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Obesity Alters Spinopelvic Alignment Changes From Standing to Relaxed Sitting: the Influence of the Soft-tissue Envelope

Open AccessPublished:May 21, 2020DOI:https://doi.org/10.1016/j.artd.2020.02.011

      Abstract

      Background

      Changes in spinopelvic and lower extremity alignment between standing and relaxed sitting have important clinical implications with regard to stability of total hip arthroplasty. This study aimed to analyze the effect of body mass index (BMI) on lumbopelvic alignment and motion at the hip joint.

      Methods

      A retrospective review of patients who underwent full-body stereoradiographs in standing and relaxed sitting for total hip arthroplasty planning was conducted. Spinopelvic parameters measured included spinopelvic tilt (SPT), pelvic incidence (PI), lumbar lordosis (LL), PI minus LL (PI-LL), proximal femoral shaft angle (PFSA), and standing-to-sitting hip range of motion. Propensity score matching controlled for age, gender, PI, and hip ostoarthritis grade. Patients were stratified into normal (NORMAL; BMI, 18.5-24.9), overweight (OW; 25.0-29.9), and obese (OB; 30.0-34.9) groups. Alignment parameters were compared using one-way analysis of variance.

      Results

      There were 84 patients in each group after propensity score matching. Standing alignment between BMI groups was similar for all parameters (P > .05) except for PFSA (P < .001). Significant differences were noted for sitting alignment between patients who are NORMAL, OW, and OB in: SPT (P = .007), PI-LL (P = .018), and LL (P = .029). PFSA between groups was not significantly different (P > .05). Significant differences were found for sitting-to-standing alignment across groups in PFSA change (P < .001), SPT change (P = .006), PI-LL change (P = .005), LL change (P = .037), and hip flexion (P < .001).

      Conclusions

      Significant differences in sitting and standing-to-sitting change in lumbopelvic alignment based on BMI suggest obese patients recruit more posterior spinopelvic tilt when sitting to compensate for soft-tissue impingement that occurs anterior to the hip joint and limiting hip flexion.

