Distal Radioulnar Joint Arthroplasty: A Masterclass in Ulnar Head Implant Technique

Introduction and Epidemiology

The distal radioulnar joint (DRUJ) is an indispensable component of forearm biomechanics, functioning as a diarthrodial trochoid (pivot) joint that facilitates pronation and supination while transmitting substantial axial and transverse loads from the carpus to the forearm. Like any synovial articulation, the DRUJ is vulnerable to a spectrum of degenerative and destructive processes. These include primary osteoarthritis, inflammatory arthropathies (most notably rheumatoid arthritis), post-traumatic arthrosis secondary to distal radius or ulnar fractures, and chronic instability resulting from untreated triangular fibrocartilage complex (TFCC) injuries.

Historically, the standard surgical interventions for end-stage DRUJ pathology involved partial or complete ablation of the distal ulna. Procedures such as the Darrach distal ulnar resection (first described in 1912), the Sauvé-Kapandji procedure (arthrodesis of the DRUJ with proximal ulnar pseudoarthrosis), and various "matched" hemiresection techniques were widely utilized. While these ablative procedures can successfully alleviate localized articular pain by removing the arthritic joint surfaces, they inherently disrupt the structural integrity and load-bearing capacity of the forearm axis.

The removal of the distal ulna eliminates the bony fulcrum around which the radius rotates. Consequently, the normal compressive muscle forces acting between the radius and ulna—driven primarily by the pronator quadratus and the brachioradialis—cause dynamic radioulnar convergence. When the forearm is rotated under such compressive loads, a palpable and severely painful grinding between the ulnar stump and the distal radius develops. This biomechanical collapse is clinically defined as radioulnar impingement.

Ulnar head implant arthroplasty was conceptualized and developed specifically to address the predictable failures and biomechanical consequences of ablative DRUJ surgery. By restoring the anatomic volume of the distal ulna, the implant acts as a structural spacer that prevents radioulnar convergence and impingement, while simultaneously restoring a stable fulcrum for forearm rotation. Early iterations in the 1970s utilized Swanson silicone caps designed merely to provide a soft buffer over the ulnar stump; however, these universally failed under the physiologic shear and compressive loads of the forearm, leading to particulate synovitis and implant fragmentation.

Contemporary designs utilize modular metallic (cobalt-chrome) or ceramic prostheses that articulate directly with the native sigmoid notch, relying on a meticulously reconstructed soft tissue envelope for stability. The epidemiological burden of DRUJ dysfunction is significant, particularly in the post-traumatic population. Distal radius fractures account for approximately one-sixth of all fractures treated in emergency departments, and up to 30% of these involve the DRUJ or result in altered radioulnar kinematics. Furthermore, the prevalence of rheumatoid arthritis affecting the wrist approaches 70% in chronic cohorts, frequently necessitating surgical intervention at the DRUJ.

As the limitations of the Darrach and Sauvé-Kapandji procedures have become starkly evident in higher-demand patients, the incidence of ulnar head implant arthroplasty has risen. It is now recognized not merely as a salvage procedure for a failed Darrach, but increasingly as a primary intervention for advanced DRUJ arthrosis in patients who require preserved forearm strength and stability.

Surgical Anatomy and Biomechanics

Osteology and Articular Geometry

The DRUJ is formed by the articulation of the convex ulnar head with the concave, shallow sigmoid notch of the distal radius. The articular surface of the sigmoid notch exhibits highly variable morphology—classified by Tolat et al. as flat, ski-slope, C-shaped, or S-shaped—and typically provides poor intrinsic osseous stability. The radius of curvature of the ulnar head is smaller than that of the sigmoid notch. This geometric mismatch dictates that the joint does not function as a simple hinge; rather, it allows for a complex combination of rotation and dorsal-palmar translation during the normal arc of forearm motion.

The ulnar head itself is not perfectly spherical. It possesses a distinct articular surface that extends approximately 105 degrees around its radial margin, interfacing with the sigmoid notch. The distal pole of the ulnar head articulates with the articular disc of the TFCC. The fovea, located at the base of the ulnar styloid, serves as the critical isometric center of rotation and the primary attachment site for the deep radioulnar ligaments.

Ligamentous Stabilizers

Because the osseous architecture provides minimal constraint, the DRUJ relies heavily on its soft tissue stabilizers. The primary stabilizer is the triangular fibrocartilage complex (TFCC), specifically the dorsal and palmar radioulnar ligaments. These ligaments originate from the dorsal and palmar margins of the sigmoid notch and converge to insert on the fovea and the base of the ulnar styloid.

