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A Preoperative Workup of a “Hip-Spine” Total Hip Arthroplasty Patient: A Simplified Approach to a Complex Problem
The Journal of Arthroplasty 34 (2019) S57eS70
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The Journal of Arthroplasty journal homepage: www.arthroplastyjournal.org
2018 AAHKS Annual Meeting Symposium
A Preoperative Workup of a “Hip-Spine” Total Hip Arthroplasty Patient: A Simplified Approach to a Complex Problem Tyler A. Luthringer, MD a, *, Jonathan M. Vigdorchik, MD b a b
Department of Orthopedic Surgery, NYU Langone Orthopedic Hospital, New York, NY Adult Reconstruction and Joint Replacement Service, Hospital for Special Surgery, New York, NY
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 December 2018 Accepted 7 January 2019 Available online 18 January 2019
Background: A large body of evidence has confirmed that patients with spinal deformity, lumbar fusion, and abnormal spinopelvic mobility are at significantly increased risk for instability, dislocation, and revision after total hip arthroplasty (THA). Methods: Achieving a stable construct in patients with pre-existing spine disease requires an understanding of basic spinopelvic parameters and the compensatory mechanisms associated with abnormal spinopelvic motion. Indicated patients with concomitant hip-spine pathology should be assessed for (1) the presence of spinal deformity and (2) the presence of spinal stiffness before undergoing THA. Preoperative imaging should include a standing anteroposterior pelvis x-ray, as well as two lateral spinopelvic radiographs in the standing and seated position. Results: Based on the presence of spinal deformity and/or spinal stiffness, patients may be categorized as one of the four groups of the “Hip-Spine Classification in THA.” A series of illustrative case examples is provided. Conclusion: A simple three-step assessment with minimal measurements will effectively identify the complex “hip-spine” THA patient at high risk for postoperative instability. Adhering to group-specific recommendations for acetabular cup position can help to further reduce the burden of instability and related revisions in this challenging population. © 2019 Elsevier Inc. All rights reserved.
Keywords: total hip arthroplasty instability spinal deformity spinopelvic mobility hip-spine relationship
Despite the overall success and high patient satisfaction after total hip arthroplasty (THA), instability remains a costly and difficult problem with negative implications on quality of life. The incidence of dislocation after primary THA is approximately 2% with use of contemporary implants, exposures, and softtissue balancing [1], yet instability remains the leading cause for revision THA in the United States [2,3]. Greater emphasis on patient risk-stratification, surgical technique, and component positioning is required to further reduce the burden of
Source of Funding: No external funds were received in support of this work. Institutional review board approval: Not applicable or required for this review. One or more of the authors of this paper have disclosed potential or pertinent conflicts of interest, which may include receipt of payment, either direct or indirect, institutional support, or association with an entity in the biomedical field which may be perceived to have potential conflict of interest with this work. For full disclosure statements refer to https://doi.org/10.1016/j.arth.2019.01.012. * Reprint requests: Tyler A. Luthringer, MD, NYU Langone Orthopedic Hospital, 301 East 17th Street, 14th Floor, New York, NY 10003. https://doi.org/10.1016/j.arth.2019.01.012 0883-5403/© 2019 Elsevier Inc. All rights reserved.
dislocation and revisions related to this complex multifactorial problem [1]. Since 1978, the range of optimal acetabular component positions to minimize dislocation risk has been defined by the “safe zones” set forth by Lewinnek et al [4]. Several authors have recently demonstrated that dislocation most often occurs within these previously defined “safe zones” [5e7], calling to question the universal application of these parameters. With use of modern implant designs, most atraumatic dislocations actually occur secondary to altered spinopelvic kinematics that lead to a functionally unsafe acetabular position [8,9]. We now recognize a shortcoming of the protective “safe zone” defined by Lewinnek et al on a single radiograph that is it assumes the acetabular component to be constant and static. A growing body of literature on the dynamic relationship between the hip, pelvis, and spine has revealed that acetabular component orientation changes with rotation of the pelvis through alterations in posture [10e13]. Furthermore, spinopelvic mobility may limit (or enhance) the magnitude of pelvic rotation that a given individual can assume in positions of sitting or standing [14,15]. Patients with decreased
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relationship on THA stability in their preoperative assessment and surgical plans. Obtaining a stable THA, particularly in patients with pre-existing spine disease, is dependent on an understanding of spinopelvic parameters and compensatory mechanisms in those with abnormal spinopelvic motion. We are typically asked, “where do I put the cup?” It is important we think to first answer the questions, “what imaging are you willing to get to find out that appropriate cup position,” and then, “what tools or landmarks are you going to use to get it there?” There are commercially available systems that can output exact predictions for functional cup position after a series of three x-rays, and there are multiple commercially available computer navigation tools, robots, smart instruments, and patient-specific instrumentation guides that can help deliver that target intraoperatively (though some require a computed tomography scan). This article outlines the authors’ simplified approach to the preoperative workup of a “hip-spine” THA candidate with the addition of two x-rays to standard clinical practice. While we recognize this as a simplified method, it can be easily reproduced and is a first great step to the understanding of the hip-spine relationship in THA. The following preoperative workup was presented as part of the 2018 American Association of Hip and Knee Surgeons Annual Meeting Symposium: “Simplifying the Concepts of the Hip-Spine Relationship for THA.” Important Definitions
Fig. 1. Anterior pelvic plane (APP) (yellow) and sacral slope (SS) (red) outlined on lateral standing radiograph.
spinal mobility due to arthritis, sagittal spinal deformity, and spinal fusion are at increased risk for functionally malpositioned acetabular components, instability, and need for revision surgery [15e19]. A history of spinal fusion was recently cited as the strongest independent risk factor for early dislocation after THA [20], imparting greater than two- and three-fold risk for dislocation and revision, respectively [21]. As hip and spine pathology often coexist [22], arthroplasty surgeons must begin to consider the implications of the hip-spine
Until recently, there has been a lack of standardization of the language used by spine and arthroplasty surgeons when defining spinopelvic parameters in the literature. This disconnect has limited the utility of much of the available evidence for community and academic hip surgeons alike. At the 2018 American Academy of Orthopedic Surgeons (AAOS) Annual Meeting, a newly conceptualized “Hip-Spine Workgroup” met for the first time to help standardize terminology across the literature and simplify the spinopelvic relationship for arthroplasty surgeons. The following terms and radiographic reference lines are used in the authors’ preferred approach to the preoperative workup of a “hip-spine” patient. The hip arthroplasty surgeon must first understand these relevant parameters to conceptualize the implications of spinopelvic mobility on THA construct stability.
Fig. 2. Neutral, anterior, and posterior pelvic tilt of the anterior pelvic plane (APP) (yellow), also termed as anterior pelvic plane tilt (APPt).
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The anterior pelvic plane (APP) is defined by the plane between the two anterior superior iliac spines (ASISs) and the pubic symphysis on a lateral pelvic radiograph (Fig. 1). On an imperfect lateral image where the ASISs are not superimposed, the superior end of the APP intersects the horizontal midpoint between the left and right ASIS (Fig. 1). The anterior pelvic plane tilt (APPt) is the measured angle created between the APP and a vertical reference line and may provide a quantitative assessment of pelvic tilt if preferred by the surgeon. The authors’ preferred method is to qualitatively assess for the presence of neutral, anterior, or posterior pelvic tilt of the APP relative to the coronal (functional) plane (Fig. 2). Is it important to note that the coronal (functional) plane is constant and should serve as the typical reference for acetabular cup inclination and anteversion. Evaluation of the APP (or APPt) is critical to goal 1 of the preoperative workupdidentification of spinal deformity. Sacral slope (SS) is defined at the angle subtended by a line parallel to the superior endplate of S1 and a horizontal reference line. The SS may be qualitatively assessed by simply gauging the slope of the superior endplate of S1 relative to the horizontal plane of the bottom of the radiograph (Fig. 1). Both APPt and SS may be used to assess spinopelvic kinematics and pelvic mobility between postures. To identify patients with spinal stiffness in goal 2 of the preoperative workup, the authors prefer to use SS. Pelvic incidenceelumbar lordosis mismatch (PI-LL mismatch) is one parameter commonly used by spine surgeons to define sagittal spine deformity. Although PI-LL mismatch may be an advanced concept in the eyes of the arthroplasty surgeon, its utility in identifying patients with “flatback deformity” is significant and worth mentioning. To understand the concept of PI-LL mismatch, PI must be explained first. PI is a morphologic parameter that remains constant throughout spinopelvic motion; it is not subject to change secondary to degenerative disease and remains unchanged throughout adulthood. PI is measured as the angle between a line drawn from the center of the femoral heads to the center of the S1 endplate and a second line drawn perpendicular to the S1 endplate (Fig. 3). PI is mathematically defined by the formula: PI ¼ SPT þ SS, where spinopelvic tilt (SPT) is an alternative measurement of pelvic tilt used more commonly by spine surgeons, as opposed to APPt described previously. SS, SPT, and APPt are all reliable means to quantify pelvic movement between sagittal postures. Throughout spinopelvic motion, PI remains constant. Any change in pelvic tilt (SPT or APPt) must be accompanied by an inverse change in SS, which directly correlates to associated changes in LL to maintain upright posture. LL (also known as the lumbar lordotic angle) is the angle subtended by two lines drawn at the superior endplates of L1 and S1 (Fig. 3). When PI-LL mismatch (defined as PI minus LL) is >10 , arthroplasty surgeons should be aware of the presence of “flatback spinal deformity.” Workup When indicating a patient with concomitant hip-spine pathology for THA, the goals of the preoperative workup are twofold: (1) to identify the presence of spinal deformity (yes or no) and (2) to identify the presence of spinal stiffness (yes or no). You can then classify each patient according to the Hip-Spine Classification in THA as follows: 1dNormal spinal alignment (defined by PI-LL ± 10 ). 2dFlatback deformity (PI-LL > 10 ). AdNormal spinal mobility (defined as >10 change in SS from stand to sit). BdStiff spine (defined as <10 change in SS from stand to sit).
