The elbow is one of the most rewarding MSK ultrasound studies because so much of what causes elbow pain sits within a centimetre or two of the skin. Tendon origins, the ulnar nerve, and the olecranon bursa are all superficial enough to image at high resolution, and several of them can be tested dynamically in ways MRI cannot match.

Work through four quadrants in the same order every time and the study takes ten minutes.

Probe and Setup

Probe: High-frequency linear, 10–18 MHz. The structures are shallow — resist the temptation to drop frequency.

The four quadrants:

Anterior — distal biceps insertion, brachialis, joint recess, median nerve
Lateral — common extensor tendon, radial collateral ligament, radiocapitellar joint
Medial — common flexor tendon, ulnar collateral ligament
Posterior — triceps insertion, olecranon bursa, posterior fat pad, ulnar nerve

Lateral: Common Extensor Tendon

This is the most common referral by a wide margin — "tennis elbow."

Position: Elbow flexed 90°, forearm pronated, hand resting palm-down. Probe in long axis over the lateral epicondyle.

Normal: Hyperechoic fibrillar tendon arising from the lateral epicondyle, roughly 3–5 mm thick, tapering distally. The extensor carpi radialis brevis is the deepest and most commonly affected component.

Lateral epicondylitis (more accurately, tendinosis):

Hypoechoic swelling at the origin with loss of the fibrillar pattern
Tendon thickening compared with the asymptomatic side — always compare
Cortical irregularity or enthesophytes at the epicondyle
Neovascularity on power Doppler, which correlates with symptomatic disease
Partial tear: focal anechoic cleft within the thickened tendon
Full-thickness tear: complete discontinuity with fluid filling the gap

Calcification within the tendon is common in chronic cases and does not by itself indicate a tear.

Tip: Scan the contralateral elbow. Tendon thickness varies enough between individuals that a single measurement means little without a comparison.

Medial: Common Flexor Tendon and UCL

Position: Elbow extended, forearm supinated, arm externally rotated.

Common flexor tendon — the medial counterpart, "golfer's elbow." Same findings, same approach, less frequently symptomatic.

Ulnar collateral ligament (UCL): The anterior bundle is the functionally important one, running from the medial epicondyle to the sublime tubercle of the ulna.

Normal: Compact hyperechoic band, typically around 4–6 mm thick.

Valgus stress testing is where ultrasound earns its place. Apply gentle valgus force while imaging the ulnohumeral joint and measure the medial joint space. Compare with the contralateral side — asymmetric gapping suggests UCL insufficiency. This is a common study in throwing athletes and the dynamic component is the entire point.

Posterior: Ulnar Nerve and Olecranon

Ulnar Nerve at the Cubital Tunnel

Position: Elbow flexed 90°, probe in short axis over the groove between the medial epicondyle and olecranon.

Normal: Oval hypoechoic fascicles in a hyperechoic sheath. Cross-sectional area at the cubital tunnel is generally under about 10 mm²; values above this raise concern for ulnar neuropathy, with larger areas correlating with more severe compression.

Technique note: Measure CSA by tracing inside the hyperechoic epineurium, not around it. Keep the probe perpendicular — obliquity inflates the measurement.

Cubital tunnel syndrome:

Nerve enlargement, most pronounced at or just proximal to the tunnel
Loss of the normal fascicular pattern; the nerve looks hypoechoic and swollen
Flattening distal to the compression point

Dynamic assessment for subluxation: Keep the probe in short axis over the nerve and slowly flex the elbow through full range. A subluxating ulnar nerve snaps over or anterior to the medial epicondyle during flexion and returns on extension. This is a purely dynamic diagnosis — a static study will miss it entirely, and it is a genuine cause of medial elbow pain and paraesthesia.

Olecranon Bursa

Normal: Not visible.

Olecranon bursitis: Fluid collection superficial to the triceps insertion. Characterize it:

Simple — anechoic, compressible, no Doppler signal
Complex — internal echoes, septations, synovial thickening, hyperaemia on Doppler

Complex bursitis with surrounding soft tissue hyperaemia and clinical erythema raises concern for septic bursitis, which changes management urgently. Say so explicitly in the report rather than describing echoes and leaving the reader to infer.

Triceps Insertion

Long axis over the olecranon. Look for tendinosis, partial tearing, and enthesophytes. Complete triceps rupture is uncommon but functionally significant and shows retraction with a fluid-filled gap.

Anterior: Distal Biceps and Joint Recess

Distal Biceps Tendon

The hardest structure in the elbow to image well, because it dives deep and posterior to insert on the radial tuberosity — running away from the probe at an angle that produces heavy anisotropy.

Approaches:

Anterior longitudinal, elbow extended and supinated — good for the proximal tendon, poor at the insertion
Medial approach using the pronator teres as a window, which improves the insertion angle
Posterior approach with the elbow flexed and pronated, imaging the tuberosity directly

Findings:

Tendinosis: hypoechoic thickening near the insertion
Partial tear: fibre discontinuity with adjacent fluid
Complete rupture: absent tendon at the tuberosity with proximal retraction and a haematoma. Clinically there is often a "reverse Popeye" deformity. Complete distal biceps rupture is usually a surgical decision, and the retraction distance matters to the surgeon — measure it.

Anisotropy warning: More false-positive distal biceps tears are called from anisotropy than from any other artifact in the elbow. Before calling a hypoechoic area a tear, angle the probe to make it fill in. If it brightens, it was anisotropy.

Joint Effusion

Position: Probe longitudinal over the posterior olecranon fossa, elbow flexed 45°.

The posterior fat pad is the most sensitive location for elbow effusion. Normally the fat pad sits within the fossa; an effusion displaces it posteriorly. Anteriorly, check the coronoid fossa recess.

Characterize as simple or complex — complex effusion with synovial hypertrophy and Doppler signal suggests inflammatory or septic arthritis.

Key Images to Capture

Common extensor tendon — long and short axis, with power Doppler
Common flexor tendon — long axis
UCL — long axis, with and without valgus stress
Ulnar nerve — short axis with CSA measurement, plus dynamic flexion clip
Olecranon bursa
Triceps insertion — long axis
Distal biceps — long axis to the tuberosity
Posterior olecranon fossa — effusion assessment
Contralateral comparison of any abnormal structure

Reporting Language

Lateral epicondylitis: "Hypoechoic thickening of the common extensor tendon origin measuring 7 mm (contralateral 4 mm) with loss of normal fibrillar architecture and moderate neovascularity on power Doppler. A 3 mm intrasubstance cleft is present without full-thickness discontinuity. Findings consistent with common extensor tendinosis with a partial-thickness tear."

Ulnar neuropathy: "The ulnar nerve measures 14 mm² in cross-section at the cubital tunnel (contralateral 7 mm²) with loss of the normal fascicular pattern. On dynamic flexion the nerve subluxes anteriorly over the medial epicondyle. Findings consistent with ulnar neuropathy at the elbow with associated nerve subluxation."

SonoBuddy's MSK reference covers shoulder, elbow, wrist, knee, and ankle protocols — available offline in the scan room.

Elbow Ultrasound: Epicondylitis, Ulnar Nerve, and Distal Biceps