Questions You Should Know about Hydraulic Prosthetic Knee Joint

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.

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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)

How It Works

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.

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.

Material Advantages

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  .

If you received your test results for the Combined Written exam only to learn that you did not pass the exam, the following information may help you focus your study for retaking the exam. Your test results notice indicates your score in each Content Domain along with the maximum score in each area. We recommend that you focus your exam study on those Content Domains where you performed the weakest. Below, along with a description of the Content Domain, are sample questions to help you determine the types of questions that you may have missed.

Patient Evaluation

Take a comprehensive patient history, including previous use of a prosthesis, diagnosis, work history, avocational activities, signs and symptoms and medical history. Perform a diagnosis-specific functional clinical examination of sensory function, range of motion, joint stability and skin integrity. Utilize knowledge of anatomy, muscle functions, normal gait parameters, pathologies, related surgical techniques and disease processes to guide assessment. Refer patient to other health care providers for intervention beyond orthotic/prosthetic scope of practice.

A trendelenburg gait occurs with weakness of the:

1. Gluteus medius

2. Gluteus maximus

3. Adductor magnus

4. Rectus femoris

An amputation that removes the midfoot and forefoot saving the talus and calcaneus is called a:

1. Modified Symes

2. Lisfranc

3. Chopart

4. Transmetatarsal

Which of the following muscles are transected in a transmetatarsal amputation?

1. Peroneus longus

2. Extensor hallicus longus

3. Tibialis posterior

4. Tibialis anterior

Which of the following is the primary flexor of the elbow?

1. Biceps brachii

2. Brachioradialis

3. Anconeus

4. Brachialis

In normal gait the hip reaches maximum flexion during:

1. Loading response

2. Initial contact

3. Midstance

4. Mid swing

The most common congenital absence of a long bone in the extremities is:

1. Fibular hemimelia

2. Proximal Focal Femoral Deficiency

3. Congenital hypoplasia of the Tibia

4. Tibial phocomelia

Formulation of the Treatment Plan

Evaluate the findings to determine a prosthetic treatment plan. Consult with physician/referral source/appropriately licensed health care provider to modify, if necessary, the original prescription and/or treatment plan. Identify design, materials and components to support treatment plan, including how the prosthesis will address the specific functional needs.

One advantage of a knee disarticulation amputation is:

1. The ability to maintain rotational control of the socket

2. The ability to bear weight on the distal end

3. The ability to match the knee center on the sound side

4. The ability to provide good cosmesis with the prosthesis

The most appropriate control system for a patient with a self-suspending transradial prosthesis would be a:

1. Figure 8 harness with cross-back strap

2. Figure 8 harness with flexible elbow hinges

3. Figure 9 harness with single control cable

4. Figure 9 harness with chest strap

Which amputation level is MOST likely to develop an equinus contracture?

1. Chopart

2. Lisfranc

3. Symes

4. Transmetatarsal

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Are you interested in learning more about Orthotic Drop Ring Lock? Contact us today to secure an expert consultation!

Which of the following is considered a pressure tolerant area on a transtibial residual limb?

1. Tibial crest

2. Medial tibial flare

3. Fibular head

4. Tibial condyle

Implementation of the Treatment Plan

Select appropriate materials/techniques in order to obtain a patient model/image. Select appropriate materials and components for prosthesis based on patient criteria to ensure optimum strength, durability and function. Complete or delegate fabrication of prosthesis including positive mould rectification. Assess/align prosthesis for accuracy in sagittal, transverse and coronal planes in order to provide maximum function/comfort. Educate patient and/or caregiver about the use and maintenance of the prosthesis. Documentation using established record-keeping techniques to verify implementation of treatment plan.

During the initial dynamic alignment of a transfemoral prosthesis with an ischial containment socket you note a lateral shift of the socket during mid-stance. What is the most likely cause?

1. Lack of ischial containment

2. Foot is too far outset

3. A/P dimension is too large

4. Prosthesis is too short

The length of the forearm section of a transradial prosthesis is determined by measuring the sound side from the:

1. Acromion to thumb tip

2. Acromion to lateral epicondyle

3. Lateral epicondyle to ulnar styloid

4. Olecranon to thumb tip

What anatomical landmark is used when establishing the height of a transtibial prosthesis?

1. Fibular head

2. Medial tibial plateau

3. Tibial condyle

4. Adductor tubercle

Flexible elbow hinges on a long transradial residual limb prosthesis allow for rotation and:

1. Provide M/L stability

2. Limit elbow extension

3. Limit elbow flexion

4. Provide suspension

When establishing the static alignment for a transfemoral prosthesis with a microprocessor hydraulic knee component, the TKA line should be:

1. Through the mechanical knee joint axis

2. 0-5mm anterior to the mechanical knee joint axis

3. 0-5mm posterior to the mechanical knee joint axis

4. 5-10mm posterior to the mechanical knee joint axis

During the initial dynamic alignment of a transtibial prosthesis there is an excessive varus moment at mid-stance. This can be resolved by:

1. Increasing the adduction of the socket

2. Increasing the abduction of the socket

3. Insetting the foot

4. Externally rotating the foot

Continuation of the Treatment Plan

Obtain feedback from patient and/or caregiver to evaluate outcome (e.g., wear schedule/tolerance, comfort, ability to don and doff, proper usage and function. Assess patient’s function and note any changes. Assess fit of prosthesis with regard to strategic contact and to anatomical relationships to prosthesis to determine need for changes relative to initial treatment goals. Address evidence of excessive skin pressures or lack of corrective forces and formulate plan to modify prosthesis accordingly. Revise treatment plan based on assessment of outcomes.

At a follow-up visit for a patient who was fit with a body powered transhumeral prosthesis, they complain that they are not able to open the terminal device with the elbow fully flexed. One possible solution to this problem is to:

1. Add a cross back strap to the harness

2. Add an elbow flexion assist component

3. Add additional rubber bands to the terminal device

4. Tighten the control attachment strap

A patient is seen for an initial follow-up after receiving a transfemoral prosthesis. They are ambulating with an abducted gait. What is the MOST likely prosthetic cause?

1. Prosthesis is too short

2. Foot is too inset

3. Prosthesis is too long

4. Knee flexion resistance is too strong

A patient is seen for follow-up for their PTB-SC prosthesis with pelite liner. The alignment of the prosthesis is good, however the patient states that the socket feels less secure recently and you note a gap between the socket and their limb at the lateral proximal brim. What is the MOST appropriate adjustment to address this problem?

1. Increase the thickness of the liner at the lateral proximal aspect

2. Increase the thickness of the liner at the medial proximal aspect

3. Increase sock ply thickness

4. Add padding to the medial tibial flare

A patient who has missed multiple follow-up appointments for their transtibial prosthesis is seen. The patient has been noncompliant with hygiene instructions and has developed a Wagner Grade 2 ulcer on the anterior distal end of their residual limb. The practitioner’s PRIMARY responsibility at this time is to:

1. Modify the prosthesis and suggest the patient follow-up with their physician

2. Modify the prosthesis and schedule a two-week follow-up appointment

3. Inform the patient to take a break from using the prosthesis and follow-up in two weeks

4. Instruct the patient to discontinue use of the prosthesis and notify the physician

These sample questions are only examples of the type of test content you will see in the exam. For additional information about how to prepare for the exam, check out the Practice Analysis Resident Guide.  Access all the Practioner Exam Prep resources available on  .