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The Spine Journal 13 (2013) 1500–1509
Clinical Study
Evaluation of pelvic morphology in the sagittal plane
Toma�z Vrtovec, PhDa,*, Michiel M.A. Janssen, MD, PhDb, Bo�stjan Likar, PhDa,Ren�e M. Castelein, MD, PhDb, Max A. Viergever, PhDc, Franjo Pernu�s, PhDa
aUniversity of Ljubljana, Faculty of Electrical Engineering, Laboratory of Imaging Technologies, Tr�za�ska cesta 25, SI-1000 Ljubljana, SloveniabUniversity Medical Center Utrecht, Department of Orthopaedics, Heidelberglaan 100, 3584 CX, Utrecht, The Netherlands
cUniversity Medical Center Utrecht, Image Sciences Institute, Heidelberglaan 100, 3584 CX, Utrecht, The Netherlands
Received 26 September 2012; revised 26 March 2013; accepted 15 June 2013
Abstract BACKGROUND CONTEXT: It is generally a
FDA device/drug
Author disclosure
close. BL: Nothing to
Medtronic (A); Resea
directly to institution/
directly to institution/
1529-9430/$ - see fro
http://dx.doi.org/10.10
ccepted that for normal subjects the angle of pelvicincidence (PI) increases during childhood and then remains unchanged throughout adolescence andadulthood. However, recent findings show that PI increases linearly throughout the lifespan due tomorphological changes of the pelvis.PURPOSE: A retrospective study aiming to determine the extent of morphological changes of thepelvis related to the age of the subjects.STUDY DESIGN: Pelvic morphology was evaluated in a normal adult population by measuringthe anatomical parameters of sagittal pelvic alignment.PATIENT SAMPLE: The final study cohort consisted of 330 subjects (mean age, 45.3 years; stan-dard deviation, 18.1 years; range, 18–87 years; 164 male and 166 female subjects).OUTCOME MEASURES: Physiologic measures, obtained as measurements of PI, sacral endplate width (S1W), and pelvic thickness (PTH).METHODS: Parameters of PI, S1W, and PTH were evaluated from computed tomography imagesof the subjects. The measured PTH was normalized according to S1W and age of the subjects, al-lowing the comparison among anatomies of different sizes. The normalized components of PTH inanteroposterior and cephalocaudal directions were computed to determine the configuration and ex-tent of changes in pelvic morphology related to subject age.RESULTS: Statistically significant correlation with both age and PI was obtained for all normal-ized parameters (except for the anteroposterior component of PTH for male subjects), and no sta-tistically significant differences were observed between the sexes. With increasing PI that occursdue to the aging process, a decrease of PTH can be observed that is manifested not only as an in-crease of the distance between the sacrum and the hip axis in the anterior direction but considerablymore as a decrease of the distance between the sacrum and the hip axis in the cephalic direction. Byconsidering these morphological changes in the pelvis simultaneously, the hip axis can move onlywithin a narrow area.CONCLUSIONS: The changes in pelvic morphology due to the aging process occur in the ante-rior direction, which may be due to the remodeling process affecting the coxal bone that results inan anterior drift of the acetabulum relative to the sacrum. More importantly, the changes are con-siderably more evident in the cephalic direction, which may be the result of the weight-bearingloads and consequent wear of acetabular cartilage. � 2013 Elsevier Inc. All rights reserved.
Keywords: Pelvic morphology; Sagittal alignment; Pelvic incidence; Pelvic thickness
status: Not applicable.
s: TV: Nothing to disclose. MMAJ: Nothing to dis-
disclose. RMC: Speaking/Teaching Arrangements:
rch Support (Investigator Salary): Synthes (D, Paid
employer). MAV: Grant: Philips Healthcare (E, Paid
employer). FP: Nothing to disclose.
The disclosure key can be found on the Table of Contents and at www.
TheSpineJournalOnline.com.
