Abstract
Background: Spinopelvic hypermobility may be secondary to a stiff osteoarthritic hip with a compliant spine. Purpose: We sought to determine if spinopelvic hypermobility resolves after total hip arthroplasty (THA) and when it resolves in patients with bilateral hip osteoarthritis (OA) undergoing staged bilateral THA. We also sought to analyze the change in spinopelvic parameters before and after the second THA. Methods: We conducted a retrospective review of 2047 THAs that were performed by 2 fellowship-trained arthroplasty surgeons from 2014 to 2018. Patients with preoperative spinopelvic hypermobility undergoing staged bilateral THA were identified. Radiographic spinopelvic parameters, including sacral slope (SS), pelvic incidence (PI), lumbar lordosis (LL), PI-LL mismatch, anterior pelvic plane tilt (APPt), and spinopelvic tilt (SPT), were measured on preoperative, 6-week postoperative, and 1-year postoperative lateral standing and sitting radiographs. Bilateral hip OA was graded using Kellgren-Lawrence criteria. Results: We identified 42 patients with preoperative spinopelvic hypermobility who underwent staged bilateral THA. Mean time (standard deviation) between surgeries was 9.4 months (±10.0). After the first THA, spinopelvic hypermobility resolved in 29% of the patients. After the second THA, it resolved in 67% at 6 weeks, increasing to 98% at 1 year postoperatively. Conclusion: Spinopelvic hypermobility resolves after staged bilateral THA in 98% of the patients, occurring most often only after the second THA. Less than one-third of the patients had resolution after the first THA, suggesting that contralateral hip OA continues to drive hip-driven spinopelvic motion. Acetabular component position targets based on functional pelvic position should incorporate these changes in spinopelvic motion with the understanding that resolution of hypermobility usually occurs after the second THA.
Keywords: total hip arthroplasty, spinopelvic mobility, hypermobility, functional cup position
Introduction
The lumbo-pelvic-hip complex moves in a coordinated fashion when a person transitions between various postural positions [12]. When moving from a standing to a sitting position, the pelvis normally rotates 20° posteriorly, measured in change in sacral slope (SS), which opens the face of the acetabulum and allows for impingement-free flexion of the femur [6,11,13]. Abnormal spinopelvic motion, categorized as either stiff or hypermobile, can have implications on optimal cup positioning during total hip arthroplasty (THA) [2,4,7,10]. A change in SS from standing to sitting (ΔSSstand-sit) ≤ 10° defines stiff spinopelvic mobility, whereas ΔSSstand-sit ≥ 30° defines spinopelvic hypermobility [6,9,19].
It is important to distinguish the effects of sagittal imbalance and abnormal spinopelvic mobility on dislocation risk. Patients with stiff mobility, due to either degenerative disk disease or long segment spine fusions, have a limited ability to rotate the pelvis posteriorly when transitioning to the seated position. This results in less acetabular anteversion, increasing the risk of anterior impingement and posterior dislocation with hip flexion [6]. Patients with sagittal imbalance, or those with flatback deformity, tend to have more posterior pelvic tilt when standing, thought to be a compensatory mechanism for maintaining upright posture in the setting of increased kyphosis. Standing posterior pelvic tilt can increase the risk of posterior impingement and anterior dislocation with hip extension [2,4].
Adjusted acetabular component targets have been recommended for patients with abnormal spinopelvic mobility; these include increasing target anteversion values for those with stiff mobility, while decreasing target anteversion values for hypermobile patients [6,14]. For example, Ike et al recommends a reduced anteversion target range of 12° to 20° in hypermobile patients, compared with 15° to 25° in normal patients [6]. The adjusted targets for hypermobile patients are meant to prevent the risk of posterior impingement and anterior dislocation that can result from excessive opening of the cup when these patients move to the sitting position.
