Stance Controlled Orthotic Knee Joints: A Report on a Collaborative Research Project
James H Campbell Ph.D., CO Introduction: The ability to provide individuals, who have reduced power of their knee extensor musculature, with a weight actuated orthotic knee joint has been acknowledged by orthotic practitioners as a desirable clinical goal. Despite significant research efforts in many countries, in terms of a practical clinical option, the goal remains unattainable. This presentation examines the current status of stance controlled orthotic knee joints by providing an overview of current, and ongoing, research activity that is being directed towards addressing this common clinical problem. During the last fifty years the only significant change for patients with quadriceps weakness who require a KAFO has been the availability and utilization of thermoplastic materials to replace metal and leather. The reason for this apparent lack of progress is based upon the assumption that the requirements of stability and progression are contradictory. The presentation will outline the design and technological barriers that currently exist and will also question if a new and improved design will lead to a definitive product that KAFO users desire. The ability of any new design to improve mobility and reduce energy expenditure is viewed as key. The importance of knee flexion in stance: The importance of knee flexion in stance was outlined by Saunders et al (1), who indicated that in the able bodied various mechanism exist that provide a reduction of the vertical and horizontal movement of the center of mass. Knee flexion in stance flattens the arc described by the center of mass during forward translation of the body over the stance foot. This mechanism is assumed to connect to a reduction of metabolic energy requirements. The energy required to walk using a KA.FO that locks the knee during stance and allows free knee motion during the swing phase of gait has been evaluated (2). The improved KAFO described in the published report reduced the metabolic requirements during gait. The authors claimed that a KAFO design that allows free motion during swing is effective in lowering the energy required for walking indicating that the principles presented in their study should be applied to all future KAFO designs and would be applicable to any individual who required a KAFO for ambulation. The KAFO used in the study uses modular electronics that sense limb loading to activate a clutch mechanism at the knee, locking and unlocking of the orthotic knee joint is done automatically and is synchronized to the gait of the individual. The goal, or challenge, is to provide users of Knee Ankle Foot Orthoses with stability whst simultaneously allowing movements that are related to forward progression (3). Clinical examples will demonstrate that the provision of stability has been the predominant factor whilst efficiency of progression is typically compromised. Addressing a basic and common question, the problem of affording knee stability in orthotic applications is not considered to be analogous to that of providing knee stability for the Trans Femoral amputee. It will be contended that the biomechanical problems encountered are fundamentally different. Anderson et al (4) in 1960 commented that many types of prosthetic knees have been designed and produced that have a mechanical lock, a friction brake, or a hydraulic control to prevent the knee from buckling, indicating that a number of mechanical knees and one hydraulic unit were commercially available. During the latter part of the 20th century, despite numerous attempts, engineers and clinicians have been unable to successfully transfer and apply this prosthetic technology in orthotics. Function of the knee joint structures: It is important to consider the function of the knee joint structures, as they relate to stability and knee motion during the gait cycle. Functional stability of the knee joint is derived from the restraint of the ligaments, the geometry of the joint and muscle activity. The activity and action of the quadriceps muscles, during loading response, is of particular interest. The quadriceps restrain knee flexion in stance and assist extension. All the vasti respond simultaneously and the gluteus maximus through its iliotibial band insertion also contributes to knee extensor stability. During the first 5% of the gait cycle the quadriceps generate peak intensity, after the first 20% of the gait cycle the quadriceps are silent. Loading response is known to have three critical events:
A Loading Response Orthotic Knee Joint has been designed to allow 18' of knee flexion during stance. The joint is shown and contains a spiral torsion spring that is preloaded to achieve an input torque of 233in-lb, the spring is manufactured from an annealed and heat treated carbon steel and has a rectangular cross section. Gait Analysis: Currently we are conducting laboratory analyses in an attempt to quantify the benefit of allowing 18o of knee flexion during the loading response part of the gait cycle. We are conducting comparative studies using the Kinemetrix Motion Analysis System, our current focus is measuring the effect on the temporal aspects of gait. The initial results from the pilot testing that has been conducted on the Loading Response Orthotic Knee Joint are promising however this joint is not designed to afford any benefit during the transition from stance phase to swing phase when the knee rapidly flexes to 40o during preswing and continues to 60o in initial swing. NASA Attempts to improve KAFO design: A further attempt (5) has been made to improve KAFO design for individuals with quadriceps weakness A selectively locking knee brace has been described and designed by engineers at the National Aeronautics and Space Administration. This orthotic knee joint relies upon mechanical actuation and uses a heel plate, spring and cable assembly, the knee locking mechanism is activated when weight is applied to the heel. Contribution made by Jonathan Naft CP: In 1998 at the American Orthotic and Prosthetic Association National Assembly, held in Chicago, Illinois, Jonathan Naft CP presented a Weight Actuated Orthotic Knee Joint. An attempt was being made to improve existing KAFO design through the realization of a computer controlled, foot force activated electromechanical knee joint. The initial/basic concept; upon sensation of foot pressure a ratcheting knee brake would engage, permitting full extension but preventing flexion. The combination of electronic foot force sensing, micro controller signal processing/control and the novel lightweight compact electromechanical knee brake actuator showed promise of overcoming the obstacles encountered in previous efforts. A preliminary prototype that was developed produced excellent results, with the group in Cleveland claiming patients walked with a more efficient and normal gait. Collaborative Project Established with Becker Orthopedic: It was evident in 1998 that progress had been made towards proof of concept, it was possible to achieve consistent operation of an orthotic joint by a microprocessor and the footplate, containing several force sensing resistors, was controlling the function of the joint. There were however general concerns over the practical issues of durability and safety, specifically there was evidence of uneven wear of the gear teeth and it was thought that the footplate design was particularly complicated. There was also the recognition that in the event of any power failure the joint would unlock. Our collaborative research efforts have been ongoing. The issues of efficiency have been investigated through analyses and experimentation of magnetic designs. Mechanical testing includes finite element analysis of mechanical stresses, physical testing of the critical components, including accelerated wear testing of the gear teeth. Outcome: The outcome is a promising new design/iteration where the joint is now configured with a passive lock. The joint has undergone fundamental changes in design and function including a redesign of the gear teeth in terms of dimension, tolerance and material. The initial logic has been reversed, if there is any power failure the joint will remain locked. As pressure is removed a ratcheting knee brake disengages permitting free flexion and extension during the swing phase of gait. This new design employs electromechanical actuation, when the joint is activated a rotating tooth plate repels and disengages the gear teeth, allowing free motion. During the gait cycle the joint is powered down during stance phase and activated during the swing phase. Acknowledgements:
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