Jon A. Jacobson, MDPeter J. L. Jebson, MDAndrew W. Jeffers, MDDavid P. Fessell, MDCurtis W. Hayes, MD

Index terms:Elbow, 422.482Muscles, US, 422.482Nerves, 422.42Ultrasound (US), 422.12989

Published online: August 21, 200110.1148/radiol.2202001723

Radiology 2001; 220:601–605

1 From the Departments of Radiology(J.A.J., D.P.F., C.W.H.) and OrthopedicSurgery (P.J.L.J., A.W.J.), University ofMichigan Medical Center, 1500 E MedicalCenter Dr, TC-2910G, Ann Arbor, MI48109-0326. Received October 27, 2000;revision requested December 18; revisionreceived January 31, 2001; accepted Feb-ruary 26. Address correspondence toJ.A.J. (e-mail: jjacobsn@umich.edu).© RSNA, 2001

Author contributions:Guarantors of integrity of entire study,J.A.J., P.J.L.J.; study concepts and de-sign, all authors; literature research,J.A.J., P.J.L.J.; clinical and experimentalstudies, J.A.J., P.J.L.J., A.W.J., D.P.F.; dataacquisition, J.A.J., P.J.L.J., A.W.J., D.P.F.;data analysis/interpretation, all authors;manuscript preparation, definition of in-tellectual content, editing, revision/re-view, and final version approval, all au-thors.

Ulnar Nerve Dislocation andSnapping Triceps Syndrome:Diagnosis with DynamicSonography—Report ofThree Cases1

Initial experience with the use of dy-namic sonography of the elbow fordiagnosing ulnar nerve dislocationand snapping triceps syndrome isreported. Cases of three consecutivepatients who underwent sono-graphic evaluation of the elbow andsubsequent open elbow surgery forsymptomatic ulnar nerve dislocationwere reviewed. Dynamic sonogra-phy of the elbow was used to aid inthe accurate diagnosis of and differ-entiation between ulnar nerve dislo-cation and snapping of the medialtriceps muscle.

Causes of medial elbow pain and/or ul-nar neuropathy are many, and they in-clude ulnar nerve compression withinthe cubital tunnel, ulnar nerve subluxa-tion or dislocation, and snapping tricepssyndrome (1,2). Ulnar nerve dislocationrepresents abnormal movement of theulnar nerve out of the cubital tunnel andover the medial epicondyle during elbowflexion (3). Snapping triceps syndromerepresents medial dislocation of the me-dial head of the triceps muscle over themedial epicondyle during elbow flexion(2). These latter two abnormalities of themedial elbow may produce a transientsnapping sensation with elbow flexionthat is palpable at physical examination(2). Because ulnar nerve dislocation withand without accompanying snapping tri-ceps syndrome may be clinically indistin-guishable, imaging is often needed toassist in the diagnosis (2). Accurate diag-nosis is essential; ulnar nerve transposi-tion surgery for ulnar nerve dislocationwithout appropriate treatment for unrec-

ognized snapping triceps syndrome willresult in persistent symptoms (4,5).

Ulnar nerve dislocation from the cubi-tal tunnel over the medial epicondyletypically occurs during elbow flexion andis reduced with elbow extension (3). Sim-ilarly, medial dislocation of the medialhead of the triceps muscle in snappingtriceps syndrome also occurs with elbowflexion and is reduced with elbow exten-sion (2). Although magnetic resonance(MR) imaging has been used with successin the evaluation of elbow abnormalities,the transient nature of ulnar nerve dislo-cation and snapping triceps syndromewith normal appearances with elbow ex-tension make routine MR imaging inad-equate for these diagnoses. NonroutineMR imaging with the elbow in variousdegrees of flexion for making the diagno-sis of snapping triceps syndrome hasbeen described (6).

