National in-hospital morbidity and mortality trends after lumbar fusion surgery between 1998 and 2008

Materials and Methods

Results

A total of 1 288 496 primary posterior lumbar fusions were performed between 1998 and 2008, during which time the number of procedures performed per year increased almost threefold, to 182 981 in 2008 (r = 0.987, p < 0.0001) (Fig. 1). A similar increase was seen in the population growth-adjusted use. Use in the civilian population increased from 23 per 100 000 in 1998 to 60 per 100 000 in 2008 (r = 0.987, p < 0.0001) (Fig. 2). A significant increase in the mean age of the patients from 52.47 to 56.85 years (β = 0.496, p < 0.0001) and comorbidity burden from 0.32 to 0.55 was seen over time (β = 0.027, p < 0.0001) (Figs 3 and 4).

Fig. 1

Graph showing the total number of lumbar fusions undertaken between 1998 and 2008 (r = 0.987; p < 0.0001).

Fig. 2

Graph showing the population growth-adjusted use of primary posterior lumbar fusions undertaken per 100 000 civilians in the United States (r = 0.987; p < 0.0001).

Fig. 3

Graph showing the increase in mean patient age at operation for each year (β = 0.496; p < 0.0001).

Fig. 4

Graph showing the increase in the mean Deyo comorbidity index between 1998 and 2008 (β = 0.027; p < 0.0001).

The mean length of stay decreased from 5.1 days in 1998 to 4.7 days in 2008 (β = -0.098, p < 0.0001) (Fig. 5). The percentage of routine discharges to the patients’ customary residence remained stable at around 70% during this time.

Fig. 5

Graph showing the mean length of hospital stay after lumbar fusion (β = -0.098; p < 0.0001).

More women than men underwent lumbar fusion at any given time point. The gender gap increased over the period from 5% in 1998 to 13% in 2008 (Pearson’s correlation coefficient (between the gender differences in proportions and time) r = 0.91, p < 0.0001) (Fig. 6). The in-hospital mortality rates after primary posterior lumbar fusion per 1000 inpatient days decreased from 0.34 in 1998 to 0.30 in 2008, but this was not significant (β = -0.009, p = 0.584) (Fig. 7).

Fig. 6

Graph showing the proportions of the genders over the study period (r = 0.91; p < 0.0001).

Fig. 7

Graph showing the in-hospital mortality rate per 1000 in-patient days (β = -0.009; p = 0.584).

Table I gives a detailed analysis of linear trends in spinal pathology. Although the proportions of procedures addressing spinal stenosis and multiple spine diagnoses (≥ 2) each significantly increased over time (p < 0.0001), degenerative disc disease, scoliosis, spondylolisthesis, trauma, tumour and other diagnosis each decreased significantly (p < 0.0001).

Table I

Linear trend analysis showing trends in percentages of spinal pathologies among patients undergoing posterior lumbar spine fusion

Pathology	1998	1999	2000	2001	2002	2003	2004	2005	2006	2007	2008	Regression coefficient	p-value
Degenerative disc disease	43.7	41.9	44.8	47.8	48.4	47.6	45.1	44.6	45.3	44.1	43.5	-0.006	< 0.0001
Spinal stenosis	10.4	9.7	10.7	10.0	9.4	9.8	10.0	9.8	9.9	10.7	11.3	0.011	< 0.0001
Scoliosis	1.7	2.1	1.9	2.0	1.5	1.2	1.4	1.4	1.2	1.2	1.1	-0.064	< 0.0001
Spondylolisthesis	6.8	7.9	7.1	6.0	6.0	6.0	5.5	5.5	5.1	4.7	4.2	-0.056	< 0.0001
Trauma	2.5	2.4	1.9	1.4	1  .4	1.7	1.9	1.3	1.6	1.6	1.2	-0.049	< 0.0001
Oncologic	0.4	0.3	0.3	0.3	0.4	0.3	0.2	0.3	0.2	0.3	0.2	-0.045	< 0.0001
Other	5.9	6.3	6.2	5.0	5.6	4.7	4.1	4.3	4.6	3.7	4.5	-0.045	< 0.0001
Multiple spinal diagnoses (≥ 2)	28.5	29.4	27.1	27.4	27.3	28.7	31.8	32.8	32.1	33.8	33.8	0.035	< 0.0001

