Scott W. Wolfe, MD, and co-investigator Joseph J. Crisco, PhD, have spent close to three decades working to unravel the mysteries of wrist kinematics, with the ultimate goal of addressing the degenerative condition known as scapholunate advanced collapse (SLAC), which occurs following traumatic disruption of the proximal carpal row.
For their work on this challenging condition, Dr. Wolfe, of Hospital for Special Surgery/Weill Medical College of Cornell University, and Dr. Crisco, of Rhode Island Hospital/Brown University, are the recipients of the 2017 Kappa Delta Elizabeth Winston Lanier Award.
In their manuscript, the authors noted, “For more than a century, researchers have sought to identify a unifying theory of wrist kinematics, but the complex mechanical behavior of the wrist has to date defied all attempts to do so.”
Relating the genesis of his interest in wrist anatomy and function, Dr. Wolfe recalled: “At the conclusion of my hand fellowship in 1990, I was left with two ‘black boxes’ of surgical mysteries in the upper limb: carpal instability and brachial plexus injury.” Both topics have remained the focal points of his research efforts.
“At a time when the rest of the orthopaedic world was investigating the power of the MRI, I recognized the dramatic evolution of computed tomography and its application to 3D visualization of small bones and joint fractures about the upper limb,” he said. He and Dr. Crisco “set out to design and test computed tomographic algorithms that could measure the complex motion of the small and irregular-shaped carpal bones in normal subjects (in vivo3D kinematics) for the first time. Our goal was to refine and automate that technology so that we could study the kinematic abnormalities that occur when the important scapholunate ligament was disrupted, and the inexorable progression to wrist arthritis. With a better understanding of normal and injured wrist kinematics, our ultimate goal was to identify improved methods to recognize and treat the injured and arthritic wrist.”
Wrist arthritis affects up to 4.8 million Americans, Dr. Wolfe noted, and “current surgical solutions do not allow normal paths of wrist motion.” His research has now reached a culmination point: a total wrist arthroplasty device recently submitted to the U.S. Food and Drug Administration. “If successful, our total wrist replacement may enable patients to return to higher levels of function with less pain,” he said.
A complex challenge
The path to development of a prosthetic solution to scapholunate instability and SLAC arthritis was one of lengthy study of wrist anatomy and the process by which injury “disrupts the finely tuned synchrony of the carpal bones and leads to SLAC.” The authors noted that “the eight carpal bones that constitute the wrist joint represent the most complex articular system in the human body—each capable of moving in different degrees or directions dependent on the position, motion, and force.”
As they embarked on their studies, two main theories of carpal kinematics prevailed (Fig. 1). Anatomically, they noted, interosseous ligaments between neighboring bones define two rows of carpal bones (proximal and distal), separated from each other by the midcarpal joint. Proponents of the “row theory” noted that the bones of the two carpal rows move in a synchronous direction regardless of the motion of the hand, and they posited that the inherently unstable proximal carpal row is guided through flexion-extension and radioulnar deviation by mechanical signals from the midcarpal joint. Adherents of the “column theory” believed that flexion-extension occurs predominantly through the linkages of the central column of the wrist (capitate-hamate-lunate), while radioulnar deviation occurs as the lateral column (scaphoid) and medial column (triquetrum) rotate around the stable central column.
Drs. Wolfe and Crisco wrote that the 3D and live-motion capture studies they have conducted “provide evidence that neither carpal bone theory is sufficiently inclusive to explain complex and ‘coupled’ motions of the wrist joint, especially given the individual variability in carpal shape and laxity.”
The clinical implication of this incomplete understanding of wrist kinematics is that surgical approaches are not wholly effective. Even prompt repair of scaphoid nonunion or of scapholunate interosseous ligament (SLIL) disruption “cannot be assured to arrest the degenerative process,” the authors write. “The so-called salvage reconstructive options for SLAC and scaphoid nonunion advanced collapse (SNAC) arthritis limit strength and mobility, are unable to restore complex or ‘coupled’ motions of the wrist, and do not prevent arthritic progression.” By their estimation, the result is that approximately 4.8 million Americans have work and recreational demands that are adversely affected by these conditions.
The authors divided their work in exploring solutions for this situation into three stages, roughly corresponding to the three decades since 1990. Initially, they focused on development of noninvasive technology to measure normal carpal bone kinematics during normal wrist motion. In the 2000s, they sought to apply that technology to understand “functional” kinematics of the normal and injured wrist. In this decade, they have worked to conceptualize and develop improved methods for treatment of SLAC and SNAC arthritis.
It’s all in the midcarpal joint
Among the major findings that emerged from their study of wrist motion was the importance of the midcarpal joint to hand and arm function. Specifically, they studied the previously identified “dart-thrower’s motion”—an important and uniquely human path of motion from radial wrist extension to ulnar wrist flexion that one would use when throwing a ball, dart, or javelin, using a hammer, casting a fly, or pouring from a pitcher. In essence, despite its unassuming name, the dart thrower’s motion is the highest functional motion of the wrist used in all ballistic upper extremity tasks; its evolution gave humans the unique ability to accurately and powerfully throw and swing tools and weapons.