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Fig. 3. Pelvic incidence (yellow) and lumbar lordosis (red) used in the assessment for PI-LL mismatch. PI-LL, pelvic incidence-lumbar lordosis.
Besides being higher risk for dislocation, the presence of spinal deformity is important for acetabular component positioning. It is important to consider that when talking about acetabular anteversion, let us say for example 20 , that is, 20 of anteversion relative to something which is 0 . Defining that 0 plane is of utmost importance. Classical methods of acetabular instrumentation define anatomic landmarks such as the transverse acetabular ligament, and current versions of computer navigation programs reference the APP, with the APP being the 0 reference. In patients with no spinal deformity, the APP tends to be neutral, aligned with the coronal plane of the body (Fig. 2). However, in the presence of a spinal deformity, as seen on the standing lateral x-ray to be defined below, the APP reference plane could be functionally defined in a different position (ie, tilted compared to the coronal plane). This makes the APP an unreliable reference for acetabular component positioning in this population. For each degree of increased posterior pelvic tilt, the surgeon can predict a concomitant increase in functional acetabular anteversion of approximately 0.7 -0.8 [11,23,24]. Therefore, the 0 reference plane needs to be the coronal plane of the body or the “functional pelvic plane” (FPP) [25,26].
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Fig. 4. Standing and sitting lateral radiographs of a patient with normal spinal pelvic mobility. When the patient sits, lumbar lordosis decreases and the pelvis “rolls back,” which is demonstrated by an increase in posterior pelvic tilt (APP, yellow) and a flattening of SS (red).
As for spinal stiffness, in this study, we are looking at the mobility of the pelvis when the patient moves from the standing to the sitting position. It is important to understand that the seated position is achieved through a combination of motion from the lumbosacral spine and femoroacetabular articulation (hip joint). When going from standing to sitting position, normal spinopelvic motion permits a decrease in LL and allows the pelvis to “roll back” (which equates to 20 -35 of posterior pelvic tilt and a decrease or flattening of SS) [10,27]. As the pelvis “rolls back,” there is more space for the femur to flex (due to increased clearance of the anterior acetabular rim) (Fig. 4). If the spine is stiff, the pelvis does not move, and the femur must flex more to achieve the seated position [28]. This preferential increase in femur flexion (relative to a somewhat static anterior acetabular rim) causes a higher risk of impingement (Fig. 5). Therefore, in the stiff
spine, more anteversion is needed in the acetabular component to compensate for limited pelvic “roll back” when seated. One caveat is that mentioned in the previous paragraph, which reminds us that anteversion is relative to the 0 reference plane. Standard history and physical examination will alert the surgeon to the possibility of spinal pathology or abnormal mobility in a given patient. Patients with normal spinopelvic anatomy stand with neutral pelvic tilt, and the ASISs are aligned over the pubic symphysis in line with the functional or coronal plane. Patients with anterior pelvic tilt will stand with increased LL; this deformity may also be noticed with the patient lying down, exhibited by increased space between the arch of the lower back and the underlying examination table. On the other hand, those with posterior pelvic tilt (decreased or more horizontal SS) tend to stand with a decreased LL and are
Fig. 5. Standing and sitting lateral radiographs of a patient with a stiff spine who underwent revision of the shown construct for asymmetric polyethylene wear, osteolysis, and posterior instability. Note the lack of “pelvic rollback” in the seated position (no change in SS, red) and the proximity of the flexing proximal femur to the anterior acetabular rim. APP (yellow).
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Fig. 6. Supine (left) and standing (right) postoperative AP pelvis radiographs of a patient with unrecognized spinal deformity (posterior pelvic tilt) who underwent primary THA templated on a supine AP pelvis. Intraoperatively, the cup was placed in 25 of anteversion relative to the APP, which increased to 41 of functional anteversion relative to the coronal plane (FPP) when standing. This patient experienced an anterior dislocation 2 wk postoperatively. AP, anteroposterior.
more likely to lie down with no space between the lower back and examination table (Fig. 2). When posterior pelvic tilt is severe, patients may be described as having a “flatback deformity.” These extremely high-risk patients commonly developed hip flexion contractures and stand with the hips and knees flexed as compensatory mechanisms to maintain forward gaze. Radiographic evaluation of THA candidates routinely begins with an anteroposterior (AP) pelvis x-ray. Even in the absence of gross
spinal pathology, the natural tilt of the pelvis varies between individuals in the standing position and may differ considerably from that while supine (the position of most preoperative AP pelvis radiographs) [11,13]. A standing AP pelvis is strongly recommended as it better represents the functional position of the pelvis during activity. Two additional lateral spinopelvic views (pelvis from L1 to below the pubic symphysis) in the sitting and standing positions are needed to systematically evaluate the lumbar spine and spinopelvic
Fig. 7. (A) Patients with posterior pelvic tilt (APP, yellow) will increase functional anteversion of the cup when standing. (B) Patients with anterior pelvic tilt (APP, yellow) will decrease functional anteversion of the cup when standing.
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Fig. 8. (A) Preoperative standing lateral image of a patient with severe posterior pelvic tilt (APP, yellow). (B) Templated cup position of 40 of inclination and 20 of anteversion relative to the APP (or traditional bony landmarks intraoperatively) leads to functional cup position of 45 of inclination and 38 of anteversion when the patient stands. (C) After accommodating for the patient’s posterior pelvic tilt in the functional (standing) position, placement of the cup in 35 of inclination and 2 of anteversion relative to the APP will lead to a cup position of 40 of functional inclination and 20 of functional anteversion relative to the coronal plane when standing.
mobility. For the lateral sitting x-ray, the authors routinely use the “relaxed” seated position with the patient sitting upright and the femurs parallel to the floor. The AP and lateral x-rays can be normal flat plate radiographs or stereoradiographs (EOS Imaging, Paris, France). Standard 36-inch cassettes are sufficient if stereoradiography is unavailable to the surgeon [10]. Goal 1: Identify Patients With Spinal Deformity Step 1: Standing AP Pelvis X-Ray The standing AP pelvis x-ray should be scrutinized for any apparent abnormalities in pelvic tilt, obliquity, or rotation. Making sure that this is a normal AP pelvis (not a low AP pelvis) including the lumbar spine is helpful in identifying lumbar degenerative abnormalities or evidence of prior surgery. Any abnormalities in the standing AP pelvis view should prompt further evaluation with additional spinopelvic imaging. The surgeon should note that functional position of the acetabular cup will be dependent on the pelvic orientation when the individual stands, which may be entirely different from that while supine. Figure 6 shows the supine and standing images of a patient who had an anterior dislocation within 2 weeks postoperatively. Initially, the construct was templated using a supine AP pelvis x-ray and accurately reproduced intraoperatively with 42 of acetabular cup inclination and 25 of anteversion. After the patient’s early anterior dislocation, a standing AP pelvis stereograph revealed the functional position of the acetabular component to be 62 of inclination and 41 of anteversion. Hence, the authors strongly recommend utilizing a standing AP pelvis x-ray when templating preoperatively. Step 2: Standing Lateral Spinopelvic X-Ray The APP is drawn on the standing lateral spinopelvic image to confirm the presence or absence of spinal deformity. The APPt (or pelvic tilt) should be identified as being neutral, tilted posterior, or tilted anterior. In patients with no deformity, the APP will be
vertical (neutral pelvic tilt) or parallel to the coronal (functional) plane of the body. Thus, the position of the cup in the functional standing position postoperatively will be the same as that referenced from the coronal plane of the body during cup implantation. As the implanted acetabular component and pelvis move as a single unit, patients with posterior pelvic tilt will increase the functional anteversion of the cup while standing postoperatively (Fig. 7a). Conversely, anterior pelvic tilt will cause patients to functionally decrease cup anteversion in the standing position (Fig. 7b). Figure 8a-c demonstrate the importance of considering APPt relative to the coronal (functional) plane of the body when planning cup position. Note that this is an extreme example for illustrative purposes. By failing to account for the patient’s posterior tilt while standing (Fig. 8a), the surgeon assumes that the APP is neutral compared with the coronal (functional) plane of the body. Thus, templated cup position of 40 of inclination and 20 of anteversion relative to the APP (or traditional bony landmarks intraoperatively) will actually lead to 45 of functional inclination and 38 of functional anteversion relative to the coronal plane when the patient stands (Fig. 8b). By accommodating for the deformity and positioning the cup in 35 of inclination and 2 of anteversion relative to the APP, the acetabular component will be in 40 of functional inclination and 20 of functional anteversion relative to the coronal plane in the standing position (Fig. 8c).
Advanced Concepts: PI-LL Mismatch Despite the fact that measuring the APP and APPt is an easy concept to look for spinal deformity, it has its flaws. The best way to look for spinal deformity is by measuring the PI minus LL (PI-LL mismatch). A PI-LL mismatch of >10 measured on the standing lateral spinopelvic image should alert the surgeon to be aware of the presence of a “flatback spinal deformity” (Fig. 9). These patients have abnormal spinopelvic mechanics from when they stand to when they sit. Therefore, in the presence of these abnormalities, surgeons should pursue functional imaging (standing
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Fig. 9. Standing and sitting lateral radiographs of a patient with flatback deformity (PI-LL mismatch >10 ). Pelvic incidence (yellow) and lumbar lordosis (red) are shown on the standing image. Sacral slope (red) is drawn along the superior endplate of S1 on the sitting image for comparison.
and sitting lateral spinopelvic views) to ensure appropriate evaluation.
segment instrumented fusions (even to S1) may still maintain spinopelvic mobility (Fig. 10c).
