The Relationship between Sacral Kyphosis and Pelvic Incidence.

Study Design: Retrospective cohort study.

Purpose: Evaluate the fixed anatomical parameter of sacral kyphosis (SK) and its relationship with pelvic incidence (PI).

Overview of Literature: Pelvic parameters determine pelvic and lumbar spinal position. Studies have defined normative values, and have evaluated the role of these parameters in clinical practice. It has been suggested that a ratio of sacral slope (SS)/PI <0.5 predisposes to spinal pathology. PI=SS+pelvic tilt (PT) and therefore for a given PI, patients with a higher SS due to an elevated SK will potentially predispose to an unfavourable SS/PI ratio.

Methods: CT measurements of SS and PI were made in 100 consecutive patients from our database. Imagings without clear landmarks were excluded. PI and SK were measured using standardised techniques. Pearson's correlation was used to assess association between PI and SK, in addition to the correlation between age and the pelvic parameters. Gender specific values for PI and SK were compared using an unpaired Student t -test.

Results: Ninety-five patients (52 females) with a mean age 51.3 years were available for analysis. A strong positive correlation between the PI and the SK was identified (Pearson's coefficient=0.636, R2 value=0.404). Neither PI nor SK had a statistically significant correlation with age ( p =0.721 and p =0.572, respectively). The mean values of both the PI and SK were statistically significantly lower in females when compared to males ( p =0.0461 and p =0.0031, respectively).

Conclusions: A strong correlation between PI and SK exists and is a reflection of different pelvic morphologies. SK partially determines SS and a relatively high SK compared to PI will result in less ability to change PT and a potentially unfavourable SS/PI ratio, which could theoretically contribute to clinical pathology.