A method for measuring joint kinematics designed for accurate registration of kinematic data to models constructed from CT data.

A method for measuring three-dimensional kinematics that incorporates the direct cross-registration of experimental kinematics with anatomic geometry from Computed Tomography (CT) data has been developed. Plexiglas registration blocks were attached to the bones of interest and the specimen was CT scanned. Computer models of the bone surface were developed from the CT image data. Determination of discrete kinematics was accomplished by digitizing three pre-selected contiguous surfaces of each registration block using a three-dimensional point digitization system. Cross-registration of bone surface models from the CT data was accomplished by identifying the registration block surfaces within the CT images. Kinematics measured during a biomechanical experiment were applied to the computer models of the bone surface. The overall accuracy of the method was shown to be at or below the accuracy of the digitization system used. For this experimental application, the accuracy was better than +/-0.1mm for position and 0.1 degrees for orientation for linkage digitization and better than +/-0.2mm and +/-0.2 degrees for CT digitization. Surface models of the radius and ulna were constructed from CT data, as an example application. Kinematics of the bones were measured for simulated forearm rotation. Screw-displacement axis analysis showed 0.1mm (proximal) translation of the radius (with respect to the ulna) from supination to neutral (85.2 degrees rotation) and 1.4mm (proximal) translation from neutral to pronation (65.3 degrees rotation). The motion of the radius with respect to the ulna was displayed using the surface models. This methodology is a useful tool for the measurement and application of rigid-body kinematics to computer models.

Forearm rotation alters interosseous ligament strain distribution.

Recent interest in reconstruction of the interosseous ligament (IOL) of the forearm has led to questions concerning optimal placement of the reconstructive graft as well as the ideal rotational position of the forearm during graft tensioning. We therefore studied the strain distribution in the IOL to determine which fibers are strained in different positions of forearm rotation. Five cadaveric human forearms were subjected to compressive axial load (simulating power grip) and the strain values across the entire IOL were measured with the forearm in neutral, supination, and pronation. The strain distribution in the IOL changed with forearm rotation. The highest overall strain was found in neutral. In neutral and pronation, higher strain was observed in the proximal region of the IOL. In supination, however, higher average strain was seen in the distal region of the IOL. These results suggest that a reconstructive graft placed in the proximal region of the IOL and tensioned in neutral rotation would provide balanced constraint in different positions of forearm rotation. A graft placed in the distal region and tensioned in forearm neutral, however, may limit forearm rotation.

Role of the forearm interosseous ligament: is it more than just longitudinal load transfer?

The objective of our study was to measure 3-dimensional force vectors (magnitude and direction) acting in the forearm when load is applied to the hand and to measure the actual force in the interosseous ligament (IOL). Fourteen cadaveric forearms were loaded to 136 N of compression while special load cells measured force vectors in the forearm. Computer forearm models were used to display the 3-dimensional force vector directions. The study results showed that the radius bears most of the load at the wrist but load on the radius at the elbow is reduced because the IOL transfers load to the ulna between the wrist and the elbow. In addition to this role in longitudinal load transfer, our measurement of 3-dimensional forces allowed identification of transverse vectors which suggest that the IOL also functions to keep the radius and ulna from splaying apart. Our results imply that the IOL participates not only in longitudinal load transfer but also in the maintenance of transverse stability of the forearm during compressive load transfer from the hand to the elbow.

Feasibility of partial A2 and A4 pulley excision: residual pulley strength.

We investigated residual digital flexor pulley strengths after 75% excision of the A2 and A4 pulleys. For direct pull-off tests, A2 and A4 pulleys from cadaveric fingers were tested by pulling on a loop of flexor digitorum profundus tendon through the pulley. For functional loading tests, fingers were positioned with the metacarpophalangeal joint flexed to 90 degrees for A2 testing, and with the proximal interphalangeal joint in 90 degrees flexion for A4 testing (with all other joints in full extension). Excision of 75% of A2 and A4 pulleys reduced pulley strengths determined by both testing methods. For the functional loading tests, which are more clinically relevant, mean tendon forces at failure after partial excision of A2 and A4 pulleys were 224 and 131 N respectively, which is sufficient to withstand flexor tendon forces expected during activities of daily living.

