Original Article J. St. Marianna Univ.Vol. 6, pp. 31–40, 2015

1 Department of Radiology, St. Marianna University School of Medicine2 Akebono Hospital3 Department of Orthopedic Surgery, St. Marianna University School of Medicine

The Radiohumeral Synovial Fold: Analysis with 3D Isotropic MR

Imaging in 80 Asymptomatic Subjects

Kaoru Kitsukawa1, Natsuki Tachizawa1, Yasuyuki Kobayashi1, Yasuo Nakajima1, Ryo Ando2, Takeshi Arai3, and Moroe Beppu3,

(Received for Publication: January 15, 2015)

AbstractPurpose: To evaluate prospectively the size of the synovial fold protruding into the radiohumeral joint using 3Disovoxel MR imaging in asymptomatic volunteers, and to find a new reference value of the synovial fold on MRimaging.Materials and methods: Eighty asymptomatic volunteers (36 women, 44 men; age range, 23–56 years; medianage, 39; 20 patients in each decade from 20s to the 50s) were examined in this study. MR imaging of both sidesof the elbow was performed using 3D isovoxel fast field-echo sequence with selective water excitation. Thewidth of the synovial fold, which is defined as the distance between the tip of the synovial fold and the outeredge of the radial head, was measured at eight locations on the radiohumeral joint. We analyzed the relationshipbetween the results and age, sex, and handedness.Results: The posterior synovial fold was largest (range, 0.2–11.6 mm; median, 6.9 mm; 95th percentile, 10.0mm). The width of the lateral synovial fold and posterolateral synovial fold was larger in patients in their 50sthan in their 20s (p < 0.05 and p < 0.01, respectively).Conclusion: Most normal synovial folds are smaller than 10.0 mm in width. Aging is a factor which influencesthe width of lateral to posterolateral synovial folds.

Key wordselbow, radiohumeral joint, synovial fold, synovial plica, MRI

Introduction

Lateral epicondylitis is the most common causeof lateral elbow pain. It is considered to be tendino‐sis, a degenerative process that is histopathologicallycharacterized by the presence of active fibroblasts,vascular hyperplasia, and disorganized collagen at theorigin of the extensor carpi radialis tendon1)2). Syno‐vial fold impingement occurring between the radialhead and capitellum is considered to be a possiblecontributing pathological factor in recalcitrant lateralepicondylitis3)4). The synovial fold of the radiohum‐eral joint may hypertrophy, resulting in not only

snapping but also posterolateral elbow impingementsyndrome5–7). Radiohumeral joint synovial folds, alsoknown as synovial fringe or plicae, are thought to bephysiological remnants of embryonic septae thatformed during development and are interposed withinthe radiohumeral joint space. Furthermore, both em‐bryonic and cadaveric anatomical studies have dem‐onstrated synovial folds to be attached to the annularligament-joint capsule complex and presented withanterior and posterior folds8). Lateral folds observedin adults usually appear to be hardened and hyali‐nized, most likely attributable to the repeated com‐pression stress caused by radial head movements dur‐

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ing the aging process8).Several reports have documented the normal

MR imaging features of the radiohumeral synovialfold9)10) and the 90th percentile for the craniocaudaldimension of the posterolateral fold was 2.6 mm9).Since the synovial fold is interposed between the cap‐itellum and radial head, we believe that its width,which is characterized by the distance between thearticular capsule and tip of the fold, might represent areference value that could potentially predict the riskof mechanical impingement. To our knowledge, thereis no previous report in the literature documenting thenormal range of width of synovial folds on MR imag‐ing. However, the boundaries of the articular capsuleand soft tissue around the radiohumeral joint are notalways clearly delineated on MR imaging and maymake accurate synovial fold width measurement adifficult task to perform. Therefore, we substitutedthe outer edge of the radial head with the articularcapsule for a better and more accurate assessment ofthe synovial fold width. Synovial fold syndrome pa‐tients have reported pronounced lateral elbow painduring elbow extension and forearm pronation11)12)

possibly due to the pronation-to-supination move‐ment of the proximal radioulnar joint which may con‐sequently affect synovial fold width. We utilized a3D isovoxel fast field echo with water selective exci‐tation (3D FFE WATS) sequence image acquisition toobtain multi-directional images of synovial fold withelbow extension and forearm pronation, and tried tosearch the location of the maximum width of syno‐vial fold within the radiohumeral joint on MR imag‐ing. Thus, the purpose of this study was to find aneasy-to-use reference value of synovial fold of the ra‐diohumeral joint on MR imaging. We also investiga‐ted the relationship between the width of the synovialfold and aging, sex, and handedness.

