studies investigating the effects of spinal fusion on adjacent levels have shown that adjacent unfused levels compensate for the loss of cervical range of motion (ROM) in fused levels [19]. Maiman et al. [20] described a finiteelement model of the cervical spine to investigate the effect of cervical spine fusion on adjacent levels. There was increased flexion-extension rotational movement of the disc in the sagittal plane especially at the upper adjacent level of the fusion. And this may contribute further to the pathologic progress.

Individualized optimization of surgical alignment has been shown to improve outcome regarding PJK [21].

Adult cervical deformity (ACD) of the spine has been shown to have a substantial negative impact on health-related measurements [20]. Therefore surgery to correct ACD can have a profound effect on improving the patient's health status. A common complication following fusion surgery is excessive kyphosis at one end of the fused construct. For example, thoracolumbar deformity correction commonly results in proximal junctional kyphosis (PJK), with reported rates as high as 40% [22]. In ACD surgery, fusions are usually extended to the upper cervical spine, which increases the likelihood of stress at the caudal part of the fusion construct, potentially leading to distal junctional kyphosis (DJK) or failure (DJF). In 2019, Oren et al. [23] introduced the utility of measurements of spinopelvic angles on prone lateral radiographs as predictors of global post-operative alignment in thoraco-lumbar deformity surgery. Similar measures are now in development for cervical deformity correction.

In this chapter, we describe our approach to the treatment of cervical deformity and the steps taken to minimize the risk of DJK post-operatively by tailoring the construction to the individual patient.

First we focus on characterization of the baseline deformity. Secondly, we assess our patients clinically. Thirdly, we simulate the correction with the use of novel in-construct measurements. The fourth step is to develop a DJK prevention strategy tailored to the individual. The last step is to perform surgery and check correction during the operation.

#### **2. Characterization of the deformity**

Ames and colleagues [23] have developed a comprehensive system of classification for cervical deformity. It defines the deformity driver and assigns severity points for four cervical parameters, the cSVA, CBVA, TS-CL, and myelopathy.

#### *Planning Cervical Deformity Surgery Including DJK Prevention Strategies DOI: http://dx.doi.org/10.5772/intechopen.94390*

The classic measure of sagittal alignment in the cervical spine is the **cervical sagittal vertical axis (cSVA)** which measures the distance between a plumb line dropped from the centroid of C2 to the posterior superior aspect of C7. Hardecker et al. defined normative values ranging from 0.5 to 2.5 cm [24]. Several studies, one of them Tang et al. [25] have shown that high post-operative cSVA correlated with poor post-operative outcomes in patients undergoing cervical fusion. A cSVA over 4 cm corresponds to a moderate disability threshold. cSVA correlates with outcome measures in patients with thoracolumbar deformity as well as myelopathy.

The **T1 slope (T1S)** has emerged as an important measurement for pre-operative planning. It is the angle formed by a line drawn along the superior endplate of T1 and a horizontal reference line at the median sagittal cervical vertebra from the CT radiographs. Knott et al. [26] predicted that when the T1 slope is higher than 25 degrees, patients had at least 10 cm of positive sagittal imbalance. Ayres et al. [27] showed that a T1 slope above 30 degrees, indicates the need to perform full-length spine radiographs to identify potential concurrent thoracolumbar (TL) deformity. The right technical conditions with the use of long X-ray cassette radiographs should be met from the beginning, as shown by Ramchandran [28, 29]. In his survey among spine surgeons, 58% opted for longer fusion constructs to the mid- or lower thoracic spine in cervical deformity, when presented with long cassette radiographs. A T1 Slope above 30 degrees was associated with worse sagittal balance and spinopelvic parameters values after corrective surgery [30]. Kim et al. showed that a high T1 slope in myelopathy patients undergoing laminoplasty predicted postoperative kyphotic alignment after laminoplasty [31].

An important marker of cervical deformity is the **C2 slope (C2S),** which correlates with **T1 Slope Minus Cervical Lordosis (TS-CL)**, one of the Ames parameters of CD. This correlation is explained by the fact that the C2 slope is a mathematical approximation of the TS-CL [32]. However, C2S is simpler and more efficient to measure since it is just one angle. A high C2S of over 20 degrees correlates with poor Health-Related Quality of Life scores [32]. These results have been further corroborated by other groups including Hyun et al. [32] who found that a TS-CL greater than 22.2 degrees corresponded to severe disability (NDI > 25) and positive cervical sagittal malalignment, defined as a C2-C7 SVA greater than 43.5 mm.

Finally, an efficient assessment of concurrent thoracolumbar deformity is necessary. A helpful singular measurement in this regard is the **T1 pelvic angle (TPA)**. It simultaneously combines the measurement of sagittal deformity (as measured by T1 spinopelvic inclination, analogous to SVA) and pelvic compensation (pelvic tilt). The TPA is the angle subtended by a line from the femoral heads to the center of the T1 vertebral body and a line from the femoral heads to the center of the superior sacral end plate. Protopsaltis et al. [33, 34] showed excellent intra- and inter-observer reliability of this measurement.

Moreover the TPA remains constant, regardless of pelvic compensatory retroversion.

To summarize, we may include these four parameters as our key alignment parameters. The cSVA correlates well with every outcome measure. The T1S gives us information about the underlying thoracolumbar deformity. TPA gives us a quick and compensatory mechanism-independent overview of global thoracolumbar deformity. C2S tells us if a patient can compensate for the cervical spine deformity.
