**2. The Kieser and Hammer classification system**

### **2.1 Type 1: global failure (implant pull-out)**

This form of ASD is seen when metalware pulls out of the vertebrae (**Figure 1**). This failure is not seen in anterior interbody constructs, unless supplementary posterior or lateral instrumentation is utilised. This is because it is recognised as persistent spinal malalignment, which requires interconnected rigid implants to remain in one position and the spine to displace from the rigid instrumentation. However, with lateral implants (plates, stapes, etc.) and a failure to restore coronal balance or posterior implants and a failure to restore sagittal balance, the metalware can pull out of the bone producing a type 1 failure.

This failure is therefore almost always due to malalignment, but can be subclassified into:


In type 1a, the bone quality is insufficient to hold the implanted device in a given configuration, with the effect that the metalware pulls out. Similarly, in type 1b the implant configuration is insufficient to stabilise and anchor in the bone and

**Figure 1.** *Type 1 (global failure).*

*An Anatomical and Pathological Classification of Thoracolumbar Adjacent Segment Disease DOI: http://dx.doi.org/10.5772/intechopen.89960*

### **Figure 2.**

*Antero-posterior X-rays of a patient with a previous L4-S1 fusion who developed type 4d ASD of L3/4 causing foraminal stenosis and therefore underwent a lateral interbody fixation and plate (a. immediate postoperative) but with failure to completely restore coronal balance and insufficient fixation resulting in implant pullout and progressive coronal imbalance with recurrence of symptomatic foraminal stenosis (b. 6 weeks postoperative). Therefore supplementary percutaneous pedicle screw insertion to augment fixation and bracing was undertaken, which resulted in sufficient fixation to permit definitive fusion (c. 6 months postoperative).*

therefore it pulls out (**Figure 2**). In most cases it is a combination of both poor bone stock and insufficient implant fixation.

In an asymptomatic patient without skin compromise, the practitioner or surgeon advises bracing to prevent further progression until the fusion has developed. In contrast, in symptomatic patients or those with progressive failure amendable to operative intervention, the treatment of type 1 failure is revision surgery with restoration of spinal alignment and supplementary bracing until fusion has developed. In addition, for each sub-classification we advocate.

1a. Bone supplementation with medical management of osteoporosis (e.g. calcium, vitamin D, bisphosphonates, etc.) and increased fixation (e.g. cemented screws, HA coated screws, sublaminar bands, etc.).

1b. Increase fixation of the operative levels. May require extension of fusion if adequate fixation of the operative levels is not possible. 1c. Both.

It should be recognised that bone supplementation takes a prolonged period of time to achieve clinical benefit and most of these patients require semi-urgent surgical intervention. Thus, in symptomatic patients with deficient bone quality, increased fixation should be provided in addition to the medical management of osteoporosis. Furthermore, bracing should be considered to supplement the spinal stability provided by the surgery until fusion has occurred.

### **2.2 Type 2: adjacent bone failure (failure of the cranial or caudal uninstrumented vertebrae)**

This form of ASD occurs when an adjacent, uninstrumented vertebrae fails, typically with a compression type fracture (**Figures 3** and **4**). This is most commonly caused by poor bone quality and/or malalignment and is therefore subclassified as:

2a. Poor bone stock. 2b. Malalignment. 2c. Combination.

Most patients have a combination of malalignment and osteoporosis but are predominantly affected by their poor bone quality. Unlike type 1 failures, these patients are rarely in need of an urgent surgical intervention. Therefore, a conservative approach can be initially trialled. Bracing as well as vertebroplasty or kyphoplasty should be considered to avoid progressive collapse and deformity. However, the clinician should recognise that vertebral body cementation may affect subsequent extension of fusion if necessitated, particularly if pedicle screws are considered necessary. Therefore, surgeons treating these patients should consider alternative fixation techniques, such as cortical trajectory screws, in the cemented vertebrae if extension is subsequently required. In those that become asymptomatic these fractures should be treated as osteoporotic compression fractures. In those that remain symptomatic and are amenable to operative intervention, the treatment should be:

2a. Bone supplementation ± bracing until fracture union is achieved. In a globally aligned spine with union of the fracture, no operative intervention is required.

2b. Deformity correction with extension of fusion and increased fixation. Increased fixation is necessary because the bone has shown evidence of weakness, even in the absence of global osteoporosis, and therefore increased fixation is necessitated.

2c. a ± b: Bone supplementation and bracing if the global malalignment is acceptable OR bone supplementation and deformity correction with extension of fusion, increased fixation and bracing if the malalignment is unacceptable.
