Minimally Invasive Surgery: An Engineering Approach

MIS benefits: shorter hospital stays, quality of life returns faster

John Pepper – Creative Orthopaedics

Minimally invasive surgery (MIS) has become quite the popular catch phrase in the industry during the past few years. MIS can encompass nearly all orthopedic techniques and is the future of our industry. This article defines MIS, explains its advantages, offers some general suggestions on how to make systems minimally invasive and identifies pitfalls to watch for and related clinical issues.

Pins indicate the edges of a wound. As these converge, the instruments working angle is greatly limited.

Orthopedic surgery involves more than just fixing skeletal problems. It requires three steps: gaining access to bones and connective or soft tissues; tissue or bone repair; and wound closure.

Because many bones and joints are deep inside the body, sometimes the surgeon must travel a significant distance through tissues to see and work on the intended site. The distance also increases when the patient is overweight or obese.

As designers, we must give surgeons the proper tools to work with these parameters. Soft tissue depths can be from several millimeters in the hand to numerous centimeters, such as in posterior lumbar fusions.

Additionally, sometimes it is advantageous for the incision to be located away from the orthopedic repair, to keep the wound away from tensioned skin. The recovery process means the incision, access tunnel and orthopedic changes all must heal simultaneously.

Advantages of MIS

The clinical importance of MIS is obvious. In my own experience with advances in IM (intramedullary) nailing, the patient has quite a different reaction to an eight-inch line of staples to close a wound versus a two cm incision covered with a Band-Aid. Smaller exposures also reduce the opportunity of postoperative infection. Hospital stays can be reduced, and the quality of life returns faster.

Minimally invasive surgery can be done by designing or redesigning instruments to work in or through smaller openings in the skin and fascia. The skin is flexible; however, the underlying fascia can be tough and may be a bigger concern for the designer. Specialized instruments are often the result.

One of the major obstacles in MIS is loss of vision. Soft tissues and blood work to close visual pathways opened in surgery. Conventional (non-MIS) techniques often use direct visualization of the boney structures.

Muscles frequently extrude through the forks of rake style retractors. The extrusion is not so much of an issue with large incisions but can easily close a large percentage of a small incision.

Newer techniques use cameras integrated either to the instruments or separately.

If designed properly, and the technique warrants, some operations can be done without direct visualization. Perhaps the simplest example is that the standard screw depth gage uses a hook to feel the opposite cortex.

Location of Instruments

Challenges of MIS are location of the instruments and position of hands, or the control means of the instruments.

In conventional surgery, the wound creates a working angle in which instruments function (see illustration on Page 30).

As the incision length decreases, the working angle gets smaller. If this is not taken into account, the tools will impinge on each other. When hand pieces collide, manipulation becomes difficult. This can be addressed by several methods; the first is to optimize instruments for external control of the technique. This can be reduced by making offsets in handles and shafts.

A hip stem tracer mill developed for MLI allows advanced cutting function through a small working channel.

The offset in the shaft allows a line of sight to the cutting zone, with hands and handle moved to one side.

Lengthening shafts has the same effect of separating the hands of the surgeon. This must be balanced with loss of control, as instruments can get too long to precisely control.

The second method to addressing instrument clutter is to take instrument function from external to internal (outside to inside the wound).

MIS tools are automating more tasks that were once done with multiple instruments.

Auto sutures are a tremendous example of this, as are vessel suture devices.

Instead of the surgeon’s hands moving the sutures and tying the knots, the mechanism performs this task, putting control of the process in a location where the hands can access, without a large incision.

A third option is to make external control of internal operations (See the hip tracer mill image). The large template is placed outside the body and, through a linkage, provides precise movement internally.

The large template resides outside, and only a small post and the cutter enter the canal.

The large template provides a magnification through the distance between the template contact and the gimbal to the bone. Precise internal operation is performed with minimal incision size.

Issues for New Implants

Many issues should be considered when developing new MIS implants and instruments. Hippocrates, the father of medicine, wrote the Oath obliging to “keep them from harm.” This has a special corollary that applies to MIS companies. We must not make a neat tool that shows well at the stockholders meeting if it does not at least equal the performance of conventional surgery.

To this end, MIS procedures should take approximately the same time as conventional techniques. The level of difficulty should be on par as well. As engineers and designers, we possess mechanical skills and aptitude that may be much greater than those of the average surgeon.

