Abstract
Introduction
We hypothesize that Negative Pressure Wound Therapy (NPWT) is associated with a lower incidence of surgical site infection (SSI) in lower extremity amputations (LEAs), a potentially devastating complication.
Methods
NSQIP database from 2011 to 2018 was queried to identify all-level LEAs. Cases using NPWT were identified. One-to-one nearest-neighbor propensity score matching was performed using a binary logistic regression on NPWT status controlling for patient comorbidities.
Results
NPWT was used in 133 of 5237 total LEAs (2.54%). Compared to propensity score-matched controls, they had significantly fewer SSIs (1.50% vs. 8.27%).
Conclusions
NPWT was associated with lower incidence of SSI.
Keywords: Negative pressure wound therapy, Surgical site infections, Lower extremity amputations
Introduction
Lower extremity amputations (LEAs) are a relatively common procedure, particularly among subsets of patients with conditions that compromise blood flow such as diabetes and peripheral vascular disease (PVD). Unfortunately, sequelae of these conditions that often necessitate LEAs also complicate recovery and wound healing, leading to increased postoperative morbidity and mortality. LEA at baseline is a particular challenge due to diminished perfusion at skin flaps.1 Complications specifically due to surgical site infections (SSIs) span from the need for prolonged dressing changes to revision amputation, and may even escalate to sepsis.2,3 Reported readmission rates after LEA in the literature range from 18 to 29%, and reported SSIs range from 13 to 28.6%.4, 5, 6, 7, 8, 9, 10 Mortality associated with LEAs is similarly high, reported at 8.8% 30 days postoperatively and rising to 47.9% by 1 year.5,10,11 As rates of nontraumatic LEA secondary to chronic conditions rise in the United States, it is imperative we investigate techniques to lower rates of SSIs, reduce subsequent morbidity and mortality, and optimize patient outcomes.12
The idea of negative pressure Wound Therapy (NPWT) has been around for centuries, and the technique was officially given a name in 1952. However, the advent of machines to perform NPWT and subsequent Medicare coverage in 2001 made its application more plausible, and thus researchers began to explore its possible utilities.13 NPWT uses sub-atmospheric pressure to reduce edema, stabilize the wound environment, and draw the wound closed, ultimately resulting in shorter closure times and decreased infection rates.14 Across various surgical disciplines, NPWT has been associated with faster wound closure, decreased complication rates, and decreased hospital stays compared to conventional closure methods.15, 16, 17, 18, 19 In the field of orthopaedics, NPWT has been utilized with varying degrees of success, depending on the procedure. In hip arthroplasties and management of traumatic open fractures, there does not appear to be a significant difference in the incidence of SSIs in cases where NPWT was used.20,21 Contrastingly, NPWT significantly reduced infection rates after both sarcoma resection and instrumented spinal fusion surgery.22,23 Perhaps most promising in a discussion on lower extremity amputations is that NPWT increases granulation tissue formation and decreases time to complete wound healing in diabetic foot ulcers and amputations.24, 25, 26
While there is support for the successes of NPWT in decreasing SSIs and improving outcomes after LEA, the majority of studies are of lower levels of evidence.27,28 The goal of this investigation was to use a nationally validated, risk-adjusted, outcomes-based database to test the following hypothesis: use of postoperative NPWT in various levels of LEA is associated with decreased risk of SSI.
Methods
Surgical outcomes data was sourced from the American College of Surgeon's National Surgical Quality Improvement Program (ACS-NSQIP) database from the year 2011–2018.29 Cases of lower extremity amputation were selected on the basis of primary operative procedure current procedural terminology (CPT) code (Table 1) with the further requirement that the procedure was performed by an orthopedic surgeon. Cases using negative pressure wound therapy (NPWT) were identified by appropriate CPT code (Table 1).
Table 1.
CPT Codes used for case identification and subgroup analysis.
| CPT | Description | Subgroup |
|---|---|---|
| 27590 | Amputation Thigh Through Femur Any Level | Closed |
| 27592 | Amputation Thigh Through Femur Open Circular | Open Circular |
| 27594 | Amputation Thigh Through Femur Secondary Closure/Scar Revision | Revision |
| 27596 | Amputation Thigh Through Femur Re-Amputation | Revision |
| 27598 | Disarticulation Knee | |
| 27880 | Amputation Leg Through Tibia and Fibula | Closed |
| 27881 | Amputation Leg Through Tibia and Fibula with Immediate Fitting Technique Including Application of First Cast | Closed |
| 27882 | Amputation Leg Through Tibia and Fibula Open Circular | Open Circular |
| 27884 | Amputation Leg Through Tibia and Fibula Secondary Closure/Scar Revision | Revision |
| 27886 | Amputation Leg Through Tibia and Fibula Re-amputation | Revision |
| 27889 | Ankle Disarticulation | |
| 28800 | Amputation Foot Midtarsal | |
| 28805 | Amputation Foot TransMetatarsal | |
| 97605 | Negative pressure wound therapy (DME) ≤ 50 cm2 | NPWT |
| 97606 | Negative pressure wound therapy (DME) > 50 cm2 | NPWT |
| 97607 | Negative pressure wound therapy (non-DME) ≤ 50 cm2 | NPWT |
| 97608 | Negative pressure wound therapy (non-DME) > 50 cm2 | NPWT |
The primary surgical outcome was 30-day postoperative surgical site infection (superficial, deep, or organ-space) with secondary outcomes including 30-day unplanned readmission and 30-day mortality. These outcomes were abstracted from the NSQIP surgical record in addition to baseline patient characteristics used for propensity score matching (Table 2). Cases with missing outcome or baseline data were excluded from the analysis.
