Site of constriction

Lumbar spinal stenosis can be distinguished, based on the site of constriction, as stenosis of the spinal canal or central stenosis, isolated stenosis of the nerve root canal or lateral stenosis, and stenosis of the intervertebral foramen (Table 87.1).

Table 87.1 Classification of Lumbar Spinal Stenosis

In stenosis of the spinal canal, the entire area of the canal, as viewed on the axial plane, is usually constricted (Fig. 87.2). In other words, both the central portion of the canal and the lateral parts, occupied by the emerging nerve roots, are constricted. Therefore, the expression stenosis of the spinal canal is more correct than that of central stenosis, which would indicate constriction only of the central area. However, the authors will use the latter term because it has become the one commonly adopted.

Fig. 87.2 MRI scan at L4–5 level showing central spinal stenosis. The spinal canal is constricted in the central portion, containing the thecal sac, as well as in the lateral portions occupied by the emerging nerve roots.

Central stenosis, except for the rare forms due to vertebral malformations, or sequelae of fractures or infections, is located at the level of the intervertebral space, where there are the anatomical structures, such as the intervertebral disc, the apophyseal joints, and the ligamenta flava, which can change with aging or disease.

The nerve root canal or radicular canal corresponds to the lateral portion of the spinal canal (Fig. 87.3). This canal, which is more of an anatomical concept than a true canal, is the semitubular structure in which the nerve root, exiting from the thecal sac, travels before entering the intervertebral foramen. As for the central form, in the last decade, the term lateral stenosis has become the most widely used for this type of stenosis.

Fig. 87.3 Lateral stenosis at L4–5. CT scan showing constriction of the nerve root canals due to degenerative changes of the facet joints.

The term lateral stenosis is often used to indicate both nerve root canal stenosis and stenosis of the intervertebral foramen. The authors believe that the intervertebral foramen, which begins and ends at the level of the medial and the lateral border of the pedicle, respectively, should be considered as a distinct anatomical entity. Therefore, stenosis of the foramen should be differentiated from the other two forms of stenosis, although it can be associated, albeit rarely, with one of the two.

Type of stenosis

Three forms of stenosis can be identified: primary, secondary and combined (see Table 87.1).

Primary forms

Central stenosis

This abnormality can be classified as congenital or developmental central spinal stenosis. Congenital stenosis, which is exceedingly rare, is due to congenital malformations of the spine.

Developmental stenosis includes achondroplastic and constitutional forms. In the former, the midsagittal and/or interpedicular diameters of the spinal canal are abnormally short and the nerve root canals are severe narrowed due to abnormal shortness of the pedicles. In constitutional stenosis the cause of the defective vertebral development is unknown. In the authors’ experience, two types of constitutional abnormality may be identified: (1) a short midsagittal diameter of the spinal canal, and (2) an exceedingly sagittal orientation and/or shortness of the pedicles (Fig. 87.4). In the latter type, the spinal canal is abnormally narrow, mainly or only, in the inter-articular diameter.

Fig. 87.4 CT images showing constitutional stenosis at L3–4 and L4–5. The articular processes are orientated more sagittally than normal and the pedicles are very short. The result is a constriction of the spinal canal more in the interpedicular, than in the sagittal, plane.

Lateral stenosis

Lateral stenosis may result from disturbances of vertebral development, particularly achondroplastic or constitutional abnormal shortness of the pedicles, even more so if associated with a trefoil configuration of the spinal canal or anomalous orientation and/or shape of the superior articular process. In this form, a primary role may be played by the intervertebral disc. A mild disc protrusion or a bulging anulus fibrosus may be enough to cause symptoms in a stenotic or only narrow radicular canal.

Stenosis of the intervertebral foramen

Primary forms are found almost exclusively in the presence of abnormally short pedicles associated with a decrease in height of the intervertebral disc (Fig. 87.5).⁴

Fig. 87.5 Effects intervertebral space narrowing on foraminal dimensions on dried vertebrae. In the presence of normal pedicle length (top and middle), even a marked decrease of the disc height does not change significantly the foraminal dimensions. However, in the presence of short pedicles (bottom), a marked decrease of the disc height causes a relevant decrease of the foraminal dimensions.

Secondary forms

Central stenosis

If the sagittal dimensions of the spinal canal are normal, or at the lower limits, and compression of the caudal nerve roots is the result of one or more acquired conditions, such as spondylotic changes of the facet joints, abnormal thickening of the ligamenta flava, and bulging of the intervertebral discs, then this form is defined as simple degenerative stenosis (Fig. 87.6).

Fig. 87.6 Simple degenerative stenosis. T2-weighted sagittal MRI showing stenosis at L3–4 and L4–5. The sagittal dimension of the thecal sac is normal in size at intervertebral levels.

