Baastrup Disease (Kissing Spine Syndrome)

Baastrup Disease (also called kissing spine syndrome) is a painful condition caused by close approximation of adjacent spinous processes with degeneration of the interspinous ligament and development of an adventitious bursa.

History:

Christian Ingerslev Baastrup, a Danish radiologist, first described this condition in 1933. He noted the characteristic radiographic finding of close approximation of spinous processes in patients with low back pain.

Epidemiology:

Associated Conditions:

Baastrup disease frequently coexists with other degenerative spine conditions. This reflects the common degenerative process affecting multiple structures.

• Facet joint arthropathy (very common)
• Disc degeneration and loss of height
• Spinal stenosis
• Spondylolisthesis

Spinous Process Anatomy

Structure:

The spinous processes are posterior projections from the vertebral arch. In the lumbar spine, they are thick, broad, and quadrilateral, projecting horizontally backward.

Normal Interspinous Space:

Interspinous Ligament

Composition:

The interspinous ligament is composed of three layers:

1. Ventral - thin, close to ligamentum flavum
2. Middle - main bulk, collagen fibers
3. Dorsal - merges with supraspinous ligament

Changes with Degeneration:

With aging and disc degeneration, the interspinous ligament undergoes changes. Loss of disc height brings spinous processes closer together, leading to increased contact pressure. The ligament may undergo myxoid degeneration, cyst formation, and eventually frank bursitis.

Interspinous Bursa

Key Concept:

The interspinous bursa is NOT congenital. It is an adventitious bursa that develops from repetitive friction between adjacent spinous processes. This distinguishes it from congenital bursae elsewhere in the body.

Development:

1. Disc degeneration leads to loss of height
2. Spinous processes approximate
3. Repetitive contact causes friction
4. Adventitious bursa develops to reduce friction
5. Bursa may become inflamed (bursitis)
6. May extend posteriorly or into epidural space (cyst formation)

Mechanism of Disease

Primary Driver:

Loss of disc height (disc degeneration) is the primary driver of Baastrup disease. As the disc loses height, the spinous processes approximate and begin to contact during extension.

Cascade of Changes:

Osseous Changes

Spinous Process Morphology:

Extension Mechanism

Why Extension Hurts:

During lumbar extension, the spinous processes are brought closer together. In Baastrup disease, this causes direct bone-on-bone contact and compression of the inflamed interspinous bursa, generating pain.

Protective Flexion:

Lumbar flexion separates the spinous processes, decompressing the interspinous space. This is why patients often prefer sitting (flexed posture) and have difficulty with prolonged standing or walking (extended posture).

Complications

Dorsal Epidural Cyst:

In some cases, the interspinous bursa may extend posteriorly into the spinal canal, forming a dorsal epidural cyst. This can cause spinal stenosis symptoms (neurogenic claudication) in addition to axial back pain.

Reported in less than 10% of cases but important to recognize on MRI as it may require surgical excision.

Imaging-Based Classification

No universally accepted classification exists, but the following imaging-based staging is useful:

Associated Findings

Often coexists with other degenerative pathology:

History

Pain Characteristics:

Important History Points:

• Duration (usually chronic, insidious onset)
• Occupation (jobs requiring prolonged standing)
• Activities that worsen symptoms (walking downhill, lying prone)
• Activities that improve symptoms (sitting, leaning forward)
• Previous spine problems or surgery

Physical Examination

Neurological Examination:

Typically normal in uncomplicated Baastrup disease. Abnormalities suggest associated stenosis, radiculopathy, or epidural cyst.

Red Flags

Differential Diagnosis of Extension-Aggravated Low Back Pain

Imaging Protocol

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MRI (Most Sensitive):

Diagnostic Injection

Interspinous Bursa Injection:

This is both diagnostic and therapeutic. Response to injection confirms Baastrup disease as the pain generator.

Laboratory Studies

Usually not required. If concern for infection or inflammatory arthropathy:

• ESR, CRP (normal in Baastrup)
• HLA-B27 (if concern for spondyloarthropathy)

The diagnostic and injection steps above rely on placing a needle into the adventitious interspinous bursa, but the topic's own account of the dorsal epidural cyst raises a question the routine work-up does not answer: how do you prove that a posterocentral epidural fluid collection genuinely arises from the interspinous bursa rather than being a facet (juxta-articular) synovial cyst or an unrelated lesion? Interspinous bursography is the technique that resolves this.

What it is:

Under fluoroscopy, a spinal needle is advanced into the interspinous space at the affected level and a small volume of iodinated contrast is injected directly into the adventitious bursa. Opacification confirms the needle lies within a true fluid-filled bursal cavity (rather than intact ligament) and outlines its extent.

Algorithm

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Treatment Algorithm

Conservative Management (First Line):

Surgical Management

Indications:

• Refractory to conservative and injection therapy
• Epidural cyst causing neurogenic symptoms
• Significant functional impairment

Disease Complications

Treatment Complications

Injection-Related:

• Infection (rare)
• Bleeding
• Temporary numbness
• Steroid side effects (if repeated)

Surgical Complications:

Long-Term Outcomes

Natural History:

Without treatment, Baastrup disease typically follows a chronic, waxing and waning course. Progressive degeneration may lead to increasing symptoms over time.

