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Sagittal Pelvic Orientation: A Comparison of Two Methods of Measurement.

In recent years, the effect of pelvic tilt upon total hip arthroplasty (THA) component positioning has become apparent. Pelvic tilt changes with posture from supine, sitting, and standing. (1-3) As the pelvic position changes in space, so does the acetabular anteversion. For each degree of posterior deviation of the anterior pelvic plane (APP), acetabular anteversion and inclination increases by 0.7[degrees] to 0.8[degrees] and 0.29[degrees], respectively. (4,5) This relationship is of primary importance because inaccurate acetabular component placement is currently the leading cause of revision THA in the USA. (6) The concept of the acetabular safe zone was described by Lewinnek to determine the optimal anteversion of the acetabular component and prevent dislocation. (7) In his paper, he referenced acetabular position against the pelvis according to the APP with 0[degrees] of pelvic tilt. The APP is defined as the plane created by the bilateral anterior superior iliac spines and the pubic symphysis (Fig. 1). The APP was chosen as a reference to pelvic tilt as it serves as an easily registered landmark intraoperatively since the pubic symphysis and anterior superior iliac spines are subcutaneous prominences. Today, the APP is still frequently used by arthroplasty surgeons to reference pelvic spatial orientation.

As further understanding of the interplay between spine and hip have developed, the common important feature is the spatial orientation of the pelvis. (8-11) Spine surgeons measure the pelvic tilt, hereby termed spino-pelvic tilt (SPT), as the angle subtended by the line connecting the center of the S1 endplate and the hip centers with the vertical (Fig. 1). The amount of pelvic tilt has been correlated to health-related quality of life (HRQoL) scores in spinal deformity literature, and spinal re-alignment aims to reduce SPT to less than 20[degrees]. (12) Changes in SPT induced by a spinal realignment surgery may change SPT by 15[degrees] to 20[degrees], resulting in changes to acetabular cup position and subsequent instability of a previously stable THA. (13)

Despite the critical importance of pelvic position to hip and spine surgeons, the measurement of pelvic tilt has remained different across the two specialties. The APP has been favored in THA due to the ease of bony landmark reference and registration for navigation. The pelvic version as defined by SPT assumes the center of rotation of the pelvis through the hip centers, whereas the APP is an unknown distance from the hip centers (Fig. 2). Several studies have called into question the accuracy and clinical relevance of APP, and current navigation systems require computed tomography (CT) of the pelvis to allow for adequate accuracy. (14-16) This is likely inadequate because CT scans are performed with the patient supine, and therefore the pelvis is not in its functional position. However, modern low-dose stereoradiographic imaging technology has made possible the three-dimensional assessment of full-body alignment in the standing position. Regardless of imaging technique, alterations in the anatomy of the iliac crest as a result of surgery (e.g., pediatric pelvic osteotomy) or trauma (e.g., pelvic ring trauma or ASIS avulsion) make these landmarks inaccurate in referencing the true anterior pelvic plane in some patients. Our study aimed to assess the relationship and the inter-rater and intra-rater reliability of measuring pelvic tilt according to the APP tilt (APPt) against spinopelvic tilt (SPT) in standing full-body alignment.


This IRB approved study utilized a single center retrospective database of patients undergoing full-body standing stereoradiographic assessment for spinal pathologies. Inclusion criteria included age 18 to 90 years with adequate quality imaging. Patients were excluded if they had any of the following: spinal instrumentation, total hip replacement, or history of pelvic bony surgery or pelvic trauma.

