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Current Concepts Review

Spine-Pelvis-Hip Relationship in the Functioning of a Total Hip Replacement

Ike, Hiroyuki MD1; Dorr, Lawrence D. MD1,a; Trasolini, Nicholas MD1; Stefl, Michael MD1; McKnight, Braden MD1; Heckmann, Nathanael MD1

Author Information
The Journal of Bone and Joint Surgery: September 19, 2018 - Volume 100 - Issue 18 - p 1606-1615
doi: 10.2106/JBJS.17.00403
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Abstract

Recent research on the functional anatomy of the hip has increased our understanding of the interrelationship of the spine, pelvis, and hip. The work of Lazennec et al. with EOS Imaging (EOS, formerly biospace med) must be credited with stimulating research in multiple centers over the last 15 years1-9. This research has furthered our knowledge of the coordinated motion of the spine-pelvis-hip joint during postural change such as from lying to standing and from standing to sitting. It has taught us that the pelvis tilts during postural changes, and because the acetabulum is part of the pelvis, pelvic motion changes the anteversion and inclination of the acetabulum as patients move around (i.e., it does not remain in the position achieved at the time of surgery) (Fig. 1). This change of the acetabular angles is the reason Lazennec et al.1-3 discussed the sagittal “functional” cup position in total hip replacement as opposed to the coronal inclination and anteversion achieved at surgery. The purposes of this Current Concepts Review were to summarize the knowledge about the interaction among the spine, pelvis, and hip as we know it and to discuss how this knowledge might affect total hip replacement. As some terms used with this new research may not be familiar to the orthopaedic surgeon, a glossary is provided in Table I.

Fig. 1
Fig. 1:
Illustrations showing normal sagittal spine-pelvis-hip motion from standing to sitting. The illustration on the left shows the normal standing position. The angles measured are sacral tilt (45°), which is an angle tangent to the end plate of S1 and a transverse line; pelvic incidence (55°), which is a measure of the anterior-to-posterior dimension of the pelvis and is an angle formed by a line from femoral head to the center of the S1 end plate and a line tangent to the S1 end plate; pelvic femoral angle (180°) describes the position of the femur relative to the pelvis and is an angle formed by a line from the center of the S1 end plate to the center of the femoral head and a second line that parallels the femoral diaphysis; and acetabular ante-inclination (35°), which is the sagittal angle of the acetabulum to a transverse line representing the transverse axis of the body. The illustration on the right shows the normal sagittal spine-pelvis-hip angles in the sitting position: the sacral tilt is 20° so there is 25° of posterior pelvic tilt; pelvic incidence (55°) is a static measurement so it remains the same as standing; the pelvic femoral angle is 125° so there is 55° of hip flexion relative to the pelvis compared with the standing position; and ante-inclination is 60° as the acetabulum opens more with both inclination and anteversion to allow clearance for the flexing femoral head. Table III has the normal values for these measurements.
TABLE I - Glossary of Terms Used
Term Definition
Acetabular opening The biological opening is the increase of inclination and anteversion when the pelvis normally tilts posteriorly during sitting. The mechanical opening is the inclination and anteversion created by the surgeon in a total hip arthroplasty.
Ante-inclination The sagittal angle of the acetabulum (or cup, with total hip replacement) that changes with the motion of the pelvis and is so named because the angle is affected by a combination of the anteversion and inclination of the cup, as shown in Figure 1.
Pelvic femoral angle The sagittal hip-femur position that is a measure of the flexion of the hip with sitting or in extension with standing in relation to the pelvic position, as shown in Figure 1.
Pelvic incidence The static measurement of the relationship of the femoral heads to the sacral end plate as shown in Figures 1, 2-A, and 2-B and Table II.
Sacral tilt (slope) The dynamic motion of the spinopelvic structure is the mobility of the first sacral end plate and measures the sagittal tilt of the pelvis during postural change as shown in Figure 1.
Spinal imbalance Abnormal spinal mobility due to hypermobility or stiffness.
Spinopelvic Lumbar spine and pelvis that move together by the connection of the sacrum to the pelvis.
Spine-pelvis-hip The coordinated mobility of the spine, pelvis, and hip (including the proximal part of the femur), as seen in Video 1.
Stiffness Pathologic stiffness occurs when the dynamic mobility of the pelvis is effectively fused as indicated by a sacral tilt mobility of <5°, which means that sagittal acetabular mobility is <5°. Surgical spinal fusions are a common cause of pathologic stiffness. Dangerous stiffness occurs when sacral tilt mobility is ≤10° and is combined with a pelvis fixed either posteriorly or anteriorly. With total hip replacement, dangerous stiffness is overcome by correct coronal cup position, whereas pathologic stiffness is not improved by cup position.

