Bone tumor near the articular cartilage is hard to remove sufficiently without damaging adjacent joint. The purpose of this study was to evaluate the feasibility of navigation-assisted surgery for saving joint in bone tumor resection or curettage. In this study, the investigators hypothesized that computer-assisted surgery is feasible method to get both enough margin and joint salvage. Each of them is important intermediate factor for either oncologic outcome or functional outcome respectively. The investigators designed this study to find what proportion of patients who underwent computer-assisted resection could get enough margins using some criteria obtained by overlapping preoperative and postoperative images. The investigators also evaluated whether computer-assisted surgery can be feasible for joint saving using some criteria including functional outcome.

Inclusion criteria:

1. First condition is that the benign tumor is located deeply in bones of extremities, or pelvis, so we anticipate the difficulty to find the tumor and to assess the adequacy of resection without the aid of computer-assisted navigation during operation.
2. Second condition is when the violation of joint or growth plate is expected during removal of tumor due to its close proximity to joint or growth plate.

In case of malignant bone tumor, the sufficient preservation of joint anatomy should be required even with resection margin of 1.5 cm away from reactive zone of tumor. The sufficient preservation of joint is defined when at least 1 cm subchondral bone is remained after tumor resection, because 1 cm subchondral bone is required for fixation with allograft in subsequent reconstruction. The sufficient preservation of joint is also defined when articular surface is removed, but more than 50% of articular surface has to be saved.

Exclusion criteria were

1. Patients who denied to perform navigation surgery after explanation of navigation surgery protocol
2. Medically high-risk patients who could not withstand long surgical time for joint reconstruction
3. Patients whose feasibility of joint preservation was not fulfill the above inclusion criteria at initial presentation, although the tumor size was decreased enough to fulfill the inclusion criteria in MRI after completion of neoadjuvant chemotherapy.