Image Acquisition

All patients underwent low dose radiation head to foot, biplanar stereoradiographic images (EOS imaging, SA, Paris, France). (17-19) The protocol included a weightbearing free-standing position of comfort with arms flexed at 45[degrees] to avoid superimposition with the spine. (20) The EOS system is a slot-scanning radiographic device consisting of two x-ray source-detector pairs, allowing simultaneous orthogonal image acquisition. (21)

Data Collection and Radiographic Analysis

Assessments of spinopelvic parameters were undertaken by two orthopaedic surgeons and two orthopaedic residents using validated software (sterEOS[R], EOS imaging SA, Paris, France). The hip centers were identified on anteroposterior and lateral projections, as was the sacral promontory. In the event of transitional anatomy (e.g., sacralized L5, lumbarized S1), the sacrum was identified after counting five lumbar vertebrae caudal to the last rib on the AP radiograph. The parameters measured included anterior pelvic plane tilt (APPt: the angle between the plane created by the bilateral anterior superior iliac spines to the pubic symphysis and the vertical plane), spinopelvic tilt (SPT: the angle between a line drawn from the hip centers to the midpoint of the sacral plate and the vertical), pelvic incidence (PI: the angle between a line drawn from the hip centers axis to midpoint of the sacral plate, and the perpendicular to the sacral plate), and sacral slope (SS: the angle between the sacral plate and the horizontal). Measurements were taken in reference to the patient plane, defined as a vertical plane passing through the hip centers. APPt was measured as positive if it tilted anteriorly and negative if tilted posteriorly.

Two-Dimensional Modelling of Changes in PT and APPt

To understand the relationship between PT and APPt, two-dimensional lateral capture images were used and rotated around the hip centers. Previously validated two-dimensional radiographic analysis software (Surgimap, Nemaris, New York, USA) (22) was used to perform the modeling. Two lines were created: one to resemble the anterior plane (from symphysis to ASIS) and another to resemble PT (from the bicoxofemoral axis to the center of the sacral plate). The image was then rotated around the axis at the hip centers in 5[degrees] increments up to 90[degrees] in clockwise and counter-clockwise directions. This was performed on all 100 patients, and the changes in SPT and APPt were plotted.

Statistical Analysis

Statistical analysis was performed with SPSS software. Pearson's correlation coefficient between APPt and SPT was assessed, in addition to intra- and inter-user correlation coefficients to assess intra- and inter-user reliability. Precision of measurement of APP and SPT by a single observer (intra-observer reliability) and between observers (interobserver reliability) was determined by calculating the 95% confidence interval (CI) between the repeated measurements and their average, which is the root mean square standard deviation. Intra-observer reliability was calculated for each of the operators in this study, and inter-observer reliability was calculated between all operators.


One thousand six hundred thirty-nine patients were identified from the database. After inclusion and exclusion criteria were applied, 1,197 patients remained. From this cohort, 100 patients were randomly selected for radiographic assessment. The mean age was 53.4 years (range: 18.1 to 81.7 years). The mean pelvic incidence was 50.22[degrees] [+ or -] 12.41[degrees], SPT 16.83[degrees] [+ or -] 10.84[degrees], sacral slope 33.99[degrees] [+ or -] 14.90[degrees], and APPt -2.94[degrees] [+ or -] 10.17[degrees]. A very strong correlation was found between APPt and SPT (R = -0.819, p < 0.001) and a strong correlation between pelvic incidence and SPT (R = 0.560, p < 0.001) and sacral slope (0.532, p < 0.001). A moderate correlation was found for APPt with pelvic incidence (R = -0.319, p [less than or equal to] 0.001) and sacral slope (R = -0.485, p < 0.001).

Values calculated for precision of measurement are presented in Tables 1 and 2. With respect to SPT, the 95% CI for the difference between repeated measurements and their mean by independent observers 1 through 4 (intra-observer precision) was calculated to be 1.34[degrees], 1.76[degrees], 1.61[degrees], and 1.05[degrees], respectively. For APP, the same 95% CI was determined to be 1.63[degrees], 2.65[degrees], 2.07[degrees], and 1.47[degrees]--a slightly numerically larger variability with a larger range than that for PT, thus suggesting less precision of measurement for APP.

Calculations for the 95% CI of SPT across all observers' measurements (inter-observer precision) was calculated to be 1.99[degrees]. For APP, this was calculated to be 2.71[degrees], or slightly less precise.