Goal 2: Identify Patients With Spinal Stiffness
Advanced Concepts: Functional X-Rays Functional x-rays to mimic the anatomic position in which THA dislocation may occur can be obtained for patients noted to have stiff spines who may be particularly at high risk for instability. Posterior dislocations tend to occur when getting up from the seated position and when the body leans forward and assumes maximum hip flexion. The anterior proximal femur ultimately impinges on the anterior rim of the cup, acting as a lever for posterior dislocation. Thus, an x-ray can be taken with the patient in a flexed seated (rise above to stand up from sitting) position, and cup placement can be planned accordingly (Fig. 11) [26]. Anterior dislocations occur in hip extension, typically when walking or getting in or out of bed. An x-ray may be taken with the patient stepping up onto a raise surface with the femur paralleling the floor, which will
Step 3: Compare Standing versus Sitting Lateral Spinopelvic X-Ray Assessment of SS on standing and sitting lateral radiographs is used to gauge the patient’s spinopelvic mobility. A change of <10 in the SS between the standing and sitting positions defines the presence of “spinal stiffness.” However, simply annotating a line along the superior S1 endplate on side-by-side standing and sitting lateral spinopelvic images offers a quick and simple visual comparison (Fig. 10a-c). The objective SS relative to a horizontal reference (and the subsequent change from standing to sitting) should be measured. It is important to recognize that patients with no history of instrumented fusion may still have stiff spines secondary to degenerative spine disease (Fig. 10b) and that patients with short
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Fig. 10. (A) Visual comparison of the sacral slope (SS, red) between the standing and sitting position in a patient with no spinal stiffness (>10 change). (B) Visual comparison of the SS (red) between the standing and sitting position in a patient with a stiff spine (<10 change). (C) Visual comparison of the SS (red) between the standing and sitting position in a patient with a history of L5-S1 fusion yet does not have spinal stiffness (>10 change). APP (yellow).
cause the pelvis to rotate posteriorly and extend the hip joint of the limb planted on the floor (Fig. 12) [26]. Excessive acetabular anteversion in this position will increase the risk of anterior dislocation. Treatment Although outside of the original scope of the American Association of Hip and Knee Surgeons symposium presentation on the preoperative assessment of hip-spine THA patients, the treatment algorithm that follows the aforementioned workup is outlined in Table 1. Group 1A patients have normal sagittal alignment (neutral pelvic tilt) and normal spinopelvic mobility (“pelvic rollback”) when going from standing to sitting. In these individuals, the FPP when standing is the same as the APP. Thus, native anatomic landmarks may be referenced intraoperatively for traditional cup positioning in 20 to 25 of anteversion. Group 1B patients also have normal sagittal alignment (neutral pelvic tilt), so FPP is again similar to the APP. However, group 1B patients have limited “pelvic
rollback” when assuming the seated position due to spinal stiffness (<10 change in SS from stand to sit). To avoid impingement of the flexing proximal femur on the anterior acetabular rim when sitting, 30 of anteversion templated on the standing AP pelvis should be targeted. Increased anteversion protects against the risk of posterior dislocation in this group. All the patients in group 2 are classified as having sagittal flatback deformity (PI-LL mismatch >10 ); these patients stand with posterior pelvic tiltdthe APP is different from the FPP. It is crucial to recognize that this posterior pelvic tilt (of the APP) will increase the functional cup anteversion relative to the coronal plane while standing (FPP). After impaction, the cup and pelvis move as a single unit. If the cup is anteverted relative to the APP (bony landmarks), when the patient stands, it will tilt posteriorly with the pelvis, causing the face of the cup to open anteriorly, increasing anteversion relative to the coronal plane (FPP). Thus, anteversion targets must be referenced from the FPP (on the standing AP pelvis) and not from the APP. Failure to do so may result in excessive functional anteversion while standing and increase the risk of
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Fig. 10. (continued).
anterior dislocation (Fig. 6). It is important to recognize that in some extreme cases, cups that are appropriately anteverted relative to the FPP may appear to have insufficient anteversion, or even retroversion, relative to the APP or traditional bony landmarks intraoperatively (Fig. 8a-c).
Group 2A patients have flatback deformity and normal spinal mobility (>10 change in SS from stand to sit); anteversion should be targeted 25 -30 from the FPP (on the stand AP pelvis). Group 2B patients have flatback deformity and a stiff spine (<10 change in SS from stand to sit) so even more anteversion is needed to protect
Fig. 11. Example of patient position and a maximum forward-flexed seated radiograph.
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Fig. 12. Example of patient position and a lateral step-up spinopelvic radiograph.
from posterior dislocation. Thirty-degrees of anteversion should be targeted on the standing AP pelvis (FPP); providers must be aware of the reference plane as the presence of spinal deformity will cause more functional cup anteversion as described previously. Group 2B individuals are at the highest risk for instability due to a very narrow safe zone of functional cup position; these patients are strong candidates for dual-mobility prostheses. Case Example 1
Table 1 The Four Categories of the Hip-Spine Classification in THA and the Necessary Acetabular Position for Each. Group
Classification
Explanation of Pathology
Treatment Recommendation
1A
Normal alignment normal mobility
Target native anatomy of 20 -25
1B
Normal alignment stiff spine (<10 change in sacral slope from stand to sit)
Normal anatomy and normal mobility (APP similar to FPP) Stiff spine, needs more anteversion relative to the APP which is similar to the FPP
2A
Flatback deformity (PI-LL>10 ) normal mobility
2Ba
Flatback deformity (PI-LL>10 ) stiff spine (<10 change in sacral slope from stand to sit)
Step 1 The supine and standing AP pelvis (Fig. 13a) x-rays show no abnormalities. Step 2 The standing lateral (Fig. 13b) shows an APP with no tilt, confirming no spinal deformity. Step 3 The sitting lateral (Fig. 13b) shows >10 change in SS, confirming no spinal stiffness. Goal 1: Spinal deformity? No. Goal 2: Spinal stiffness? No. Action: proceed with standard THA component position of 40 of inclination and 20 -25 of anteversion. This patient underwent uncomplicated primary THA with imageless computer navigation to achieve traditional component
Beware, the FPP is different from the APP (posterior pelvic tilt from the spinal deformity causes more functional cup anteversion) FPP is different from the APP and a stiff spine, so you need more anteversion, but be aware of the reference because the spinal deformity will cause more functional cup anteversion
Target 30 anteversion on the standing AP pelvis. Note the higher target due to the stiff spine to protect from a posterior dislocation. Target 25 -30 anteversion on the standing AP pelvis (to the functional pelvic plane)
Target 30 on the standing AP pelvis (to the FPP). Note the higher target due to the stiff spine to protect from a posterior dislocation.
Anteversion targets refer to Murray's radiographic definition. AP, anteroposterior; APP, anterior pelvic plane; FPP, functional pelvic plane; PI-LL, pelvic incidenceelumbar lordosis. a Most difficult to treat, very narrow window of safe zone.
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Fig. 13. (A) Case example 1: supine (left) and standing AP (right) pelvis x-rays reveal no abnormalities. (B) Case example 1: standing lateral x-ray demonstrating an APP with no tilt (yellow), and sitting lateral demonstrating >10 change in SS (red). (C) Case example 1: postoperative standing AP pelvis and cross-table lateral x-ray of the operative right hip. AP, anteroposterior.
position targets. The patient has experienced no episodes of instability within the first 4 years postoperatively (Fig. 13c). Case Example 2 Step 1 The supine and standing (Fig. 14a) AP pelvis x-rays are noted to be very similar, suggesting the absence of deformity. Step 2 The APP is drawn on the lateral standing x-ray. Very minimal posterior tilt is noted, confirming that there is no spinal deformity (Fig.14b).
Step 3 The SS is compared on the standing versus the sitting lateral images, noting minimal (<10 ) change between positions, confirming the presence of a stiff spine (Fig. 14b). Goal 1: Spinal deformity? No. Goal 2: Spinal stiffness? Yes. Action: add anteversion. This patient underwent primary THA with 30 of targeted anteversion (relative to functional coronal plane when standing) achieved intraoperatively with imageless computer navigation. The patient is doing well without episodes of instability 5 years postoperatively (Fig. 14c).
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Fig. 14. (A) Case example 2: minimal difference is noted between the supine (left) and standing (right) AP pelvis x-rays. (B) Case example 2: minimal posterior tilt of the APP (yellow) and <10 change in SS between the standing and sitting lateral x-rays (red). (C) Case example 2: postoperative standing AP pelvis and cross-table lateral x-ray of the operative right hip. AP, anteroposterior.
Case Example 3 Step 1 Compared with the supine AP pelvis, the standing AP pelvis xray (Fig. 15a) is noted to look like an outlet view (posterior pelvic tilt). Therefore, additional hip-spine imaging is warranted. Step 2 The APP is drawn on the lateral standing x-ray (Fig. 15b) showing posterior pelvic tilt signifying spinal deformity. Step 3 No change in SS on the standing versus the sitting lateral (Fig. 15c) images, confirming the presence of a stiff spine. Goal 1: Spinal deformity? Yes. Goal 2: Spinal stiffness? Yes.