What Is Normal Spine-Pelvis-Hip Motion?

The position of the spine, pelvis, and hip balances the mass of the trunk above it, and the mobility of these articulations allows for coordinated motion during activities such as moving from standing to sitting or bending forward at the waist1,10,11. This spine-pelvis-hip relationship can be captured on a lateral radiograph that includes the L3 vertebra to the proximal part of the femur1,4. The hip is influenced most by the lower 3 lumbar vertebrae, so this is all of the lumbar spine that is included in the radiograph5.

When standing, the pelvis is tilted anteriorly, the lumbar spine is in lordosis, and the legs are extended (Fig. 1). This balances the trunk above the pelvis, and positions the acetabulum over the femoral head1,2,7 (Video 1). When sitting, the hip does not simply flex 90°. Rather, the pelvis tilts posteriorly as the spine becomes less lordotic, and the hip flexes (Fig. 1, Video 1). The acetabulum is part of the pelvis so when the pelvis tilts the acetabulum tilts as well. When the pelvis tilts posteriorly, there is an increase in the anteversion and inclination of the acetabulum. This change is termed the biological opening of the acetabulum because it occurs during normal posterior tilt of the pelvis when sitting. The sagittal acetabular angle viewed on the lateral spine-pelvis-hip radiograph is termed ante-inclination because its angle is affected by the changes in both anteversion and inclination4. Lazennec et al.1 considered this sagittal cup position to be the operative inclination described by Murray11; however, in a laboratory study, Kanawade et al.4 showed that it was a combination of inclination and anteversion and thus was named ante-inclination to reflect both angles. The normal range for standing ante-inclination has been reported to be 41° to 63° (mean and standard deviation, 52° ± 11°)5.

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The acetabular change as the pelvis tilts posteriorly during sitting accommodates the necessary hip flexion and internal rotation1,2,12,13. The posterior pelvic tilt from standing to sitting is normally 20°, and the femur flexes only 55° to 70° to accomplish sitting (Fig. 1, Video 1)1,12. Bending forward from the waist to pick up an object on the floor requires increased flexion of the hip to 85° combined with internal rotation of 12°13. When a patient is supine, the pelvis tilts anteriorly and the lumbar lordosis is increased from standing by only 3° to 5°; thus, the average pelvic arc of motion from lying down to standing is <5°, which is the reason why research has focused on the movement from standing to sitting.

In normal patients, there is variance in the amount of pelvic tilt, the degree of lordosis in the lumbar spine, and the location of the femoral heads underneath the spine. These variations may be explained by differences in pelvic incidence, an angle described by Legaye et al.14. It is a fixed measurement of the anterior to posterior dimension of the pelvis (Philippot et al. called it the anatomic parameter7) that determines the position of the femoral heads in relation to the spine. It is for this reason that spine surgeons use this measurement to determine an optimal spinal fusion position. It is not known why people have differences in standing pelvic tilt, and different degrees of pelvic incidence, but this variance affects hip motion. The 3 variants of pelvic incidence, and how they change pelvic position and the hip or femoral position, are shown in Figure 1 (normal pelvic incidence), Figure 2-A (high pelvic incidence), and Figure 2-B (low pelvic incidence). The difference in dynamic spine-pelvis-hip motion with each pelvic incidence is shown in Table II.