Intra-user reliability was stronger for SPT (R = 0.964) than for APP (R = 0.926), and inter-user reliability was stronger for SPT (R = 0.961) than for APP (0.921).

Two-dimensional modeling of changes in APP and SPT when rotating the lateral image around the hip centers revealed that for each 1[degrees] increase in SPT, there was a reciprocal 1[degrees] decrease in APPt. The mean difference between the SPT and APPt was 13.98[degrees] [+ or -] 7.04[degrees]--i.e., APPt = 13.98 - SPT ([R.sup.2] = 0.548), (Fig. 3).


Accurate acetabular component positioning is important for several reasons. Dislocation rates have been shown to decrease when the acetabulum is placed within the "safe zone" of 40[degrees] [+ or -] 10[degrees] inclination and 15[degrees] [+ or -] 10[degrees] anteversion in reference to the anterior pelvic plane. Excessive anteversion or inclination has important tribology changes with increased wear and osteolysis in metal-on-polyethylene bearings (7,23,24); increased metal ions, failure and pseudo-tumor formation in metal-on-metal bearings (25-28); and squeaking in ceramic bearings. (29) Despite the importance placed upon acetabular positioning, recent literature has suggested that accurate positioning is achieved in only 50% to 70% of acetabulae. (30,31) Computer navigation has increased the accuracy of acetabular placement; nonetheless, the gold standard for measurement of position has been the CT of the pelvis, a scan taken supine and therefore not in a functional position. Importantly, navigation uses the APP as the reference for pelvic tilt, and component position is determined relative to it.

Pelvic tilt-adjusted acetabular placement has been examined in recent years with the hope of achieving more accurate component placement and avoiding instability. (4,32-34) Several reports have called into question the accuracy of APP in describing pelvic spatial orientation, however. (1,14,35-38) Barbier and coworkers (35) demonstrated an ICC of 0.93 for measurement of acetabular component anteversion and inclination using standing EOS scans; however, the correlation between intraoperative navigation-measured anteversion and actual anteversion was only 0.36. For inclination, the correlation was only 0.38. Given the lack of measurement error on EOS, this was thought to be due to inaccuracy in registration of the APP intraoperatively. Other studies have also confirmed the accuracy of the measurement of acetabular orientation using EOS imaging. (39) Regardless of imaging technique, alterations in the anatomy of the iliac crest as a result of surgery (Fig. 4) or trauma make these landmarks inaccurate in referencing the pelvic tilt from the APP in some patients. Such abnormalities do not exist when using the sacral endplate as a reference, as when measuring SPT.

Errors in placement of the acetabular component come from errors in planning, errors in registration (navigated or non-navigated reference landmarks), or intra-operative error (error in placement or in the navigation system).

Measurement of the APP in the planning phase may contribute to error in acetabular positioning. Our results confirm that measurement of APP is slightly less reliable than measurement of SPT using sterEOS[R] software. This may partly be due to difficulty in identifying the ASIS on a lateral film due to overlying bowel gas; however, we believe that SPT would be a more accurate measure of pelvic spatial orientation and, therefore, a more accurate reference to anteversion because the axis of SPT is the hip center of rotation.

In an attempt to increase the accuracy of APP measurement, CT scans are used to better delineate bony landmarks. However, these are taken supine while the functional positions of the pelvis are in standing or sitting. Some studies have claimed that pelvic tilt does not change from lying to sitting; however, in the presence of degenerative disc disease or spinal deformity, patients will increase pelvic tilt to maintain upright posture. Therefore, measurement in the supine position may not represent that in the seated or standing position (Figs. 5 and 6). Philippot examined changes in APP, SPT, and "Pelvi-Lewinnek Angle" (SPT-APPt) and found that while APP and SPT increased with sitting, the Pelvi-Lewinnek angle remained 12[degrees]; however, the variability in this angle was not commented upon. (2) The constant Pelvi-Lewinnek angle is supported by our data revealing that changes in the rotation of the pelvis in the sagittal plane results in changes in APPt and SPT in a 1:1 ratio. Our data showed a similar mean difference in standing (SPT = 13.98[degrees] - APPt [+ or -] 7.04[degrees]). Although the mean was similar, the relationship between SPT and APPt (i.e., Pelvi-Lewinnek angle) is very variable.

