Baastrup disease (kissing spines) is a degenerative condition resulting from repetitive contact and friction between adjacent spinous processes. Neoarthrosis (new joint surface formation) and interspinous bursitis develop at spinous process tips. First described in 1933 by Christian Ingerslev Baastrup. CT shows reactive sclerosis, flattening, small osteophytes, and sometimes interspinous vacuum phenomenon (gas accumulation) at spinous process tips. MRI characteristically shows STIR/T2 hyperintense edema and/or fluid (bursitis) in the interspinous space. L4-5 is the most commonly affected level, followed by L3-4. Increased lumbar hyperlordosis, aging, and degenerative disc disease are predisposing factors. Usually detected as an incidental finding but may be a source of mechanical back pain in some patients — pain increasing with extension is typical. Diagnostic block and treatment with interspinous injection is possible.
Age Range
50-85
Peak Age
65
Gender
Equal
Prevalence
Common
The pathophysiology of Baastrup disease is based on biomechanical stress and degenerative changes. In the lumbar spine, intervertebral disc degeneration reduces disc height — this brings spinous processes closer together. Lumbar hyperlordosis (aging, obesity, anterior disc loss) increases this approximation. With repetitive extension movements, adjacent spinous process tips rub against each other — mechanotransduction activates osteoblasts and reactive bone sclerosis develops. Spinous process tips flatten and neoarthrosis surfaces form. Chronic friction leads to periosteal irritation and inflammatory response — interspinous bursitis develops. Bursa fluid appears as T2/STIR hyperintense signal on MRI: free water molecules produce bright signal with long T2 relaxation time. Sclerosis on CT reflects high X-ray absorption in bone tissue with increased mineral density — dense lamellar bone replaces trabecular bone and appears hyperdense. Vacuum phenomenon (interspinous gas) is nitrogen gas drawn in by negative pressure — detected as very low density (-800 to -1000 HU) on CT.
Contact between adjacent spinous processes — approximation of spinous process tips, sclerosis, flattening, and neoarticulation on sagittal CT. Interspinous distance is lost or markedly narrowed.
Bilateral symmetric reactive sclerosis is seen at adjacent spinous process tips — normally pointed spinous process tips are flattened with smooth sclerotic cortical surface. Neoarticulation surfaces appear as opposing flattened, bright sclerotic surfaces. Small osteophytes may develop at lateral margins of the interspinous space. Sagittal reformation is the most useful plane for diagnosis.
Report Sentence
Reactive sclerosis, flattening, and approximation at adjacent spinous process tips at L4-5 level, consistent with Baastrup disease.
Hyperintense fluid collection and/or bone marrow edema in the interspinous space on STIR/T2 fat-sat sequences. Bursa fluid appears as a well-defined, oval/fusiform, homogeneously hyperintense collection. Bone marrow edema (T1 hypointense, STIR hyperintense) at surrounding spinous process tips may accompany. Indicates active inflammatory phase and correlates with symptomatic Baastrup.
Report Sentence
Hyperintense fluid collection in L4-5 interspinous space and surrounding spinous process bone marrow edema on STIR, consistent with active interspinous bursitis (Baastrup).
In some cases, gas-density collection (vacuum phenomenon) may be seen in the interspinous space. Detected as very low density (-800 to -1000 HU). This finding indicates advanced degeneration of the neoarticulation. Mechanism similar to disc vacuum phenomenon — negative pressure created by extension movement draws in nitrogen gas.
Report Sentence
Vacuum phenomenon in L4-5 interspinous space supporting advanced degenerative Baastrup changes.
Low signal at spinous process tips on T1 (edema = water replacing fat) and/or high signal (fatty degeneration = chronic change) may be seen. In acute/active phase: T1 hypointense + STIR hyperintense (edema). In chronic phase: T1 hyperintense (fatty infiltration). These changes show a pattern similar to Modic changes in vertebral bodies.
Report Sentence
STIR hyperintense bone marrow edema at adjacent spinous process tips supporting active Baastrup pathology.
Baastrup disease may occur in isolation but is frequently accompanied by other degenerative changes: disc degeneration (T2 signal loss), facet arthrosis (hypertrophy, effusion), ligamentum flavum thickening, spinal stenosis. These accompanying findings indicate that the biomechanical load is part of the overall degenerative process.
Report Sentence
In addition to Baastrup changes, disc degeneration and facet arthrosis are present at the same level, with multilevel degenerative changes.
Criteria
Sclerosis and flattening present on CT. No active edema/bursitis on STIR. No back pain or unrelated.
Distinct Features
No treatment needed. Documentation in report sufficient. Degenerative aging finding.
Criteria
Interspinous edema/fluid present on STIR. Bone marrow edema accompanies. Back pain increasing with extension.
Distinct Features
Confirmed with diagnostic block (interspinous injection). Steroid injection effective for treatment.
Criteria
Prominent neoarticulation surfaces. Vacuum phenomenon present. Multiple level involvement. Advanced sclerosis.
Distinct Features
Surgery may be considered (interspinous distractor). Usually associated with advanced age and severe degenerative changes.
Distinguishing Feature
Facet arthropathy shows sclerosis, hypertrophy, and effusion at posterolateral joint surfaces — lateral location. Baastrup shows sclerosis at posterior midline spinous process tips. Both may coexist but injection tests are needed to differentiate pain source.
Distinguishing Feature
Spondylolisthesis shows anterior slippage of the vertebral body — translation on sagittal CT. No translation in Baastrup; pathology is limited to posterior elements. However, both may be different manifestations of segmental instability.
Distinguishing Feature
Synovial cyst originates from facet joint and extends into spinal canal — posterolateral location, cystic structure. Interspinous bursitis is confined between spinous processes at posterior midline, does not extend into spinal canal. Peripheral enhancement may be present in synovial cyst.
Urgency
routineManagement
conservativeBiopsy
Not NeededFollow-up
no-follow-upBaastrup disease is usually an incidental finding and mostly does not require treatment. Conservative treatment is first-line for symptomatic cases (mechanical back pain increasing with extension): NSAIDs, physical therapy (core strengthening, postural correction), activity modification. Diagnostic block (fluoroscopy or US-guided interspinous steroid injection) is used both as diagnostic test and treatment — pain reduction >50% with injection confirms diagnosis. Surgery (interspinous distractor placement or partial spinous process resection) is rarely considered for refractory cases. Baastrup changes should be described in reports but clinical significance is limited if no active edema/bursitis on STIR.
Baastrup disease is often incidental but may cause low back pain. Conservative treatment (NSAIDs, physical therapy) is first-line. Interspinous bursa steroid injection may be performed for refractory pain. Surgery (interspinous distractor or spinous process resection) is rarely needed.