Concurrent spine and hip disease is common. Spinal deformities can restrict lumbar range of motion and lumbar lordosis, leading to pelvic obliquity and increased pelvic tilt. A comprehensive preoperative workup and component templating ensure appropriate compensation for altered pelvic parameters for implantation of components according to functional positioning. Pelvic obliquity from scoliosis must be measured to calculate appropriate leg length. Cup positioning should be templated on standing radiograph to limit impingement from cup malposition. In spinal deformity, the optimal position of the cup that accommodates pelvic parameters and limits impingement may lie outside the classic parameters of the safe zone.
Key points
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Spinal deformities can significantly restrict lumbar range of motion and lumbar lordosis, leading to pelvic obliquity and increased pelvic tilt.
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A comprehensive preoperative workup and component templating are essential to ensure appropriate compensation for altered pelvic parameters for implantation of components according to functional positioning.
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Pelvic obliquity from scoliosis must be measured to calculate appropriate leg length.
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Cup positioning should be templated on standing radiograph to limit potential impingement from cup malposition.
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In cases of spinal deformity, the optimal position of the cup that accommodates pelvic parameters and limits impingement may lie outside the classic parameters of the safe zone.
Introduction
Osteoarthritis (OA) is a systemic and inevitable disease of aging that can affect the hip and spine concurrently, producing substantial pain and disability. The degenerative changes of end-stage arthritis in the hip and spine can significantly alter body kinematics and spinopelvic alignment. Accordingly, the diagnosis and treatment of patients with simultaneous hip and spine disease require consideration of the relative contributions of each region to clinical symptoms, postural balance, and locomotion.
Introduction
Osteoarthritis (OA) is a systemic and inevitable disease of aging that can affect the hip and spine concurrently, producing substantial pain and disability. The degenerative changes of end-stage arthritis in the hip and spine can significantly alter body kinematics and spinopelvic alignment. Accordingly, the diagnosis and treatment of patients with simultaneous hip and spine disease require consideration of the relative contributions of each region to clinical symptoms, postural balance, and locomotion.
Lumbar range of motion
Lumbar disease, low back pain (LBP), and guarding from fear of LBP cause a significant reduction in lumbar range of motion (ROM). In addition, McGregor and colleagues showed that patients with disk prolapse, degenerative disk disease, and stenosis have a predictable reduction in lumbar ROM in flexion/extension, lateral bending, and rotation compared with age-matched controls. In more obvious cases, patients with ankylosing spondylitis and those undergoing multilevel spinal fusion procedures can have profound, fixed reductions in lumbar ROM.
To complete activities of daily living (ADLs) that demand spinopelvic motion, patients with lumbar disease compensate for decreased lumbar motion from stiffness or fusion by increasing motion through the hip joints.
Spinopelvic motion and pelvic parameters
Spinopelvic flexion and extension are the sum of intrinsic motion from the hip joints and extrinsic motion from the lumbosacral joints. Duval-Beaupere and colleagues introduced the concept of pelvic incidence (PI) to describe the anatomic relationship between the alignment and motion of the spine and pelvis. PI, defined by the angle between a line drawn from the center of the femoral head to the midpoint of the S1 end plate on a lateral radiograph ( Fig. 1 ), can vary significantly from person to person but remains an anatomic constant for each individual after puberty throughout all pelvic ROM. Sacral slope (SS), defined by the angle between a line parallel to the S1 end plate and a horizontal line, is a measure of the inclination of the pelvis base and is variable throughout spinopelvic motion, increasing from the sitting position to standing and from standing to lying supine. Pelvic tilt (PT), defined by the angle between a line drawn from the midpoint of the S1 end plate and a vertical line, is a measure of the position of the pelvis relative to the acetabulum and varies reciprocally with SS, decreasing, or becoming less posterior, from the sitting position to standing and further decreasing from standing to lying supine. The relationship between the geometric measures is PI = SS + PT.
Pelvic parameters and spine measures are connected by a linear relationship between SS and lumbar lordosis. This relationship was initially described by Stagnara and colleagues in 1982 and subsequently confirmed by other investigators showing a correlation of r = 0.84 to 0.86 between the measures. The implication of this relationship is that as lumbar lordosis decreases, SS concurrently decreases and PT increases, retroverting the pelvis.
Influence of body position on pelvic parameters
Although PI remains constant throughout spinopelvic motion, SS and PT significantly vary with body position. While lying supine, lumbar lordosis and SS (often exceeding 45°) are accentuated, the pelvis is tilted forward, and acetabular version and abduction are low to permit maximal hip extension. Moving from lying supine to standing, the pelvis tilts slightly backward and SS decreases slightly (between 35° and 45°), and acetabular version and abduction increase. Moving from standing to sitting, the pelvis is further tilted backward and the SS decreases to 25° or less, and acetabular version and abduction further increase to permit further hip flexion.