      Keywords

      Introduction

      Hip instability after total hip arthroplasty (THA) is a leading cause of revision [
      • Lee S.H.
      • Lim C.W.
      • Choi K.Y.
      • Jo S.
      Effect of spine-pelvis relationship in total hip arthroplasty.
      ]. Dislocation after THA is correlated with reduced quality of life and added health-care costs [
      • Ullmark G.
      The unstable total hip arthroplasty.
      ]. In maintaining hip stability, lumbopelvic alignment plays a crucial role. The pelvis acts as a regulator of sagittal plane alignment, and its positioning varies according to spinal alignment [
      • Schwab F.
      • Patel A.
      • Ungar B.
      • Farcy J.-P.
      • Lafage V.
      Adult spinal deformity—postoperative standing imbalance.
      ]. Patients with abnormal spinopelvic alignment or mobility, especially in combination with poor acetabular cup positioning or soft-tissue abnormalities, are at an increased risk of dislocation [
      • Heckmann N.
      • Mcknight B.
      • Stefl M.
      • Trasolini N.A.
      • Ike H.
      • Dorr L.D.
      Late dislocation following total hip arthroplasty.
      ]. For these patients, the traditional safe zone for acetabular cup implantation positioning may not be appropriate [
      • Maratt J.D.
      • Esposito C.I.
      • Mclawhorn A.S.
      • Jerabek S.A.
      • Padgett D.E.
      • Mayman D.J.
      Pelvic tilt in patients undergoing total hip arthroplasty: when does it matter?.
      ,
      • Delsole E.M.
      • Vigdorchik J.M.
      • Schwarzkopf R.
      • Errico T.J.
      • Buckland A.J.
      Total hip arthroplasty in the spinal deformity population: does degree of sagittal deformity affect rates of safe zone placement, instability, or revision?.
      ]. Instead, adjusting cup positioning based on spinopelvic alignment may provide optimum stability after THA [
      • Maratt J.D.
      • Esposito C.I.
      • Mclawhorn A.S.
      • Jerabek S.A.
      • Padgett D.E.
      • Mayman D.J.
      Pelvic tilt in patients undergoing total hip arthroplasty: when does it matter?.
      ,
      • Lum Z.C.
      • Coury J.G.
      • Cohen J.L.
      • Dorr L.D.
      The current knowledge on spinopelvic mobility.
      ,
      • Rivière C.
      • Lazennec J.-Y.
      • Straeten C.V.D.
      • Auvinet E.
      • Cobb J.
      • Muirhead-Allwood S.
      The influence of spine-hip relations on total hip replacement: a systematic review.
      ].
      Implant dislocation tends to occur during changes in posture, most commonly when rising from a chair [
      • Lee S.H.
      • Lim C.W.
      • Choi K.Y.
      • Jo S.
      Effect of spine-pelvis relationship in total hip arthroplasty.
      ]. Thus, a comprehensive understanding of THA stability includes examining alignment not only during standing positions but also during postural changes. A combination of acetabular and femoral orientation impacts the distance between the rim of the acetabulum and the proximal femur and thus affects the chance of anterior femoroacetabular impingement and risk of posterior dislocation in sitting [
      • Esposito C.I.
      • Miller T.T.
      • Kim H.J.
      • et al.
      Does degenerative lumbar spine disease influence femoroacetabular flexion in patients undergoing total hip arthroplasty?.
      ]. Acetabular orientation is altered by changes in spinopelvic tilt (SPT) [
      • Maratt J.D.
      • Esposito C.I.
      • Mclawhorn A.S.
      • Jerabek S.A.
      • Padgett D.E.
      • Mayman D.J.
      Pelvic tilt in patients undergoing total hip arthroplasty: when does it matter?.
      ]. Patients with dislocations tend to have different standing-to-sitting spinopelvic alignment changes, including changes in pelvic tilt and spine flexion, compared with normal patients, along with altered acetabular component orientation [
      • Esposito C.I.
      • Carroll K.M.
      • Sculco P.K.
      • Padgett D.E.
      • Jerabek S.A.
      • Mayman D.J.
      Total hip arthroplasty patients with fixed spinopelvic alignment are at higher risk of hip dislocation.
      ].
      Changes in spinopelvic alignment may be attributed to multiple causes. Owing to the coordinated nature of spinopelvic motions, a limit in motion on one spine segment tends to increase mobility in other spine segments and in pelvic tilt to maintain spinopelvic “balance” [
      • Lee S.H.
      • Lim C.W.
      • Choi K.Y.
      • Jo S.
      Effect of spine-pelvis relationship in total hip arthroplasty.
      ]. Spine diseases, including degenerative disc disease (DDD), degenerative spondylolisthesis, and lumbar fusion, have been associated with abnormal spinopelvic alignment and mobility [
      • Esposito C.I.
      • Miller T.T.
      • Kim H.J.
      • et al.
      Does degenerative lumbar spine disease influence femoroacetabular flexion in patients undergoing total hip arthroplasty?.
      ,
      • Day L.M.
      • Delsole E.M.
      • Beaubrun B.M.
      • et al.
      Radiological severity of hip osteoarthritis in patients with adult spinal deformity: the effect on spinopelvic and lower extremity compensatory mechanisms.
      ,
      • Barrey C.
      • Jund J.
      • Noseda O.
      • Roussouly P.
      Sagittal balance of the pelvis-spine complex and lumbar degenerative diseases. A comparative study about 85 cases.
      ].
      While the propensity for concomitant hip and spine disease has been well established, there is an even stronger association between obesity and osteoarthritis of the lower extremity [
      • Anandacoomarasamy A.
      • Caterson I.
      • Sambrook P.
      • Fransen M.
      • March L.
      The impact of obesity on the musculoskeletal system.
      ,
      • Bourne R.
      • Mukhi S.
      • Zhu N.
      • Keresteci M.
      • Marin M.
      Role of obesity on the risk for total hip or knee arthroplasty.
      ]. Morbidly obese patients require THA a decade faster than patients of normal weight [
      • Changulani M.
      • Kalairajah Y.
      • Peel T.
      • Field R.E.
      The relationship between obesity and the age at which hip and knee replacement is undertaken.
      ]. Prior studies have demonstrated an obesity-associated relative risk for dislocation after THA [
      • Chee Y.H.
      • Teoh K.H.
      • Sabnis B.M.
      • Ballantyne J.A.
      • Brenkel I.J.
      Total hip replacement in morbidly obese patients with osteoarthritis: results of a prospectively matched study.
      ,
      • Kim Y.
      • Morshed S.
      • Joseph T.
      • Bozic K.
      • Ries M.D.
      • Rothman R.H.
      Clinical impact of obesity on stability following revision total hip arthroplasty.
      ,
      • Davis A.M.
      • Wood A.M.
      • Keenan A.C.M.
      • Brenkel I.J.
      • Ballantyne J.A.
      Does body mass index affect clinical outcome post-operatively and at five years after primary unilateral total hip replacement performed for osteoarthritis? A multivariate analysis of prospective data.
      ], yet the exact causality of the relationship between obesity and THA instability remains unclear. Information regarding BMI and spinopelvic alignment measurements is scarce and inconsistent [
      • Changulani M.
      • Kalairajah Y.
      • Peel T.
      • Field R.E.
      The relationship between obesity and the age at which hip and knee replacement is undertaken.
      ,
      • Vismara L.
      • Menegoni F.
      • Zaina F.
      • Galli M.
      • Negrini S.
      • Capodaglio P.
      Effect of obesity and low back pain on spinal mobility: a cross sectional study in women.
      ,
      • Romero-Vargas S.
      • Zárate-Kalfópulos B.
      • Otero-Cámara E.
      • et al.
      The impact of body mass index and central obesity on the spino-pelvic parameters: a correlation study.
      ,
      • Boulay C.
      • Tardieu C.
      • Hecquet J.
      • et al.
      Sagittal alignment of spine and pelvis regulated by pelvic incidence: standard values and prediction of lordosis.
      ].
      In this study, we analyze spinopelvic alignment parameters among patients with different BMI in relaxed standing and sitting positions, as well as the change between positions. The purpose of this study was to investigate the effect of obesity on lumbopelvic alignment and motion at the hip joint. Pelvic tilt will be referenced in this article by the term “spinopelvic tilt” rather than anterior pelvic plane tilt because of the improved accuracy in measurement [
      • Buckland A.J.
      • DelSole E.M.
      • George S.G.
      • et al.
      Sagittal pelvic orientation: a comparison of two methods of measurement.
      ].

