Considering how fundamental knowledge of muscle and its anatomy is to orthopaedics, Richard L. Lieber, PhD, found it “inconceivable” when he realized by looking at past textbooks that, prior to the 1990s, scant attention was paid to the basic architecture of muscle and its microscopic qualities.
In the paper, he notes that because orthopaedic surgeons treat musculoskeletal deformities, “it is fitting that orthopaedic researchers would be highly invested in studies of these tissues. It is incumbent on the orthopaedic community to continue to pursue a basic understanding of skeletal muscle design and plasticity, as it almost certainly relates to the very common problems of low back pain, sarcopenia, and fatty infiltration of muscle after injury.”
His work has led to important discoveries about the relationship between sarcomere length and muscle function, and how manipulation of clinical length of muscle corresponds to length changes at the microscopic level. This knowledge has current applications for procedures such as tendon transfer around the hand and wrist, but has implications for muscle systems throughout the body.
In an interview with the AAOS Now, Dr. Lieber discussed his work and its applications to clinical practice.
AAOS Now: How would you characterize your work overall?
Dr. Lieber: We studied the mechanical properties and the structural properties of muscle. What we found is that even though muscles in the human body are basically made out of the same material, it is the organization of that material that gives a muscle highly unique properties. Muscles can be said to be made up of building blocks like bricks. You can arrange bricks in a million different patterns to make structures that have very different properties. In finger grip, for example, the muscle can move a considerable distance. The muscles in the back serve to stabilize and hold steady, though they are made up of the same kinds of cells and proteins as the finger muscles.
AAOS Now: How did you use laser light to learn about muscle?
Dr. Lieber: It’s a challenge to figure out how muscles do what they do. We developed a tool we can take in the operating room, mostly for hand surgery. The laser light gets scattered by the muscle in a way that tells us the underlying microscopic properties of the muscle. It’s like a high-tech ruler that allows us to measure microscopic properties. We did a simple experiment where we would move a wrist joint back and forth and would measure the muscle to try to understand how the muscle was designed to have specific properties. Another example is that we took six cadavers and did microscopic analysis of wrist muscles, back muscles, and shoulder muscles. We found, everywhere we looked, each type of muscle had its own specialization at a micro-anatomical level. Many of these studies were actually very simple.
AAOS Now: What should surgeons know about sarcomeres?
Dr. Lieber: Surgeons learned about some of these things, such as sarcomeres, in medical school, but they probably quickly forgot them. A sarcomere is essentially a small, microscopic machine. It is the fundamental force generator in muscle. Sarcomeres are the bricks. And they are very sensitive to their length. If a sarcomere gets stressed too much, or is allowed to get too short, it doesn’t generate any force. If a surgeon moves a muscle from one place to another, which happens in tendon transfer, total joint replacement, or back surgery, and changes the length of a muscle, that changes the way the muscle functions. Some muscles are extremely sensitive to length changes, and some muscles don’t care very much. We developed tools to measure sarcomere length.
AAOS Now: What is the function of the laser?
Dr. Lieber: It is basically a surrogate for a microscope. Laser light is special. It has a wavelength of about a micron, so it interacts with sarcomeres, which are also about a micron or two in diameter. So we get data from the tool. No one had ever applied it to humans, and no one had ever applied it to surgery. In children who have cerebral palsy, the design of the muscle gets ruined because of the brain. It has sarcomere lengths that are out of whack. That has surgical implications because potentially a surgeon can go in and sort of fix it.
AAOS Now: How have you applied this knowledge clinically?
Dr. Lieber: The laser is fairly difficult to use; it has a steep learning curve. We’ve established guidelines and grids, along the lines of “If you want to do a Brand tendon transfer in the forearm with the flexor carpi ulnaris, we recommend you stretch it this many centimeters beyond its resting length.” Based on the studies we’ve done, that will give them a certain sarcomere length.
We’ve sold about a dozen of the laser tools. In general, people just want an estimate; they want to know, for this muscle, how much should they pull. A change of 1 cm in one muscle may correspond to a sarcomere length change of 3 cm in another. We’ve given them the microscopic to macroscopic conversion.
If a patient has a cut radial nerve, which goes to the muscle that extends the wrist and fingers, the wrist and fingers are paralyzed. A typical procedure is to take one of the wrist flexor muscles and wrap it around the wrist. In the tendon transfer, when the flexor carpi ulnaris is reattached to the wrist extensors, it should be sutured in 1 cm longer than its slack length. That will correspond to a sarcomere length of about 3 microns. That’s about right for that particular system.
Each system has its own recommendation. For a while, we thought they would all be about the same. But it’s amazing: the most active and passive properties of muscle are highly specialized to a particular kind of function, so we have to study each system. We’ve done a good job around the human wrist and fingers.
In the leg, we’ve done just the beginning of procedures around the hip. We have measured gluteal muscles and internal and external rotators of the hip. We have shown that when you use different components, you end up with different function.
Muscles are very length-sensitive, probably more than you would think. They should not be stretched very much if you want them to keep functioning. If you do a surgery and the muscle is very tight, the odds are it’s not going to be generating much force; almost no muscle functions under very high tension.
Coauthors with Dr. Lieber are Samuel R. Ward, PT, PhD; and Jan Fridén, MD, PhD.
Disclosures: Dr. Lieber: Allergan, Halozyme, Mainstay Medical, Wolters Kluwer Health–Lippincott Williams & Wilkins; Dr. Ward—Orthopaedic Research Society, American Physical Therapy Association, American Society of Biomechanics, Journal of Orthopaedic and Sports Physical Therapy. Dr. Fridén reported no conflicts.