In multiple studies, Lazennec and colleagues documented the variable radiographic position of total hip arthroplasty (THA) cups on anteroposterior (AP) and lateral films in the sitting and standing positions. These investigators reported changes in the AP inclination of the cup from 49° to 52° in the standing position to 57° to 64° in the seated position and changes in the sagittal inclination of the cup from 36° to 47° standing to 51° to 58° in the seated position. Patients with limited lumbar ROM from lumbar disease or resultant fusion show little variation in pelvic orientation based on position. That is, there is little change in SS, PT, or acetabular version or abduction with body positioning, inherently limiting the ability to accommodate additional hip flexion when seated or extension when standing or lying supine.
Coronal and sagittal alignment and spinopelvic mechanics
Coronal imbalance is governed by scoliosis and pelvic obliquity. Although idiopathic adolescent scoliosis and degenerative scoliosis arise de novo, compensatory lumbar and thoracic scoliosis can also arise from limb length deformity secondary to multiple causes, including end-stage degenerative changes in the hip. Uncorrected pelvic obliquity accelerates degenerative changes in the lumbosacral spine and can significantly affect normal gait.
Sagittal alignment is a measure of an individual’s adaptation to variations in anatomy and disease to produce a functional and balanced posture. Sagittal balance is the net effect of thoracic kyphosis and lumbar lordosis and can be influenced by axial musculature, spinal disease, and surgical interventions.
Degenerative changes in the spine are manifested through hypertrophy and cystic changes of the facets, degeneration of intervertebral disks, osteophytosis of the vertebra, and atrophy of spinal extensor muscles, which lead to a net effect of hypolordosis. Compared with age-matched control individuals, patients with degenerative spine disease and LBP have a significant reduction in lumbar lordosis and SS and significant increase in PT (or posterior tilt).
Lumbar spine fusion can provide reliable and durable stabilization and symptom relief for degenerative spinal conditions, including spondylolisthesis and scoliosis. The advent of interbody fusion techniques from the anterior, lateral, transforaminal, and posterior approaches has increased the frequency of the procedures and provided new strategies to treat complex deformities.
The goals of spinal deformity surgery are stabilization of unstable segments and restoration of lumbar lordosis and coronal and sagittal balance. The restoration of lumbar lordosis is influenced by multiple factors, including operative table and positioning, osteotomies, and single or combined anterior and posterior approaches. Failure to restore appropriate lordosis can result in positive sagittal balance, chronic LBP, and accelerated adjacent segment disease.
The term flatback syndrome was coined by Moe and Denis in 1977 to describe a fixed, iatrogenic positive sagittal imbalance resulting from spinal fusion. Although initially resulting nearly exclusively from Harrington rod constructs, the increase in interbody fusion surgery over the last 2 decades has led to an increase in reports of flatback syndrome. Poor sagittal alignment can result from inadequate restoration of lordosis in patients with significant preoperative positive sagittal balance or from iatrogenic causes, including hypolordotic positioning on the operative table and undercontouring of rods.
The compensatory mechanisms to restore postural balance in patients with lumbar hypolordosis include thoracic hyperextension (or loss of kyphosis), forward tilt of the trunk, knee flexion, hip extension, and increased or posterior PT. The net compensatory effect is positive sagittal balance and relative retroversion of the pelvis, resulting in excessive acetabular anteversion.
Impingement and instability
The combined effect of hypolordosis and posterior PT and limited lumbar ROM in patients with degenerative spine disease and deformity decrease the functional ROM of the hip before impingement occurs. In the standing position, the compensation of increased posterior PT for hypolordosis increases anteversion and abduction of the acetabulum and forces the hips into a hyperextended position and can cause posterior hip impingement and create anterior instability. Conversely, in the seated position, a rigid lumbar spine is unable to contribute to combined spinopelvic flexion and the hip joints are hyperflexed, leading to potential posterior instability.
Indications/contraindications
The well-recognized poor correlation between radiographic severity of hip arthritis and clinical signs and symptoms poses a significant diagnostic challenge when patients present with concurrent LBP, hip pain, or leg pain and have radiographic polyarticular disease. The absolute indications for THA do not differ in this population; however, the overlap of symptoms between lumbar and hip disease makes it difficult to assess the respective effects that each anatomic location has on morbidity and disability.
