15.1 Design Rationale

The PIP-R (MatOrtho, Leatherhead, United Kingdom) is a two-part cobalt-chromium and polyethylene joint replacement for the proximal interphalangeal (PIP) joints of the fingers. It came about as a result of a photographic study of the morphology of cadaveric PIP joints which had been finely sectioned in two planes. This suggested that the effect of the radial or ulna tilt of the condyles coupled with a different radius of curvature in each digit leads to a degree of axial rotation of the middle phalanx relative to the proximal phalanx as the digit flexed. ¹ Other designs of PIP replacement at the time did not allow for this, leading the authors to suggest that this might be the reason for the variability in outcomes following replacement surgery. As a result, the senior author of the paper was involved in the design of the PIP-R.

15.2 Design Features

The implant initially came as three parts: a proximal stem, a distal stem, and the polyethylene insert, which had to be assembled onto the middle phalanx component at the time of insertion. This changed to a preassembled middle phalanx component soon after due to difficulties in carrying out this maneuver in the operating theater under sterile conditions. The implant comes in a box containing both components packed separately. There are five sizes, from a size 7 component on both sides to a size 11 on both sides based on the radio-ulna width of the articular surface. There are “intermediate” combinations allowing for differing sizes on both sides of the joint, with the larger part proximally (7/8, 8/9, 9/10, and 10/11) giving nine possible combinations.

The proximal component has a bicondylar bearing surface with a greater degree of surface anteriorly copying the natural joint to potentially allow over 90 degrees of flexion. It is also flared anteriorly as is the native proximal phalangeal head. The stem is anatomically shaped to fit the medullary canal of the proximal phalanx based on the data from the aforementioned study. The stem is coated in calcium hydroxyapatite to allow for bone ongrowth for long-term stability, and early stability is provided by the press fit into the prepared medullary canal and the cut end of the proximal phalanx (Fig. 15.1).

The distal component has a smaller thinner stem than the other contemporary implants, again based on the morphological study. It is also coated in hydroxyapatite. This component is designed to sit on the subchondral cancellous bone within the cortical rim of the middle phalanx in order to avoid impinging on the collateral ligaments which can be better preserved during surgery. An added benefit of this is that any defect in the middle phalangeal base can be bone grafted, if contained by the cortical margin (Fig. 15.1).

The polyethylene articular surface is contoured to the proximal component providing a large contact area to reduce the contact loads. It also rotates on the middle phalangeal component allowing for the rotation that is seen to occur in the PIP joint during flexion of the digit. It can rotate through 360 degrees but this is not of clinical relevance. This rotation also compensates for any unintentional malrotation of either component at the time of implantation, which may be caused by a previous fracture, etc. (Fig. 15.1).

The implant entered clinical use in 2006, with the first joints being implanted at Wrightington Hospital, United Kingdom by the principal designer. The PIP-R can be inserted by a dorsal, lateral, or volar approach, with the instrumentation working well to prepare the bones via all three approaches. The only exception is that the device used to check the flexion and extension gaps which has a handle dorsally gets in the way of the extensor tendon falling into place in the lateral approach and cannot be used with the volar approach at all.