However, the above recommendations assume that spinopelvic mobility is constant following THA. If the patient’s hypermobility resolves after hip replacement, there may not be enough functional anteversion of the cup, which could result in anterior impingement leading to posterior dislocation in the sitting position. Prior studies have shown that changes in spinopelvic mobility can occur somewhat unpredictably following THA [8,17,18]. This may depend on whether preoperative spinopelvic motion is hip-driven or lumbar spine-driven, as spinopelvic hypermobility assessed on lateral standing and sitting radiographs may represent normal lumbar spine motion or be secondary to a stiff osteoarthritic hip. We believe that lumbar spine-driven hypermobility will not change after THA but rather that hip-driven spinopelvic motion will return to being lumbar-spine driven only. Therefore, hip flexion will improve and hip flexion contractures will resolve, which may change postoperative pelvic motion and functional acetabular component positioning. It is unknown whether or not preoperative spinopelvic mobility resolves after THA; nor is it known when it resolves in patients with bilateral hip osteoarthritis (OA) undergoing staged THA. For this reason, the primary aim of our study was to determine if spinopelvic hypermobility resolved (ΔSSstand-sit < 30°) in these patients after staged bilateral THA. Our secondary goal was to analyze the change in spinopelvic parameters before and after the second THA; our hypothesis was that preoperative spinopelvic hypermobility would resolve only after both hips are replaced.
Methods
We performed a retrospective review of 2047 THAs that occurred from 2014 to 2018 and were performed by 2 fellowship-trained arthroplasty surgeons. Our inclusion criteria consisted of patients who had (1) bilateral OA or avascular necrosis (AVN) at the time of their first THA, (2) staged bilateral THAs within 3 years of each other, (3) preoperative hypermobility (ΔSSstand-sit ≥ 30°) before the first THA, and (4) complete standing and relaxed-seated lateral stereoradiographs (EOS Imaging, Paris, France) at both preoperative visits (before first and second THA), at 6 weeks, and at least 1 year postoperatively after the second THA. We excluded patients with unilateral OA who underwent primary unilateral THA, revision or conversion THAs. There were 1963 patients who met the exclusion criteria (1543 not hypermobile, 362 unilateral, 58 bilateral without complete imaging). Ethics review board approval and informed consent were not required as this study was a retrospective chart review with no risk to participants.
A total of 42 patients (84 hips) undergoing staged bilateral THA with preoperative spinopelvic hypermobility were identified. All THAs were performed using the posterior approach and with assistance of either sensor-based computer navigation or robotics. Mean age at time of first surgery was 55.8 ± 9.7 years, and 57% (24/42) of patients were male (Table 1). Mean body mass index (BMI) was 31.0 ± 7.0 kg/m2. Dual mobility liners were placed in 2/84 hips (2%), and no patients had spine fusion before or after both THAs. A total of 38 patients (90%) had bilateral OA at the time of their first THA, whereas 4 patients (10%) had bilateral AVN. Of the patients with OA, 35/38 patients (92%) had severe OA (KL grade 3–4) in bilateral hips at the time of their first THA. Mean time between surgeries was 9.4 months (standard deviation: ± 10.0 months, range: 1–37 months). The median follow-up time was 21 months (range: 11–49 months). No patients had dislocations or underwent reoperations before time of last follow-up.
Table 1.
Patient demographics and clinical characteristics of study population.
