Does timing of lumbar fusion affect dislocation rate after total hip arthroplasty?

Abstract

Introduction

The impact of lumbar spinal fusion (LSF) on pelvic mobility and increased risk for THA dislocation are recognized. However, there is still controversy on whether THA should be performed prior or following LSF. This study aims to compare the rates of hip dislocation in patients undergoing THA prior to or following LSF.

Methods

We retrospectively reviewed 109 primary THA and LSF. There were 34 men and 75 women with a mean age of 66.9. The cohort was divided into 2 groups: 1) THA prior to lumbar fusion (n = 42) and 2) THA following lumbar fusion (n = 67). Radiographic parameters including acetabular component abduction, anteversion, pelvic incidence (PI), sacral slope, standing lumbar lordosis (LL) and PI-LL mismatch were determined for each patient. The surgical approach and THA parameters were also recorded and compared between the 2 groups. Patients with fracture, malignant disease, and prior hip hardware were excluded.

Results

The mean follow up was 14.7 months. Overall, 8 patients (7.3%) had a postoperative hip dislocation (0 in group 1 (0%), and 8 in group 2 (11.9%) (p = 0.022). The mean cup abduction and anteversion in patients that dislocated was 37.7° and 23.4° respectively while patients who did not dislocate had an average cup abduction of 37.6° (p = 0.970) and anteversion of 25.9° (p = 0.367). Patients who dislocated had decreased lumbar lordosis (p = 0.022) and higher PI-LL mismatch (p = 0.0004) compared to that did not dislocate. There were no other significant differences in the spinopelvic parameters between the 2 groups. Neither surgical approach nor dual mobility articulations use had a significant impact on postoperative dislocations.

Conclusion

Higher rates of hip dislocation were observed in patients undergoing primary THA following LSF. Dislocators had decreased lumbar lordosis and increased PI-LL mismatch. Patients who require both spinal fusion and THA should undergo hip arthroplasty first to minimize the risk of postoperative instability.

1. Introduction

Despite improvements in instrumentation and prosthetic design, instability following total hip arthroplasty (THA) remains a leading cause for revision.¹ While there is agreement that acetabular component position is a critical factor that contributes to hip joint stability, there has also been increasing recognition that the traditional safe zone described by Lewinnek is not universally protective against dislocation.² Abdel et al. reported that in a contemporary series of primary THAs, 58% of their postoperative hip dislocations occurred in patients with acetabular component position within 40° ± 10° of abduction and 15° ± 10° of anteversion.² At the same time, the effect and impact of the spinopelvic relationship on the functional position of the acetabular component have also been accepted.³ Stefl and colleagues found that patients with prior spinal fusions (LSF) and spinal imbalance were at increased risk of prosthetic impingement if the acetabular component was placed in this default cup orientation.⁴ Patients undergoing THA following prior lumbar fusion have been shown to be at increased risk of postoperative dislocations.⁵ Buckland et al. also reported that increasing levels of lumbar fusion, increased the risk of subsequent dislocation compared to controls.⁶ However, the prevalence of concurrent spinal and hip arthritis is common. Zachary and colleagues found there is a correlation between spinal fusion and progression on hip osteoarthritis at an average of 2 years.⁷ Similarly, Warashina et al. found a significant correlation between severity of spinal pathology and severity of hip degenerative disease.⁸ and the question of whether to proceed with lumbar fusion prior to THA or vice-versa remains unanswered. Parilla and colleagues found no differences in dislocation rates between patients with THA prior or following LSF.⁹ Malkani et al. showed using the Medicare database that patients with prior LSF to THA had 106% increased risk of dislocation compared to patients with LSF at least 5 years following THA.¹⁰ Finally, Liu et al. showed a relative lack of consensus among hip and spine surgeons on the preferred sequence of treatment across various clinical scenarios involving coexisting hip and spine disease.¹¹ Therefore, the purpose of this study is to 1) compare the rates of postoperative hip dislocation in patients undergoing THA prior or after lumbar spinal fusion and 2) compare the characteristics of patients with hip instability compared to those without.