Very often, however, degenerative spondylolisthesis of the cranial vertebra of the motion segment is also present at one or, occasionally, two or more levels (Fig. 87.7). Degenerative spondylolisthesis is consistently responsible for narrowing of the spinal canal, but may not cause lateral or central stenosis. This is because the presence, type, and severity of stenosis is related to several factors, such as the constitutional dimensions of the spinal canal, the orientation (more or less sagittal), and the severity of degenerative changes of the facet joints, and the amount of vertebral slipping, which may in some cases play a minor role. For example, a grade I spondylolisthesis in a patient with a constitutionally large spinal canal produces no significant narrowing of the canal, would be categorized as no stenosis. In contrast, the same or even lesser grade of spondylolisthesis in a patient with a primarily narrow canal can be associated with clinically significant stenosis. The type of stenosis, that is whether stenosis is central or lateral, depends on the orientation of the articular processes and the length of the pedicles. Usually, stenosis initially presents as lateral and then central in later stages. Instability, that is hypermobility on flexion–extension radiographs, is one of the main characteristics of degenerative spondylolisthesis. However, in many cases there is no appreciable hypermobility of the slipped vertebra. The authors consider the latter condition as a potential instability, which can become unstable as a result of surgery. Such a scenario may arise following removal of a large part of one or both facet joints, unilateral or bilateral discectomy, or when destabilizing factors unable to stabilize a normal vertebra intervene, such as disc degeneration or severe degenerative changes of the facet joints. In degenerative spondylolisthesis, the intervertebral disc often bulges into the intervertebral foramen to cause stenosis. However, true stenosis of the foramen is rarely present as the foramen becomes larger in the sagittal dimensions in the presence of slipping of the cranial vertebra.

Fig. 87.7 Plain radiographs of a patient with severe degenerative spondylolisthesis of L5 and mild slipping of L4.

A particular form of acquired stenosis is the type that is associated with degenerative scoliosis. In this instance a role may be played by the scoliotic curve in tandem with the pure degenerative changes of the facet joints and the intervertebral discs.

Other forms of secondary stenosis include late sequelae of fractures or infectious diseases of the spine, which, however, are rare conditions. Rarely, stenosis can occur secondary to systemic bone diseases such as Paget’s disease. Paget’s disease can lead to an increase in volume and/or deformation of one or more vertebral components.

Lateral stenosis

Most often, this form of stenosis is degenerative in nature. Usually, degenerative stenosis involves only the lateral portions of the spinal canal in the initial stages and become central in more advanced stages when spondylotic changes becomes more severe. This is particularly true for degenerative spondylolisthesis.

Lateral stenosis can occur due to a cyst of the facet joint when it compress the emerging root in the nerve root canal (Fig. 87.8).

Fig. 87.8 Lateral stenosis due to a synovial cyst of the left facet joint at L4–5 level.

Stenosis of the neuroforamen

Stenosis of the intervertebral foramen is rarely encountered, especially as an isolated condition. In most cases, foramenal stenosis is associated with a central or lateral stenosis. At times, root compression occurs when there is a lateral disc herniation or disc bulge in the presence of advanced degenerative changes of the articular processes.

Advanced age

Surgical decompression may offer significant relief of symptoms also to patients older than 70 years.^5–8 In the authors’ experience, there is no significant difference in the results of surgery between the patients in early senile age and those aged 80–90 years old, provided the stenosis is severe and the patient’s general health is satisfactory.

Comorbidity

In one study,⁷ a high rate of comorbid illnesses was found to be inversely related to the rate of satisfactory results after surgery. Another study⁹ compared the long-term results of surgery in 24 diabetic and 22 nondiabetic patients. In the diabetic group there was a 41% rate of satisfactory results, compared with 90% in the nondiabetic group. Different results, however, were observed in a similar study,¹⁰ in which the outcome was satisfactory in 72% of the diabetic and 80% of the nondiabetic patients. Neither the duration of the diabetes before surgery nor its type correlated with the outcome. A mistaken preoperative diagnosis was the main cause of failure in diabetic patients. In many of the failures, diabetic neuropathy or angiopathy had elicited symptoms that had been confused with pseudoclaudication.

Previous surgery

Surgery for spinal stenosis tends to give less satisfactory results in patients who had previously undergone decompressive procedures in the lumbar spine.^7,11–14 This is particularly true when stenosis is at the same level or levels at which the previous surgery for disc herniation or stenosis had been performed.¹⁵

Type of stenosis

There is no significant difference in outcomes between the various types of central stenosis. However, patients with degenerative or combined stenosis at a single level are the best candidates for surgery because they tend to have a better outcome than those with stenosis at multiple levels. In patients with constitutional stenosis, the intervertebral disc may play a significant role in the compression of the neural structures. When the disc bulges considerably in the spinal canal, but it is not truly herniated, it may be difficult to eliminate the anterior compression of the neural structures. Consequently, less satisfactory outcomes may be obtained when compared with the cases in which the neural compression is caused exclusively by the articular processes. Patients with lateral stenosis, particularly at a single level, tend to have better results than those with central stenosis.