Introduction: Total disc arthroplasty’s (TDA) fall into two groups – constrained ball and socket and sliding core devices. It is commonly theorized that sliding core devices offer the advantage of being able to adapt to varying centres of rotation (COR) of the functional spinal unit (FSU), however no rigorous justification has, so far, been tendered for this. Despite the perceived advantage, differing clinical results have been reported in the lumbar spine, possibly with better results with ball and socket devices. Furthermore abnormal motion with a large hysteresis effect has been identified in in vitro flexibility testing with a physiological preload in the lumbar spine. The purpose of this paper was to develop an understanding of the kinematics of sliding core TDA’s, their ability to match variable COR’s of a normal FSU, and to gain an understanding of theoretical load displacement behaviour when implanted. Methods: The motion of a biconvex sliding core prosthesis was observed to define the motion as a linked kinematic chain. By the use of sequential multiplication by appropriate transformation matrices that described this kinematic chain, equations for the position and orientation of the upper vertebrae were established. By a similar method equations for the position and orientation of the upper vertebrae were developed for a physiological simple rotation around the FSU COR. Attempts were made to solve these two sets of equations simultaneously to see if motion of the biconvex core prosthesis could match either the position, orientation or both position and orientation of the normal physiological motion. Functions defining the length of the load vector through the COR were obtained. By considering a physiological load in the direction of this vector, a function describing potential energy was defined. This was further modified by the addition of ligament constraints with a “J” shaped non linear load displacement behaviour that approximated normal ligament stiffness. Sensitivity analysis was then performed to establish the behaviour of the prosthesis under differing loads, ligament strains and malplacements and the outcomes were compared to published in vitro results. Results: The motion of the device could be modeled as a ‘two bar linkage’. Attempts to find simultaneous solutions for the equations for the two bar linkage and physiological movement showed that a solution was possible when matching either position or orientation but not both. The biconvex core prosthesis best approximated the normal motion by a change in the length of the vector joining the FSU COR to the vertebrae above. When the potential energy caused by this change in length was plotted as a two dimensional surface, a ‘saddle shape’, indicating an unstable high energy equilibrium position at neutral was found. The addition of functions to simulate ligament structures showed a ‘metastable’ energy surface with two stable minimum equilibrium positions with an intervening unstable high energy equilibrium position. Sensitivity analysis showed that the prosthesis could adapt quite well to changes in vertical position of the FSU COR though had limited ability to adapt to anteroposterior malplacement. Discussion: The theoretical potential energy function for a biconvex core prosthesis predicts significant hysteresis with a high energy unstable central position. The equations predict abnormal load behaviour that is similar to observed in vitro testing. This may explain the difference in clinical results

Robotic surgical systems reduce the cognitive workload of the surgeon by assisting in guidance and operational tasks. As a result, higher precision and a decreased surgery time are achieved, while human errors are minimised. However, most of robotic systems are expensive, bulky and limited to specific applications. In this paper a novel semi-automatic robotic system is evaluated, that offers the high accuracies of robotic surgery while remaining small, universally applicable and easy to use. The system is composed of a universally applicable handheld device, called Smart Screwdriver (SSD) and an application specific kinematic chain serving as a tool guide. The guide mechanism is equipped with motion screws. By inserting the SSD into a screw head, the screw is identified automatically and the required number of revolutions is executed to achieve the desired pose of the tool guide. The usability of the system was evaluated according to IEC 60601-1-6 using pedicle screw implementation as an example. The achieved positioning accuracies of the drill sleeve were comparable to those of fully automatic robotic systems with −0.54 ± 0.93 mm (max: − 2.08 mm) in medial/lateral-direction and 0.17 ± 0.51 mm (max: 1.39 mm) in cranial/caudal- direction in the pedicle isthmus. Additionally, the system is cost-effective, safe, easy to integrate in the surgical workflow and universally applicable to applications in which a static position in one or more DOF is to be adjusted

Introduction. Post-operative gait abnormalities are recognized following total hip arthroplasty (THA). Despite global improvement in functional outcome, gait abnormality persists for a decade or more. In this study 3-dimensional gait analysis (3DGA) was performed using a portable system with Inertial Measurement Units (IMUs) to quantify this abnormality. Methods. The gait of 55 patients with monarthrodial hip arthrosis was measured pre-operatively and at one year post-surgery. Patients with medical co-morbidity or other conditions affecting their gait were excluded. Six IMUs were aligned at the level of the anterior superior iliac spines, mid-thigh and mid-leg. Data was analysed using proprietary software. Each patient underwent a conventional THA using a posterolateral approach. 92 healthy individuals were assessed for comparison. Results. Pre-operative movement in the sagittal plane of the ipsilateral hip (mean range 20.4) and the contra-lateral non-diseased hip (35.3 degrees) was reduced compared to the control group (40.5 degrees), (P<0.001). The pre-operative movement of both knees was reduced compared with normal (P<0.001). Pelvic movement on the ipsilateral side was increased. After one year ipsilateral hip movement significantly improved (Mean range 28.9 deg SD 6.6) but did not reach normal values (P<0.001). Movement measured in the contralateral hip was further reduced with a mean difference of −5.25 degrees (95% CI −8.06 to −2.43). Knee movement on both sides increased but not to normal values (p<0.001). There was increased coronal movement bilaterally at the thigh and calf one year after surgery. Discussion and Conclusion. Gait after routine THA does not return to normal. Unilateral hip pathology causes bilateral gait abnormality affecting the entire kinematic chain. This portable technology allows practical assessment of gait in the outpatient setting and will enable identification of key aspects of gait abnormality to target during rehabilitation following THA