2. Materials and methods

We retrospectively reviewed all the clinical records of patients undergoing primary THA with a concurrent LSF performed at our institution between 2013 and 2019. Ethical approval for this study was given by our institution's internal review board. A waiver of informed consent was obtained since all patient identifiers were excluded during data collection and the study does not pose any significant risk to the patients. Only patients older than 18 years of age with spinal fusion performed by our spinal or neurosurgeons were included. Patients with prior hip joint hardware, neuromuscular conditions, substance abuse, and less than 12 months follow up were excluded. In addition, we only included patients with adequate hip and spine imaging to allow measurement of various spinopelvic parameters. Following exclusions, there were 15 men and 19 women with a mean age of 56.4 years. The indication for THA was 103 for osteoarthritis, 5 for osteonecrosis, and 1 for hip dysplasia.

Patient were divided into 2 groups: 1) patients with THA following LSF (n = 67) and 2) patients with LSF following THA (n = 42). The demographic data such as age, sex, race for each patient were recorded. In addition, the dates of both THA and LSF and their temporal relationship as well as surgical approach, acetabular component size, head size including dual mobility articulation, and femoral component offset were also abstracted from the medical records. Postoperatively, the presence of absence of hip dislocation and subsequent revision surgery were noted for each patient. Radiographic analysis included acetabular component abduction and anteversion measured from the AP pelvis and cross table lateral radiographs respectively. The pelvic incidence (PI), sacral slope, standing lumbar lordosis (LL), and PI-LL mismatch were determined using a standing lumbosacral radiograph.

A priori power analysis showed that a minimum sample size of 137 in each group was required reach 80% power to detect a minimally clinically significant difference in dislocation rate of 2%. Continuous variables were compared using a one-way analysis of variance (ANOVA) among the 2 groups while categorical variables were compared using the Fisher exact test. Statistical significance was set at p < 0.05. All calculations were performed using MATLAB version 9.6 (MathWorks, Massachusetts).

3. Results

A total of 105 patients underwent primary THA and lumbar spinal fusion at our institution. 8 were lost to follow up prior to 12 months and were excluded from the final analysis, leaving 97 patients (109 THAs). The mean follow-up in this group was 14.6 months. 67 patients had LSF following THA (Group I) while 42 patients had THA following LSF (Group II). The median number of fused lumbar levels in group I was 2 (range 1–16) compared to 2 (range 1–16) in group 2 (p = 0.3667). The average time prior to THA following LSF was 21.1 months (range 1–70) while the mean time to LSF following THA was 36.1 months (range 1–345) in the second cohort. There were no significant differences between the demographic characteristics between these 2 groups except for BMI (Table-1).

Table 1.

Demographic, BMI and levels fused comparison between groups 1&2.

	THA following LSF (Group 1)	LSF following THA (Group 2)	p value
No. of patients	67	42
Age	68.1	64.9	0.114
Male	19	15	0.525
Female	48	27
White	51	34	0.657
Black	15	8
Asian	0	0
Hispanic	1	0
BMI	28.8	33.1	0.008
Levels Fused	2.43	2.97	0.3667

Eight patients (7.3%) with LSF experienced at least 1 dislocation following primary THA. All of these patients had LSF following primary THA. No patients with THA following LSF experienced postoperative instability. The mean time to dislocation was 2.04 months (range 0.54 months–4.89 months). There were no significant differences in acetabular component abduction, anteversion, and spinopelvic parameters between these 2 groups (Table-2). Additionally, there were no differences in the distribution of femoral head size, dual mobility articulations, hip stem offset, lateral or posterior surgical approach between the 2 cohorts (Table-3). There was an increased number of direct anterior approach performed in the group of patients with THA performed prior to LSF (p = 0.006).

Table 2.

Dislocation and spino-pelvic parameter comparisons between groups 1&2.