* Corresponding author. University of Ljubljana, Faculty of Electrical
Engineering, Laboratory of Imaging Technologies, Tr�za�ska cesta 25, SI-
1000 Ljubljana, Slovenia. Tel.: þ386 1-4768-327; fax: þ386 1-4768-279.
E-mail address: [email protected] (T. Vrtovec)
nt matter � 2013 Elsevier Inc. All rights reserved.
16/j.spinee.2013.06.034
1501T. Vrtovec et al. / The Spine Journal 13 (2013) 1500–1509
Introduction
Pelvic morphology has a significant role in the spinopel-vic relationship, as it is considered to influence the spinalbalance and postural equilibrium, and has been recognizedas an essential component in the regulation of sagittal planealignment [1–6]. In contrast to positional parameters, suchas the sacral slope or pelvic tilt, pelvic morphology is bestevaluated by anatomical parameters of pelvic alignment inthe sagittal plane, which are constant and unchanged foreach individual subject and describe the position of the sa-crum in relation to the pelvis [7,8].
Since its introduction by Duval-Beaup�ere et al. [9], the an-gle of pelvic incidence (PI) has gained large attention and ac-ceptance as one of the key parameters in the complexframework of sagittal spinal alignment and related defor-mities [10–13]. By being an anatomical parameter, PI de-scribes structural characteristics of the sacrum and pelvisthat are not modified by posture [1,14] and can be thereforeobserved and compared among subjects in either standing, sit-ting, or supine position. For normal subjects, it is generally ac-cepted that there is no difference in PI between male andfemale subjects, that PI increases during childhood and thenremains unchanged throughout adolescence and adulthood[2,15], and that PI is not significantly influenced by normaldegenerative changes of the hips, sacrum, and sacroiliac joints[16]. However, Mendoza-Lattes et al. [17] recently suggestedthat PI continues to increase linearly even after skeletal matu-rity and throughout the lifespan as a result of the increasingdistance between the sacrum and the femoral heads, suggest-ing amorphological change of the pelvis. If the femoral headsare in a relatively more anterior position, then PI is conse-quently going to present larger angular values.
The purpose of this study was to observe whether thereare morphological changes of the pelvis related to the ageof the subjects as described by Mendoza-Lattes et al.[17]. By measuring the anatomical parameters of sagittalpelvic alignment in a normal adult population, the configu-ration of pelvic morphology and the extent of changes thatoccur due to the aging process are evaluated.
Fig. 1. Pelvic parameters in the sagittal plane are used to evaluate pelvic
morphology.
Materials and methods
Subjects
The initial cohort consisted of 430 subjects who receiveda computed tomography (CT) scan of the pelvis or abdo-men in the emergency room of the (University MedicalCenter Utrecht, the Netherlands) during a 5-year period(2005–2010) for reasons such as trauma or acute abdominalpathology. Computed tomography scans (Brilliance 16 and64 scanners, Philips Healthcare, The Netherlands) were ax-ially reconstructed three-dimensional (3D) images (pixelsize, 0.4–1.0 mm; slice thickness, 3.0–4.0 mm), showingthe femoral heads, the whole pelvis, and at least the lowerlumbar region of the spine. Subject height, weight, or other
body parameters were not recorded for scan acquisition,and the original DICOM files were further anonymizedby removing all confidential information and retaining onlysubject sex and age. The following exclusion criteria wereapplied to the initial cohort: age!18 years (skeletal matu-rity not reached) and ageO90 years (small population ratio,inadequate image quality due to bone aging); prior hip orspine surgery; any clinical or radiological history or evi-dence for pathology or trauma of the spine, pelvis, or fe-mur; evidence for anatomical anomalies (eg, abnormalnumber of lumbar vertebrae); syndromes associated withdisorder of growth; any form of psychomotor retardation;poor quality of the scan (eg, pelvis not fully included, arti-facts); pregnancy. As a result, 100 subjects were excludedfrom the initial cohort and the remaining 330 subjects rep-resented the final study cohort.
