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## Long-term back pain after a single-level discectomy for radiculopathy: incidence and health care cost analysis

### Object

The most common spinal procedure performed in the US is lumbar discectomy for disc herniation. Longterm disc degeneration and height loss occur in many patients after lumbar discectomy. The incidence of mechanical back pain following discectomy varies widely in the literature, and its associated health care costs are unknown. The authors set out to determine the incidence of and the health care costs associated with mechanical back pain attributed to segmental degeneration or instability at the level of a prior discectomy performed at their institution.

### Methods

The authors retrospectively reviewed the data for 111 patients who underwent primary, single-level lumbar hemilaminotomy and discectomy for radiculopathy. All diagnostic modalities, conservative therapies, and operative treatments used for the management of postdiscectomy back pain were recorded. Institutional billing and accounting records were reviewed to determine the billed costs of all diagnostic and therapeutic measures.

### Results

At a mean follow-up of 37.3 months after primary discectomy, 75 patients (68%) experienced minimal to no back pain, 26 (23%) had moderate back pain requiring conservative treatment only, and 10 (9%) suffered severe back pain that required a subsequent fusion surgery at the site of the primary discectomy. The mean cost per patient for conservative treatment alone was $4696. The mean cost per patient for operative treatment was$42,554. The estimated cost of treatment for mechanical back pain associated with postoperative same-level degeneration or instability was $493,383 per 100 cases of first-time, single-level lumbar discectomy ($4934 per primary discectomy).

### Conclusions

Postoperative mechanical back pain associated with same-level degeneration is not uncommon in patients undergoing single-level lumbar discectomy and is associated with substantial health care costs.

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## Oral Presentations

### 2010 AANS Annual Meeting Philadelphia, Pennsylvania May 1–5, 2010

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## Utility of minimum clinically important difference in assessing pain, disability, and health state after transforaminal lumbar interbody fusion for degenerative lumbar spondylolisthesis

### Object

Outcome studies for spine surgery rely on patient-reported outcomes (PROs) to assess treatment effects. Commonly used health-related quality-of-life questionnaires include the following scales: back pain and leg pain visual analog scale (BP-VAS and LP-VAS); the Oswestry Disability Index (ODI); and the EuroQol-5D health survey (EQ-5D). A shortcoming of these questionnaires is that their numerical scores lack a direct meaning or clinical significance. Because of this, the concept of the minimum clinically important difference (MCID) has been put forth as a measure for the critical threshold needed to achieve treatment effectiveness. By this measure, treatment effects reaching the MCID threshold value imply clinical significance and justification for implementation into clinical practice.

### Methods

In 45 consecutive patients undergoing transforaminal lumbar interbody fusion (TLIF) for low-grade degenerative lumbar spondylolisthesis-associated back and leg pain, PRO questionnaires measuring BP-VAS, LPVAS, ODI, and EQ-5D were administered preoperatively and at 2 years postoperatively, and 2-year change scores were calculated. Four established anchor-based MCID calculation methods were used to calculate MCID, as follows: 1) average change; 2) minimum detectable change (MDC); 3) change difference; and 4) receiver operating characteristic curve analysis for two separate anchors (the health transition index [HTI] of the 36-Item Short Form Health Survey [SF-36], and the satisfaction index).

### Results

All patients were available at the 2-year follow-up. The 2-year improvements in BP-VAS, LP-VAS, ODI, and EQ-5D scores were 4.3 ± 2.9, 3.8 ± 3.4, 19.5 ± 11.3, and 0.43 ± 0.44, respectively (mean ± SD). The 4 MCID calculation methods generated a range of MCID values for each of the PROs (BP-VAS, 2.1–5.3; LP-VAS, 2.1–4.7; ODI, 11–22.9; and EQ-5D, 0.15–0.54). The mean area under the curve (AUC) for the receiver operating characteristic curve from the 4 PRO-specific calculations was greater for the HTI versus satisfaction anchor (HTI [AUC 0.73] vs satisfaction [AUC 0.69]), suggesting HTI as a more accurate anchor.