Table 2.
Propensity score variables and balance statistics.
| Variable | All Lower Ext Amputation | Matched Controls | NPWT | p-value |
|---|---|---|---|---|
| Patient Characteristics | ||||
| Age | 59.90 | 59.65 | 54.92 | 0.007 |
| Renal Failure | 3.80% | 3.76% | 9.02% | 0.080 |
| NSQIP Mortality Probability | 3.78% | 3.53% | 5.08% | 0.129 |
| NSQIP Morbidity Probability | 13.65% | 15.46% | 15.18% | 0.802 |
| Female Gender | 30.40% | 27.82% | 24.81% | 0.579 |
| Diabetes | 61.47% | 67.67% | 60.15% | 0.203 |
| Totally dependent functional status | 4.41% | 2.26% | 3.01% | 0.703 |
| COPD | 8.97% | 12.78% | 5.26% | 0.033 |
| CHF | 6.59% | 3.76% | 5.26% | 0.556 |
| Hypertension | 68.67% | 70.68% | 62.41% | 0.154 |
| Dialysis dependent | 11.51% | 13.53% | 9.77% | 0.341 |
| Disseminated cancer | 1.28% | 3.76% | 0.75% | 0.100 |
| Weight loss | 2.46% | 0.75% | 3.76% | 0.100 |
| Preoperative blood transfusion | 8.13% | 11.28% | 7.52% | 0.295 |
| Preoperative sepsis | 25.28% | 37.59% | 42.11% | 0.454 |
| Chronic steroid/immunosuppressant use | 5.88% | 5.26% | 1.50% | 0.091 |
| Open wound preoperatively | 62.38% | 68.42% | 66.92% | 0.794 |
| Infection present preoperatively | ||||
| Superficial | 1.32% | 0.00% | 0.00% | ∞ |
| Deep | 1.57% | 1.50% | 1.50% | 1.00 |
| Organ space | 2.65% | 2.26% | 2.26% | 1.00 |
| Surgical characteristics | ||||
| Emergency surgery | 14.68% | 18.05% | 9.02% | 0.032 |
| ASA classification≥3 | 88.01% | 92.48% | 89.47% | 0.394 |
| Operative time (min) | 77.07 | 78.37 | 77.38 | 0.895 |
| Intra/postoperative blood transfusion | 16.38% | 18.05% | 15.04% | 0.511 |
| Postoperative wound classification | ||||
| Clean/contaminated | 7.12% | 10.53% | 9.77% | 0.840 |
| Contaminated | 10.52% | 11.28% | 12.03% | 0.849 |
| Dirty/infected | 38.27% | 58.65% | 59.40% | 0.901 |
Propensity score matching was used to address the potential for bias related to treatment selection. Namely, the propensity score matching sought to compare patients who received NPWT to patients with similar comorbidities and surgical risk characteristics. Propensity scores were estimated for each case using a binary logistic regression model with 1:1 greedy, nearest-neighbor matching performed without replacement. Cases were matched exactly on primary procedure CPT code and preoperative wound infection status. Importantly, since open amputations are oftentimes part of a larger plan to carry out the amputations in two or more stages secondary to infection, our model was controlled for preoperative infection. Outcomes and baseline characteristic balance were evaluated using two-sided t-tests with α = 0.05. Subgroup analysis was performed on circular open, closed, and revision/re-amputation as identified by the appropriate primary procedure CPT code. Statistical analysis was performed in R version 3.6.0 with the MatchIt library.30
Results
5237 LEA patients were identified in the NSQIP-ACS database from 2011 to 2018. There were no cases missing required covariates and all were included for analysis without imputation. These patients had a mean age of 59.9 years and 30.4% were female. Surgical site infection occurred in 178 cases (3.4%). NPWT was used in 133 total cases (2.5%) which were matched to an equal number of propensity-score matched controls. Baseline characteristics between the groups were comparable, demonstrating nonsignificant p-values for all matched covariates with the exception of age (59.7 vs. 54.9, p = 0.007) and emergency surgery (18.0% vs 9.0%, p = 0.03) (Table 2). These covariates had a nonsignificant effect on treatment selection in the propensity score model.
