Section 2 Arthroplasty of Finger Joints

Martin Richter

Abstract

The CapFlex-PIP surface replacement is an unconstrained modular system with different sizes made of cobalt-chrome coated with titanium and one polyethylene gliding surface. Advantages of this new implant are minimal primary bone resection and high lateral stability of the prosthesis. The preferred implantation technique is from dorsal. The technique and aftercare are explained in detail. The results are promising. In the author’s own series (n = 23), there was a mean ROM of 71 degrees for the PIP joint after 1 year and no signs of loosening.

Key words

proximal interphalangeal joint – replacement – arthroplasty – cementless – surface gliding implant – prosthesis – CapFlex implant

14 Third-Generation PIP Arthroplasty: CapFlex-PIP

14.1 Introduction

The surgical treatment of symptomatic proximal interphalangeal (PIP) osteoarthritis has been a challenge for hand surgeons for many years. The history of total joint replacement of the PIP joint began in 1966 with the introduction of the silicone spacer. Prosthetic fractures, collateral ligament instability, and silicone synovitis have caused recurring problems with these prostheses. The next generation of prostheses was introduced by Linscheid and Dobyns in 1979. ¹ The unconstrained anatomically shaped PIP prosthesis with stems for the medullary cavity was first made of cobalt-chromium type, next titanium, and finally pyrocarbon. The shape remains largely unchanged. Loosening, migration, and dislocation were the main complications.

As part of the third generation, the CapFlex-PIP prosthesis (KLS Martin Group, Tuttlingen, Germany) is also a surface prosthesis; however, the stems reaching into the shaft are minimal and bone resection is less.

14.2 Characteristic of Implant

The CapFlex prosthesis consists of two unconstrained components shaped to match the ends of the contiguous proximal and middle phalanges (Fig. 14.1). The proximal component is made of a cobalt-chromium alloy with polished distal articular surface replicating “normal” condylar anatomy. The proximal part of the component head is roughened and coated with titanium to encourage osteointegration. ² Schindele et al confirmed osteointegration in a CapFlex prosthesis removed in 2016 due to soft tissue complications. ³ For stabilization prior to osteointegration, the proximal component has two small ministems that secure the metal cap on the head of the proximal phalanx.

**Fig. 14.1** The CapFlex prosthesis consists of two anatomically shaped unconstrained components.

The distal component is made of three different materials: the proximal gliding surface is made of ultra-high-molecular-weight polyethylene (UHMWPE) sitting on a cobalt-chrome body coated distally with roughened titanium.

The CapFlex prosthesis is a modular system with three different sizes for each component: small, medium, and large. They can be linked in seven different combinations: each size with its equivalent, e.g., small with small of one size different, i.e., small with medium or medium with small or large. The commonest combination is a medium (M) proximal component and a medium (M) or large (L) distal (middle phalanx) component. The distal components are available in three different thicknesses: 2.1 mm, 3.0 mm, and 4.4 mm. The different thicknesses help balance the joint soft tissues at the end of the procedure.

Due to the limited bone resection, the collateral ligaments can “always” be preserved, which helps maintain joint stability.

The prosthesis can be inserted via a dorsal or a volar approach. The instruments are designed for both approaches.

14.3 Indication and Contraindication

The main indication for the CapFlex prosthesis is degenerative osteoarthritis. ³ Good bone quality is a prerequisite for the use of the prosthesis, as the small components need bony support. The modern antirheumatic medication, which often leads to osteoarthritis-like progressions in rheumatoid arthritis (RA) patients, may increase the indications in patients with RA. I have treated a patient with RA with a CapFlex prosthesis in their index finger PIP joint which already had a Swanson MCP joint arthroplasty.

There is likely to be role in treating posttraumatic arthritis but it is less common and the results are likely to be worse.

Intact functioning collateral ligaments are essential as the implants have only limited joint stability on their own. If there is some instability, then one or both collateral ligaments can be shortened or re-sited. This is often necessary in cases with primary deviations of the joint. In the group of our patients the results regarding the straight finger axis were encouraging, since due to the good bone support there is probably little tension on the shortened collateral ligaments.

As with PIP prostheses in general, it should be noted that the actual indication is the relevant pain, as a normal range of motion is not to be expected. Whether an expected improvement of 10 to 40 degrees for the patient justifies the operation should be examined in each individual case. I used a CapFlex prosthesis with a musician who had an almost stiff, nonpainful PIP joint of the index finger due to degenerative osteoarthritis. After surgery the patient was able to play again her clarinet due to the improvement of motion, so that she was satisfied with the treatment. However, these are individual decisions!