Measurement of pelvic parameters
To measure pelvic parameters that allow the evaluationof pelvic morphology, the following anatomical referenceswere determined in the CT scan of each subject: the centerof the left femoral head, the center of the right femoralhead, the center of the sacral end plate, and the inclinationof the sacral end plate (Fig. 1). Multiplanar 3D image ref-ormation was then performed to obtain the superposition of
1502 T. Vrtovec et al. / The Spine Journal 13 (2013) 1500–1509
the femoral heads in the sagittal view, so that all anatomicalstructures were completely in line with the hip axis. In suchperfect sagittal view, the following anatomical pelvic pa-rameters were measured:
� Pelvic incidence: The angle between the line orthog-onal to the sacral end plate inclination and the lineconnecting the center of the sacral end plate withthe hip axis [9].
� Sacral end plate width (S1W): The length of the sa-cral end plate in the direction of its inclination.
� Pelvic thickness (PTH): The distance between thecenter of the sacral end plate and the hip axis [9].
By considering the measured PI and PTH within an or-thogonal triangle, the anteroposterior (PTHA-P) and cepha-locaudal (PTHC-C) components of PTH were in additioncalculated as (Fig. 1):
PTHA�P5PTH� sinðPIÞ;PTHC�C5PTH� cosðPIÞ: ð1Þ
The measured pelvic parameters are constant for the ob-served subject, do not change with subject position and/ororientation, and can be therefore measured from CT imagesacquired in patient supine position. To determine the exactlocation of the aforementioned anatomical references in3D images and therefore measure the previously describedpelvic parameters in CT scans, we used a computerizedmethod based on image processing techniques that was in-troduced and validated in a recently published study [18].
Evaluation of pelvic morphology
Although the measured pelvic parameters are constantfor each individual subject, it is not appropriate to directlycompare distance parameters (ie, S1W and PTH) amongsubjects because they depend on individual body size. Asa result, for two subjects with equal anatomy shape but dif-ferent anatomy size (ie, equal PI but different S1W andPTH), distance parameters may be different and thereforehave to be normalized before they are compared. As weare interested in changes of PTH and its components, thenormalization is performed according to S1W, for whichwe assume that it reflects anatomy size. The normalizedpelvic thickness (nPTH) and its normalized anteroposterior(nPTHA-P) and cephalocaudal (nPTHC-C) components aretherefore defined for each subject as:
nPTHð%Þ5PTH�mS1WðageÞS1W
� 100%
aPTH;
nPTHA�Pð%Þ5PTHA�P �mS1WðageÞS1W � 100%
aPTHA�P;
nPTHC�Cð%Þ5PTHC�C �mS1WðageÞS1W
� 100%
aPTHC�C;
ð2Þ
where mS1W(age) is the modeled expected average S1Wfor the observed age of the subject and aPTH, aPTHA-P,and aPTHC-C are the average PTH, average anteroposteriorPTH component, and average cephalocaudal PTH compo-nent across population, respectively. The effects of suchnormalization are twofold:
� After dividing by aPTH, aPTHA-P, or aPTHC-C andmultiplying by 100%, the resulting value representsthe percentage of variation of the observed parameteraround its average in the studied population.
� After dividing by S1W and multiplying bymS1W(age), the influence of different anatomy sizeis removed. The average S1W is expected to increasewith subject age because the sacral end plate adapts tothe weight-bearing loads. Normalization of PTH istherefore not performed according to the averageS1W but the modeled expected average S1W forthe observed age of the subject (ie, mS1W(age)).
Statistical analysis
The obtained measurements were ordered according toage and sex of the subjects and analyzed in terms of meanand standard deviation (SD). Regression lines between anytwo observed variables were determined by the random sam-ple consensus method (assumed ratio of outliers, 25%; num-ber of iterations, 1,000). Pearson correlation analysis wasperformed to search for statistically significant correlationsbetween a selected measurement and age or PI. The inde-pendent samples t test was used to evaluate statistical differ-ences between the sexes (level of significance a50.05).
