Direct Anterior Approach in the Lumbar-Sacral Fusion Patient: EOS Considerations

Jenna A. Bernstein

Roy I. Davidovitch

Aaron J. Buckland

James Slover

Introduction

The coexistence of degenerative hip and spinal pathology has been well documented,¹^,² with up to 40% of patients with hip osteoarthritis undergoing primary THA having lumbar spine disease related to age.³ It is important to understand how deformity or arthritis in either the lumbar spine or hip can impact the other and the implications of treatment to the lumbar spine or hip may have on one another. Examining and understanding the influence of the spine-hip relationship (SHR) on outcomes after THA are important to optimizing treatment.

There has been a recent explosion in the literature outlining the relationship between postural change orientation in the spine and pelvis. The dynamic and compensatory relationships between the two are important to allow for a safe functional range of motion in replaced hips as well as in native hips. Changes in alignment or functional alterations to the hip or the lumbar spine can lead to an increased risk of complications after THA,⁴ the most common of which is instability. Most of the studies that examine the effect of the SHR on postoperative THA instability have been completed without specific regard to surgical approach. There are important differences that should be understood and integrated into the decision making that may vary by surgical approach, including the direct anterior approach (DAA).

This chapter examines the conventional knowledge of THA instability and how that may apply to the DAA approach. We examine what is known about the SHR, which patients are concerning for pathology, and how to evaluate them preoperatively. Recommendations are made based on the literature on what to do when identifying patients with a concerning or pathologic SHR. Finally, we address considerations specific to DAA surgeons regarding the SHR. At the end of the chapter, there is a simple, stepwise approach to incorporate this into your practice.

Epidemiology

Instability is one of the most common problems in THA, and dislocation is the most common early surgical complication after primary THA. It is also the number one short-term complication requiring reoperation within the first 2 years postoperatively, with an incidence of 0.2% to 1.7% reported in the literature.⁵ Factors that contribute to the increased incidence of THA dislocation include patient factors (female sex, previous ipsilateral hip surgery, and neuromuscular weakness), surgical factors (component malposition, utilization of the posterior approach, osteotomy, and avulsion of the greater trochanter), and design factors (a small prosthetic femoral head diameter).⁶

There remains a significant group of patients whose etiology for instability is still unknown (17%).⁶ Impingement is described as the most common mechanism of hip dislocation.⁷^,⁸ The best treatment for THA dislocation is prevention, and understanding how to properly position components is an important aspect of obtaining a good outcome. Patients with lumbosacral fusion are at an increased risk of dislocation after THA. Lumbar fusion before THA may be a risk factor for dislocation leading to an increased rate of revision THA,⁹ although this has been disputed in other studies.¹⁰ There is a significant overlap between patients who have both lumbar spine pathology and hip osteoarthritis who are indicated for THA.¹^,² One study found that, of patients with spinopelvic fusions, 4.6% had concurrent THA, and of patients with THA, 0.1% had concurrent lumbar spine fusions.¹¹ Between 2002 and 2014, there was a 293% increase in the number of patients with prior lumbar fusion undergoing THA.⁹ Patients with THA and concomitant spinal deformity have a high rate of THA instability despite having an acetabular cup position in the traditional safe zone.¹²

Multiple studies have tried to define the reason for this risk of instability, and with these studies, the traditional knowledge about the acetabular safe zone has come into question. In 1978, Lewinnek et al¹³ established the original safe zone for acetabular cup position as 15° ± 10° of anteversion and 40° ± 10° of inclination as a benchmark to optimize THA stability.¹³ Abdel et al¹⁴ have called this into question, finding that the majority (58%) of dislocated THAs were actually within the Lewinnek safe zone, and highlighted the importance of patient-specific cup positioning targets. Similarly, Esposito et al¹⁵ found that
there was no difference between the cup abduction angle and anteversion between patients who were early dislocators and nondislocators, again suggesting that there are likely additional factors, outside of component position, that play a role in hip instability. Many surgeons now agree that the ideal cup position for some patients may lie outside the Lewinnek safe zone, especially in patients with abnormal pelvic tilt (PT) and posture, such as those with concomitant spine pathology.¹⁶^,¹⁷