The in-hospital complication rates after primary posterior lumbar fusion per 1000 inpatient days were also calculated. No significant changes in rates of peri-operative complications were observed for cerebral (β = 0.035, p = 0.051) and myocardial infarction (β = -0.007, p = 0.527); however, cardiac complications (other than myocardial infarction) (β = 0.059, p < 0.0001), pneumonia (β = 0.063, p < 0.0001), pulmonary embolism (β = 0.060, p < 0.0001), sepsis (β = 0.064, p < 0.0001) and shock/cardiorespiratory arrest each increased significantly. Detailed analysis of the rates of peri-operative major complications linear trends adjusted per 1000 in-hospital days is given in Table II.

Table II

Linear trend analysis showing the rate per 1000 in-hospital days of peri-operative complications after lumbar spinal fusion surgery

Complication	1998	1999	2000	2001	2002	2003	2004	2005	2006	2007	2008	Regression coefficient	p-value
Cerebral infarction	0.34	0.28	0.26	0.28	0.23	0.27	0.28	0.41	0.33	0.30	0.42	0.035	0.051
Sepsis	0.58	0.62	0.54	0.58	0.55	0.75	0.89	0.70	0.93	0.84	1.03	0.064	< 0.0001
Shock/cardiac arrest	0.16	0.13	0.21	0.21	0.30	0.28	0.15	0.28	0.26	0.21	0.29	0.046	0.02
Acute myocardial infarction (AMI)	0.65	0.75	0.88	0.73	0.51	0.77	0.80	0.63	0.62	0.69	0.73	-0.007	0.527
Cardiac complications (non-AMI)	5.13	5.16	4.99	5.37	5.42	5.80	6.83	6.99	6.88	8.16	8.44	0.059	< 0.0001
Pneumonia	1.88	2.01	1.99	2.24	2.40	2.55	3.05	2.83	3.30	3.00	3.57	0.063	< 0.0001
Pulmonary embolism	0.31	0.47	0.47	0.41	0.48	0.38	0.61	0.63	0.53	0.65	0.69	0.059	< 0.0001

Discussion

We analysed nationally representative data on linear trends of major morbidity and mortality in primary posterior lumbar fusion surgery between 1998 and 2008. Over this period the total number of operations increased significantly. Patient age and comorbidity burden also increased over time. During the ten-year study period the length of hospital stay decreased, whereas routine discharges to patients’ customary residence remained unchanged. Adjusted in-hospital mortality also remained unchanged, whereas the incidence of most peri-operative major complications increased.

Our data demonstrate a mean increase in the number of primary posterior lumbar fusions of 11.6% per year. Although regional differences in the rate of lumbar fusion in the United States have been described previously,¹⁵ our data confirm an absolute, as well as a population growth-adjusted, increase in these procedures. This increase has been reported previously.^2,16,17 Further analysis of types of fusion demonstrated a 15-fold increase in complex spinal fusions between 2002 and 2007.¹⁸ This constant increase in the total number and complexity of lumbar spinal fusions might be attributed to advances in implant design and manufacture, surgical techniques and management strategies. Although the surgical indications for fusion may remain similar, new technologies and improvements in peri-operative management allow surgeons to perform complex operations on patients with higher comorbidity and advanced age.

We found an increase in mean patient age of over four years within one decade. This may be attributable to the ageing population in general as well as to a steeper increase in lumbar fusions in elderly people.^2,18 The comorbidity burden also increased between 1998 and 2008. Age and the presence of comorbidities have previously been linked to increased rates of complications and mortality after surgical procedures.^3,4,19 In this context, our finding of increasing mean patient age over time must be viewed with concern.

A reduction in length of hospital stay, as shown in our study, has been described in other fields of spinal surgery,²⁰ possibly due to improved peri-operative management strategies, pain control and improved rehabilitation regimes. In addition, increasing financial pressure by insurance carriers is likely to be a significant factor. However, the rates of patient discharge to their customary residence remained stable over the ten-year study period, suggesting that advances in post-operative care may be partially responsible for this trend.

