Bone density of first and second segments of normal and dysmorphic sacra.

BACKGROUND: Iliosacral screw fixation is safe and effective but can be complicated by loss of fixation, particularly in patients with osteopenic bone. Sacral morphology dictates where iliosacral screws may be placed when stabilizing pelvic ring injuries. In dysmorphic sacra, the safe osseous corridor of the upper sacral segment (S1) is smaller and lacks a transsacral corridor, increasing the need for fixation in the second sacral segment (S2). Previous evidence suggests that S2 is less dense than S1. The aim of this cross-sectional study is to further evaluate bone mineral density (BMD) of the S1 and S2 iliosacral osseous pathways through morphology stratification into normal and dysmorphic sacra. MATERIALS AND METHODS: Pelvic computed tomography scans of 50 consecutive trauma patients, aged 18 to 50 years, from a level 1 trauma center were analyzed prospectively. Five radiographic features (upper sacral segment not recessed in the pelvis, mammillary bodies, acute alar slope, residual S1 disk, and misshapen sacral foramen) were used to identify dysmorphic characteristics, and sacra with four or five features were classified as dysmorphic. Hounsfield unit values were used to estimate the regional BMD of S1 and S2. Student's t-test was utilized to compare the mean values at each segment, with statistical significance being set at p < 0.05. No change in clinical management occurred as a result of inclusion in this study. RESULTS: A statistical difference in BMD was appreciated between S1 and S2 in both normal and dysmorphic sacra (p < 0.0001), with 28.4% lower density in S2 than S1. Further, S1 in dysmorphic sacra tended to be 4% less dense than S1 in normal sacra (p = 0.047). No difference in density was appreciated at S2 based on morphology. CONCLUSIONS: Our results would indicate that, based on BMD alone, fixation should be maximized in S1 prior to fixation in S2. In cases where S2 fixation is required, we recommend that transsacral fixation should be strongly considered if possible to bypass the S2 body and achieve fixation in the cortical bone of the ilium and sacrum. LEVEL OF EVIDENCE: Level III.

Pelvic incidence: a fixed value or can you change it?

BACKGROUND CONTEXT: There has been renewed interest in the pelvic vertebrae by spinal surgeons recently. Those involved in working with patients with adult spinal deformity focus on the position of the fused spine as it relates to the pelvis, and determine success or failure by specific numbers for given pelvic parameters. The pelvic parameters that are commonly measured for these patients are pelvic tilt, sacral slope, and pelvic incidence (PI). Out of the three, PI has always been considered to be the fixed measurement, whereas pelvic tilt and sacral slope have the capacity to change in relation to external forces. The assumption that the PI does not change has not been proven in a healthy, asymptomatic population. PURPOSE: This study aimed to investigate the differences in PI between three pelvic positions used in common functional activities: resting baseline pelvic posture, maximal anterior pelvic rotation, and maximal posterior pelvic rotation. STUDY DESIGN/SETTING: This was a randomized, prospective study of 50 healthy, asymptomatic, individuals who were recruited from the vicinity of our institution. PATIENT SAMPLE: Fifty patients (16 men with a mean age of 26.5±12.1 years; 34 women with a mean age of 27.2±10.8 years) were recruited for this study. Initial screening occurred by telephone. The inclusion criteria consisted of participants being between 18 and 79 years of age, no previous history of spine, pelvic, or lower extremity pain which had lasted longer than 48 hours, or history of any disorder in the spine, pelvis, or lower extremity that had required medical care. Female patients could not be pregnant at the time of participation. OUTCOME MEASURES: Changes in PI were assessed by examining the differences between the values of the PI with each change in pelvic position: resting to maximal anterior pelvic rotation and resting to maximal posterior pelvic rotation. Inter-rater reliability was assessed using Cronbach's alpha. METHODS: This study was funded by a Small Exploratory Grant from the Scoliosis Research Society. All subjects had an initial posterior-anterior and lateral radiograph taken in their resting pelvic position. If no spinal deformity was noted, each subject was instructed to maximally rotate their pelvis anteriorly and an immediate lateral radiograph was taken. The subject was then instructed to maximally rotate their pelvis posteriorly and an immediate lateral radiograph was again taken. Radiographic measurements of PI were independently measured by a board-certified, fellowship trained orthopedic spine surgeon and a board-certified musculoskeletal radiologist after defining and agreeing to the specific manner of measurement. RESULTS: Pelvic incidence values changed in 44 of 50 subjects (88%) when they maximally anteriorly rotated their pelvis from the resting pelvic position. The mean change was 2.9°, with 23 of 50 subjects (46%) changing ≥3°. Pelvic incidence values changed in 40 of 50 subjects (80%) when they maximally posteriorly rotated their pelvis from the resting position. The mean change was 2.82° with 27 of 50 subjects (54%) changing by ≥3°. CONCLUSIONS: This study demonstrated that for a high percentage of the healthy subjects who participated, the PI changed when the subjects varied their pelvic position. This questions the assumption that PI is a fixed parameter and suggests a potential functional motion at the sacroiliac joint. It also supports the idea that intentionally changing one's posture could lead to a change in PI, an idea that could have ramifications in surgical cases.