	THA following LSF (Group 1)	LSF following THA (Group 2)	p value
Dislocation	8	0	0.022
Acetabular anteversion	26	25.4	0.782
Acetabular abduction	38.7	35.8	0.054
Pelvic incidence	67.8	64.3	0.157
Sacral slope	45.6	43.5	0.399
Standing LL	52.4	53.8	0.603
PI-LL mismatch	15.5	10.4	0.346

Table 3.

Hip offset, head size and surgical approach comparisons between groups 1&.

	THA following LSF (Group 1)	LSF following THA (Group 2)	p value
Hip-stem offset	58.1	58.4	0.876
Femoral head:
Dual mobility	10	1	0.161
28	12	7
32	22	20
36	21	12
40	1	2
Approach:
Direct Anterior	18	2	0.006
Lateral	18	20
Posterior	31	20

Flat-back deformity and sagittal spinopelvic imbalance was common in patients with THA and LSF who experienced hip dislocation. In a subgroup analysis of patients who underwent THA following LSF with postoperative instability compared to those who did not dislocate, there was no difference in acetabular component position, femoral head size distribution, hip stem offset, and surgical approach. (Table-4). However, there was decreased standing lumbar lordosis (43.5 vs. 53.8, p = 0.022) and increased PI-LL mismatch (31.2 vs. 12.1, p = 0.0004) in patients who dislocated postoperatively compared to patients who did not dislocate.

Table 4.

Spinopelvic parameters and THA factors stratified by dislocators vs non-dislocators.

	Dislocation	Non-dislocators	p value
Prior LSF	8	59
Post THA LSF	0	42	0.022
Acetabular anteversion	23.4	25.9	0.367
Acetabular abduction	37.7	37.6	0.97
Pelvic Incidence	73	66	0.138
Sacral slope	46.8	44.6	0.634
Standing LL	43.5	53.8	0.022
PI-LL mismatch	31.2	12.1	0.0004
Femoral Head:
Dual mobility	1	10	0.901
28	2	18
32	2	40
36	3	30
40	0	3
Hip-stem offset	60.7	58.1	0.452
Approach:
Direct Anterior	1	19	0.238
Lateral	1	37
Posterior	6	45

4. Discussion

The presence of concurrent spinal disease in patients with hip arthritis is not uncommon, but there is controversy on sequence of whether THA or spinal treatment should be first performed.⁹ Lumbar spinal fusions can significantly impact the spinopelvic relationship and thus, affect the functional position of the acetabular component.² Therefore, the purpose this study is to 1) compare the rates of postoperative hip dislocation in patients undergoing THA prior or after lumbar spinal fusion and 2) compare the characteristics of patients with hip instability compared to those without.

This study has several limitations. First, the retrospective study design makes it susceptible to shortcomings such as selection and recall bias. Second, our relatively small sample size and patients undergoing THA through various surgical approaches and using a variety of implants can further confound the analysis. However, while the inclusion of only patients undergoing LSF within our institution decreased our sample size, it minimizes variability of surgical technique and improves consistency of spinal alignment as demonstrated by the similarities in spinopelvic parameters between the 2 groups of patients. We felt that this was the best way to isolate the true impact of the temporal sequence of LSF on dislocation following THA. Furthermore, despite being underpowered, a post hoc power analysis of the disparity in dislocation rate between the 2 groups showed that our results are 41.5% predictive and therefore in our cohorts with demographics, spinopelvic parameters, surgical approach and implant selection, we feel that these observations are still valid. Third, this study included patients with minimum 12 months follow up and 8 patients (7.6%) were excluded due to lost to follow up prior to this time period. This can underestimate the number of patients who developed postoperative dislocation and impact our analysis. However, since the number of patients lost to follow up is relatively small and most postoperative hip dislocations (if any) occur within the first 9 months,¹² the impact of these factors are minimized. Fourth, THA component and spinopelvic measurements were made on plain radiographs which may be influenced by radiographic technique and limb rotation. While all hip and spine images were acquired at our center and therefore the variability of technique is reduced, CT scans would provide a more accurate method of acquisition for some of these measurements. Finally, functional analysis of spinopelvic relationship was not performed in any of our patients prior to THA and it was impossible to standardize or capture the hip surgeon's approach to patients with prior LSF undergoing THA. Therefore, we are unable to comment on whether the postoperative dislocations in this group could have been prevented with additional work up and technical adjustments. While this is a limitation, it is beyond the scope of this study. Future studies including functional spinopelvic analysis with standardized surgical approach and implant selection are needed to definitively answer the question of whether LSF or THA should be performed first and vice versa.