Normative data suggests that asymptomatic individuals have a mean SPT of 11[degrees] to 15[degrees] ([+ or -] 6[degrees]). (9,40-44) Worsening health related quality of life (HRQoL) scores (ODI, SRS, PCS, and MCS) have been associated with higher SPT values. (45) Severe disability in spinal deformity (defined as ODI > 40) is associated with SPT greater than 22[degrees]. (45) We are unaware of any literature discussing the normative values of the APPt in asymptomatic individuals nor its relevance to HRQoL outcomes.

Intraoperative registration during computer navigated THA may be inaccurate due to soft tissue coverage of bony landmarks. The public symphysis typically has more soft tissue coverage than the ASIS, falsely increasing posterior pelvic tilt (increasing SPT) and causing inadvertent error in acetabular anteversion. (16,34,46) Anterior pelvic plane registration in the lateral decubitus position is limited by access to both ASISs or by pelvic supports obscuring the pubic symphysis. Percutaneous- and ultrasound-assisted registration of the APP may be performed to improve accuracy of registration. (47) The concern with percutaneous APP registration is with added incisions (including in the groin region for the pubic symphysis), which may increase the risk of infection.

The effect of acetabular position between sitting and standing is governed by the mobility in the lumbosacral joints. With flattening of lordosis comes increased SPT (posterior APP tilt). Similarly changes in spinal alignment can cause changes in acetabular position via a change in pelvic tilt, which is a compensatory mechanism for sagittal imbalance (Fig. 6). The critical interplay between spine and hip is displayed in spinal mobility, spinal deformity, and its effect on the pelvis. As such, common terminology should be used between hip and spine surgeons to further investigate this inter-relationship and better plan the effect of hip and spine procedures upon each other. The definition of pelvic tilt in spinal deformity is in fact the same as SPT; however, APP has been favored by hip surgeons due to ease of registration or referencing of the ASIS intraoperatively.

While CT scans of the pelvis have been used to increase the accuracy of APP measurement in computer navigated THA, these expose the patient to excessive radiation doses. Alternatively, pelvic tilt can be measured reliably and accurately by the measurement of SPT in modern full-body low dose stereoradiographs or with standard lateral radiography, both of which deliver only a fraction of the dose of a CT scan. Standing full-body scans in particular allow a better understanding of the patient's functional position, and appreciation of both spinal and hip pathology as a contribution to changes in pelvic tilt.

At this time, it remains unknown whether SPT or APPt is a more accurate reference for accurate acetabular placement. This study suggests that SPT may be a better measurement of pelvic tilt as it has increased accuracy of measurement, although the improvement in accuracy is small and the clinical significance of this is unknown. Using SPT as a common measurement may also help aid further understanding of the interrelationship between the hip and spine. Further research between spino-pelvic relationships with postural changes is needed to identify the effect of posture on pelvic mobility and acetabular position. Understanding this will help to optimize acetabular placement based on a patient specific morphology.

Study Limitations

The limitations of this study are in its basis upon precision of measurement without direct clinical correlation. Although it appears that SPT is more reliably and precisely measured than APP on full body stereoradiography, the clinical relevance of improved reliability has yet to be demonstrated. Similarly, it is not known which measurement is more relevant for determining acetabular orientation, although we speculate for the above reasons that SPT is a superior measurement. Future study of this subject must involve comparison of postoperative acetabular component version when THA has been planned or navigated using APP and SPT as independent references for pelvic orientation. Only then will we understand the way that each measurement truly influences component positioning. The degree of pelvic tilt present within the spinal deformity population may not resemble most patients requiring total hip replacement; however, it enabled us to measure a greater range of pelvic tilt measurements to compare APPt and SPT.