Action: add anteversion, but be careful of your 0 reference plane. Consider the use of dual-mobility prosthesis. This patient underwent primary THA with 30 of targeted anteversion (to the functional coronal plane in standing) achieved intraoperatively with robotic-assisted navigation. A dual-mobility device was used given the high-risk nature of having a spinal deformity and spinal stiffness. The patient is doing well with no episodes of instability 5 years postoperatively (Fig. 15d). Discussion Patients with concomitant hip and spine pathology must be appropriately assessed for the presence of deformity and abnormal spinopelvic mobility when planning for THA. Adult spinal deformity patients have been shown to have an 8% dislocation rate after THA compared to 1.5% in normal controls [17]. Spine stiffness, whether secondary to instrumented or biologic fusion (spondylosis), imparts an equally great risk of instability after THA. A Medicare database study of 2912 patients who had undergone both
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Fig. 15. (A) Case example 3: supine (left) and standing (right) AP pelvis x-rays are compared; the standing AP pelvis resembles an outlet view, suggesting posterior pelvic tilt. (B) Case example 3: the APP (yellow) is drawn on the lateral standing x-rays, demonstrating 16 of posterior pelvic tilt. (C) Case example 3: side-by-side comparison of the standing (left) versus sitting (right) lateral images demonstrating no change in SS (red). (D) Case example 3: postoperative standing AP pelvis and cross-table lateral x-ray of the operative left hip with a dual-mobility prosthesis and 29 of targeted acetabular anteversion (relative to the functional coronal plane while standing). AP, anteroposterior.
lumbar spinal fusion and THA demonstrated that the risk of dislocation increases with the number of vertebral levels fused (or increase in spinal stiffness) [16]. The dislocation rate of patients with a 1-2 level and 3-7 level lumbar fusion was found to be 2.73% and 4.62%, respectively, compared with 1.55% in patients without spinal fusion [16]. The preoperative imaging protocol described in this study has been implemented as part of a new risk assessment score and treatment algorithm for instability-prone patients undergoing primary THA [29]. In a series of 1009 cases, patients were designated as being at low, intermediate, or high risk for dislocation. Those with history of spinal fusion of three or more levels and patients with spinal deformity (defined at PI-LL mismatch >10 ) with no change in SS between standing and sitting positions were identified as being at high risk (n ¼ 192). After implementation of the imaging protocol and a risk-stratified treatment algorithm, dislocation rates decreased from 3.1% to 0.5% among the high-risk cohort [29]. A similar approach to the evaluation of the spine has been applied to patients undergoing revision THA for recurrent instability [30]. One hundred eleven patients were prospectively analyzed for the presence of spinal deformity and spinal stiffness with the previously performed spinopelvic imaging and scored according to the “Hip-Spine Classification in Revision THA.” Following a classification-based surgical treatment algorithm, 2year dislocation-free survivorship was 97% among the protocol group compared to 84% in matched controls [30]. Furthermore, 77% of the inappropriately positioned acetabular components identified by the spinopelvic imaging protocol would have gone unrecognized by supine AP pelvis imaging alone [30]. To further reduce the burden of THA instability and related revisions, indicated patients should be assessed with functional spinopelvic imaging and a standing AP pelvis radiograph. Spinal deformity and spinal stiffness can be easily identified preoperatively by the untrained eye with the addition of two lateral spinopelvic x-rays (sitting and standing) and minimal measurements. This three-step assessment has been effective in identifying and
managing high-risk THA candidates and has become our standardof-care in the preoperative setting. References [1] Goel A, Lau EC, Ong KL, Berry DJ, Malkani AL. Dislocation rates following primary total hip arthroplasty have plateaued in the Medicare population. J Arthroplasty 2015;30:743e6. https://doi.org/10.1016/j.arth.2014.11.012. [2] Bozic KJ, Kurtz SM, Lau E, Ong K, Vail TP, Berry DJ. The epidemiology of revision total hip arthroplasty in the United States. J Bone Joint Surg Am 2009;91:128e33. https://doi.org/10.2106/JBJS.H.00155. [3] Gwam CU, Mistry JB, Mohamed NS, Thomas M, Bigart KC, Mont MA, et al. Current epidemiology of revision total hip arthroplasty in the United States: national inpatient sample 2009 to 2013. J Arthroplasty 2017;32:2088e92. https://doi.org/10.1016/j.arth.2017.02.046. [4] Lewinnek GE, Lewis JL, Tarr R, Compere CL, Zimmerman JR. Dislocations after total hip-replacement arthroplasties. J Bone Joint Surg Am 1978;60:217e20. [5] Abdel MP, von Roth P, Jennings MT, Hanssen AD, Pagnano MW. What safe zone? The vast majority of dislocated THAs are within the Lewinnek safe zone for acetabular component position. Clin Orthop Relat Res 2016;474:386e91. https://doi.org/10.1007/s11999-015-4432-5. [6] Esposito CI, Gladnick BP, Lee Y-Y, Lyman S, Wright TM, Mayman DJ, et al. Cup position alone does not predict risk of dislocation after hip arthroplasty. J Arthroplasty 2015;30:109e13. https://doi.org/10.1016/j.arth.2014.07.009. [7] Seagrave KG, Troelsen A, Malchau H, Husted H, Gromov K. Acetabular cup position and risk of dislocation in primary total hip arthroplasty A systematic review of the literature. Acta Orthop 2017;88:10e7. https://doi.org/10.1080/ 17453674.2016.1251255. Y. Mathematical evaluation of [8] Sariali E, Lazennec JY, Khiami F, Catonne jumping distance in total hip arthroplasty: influence of abduction angle, femoral head offset, and head diameter. Acta Orthop 2009;80:277e82. https://doi.org/10.3109/17453670902988378. [9] Miki H, Kyo T, Kuroda Y, Nakahara I, Sugano N. Risk of edge-loading and prosthesis impingement due to posterior pelvic tilting after total hip arthroplasty. Clin Biomech 2014;29:607e13. https://doi.org/10.1016/j.clinbiomech.2014.05.002. [10] Kanawade V, Dorr LD, Wan Z, Wan Z. Predictability of acetabular component angular change with postural shift from standing to sitting position. J Bone Joint Surg Am 2014;96:978e86. https://doi.org/10.2106/JBJS.M.00765. [11] Maratt JD, Esposito CI, Mclawhorn AS, Jerabek SA, Padgett DE, Mayman DJ, et al. Pelvic tilt in patients undergoing total hip arthroplasty: when does it matter? J Arthroplasty 2015;30:387e91. https://doi.org/10.1016/ j.arth.2014.10.014. [12] Stefl M, Lundergan W, Heckmann N, Mcknight B, Ike H, Murgai R, et al. Spinopelvic mobility and acetabular component position for total hip arthroplasty. Bone Joint J 2017;99-B:37e45. https://doi.org/10.1302/0301-620X.99B1. [13] Uemura K, Takao M, Otake Y, Koyama K, Yokota F, Hamada H, et al. Basic science change in pelvic sagittal inclination from supine to standing position
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T.A. Luthringer, J.M. Vigdorchik / The Journal of Arthroplasty 34 (2019) S57eS70 before hip arthroplasty. J Arthroplasty 2017;32:2568e73. https://doi.org/ 10.1016/j.arth.2017.03.015. Lazennec JY, Clark IC, Folinais D, Tahar IN, Pour AE. What is the impact of a spinal fusion on acetabular implant orientation in functional standing and sitting positions? J Arthroplasty 2017;32:3184e90. https://doi.org/10.1016/ j.arth.2017.04.051. Esposito CI, Carroll KM, Sculco PK, Padgett DE, Jerabek SA, Mayman DJ. Total hip arthroplasty patients with fixed spinopelvic alignment are at higher risk of hip dislocation. J Arthroplasty 2018;33:1449e54. https://doi.org/10.1016/ j.arth.2017.12.005. Buckland AJ, Puvanesarajah V, Vigdorchik J, Schwarzkopf R, Jain A, Klineberg EO, et al. Dislocation of a primary total hip arthroplasty is more common in patients with a lumbar spinal fusion. Bone Joint J 2017;99-B: 585e91. https://doi.org/10.1302/0301-620X.99B5.BJJ-2016-0657.R1. Delsole EM, Vigdorchik JM, Schwarzkopf R, Errico TJ, Buckland AJ. Total hip arthroplasty in the spinal deformity population: does degree of sagittal deformity affect rates of safe zone placement, instability, or revision? J Arthroplasty 2017;32:1910e7. https://doi.org/10.1016/j.arth.2016.12.039. Perfetti DC, Schwarzkopf R, Buckland AJ, Paulino CB, Vigdorchik JM. Prosthetic dislocation and revision after primary total hip arthroplasty in lumbar fusion patients: a propensity score matched-pair analysis. J Arthroplasty 2017;32: 1635e1640.e1. https://doi.org/10.1016/j.arth.2016.11.029. Sultan AA, Khlopas A, Piuzzi NS, Chughtai M, Sodhi N, Mont MA. The impact of spino-pelvic alignment on total hip arthroplasty outcomes: a critical analysis of current evidence. J Arthroplasty 2017;33:1606e16. https://doi.org/ 10.1016/j.arth.2017.11.021. Gausden EB, Parhar HS, Popper JE, Sculco PK, Rush BNM. Risk factors for early dislocation following primary elective total hip arthroplasty. J Arthroplasty 2018;33:1567e1571.e2. https://doi.org/10.1016/j.arth.2017.12.034. An VV, Phan K, Sivakumar BS, Mobbs RJ, Bruce WJ. Prior lumbar spinal fusion is associated with an increased risk of dislocation and revision in total hip arthroplasty: a meta-analysis. J Arthroplasty 2018;33:297e300. https:// doi.org/10.1016/j.arth.2017.08.040.
