Clinical and Radiological Safety of Retained Implantable Doppler Devices Used for Free Flap Monitoring

Abstract

Introduction:

Implantable Doppler devices are reliable adjuncts used for free flap monitoring. Occasionally, the probe/wire is not removed and remains in the soft tissues. The clinical safety of the retained probes and safety and compatibility with magnetic resonance imaging (MRI) have not been studied. We present a series of retained implantable Doppler probes examining clinic outcomes, safety and compatibility with MRI, and effect on MRI image quality.

Methods:

A retrospective review was conducted of patients who had an implantable Doppler device for free flap monitoring between July 2007 and August 2018. Routine post-operative imaging was reviewed for all patients to identify incidental findings of a retained probe. A subset of patients with retained implantable Doppler probes who underwent MRI was identified. Magnetic resonance images were reviewed to detect any degradation of image quality.

Results:

A total of 323 patients who had an implantable Doppler device placed were reviewed 18 (5.6%) patients were identified with a retained probe and were included in this study. Mean age was 49 years with mean follow-up of 34.4 months. One potential device-related complication occurred in 1 (5.6%) patient. A total of 32 MRI scans were performed in 8 patients with retained devices, including 6 patients who underwent a total of 21 MRIs of the surgical site. There were no complications related to the MRI scans, and we found no significant degradation of image quality.

Conclusion:

Retained implantable Doppler probes were not associated with substantial adverse clinical outcomes nor affected MRI image quality of the surgical site.

Introduction

Rigorous and close monitoring of free flaps is crucial for early identification of microvascular complications allowing prompt intervention and potentially higher salvage rates. ^{1 -4} Although clinical evaluation remains the gold standard for post-operative free flap monitoring, multiple devices are available for adjunctive monitoring. In 1988, Swartz et al first presented an implantable Doppler device for flap monitoring. ^2,5 The Cook-Swartz (Cook Medical LLC) is a removable 20 MHz ultrasonic Doppler crystal probe within a nonremovable 8 × 5 mm silicone cuff film, which is wrapped around the vessel to be monitored. The probe is connected to a thin metallic wire, which exits the wound and is connected to a portable monitor at the patient bedside. ⁶ The device provides continuous blood flow information, and when vascular monitoring is no longer necessary, the implantable Doppler is removed by simply applying gentle traction. The probe is designed to detach from the cuff with 50 g of tension. ⁵ Removing the Cook-Swartz Doppler probe occasionally requires the use of more force, and in these cases, the metallic filament may be cut at the skin level and a portion of the wire is retained in the soft tissues. However, there is a concern regarding the foreign material left inside the wound. ^5,7 A significant number of patients undergoing free tissue transfer may need magnetic resonance imaging (MRI) surveillance for recurrent malignancy or osteomyelitis, and the presence of retained metal probes may raise some level of concern for MRI safety and image quality due to a retained metallic body at the surgical site.

We present a series of retained implantable Doppler devices examining clinical outcomes, MRI safety, and image quality. To the best of our knowledge, no previous study has addressed post-operative clinical follow-up of patients with implantable Doppler device retained or MRI safety and changes in imaging quality.

Methods

Following institutional review board approval, a retrospective chart review was conducted of patients who underwent microvascular free tissue transfer and placement of an implantable Doppler probe from July 2007 to August 2018. Any available post-operative imaging of the surgical site was reviewed for all patients to identify incidental findings of a retained metal probe. Data related to patient demographics, post-operative complications, and surgical site complications. All subsequent MRI scans performed with a retained probe in place were reviewed for documentation of adverse events. The images were reviewed by a senior radiologist (M.F.) to detect any degradation of image quality. Descriptive statistics was reported in mean (range) and frequency (percentage).

Results

During the study period, implantable Doppler probes were placed in 323 free tissue transfer patients. Eighteen (5.6%) patients were identified with an incidental radiographic finding of a retained probe and were subsequently included in the analysis. Ten (55.5%) patients were male, 8 (45.5%) were female. The mean age at surgery 48.8 (range 25-67) years with a mean follow-up duration of 34.4 (range 2-122) months. The retained device was detected on various imaging modalities, including plain radiograph, mammogram, and computed tomography scans (Figures 1 and 2). The mean time from surgery to the earliest radiographic appearance was 20.8 (range 1.2-65.3) months. Of the patients included in this study, free tissue transfer was performed for esophageal reconstruction (n = 5), breast reconstruction (n = 5), extremity reconstruction (n = 5), and facial reconstruction (n = 3). Reconstruction was performed following tumor extirpation (n = 14) or reconstruction of traumatic defects (n = 4). Patient characteristics and post-operative complications are described in Table 1.

