Computer-Assisted Planning and Patient-Specific Instruments for Bone Tumor Resection within the Pelvis: A Series of 11 Patients

Abstract

Pelvic bone tumor resection is challenging due to complex geometry, limited visibility, and restricted workspace. Accurate resection including a safe margin is required to decrease the risk of local recurrence. This clinical study reports 11 cases of pelvic bone tumor resected by using patient-specific instruments. Magnetic resonance imaging was used to delineate the tumor and computerized tomography to localize it in 3D. Resection planning consisted in desired cutting planes around the tumor including a safe margin. The instruments were designed to fit into unique position on the bony structure and to indicate the desired resection planes. Intraoperatively, instruments were positioned freehand by the surgeon and bone cutting was performed with an oscillating saw. Histopathological analysis of resected specimens showed tumor-free bone resection margins for all cases. Available postoperative computed tomography was registered to preoperative computed tomography to measure location accuracy (minimal distance between an achieved and desired cut planes) and errors on safe margin (minimal distance between the achieved cut planes and the tumor boundary). The location accuracy averaged 2.5 mm. Errors in safe margin averaged -0.8 mm. Instruments described in this study may improve bone tumor surgery within the pelvis by providing good cutting accuracy and clinically acceptable margins.

Figure 1

Preoperative planning for patient number 2. Preoperative CT images of the patient were segmented to construct the 3D virtual models of the patient and the tumor. The resection strategy consisted of one target plane defining the desired resection plane with a 6 mm safe margin.

Figure 2

Bone models and PSI produced by additive manufacturing for patient number 6. (a) Bone model of the patient enables the visualization of the desired resection strategy and the tumor specimen to be resected. (b) PSI is equipped with flat surfaces to indicate the desired resection planes, holes to be pinned temporarily on the bone using Kirschner wires. (c) PSI has a position of best fit on the bone model. Calibration marks are engraved on the edge to provide visual control of the cutting depth. (d) Associated with a calibration mark direction lines indicate the depth of cutting. (e) The depth is measured from the outer edge of PSI to the deepest bone structure. (f) The direction lines engraved onto the flat surfaces of PSI.

Figure 3

Intraoperative situation for patient number 11. (a) PSI is designed using computer-aided-design software. (b) PSI are sterilizable to be manipulated by the surgeon in the operating room. PSI is positioned on the bone and temporarily fixed using Kirschner wires. (c) Cuts are initiated with the oscillating saw guided by the flat surfaces of the PSI.

Figure 4

Quantitative evaluation of bone cuts for patient number 2. Postoperative 3D virtual model of the patient was constructed from the postoperative CT images and registered to the preoperative 3D model. The achieved cut plane was manually identified and compared to the desired cut plane. See text for details on the computation of location accuracy parameter L and surgical margin SM.