[22] Eneqvist T, Nemes S, Brisby H, Fritzell P, Garellick G, Rolfson O. Lumbar surgery prior to total hip arthroplasty is associated with worse patient-reported outcomes. Bone Joint J 2017;99-B:759e65. https://doi.org/10.1302/0301620X.99B6.BJJ-2016-0577.R2. [23] Lembeck B, Mueller O, Reize P, Wuelker N. Acta orthopaedica pelvic tilt makes acetabular cup navigation inaccurate Pelvic tilt makes acetabular cup navigation inaccurate. Acta Orthop 2005;76:517e23. https://doi.org/10.1080/ 17453670510041501. [24] Zhu J, Wan Z, Dorr LD. Quantification of pelvic tilt in total hip arthroplasty. Clin Orthop Relat Res 2010;468:571e5. https://doi.org/10.1007/s11999-0091064-7. [25] Tezuka T, Heckmann ND, Bodner RJ, Dorr LD. Functional safe zone is superior to the Lewinnek safe zone for total hip arthroplasty: why the Lewinnek safe zone is not always predictive of stability. J Arthroplasty 2019;34:3e8. https:// doi.org/10.1016/j.arth.2018.10.034. J, Walter LR, et al. Vari[26] Pierrepont J, Hawdon G, Miles BP, O’Connor B, Bare ation in functional pelvic tilt in patients undergoing total hip arthroplasty. Bone Joint J 2017;99-B:184e91. https://doi.org/10.1302/0301-620X.99B2.BJJ2016-0098.R1. [27] Lum ZC, Coury JG, Cohen JL, Dorr LD. The current knowledge on spinopelvic mobility. J Arthroplasty 2018;33:291e6. https://doi.org/10.1016/j.arth.2017.08.013. [28] Esposito CI, Miller TT, Kim HJ, Barlow BT, Wright TM, Padgett DE, et al. Does degenerative lumbar spine disease influence femoroacetabular flexion in patients undergoing total hip arthroplasty? Clin Orthop Relat Res 2016;474: 1788e97. https://doi.org/10.1007/s11999-016-4787-2. [29] Vigdorchik JM, Elbuluk AE, Carroll KM, Mayman DJ, Iorio R, Buckland A, et al. A new risk-assessment score and treatment algorithm for patients at high-risk of dislocation following total hip arthroplasty. American Academy of Orthopaedic Surgeons 2018 Annual Meeting; Mar 6-10; New Orleans. [30] Vigdorchik JM, Eftekhary N, Elbuluk AE, Abdel MP, Buckland A, Schwarzkopf R, et al. Evaluation of the spine is critical in patients with recurrent instability after total hip arthroplasty. American Association of Hip and Knee Surgeons 2018 Annual Meeting; Nov 1-4: Dallas.
Contents lists available at ScienceDirect
The Journal of Arthroplasty journal homepage: www.arthroplastyjournal.org
2018 AAHKS Annual Meeting Symposium
A Preoperative Workup of a “Hip-Spine” Total Hip Arthroplasty Patient: A Simplified Approach to a Complex Problem Tyler A. Luthringer, MD a, *, Jonathan M. Vigdorchik, MD b a b
Department of Orthopedic Surgery, NYU Langone Orthopedic Hospital, New York, NY Adult Reconstruction and Joint Replacement Service, Hospital for Special Surgery, New York, NY
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 December 2018 Accepted 7 January 2019 Available online 18 January 2019
Background: A large body of evidence has confirmed that patients with spinal deformity, lumbar fusion, and abnormal spinopelvic mobility are at significantly increased risk for instability, dislocation, and revision after total hip arthroplasty (THA). Methods: Achieving a stable construct in patients with pre-existing spine disease requires an understanding of basic spinopelvic parameters and the compensatory mechanisms associated with abnormal spinopelvic motion. Indicated patients with concomitant hip-spine pathology should be assessed for (1) the presence of spinal deformity and (2) the presence of spinal stiffness before undergoing THA. Preoperative imaging should include a standing anteroposterior pelvis x-ray, as well as two lateral spinopelvic radiographs in the standing and seated position. Results: Based on the presence of spinal deformity and/or spinal stiffness, patients may be categorized as one of the four groups of the “Hip-Spine Classification in THA.” A series of illustrative case examples is provided. Conclusion: A simple three-step assessment with minimal measurements will effectively identify the complex “hip-spine” THA patient at high risk for postoperative instability. Adhering to group-specific recommendations for acetabular cup position can help to further reduce the burden of instability and related revisions in this challenging population. © 2019 Elsevier Inc. All rights reserved.
Keywords: total hip arthroplasty instability spinal deformity spinopelvic mobility hip-spine relationship
Despite the overall success and high patient satisfaction after total hip arthroplasty (THA), instability remains a costly and difficult problem with negative implications on quality of life. The incidence of dislocation after primary THA is approximately 2% with use of contemporary implants, exposures, and softtissue balancing [1], yet instability remains the leading cause for revision THA in the United States [2,3]. Greater emphasis on patient risk-stratification, surgical technique, and component positioning is required to further reduce the burden of
Source of Funding: No external funds were received in support of this work. Institutional review board approval: Not applicable or required for this review. One or more of the authors of this paper have disclosed potential or pertinent conflicts of interest, which may include receipt of payment, either direct or indirect, institutional support, or association with an entity in the biomedical field which may be perceived to have potential conflict of interest with this work. For full disclosure statements refer to https://doi.org/10.1016/j.arth.2019.01.012. * Reprint requests: Tyler A. Luthringer, MD, NYU Langone Orthopedic Hospital, 301 East 17th Street, 14th Floor, New York, NY 10003. https://doi.org/10.1016/j.arth.2019.01.012 0883-5403/© 2019 Elsevier Inc. All rights reserved.
dislocation and revisions related to this complex multifactorial problem [1]. Since 1978, the range of optimal acetabular component positions to minimize dislocation risk has been defined by the “safe zones” set forth by Lewinnek et al [4]. Several authors have recently demonstrated that dislocation most often occurs within these previously defined “safe zones” [5e7], calling to question the universal application of these parameters. With use of modern implant designs, most atraumatic dislocations actually occur secondary to altered spinopelvic kinematics that lead to a functionally unsafe acetabular position [8,9]. We now recognize a shortcoming of the protective “safe zone” defined by Lewinnek et al on a single radiograph that is it assumes the acetabular component to be constant and static. A growing body of literature on the dynamic relationship between the hip, pelvis, and spine has revealed that acetabular component orientation changes with rotation of the pelvis through alterations in posture [10e13]. Furthermore, spinopelvic mobility may limit (or enhance) the magnitude of pelvic rotation that a given individual can assume in positions of sitting or standing [14,15]. Patients with decreased
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relationship on THA stability in their preoperative assessment and surgical plans. Obtaining a stable THA, particularly in patients with pre-existing spine disease, is dependent on an understanding of spinopelvic parameters and compensatory mechanisms in those with abnormal spinopelvic motion. We are typically asked, “where do I put the cup?” It is important we think to first answer the questions, “what imaging are you willing to get to find out that appropriate cup position,” and then, “what tools or landmarks are you going to use to get it there?” There are commercially available systems that can output exact predictions for functional cup position after a series of three x-rays, and there are multiple commercially available computer navigation tools, robots, smart instruments, and patient-specific instrumentation guides that can help deliver that target intraoperatively (though some require a computed tomography scan). This article outlines the authors’ simplified approach to the preoperative workup of a “hip-spine” THA candidate with the addition of two x-rays to standard clinical practice. While we recognize this as a simplified method, it can be easily reproduced and is a first great step to the understanding of the hip-spine relationship in THA. The following preoperative workup was presented as part of the 2018 American Association of Hip and Knee Surgeons Annual Meeting Symposium: “Simplifying the Concepts of the Hip-Spine Relationship for THA.” Important Definitions
Fig. 1. Anterior pelvic plane (APP) (yellow) and sacral slope (SS) (red) outlined on lateral standing radiograph.
spinal mobility due to arthritis, sagittal spinal deformity, and spinal fusion are at increased risk for functionally malpositioned acetabular components, instability, and need for revision surgery [15e19]. A history of spinal fusion was recently cited as the strongest independent risk factor for early dislocation after THA [20], imparting greater than two- and three-fold risk for dislocation and revision, respectively [21]. As hip and spine pathology often coexist [22], arthroplasty surgeons must begin to consider the implications of the hip-spine
Until recently, there has been a lack of standardization of the language used by spine and arthroplasty surgeons when defining spinopelvic parameters in the literature. This disconnect has limited the utility of much of the available evidence for community and academic hip surgeons alike. At the 2018 American Academy of Orthopedic Surgeons (AAOS) Annual Meeting, a newly conceptualized “Hip-Spine Workgroup” met for the first time to help standardize terminology across the literature and simplify the spinopelvic relationship for arthroplasty surgeons. The following terms and radiographic reference lines are used in the authors’ preferred approach to the preoperative workup of a “hip-spine” patient. The hip arthroplasty surgeon must first understand these relevant parameters to conceptualize the implications of spinopelvic mobility on THA construct stability.
Fig. 2. Neutral, anterior, and posterior pelvic tilt of the anterior pelvic plane (APP) (yellow), also termed as anterior pelvic plane tilt (APPt).