| Variable | Group | Overall | First surgery | Second surgery | P value | |||
|---|---|---|---|---|---|---|---|---|
| Mean or N |
SD or % |
Mean or N |
SD or % |
Mean or N |
SD or % |
|||
| Age at surgery | 56.2 | 9.7 | 55.8 | 9.7 | 56.5 | 9.8 | .721 | |
| BMI | 31.0 | 6.9 | 31.0 | 7.0 | 31.1 | 6.9 | .966 | |
| Interval time (months) | 9.4 | 10.0 | . | . | 9.4 | 10.0 | NA | |
| Gender | M | 24 | 57% | 24 | 57% | . | . | NA |
| F | 18 | 43% | 18 | 43% | . | . | ||
| Computer navigation | N | 42 | 50% | 22 | 52% | 20 | 48% | .663 |
| Y | 42 | 50% | 20 | 48% | 22 | 52% | ||
| Robotics | N | 42 | 50% | 20 | 48% | 22 | 52% | .663 |
| Y | 42 | 50% | 22 | 52% | 20 | 48% | ||
| Dual mobility | N | 82 | 98% | 41 | 98% | 41 | 98% | 1.000 |
| Y | 2 | 2% | 1 | 2% | 1 | 2% | ||
| Spine fusion | N | 84 | 100% | 42 | 100% | 42 | 100% | NA |
| Y | 0 | 0% | 0 | 0% | 0 | 0% | ||
| Reoperation | N | 82 | 100% | 41 | 100% | 41 | 100% | NA |
| Y | 0 | 0% | 0 | 0% | 0 | 0% | ||
| Spine Grade preop | 0 | 32 | 40% | 16 | 40% | 16 | 39% | .910 |
| 1A | 44 | 54% | 22 | 55% | 22 | 54% | ||
| 2A | 5 | 6% | 2 | 5% | 3 | 7% | ||
| Pelvic mobility preop | Normal | 12 | 14% | 0 | 0% | 12 | 29% | .000 |
| Hypermobile | 72 | 86% | 42 | 100% | 30 | 71% | ||
| KL Grade—Surgical Side | Low Grade (0,1,2) | 0 | 0% | 0 | 0% | 0 | 0% | NA |
| Severe (3,4) | 76 | 100% | 38 | 100% | 38 | 100% | ||
| KL Grade—contralateral side | Low Grade (0,1,2) | 45 | 56% | 3 | 8% | 42 | 100% | .000 |
| Severe (3,4) | 35 | 44% | 35 | 92% | 0 | 0% | ||
| PI-LL balance preop | PI-LL < −10 = Hyperlordosis | 32 | 40% | 16 | 40% | 16 | 39% | .910 |
| −10 > PI-LL > 10 = Balanced | 44 | 54% | 22 | 55% | 22 | 54% | ||
| PI-LL > 10 = Lumbar Flatback | 5 | 6% | 2 | 5% | 3 | 7% | ||
BMI body mass index, KL Kellgren-Lawrence, PI-LL pelvic incidence—lumbar lordosis, SD standard deviation.
Spinopelvic measurements were completed by one author (E.W.), a 4-year medical student, on standing and sitting lateral EOS radiographs at all time points. These measurements included standing sacral slope (SSstand), pelvic incidence (PI), lumbar lordosis (LL), PI-LL mismatch (PI minus LL), spinopelvic tilt (SPT), and anterior pelvic plane tilt (APPt) on the standing lateral radiograph. Seated sacral slope (SSseated) was measured on the sitting radiograph. Fig. 1 shows an example of how the spinopelvic measurements were taken. We considered a 6.8° difference in SS between pre- and post-THA measurements to be a clinically significant change in pelvic tilt, as this is equivalent to a 5° change in postoperative cup anteversion (1° change in pelvic tilt = 0.7º–0.8° change in functional cup anteversion) [16]. The difference in SS between the standing and sitting positions (ΔSSstand-sit) at each time point was calculated, which determined the patient’s spinopelvic mobility category as follows: stiff (ΔSSstand-sit ≤ 10), normal (ΔSSstand-sit = 10–30), hypermobile (ΔSSstand-sit ≥30). Spine grade was determined according to the hip-spine classification described by Luthringer and Vigdorchik [15]. At all time points, bilateral OA severity was graded using the Kellgren-Lawrence (KL) scale. Osteoarthritis was categorized as either low-grade OA (LOA, KL grade 0–2) or severe OA (SOA, KL grade 3–4). Information on clinical follow-up—including dislocations, reoperations, and revisions—were collected.