### Conclusions

The TLIF-specific MCID is highly variable based on calculation technique. The MDC approach with the SF-36 HTI anchor appears to be most appropriate for calculating MCID because it provided a threshold above the 95% CI of the unimproved cohort (greater than the measurement error), was closest to the mean change score reported by improved and satisfied patients, and was least affected by the choice of anchor. Based on the MDC method with HTI anchor, MCID scores following TLIF are 2.1 points for BP-VAS, 2.8 points for LP-VAS, 14.9 points for ODI, and 0.46 quality-adjusted life years for EQ-5D.

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## Comparative analysis of perioperative surgical site infection after minimally invasive versus open posterior/transforaminal lumbar interbody fusion: analysis of hospital billing and discharge data from 5170 patients

### Object

Surgical site infection (SSI) after lumbar fusion results in significant patient morbidity and associated medical resource utilization. Minimally invasive (MI) techniques for posterior/transforaminal lumbar interbody fusion (P/TLIF) were introduced with the goals of smaller wounds, less tissue trauma, reduced blood loss, and quicker postoperative recovery, while maintaining comparable surgical results. Studies with sufficient power to directly compare the incidence of SSI following MI versus open P/TLIF procedures have been lacking. Furthermore, the direct medical cost associated with the treatment of SSI following the P/TLIF procedure is poorly understood and has not been adequately assessed. Thus, the aim in the present study was to determine the incidence of perioperative SSI in patients undergoing MI versus open P/TLIF and the direct hospital cost associated with the diagnosis and management of SSI after P/TLIF as reported in a large administrative database.

### Methods

The authors retrospectively reviewed hospital discharge and billing records from the Premier Perspective Database for 2003 to 2009 to identify patients undergoing 1- or 2-level MI or open P/TLIF for lumbar spondylotic disease, disc degeneration, or spondylolisthesis. The ICD-9-CM procedure codes were used to identify patients undergoing P/TLIF and those experiencing SSI. Infection-related costs were obtained from the total costs incurred by the hospital for SSI-related care provided during inpatient or hospital outpatient encounters.

Five thousand one hundred seventy patients undergoing P/TLIF were identified. Demographic profiles, including the Charlson Comorbidity Index, were similar between MI and open cohorts. Overall, 292 patients (5.6%) experienced an SSI with a mean direct cost of $15,817 per SSI. For 1-level MI versus open P/TLIF, the incidence of SSI (38 [4.5%] vs 77 [4.8%], p = 0.77) and the mean SSI-associated cost per P/TLIF ($684 vs $724, p = 0.680) were similar. For 2-level MI versus open P/TLIF, the incidence of SSI (27 [4.6%] vs 150 [7.0%], p = 0.037) and mean SSI-associated cost per P/TLIF ($756 vs $1140, p = 0.030) were both significantly lower among MI-treated patients. In a multivariate model that accounted for differences in demographics and patient severity, open fusion was associated with a strong trend of increased incidence of SSI as compared with MI fusion (OR 1.469, 95% CI 0.959–2.250). ### Conclusions In this multihospital study, the MI technique was associated with a decreased incidence of perioperative SSI and a direct cost savings of$38,400 per 100 P/TLIF procedure when used in 2-level fusion. There was no significant difference in the incidence of SSIs between the open and MI cohorts for 1-level fusion procedures. The results of this study provide further evidence of the reduced patient morbidity and health care costs associated with MI P/TLIF.

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## Ability of electromyographic monitoring to determine the presence of malpositioned pedicle screws in the lumbosacral spine: analysis of 2450 consecutively placed screws

### Object

Pedicle screws provide efficient stabilization along all 3 columns of the spine, but they can be technically demanding to place, with malposition rates ranging from 5% to 10%. Intraoperative electromyographic (EMG) monitoring has the capacity to objectively identify a screw breaching the medial pedicle cortex that is in proximity to a nerve root. The purpose of this study is to describe and evaluate the authors' 7-year institutional experience with intraoperative EMG monitoring during placement of lumbar pedicle screws and to determine the clinical utility of intraoperative EMG monitoring.

