The manner in which this clearance is determined can be described as follows. The existing crank arm is cut somewhere along its length to insert a free-moving hinge that will permit the outer, cut-off part of the crank to swing down. Clearance is determined by observing the distal aspect of the traced segments, more specifically, the plantar aspect of the foot where it meets the pedal. The tracing is rotated to a position that lines up this distal aspect of the traced segments anteroposteriorly with the main chain wheel axis. At this point it is important to note whether this part of the foot lies superior or inferior to the main chain wheel axis. This will indicate the clearance required at the 12 o’clock position, and allows determination of the hinge ratio based on the length of the original crank arm. The hinge ratio simply describes where the free-moving hinge must be inserted along the original crank arm. Using a ratio based on the original length helps ensure the crank arm does not become longer than its original length. This is important for the safety of the rider when the pedal is in the six o’clock position in the PRC (closest to the ground). To summarize this concept, if the plantar surface of the foot lies inferior to the main chain wheel axis once rotated on the tracing, the hinge ratio would be such that the inner crank arm would be shorter, and the outer crank arm would be longer. The opposite would be the case in the event that the plantar surface of the foot lies superior to the main chain wheel axis once rotated on the tracing. In this case, the clearance required above the main chain wheel axis was determined to be 0 cm, which indicated that a hinge ratio of 50:50 would be required to allow the rider to complete a full PRC.
For fabrication, an existing crank arm was cut, milled, and attached to an adapter with a hinge at the appropriate location to achieve proper foot and pedal clearance (Figure 3). Effectively, the pedal would be at the level of the main chain wheel axis at the 12 o’clock position in the PRC. A simple shoulder screw with no bearing served as a hinge with a secured set-screw because it was uncertain whether the hypothesized pedal clearance was correct. Upon testing of the hinged crank arm, and attempting to keep the ankle joint at neutral, it appeared that the knee angles remained less than 90°. The next step was to determine whether this ratio would be suitable for the subject. Fortunately, this bicycle adaptation is interchangeable among bicycles, making it possible for the authors to simply transport the part to the subject. Other equipment used for the trial run included a bicycle trainer to allow a safe trial period, clearance blocks in the event the foot could have been higher than the level of the main chain wheel axis, and a digital video camera to document the trial run.
The initial trial run was a success. After the seat was set to the proper height for efficient cycling (such that the extended knee was slightly flexed when the subject was sitting on the bike seat), the level of hinge was appropriate to provide enough clearance that would allow a complete PRC. Initially, the subject pedaled with an abrupt jerky movement on the affected side because of the different pattern for pedaling required with the adapted pedal. He stated he felt comfortable on the trainer (Figure 4) and was eager to ride outside. The subject found that constant pressure on the pedal eliminated the jerky movements, and he rapidly adopted the new pedaling pattern. It was interesting to note that the maximum knee flexion angle on the affected side did not reach 90°, whereas on the sound limb it approached 135°.
After the adapted bicycle was used for a 2-month period, approximately 2 hours a day, the crank arm broke where the original crank attached to the hinge. A more durable hinged crank arm, including a needle bearing around the hinge axis (intended for longer term use), was then fabricated and sent to the subject. He now enjoys the activity of cycling and the increased level of independence.
Patients with limited knee flexion have experienced limitations in cycling for years. Pegs have been used in the past in place of a crank to allow the person with limited knee flexion to ride with the sound limb producing some power but at the expense of zero force production on the affected side. The hinged crank arm enables the rider to produce some amount of power on the affected side, although it may be reduced. This design enables dynamic involvement of the affected limb while cycling. This has a positive effect on the activity because both legs help to provide stability and power while riding.
It has been shown that physical activity reaps benefits for amputees.3 Professionals in orthotics and prosthetics have a duty to ensure their clients are aware of the options that exist for them to participate in the healthiest lifestyle possible. When physical restrictions limit client activity, it can be very rewarding to the practitioner and to the client to seek unique solutions that serve to overcome these limitations. Additional tracking of individuals now riding as a result of the hinged crank arm will help determine future design considerations.
The hinged crank arm adaptation remains independent of prostheses and allows clients with limited knee flexion, such as those having undergone Van Nes rotationplasty surgery, to return to cycling.
The author thanks William Buston (co-developer), Scott Knight (co-developer), Tim Inglis, Craig Smith & Smith Prosthetic Services, Dan Blocka, and Gord Ruder.
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3. Kegel, B. Adaptations for sports and recreation. In: In: Bowker JH, Michael JW, eds. Atlas of Limb Prosthetics,
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Keywords:© 2005 American Academy of Orthotists & Prosthetists
cycling; limited knee flexion; Van Nes rotationplasty