Ulna

Original Clinical Contribution: Authored for the OrthopaedicsOne Hand and Wrist Specialty Section. Preserved, digitized, and updated to modern evidence-based standards.

1. Overview & Articular Relationships

The ulna is the medial long bone of the forearm, situated parallel to the radius in the anatomical position. Serving as the primary stabilizing post of the upper extremity, the ulna provides a rigid osseous foundation around which the radius rotates during pronation and supination, while directly transferring axial loads across the forearm and upper limb. The ulna participates in four major anatomical articulations:

• Humero-Ulnar Articulation: A true ginglymus (hinge) joint formed between the spool-shaped humeral trochlea and the deep, semicircular greater sigmoid (trochlear) notch of the proximal ulna. This joint provides intrinsic structural stability to the elbow, allowing a normal flexion-extension arc of 0 degrees to 140 degrees (with a functional arc of 30 degrees to 130 degrees), constrained by a physiological valgus carrying angle averaging 10 to 15 degrees in males and 15 to 20 degrees in females.
• Proximal Radioulnar Joint (PRUJ): A trochoid (pivot) joint formed between the lesser sigmoid (radial) notch on the lateral aspect of the coronoid process and the circumferential articular cartilage rim of the radial head. Encircled and stabilized by the fibrocartilaginous annular ligament, the PRUJ permits axial spin of the radial head during forearm rotation.
• Distal Radioulnar Joint (DRUJ): A pivot joint formed between the convex ulnar head (“seat”) and the concave sigmoid notch of the distal radius. Accommodating a total rotational arc of approximately 150 degrees (80 degrees of pronation and 70 degrees of supination), the DRUJ undergoes dynamic translation, with the ulnar head moving dorsally relative to the radius in full pronation and volarly in full supination.
• Radio-Ulnar Syndesmosis (Interosseous Membrane): An extensive fibrous syndesmosis connecting the interosseous crests of the radius and ulna along their diaphyseal shafts. The central band of the interosseous membrane (IOM)—oriented obliquely from proximal-radial to distal-ulnar at an angle of 21 degrees—is approximately 3.5 times thicker than adjacent membranous tissue and provides critical longitudinal stability, resisting proximal migration of the radius following axial loading or radial head excision.

2. Proximal Ulna Morphology & Functional Landmarks

The proximal ulna is specialized for joint congruency, load absorption, and muscular lever mechanics. It is dominated by two prominent processes—the posterior olecranon and the anterior coronoid—which together form the articular concavity of the elbow joint:

The Olecranon Process

The olecranon process is a massive, curved posterior projection capping the proximal ulna. Its superior and posterior surfaces serve as the broad insertion site for the triceps brachii tendon, creating a substantial mechanical lever arm for active elbow extension. The posterior aspect is entirely subcutaneous and shielded by the olecranon bursa. Anteriorly, its smooth, articular surface forms the superior half of the greater sigmoid cavity. In full elbow extension, the olecranon tip seats snugly into the olecranon fossa of the distal posterior humerus, providing a rigid bony block that prevents physiological hyperextension.

The Coronoid Process

The coronoid process is an anterior triangular bracket projecting horizontally from the proximal ulnar shaft. It serves as the indispensable primary anterior and varus osseous buttress of the elbow. In flexion, its acute beak engages the coronoid fossa on the anterior humerus. Biomechanically, loss of more than 50% of the coronoid height results in gross posterior subluxation of the elbow joint, particularly under axial and rotatory load. The coronoid is subdivided into three distinct anatomic regions:

• Coronoid Tip: Non-articular superior margin where the anterior elbow joint capsule and deep fibers of the brachialis insert.
• Anteromedial Facet: A crucial osseous projection that extends medially and supports the medial trochlear groove. Fractures involving this facet produce severe posteromedial rotatory instability (PMRI) and varus collapse.
• Sublime Tubercle: A prominent osseous ridge situated on the distal margin of the anteromedial facet, serving as the definitive insertion footprint for the anterior bundle of the medial collateral ligament (AMCL), the primary restraint to elbow valgus stress.

Trochlear (Semilunar) Notch & Radial Notch

The greater sigmoid notch (trochlear or semilunar notch) is a deep articular semicircular concavity spanning 180 degrees, bounded superiorly by the olecranon and inferiorly by the coronoid process. It is divided longitudinally by a smooth median ridge that conforms to the central groove of the humeral trochlea. Transversely, the articular cartilage is interrupted by an inconsistent non-articular transverse groove (“bare area”) representing the developmental junction between the olecranon and coronoid ossification centers.

