Schmorl's nodes

Introduction

Schmorl’s nodes are nothing but protrusions of disc material into the surface of the vertebral body i.e. intraosseous disk herniation. It is named after German pathologist Christian Georg Schmorl (1861-1932). Schmorl's node can be detected radiographically but it is imaged better by CT or MRI. MRI is not only useful in detecting the recently developed Schmorl's nodes but also in differentiating between symptomatic and asymptomatic Schmorl's nodes (4). With reference to Schmorl’s nodes discs in MRI are generally noted for size, location, margins, internal and surrounding T1/T2 signal, adjacent disc herniation or bulge, concentric ring, underlying fracture including malignancy, infection, or prior disc surgery.

The migrating disc material of a Schmorl’s node when comes in contact with the marrow of vertebra it leads to inflammation. Many times protrusions are also associated with necrosis of the vertebral bone. Whether these protrusions and inflammation cause the necrosis, or the cartilage that migrates into vertebra become necrotic due to other conditions, is under investigation.

Few researchers claim that Schmorl’s node may be present by birth. However, the common belief is that it develops following back trauma (especially axial compressive force for example via a direct fall on buttocks), although this is incompletely understood. Clinically they may or may not be symptomatic. However, their etiological significance for back pain is controversial. Let us review it in a greater depth.
How Schmorl’s node manifests?

Clinical pain syndromes associated with acute, traumatic Schmorl's nodes (SNs) is limited. Wagner et al (2000) reported most of the Schmorl’s nodes could be traced to episodes of significant, sudden-onset, localized, nonradiating back pain and tenderness.

When the Schmorl's nodes produce pain? An explanation:

Under MRI, Schmorl's nodes reveal vascularization of the nodes and associated bone marrow edema (3). Takahashi et al (1995) postulated that after fracture healing and subsidence of inflammation, the Schmorl's nodes become asymptomatic, in analogy with old vertebral compression fractures. However many cases complain of persistence lower back pain.

Stäbler et al (3) reviewed MRI of 372 patients for Schmorl's nodes, vascularization of Schmorl's nodes, and associated bone marrow edema of lumbar and thoracic spine location. They tried to correlate the size of the node to clinical complaints. Diameter wise sizes of the Schmorl’s node vary. Two distinctly different varieties are marked on Gado-MRI investigations.

Type 1: Vascularized Schmorl's nodes without bone marrow edema

Type 2: Vascularized Schmorl's nodes surrounded by bone marrow edema

The diameter of the Schmorl’s node increases from type 1 through to type 2. The mean diameter of type 1 Schmorl’s node is found to be around 6.4mm where type 2 Schmorl’s node is around 8.2mm. However this study also revealed that subjects having Schmorl’s node size yet asymptomatic have a diameter of the node around 5.2mm. Once Schmorl’s node is associated with bone marrow edema there is a tendency of back pain. Such people have a Schmorl’s node diameter of around 7.9mm.

Similarly, Takahashi et al (4) reported, in all symptomatic cases, the vertebral body marrow surrounding the Schmorl's node was seen as low signal intensity on T1-weighted images and as high signal intensity on T2-weighted images. It was confirmed by histological examination that the MRI findings indicated the presence of inflammation and oedema in the vertebral bone marrow. These MRI findings were not seen in asymptomatic individuals. Inflammatory changes in the vertebral body marrow induced by intraosseous fracture and biological reactions to intraspongious disc materials might cause pain.

Characteristics of Schmorl's nodes:

Wu HT et al reported lumbar Schmorl’s nodes are generally located at the central or outer third of the endplate. They also reported majority of Schmorl's nodes are associated with disc bulging. However, a small proportion of Schmorl's nodes are also associated with disc herniation. fracture, tumor, or infection (2).

1. More than 80 percent Schmorl's nodes have well-defined margins.

2. Another common finding is concentric rings in the marrow surrounding the node. As reported by Wu et al (2006) this feature has 72% negative predictive value for absence of infection, tumor and fracture.

Time-course changes in edematous Schmorl's nodes:
Wagner et al reported many Schmorl’s nodes not immediately apparent as Schmorl’s nodes manifested only as vertebral body edema representing endplate fracture evolve into classical chronic Schmorl’s nodes at follow-up in MR imaging studies. These authors also reported enhancement of the invaginated disk material and enhancement of the surrounding vertebral body.

According to Wu et al although most remain unchanged, a relatively large minority of edematous Schmorl's nodes evolve in size and signal over a relatively short time. The statistics is as follows: In almost half cases node size of Schmorl's nodes does not change in time course. In almost quarter of the cases increased size occurs. Some evolve to form well-defined concentric rings in the surrounding marrow that appear to be analogous to degenerative changes of endplates. Concentric ring formation has a high negative predictive value for "idiopathic" Schmorl's nodes without underlying fracture, infection, or malignancy (2).

References:
1. Wagner AL et al; AJNR Am J Neuroradiol. 2000 Feb;21(2):276-81.
2. Wu HT et al; Skeletal Radiol. 2006 Apr;35(4):212-9. Epub 2006 Feb 10.
3. Stäbler A et al; AJR Am J Roentgenol. 1997 Apr;168(4):933-8.
4. Takahashi K et al; Eur Spine J. 1995;4(1):56-9.

My Class on disc pain-2 (Schmorl’s node- a unusual disc pain source!)