Baastrup Disease (Kissing Spine Syndrome)

Baastrup disease, the kissing spine syndrome, is a painful degenerative condition caused by close approximation and impingement of adjacent spinous processes, with degeneration of the interspinous ligament between them and development of an adventitious bursa.

History. Christian Ingerslev Baastrup, a Danish radiologist, described it in 1933. He noted the close approximation of the spinous processes on the radiographs of patients with low back pain.

Who. Patients are typically over 60 years, and the finding becomes more common with age. There is no consistent gender predilection: Maes (2008) found no gender association and Filippiadis (2015) states there is none.

How common. In a symptomatic cohort of 539 patients aged 7-89 having lumbar MRI, Maes (2008) found it on 8.2% of scans, rising with age. No figure exists for an asymptomatic population, so the proportion of these findings that are painful is unknown. Only a minority of radiographic findings are held to be symptomatic, but without that comparison the fraction has never been measured.

Where. L4-L5 is the most commonly affected level (Filippiadis 2015). In most cases only one level is affected, though multilevel disease occurs.

What travels with it. Baastrup disease frequently coexists with other degenerative conditions of the spine, reflecting a common degenerative process that affects multiple structures.

In the Maes (2008) MRI series, Baastrup changes were significantly associated with older age, central canal stenosis, disc bulging and anterolisthesis, and not with disc degeneration, herniation, Modic changes, scoliosis or gender.

The spinous processes. Posterior projections from the vertebral arch. In the lumbar spine they are thick, broad and quadrilateral, projecting horizontally backward, and their shape changes down the column.

The interspinous space. Normally it is filled with fat and spanned by two ligaments: the interspinous ligament, which connects adjacent spinous processes, and the supraspinous ligament, which runs along their tips. The space holds only a potential bursa.

The interspinous ligament is composed of three layers:

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

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

The driver. Loss of disc height from disc degeneration is the primary driver. As the disc loses height with ageing, the spinous processes approximate and begin to contact during extension, and the sequence below follows.

The bone. Repeated contact remodels the opposing spinous processes.

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

Why flexion helps. Lumbar flexion separates the spinous processes and decompresses the interspinous space. This is why patients often prefer sitting, a flexed posture, and have difficulty with prolonged standing or walking, which hold the spine extended.

The dorsal epidural cyst. In some cases the interspinous bursa may extend forward into the spinal canal as a dorsal epidural cyst. It can cause spinal stenosis symptoms, neurogenic claudication, in addition to axial back pain, and it is important to recognise on MRI because it may require surgical excision.

How often is not known. Dorsal epidural cysts are described only in case reports and small series; Chen (2004) reported 10 patients, with no denominator. The "less than 10%" figure in the Filippiadis review refers to how often MRI shows interspinous bursitis in symptomatic patients, not to cysts.

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

The pain. Midline low back pain, aching and sometimes sharp, usually chronic and of insidious onset. Extension makes it worse: standing, walking, walking downhill, arching the back, lying prone. Flexion relieves it: sitting, bending or leaning forward. That is the opposite of the pattern in disc herniation.

What it does not do. It usually does not radiate, though there may be local radiation, and there are no neurological symptoms unless an epidural cyst has formed.

The rest of the history. Ask about occupation, particularly jobs requiring prolonged standing, and about previous spine problems or surgery.

Examination. Inspection may show an exaggerated lumbar lordosis or a flexed antalgic posture. The patient is tender over the spinous processes in the midline, which may feel prominent; paramedian tenderness suggests facet involvement. Extension is limited by pain and flexion is typically full, though associated degeneration may add stiffness.

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

The differential. Among the causes of extension-aggravated low back pain, the location of tenderness is what differentiates them. Interspinous ligament sprain also belongs on the list, and Baastrup disease may coexist with other pathology.

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Radiographs are first line, and the lateral view is essential. It shows the kissing spinous processes, sclerosis of their tips and loss of disc height at the affected levels. Its role is screening, showing the bony contact.

CT is best for bony detail: sclerosis, flattening and enlargement of the spinous processes, cystic change within them, and the anatomy needed for surgical planning.

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MRI is the most sensitive test for active disease, and the one that shows the bursa.

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The diagnostic injection. An injection into the interspinous bursa is both diagnostic and therapeutic, and a response confirms Baastrup disease as the pain generator. With the patient prone and under fluoroscopic or ultrasound guidance, a needle is placed into the interspinous space at the affected level or levels, and local anaesthetic and steroid are injected. The response is read in three ways:

• Good relief - Baastrup disease confirmed as the pain source
• Partial relief - there may be coexisting pathology
• No relief - consider another diagnosis

Blood tests are usually not required. If infection or inflammatory arthropathy is a concern, check the ESR and CRP, which are normal in Baastrup disease, and HLA-B27 if spondyloarthropathy is suspected.

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 it.

The technique. 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 that the needle lies within a true fluid-filled bursal cavity rather than intact ligament, and outlines its extent. It is usually combined with the therapeutic injection, so that once the bursa is opacified the local anaesthetic and corticosteroid are delivered into a confirmed target.