Figure 1.

A 56-year-old male patient underwent free jejunal flap for esophageal reconstruction. The surgical team attempted to remove the implantable Doppler device on the 10th post-operative day. An esophagram performed on the 35th post-operative day showed the implantable Doppler device retained (arrow).

Figure 2.

A 31-year-old female patient underwent sarcoma resection in the right ankle followed by free latissimus dorsi flap and split thickness skin graft. The surgical team attempted to remove the implantable Doppler device on the 49th post-operative day; however, the wire fractured just below the skin level. An x-ray performed at 3.6 months showed the retained wire (arrow).

Table 1.

Patient Characteristics and Post-Operative Complications.

Characteristic
Total number of patients with retained device	18 (100%)
Gender
Male	10 (55.5%)
Female	8 (44.5%)
Mean age (years old)	48.8 (25-67)
Follow-up after surgery (months)	34.4 (2-122.4)
Type of reconstruction
Esophageal reconstruction	5 (27.7%)
Breast reconstruction	5 (27.7%)
Extremity reconstruction	5 (27.7%)
Facial reconstruction	3 (16.6%)
Indication for free tissue transfer
Oncologic resection	14 (77.8%)
Trauma/osteomyelitis	4 (22.2%)
Mean time to radiological identification of the device after surgery (months)	20.8 (1-65)
Post-op interval until attempted removal (days)	36 (5-165)
Site of post-operative MRI
Surgical site (retained probe)	21 (0-9)
Other site	11 (0-5)
Post-operative complications
Related to probe	1
Unrelated to retained probe	5

Abbreviation: MRI, magnetic resonance imaging.

Clinical Outcomes

In this series, removal of the device was attempted at a mean of 36 (range, 5-165) days following surgery. Potential device–related complications occurred in only 1 (5.5%) patient, who underwent lower extremity reconstruction after oncologic resection and subsequently developed chronic draining sinus 14 months after surgery. The etiology of the draining sinus was attributed to the retained filament and was successfully treated with removal of the retained filament. Five (27.7%) patients developed post-operative complication presumed unrelated to the Cook-Swartz device, such as minor superficial wound dehiscence, bloodstream infection, post-operative bleeding, arterial line infection, and osteomyelitis 10 years after a total knee arthroplasty. One (5.5%) patient who underwent breast reconstruction presented with shoulder pain 8 months the initial surgery, and an MRI was requested as part of her workup. Before the MRI was performed, the retained implantable Doppler device was localized with mammogram and surgically removed due to concerns about MRI safety and potential effects on imaging quality (Figure 3).

Figure 3.

A 44-year-old woman had a bilateral DIEP flap for breast reconstruction. Post-operatively, a mammogram and an ultrasound demonstrated the presence of the implantable Doppler wires (arrow). The device was surgically removed due to concerns about MRI safety and potential effects on imaging quality. MRI indicates magnetic resonance imaging.

Radiologic Outcomes

A total of 32 MRIs were performed in 8 patients (44.4%) with retained devices. Six of 8 patients underwent a total of 21 MRIs of the surgical site. In these 8 patients, the number of MRI exams per patient ranged from 1 to 9 exams per patient. One patient initially had one MRI postponed due to concerns related to the retained device but later underwent the exam with no reported MRI-related complications. No patient who underwent MRI with a retained probe experienced any known MRI-related complications due to device heating or motion. Upon independent review of the quality of MRIs images by a senior musculoskeletal radiologist (M.F.), no significant image quality degradation was noted. In some patients, the retained device was not even detected on the MRI but was evident on other imaging studies such as x-ray, computed tomography scan, or mammogram.

Discussion

In this study, we present a series of 32 patients with a retained component of an implantable Doppler device with only 1 required intervention and without adverse events related to the MRI scan. When MRI was performed, the quality of the obtained images did not seem to be affected by the retained device as evident by lack of image quality degradation.