Figs. 2-A and 2-B These figures show the different morphologies for high and low pelvic incidence (PI; see Figure 1 for normal PI).

Fig. 2-A
Fig. 2-A:
Preoperative standing and sitting lateral spine-pelvis-hip radiographs of a hip with high PI (73°) so the femoral heads are anterior to S1. This figure is also an example of a patient who has fixed anterior tilt of the pelvis, so the sacral tilt (ST) is 44° with standing and 39° with sitting (sitting tilt must be <30° to qualify as posterior tilt). Therefore, the acetabular ante-inclination (AI) with standing (57°) is high and with sitting (52°) is normal (but this is pathologic stiffness because the difference is 5°). The standing pelvic femoral angle (PFA) is high at 201°, but posterior impingement does not occur because of fixed anterior tilt and the anterior position of the femoral heads. Sitting PFA of 122° is normal.
Fig. 2-B
Fig. 2-B:
Postoperative standing and sitting lateral spine-pelvis-hip radiographs of a patient with low pelvic incidence (PI = 35°) so the femoral heads are directly below S1. This is a typical radiograph for low PI, in which the hip is geometrically fixed in posterior tilt (see Table II). This figure is also an example of pathologic stiffness because the change in sacral tilt (ST) motion is 3°. Likewise, the change in ante-inclination (AI) mobility is 3° (38° to 41°) because this mirrors the ST. The pelvic femoral angle (PFA) is normal with standing but shows increased flexion with sitting (109°), which is typical in patients with low PI because the femoral heads are posterior, and more flexion is needed to sit because the pelvis has limited posterior tilt even when not stiff like this one. The intraoperative cup angles were an inclination of 45°, anteversion of 21°, and combined anteversion of 33°, which keeps the head centered in both the standing and sitting positions, providing mechanical stability; however, if the leg length and/or offset were shortened, this hip would have osseous impingement.
TABLE II - Characteristics of Pelvic Incidence (PI)*
Spinopelvic Mobility Low PI Normal PI High PI
Sacral slope
 Standing 30.1° ± 8.0° 38.4° ± 7.2° 44.1° ± 11.1°
 Sitting 14.4° ± 11.5° 18.5° ± 8.4° 23.2° ± 12.1°
 Change 14.3° ± 9.8° 20.3° ± 10.4° 22.4° ± 11.8°
Pelvic femoral angle
 Standing 180.2° ± 9.1° 186.8° ± 9.0° 195.7° ± 10.0°
 Sitting 111.2° ± 14.0° 124.5° ± 12.9° 133.8° ± 12.7°
 Change 69.0° ± 17.1° 62.3° ± 13.5° 62.1° ± 13.2°
Ante-inclination
 Standing 33.7° ± 8.9° 34.1° ± 8.3° 35.6° ± 8.3°
 Sitting 49.2° ± 12.4° 53.7° ± 8.8° 56.1° ± 8.6°
 Change 15.5° ± 10.5° 19.6° ± 9.2° 20.5° ± 11.6°
*The values are given as the mean and the standard deviation. Hips with low PI have a low standing mean sacral slope, which means that many hips with low PI are fixed in posterior tilt as their normal pattern. Therefore, hips with low PI require more flexion to sit, which is reflected in the low sitting pelvic femoral angle mean of 111°. As PI progresses from low to high, the pelvis tilts more anteriorly with standing and the femoral heads move further anteriorly in the pelvis because the spine has more lordosis. With high PI, the femur flexes less so the risk of impingement is less with high PI. It is important to note the PI of the patient because the cup inclination and anteversion need to be adjusted for the flexibility of the pelvis and femur.