      Material and methods

      Inclusion and exclusion criteria

      An institutional review board–approved retrospective review of patients visiting a single academic institution undergoing unilateral THA, who had full-body stereoradiographs in both the standing and relaxed seated positions for preoperative planning, was conducted.
      Only patients with BMI between 18.5 and 35 were included to encompass 3 categories of BMI as per World Health Organization standards [
      Physical status: the use and interpretation of anthropometry. Report of a WHO Expert Committee.
      ]. Underweight and morbidly obese patients were excluded because of lack of patients (n < 20 for each category). Patients with poor visualization of lumbar spine or femoral heads, lumbosacral transitional anatomy, or history of either hip arthroplasty or lumbar fusion were excluded because of the previously published literature demonstrating changes in sit-stand alignment.

      Patient demographics

      A total of 466 patients met the inclusion and exclusion criteria, of which 37.55%, 38.20%, and 24.25% were categorized into NORMAL, overweight, and obese groups, respectively. No differences were found in age (P = .430), standing PI (P = .185), and proportion of lumbar flatback deformity and DDD (P = .559). The proportion of DDD (42.86%, 45.51%, and 45.13%) and lumbar flatback (14.29%, 18.54%, and 19.47%) for NORMAL, overweight, and obese groups, respectively, were not statistically significant (P = .559). Proportion of severe hip osteoarthritis (37.71%, 44.38%, and 53.98%, P = .005) and female gender (75.88%, 53.76%, and 52.29%, P < .000) were statistically significant between groups.
      Owing to the demographic differences between groups, propensity score matching (PSM) was performed. After PSM, there were 84 patients in each group. The average age was 60.07 ± 13.98, 60.55 ± 15.11, and 60.36 ± 12.25 years and average standing PI was 54.26 ± 13.11, 55.82 ± 11.86, and 54.60 ± 11.85 for NORMAL, overweight, and obese patients, respectively. There were 61.90%, 57.14%, and 60.71% female patients in NORMAL, overweight, and obese groups, respectively. Proportion of severe hip osteoarthritis was 49.41%, 54.88%, and 49.41%. No differences were found in the proportion of DDD (39.29%, 50.00%, and 46.43%) and lumbar flatback deformity (19.05%, 19.05%, and 17.86%) across groups after PSM (P = .647).

      Image acquisition

      All patients underwent low-dose radiation, head-to-foot, biplanar stereoradiographic images (EOS imaging, Paris, France). This is a slot-scanning radiographic device consisting of 2 radiograph source-detector pairs, allowing simultaneous orthogonal image acquisition. The standardized protocol included a weight-bearing free-standing position of comfort in standing and unsupported sitting position on a radiolucent chair, both with arms flexed at 45 degrees and fingers on clavicles to prevent superimposition of the upper extremities on the spine [
      • Horton W.C.
      • Brown C.W.
      • Bridwell K.H.
      • Glassman S.D.
      • Suk S.-I.
      • Cha C.W.
      Is there an optimal patient stance for obtaining a lateral 36" radiograph? A critical comparison of three techniques.
      ]. Owing to the field of view of the EOS, the lower extremity distal to the proximal femur was unable to be captured in the sitting position.