On the lateral aspect of the coronoid process lies the lesser sigmoid cavity (radial notch), a narrow, oblong, concave articular facet lined with hyaline cartilage that articulates with the circumferential rim of the radial head. Directly distal to the radial notch lies the supinator crest and fossa, which anchors the lateral ulnar collateral ligament (LUCL)—the essential primary soft-tissue constraint against posterolateral rotatory instability (PLRI).

3. Muscular & Ligamentous Attachments

The ulna provides extensive origin and insertion footprints for fourteen major muscles of the arm, forearm, and hand, serving as the central anchor for both flexor-pronator and extensor-supinator kinetic chains:

Ligamentous Attachments & Capsular Constraints

In addition to muscular attachments, the ulna serves as the primary osseous anchor for the stabilizing ligamentous complexes of the elbow and wrist:

• Medial Collateral Ligament (MCL / UCL) Complex: The anterior bundle inserts directly onto the sublime tubercle on the anteromedial coronoid process, functioning as the primary static restraint to valgus stress from 20 degrees to 120 degrees of flexion. The posterior bundle inserts broadly along the medial margin of the olecranon, tensioning predominantly in deep flexion beyond 90 degrees. The transverse ligament (Cooper ligament) spans between the olecranon and coronoid, contributing minimal intrinsic stability.
• Lateral Collateral Ligament Complex: The lateral ulnar collateral ligament (LUCL) originates from the lateral humeral epicondyle and inserts onto the crista supinatoris of the ulna. It acts as the primary stabilizer against posterolateral rotatory subluxation. The annular ligament attaches to the anterior and posterior margins of the lesser sigmoid notch, encircling the radial head.
• Triangular Fibrocartilage Complex (TFCC): At the distal ulna, the TFCC suspends the ulnocarpal joint. The deep fibers of the dorsal and palmar radioulnar ligaments (ligamentum subcruentum) insert securely into the ulnar fovea at the base of the styloid process, while the superficial fibers insert onto the ulnar styloid tip and capsule.

4. Shaft Geometry, Surface Anatomy & Biomechanics

The ulnar shaft (corpus ulnae) transitions from a robust, trihedral cross-section proximally to a smooth, slender, cylindrical contour distally. It presents three distinct borders and three surfaces:

• Posterior Border (Margo Posterior): Completely subcutaneous throughout its entire longitudinal length, spanning uninterruptedly from the apex of the olecranon to the tip of the ulnar styloid process. This prominent subcutaneous border is easily palpable along the dorsal forearm, making it a critical anatomical landmark for surgical approaches and physical examination.
• Interosseous Border (Margo Interosseus): A sharp lateral crest providing the continuous linear attachment for the forearm interosseous membrane.
• Anterior Border (Margo Anterior): Rounded margin originating beneath the coronoid process and terminating distally along the base of the ulnar head.

The internal architecture of the ulnar shaft consists of a dense cortical envelope enclosing a narrow medullary canal. Cortical thickness is maximized along the interosseous crest and posterior subcutaneous surface to resist bending and torsional stresses during upper extremity loading. Proximally, dense trabecular arches radiate from the cortical walls into the olecranon and coronoid processes along lines of principal mechanical compression and tension.

Forearm Curvature & Axial Load Transmission

Although historically depicted as a straight osseous strut, anatomical 3D modeling confirms that the ulna exhibits a distinct proximal dorsal bow averaging 4.2 degrees (with its apex directed dorsally at the junction of the proximal and middle thirds), accompanied by a subtle lateral bow. Restoration of these physiological curvatures during fracture reduction is mandatory to avoid rotational impingement against the radial bow during full pronation and supination.

In the neutral wrist position, axial compressive loads across the carpus are distributed disproportionately: the distal radius transmits approximately 80% of the total load, while the distal ulna transmits 20% across the TFCC and ulnocarpal space. However, this load partition is highly sensitive to rotational position and ulnar variance. In full pronation, the ulnar load transmission fraction increases to over 35%. A positive ulnar variance of 2.5 mm increases ulnar load transmission to greater than 40%, whereas negative ulnar variance of 2.5 mm reduces ulnar load to approximately 4%.