While the pelvic incidence is a static measurement unaffected by a pelvic change in position, the most reproducible measurement used to assess dynamic motion of the pelvis (the positional parameter described by Philippot et al.7) is sacral tilt (also called sacral slope)1,3-5,7. The sacral tilt is normally 40° while standing and decreases to 20° when sitting, representing 20° of pelvic motion between standing and sitting. The femur is extended relative to the pelvis when standing and flexes in relation to the pelvis when sitting. The pelvic femoral angle can be used to measure the position of the femur relative to the pelvis when standing (180°) and sitting (125°), and differences between the standing and sitting values can be used to assess the dynamic motion of the femur. The normal values for these commonly assessed measurements of spinopelvic mobility are listed in Table III.

TABLE III - Normal Radiographic Spinopelvic Values*
Standing Sitting Change
Pelvic incidence 53° ± 11° 53° ± 11°
Sacral slope 40° ± 10° 20° ± 9° 11°-29°
Pelvic femoral angle 180° ± 15° 125° ± 15° 50°-75°
Ante-inclination 35° ± 10° 52° ± 11°
*Data are from Stefl et al.5.
The values are given as the mean and the standard deviation.
Change is the difference between standing and sitting; pelvic incidence is a static anatomic measurement, so it does not differ between standing and sitting. The other 3 measurements are dynamic (positional parameters) so they differ between standing and sitting.

What Happens with Abnormal Spine-Pelvis-Hip Motion?

Because motion of the spine, pelvis, and hip is coordinated during postural changes, any disease that affects the mobility of one will often affect the others. If one part of a mobile segment stiffens (i.e., reduced motion), then the other sections of the segment must accommodate for this by becoming more mobile. Orthopaedic surgeons have long understood that, after a spine fusion, the vertebral segment cephalad to the fusion has more stress because it must move more. This same stiff spine can force the femoral side of the hip to flex more with sitting, or extend more with standing, and this excessive hip motion can cause impingement of the greater trochanter on the pelvis1,2,5.

The pelvis is connected to the spine by the lumbosacral joint (the pelvis has been termed an accessory vertebra). The motion of the spine and pelvis is called spinopelvic mobility, and abnormal motion results in an unbalanced spine and pelvis1,2,4-8. The patterns of spinopelvic mobility abnormalities have been categorized by 2 studies5,6 into 2 types: too much motion (hypermobility) and reduced motion (stiffness). Hypermobility is defined as pelvic motion of ≥30° between standing and sitting. Hypermobility may be a normal variant, and it is found mostly in younger patients and women (Fig. 3-A). No adverse consequences of hypermobility have been identified. Hypermobility provides an advantage for patients because it requires less hip motion during postural change, and therefore is associated with the lowest risk of impingement. Hypermobility is considered to be unbalanced when it is a result of the lumbar spine tilting into kyphosis with sitting (the Stefl kyphosis5 or the Phan flexible and unbalanced category6) (Fig. 3-B). The kyphosis pattern is represented by a seated sacral tilt of <10° and is considered severe when <5°. This pattern is most commonly associated with 3 conditions: stiff hips that have flexion of ≤50°, which forces increased posterior tilt of the pelvis during sitting; patients with a body mass index (BMI) of >40 kg/m2, who have a large trunk mass that forces increased posterior tilt of the pelvis with sitting to balance their body8; and patients with neuromuscular imbalance2.

Figs. 3-A and 3-B Illustrations of high and low pelvic incidence (normal pelvic incidence is shown in Figure 1).

Fig. 3-A
Fig. 3-A:
Illustration of high pelvic incidence (70°) that often has hypermobility of sacral tilt (50° of sacral tilt with standing to 20° with sitting, which is 30° of motion). Pelvic femoral angle motion is from 184° to 155°, which is 29°, and is low because pelvic posterior motion is above normal.
Fig. 3-B
Fig. 3-B:
Illustration of kyphosis, or a flexible unbalanced hip, which means the sacral tilt is posteriorly tilted to <5° and is 0° in this illustration. The spine tilts backward. This can occur with any pelvic incidence, so it is inconsequential. With this amount of posterior tilt, the acetabular inclination and anteversion increase so much with sitting that the ante-inclination is very vertical—in this illustration, it is 70° with sitting. Because no hip surgeon wants a cup at 70°, the dotted lines outline the ante-inclination of 50° if the intraoperative cup angles are between 35° and 40° and anteversion is 15° to 20° (Table IV). With standing, the cup outline is 30°, with osseous ante-inclination of 45°. The pelvic femoral angle moves from 210° to 135°.

