Question

What are designs/devices being used today for finger joint replacements (MCP) *include about 3 or more devices*

1. Description of each device (mechanics involved)

2.FDA class

3. Which current design is the most used today

Please include citations from the information obtained

Thank you

Answer 1

MCP Joint Implant Arthroplasty Stability, recurring deformity, loosening, and tendon balancing are the primary challenges facing the design of a replacement for the MCP joint.5,20 A common problem in MCP total joint designs has been the appropriate location of the center of rotation for the metacarpal head component.5 Incorrect placement of the center of rotation hinders joint flexion and extension. If the center of rotation of an MCP joint prosthesis is placed too dorsal, digital extension becomes difficult but flexion is enhanced. Placement of the center of rotation in a palmar direction may limit digital flexion but may enhance digital extension.5 In the native joint, the center of rotation of the MCP joint in relation to the metacarpal head is not fixed because the sagittal contour of the head is elliptical. The movements of the normal MCP joint produce both abduction and adduction, along with some rotation.21 Finally, three-dimensional models of the hand have shown that internally transmitted compression joint forces can range to as high as six times the externally applied pinch force.21 Theoretically, the design of a prosthetic joint would be superior if the design closely approached the normal anatomic configuration. Such a design would allow the sliding and rotational movements typically observed. However, shortcomings of an anatomically configured design are the potential for instability or subluxation, particularly when ligamentous incompetence is present. The MCP PyroCarbon Total Joint Prosthesis (Ascension Orthopedics, Austin, TX) is an unlinked MCP joint implant. The pyrolytic carbon coating is applied to a high-strength graphic substrate to create an implant that is highly compatible with living tissue.22 The components have offset intramedullary stems, which support hemispheric articulating surfaces ). The offset intramedullary stems presumably help neutralize ulnarly directed forces. These articulating surfaces resemble, but do not anatomically replicate, the metacarpal head and the articular base of the proximal phalanx. The implant is very effective in implant-bone load transfer because of an elastic modulus similar to that of cortical bone.22 The pyrolytic carbon material has been shown to be very stable in a primate model, producing no wear, wear debris, or inflammatory reaction. The low profile of the MCP PyroCarbon Total Joint Prosthesis is designed to preserve the collateral ligaments. Based on the same design concepts used for the development of the SR PIP Finger Prosthesis, the SR MCP Finger Prosthesis (Avanta) is a minimally constrained, unlinked design that attempts to reestablish the anatomic geometry of the metacarpal head. The metacarpal component is made of CoCr; the proximal phalanx component is manufactured of UHMW polyethylene (Fig. 7). The metacarpal head component is elliptical in an attempt to approximate the changing center of rotation in the natural MCP joint. Furthermore, the metacarpal head prosthesis has volar flanges, thereby enhancing surface contact in flexion. This enhanced contact in flexion increases radioulnar stability.19 This prosthesis has been designed to help compensate for the soft-tissue imbalance often encountered at the MCP joint in the rheumatoid patient. The dorsal lip of the proximal phalangeal component has been extended to prevent palmar subluxation of the joint. Additionally, the metacarpal component has a central raised portion designed to inhibit ulnar drift. The metacarpal head also is offset radially on its stem to help decrease ulnarly directed moments.7 Perhaps more important than any other stabilizing design feature, the low-profile nature of the prosthesis retains the origin and insertion of the collateral ligaments. Therefore, the MCP joint surface replacement arthroplasty ultimately may be appropriate for both osteoarthritis and rheumatoid arthritis. However, certain conditions encountered in patients with rheumatoid arthritis, such as severe bone erosion and collateral ligament incompetence, may create limitations for the use of this device. Several other MCP joint prostheses recently have been developed. The Saffar implant is a noncemented, semiconstrained titanium-polyethylene MCP joint prosthesis with a central articulating crest for stability. The Digitale MCP prosthesis has titaniumcoated, anatomically shaped, stainless steel press-fit stems designed to stimulate bony ingrowth. The Mathys MCP RM Finger System (Mathys, Bettlach, Switzerland) uses a polyacetal resin proximal component and a polyester distal component. This prosthesis has the unique feature of a screwexpanded intramedullary fixation for enhanced intramedullary fit21 (Fig. 8). The DJOA3 MCP joint implant (Fig. 2) studied by Condamine et al10 has a spherical stainless steel head and a cylindrical polyethylene proximal phalangeal component.

The principal shortcoming of previous metallic, metalloplastic, and singlecomponent polymeric plastic-hinged designs was the amount of bone resection required for implantation. The extent of resection frequently violated the origin and insertion of the collateral ligaments. The two primary stabilizing factors of the PIP joint are the bicondylar geometry of the articulation and the collateral ligaments.12,13 The extensor mechanism also may be considered a stabilizer.12,13 In the absence of the two primary stabilizers, the stems of the monoaxial-hinged design of the first-generation PIP joint arthroplasty bore high loads, which frequently resulted in loosening, corticalpenetration,andsubsidence.1,4-6,12,14 Subsequent hinged or fully constrained linked designs were unable to ameliorate these shortcomings. The natural flexibility of the Swanson Silastic spacer offers greater longevity compared with previous metallic-hinged designs. The hinge resists prolonged cyclic loading but is prone to fracture at the stem-hinge junction. However, these implants continue to function after breakage in rheumatoid patients. The Swanson Finger Joint Implant (Wright Medical Technology, Arlington, TN) is the most commonly used PIP joint arthroplasty device, but it is generally not recommended for the index or long fingers of active individuals.9,15 The generous resection of the proximal phalangeal head required by the Swanson Silastic spacer sacrifices the radial and ulnar collateral ligaments of the PIP joint. Resection of the collateral ligaments leaves the Silastic implants of the index and long digits vulnerable to pinch stresses. External pinch forces of 70 N are considered normal, with resultant forces on the PIP joint postulated to be as high as six times the externally applied force.6 A successful arthroplasty must be able to sustain these transmitted forces. Therationalebehindnew-generation arthroplasty of the PIP joint is that a minimally constrained, unlinked prosthesis with an anatomic center of rotationwouldbalanceforcesactingacross the joint. In theory, preservation of bone stock and collateral ligaments lends enhanced stability to the arthroplasty beyondthatwhichcanbeaccomplished withaSilasticspaceralone.Also,greater durability can be expected compared with earlier hinged designs. The anatomic configuration, in combination with retention of the collateral ligaments and PIP joint capsule, should reduce axial torque from the boneprosthesis interface.12 Ash and Unsworth16 demonstrated that an anatomically designed PIP joint surface replacement arthroplasty could withstand pinch force >65 N. They also showed that an ultra-high–molecularweight (UHMW) polyethylene material for both weight-bearing surfaces could produce wear rates similar to those of metal-on-polymer.