Bone-specific kinetic model to quantify periosteal and endosteal blood flow using indocyanine green in fluorescence guided orthopedic surgery

Use of intra-operative fluorescence imaging in periprosthetic joint infection: State of the art and future perspectives

BACKGROUND

In periprosthetic joint infections (PJIs), the surgeon's role becomes pivotal in addressing the infection locally, necessitating the surgical removal of infected and necrotic tissue. Opportunity to enhance the visualization of infected tissue during surgery could represent a game-changing innovation.

OBJECTIVE

The aim of this narrative review is to delineate the application of intraoperative fluorescence imaging for targeting infected tissues in PJIs.

METHODS

A systematic review, adhering to the Preferred Reporting Items for Systematic Review and Meta-Analyses (PRISMA) guidelines, was carried out. The search included multiple online database; MEDLINE, Scopus, and Web of Science. For data extraction the following were evaluated: (i) diagnosis of musculoskeletal infection; (ii) use of intraoperative fluorescence imaging; (iii) infected or necrotic tissues as target.

RESULTS

Initially, 116 studies were identified through online database searches and reference investigations. The search was narrowed down to a final list of 5 papers for in-depth analysis at the full-text level. Subsequently, 2 studies were included in the review. The study included a total of 13 patients, focusing on cases of fracture-related infections of the lower limbs.

CONCLUSION

The primary and crucial role for orthopedic surgeons in PJIs is the surgical debridement and precise removal of necrotic and infected tissue. Technologies that enable clear and accurate visualization of the tissue to be removed can enhance the eradication of infections, thereby promoting healing. A promising avenue for the future involves the potential application of intraoperative fluorescence imaging in pursuit of this objective.

Mapping estimates of vascular permeability with a clinical indocyanine green fluorescence imaging system in experimental pancreatic adenocarcinoma tumors

Significance

Pancreatic cancer tumors are known to be avascular, but their neovascular capillaries are still chaotic leaky vessels. Capillary permeability could have significant value for therapy assessment, and its quantification might be possible with macroscopic imaging of indocyanine green (ICG) kinetics in tissue.

Aim

The capacity of using standard fluorescence surgical systems for ICG kinetic imaging as a probe for capillary leakage was evaluated using a clinical surgical fluorescence imaging system, as interpreted through vascular permeability modeling.

Approach

Xenograft pancreatic adenocarcinoma models were imaged in mice during bolus injection of ICG to capture the kinetics of uptake. Image analysis included ratiometric data, normalization, and match to theoretical modeling. Kinetic data were converted into the extraction fraction of the capillary leakage.

Results

Pancreatic tumors were usually less fluorescent than the surrounding healthy tissues, but still the rate of tumor perfusion could be assessed to quantify capillary extraction. Model simulations showed that flow kinetics stabilized after about 1 min beyond the initial bolus injection and that the relative extraction fraction model estimates matched the experimental data of normalized uptake within the tissue. The kinetics in the time period of 1 to 2 min post-injection provided optimal differential data between AsPC1 and BxPC3 tumors, although high individual variation exists between tumors.

Conclusions

ICG kinetic imaging during the initial leakage phase was diagnostic for quantitative vascular permeability within pancreatic tumors. Methods for autogain correction and normalized model-based interpretation allowed for quantification of extraction fraction and difference identification between tumor types in early timepoints.

Utility of preoperative MRI for assessing proximal fragment vascularity in scaphoid nonunion

Aims

The benefit of MRI in the preoperative assessment of scaphoid proximal fragment vascularity remains controversial. The purpose of this study is to compare preoperative MRI findings to intraoperative bleeding of the proximal scaphoid.

Methods

A retrospective review of 102 patients who underwent surgery for scaphoid nonunion between January 2000 and December 2020 at a single institution were identified. Inclusion criteria were: isolated scaphoid nonunion; preoperative MRI assessing the proximal fragment vascularity; and operative details of the vascularity of the proximal fragment with the tourniquet deflated. MRI results and intraoperative findings were dichotomized as either 'yes' or 'no' for the presence of vascularity. A four-fold contingency table was used to analyze the utility of preoperative MRI with 95% confidence intervals. Relative risk was calculated for subgroups to analyze the association between variables and MRI accuracy.

Results

Preoperative MRI identified 55 proximal scaphoid fragments with ischaemia and 47 with vascularized proximal fragments. After the proximal fragment was prepared, the tourniquet was deflated and assessed for bleeding; 63 proximal fragments had no bleeding and 39 demonstrated bleeding. MRI was not reliable or accurate in the assessment of proximal fragment vascularity when compared with intraoperative assessment of bleeding. No patient or MRI factors were identified to have a statistical impact on MRI accuracy.

