Progressive destruction of the metacarpophalangeal (MP) joint can be the source of significant pain, difficulty with performance of daily activities, and aesthetic deformity in the hand (1,2). Although in the majority of cases MP joint arthritis is a manifestation of rheumatoid arthritis (RA), other pathologic processes such as osteoarthritis (OA) as well as traumatic arthritis can also lead to MP joint destruction (1,2,3). MP joint arthroplasty aims at addressing all sequelae of progressive arthritis by restoring joint range of motion, alleviating pain, and correcting joint deformities (2,3). The MP joint plays a critical role in the function of the hand and the rest of the upper extremity, making its repair necessary when mobility is limited by arthrosis (3). At the same time, the complexity of the joint balancing the action of intrinsic and extrinsic muscles of the hand and offering motion in three different planes has proven designing the perfect implant prosthesis to be a very difficult task (3,4,5). The Swanson silicone implant first designed half a century ago still remains the point of reference for all MP implants attesting to the challenges involved in MP implant arthroplasty (4,5).

RA and the complications from it remain the most common cause of MP joint arthritis requiring joint implant arthroplasty (6). Affecting approximately 1.3 million people in the United States, RA is the most common cause of arthritis following OA (1,2,4,5,6). The disease process almost routinely affects the MP joint producing typical deformities that directly affect hand function and produce significant disability (6). Patients with RA will present with pain, edema, and limited range of motion at the MP joint (1). The disease process typically leads to progressive ulnar deviation and volar subluxation of the proximal phalanx with subsequent destruction of the articular surfaces at the MP joint (4,6). Rheumatic synovitis produces several anatomic changes that contribute to the typical ulnar deviation of the fingers: carpal collapse leading to radial deviation of the carpus, weakening of the MP radial sagittal band causing ulnar subluxation of the extensor mechanism, damage to the accessory collateral ligaments and volar plate causing volar and ulnar displacement of the flexor tendon sheath, and interosseous muscle contractures creating a volar-directed force to the MP joint (4).

RA treatment is primarily nonoperative and involves splinting of the affected joints, antirheumatic medications, and corticosteroid injections (1). First-generation disease-modifying antirheumatic medications such as corticosteroids, methotrexate, and azathioprine have been recently replaced by drugs targeting interleukin-1 and TNF-a (1,6). Despite the recent advances in the pharmacologic treatment for RA and the efficacy of new-generation drugs in delaying disease progression, the vast majority of patients affected will eventually develop worsening hand function rendering up to 13% of them completely disabled 10 years after diagnosis (6,7). Despite the proven effects of MP joint arthroplasty in alleviating pain and significantly improving hand function in patients with
debilitating arthritis, there is no consensus between hand surgeons and rheumatologists regarding the appropriate indications and timing of surgery (6,8). Of note, in a recent nationwide study, only 34% of rheumatologists supported that MP arthroplasty improves hand function in patients with advanced rheumatoid MP arthritis as opposed to 82.5% of hand surgeons who participated in the study (8).

Although not as common as RA, OA of the MP joint also represents a common cause of MP arthritis (1,9). Either idiopathic or as a result of repetitive joint trauma, OA usually affects the MP joints of the index and long finger (9). Patients present with pain and range-of-motion limitation in the affected joints, while radiographic imaging confirms the diagnosis, demonstrating joint space narrowing, subchondral sclerosis, and formation of osteophytes (9). Similarly to RA, nonoperative management is the mainstay of treatment for OA, to include oral or topical nonsteroidal anti-inflammatory medications as well as corticosteroid injections (1,9). Besides pharmacologic treatment, static and dynamic splinting plays an important role in OA treatment as it has been proven to improve range of motion, strength, as well as dexterity in the affected joints (1). Surgical options are reserved for patients who fail nonoperative management and include joint debridement, arthrodesis, and arthroplasty, which is typically less complicated compared to arthroplasty in RA patients as the ligamentous structures supporting the joint are largely unaffected by the disease process (3).

Although the thumb MP joint is more a uniaxial hinge joint, the finger MP joints are a complex condylar-type joint allowing motion in three different planes: flexion-extension, abductionadduction, and circumduction (1,4,5). The articular surfaces of the metacarpal head and proximal phalanx base are unique in their shape and articulation, which, in combination with the supporting collateral ligaments and volar plate, give the joint its range-of-motion properties (4,5). The metacarpal head is asymmetric in both coronal and sagittal planes (4), while the radial condyle is larger in size than the ulnar one on the coronal plane resulting in the ulnar deviation of the phalanx upon flexion, which is more pronounced in the index and middle finger, thus helping in hand grasp (4). Furthermore, both the radial and ulnar condyles of the metacarpal head are broader in the volar surface, creating a cam effect tightening the collateral ligaments upon MP joint flexion (4,5). As a result, joint flexion decreases the ability of longitudinal rotation, while in maximal flexion, the MP joint behaves more as a hinge joint (4,5). In contrast, in the extended position, the collateral ligaments are not taut, allowing for abduction-adduction and rotation (5). The radial collateral ligament runs a more oblique course compared to its ulnar counterpart allowing ultimately for increased ulnar deviation of the proximal phalanx during flexion and supination of the MP joint (4,5).