Reviewing the Manufacturer and User Facility Device Experience available on Food and Drug Administration (FDA) website, the authors identified a total of 28 reports related with Cook-Swartz complications. ⁸ The most frequent report was related to Cook-Swartz early detachment (13 reports), all requiring reoperation to inspect the flap and place another implantable Doppler. The second most common complication was malfunctioning device (8 reports), in which 6 of these cases required reoperation. The third most frequent complication was retained wire (5 reports), in which 1 patient developed subsequently infection requiring reoperation to remove the retained filament. The least frequent complication was failure to disconnect the implantable Doppler resulting in injury to the anastomosed vessels (2 reports), both cases required reoperation. ⁸ Based on our small series, the retained component of the implantable Doppler device does not seem to pose major clinical or radiological risks. We are not aware of other studies evaluating the clinical safety of retained implantable Doppler devices and a larger series is necessary to challenge or validate our findings.

The compatibility of the retained metal component with MRI is a concern, and the clinical consequences of probe/wire heating are largely unknown. The presence of metallic objects may limit MRI diagnostic utility as well as its safety. In our study, the majority of patients in whom part of the implantable Doppler device was retained are patients who underwent reconstructions following oncologic resections and tumor extirpation. Magnetic resonance imaging is a crucial surveillance method, and the inability to obtain MRI may negatively impact and hinder the required cancer surveillance. ⁹ Therefore, evaluating the safety of MRI and the impact of retained implantable Doppler devices on MRI imaging quality may help to address some of these concerns.

Some implanted devices, such as the implantable Doppler studied in this paper, are considered unsafe for MRI. ^10,11 The major recognized mechanical risk associated with MRI scanner is the presence of ferromagnetic devices, including biomedical implants. The static field produces attractive and rotational forces on these devices, whose magnitude is related to their mass and distance from the bore entrance. ^9,12,13 Another concerning aspect is that some implants, such as guide wires, can produce heating due to their interaction with the radiofrequency field. ^9,14,15 Even though no major complication was reported in patients with the implantable Doppler who underwent MRI in our study, the retained implantable Doppler eventually may heat during the MRI exam. We did not find studies reporting the implantable Doppler’s magnetic field interactions. Other implantable devices, such as coils, stents, filters, and vascular grafts have demonstrated magnetic field interactions; however, 6 weeks after implantation, these devices are incorporated in tissue due to tissue ingrowth and other mechanisms. ¹¹ According to Shellock, it is unlikely that implants will move or dislodge due to MRI exposure, and there is no reported case of these types of implants heating excessively due to MRI exposure. ^11,16

Regarding MRI quality, metallic devices can produce artefact, which consists of a feature appearing in an image which actually is not present in the real object. ¹⁷ This occurs due to magnetic differences between the metal device and the adjacent tissue. ^17,18 Multiple factors may influence the artefact formation such as size of the implant itself, the MRI field strength (1.5 Tesla vs 3 Tesla), the MRI protocol, and sequence parameters. ^18,19 In the present study, no significant degradation of MRI quality was perceived by a senior radiologist. The small size of the implantable Doppler and the round symmetric cross-sectional area of the metal implant likely limits the magnitude of artefact formation. ⁹

The present study has several limitations. Identification of retained probes was based on their presence on post-operative imaging; however, many patients who underwent free flap reconstruction with implantable Doppler monitoring did not have any imaging exams of the surgical site performed post-operatively. The prevalence of retained devices reported in our study (5.6%) is therefore likely an underestimate. Additionally, there is a relatively small sample size of patients who underwent MRI scans with a retained device in place and our study lacks a control cohort. Further studies of implantable Doppler properties, such as the radiofrequency field and heating characteristics should be performed before the implantable Doppler device can be considered absolutely safe in patients undergoing MRI scans with a retained device. Despite these limitations, this study highlights critical clinical and radiological aspects of retained implantable Doppler devices that have not been addressed in previous studies.

Conclusion

Retained implantable Doppler devices were not associated with substantial adverse clinical outcomes following free tissue transfer for extremity, face, breast, and esophageal reconstruction. Retained filaments did not affect MRI image quality of the surgical site. Additionally, no patient who underwent MRI with a retained probe experienced any MRI-related complications due to heating or motion. Nevertheless, the implantable Doppler device may heat during MRI, and if this imaging exam is to be considered in situations with a known retained probe, we recommend that patients should be awake and communicative for the study in order to be able to report any complication.