Materials and methods

Asymptomatic subjectsThis study obtained institutional review board

approval and informed consent from each volunteer(the approval number of 1453). Eighty asymptomaticvolunteers (36 women, 44 men; age range, 23–56years; mean age, 38; median age, 39) were prospec‐tively examined in this study. In order to investigateand compare synovial fold sizes, all patients (n=80)were selected and categorized into 4 age groups of10-year increments: 20 (n=20), 30 (n=20), 40 (n=20),and 50 (n=20) years of age. All eighty elbows under‐went bilateral examination making a total of n=160

elbows for this present study. Two subjects were left-handed (2.5%) and 78 were right-handed (97.5%).The inclusion criteria were as follows: (1) no elbowpain, (2) no recent (less than 3 months) visit tohealthcare professional for pain involving elbow and(3) never been involved in any sport activity beyond arecreational level. On physical examination, no sub‐ject reported either signs of tenderness at the elbowregion or positive pain provocative tests for lateralepicondylitis after applying the Thomsen and middlefinger extension test. Fringe impingement test forsynovial impingement syndrome was also negative inall patients12).

MR imagingMR imaging was performed with a 1.5-T mag‐

net (Gyroscan Intera/Achiva; Philips Medical Sys‐tems, Best, the Netherlands) using the Microscopycoil (inner diameter, 47 mm; Philips Medical Sys‐tems). The subjects were examined in the supine po‐sition with the elbow extended and forearm pronated.The Microscopy coil was placed on the lateral dorsalaspect of the radiohumeral joint. One side was exam‐ined, and then the other. Coronal T2-weighted images(repetition time, 3600 ms; echo time, 110 ms; field ofview, 80 mm; number of averaging, 4; slice thick‐ness, 2 mm; and matrix, 203 x 256) and 3D FFEWATS sequence (repetition time, 14.7 ms; echo time,7.1 ms; flip angle, 40 degrees; field of view, 60 mm;number of averaging, 2; resolution, 0.5 x 0.5 x 0.5mm; and matrix, 120 x 120) were obtained. No intra‐venous or intra-articular contrast was provided.

Quantitative analysis of MR imageQuantitative measurement of MR imaging was

performed by a radiologist using 3D workstation(Ziosoft Inc, Tokyo, Japan). 3D FFE WATS imagingwith multiplanar reformats was obtained for measure‐ment of the width of the synovial fold of the radio‐humeral joint. We measured the width of the synovialfold at eight locations; anterior (A), anteromedial(AM), medial (M), posteromedial (PM), posterior(P), posterolateral (PL), lateral (L), and anterolateral(AL) (Fig. 1). To measure the width of the synovialfold, the outer edge of the radial head was used insubstitution for the position of the articular capsule.We defined that the distance between the outer edgeof the radial head and the tip of the synovial fold isthe width of synovial fringe (Fig. 2). The width of thelateral synovial fold was measured on coronal T2-weighted image, which corresponds to the lateral

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Figure 1. Coronal T2 weighted image (a) shows the reference line of reformatted axial 3D FFE

WATS image at the level of proximal radioulnar joint (b). Four lines divide the articular

surface of the radial head into eight equal areas (b). On these lines, the eight locations on

the radial head are defined as anterior (A), anteromedial (AM), medial (M), posteromedial

(PM), posterior (P), posterolateral (PL), lateral (L), and anterolateral (AL). Reformatted

oblique sagittal images of the radiohumeral joint demonstrate synovial folds at eight loca‐

tions (arrows in c to f). **= biceps tendon, *= brachialis tendon.

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MRI of radiohumeral synovial fold 33

Figure 2. For measuring the width of the synovial fold at

eight locations, the outer edge of the radial head

is used in substitution for the position of the ar‐

ticular capsule. We draw a projection line on the

outer articular surface of the proximal radioul‐

nar joint (solid line) perpendicular to the articu‐

lar surface of the radiohumeral joint at the radial

head (broken line). This projection line is con‐

sidered the outermost edge of the radial head.

