The influence of UKA Installation Error of Joint Line on Contact Mechanics and Kinematics of Knee Joint / 医用生物力学

Objective Aiming at the medial prosthetic loosening failure and lateral cartilage degeneration after unicompartmental knee arthroplasty ( UKA), the effects of prosthetic installation errors of joint line in UKA on knee contact mechanics and kinematics during different physiologic activities were studied using musculoskeletal multi-body dynamic method. Methods Taking the medial natural joint line as 0 mm error, six installation errors ofjoint line including ±2 mm, ±4 mm and ±6 mm were considered respectively, and seven musculoskeletal multi body dynamic models of medial UKA were established, to comparatively study the variations in knee contact mechanics and kinematics during walking and squatting. Results At 70% of walking gait cycle, compared with 0 mm error, the medial prosthetic contact force was increased by 127. 3% and the contact force of the lateral cartilage was decreased by 12. 0% under 2 mm elevation in joint line, the medial prosthetic contact force was close to 0 N, but the lateral cartilage contact forces were increased by 10. 1% under 4 mm reduction in joint line. The tibiofemoral total contact forces were increased by 19. 7% and decreased by 14. 2% under 2 mm elevation and 2 mm reduction in joint line, respectively. At the 100°knee flexion during squatting, compared with 0 mm error, the medial prosthetic contact force and the tibiofemoral total contact force increased by 31. 6% and 11. 1% under 2 mm elevation in joint line, and decreased by 24. 5% and 8. 5% under 2 mm reduction in joint line, respectively. The change in the lateral cartilage contact force was not marked. Moreover, at 70% of walking gait cycle, the varus angle decreased, the internal rotation and the anterior translation increased along with the elevation of joint line in UKA, while it was just the opposite along with the reduction of joint line in UKA. The trends of the varus valgus movement and anterior-posterior translation during squatting were consistent with those during swing phase of walking, but the trend of the internal-external rotation was opposite. Conclusions In order to reduce the risk of medial prosthetic loosening failure and lateral cartilage degeneration, it is recommended that the installation error of joint line in UKA should be controlled in the range of -2 mm to +2 mm. This study provides theoretical basis for UKA clinical failure caused by changes in joint line

Effect of prosthetic joint line installation height errors on insert wear in unicompartmental knee arthroplasty / 生物医学工程学杂志

The clinical performance and failure issues are significantly influenced by prosthetic malposition in unicompartmental knee arthroplasty (UKA). Uncertainty exists about the impact of the prosthetic joint line height in UKA on tibial insert wear. In this study, we combined the UKA musculoskeletal multibody dynamics model, finite element model and wear model to investigate the effects of seven joint line height cases of fixed UKA implant on postoperative insert contact mechanics, cumulative sliding distance, linear wear depth and volumetric wear. As the elevation of the joint line height in UKA, the medial contact force and the joint anterior-posterior translation during swing phase were increased, and further the maximum von Mises stress, contact stress, linear wear depth, cumulative sliding distance, and the volumetric wear also were increased. Furthermore, the wear area of the insert gradually shifted from the middle region to the rear. Compared to 0 mm joint line height, the maximum linear wear depth and volumetric wear were decreased by 7.9% and 6.8% at -2 mm joint line height, and by 23.7% and 20.6% at -6 mm joint line height, the maximum linear wear depth and volumetric wear increased by 10.7% and 5.9% at +2 mm joint line height, and by 24.1% and 35.7% at +6 mm joint line height, respectively. UKA prosthetic joint line installation errors can significantly affect the wear life of the polyethylene inserted articular surfaces. Therefore, it is conservatively recommended that clinicians limit intraoperative UKA joint line height errors to -2-+2 mm.

Mechanical analysis on laparoscopic instruments when contacting with biological tissues / 医用生物力学

Objective To establish the contact deformation model of biological tissues contacting with endoscopic instruments, and to make mechanical analysis on contact stress and strain. Methods Based on Kelvin-Voigt model and Hertz contact theory, the contact deformation model of instruments (with both wedge-shaped teeth and cylinder-shaped teeth) contacting with biological tissues was established, and the variation of contact stress and strain changing with time in different endoscopic instruments were obtained through finite element analysis method and bio-impedance measurement. Results Endoscopic instruments with different structures of the teeth could cause different strain and stress on tissues during laparoscopic grasping. The stress of the instrument with wedge-shaped teeth on tissues was largest, while that with cylinder-shaped teeth was smallest, and that of instrument with hybrid structure of wedge-shaped and cylinder-shaped teeth was in between. Conclusions The hybrid structure of wedge-shaped and cylinder-shaped teeth can effectively reduce the peak pressure during laparoscopic grasping, thus prevent less tissue damage caused by wedge-shaped teeth, and enhance the grasping ability with cylinder-shaped teeth. This study provides an important reference for the safety use and better design of laparoscopic instruments in clinic.

Contact mechanics performance of hip joint replacements with different material combinations / 医用生物力学

Objective To study the contact performance of hip joint replacements with different material combinations typically used in clinic. Methods The finite element model of spherical conformal contact of hip joint replacements was developed to analyze the different contact mechanics performance of hip joint replacements with different material combinations by simulation test under a vertical constant load within one normal walking gait. The corresponding wear prediction was also evaluated. Results The contact mechanics performances of hip joint replacements with the material combination of metal-on-metal (MOM), metal-on-ceramic (MOC), ceramic-on-ceramic (COC), metal-on-polyethylene (MOP) and ceramic-on-polythene (COP) were analyzed. The results obtained from the simulations showed that contact stress was decreased in the order of COC＞MOC＞MOM＞COP＞MOP, while contact area was decreased in the order of MOP＞COP＞MOM＞MOC＞COC. Conclusions Hip joint replacement with the material combination of high elastic modulus and low Poisson’s ratio has smaller deformation, but could induce larger local stress, while flexible material polythene of low elastic modulus and high Poisson’s ratio might have smaller concentrated contact stress, but cause larger deformation and edge contact phenomenon on the acetabulum cup of hip joint replacements. In addition, hip joint replacements with MOC and MOM material combination may have lower contact bearing performance, which provides references for material choices of typical artificial hip joints in clinic.

Contact mechanics of non-spherical hip joint replacements / 医用生物力学

Objective Due to the limitation of manufacturing techniques, possible design optimization selecting and influence of its wearing in clinical application, the bearing surface of hip joint replacements is presented as non-spherical geometry, and the finite element method can be used to study the contact mechanics behavior in such kind of non-spherical hip joint replacement. MethodsThe reconstructing of non-spherical articular surface based on spherical-grid-data model (SGDM) was developed to investigate the effect of contact mechanics of an ellipsoidal head against a spherical cup in a typical metal-on-metal hip joint replacement. Results The maximum contact pressure of the non-spherical bearing was decreased effectively, and meanwhile the contact area was increased when curvature radius of the ellipsoidal head around the centre of the contact zone was increased, while the effects of the cup inclination angle on the maximum contact pressure and contact area of the non spherical bearing under the same load showed relatively small, but the contact pressure distributions were different. Conclusions A well-controlled non-sphericity can improve the magnitude and distribution of contact pressures on metal-on-metal hip joint replacements. In addition, the developed model and evaluation method in this paper can be used for simulation of dynamic contact and wear prediction of non-spherical hip joint replacements.