Spinopelvic tilt (SPT) measurement has a better intra- and inter-user reliability than anterior pelvic plane tilt (APPt) and enables measurement of pelvic tilt in all patients regardless of prior pelvic surgery or fracture. The relationship between APPt and SPT is variable between patients. Further investigation should be directed toward assessing the accuracy of each of these planes for referencing in total hip replacement.

Disclosure Statement

None of the authors have a financial or proprietary interest in the subject matter or materials discussed, including, but not limited to, employment, consultancies, stock ownership, honoraria, and paid expert testimony.


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Aaron J. Buckland, M.B.B.S., F.R.A.C.S., Edward M. DelSole, M.D., Stephen G. George, M.D., Shaleen Vira, M.D., Virginie Lafage, Ph.D., Thomas J. Errico, M.D., and Jonathan Vigdorchik, M.D.

Aaron J. Buckland, M.B.B.S., F.R.A.C.S., Edward M. DelSole, M.D., Stephen G. George, M.D., Shaleen Vira, M.D., Virginie Lafage, Ph.D., Thomas J. Errico, M.D., and Jonathan Vigdorchik, M.D., NYU Langone Medical Center, Department of Orthopaedic Surgery, New York, New York.

Correspondence: Edward M. DelSole, M.D., NYU Langone Medical Center, Department of Orthopaedic Surgery, 301 East 17th Street, New York, New York 10003;

Caption: Figure 1 The anterior pelvic plane tilt (APPt) is defined as the angle between the plane created by the bilateral anterior superior iliac spine to the pubic symphysis (A) and the coronal vertical plan (B). Spinopelvic tilt (SPT) is defined as the angle between a line drawn from the hip centers to the midpoint of the sacral plate (C) and the vertical (D).

Caption: Figure 2 Lateral radiograph displaying both anterior pelvic plane (APP) and pelvic tilt. "PI" denotes "pelvic incidence."

Caption: Scatter-plot showing the relationship between the anterior pelvic plan (APP) and spinopelvic tilt (SPT).

Caption: Figure 4 Anteroposterior radiograph of the pelvis in a patient following bilateral innominate osteotomy. Note the alteration in anatomy of the ilium and subsequently the relationships of the anterior superior iliac spine to the symphysis pubis. In this setting, the anterior pelvic plane may not be easily determined or reliable as a measure of spatial orientation.

Caption: Figure 5 Supine plain radiograph of the pelvis showing an acetabulum with acceptable anteversion in the supine position

Caption: Figure 6 Standing stereoradiograph of the same patient as Figure 5. Note the increased pelvic tilt to account for sagittal spinal deformity and resultant increase in acetabular anteversion.
Table 1 Intra-Observer Precision Measurements Reported as the Root Mean
Squared Standard Deviation of the Mean for Each Observer's Measurements
Which Represents the 95% CI for Each Observer's Measurements (The 95% CI
Tended to Widen with the Measurement of the APP Suggesting Lower
Precision for Each Observer)

                       Root Mean Square
                       Standard Deviation
Measurement  Observer  (RMSSD, Degrees)

SPT          1         1.34
             2         1.76
             3         1.61
             4         1.05
APP          1         1.63
             2         2.65
             3         2.07
             4         1.47

Table 2 Inter-Observer Precision Measurements Reported as the Root Mean
Squared Standard Deviation of the Mean for Each Observer's Measurements
Which Represents the 95% CI for Each Observer's Measurements (The 95% CI
Tended to Widen with the Measurement of the APP Suggesting Lower
Precision for Each Observer)

             Root Mean Square Standard Deviation
Measurement  (RMSSD, Degrees)

PT           1.99
APP          2.71
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Author:Buckland, Aaron J.; DelSole, Edward M.; George, Stephen G.; Vira, Shaleen; Lafage, Virginie; Errico,
Publication:Bulletin of the NYU Hospital for Joint Diseases
Date:Oct 1, 2017
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