      Radiographic measurements

      Spinopelvic parameters measured included SPT (angle between a line from the bicoxofemoral axis to the midpoint of the sacral endplate and a vertical line), pelvic incidence (PI: angle between a line from the bicoxofemoral axis to the midpoint of the sacral endplate and the line perpendicular to the sacral endplate), pelvic incidence minus lumbar lordosis (PI-LL), and L1 to S1 lumbar lordosis (LL: angle between the superior endplate of L1 and the superior endplate of S1) (Fig. 1). Segmental lumbar lordosis (between the superior endplate of the upper vertebra and superior endplate of the lower vertebra) at levels L1-L4 and L4-S1 were also measured. An increase in SPT denotes an increase in posterior pelvic tilt. PI-LL was included because it is an important measurement of spinopelvic alignment that has been shown to predict worse clinical outcomes and quality of life when greater than 10° (flatback) [
      • Schwab F.
      • Patel A.
      • Ungar B.
      • Farcy J.-P.
      • Lafage V.
      Adult spinal deformity—postoperative standing imbalance.
      ]. Furthermore, increasing SPT (pelvic retroversion) has a strong relationship with PI-LL mismatch in adult spinal deformity literature, functioning as a “compensatory mechanism” for lumbar flatback deformity. In addition, proximal femoral shaft angle (PFSA: the angle between the axis of the femoral shaft and the vertical) and hip flexion (PFSA change between sitting and standing radiographs minus SPT change) were measured for each patient. Severity of hip osteoarthritis was graded using the Kellgren-Lawrence system [
      • Kellgren J.H.
      • Lawrence J.S.
      Radiological assessment of osteo-arthrosis.
      ]. Lumbar spines were considered degenerative if disc height loss is >50% and facet arthropathy or spondylolisthesis was present at either radiograph and flatback if patient presented degenerative criteria and PI-LL >10°.
      Figure thumbnail gr1
      Figure 1Sagittal spinopelvic alignment parameters measured in each patient are illustrated. Measured parameters include SPT, PI, L1 to S1 LL, and PFSA. Segmental lumbar lordosis was evaluated at levels L1-L4 (L4-L4) and L4-S1 (L4-S1). Pelvic tilt was referenced as SPT rather than anterior pelvic plane tilt (APPt) because of the improved accuracy in measurement [
      • Buckland A.J.
      • DelSole E.M.
      • George S.G.
      • et al.
      Sagittal pelvic orientation: a comparison of two methods of measurement.
      ].

      Statistical analysis

      PSM was performed to control for PI, age, gender, and hip ostoarthritis grade. Patients were stratified into 3 groups: normal (NORMAL; BMI, 18.5-24.9), overweight (25.0-29.9), and obese (30.0-34.9). Alignment parameters were compared using one-way analysis of variance, followed by the Turkey HSD test.

      Results

      Radiographic measurements

      Standing alignment parameters (SPT, PI, PI-LL, LL) were similar between BMI groups (P > .05) except for standing PFSA, which was statistically significant among groups (NORMAL, 6.56 ± 4.11; overweight, 7.04 ± 4.70; obese, 9.74 ± 5.17, P < .001) (Table 1). Post-hoc Tukey test indicated the significant difference occurred in all groups except between NORMAL and overweight groups. A trend toward increased standing PFSA was observed among groups with increasing BMI.
      Table 1Mean spinopelvic alignment parameters with standard deviations for normal, overweight, and obese patients in standing, sitting, and sitting-to-standing positions.
      Spinopelvic measuresBMI categoryP value
      Normal (n = 84)Overweight (n = 84)Obese (n = 84)
      Standing
       PFSA6.56 ± 4.117.04 ± 4.709.74 ± 5.17.000
       SPT14.97 ± 8.2816.19 ± 7.6215.74 ± 8.56.615
       PI54.26 ± 13.1155.82 ± 11.8654.60 ± 11.85.687
       PI-LL0.15 ± 12.67−0.04 ± 10.570.50 ± 11.44.954
       L1-L419.00 ± 10.5521.87 ± 8.4421.78 ± 8.58.075
       L4-S135.10 ± 8.4433.99 ± 8.6532.33 ± 8.16.101
       LL54.11 ± 12.9855.86 ± 11.8054.11 ± 11.98.564
      Sitting
       PFSA96.59 ± 3.8995.52 ± 4.0195.67 ± 4.20.181
       SPT25.57 ± 11.6526.95 ± 10.7130.88 ± 11.14.007
       PI55.37 ± 12.0656.39 ± 12.0356.05 ± 12.22.857
       PI-LL16.95 ± 15.4517.86 ± 14.0323.04 ± 15.20.018
       L1-L414.18 ± 11.9815.72 ± 10.9412.09 ± 11.14.118
       L4-S124.22 ± 9.5222.81 ± 9.6620.93 ± 8.62.072
       LL38.40 ± 16.2538.53 ± 13.6533.02 ± 15.68.029
      Sitting-standing change
       PFSA90.02 ± 5.0388.47 ± 5.8085.94 ± 6.26.000
       SPT9.60 ± 11.6910.75 ± 12.0715.14 ± 11.16.006
       PI0.15 ± 9.980.56 ± 9.321.45 ± 7.24.631
       PI-LL15.86 ± 12.7817.90 ± 14.3222.54 ± 13.42.005
       L1-L4−4.82 ± 7.02−6.16 ± 8.22−9.69 ± 8.61.000
       L4-S1−10.89 ± 8.39−11.18 ± 10.77−11.40 ± 9.33.941
       LL−15.71 ± 12.82−17.34 ± 14.68−21.09 ± 14.06.037
       Hip Flexion80.42 ± 12.5277.72 ± 15.2970.87 ± 14.25.000
      Bold indicates statistically significant P-values (<.05).
      Significant differences were found for sitting alignment for NORMAL, overweight, and obese patients in SPT (NORMAL, 25.57 ± 11.65; overweight, 26.95 ± 10.71; obese, 30.88 ± 11.14, P = .007), PI-LL (NORMAL, 16.95 ± 15.45; overweight, 17.86 ± 14.03; obese, 23.04 ± 15.20, P = .018), and LL (NORMAL, 38.40 ± 16.25; overweight, 38.53 ± 13.65; obese, 33.02 ± 15.68, P = .029) (Table 1). No significant differences were noted for sitting PFSA between groups (P >.05). Post-hoc Tukey test demonstrated statistically significant differences between all groups except between NORMAL and overweight groups for both SPT and PI-LL. Significant differences were only found between NORMAL and obese patients for SPT and PI-LL. The difference in LL between NORMAL and obese was close to significance (P = .059) but was not significant because of the methodologies of the Tukey test.
      From standing to sitting, groups of increasing BMI were associated with significantly greater changes in SPT (NORMAL, 9.60 ± 11.69; overweight, 10.75 ± 12.07; obese, 15.14 ± 11.16, P = .006), PI-LL (NORMAL, 15.86 ± 12.78; overweight, 17.90 ± 14.32; obese, 22.54 ± 13.42, P = .005), and LL (NORMAL, -15.71 ± 12.82; overweight, −17.34 ± 14.68; obese, −21.09 ± 14.06, P = .037). Significantly smaller changes in PFSA (NORMAL, 90.02 ± 5.03; overweight, 88.47 ± 5.80; obese, 85.94 ± 6.26, P < .001) and hip flexion (NORMAL, 80.42 ± 12.52; overweight, 77.72 ± 15.29; obese, 70.87 ± 14.25, P < .001) occurred across groups of increasing BMI (Table 1). Results of standing to sitting alignment are highlighted in Figure 2, Figure 3. Post-hoc Tukey demonstrated statistically significant differences between all BMI groups for PFSA and SPT changes except between NORMAL and overweight groups. Significant differences in PI-LL and LL changes were only found between NORMAL and obese groups. Analysis of standing-to-sitting change in segmental lumbar lordosis revealed that only lordosis at L1-L4 (NORMAL, −4.82 ± 7.02; overweight, −6.16 ± 8.22; obese, −9.69 ± 8.61, P < .001) was significantly different across groups (Fig. 4).
      Figure thumbnail gr2
      Figure 2Standing and sitting lateral radiographs comparing obese and normal patients are shown. (a) Standing alignment of a patient with normal BMI. (b) Standing alignment of a patient with obese BMI. (c) Sitting alignment of a patient with normal BMI. (d) Sitting alignment of a patient with obese BMI. In the present study, standing alignment was found to be similar for patients with obese BMI and normal BMI. However, sitting alignment and change in alignment from sitting to standing was found to be significantly different. Obese patients recruit more pelvic tilt while sitting than normal BMI patients to compensate for greater soft-tissue impingement anterior to the hip, which limits hip flexion.
      Figure thumbnail gr3
      Figure 3The relationship between BMI categories and change in sitting-standing SPT is demonstrated. A histogram was chosen over a scatter plot because of the large amount of noise in the data set that made it difficult to discern any patterns among individual data points. Compared with other BMI groups, obese patients tend to comprise larger proportions of patients with greater sitting-standing SPT changes. The proportions of overweight and normal-weight patients in each sitting-standing SPT category do not show well-defined trends.
      Figure thumbnail gr4
      Figure 4Change in segmental lumbar lordosis from standing to sitting was analyzed across BMI groups. While all groups had similar motion at the lower segments (L4-S1) of the lumbar spine, motion at the upper segments (L1-L4) was significantly increased across groups of increasing BMI. The increased motion is likely due to soft-tissue impingement around the hip in the sitting position.