5. Distal Ulna & The Ulnocarpal / TFCC Complex

The distal ulna comprises the ulnar head and the styloid process, functioning as the central pivot for forearm rotational mechanics and the suspensory pole for the medial carpus:

• Ulnar Head (“Seat”): A rounded, semi-cylindrical articular prominence covered by hyaline cartilage over a 270-degree arc. Its convex peripheral articular surface articulates with the concave sigmoid notch of the distal radius at the DRUJ, while its distal pole faces the proximal articular disc of the TFCC.
• Ulnar Styloid Process: A non-articular, conical bony projection extending distally from the posteromedial aspect of the ulnar head. It provides attachment for the superficial limb of the radioulnar ligaments, the ulnar collateral ligament of the wrist, and the sheath of the extensor carpi ulnaris (ECU) subsheath within the sixth dorsal extensor compartment.
• Ulnar Fovea: A deep groove or depression located at the base of the styloid process between the ulnar head and the styloid. The fovea represents the true isometric center of rotation of the forearm and anchors the deep radioulnar ligaments (ligamentum subcruentum). Disruption of this foveal insertion produces severe, disabling DRUJ instability.

Ulnar Variance & Clinical Implications

Ulnar variance refers to the relative longitudinal length difference between the articular surfaces of the distal radius and the distal ulna, measured on a standard PA wrist radiograph taken with the shoulder abducted 90 degrees, elbow flexed 90 degrees, and forearm in neutral rotation:

• Neutral Variance: The distal cortical articular margin of the ulnar head is level with the medial cortical corner of the radial sigmoid notch (variance = 0 mm).
• Positive Ulnar Variance: The ulnar head projects distally beyond the radial articular margin. Positive variance increases dynamically during pronation and power grip. Pathologically, chronic positive variance leads to ulnocarpal impaction syndrome (ulnar abutment), characterized by progressive attrition and tear of the central TFCC disc (Palmer Class 2), chondromalacia of the lunate and triquetrum, and lunotriquetral interosseous ligament perforation.
• Negative Ulnar Variance: The ulnar head lies proximal to the radial sigmoid notch. Negative variance increases shear stress across the radiolunate joint and is strongly associated with Kienbock disease (avascular necrosis of the lunate) and scapholunate ligament dissociation.

6. Radiographic Evaluation & Advanced Imaging Parameters

Accurate radiographic and cross-sectional assessment of the ulna requires systematic adherence to established imaging rules and standardized projection parameters:

Plain Radiography Rules

• Rule of Twos (Two Views, Two Joints): Forearm trauma necessitates true AP and lateral views capturing both the elbow and wrist joints on a single long cassette. Any isolated fracture of the ulnar shaft must prompt meticulous scrutiny of the radial head and DRUJ to exclude Monteggia or Galeazzi-equivalent injuries.
• Radiocapitellar Line: On all plain radiographic projections of the elbow (AP, lateral, and obliques), a line drawn through the center of the radial shaft and head must pass directly through the center of the humeral capitellum. Deviation signifies occult radial head dislocation.
• True Lateral Elbow: Obtained with the elbow flexed to 90 degrees and the forearm in neutral rotation. Evaluates trochlear notch congruency, coronoid height, and displacement of anterior and posterior fat pads (the “sail sign,” indicative of acute intra-articular hemarthrosis).
• Standard Neutral PA Wrist: Shoulder abducted 90 degrees, elbow flexed 90 degrees, forearm flat on the cassette in neutral rotation. This standardized view prevents artifactual alteration of ulnar variance (pronation induces apparent positive variance, while supination creates apparent negative variance). A pronated grip view can be obtained to quantify dynamic positive variance under load.

Advanced Cross-Sectional Imaging

• Computed Tomography (CT): Multiplanar 2D and 3D reconstructed CT is the gold standard for characterizing intra-articular proximal ulna fractures, defining coronoid anteromedial facet fragment morphology, and quantifying DRUJ subluxation using the epicenter method or radioulnar ratio.
• Magnetic Resonance Imaging (MRI): High-resolution 3T MRI or direct MR arthrography provides definitive visualization of TFCC tears, distinguishing between peripheral reparable foveal avulsions (Palmer 1B) and central degenerative perforations (Palmer 2), while evaluating tears of the LUCL and intrinsic carpal interosseous ligaments.