Given the limitations of routine MRimaging in these clinical settings, we ex-plored the use of sonography to assist inthe diagnosis of ulnar nerve dislocationand snapping triceps syndrome. The pur-pose of our study was to report our initialexperience with the use of dynamicsonography of the elbow to aid in thediagnosis of ulnar nerve dislocation andsnapping triceps syndrome.

Sonographic Procedure

Preoperative sonography of the elbowwas completed by a musculoskeletal radi-ologist (J.A.J.) who was experienced inmusculoskeletal sonography. The radiol-ogist was given a clinical history of pos-sible ulnar nerve dislocation and/orsnapping triceps syndrome. A 10-MHzlinear transducer (Advanced TechnologyLaboratories, Bothell, Wash) was used. Aliberal amount of acoustic transmissiongel was used in place of a standoff pad.

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For sonographic evaluation of the cubi-tal tunnel, the patient sat on a stool andplaced the extended elbow on the exami-nation table, with slight external rotationof the shoulder to enable visualization andpalpation of the medial epicondyle andolecranon. The transducer was placed inthe transverse plane over the posterome-dial aspect of the elbow; one end of thetransducer was placed over the olecranonprocess and the other end was placed overthe medial epicondyle (Fig 1a).

At sonography, the position of the ulnarnerve with respect to the medial epicon-dyle was noted (Fig 1b, 1c), and the cross-sectional area of the ulnar nerve was mea-sured in the transverse plane at the level ofthe medial epicondyle in the cubital tun-nel. The ulnar nerve was identified on thebasis of its parallel hypoechoic neuronalfascicles separated by hyperechoic connec-tive stroma (7). This produced a speckledechogenicity that appeared hypoechoicrelative to the surrounding hyperechoic fatin an imaging plane transverse to the ulnarnerve (Fig 1b).

The normal speckled appearance of theulnar nerve may be less apparent if theultrasound beam is not perpendicular tothe nerve fibers due to anisotropy. Iden-tification of the ulnar nerve may be con-firmed with scanning proximal and distalto the cubital tunnel and noting the ex-pected location and echotexture of theulnar nerve. Color or power Dopplersonography may also be used to differen-tiate the ulnar nerve from a vascularstructure, although the latter is typicallysmaller in diameter when compared withthe ulnar nerve.

Once the ulnar nerve was identified inthe transverse plane, the patient then ac-tively flexed the elbow, with the trans-ducer remaining stationary relative tothe medial epicondyle (Fig 1d). With dy-namic imaging throughout elbow flex-ion, the positions of the ulnar nerve andmedial head of the triceps muscle wereassessed relative to the medial epicondyle(Fig 1e). The triceps muscle was identifiedon the basis of its predominantly hypo-echoic echotexture with internal hyper-echoic fibroadipose septa. Imaging con-tinued with active elbow extension in thesame imaging plane. Typically, a focusedsonographic examination of the cubitaltunnel region can be completed in lessthan 5 minutes. Imaging of the contralat-eral elbow was not performed.

At sonography, a diagnosis of ulnarnerve dislocation was made when the ul-nar nerve dislocated over the apex of themedial epicondyle with elbow flexion.The apex of the medial epicondyle was

defined as the point where the two nearlyflat cortical surfaces of the humerusformed an angle of approximately 80°–100°; sonographic identification wasaided by finding the common flexor ten-don origin located immediately anteriorto the apex of the medial epicondyle (Fig1c). Of note, the common flexor tendontypically appears relatively hypoechoicin the transverse plane with elbow exten-sion due to anisotropy (Fig 1b), whereaswith elbow flexion, a fibrillar hypere-choic echotexture can be appreciated (Fig1e). The diagnosis of snapping tricepssyndrome was made when the hypo-echoic medial muscle belly of the tricepsmuscle dislocated over the apex of themedial epicondyle with elbow flexion.Ulnar nerve dislocation and snapping tri-ceps syndrome were excluded if the ulnarnerve and medial head of the tricepsmuscle remained posterior to the apex ofthe medial epicondyle during full activeelbow flexion (Fig 1e).