Spinopelvic stiffness is defined as ≤10° of motion at the spine-pelvis junction between the standing and seated positions (i.e., change in sacral tilt between standing and sitting positions) (Figs. 2-A and 2-B). This immobility is almost always caused by lumbar degenerative disc disease, facet spondylosis of the lumbar spine, lumbar fusion, or ankylosing spondylitis1,7,8. Hip and spine surgeons recognize that degenerative changes of the spine and hip osteoarthritis commonly coexist. Two studies found that approximately 40% of patients with hip osteoarthritis undergoing primary total hip replacement had degenerative disc disease of the lumbar spine that was related to age5,8. In our patients, 30% of those who were <60 years old had radiographic stiffness of the spine compared with 55% of those who were ≥60 years old. Stiffness can occur in 3 patterns as described by Stefl et al.5. In the first pattern, the pelvis tilts posteriorly from standing to sitting ≤10°, such that the sacral tilt crosses a value of 30° (i.e., a sacral tilt of 34° while standing and sacral tilt of 26° while sitting). This pattern is characterized by decreased pelvic motion, but is not fixed anteriorly or posteriorly. The second pattern, termed stuck standing, is loss of posterior tilt when sitting so that the pelvis is fixed in anterior tilt (sitting sacral tilt is >30°). In the study by Phan et al.6, this pattern is called rigid and balanced (Fig. 2-A). In the third pattern, termed stuck sitting, the pelvis is fixed in posterior tilt and never tilts anteriorly with standing (standing sacral tilt is <30°). Phan et al.6 called this pattern rigid and unbalanced (Fig. 2-B). The patterns of fixed posterior tilt and fixed anterior tilt can be combined with marked stiffness of the pelvis, defined as a <10° change in sacral tilt during postural changes, which compounds these fixed patterns and increases the risk of impingement and dislocation.

There is a 0.8° change in acetabular anteversion for each degree of pelvic tilt15 so the amount of functional anteversion can be calculated when sitting based on pelvic motion. Lazennec et al.16 used computed tomography (CT) scans to measure 15.6° of increased acetabular anteversion with the normal 20° of posterior pelvic tilt with sitting, which confirmed this calculation. Since the acetabulum anteverts more with sitting to contain the flexed and internally rotated femoral head, the loss of this anteversion increases the risk of impingement. After total hip replacement, dislocation can occur because of the loss of functional anteversion, resulting in anterior impingement.

If the stiffness is so severe that the spinopelvic motion between sitting and standing is <5°, the spinopelivic junction is essentially fused, either biologically from degenerative changes or surgically. Stefl et al.5 termed this condition pathologic stiffness. As noted by Phan et al.6, this is an extreme version of the rigid and unbalanced pattern, whereby the immobile pelvis and acetabulum transfers all motion for postural changes to the hip, creating the highest possible risk for impingement (Figs. 2-A and 2-B). The dangerous stiffness described by Stefl et al. is less severe because there is up to 10° of spinopelvic motion (i.e., the difference between standing and sitting sacral tilt is 5° to 10°), but this stiffness is often present with a pelvis fixed in either posterior or anterior tilt. In the next section, the consequences of these different patterns of stiffness in relation to total hip replacement are addressed.

