Our next speaker is going to be on the biomechanics of the scaphalunae ligament. And no one has done more in this and really has done some of the key landmark articles. And that's Dr. Walter Short from Syracuse. Dr. Short. Thank you. Today I'm going to be talking about biomechanics and pathomechanics of the scaphalunae joint. The understanding of biomechanics is certainly elusive. There are several possible factors that determine the biomechanics of the scaphalunae interval. These include the ligaments that Dr. Berger just talked about, the bony geometry, the direction that the wrist is moving, and the transmitted forces from either the muscle or an external load may have things to do with the biomechanics. There are several named ligaments of the scaphalunae joint. Dr. Berger has gone over a few of those. I'll just name them because we have tested several of these. These are the scaphalunae ligament, which is the most studied and the strongest dorsally, radial scaphocapitate, the scaphotrapezoid trapezoid, the long radiolunate, short radiolunate, and the ulnolunate ligament. Dorsally, there's a dorsal intercarpal ligament and the dorsal radiocarpal ligament. All of these named ligaments either attach or go over the scaphoid or lunate. The mechanical properties of the dorsal scapholunate ligament are the strongest, but the material properties of all of these other named ligaments are similar to other similar-sized ligaments in the body. There are several methodologies to evaluate scapholunate biomechanics. These include pressure-sensitive films such as Fujifilm, electromagnetic sensors, computer models to determine proximity of carpal bones, pressure-sensitive inks, which are used in tactile sensors, and optical motion sensors. In 1995, myself and Fred Werner used a combination of a wrist joint simulator, which moves a wrist, pressure-sensitive tactile sensors to evaluate pressures in the wrist, and electromagnetic tracking devices to monitor motion of the carpal bones to evaluate the role of the scapholunate ligament. This shows the wrist joint simulator. The wrist tendons are moved by hydraulic cylinders, which are controlled by a computer model to cyclically move the wrist reproducibly. These are electromagnetic sensors, which are indirectly attached to the carpal bones. As you can see here, through carbon fiber rods. And right here is a tactile sensor, which is inserted into the radial carpal joint to measure pressures. We concluded, first of all, that the position of the scaphoid and lunate is dependent upon wrist position and direction of movement. Thus, as you can see here, when the wrist is moving in radial ulnar deviation, if the wrist is in zero degrees of radial ulnar deviation, and if you're moving into ulnar deviation, the carpal bone is in one position. If you're in zero degrees of radial ulnar deviation and going into radial deviation, that same bone is in a different position. After you're sectioning the scapholunate ligament, we found that the scaphoid flexed more and pronated, while the lunate extended. Also, the pressures in the wrist are redistributed. This compares the intact specimen and after sectioning the scapholunate and STT ligament. And you can see that there's a redistribution of forces and a concentration of forces in the radioscaphoid fossa. We also did analysis of serial sectioning of multiple ligaments to see if there was any difference in which order you section the ligaments. And we also did 1,000 cycles of motion after complete sectioning of multiple ligaments to determine if repetitive activities after wrist damage affected the result. We found that after scapholunate ligament sectioning, there was an increased scaphoid ulnar deviation lunate extension, while after sectioning the scapholunate, STT, and RSC, there was an increased scaphoid flexion during wrist flexion extension, which increased after cyclic motion. We also evaluated whether the order of ligament sectioning changes the result, and we found that sectioning the RSC or STT caused no measurable changes in the scaphoid or lunate biomechanics. Additional sectioning of the scapholunate ligament after these other two ligands resulted in increased scaphoid flexion and ulnar deviation and lunate extension. If the scapholunate ligament was sectioned first before the RSC or STT, there was a similar result. We concluded from this series of experiments that the scapholunate ligament is a primary stabilizer, the RSC and STT are secondary restraints, but cyclic motion after sectioning these ligaments caused increased instability. We also evaluated the scapholunate gap since that seems to be an important clinical tool in determining instability. In this study, we evaluated whether sectioning the STT or RSC, we found that there was no change in the minimum distance between the scaphoid and lunate. Scapholunate ligament sectioning increased the scapholunate gap, but the gap was dependent upon the wrist position. This is a computer animation. Anyway, the animation doesn't play, but it shows that in wrist flexion the gap between the scaphoid and lunate increases, whereas in wrist extension the gap is minimal. Looking at two-dimensional X-rays and comparing it to the three-dimensional image, we found that although in a two-dimensional X-ray, A appears to be the minimum distance, where actually if you look at it in three dimensions, this A line is actually a spurious measurement. We did additional ligament sectioning, including the DRC and DIC, and after all of these experiments, we found that only after the scapholunate ligament is cut are there changes in carpal motion. Scapholunate ligament sectioning after DRC and DIC caused large increases in scaphoid flexion, ulnar deviation, and lunate extension. And this magnitude was greater than any other ligament sectioning series and was up to 18 degrees of angular change. The scapholunate gap only increases after scapholunate ligament sectioning, but not after DRC and or DIC sectioning. Preliminary studies have also been done on the long radiolunate and the short radiolunate after sectioning the SLIL, DIC, and DRC, and this resulted in further scaphoid and lunate instability. We also have done studies to compare the relative importance of the dorsal versus the volar component of the scapholunate ligament. We did sectioning of the dorsal or volar and then serial sectioning of the entire ligament. We found that there was increased scaphoid flexion, ulnar deviation, and lunate extension following dorsal scapholunate ligament sectioning compared to volar scapholunate sectioning. In summary, we thought that the scapholunate ligament is a primary stabilizer. The remaining examined ligaments are secondary restraints. The DIC and DRC are relatively more important than the RSC and STT based upon changes after ligament sectioning. Cyclic motion accentuates these change, and the dorsal scapholunate ligament is relatively more important than the volar SLIL in stabilizing the scapholunate interval. We also have done some studies to evaluate bony geometry using the same methodology and CAD models. We found that a deeper radioscaphoid fossa and a greater volar tilt imparted more stability after ligament sectioning. A larger curvature of radius in sagittal and coronal planes is more stable. Others have also evaluated wrist instability and comparing type 1 to type 2 lunates. Patients with a DZ deformity have a more preponderance of type 1 lunate. Vegas and Patterson have also done biomechanical studies using optical trackers and found that the dorsal scapholunate ligament is an important stabilizer of the wrist. Cate and Wolf have studied both intrinsic and extrinsic ligaments and found that sectioning the scapholunate ligament results in an immediate change in kinematics of the wrist. Werner has also evaluated out-of-plane motions that are not purely flexion, extension, or radial under-deviation, which is most of our daily wrist motion functioning. He found that the function of these various ligaments have not been fully tested and may become important stabilizers if you study out-of-plane motion. In summary, the knowledge of the scapholunate biomechanics is very incomplete. Scapholunate ligament appears to be the primary stabilizer. Secondary stabilizers appear to be incapable of stabilizing the joint without an intact scapholunate ligament. Bony geometry also imparts stability. Reconstruction of the scapholunate ligament may restore normal biomechanics or near-normal biomechanics even if other ligaments are damaged while the wrist is moving in flexion, extension, or radial under-deviation. The contribution of the secondary restraints in other planes of motion is unknown. Thank you.

biomechanics

pathomechanics

scapholunate joint

ligaments

bony geometry