7. Physical Examination & Provocative Clinical Tests

Physical examination of the ulna begins with systematic visual inspection and palpation along its entire subcutaneous border, followed by targeted provocative maneuvers:

• Inspection & Alignment: Evaluation of the carrying angle in full extension to detect abnormal cubitus valgus or post-traumatic cubitus varus (“gunstock deformity”). The dorsal wrist is inspected for abnormal ulnar prominence, swelling, or the “piano key” sign seen with dorsal DRUJ dislocation.
• Palpation: Palpation proceeds from the olecranon tip along the entire posterior subcutaneous border down to the ulnar styloid, checking for step-offs, point tenderness, or bursal thickening. The cubital tunnel is palpated posterior to the medial epicondyle to assess for ulnar nerve tenderness or anterior subluxation during flexion.
• Range of Motion: Active and passive arcs must be documented for elbow flexion-extension (normal 0 to 140 degrees) and forearm pronation-supination (normal 80 degrees pronation, 85 degrees supination).

Provocative Clinical Maneuvers

• DRUJ Ballottement / Piano Key Test: The examiner stabilizes the distal radius with one hand while grasping the ulnar head with the other, translating the ulna anteroposteriorly in pronation, supination, and neutral. Excessive translation, painful clicking, or lack of a firm mechanical endpoint denotes complete disruption of the radioulnar ligaments.
• Ulnar Fovea Sign: Direct, firm thumb pressure is applied into the soft-tissue sulcus between the ulnar styloid process, the flexor carpi ulnaris tendon, and the volar surface of the triquetrum. Reproduction of exquisite focal pain demonstrates high diagnostic sensitivity (95%) and specificity (87%) for foveal disruptions of the TFCC or ulnotriquetral ligament complex.
• Ulnocarpal Stress Test: An axial compressive load is applied to the wrist while passively deviating the carpus ulnarly and rotating through pronation and supination. Elicitation of mechanical pain or crepitus indicates ulnocarpal impaction or chondromalacia.
• ECU Synergy Test: The patient rests the forearm supinated on a flat surface and is asked to actively abduct the thumb against manual resistance. This maneuver selectively recruits the extensor carpi ulnaris without wrist extension; reproduction of ulnar-sided wrist pain confirms ECU tenosynovitis or subluxation within the sixth extensor compartment.
• Elbow Stability Tests: The moving valgus stress test and milking maneuver assess anterior bundle MCL integrity at the sublime tubercle, while the lateral pivot-shift test and posterolateral rotatory drawer test evaluate LUCL competence at the supinator crest.

8. Embryology & Developmental Anatomy

The embryological morphogenesis and ossification timetable of the ulna follow a highly regulated spatiotemporal cascade:

• Embryonic Origin: The upper limb bud emerges from the lateral plate somatic mesoderm at approximately the fourth gestational week under the signaling control of the apical ectodermal ridge (AER) and zone of polarizing activity (ZPA). By the sixth week, mesenchymal condensation forms a continuous chondrogenic model of the radius and ulna.
• Primary Ossification Center: Endochondral ossification initiates centrally within the ulnar diaphysis during the eighth gestational week, advancing bidirectionally toward the proximal and distal cartilaginous epiphyses.
• Proximal Secondary Ossification Center (Olecranon): A secondary traction apophysis appears within the cartilaginous olecranon between 9 and 10 years of age. Fusion with the ulnar shaft occurs between 15 and 17 years. Occasionally, a separate accessory ossification center fails to fuse, persisting into adulthood as a benign bipartite olecranon (patella cubiti) that must not be confused with an acute fracture.
• Distal Secondary Ossification Center (Head and Styloid): The distal epiphyseal center appears between 5 and 6 years of age and fuses with the shaft between 17 and 20 years. This distal physis is responsible for approximately 80% of total longitudinal ulnar growth (and approximately 40% of overall forearm longitudinal elongation).

9. Congenital & Developmental Anomalies

Developmental disruptions during embryonic limb budding produce characteristic ulnar dysplasias and longitudinal deficiency syndromes:

• Ulnar Longitudinal Deficiency (Ulnar Hemimelia): A postaxial longitudinal deficiency occurring in approximately 1 in 100,000 live births, characterized by partial or complete hypoplasia of the ulna. Classified into four morphological types (Bayne and Flatt classification): Type I (mild ulnar hypoplasia with both physes present), Type II (partial ulnar aplasia with distal fibrocartilaginous anlage tethering, the most common variant), Type III (total absence of the ulna), and Type IV (radiohumeral synostosis). The unyielding fibrocartilaginous ulnar anlage acts as a rigid tether, causing severe radial bowing, carpal coalition, absent postaxial digits (ring and small fingers), and progressive radial head dislocation.
• Madelung Deformity: A developmental growth arrest of the volar and medial aspect of the distal radial physis, frequently associated with Leri-Weill dyschondrosteosis and SHOX gene haploinsufficiency. A thick, anomalous fibrous band—the Vickers ligament—tethers the volar-ulnar distal radius to the lunate, producing pronounced volar tilt and inclination of the distal radius. This produces relative distal ulnar overgrowth, prominent dorsal displacement of the ulnar head, and secondary DRUJ dislocation.
• Multiple Hereditary Exostoses (MHE): Autosomal dominant disorder linked to EXT1 and EXT2 mutations. Juxta-physeal osteochondromas frequently arise at the distal ulna, causing tethering and premature physeal arrest. Ulnar shortening leads to secondary radial bowing, carpal slip, and progressive radial head dislocation (Masada classification Types I-III).