Prognosis:

With appropriate treatment (conservative, injection, or surgery), prognosis is generally good. Most patients achieve adequate pain control with conservative measures.

Spinous Process Excision Protocol

Treatment Outcomes by Modality

Prognostic Factors

Patient Satisfaction

• Conservative management: 50-60% satisfied
• Post-injection: 60-70% good/excellent
• Post-surgery: 70-85% good/excellent pain relief

Baastrup disease has no dedicated society guideline; it is managed under the broader frameworks for non-specific and degenerative low back pain. There is no condition-specific implant registry. The themes below apply worldwide.

Global Epidemiology

Society Guidance: How Baastrup Fits Low Back Pain Pathways

The common thread across societies: conservative first, confirm the level with an image-guided block, operate only on concordant refractory disease.

High- vs Limited-Resource Practice

• Well-resourced settings: MRI (sagittal T2/STIR) to confirm bursitis and exclude mimics, fluoroscopic/ultrasound-guided infiltration, and endoscopic or open spinous process surgery available.
• Limited-resource settings: Diagnosis often rests on lateral radiograph plus the clinical extension-pain pattern and a landmark-guided (non-image) infiltration as both test and treatment. Open partial spinous process excision is the realistic surgical option where endoscopy is unavailable.

Areas of Uncertainty / Controversy

• Symptomatic attribution: Because kissing spines are frequently incidental, the chief controversy is whether Baastrup disease is a true pain generator or a bystander. A positive diagnostic interspinous block is the only reliable arbiter.
• No accepted classification: The Grade 1-3 imaging staging used here is descriptive, not validated against outcomes.
• Surgical evidence is weak: All operative data are small case series or reports (Corr 2023, Lin 2021); no comparative trials exist, so surgery remains a last resort for image-confirmed, refractory, isolated disease.
• Interspinous spacers: Devices such as X-STOP have been withdrawn in several markets; evidence for spacers in isolated Baastrup disease (as opposed to stenosis) is minimal.

Baastrup disease ("kissing spine syndrome") is a degenerative condition caused by close approximation and impingement of adjacent spinous processes, most commonly at L4-L5. Pain is worse with extension (standing, walking downhill) and improved with flexion—the opposite pattern of disc herniation. An interspinous bursa develops from repetitive friction and appears as T2 hyperintensity on MRI between spinous processes. CT shows sclerosis and flattening of the articulating spinous process surfaces. Distinguish from facet arthropathy (paramedian tenderness) and discogenic pain (flexion-aggravated). Injection into the interspinous bursa is both diagnostic and therapeutic. Conservative management is first-line; refractory cases may require partial spinous process excision.

The surgical options above name interspinous process devices (X-STOP, Coflex, Wallis, DIAM) but do not explain why a spacer is a biomechanically logical idea for kissing spines, nor why it so often disappoints in this specific disease.

The rationale:

Baastrup pain is generated by direct spinous-on-spinous contact and compression of the inflamed bursa during extension. An interspinous spacer is placed between the two spinous processes to:

• Distract the interspinous space, physically separating the kissing surfaces so they no longer impinge.
• Limit segmental extension at that level, blocking the very movement that provokes pain while preserving flexion.

In principle this directly reverses the mechanism, which is why spacers were trialled and why they can simultaneously help coexisting dynamic (extension-dependent) stenosis.

1. Baastrup CI. On the spinous processes of the lumbar vertebrae and the soft tissues between them, and on pathological changes in that region. Acta Radiologica. 1933;14(1):52-55. (Historical primary source; predates PubMed indexing.)
2. Maes R, Morrison WB, Parker L, Schweitzer ME, Carrino JA. Lumbar interspinous bursitis (Baastrup disease) in a symptomatic population: prevalence on magnetic resonance imaging. Spine (Phila Pa 1976). 2008;33(7):E211-5. PMID: 18379391. doi:10.1097/BRS.0b013e318169614a
3. Filippiadis DK, Mazioti A, Argentos S, et al. Baastrup's disease (kissing spines syndrome): a pictorial review. Insights Imaging. 2015;6(1):123-128. PMID: 25582088. doi:10.1007/s13244-014-0376-7
4. Chen CKH, Yeh L, Resnick D, et al. Intraspinal posterior epidural cysts associated with Baastrup's disease: report of 10 patients. AJR Am J Roentgenol. 2004;182(1):191-194. PMID: 14684538. doi:10.2214/ajr.182.1.1820191
5. Filippiadis DK, Velonakis G, Malagari A, et al. Fluoroscopy-guided infiltration for pain reduction in patients with Baastrup's disease: clinical experience and results. Skeletal Radiol. 2015;44(9):1327-31. PMID: 25930945. doi:10.1007/s00256-015-2154-0
6. Lin WT, Xie FQ, Lin SH, et al. Full-endoscopic approach for chronic low back pain from Baastrup's disease: interspinous plasty. Orthop Surg. 2021;13(3):1102-1110. PMID: 33783125. doi:10.1111/os.12988
7. Corr F, Grimm D, Rothoerl RD. Partial spinous process decompression in Baastrup's disease: a case report and literature review. Cureus. 2023;15(1):e34070. PMID: 36843812. doi:10.7759/cureus.34070