Conclusion

Current preoperative MRI protocols and diagnostic criteria do not provide a high degree of correlation with observed intraoperative assessment of proximal fragment bleeding. While preoperative MRI may assist in surgical planning, intraoperative assessment remains the best means for assessing proximal fragment vascularity in scaphoid nonunion. Future efforts should focus on the development of objective measures of osseous blood flow that may be performed intraoperatively.

High-energy open-fracture model with initial experience of fluorescence-guided bone perfusion assessment

High-energy orthopedic injuries cause severe damage to soft tissues and are prone to infection and healing complications, making them a challenge to manage. Further research is facilitated by a clinically relevant animal model with commensurate fracture severity and soft-tissue damage, allowing evaluation of novel treatment options and techniques. Here we report a reproducible, robust, and clinically relevant animal model of high-energy trauma with extensive soft-tissue damage, based on compressed air-driven membrane rupture as the blast wave source. As proof-of-principle showing the reproducibility of the injury, we evaluate changes in tissue and bone perfusion for a range of different tibia fracture severities, using dynamic contrast-enhanced fluorescence imaging and microcomputed tomography. We demonstrate that fluorescence tracer temporal profiles for skin, femoral vein, fractured bone, and paw reflect the increasing impact of more powerful blasts causing a range of Gustilo grade I-III injuries. The maximum fluorescence intensity of distal tibial bone following 0.1 mg/kg intravenous indocyanine green injection decreased by 35% (p < 0.01), 75% (p < 0.001), and 87% (p < 0.001), following grade I, II, and III injuries, respectively, compared to uninjured bone. Other kinetic parameters of bone and soft tissue perfusion extracted from series of fluorescence images for each animal also showed an association with severity of trauma. In addition, the time-intensity profile of fluorescence showed marked differences in wash-in and wash-out patterns for different injury severities and anatomical locations. This reliable and realistic high-energy trauma model opens new research avenues to better understand infection and treatment strategies. Level of evidence: Level III; Case-control.

Use of Freshly Amputated Human Limbs for Pre-Clinical Evaluation of Molecular-Targeted Fluorescent Probes

We have co-developed a first-in-kind model of fluorophore testing in freshly amputated human limbs. Ex vivo human tissue provides a unique opportunity for the testing of pre-clinical fluorescent agents, collection of imaging data, and histopathologic examination in human tissue prior to performing in vivo experiments. Existing pre-clinical fluorescent agent studies rely primarily on animal models, which do not directly predict fluorophore performance in humans and can result in wasted resources and time if an agent proves ineffective in early human trials. Because fluorophores have no desired therapeutic effect, their clinical utility is based solely on their safety and ability to highlight tissues of interest. Advancing to human trials even via the FDA's phase 0/microdose pathway still requires substantial resources, single-species pharmacokinetic testing, and toxicity testing. In a recently concluded study using amputated human lower limbs, we were able to test successfully a nerve-specific fluorophore in pre-clinical development. This study used systemic administration via vascular cannulization and a cardiac perfusion pump. We envision that this model may assist with early lead agent testing selection for fluorophores with various targets and mechanisms.

Automated motion artifact correction for dynamic contrast-enhanced fluorescence imaging during open orthopedic surgery

Indocyanine green (ICG)-based dynamic contrast-enhanced fluorescence imaging (DCE-FI) can objectively assess bone perfusion intraoperatively. However, it is susceptible to motion artifacts due to patient's involuntary respiration during the 4.5-minute DCE-FI data acquisition. An automated motion correction approach based on mutual information (MI) frameby-frame was developed to overcome this problem. In this approach, MIs were calculated between the reference and the adjacent frame translated and the maximal MI corresponded to the optimal translation. The images obtained from eighteen amputation cases were utilized to validate the approach and the results show that this correction can significantly reduce the motion artifacts and can improve the accuracy of bone perfusion assessment.

Fluorescence-guided and molecularly-guided debridement: identifying devitalized and infected tissue in orthopaedic trauma

Following orthopaedic trauma, bone devitalization is a critical determinant of complications such as infection or nonunion. Intraoperative assessment of bone perfusion has thus far been limited. Furthermore, treatment failure for infected fractures is unreasonably high, owing to the propensity of biofilm to form and become entrenched in poorly vascularized bone. Fluorescence-guided surgery and molecularly-guided surgery could be used to evaluate the viability of bone and soft tissue and detect the presence of planktonic and biofilm-forming bacteria. This proceedings paper discusses the motivation behind developing this technology and our most recent preclinical and clinical results.

