Total disc arthroplasties change the kinematics of functional spinal units during lateral bending.

BACKGROUND: Information about kinematics in different functional spinal units before and after total disc arthroplasties is necessary to improve prostheses and determine indications. There is little information about the nonstationary instantaneous helical axis of rotation under lateral bending in the cervical spine before and after total disc arthroplasty. METHODS: Kinematic analyses were performed with an established measuring apparatus on 8 human functional spinal units (C3/C4, C5/C6) under intact conditions and after total disc arthroplasty with two different types of prostheses: Bryan and Prestige. The instantaneous helical axis, migration, and stiffness of the segments were calculated. FINDINGS: The instantaneous helical axis direction was always inclined ventrally. Ventral inclination was significantly higher in segment C3/C4 than in segment C5/C6 under all conditions (p < 0.001). Both types of arthroplasties significantly increased ventral inclination compared to intact conditions. In both segments, the path length of the instantaneous helical axis' migration was significantly longer after total disc arthroplasty with Bryan (p = 0.001) and shorter after Prestige (p < 0.001) prostheses than under intact conditions. After both types of arthroplasties, the migration path length was significantly longer and the stiffness was significantly lower in segment C3/C4 than in segment C5/C6. INTERPRETATION: Both types of arthroplasties changed the kinematics of both segments during lateral bending. Altered instantaneous helical axis migration, greater ventral inclination and less stiffness after both arthroplasties indicate unphysiological motion. Both arthroplasties had greater impact on segment C3/C4 than on segment C5/C6 in terms of hypermobility. Increased translational motion after total disc arthroplasty with a Bryan prosthesis might be caused by the prosthetic design.

Kinematics of cervical segments C5/C6 in axial rotation before and after total disc arthroplasty.

PURPOSE: The kinematical properties of C5/C6 segments in axial rotation are evaluated before and after total disc arthroplasty (TDA) with PRESTIGE®-and BRYAN® Cervical Disc (Medtronic) under flexion/extension as parameters and compared with those of C3/C4. METHODS: Eight human segments were stimulated by triangularly varying, axially directed torque (T z(t)) under compressing static axial preloads. Using a 6D-measuring device with high resolution, the response of segmental motion was characterized by the instantaneous helical axis (IHA). The position, direction, and migration path length of the IHA were measured before and after TDA (parameter: position of the axially directed preload). RESULTS: The periodic torque T z(t) generated IHA migrations whereupon the IHA direction was constantly rotated to the dorsal by ≈15.5°. After TDA, the IHA0 (neutral positions) were significantly shifted to the dorsal (PRESTIGE®: 4.3 mm, BRYAN®: 7.0 mm) just as the points of balance of the entire IHA migration paths. CONCLUSIONS: Due to the configuration of the vertebral joints and their interaction with the intervertebral disc, the IHA migrates during the axial rotation within a distinct domain of each C5/C6-segment. Implantation of the PRESTIGE® and BRYAN® prostheses significantly alters these kinematical properties by dorsal displacements of the domains. Statistically TDA of C3/C4 and of C5/C6 are not correlated. Under axial rotation of the cervical spine, additional lateral and/or ventral/dorsal displacements are produced by TDA. Consequently, adjacent level disease (ALD) may be mechanically stimulated.

The description of the human knee as four-bar linkage.

PURPOSE: We investigate the dependence of the kinematics of the human knee on its anatomy. The idea of describing the kinematics of the knee in the sagittal plane using four-bar linkage is almost as old as kinematics as an independent discipline. We start with a comparison of known four-bar linkage constructions. We then focus on the model by H. Nägerl which is applicable under form closure. METHODS: We use geometry and analysis as the mathematical methods. The relevant geometrical parameters of the knee will be determined on the basis of the dimensions of the four-bar linkage. This leads to a system of nonlinear equations. RESULTS: The four-bar linkage will be calculated from the limits of the constructively accessible parameters by means of a quadratic approximation. CONCLUSIONS: By adapting these requirements to the dimensions of the human knee, it will be possible to obtain valuable indications for the design of an endoprosthesis which imitates the kinematics of the natural knee.

Characteristic points and cycles in planar kinematics with application to the human gait.

PURPOSE: We present a novel method to process kinematical data typically coming from measurements of joints. This method will be illustrated through two examples. METHODS: We adopt theoretical kinematics together with the principle of least action. We use motion and inverse motion for describing the whole experimental situation theoretically. RESULTS: By using the principle of least action, the data contain information about inherent reference points, which we call characteristic points. These points are unique for direct and inverse motion. They may be viewed as centers of the fixed and moving reference systems. The respective actions of these characteristic points are analytically calculated. The sum of these actions defines the kinematical action. This sum is by design independent of the choice of reference system. The minimality of the kinematical action can be used again to select numerically one representative cycle in empirically given, approximately periodic motions. Finally, we illustrate the theoretical approach making use of two examples worked out, hinge movement and the sagittal component of the movement of a human leg during gait. CONCLUSIONS: This approach enables automatic cycle choices for evaluating large databases in order to compare and to distinguish empirically given movements. The procedure can be extended to three dimensional movements.

