Shoulder replacement has become a very successful procedure. If you have done some reading, or browsed content on this topic, you will have come across the fact that, currently, the most common type of shoulder replacement used is called reverse shoulder replacement.
A reverse shoulder replacement contains several parts. Most commonly, a disc-like metallic part, called the baseplate, is attached with screws to the bony socket (glenoid) of the shoulder joint. A metallic spherical part is then connected to the baseplate. On the other side of the joint, the humerus bone receives a metallic stem that the surgeon fits inside the bone to receive a plastic liner that glides over the surface of the glenoid sphere (or glenosphere).
All Shoulders Are Different!
Humans are all different: the color of our eyes, how tall or short we are, the shape of our face… Those differences apply to our shoulders too: a tall person will have larger shoulder bones, whereas a short person has smaller bones. Also, the conditions that damage the shoulder joint can lead to loss of bone structure in different locations. For these reasons, industry companies that make shoulder replacement parts produce them in multiple sizes and shapes, or even custom for each shoulder. The surgeon can then pick and choose which replacement parts will fit best any given shoulder.
But how do surgeons know what parts to choose and where to place them?
Technological advancements have really helped surgeons optimize the shoulder replacement procedure. Decades ago, implant options were more limited, and surgeons mostly made decisions based on radiographs. Preoperative planning computer software has dramatically revolutionized this field. Of the multiple image tests that can be utilized to assess a joint, computed tomography (CT) is best for bones. The image files acquired with CT can be imported into software packages that display three dimensional renderings of the shoulder. They also allow the surgeon to move overlays of the various implants’ shapes and sizes on the screen until the best combination and position of implants for each shoulder is determined.
In my opinion, the mental exercise of planning implant size and placement with preoperative planning software makes me a better surgeon: I have noticed I am better prepared to manage the nuances that each and every individual shoulder replacement presents. These software packages allow adjustments of implant position to the millimeter and the degree. However, it can be difficult to apply these preoperative plans at the time of the procedure in the operating room. Traditionally, surgeons use their skills to try to replicate the plan, but the accuracy and precision of human beings is not perfect, and occasionally the implants land in suboptimal positions. That is why the next revolution in this field is the adoption of digital enabling technologies!
Digital Enabling Technologies
From plan to execution! Everyone in business knows that sometimes there are wonderful plans that never get properly executed. In shoulder replacement surgery, the planning software just discussed do allow to formulate the best surgical plan for each patient, each shoulder. Over the last few years, various digital technologies have been developed to enable the surgeon to properly execute the surgical plan; they include shoulder-matched implants, shoulder-specific instruments or guides (also known as patient-specific guides), optical navigation with or without augmented reality, mixed reality visualization and navigation, robot-enabled surgery, and smart navigated instruments. Robot-enabled surgery has been widely adopted in the field of hip and knee replacement. Last year, it was released for shoulder replacement as well.
What Exactly is a Surgical Robot?
Robots have been used outside the field of Medicine for decades! A robot can be defined as a programmable machine that is able to perform physical and other tasks traditionally conducted by humans in an autonomous or semiautonomous fashion. Robots receive inputs through sensors, have actuators that move to perform tasks, and a computer that can be programmed and respond to inputs by controlling the actuators.
Robots have been adopted by mankind for one of two reasons. First, robots can perform tasks relentlessly provided they are powered and do not experience malfunction: they are not subject to physical, mental or emotional burnout. One example would be robots on a car assembly line. Second, certain robots can accomplish tasks in a way that humans cannot, because they are (a) more accurate or precise, (b) stronger and/or faster, or (c) can function in environments adverse to humans. One example would be robots that can function in a toxic environment.
In Orthopedic Surgery, companies have developed robots with improved accuracy and precision compared to humans. But in addition, certain robots offer additional attractive features: haptic boundaries and feedback, assessment of motion and soft-tissue balance, data acquisition, and the use of smaller end effector tools that may allow surgery through a minimally invasive approach. At the time of release of this post, the United States Food and Drug Administration has cleared two robots for shoulder replacement: Mako (Stryker) and RoSA (Zimmer-Biomet). In this post we describe how Mako is used to prepare the glenoid for implantation of a reverse baseplate.
Robot Assisted Baseplate Preparation, Step by Step
Currently, the Mako robotic system is used to prepare the bone on the shoulder socket (glenoid) so that the reverse baseplate will be positioned exactly as planned.
Feeding the plan to the robot
The first step when using Mako robotic assistance to implant a reverse prosthesis is to upload the plan into the computer of the robot. The plan can be visualized in any of the two screens of the system and modified if needed. The name, date of birth, and other patient identifiers are displayed and confirmed by the surgeon as well for safety reasons.
Bone registration
Surgical exposure allows the surgeon to identify and visualize two important landmarks: the glenoid face and the coracoid. A small screw is inserted at the base of the coracoid; the screw has a little divot that serves as a fiducial marker (from the Latin word fiducia, meaning “trust” or “confidence”), which can be used at any time of the procedure to inform the robot of the position of the scapula in space. Additionally, a clamp is secured to the coracoid to fix an array with five reflective discs that can be continuously tracked by the Mako optical camera system.
Next, the computer software prompts the surgeon to identify various landmarks on the glenoid and coracoid using a bone registration probe with similar reflective discs. This process, called bone registration, essentially maps the shoulder actual bone anatomy to the preoperative CT-based three-dimensional rendering, ensuring that the robotic system has a highly accurate reference of the shoulder position before proceeding with bone preparation.
Robot registration
The robotic arm is then positioned in the operating room at a location that will ensure that the effector end can reach in space the bone to be prepared. The effector end, also called minimally invasive care system (MICS), is registered with a second array of reflective fiducial marker discs that is used any time a new cutting tool is attached. Once the robot is registered and the adequate position of the cutting tool is confirmed with the array, Mako is ready for bone preparation.
Bone preparation
Currently, Mako prepares the surface and central aspect of the glenoid vault by removing bone with a bur inserted into the MICS. Bone to be removed is depicted in blue on the three-dimensional rendering. As bone is removed, the blue color disappears. Mako provides haptic boundaries: if the surgeon tries to accidentally remove bone beyond what was planned, the robotic unit vibrates and quickly stops. When bone removal from the face is complete, the program is advanced to removal of bone at the vault to allow insertion of the boss of the baseplate. If use of a central screw is desired by the surgeon, a different cutting tool (called router) is registered and used to create the drill hole for the central screw. At that point, the anticipated rotational position of the baseplate is marked, and the robotic portion of the procedure is finalized. The definitive baseplate is then implanted and secured with additional screws.
If you are interested in watching a video of how the glenoid is prepared to receive a baseplate using the Mako robotic system, please click here.
A Look Into the Future!
We certainly live exciting times! Robot-assisted surgery with the system described here allows safe bone preparation with accuracy and precision above and beyond human performance. But this is just the beginning: robot-enabled shoulder replacement will be extended to other portions of the procedure. The ability of the robotic system to measure and collect soft-tissue tension and intraoperative motion is unparalleled. Because the effector ends are small, Mako opens the possibility to perform the operation through minimally invasive surgery, with much less muscle damage and no need for postoperative protection. All data collected can be used for analytics. However, robots are expensive, and the true impact on outcomes after surgery cannot be established just yet. As we gain more experience with robots, we will be able to answer these questions and more!