Surgical Diagnosis

Although the sonographic results wereavailable to the orthopedic surgeon(P.J.L.J.), the decision to proceed withsurgical treatment was based on the clin-ical history, physical examination find-ings, and results of nerve conductionanalysis and electromyography.

Surgery was performed by the orthope-dic surgeon who was experienced in elbowsurgery. At open elbow surgery, dislocationof the ulnar nerve was diagnosed if theulnar nerve was abnormally located overthe medial epicondyle with elbow flexion.The diagnosis of snapping triceps syn-drome was made when visible and palpa-ble snapping of the medial triceps musclewas noted over the medial epicondyle dur-ing elbow flexion. Surgical treatment wasperformed for each abnormality, if present.

Case Reports

Institutional review board approval wasobtained to retrospectively search patientrecords, and informed consent was not re-quired. By using the patient informationdatabase of one orthopedic surgeon, threeconsecutive patients were identified ashaving (a) open elbow surgery for symp-tomatic ulnar nerve dislocation with orwithout snapping triceps syndrome and(b) a preoperative sonographic examina-tion of the elbow between June 1999 andJuly 2000. In addition, institutional reviewboard approval and informed consent wereobtained to image the elbow of a healthyvolunteer with sonography.

Case 1

A 52-year-old woman had clinical andelectromyographic findings consistentwith ulnar neuritis and a dislocating ul-nar nerve of the right elbow. Sono-graphic findings demonstrated that theulnar nerve was in a normal positionwith elbow extension (Fig 2a), and it was0.24 3 0.21 cm in cross section (area,0.05 cm2). With active elbow flexion, theulnar nerve dislocated over the apex ofthe medial epicondyle and superficial tothe common flexor tendon origin (Fig2b), which produced a painful snap thatwas felt through the transducer. The hy-poechoic medial head of the triceps mus-cle remained posterior to the medial epi-condyle and was visualized only in thecubital tunnel with elbow flexion. Thereal-time sonographic finding was inter-preted as an isolated ulnar nerve disloca-tion. The diagnosis was confirmed byfindings at surgery.

Case 2

A 34-year-old woman had a snappingsensation and ulnar nerve symptoms con-sistent with compression of the ulnarnerve at the elbow (cubital tunnel syn-drome). Sonographic findings demon-strated that the ulnar nerve was posteriorto the medial epicondyle apex with elbowextension (Fig 3a). In addition, with elbowextension, the medial head of the tricepsmuscle was identified within the cubitaltunnel. The cubital tunnel appearedcrowded because both the medial head ofthe triceps muscle and the ulnar nervewere near the medial epicondyle apex.

The ulnar nerve was 0.6 3 0.25 cm at thelevel of the epicondyle (area, 0.15 cm2).With elbow flexion (Fig 3b), abnormal dis-location of the ulnar nerve and the medialhead of the triceps muscle occurred overthe apex of the medial epicondyle and su-perficial to the common flexor tendon or-igin, which produced two painful snapsthat were felt through the transducer. Thedislocated ulnar nerve and medial head ofthe triceps muscle returned to the normalposition with elbow extension. Both themedial head of the triceps muscle and theulnar nerve remained in contact with eachother throughout dislocation and reloca-tion. The real-time sonographic findingswere interpreted as ulnar nerve dislocationand snapping triceps syndrome. Thesefindings were confirmed by findings atopen elbow surgery.