The distance between the projection line and the

tip of the synovial fold protruding articular cav‐

ity is regarded the width of the synovial fold (*).

This is the same image as Fig. 1e.

Table 1. The Width of the Synovial Fold of the Radiohumeral Joint at Eight Locations

synovial fold on reformatted coronal 3D FFE WATSimage, to access intersequence reliability. All meas‐urements were performed with electronic calipers.

Statistical analysisIntersequence reliability was used to clarify that

the measurement of the width of the synovial fold

was not influenced by the sequence of MR imaging.Statistical correlation in measurement of the size ofthe lateral synovial fold between T2-weighted and3D FFE WATS sequence imaging was analyzed bycorrelation coefficient. The frequency and width ofthe synovial fold in eight locations were noted.Means, medians, ranges, and 10th and 95th percen‐tiles were calculated. The relationships between thewidth of the synovial fold and decade of age were an‐alyzed using the Steel test. The Mann-Whitney U testwas used to assess the relationship between the widthof the synovial fold and sex and handedness. A p-value of less than 0.05 was considered to indicate astatistically significant difference. Statistical calcula‐tions were performed using JMP 9 (SAS InstituteInc.).

Results

Lateral synovial fold width measurement be‐tween T2-weighted and 3D FFE WATS sequenceimaging displayed a strong correlation (r = 0.890).The anterior, anterolateral, lateral, posterolateral, pos‐terior, and anteromedial synovial folds were presentin all examined elbows. The medial synovial fold wasidentifiable only in 9 (5.6 %) out of 160 elbows. Theposteromedial synovial fold was observed in 102(63.7 %) elbows. The mean, median, minimum andmaximum width, and 10th and 95th percentile of thesynovial fold of the radiohumeral joint at eight loca‐tions (A, AM, M, PM, P, PL, L, and AL) are presen‐ted in Table 1. The posterior synovial fold is largest,with a median width of 6.9 mm (range, 0.2–11.6mm), followed by the posterolateral fold, with a me‐dian width of 6.7 mm (range, 2.6–9.4 mm) and thelateral synovial fold, with a median width of 4.7 mm(range, 0.2–8.8 mm). The 95 percentile of the poste‐

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rior fold was 10.0 mm.Statistical analysis for the relationship between

the width of the synovial fold and age group was per‐formed at seven locations (A, AM, PM, P, PL, L, andAL) because of the small number of measurable me‐dial synovial folds (3 in patients in their 20s, 3 intheir 40s, and 3 in their 50s). Synovial folds exhibiteda statistically significant difference in size betweenthe 20 and 50 year-old groups with the lateral (p <0.05) and posterolateral folds (p < 0.01) being con‐siderably larger in the latter group (Fig. 3). Further‐more, no statistically significant difference betweensynovial fold size and sex or handedness was found(Table 2).

Discussion

In this study we evaluated 160 elbows, found thepresence of synovial folds in the radiohumeral joint,and, also in accordance with previous reports8)12)13),revealed that the posterior and posterolateral foldswere respectively the largest synovial fold detectedon MR imaging. Cadaveric and embryonic investiga‐tion by Isogai et al. concluded that synovial foldswere found to be attached to the annular ligament-joint capsule complex and to protrude within the radi‐ohumeral interspace. Furthermore, the lateral foldswere located on the proximal edge of the annular lig‐ament and present in adults only while the posteriorfolds were generally longer, wider, extended morelaterally, and were interposed more deeply within theradiohumeral joint than the anterior folds8). In a sepa‐rate study on 49 fresh cadaveric elbows, Koh et al. re‐ported the presence of radiohumeral synovial folds inall their specimens, with the posterior folds beinglarger than the anterior13). Similarly, Ando et al. docu‐mented the presence of synovial folds within the radi‐ohumeral joint and, in 70 % of all cases, found largersynovial folds in the posterolateral region12).

