In order to try to present the topic in a practical way, the main surgeries in each segment of the spine will be arranged, with examples of the expected findings after the different surgeries.

Subsequently, possible complications that may arise will be reviewed.

1. CERVICAL SPINE.

A. ACDF (Anterior cervical discectomy and fusion):

ACDF are the acronym of anterior cervical discectomy and fusion. This is the main surgical technique for the treatment of foraminal or central stenosis at the cervical spine.

This is a surgery performed by an anterior approach (Smith-Robertson Approach). In normal conditions, the anterior approach to the cervical spine is easier than the posterior approach, as there is access between the different anatomical structures.

The intervention is performed through a small incision in the anterior part of the neck, and the different structures (mainly muscular, taking special care with the vessels) are separated until reaching the level that is the object of the surgery. [Figure 1]

The intervention consists of two main parts: the first is a discectomy of the affected level. The second is the fusion. For this, either bone grafts or implants are used where the disc was originally located, and the upper and lower vertebrae are subsequently stabilised with screws and plates. [Figure 2]

This procedure can be carried out on various consecutive segments, if it is required.

In the cervical spine, it is important to replace the disc with a graft or implant to avoid deformity and kyphosis (which is not essential in the lumbar spine).

B. CORPECTOMY:

This procedure is usually performed in cervical burst fractures in a traumatic context. This type of fracture is characterised by the destruction of several fragments of the vertebral body, with eventual retropulsion of these towards the canal.

Given the involvement of the vertebral body, it is decided to "resect" the fragments, with replacement of the vertebral body with an autologous bone graft (such as can be obtained from the iliac crest) or an implant, with fixation with plates and screws. [Figure 3]

An anterior approach is performed, similar to that described in the previous section, until the affected level is located. In more complex cases, a joint anterior and posterior approach may be required.

C. ODONTOID FRACTURES:

As a general rule, most type II fractures in the Anderson and D'Alonzo classification are susceptible to surgical treatment, as they are the most unstable. For type I and III fractures, conservative treatment may be enough.

As radiologists we should be aware that there are two types of approach, choosing one or the other depending on the different characteristics of the patient and the fracture (which are not the subject of this review):

In general, the anterior is more commonly used.

- Anterior: In acute type II fractures, this technique is the best option due to its simplicity and the preservation of the normal movement of atlantoaxial rotation. Anterior cannulated screw fixation has become established as the most commonly used surgical technique in unstable odontoid fractures in older adult patients. [Figure 4]

Surgical incision is made in the skin at C5-C6 level. The Smith-Robertson approach is performed to expose the cervical prevertebral space; the prevertebral fascia and long neck muscles are elevated bilaterally to expose the antero-inferior C2 vertebral body.

- Posterior: Within this approach there are variants, of which the most commonly used is the posterior C1-C2 instrumentation with screws and rods. [Figure 5]

2. LUMBAR SPINE.

A. SPINAL FUSION SURGERIES: [Figure 6]

They aim to remove a pathological disc that affects the neural elements or spinal canal, and replace it with an interbody spacer (or bone graft). This makes it possible to restore the height of the intervertebral space, and at the same time avoid alterations in the dynamics of the spine, which could occur in procedures where the disc is not replaced by any element (thus aiming to stabilise and maintain alignment).

As a potential disadvantage, we could lose mobility of the fixed segments.

Common indications for lumbar fusion include spondylolisthesis, both lytic and degenerative; spinal deformity (scoliosis/kyphosis), advanced degenerative disc disease with disabling low back pain, among others.

The most commonly used devices in these surgeries are rods and plates, transpedicular screws and interbody spacers. In the post-surgery imaging tests, the intergrity of the devices should be assessed, as well as their proper position.

Some examples of this type of surgery at the lumbar level are:

-ALIF (Anterior lumbar interbody fusion): This is mainly used for surgery on the L5-S1 intersomatic space, which is the most accessible via the anterior approach. Access to the spine is between the iliac vessels, below the aortic bifurcation and the inferior cava. [Figure 7]

-DLIF (Direct Lateral Interbody Fusion)/OLIF (Oblique Lumbar Interbody Fusion): Both are similar in imaging, the difference lies in the technique used by the surgeon. In OLIF the approach will be anterior to the psoas, whereas in DLIF the approach is through the psoas (requires neuromonitoring to reduce the risk of intrapsoas nerve root injury).  [Figure 8]

B. SPINAL SIMPLE DECOMPRESSIVE SURGERIES (WITHOUT FUSION).

In general, its objective is to release or decompress neural elements that are causing the patient's symptoms. In this section there are several surgeries, which can be performed in isolation or in combination (depending on the findings, both in the pre-surgical tests and intraoperatively).

-Discectomy: Resection of the herniated disc that is causing the compressive symptoms.

-Laminectomy/hemilaminectomy/ facetectomy: Resection of all (laminectomy) or part of the lamina of the vertebral body (hemilaminectomy). The main purpose is to relieve central stenosis or to give greater exposure to a herniated disc during discectomy. [Figure 9]

In the case of facetectomy, the entire facet joint is also resected, so that this procedure is particularly useful when foraminal, lateral recess and central stenoses are present.

- Interspinous device (DIAM ®): they are devices placed between the spinous process. They are used in canal stenosis or foraminal stenosis. [Figure 10]

C. SPECIAL CASES:

Combination of transpedicular screws and possible vertebroplasty (by interventional radiology).

Unstable burst fractures often require surgical stabilisation. One of the methods currently used is the percutaneous approach with transpedicular rods and screws, which are placed in the spaces above and below the injured vertebral body. This technique is also possible when there is inestibility for another reason (e.g. in metastatic vertebrae).

Sometimes it is possible to perform combined approaches with interventional radiologists, associating vertebroplasty of the affected vertebral body. [Figure 11]

3. COMPLICATIONS. [Figure 12]

A. IMPLANT-RELATED COMPLICATIONS:

-Implant rupture: they occur secondary to metal fatigue due to spinal movement.

-Implant movement: The interbody spacer may move, invading the spinal canal or compressing neural structures. We considere that the interbody spacer is  well positioned, if the distance between the radiopaque marker on the posterior margin of the graft and the posterior margin of the vertebral body is equal to or greater than 2 mm. [Figue 13]

-Malposition: An example is malposition in transpedicular screw placement. The position of the screw is considered optimal when it passes through the central face of the pedicle and is aligned parallel to the upper endplate. Abnormal orientation of the screw can lead to nerve compression if it invades the foramen. [Figure 14]

B. ADJACENT LEVEL SYNDROME:

This is the development of pain or disability in patients who have previously undergone fusion surgery. Degenerative changes are observed in the spaces immediately above or below the operated level. 

Clinical assessment is important in its diagnosis, as there may be degenerative changes on imaging tests without clinical signs.

C. PSEUDARTHROSIS [Figure 15]:

As in other regions of the anatomy, this is the failure of the implanted elements to fuse. In imaging tests (simple X-ray or CT), a radiolucent line is visualised adjacent to the bone graft material.

D. POSTOPERATIVE COLLECTIONS:

-Seroma /Abscesses: its imaging characteristics do not differ from others located in other locations.  In the case of abscess, we will see a peripheral enhancing collection after intravenous contrast administration. [Figure 16]

In case of infection it can lead to spondylodiscitis. [Figure 17]

- Hematoma: high density collection on CT scan. [Figure 18]

E. COMPLICATIONS RELATED TO THE PROCEDURE:

- CSF fistula: is caused by accidental rupture of the dura mater, with CSF leaking out around the surgical wound. [Figure 19]