10. Traumatic Injuries & Pathologic Conditions

The ulna is subject to high-energy fracture-dislocations, direct blunt trauma, and chronic overuse syndromes across all demographic populations:

Olecranon Fractures

Olecranon fractures result from direct impaction onto the flexed elbow or eccentric avulsion during forceful triceps contraction. Disruption of the articular surface alters the greater sigmoid notch, producing extensor mechanism failure and articular incongruity. The Mayo Classification guides clinical management:

• Mayo Type I (Non-Displaced): Type IA (non-comminuted) and Type IB (comminuted). Managed nonoperatively with immobilization in 45 to 60 degrees of flexion, progressing to early protected active motion.
• Mayo Type II (Displaced, Stable Elbow): Type IIA (simple transverse or oblique non-comminuted) and Type IIB (comminuted). The elbow joint remains intrinsically stable without associated subluxation. Type IIA fractures are traditionally managed with tension band wiring (TBW) using Kirschner wires or modern cable-pin systems, converting triceps tensile forces into compressive articular forces. Type IIB comminuted fractures require pre-contoured anatomical locked plate osteosynthesis to avoid articular collapse.
• Mayo Type III (Displaced with Elbow Instability): Fractures accompanied by humeroulnar dislocation or subluxation (trans-ulnar fracture-dislocation). These represent severe high-energy patterns requiring rigid anatomic locked plate fixation, often combined with coronoid or collateral ligament reconstruction.

Coronoid Process Fractures

Coronoid fractures are critical hallmarks of elbow instability, commonly occurring in terrible triad injuries (coronoid fracture, radial head fracture, and elbow dislocation) or varus posteromedial rotatory instability (PMRI). The Regan-Morrey classification categorizes fractures by height (Type I tip <10%, Type II 10% to 50%, Type III basal >50%). The anatomically oriented O’Driscoll classification provides greater prognostic utility:

• O’Driscoll Type 1 (Tip Fractures): Involves the transverse tip, subdivided into Subtype 1 (≤2 mm) and Subtype 2 (>2 mm). Typically avulsed by the anterior joint capsule during posterior elbow subluxation.
• O’Driscoll Type 2 (Anteromedial Facet Fractures): Subdivided into Subtype 1 (anteromedial rim), Subtype 2 (rim and tip), and Subtype 3 (rim and sublime tubercle). Highly unstable; under physiological varus loading, loss of this medial buttress allows the medial trochlea to drop into the defect, producing progressive PMRI and rapid post-traumatic osteoarthritis. Demands rigid buttress plating or retrograde screw fixation.
• O’Driscoll Type 3 (Basal Coronoid Fractures): Fracture through the base involving >50% of coronoid body, producing gross posterior humeroulnar dislocation requiring stable plate osteosynthesis.

Monteggia Fracture-Dislocations

The Monteggia lesion is defined as a fracture of the ulnar shaft accompanied by dislocation of the proximal radioulnar joint (radial head). The Bado Classification categorizes these lesions based on the direction of radial head dislocation:

• Bado Type I (Anterior Dislocation, 60%): Anterior dislocation of the radial head with anteriorly apexed fracture of the ulnar diaphysis.
• Bado Type II (Posterior Dislocation, 15%): Posterior or posterolateral dislocation of the radial head with posteriorly apexed ulnar fracture. Frequently associated with coronoid fractures and poor clinical prognosis.
• Bado Type III (Lateral Dislocation, 20%): Lateral or anterolateral dislocation of the radial head with ulnar metaphyseal fracture; highly prevalent in pediatric cohorts.
• Bado Type IV (Both Bones Fracture, 5%): Fracture of both radius and ulna with anterior dislocation of the radial head.