In this study, the 95 percentile of the posteriorfold width measured 10.0 mm on MR imaging andrepresented the largest synovial fold. Hence, ‘normal’synovial fold size commonly measures less than 10mm in width. We believe that this is the first report ofthe width of the synovial fold being used for refer‐ence value of the size of the synovial fold on MRimaging and could be easily applied in clinical prac‐tice. The accurate measurement of the posterior syno‐vial fold appears to vary widely, and some authors re‐ported it as ranging from 2.1 to 11.0 mm (mean, 4.4mm)8) while others stated 5.2 ± 1.3 mm13). Our resultswere slightly larger (range, 0.2–11.6 mm; mean, 6.8

mm). We attribute this discrepancy to the lack of aclear and easy method where the junction betweenthe synovial fold and articular capsule could be pre‐cisely identified on MR imaging. However, ana‐tomic8)12) and arthroscopic14) assessments showedcontinuity between the synovial fold and the articularcapsule on the proximal edge of the annular ligament,which may not always be readily and accurately iden‐tifiable on MR imaging. To this end, during ourmeasurement process, and for the purpose of practi‐cality, we replaced the articular capsule by the articu‐lar surface of the radial head at the proximal radioul‐nar joint as the outer edge of the synovial fold. In thisway, the latter could be easily recognized on MRimaging and readily measurable, despite the fact thatit may cause a slight overestimation of the synovialfold size.

Normal MRimaging reveals the synovial foldsas a hypointense structure protruding within the radi‐ohumeral articular cavity. There is a significant over‐lap in size between symptomatic and asymptomaticfolds9)15). Husarik et al. assessed the size of the syno‐vial fold in 60 asymptomatic subjects. They meas‐ured the size of the posterolateral fold in craoniocau‐dal diameter on a sagittal image and in mediolateraldiameter on a coronal image. The median size of theposterolateral plica was 4.3 x 1.9 x 3.9 mm (sagittal xcraniocaudal x mediolateral dimensions). Because ofhigh prevalence of the posterolateral fold in asympto‐matic subjects, they speculated that the mere pres‐ence of a posterlateral fold may not explain clinicalsymptoms and that the size of the synovial foldshould be considered. They proposed a cutoff valueof 3 mm for thickened folds9). A recent study com‐paring synovial fold thickness between synovialfringe syndrome and control subjects revealed a sig‐nificant correlation between the presence of plicaethicker than 2.6 mm and synovial fold syndrome10).

We performed MR imaging with the forearms inthe pronated position and elbows fully extended un‐der the assumption that this would increase the ten‐sion of the anterior articular capsule, hence displac‐ing and trapping the synovial fold within theradiohumeral joint. Pronation of the forearm couldproduce tension in the muscles overlying the articularcapsule at the radiohumeral joint11). We believe thatthis position would allow a larger protrusion of thesynovial fold and better visualization on imagingsince this study is intended to investigate and definethe normal range, especially the upper limit of thenormal range of the size of synovial folds.

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MRI of radiohumeral synovial fold 35

Another notable result of this study is that thelateral and posterolateral synovial folds appeared tobe larger in subjects in their 50s than in their 20s. Iso‐gai et al. reported that, although the anterior and pos‐terior folds could be identified in both embryos andadults, lateral folds could in no instance be identifiedin embryos. Furthermore, anterior folds were shorterand narrower in the adult population, while the poste‐rior folds were generally longer and wider, extendedmore laterally, and interposed deeply into the radio‐humeral joint than were anterior folds. The authorssuggest that the lateral fold formation may be the re‐sult of repeated extension-compression stress exerted

on the elbow as an adaptive mechanism allowing forfull range joint movement, despite the alterationscaused by aging elbow joint and surrounding support‐ing tissues8). Our results concur with this theory.

Contrary to our expectations, neither handednessnor gender manifested any statistically significant dif‐ference in synovial fold size, although we speculatedthat a possible difference in size might exist due tothe increased amount of mechanical stress sustainedby the dominant side as opposed to the non-dominantside. However, Husaik et al. reported that the cranio‐caudal dimension of the posterolateral plica was sig‐nificantly larger in men than in women, and no rela‐

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tionship was found between age and the size of thesynovial fold9). The reason for the difference betweentheir results and our results is unclear. However, thedifference in size parameters might be part of the rea‐son; Husaik used thickness while we used width.

