Abstract
CASN is generally good at bone morphing and sizing, assisting with component orientation, gap balancing and providing reasonably accurate alignments of limb and components alike. However, such routine navigation technique fails to use the full potential of the registered information. Current technique provides reasonable static stability information in the coronal plane, but with axial and sagittal planes less well considered. A more dynamic approach seems to be necessary to define ‘potential envelopes of motion’, seeming to be the best possible way in which CASN will finally show fundamental improvements over ‘conventional’ technique.
Enhanced dynamic assessment using an upgraded CASN system (Brainlab) is now capable of improved ROM analysis and contact point observations. This consists of storing dynamic information including a) epicondylar axis motion, b) valgus and varus alignments, c) antero-posterior shifts, as well as d) flexion and extension gaps. Tracking values for both tibiofemoral and patellofemoral motion can also be obtained after performing registration of the prosthetic trochlea.
Observations can be made using a set of standardised dynamic tests. Firstly, the lower leg can be placed in neutral alignment and the knee put through a flexion-extension cycle. Secondly the test can be repeated but with the lower leg being placed into varus and internal rotation. The third test can be performed with the lower leg in valgus and external rotation. Also a new passive technique of ‘Drop and Push Testing’ into a) flexion and b) extension is giving new information which may prove useful a) in terms of over-stuffing of the extensor mechanism and tightness of flexion gap and b) provision of hyperextension to assist gait. Upgraded software prompts can improve workflows to facilitate optimisation of joint dynamics.
Twenty total knee arthroplasties have been studied using these techniques with particular reference to the patterns of instability found. Marked intra-operative variation in the stability characteristics of the trial implanted joints has been quantified before corrections have been made and final assessments performed. These corrections have also been analysed in terms of change in antero-posterior translations, rotations and contact points. Edge loading and excessive paradoxical motions have been identified and corrective measures carried out, thereby improving PCL tensioning. Component rotations, tibial slope angles, insert thicknesses and femoral sizing have had to be adjusted to optimise range of motion and stability characteristics. Certain cases have been identified where use of more congruent or even stabilised components was considered necessary. Patellar tracking has also been observed during such dynamic tests and appropriate adjustments made to components and soft tissue balancing.
In summary, this study has enabled intra-operative observation, classification and quantification of patterns of instability in 6 degrees of freedom using simple stress tests followed by appropriate adjustments.
