aDepartment of Orthopaedic Surgery, Mayo Clinic College of Medicine, 200 1st Street SW, Rochester, MN 55905, USA bDepartment of Orthopaedic Surgery, Division of Education, Mayo Clinic, 4500 San Pablo Road,
Jacksonville, FL 32224, USA
In 1940, Burman reported the use of a Vi-tallium cap for proximal interphalangeal joint (PIPJ) arthroplasty . This and other early digital implants used concepts similar to those used in the highly successful implant arthroplasty of the lower extremity. In 1959, Brannon and Klein  from Lackland Air Force Base, Texas published the first series of a digital total joint replacement. They reported encouraging results with the use of a hinged prosthesis initially used for traumatic PIPJ injuries. Two years later, Flatt  reported on the use of a more rotationally stable form of the Brannon prosthesis. This prosthesis was indicated in patients who had arthritis [3,4].
In 1979, Linscheid and Dobyns developed a prototype of a PIPJ prosthesis, which they termed "surface replacement arthroplasty (SRA).'' This prosthesis intended to preserve the collateral ligaments and unload the component stems . Various modifications of this prosthesis have occurred with the most recent modification referred to as the PIP-SRA (Avanta, SBI, New York, New York). Subsequently, a variety of designs were developed including the Keesler, the Hagert, and the Sibly-Unsworth [4,6].
Biomechanics and prosthetic design
Restoring motion and alleviating pain are the primary goals of PIPJ replacement arthroplasty. Because of early design failures, total joint
* Department of Orthopaedic Surgery, Mayo Clinic, 4500 San Pablo Road, Jacksonville, FL 32224. E-mail address: [email protected]
arthroplasty for the PIPJ has been slow to develop. These first-generation hinged designs failed because of nonanatomic centers of rotation, a high hinge mechanism coefficient of friction, metallic implant debris, and ultimately breakage [5,6]. A second generation of hinged prostheses was developed with a "ball and socket'' design, attempting to add adduction and abduction motion to flexion and extension . These designs were fraught with complications, including proximal phalangeal component failure, hypertrophic bone formation, poor motion, and instability [5,7].
The principal shortcoming of previous metallic or metalloplastic hinged designs has been the amount of bone resection needed for implantation. The primary stabilizing factors of the PIPJ is the bicondylar geometry of the articulation and the radial and ulnar collateral ligaments [8,9]. The extensor mechanism is also considered a stabilizer [8,9]. The bone resection required for insertion of the first and second generation devices violated the origin and insertion of the PIPJ collateral ligaments. The monoaxial hinged design of the first generation PIPJ arthroplasty created high loads borne by the component stems. Loosening, cortical penetration, and subsidence were among the complications encountered [2-4,6,8,10]. Subsequent constrained linked designs were unable to improve on these shortcomings.
The newest generation of PIPJ arthroplasty can be termed "surface replacement arthroplasty.'' The rationale behind this design is that a minimally constrained, unlinked prosthesis with an anatomic center of rotation can better balance forces acting across the joint. Preserving bone stock and collateral ligament origins and insertions enhances stability of the arthroplasty. This stability is particularly important when index and long finger PIPJ arthroplasty is considered. Sustained pinch forces of over 70N are often encountered between the thumb and index fingers and the thumb and ring fingers. Resultant forces on the PIPJ can be as high as six times this sustained, externally applied force . A successful arthroplasty, therefore, must withstand these transmitted forces. In theory, greater durability from the surface replacement arthroplasty can be expected compared with previous hinged designs. The anatomic configuration of the prosthesis and the retention of the collateral ligaments and PIPJ capsule should reduce axial torque at the bone/prosthesis interface . Ash and Unsworth  have demonstrated that an anatomically designed PIPJ surface replacement ar-throplasty could withstand pinch forces in excess of 65N. They have also shown that an ultra-high molecular weight polyethylene material for both weight- bearing surfaces could produce wear rates similar to metal on polymer .
One such PIPJ surface replacement implant (SBI), has a stemmed, bicondylar proximal phalangeal component milled from a CoCr alloy. The middle phalangeal component of this PIPJ implant is machined from ultra-high-molecular-weight polyethylene. The polyethylene articular surface is supported by a thin titanium backing and a broad stem. The articular surfaces of the proximal and middle phalangeal components are matched and are congruent. The component stems are designed to fit the internal contours of the medullary canal. The low profile design of the PIPJ surface replacement arthroplasty requires less bone removal, thereby preserving the integrity of the lateral collateral ligaments (Figs. 1, 2A-C).
Based on anthropomorphic data, four component sizes have been made. Currently, the PIPJ surface replacement arthroplasty is approved for advanced osteoarthritis, post-traumatic arthritis, and for revision arthroplasty of the PIPJ. Use of this prosthesis is less desirable in settings of pronounced bone loss or where the collateral ligaments are incompetent.
The Ascension PIP joint prosthetic replacement (Ascension Orthopedics, Austin, Texas) is a pyrolytic carbon implant with an anatomic design (Fig. 3). The device is indicated for implantation in patients with osteoarthritis or post-traumatic arthritis of the PIP joint. The pyrolytic carbon is a coating material formed by heating propane to 1300° Celsius. The hydrocarbon gas is then applied to a graphite substrate to create the pyrocarbon implant . Two attractive features of pyrocarbon are an elastic modulus similar to cortical bone, creating a favorable situation for load transfer; and a low incidence of wear debris or soft-tissue reaction .
Other newer designs have focused more on improved intermedullary fixation rather than anatomic configuration of the articular surfaces [5,14,15]. These designs include the Saffar (Dimso
Fig. 1. Small Bone Innovations PIPJ surface replacement arthroplasty. (Courtesy of Avanta Orthopedics/ Small Bone Innovations, New York, NY.)
Fig. 3. Ascension PIPJ pyrolytic carbon implant. (Courtesy of Ascension Orthopedics, Austin, TX.)
S.A., Mernande, France), the Digitos (Osteo A.G., Selzach, Switzerland), the DJOA3 (Landos, Chaumont, France) and the Wecko Fingergrundgelenk prosthesis (Implant Service, Hamburg, Germany). The Saffar and the DJOA3 prosthesis have a prominent stabilizing midline crest between the proximal and distal components and are considered "semiconstrained" by the manufacturers. The DJOA3 (Fig. 4) is composed of a stainless steel proximal component and a polyethylene distal component. Insertion of the prosthesis does not require preservation of the collateral ligaments. The Saffar prosthesis has a similar design that includes a noncemented, semiconstrained, titanium-polyethylene prosthesis . The modular Digitos prosthesis (Fig. 5) is a fully constrained prosthesis designed specifically for unstable joints missing the collateral ligaments. In essence, it is a second-generation device. Similarly, the Wecko Fingergrundgelenk prosthesis is a constrained design that fits into the inter-medullary bone sleeve (Fig. 6).
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