Baastrup's Disease: Causes, Contributing Factors, and Pathophysiology

What is the cause of Baastrup's disease?

Baastrup's disease, commonly known as "kissing spine syndrome," is primarily caused by chronic mechanical stress and degenerative changes that lead to the abnormal approximation and contact of adjacent vertebral spinous processes, exacerbated by factors such as age, specific postures, and repetitive spinal movements.

Understanding Baastrup's Disease

Baastrup's disease is a musculoskeletal condition characterized by the pathological approximation and contact between the spinous processes of adjacent vertebrae in the spine. This close contact leads to friction, irritation, and degenerative changes in the involved bony structures and surrounding soft tissues. While it can occur at any level of the spine, it is most frequently observed in the lumbar region (lower back), particularly at L4-L5 and L5-S1 levels, where the spine experiences significant load and movement.

The Primary Cause: Repetitive Mechanical Stress

The fundamental cause of Baastrup's disease is chronic and repetitive mechanical stress on the spinous processes. This stress often arises from movements and postures that promote excessive spinal extension, leading to the "kissing" or impingement of these bony projections. Over time, this repeated friction and compression initiate a cascade of degenerative changes.

Contributing Factors and Risk Elements

While mechanical stress is the direct trigger, several factors contribute to its development and progression:

• Age-Related Degeneration: As individuals age, the intervertebral discs naturally lose height and hydration, reducing the space between vertebrae. This disc degeneration is a significant predisposing factor, as it brings the spinous processes closer together, increasing the likelihood of contact. Ligamentous laxity or calcification can also play a role.
• Hyperlordosis: An exaggerated inward curve of the lower back (lumbar lordosis) significantly increases the posterior tilt of the vertebrae, thereby reducing the space between the spinous processes and promoting their contact. This can be due to:
  • Poor Posture: Sustained postures that accentuate lumbar lordosis, such as prolonged sitting with an anterior pelvic tilt or standing with an exaggerated sway back.
  • Muscle Imbalances: Weak core musculature (abdominal muscles, glutes) combined with tight hip flexors and erector spinae can contribute to hyperlordosis.
• Occupational and Activity-Related Stress: Professions or activities that involve repetitive spinal extension, prolonged standing, heavy lifting, or high-impact movements can accelerate the degenerative process. Examples include:
  • Athletes in sports requiring hyperextension (e.g., gymnastics, weightlifting, diving).
  • Occupations requiring repetitive bending backward or prolonged standing.
• Obesity: Excess body weight increases the load on the lumbar spine, potentially contributing to disc degeneration and altering spinal mechanics, which can exacerbate the contact between spinous processes.
• Previous Spinal Trauma or Surgery: Prior injuries or surgical interventions in the spine can alter spinal alignment and biomechanics, potentially predisposing individuals to Baastrup's disease by changing the normal spacing or movement patterns.
• Genetic Predisposition: While not a direct cause, individual anatomical variations in spinal curvature or the shape of spinous processes may subtly increase susceptibility in some individuals.

Anatomical Basis: The Spinous Processes and Interspinous Ligaments

To understand the cause, it's crucial to appreciate the anatomy. The spinous processes are the bony projections that extend posteriorly from the vertebral arch, forming the palpable "bumps" down the center of the back. Normally, these processes are separated by the interspinous ligaments, which are fibrous bands connecting adjacent spinous processes, providing stability and limiting excessive spinal flexion. In Baastrup's disease, the space normally occupied by these ligaments narrows, leading to the actual contact of the bony processes.

Pathophysiology: The Degenerative Cascade

The development of Baastrup's disease involves a progressive degenerative cascade:

1. Reduced Interspinous Space: This is often initiated by age-related disc height loss or sustained hyperlordosis.
2. Repetitive Contact and Friction: As the spinous processes approximate, repetitive movements, especially extension, cause them to rub against each other.
3. Inflammation and Microtrauma: This constant friction leads to local inflammation, irritation of the periosteum (the membrane covering the bone), and microtrauma to the bone and surrounding soft tissues.
4. Bone Remodeling and Osteophyte Formation: In response to the chronic stress and inflammation, the body attempts to adapt by remodeling the bone. This often results in the formation of osteophytes (bone spurs) on the surfaces of the spinous processes. These osteophytes can further exacerbate the impingement.
5. Bursitis and Cyst Formation: Over time, an adventitious bursa (a fluid-filled sac similar to those found in joints) may form between the contacting spinous processes as a protective mechanism against friction. This bursa can become inflamed (bursitis) and contribute significantly to pain. In some cases, intra-osseous cysts can also form within the spinous processes.
6. Ligamentous Changes: The interspinous ligaments, subjected to abnormal stress, may undergo hypertrophy (thickening), calcification, or degeneration, further contributing to the pathology.
7. Pain Generation: The pain associated with Baastrup's disease arises from various sources, including inflammation of the periosteum, irritation of the bursa, local nerve compression or irritation, and the degenerative changes within the bone itself.

Clinical Manifestations and Diagnosis

Patients typically experience localized pain and tenderness in the affected area, which often worsens with spinal extension (e.g., standing upright for prolonged periods, arching the back). Diagnosis is primarily made through imaging studies, particularly lateral view X-rays, which clearly show the close approximation or "kissing" of the spinous processes and the presence of osteophytes. MRI can provide more detailed information on soft tissue involvement, such as bursal formation and inflammation.

Management and Prevention

Understanding the causes of Baastrup's disease is crucial for its management and prevention. Treatment strategies often focus on alleviating symptoms and addressing the underlying mechanical factors. This includes:

• Conservative Management: Physical therapy to improve posture, strengthen core muscles, stretch tight hip flexors, and reduce lumbar lordosis.
• Pain Management: Anti-inflammatory medications, local injections (steroids, anesthetics) into the affected area or bursa.
• Lifestyle Modifications: Avoiding aggravating activities, maintaining a healthy weight, and ergonomic adjustments.
• Surgical Intervention: In severe, refractory cases, surgical removal of the impinging spinous processes or osteophytes (spinous process osteotomy or partial removal) may be considered to create more space.

Prevention largely revolves around maintaining good spinal mechanics, practicing proper posture, engaging in regular core-strengthening exercises, and avoiding activities that excessively stress the lumbar spine in extension.

Conclusion

Baastrup's disease is a condition rooted in the biomechanics of the spine, primarily driven by repetitive mechanical stress and age-related degenerative changes that lead to the abnormal contact of vertebral spinous processes. While aging and disc degeneration are significant predisposing factors, poor posture, specific occupational demands, and certain athletic activities can exacerbate the condition. Understanding these multifactorial causes is key to both effective treatment and preventive strategies, emphasizing the importance of proper spinal mechanics and long-term spinal health.