This work describes the design and capabilities of the Purdue Knee Simulator: Mark II and a sagittal-plane model of the machine. This five-axis simulator was designed and constructed to simulate dynamic loading activities on either cadaveric knee specimens or total knee prostheses mounted on fixtures. The purpose of the machine was to provide a consistent, realistic loading of the knee joint, allowing the kinematics and specific loading of the structures of the knee to be determined based on condition, articular geometry, and simulated activity. The sagittal-plane model of the knee simulator was developed both to predict the loading at the knee from arbitrary inputs and to generate the necessary inputs required to duplicate specified joint loading. Measured tibio-femoral compressive force and quadriceps tension were shown to be in good agreement with the predicted loads from the model. A controlled moment about the ankle-flexion axis was also shown to change the loading on the quadriceps.
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For patients who have lost a leg, regaining mobility is a top priority. However, development of more true-to-nature prosthetic limbs and joints has come an amazingly long way from the often thought of peg leg and hook hand. Over the past several years, medical prosthetic technology has advanced drastically, allowing for patients to regain the normalcy in their life that they would not have had access to in the past.
Fig. 1a (left) and b (right) &#; Straight and bent images of the hydraulic prosthetic knee joint are seen. Otto Bock Healthcare GmbH, Duderstadt, Germany, a company with 90 years of orthopedic experience, has developed a more natural-feeling prosthetic knee joint to not only allow more comfortable and natural ambulation, but also provide a long-lasting, durable solution so patients can spend less time at the orthopedic mechanic, and more time living life as usual.
Otto Bock has developed their 3R60 and 3R78 prosthetic knee joints with their patented &#;EBS,&#; or &#;Ergonomically Balanced Stride&#;, system, which allows for an individually adjustable stance, reducing the strain on the hips and spine of the wearer. Suitable for patients up to 275 pounds, higher than with many traditional prosthetics, the joint is able to be adapted to patient weight and activity for additional comfort and stability, especially on uneven terrain, and four modular connectors make it an appropriate prosthetic for various levels of injury. The joint system uses special engineered components and features to mimic the motion enabled by a biological knee joint, unlike monocentric, single-jointed traditional prosthetics. (See Figures 1a and b)
In the joint itself, needle and bronze bearings, as well as PTFE guide tape, were replaced with plastic components from igus®. These plastic components are extremely light, do not require external lubrication, and also possess very low coefficients of friction, with excellent wear-resistance. According to Otto Bock, these plastic components allowed for the further development of their EBS joint systems to where they are today. The EBS joint system is lightweight and shock absorbing, allowing for more comfortable walking, and is extremely low maintenance thanks to low-wear, media-resistant, and corrosion-free components, meaning fewer trips to the orthopedic mechanic. The plastic bearings used in the joint were selected for special characteristics specific to the demands in a prosthetic knee joint. There are bearings made of different varieties of plastic material, including iglide® Q, which was selected because it is extremely resilient under high loads and exceptionally shock absorbing, making it an excellent buffer for impact loads. The material is also resistant to many chemicals, including solvents, oils, and weak acids. The bearings also have a low moisture absorption rate, letting the user walk in all weather without increased friction. The shock absorbing qualities of the plastic material, coupled with swing phase hydraulics, act as a substitute for cartilage in a biological knee, cushioning the force of two to four times the patient&#;s body weight that occurs with each step.
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In prosthetic knee joints that utilize metal bearing systems, dirt, dust, pet hair, and other common debris is attracted to the external lubrication, therefore impeding the friction surface which can lead the knee to seize up and the patient to stumble or fall. The specialized plastic bearings in the EBS system are self-lubricating, using microscopic particles of solid lubricant included throughout the plastic material. Without external bearing lubricant, dirt and debris from everyday wear is not attracted to the joint, and any dirt is simply embedded into the plastic material with no noticeable increase in friction.
Patients using the EBS prosthetic joint are able to walk more naturally and comfortably than those with traditional prosthetics. The EBS joint gives a higher ground clearance in the swing phase of a patient walking due to the polycentric nature of the system. Polycentric knees, with more than one axis of motion, are more stable than single axis systems, which are essentially a hinge. The polycentric system gives a much more &#;knee-like&#; feel, and with added hydraulic power in polycentric joints like the 3R60, patients are able to have more variability in the walking speed and function, but at a price.
Typically, polycentric knee joints are heavier and more expensive than single-axis joints, even more so with added weight and cost of added hydraulics. Otto Bock&#;s joint systems, however, have compensated for the added price and weight by substituting plastic components that are very light weight and low cost, too. At only 1.8 pounds, the joint offers a joint flexion angle of 175 degrees, limited only by shaft connections/forms or optional cosmetic cover. The mini hydraulic system utilizes piston rings with separately adjustable flexion and extension to control the behavior of the joint during the swing phase of the walking motion. A rubber bumper is compressed during stance phase flexion, generating the corresponding movement resistance, which is adjustable per patient requirements. The hydraulic components absorb subsequent stance phase extension. The five-axis polycentric design structure of the EBS joint allows for separation between the swing and stance phases. In the stance phase, flexion is made possible by interplay between the lower front axis as the joint&#;s center of rotation with the EBS&#;s pivoting mount and the posterior link of the polycentric structure. The EBS pivoting mount also visualizes the degree to which the EBS function is used.
Piston rings are applied in the mini hydraulic unit in the top and bottom pistons located in the pneumatic cylinders. The piston rings replaced PTFE guide tape, used to create bands. When producing the knee joints, each piece of PTFE tape, supplied as a large coil, had to be cut to the appropriate size for the particular joint, then packed and stored in strips, inevitably with some error in sizing. If the strip was too long, it had to be cut again. Cropping on the cutting edge could leave the band too short, keeping it from properly fitting into the driving band groove. Instead of the lengthy and error-prone process of sizing, cutting, and fitting the PTFE tape, the piston rings simply snap onto the piston, greatly reducing the production costs and lowering the total cost of the joint. When tested within Otto Bock EBS joints, barely any perceptible wear was reported on the piston rings, even under maximum stresses, and according to the company, increased the quality, functionality, and durability of their overall system.
This article was written by Ellen Rathburn, Technical Copywriter for igus, Inc. East Providence, RI. For more information on Otto Bock, visit http://info.hotims.com/-189 . For more information on igus, visit http://info.hotims.com/-163 .
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