Hip-spine syndrome, first coined by Offierski and MacNab in 1983, is defined as the concurrent existence of OA and degenerative lumbar spinal stenosis. Spine disease can present with pain and symptoms that mimic many common disorders, including hip arthritis, trochanteric bursitis, iliopsoas impingement, sacroiliac arthritis, and piriformis syndrome. Similarly, studies have shown that THA can result in significant improvement in lumbar spine pain. Although multiple studies have shown high sensitivity and specificity using diagnostic anesthetic hip injections to distinguish between lumbar disease and hip disease when the anatomic source of pain is ambiguous, more often, the existence of concurrent hip and lumbar spine disease is only realized after pain persists after operative treatment of the spine or the hip.
Currently, there exist no evidence-based guidelines regarding the order or operative treatment of concurrent hip and spine disease. Most surgeons recommend treating the most symptomatic disease first; however, the acetabular cup positioning and stability are dependent on lumbar lordosis, SS, and PT, which can all be altered by spine procedures. In a recent study, it was shown that patients with lumbar spine disease undergoing THA had significantly less improvement in pain and functionality and greater medical charges and longer episodes of care after THA than did patients without lumbar spine disease.
Surgical procedure
Preoperative Planning
Understanding the effect of PT on acetabular cup positioning is the crux of THA preoperative planning in patients with concomitant lumbar spine disease ( Fig. 2 ). The traditional radiographic AP pelvis plane was described by Murray and is based on coronal plane projection of the pelvis. Conversely, computer navigation is based on the anterior pelvic plane (APP) defined by the plane between the anterior superior iliac spines and the pubic symphysis. When the APP and coronal plane of the standing body are equal, the pelvis has no tilt and any navigation based on the APP is reflective of a patient’s functional anatomy. This alignment occurs in only a few patients. Specifically, only 6.1% of 477 patients were found to have zero PT. Most patients fell in a narrow range close to neutral PT, with 83.9% of patients having less than 10° of tilt. However, the outlier patients would have a cup implanted correctly according to anterior reference points, but functionally, the acetabular cup would be malpositioned, leading to possible dislocation and impingement in extreme positions.
A similar study of 138 THA primary patients reported that 17% of patients had greater than 10° of PT. Because of the significant percentage of outliers, computed tomography (CT) navigation measurements based on the APP are not directly comparable with AP radiographs. Also, the approximate 16.5% of patients with a significant PT may be the outliers who have component malposition on CT despite excellent cup position on AP pelvic radiographs as a result of the lack of sagittal alignment influence on AP radiographs.
In simplistic terms, an increase in PT changes the normal AP pelvis view to modified outlet view. This finding is not parallax, but a true dynamic change in orientation of the acetabulum. Studies have quantified the impact of PT on acetabular inclination as approximately each degree of posterior tilt leads to a 0.7° increase in acetabular inclination. Similarly, anatomic version progressively increases from supine to standing to sitting, from 24.2° to 31.7° to 38.8°, respectively, with a mean of 7.1° increase in version from standing to sitting in relation to a 14.5° increase in posterior tilt. This linear relationship also holds with the lateral radiographic parameter of anteinclination, defined as the sagittal cup position based on change in standing to sitting on lateral radiographs. The importance of analyzing and considering acetabular version and abduction in the standing position for THA templating was highlighted by Tiberi and colleagues, who showed that 43% of patients had greater than a 5° change in acetabular inclination and 53% patients had great than a 5° change in version between supine and standing films.
Understanding the normal effect of PT on cup position is crucial to understanding the implications of stiff or hypermobile spinopelvic motion. Pelvic stiffness is defined as the difference in PT between standing and sitting on lateral radiographs. Normal pelvises have a 20° to 35° increase in PT, stiff pelvises have less than 20°, and hypermobile greater than 35°. The variation between stiff and hypermobile pelvises results in a minimum of 15° difference in functional cup positioning. This variation may lead to malpositioning of the cup at certain positions during ADLs, leading to potential impingement, subluxation, dislocations, or accelerated wear.
Functional consequences of pelvic tilt on acetabular cup position
Acetabular cup position is dynamic to accommodate hip extension while standing or lying supine and hip flexion while sitting. Limited lumbar spine motion forces the body to compensate with increased hip extensions or flexion to achieve a functional position. Extreme flexion/extension can then lead to impingement, subluxation, and dislocation despite excellent cup position on an AP radiograph. Classically, the safe zone described by Lewinnek and colleagues in 1978 has been used to define ideal cup position of 40° ± 10°and 15° ± 10°. Major limitations to this definition include the lack of consideration for lumbar spine dynamics, implant design and wear, and relatively small sample size of 300 patients. Most importantly, these parameters are based on two-dimensional coronal imaging and do not account for three-dimensional position of the cup.