Fig. 1.
Example of spinopelvic measurements on lateral standing (left) and sitting (right) EOS imaging.
Statistical Analysis
Differences in radiographic parameters over each time point were compared using a repeated measures analysis of variance (ANOVA). Post-hoc pairwise comparisons were conducted to evaluate differences between time points, which were adjusted to control for any type 1 error from multiple comparisons. A series of McNemar’s tests was used to see if there was a change in the proportion of patients who were hypermobile based on ΔSSstand-sit. Standard descriptive statistics were reported with means and standard deviations for the continuous variables, and counts and percentages for the categorical variables. All statistical tests used a significance level of 0.05.
Results
In this series of patients, spinopelvic hypermobility is reduced to clinically safe normal ranges (10° < ΔSSstand-sit < 30°) at 6 weeks following the second THA and this improvement continues out to 1 year. Prior to undergoing the second THA, 29% (12/42) of patients had resolution of spinopelvic hypermobility on preoperative sitting and standing radiographs (Table 2). Six weeks after the second THA, 67% (28/42) of patients were no longer hypermobile on sitting and standing radiographs. 1 year after the second THA, 98% (41/42) of patients had resolution of spinopelvic hypermobility.
Table 2.
Change in spinopelvic measures over time between first and second THA.
| Outcome | Time | First surgery | Second surgery | P value [B] | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Mean | SEM | 95% L | 95% U | Mean | SEM | 95% L | 95% U | |||
| PI | Pre | 57.3 | 10.6 | 54.2 | 60.5 | 57.3 | 10.9 | 54.2 | 60.4 | .990 |
| 6W | 57.1 | 10.7 | 53.3 | 61.0 | 57.9 | 10.7 | 54.8 | 61.0 | .761 | |
| 1Y | 57.5 | 10.8 | 54.4 | 60.6 | ||||||
| P-value [W] | .985 | .965 | ||||||||
| LL | Pre | 62.9 | 10.3 | 59.8 | 66.0 | 63.3 | 9.8 | 60.2 | 66.3 | .868 |
| 6W | 64.5 | 11.8 | 60.7 | 68.3 | 64.1 | 9.3 | 61.0 | 67.1 | .861 | |
| 1Y | 62.0 | 9.8 | 58.9 | 65.0 | ||||||
| P-value [W] | .296 | .638 | ||||||||
| PI-LL | Pre | −5.4 | 11.3 | −8.9 | −2.0 | −5.6 | 11.7 | −8.9 | −2.2 | .964 |
| 6W | −7.1 | 11.9 | −11.3 | −2.9 | −5.6 | 12.0 | −9.1 | −2.2 | .598 | |
| 1Y | −4.4 | 11.4 | −7.8 | −1.0 | ||||||
| P-value [W] | .430 | .854 | ||||||||
| SPTstand | Pre | 11.3 | 9.2 | 8.7 | 13.8 | 10.8 | 8.4 | 8.3 | 13.3 | .808 |
| 6W | 11.2 | 8.7 | 8.1 | 14.4 | 13.2 | 9.0 | 10.7 | 15.7 | .340 | |
| 1Y | 15.4 | 8.6 | 12.9 | 18.0 | ||||||
| P-value [W] | .571 | .041 | ||||||||
| SPTseated | Pre | 49.7 | 10.3 | 46.6 | 52.8 | 44.2 | 10.4 | 41.2 | 47.3 | .013 |
| 6W | 43.4 | 10.7 | 39.7 | 47.2 | 39.0 | 11.6 | 35.9 | 42.1 | .074 | |
