What is Pelvic Incidence (PI)?
Pelvic Incidence (PI) is a morphological parameter that defines the orientation of the sacrum relative to the femoral heads. Geometrically, it is the angle between a line perpendicular to the sacral endplate and a line connecting the midpoint of the sacral endplate to the center of the bicoxofemoral axis. Unlike positional parameters like pelvic tilt and sacral slope, PI is considered a fixed, anatomical measurement inherent to an individual’s pelvic structure, largely independent of posture in standing or sitting positions. It acts as a fundamental determinant of an individual's spinopelvic sagittal alignment, directly influencing the angles of other compensatory spinal curves, such as lumbar lordosis (LL).
The Change of Pelvic Incidence During Growth
During childhood and adolescence, pelvic incidence changes with age as the body grows and adapts to bipedal locomotion. Multiple studies confirm a progressive increase in PI from birth until skeletal maturity is reached, typically around age 10 for stabilization.
- Neonatal Period: In newborns, the sacrum is relatively flat, and PI values are significantly lower, sometimes averaging below 30°.
- Early Childhood: As a child learns to walk and develops an erect posture, biomechanical stresses influence pelvic remodeling. The sacrum becomes more curved and positions itself further from the iliac crests, leading to an increase in PI. Studies show PI values rise to approximately 40-44° in early childhood.
- Adolescence and Puberty: The most notable increase in PI occurs during the pubertal growth spurt, as the pelvis and spine undergo rapid developmental changes. This is a critical period for establishing the final, mature spinopelvic balance. By late adolescence, PI values approach adult averages, stabilizing into what is considered its fixed, adult value.
Pelvic Incidence in the Aging Adult
For decades, it was widely accepted that PI remained constant once skeletal maturity was achieved. However, recent research on aging populations has introduced nuances to this understanding.
Historically, PI is considered a constant in adults, but studies on degenerative spine patients and older asymptomatic individuals reveal potential for small, age-related changes. The changes are not due to inherent pelvic remodeling but rather potential measurement artifacts or subtle adaptations due to degenerative spinal conditions. One study found a significant increase in PI among symptomatic patients over 45 years old compared to younger adults, though the trend was less significant in asymptomatic subjects. The debate continues, with some studies confirming a slight increase with age while others find little to no change in healthy aging spines. Factors like degenerative changes in the sacroiliac joint or changes in radiographic technique could also play a role in observed differences.
Why the Stability of Pelvic Incidence Matters
The stability of PI in adulthood is paramount for maintaining sagittal balance. It acts as an anatomical anchor around which the rest of the spine adjusts. When the lumbar lordosis (the inward curve of the lower back) decreases with age due to disc degeneration, the pelvis compensates by rotating backward (pelvic retroversion). This process, in which pelvic tilt (PT) increases, allows the body to keep its center of gravity aligned over the hips.
However, this compensatory mechanism is finite. The capacity for compensation is directly related to the individual's PI value; a higher PI allows for more potential compensatory pelvic retroversion. When the spine and pelvis can no longer compensate, sagittal imbalance occurs, leading to a forward-leaning posture, pain, and disability.
Comparison of Pelvic Incidence Across Different Life Stages
| Characteristic | Childhood & Adolescence | Adulthood | Advanced Age & Spinal Degeneration |
|---|---|---|---|
| PI Change | Increases significantly with age during development. | Generally considered a fixed anatomical constant. | Potential for minor increases or changes due to spinal pathology. |
| Pelvic Role | Undergoes active remodeling and development to accommodate bipedalism. | Acts as a stable base for the spine, influencing overall sagittal alignment. | Compensatory mechanism (pelvic tilt) is used to offset degenerative spinal changes. |
| Average PI Value | Starts low (neonatal: ~27°) and rises to adult levels during growth. | Stabilizes around 45-55° on average, but with wide individual variation (33-85°). | Can show higher mean values in some patient populations with degeneration. |
| Spinal Alignment | Lumbar lordosis (LL) increases to correlate with PI as child matures. | A mismatch between PI and LL can indicate pathology. | Compensation for decreasing LL through pelvic retroversion occurs until capacity is exhausted. |
Conclusion
While the conventional wisdom holds that pelvic incidence is a fixed anatomical angle once skeletal maturity is reached, the full story is more nuanced. PI increases dramatically during childhood and adolescence as the body develops bipedal posture. In adulthood, it functions as a stable, individual parameter. However, research in older and symptomatic populations suggests that subtle PI changes may occur in conjunction with degenerative spinal disease, or represent measurement variability. This highlights the importance of not viewing PI as an isolated value but as a dynamic component of the entire spinopelvic system that can be influenced by the ongoing process of aging and associated spinal changes.
Key Takeaways
- Significant Change During Growth: Pelvic incidence (PI) increases significantly from infancy to skeletal maturity, with the most notable growth spurt occurring during puberty.
- Adult Stability: Once skeletal maturity is reached, PI is generally considered a fixed anatomical parameter unique to each individual, independent of posture.
- Age-Related Influence: Recent studies on older, symptomatic populations indicate minor increases in PI may occur with age, potentially influenced by spinal degeneration rather than inherent pelvic changes.
- Compensatory Mechanism: An individual's PI determines their capacity to compensate for age-related spinal changes. Those with higher PI can tolerate more lumbar lordosis loss before developing sagittal imbalance.
- Imaging Factors: Accurate PI measurement is critical and can be influenced by imaging techniques (CT vs. X-ray) and patient positioning, especially pelvic rotation, underscoring the need for careful radiological interpretation.
- Surgical Relevance: The understanding of PI and its potential age-related influences is crucial for surgical planning in adult spinal deformity, where age-adjusted alignment goals are considered.