Patients with spinal deformity are vulnerable to dislocation both anteriorly and posteriorly, regardless of the direction of the surgical approach. Patients undergoing anterior approach surgery have a near equal incidence of anterior and posterior dislocation, whereas patients undergoing posterior approach surgery are more predisposed to posterior dislocation. Patients with lumbar spine rigidity and compensatory PT are at risk for instability anteriorly with requisite hip hyperextension while standing or lying supine and posteriorly with the requisite hyperflexion while sitting. In addition, increased PT leading to verticalization of the cup is a risk factor for increased surface wear and liner fracture.
The compensatory position of the pelvis and acetabulum for lumbar rigidity and deformity may explain why most (58%) dislocated THAs have cups placed in the safe zone, leading investigators to recommend advanced analysis of dynamic factors.
Pelvic Obliquity
Coronal imbalance must be addressed during THA planning to determine proper leg length. Pelvic obliquity resulting from severe degenerative changes of the hip joint can be corrected by templating the prosthesis to match the ipsilateral hip. However, pelvic obliquity caused primarily by scoliosis can be more challenging. In their 2015 study, Abe and colleagues showed improvement in pelvic obliquity in 79% of patients with preoperative scoliosis obliquity through intentional leg lengthening with THA, although their algorithm to determine amount of lengthening was not given. The amount of leg lengthening is inherently limited by soft tissue compliance and potential neural stretch injury, but any plan to intentionally lengthen the leg to address pelvic obliquity resulting from scoliosis should also factor the potential for subsequent spine surgery to directly correct the spinal deformity. That is, a THA with intentional lengthening of the leg to compensate for pelvic obliquity could result in iatrogenic pelvic obliquity in the opposite direction from leg length discrepancy if the spinal deformity is corrected.
In their 2013 series, Meftah and colleagues described their method to address pelvic obliquity from fixed spinal deformity during THA templating for the goal of orienting the cup in a position of function. These investigators described drawing a horizontal line that crosses the teardrop and planning their cup abduction according to that line, noting the relative position of the lateral edge of the cup to the most lateral margin of the acetabulum to use for comparison intraoperatively. Although this technique does not alter pelvic obliquity, it ensures cup orientation according to functional positioning.
Clinical assessment
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Thorough history and physical to identify potential concurrent spine disease, including overlapping symptoms
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Obtain a standing AP radiograph of the pelvis
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Obtain standing and sitting lateral radiographs in all patients with lumbar spine symptoms, previous spine surgery, pelvic obliquity, or PT irregularity on AP pelvis radiograph
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Calculate dynamic PT between sitting and standing to determine lumbar rigidity
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Calculate pelvic obliquity according to method of Meftah and colleagues
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Template arthroplasty components, accounting for pelvic obliquity and PT and balancing the risk of anterior impingement in hip flexion with a flattened cup and posterior impingement in hip extension with a vertical cup
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If using CT navigation, consider converting the CT-based APP cup positioning to a functional positioning based on preoperative sitting and standing radiographs
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Counsel patients regarding increased risk of THA given concurrent spine disease
Operative considerations
Intraoperative imaging has been shown to be a quick, reliable, readily available option to evaluate cup position with limited cost, and almost half of all cases required an adjustment to either the acetabular or femoral component. However, it is important to interpret the intraoperative radiograph in relation to the preoperative standing and sitting radiographs, to account for potential variances in the plane of the AP images.
Although there are no published data, it may be reasonable to consider a constrained liner, larger heads, or even bipolar or tripolar heads for primary THA in cases of extreme PT and lumbar rigidity.
Complications and management
The principal surgical complications after THA include periprosthetic joint infection, dislocation, periprosthetic fracture, component failure or loosening, osteolysis or polyethylene wear, and revision THA. Analysis of a large cohort of patients from the Medicare database from 2005 to 2012 showed that patients undergoing THA with coexisting spinal deformities including kyphosis and thoracic and lumbar scoliosis have a significantly increased relative risk (RR) of several postoperative medical ( Fig. 3 ) and surgical ( Fig. 4 ) complications compared with patients without spinal deformities. At 90 days after THA, patients with spinal deformity have a significantly increased risk of prosthetic hip dislocation (RR = 1.86), prosthetic component failure (RR = 1.77), periprosthetic fracture (RR = 1.78), and early revision THA (RR = 1.51) compared with control patients without spine deformity (see Fig. 4 ). At 2 years after THA, patients with spine deformity have a significantly higher RR of all surgical complications measured (see Fig. 4 ).