The aforementioned characteristics of the MP joint make the design and production of an implant for MP arthroplasty both an engineering challenge and a manufacturing challenge. Besides the restoration of the normal range of motion, which is complex in the case of the MP joint, the ideal implant must also provide adequate joint stability through optimal mechanical interaction between articular contours and dynamic support of the muscles and tendons investing the joint (5). In the MP joint, the articular surface structure, volar plate, capsule, and ligaments play a significant role in joint stability and function; release of the collateral ligaments at the time of native joint excision will result on ulnar bowstringing of the flexor tendons, resulting eventually in flexion contracture and ulnar deviation of the fingers (1,5,7,10,11). Another challenge for implant-bone interface is the fixation or setting of the device to native bone construct. The ideal implant would ensure immediate fixation of the stems to the endosteal surface of the medullary canals upon placement, which would later be further reinforced by bone ingrowth (2,5). The different sizes and contours or medullary canals between different metacarpals and less so the proximal phalanges make it difficult to design a single-implant device model with stems ensuring a perfect fit for all joints. Also, the fine structure of metacarpals and phalanges and the relative large fill volume of these small joint implants limit these small jointimplant interfaces to withstand the high mechanical stresses. Larger joint bone-implant material interfaces demonstrate a more optimal implant-bone interaction (5). Along the same lines, in order to accommodate differences in hand size between patients as well as different fingers and joint laterality, an implant solution should be produced with several models available ensuring optimal fit, making development and production even harder to be marketable for manufacturing companies (5).

Since their first employment in the 1950s, available prostheses for MP joint arthroplasty have significantly evolved. The available MP joint replacement systems can be divided in four different categories; hinge joint replacements, flexible one-piece joint replacements, the newly developed surface replacement joint implants, and the absorbable interposition implants (12,13,14). Hinged joint prostheses were the first MP joint implants to be developed (5). Although initially simple, singleaxis metal prostheses, they subsequently developed into metaloplastic designs consisting of a metal element articulating with a polyethylene bearing (1,2,5). Hinged joint implants were plagued with a nonanatomic center of rotation as well as a high coefficient of friction at the hinge mechanism (2,3,5,13). Bone resorption resulting in finger shortening and subsequent loss of range of motion, prosthetic loosening, and implant material failure were the most common complications associated with hinged joints and the reason they fell out of favor among hand surgeons (1,2,5,15). Flexible single-piece MP joint implants led the next step in evolution of implant arthroplasty (2,3,5).
Although initially designed as composite implants with a combination of silicone and other materials, the silicone implant designed by Swanson was the one that became the implant of choice for several decades and remains to date the point of reference for all other implants (1,2,3,5). Finally, the development of newer materials and the application of concepts initially developed for and applied to large joint arthroplasty led to the design of surface replacement MP joint implants, based on the articulation of the metacarpal head and proximal phalanx base with the interposition of wearresistant materials in an effort to replicate normal MP joint mechanics and function (2,13).

Single-piece silicone implants were first described by Swanson as a treatment option for the management of patients with arthritis involving the MP joint (16). The Swanson implant, essentially a silicone spacer for the MP joint, was designed to correct the alignment of the MP joint but most importantly to provide joint stability and promote the formation of a fibrous capsule around the MP joint (5,17,18). Since its introduction, the Swanson implant has been extensively used in MP joints affected by RA as well as OA making it the most widely used and studied implant for MP joint arthroplasty (1,4,5,17,18,19). Initially, the implant was manufactured from conventional silicone elastomer. This material was subsequently replaced by high-performance elastomer in order to decrease the incidence of material failure (19,20). This improved silicone material, Flexspan, is used up to this day (19). Additionally, in an effort to delay or even completely prevent implant fractures, metal grommets were introduced to the design of the original Swanson implant, protecting the prosthesis from damage by the bone at the points of insertion in the medullary canals (19). Of note, implant fracture does not necessarily mean loss of joint function, especially if encapsulation of the prosthesis has already occurred (18,19). Functionally, Swanson implant arthroplasty shifts the arc of motion across the MP joint to a more extended position, resisting flexion as well as ulnar deviation at the level of the MP joint (17,18,19). This functional benefit though comes with the drawback of compromised active MP joint flexion, affecting mostly the function of the ring and small finger (17).

In an effort to improve functional outcomes of single-piece silicone MP joint implant arthroplasty, newer implant designs were developed. The Sutter (Avanta) implant was first introduced in the late 1980s, featuring a center of flexion located volar to its longitudinal axis, as well as sharp, angled borders between the stems and hinge area, differentiating it from the Swanson implant (Fig. 25-1) (21). Despite the initial enthusiasm with its use, computer modeling later calculated higher mechanical
stresses developing across the Sutter implant, which could account for the increased rate of implant fracture rates observed when compared to the Swanson one, although the use of the Sutter implant resulted in greater improvement in range of motion (21,22). Newer designs in the options for MP joint arthroplasty included the introduction of preflexed implants, as the NeuFlex implant. The NeuFlex silicone implant, first introduced in 1998, featured an “anatomically neutral” preformed 30% angle—simulating the relaxed position of the hand—as well as a hinged design. The singlepiece NeuFlex implant better duplicates the anatomic center of MP joint flexion and better mimics the resting semiflexed MP joint position, resulting in decreased mechanical stress (18,19). When compared to the Swanson implant postoperatively, the NeuFlex implant has demonstrated a greater MP joint range of motion and increased longevity (17,18,19). Despite minor differences in the silicone polymer used or the implant design, silicone implants are by far the most commonly used implants in MP joint arthroplasty improving function, decreasing pain, and correcting the appearance of the hand (19,23). Although selectively used in cases of OA as well as traumatic arthritis of the MP joint, the main indication for the use of silicone MP joint implants is RA (1,2,3,4,23). Patients with advanced RA, where ligamentous and soft-tissue support to the MP joint is severely compromised as a result of the disease process, are better candidates for silicone implant arthroplasty as opposed to younger, more active patients with either earlier-stage disease or greater functional demands from their hands (1,2,3,4,19).