Distal Radioulnar Joint Prosthesis

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RATIONALE AND BASIC SCIENCE PERTINENT TO THE PROCEDURE

Two primary forces act on the forearm: the force of gravity and axial load. Axial load is represented by many tasks, including pushing and gripping. The function of the forearm is the transfer of loads while changing position in space. One component of this function is the gripping of an object. To grip, the extrinsic muscles of the hand contract, creating intrinsic axial load. This load passes from the carpus to the radius and on to the humerus. A second component of function is the passing of extrinsic axial loads. These are represented by actions such as pushing to open a door. These loads are supported by the hand and are transferred to the radius through the carpus. The radius, supported by the ulna, transfers the axial load onto 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 throughout its range of motion, including 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.

∗ This adapted chapter has been previously published in Fractures and Injuries of the Distal Radius and Carpus by David J. Slutsky and A. Lee Osterman.

The radius and ulna are connected through the bicondylar radioulnar joint. This consists of two hemi-joints: the proximal radioulnar joint and the distal radioulnar joint (DRUJ). The focus of this chapter is the DRUJ. In addition to trauma, dysfunction of the DRUJ can be the result of congenital abnormalities, degenerative arthritis, inflammatory arthritis, and neoplasm. It must also be noted that damage to the proximal radioulnar joint, with nonreparable fractures leading to excision of the radial head, allows proximal migration of the radius. The proximal migration affects the function of the distal radioulnar joint, eventually leading to its disruption. 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 distal radius fractures. These fractures are common, accounting for 60% of all fractures treated in the emergency department.

Essex-Lopresti and Galeazzi fractures always involve the distal radioulnar joint. Whenever possible, anatomic structures of the DRUJ should be repaired to allow a stable, congruent joint. In fractures, the malunion may be corrected, and, if necessary, the ulna may be shortened to change contact between the sigmoid notch and the seat of the ulna. Ulnar shortening may also help to correct some element of ulnar impaction. Shortening the ulna by 2.5 mm in cases of localized post-traumatic and degenerative DRUJ arthritis, as well as in some instances of instability with grinding, has been an effective treatment. The shortened ulna changes the point of contact between the sigmoid notch and the seat of the ulna, aligning good cartilage while also tensioning the triangular fibrocartilage (TFC), improving stability. In cases of instability without loss of cartilage or grinding, reconstruction of the ligaments of the TFC may restore the integrity of the DRUJ.

Mechanical derangement of the distal radioulnar articulation has serious effects on its biomechanical properties. The etiologic spectrum for this derangement can range from early arthritic erosion to the mutilation that accompanies resection of part or all of the distal ulna. The latter situation, as encountered after Darrach, Sauvé-Kapandji, Bowers, Watson, and wide excision of the distal ulna procedures, can lead to conditions known as ulnar impingement and/or ulnar subluxation. These conditions occur because the support once provided by the ulnar head to the radius is no longer present. Without support, the distal end of the radius moves free of the distal ulna, falling against the ulnar shaft or subluxing volarly or dorsally while the ulna is unable to restrain its motion. 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 opposite is true.

The ulna is the support of the radius, and, consequently, loss of distal support results in the radius “dropping” and impinging on the distal end of the resected ulna. This is particularly enhanced with weight bearing in the neutral position. With an intact DRUJ, 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.

Absence of an intact DRUJ, as occurs after any resection procedures involving the head of the ulna, causes the radius to make contact with the remaining distal end of the ulna and “hitch a ride” at this point. Radioulnar impingement may not be evident on regular posteroanterior radiograms, since the forearm is resting on the x-ray table and therefore is not loaded in this position. A horizontally 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. 33-1 ). Procedures describing wider excision of the ulna only exacerbate this impingement and subluxation as the shortened lever arm of the ulna meets the radius with greater force.

With this understanding of the anatomy of the ulna and its relation to the radius, it is not surprising that ulnar head replacements or hemiarthroplasties are unable to provide an adequate transfer of force when the forearm is loaded. These implants—whether unipolar or bipolar—must rely on soft tissues to maintain their reduction. Relying on biologic soft tissues to restrain nonbiologic implants has seen questionable results at best. The unipolar implants suffer another drawback if they remain constrained in the proper anatomical location: with time they tend to wear into the opposing surface, causing pain.

Considering the concerns listed above, a prosthesis that replaces the entire DRUJ, fulfilling the joint’s functions of transmitting forces during lifting as well as permitting pronosupination, has been devised. The Scheker DRUJ prosthesis (APTIS, Louisville, Kentucky) has been designed with the specific intention of addressing the issues of transmission of lifting forces as well as permitting stable pronosupination while allowing the radius its normal migration against the ulna. This is accomplished by replacing the function of the sigmoid notch, the ulnar head, and the stabilizing characteristics of the TFC. Replacing all three of the critical aspects of the DRUJ eliminates the guesswork that follows soft tissue and partial replacement procedures. This also allows a much quicker return of pronosupination and the stresses of bearing of weight. Total prosthetic joints in general have been considered a last resort after all other options, owing to their destructive nature on the anatomy involved. If this total joint ever requires removal, the anatomy will be left as if one of the aforementioned soft tissue procedures had been performed.

INDICATIONS

The Scheker DRUJ prosthesis may be considered for a wide range of indications.