In this study, all of the dislocations were recorded in patients who underwent THA following prior LSF. There were no dislocations in patients with THA prior to LSF. Our overall dislocation rate of 7.3% in this population is consistent with 7.4% reported by Berry et al. in the Medicare population.¹⁰ The observation that THA following LSF is associated with increased rates of instability compared to LSF following THA is also consistent with some prior studies. Malkani and colleagues showed that patients with THA following LSF had 106% increased risk of dislocation compared to those undergoing LSF following THA. While revision for instability was common in both groups, 26.6% of patients with prior LSF prior to THA were subsequently revised for instability.¹² However, Parilla et al. could not demonstrate a difference in dislocation rates between these 2 groups in an analysis of institutional results.⁹ Their study lacked analysis of spinopelvic parameters for nearly 2/3 of their patients. Lumbar fusion prior or after THA can impact functional cup orientation and lead to impingement and dislocation5. A simplistic explanation of why patients with THA prior to LSF have fewer dislocations may be due to the maturation of the soft tissue envelope around the hip prosthesis that is critical to hip joint stability.¹³ However, excessive alterations of the spinopelvic relationship after successful THA can also be a source of dislocation.¹⁴ Consequently, this data suggests that when possible, primary THA should be performed first prior to lumbar spinal fusion in order to minimize postoperative dislocations.

Our results also highlight the need for additional preoperative planning in this population that is at increased risk of dislocations. Patients who experienced dislocations in this study had decreased lumbar lordosis and increased spinopelvic sagittal imbalance compared to patients who did not dislocate. McKnight et al. postulated that when lumbosacral motion becomes pathologic, patients rely on greater hip motion to reach either standing or sitting positions which in turn increases the risk for impingement.¹⁵ Also, Buckland and colleagues showed that the flat back deformity was associated with spine hypo-mobility and the greatest spinopelvic tilt and PI-LL mismatch.¹⁶ This suggest that functional preoperative planning to investigate spinopelvic abnormalities and adjustments to the acetabular component may ultimately reduce the risk of instability. However, Nam et al., demonstrated that pelvic motion following THA can be highly variable and unpredictable, so targeting a patient specific safe zone based on preoperative images alone may not be possible.¹⁷ Perhaps a combination of larger femoral heads including the use dual mobility articulations along with functional spinopelvic analysis in patients with severe spinopelvic abnormalities are needed to minimize postoperative instability. Chalmers and colleagues reported no dislocations in a group of patients with prior spinal fusion at a mean of 3 years follow up treated with a dual mobility articulation.¹⁸

In summary, in our series of primary THA using contemporary implants in patients with LSF, the dislocation was 7.3% consistent with other published results. All the dislocations occurred in patients who had THA following LSF and those who experienced a dislocation had significantly decreased lumbar lordosis and increased spinopelvic sagittal imbalanced compared to non-dislocators. Based on these results, whenever possible, THA should be performed prior to LSF and consideration should be given to changing acetabular component orientation and/or maximizing effective head size in patients with significant spinopelvic deformities.

Author contribution

GA and GL wrote the paper, CN and EH contributed unpublished reagents, GA, GL and CN assisted with the analysis, EH assisted with study design.

Declaration of competing interest

Dr. Charles Nelson: Paid consultant for Zimmer Biomet, Board member-ABOS.

Dr. Eric Hume: No conflicts of interest.

Dr. Gwo-Chin Lee: Paid consultant for Stryker, Corin, Heron Therapeutics. Research support from Smith and Nephew, Depuy; Medical publication editorials: Journal of Arthroplasty, Clinical Orthopaedics and Related Research, Journal of Bone and Joint Surgery, Bone and Joint Journal.