14.4 Results in the Literature

Since the CapFlex prosthesis is still quite new on the market and no aggressive but cautious introduction was made, only a few studies and results are available. The greatest experience for the CapFlex prosthesis is certainly in the Schulthess Klinik in Zurich (Switzerland), where the prosthesis was codeveloped. A publication from 2017 refers to 50 cases with a follow-up of 1 year. The motion of the PIP joint improved during this period from 43.4 to 55.9 degrees on average. The pain measured with the VAS (0–10) decreased from 6.5 to 2.2 points. There was one revision surgery due to radial collateral ligament instability and four secondary tenolyses. ⁴

In 100 patients with 104 fingers, 1 year after surgery they found decreasing pain at rest from 4.3 to 1.3 on the VAS and during activities from 6.5 before to 2.3 points on the VAS after surgery. A minimal important change (MIC) of 1.2 for pain at rest and 2.8 for pain during activities was estimated for patients being satisfied with the procedure.⁵ The brief MHQ increases after 1 year from 45 to 71 points. ⁵

Comparing the approach using the CapFlex prosthesis 2 years after surgery in 100 patients, they found the best results for the dorsal tendon split approach with increase of active PIP-ROM from 40 to 61 degrees which corresponds to an improvement of 21 degrees compared with the preoperative value. ⁶

14.5 Author’s Own Experience and Preferred Technique

In our department, the CapFlex prosthesis has been implanted since the end of the year 2015. By July 2019, we had implanted 43 prostheses in 38 patients.

14.5.1 Author’s Preferred Technique

All our patients were operated from a dorsal approach. In my opinion, this gives the best view and allows for accurate implant alignment which is very important to optimize function and avoid instability such as a swan-neck deformity. I use a dorsal, slightly curved off, longitudinal incision centered over the PIP joint. The joint is opened via a longitudinal tendon-splitting incision raising the central slip tendon off the base of the middle phalanx in two halves (Fig. 14.2). The incision begins distally in the tendon-free triangle beyond the central slip insertion and runs proximally to the middle of the proximal phalanx. After exposing the joint by reflecting the divided halves of the tendon laterally I remove osteophytes to better define the joint margin which needs to be exposed fully especially volarly. Around 3 to 4 mm of the head of the proximal phalanx must be resected so the distal part of the origin of the collateral ligaments must be released carefully with a scalpel. A small elevator is now used to create a blunt canal volar to the proximal phalanx, effectively deep to the volar plate, to insert the saw gauge for head resection. The extent of bony resection can be set in mm, typically 3 to 4 mm. The saw cut should be performed perpendicular to the long axis of the proximal phalanx (Fig. 14.3) if anything aiming slightly volar to help optimize PIP joint flexion; a dorsally angulated cut may increase the risk of a swan-neck deformity.

**Fig. 14.2** Dorsal approach with central split of the extensor tendon.

**Fig. 14.3** Resection of 3 to 4 mm of the head of the proximal phalanx using the adjustable saw guide.

The size of the proximal component is then determined. In order not to irritate the collateral ligaments, a “as small as possible” component is preferred. An appropriate instrument allows for sizing and creation of small holes for the implant ministems. After that a 45-degree volar and dorsal angulated saw cut are made with the help of another saw guide to accommodate the shape of the proximal implant (Fig. 14.4). Because there is limited bone these cuts need to be performed with great care, not least to avoid introducing a rotational deformity. The corresponding trial prosthesis can then be inserted. Like the subsequent prosthesis, it is hammered in place and confirmed with perioperative radiographs. If the first is not optimal, it should be corrected. In my experience, a well-aligned proximal implant is extremely important for the later result.

**Fig. 14.4** Saw guide for the 45-degree saw cuts of the head of the prominal phalanx.

The preparation for the insertion of the distal component begins with the planar resection of the joint surface of the base of the middle phalanx. I typically use a ronguer for this step. Next I measure the depth of the distal component (Fig. 14.5). The depth should not be too great leading to tightness, as this increases the risk of swan-neck deformity although it is tempting to put in a thick implant to maximize the stability. The instrument used to determine the height should be placed in the resection space without much tension. Now the size of the distal implant is determined, usually one size larger than the proximal implant. The instrument to measure the size of the implant aids preparation of the three holes for the ministems of the distal component. The appropriate trial component is now hammered into the base of the middle phalanx (Fig. 14.6) and is checked radiologically. Once satisfied, the definitive prostheses are hammered into place for a press-fit (Fig. 14.7).

**Fig. 14.5** Measuring the height of the distal component.

**Fig. 14.6** **(a-c)**Inserting the distal trial component.

**Fig. 14.7** After replacement with the original prosthesis components.

If the collateral ligament is loose on one side, as is typical if there is preoperative coronal deviation of the PIP joint, the lax collateral ligament is tightened with a suture. I insert an “X”-shaped 4 × 0 PDS suture centered on the PIP joint line (Fig. 14.8). This technique allows the ligament to be tensioned without restricting the range of motion. I use a continuous 4 × 0 PDS crisscross suture to appose the two halves of the extensor mechanism. I usually release the tourniquet and wait for some minutes until most of the bleeding stops, helped by electrocautery aiming to minimize the postoperative swelling. The operated finger is immobilized with a neighboring finger on an “intrinsic plus splint.”