      Discussion

      A body of evidence has demonstrated the increased risk of dislocation and revision in patients undergoing THA with adult spinal deformity [
      • Delsole E.M.
      • Vigdorchik J.M.
      • Schwarzkopf R.
      • Errico T.J.
      • Buckland A.J.
      Total hip arthroplasty in the spinal deformity population: does degree of sagittal deformity affect rates of safe zone placement, instability, or revision?.
      ,
      • Sultan A.A.
      • Khlopas A.
      • Piuzzi N.S.
      • Chughtai M.
      • Sodhi N.
      • Mont M.A.
      The impact of spino-pelvic alignment on total hip arthroplasty outcomes: a critical analysis of current evidence.
      ], fixed spinopelvic alignment, [
      • Esposito C.I.
      • Carroll K.M.
      • Sculco P.K.
      • Padgett D.E.
      • Jerabek S.A.
      • Mayman D.J.
      Total hip arthroplasty patients with fixed spinopelvic alignment are at higher risk of hip dislocation.
      ] history of lumbar fusion [
      • Buckland A.J.
      • Puvanesarajah V.
      • Vigdorchik J.
      • et al.
      Dislocation of a primary total hip arthroplasty is more common in patients with a lumbar spinal fusion.
      ,
      • Perfetti D.C.
      • Schwarzkopf R.
      • Buckland A.J.
      • Paulino C.B.
      • Vigdorchik J.M.
      Prosthetic dislocation and revision after primary total hip arthroplasty in lumbar fusion patients: a propensity score matched-pair analysis.
      ,
      • Barry J.J.
      • Sing D.C.
      • Vail T.P.
      • Hansen E.N.
      Early outcomes of primary total hip arthroplasty after prior lumbar spinal fusion.
      ], and noninstrumented spinal disease [
      • Delsole E.M.
      • Vigdorchik J.M.
      • Schwarzkopf R.
      • Errico T.J.
      • Buckland A.J.
      Total hip arthroplasty in the spinal deformity population: does degree of sagittal deformity affect rates of safe zone placement, instability, or revision?.
      ]. Several studies have also demonstrated an increased risk of dislocation after primary [
      • Chee Y.H.
      • Teoh K.H.
      • Sabnis B.M.
      • Ballantyne J.A.
      • Brenkel I.J.
      Total hip replacement in morbidly obese patients with osteoarthritis: results of a prospectively matched study.
      ,
      • Davis A.M.
      • Wood A.M.
      • Keenan A.C.M.
      • Brenkel I.J.
      • Ballantyne J.A.
      Does body mass index affect clinical outcome post-operatively and at five years after primary unilateral total hip replacement performed for osteoarthritis? A multivariate analysis of prospective data.
      ] and revision THA [
      • Kim Y.
      • Morshed S.
      • Joseph T.
      • Bozic K.
      • Ries M.D.
      • Rothman R.H.
      Clinical impact of obesity on stability following revision total hip arthroplasty.
      ] in obese compared with normal-weight patients. Existing literature on the relationship between obesity and spinopelvic kinematics, and its potential contribution to THA instability, is inconsistent [
      • Changulani M.
      • Kalairajah Y.
      • Peel T.
      • Field R.E.
      The relationship between obesity and the age at which hip and knee replacement is undertaken.
      ,
      • Vismara L.
      • Menegoni F.
      • Zaina F.
      • Galli M.
      • Negrini S.
      • Capodaglio P.
      Effect of obesity and low back pain on spinal mobility: a cross sectional study in women.
      ,
      • Romero-Vargas S.
      • Zárate-Kalfópulos B.
      • Otero-Cámara E.
      • et al.
      The impact of body mass index and central obesity on the spino-pelvic parameters: a correlation study.
      ,
      • Boulay C.
      • Tardieu C.
      • Hecquet J.
      • et al.
      Sagittal alignment of spine and pelvis regulated by pelvic incidence: standard values and prediction of lordosis.
      ]. This study analyzed the effect of obesity on lumbopelvic alignment between standing and relaxed sitting postures.
      Spinopelvic alignment plays a crucial role in maintaining THA stability. Because of the coordinated motion of the spine-pelvis-hip, patients with stiff spines require increased mobility of the hip joint from standing to sitting, with less change in SPT. These patients are thus at increased risk of anterior impingement and posterior dislocation while sitting [
      • Heckmann N.
      • Mcknight B.
      • Stefl M.
      • Trasolini N.A.
      • Ike H.
      • Dorr L.D.
      Late dislocation following total hip arthroplasty.
      ]. On the contrary, patients with stiff hip joints require increased mobility of the spine from standing to sitting, which may account for the relatively high rates of back pain in osteoarthritic patients [
      • Heckmann N.
      • Mcknight B.
      • Stefl M.
      • Trasolini N.A.
      • Ike H.
      • Dorr L.D.
      Late dislocation following total hip arthroplasty.
      ,
      • Staibano P.
      • Winemaker M.
      • Petruccelli D.
      • Beer J.D.
      Total joint arthroplasty and preoperative low back pain.
      ].
