Delineate the tumor on each MRI slice where the tumor is visible (Fig. 1-A), using the open-source software ITK-SNAP 1.6.0.1 (www.itksnap.org) (Fig. 1-B). Ideally, this important step is performed by the surgeon together with the radiologist.

Perform co-registration of MRI to computed tomography (CT), allowing fusion of the delineated tumor volume with the CT.

Define the target planes for tumor resection. Two cutting planes are positioned with a haptic device. Custom-made software locates the planes with a 1-cm safe margin related to the tumor (Fig. 1-C).

Select the allograft. All available pelvic allografts in the bone bank are scanned. Allograft CT-to-patient CT registration allows you to select the best matching allograft (Fig. 1-D).

Transfer the cutting planes to the allograft. The same allograft CT-to-patient CT registration as was used for allograft selection is utilized to transfer the cutting planes to the graft.

Use general anesthesia augmented with an epidural catheter. Administer prophylactic antibiotics before the surgical incision and insert a Foley urinary catheter.

Place the patient in a lateral decubitus position on a radiolucent operating table (Fig. 2) and stabilize him or her in place with cloth rolls. The operating table is moved if you need to turn the patient intraoperatively.

Combine ilioinguinal with iliocrural and obturator surgical approaches to extend to the external iliac fossa and obturator area. Isolate and mobilize the femoral vessels and nerve, iliopsoas, and spermatic cord. Ligate and section the obturator vessels and nerve. Lift the gluteus medius and minimus muscles from the external iliac fossa. Dissect the gluteal pedicle at the sciatic notch as well as the sciatic nerve. Detach the tendons of the gluteus medius and minimus from the greater trochanter. Detach the hamstrings from the ischial tuberosity with an osseous fragment to facilitate reinsertion.

For patient-to-CT registration, the dynamic reference basis of the navigation system is fixed to the iliac crest with use of a rotational stabilizer and is tracked by the localizer (NDI; Waterloo, Ontario, Canada). Via palpation of the exposed ilium, patient-to-CT registration is obtained with a surface-based registration algorithm.

Define the oscillation plane of the oscillating saw using the calibration unit provided by the navigation system and an additional calibration device that we specifically designed for the oscillating saw.

Mark the navigation planes with the navigation pointer and a surgical pencil; provide a start line for the saw.

Perform the osteotomies using the navigation system to guide the saw blade via direct visualization of the bone cutting on a monitor screen; follow the predefined target planes (Fig. 3). Perform an extemporaneous biopsy of the bone marrow at the osteotomy site to check if the margin is free of tumor.

For graft-to-CT registration, fix the dynamic reference basis of the navigation system to the graft. Via palpation of the allograft surface, graft-to-image registration is obtained with a surface-based registration algorithm.

Mark the navigation planes with the navigation pointer.

Perform the osteotomies with the navigating saw, following the same target planes as you used to cut the iliac bone for the tumor resection (Fig. 4).

Place the graft in the bone defect to ensure a good fit. Fix the graft initially with Kirschner wires (Fig. 5).

Fix the graft with molded reconstruction plates and lag screws (Figs. 5 and 7). Use an anterior plate on the pubic symphysis, an endopelvic plate and an exopelvic plate on the posterior column, and two lag screws from the anterior border of the iliac bone.

Ream the graft acetabulum and cement a dual-mobility POLARCUP acetabular prosthesis (Plus Orthopedics Belgium Sprl, Nivelles, Belgium) in place. Osteotomize the patient's femoral neck and cement a CMK21 femoral stem (Smith & Nephew, Brussels, Belgium) in place (Fig. 6).

After hip reduction, reinsert the tendons of the gluteus medius and minimus to the greater trochanter. Reinsert all detached tendons and elevated muscles with transosseous absorbable sutures to the iliac bone (Fig. 6).

Limb salvage surgery is preferable to amputation as it better restores patient function and walking ability.

Patients with a malignant bone tumor involving Enneking zone II of the iliac bone are good candidates for pelvic reconstruction⁷. Most tumors involving only zone I or III do not necessitate reconstruction.

Computer navigation is especially useful for pelvic bone tumors in a difficult location (zone II).

Incurable distant metastases preclude this extensive surgery. Palliative surgery is preferred in those cases.

Limb-salvage surgery is not recommended if the sciatic nerve or important vascular pedicles have been invaded by tumor or if there is massive skin invasion.

Persisting infection of the surgical wound or of the biopsy site contraindicates allograft or prosthesis insertion.

Tumor irradiation (>30 Gy) or pathological fracture with tumor dissemination to the soft tissue may represent relative contraindications.

Fix the dynamic reference basis rigidly into the bone; if it moves, the accuracy of the patient-to-CT registration is lost.

Ensure a free line of sight between the localizer and dynamic reference basis and navigation tools.

Provide cutting planes accessible through the surgical exposure, taking into account the dimensions of the surgical saw.

Avoid bending the saw blade during bone cutting. Bending can generate a difference between the saw-blade position displayed on the navigation screen and the actual position at the osseous surface.

Take great care to protect femoral and sciatic nerves as well as femoral and gluteal vascular pedicles during the procedure.

Tumor delineation: Both CT and MRI are essential for preoperative planning of tumor surgery. CT reveals precise osseous details, while MRI is superior in delineating the intraosseous and extraosseous tumor extensions, particularly in the soft tissues. We prefer MRI to delineate the tumor, with secondary transfer to CT by co-registration.

Allograft selection: To select the best-fitting allograft among the available iliac bone allografts from the local bone bank, we performed a CT-based registration between the iliac bone of the patient and different available allografts¹⁷. The selection criterion is the best congruency of the two iliac bones and their acetabula.

Preoperative simulation: Surgery simulation with synthetic models of the patient's pelvis and the graft allowed us to choose the best location of the dynamic reference basis, thereby increasing navigation tools’ visibility¹⁴. The target plane feasibility (i.e., whether or not the saw blade could reasonably reach the target plane through the planned surgical approach) was also checked.

Preoperative testing of the allograft fit: Navigated simulation of the cutting of the patient's iliac bone and of the allograft in a synthetic model allowed the fit of the allograft to be checked preoperatively¹⁴.

Direct navigation of the saw blade: This is more advantageous than navigation to align cutting blocks. Cutting blocks are bulky instruments. Registration of the entire saw as one rigid body is faster than using cutting blocks, and the accuracy is improved¹⁸.