The “first choice” partial ulnar head prosthesis has been designed to match as best as possible to the shape and the biomechanical properties of the native ulnar head. In contrast to the total ulnar head implants, it requires only minimal bone resection and hereby preserves the soft tissue stabilizers of the distal radioulnar joint (DRUJ). On the other hand, the indication is limited to primary salvage procedures of the DRUJ. The surgical technique for the insertion of this one-piece implant is less forgiving than that for the total head prostheses as it requires an oblique and curved osteotomy at a predetermined level and, more importantly, at the anatomically correct rotatory orientation. This defines the position of the prosthesis and its biomechanical behavior. The results of this procedure compare favorably with total ulnar head replacements. Our experience in 21 cases has been free of complications and shows better pain relief and strength than that with ulnar head replacements, probably due to the improved stability of the DRUJ, however, at the expense of some of the range of motion, namely supination. This needs further attention in future, but we consider the benefits of this prosthesis outweigh the disadvantages.
Key wordsdistal radioulnar joint – arthroplasty – prosthesis – resurfacing
32 First Choice Distal Radioulnar Joint Arthroplasty
Most salvage procedures of the distal radioulnar joint (DRUJ) include the ablation of the ulnar head, e.g., the Darrach’s procedure, hemiresection arthroplasty, or Sauvé-Kapandji. With the absence of the ulnar head or part of it, and due to the complete absence of separating forces, the residual distal end of the ulna will press upon the radius—this convergence may be markedly symptomatic, but in most patients it is not. Moreover, this convergence will lead to laxity of the radioulnar ligaments causing sagittal as well as coronal instability.
Replacement of the ulnar head was designed to address this and provide a more anatomical reconstruction of the DRUJ. Following failure of silicone ulnar head replacements 1 , metallic or ceramic replacements were introduced. 2 Biomechanical testing confirmed the prevention of convergence of the distal radius and ulna 3 and clinical testing showed reliable outcomes. 2 Nonetheless ulnar head replacements are not designed anatomically: prosthetic ulna heads are effectively spheres with the ends removed, designed to lie along the axis of the ulna. Ulna head prostheses are not stabilized by ligaments attaching to them, but by encapsulation creating a soft tissue envelope around the implant. If the soft tissue envelope is inadequate there may be symptomatic DRUJ instability. In contrast, in normal anatomy the sphere of the ulnar head lies eccentric to the long axis of the ulna. 4 , 5 This is necessary to correctly tension the radioulnar ligaments and the interosseous membrane in order to maintain the stability of the DRUJ and the forearm unit. The First Choice replacement is an “anatomic” ulnar head designed to lie eccentric to the axis of the ulna and preserve the insertion of the radioulnar ligaments in order to ensure greater DRUJ stability 6 , 7 although in my experience the “critical” foveal attachment of the triangular fibrocartilage complex (TFCC) cannot be preserved during the operative procedure.
32.2 Implant Characteristics
The “First Choice” partial ulnar head prosthesis is a one-piece implant constructed of Cobalt-Chromium alloy. The distal articulating surface is highly polished while the proximal stem surface is roughened with an aluminum oxide grit blast to optimize the chance of bone ingrowth following press-fit fixation (Fig. 32.1). There are four different head sizes (145, 160, 175, 190 mm diameter) and three different stem diameters (45, 55, 65 mm), giving 12 different implants. The head corresponds to 220° (of 360°) of the head circumference with an oblique collar orientation. This sits on an eccentrically placed 5.5 cm long slightly conical stem. The aim of the design is to replace the articulating surface only, sparing the ulnar styloid and its ligament attachment and the sulcus for the extensor carpi ulnaris tendon.
For salvage procedures, there is also a modular standard total head implant with the same stem-design and sizes meaning the same reamers can be used. There are three different collar heights (3, 11, 20 mm), and three differently sized heads (160, 175, 190 mm) with a conical Morse taper coupling. The heads, as in the uHead ulnar head system introduced by Small Bone Innovations (now Stryker), feature holes for the attachment of local soft tissues.
32.3 Surgical Technique
Preoperatively transparent templates are used to assess the appropriate size of the stem and the head (Fig. 32.2). For the latter, if available, axial CT-scan sections can be used for potentially more precise measurement. It is particularly important to measure ulnar variance as this will determine the amount of resection. The surgical aim is for a slightly more negative ulnar variance than normal for the patient.
We prefer to expose the DRUJ capsule through the fifth extensor tendon compartment. A dorsal rectangular capsulotomy is performed proximal to the dorsal radioulnar ligament preserving the TFC insertion to the ulnar styloid and leaving a capsular rim on the dorsum of the sigmoid notch of the radius for later closure. We have found that sparing the foveal attachment of the TFC is not possible: with this structure intact, adequate dislocation of the ulnar head is mostly impossible, and in any case the reamer would damage/destroy this attachment. The ulnar head is dislocated dorsally with the forearm pronated and the wrist flexed. The ulnar head is held displaced with one or two Hohman retractors. When starting to prepare the medullary canal it is important to remember the eccentricity of the ulnar head relative to the medullary canal. In particular, the entrance of the awl should not be in the center of the ulnar head, but more ulnar, almost at the fovea (Fig. 32.3). Care must be given not to advance the correctly sized reamer deeper than based upon the measurements of the preoperative ulnar variance (Fig. 32.4). Next, and in my opinion the most important step is to make a correctly oriented, oblique, incomplete bone cut relative to the long axis of the ulna in contrast to the complete, perpendicular bone cut made for total ulnar head replacements. The resection guide clips onto the reamer in one single position, so the reamer handle needs to be aligned precisely with the long axis of the ulna, which is not that easy to determine; the location of the ulnar styloid to guide the bone cut is critical. The resection guide is secured into the native ulnar head with K-wires and the angled osteotomy is performed paying attention not to overcut the corners, as this risks a distal ulna fracture (Fig. 32.5). After completion of the osteotomy with a longitudinal bone cut preserving the ulnar styloid, the hemiresected section of the ulnar head is removed and used to determine the optimal prosthetic head size. Next, the trial stem and ulnar head are inserted Fig. 32.6. Following reduction of the joint, check the congruity of the head in the sigmoid fossa of the distal radius and the stability of the DRUJ. Sometimes, especially with preoperative positive ulnar variance, the sigmoid fossa needs to be leveled with a large spherical burr. Then the correct position and fit of the trial implant is verified. Once the trial is deemed correct, the trial implant is replaced by the definitive prosthesis (Fig. 32.7, Fig. 32.8); this needs careful positioning and rotational control while hammering it in until there is a good tight fit between the prosthetic head and the ulna. The capsule and the fifth extensor tendon compartment are closed separately with interrupted 3–0 resorbable sutures. Postoperatively, the forearm rotation is held in an above elbow plaster cast in neutral/slight supination for 3 weeks. After this, the plaster cast is removed and the forearm and wrist are mobilized nonweight bearing for another 3 weeks. At 6 weeks if the implant appears stable clinically and radiologically, strengthening and progressive weight bearing are initiated with hand therapy support. Further clinical and radiological follow-up is undertaken at 6 months and 1 year, then 2, 5, and 10 years postoperatively.