Clinical Consequences of Spinopelvic Imbalance

The clinical consequences of spinopelvic imbalance can affect both the lumbar spine and the hip. The management of degenerative diseases of the intervertebral disc and of spinal stenosis have been well described and are not the focus of our review. This review focuses on newer research related to the relationship between spinopelvic alignment (and motion) and negative outcomes following total hip replacement. Deleterious consequences are often the result of decreased spinopelvic motion, and the only hypermobility imbalance to cause complications is the kyphosis pattern (i.e., the flexible and unbalanced pattern described by Phan et al.6) (Figs. 3-B and 4). Lazennec et al. were the first, to our knowledge, to describe the effects of spine and pelvic mobility on the hip, and we refer readers interested in this subject to their studies1-3.

Fig. 4
Fig. 4:
Postoperative lateral spine-pelvis-hip radiograph made with the patient sitting shows a kyphotic pattern (sitting sacral tilt [ST] = 4°). The pelvic incidence (PI) is 56°, which is normal. The sitting ante-inclination (AI) is high (77°) because the pelvis has tilted posteriorly so much with a cup that has coronal radiographic inclination of 54° and anteversion of 18°. Dropout dislocation occurred in the first week postoperatively as the patient sat in a deep soft couch and shifted position. The vertical cup position provided no inferior mechanical block, so the head dropped out (in biomechanical language, it exceeds its jump distance). The cup inclination for a patient with lumbar kyphosis should be no more than 40° (Table IV).

Surgeons are accustomed to viewing the hip on standard anteroposterior radiographs, which assess the acetabulum in the coronal plane. However, the use of lateral radiographs and the assessment of sagittal motion of the hip during postural change are relatively new1-3. The consequences of spinal imbalance on hip function in a nonarthritic hip have not been fully studied. It has been suggested that patients with low pelvic incidence are at greater risk for osseous impingement of the greater trochanter on the ilium because more hip motion is routinely observed in such individuals (Table II)3. In the past 5 years, research has focused on the consequences of spinal imbalance after a total hip replacement, so we focused on this area.

The interest in studying the sagittal cup position is, first, to understand its relationship to the cup position seen on the anteroposterior radiograph and to determine if understanding the sagittal cup position would be helpful in identifying the best intraoperative cup position2,5,17,18. Second, the coronal cup position has not been reliable in predicting postoperative dislocation following total hip replacement, and thus there is interest in whether the sagittal radiographs will be better. If the sagittal view is better, then we need to understand the impact of spinopelvic abnormalities that it may demonstrate. Phan et al.6 emphasized the influence of spinal imbalance on anteversion. This is important because anteversion has been labeled as the most important implant parameter6,19-21, and anteversion changes substantially for each degree of pelvic change (as discussed above). In almost all such studies, investigators have researched the potential for spinal imbalance to influence impingement with total hip replacement1-9. Surgeons cannot easily diagnose hip impingement following total hip replacement because it is a clinical diagnosis, and no currently available imaging or computerized technique can reliably identify impingement22. Dislocation is the most recognized consequence of impingement. It occurs when the impingement, either component or osseous, is severe enough that the mechanical constraint of the implants, and the biological constraint of the capsule and muscle tension, cannot prevent escape at the egress site19. A second cause is when the cup position is too vertical, and impingement occurs with excessive hip flexion so that the femoral head has no cup constraint inferiorly and may dislocate posteriorly (Fig. 3-B)19,23. In biomechanical terms, the head has exceeded its jump distance23. In primary total hip replacement done by experienced surgeons, the cup inclination and anteversion are maintained within a 10° range, and stability and wear are optimal20,24. The dislocation is more commonly caused by decreased leg length or offset25. The association between dislocation and coronal cup position has been unpredictable26.

In primary hip replacement, the highest risk for impingement from spinal imbalance is pathologic stiffness (rigid and unbalanced hips, according to Phan et al.6). The effect on total hip replacement stability in the presence of a spinal fusion, which creates this imbalance, has been studied27-29. These investigations all noted a higher rate of dislocation in patients with fused spines. The patients with a fused spine provided the clearest data to date on the impact of spinal imbalance on impingement. However, other complications also occur because of impingement. Pain is a known consequence of impingement but may be difficult to identify30; wear debris and fluid collections around the joint may be caused by impingement and may be destructive31,32; and loosening of components may occur from severe impingement, which may explain the recent findings of loosening of the femoral components at 3 to 5 years after the direct anterior approach33,34.

