Introduction: Electromagentic Navigation(EM) system has been introduced in total knee arthroplasty to increase the accuracy of lower limb alignment and positioning of the implant. EM navigation systems offer several potential advantages over their infrared counterparts. To our best knowledge, there have been scarce clinical results reported In order to obtain optimal results, a certain period of learning curve may be necessary. We have compared our first 100 cases of total knee arthroplasty to our last 100 cases in order to verify the clinical accuracy, efficacy and learning curve.

Methods: From July 2006 to November 2007, 138 patients underwent 200 serial primary TKA operations by a single surgeon with the assistance of Electromagnetic Navigation system. The 200 TKA cases were divided into two groups; the first 100 and the next 100 cases. We have compared the deviation in postoperative mechanical axis and angles of femoral and tibial component position(α°, β°, and γ°) in addition to the outlier percentage of post-mechanical axis between the two groups. We used the independent sample t-test to verify our results.

Results: The deviation in angle of postoperative mechanical axis was significantly lower in the last group than the first group; 2.0633 vs. 2.6944. (p=0.0145) respectively. The deviation of α° was significantly lower in the last group than the first group; 1.1597 vs. 1.6778. (p=0.005) respectively. The deviation of β ° was lower in the last group than the first group; 1.3475 vs. 1.2115, but this value was not significant. (p=0.849). The results of the value γ° proved to be more towards extension in the first group and more towards flexion in the last group, yet these values were not significant (p=0.159). The outlier percentages of postop-mechanical axis between two groups were significantly different.

Discussion and Conclusion: The navigation system most often used in studies is an optical system with an infrared camera. Many authors have reported the efficacy of optical navigation system. It has been known to increase the accuracy of lower limb alignment and positioning of the implant while decreasing the outlier percentage of postoperative mechanical axis. The large transmitter, however, for this system requires bicortical pins, which would result in stress fracture through the drill-holes in bone. In addition, another skin incision is needed for the transmitter. A new navigation technique using electromagnetic signals has been introduced with advantages including small transmitter size, although its signal is often distorted by metal devices used in the operative field. Our hypothesis was therefore that the EM system could lead to better alignment of the leg and positioning of implants than traditional method with comparable learing curve. In summary, the EM navigation system can lead to better alignment of the mechanical axis of leg and positioning of femoral implants in coronal view compared to conventional method, although it can’t prevent outliers in all case. And also our experience suggests that in order to obtain such results, however, a certain period of learning curve may be necessary. Several valuable surgical tips specific to this technology were obtained enduring our learning curve and will be presented.

Introduction: Avascular necrosis (AVN) of bone is a process that is characterized pathologically by bone marrow ischemia and eventual death of trabecular bone. Following the development of the disease with the remodeling process, the microstructure and corresponding mechanical properties of the trabecular bone changes in different regions with different intensities. Eventually, the lesion leads to collapse of the femoral head and destruction of the hip joint. The most striking finding is the direct relationship between mechanical stress and the progressive collapse of the necrotic region. This study investigated the differences in the mechanical properties from the trabecular bone of the different regions in AVN of the femoral head using microfinite element models.

Materials and Methods: A 20mm cylindrical core sample was obtained from the necrotic zone of the human femoral head with pre-collapse disease throughout the overall head under the fluoroscope and then was scanned using Micro-CT. Region of interest (ROI) was determined in the necrotic, the reactive, and the sub-reactive zone respectively, which were created with the hexahedron mesh model; finite element analysis was performed.

Results: The histomorphology and FE-analysis of three zones revealed that the parameters of Tb. Th, BV/TV, reaction force, ultimated stress, and elastic modulus increase obviously in the reactive zone.

Discussion: The authors conclude that obvious increases of the parameters and the stress concentration in the reactive zone are due to the adaptive remodeling of trabeculae in the boundary zone between the necrotic and the normal zone.