Case 3

A 17-year-old girl presented with apainful snapping sensation at the me-

602 z Radiology z September 2001 Jacobson et al

dial aspect of the right elbow duringelbow flexion. The patient had previ-ously undergone decompression and

anterior ulnar nerve transposition sur-gery but had continued to have medialelbow pain. With transposition surgery,

the ulnar nerve is surgically relocatedanterior to the medial epicondyle fortreatment of cubital tunnel syndrome.Sonography depicted a normal-appear-ing ulnar nerve, located anterior to themedial epicondyle, that was 0.5 3 0.25cm (area, 0.125 cm2). With elbow ex-tension, the triceps muscle was identi-fied within the cubital tunnel. With el-

Figure 1. (a) Photograph of the elbow in an asymptomatic healthy volunteerdemonstrates positioning of the transducer between the medial epicondyle (blackdot, arrowhead) and olecranon (black dot, arrow) with the elbow in extension for asonographic examination. Right side of image is proximal, and left side is distal.Corresponding (b) transverse sonogram and (c) illustration show the normalulnar nerve (straight arrow) posterior to the medial epicondyle apex (arrowhead).Note olecranon (curved arrow) and common flexor tendon origin (F). The brackets inc indicate the sonographic field of view as depicted in b. Left side of image isposterior, and right side of image is anterior. (d) Photograph shows elbow flexion,and the transducer remains stabilized to the medial epicondyle (black dot, arrow-head) while the olecranon (black dot, arrow) moves distally away from the trans-ducer. Right side of image is proximal and left is distal. (e) Corresponding transversesonogram shows the ulnar nerve (solid arrow) and medial head of the triceps (openarrows) in normal position posterior to the medial epicondyle apex (arrowhead).Note common flexor tendon origin (F). Left side of image is posterior, and right sideof image is anterior.

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bow flexion, the medial head of thetriceps muscle dislocated over the apexof the medial epicondyle, which causeda painful snap that was felt through thetransducer. The dislocated medial tri-ceps muscle reduced with elbow exten-sion. Findings at open elbow surgeryconfirmed the presence of snapping tri-ceps syndrome.

Discussion

Our results show that dynamic sonogra-phy of the elbow can be used to help dem-onstrate abnormal dislocation of the ulnarnerve, with and without snapping tricepssyndrome. The dynamic imaging allowscontinual visualization of the ulnar nerveand triceps muscle throughout active elbow

flexion and extension. Knowledge and ac-curate diagnosis of ulnar nerve and/or me-dial triceps muscle dislocation as causes formedial elbow snapping are important sothat proper surgical treatment may be pre-scribed. This point is emphasized in patient3 of this study; this subject required openelbow surgery twice because the diagnosisof snapping triceps syndrome was not ini-

Figure 2. Case 1. Isolated ulnar nerve dislocation. (a) Transverse sonogram shows the ulnar nerve (straight arrow), which is in its normal positionposterior to the medial epicondyle apex (arrowhead), with the elbow in extension The olecranon is indicated by the curved arrow. F 5 commonflexor tendon origin. (b) Transverse sonogram shows that with elbow flexion, the ulnar nerve (solid arrow) is dislocated anteriorly over the medialepicondyle apex (arrowhead) and superficial to the common flexor tendon origin (F). Note separation of the medial head of the triceps muscle (openarrows) from the dislocated ulnar nerve. Left side of image is posterior, and right side of image is anterior.

Figure 3. Case 2. Snapping triceps syndrome. (a) Transverse sonogram shows the ulnar nerve (solid arrow) slightly medially displaced, althoughposterior to the medial epicondyle apex (arrowhead). Note depiction of the medial head of the triceps muscle (open arrows) within the cubitaltunnel with elbow extension. F 5 common flexor tendon origin. (b) Transverse sonogram shows that with elbow flexion, both the ulnar nerve (solidarrow) and medial head of the triceps muscle (open arrows) are displaced over the medial epicondyle (arrowhead) and are superficial to the commonflexor tendon origin (F). Note that the ulnar nerve and medial head of the triceps muscle remain in close continuity. Left side of image is posterior,and right side of image is anterior.

604 z Radiology z September 2001 Jacobson et al

tially made in conjunction with presumedulnar nerve compression.

