In this exclusive article for Robotics and Automation News, Chris Wagner, head of advanced surgical systems at Cambridge Consultants, takes a microscopic look at the developments in medical robotics

This is an exciting time for medical robotics, as there is a proliferation of systems on the market or in development.

Intuitive Surgical’s da Vinci, the long-time market leader for robotic laparoscopic procedures, now has potential competition from Medtronic, Verb Surgical (backed by Google and J&J), Auris Surgical, Transenterix’s ALF-X, and Titan Medical’s Sport system.

Similarly, the orthopaedics robot market is active with Stryker’s Mako platform, Think Surgical’s TCAT system, Mazor Robotics Renaissance system for spine surgery, and Blue Belt Technologies (now owned by Smith & Nephew) Navio system.

One interesting observation is of the physical size of the various systems as compared to the active operating volume of the robot. 

The operating volume of the da Vinci system, for example, is an approximately 10-20 cm sphere, where its footprint is a large fraction of a square meter. Similar ratios hold for Mako’s and Think Surgical’s systems.

This is interesting because larger systems are more difficult to integrate into a clinical setting that is already space limited by clinical staff and other surgical equipment.

Further, workflow options are reduced, as setup and teardown either limit other tasks near the patient, or take up a large part of the room of if the setup tasks (for example, sterile preparation) take place away from the patient. Finally, in a coarse sense, size is correlated with cost of goods, which further limits adoption.

Do large medical robots have to be the norm? For robots that are designed to address only a localised part of the anatomy, such as an eye surgery robot, how small should it be?

Here we consider a few design pressures that push robots to be larger, with the idea of challenging the next crop of robot designs to find solutions to these challenges, thus creating smaller robots that might be more easily adopted.

Mechanical ground 

Robots need to develop significant forces to manipulate tissue. There are only several paths, however, to reference this force back to mechanical ground. Using the actual ground (that is, robot mounted on a cart) has the advantage that the floor is standardised, and your robot will likely already have a connection to the ground for transport.

The drawback is that the actual ground is usually the farthest mechanical ground away from the patient, so the robot needs to have sufficient footprint, stiffness, and corresponding size to reach the patient and resist all applied tool forces.

There are other mechanical grounding options with their own set of tradeoffs. The Laprotek laparoscopic system from endoVia (acquired by Hansen Medical) used an OR [operating room] table mounted system.

Challenges include interference with other bed-mounted tools, sterility management, and dealing with large bed tilt angles. Another option is the patient themselves.

Mazor’s Renaissance spine surgery system mounts mechanically to the spine, and is a notable exception to the large robots listed above. The tradeoff is that the bone screws required increase tissue trauma.

A final grounding option is the surgeon, if the surgeon is the one holding the robotic tool.  An example of this approach is the Navio system for joint replacement, which enables and disables the cutting action of a surgeon held burr depending on its current position. This results in a small handheld component, though limited in the types of robotic guidance.

Dynamic workspace adjustment 

One obvious reason for the large size of medical robots is that they need to be compatible with a number of operating room setups, flexible to patient size and positioning, and able to adjust setups during the surgery.

To enable this flexibility, additional degrees of freedom and larger ranges of motion are used. As a consequence, the mechanical components need to be larger and stiffer, which causes the knock-on effect of having larger actuators to carry more distal ends of the system.

Hence, the design challenge to make smaller robotic systems comes with interesting tradeoffs for usability (for example, smaller robot for easier setup, or larger, but motorised axes).

Internal access through a port

This applies to any robotic procedure that accesses internal structures through a port using rigid tools, such as with laparoscopic procedures using da Vinci, or neurosurgery procedures such as with Renishaw’s neuromate stereotactic robot.

These systems ensure a pivot point (called a remote center of motion) behaviour at the port, so there are no sideways tissue forces at the pivot. 

This forces a coupling between potentially small motions inside the body and large ranges of motion outside the body through the pivot effect, which then can force a large range of motion requirement on the robot.

Application to eye surgery and beyond

Returning to our eye surgery robot example, how big should a robot that performs eye surgery be?

Advanced tools for eye surgery itself is an important issue – there are insufficient surgeons available to treat the 20 million people worldwide with curable blindness from cataracts, for example – but shouldn’t a small robot address this representative small anatomy (approximately 25 mm diameter sphere)?

If we look at our above design pressures, we also see that maybe there are ways around them.

Most eye surgery is carried out with the patient on an OR table in a consistent position, so a table-mounted robot should be an option. The workspace is consistent, with small variations in patient size and no repositioning requirements during the surgery.

Access through a port with a straight instrument is still a design constraint for retinal procedures, so care must be taken with mechanism design.

Steps in this size reduction direction are promising. Early stage eye robot company Preceyes is developing a system that is small enough to share the operative space with a surgeon.

Eye robots in the research community, such as the retinal surgery system from the University of Utah, achieve an even smaller footprint yet still address key clinical issues such as sterility and tool exchange.

Hopefully, this trend continues and we will see a wider adoption of clinical robotics and the associated patient benefits. 

Christopher Wagner is head of advanced surgical systems at Cambridge Consultants. His specialities include robotics, haptics, electronics, mechanical design, software engineering.