Results
Subjects
The final study cohort of 330 subjects (mean age, 45.3years; SD, 18.1 years; range, 18–87 years) was divided ac-cording to sex into the group of 164 male subjects (meanage, 44.3 years; SD, 18.0 years; range, 18–87 years) andthe group of 166 female subjects (mean age, 46.2 years;SD, 18.1 years; range, 18–86 years). No statistically signif-icant differences in age were found between the sexes(pO.05).
Measurement of pelvic parameters
Pelvic parameters were measured by a computerizedmethod that was introduced and validated in a recently pub-lished study [18] and proved to be highly consistent withmanual measurements (SD of up to 1.8 mm for distancesand 3.5� for angles). Fig. 2 shows examples of measure-ments, whereas Fig. 3 and Table 1 show the obtained mea-surements for PI, S1W, and PTH. The correlation of eachparameter with age was relatively low but statistically
Fig. 2. Measurement of pelvic parameters in three dimensions is performed by applying a computerized method to computed tomography images. The re-
sults for (Left) a 48-year-old male and (Right) a 31-year-old female subject are shown in the perfect sagittal view for the maximal intensity projection (top)
and midsagittal cross-section (bottom).
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significant (p!.05). The distribution of S1W measurementsshows a distinctive difference between male and female sub-jects, which was expected because of anatomy size, and alsoa distinctive increasing tendency of the average S1W withage (Fig. 3, Middle), which was expected because of the ad-aptation of the sacral end plate to the weight-bearing loads.Because a statistically significant difference between thesexes was found for S1W (p!.0001), the expected averageS1W for the observed age of the subject (ie, mS1W(age))was modeled separately for male and female subjects. Theobtained linear regression model using random sample con-sensus was mS1W(age)50.08�ageþ33.25 for male sub-jects and mS1W(age)50.11�ageþ28.19 for femalesubjects. Themodeled expected average S1W for the two ex-amples shown in Fig. 2 were therefore mS1W(48)5
0.08�48þ33.25537.1 mm for the 48-year-old male subject(Fig. 2, Left) and mS1W(31)50.11�31þ28.19531.6 mmfor the 31-year-old female subject (Fig. 2, Right). On thecontrary, no significant differences were found betweenthe sexes for PTH and its components PTHA-P and PTHC-C
(pO.05); therefore, the average values aPTH5104.9, aP-THA-P575.9, and aPTHC-C570.1 mm were computed to-gether for male and female subjects.
Evaluation of pelvic morphology
The measured PTH and its components PTHA-P andPTHC-C were normalized according to their average values(ie, aPTH, aPTHA-P, and aPTHC-C, respectively) and ac-cording to the modeled expected average S1W for the
Fig. 3. (Top) PI, (Middle) S1W, and (Bottom) PTH in relation to age for
male and female subjects with superimposed regression lines. PI, pelvic in-
cidence; S1W, sacral end plate width; PTH, pelvic thickness.
Table 1
Pelvic incidence, S1W, and PTH, as measured for all subjects (N5330)
and separately for male (N5164) and female (N5166) subjects
Pelvic parameter Mean SD
Vs. age*
pyr p
PI ( �) 47.6 10.2 þ0.25 !.0001 O.05
Male 47.3 9.5 þ0.16 !.05
Female 47.9 10.8 þ0.32 !.0001
S1W (mm) 35.2 4.4 þ0.38 !.0001 !.0001
Male 37.0 4.1 þ0.37 !.0001
Female 33.4 4.0 þ0.51 !.0001
PTH (mm) 104.9 8.6 �0.26 !.0001 O.05
Male 104.5 8.3 �0.27 !.05
Female 105.4 8.9 �0.25 !.05
PI, pelvic incidence; S1W, sacral end plate width; PTH, pelvic thick-
ness; SD, standard deviation.