The investigators identified “important coupledmotion paths of combined flexion-extension and radio-ulnar deviation that occur almost exclusively at the midcarpal joint.” With the 3D technologies they developed to measure wrist kinematics during active tasks, they demonstrated that surgical restriction of midcarpal motion degrades functional performance.
In their paper, Drs. Wolfe and Crisco identified the following as their top 10 discoveries in wrist kinematics:
Computed tomographic analysis of in vivo motionof the carpus
Markerless registrationfor noninvasive measurement of carpal kinematics
Identification of minimal in vivomotion of the proximal carpal row during the dart thrower’s motion
Recognition that midcarpal motion is necessary for most occupational, recreational, and daily activities
Noninvasive 3D measurement of coupled motion of the wrist
Proof that restriction of coupled motion of the wrist leads to impairment of function
Recognition that the mechanical (and in essence functional) axes of the wrist are oblique to its anatomically defined axes
Development of a hybrid wrist coordinate system following surgical alteration
Proof that functional performance is impaired when the midcarpal joint is altered
Support for the fact that midcarpal joint replacement improves range of motion and outcomes
A better prosthesis
In pointing to the need for an effective arthroplasty solution for diseased wrists, the authors noted the shortcomings of current surgical approaches. “While effective at controlling pain and providing stability, most current SLAC and SNAC reconstructive procedures do so at the expense of wrist motion, and specifically, coupled wrist motion,” they wrote.
Current surgical methods can be categorized in three groups: ablative, arthrodesis, or arthroplasty. “Effective ablative procedures include radial styloidectomy, partial scaphoidectomy, or proximal row carpectomy, and are indicated in early SLAC or SNAC arthritis to control localized arthritis limited to the radiocarpal joint. Since they do not restore normal carpal kinetics or kinematics, ablative procedures are considered temporizing, and associated with progressive instability and degenerative arthritis,” they explained.
Partial wrist arthrodesis eliminates either the radiocarpal or midcarpal joints, and complete arthrodesis eliminates both. Complete radiocarpal fusion “bypasses the opportunity to utilize healthy remaining joints in early stage SLAC or SNAC wrists, and while straightforward, durable and effective for pain control, is associated with functional limitations in personal hygiene, work in tight spaces, and picking up small objects,” the authors wrote. “These deficits are further compounded when the ability to compensate for absent wrist motion is diminished by ipsilateral degenerative disease of the digits, elbow, or shoulder.”
Efforts to develop an effective total wrist arthroplasty (TWA) option have been underway for some five decades. Yet so far, the authors noted, no system has been able to demonstrate comparable outcomes to other prosthetic joints. Previous designs have been plagued by complications including distal component loosening, fracture, dislocation, and particulate wear and are not generally indicated in younger patients with posttraumatic or degenerative arthritis.
Through computer modeling of existing prosthetic designs, the researchers found that the designs do not permit a 1:1 dart thrower’s arc of coupled wrist motion without disengagement. They theorized that wrist arthroplasty could become a durable solution if distal component failure could be minimized and coupled wrist motion restored. They noted that none of the current TWA products were designed to enable midcarpal joint motion.
Based on their understanding of coupled dart thrower’s motion and its dependence on the full mobility of the midcarpal joint, Drs. Wolfe and Crisco designed and tested the first midcarpal TWA device.
The polyethylene on cobalt-chrome articular surface of the device (Fig. 2) was based on the averaged contours of the midcarpal joint in 25 healthy subjects. “The proximal component was designed to replacethe proximal carpal row anatomically, with the exception of the distal third of the scaphoid, which was removed to prevent impingement,” the authors wrote. “All extrinsic dorsal and volar ligament origins as well as the posterior interosseous nerve capsular innervation were preserved during joint preparation, and the radial subchondral plate was maintained to ensure restoration of carpal height and prevent interference with the radioulnar joint.
“To mimic other successful joint arthroplasties (eg, knee, hip, shoulder) and minimize particulate wear, the polyethylene component was moved to the convex (radial) component,” their paper continued. “The distal component articulating surface was designed to anatomically replace the capito-hamate midcarpal surface and restore the anatomic center of rotation.”
The authors are currently following a cohort of 20 patients in the United Kingdom who have received a monobloc midcarpal cobalt-chrome hemiarthroplasty version of their device. (Dr. Wolfe noted that wrist hemiarthroplasty is not FDA-approved for use in the United States.) In the nine patients for whom they reported results at 6 months, statistical improvements in Mayo and DASH (Disabilities of the Arm, Shoulder, and Hand) scores were seen, and five of seven working patients had returned to their previous work. At 2.8 years, all but one patient maintained improvement in range of motion, grip strength, and Mayo and DASH scores. The authors have submitted for publication their 4-year follow-up study on all 20 patients, which demonstrates continued clinical success.
“Whether these encouraging results can be attributed to the midcarpal articulating design can only be speculated,” the authors wrote. But based on these promising results, they seek to introduce a modular midcarpal TWA device this year.
They concluded: “The future is bright indeed; advances in diagnostic imaging will enable earlier diagnosis of ligament injury, kinematic dysfunction, and cartilage loss; while advances in our kinematic understanding and materials will enable anatomic and effective means to prevent or reverse the profound functional losses due to SLAC and SNAC arthritis.”
Terry Stanton is the senior science writer for AAOS Now. He can be reached at firstname.lastname@example.org