Elbow synovial fringe syndrome occurs by im‐pingement of the synovial fold in radiohumeral jointand can be clinically confused with lateral epicondy‐litis3)4)15). Mullett et al. analyzed 30 patients who hadrecalcitrant symptoms of lateral epicondylitis andsynovial fold impingement in radiohumeral joint atarthroscopic surgery. They observed a collar-likeband of radiocapitellar capsular complex that sublux‐

ated into the radiocapitellar joint with manipulationunder arthroscopy. Most of these patients experiencedsome pain relief due to resection of this collar-likeband structure4). The synovial fold is a distinct entityfrom the annular ligament and is contiguous with thejoint capsule, which fuses with the common extensortendon forming an enthesis at the lateral epicondyle.This explains the deteriorating effect caused by lat‐eral epicondylitis of the common extensor tendonwhich may consequently involve the synovial foldand hence induce hypertrophic changes14). Similarly,Ando et al. confirmed a histological congruity be‐tween synovial fold and the articular capsule, sug‐

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MRI of radiohumeral synovial fold 37

Table 2. Relationship Between Size of Synovial Fold and Sex and Handedness at Eight Locations

gesting that synovial fold invagination may have a di‐rect implication in the pathological conditions oflateral epicondylitis12). Nerve fibers were found in thedeep part of the synovial fold close to the attachmenton the capsule, which accounts for the associatedpain. Five synovial folds from five operated patientswith lateral epicondylalgia showed hypertrophic andincreased number of nerve fibers16). Debridement ofthe extensor carpi radialis brevis tendon1) with syno‐vial fold excision4)12)17) is the accepted treatmentmethod for recalcitrant lateral epicondylitis. How‐ever, a recent report argues that the debridement ofthe extensor carpi radialis brevis tendon accompaniedby posterior synovial fold excision does not provideany additional symptomatic improvement when com‐pared with the debridement of only the extensor carpiradialis brevis tendon18). The relationship between lat‐eral epicondylitis and synovial fold syndrome has notyet been fully elucidated. If there is additional refer‐ence value other than the thickness of the synovialfold in the radiohumeral joint on MR imaging, itmight be useful for further investigation of the clini‐cal importance of synovial folds.

The current study was performed using 3D iso‐tropic resolution MR sequence, allowing for the eval‐uation of the synovial fold of the radiohumeral jointin multiple directions. This method may potentiallyrepresent a novel MR imaging approach to obtaininga measurable reference for the larger synovial foldsthat protrude into the radiohumeral joint space.

Within a single imaging acquisition, isovoxel 3D se‐quence imaging with secondary multiplanar reforma‐tion promptly produces all imaging planes, consider‐ably reducing examination time. This techniqueincludes gradient echo sequence and, more recently,fast spin echo sequences with variable flip-angle thathave been successfully applied in evaluation of themusculoskeletal system19–24), the ankle, and the foot.After the advent of 3T MR scanner, enhanced signal-to-noise ratio, higher spatial resolution, and thegreater contrast-noise ratio of intrinsic joint structuresat higher field strength are possible supplementaryways to improve the diagnostic ability of 3D iso‐tropic resolution imaging24)25).

There were limitations in this study. First, ourmeasurements of the width of the synovial fold maynot represent the actual width of the fold itself. Therewas slight overestimation of the synovial fold size onMR imaging, compared to the results from the cadav‐eric study8)13). Second, we performed MR imagingwith elbows fully extended and pronated forearms,which could not be the standard MR imaging posi‐tioning for the elbow. Performing elbow MR imagingwith the elbow extended and the forearm in a neutralor supinate position may underestimate width of thesynovial fold because the synovial fold might bepulled back from the radiohumeral joint space in thisposition. Lastly, if there had been capsular distensionby fluid, the synovial fold may have been pushed outfrom the radiohumeral joint space, causing underesti‐

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mation in our measurement method.In conclusion, we measured the distance be‐

tween the outer articular surface of the radial head inthe proximal radioulnar joint and the tip of the syno‐vial fold of the radiohumeral joint on MR imagingusing 3D FFE WATS sequence. Most normal syno‐vial folds measure 10.0 mm or less. This could beused as a new reference value of the size of the syno‐vial fold for evaluation of lateral elbow pain. The lat‐eral and posterolateral synovial folds become widerwith age.

Acknowledgments

We are grateful to Dr. Ayako Muto for her con‐tribution to volunteer recruitment and data collectionand Dr. Brandon Lohman for his enormous help inpreparing this manuscript.

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