Hospital for Special Surgery Perspectives

Experts Unravel the 'Mysteries of Wrist Motion'

Anya Romanowski, MS, RD


October 05, 2020

Editorial Collaboration

Medscape &

The wrist joint comprises eight carpal bones and is the most complex articular system in the human body. Each bone is capable of moving in different degrees or directions dependent on the position, motion, and force generation of the hand in space. Any instability of the wrist caused by an injury can produce disabling symptoms and may lead to debilitating, degenerative arthritis.

Scott W. Wolfe, MD, an orthopedic surgeon at Hospital for Special Surgery (HSS), and co-investigator Joseph (Trey) J. Crisco, PhD, director of bioengineering in the department of orthopedics at the Warren Alpert Medical School, have spent nearly three decades studying wrist kinematics in order to gain a better perception of how to address a degenerative condition referred to as SLAC (scaphoid lunate advanced collapse).

Scott W. Wolfe, MD

At an American Academy of Orthopaedic Surgeons (AAOS) annual meeting, Wolfe and Crisco received the Kappa Delta Elizabeth Winston Lanier Award for their paper "Kinematics of the Normal and Injured Wrist: The Importance of the Midcarpal Joint."

Medscape spoke with Dr Wolfe to learn more about recent developments in wrist kinematics and arthroplasty.

In an interview at AAOS, you mentioned that there are two "black boxes" of surgical mysteries in the upper limb — carpal instability and brachial plexus injury — and that both of these have been the focus of your research efforts.

That's correct. It's been 30 years since Trey Crisco and I met. We combined our talents to tackle the mysteries of wrist motion, and in particular, this entity called SLAC arthritis. Trey came from a predominantly basic science background in spine. I had a lot of training in wrist injury but noticed that there were many deficiencies in our ability to treat this effectively. Together, we determined that we really needed to understand how the wrist worked and how it moved in order for us to be able to diagnose and treat it better. That was back in 1990.

As we set out on our journey, we found out that we didn't really have a great way to do that. There were no tools to look at how wrists moved in a living human being, and most of the studies at the time were done on cadavers. Right away, that's a limitation because the muscles aren't working in a cadaver so you don't get the normal forces and so on. So we determined that we were going to develop a technique that would allow us to measure how all of the eight different bones in the wrist moved in a live patient during wrist motions.


We started with two-dimensional studies of wrist motion. It took the better part of 10 years to develop the computer algorithms to be able to detect the contours of all of the small bones that are in the wrist, and then show how they moved both as a unit but also independently of each other. By the end of the first decade, we had published several papers on how the carpus works, how it moves, and what happens in the normal wrist and in the abnormal wrist. We were on the right track, and that was really the first time that anyone had been able to map carpal bone motion in humans.

We began in a very controlled environment with CT, having people move their wrist inside the CT gantry and analyzing how the wrist bone contours moved in relation to each other with our computer algorithms. Our ultimate goal was to be able to do that in three dimensions, visualize the actual bones in 3D, "register" their 3D contours independently, and then precisely track each bone independently in all of its motions, translations, and rotations. That involved using a program similar to one that the Department of Defense uses to track and identify incoming high-speed aircraft. Then we could begin to look at that process in injured patients as well as in normal patients.

You and Dr Crisco came up with a technique as an innovative approach in studying wrist kinematics.

Right. The technique that we came up with is called "markerless bone registration." That means that without using markers, we were able to identify how these tiny little irregular bones move — not just in two dimensions but in three dimensions. Each of them move in flexion, extension, radial, ulnar deviation, and complex rotations (or what we call coupled rotations, which are the nontraditional planes of motion). Coupled motion cannot be identified by the orthopedic planes of the anteroposterior or lateral planes of an x-ray but exists in composite planes of motion, such as what would occur when stirring a pot or throwing. That was a tough issue to tackle, and we did that predominantly over the second decade.