Sterically Shielded Hydrophilic Analogs of Indocyanine Green

A modular synthetic process enables two or four shielding arms to be appended strategically over the fluorochromes of near-infrared cyanine heptamethine dyes to create hydrophilic analogs of clinically approved indocyanine green. A key synthetic step is the facile substitution of a heptamethine 4'-Cl atom by a phenol bearing two triethylene glycol chains. The lead compound is a heptamethine dye with four shielding arms, and a series of comparative spectroscopy studies showed that the shielding arms (a) increased dye photostability and chemical stability and (b) inhibited dye self-aggregation and association with albumin protein. In mice, the dye cleared from the blood primarily through the renal pathway rather than the biliary pathway for ICG. This change in biodistribution reflects the much smaller hydrodynamic diameter of the shielded hydrophilic ICG analog compared to the 67 kDa size of the ICG/albumin complex. An attractive feature of versatile synthetic chemistry is the capability to systematically alter the dye's hydrodynamic diameter. The sterically shielded hydrophilic ICG dye platform is well-suited for immediate incorporation into dynamic contrast-enhanced (DCE) spectroscopy or imaging protocols using the same cameras and detectors that have been optimized for ICG.

Spatial and temporal patterns in dynamic-contrast enhanced intraoperative fluorescence imaging enable classification of bone perfusion in patients undergoing leg amputation

Dynamic contrast-enhanced fluorescence imaging (DCE-FI) classification of tissue viability in twelve adult patients undergoing below knee leg amputation is presented. During amputation and with the distal bone exposed, indocyanine green contrast-enhanced images were acquired sequentially during baseline, following transverse osteotomy and following periosteal stripping, offering a uniquely well-controlled fluorescence dataset. An unsupervised classification machine leveraging 21 different spatiotemporal features was trained and evaluated by cross-validation in 3.5 million regions-of-interest obtained from 9 patients, demonstrating accurate stratification into normal, suspicious, and compromised regions. The machine learning (ML) approach also outperformed the standard method of using fluorescence intensity only to evaluate tissue perfusion by a two-fold increase in accuracy. The generalizability of the machine was evaluated in image series acquired in an additional three patients, confirming the stability of the model and ability to sort future patient image-sets into viability categories.

Near-Infrared Fluorescence with Indocyanine Green to Assess Bone Perfusion: A Systematic Review

Background: Adequate perfusion of a bone flap is essential for successful reconstruction of osseous defects. Unfortunately, complications related to inadequate bone perfusion are common. Near-infrared fluorescence (NIRF) imaging enables intraoperative visualization of perfusion. NIRF has been investigated in reconstructive surgery to aid the surgeon in clinical perioperative assessment of soft tissue perfusion. However, little is known on the beneficial use of NIRF to assess bone perfusion. Therefore, the aim of this review was to search for studies evaluating NIRF to assess bone perfusion. Methods: A systematic review, according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guideline, was performed. Studies up to October 2021 were included. We extracted data regarding the study population, size and design, reported objective fluorescence parameters and the methodology used for fluorescence imaging and processing. Results: Ten articles were included. Studies reported unevenly on the protocol used for NIRF imaging. Five studies reported objective parameters. Absolute and relative perfusion parameters and parameters derived from maximum fluorescence were reported. The clinical significance of these parameters has not been evaluated in humans. Conclusion: The evidence on bone perfusion as measured with NIRF is limited. More clinical studies are required.

ICG-based dynamic contrast-enhanced fluorescence imaging guided open orthopaedic surgery: pilot patient study

Forty two patients with high energy open fractures were involved into the study to investigate whether an indocyanine green (ICG)-based dynamic contrast-enhanced fluorescence imaging (DCE-FI) can be used to objectively assess bone perfusion and guide surgical debridement. For each patient, fluorescence images were recorded after 0.1 mg/kg of ICG was administered intravenously. By utilizing a bone-specific kinetic model to the video sequences, the perfusion-related metrics were calculated. The results of this study shown that the quantitative ICG-based DEC-FI can accurately assess the human bone perfusion during the orthopedic surgery.

Evaluation of bone perfusion during open orthopedic surgery using quantitative dynamic contrast-enhanced fluorescence imaging

In this study, an indocyanine green (ICG)-based dynamic contrast- enhanced fluorescence imaging (DCE-FI) technique was evaluated as a method to provide objective real-time data on bone perfusion using a porcine osteotomy model. DCE-FI with sequentially increasing injury to osseous blood supply was performed in 12 porcine tibias. There were measurable, reproducible and predictable changes to DCE-FI data across each condition have been observed on simple kinetic curve-derived variables as well variables derived from a novel bone-specific kinetic model. The best accuracy, sensitivity and specificity of 89%, 88% and 90%, have been achieved to effectively differentiate injured from normal/healthy bone.