A new methodology to measure load transfer through the forearm using multiple universal force sensors.

Previous approaches to measuring forces in the forearm have made the assumption that forces acting in the radius and ulna are uniaxial near the wrist and elbow. To accurately describe forces in the forearm and the forces in the interosseous ligament, we have developed a new methodology to quantitatively determine the 3-D force vectors acting in forearm structures when a compressive load is applied to the hand. A materials testing machine equipped with a six degree-of-freedom universal force-moment sensor (UFS) was employed to apply a uniaxial compressive force to cadaveric forearms gripped at the hand and humerus. Miniature UFSs were implanted into the distal radius and proximal ulna to measure force vectors there. A 3-D digitizing device was used to measure transformations between UFS coordinate systems, utilized for calculating the force vectors in the distal ulna, proximal radius, and the interosseous ligament (IOL). This method was found to be repeatable to within 3 N, and accurate to within 2 N for force magnitudes. Computer models of the forearm, generated from CT scans, were used to visualize the force vectors in 3-D. Application of this methodology to eight forearm specimens showed that the radius carries most of the load at the wrist while force in the IOL relieves load acting in the radius at the mid-forearm. For a 136 N applied hand force, the force in the IOL was 36 + 21 N. Advantages of this methodology include the determination of 3-D force vectors, especially those in the IOL, as well as computer generated 3-D visualization of results.

Feasibility of partial A2 and A4 pulley excision: effect on finger flexor tendon biomechanics.

We investigated the effect of partial excision of the A2 and A4 digital pulleys, separately and in combination, on finger angular rotation and the energy for finger flexion. Statistically significant decreases in angular rotation resulted only after 50% and 75% excision of A2, A4, or A2 and A4 in combination. Work of flexion trends were weak and none of the changes were statistically significant. Although optimal finger function relies on the integrity of the A2 and A4 pulleys to maintain the efficiency of the digital flexor system, these data suggest that the A2 and A4 pulleys can be excised up to 25%, either separately or in combination, without significant effects on angular rotation. Decreases in total angular range of motion after 50% and 75% pulley excision were small, even for combined pulley excision (9 degrees +/- 3 degrees and 15 degrees +/- 5 degrees [mean +/- SD], respectively), and may be clinically acceptable.

Tensile properties of the interosseous membrane of the human forearm.

The interosseous membrane is a structure deep in the forearm that joins the radius and the ulna. It is made up of membranous and ligamentous regions. Two main ligamentous structures have been described: a prominent central fiber group, the "central band," and a smaller proximal fibrous band, the "oblique cord." Many authors believe that the central band plays a biomechanical role in the normal and fractured forearm and that it may function much like a ligament. The objective of this study was to determine the tensile properties of the central band. Eighteen fresh frozen forearms from cadavers (45-70 years of age, both sexes) were used. A fiber bundle of the central band was subjected to a uniaxial tensile test to failure in a materials testing machine, and its tensile properties were calculated. Stiffness, ultimate load, and energy absorbed to failure were expressed as a function of specimen width. The central band structure had a stiffness of 13.1 +/- 3.0 N/mm per mm width and an ultimate load of 56.6 +/- 15.1 N per mm width (mean +/- SD). The tissue of the central band displayed a modulus of 608.1 +/- 160.2 MPa, ultimate tensile strength of 45.1 +/- 10.3 MPa, and strain at failure of 9.0 +/- 2.0%. This study demonstrated that the central band is comprised of strong tissue. The material properties of the central band compare with those of patellar tendon: modulus is 120% and ultimate tensile strength is 84% that of patellar tendon. As a structure, the interosseous membrane is stiff and capable of bearing high loads. Although load distribution across the central band is unknown, a 1.7 cm wide, evenly loaded homogenous portion of the central band would possess a stiffness comparable with that of the anterior cruciate ligament. The results of this study provide a basis for future analyses of radioulnar stability and load transfer.