23.1 Introduction

The interphalangeal (IP) joint of the thumb is, after the thumb saddle (or carpometacarpal [CMC-1]) joint, the second most important joint in the axis of the first ray. The CMC-1 joint positions the thumb in its three-dimensional space for retropulsion, opposition, and ab- or adduction, while the metacarpophalangeal (MCP-1) joint fixes objects toward the long fingers. The IP joint at the end of the motion axis is responsible for the precise adjustment of the thumb. Pain-free mobility at the end of the thumb is important for grasping small objects, especially for pinching to the index and middle fingers, while stability is indispensable in fixing those objects. Jemec et al highlighted the existence of IP joint rotational mobility to optimize fine and precise motor skills. ¹

23.1.1 Treatment Strategies at the Thumb IP Joint

Posttraumatic destruction or primary and secondary osteoarthritis of the thumb IP joint is rare, but leads to considerable pain and limitations in functional activities. Conservative treatment (e.g., local treatment, painkillers, and steroid injections) is the first step in dealing with the associated symptoms. Following unsuccessful conservative treatment, arthrodesis of the destructed joint is the preferred operative procedure to manage this condition. ^{2 , 3} Arthrodesis achieves a pain-free situation, but with a limitation in functional activity, particularly in the precise pinching and picking up of small objects from the table (i.e., paper clips) or between the radial-sided fingers. Turning small objects (i.e., a Swiss fondue fork) between the thumb and index finger and holding a pen while writing requires enough mobility at the distal joints of both digits (Fig. 23.1).

23.2 Characteristics of Possible Implants for Thumb IP Joint Replacement

Possible implants for IP joint replacement are those that fit well into the destructed joint and have the potential to provide sufficient stability, especially for the pinch to the index finger at the ulnar side. Destructed and unstable IP joints with insufficient ulnar collateral ligaments or huge bone loss (due to inflammatory disease or of posttraumatic origin) are a contraindication for joint replacement at this level.

Based on our experience, we use the original Swanson silicone spacer (Wright Medical Group N.N.) or a metal-polyethylene nonconstrained surface replacement, which was originally developed and designed for the PIP joint (CapFlex, KLS Martin Group, Tuttlingen, Germany). ^{12 , 13 , 14} The latter comprises a cobalt-chrome alloy element and the corresponding surface partner is made of ultra-high molecular weight polyethylene (UHMW-PE). The back of both elements are coated with titanium to promote osteointegration (Fig. 23.2).

23.3 Indications and Contraindications for Thumb IP Joint Replacement

IP joint replacement is possible for any destructed joint where mobility is needed. Especially in the case of simultaneously arthrodesis at the more proximal MCP- or CMC-1 levels, sustaining mobility at the most distal end of the thumb is important. Prerequisites for replacement of the thumb IP joint are a stable joint with sufficient collateral ligaments and tendons, especially at the ulnar side, and no severe bone loss at the corresponding base or head of the first phalanx.

Contraindications include severe bone destruction with huge defects, chronic instability, or luxation of the joint, and chronic or acute infections or skin lesions at the involved thumb.

23.4 Published Outcomes and Our Own Results

Only two case reports exist in the literature to date. We recently presented one case with a Swanson silicone arthroplasty at one IP joint and a CapFlex surface replacement on the contralateral side. The postoperative follow-up time points for the silicone and metal-PE implants were 6 and 4 years, respectively. ¹¹ The second publication presented the outcome of a pyrocarbon implant in a 15-year-old boy with posttraumatic destruction and a follow-up for 22 months. ¹⁰

The prevalence of destructed thumb IP joints is low, and in our clinic, we can only report on a small series of patients with silicone and surface-gliding implant for IP joint replacements. Silicone has the disadvantage of being a flexible material, which contributes to a higher associated risk of implant breakage; therefore, our team no longer uses this implant type. We have observed good postoperative stability of CapFlex-PIP at the PIP level, and now use this implant for the IP joint. Although this implant is specifically made available for its application and treatment of the PIP joint, we have experience with the CapFlex-PIP as an off-label implant in the treatment of IP joints. All IP joint-treated patients are documented in a clinical registry in conjunction with our patient series receiving PIP arthroplasty.

Currently, we have data stemming from a small series of eight patients treated with a noncemented surface replacement at the IP joint and a minimum follow-up of 1 year. The patient mean age is 66.4-year-old and six patients are female. The dominant hand was affected in five patients, two had an affected nondominant hand, and the last patient is ambidextrous (i.e., right and left hand were affected in five and three patients, respectively). We reported one complication involving dorsal luxation of the joint within the first 6-week postoperative control period. Because the affected patient was dissatisfied with the outcome and a closed reduction procedure was not possible, open reduction with temporary K-wire fixation through the soft tissue was performed. A further 6 months after the second intervention, subluxation of the joint reoccurred and the patient underwent a conversion to arthrodesis.

The progression of the remaining seven patients could be followed up to a minimum period of 1 year. Active range of motion was 40 degrees (range, 10–60 degrees), which was slightly lower compared to the mean baseline value of 56 degrees. Mean postoperative pain at rest as measured on a visual analog scale decreased from 6 (baseline) to 2.5, and pinch grip to the index finger increased from a preoperative mean value of 5.5 to 6.7 kg at 1 year postsurgery. Mean baseline grip strength also increased from 18.4 to 21.8 kg at follow-up, and the mean brief Michigan Hand Outcomes Questionnaire score at baseline (where 100 indicates good overall function) increased from 43.5 to 57.1 points 1 year after surgery. All implants remained stable throughout the mean 1-year follow-up period without any radiographic signs of migration, osteolysis, or loosening.