| 1Y | 34.8 | 8.4 | 31.8 | 37.9 | ||||||
| P-value [W] | .039 | .000 | ||||||||
| APPt | Pre | 3.7 | 7.5 | 1.4 | 6.0 | 4.1 | 8.3 | 1.8 | 6.4 | .795 |
| 6W | 4.9 | 9.3 | 2.1 | 7.7 | 2.1 | 7.1 | −0.15 | 4.4 | .136 | |
| 1Y | −0.34 | 7.7 | −2.6 | 1.9 | ||||||
| P-value [W] | .664 | .027 | ||||||||
| SSstand | Pre | 46.1 | 8.1 | 43.6 | 48.5 | 46.5 | 8.6 | 44.1 | 49.0 | .814 |
| 6W | 45.9 | 9.5 | 42.9 | 48.9 | 44.7 | 7.6 | 42.2 | 47.2 | .549 | |
| 1Y | 42.1 | 7.5 | 39.6 | 44.5 | ||||||
| P-value [W] | .436 | .041 | ||||||||
| SSseated | Pre | 7.6 | 8.8 | 4.8 | 10.4 | 13.1 | 8.8 | 10.3 | 15.9 | .007 |
| 6W | 13.7 | 9.8 | 10.3 | 17.2 | 18.9 | 10.1 | 16.1 | 21.7 | .022 | |
| 1Y | 22.7 | 10.3 | 19.9 | 25.5 | ||||||
| P-value [W] | .027 | .000 | ||||||||
| ΔSSstand-sit | Pre | 38.5 | 7.0 | 35.9 | 41.0 | 33.4 | 9.4 | 30.9 | 36.0 | .006 |
| 6W | 32.2 | 11.2 | 29.0 | 35.3 | 25.8 | 8.9 | 23.3 | 28.4 | .002 | |
| 1Y | 19.4 | 7.5 | 16.9 | 22.0 | ||||||
| P-value [W] | .005 | .000 | ||||||||
APPt anterior pelvic plane tilt, LL lumbar lordosis, PI pelvic incidence, SEM standard error measurement, SPT spinopelvic tilt, SS sacral slope, ΔSSstand-sit change in sacral slope from standing to sitting position, THA total hip arthroplasty.
The following spinopelvic mobility changes were noted. Preoperative to the first THA, mean ΔSSstand-sit was 38.5° ± 7.0, which decreased to 33.4° ± 9.4 preoperative to the second THA (P = .006). Six weeks postoperative to the second THA, mean ΔSSstand-sit decreased to 25.8° ± 8.9 and continued to decrease to 19.4° ± 7.5 at 1 year after the second THA (P < .001). Both SSstand and APPt did not change significantly between the 2 preoperative visits, but then decreased from 46.5° ± 8.6 to 42.1° ± 7.5 for SSstand (P = .041) and from 4.1° ± 8.3 to -0.3° ± 7.7 for APPt (P = .027) at 1-year after the second THA. Ultimately, SSseated increased from 7.6° ± 8.8 at the first preoperative visit to 13.1° ± 8.8 at the second preoperative visit (P = .007), and then increased to 18.9° ± 10.1 at 6 weeks and then to 22.7° ± 10.3 at 1 year following the procedure on the second side (P < .001). The trends of hypermobility, ΔSSstand-sit, SSseated, and APPt over time are shown in Figs. 2–5. There was no significant effect of time interval between staged THAs on spinopelvic parameters. However, values are reported as adjusted for time interval.
Fig. 2.
Percentage of patients with spinopelvic hypermobility over time.
Fig. 3.
Change in sacral slope from standing to sitting (ΔSSstand-sit) over time.
Fig. 4.
Change in seated sacral slope (SSseated) over time.
Fig. 5.
Change in standing anterior pelvic plane tilt (APPt) over time.