      To our knowledge, our study is the first to examine sitting-standing changes in spinopelvic parameters between groups of different BMIs. Changes in spinopelvic alignment parameters of obese patients follow similar trends to patients with severe hip osteoarthritis. Buckland et al. [
      • Buckland A.J.
      • Steinmetz L.
      • Zhou P.
      • et al.
      Spinopelvic compensatory mechanisms for reduced hip motion (ROM) in the setting of hip osteoarthritis.
      ] found greater magnitude of standing-to-sitting changes in SPT, PI-LL, and LL in patients with severe osteoarthritis than in patients with low-grade osteoarthritis. It was postulated that osteoarthritic patients have reduced hip range of motion and compensate by increasing SPT recruitment and increasing changes in LL and TL kyphosis. The mechanism underlying changes in spinopelvic parameters of patients with osteoarthritis may be similar to that in obese patients, with the exception that the restricted hip range of motion stems from extraarticular soft-tissue impingement in obese patients, rather than periarticular bony impingement in severe osteoarthritic patients. This is supported by the reduced hip flexion exhibited by obese patients compared to other groups. Soft-tissue impingement in obese patients may also lead to reduced femoral extension, indicated by the relatively greater standing PFSA. In contrast to patients with restricted hip motion, patients with stiff spines have standing femoral hyperextension and increased femoral motion [
      • Heckmann N.
      • Mcknight B.
      • Stefl M.
      • Trasolini N.A.
      • Ike H.
      • Dorr L.D.
      Late dislocation following total hip arthroplasty.
      ,
      • Esposito C.I.
      • Miller T.T.
      • Kim H.J.
      • et al.
      Does degenerative lumbar spine disease influence femoroacetabular flexion in patients undergoing total hip arthroplasty?.
      ].
      Loss of hip motion may lead to increased requirement for spine mobility in obese patients. This is supported by the relatively large increase in standing-to-sitting LL magnitude. Increased LL change drives the greater SPT change observed in obese patients. SPT is influenced by both spinal and femoral alignment because of the position of the pelvis between the spine and hip [
      • Esposito C.I.
      • Miller T.T.
      • Kim H.J.
      • et al.
      Does degenerative lumbar spine disease influence femoroacetabular flexion in patients undergoing total hip arthroplasty?.
      ]. Demonstrating this relationship, patients with limited spine mobility are not able to adequately increase SPT when transitioning from standing to sitting and thus need to recruit more hip flexion to move the femur from a vertical to horizontal position than healthy patients [
      • Esposito C.I.
      • Carroll K.M.
      • Sculco P.K.
      • Padgett D.E.
      • Jerabek S.A.
      • Mayman D.J.
      Total hip arthroplasty patients with fixed spinopelvic alignment are at higher risk of hip dislocation.
      ]. In addition, the relatively increased motion of the upper lumbar segments in obese patients from standing to sitting, indicated by the greater change in L1-L4 than that of L4-S1, is likely due to soft-tissue impingement around the groin crease in the sitting position.
      Applying our results clinically, obesity may provide a protective mechanism against risk of prosthetic impingement. Each increase in degree of SPT is concomitantly associated with 0.7° of acetabular component anteversion [
      • Maratt J.D.
      • Esposito C.I.
      • Mclawhorn A.S.
      • Jerabek S.A.
      • Padgett D.E.
      • Mayman D.J.
      Pelvic tilt in patients undergoing total hip arthroplasty: when does it matter?.
      ]. Increased acetabular anteversion has been shown to decrease the risk for anterior impingement and, ultimately, posterior dislocation [
      • Biedermann R.
      • Tonin A.
      • Krismer M.
      • Rachbauer F.
      • Eibl G.
      • Stöckl B.
      Reducing the risk of dislocation after total hip arthroplasty.
      ]. Thus, in planning THA, BMI may be an important factor in evaluating spinopelvic alignment and, ultimately, risk of impingement. Although obesity may protect against prosthetic impingement, obesity may be associated with overall increased THA instability risk, [
      • Chee Y.H.
      • Teoh K.H.
      • Sabnis B.M.
      • Ballantyne J.A.
      • Brenkel I.J.
      Total hip replacement in morbidly obese patients with osteoarthritis: results of a prospectively matched study.
      ,
      • Kim Y.
      • Morshed S.
      • Joseph T.
      • Bozic K.
      • Ries M.D.
      • Rothman R.H.
      Clinical impact of obesity on stability following revision total hip arthroplasty.
      ,
      • Davis A.M.
      • Wood A.M.
      • Keenan A.C.M.
      • Brenkel I.J.
      • Ballantyne J.A.
      Does body mass index affect clinical outcome post-operatively and at five years after primary unilateral total hip replacement performed for osteoarthritis? A multivariate analysis of prospective data.
      ] which may be attributed to multiple alternate factors. For example, Callanan et al. [
      • Callanan M.C.
      • Jarrett B.
      • Bragdon C.R.
      • et al.