The coronal cup position implanted at surgery directly influences the sagittal cup position (ante-inclination) measured on the lateral spinopelvic radiographs. It is possible that sagittal cup measurements can provide a more predictable “safe zone” for optimal cup position (the combination of anteversion and inclination) than coronal safe zones such as those described by Lewinnek et al.35. As long ago as 1990, McCollum and Gray, in their study on dislocation, recommended lateral radiographs of the cup36. Further research and validation is necessary before that can be accepted in routine clinical practice; however, research has been done to determine coronal cup positions that keep the cup angle in the normal sagittal range10,17,18. Multiple studies have shown that normal spine-pelvis-hip mobility (flexible and balanced according to Phan et al.6) can have a wider range of coronal cup positions without failure from instability10,24,26,37; however, a component range of 10° seems optimal (Table IV). A 10° range duplicates the findings in the laboratory that were ideal for stability and wear20. In addition, McCarthy et al.38 showed that, for functional activities such as bending, low sitting, and squatting, a smaller safe zone than that described by Lewinnek et al.35 is required. One problem with accepting cups outside even the broad zones described by Lewinnek et al. is that, although the cups have been tested for instability26, there have been no studies, to our knowledge, on the longevity of cups outside this zone. Goyal et al.24 reported dislocation and adverse wear in 1% of their patients at 12 years after total hip arthroplasty performed by experienced surgeons. However, data from high-volume centers with skilled surgeons are not always consistent with data from the larger orthopaedic community, which have shown that 15.8% to 32.4% of patients were readmitted because of dislocation after primary total hip replacement39-41. Dislocation also ranks as the second most common mechanical cause of revision after primary total hip replacement, with a rate of 12.2% of revisions39. In addition, the increased prevalence of dislocations after total hip replacement in patients with a spinal fusion provide the basis for the development of a sagittal “safe zone.”27-29 Therefore, orthopaedic surgeons need to gain further insight into the spine-pelvis-hip relationship to minimize such cases of postoperative instability.

TABLE IV - Ideal Cup Position by Spinopelvic Mobility*
Inclination Anteversion Combined Anteversion
Normal 35°-45° 15°-25° 25°-45°
Stiff 45°-50° 20°-25° 35°-45°
Kyphotic 35°-40° 15°-20° 25°-35°
Hypermobile 35°-40° 12°-20° 25°-35°
*Cup anteversion is dependent on combined anteversion, which must be higher for stiff imbalance and lower for hypermobile hips to keep sitting ante-inclination within its normal range. In hips that are retroverted, it is difficult to achieve cup anteversion exceeding 12° to 15° so combined anteversion becomes critical in achieving stability for those hips. The range for each of these patterns is within 10°, and it is difficult to achieve this precision at surgery without some form of navigation. However, these would be the ideal coronal cup angles for these patterns to keep the sagittal ante-inclination in its normal range. Total hip replacement has done so well for so many years because these cup angle numbers are within the cup positions that most surgeons strive to achieve at surgery.
Inclination of 50° is reserved for elderly patients.

For new meaningful safe zones to be recommended, we need studies that assess the stability and survival of acetabular cups placed outside the currently recommended ranges of inclination and anteversion. Until those data are available, the cup positions currently suggested for the different spinal imbalances can be used (Table IV). For patients with stiff spinopelvic motion, the acetabular cup requires more coronal inclination and anteversion; and, for hips with hypermobility, the cup needs less coronal inclination and anteversion. Primary total hip replacement has been successful because surgeons have targeted cups at 40° to 45° of inclination and 15° to 20° of anteversion, and these positions keep the cup in the sagittal safe zone even with most spinopelvic abnormalities. Stefl et al.5 observed dislocations when cup inclination was <35° and/or anteversion was <15°, and Tang et al.17 observed cup coverage deficiencies at these low angles.