We looked at these coupled motions and found that a specific plane of motion called the "dart-thrower's motion" (DTM) was particularly important. That motion is used in everyday activities (household, sporting, and occupational activities) and combines the positions of radial extension to ulnar flexion (like throwing a dart). This plane is diagonal, or oblique to the standard orthopedic posteroanterior and lateral planes.

Did you find that women, men, and children have similar motions with their hands?

The motions were remarkably consistent between the genders. We didn't study children because of institutional review board considerations. The most important finding was that a whole set of bones, called the proximal carpal row, stayed virtually still during that complex motion.

We postulated that this was actually a developmental change in evolution that enabled early hominids to begin using their hands for throwing, hunting, battling, protecting their young, etc. In the transition from predominantly vegetarian arboreal primates to bipedal carnivorous primates came the release of the hand to be able to use it as a hunting instrument, a weapon, or for protection. To put this in context, this DTM plane is essential to nearly all precision activities that humans do, combining force and accuracy — which ultimately includes a wide variety of sports. It is likely that the DTM is unique to humans and probably has more evolutionary significance than the opposable thumb.

That was a big discovery in the mid-2000s. From that, we looked at where that motion occurred, and it turns out that it occurs predominantly in a single joint in the wrist called the midcarpal joint. That's the junction between the proximal row and the distal carpal row.

From that discovery, we have tried to advance treatments for SLAC arthritis that preserve the midcarpal joint. And we're not alone. Many people across the globe agree with this theory of dart-throwing and midcarpal motion, and are coming up with techniques to leverage that understanding.

So with that, we were able to devise different ways of reconstructing the wrist to preserve that midcarpal joint and allow people to have a more complete coupled motion path in the activities that they do. And the last stage is to develop a wrist prosthesis, a wrist arthroplasty that uses that plane of motion.

You have created a wrist prosthesis that has received FDA approval in March. One version had previously received the CE mark in the UK, right?

Yes. There are actually two prostheses; the first is the hemiarthroplasty, which is the radial component of the total wrist arthroplasty. This is a single component that sits inside the radius, and its articulating surface is shaped exactly like the proximal surface of the human midcarpal joint. The native distal carpal row sits inside of it and is minimally constrained by the prosthesis. The proximal side of the joint is a cobalt chrome surface, and it articulates, interestingly, with the cartilage of the native midcarpal joint.

The KinematX Total Wrist Implant

We received an approval for that in Europe almost a decade ago with the CE mark and in Australia more recently, but not in the United States because the FDA does not have a predicate for radius hemiarthroplasties. We designed a single-surgeon, single-hospital, single–prosthetic design study in London, and we published the results of our first 20 patients with an average of 4 years of follow-up. We have patients who are up to 7 years out and are still playing golf regularly with this prosthesis and are enjoying an improved range of motion. The published paper demonstrates statistically significant increases in the range of motion in every plane and in patient-related outcome scores (the Mayo score and the DASH score). That was very encouraging for our first trial.

You mentioned a second device that was pending approval.

Yes. The idea is that the hemiarthroplasty is not going to last forever. Cobalt chrome against native cartilage has a finite life span, though we do not know how long. We know and anticipate that, but at the same time, we need to have a rescue for that. The rescue must be able to convert the hemiarthroplasty into a total wrist arthroplasty. We have designed and built that solution. Our components now are modular, such that the ultimate plan would be for a surgeon to implant a metal proximal component and then later (when and if the joint wears out) switch out the metal articulated portion for a polyethylene portion, and put in a new metal carpal component in place of the native distal carpal row, thereby creating a total wrist replacement.

In a previous interview with Dr Mathias P. Bostrom (chief of the arthroplasty service at HSS), he described to me that anything you put in the body will corrode over time and will need to be replaced. It's like putting something into sea water, and we always have to be prepared for that.

It is not a self-renewing process. It has a finite lifetime and we have to be prepared for that. Knowing that, we developed this modular componentry so that we can switch out the articulating surfaces as they wear.

Any additional comments on the prosthesis studies? Are you working on any other research that you would like to share with us?

