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

BACKGROUND: To improve and achieve adequate bony surgical margins, surgeons may consider computer-aided navigation a promising intraoperative tool, currently applied to a relatively few number of patients in whom freehand resections might be challenging. Placing fiducials (markers) in the bone, identifying specific anatomical landmarks, and registering patients for navigated resections are time consuming. To reduce the time both preoperatively and intraoperatively, skin fiducials may offer an efficient and alternative method of navigation registration. QUESTIONS/PURPOSES: (1) Does preoperative navigation using skin fiducials for registration allow the surgeon to achieve margins similar to those from bone fiducial registration in a simulated lower extremity tumor resection model in cadavers? (2) Does the use of preoperative navigation using skin fiducials for registration allow the surgeon to achieve similar bony margins in pelvic resections of simulated tumors as those achieved in long-bone resections using only skin fiducials for navigation in a cadaver model?
METHODS: Simulated bone tumor resections were performed in three fresh-frozen cadavers with intact pelvic and lower-extremity anatomy using navigation guidance. We placed 5-cm intraosseous cement simulated bone tumors in the proximal/distal femur (n = 12), and proximal/distal tibia (n = 12) and pelvis (supraacetabular; n = 6). After bone tumor implantation, CT images of the pelvis and lower extremities were obtained. Each planned osseous resection margin was set at 10 mm. Navigation registration was performed for each simulated tumor using bone and skin markers that act as a point of reference (fiducials). The simulated bone tumor was resected based on a resection line that was established with navigation, and the corresponding osseous margins were calculated after resection. These margins were determined by an orthopaedic surgeon who was blinded to resection planning by the removal of cancellous bone around the cement simulated tumor. The shortest distance was measured from the cement to the resection line. Smaller mean differences between planned and postoperative margins were considered accurate. Independent t-tests were conducted to assess measurement differences between planned and postoperative margins at the 95% CI. Bland-Altman analyses were conducted to compare the deviation in margin difference between planned and postoperative margins in skin and bone fiducial registration, respectively.
RESULTS: In all, 84 total resection margins were measured with 48 long bone and 20 pelvic obtained with skin fiducials and 16 long bone obtained with bone fiducials. The planned mean margin was 10 mm for all long bone and pelvic resections. We found that skin fiducial and bone fiducial postoperative margins had comparable accuracy when resecting long bones (10 ± 2 mm versus 9 ± 2 mm, mean difference 1 [95% CI 0 to 2]; p = 0.16). Additionally, skin fiducial long bone postoperative margins were comparable in accuracy to pelvic supraacetabular postoperative margins obtained with skin fiducials (10 ± 2 mm versus 11 ± 3 mm, mean difference -1 mm [95% CI -3 to 1]; p = 0.22). When comparing the deviation in margin difference between planned and postoperative margins in skin and bone fiducial registration, 90% (61 of 68) of skin fiducial and 100% (16 of 16) bone fiducial postoperative margins fell within 2 SDs.
CONCLUSIONS: In this pilot study, skin fiducial markers were easy to identify on the skin surface of the cadaver model and on CT images used to plan margins. This technique appears to be an accurate way to plan margins in this model, but it needs to be tested thoroughly in patients to determine if it may be a better clinical approach than with bone fiducials. CLINICAL RELEVANCE: The margins obtained using skin fiducials and bone fiducials for registration were similar and comparable in this pilot study with a very small effect size. Boundaries of the simulated tumors were not violated in any resections. Skin fiducials are easier to identify than bone fiducials (anatomic landmarks). If future clinical studies demonstrate that margins obtained using skin fiducials for registration are similar to margins obtained with anatomical landmarks, the use of navigation with skin fiducials instead of bone fiducials may be advantageous. This technique may decrease the surgeon's time used to plan for and localize registration points and offer an alternative registration technique, providing the surgeon with other registration approaches.