Understanding the Normal Spine-Hip Relationship

Spinopelvic mobility in a normal patient is coordinated to allow balance of the mass of the trunk with hip motion when changing position, such as from standing to sitting. A normal SHR is one in which a healthy flexible lumbar spine and lumbar-pelvic complex interact with healthy flexible hips. The first thing to understand is that spinopelvic motion has a goal of allowing the center of gravity of the trunk to be positioned so that it is supported by the femoral heads relative to the pelvis and maintaining balance with minimal muscular effort.¹⁸^,¹⁹

In order to fully understand a patient’s specific SHR, some of the important parameters in the SHR must be defined. Discussed here are the angles that are pertinent to describing the SHR, to evaluating patients, and to prescribing ways to improve the THA outcomes. Sacral slope (SS), PT, and lumbar lordosis (LL) are called functional or dynamic parameters, and their value varies with body position. SS is the angle between the superior plate of the first sacral vertebra and a horizontal line and is normally 40° ± 10° when standing and 20° ± 9° when sitting, normally with a 20 (11-29) degree arc of pelvic motion between standing and sitting.³ The anterior pelvic plane (APP) is seen on a lateral radiograph and is the line between the midpoint between the pubic tubercles and the two anterior superior iliac spines. The functional pelvic plane is defined as a plane between the two anterior superior iliac spines that is parallel to the torso (or the coronal plane of the body). The angle between these points is defined as the APP-PT angle; it measures pelvic rotation between sit to sand and can be used to quantify pelvic rotation by hip surgeons.²⁰

Spinopelvic tilt (SPT) is the angle between the vertical and the line connecting the midpoint of the sacral plate to the axis of the femoral heads.²⁰^,²¹ Spine surgeons use SPT to measure spinopelvic positioning, and it increases as the pelvis rotates posteriorly when going from standing to sitting.²⁰ Pelvic incidence (PI) is a morphologic parameter, and it is constant for an individual independent of position. PI can be used to estimate the patient’s physiologic range of spinopelvic motion in the sagittal plane³^,²⁰^,²¹^,²² (Figure 20.1).

FIGURE 20.1 Lumbar spine angles: 1, SS; 2, SPT/PT; 3, LL; 4, PI; 5, acetabular tilt; and 6, APP-PT angle.

These values are linked by the equation PI = SS + PT, which demonstrates that, with pelvic rotation as the SS changes, there is a proportional inverse change in pelvic tilt. In addition, it has been shown that the overall LL should be within 10° of PI and a significant mismatch may indicate deformity and sagittal imbalance.²³ Acetabular tilt is a reflection of surgical anteversion and is defined as the angle from the horizontal and a line tangent to the anterior and posterior edges of the acetabulum or
the acetabular cup on a sagittal radiograph.¹⁶ This is also called anteinclination because it is a combination of both anteversion and inclination, with a normal range of 41° to 62°.³ An anteinclination >75° puts the patient at higher risk for dislocation.²⁰

Anterior tilt of the pelvis is defined as the upper portion of the pelvis tilting forward, whereas posterior tilt is the upper portion of the pelvis tilting backward.⁴^,¹⁶^,¹⁸ Anterior tilt of the pelvis also corresponds to a reduced SPT.²⁰ A surrogate measurement of pelvic tilt change is the change in SS when changing positions; given the fact that PI is a fixed parameter, for every 1° increase in SPT, there is similarly a 1° reduction in SS. In normal motion, a standing position corresponds to the relative anterior tilt of the pelvis and a neutral APP-PT (ie, a vertical APP). In a sitting position, the pelvis tilts posteriorly, the SS decreases, the acetabular tilt increases, and there is increased acetabular anteversion.⁶^,¹⁶^,¹⁸ This is coupled with a reduction in LL (Figure 20.2). A normal change in SS from the seated to standing position is 20° to 40°.⁶

FIGURE 20.2 Sitting and standing radiographs in a patient with a normal lumbar spine.