* Correlation coefficient r and significance level p in relation to age.y Significance level p between male and female subjects.
1504 T. Vrtovec et al. / The Spine Journal 13 (2013) 1500–1509
observed age of the subject (ie, mS1W(age)). Fig. 4 andTable 2 show the obtained results in relation to the age ofthe subjects. Statistically significant correlation with agewas obtained for all parameters, except for nPTHA-P formale subjects. Fig. 5 and Table 2 show the obtained resultsin relation to PI of the subjects. Statistically significant cor-relation with PI was obtained for all parameters. By com-paring the resulting values between the sexes, nostatistically significant differences were observed for allthree parameters (pO.05).
Discussion
For normal subjects, it is generally accepted that PI in-creases during the first months of life, becomes stable ataround 10 years of age, and then remains unchangedthroughout adolescence and adulthood. However, it maybe altered by pathologic processes that modify the shapeof the sacrum or the position of the acetabulae within thepelvis [15], such as spondylolisthesis or spondylolisis[2,19–23], scoliosis [1,24–26], and other spinopelvic pa-thologies [5,27–32]. Studies were conducted to determinethe difference in PI between male and female subjects, re-porting in general no relationship between PI and sex[6,9,14–16,20,33–35] but in some cases higher but not sta-tistically significant values for male [1,17,36] or female[33,37] subjects. On the contrary, Vialle et al. [4] reportedstatistically significant differences between the sexes, withhigher PI for female subjects. In examining the relationshipbetween PI and age of the subjects, studies reported no re-lationship in fetuses [15], adults [4,15,20], elderly popula-tion [36], or various age groups [38], but PI was found tobe higher in normal elderly [36,38] than normal adult pop-ulation [1,2,4,6,9,14,35], and a relationship between PI andage was found for children [15,19,33,37] and adolescents[33,37]. Recent studies also reported that aging or degener-ative processes, manifested as a progressive loss in theheight of intervertebral discs, induce compensation in thesagittal balance by a backward rotation (ie, retroversion)of the pelvis [39], which is for an individual limited byPI that remains constant [40]. When considering the sizeof the pelvis, a statistically significant difference was re-ported between the sexes for PTH [41] and a significantnegative correlation between PTH and PI [42], meaningthat an increase in PI is coupled with a decrease in PTH.
In contrast to the generally accepted views, by compar-ing the results of different studies Mendoza-Lattes et al.[17] recently concluded that PI increases linearly through-out the lifespan of normal subjects due to a remodeling pro-cess affecting the coxal bone (ie, the hip bone consisting of
Fig. 4. (Top) nPTH, (Middle) nPTHA-P, and (Bottom) nPTHC-C in relation
to age for male and female subjects with superimposed regression lines.
nPTH, normalized pelvic thickness; nPTHA-P, normalized anteroposterior
component of pelvic thickness; nPTHC-C, normalized cephalocaudal com-
ponent of pelvic thickness.
Table 2
Normalized pelvic thickness, nPTHA-P, and nPTHC-C, as evaluated for all
subjects (N5330) and separately for male (N5164) and female (N5166)
subjects
Pelvic parameter SD
Vs. age* Vs. PIy
pzr p r p
nPTH (%) 13.2 �0.18 !.05 �0.27 !.0001 O.05
Male 11.7 �0.19 !.05 �0.20 !.05
Female 11.7 �0.19 !.05 �0.39 !.0001
nPTHA-P (%) 17.8 þ0.11 !.05 þ0.70 !.0001 O.05
Male 17.2 þ0.01 O.05 þ0.75 !.0001
Female 16.6 þ0.22 !.05 þ0.74 !.0001
nPTHC-C (%) 27.1 �0.27 !.0001 �0.87 !.0001 O.05
Male 23.9 �0.21 !.05 �0.86 !.0001
Female 28.0 �0.33 !.0001 �0.91 !.0001
nPTH, normalized pelvic thickness; nPTHA-P, normalized anteroposte-
rior component of pelvic thickness; nPTHC-C, normalized cephalocaudal
component of pelvic thickness; PI, pelvic incidence; SD, standard devia-
tion around the 100% mean.