Does total disc arthroplasty in C3/C4-segments change the kinematic features of axial rotation?

We analyze how kinematic properties of C3/C4-segments are modified after total disc arthroplasty (TDA) with PRESTIGE(®) and BRYAN(®) Cervical Discs. The measurements were focused on small ranges of axial rotation (<0.8°) in order to investigate physiologic rotations, which frequently occur in vivo. Eight human segments were stimulated by triangularly varying, axially directed torque. By using a 6D-measuring device with high resolution the response of segmental motion was characterised by the instantaneous helical axis (IHA). Position, direction, and migration rate of the IHA were measured before and after TDA. External parameters: constant axially directed pre-load, constant flexional/extensional and lateral-flexional pre-torque. The applied axial torque and IHA-direction did not run parallel. The IHA-direction was found to be rotated backwards and largely independent of the rotational angle, amount of axial pre-load, size of pre-torque, and TDA. In the intact segments pre-flexion/extension hardly influenced IHA-positions. After TDA, IHA-position was shifted backwards significantly (BRYAN-TDA: ≈8mm; PRESTIGE-TDA: ≈6mm) and in some segments laterally as well. Furthermore it was significantly shifted ventrally by pre-flexion and dorsally by pre-extension. The rate of lateral IHA-migration increased significantly after BRYAN-TDA during rightward or leftward rotations. In conclusion after the TDA the IHA-positions shifted backwards with significant increase in variability of the IHA-positions after the BRYAN-TDA more than in PRESTIGE-TDA. The TDA-procedure altered the segment kinematics considerably. TDA causes additional translations of the vertebrae, which superimpose the kinematics of the adjacent levels. The occurrence of adjacent level disease (ALD) is not excluded after the TDA for kinematical reasons.

Construction-conditioned rollback in total knee replacement: fluoroscopic results.

Firstly, the way of implementing approximatively the initial rollback of the natural tibiofemoral joint (TFJ) in a total knee replacement (AEQUOS G1 TKR) is discussed. By configuration of the curvatures of the medial and lateral articulating surfaces a cam gear mechanism with positive drive can be installed, which works under force closure of the femoral and tibial surfaces. Briefly the geometric design features in flexion/extension are described and construction-conditioned kinematical and functional properties that arise are discussed. Due to a positive drive of the cam gear under the force closure during the stance phase of gait the articulating surfaces predominantly roll. As a result of rolling, a sliding friction is avoided, thus the resistance to motion is reduced during the stance phase. Secondly, in vivo fluoroscopic measurements of the patella tendon angle during flexion/extension are presented. The patella tendon angle/ knee flexion angle characteristic and the kinematic profile in trend were similar to those observed in the native knee during gait (0°-60°).

Physiologically shaped knee arthroplasty induces natural roll-back.

After total knee replacement the persistence of pain represents a significant problem. In this study, a novel knee arthroplasty (Aequos G1 knee arthroplasty) is investigated that was designed to replicate main features of human knee morphology to reduce the periodically occurring pain after knee replacement. Previous work showed theoretically that this arthroplasty design may reconstruct the four-bar linkage mechanism as it occurs in human knee by contriving a convex lateral tibial compartment and a sagittal offset of the centre of the medial and lateral femur condyles - inducing a roll-back mechanism as it exists in human. The aim of this study was to determine whether this potential roll-back mechanism can be confirmed by in-vivo measurements. This retrospective study showed that the patellar tendon angle decreases during flexion of 0.21° per degree of flexion on average in the 16 knees studied. This amount is similar to physiological knee kinematics and in contrast to existing results in the literature after implantation of conventional total knee replacements which lack physiological knee kinematics. The results suggest that physiological motion after implantation of the Aequos G1 knee arthroplasty occurs during loaded motion up to approximately 45° knee flexion.

Mathematical study on the guidance of the tibiofemoral joint as theoretical background for total knee replacements.

The mathematical approach presented allows main features of kinematics and force transfer in the loaded natural tibiofemoral joint (TFJ) or in loaded knee endoprostheses with asymmetric condyles to be deduced from the spatial curvature morphology of the articulating surfaces. The mathematical considerations provide the theoretical background for the development of total knee replacements (TKR) which closely reproduce biomechanical features of the natural TFJ. The model demonstrates that in flexion/extension such kinematic features as centrodes or slip ratios can be implemented in distinct curvature designs of the contact trajectories in such a way that they conform to the kinematics of the natural TFJ in close approximation. Especially the natural roll back in the stance phase during gait can be reproduced. Any external compressive force system, applied to the TFJ or the TKR, produces two joint reaction forces which--when applying screw theory--represent a force wrench. It consists of a force featuring a distinct spatial location of its line and a torque parallel to it. The dependence of the geometrical configuration of the force wrench on flexion angle, lateral/medial distribution of the joint forces, and design of the slopes of the tuberculum intercondylare is calculated. The mathematical considerations give strong hints about TKR design and show how main biomechanical features of the natural TFJ can be reproduced.