In the sitting motion, the pelvis is tilted backward, and the SS is decreased. The acetabular tilt is increased, and there is decreased lordosis of the lumbar spine. In the standing position, the pelvis is tilted forward, the SS is increased, the acetabular tilt is decreased, and there is increased lordosis of the lumbar spine.

Risks for Instability

Pathology Within the Spine-Hip Relationship

When a part of the SHR becomes abnormal, a clinically deleterious compensatory mechanism occurs in response. When there is increased stiffness of the lumbar spine, the loss of pelvic mobility leads to reduced pelvic tilting motion when changing position and leads to compensatory mechanisms within the lumbar spine-pelvis-hip complex¹⁸ that can lead to impingement and dislocation. It is important to recognize that stiff spines are a spectrum, from degenerative spines, which are a little stiff, to deformity, which are yet stiffer, and then fusion, which are the stiffest.

In the case of THA patients, we mostly see coexisting issues when there is age-related degeneration of the spine. This results from degenerative disk disease and osteoporotic vertebral collapse, which can affect a patient’s sagittal balance. Most commonly this leads to loss of LL, which reduces lumbar flexibility. As lordosis reduces (lumbar flatback), patients compensate by increasing SPT (posterior tilt of the pelvis) in standing via hip extension.²⁴^,²⁵^,²⁶ Because some PT has already been recruited in standing, less increase in SPT (posterior tilt) occurs when transitioning from standing to sitting. Furthermore, the reduction in lumbar flexibility results in less PT change between standing and sitting. This resultant change in SPT results in less acetabular anteversion increase in sitting, providing less posterior coverage and less anterior clearance. The result is an increased likelihood of anterior femoroacetabular impingement and posterior dislocation.²⁵

If the patient has hip osteoarthritis as the primary problem and decreasing mobility with decreased hip extension (or in the setting of hip arthrodesis/ankyloses), the patient has anterior PT (reduced SPT) in standing. The lumbopelvic complex compensates with increasing LL of the spine, which causes accelerated degeneration of the spine and increased pain.¹⁸^,²⁴ The loss of intrinsic hip extension can be examined clinically by the Thomas test as well as by examining hip extension on the standing lateral radiograph.²⁴ A patient with hip osteoarthritis and flatback may be unable to maintain an upright posture, and THA to restore hip extension can enable these patients to compensate for their spinal deformity. Interestingly, the DAA may be a very effective way to manage the anterior hip capsular contracture as a key part of the initial surgical exposure and osteotomy in these patients who have hip flexion contractures secondary to spinal disease. Patients with stiff hips have also been demonstrated to have a greater change in SPT from standing to sitting.²⁷

Lumbar Fusion and the Effect on the Spine-Hip Relationship

Patients with ankylosis or iatrogenic lumbar fusion are at increased risk for impingement and dislocation because
restricted pelvic movement does not allow the acetabulum antevert due to posterior PT when transitioning to sitting. Onggo et al²⁸ performed a meta-analysis in 2020 in which they included 10 studies in the review with a total of 28,396 spinal fusion patients and 1,578,687 non-spinal fusion patients. Their study showed that patients with a spinal fusion were at a higher risk of hip dislocation (2.2×), all-cause revision (3.6×), and all complications (2.8×). These numbers were all higher when the registry data were removed, which brought up concerns for underreporting in national administrative databases.²⁸ York et al²⁹ looked at a 5-year retrospective review of THA cases done through the posterior approach and grouped them into those with or without lumbar fusions. They found that the dislocation rate was significantly higher in fusion patients (29% vs 4%, P = .009) with a relative risk of dislocation of 4.77 (P ≤ .0001). In addition, dislocators in the fusion group were at greater risk of requiring subsequent revision compared with dislocators in the nonfusion group (relative risk = 3.24, P = .003).³⁰ Perfetti et al³⁰ also demonstrated that, compared with patients with no prior spinal fusion, those with lumbar fusion had increased rates of THA dislocation in the first 12 months postoperatively (0.4% vs 3.0%, P < .001) and revision (control 0.9% vs fusion 3.9%, P < .001). At 12 months, fusion patients were 7.19 times more likely to dislocate their THA and 4.64 times more likely to undergo revision.³⁰

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