What it shows.

• The bursa as a discrete, contrast-filling cavity, the anatomical substrate of the Grade 3 bursitic stage
• In patients with a dorsal epidural cyst, contrast tracking from the bursa anteriorly into the posterocentral epidural space, documenting a direct connection. Chen and colleagues used bursography, alone or combined with CT, to establish this channel in their series of ten patients
• On post-bursography CT, the channel into the canal, which CT displays best when the plain fluoroscopic images are equivocal

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The principle. Conservative management is first line, and most patients respond to conservative measures and injection therapy. Surgery is reserved for refractory cases.

Algorithm

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Conservative care is the starting point for every patient.

Injection. Indicated when conservative management has failed after 6-12 weeks, for diagnostic confirmation, or as a therapeutic trial, using the technique described under Investigations. The only published series (Filippiadis 2015, n=55) reported near-complete relief, mean pain falling from 8.18 to 0.62 on the NVS at 1 year, but it was uncontrolled: treat it as evidence of safety and diagnostic utility rather than a measured effect size. In that series 22% needed a second injection within 7-10 days, and the duration of relief is variable.

When to operate. The choice of surgical technique depends on associated pathology and surgeon preference. When an epidural cyst is present, surgery is often required for decompression.

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

Spinous process excision. Either partial excision of just the articulating surfaces or complete resection of the spinous process. Through a midline posterior approach the interspinous bursa is removed and enough of the spinous process excised to eliminate contact, preserving the supraspinous and interspinous ligaments if possible. Morbidity is minimal; the 70-85% good-to-excellent figure for pain relief is a conventional estimate rather than a measured rate (see Outcomes).

Endoscopic interspinous plasty. A full-endoscopic approach removes the interspinous bursa tissue and resects part of the spinous process with minimal soft-tissue disruption. It is minimally invasive, recovery is faster, and it can be an outpatient procedure. It is an emerging technique with promising early results, to be considered for isolated Baastrup disease, though the evidence is a three-patient series (Lin 2021).

Interspinous spacers are the third option, discussed in the section that follows.

Of the disease. Beyond the dorsal epidural cyst, ongoing degeneration may affect other levels, and Baastrup disease may develop at them.

Of injection. Infection, which is rare, bleeding, temporary numbness, and steroid side effects if injections are repeated.

Of surgery. Each complication and how it is prevented or managed:

Natural history. Without treatment Baastrup disease typically follows a chronic, waxing and waning course, and progressive degeneration may lead to increasing symptoms over time. With appropriate treatment, whether conservative, injection or surgery, the prognosis is generally good, and most patients achieve adequate pain control with conservative measures.

After spinous process excision:

Read these as teaching figures. Every success-rate range in this section is a conventional teaching figure, not a measured rate from a defined cohort. No comparative or controlled outcome data exist for Baastrup disease: the surgical evidence is case reports and small series (Lin 2021 n=3, Corr 2023 n=1), and the one injection series (Filippiadis 2015 n=55) was uncontrolled.

Satisfaction is estimated the same conventional way: 50-60% satisfied after conservative management, 60-70% good or excellent after injection, and 70-85% good or excellent pain relief after surgery.

Prognostic factors. Isolated disease without stenosis, single-level involvement, a good response to the diagnostic injection and no epidural cyst are favourable. Multilevel involvement, associated spinal stenosis or facet arthropathy, workers' compensation claims and a poor response to injection are unfavourable.

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.

The rationale. Baastrup pain is generated by direct spinous-on-spinous contact and compression of the inflamed bursa during extension. An interspinous process device (IPD) is placed between the two spinous processes to prevent that contact and maintain the interspinous space:

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

In principle this directly reverses the mechanism, which is why spacers were trialled and why they can also help coexisting dynamic (extension-dependent) stenosis. They are minimally invasive and reversible. Examples are X-STOP, Coflex, Wallis and DIAM. Several early devices such as X-STOP have been withdrawn in multiple markets, X-STOP itself in many countries.

Why they often fail in Baastrup disease.

• Spinous process fracture is the dominant failure mode. The device transmits load through the spinous process, which in Baastrup disease is already remodelled, sclerotic and frequently osteoporotic in the elderly typical patient. A poor bony anchor fractures under the distraction load, and the fracture reproduces or worsens midline pain.
• Migration, loosening and heterotopic ossification, with loss of the initial distraction over time, among other device-related complications.
• The bursa is not removed. A spacer that separates the tips can still leave the symptomatic adventitious bursa in situ, so relief may be incomplete compared with excising the bursa and the articulating bone.

The evidence. Outcomes are variable, and the evidence for spacers in isolated Baastrup disease, as opposed to neurogenic claudication from stenosis, is minimal. The sclerotic Baastrup spinous process is a particularly unfavourable host for a load-bearing implant, and most surgeons therefore favour partial spinous process excision with bursectomy for refractory isolated disease.

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