During pronation, the dorsal radioulnar ligament becomes taut, preventing palmar subluxation of the ulnar head relative to the radius. Conversely, during supination, the palmar radioulnar ligament tightens to prevent dorsal subluxation. Secondary stabilizers include the extensor carpi ulnaris (ECU) subsheath, the pronator quadratus, the interosseous membrane (specifically the distal oblique bundle), and the ulnocarpal ligaments (ulnolunate and ulnotriquetral). The ECU subsheath is particularly vital; it acts as a dynamic dorsal restraint, and its preservation or reconstruction is absolutely critical during ulnar head arthroplasty to prevent postoperative dorsal instability.

Forearm Kinematics and Load Transmission

The forearm functions as a closed kinematic chain, articulated proximally at the proximal radioulnar joint (PRUJ) and distally at the DRUJ. The axis of rotation passes from the center of the radial head proximally to the fovea of the ulnar head distally. During normal pronosupination, the radius rotates around the fixed ulna.

Axial load transmission across the wrist is distributed such that approximately 80% passes through the radiocarpal joint and 20% passes through the ulnocarpal joint via the TFCC. However, this ratio is highly sensitive to ulnar variance. A positive ulnar variance of merely 2.5 mm can increase the ulnar axial load transmission to over 40%, accelerating degenerative changes at the DRUJ and ulnocarpal articulation. When the ulnar head is resected (as in a Darrach procedure), this load-bearing capacity is abolished, transferring excessive stress to the radius and the interosseous membrane, ultimately leading to proximal migration of the radius and radioulnar convergence. Ulnar head arthroplasty restores the anatomic length and volume of the ulna, thereby re-establishing the physiological distribution of forces and the mechanical fulcrum necessary for powerful forearm rotation.

Indications and Contraindications

The decision to proceed with ulnar head implant arthroplasty requires a meticulous assessment of the patient's pathology, structural anatomy, and functional demands. The procedure is primarily indicated for patients with end-stage DRUJ arthrosis or instability who have failed conservative management and require preservation of forearm strength and lifting capacity.

Primary Indications

1. Failed Ablative Procedures: This is the most common indication. Patients who have undergone a Darrach resection or Sauvé-Kapandji procedure and subsequently developed painful radioulnar convergence (impingement syndrome) are prime candidates. The implant acts as a spacer to separate the radius and ulna.
2. Primary or Post-Traumatic Osteoarthritis: In cases of severe cartilage loss at the DRUJ with an intact sigmoid notch, hemiarthroplasty can restore smooth kinematics. Post-traumatic cases often involve malunited distal radius fractures that have permanently altered DRUJ congruity.
3. Inflammatory Arthropathy: Rheumatoid arthritis frequently destroys the DRUJ. While Darrach resections were historically favored in low-demand rheumatoid patients, modern ulnar head implants offer superior functional outcomes, provided the soft tissue envelope is adequate for reconstruction.
4. Chronic Irreparable DRUJ Instability: In cases where the TFCC is completely deficient and the joint is severely arthritic, soft tissue reconstruction alone will fail. Arthroplasty combined with ligamentous reconstruction provides a stable fulcrum.

Contraindications

Absolute contraindications include active local or systemic infection, neuropathic arthropathy (Charcot joint), and severe medical comorbidities precluding surgery.

Relative contraindications are heavily dependent on the structural integrity of the surrounding anatomy. A grossly incompetent soft tissue envelope that cannot be reconstructed is a major contraindication, as the prosthesis will inevitably dislocate. Severe, unaddressed radiocarpal arthritis is also a contraindication; if the radiocarpal joint is painful and arthritic, isolated DRUJ arthroplasty will not yield a satisfactory clinical result. Similarly, profound proximal radioulnar joint (PRUJ) pathology must be evaluated, as normal forearm rotation requires two functional articulations. Finally, inadequate ulnar bone stock (e.g., massive proximal resection in a failed Darrach) may preclude the secure fixation of the implant stem.

Operative Versus Non Operative Management Parameters

Pre Operative Planning and Patient Positioning

Thorough preoperative planning is paramount to the success of ulnar head implant arthroplasty. The surgeon must accurately assess the bony geometry, the extent of soft tissue deficiency, and the correct sizing of the implant to restore normal kinematics without overstuffing the joint.

Clinical Evaluation

The physical examination should quantify the arc of pronation and supination, noting any crepitus, mechanical blocks, or pain at the extremes of motion. The "piano key" sign (dorsal prominence and ballotability of the ulnar head) indicates gross DRUJ instability. Grip strength should be measured dynamically using a Jamar dynamometer, as patients with radioulnar impingement typically exhibit a precipitous drop in grip strength when the forearm is fully pronated due to the compressive forces driving the radius into the ulnar stump.

Radiographic and Advanced Imaging

Standard posteroanterior (PA) and lateral radiographs of the wrist and forearm are mandatory. The PA view must be taken with the shoulder abducted 90 degrees, elbow flexed 90 degrees, and forearm in neutral rotation (zero-rotation view) to accurately assess ulnar variance.