I don't like to refer to the KinematX Total as a culmination of our work but really the beginning of the next phase. We have already applied for NIH grant funding to instrument these arthroplasties so that we can get real-time data on motion and wear patterns during patient activities. There will be wireless transmitters that will track and record both motion and loading patterns.

We are also studying other total wrist designs that have been previously implanted, using a technique called biplanar fluoroscopy that measures motion during actual activities. In a biplanar fluoroscopy scenario, a patient's prosthetic device is registered in 3-space and then automatically identified within a patient's wrist as they perform an activity. The computer can track and analyze the prosthetic component's motion throughout the activity. These studies have yielded data demonstrating that previous designs have suboptimal kinematic properties, such as a highly variable center of rotation.

In another interview, Dr Todd J. Albert (surgeon-in-chief emeritus at HSS) described sensors in devices that can actually monitor patients' motions as they are performing their activities of daily living, and then determine which motions are wearing out the devices the most.

Exactly — those were developed for the knee several years ago. As you can imagine, we have to miniaturize these components so that they would be small enough for the wrist.

The wrist application would be a first. No one has used this technology in the wrist or in any of the small bones. This design would be both a research and a clinical device that would help us better understand how the wrist works and how we use our hands on a daily basis — a "tracker," if you will. It will also allow surgeons to better customize treatment to their patients' individual occupational and recreational demands.

What does the future hold in terms of technology tracking wrist motion, including repetitive motions (ie, when using electronic handheld devices)?

We have developed the technology to be applied to real-time keyboarding as well. Whereas before we had to position patients within a CT scanner and move wrists through a constrained range of motion within a jig, we now have developed motion analysis instrumentation that's external to the hand and wrist to enable accurate tracking of wrist, hand, and digital motion.

In a gait lab scenario, we can have a subject with some markers affixed to their external wrist, and we use 10 cameras to record data as they throw darts or baseballs, use a hammer, and perform heavy occupational activities in our laboratory, while simultaneously monitoring how the wrist is moving in those motions.

So we've gone back to our first cohort of patients who had the hemiarthroplasty wrist implant and analyzed how they actually used their wrists in activities such as throwing and hammering. The reason why we feel that this implant is particularly novel is that it enables those coupled motions, like the DTM, which most of our current reconstructive options don't allow. In fact, most current solutions ablate or remove that midcarpal joint. The new device re-creates and reconstructs that particular, very important joint, and this recently completed study demonstrates that our midcarpal design enables coupled motions, including dart-throwing and hammering.

Any final comments you wish to share with other orthopedists on hand injuries and procedures?

We are approaching a ceiling effect on our ability to treat scapholunate ligament injury effectively. We are limited by our inability to effectively reconstruct the complexity of this three-dimensional ligament and the array of surrounding stabilizers of the carpus.

SLAC wrist arthritis will never go away, and current SLAC wrist solutions seem inadequate for highly active individuals. The next phase will be to identify an individual's needs and expectations prior to surgery, and to try to match them with a customized solution that will best meet those needs in a durable fashion.

Scott W. Wolfe, MD, is chief emeritus of the hand and upper extremity service, and attending orthopedic surgeon at the Hospital for Special Surgery in New York City.

Scott W. Wolfe, MD, has disclosed the following relevant financial relationships:

Serve(d) as a director, officer, partner, employee, advisor, consultant, or trustee for: TriMed, Inc.; Elsevier, Inc.; Extremity Medical

Serve(d) as a speaker or a member of a speakers bureau for: TriMed, Inc..

Received income in an amount equal to or greater than $250 from: Elsevier, Inc.; Extremity Medical, LLC; TriMed, Inc.

Follow Medscape on Facebook, Twitter, Instagram, and YouTube


Comments on Medscape are moderated and should be professional in tone and on topic. You must declare any conflicts of interest related to your comments and responses. Please see our Commenting Guide for further information. We reserve the right to remove posts at our sole discretion.