Significant gender differences were observed, with more women undergoing lumbar fusion at any given time. This gender gap increased over time to reach a level of 12.6% weighted towards women in 2008. Such increased rates in women undergoing primary and revision lumbar fusion have previously been shown.^4,18,21

Unadjusted in-hospital mortality rates decreased from 0.17% in 1998 to 0.12% in 2008, and similar mortality rates were reported by Shen, Silverstein and Roth.³ These researchers also concluded that medically complicated and elderly patients are at an increased risk for in-hospital mortality and early post-operative complications. Fu et al reviewed over 10 000 lumbar operations and reported that all patients who died following these operations were over 60 years old.²² However, after adjusting for length of stay, the decrease in in-hospital mortality was not found to be significant. Our data represent unchanged adjusted in-hospital mortality, even though the mean age and comorbidity burden increased from 1998 to 2008. Shifts in the patient population and improvements in management might explain this stability of mortality rates. Although mortality risk might be increased in the elderly, this is counterbalanced by a significantly decreased mortality rate among younger patients.³ Furthermore, advances in peri-operative care might counterbalance the increased risk of peri-operative mortality posed by an older and medically unfit patient population. Our findings clearly underline the importance of ongoing research in optimising peri-operative management strategies. As there is a trend towards increasing comorbidity and age among patients undergoing lumbar fusion, future efforts to improve patient care, targeting multiple peri-operative factors, will have to be pursued in order to ensure satisfactory outcomes.

The analysis of major peri-operative complications is shown in Table II. The most significant increase was seen in the incidence of sepsis, cardiac complications other than acute myocardial infarction, pneumonia and pulmonary embolism. This increase in pulmonary and cardiac complications has already been reported in patients undergoing cervical fusion.²⁰ Although pulmonary embolism is relatively rare following lumbar surgery and routine screening for detection of asymptomatic thrombosis might not be indicated,²³ our data show a significant increase in pulmonary embolism over the past decade, and these findings are comparable with previously published data.²⁴ Owing to the shift towards elderly patients, prospective studies should re-evaluate the risk of pulmonary embolism in patients undergoing lumbar surgery. It is possible that the increase in pneumonia can be attributed to the increase in the mean age and comorbidity burden. Interestingly, a retrospective analysis showed that > 90% of the post-operative deaths occurring after lumbar surgery were due to respiratory complications, myocardial infarction or sepsis.²¹

Our findings must be interpreted according to the limitations associated with the analysis of the NIS database. The clinical information gained from the NIS is limited. For example, the exact procedural details are unknown. This is important, as it is known that posterolateral, PLIF and TLIF procedures differ significantly in their risk–benefit profile.²⁵ Furthermore, the database only allows interpretation of inpatient complications and mortality rates. The rates of peri-operative complications are likely to be higher than shown in this study because post-operative complications and deaths outside the hospital are not captured. Additionally, possible coding bias has to be considered. Using the ICD-9-CM coding system leaves a large number of alternative code options for each entity.

In general, there is a lack of detailed information relating to spinal surgery in the available databases, and thus it seems prudent that a specialty-specific registry be pursued. With such an approach more detailed and important information could be captured that might lead to opportunities to study outcomes and linear trends in a more comprehensive manner.

In conclusion, this study confirms a nationwide increase in the number of primary posterior lumbar fusions. Generally, patients presented at an older age and had an increased comorbidity burden over time. However, adjusted mortality rates remained stable between1998 and 2008, whereas non-myocardial infarction cardiac, pulmonary and septic complications increased. These findings suggest that although advances in peri-operative medicine may have a positive impact on the extreme outcome of mortality, the increases in many major complications reflect the increasing demands on the peri-operative health-care team brought about by increases in comorbidity burden and advanced age. Future endeavours in the peri-operative arena will need to focus on interventions targeted to maintain adequate outcomes in a patient population that is increasingly medically unfit and elderly.