CHAPTER 37 Distal Radioulnar Joint Prosthesis
The forearm has two main forces acting on it: axial load and the force of gravity. Axial load can be represented by a multitude of tasks, including pushing and gripping. Most of the forearm’s function can be separated into two components: gripping and load transfer. To grip, the extrinsic muscles of the hand contract, creating axial load passing from the carpus to the radius and to the humerus. Loads are transferred from the hand to the radius and, while supported by the ulna, on to the humerus. These two functions are completely different, and yet they combine to form the normal function of the forearm. The ulna, acting as the axis of the forearm, supports the radius through both flexion and extension of the elbow. The support provided by the ulna allows the radius to rotate in space as it transfers axial load to the humerus.1,2
The two bones of the forearm are connected through the radioulnar joint. This bicondylar joint is divided into two parts: the proximal radioulnar joint and the distal radioulnar joint. The focus of this chapter is injury sustained over the distal radioulnar joint (DRUJ). Other than trauma, dysfunction of the DRUJ can be the result of congenital abnormality, degenerative arthritis, inflammatory arthritis, and neoplasm. It must also be noted that excision of the radial head, which allows proximal migration of the radius, will affect the function of the DRUJ, eventually leading to its destruction. All these conditions can create mechanical derangement of the distal radioulnar articulation with serious effects on its biomechanical properties. The DRUJ is involved in approximately 30% of all the distal radius fractures and in all of the Essex-Lopresti and Galeazzi fractures. Distal radius fractures are common, accounting for 60% of all fractures treated in the emergency department.3
When anatomical structures of the DRUJ can be repaired to allow a stable, congruent joint, all efforts should be made to do so. In fractures, the malunion may be corrected4,5 and, if necessary, the ulna may be shortened to change contact between the sigmoid notch and the seat of the ulna, which may help to correct some element of ulna impaction.6 Post-traumatic and degenerative DRUJ arthritis, as well as some instances of instability with grinding,7 may be corrected by shortening the ulna by 2.5 mm. The shortened ulna changes the point of contact between the radius and ulna head while also tensioning the triangular fibrocartilage (TFC), which improves stability. In cases of instability without grinding, reconstruction of the ligaments of the TFC may restore function of the DRUJ.8
Mechanical derangement of the distal radioulnar articulation has serious effects on its biomechanical properties. The etiological spectrum for this derangement can range from early arthritic erosion to the mutilation that accompanies resection of part or all of the distal ulna.9 The latter situation, as encountered after Darrach, Watson, Bower, and Sauvé-Kapandji procedures, can lead to a condition known as “ulnar impingement.” This condition occurs because the now unsupported distal end of the radius falls against the adjacent ulnar shaft. It is important to note at this stage that the term “ulnar impingement” conjures up the picture of an ulna that moves and impinges against the radius, but, in reality, the contrary is true.10
The ulna is the support of the distal radius and, consequently, loss of this support results in the radius “dropping” and impinging on the distal end of the resected ulna. This is particularly enhanced during lifting of weight in the neutral position. The distal end of the radius rides on top of the head of the ulna while lifting objects with the forearm in the neutral position.11–13
Absence of an intact DRUJ, as occurs after any of the previously mentioned resection procedures, causes the radius to make contact with the distal end of the ulna and “hitch a ride” at this point. This impingement may not be evident on regular posteroanterior radiographs, because the forearm is not loaded in this position. A laterally shot posteroanterior view obtained when the person is holding a weight against gravity with the forearm in the neutral position characteristically brings out the impingement (Fig. 37-1). With this understanding of the anatomy of the ulna, it is not surprising that ulnar head replacements or hemi-arthroplasties are unable to provide an adequate transfer of force when the forearm is in the loaded position.
Patient Selection
Ideal candidates are those individuals in whom the DRUJ cannot be reconstructed. Other requirements are good bone stock with no history of infection in the area, no systemic disease, and no allergy to nickel. Patients may receive the prosthesis at any time after the skeleton is mature; the timing depends on the individual. In some cases, an individual may sustain a fracture that destroys the DRUJ and it is possible to do an immediate reconstruction. However, because it is not yet available in every institution, a delay may be required to obtain the prosthesis.
Surgical Technique
The Prosthesis
The DRUJ prosthesis is a semi-constrained ball-and-socket joint comprising a radial component and an ulnar component (Fig. 37-2).
The radial component (Fig. 37-3) provides the socket for the joint and consists of two parts that are assembled intraoperatively. The main part is shaped in the form of a plate with a hemi-socket on the distal end. The body of the plate, with its five screw holes, is contoured to fit against the distal 6 to 7 cm of the interosseous crest, in the area of the sigmoid notch. The plate is fixed to the radius by two means. The first is a peg that is driven into the distal radius in an ulnoradial direction, whereas the second method of fixing the plate involves the use of five specially designed 3.5-mm cortical screws. The hemi-socket, which is part of the plate, is directed ulnarward and is designed to receive the ultra-high-molecular-weight polyethylene ball (UHMWP) of the ulnar component. The other half of the socket, a cover, is separate from the radial component and is fixed to its counterpart on the plate by means of two screws. This assembly encloses the UHMWP ball. The radial component is available in two sizes, small (size 20) and large (size 30), which fit with the corresponding sizes of the ulnar component.
A fluted stem and the UHMWP ball (Fig. 37-4) make up the ulnar component. The ball is placed on the distal end of the stem; this combination replaces the articular surface of the ulnar head. The ulnar stems measure 11 cm in interosseous length, and the distal one third is plasma coated to allow bony ingrowth. A gentle flare at its distal end is present to provide better fixation. The stem is also fluted and slightly tapered to provide rotatory stability and ease of insertion, respectively. The most distal end of the stem bears a highly polished peg or pivot that fits into the hole of the UHMWP ball. The stems are provided in two diameters, 4.5 mm and 5.0 mm, and have correspondingly sized UHMWP balls. Each size couples with the small and large radial components, respectively.
FIGURE 37-4 The ulnar component comprises a fluted stem and an ultra-high-molecular-weight polyethylene (UHMWP) ball.
Preoperative graded radiographs are used to determine whether the large or small prosthesis is most likely needed (Fig. 37-5). Digital radiographs can be used if they have accurate measuring capability. The ulna stem size is determined by measuring the narrowest intramedullary diameter of the ulna in its distal 11 cm. A size is chosen that fits this intramedullary diameter best, leaving behind a minimum of 2 mm of cortical bone surrounding the flare of the distal stem. The length of the stem’s extraosseous neck, if one is needed, can also be determined preoperatively with the help of a radiographic measuring template (Fig. 37-6).
