CORR Insights®: Are Skin Fiducials Comparable to Bone Fiducials for Registration When Planning Navigation-assisted Musculoskeletal Tumor Resections in a Cadaveric Simulated Tumor Model?

Where Are We Now?

Pre-operative radiologic imaging and intra-operative fluoroscopy, although useful, cannot reliably achieve accurate planned tumor margins [2]. The desire to improve both oncologic and functional outcomes following tumor resection of the pelvis and sacrum (as well for some long bone tumors) has led to the investigation, adoption, and refinement of computerized navigation technology [7, 8, 10, 11]. Current navigation strategies for bone tumor resection use bone fiducial markers to register pre-operative images to specific patient landmarks. A bone fiducial is usually a pin that is placed into a distinct and recognizable osseous landmark that allows the surgeon to register a patient to uploaded images for the purpose of live (real-time) navigation. This technique is invasive for patients as these pins are placed into specific areas of bone; a process that is time intensive to ensure that the landmarks are properly identified and matched to pre-operative imaging.

In the current study, Zamora and colleagues [13] developed a cadaveric tumor model to determine whether skin fiducial markers were as helpful as bone fiducial markers in achieving desired tumor resection margins in long bones and the pelvis. The accuracy of margins achieved by skin and bone fiducial markers were comparable in long bone resections. Furthermore, the use of skin fiducial markers resulted in comparable margins in both long bone and pelvic resections [13].

Where Do We Need To Go?

Preservation of the superior weight-bearing dome of the pelvis while achieving negative tumor margins is of utmost importance in achieving acceptable oncologic and functional outcomes. Bone resection can be minimized in this region without compromising tumor margins [4, 9]. I would have imagined there would be a demonstrable learning curve associated with computer-assisted navigation, but during the course of one retrospective study, registration time did not improve in those instances in which bone fiducials were used [6]. Therefore, a technique, such as the use of skin fiducials, that limits registration time, is non-invasive, reproducible, reliable, and comparable to bone fiducial markers is needed.

Skin fiducial markers can result in acceptable planned tumor resection for simulated soft-tissue tumors in a cadaver model [5]. Using computerized navigation based on pre-operative MRIs, tumors were resected with pre- and post-operative measurement agreement observed with 94.9% (93/98) of resections occurring within two standard deviations of the mean measurement difference [5]. Skin fiducial marker registration may represent an improvement in navigation that along with other refinements in technique may affect ease of use and wider spread use of this method. The goal is to continually improve resection accuracy and reliability, limit operative time, cost and ultimately, patient outcomes.

How Do We Get There?

Accurate pre-operative imaging can be used for patient registration and intra-operative computerized image guidance, as well as to develop new three-dimensional (3-D) tools to assist in making accurate and implant-specific osteotomies. Pre-operative imaging can be used to create custom 3-D printed implants or accurate allograft matches [14]. For example, cutting guides are 3-D printed to assist in creating precise osteotomies that match 3-D printed implants or intercalary allografts to accurately reconstruct post-resection defects. This technique allows for joint preservation surgery in select cases without compromising tumor margins. It also aids in the preservation of joints and even physes in children that allows for durable near-normal function, growth and development. In one study, comparing conventional resection to computer-assisted navigation and patient-specific instruments used for joint-sparing surgery, patient-specific instruments resulted in greater resection accuracy and was technically easier and faster to use [1]. Pre-operative planning, intra-operative computer-assisted navigation, and computer-generated designs for 3-D printed tools and implants continue to improve. In developing new techniques and skills in resecting and reconstructing bone, efforts to preserve soft tissue for implant coverage and attachment is equally prescient. Presently the cost outlay for navigation equipment and software is substantial. It is cumbersome to set up and use in an already crowded operating theatre and requires experienced personnel to operate. So too is the time and effort in planning three dimensional cutting guides and implants not to mention the considerable expense. These issues must be addressed in moving the field of precision surgery forward.

To this end, another exciting and emerging technology is the use of augmented reality that is based on a software platform for tablets, eliminating costly and bulky equipment. In comparing augmented reality to conventional resection, one study found improved resection accuracy [3]. Although preliminary and only done in animal models to date, these results are encouraging and require further evaluation. Investigational human studies are needed to prove efficacy and safety. It is important to understand the depth and potential of these new technologies in treating complex diseases but equally important to recognize the limitations as well.

These advances are exciting and are clearly useful adjuncts to tumor resection surgery, but are not surrogates for careful soft-tissue dissection and intra-operative adjustments for improved resection and subsequent reconstruction. Furthermore, these methods are not mutually exclusive but rather, together or in part, represent an evolving surgical paradigm utilizing advanced technology, thoughtful surgical planning, and meticulous surgical technique to achieve improved oncologic and functional outcomes. The use of skin fiducials is one such potential modification that may prove to be an improvement in computerized-assisted surgery.