The normal wrist is illustrated in FIG. 3 of the drawings forming part of this specification and comprises three sets of bones: the forearm, the carpals and the metacarpal bones in the hand. The carpals are the bones most closely associated with the motion of the wrist. In the forearm there are two bones, the radius 2 and the ulna 4. In the hand there are eight carpals, which are divided into rows, the proximal and the distal. The proximal row comprises a navicular 6, a lunate (not shown), a triquetrum 8 and a pisiform 10. The distal row comprises a trapezium and a trapezoid 12, a capitate 14 and a hamate. There are five metacarpals consecutively numbered 20 to 24 from the thumb through the last finger.
Wrist movement is divided between the radiocarpal and midcarpal joints of the wrist in a relatively complex manner, displacement of the carpal bones being necessary for motion. The configuration of each row of bones changes according to the position of the hand. Although both joints contribute to all hand motions palmar flexion is predominantly a midcarpal motion and dorsiflexion is radiocarpal. Ulnar deviation also occurs at the radiocarpal joint while radial deviation takes place at the midcarpal level. Anatomical distortion of the carpal bones or loss of integrity of their ligaments or secondary stiffness affects the joint and results in wrist disability.
The carpal bones are held together by ligaments. Collateral ligaments provide lateral support of the wrist, while palmar radiocarpal and dorsal radiocarpal ligaments maintain support of the carpal area. These ligaments define a symmetrical pattern due to insertions into the scaphoid, lunate, triquetrum and capitate bones. It is important that the integrity of the radiocarpal and ulnocarpal bands of ligaments be maintained in carpal bone surgery and that these ligaments not be interferred with or impinged on by the implant.
Currently, there are devices available for either total or partial replacement of the wrist joint. These devices use one or more of the possible types of mechanical articulations available, which are the hinge, ball and socket, or runners in grooves. Most devices use intramedullary stems and acrylic bone cement to secure the prosthesis to bone. Presently available prostheses have components constructed from several types of biologically inactive metals and are designed to articulate with other components constructed from a plastic, such as high density polyethylene.
Several methods or techniques are used to insure that the components remain articulated, and these methods include the use of the soft tissues existent at the time of implantation, the use of pins or screws to hold articular surfaces together, and the use of bayonet type locks. Some types of the available prostheses are quite simple while other types comprise complex mechanical systems, with both types having attendant or inherent disadvantages. The principal disadvantage of the simple prostheses is that they may not reproduce the full range of motion of the joint, while the principal disadvantage of the complex prostheses is the potential complexity of surgery and increased chance of failure, typical causes of such failure including fracture of bone during reaming and breakage of the implant itself. Additionally, the prosthesis components, such as pins, screws or intramedullary stems work loose following implantation. In the event of failure fusion may be required, but such surgery may be difficult if a great deal of bone has been removed.
It has been the principal object of my invention to provide a prosthesis for replacement of the wrist joint having new and improved construction and operative relation of its parts, which will provide maximal motion in dorsiflexion and palmar flexion and ulnar and radial deviation, compatible with satisfactory stability and with possible reproduction of the normal range of wrist motion.