Frédéric Schuind

Flexor tendon forces: In vivo measurements

S-shaped force transducers were developed for measurement of the forces along intact tendons. After calibration, the transducers were applied to the flexor pollicis longus and flexor digitorum superficialis and profundus tendons of the index finger in five patients operated on for treatment of carpal tunnel syndrome. The tendon forces generated during passive and active motion of the wrist and fingers were recorded. For pinch function, the amount of the applied load was measured with a special pinch meter. Tendon forces in the range of 0.1 to 0.6 kgf were measured during passive mobilization of the wrist. Tendon forces up to 0.9 kgf were present during passive mobilization of the fingers. Tendon forces up to 3.5 kgf were present during active unresisted finger motion. Tendon forces up to 12.0 kgf were recorded during tip pinch, with a mean applied pinch force of 3.5 kgf. These results have potential application in determining the amount of force that a tendon repair would have to resist during passive as well as active postsurgical mobilizations.

The distal radioulnar ligaments: A biomechanical study

The mechanical roles of the triangular fibrocartilage have been examined in three experiments. Kinematic analysis by a stereophotogrammetric method revealed that the palmar radioulnar ligament was taut in supination and that the dorsal radioulnar ligament was taut in pronation. In full pronation, the palmar radioulnar ligament decreased to an average of 71% of its length in tension. In full supination, the dorsal radioulnar ligament decreased to an average of 90% of its length. Mechanical testing of the triangular fibrocartilage under axial load disclosed a significant laxity (mean: 10.4 mm), which was decreased in pronation. Transverse loading tests demonstrated that the triangular fibrocartilage is less stiff in neutral forearm rotation. Study of the material properties of the palmar and dorsal parts of the triangular fibrocartilage showed these structures to be strong ligaments with material properties similar to those of the radiocarpal ligaments.

Force and pressure transmission through the normal wrist. A theoretical two-dimensional study in the posteroanterior plane

Force transmission through the wrist in the normal population was investigated using the rigid body spring modeling (RBSM) technique (assuming carpal bones are rigid bodies interposed by series of springs simulating articulating cartilage and constraining ligaments). One-hundred and twenty normal wrist posteroanterior X-rays of adults (evenly divided to represent both genders and two age groups) provided the anatomical data. Reaction forces between the carpal bones were modeled using a system of compression linear springs, representing cartilage and subchondral bone, and of tensile linear springs, representing ligaments. The spring constants were determined based on the material properties of wrist cartilage and ligaments. Assumed axial loads were applied along the metacarpals to simulate a grasp strength of 10 N with active stabilization of the wrist in neutral position. The force transmission ratio at the radio-ulno-carpal joint was 55% through the radio-scaphoid and 35% through the radio-lunate joints. The remaining 10% of the load was passing through the triangular fibrocartilage with minor differences between genders. Among the intercarpal joints, a large percentage of the load of the wrist was transmitted to the scaphoid. The peak pressure was highest at the proximal pole of the radio-scaphoid, with a radio-scaphoid versus radio-lunate peak pressure ratio of 1.6. The most important ligaments in terms of load transmission were those opposing ulnar translation of the carpus. The wrist morphology had little influence on the magnitude and pattern of load distribution. There was no effect of age on wrist force distribution.

Loose-hinge total elbow arthroplasty: An experimental study of the effects of implant alignment on three-dimensional elbow kinematics

A previous study suggested that the kinematics of a loose-hinge total elbow arthroplasty (TEA) are those of a truly semiconstrained joint. This study addresses the effects of malposition of the implant. The three-dimensional elbow kinematics during simulated active motion were studied in six cadaver specimens using an electromagnetic tracking device. In addition to simulated active elbow flexion, flexion arcs were obtained under an elbow varus or valgus moment, to calculate the structural varus-valgus laxities. The results after four different Coondrad-Morrey TEA positions of implantation were compared with those of the intact elbow. The flexion-extension amplitudes were not significantly decreased after TEA implantation, except with external rotation of the ulnar component, which resulted in a loss of extension. In the intact elbow and after TEA implantation in any position, the mean varus-valgus deviations throughout elbow flexion were in a narrower range than the structural limits imposed by the ligaments (intact elbow) or the TEA hinge design. With internal malrotation of the humeral component over 10 degrees, however, the valgus structural limit was reached and, conversely, the varus limit with external rotation over 10 degrees. The clinical improvement observed with the semiconstrained TEA is derived from the benefits of the less constrained articulation. The proximodistal changes of TEA implantation have no consequence on the kinematic pattern. Rotational malpositioning of either humeral or ulnar component should be avoided, the first because it changes the kinematic pattern toward the structural limits of the implant and, therefore, may lead to excessive stresses at the bone-cement-implant interfaces and to early loosening, and the latter because it causes loss of extension.

Small external fixation devices for the hand and wrist

Small external fixators have been developed for use in the hand and wrist. The main indications are stabilization of open or infected fracture or non-union of the metacarpals and phalanges; length maintenance in segmental bone loss; and distraction-lengthening of the fingers. The device may be used also for articular or periarticular fracture, closed diaphyseal fracture in adults, fracture in children, various arthrodeses, or corrective osteotomies.

Changes of elbow muscle moment arms after total elbow arthroplasty

The changes of muscle moment arms on the elbow during simulated active motion were calculated from the simultaneous kinematic and tendon excursion in six cadaver specimens. The results after four different Coonrad-Morrey total elbow arthroplasty positions of implantation were compared with those of the intact elbow. With optimal total elbow arthroplasty implantation, the general pattern of change of the muscle moment arms with elbow flexion resembled that of the intact elbow, but with increased flexor dominance over the extensors with elbow flexion. No significant modification of the muscle moment arms was related to the changes of positioning of the implants, except with 60° elbow flexion, a slight increase of the brachialis muscle moment arms with distal placement of the humeral component. The malplacements of the humeral component in varus/valgus or in internal/external rotation were found to significantly affect the muscle moment arms.

Changes in wrist and forearm configuration with grasp and isometric contraction of elbow flexors.

The purpose of this study was to determine the changes in wrist and forearm configuration with grasp and elbow flexor contraction by analysis of resting and grasping x-ray films of 15 normal adults and forearm scanograms of 10 other normal subjects. With grasp, a significant proximal migration of the radius (averaging 0.9 mm), a decrease in the carpal height ratio, and an increase in the lunate uncovering index were observed. With flexor contraction, there was a significant decrease in the forearm interosseous space.