      The John Charnley Award: risk factors for cup malpositioning: quality improvement through a joint registry at a tertiary hospital.
      ] found that BMI increases risk of cup malpositioning for abduction only or for abduction and version combined. It was postulated that the inaccuracy was due to increased adipose tissue, leading to reduced incision size field and difficulty locating anatomic landmarks. However, when controlling for other factors that influenced instability risk (high-volume surgeon and standard posterolateral approach), McArthur et al. [
      • Mcarthur B.A.
      • Vulcano E.
      • Cross M.
      • Nguyen J.
      • Valle A.G.D.
      • Salvati E.
      Acetabular component orientation in total hip arthroplasty: the impact of obesity.
      ] found no correlation between obesity and risk of malpositioning. Weight loss preoperatively may also help reduce the risk of prosthetic dislocation.
      Owing to the novelty of our field of study, we were unable to compare many of our findings to existing literature. In agreement with previous works, all groups adjusted to a change in alignment from standing to sitting using similar mechanisms, exhibiting increased SPT [
      • Schwab F.
      • Patel A.
      • Ungar B.
      • Farcy J.-P.
      • Lafage V.
      Adult spinal deformity—postoperative standing imbalance.
      ,
      • Carvalho D.E.D.
      • Soave D.
      • Ross K.
      • Callaghan J.P.
      Lumbar spine and pelvic posture between standing and sitting: a radiologic investigation including reliability and repeatability of the lumbar lordosis measure.
      ,
      • Lazennec J.-Y.
      • Brusson A.
      • Rousseau M.-A.
      Hip–spine relations and sagittal balance clinical consequences.
      ] and decreased LL [
      • Schwab F.
      • Patel A.
      • Ungar B.
      • Farcy J.-P.
      • Lafage V.
      Adult spinal deformity—postoperative standing imbalance.
      ]. However, previous reports on standing spinopelvic alignment in obese patients are conflicting. Corresponding to our results, Jalai et al. [
      • Jalai C.M.
      • Diebo B.G.
      • Cruz D.
      • et al.
      The impact of obesity on compensatory mechanisms in response to progressive sagittal malalignment.
      ] found no differences in standing SPT, PI-LL, and LL between obese and nonobese patients. Horn et al. [
      • Horn S.R.
      • Segreto F.A.
      • Ramchandran S.
      • et al.
      The influence of body mass index on achieving age-adjusted alignment goals in adult spinal deformity corrective surgery with full-body analysis at 1 year.
      ] also found no differences in standing SPT but significantly higher standing PI-LL in obese patients than in normal-weight patients. In contrast to our results, there are reports of both higher [
      • Youdas J.W.
      • Garrett T.R.
      • Egan K.S.
      • Therneau T.M.
      Lumbar lordosis and pelvic inclination in adults with chronic low back pain.
      ,
      • Kulcheski A.L.
      • Graells X.S.
      • Benato M.L.
      • Baretta G.
      Evaluation of angular sagittal balance in obese patients.
      ,
      • Araújo F.
      • Lucas R.
      What do we know about the determinants of sagittal standing posture?.
      ] and lower [
      • Boulay C.
      • Tardieu C.
      • Hecquet J.
      • et al.
      Sagittal alignment of spine and pelvis regulated by pelvic incidence: standard values and prediction of lordosis.
      ] standing SPT in obese patients than in normal-weight patients. Several studies have also demonstrated increased standing LL in obese patients compared with that in normal-weight patients [
      • Boulay C.
      • Tardieu C.
      • Hecquet J.
      • et al.
      Sagittal alignment of spine and pelvis regulated by pelvic incidence: standard values and prediction of lordosis.
      ,
      • Onyemaechi N.
      • Anyanwu G.
      • Obikili E.
      • Onwuasoigwe O.
      • Nwankwo O.
      Impact of overweight and obesity on the musculoskeletal system using lumbosacral angles.
      ].
      Comparing existing reports on sitting-to-standing spinopelvic alignment, the correlation between reduced hip flexion and obesity has been validated by Yeung et al. [
      • Yeung E.
      • Jackson M.
      • Sexton S.
      • Walter W.
      • Zicat B.
      • Walter W.
      The effect of obesity on the outcome of hip and knee arthroplasty.
      ]. Furthermore, Sparrey et al. claimed that BMI does not appear to correlate with LL but limits lumbar range of motion, especially in sitting positions [
      • Maratt J.D.
      • Esposito C.I.
      • Mclawhorn A.S.
      • Jerabek S.A.
      • Padgett D.E.
      • Mayman D.J.
      Pelvic tilt in patients undergoing total hip arthroplasty: when does it matter?.
      ,
      • Sparrey C.J.
      • Bailey J.F.
      • Safaee M.
      • et al.
      Etiology of lumbar lordosis and its pathophysiology: a review of the evolution of lumbar lordosis, and the mechanics and biology of lumbar degeneration.
      ,
      • Lord M.J.
      • Small J.M.
      • Dinsay J.M.
      • Watkins R.G.
      Lumbar Lordosis.
      ].