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The anterior pelvic plane (APP) is defined by the plane between the two anterior superior iliac spines (ASISs) and the pubic symphysis on a lateral pelvic radiograph (Fig. 1). On an imperfect lateral image where the ASISs are not superimposed, the superior end of the APP intersects the horizontal midpoint between the left and right ASIS (Fig. 1). The anterior pelvic plane tilt (APPt) is the measured angle created between the APP and a vertical reference line and may provide a quantitative assessment of pelvic tilt if preferred by the surgeon. The authors’ preferred method is to qualitatively assess for the presence of neutral, anterior, or posterior pelvic tilt of the APP relative to the coronal (functional) plane (Fig. 2). Is it important to note that the coronal (functional) plane is constant and should serve as the typical reference for acetabular cup inclination and anteversion. Evaluation of the APP (or APPt) is critical to goal 1 of the preoperative workupdidentification of spinal deformity. Sacral slope (SS) is defined at the angle subtended by a line parallel to the superior endplate of S1 and a horizontal reference line. The SS may be qualitatively assessed by simply gauging the slope of the superior endplate of S1 relative to the horizontal plane of the bottom of the radiograph (Fig. 1). Both APPt and SS may be used to assess spinopelvic kinematics and pelvic mobility between postures. To identify patients with spinal stiffness in goal 2 of the preoperative workup, the authors prefer to use SS. Pelvic incidenceelumbar lordosis mismatch (PI-LL mismatch) is one parameter commonly used by spine surgeons to define sagittal spine deformity. Although PI-LL mismatch may be an advanced concept in the eyes of the arthroplasty surgeon, its utility in identifying patients with “flatback deformity” is significant and worth mentioning. To understand the concept of PI-LL mismatch, PI must be explained first. PI is a morphologic parameter that remains constant throughout spinopelvic motion; it is not subject to change secondary to degenerative disease and remains unchanged throughout adulthood. PI is measured as the angle between a line drawn from the center of the femoral heads to the center of the S1 endplate and a second line drawn perpendicular to the S1 endplate (Fig. 3). PI is mathematically defined by the formula: PI ¼ SPT þ SS, where spinopelvic tilt (SPT) is an alternative measurement of pelvic tilt used more commonly by spine surgeons, as opposed to APPt described previously. SS, SPT, and APPt are all reliable means to quantify pelvic movement between sagittal postures. Throughout spinopelvic motion, PI remains constant. Any change in pelvic tilt (SPT or APPt) must be accompanied by an inverse change in SS, which directly correlates to associated changes in LL to maintain upright posture. LL (also known as the lumbar lordotic angle) is the angle subtended by two lines drawn at the superior endplates of L1 and S1 (Fig. 3). When PI-LL mismatch (defined as PI minus LL) is >10 , arthroplasty surgeons should be aware of the presence of “flatback spinal deformity.” Workup When indicating a patient with concomitant hip-spine pathology for THA, the goals of the preoperative workup are twofold: (1) to identify the presence of spinal deformity (yes or no) and (2) to identify the presence of spinal stiffness (yes or no). You can then classify each patient according to the Hip-Spine Classification in THA as follows: 1dNormal spinal alignment (defined by PI-LL ± 10 ). 2dFlatback deformity (PI-LL > 10 ). AdNormal spinal mobility (defined as >10 change in SS from stand to sit). BdStiff spine (defined as <10 change in SS from stand to sit).
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Fig. 3. Pelvic incidence (yellow) and lumbar lordosis (red) used in the assessment for PI-LL mismatch. PI-LL, pelvic incidence-lumbar lordosis.
Besides being higher risk for dislocation, the presence of spinal deformity is important for acetabular component positioning. It is important to consider that when talking about acetabular anteversion, let us say for example 20 , that is, 20 of anteversion relative to something which is 0 . Defining that 0 plane is of utmost importance. Classical methods of acetabular instrumentation define anatomic landmarks such as the transverse acetabular ligament, and current versions of computer navigation programs reference the APP, with the APP being the 0 reference. In patients with no spinal deformity, the APP tends to be neutral, aligned with the coronal plane of the body (Fig. 2). However, in the presence of a spinal deformity, as seen on the standing lateral x-ray to be defined below, the APP reference plane could be functionally defined in a different position (ie, tilted compared to the coronal plane). This makes the APP an unreliable reference for acetabular component positioning in this population. For each degree of increased posterior pelvic tilt, the surgeon can predict a concomitant increase in functional acetabular anteversion of approximately 0.7 -0.8 [11,23,24]. Therefore, the 0 reference plane needs to be the coronal plane of the body or the “functional pelvic plane” (FPP) [25,26].
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Fig. 4. Standing and sitting lateral radiographs of a patient with normal spinal pelvic mobility. When the patient sits, lumbar lordosis decreases and the pelvis “rolls back,” which is demonstrated by an increase in posterior pelvic tilt (APP, yellow) and a flattening of SS (red).
As for spinal stiffness, in this study, we are looking at the mobility of the pelvis when the patient moves from the standing to the sitting position. It is important to understand that the seated position is achieved through a combination of motion from the lumbosacral spine and femoroacetabular articulation (hip joint). When going from standing to sitting position, normal spinopelvic motion permits a decrease in LL and allows the pelvis to “roll back” (which equates to 20 -35 of posterior pelvic tilt and a decrease or flattening of SS) [10,27]. As the pelvis “rolls back,” there is more space for the femur to flex (due to increased clearance of the anterior acetabular rim) (Fig. 4). If the spine is stiff, the pelvis does not move, and the femur must flex more to achieve the seated position [28]. This preferential increase in femur flexion (relative to a somewhat static anterior acetabular rim) causes a higher risk of impingement (Fig. 5). Therefore, in the stiff
spine, more anteversion is needed in the acetabular component to compensate for limited pelvic “roll back” when seated. One caveat is that mentioned in the previous paragraph, which reminds us that anteversion is relative to the 0 reference plane. Standard history and physical examination will alert the surgeon to the possibility of spinal pathology or abnormal mobility in a given patient. Patients with normal spinopelvic anatomy stand with neutral pelvic tilt, and the ASISs are aligned over the pubic symphysis in line with the functional or coronal plane. Patients with anterior pelvic tilt will stand with increased LL; this deformity may also be noticed with the patient lying down, exhibited by increased space between the arch of the lower back and the underlying examination table. On the other hand, those with posterior pelvic tilt (decreased or more horizontal SS) tend to stand with a decreased LL and are
Fig. 5. Standing and sitting lateral radiographs of a patient with a stiff spine who underwent revision of the shown construct for asymmetric polyethylene wear, osteolysis, and posterior instability. Note the lack of “pelvic rollback” in the seated position (no change in SS, red) and the proximity of the flexing proximal femur to the anterior acetabular rim. APP (yellow).
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Fig. 6. Supine (left) and standing (right) postoperative AP pelvis radiographs of a patient with unrecognized spinal deformity (posterior pelvic tilt) who underwent primary THA templated on a supine AP pelvis. Intraoperatively, the cup was placed in 25 of anteversion relative to the APP, which increased to 41 of functional anteversion relative to the coronal plane (FPP) when standing. This patient experienced an anterior dislocation 2 wk postoperatively. AP, anteroposterior.
more likely to lie down with no space between the lower back and examination table (Fig. 2). When posterior pelvic tilt is severe, patients may be described as having a “flatback deformity.” These extremely high-risk patients commonly developed hip flexion contractures and stand with the hips and knees flexed as compensatory mechanisms to maintain forward gaze. Radiographic evaluation of THA candidates routinely begins with an anteroposterior (AP) pelvis x-ray. Even in the absence of gross
spinal pathology, the natural tilt of the pelvis varies between individuals in the standing position and may differ considerably from that while supine (the position of most preoperative AP pelvis radiographs) [11,13]. A standing AP pelvis is strongly recommended as it better represents the functional position of the pelvis during activity. Two additional lateral spinopelvic views (pelvis from L1 to below the pubic symphysis) in the sitting and standing positions are needed to systematically evaluate the lumbar spine and spinopelvic
Fig. 7. (A) Patients with posterior pelvic tilt (APP, yellow) will increase functional anteversion of the cup when standing. (B) Patients with anterior pelvic tilt (APP, yellow) will decrease functional anteversion of the cup when standing.
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Fig. 8. (A) Preoperative standing lateral image of a patient with severe posterior pelvic tilt (APP, yellow). (B) Templated cup position of 40 of inclination and 20 of anteversion relative to the APP (or traditional bony landmarks intraoperatively) leads to functional cup position of 45 of inclination and 38 of anteversion when the patient stands. (C) After accommodating for the patient’s posterior pelvic tilt in the functional (standing) position, placement of the cup in 35 of inclination and 2 of anteversion relative to the APP will lead to a cup position of 40 of functional inclination and 20 of functional anteversion relative to the coronal plane when standing.