Amputations that received NPWT postoperatively had significantly decreased rates of surgical site infection (1.5% vs. 8.3%, p = 0.011), in particular leading to significantly reduced rates of superficial and deep infection. The NPWT group also had fewer unplanned 30-day readmissions (11.2% vs. 15.0%, p = 0.366) and decreased 30-day mortality (3.0% vs. 6.8%, p = 0.156). Notably, postoperative length of stay was increased in patients receiving NPWT (11.2 vs. 9.8 days) (Table 3a).
Table 3a.
Results for all lower extremity amputation.
| All LE Amputation |
Matched Controls | NPWT | p-value | ||||
|---|---|---|---|---|---|---|---|
| N | 5237 | 133 | 133 | ||||
| Surgical Site Infection | 178 | (3.40%) | 11 | (8.27%) | 2 | (1.50%) | 0.011 |
| Superficial | 107 | (2.04%) | 4 | (3.01%) | 0 | (0.00%) | 0.045 |
| Deep | 62 | (1.18%) | 7 | (5.26%) | 1 | (0.75%) | 0.032 |
| Organ Space | 15 | (0.29%) | 1 | (0.75%) | 0 | (0.00%) | 0.319 |
| Unplanned Reoperation | 397 | (7.58%) | 16 | (12.03%) | 16 | (12.03%) | 1.000 |
| 30-Day Readmission | 620 | (11.84%) | 20 | (15.04%) | 16 | (12.03%) | 0.475 |
| Unplanned readmission | 597 | (11.40%) | 20 | (15.04%) | 15 | (11.28%) | 0.366 |
| Mean Length of Stay | 9.50 | 9.81 | 11.21 | 0.230 | |||
| Mean Postoperative LoS | 5.67 | 5.29 | 8.18 | 0.093 | |||
| Death (≤30 days postop) | 13 | (0.25%) | 9 | (6.77%) | 4 | (3.01%) | 0.156 |
Subgroup analysis demonstrated the benefit of NPWT in rates of surgical site infection for open circular (3.3% vs. 10.0%, p = 0.310), closed (2.1% vs 8.5%, p = 0.173), and revision/reamputation (0.0% vs. 15.0%, p = 0.083), although this study was not statistically powered for this analysis (Table 3b).
Table 3b.
Results for subgroup analysis.
| Open Circular | Matched Controls | NPWT | p-value | ||||
|---|---|---|---|---|---|---|---|
| N | 244 | 30 | 30 | ||||
| Surgical Site Infection | 6 | 2.46% | 3 | 10.00% | 1 | 3.33% | 0.310 |
| Superficial | 1 | 0.41% | 1 | 3.33% | 0 | 0.326 | |
| Deep | 4 | 1.64% | 2 | 6.67% | 0 | 0.161 | |
| Organ Space | 1 | 0.41% | 0 | 0.00% | 1 | 3.33% | 0.326 |
| Unplanned Readmission | 30 | 12.30% | 7 | 23.33% | 2 | 6.67% | 0.074 |
| Mean Postoperative LoS | 7.73 | 9.13 | 11.27 | 0.336 | |||
| Death | 18 | 7.38% | 2 | 6.67% | 2 | 6.67% | 1.000 |
| Revision | Matched Controls | NPWT | p-value | ||||
| N | 662 | 20 | 20 | ||||
| Surgical Site Infection | 29 | 4.38% | 3 | 15.00% | 0 | 0.083 | |
| Superficial | 17 | 2.57% | 0 | 0 | ∞ | ||
| Deep | 12 | 1.81% | 3 | 15.00% | 0 | 0.083 | |
| Organ Space | 1 | 0.15% | 0 | 0 | ∞ | ||
| Unplanned Readmission | 83 | 12.54% | 5 | 25.00% | 2 | 10.00% | 0.223 |
| Mean Postoperative LoS | 3.99 | 4.65 | 7.00 | 0.179 | |||
| Death | 10 | 1.51% | 3 | 15.00% | 0 | 0.083 | |
| Closed Amputation | Matched Controls | NPWT | p-value | ||||
| N | 3320 | 47 | 47 | ||||
| Surgical Site Infection | 125 | 3.77% | 4 | 8.51% | 1 | 2.13% | 0.173 |
| Superficial | 77 | 2.32% | 3 | 6.38% | 0 | 0.083 | |
| Deep | 40 | 1.20% | 1 | 2.13% | 1 | 2.13% | 1.000 |
| Organ Space | 12 | 0.36% | 0 | 0 | ∞ | ||
| 0 | 0 | ||||||
| Unplanned Readmission | 375 | 11.30% | 7 | 14.89% | 6 | 12.77% | 0.768 |
| Mean Postoperative Length of Stay | 5.86 | 1.96 | 7.23 | 0.235 | |||
| Death | 137 | 4.13% | 3 | 6.38% | 1 | 2.13% | 0.313 |
Discussion
Patients requiring LEA oftentimes have multiple comorbidities complicating wound care, such as diabetes and PAD. Therefore, based on current data supporting the use of NPWT in diabetic and vascular lower extremity wound care, we hypothesized that NPWT may be associated with decreased SSIs in LEA. Our investigation found that use of NPWT was associated with a significantly reduced prevalence of surgical site infections compared to matched controls. These patients also had fewer unplanned 30-day readmissions and lower 30-day mortality; while this was not statistically significant, we argue that the 25% reduction in 30-day readmissions and the 50% reduction in 30-day mortality is clinically relevant.
