###### Abstract  
Primary bone sarcomas are rare tumors and surgical resection in combination with chemo and radiation therapy is the mainstay of treatment. Some specific anatomical sites still represent a reconstructive challenge due to their complex three-dimensional anatomy. In recent years, patient specific instruments along with 3D printing technology has come to represent innovative techniques in orthopaedic oncology. We retrospectively reviewed 23 patients affected by primary bone sarcoma treated with patient-specific instruments and 3D printing custom made prostheses. At follow up after approximately two years, the infection rate was 26%, mechanical complication rate 13%, and local recurrence rate 13% (with a five-years implant survival rate of 74%). Based on our experience, patient-specific instruments and 3D custom-made prostheses represents a reliable and safe technique for improving the accuracy of resection of primary bone tumour, with a particular use in pelvic surgery ameliorating functional results.  
computer assisted surgery; 3D printing; bone sarcomas +
Footnote †: journal: Medical  
In recent years, \"image based\" computer assisted surgery (CAS) has been developed along with 3D printing technology and now provides opportunities for personalizing reconstruction with custom-made prostheses [2; 3; 4; 5] and improving accuracy in bone cutting using patient-specific instruments (PSI) [6].  
Orthopaedic surgery and oral and maxill surgery were among the first specialties to adopt this technology [7; 8; 9; 10; 11; 12; 13; 14] and the advent of electron beam melting (EBM) rapid prototyping (RP) techniques have improved the process which directly produces a physical object with a defined structure and shape based on virtual/mathematical model data, ensuring personalized and anatomical reconstructions after bone tumour resection. Rapid prototyping was first used in the late 1990s and was developed to apply the precision and functionality of computer assisted design (CAD) to manufacturing [15]. With this technology, a prototype could be quickly produced and accurately represents the engineers draft [16; 17]. Furthermore, if changes needed to be made, the engineer could easily digitally work on the draft and print the redesigned part. The EBM is the optimal technology to fabricate metallic components with complex shapes and porous structures and has a pivotal role either in the PSI technology and prosthesis manufacturing.  
PSI technology has been used during the last decade in maxillofacial surgery, total knee arthroplasty, hip resurfacing, pedicle screw insertion, pelvic, and long-bones osteotomy. Evaluation tests have shown a high accuracy when compared to standard methods with reduced surgery time [18; 19; 20; 21; 22; 23; 24; 25]. A case example is reported in Figure 1.  
The aim of this article is to verify the oncological safety of PSI and 3D custom made prostheses in a selected group of patients affected by primary bone sarcomas.  
Figure 1: Case #10, Male 36 years, low grade osteosarcoma, 3D-virtual model of (**a**) low grade osteosarcoma of the left glenoid, (**b**) PSI cutting guide, (**c**) custom made prosthesis, (**d**) pre-operative plain X-ray, (**e**) surgical specimen and custom-made prostheses, and (**f**) plain X-ray (three years follow up).