An important confounding factor to consider when including spinopelvic mobility into preoperative planning for primary total hip replacement is that the postoperative spinopelvic mobility may change from the preoperative mobility9,42-44. One reason for this postoperative change is that the preoperative contractures of the hip, which reduce its motion, affect the pelvic mobility, and after these contractures are released at surgery, the postoperative mobility of both the hip and the pelvis may change. The preoperative measurements of sacral tilt may change as much as 20° postoperatively9,42,44. In patients with normal spine-pelvis-hip mobility, Sariali et al.44 found an average change of 5° with standing and 3° with sitting after total hip replacement; however, 46% of patients while standing and 33% of those sitting had a change of >5°. What has not been determined is how often these changes cause the sagittal position of the cup to fall outside its normal ante-inclination range. In other words, the change in pelvic mobility may be clinically insignificant if it does not force the sagittal cup position out of its normal range. Future research to establish the clinical relevance of the relationship between the change in postoperative pelvic mobility and the sagittal cup position is required. If this research validates the sagittal cup position as a predictable measure of hip stability, it will be of benefit to hip surgeons and their patients.

The impact of spinal imbalance is greatest with revisions, with late dislocations, and in elderly patients. The rate of dislocation resulting in readmission was reported to be 4.4% after revision total hip replacement compared with 0.7% after primary total hip replacement in 1 study39. In our study of patients 10 years after primary total hip replacement, 60% of the patients had spinal stiffness compared with 20% of those undergoing primary total hip replacement5,45. This is consistent with the findings of Tamura et al.46 and Okanoue et al.47 , who both found increased tilt of the pelvis 10 years after primary total hip replacement. Dislocation following revision surgery and late dislocations following primary total hip replacement are more often affected by spine disease as these complications are more likely to occur in older patients8,45-48. In our unpublished data from 20 late dislocations, we found that 18 patients had spinopelvic stiffness as well as soft-tissue changes that combined to cause unstable impingement. In our patients who had late dislocation, the most common imbalance was fixed anterior tilt and stiffness (i.e., stuck standing or rigid and unbalanced) that resulted in posterior dislocation. These older patients had spinopelvic changes that were fixed with no postoperative change in sacral tilt after revision surgery. These patients commonly have soft-tissue weakness—capsular laxity and muscle weakness, especially of the abductors—that, when combined with an unbalanced spine, creates a high risk for dislocation48. In most of these patients, we use constrained liners or dual mobility articulations at revision total hip replacement to enhance the mechanical stability of the articulation.

We suggest that standing and sitting lateral spine-pelvis-hip radiographs be made before surgical treatment involving revision total hip replacement, especially for acute or late dislocation, and primary total hip replacement in patients with spinal fusion or known spine disease (Table IV). In patients with severe spinopelvic stiffness, the surgeon must pay attention to the cup position and consider increased constraint of the articulation. When surgical fusions of the lower lumbar spine are present, awareness of the increased risk of dislocation should prompt the surgeon to pay close attention to cup position as well as biomechanical reconstruction of offset and hip length and to consider the use of constraint or a dual mobility articulation.

Note: The authors thank Patricia Paul for manuscript preparation, Michael Smith for audiovisual assistance, and Dr. Russell Bodner for contributions to the concepts discussed in this manuscript.

Investigation performed at the Department of Orthopaedic Surgery, Keck Medical Center of the University of Southern California, Los Angeles, California

Disclosure: Funding provided by the Dorr Research and Education Institute. On the Disclosure of Potential Conflicts of Interest forms, which are provided with the online version of the article, one or more of the authors checked “yes” to indicate that the author had a relevant financial relationship in the biomedical arena outside the submitted work (http://links.lww.com/JBJS/E882).

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