Patients commonly present with hip and lumbar spine arthritis, and in light of the aging United States population, the frequency of “hip-spine syndrome” continues to rise [12, 18, 25]. Surgical interventions that address these conditions, including THA and lumbar fusion, also have increased in frequency [1, 21]. In 2013, the lifetime probability of undergoing THA was 8.3% in males and 16% in females, and it is estimated that up to 2% of patients undergoing THA have had a previous lumbar fusion [1, 25]. Patients with hip-spine syndrome may not necessarily experience symptoms commonly associated with either of their spine or hip disorders, but they are prone to the mechanical interplay between the spine and the pelvis via the hip [3, 4, 7, 18, 30]. Patients with spinal disorders might present with spinal sagittal malalignment . This malalignment commonly affects the pelvis, introducing atypical pelvic kinematics such as pelvic tilt (pelvic sagittal retroversion and anteversion with positive and negative sagittal malalignment, respectively) [11, 22]. As such, patients with hip-spine syndrome are difficult to address operatively as the varied morphologic features and version of the pelvis have been shown to affect the position of the acetabular component in the sagittal plane [17, 29].
Some studies have shown that correction of spinal sagittal malalignment changes pelvic and acetabular version postoperatively, which may further affect the integrity of a prior THA [5, 14, 16]. However, when a THA is performed in patients with prior lumbar fusion, these patients might experience an increased risk of hip dislocation and subsequent revision THA [2, 20, 25]. To our knowledge, there is a paucity of studies  that have used longitudinal data with a substantial sample size to investigate the associated outcomes of spinal fusion in patients who have had a previous THA.
Therefore, using New York State’s Department of Health Statewide Planning and Research Cooperative System (SPARCS), we asked: Is short or long spinal fusion associated with an increased rate of postoperative complications in patients who had a prior THA?
Patients and Methods
We performed a retrospective analysis of the SPARCS database from 2009 to 2013. The SPARCS database is an industry and governmental collaboration that was used to create an accessible source for patient-level discharge information and is available through the New York State Department of Health’s Bureau of Health Informatics. The database collects information from all patients seen in the state of New York and includes data from outpatient and inpatient visits including emergency department visits, ambulatory surgery, and hospital admissions . To ensure accuracy, the SPARCS database is routinely vetted for quality by the New York State Department of Health, which is accomplished through collaborations with the Vital Statistics Birth Registry, the Vital Statistics Death Registry, and trend-analysis conducted by the Bureau of Biometrics. Owing to the deidentified nature of the dataset, the current study was exempted from human-subjects review by SUNY Downstate Medical Center’s institutional review board.
Patients were identified on the basis of the ICD-9-clinical modification (CM) codes for an elective THA from 2009 to 2011. Patients who had a THA after 2011 were excluded to ensure a minimum of 2 years followup. Elective THAs were defined as procedures without concomitant diagnoses of traumatic hip fracture, pathologic fracture, malunion of fracture, and loosening of the prosthesis. Of patients who had THAs, those who had procedural codes corresponding to a two- to three-thoracolumbar vertebrae fusion subsequent to the THA were identified as being in the “Short Fusion” group, while those with a fusion greater than four thoracolumbar vertebrae subsequent to the THA were identified as being in the “Long Fusion” group. Revision THAs were identified using ICD-9-CM codes specific for THA revision, while contralateral THAs were identified on the basis of a primary THA occurring subsequent to a patient’s previous THA. A complete list of ICD-9-CM codes corresponding to these procedures is provided (see Table, Supplemental Digital Content 1).
Demographics obtained from the SPARCS dataset for analysis included age, sex, race, insurance provider, Deyo/Charlson index, and total hospital charges for all visits between 2009 and 2013. The Deyo/Charlson index is a comorbidity index that allows for calculation of a comorbidity score from an administrative database that uses ICD-9-CM codes . A total of 49,920 patients met the inclusion criteria of the study. Patients who underwent a spinal procedure (short versus long fusion) were comparable in age. However, patients who did not undergo a spinal procedure were older than patients who had short fusion (65 ± 12.4 years versus 63 ± 10.7 years; p < 0.001). There was no variation with respect to the distribution of sex and race across the three groups. Patients who underwent long fusions were insured more frequently under Medicare, while those without a subsequent spinal procedure were insured more frequently by a private insurance company. Finally, patients with a history of long fusion had a substantially greater total cost relative to the patients who underwent a short fusion or no spinal procedure (Table 1).
Logistic regression was used to determine our primary endpoint: the independent predictors of postoperative dislocation, total surgical complications (wound disruption, wound irrigation, and postoperative dislocation), contralateral THA, and revision THA. Each analysis included only entries with valid data to account for the possible presence of missing data: if data points were missing they were not included in the analysis. The missing data pertained to 0.3% of the total dataset lacking entries for race; however, owing to the retrospective nature of the dataset, we were unable to verify whether the data were missing owing to failure to collect it by the provider or failure to report it by the patient. The threshold for statistical significance was set at α = 0.05. All statistical analyses were performed using SPSS Statistics, Version 24.0 (IBM Corporation, Armonk, NY, USA).