### Methods

The authors retrospectively studied 2450 consecutive lumbar pedicle screws placed in 418 patients from June 2002 through June 2009. All screws were inserted using a free-hand technique and anatomical landmarks, stimulated at 10.0 mA, and evaluated with CT scanning within 48 hours postoperatively. Medial pedicle screw breach was defined as having greater than 25% of the screw diameter extend outside of the pedicle, as confirmed on CT scanning or intraoperatively by a positive EMG response indicating a medial breach. The sensitivity and specificity of intraoperative EMG monitoring in detecting the presence of a medial screw breach was evaluated based on the following definitions: 1) true positive (a positive response to EMG stimulation confirmed as a breach intraoperatively or on postoperative CT scans); 2) false positive (positive response to EMG stimulation confirmed as a correctly positioned screw on postoperative CT scans); 3) true negative (no response to EMG stimulation confirmed as a correctly positioned screw on postoperative CT scans); or 4) false negative (no response to EMG stimulation but confirmed as a breach on postoperative CT scans).

### Results

One hundred fifteen pedicle screws (4.7%) showed positive stimulation during intraoperative EMG monitoring. At stimulation thresholds less than 5.0, 5.0–8.0, and > 8.0 mA, the specificity of a positive response was 99.9%, 97.9%, and 95.9%, respectively. The sensitivity of a positive response at these thresholds was only 43.4%, 69.6%, and 69.6%, respectively. At a threshold less than 5.0 mA, 91% of screws with a positive EMG response were confirmed as true medial breaches. However, at thresholds of 5.0–8.0 mA or greater than 8.0 mA, a positive EMG response was associated with 89% and 100% false positives (no breaches), respectively.

### Conclusions

When using intraoperative EMG monitoring, a positive response at screw stimulation thresholds less than 5.0 mA was highly specific for a medial pedicle screw breach but was poorly sensitive. A positive response to stimulation thresholds greater 5.0 mA was associated with a very high rate of false positives. The authors' experience suggests that pedicle screws showing positive stimulation below 5.0 mA warrants intraoperative investigation for malpositioning while responses at higher thresholds are less reliable at accurately representing a medial breach.

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## Cost-effectiveness of transforaminal lumbar interbody fusion for Grade I degenerative spondylolisthesis

### Object

Transforaminal lumbar interbody fusion (TLIF) for spondylolisthesis-associated back and leg pain is associated with improvement in pain, disability, and quality of life. However, given the rising health care costs associated with spinal fusion procedures and varying results of recent cost-utility studies, the cost-effectiveness of TLIF remains unclear. The authors set out to assess the comprehensive costs of TLIF at their institution and to determine its cost-effectiveness in the treatment of degenerative spondylolisthesis.

### Methods

Forty-five patients undergoing TLIF for Grade I degenerative spondylolisthesis–associated back and leg pain after 6–12 months of conservative therapy were included. The authors assessed the 2-year back pain visual analog scale (VAS) score, leg pain VAS score, Oswestry Disability Index, and total back-related medical resource utilization, missed work, and health-state values (quality-adjusted life years [QALYs], calculated from EQ-5D with US valuation). Two-year resource use was multiplied by unit costs based on Medicare national allowable payment amounts (direct cost), and patient and caregiver workday losses were multiplied by the self-reported gross-of-tax wage rate (indirect cost). The mean total 2-year cost per QALY gained after TLIF was assessed.

Compared with preoperative health states reported after at least 6 months of medical management, a significant improvement in back pain VAS score, leg pain VAS score, and Oswestry Disability Index was observed 2 years after TLIF, with a mean 2-year gain of 0.86 QALYs. The mean ± SD total 2-year cost of TLIF was $36,836 ±$11,800 (surgery cost, $21,311 ±$2800; outpatient resource utilization cost, $3940 ±$2720; indirect cost, $11,584 ±$11,363). Transforaminal lumbar interbody fusion was associated with a mean 2-year cost per QALY gained of $42,854. ### Conclusions Transforaminal lumbar interbody fusion improved pain, disability, and quality of life in patients with degenerative spondylolisthesis–associated back and leg pain. The total cost per QALY gained for TLIF was$42,854 when evaluated 2 years after surgery with Medicare fees, suggesting that TLIF is a cost-effective treatment of lumbar spondylolisthesis.

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## Editorial: Cost-effectiveness of transforaminal lumbar interbody fusion

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## Cerebrospinal shunt infection in patients receiving antibiotic-impregnated versus standard shunts

### Object

Cerebrospinal fluid shunt infections are associated with significant morbidity and mortality in the treatment of adult and pediatric hydrocephalus. Antibiotic-impregnated shunt (AIS) catheters have been used with the aim of reducing shunt infection. While many studies have demonstrated a reduction in shunt infection with AIS, this reported efficacy has varied within the literature.