Although use of NPWT in orthopaedics remains controversial and procedure-dependent, literature supports the use NPWT in other surgical subspecialties. Not only do these investigations support patient morbidity and mortality improvements, but also some advocate that NPWT is cost-effective despite the fact that its unit cost may be perceived as high. Its cost-effectiveness may be attributable to fewer dressing changes, less nursing time, and decreased need for future surgical interventions secondary to complications.31,32 Further, variations of NPWT machinery continue to hit the market in order to improve cost-effectiveness of this intervention.33,34 It is important to note, however, that postoperative length of stay was increased in the NPWT group in our study; while this was not our primary outcome, it does need to be considered when assessing the cost-effectiveness of the intervention.
Effectively managing complex lower extremity incisions after amputation accelerates rehabilitation and time to prosthetic fitting.1 Therefore, optimal wound care after an amputation can minimize recovery time and improve quality of life in amputees. In their meta-analysis, Semsarzadeh et al.35 found a significantly lower rate of SSIs and lower rate of wound dehiscence in LEA patients who received NPWT versus their control group. Zayan et al.1 found a very low rate of wound complications in a cohort of 25 amputation patients (21 of which were LEA) treated with NPWT, with only one patient experiencing a SSI which quickly resolved with oral antibiotics. Our study found a greater than five-fold decrease in SSIs in patients who received NPWT compared to controls, consistent with both of these high-quality studies in the current literature. In 2017, an international expert panel on NPWT recommended this treatment for patients at high risk for surgical site complications; notable risk factors included diabetes, obesity, tobacco use, corticosteroid use, and high-tension wounds.36 Given the likelihood that patients who require LEAs have at least one of these risk factors, this is particularly pertinent to the findings of our investigation. These findings, along with those reported in previous literature, support that NPWT should be considered for routine utilization in subsets of patients undergoing LEA secondary to chronic diseases in order to decrease SSIs. Additionally, in deciding what level of amputation, the surgeon must consider the degree to which the remainder of the appendage can serve as a residual limb. Therefore, optimizing wound care to ensure a well-healed, non-tender residual limb is pertinent at all levels.
There are several limitations to this study. Firstly, our sample size may not fully capture usage of NPWT in the orthopaedic LEA patient population. Medicare reimbursements for NPWT have decreased in recent years, which decreases financial incentives for accurately billing for their use. This may have contributed to our lower sample size of only 133 LEA cases using NPWT over an eight-year time period. Additionally, an imbalance in the propensity score matching characteristics for age and emergent surgery type may have introduced bias into our model. As with most studies that utilize a national database, this study may have a biased patient population as patients were identified exclusively by CPT codes. Furthermore, complication outcomes were also tracked using ICD-9 codes, introducing the possibility of coding bias into outcomes analysis. Despite the aforementioned limitations, our findings are consistent with previous studies that investigated NPWT use after LEA and represent a step forward in amputee wound care. Future research should aim to test our hypothesis in a prospective, multi-arm-controlled trial, that is powered to investigate specific LEA patient populations with compromised blood flow at baseline such as diabetes and PAD.
Conclusion
Patients managed with NPWT after LEA are significantly less likely to develop a SSI compared to those who aren't managed with NPWT. NPWT may also lead to fewer 30-day readmissions and lower 30-day mortality in LEA patients. Although the unit cost of NPWT may be high, its ability to decrease the need for dressing changes, nursing staff time, SSI, and re-operation makes it a cost-effective management option. Furthermore, its promise of potential improvements in patient care and time to postoperative mobilization support NPWT should be considered more frequently for implementation in wound care.
Conflict of Interest Statement
The authors have no personal or institutional interest with regards to the authorship and/or publication of this manuscript. No funding or grants were obtained.
Ethical review committee statement
This study did not include the use of identifiable human data or animals and was therefore exempt from review by the institutional review board.
Research site
This investigation was performed at Rutgers New Jersey Medical School, in Newark, New Jersey.
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