Our designing surgeons will likely share our ability to operate and understand complex mechanism intuitively, whereas the average surgeon may not possess this ability. As with all surgical instruments, we need to eliminate the fiddle factor—with MIS hardware, this need is acute.

One other pitfall that must be avoided is the assumption that we can design components solely with the use of plastic bone models.

These are a great assistance, and every research and development department should have a skeleton in it. The use of the bone models lulls the designer into thinking that if a procedure works on the bone model, it is ready to go to market. I once designed a universal intramedullary nail extractor for a major orthopedic company. I had developed a tapered thread profile that would grab the internal threads that were present on the proximal end of all rods. It worked great in the lab.

However, the designing surgeon commented, “That is great; put a slab of bacon over it, and if it cuts through, it is ready to use.”

During testing, the extractor would not penetrate a pound of bacon. A modification to the leading edge allowed it to cut the scar tissue and any ingrowth present, and the device is now performing well in vivo.

The trap was to assume that all I had to deal with was bone. Bone model companies are offering more complex models with muscles. We should always be aware of the tissues we will need to pass through to get to our working area.

Anterior lumbar fusion devices need to consider manipulation of the great vessels to get to the disc space. Moving the descending aorta is not a task to be taken lightly. Each procedure has its special challenges, and we must know them. It is best to learn by going to the operating room to see this firsthand.

Blood management is another important function in MIS surgery. One must assume that blood will flow into the wound constantly for the duration of the case. Conventional technique uses a suction wand.

While this instrument is highly effective, it often requires that a hand be used with it the entire time to empty the small pools that form at the base of the wound. The resulting problem is compromised visibility as the hand and wand take up valuable space in the instrument control zone just outside the incision.

Suction can be integrated into the existing instruments to reduce complexity of the system and eliminate the need for an extra hand.

Instrument Consolidation

This brings us to an important principle in the design of MIS instruments: instrument consolidation. If one tool can perform the function of two, the technique becomes more manageable. The entry portal tool I designed for an intramedullary nail system employed this in its design. There was a desire to have circumferential tissue protection to protect muscle from rotating reamers.

The backstroke of the reamers pulled several milliliters of blood per pass out of the bone canal and onto the floor.

These cases were very messy. The entry portal tool was designed as a tissue protection tube with a perpendicular suction port. The tube formed a gasket with the fascia and directed the blood to go through it. The suction port was movable to allow gravity assist in left and right application.

This tool was the key to the three cm incision intramedullary nail. The principle behind this can be used in many other orthopedic applications; in fact, many retractors for spinal fusion presently employ this.

Using MIS entry to the site can benefit from new ideas. In planning the surgical approach, it may be that the elimination of one large incision with one small one is impractical.

The solution is the use of a second portal. The sum of the two may well be less than the initial. The second portal gives advantages, especially with camera placement. This is done in the shoulder and other joints.

MIS Entry Tools

While directing MIS entry tools, it is wise to consider the direction of the muscle fibers and bellies.

Going between muscle bellies is easy. Splitting muscles along their natural fiber direction (usually along the muscle’s long axis) is relatively easy.

Pushing an instrument through the fibers perpendicular to their direction is difficult and may cause injury to the muscle and postoperative pain.

One can shop for ideas by viewing other surgical disciplines. Laparoscopic and cardiovascular surgeries are two advanced fields with interesting tools that those of us in the orthopedic industry can use for inspiration.

The intellectual property for these may be narrow enough that we can copy or reverse engineer features without infringing on them. As has become the normal state of affairs, a good patent search and patent protection will be important to ensure profitable business free from expensive legal pitfalls.

The MIS development team will be different from earlier development teams. For reasons described in the anatomic section, it is more critical than ever to involve one or more surgeons in the team. The surgeons have the experience and knowledge of anatomy few of us possess.

We need to check all our assumptions with them as early as possible in the design cycle.

With increasing device complexity, the project director may require an expanded team and longer design cycles.

The net benefit from the MIS tools is great, but we should realize it would come at a cost.

In summary, the orthopedic device market is competitive.

Surgeons and patients embrace new effective technology.

The bar is set quite high for us.

We will need to do a good job in developing MIS to stay in the forefront of orthopedic surgery and remain market leaders.

John Pepper, MS, PE, is the founder of Creative Orthopaedics LLC, specializing in orthopaedic device design. He can be reached at (203) 699- 8475 or [email protected].