Perspective on optical imaging for functional assessment in musculoskeletal extremity trauma surgery

SIGNIFICANCE

Extremity injury represents the leading cause of trauma hospitalizations among adults under the age of 65 years, and long-term impairments are often substantial. Restoring function depends, in large part, on bone and soft tissue healing. Thus, decisions around treatment strategy are based on assessment of the healing potential of injured bone and/or soft tissue. However, at the present, this assessment is based on subjective clinical clues and/or cadaveric studies without any objective measure. Optical imaging is an ideal method to solve several of these issues.

AIM

The aim is to highlight the current challenges in assessing bone and tissue perfusion/viability and the potentially high impact applications for optical imaging in orthopaedic surgery.

APPROACH

The prospective will review the current challenges faced by the orthopaedic surgeon and briefly discuss optical imaging tools that have been published. With this in mind, it will suggest key research areas that could be evolved to help make surgical assessments more objective and quantitative.

RESULTS

Orthopaedic surgical procedures should benefit from incorporation of methods to measure functional blood perfusion or tissue metabolism. The types of measurements though can vary in the depth of tissue sampled, with some being quite superficial and others sensing several millimeters into the tissue. Most of these intrasurgical imaging tools represent an ideal way to improve surgical treatment of orthopaedic injuries due to their inherent point-of-care use and their compatibility with real-time management.

CONCLUSION

While there are several optical measurements to directly measure bone function, the choice of tools can determine also the signal strength and depth of sampling. For orthopaedic surgery, real-time data regarding bone and tissue perfusion should lead to more effective patient-specific management of common orthopaedic conditions, requiring deeper penetrance commonly seen with indocyanine green imaging. This will lower morbidity and result in decreased variability associated with how these conditions are managed.

Endosteal and periosteal blood flow quantified with dynamic contrast-enhanced fluorescence to guide open orthopaedic surgery

Due to the lack of objectively measurable or quantifiable methods to assess the bone perfusion, the success of removing devitalized bone is based almost entirely on surgeon's experience and varies widely across surgeons and centers. In this study, an indocyanine green (ICG)-based dynamic contrast-enhanced fluorescence imaging (DCE-FI) has been developed to objectively assess bone perfusion and guide surgical debridement. A porcine trauma model (n = 6 pigs × 2 legs) with up to 5 conditions of severity in loss of flow in each, was imaged by a commercial fluorescence imaging system. By applying the bone-specific hybrid plug-compartment (HyPC) kinetic model to four-minute video sequences, the perfusion-related metrics, such as peak intensity, total bone blood flow (TBBF) and endosteal bone blood flow to TBBF fraction (EFF) were calculated. The results shown that the combination of TBBF and EFF can effectively differentiate injured from normal bone with the accuracy, sensitivity and specificity of 89%, 88% and 90%, respectively. Our subsequent first in human bone blood flow imaging study confirmed DCE-FI can be successfully translated into human orthopaedic trauma patients.

Intraoperative fluorescence perfusion assessment should be corrected by a measured subject-specific arterial input function

SIGNIFICANCE

The effects of varying the indocyanine green injection dose, injection rate, physiologic dispersion of dye, and intravenous tubing volume propagate into the shape and magnitude of the arterial input function (AIF) during intraoperative fluorescence perfusion assessment, thereby altering the observed kinetics of the fluorescence images in vivo.

AIM

Numerical simulations are used to demonstrate the effect of AIF on metrics derived from tissue concentration curves such as peak fluorescence, time-to-peak (TTP), and egress slope.

APPROACH

Forward models of tissue concentration were produced by convolving simulated AIFs with the adiabatic approximation to the tissue homogeneity model using input parameters representing six different tissue examples (normal brain, glioma, normal skin, ischemic skin, normal bone, and osteonecrosis).

RESULTS

The results show that AIF perturbations result in variations in estimates of total intensity of up to 80% and TTP error of up to 200%, with the errors more dominant in brain, less in skin, and less in bone. Interestingly, error in ingress slope was as high as 60% across all tissue types. These are key observable parameters used in fluorescence imaging either implicitly by viewing the image or explicitly through intensity fitting algorithms. Correcting by deconvolving the image with a measured subject-specific AIF provides an intuitive means of visualizing the data while also removing the source of variance and allowing intra- and intersubject comparisons.

CONCLUSIONS

These results suggest that intraoperative fluorescence perfusion assessment should be corrected by patient-specific AIFs measured by pulse dye densitometry.