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Patients who experience pain and weakness in the DRUJ that are not improved by nonoperative treatment
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Patients with an unreconstructible DRUJ due to irreducible instability of the ulnar head with radiographic evidence of erosive changes
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Severe fractures of the ulnar head or neck not allowing good reduction and proper fixation
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Fractures of the distal radius involving the sigmoid notch with loss of articular surface
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Loss of cartilage involving the ulnar head or sigmoid notch secondary to disease or chronic instability
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Failed ulnar head arthroplasties including both soft tissue and implant

Bone stock must be of good quality with no history of infection in the area, no systemic disease, and no allergy to nickel.

Ideal Candidate; Age; Timing; Time Limits

Patients may receive the prosthesis at any time after the skeleton is mature; the timing depends on initial injury and 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. The bone stock and the patient’s ability to comply are the determining factors concerning the upper limits of age.

CONTRAINDICATIONS

Severe osteoporosis, unresolved osteomyelitis, and systemic disease that debilitate the patient both physically and mentally contraindicate the use of the prosthesis. The use of the implant also is contraindicated when bone, musculature, tendons, or adjacent soft tissue is compromised by disease or infection and would not provide adequate support or fixation for the prosthesis. The implant should not be used in patients who have not reached skeletal maturity or who do not have the ability to comply with the rigors of having a total joint implant.

PREOPERATIVE PLANNING

Preoperative graded radiographs are used with measuring templates to determine the size of the prosthesis most likely needed ( Fig. 33-2 ). Digital radiographs can be used if they have accurate measuring capability. By measuring the narrowest intramedullary diameter of the ulna in its distal 11 cm, the ulnar stem size is determined. A size is chosen that fits this intramedullary diameter best, leaving behind a minimum of 2 mm of cortical bone surrounding the stem. The length of the stem’s extraosseous neck, if one is needed, can also be determined preoperatively ( Fig.33-3 ). The template also allows determination of positioning for the radial plate and ulnar stem before the procedure.

SURGICAL TECHNIQUE

Device Description

The DRUJ prosthesis is a semiconstrained ball-and-socket joint comprising a radial component and an ulnar component ( Fig. 33-4 A and B).

The radial component 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 3.5-mm cortical screws. The hemi-socket part of the plate is directed ulnarward and is designed to receive the ultra-high–molecular-weight polyethylene ball of the ulnar component. This recreates the relation of the sigmoid notch to the ulnar head. The other half of the socket, a cover, is fixed to its counterpart, the plate, by means of two screws. This assembly encloses the ultra-high–molecular-weight polyethylene ball. When the cover is secured, it recreates the function of the TFC by preventing the ball-and-socket interface from dislocating. 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 an ultra-high–molecular-weight polyethylene ball make up the ulnar component. The ball is placed on the distal end of the stem; this combination replaces the function of the articular surface of the ulnar head. The ulnar stem measures 11 cm in interosseous length, and the distal third is titanium 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 prevent rotation inside the ulna and to provide stability and ease of insertion, respectively. The most distal end of the stem bears a highly polished peg or pivot, which fits into the hole of the ultra-high–molecular-weight polyethylene ball. The stems are provided in two diameters, 4.5 mm and 5.0 mm, and have correspondingly sized ultra-high–molecular-weight polyethylene balls. Each size couples with the small and large radial components, respectively.

Fully assembled, this prosthesis is semiconstrained. The device allows a full range of pronosupination while preventing any possible dislocation. This is accomplished by the sliding ball-and-socket design. At no time does the prosthesis meet a stopping point, thus preventing the fatigue seen in other implant designs. The patient’s anatomy, not the device, defines full pronation and supination.

Instrumentation and Patient Position

The procedure should be treated as a total joint replacement by using all accepted precautions and methods of prepping and draping for a total joint replacement. The procedure can generally be accomplished under axillary block. Steri-Drape Wound Edge Protector (3M, St. Paul, Minnesota) or other protective plastic is recommended to reduce contact between the skin and the implant. A tourniquet is always used with a pressure setting of about 250 mm Hg (approximately 100 mm Hg higher than the patient’s systolic pressure). The patient is positioned supine with the extremity placed fully pronated on a hand table. If the patient cannot fully extend the elbow or if the shoulder has limited range of motion, the procedure may be accomplished with the forearm pronated in a vertical plane.

Incision Placement and Dissection

With the forearm in full pronation an 8- to 9-cm incision is made above the interval between extensor carpi ulnaris (ECU) and extensor digiti quinti minimi (EDQM) on the dorsal lateral aspect of the distal ulna, turning radially an additional 2 cm just distal to the head of the ulna to form a hockey-stick shape ( Fig. 33-5 ). If the patient has had surgery in the area, the previous incision is incorporated into the exposure. Care should always be taken to protect the dorsal sensory branch of the ulnar nerve. After initial skin incision, a soft tissue flap incorporating both fascia and a 3-mm band of the proximal extensor retinaculum should be planned to later provide a barrier between the prosthesis and the ECU and EDQM tendons. The design of the flap is ulnarly based, extending above the radius and generally rectangular in shape ( Fig. 33-6 ). If the tissues are disturbed from injury or previous surgery, a barrier of some form should be used. Dissection can then follow the interval between the EDQM and ECU. The ECU tendon sheath is released from the ulnar head to the distal insertion of the ECU. Approaching the ulna between the ECU and the EDQM, the EDQM is elevated from the ulna and interosseous membrane. The posterior interosseous nerve is divided as it rests on the interosseous membrane to prevent avulsion from the extensor muscles to the thumb and the index and small fingers. The extensor mass is elevated to expose the interosseous crest of the radius.