Medical Design Technology

equipment as needed. While pillars can lift the same loads as actuators, their guiding functionality, inherent in the profile tubes, enables them to accommodate offset and moment loads without the need to design an additional support and guiding frame.

Some pillars are multi-stage, enabling vertical movement in two or more stages. While a single-stage pillar has one screw that pushes two tubes apart to lift a load, a double-stage pillar has a double acting mechanism (with two screws) that pushes three separate stages apart (one stage pushes against the second and the second pushes against the third).

Telescopic pillars are able to achieve a high stroke to retracted height ratio, which is particularly relevant for surgical tables (Figure 2) and procedure chairs. This ratio allows a table or chair to be positioned very low (easier access for patients) and travel very high (ergonomic benefits for medical staff). Entry height capabilities have become a matter of regulation—evidenced by the Americans with Disabilities Act (ADA) requiring equipment with an entry height of 17-19 inches.

Scissor mechanisms can also be used for patient lifting functions, especially in procedure chairs. One advantage is a low retracted height with high stroke, but the downside is poor stiffness characteristics, and a much more complex chair design with a high number of parts, extra cost, and added assembly time. For surgical tables requiring a small footprint of the central frame, a scissor mechanism with a corresponding load and stiffness is simply too large to be a viable option. In contrast, pillars can be supplied as “plug-and-play” systems without the issues associated with complex assembly, performance, and size limitations.

Competing with electromechanical
actuator and pillar technologies, hydrau-
lic systems are often used to perform
positioning functions. While hydraulic
systems, in general, are compact and
provide high power density relative to
their size, several disadvantages make
them a less favorable option for new de-
velopments. Poor positioning capability,
slow speeds, high service requirements,
and the ever-present risk of oil leakage
have largely relegated hydraulic systems
to the second choice behind electrome-
chanics.

Checklist for Optimized System
Design

Optimized patient positioning systems
will rely upon a designer’s insights into
the performance and safety needs of an
application, and how potential issues
can be resolved at the outset. Here's a
checklist of basic objectives to help guide
the decision-making process:
Ascertain load requirements. Elec-
tromechanical actuators and pillars for
chairs or tables must be able to support
imposed patient loads, and load capaci-
ties will vary depending on the required
function. For example, when designing
a procedure chair to
handle up to 600 lb. pa-
tients, the load capacity
of the actuators can vary
between 2000 N for leg
adjustments, and up to
6000-8000 N for back
adjustment, and Tren-
delenburg (longitudinal
tilting of the body axis).

An important factor to consider when designing the lifting function with a telescopic pillar is the distance of the center of gravity in relation to the center of the pillar, since the offset load introduced by this can introduce a significant additional load. A pillar load capacity of 4000 N is generally sufficient to handle a 600 lb. patient, while 6000 N would be more suitable for patient loads up to 1000 lbs.

Safety first. The me-
chanical and electrical
safety of the equipment
is a key requirement that should not be
taken lightly. Depending on the chair or
table, a first failure safety factor of 2. 5 to
four times maximum load (up to which
there can be no collapse to harm the
patient) has to be implemented. Actu-
ators and pillars certified according to
the medical safety norm IEC 60601-1
should be chosen to reduce the risk to
the patient and to reduce final system
certification time.

Minimize system “play.” Overall system stiffness and low-play design can be a key requirement when designing tables and chairs for critical procedures, such as eye surgery. Play can be introduced from the actuator (overall construction, screw design, brakes, and/or gears), as well as from attachment points and hinges. Additionally, equipment stiffness can be affected, resulting in uncontrolled shaking or wobbling. All of these adverse

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