Shaping Patient Specific Surgical Guides for Arthroplasty to Obtain High Docking Robustness

Patient specific surgical guides (PSSGs) are used in joint replacement surgery to simplify the surgical process and to increase the accuracy in alignment of implant components with respect to the bone. Each PSSG is fabricated patient specifically and fits only in the planned position on the joint surface by the matching shape. During surgery, the surgeon holds the PSSG in the planned position and the incorporated guidance is used in making the essential cuts to fit the implant components. The shape of the PSSG determines its docking robustness (i.e., the range of forces that the surgeon may apply without losing the planned position). Minimal contact between the PSSG and the joint surface is desired, as this decreases the likelihood of interposition with undetected tissues. No analytical method is known from literature where the PSSG shape can be optimized to have high docking robustness and minimal bone-guide contact. Our objective is to develop and validate such an analytical method. The methods of motion restraint, moment labeling and wrench space-applied in robotic grasping and workpart fixturing-are employed in the creation of this new method. The theoretic approach is utilized in an example by optimizing the PSSG shape for one joint surface step-by-step. The PSSGs that arise from these optimization steps are validated with physical experiments. The following design tools for the analytical method are introduced. The optimal location for bone-guide contact and the application surface where the surgeon may push can be found graphically, respectively, by the use of the wrench space map and the application angle map. A quantitative analysis can be conducted using the complementary wrench space metrics and the robustness metric R. Utilization of the analytical method with an example joint surface shows that the PSSG's shape can be optimized. Experimental validation shows that the standard deviation of the error between the measured and calculated angular limits in the docking force is only 0.7 deg. The analytical method provides valid results and thus can be used for the design of PSSGs