The cardinal surgical principle in Monteggia injuries is rigid anatomical reduction and compression plating of the ulna, restoring length, alignment, and the proximal dorsal bow. In more than 90% of acute cases, accurate anatomical reduction of the ulna achieves spontaneous, concentric reduction of the radial head.

Isolated Shaft (“Nightstick”) Fractures & Pathologies

Isolated fractures of the ulnar shaft classically occur when an individual raises their arm to parry a direct blow (“nightstick” mechanism). Nondisplaced or minimally displaced fractures (<50% displacement, <10 degrees of angulation) can be managed nonoperatively in a functional forearm brace. Displaced fractures require open reduction and internal fixation (ORIF) with a 3.5 mm dynamic compression plate to eliminate rotational deformity and prevent symptomatic nonunion.

Additional common clinical pathologies involving the ulna include:

• Ulnar Impaction Syndrome: Progressive ulnocarpal abutment in positive ulnar variance, managed surgically by ulnar shortening osteotomy (USO) or arthroscopic wafer resection of the distal 2 to 3 mm of the ulnar dome.
• DRUJ Instability & Galeazzi-Equivalent Lesions: Disruption of the distal radioulnar joint resulting from TFCC foveal avulsion or ulnar styloid base fracture, requiring foveal transosseous re-anchoring or locked styloid osteosynthesis.
• Olecranon Bursitis: Acute or chronic inflammation of the subcutaneous bursa overlying the olecranon tip, managed by aspiration, bursal compression, and targeted antibiotic therapy for septic presentations.

11. Evidence-Based Synthesis

Modern clinical decision-making is grounded in high-quality randomized controlled trials and prospective registry analyses, corroborated by seminal biomechanical and anatomical descriptions.

12. References

• Nieboer MJ, Cancio-Bello A, Mallett KE et al. Trans-ulnar fracture dislocations of the elbow: a systematic review and clarification of classification systems. J Shoulder Elbow Surg. 2024. doi:10.1016/j.jse.2023.10.014 [Level I: Systematic Review / Meta-Analysis]
• Joshi MA, Le M, Campbell R et al. Surgery for Olecranon Fractures in the Elderly (SOFIE): Results of the SOFIE Randomized Controlled Trial. J Bone Joint Surg Am. 2025. doi:10.2106/JBJS.24.00655 [Level I: Randomized Controlled Trial]
• Rao C, Hu H, Tan R. Comparative Analysis of Clinical Effects of the Cable-Pin System and Kirschner Wire Tension Band (TBW) Internal Fixation in the Treatment of the Olecranon Fracture. Contrast Media Mol Imaging. 2022. doi:10.1155/2022/3073121 [Level I: Randomized Controlled Trial]
• Erpala F, Ozturk T. “Snapping” of the extensor carpi ulnaris tendon in asymptomatic population. BMC Musculoskelet Disord. 2021. doi:10.1186/s12891-021-04271-z [Level I: Randomized Controlled Trial]
• Liang C, Li CY, Qin XS et al. Biomechanical comparison of the new cross-locking intramedullary nail with tension band wiring for transverse olecranon fractures. Injury. 2023. doi:10.1016/j.injury.2022.07.010 [Level I: Randomized Controlled Trial]

13. Master Class Pro-Tip

When performing internal fixation or reconstructive surgery of the ulna, anatomical restoration of length, axial alignment, and both the sagittal and coronal bows is paramount to preserve unrestricted forearm pronation and supination. In proximal fracture-dislocations (Monteggia and trans-ulnar variants), failure to restore the physiological proximal dorsal ulnar bow (averaging 4.2 degrees with apex dorsal located approximately one-third the distance from the olecranon tip) leads to persistent radial head subluxation and secondary radiocapitellar joint degeneration. In coronoid process fractures, always scrutinize the anteromedial facet: fractures involving this biomechanical buttress produce profound posteromedial rotatory instability (PMRI) under physiological varus stress and must be rigidly stabilized with a buttress plate or retrograde screw fixation to protect the anterior bundle of the medial collateral ligament (AMCL) at the sublime tubercle. Finally, following anatomical ulnar shaft osteosynthesis, always dynamically evaluate distal radioulnar joint (DRUJ) stability through full pronation and supination: persistent dorsal instability of the ulnar head signifies concurrent deep foveal avulsion of the triangular fibrocartilage complex (TFCC) or an associated Galeazzi-equivalent injury demanding direct transosseous foveal refixation.

Master Class Surgical Pearl: Technical Precision in Ulnar Reconstruction and DRUJ Kinematics