Discussion
Multiple studies have emphasized the importance of functional acetabular component positioning over static traditional targets based on the Lewinnek safe zone [1,6,15,20]. Functional acetabular component placement incorporates patient-specific pelvic positioning in the standing, sitting, supine, and occasionally hyper-flexed seated positions. For patients with spinopelvic hypermobility, current recommendations include decreasing target anteversion values. However, little is known about the effect of unilateral or bilateral hip OA on spinopelvic motion [6,14]. Changes in spinopelvic mobility following unilateral THA have so far been unpredictable. Stefl et al found that 57% of hypermobile patients resolved to normal mobility at last follow-up after THA (from 6 weeks to 1 year); the percentage of patients with normal mobility increased from 54% to 80% after THA [19]. Nam et al found a mean difference in ΔSSstand-sit of 1.9° ± 12.3° from preoperatively to postoperatively, suggesting that THA had minimal impact on pelvic mobility [17]. However, there was large variability, with values ranging from -20.2° to +23.3°. Furthermore, Kanawade et al found no significant difference in ΔSSstand-sit after THA, but their results also showed large variability [9]. A recent study found pelvic mobility, or median ΔSSstand-sit, decreased from 19.0° preoperatively to 16.0° postoperatively, which was statistically significant [5].
In our cohort of 42 patients undergoing staged THA, we found that spinopelvic hypermobility resolved in 98% (41/42) of the patients at 1 year after the second THA. Only 29% of patients had resolution of hypermobility after the first THA, which suggests that contralateral hip OA continues to drive hip-driven spinopelvic motion. Spinopelvic hypermobility resolves in nearly all patients (98%) at 1 year post second THA. Interestingly, of the 30 patients who continued to have spinopelvic hypermobility after the first THA, 29 of them (97%) eventually resolved their hypermobility once the contralateral hip OA was addressed. These findings strongly suggest that patients can have persistent hip-driven spinopelvic hypermobility until both hips are replaced. Furthermore, when planning the contralateral side of a persistently hypermobile patient who has had prior hip replacement, it is safe to assume that spinopelvic hypermobility will resolve after the second THA.
Our study does include some limitations. Due to our small sample size, there was some variability in the time interval between the 2 THAs and therefore we could not complete a sub-analysis more closely evaluating the effect of time between THAs. Our standard deviation of time between surgeries was large due to a few outliers who underwent their second THA more than 2 years after their first procedure. However, 93% of the patients in our cohort had severe bilateral OA (KL grades 3–4) or AVN at the time of their first THA, indicating that severe disease was present in both hips. Patient-reported outcome measures (PROMs) were not collected, so the clinical effect of resolution of hypermobility could not be assessed. Another limitation was that only 1 author completed all the measurements and, therefore, interobserver reliability was not assessed. Finally, we did not analyze the patients’ postoperative cup position or measure them on x-ray to determine the exact change or whether any fell out of the targeted safe zone. Instead, we used a formula to make an assumption of the change in postoperative functional cup anteversion based on the amount of pelvic tilt change.
The 1 patient whose hypermobility did not resolve was a 32-year-old male with AVN. This patient, who had severe bilateral hip flexion contractures and a preoperative APPt of 30º, continued to have spinopelvic hypermobility, with a ΔSSstand-sit of 41° at 1 year. Instead of classifying this as hypermobile, given this unique situation it is probable that the definition of normal mobility needs to be expanded.
SSseated increased 15.0° from the first preoperative visit to the 1-year postoperative visit following the second side. The significant increase in SSseated (or seated pelvic tilt), amounted to a decrease in seated functional cup anteversion that reached clinical significance. When the pelvis rotates anteriorly, as evidenced by an increasing SS, functional anteversion of the cup decreases by 0.7° to 0.8° for every 1° of pelvic tilt [16]. Therefore, from the preoperative visit before the first THA to the last postoperative visit at 1 year following the second THA, mean postoperative cup anteversion decreased by 11.1° with a maximum decrease of 23.7°. There were 35 patients (83%) who had a clinically significant decrease in seated functional anteversion of greater than 5° over this time span due to the pelvic recoil observed in the sitting position following THA.
APPt, (or standing pelvic tilt), changed from 4.1° of anterior tilt at the second THA’s preoperative visit to -0.3° at 1-year postoperative to the second THA. These results suggest that following bilateral THA, patient’s pelvic tilt will predictably return to neutral in the standing position after hip flexion contractures are resolved.