Computed Tomography (CT) is highly recommended for preoperative templating. Axial cuts through the DRUJ provide critical information regarding the morphology of the sigmoid notch. If the sigmoid notch is severely eroded, flattened, or dysplastic, a standard hemiarthroplasty may be prone to subluxation, and a total DRUJ arthroplasty (incorporating a radial sigmoid notch component) may be required. CT also aids in assessing the medullary canal diameter of the ulna for stem sizing.

Templating and Implant Selection

Digital or acetate templating is performed on the PA and lateral radiographs. The goal is to select an ulnar head size that matches the native articular volume. Over-sizing the implant will lead to excessive tension on the soft tissue reconstruction, restricted rotation, and accelerated wear of the sigmoid notch. Under-sizing can result in instability and persistent convergence. The stem length and diameter must be planned to ensure adequate diaphyseal engagement, particularly in salvage cases where the proximal ulna may be osteopenic or deformed.

Patient Positioning and Anesthesia

The procedure is typically performed under regional anesthesia (supraclavicular or axillary brachial plexus block) supplemented with intravenous sedation or general anesthesia.
1. The patient is positioned supine on the operating table.
2. The operative extremity is placed on a radiolucent hand table.
3. A well-padded upper arm tourniquet is applied.
4. The arm is prepped and draped in a standard sterile fashion, allowing for full, unencumbered pronation, supination, and elbow flexion.
5. Intraoperative fluoroscopy (C-arm) must be positioned to allow for orthogonal views of the wrist and forearm without compromising the sterile field.

Detailed Surgical Approach and Technique

The surgical execution of ulnar head implant arthroplasty demands meticulous soft tissue handling. The success of the procedure relies as much on the reconstruction of the ligamentous envelope as it does on the precise positioning of the metallic implant.

Surgical Approach and Dissection

A dorsal longitudinal incision is made over the distal ulna, typically centered over the fifth extensor compartment, extending from the ulnar styloid proximally for approximately 6 to 8 centimeters. The subcutaneous tissues are sharply dissected, taking great care to identify and protect the dorsal sensory branches of the ulnar nerve, which frequently cross the operative field from volar to dorsal.

The extensor retinaculum is exposed. The fifth extensor compartment is opened longitudinally, and the extensor digiti minimi (EDM) is retracted radially. The sixth extensor compartment, containing the extensor carpi ulnaris (ECU), is identified. It is absolutely critical to preserve the ECU subsheath. The subsheath is a distinct fibrous tunnel beneath the retinaculum that stabilizes the ECU tendon; violating it will lead to postoperative ECU subluxation and loss of a primary dynamic stabilizer of the DRUJ. The capsule of the DRUJ is incised longitudinally, creating dorsal and palmar flaps for later repair.

Osteotomy and Joint Preparation

In cases of primary arthrosis, the ulnar head is exposed. An osteotomy is planned at the anatomic neck of the ulna. The level of the cut is determined by the specific implant system being utilized, often referencing the distal margin of the sigmoid notch. The osteotomy is performed using an oscillating saw under continuous saline irrigation to prevent thermal necrosis.

In salvage cases (failed Darrach), the distal ulnar stump is identified. It is frequently encased in dense, scarred pseudocapsule. This scar tissue must be carefully excised to mobilize the stump and expose the medullary canal, while preserving any remnants of the TFCC or ulnocarpal ligaments distally.

Medullary Canal Preparation and Trialing

The ulnar medullary canal is sequentially broached and reamed to accommodate the implant stem. The canal must be prepared in the correct rotational alignment. Most modern systems utilize a non-cylindrical stem (e.g., fluted or rectangular) to provide rotational stability.

Once the canal is prepared, trial components are inserted. The trial head is placed onto the stem, and the joint is reduced. Fluoroscopy is utilized to confirm that the ulnar variance is neutral relative to the radius and that the implant is centered within the sigmoid notch. The forearm is taken through a full arc of pronation and supination. The surgeon must meticulously assess for:
1. Impingement: Does the implant abut the radius smoothly, or is there catching?
2. Tracking: Does the head remain engaged in the sigmoid notch throughout the arc of motion?
3. Stability: Is there excessive dorsal or palmar translation?
4. Tension: Can the capsular flaps be approximated over the implant without excessive tension?

Implant Insertion

Following satisfactory trialing, the definitive components are selected. The stem may be press-fit or cemented, depending on bone quality and implant design. In cases of severe osteopenia or an extensively widened canal from a prior failed procedure, polymethylmethacrylate (PMMA) bone cement is utilized to ensure rigid fixation. The definitive modular head is then impacted onto the Morse taper of the stem.

Soft Tissue Reconstruction

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Clinical & Radiographic Imaging