Effects of micropipette handle diameter and inclusion of finger rest on basilar thumb joint contact mechanics.

Micropipette users commonly experience problems in the hand and upper limbs. Mechanical factors are thought to contribute to osteoarthritis (OA) initiation and progression in the basilar thumb joint. Finite element analysis can be used to examine the effects of micropipette design on contact mechanics measures within the basilar thumb joint. This pilot study examined the effect of micropipette handle diameter (12 mm, 25 mm, and 40 mm) and the presence of a finger rest on contact area, contact force, and peak contact pressure in the basilar thumb joint. All contact mechanics measures decreased with increasing handle diameter with significant differences between the 12 mm diameter and the 40 mm diameter handles (contact area down about 30 mm2, contact force down about 15 N, and peak pressure down about 1 MPa). Decreasing contact mechanics measures with increasing diameter matched our expectations that contact measures would decrease with a more open grip. Contact mechanics measures were higher (p < 0.05) with a finger rest for contact area and force. We expected peak contact pressure and contact area to decrease with the presence of a finger rest. The unexpected outcome may have been due to non-randomized testing order and fatigue during testing.

Effects of Internal Fluid Pressure on Stresses in Subchondral Bone Cysts of the Medial Femoral Condyle.

The etiology of subchondral bone cysts (SBCs) is not fully understood. Mechanical trauma and fluid pressure are two mechanisms believed to cause their formation and growth. The equine stifle joint provides a natural animal model for studying SBCs. Computed tomography images of an extended yearling cadaveric stifle joint were segmented using ScanIP to isolate bones and relevant soft tissues. Three model geometries were created to simulate cyst sizes of approximately 0.03 cm3 (C1), 0.5 cm3 (C2), and 1 cm3 (C3). A uniform pressure resulting in 3000 N force was applied at the proximal end of the femur. Two types of simulations, filled-cyst and empty-cysts with uniform pressure loads, were used to simulate fluid pressurization. Our models suggest that shear stresses are likely the cause of failure for the subchondral bone and not pressurized fluid from the joint. Bone stresses did not begin to increase until cyst pressures were greater than 3 MPa. For all cyst sizes, fluid pressure must rise above what is likely to occur in vivo in order to increase bone shear stress, shown to be most critical. Synovial fluid pressure acts upon a porous trabecular bone network, soft tissue, and marrow, so the continuum nature of our model likely overestimates the predicted effects of fluid pressures.

The Key Pinch Stress Radiograph to Evaluate Dorsal Subluxation in the Basilar Thumb Joint.

The basilar thumb joint is the joint second most commonly affected by osteoarthritis (OA) in the hand. Evaluation of dorsal subluxation of the thumb during a functional task such as key pinch could help assess OA risk. The objectives of this study were to determine the best imaging angle for measuring thumb dorsal subluxation during key pinch and to compare subluxation to corresponding OA grades on the Eaton-Glickel, Outerbridge, and International Cartilage Repair Society scales. Eleven cadaveric forearm specimens were rigged to simulate key pinch. A mobile c-arm captured anteroposterior (AP) view images of the hand and was rotated in 5 deg increments toward the ulnar aspect of the arm up to 60 deg. Dorsal subluxation was measured on each image and compared to determine which angle captured maximum subluxation. The resulting best imaging angle was used for comparisons between dorsal subluxation of the thumb and OA grades for the basilar thumb joint. The max subluxation was in the AP view for most specimens. There was a significant correlation between subluxation and the Eaton-Glickel grade (p = 0.003, R2 = 0.779), but not with either Outerbridge grades (p = 0.8018) or International Cartilage Repair Society grades (p = 0.7001). Our results indicate that dorsal thumb subluxation during key pinch should be measured in the AP view of the forearm/hand. Dorsal thumb subluxation during key pinch had a significant correlation with the Eaton-Glickel radiographic measure of OA but not with more accurate visual classifications of OA.

The Effect of the Joint Capsule and Anterior Oblique Ligament on Dorsal Subluxation of the First Metacarpal During Key Pinch.

Laxity of the anterior oblique ligament (AOL) and/or the dorsoradial ligament (DRL) are believed to contribute to the progression of osteoarthritis in the trapeziometacarpal joint through increased dorsal subluxation. Stress radiographs during functional tasks, such as key pinch, can be used to evaluate such joint instability. Cadaveric experiments can explore joint contact pressures as well as subluxation under varying conditions, to gain knowledge about joint mechanics. The disturbance of supporting tissues, such as the joint capsule, during experiments may affect the recorded stability of the joint. To evaluate potential effects of opening the joint capsule and severing the AOL, eleven cadaveric specimens were rigged to simulate key pinch. An anteroposterior (AP) radiograph of the hand was recorded for each specimen while intact, after partially opening the joint capsule and after sectioning the AOL. First metacarpal subluxation levels were compared between the intact joint, partially open joint capsule, and sectioned AOL. Neither opening the joint capsule nor cutting the AOL had a statistically significant effect on metacarpal subluxation. The results indicate that partially opening the joint capsule has a negligible effect on joint mechanics and support recent studies that postulate that the AOL plays a less substantial role in preventing subluxation.