mobility. For the lateral sitting x-ray, the authors routinely use the “relaxed” seated position with the patient sitting upright and the femurs parallel to the floor. The AP and lateral x-rays can be normal flat plate radiographs or stereoradiographs (EOS Imaging, Paris, France). Standard 36-inch cassettes are sufficient if stereoradiography is unavailable to the surgeon [10]. Goal 1: Identify Patients With Spinal Deformity Step 1: Standing AP Pelvis X-Ray The standing AP pelvis x-ray should be scrutinized for any apparent abnormalities in pelvic tilt, obliquity, or rotation. Making sure that this is a normal AP pelvis (not a low AP pelvis) including the lumbar spine is helpful in identifying lumbar degenerative abnormalities or evidence of prior surgery. Any abnormalities in the standing AP pelvis view should prompt further evaluation with additional spinopelvic imaging. The surgeon should note that functional position of the acetabular cup will be dependent on the pelvic orientation when the individual stands, which may be entirely different from that while supine. Figure 6 shows the supine and standing images of a patient who had an anterior dislocation within 2 weeks postoperatively. Initially, the construct was templated using a supine AP pelvis x-ray and accurately reproduced intraoperatively with 42 of acetabular cup inclination and 25 of anteversion. After the patient’s early anterior dislocation, a standing AP pelvis stereograph revealed the functional position of the acetabular component to be 62 of inclination and 41 of anteversion. Hence, the authors strongly recommend utilizing a standing AP pelvis x-ray when templating preoperatively. Step 2: Standing Lateral Spinopelvic X-Ray The APP is drawn on the standing lateral spinopelvic image to confirm the presence or absence of spinal deformity. The APPt (or pelvic tilt) should be identified as being neutral, tilted posterior, or tilted anterior. In patients with no deformity, the APP will be
vertical (neutral pelvic tilt) or parallel to the coronal (functional) plane of the body. Thus, the position of the cup in the functional standing position postoperatively will be the same as that referenced from the coronal plane of the body during cup implantation. As the implanted acetabular component and pelvis move as a single unit, patients with posterior pelvic tilt will increase the functional anteversion of the cup while standing postoperatively (Fig. 7a). Conversely, anterior pelvic tilt will cause patients to functionally decrease cup anteversion in the standing position (Fig. 7b). Figure 8a-c demonstrate the importance of considering APPt relative to the coronal (functional) plane of the body when planning cup position. Note that this is an extreme example for illustrative purposes. By failing to account for the patient’s posterior tilt while standing (Fig. 8a), the surgeon assumes that the APP is neutral compared with the coronal (functional) plane of the body. Thus, templated cup position of 40 of inclination and 20 of anteversion relative to the APP (or traditional bony landmarks intraoperatively) will actually lead to 45 of functional inclination and 38 of functional anteversion relative to the coronal plane when the patient stands (Fig. 8b). By accommodating for the deformity and positioning the cup in 35 of inclination and 2 of anteversion relative to the APP, the acetabular component will be in 40 of functional inclination and 20 of functional anteversion relative to the coronal plane in the standing position (Fig. 8c).
Advanced Concepts: PI-LL Mismatch Despite the fact that measuring the APP and APPt is an easy concept to look for spinal deformity, it has its flaws. The best way to look for spinal deformity is by measuring the PI minus LL (PI-LL mismatch). A PI-LL mismatch of >10 measured on the standing lateral spinopelvic image should alert the surgeon to be aware of the presence of a “flatback spinal deformity” (Fig. 9). These patients have abnormal spinopelvic mechanics from when they stand to when they sit. Therefore, in the presence of these abnormalities, surgeons should pursue functional imaging (standing
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Fig. 9. Standing and sitting lateral radiographs of a patient with flatback deformity (PI-LL mismatch >10 ). Pelvic incidence (yellow) and lumbar lordosis (red) are shown on the standing image. Sacral slope (red) is drawn along the superior endplate of S1 on the sitting image for comparison.
and sitting lateral spinopelvic views) to ensure appropriate evaluation.
segment instrumented fusions (even to S1) may still maintain spinopelvic mobility (Fig. 10c).
Goal 2: Identify Patients With Spinal Stiffness
Advanced Concepts: Functional X-Rays Functional x-rays to mimic the anatomic position in which THA dislocation may occur can be obtained for patients noted to have stiff spines who may be particularly at high risk for instability. Posterior dislocations tend to occur when getting up from the seated position and when the body leans forward and assumes maximum hip flexion. The anterior proximal femur ultimately impinges on the anterior rim of the cup, acting as a lever for posterior dislocation. Thus, an x-ray can be taken with the patient in a flexed seated (rise above to stand up from sitting) position, and cup placement can be planned accordingly (Fig. 11) [26]. Anterior dislocations occur in hip extension, typically when walking or getting in or out of bed. An x-ray may be taken with the patient stepping up onto a raise surface with the femur paralleling the floor, which will
Step 3: Compare Standing versus Sitting Lateral Spinopelvic X-Ray Assessment of SS on standing and sitting lateral radiographs is used to gauge the patient’s spinopelvic mobility. A change of <10 in the SS between the standing and sitting positions defines the presence of “spinal stiffness.” However, simply annotating a line along the superior S1 endplate on side-by-side standing and sitting lateral spinopelvic images offers a quick and simple visual comparison (Fig. 10a-c). The objective SS relative to a horizontal reference (and the subsequent change from standing to sitting) should be measured. It is important to recognize that patients with no history of instrumented fusion may still have stiff spines secondary to degenerative spine disease (Fig. 10b) and that patients with short
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Fig. 10. (A) Visual comparison of the sacral slope (SS, red) between the standing and sitting position in a patient with no spinal stiffness (>10 change). (B) Visual comparison of the SS (red) between the standing and sitting position in a patient with a stiff spine (<10 change). (C) Visual comparison of the SS (red) between the standing and sitting position in a patient with a history of L5-S1 fusion yet does not have spinal stiffness (>10 change). APP (yellow).
cause the pelvis to rotate posteriorly and extend the hip joint of the limb planted on the floor (Fig. 12) [26]. Excessive acetabular anteversion in this position will increase the risk of anterior dislocation. Treatment Although outside of the original scope of the American Association of Hip and Knee Surgeons symposium presentation on the preoperative assessment of hip-spine THA patients, the treatment algorithm that follows the aforementioned workup is outlined in Table 1. Group 1A patients have normal sagittal alignment (neutral pelvic tilt) and normal spinopelvic mobility (“pelvic rollback”) when going from standing to sitting. In these individuals, the FPP when standing is the same as the APP. Thus, native anatomic landmarks may be referenced intraoperatively for traditional cup positioning in 20 to 25 of anteversion. Group 1B patients also have normal sagittal alignment (neutral pelvic tilt), so FPP is again similar to the APP. However, group 1B patients have limited “pelvic
rollback” when assuming the seated position due to spinal stiffness (<10 change in SS from stand to sit). To avoid impingement of the flexing proximal femur on the anterior acetabular rim when sitting, 30 of anteversion templated on the standing AP pelvis should be targeted. Increased anteversion protects against the risk of posterior dislocation in this group. All the patients in group 2 are classified as having sagittal flatback deformity (PI-LL mismatch >10 ); these patients stand with posterior pelvic tiltdthe APP is different from the FPP. It is crucial to recognize that this posterior pelvic tilt (of the APP) will increase the functional cup anteversion relative to the coronal plane while standing (FPP). After impaction, the cup and pelvis move as a single unit. If the cup is anteverted relative to the APP (bony landmarks), when the patient stands, it will tilt posteriorly with the pelvis, causing the face of the cup to open anteriorly, increasing anteversion relative to the coronal plane (FPP). Thus, anteversion targets must be referenced from the FPP (on the standing AP pelvis) and not from the APP. Failure to do so may result in excessive functional anteversion while standing and increase the risk of
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Fig. 10. (continued).
anterior dislocation (Fig. 6). It is important to recognize that in some extreme cases, cups that are appropriately anteverted relative to the FPP may appear to have insufficient anteversion, or even retroversion, relative to the APP or traditional bony landmarks intraoperatively (Fig. 8a-c).
Group 2A patients have flatback deformity and normal spinal mobility (>10 change in SS from stand to sit); anteversion should be targeted 25 -30 from the FPP (on the stand AP pelvis). Group 2B patients have flatback deformity and a stiff spine (<10 change in SS from stand to sit) so even more anteversion is needed to protect
Fig. 11. Example of patient position and a maximum forward-flexed seated radiograph.
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Fig. 12. Example of patient position and a lateral step-up spinopelvic radiograph.
from posterior dislocation. Thirty-degrees of anteversion should be targeted on the standing AP pelvis (FPP); providers must be aware of the reference plane as the presence of spinal deformity will cause more functional cup anteversion as described previously. Group 2B individuals are at the highest risk for instability due to a very narrow safe zone of functional cup position; these patients are strong candidates for dual-mobility prostheses. Case Example 1
Table 1 The Four Categories of the Hip-Spine Classification in THA and the Necessary Acetabular Position for Each. Group
Classification
Explanation of Pathology
Treatment Recommendation
1A
Normal alignment normal mobility
Target native anatomy of 20 -25
1B
Normal alignment stiff spine (<10 change in sacral slope from stand to sit)
Normal anatomy and normal mobility (APP similar to FPP) Stiff spine, needs more anteversion relative to the APP which is similar to the FPP
2A
Flatback deformity (PI-LL>10 ) normal mobility
2Ba
Flatback deformity (PI-LL>10 ) stiff spine (<10 change in sacral slope from stand to sit)
Step 1 The supine and standing AP pelvis (Fig. 13a) x-rays show no abnormalities. Step 2 The standing lateral (Fig. 13b) shows an APP with no tilt, confirming no spinal deformity. Step 3 The sitting lateral (Fig. 13b) shows >10 change in SS, confirming no spinal stiffness. Goal 1: Spinal deformity? No. Goal 2: Spinal stiffness? No. Action: proceed with standard THA component position of 40 of inclination and 20 -25 of anteversion. This patient underwent uncomplicated primary THA with imageless computer navigation to achieve traditional component
Beware, the FPP is different from the APP (posterior pelvic tilt from the spinal deformity causes more functional cup anteversion) FPP is different from the APP and a stiff spine, so you need more anteversion, but be aware of the reference because the spinal deformity will cause more functional cup anteversion
Target 30 anteversion on the standing AP pelvis. Note the higher target due to the stiff spine to protect from a posterior dislocation. Target 25 -30 anteversion on the standing AP pelvis (to the functional pelvic plane)
Target 30 on the standing AP pelvis (to the FPP). Note the higher target due to the stiff spine to protect from a posterior dislocation.
