**2.4 Type 4: IVD failure**

This is a common form of ASD and is most commonly seen as a late complication of fusion (**Figure 7**). It may be related to inherent disc degeneration that would have occurred whether fusion was performed or not. However, it may also be caused by:

4a. Acute hyper-load which presents as an acute IVD prolapse. 4b. Diffusion insufficiency and/or chronic excessive loading which presents with progressive IVD desiccation.

Hyperload is caused by the rigidity imparted by the fusion. Typically, with circumferential fusions at the most cranial level this causes acute hyperload and acute disc prolapse. In contrast, isolated posterior or isolated interbody devices cause chronic overload as the fusion develops and micromotion of the fusion construct reduces. This causes progressive disc failure of the adjacent level (**Figure 8**). Similarly, implants that induce rigidity of the end-plates, such as subchondral pedicle screws limit the usual motion of the endplate. This motion, which mimics that of a trampoline, aids diffusion of nutrients into, and waste products out of the IVD. Thus, limiting its motion affects disc health by affecting its nutrient supply.

**Figure 7.** *Type 4 failure (IVD failure).*

### **Figure 8.**

*Sagittal (a) and axial (b) MRI sequences of a type 4b failure above a L4/5 circumferential fusion. The IVD prolapse caused central stenosis and neurogenic claudication.*

The treatment of this form of ASD depends on the symptoms and is the same for both 4a and 4b:

a.If neural compromise without instability: decompression alone.

b.If neural compromise with instability: decompression and single level fusion.

c.If discogenic pain or instability: single level fusion.

In patients with malalignment, deformity correction should be considered.