The mechanism of hip-driven spinopelvic hypermobility can be explained by a stiff osteoarthritic hip that restricts hip flexion causing the pelvis to roll back posteriorly in order to achieve a sitting position. After undergoing THA, hip stiffness resolves and patients are better able to flex their femurs when sitting, no longer requiring compensation through the pelvis. A hip flexion contracture can also contribute to hip-driven hypermobility causing increased anterior pelvic tilt when standing, leading to an increase in measured spinopelvic motion when transitioning from the standing to the sitting position. Fukushima et al found that women with developmental dysplasia of the hip had significantly greater standing SS and LL [3]. Their results suggest that acetabular dysplasia can lead to lumbar hyperlordosis and anterior pelvic inclination, which may be a source of spinopelvic hypermobility in these patients. Nevertheless, our results suggest that increased pelvic rollback when seated due to a stiff hip is the predominant source of hip-driven hypermobility. This radiographic observation correlates with clinical observations that a stiff hip in flexion is more common than hip flexion contractures.
Hypermobile patients are believed to be at risk of “dropout dislocation,” the idea that a vertical cup position greater than 65° to 75° ante-inclination in the sitting position can increase the risk of the femoral head exceeding the jump distance of the cup’s inferior edge during flexion, falling out posteriorly [9,14,19]. The increased pelvic mobility seen in these patients when transitioning from standing to sitting is thought to result in a more vertical cup while seated, increasing the risk of dropout dislocation. Thus, lower anteversion targets of 12° to 20° instead of 15° to 25° have been recommended for hypermobile patients to prevent them from having too vertical of a cup when sitting [6]. Since we found that patients predictably resolve their hypermobility after bilateral THA, we do not recommend adjusted anteversion targets. Once both hips are replaced and hypermobility resolves, pelvic rollback in the seated position should return to patient-specific lumbar spine-driven motion. However, if a patient has severe OA of the contralateral hip, hypermobility can persist until the other hip is treated. Yet, even in this case we do not recommend following the adjusted target recommendations. If the cup is placed at lower anteversion values and the patient’s hypermobility corrects postoperatively as expected, this can result in not enough anteversion in the seated position (due to the pelvis recoiling forward) and may lead to anterior impingement, posterior undercoverage, and increased risk of posterior dislocation.
In conclusion, spinopelvic hypermobility resolves in 98% of the patients 1 year after staged bilateral THA. However, 71% of the patients will have persistent hypermobility after the first THA when severe OA is present on their contralateral side. Acetabular anteversion targets should not be adjusted lower in patients with spinopelvic hypermobility, as hypermobility is hip-driven and will resolve after the second THA. Pelvic recoil in the sitting position after the second THA could lead to posterior undercoverage and posterior dislocation, particularly in patients undergoing posterior-approach THA.
Footnotes
Declaration of Conflicting Interests: The author(s) declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: Eric N. Windsor, MS, declares he has nothing to disclose. Peter K. Sculco, MD, reports relationships with Intellijoint Surgical, Depuy Synthes, and EOS Imaging outside the submitted work. David J. Mayman, MD, reports relationships with Imagen, Insight, Orthalign, Wishbone, and Smith and Nephew outside the submitted work. Jonathan M. Vigdorchik, MD, reports relationships with Corin, Intellijoint Surgical, Medacta, Motion Insights, and Zimmer outside the submitted work. Seth A. Jerabek, MD, reports relationships with Imagen and Stryker outside the submitted work.
Funding: The author(s) received no financial support for the research, authorship, and/or publication of this article.
Informed Consent: Informed consent was waived for all patients included in this study.
Level of Evidence: Level IV, therapeutic study.
Required Author Forms: Disclosure forms provided by the authors are available with the online version of this article as supplemental material.
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