Anteversion targets refer to Murray's radiographic definition. AP, anteroposterior; APP, anterior pelvic plane; FPP, functional pelvic plane; PI-LL, pelvic incidenceelumbar lordosis. a Most difficult to treat, very narrow window of safe zone.
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Fig. 13. (A) Case example 1: supine (left) and standing AP (right) pelvis x-rays reveal no abnormalities. (B) Case example 1: standing lateral x-ray demonstrating an APP with no tilt (yellow), and sitting lateral demonstrating >10 change in SS (red). (C) Case example 1: postoperative standing AP pelvis and cross-table lateral x-ray of the operative right hip. AP, anteroposterior.
position targets. The patient has experienced no episodes of instability within the first 4 years postoperatively (Fig. 13c). Case Example 2 Step 1 The supine and standing (Fig. 14a) AP pelvis x-rays are noted to be very similar, suggesting the absence of deformity. Step 2 The APP is drawn on the lateral standing x-ray. Very minimal posterior tilt is noted, confirming that there is no spinal deformity (Fig.14b).
Step 3 The SS is compared on the standing versus the sitting lateral images, noting minimal (<10 ) change between positions, confirming the presence of a stiff spine (Fig. 14b). Goal 1: Spinal deformity? No. Goal 2: Spinal stiffness? Yes. Action: add anteversion. This patient underwent primary THA with 30 of targeted anteversion (relative to functional coronal plane when standing) achieved intraoperatively with imageless computer navigation. The patient is doing well without episodes of instability 5 years postoperatively (Fig. 14c).
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Fig. 14. (A) Case example 2: minimal difference is noted between the supine (left) and standing (right) AP pelvis x-rays. (B) Case example 2: minimal posterior tilt of the APP (yellow) and <10 change in SS between the standing and sitting lateral x-rays (red). (C) Case example 2: postoperative standing AP pelvis and cross-table lateral x-ray of the operative right hip. AP, anteroposterior.
Case Example 3 Step 1 Compared with the supine AP pelvis, the standing AP pelvis xray (Fig. 15a) is noted to look like an outlet view (posterior pelvic tilt). Therefore, additional hip-spine imaging is warranted. Step 2 The APP is drawn on the lateral standing x-ray (Fig. 15b) showing posterior pelvic tilt signifying spinal deformity. Step 3 No change in SS on the standing versus the sitting lateral (Fig. 15c) images, confirming the presence of a stiff spine. Goal 1: Spinal deformity? Yes. Goal 2: Spinal stiffness? Yes.
Action: add anteversion, but be careful of your 0 reference plane. Consider the use of dual-mobility prosthesis. This patient underwent primary THA with 30 of targeted anteversion (to the functional coronal plane in standing) achieved intraoperatively with robotic-assisted navigation. A dual-mobility device was used given the high-risk nature of having a spinal deformity and spinal stiffness. The patient is doing well with no episodes of instability 5 years postoperatively (Fig. 15d). Discussion Patients with concomitant hip and spine pathology must be appropriately assessed for the presence of deformity and abnormal spinopelvic mobility when planning for THA. Adult spinal deformity patients have been shown to have an 8% dislocation rate after THA compared to 1.5% in normal controls [17]. Spine stiffness, whether secondary to instrumented or biologic fusion (spondylosis), imparts an equally great risk of instability after THA. A Medicare database study of 2912 patients who had undergone both
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Fig. 15. (A) Case example 3: supine (left) and standing (right) AP pelvis x-rays are compared; the standing AP pelvis resembles an outlet view, suggesting posterior pelvic tilt. (B) Case example 3: the APP (yellow) is drawn on the lateral standing x-rays, demonstrating 16 of posterior pelvic tilt. (C) Case example 3: side-by-side comparison of the standing (left) versus sitting (right) lateral images demonstrating no change in SS (red). (D) Case example 3: postoperative standing AP pelvis and cross-table lateral x-ray of the operative left hip with a dual-mobility prosthesis and 29 of targeted acetabular anteversion (relative to the functional coronal plane while standing). AP, anteroposterior.
lumbar spinal fusion and THA demonstrated that the risk of dislocation increases with the number of vertebral levels fused (or increase in spinal stiffness) [16]. The dislocation rate of patients with a 1-2 level and 3-7 level lumbar fusion was found to be 2.73% and 4.62%, respectively, compared with 1.55% in patients without spinal fusion [16]. The preoperative imaging protocol described in this study has been implemented as part of a new risk assessment score and treatment algorithm for instability-prone patients undergoing primary THA [29]. In a series of 1009 cases, patients were designated as being at low, intermediate, or high risk for dislocation. Those with history of spinal fusion of three or more levels and patients with spinal deformity (defined at PI-LL mismatch >10 ) with no change in SS between standing and sitting positions were identified as being at high risk (n ¼ 192). After implementation of the imaging protocol and a risk-stratified treatment algorithm, dislocation rates decreased from 3.1% to 0.5% among the high-risk cohort [29]. A similar approach to the evaluation of the spine has been applied to patients undergoing revision THA for recurrent instability [30]. One hundred eleven patients were prospectively analyzed for the presence of spinal deformity and spinal stiffness with the previously performed spinopelvic imaging and scored according to the “Hip-Spine Classification in Revision THA.” Following a classification-based surgical treatment algorithm, 2year dislocation-free survivorship was 97% among the protocol group compared to 84% in matched controls [30]. Furthermore, 77% of the inappropriately positioned acetabular components identified by the spinopelvic imaging protocol would have gone unrecognized by supine AP pelvis imaging alone [30]. To further reduce the burden of THA instability and related revisions, indicated patients should be assessed with functional spinopelvic imaging and a standing AP pelvis radiograph. Spinal deformity and spinal stiffness can be easily identified preoperatively by the untrained eye with the addition of two lateral spinopelvic x-rays (sitting and standing) and minimal measurements. This three-step assessment has been effective in identifying and
managing high-risk THA candidates and has become our standardof-care in the preoperative setting. References [1] Goel A, Lau EC, Ong KL, Berry DJ, Malkani AL. Dislocation rates following primary total hip arthroplasty have plateaued in the Medicare population. J Arthroplasty 2015;30:743e6. https://doi.org/10.1016/j.arth.2014.11.012. [2] Bozic KJ, Kurtz SM, Lau E, Ong K, Vail TP, Berry DJ. The epidemiology of revision total hip arthroplasty in the United States. J Bone Joint Surg Am 2009;91:128e33. https://doi.org/10.2106/JBJS.H.00155. [3] Gwam CU, Mistry JB, Mohamed NS, Thomas M, Bigart KC, Mont MA, et al. Current epidemiology of revision total hip arthroplasty in the United States: national inpatient sample 2009 to 2013. J Arthroplasty 2017;32:2088e92. https://doi.org/10.1016/j.arth.2017.02.046. [4] Lewinnek GE, Lewis JL, Tarr R, Compere CL, Zimmerman JR. Dislocations after total hip-replacement arthroplasties. J Bone Joint Surg Am 1978;60:217e20. [5] Abdel MP, von Roth P, Jennings MT, Hanssen AD, Pagnano MW. What safe zone? The vast majority of dislocated THAs are within the Lewinnek safe zone for acetabular component position. Clin Orthop Relat Res 2016;474:386e91. https://doi.org/10.1007/s11999-015-4432-5. [6] Esposito CI, Gladnick BP, Lee Y-Y, Lyman S, Wright TM, Mayman DJ, et al. Cup position alone does not predict risk of dislocation after hip arthroplasty. J Arthroplasty 2015;30:109e13. https://doi.org/10.1016/j.arth.2014.07.009. [7] Seagrave KG, Troelsen A, Malchau H, Husted H, Gromov K. Acetabular cup position and risk of dislocation in primary total hip arthroplasty A systematic review of the literature. Acta Orthop 2017;88:10e7. https://doi.org/10.1080/ 17453674.2016.1251255. Y. Mathematical evaluation of [8] Sariali E, Lazennec JY, Khiami F, Catonne jumping distance in total hip arthroplasty: influence of abduction angle, femoral head offset, and head diameter. Acta Orthop 2009;80:277e82. https://doi.org/10.3109/17453670902988378. [9] Miki H, Kyo T, Kuroda Y, Nakahara I, Sugano N. Risk of edge-loading and prosthesis impingement due to posterior pelvic tilting after total hip arthroplasty. Clin Biomech 2014;29:607e13. https://doi.org/10.1016/j.clinbiomech.2014.05.002. [10] Kanawade V, Dorr LD, Wan Z, Wan Z. Predictability of acetabular component angular change with postural shift from standing to sitting position. J Bone Joint Surg Am 2014;96:978e86. https://doi.org/10.2106/JBJS.M.00765. [11] Maratt JD, Esposito CI, Mclawhorn AS, Jerabek SA, Padgett DE, Mayman DJ, et al. Pelvic tilt in patients undergoing total hip arthroplasty: when does it matter? J Arthroplasty 2015;30:387e91. https://doi.org/10.1016/ j.arth.2014.10.014. [12] Stefl M, Lundergan W, Heckmann N, Mcknight B, Ike H, Murgai R, et al. Spinopelvic mobility and acetabular component position for total hip arthroplasty. Bone Joint J 2017;99-B:37e45. https://doi.org/10.1302/0301-620X.99B1. [13] Uemura K, Takao M, Otake Y, Koyama K, Yokota F, Hamada H, et al. Basic science change in pelvic sagittal inclination from supine to standing position
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