      Limitations

      Although we minimized confounding variables by matching lumbar flatback and DDD rates across groups, as well as hip ostoarthritis grade, other characteristics associated with spine diseases may exist between groups. In addition, other unaccounted spine conditions may correlate with spinopelvic alignment changes such as disc herniation [
      • Barrey C.
      • Jund J.
      • Noseda O.
      • Roussouly P.
      Sagittal balance of the pelvis-spine complex and lumbar degenerative diseases. A comparative study about 85 cases.
      ] or laminectomy [
      • Maratt J.D.
      • Esposito C.I.
      • Mclawhorn A.S.
      • Jerabek S.A.
      • Padgett D.E.
      • Mayman D.J.
      Pelvic tilt in patients undergoing total hip arthroplasty: when does it matter?.
      ] which cannot be accounted for in this analysis.
      BMI does not distinguish between soft-tissue type and distribution. Given that soft-tissue impingement in obese patients could be a contributing factor to changes in spinopelvic parameters, future studies may be able to study more direct relationships by stratifying patients based on soft-tissue distribution instead of BMI.
      Moreover, our study only analyzed alignment parameters at one cross-section of time. In the future, analysis of spinopelvic alignment parameters over time, including post-THA, would allow us to track progressive changes in alignment which may help us better understand hip instability.

      Conclusions

      Significant differences were noted in sitting alignment and the change in alignment from sitting to standing based on BMI. The results of this study suggest that obese patients recruit more posterior PT when transitioning from standing to sitting to compensate for soft-tissue impingement that occurs anterior to the hip joint during hip flexion.

      Appendix A. Supplementary data

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