Dislocation of the ulnar nerve from thecubital tunnel of the elbow has been re-ported (3) in approximately 16% of healthysubjects. One proposed cause is congenitalabsence of the cubital tunnel retinaculum, afibrous band extending from the olecranonprocess to the medial epicondyle and form-ing the roof of the cubital tunnel (1). Dislo-cation of the ulnar nerve over the medialepicondyle may cause ulnar nerve irritationdue to friction as it passes over the medialepicondyle (3). An abnormal ulnar nervelocation over the medial epicondyle alsoplaces the nerve at risk for direct injury (3).

To our knowledge, snapping triceps syn-drome was first described as a distinct entityin 1970 (4). Although not uncommon, theprevalence of this syndrome is not known,possibly due to ambiguous clinical findingsthat cause it to remain unrecognized (2).The clinical presentation is variable and in-cludes medial elbow pain, snapping, neu-ropathy, or a combination of symptoms (2).Two palpable “snaps” may be detected clin-ically, with the first representing disloca-tion of the ulnar nerve and the second rep-resenting dislocation of the medial head ofthe triceps muscle (2).

The cause of snapping triceps syndromeis unknown. However, both acquired andcongenital causes have been implicated (2).For example, bodybuilding may increasethe bulk of the triceps muscle, which causesmedial dislocation (2). Posttraumatic osse-ous abnormalities may also be associatedwith dislocation of the ulnar nerve and tri-ceps muscle (2). Congenital accessory tri-ceps tendon and abnormal medial tricepsmuscle have also been suggested as causes(2). Symptoms may be exacerbated withoverhead activities, such as pitching andweight lifting (2).

The MR imaging findings of snapping tri-ceps syndrome have been described (6). Itshould be noted, however, that elbow flex-ion is required to demonstrate both disloca-tion of the ulnar nerve and dislocation ofthe medial head of the triceps muscle (6).Dynamic sonography permits continual vi-sualization of the ulnar nerve and medialhead of the triceps muscle throughout ac-tive elbow flexion and extension. This dy-namic evaluation with sonography is oneadvantage, as opposed to static evaluationwith routine MR imaging. In addition, anypalpable snap can be directly correlatedwith the sonographic findings to confirmthe diagnosis. This is also important be-cause not all cases of ulnar nerve and me-dial triceps muscle dislocation are symp-tomatic.

The sonographic results in this study

demonstrated that ulnar nerve dislocationand snapping triceps syndrome were cor-rectly diagnosed during elbow flexion (Figs2, 3). We used the apex of the medial epi-condyle, located immediately posterior tothe common flexor tendon origin, as thelandmark to diagnose abnormal dislocation(Fig 1c, 1e). In each of our cases, the ulnarnerve and/or medial head of the tricepsmuscle dislocated medial to the epicondyleapex and superficial to the common flexortendon origin during elbow flexion (Figs 2b,3b).

We have noted that when ulnar nervedislocation is secondary to snapping tricepssyndrome, the medial head of the tricepsmuscle and the ulnar nerve remain in closecontinuity as they dislocate over the medialepicondyle (Fig 3b). In contrast, in the caseof isolated ulnar nerve dislocation, the me-dial head of the triceps muscle and the ul-nar nerve separate from each other (Fig 2).Mild medial displacement of the tricepsmuscle normally occurs with elbow flexion;however, the muscle remains posterior tothe medial epicondyle (Fig 1e) (6). In addi-tion, in the two cases of snapping tricepssyndrome, we noted visualization of themedial triceps muscle within the cubitaltunnel with elbow extension (Fig 3a). Thisfinding was not present in the healthy vol-unteer (Fig 1) and in the patient with iso-lated ulnar nerve dislocation (Fig 2).

With ulnar nerve dislocation, the abruptmovement of the dislocating ulnar nervebetween the ultrasound transducer and themedial epicondyle produced a snappingsensation felt through the transducer. Onepotential pitfall is that excessive transducerpressure may inhibit the ulnar nerve fromdislocating, thus resulting in misdiagnosis.This can be avoided by intermittently de-creasing transducer pressure on the soft tis-sues throughout the dynamic examination.