* Correlation coefficient r and significance level p in relation to age.y Correlation coefficient r and significance level p in relation to PI.z Significance level p between male and female subjects.
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the ilium, ischium, and pubis), which results in an anteriordrift of the acetabulum relative to the sacroiliac joint. Theyexplained the gradual increase of PI throughout the lifespanby an increase of the sacropelvic translation [43], defined asthe horizontal distance between the reference verticalthrough the center of the femoral head and the referencevertical through the posterior corner of the sacral end plate.
The horizontal distance between the sacrum and the hipaxis can also be described by the pelvic overhang [9], ifthe distance is measured against the reference verticalthrough the center of the sacral end plate, or by the sacro-femoral distance [44], if the distance is measured againstthe reference vertical through the anterior corner of the sa-cral end plate. As a result, if the femoral heads move toa relatively more anterior position, PI is consequently goingto present larger angular values.
The results of our study confirm the conclusions ofMendoza-Lattes et al. [17] and also bring new insight intothe evaluation of pelvic morphology. The increase of thehorizontal distance between the sacrum and the hip axis,as suggested by Mendoza-Lattes et al. [17], implies an in-crease of the straight distance between the sacrum andthe hip axis, represented by PTH. However, our studyshows that with increasing PI that occurs due to the agingprocess (Fig. 3, Top), PTH decreases (Fig. 4, Top, andFig. 5, Top). The decrease of PTH is manifested not onlyas an increase of the distance between the sacrum and thehip axis in the anterior direction (ie, PTHA-P; Fig. 4, Mid-dle, and Fig. 5, Middle) but considerably more as a decreaseof the distance between the sacrum and the hip axis in thecephalic direction (ie, PTHC-C; Fig. 4, Bottom, and Fig. 5,Bottom). The horizontal distance between the sacrum andthe hip axis can be, to some extent, compared withPTHA-P in our study. Our results show that PTHA-P in gen-eral increases with age for female subjects but slightly de-creases with age for male subjects. The same tendency canbe observed for PTHC-C, which manifests a stronger de-crease for female than for male subjects. Nevertheless, byconsidering these morphological changes in the pelvis si-multaneously (ie, increase of PI, decrease of PTH, increaseof PTHA-P, and decrease of PTHC-C), the hip axis can moveonly within a narrow area (Fig. 6). These changes therefore
Fig. 5. (Top) nPTH, (Middle) nPTHA-P, and (Bottom) nPTHC-C in relation
to PI for male and female subjects with superimposed regression lines.
nPTH, normalized pelvic thickness; nPTHA-P, normalized anteroposterior
component of pelvic thickness; nPTHC-C, normalized cephalocaudal com-
ponent of pelvic thickness; PI, pelvic incidence.
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occur in the anterior direction, which may be due to the re-modeling process affecting the coxal bone that results in ananterior drift of the acetabulum relative to the sacrum. Moreimportantly, the changes are considerably more evident inthe cephalic direction, which may be the result of theweight-bearing loads and consequent wear of acetabularcartilage.