### Methods

The authors performed a systematic literature review to identify all published articles comparing the incidence of CSF shunt infection in AIS versus non-AIS catheters. The incidence of infection for AIS versus non-AIS catheters was calculated using the Mantel-Haenszel common odds ratio, and baseline demographics were compared between AIS and non-AIS cohorts.

### Results

Twelve AIS versus non-AIS cohort comparisons were identified in the literature (5 pediatric hydrocephalus, 3 adult hydrocephalus, and 4 mixed populations). In a total of 5613 reported shunt procedures (2664 AISs vs 2949 non-AISs), AISs were associated with a reduction in shunt infection (3.3% vs 7.2%; OR 0.439, p < 0.0001). In 787 shunt procedures for adult hydrocephalus (427 AIS vs 360 non-AIS), AISs were associated with reduction in shunt infection (0.9% vs 5.8%; OR 0.153, p < 0.0001). In 1649 shunt procedures for pediatric hydrocephalus (854 AIS vs 795 non-AIS), AISs were associated with reduction in shunt infection (5.0% vs 11.2%; OR 0.421, p < 0.0001).

### Conclusions

The authors' systematic review of the literature demonstrates that AIS catheters are associated with a significant reduction over non-AIS catheters in the reported incidence of CSF shunt infection in adult and pediatric populations. The AIS catheters do not appear to be associated with an increased incidence of antibiotic-resistant microorganisms. Prospective, randomized trials are needed to firmly assess and confirm this apparent difference in infection incidence.

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## Editorial: Pooling data on antibiotic-impregnated shunts

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## Long-term outcomes of revision fusion for lumbar pseudarthrosis

### Object

The number of low-back fusion procedures for the treatment of spine disorders has increased steadily over the past 10 years. Lumbar pseudarthrosis is a potential complication of lumbar arthrodesis and can be associated with significant pain and disability. The aim of this study was to assess, using validated patient-reported outcomes measures, the long-term effectiveness of revision arthrodesis in the treatment of symptomatic pseudarthrosis.

### Methods

This is a retrospective study of 47 patients who underwent revision lumbar arthrodesis for pseudarthrosis-associated back pain. Baseline 2-year outcomes were assessed using the following: visual analog scale (VAS) for back pain, Oswestry Disability Index (ODI), Zung Self-Rating Depression Scale, time to narcotic independence, time to return to work, EuroQol health-state utility, and physical and mental quality of life (Short Form [SF]–12 Physical and Mental Component Summary scores).

### Results

The mean duration of time between prior fusion and development of symptomatic pseudarthrosis was 2.69 years. Bone morphogenetic protein was used in 4 cases (8.5%) of revision arthrodesis. A significant improvement in VAS back pain (7.31 ± 0.81 vs 5.06 ± 2.64, p = 0.001), ODI (29.74 ± 5.35 vs 25.42 ± 6.0, p = 0.001), and physical health SF-12 (23.83 ± 6.89 vs 27.85 ± 8.90, p = 0.001) scores was observed when comparing baseline and 2-year post–revision arthrodesis scores, respectively, with a mean cumulative 2-year gain of 0.35 quality-adjusted life years. The median time to narcotics independence was 12.16 (interquartile range 1.5–24.0) months and the median time to return to work was 4 months (interquartile range 3–5 months). By 2 years after revision surgery, no patients had experienced pseudarthrosis. The SF-12 Mental Component Summary (44.72 ± 7.90 vs 43.46 ± 7.51, p = 0.43) and Zung Self-Rating Depression Scale scores (39.36 ± 7.48 vs 41.39 ± 10.72, p = 0.37) were not significantly improved by 2 years.

### Conclusions

The authors' study suggests that revision lumbar arthrodesis for symptomatic pseudarthrosis provides improvement in low-back pain, disability, and quality of life. Revision lumbar arthrodesis should be considered a viable treatment option for patients with pseudarthrosis-related back pain. Mental health symptoms from pseudarthrosis-associated back pain may be more refractory to revision surgery.