Controlling for the potential confounding variables of sex, Deyo/Charlson index, and fusion length, we found that patients who underwent short and long spinal fusions had greater odds of having a postoperative THA dislocation (odds ratio [OR] = 4.4, 95% CI, 2.7-7.2, p = 0.002; OR = 2.2, 95% CI, 1.4-3.6, p < 0.001, respectively). Similarly, short and long spinal fusions were associated with increased surgical complication rates (OR = 2.8, 95% CI, 2.1-3.8, p < 0.001; OR = 5.3, 95% CI, 3.8-7.4, p < 0.001, respectively). A spinal fusion was not associated with increased odds of undergoing a future contralateral THA; however, short and longs fusion were associated with increased odds of undergoing revision THA (OR = 2, 95% CI, 1.4-2.8, p < 0.001; OR = 3.2, 95% CI, 2.1-4.8, p < 0.001, respectively) (Table 2).
Published studies have only begun to delve into the complexity of patients with hip-spine syndrome [7, 20, 25]. Chief among these investigations is characterizing the potential association of deleterious consequences with concomitant hip and spine disorders, which may modulate outcomes and inform further analysis. Some studies have shown an increased risk of surgical complications with THA in patients with prior spinal fusion, with dislocation rates as high as 20% [2, 20, 25]. Perfetti et al.  reported that patients with prior lumbar fusion were seven times more likely to have a hip dislocation and more than four times more likely to require revision surgery than patients without fusion at 1 year of followup. Sing et al.  examined Medicare data and also found an increased risk of complications in patients with THA and prior lumbar fusion at 2 years followup. After controlling for potential cofounding variables, we sought to determine whether short or long spinal fusion was associated with greater rates of postoperative complications for patients with a prior THA. We found an increased risk of complications, hip dislocation, and rates of subsequent revision THA in patients who underwent post-THA spinal fusions. In addition, we found that these risks were greater in patients with longer fusions.
Our study had several limitations. Most importantly, owing to the nature of the SPARCS database, we could not assess whether spinal deformity or spinal fusion drives the association we noted in this study. Further, as this was a retrospective database study that was dependent on ICD-9-CM billing codes, we were unable to evaluate procedural details and characteristics of the THA (eg, bearing diameter, orientation of the cup and stem) and the spinal fusion (eg, magnitude of realignment, details regarding the construct and surgical techniques). In addition, despite the potential association of spinal malalignment with varied direction of postoperative dislocation, we were unable to evaluate the direction of dislocation owing to intrinsic limitations of the ICD-9-CM billing codes and the lack of radiographic imaging results. Additionally, as the SPARCS database is unable to follow patients who sought care outside New York, any patients who left the state no longer would be represented in the dataset. Finally, the study did not provide a definitive mechanism behind why subsequent spinal fusion can lead to postoperative complications after THA. Despite these limitations, this study provided additional insight in the complexity of patients with hip and spine disorders and the associated complications.
The results of this study must be interpreted with an understanding of the dynamic relationship between acetabular cup position and spinopelvic parameters. Among several interplays, spinal fusion and spinal malalignment were found to affect the pelvic and acetabular position. Loss of spinal motion after fusion may in part lead to abnormal loads across the hip, placing stress on THA components, which may lead to subsequent wear and early loosening [4, 10, 13, 15, 20, 25, 27, 31]. Similarly, in patients with positive spinal malalignment and rigid spines, such as in patients with ankylosing spondylitis, this relationship is lost, resulting in pelvic hyperextension, which exaggerates the acetabular anteversion . This is especially important as several studies have established the effect of cup malposition on implant longevity [19, 23, 28]. As a result, the pelvis can be conceptualized as an extension of the lumbar spine, so much so that the term “pelvic vertebrae”, initially proposed by Dr. Jean Dubousset , refers to the hip as the next adjacent motion segment [13, 20]. We postulate that these dynamics played a role in the increased revision and dislocation rates seen among patients who have had THA and then underwent spinal fusion, especially the increased revision and dislocation rates with longer fusion constructs which might be a proxy for realignment surgeries.
Regarding the question of whether we should do the spine or hip first, prospective studies should be designed to examine the outcomes of the two surgeries in opposite order to assess whether the sequence matters or whether the conventional wisdom of doing the hip before the spine is correct. Such studies require radiographic data for the entire spinopelvic region in sagittal and coronal planes to analyze and assess the effect of sagittal and coronal spinal alignment on THA-related complications. This is, in part, challenging owing to the different radiographic protocols of spine versus hip surgeons. If a patient is indicated for both surgeries and patients with a spinal deformity really do need specific THA component positioning based on their alignment, one unexplored option would be to do both surgeries in a staged fashion, with the hip surgeon being aware of the spinal realignment plan of the spine surgeon.
Patients with concomitant spine and hip disorders must be considered as complex cases regarding THA. The results of our study suggest that the relationship between spinal disorders and THA must be studied further to determine whether the association of poor outcomes is similar in patients who undergo spinal fusion or realignment versus nonoperative management of their spinal conditions. We also documented an association between short and long fusions and postoperative dislocation, reoperation, and revision in patients with THAs. Given the observed risk for THA-specific complications in this population, we recommend further studies to evaluate whether there is an ideal order to perform theses surgeries to optimize postoperative outcomes and minimize complications.
We thank Jonathan Elysée BS (Spine Service, Hospital for Special Surgery, New York NY), Joshua D. Lavian BS, Frank A. Segreto BS, Steven Burekhovich BS, Barrett Torre BA, and Neil V. Shah MD MS (all from the Department of Orthopaedic Surgery, SUNY Downstate Medical Center, Brooklyn, NY) for their valuable contributions to this project.
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