The main shortcoming of the performed study is that it iscross-sectional and therefore the differentiation betweencause and effect is difficult. However, performing a longitu-dinal study and therefore radiographically screening a largenumber of normal individuals throughout their lifespan isnot feasible and would be questionable in terms of costsand medical ethics. Regarding cause and effect, there isno doubt that the increasing PI is the effect, while the causeis represented by the morphological changes of the pelvis,observed through pelvic parameters. It is, however, impor-tant to note that assuming the size of anatomy is reflected inthe size of the sacrum, and therefore S1W, can be treated asadequate only for cases of normal anatomy, or anatomy thatis considered normal. In the case of severe abnormalities orpathologies, for example, dome-shaped sacral end plate inthe case of spondylolysis [45], sacral dysplasia in the caseof spondylolisthesis [46] or even sacral insufficiency frac-tures [47], the corresponding expected values mS1W(age)would have to be modeled according to the studied popula-tion. Although the observed cohort of subjects was assumedto represent a normal population, several subjects were ex-cluded from the original cohort due to several criteria (ie,age, clinical, or radiological history or evidence for pathol-ogy or trauma, evidence for anatomical anomalies, and soon), and imposing of additional criteria may result in fur-ther exclusion of subjects. Unfortunately, criteria such asactivity (eg, sedentary vs. mobile lifestyle, practicingsports, and so on), body mass index, or other physiologicalcharacteristics were not recorded along with other subjectinformation but may nevertheless influence the sagittal pel-vic alignment [19,34]. However, the natural biological var-iability of the human anatomy is relatively large, which isalso reflected in the large variability of the measured pa-rameters that describe pelvic morphology. Ascertaininga truly normal and asymptomatic population therefore re-mains poorly defined and represents a limitation of thepresent as well as of the aforementioned studies.
In contrast to current routine clinical protocols, themeasurements in this study were obtained in 3D from vol-umetric CT images of the pelvis. This does not imply thatpelvic morphology should not be evaluated in two dimen-sions from plain sagittal radiographic images; however,computerized measurements in 3D proved to be highlyconsistent and reliable, especially because they were ob-tained from sagittal views after multiplanar reformation ac-cording to the hip axis. In such perfect sagittal views, allanatomical structures were observed completely in linewith the hip axis and were therefore not biased by the pro-jective nature of radiographic image acquisition [18]. Be-cause CT images are acquired in subject supine position,the obtained findings cannot be transferred to positionalpelvic parameters (eg, sacral slope, pelvic tilt, sacropelvictranslation, pelvic overhang, sacrofemoral distance) butneed to be contained within anatomical pelvic parameters(eg, PI, PTH) that are invariant against arbitrary patient po-sition and/or orientation.
Fig. 6. (A) If PI increases, the hip axis must move into the shown area. (B) If PTH decreases, the hip axis must move into the shown area. (C) If PTHA-P
increases and PTHC-C decreases, the hip axis must move into the shown area. (D) If all changes occur simultaneously, the hip axis must be located within the
intersection of these areas, that is, within the shown area. PI, pelvic incidence; PTH, pelvic thickness; PTHA-P, anteroposterior component of pelvic thickness;
PTHC-C, cephalocaudal component of pelvic thickness.
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Conclusion
Despite the previously described shortcomings, thisstudy revealed that there are indications for morphologicalchanges in the pelvis that continue to occur throughout thelifespan and reflect the effort of the human body to main-tain adequate sagittal balance. The pelvis is also subjectto a posteriorly inferiorly directed load that leads to bonyremodeling changes according to Wolff’s law [48,49],which asserts that bone in a healthy person will adapt tothe loads it is placed under to maximize mechanical effi-ciency. According to the Utah paradigm of skeletal physiol-ogy [50–52], which is a refinement of Wolff’s law, bonegrowth and bone loss are stimulated by the local mechani-cal elastic deformation of bone. The adaptation (feedbackcontrol loop) of bone according to the maximum forces isconsidered to be a lifelong process; hence, bone adapts
its mechanical properties according to the needed mechan-ical function—bone mass, geometry, and strength are there-fore adapted according to the everyday usage and needs.
Acknowledgments
This work has been supported by the Ministry of Educa-tion, Science, Culture and Sport, Slovenia, under grantsP2-0232, J7-2264, L2-7381, and L2-2023, and partly byBiomet and the Anna Foundation.
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