Sonography has been used to detect ulnarnerve dislocation in healthy volunteers andhas been used in the evaluation of ulnarneuritis and ulnar nerve compression in thecubital tunnel (8–10). It has been shown (9)that ulnar nerve cross-sectional area in-creases in the cubital tunnel in control sub-jects, and an area greater than 0.075 cm2 atthe level of the epicondyle indicates cubitaltunnel syndrome. It has also been shown(8) that the ulnar nerve flattens during el-bow flexion, and this flattening is mostmarked with ulnar nerve subluxation. Inthis study, the ulnar nerve cross-sectionalarea was 0.05 cm2 in patient 1, 0.15 cm2 inpatient 2, and 0.125 cm2 in patient 3.

By using 0.075 cm2 as the threshold valuefor area, the second and third patients metthe sonographic criterion for cubital tunnelsyndrome (9). Both patients had symptoms

of ulnar nerve compression at the elbow.Although the first patient met the electro-diagnostic criterion for ulnar nerve com-pression, the nerve itself was not enlarged atsonography. It is unclear why this discrep-ancy was present. It is possible that the con-servative treatment prior to surgery reducedthe ulnar nerve irritation and decreasednerve size to less than the threshold value.

The limitations of this study include thesmall number of subjects. This is primarilybecause the intraoperative surgical findingswere used as the standard of reference andbecause snapping triceps syndrome is anuncommon and often unrecognized clini-cal condition. Further studies are requiredto confirm our findings in a larger subjectpopulation. In addition, interobserver vari-ability could not be calculated because onlyone radiologist performed each sono-graphic examination.

In summary, dynamic sonography of theelbow was used to aid in the diagnosis ofand differentiation between ulnar nerve dis-location and snapping of the medial tricepsmuscle. Although accurate in this small se-ries, a larger prospective study would berequired to determine the sensitivity andspecificity of sonography to assist in mak-ing these diagnoses.

References1. O’Driscoll SW, Horii E, Carmichael SW, Mor-

rey BF. The cubital tunnel and ulnar neurop-athy. J Bone Joint Surg Br 1991; 73:613–617.

2. Spinner RJ, Goldner RO. Snapping of the me-dial head of the triceps and recurrent dislo-cation of the ulnar nerve. J Bone Joint SurgAm 1998; 80:239–247.

3. Childress HM. Recurrent ulnar-nerve disloca-tion at the elbow. Clin Orthop 1975; 108:168–173.

4. Rolfsen L. Snapping triceps tendon with ulnarneuritis. Acta Orthop Scand 1970; 41:71–76.

5. Haws M, Brown RE. Bilateral snapping tricepstendon after bilateral ulnar nerve transposi-tion for ulnar nerve subluxation. Ann PlastSurg 1995; 34:550–551.

6. Spinner RJ, Hayden FR, Hipps CT, GoldnerRD. Imaging of snapping triceps. AJR Am JRoentgenol 1996; 167:1550–1551.

7. Silvestri E, Martinoli C, Derchi LE, BertolottoM, Chiaramondia M, Rosenberg I. Echotex-ture of peripheral nerves: correlation be-tween US and histologic findings and criteriato differentiate tendons. Radiology 1995;197:291–296.

8. Okamoto M, Abe M, Shirai H, Ueda N. Mor-phology and dynamics of the ulnar nerve inthe cubital tunnel: observation by ultrasonog-raphy. J Hand Surg [Br] 2000; 25:85–89.

9. Chiou HJ, Chou YH, Cheng SP, et al. Cubitaltunnel syndrome: diagnosis by high-resolu-tion ultrasonography. J Ultrasound Med1998; 17:643–648.

10. Puig S, Turkof E, Sedivy R, Ciovica R, Lang S,Kainberger FM. Sonographic diagnosis of re-current ulnar nerve compression by ganglioncysts. J Ultrasound Med 1999; 18:433–436.

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