Pelvic parameters are a “very hot topic” in spine, according to Howard Place, (St Louis, USA), lead author of a “Best Paper”-winning study on pelvic incidence, presented at the Annual Meeting of the North American Spine Society (NASS, 26-29 October; Boston, USA). Classification systems for conditions such as adult spinal deformity and spondylolisthesis increasingly use pelvic parameters as part of their calculations. Place and colleagues sought to investigate the reliability of accepting pelvic incidence as a “fixed” value by assessing its variability among healthy volunteers with no history of spinal abnormality. The team observed pelvic incidence changes in more than 80% of their subjects, bringing into question the basic utility of the value.
Pelvic incidence is calculated by combining the angles of a patient’s sacral slope and pelvic tilt, where these components are variable. “The angle of incidence was constructed to enable reproducible analysis of the anatomical characteristics of the pelvis in the sagittal plane,” Jean-Charles Le Huec et al state in a September 2011 European Spine Journal article. “Since the value of incidence is fixed for any given patient, the sum of pelvic tilt and sacral slope is a constant value: when one increases, the other necessarily decreases,” they explain.
The value is used widely among classification systems, including the Scoliosis Research Society-Schwab adult spinal deformity and Spine Deformity Study Group spondylolisthesis classifications, and informs decisions regarding implant placement and treatment alternatives. Place and colleagues’ results, however, challenge this conventional wisdom, “Sacral slope and pelvic tilt seem to be the variables to which we all pay attention…but is pelvic incidence really fixed?” Place asked the NASS audience.
Posterior-to-anterior and lateral scoliosis radiographs were taken of 50 healthy adult volunteers in a resting, standing posture, as well as maximal anterior and maximal posterior pelvic rotation totalling 150 total radiographs. Pelvic parameters were assessed by a musculoskeletal radiologist and a fellowship-trained orthopaedic spinal surgeon. The radiographs were reassessed by the surgeon to calculate intraobserver reliability.
Using Chronbach’s alpha to measure interobserver reliability, the team found “excellent” consistency (0.971). Similarly, intraclass correlation coefficients showed intraobserver reliability of an excellent >0.94 for all measurements.
From resting baseline to maximal anterior pelvic rotation, the researchers found that 88% of volunteers experienced a changed pelvic incidence, with a mean of 2.86 degrees. Almost half of subjects (46%) experienced a change of up to three degrees, with 22% changing by greater than five degrees, positively or negatively. “Forty-four out of fifty people changed with anterior pelvic rotation,” Place stated.
Similarly, change in pelvic incidence was observed in the vast majority of cases when measured from resting baseline to maximal posterior pelvic rotation. Eighty per cent of volunteers experienced a change in pelvic incidence, averaging 2.76 degrees. Over half (54%) of these individuals experienced a change greater than three degrees, while over a quarter (26%) changed by more than five degrees. “This is not a huge change,” Place stated. “But it is a change, and certainly makes us think about whether the pelvic vertebrate is a fixed value.”
“Though 80% or more of normal subjects changed their numeric value, the changes for some were as small as one or two degrees,” Place told Spinal News International. “The significant finding though, is that almost 50% (depending on the direction of pelvic motion) changed greater than or equal to three degrees and almost 25% (again depending on direction) changed greater than or equal to five degrees. Depending on your tolerance for measurement error—which we believe to be less than two degrees for this dataset—these are significant changes which cannot be dismissed as measurement error.”
“There was no real consistency in the direction for which they changed” according to either anterior or posterior rotational calculations, Place stated. Whilst the vast majority of individuals (>80%) experienced pelvic incidence changes, it was not possible to measure patterns in these changes amongst the group.
“Pelvic parameters are considered key for so many things we are doing in adult spinal deformity right now,” Place said. “We talk about the ‘pelvic vertebrate’ and the pelvic morphology [to] really define how people are. We assume, again, that this is a stable, fixed platform…that change is what happens above the pelvis, and not what happens [at the pelvis]. But what if the base changes?” If pelvic incidence can change easily and frequently, Place asked the audience, “is this really a good value for us to be measuring?”
“This has implications for patient classification schemes,” he told Spinal News International. “It means that a standardised, standing pelvic position should be established for patients, since the same patient on the same day can change their pelvic incidence and thus be reclassified.”
Concluding, Place said, “Our data really suggests that a normal person can change the ‘pelvic vertebrate’ not just in its relationship in space, but also its own shape. The implications of a mobile pelvic incidence could have consequences for implant placement, classification schemes, as well as for some treatment alternatives, he explained.
“This research suggests that maybe our long-held belief that the pelvic incidence is a fixed value may not be true. It suggests that we should recognise that the pelvis does adapt to posture and the stresses on our spine,” Place told Spinal News International. “It also may help open the eyes of some surgeons as to the importance of the sacroiliac joint in spinal disorders.”
The next step for this area of research is independent validation, Place told Spinal News International. “This information should be repeated by other investigators,” he said. “In addition, this information needs to be applied to patients to determine if they exhibit the same change in pelvic incidence that healthy individuals do.”
“As a group,” Place affirmed, “we should stop thinking about pelvic incidence as a fixed value, [but] as something that can be changed.”