MRI features of posterior ankle impingement syndrome in ballet dancers

Clinical Radiology (2004) 59, 1025–1033

MRI features of posterior ankle impingement syndrome in ballet dancers: a review of 25 cases K.A.L. Peacea,*, J.C. Hilliera, A. Hulmeb, J.C. Healya Departments of aClinical Radiology, and bOrthopaedics, Chelsea and Westminster Hospital, London UK Received 28 September 2003; received in revised form 6 February 2004; accepted 9 February 2004

KEYWORDS Ankle; Athletic injuries; Magnetic resonance (MR)

AIM: To describe the magnetic resonance imaging (MRI) features of posterior ankle impingement syndrome (PAIS) in classical ballet dancers. MATERIALS AND METHODS: A retrospective review was undertaken of 25 MRI examinations of the ankle performed on 23 ballet dancers over a 26-month period. Images were examined for the presence of osseous and soft-tissue anatomical variants at the posterior ankle and imaging signs of PAIS. All patients presented with symptoms and signs suggestive of PAIS including posterior ankle pain, swelling and stiffness during plantar flexion. RESULTS: Anatomical variants predisposing to PAIS including as os trigonum and tuberosity arising from the superior calcaneum were clearly depicted. The most common imaging feature of PAIS in our series was high T2 signal posterior to the talocalcaneal joint indicating synovitis ðn ¼ 25Þ: Thickening of the posterior capsule ðn ¼ 13Þ and tenosynovitis of flexor hallucis longus ðn ¼ 17Þ were also common. An os trigonum was an infrequent finding ðn ¼ 7Þ: Bone marrow oedema, commonly in the posterior talus ðn ¼ 10Þ or in a patchy distribution ðn ¼ 10Þ was often noted. CONCLUSION: MRI is a useful diagnostic tool in PAIS, and in the present series, clearly demonstrates the anatomical variants and range of osseous and soft-tissue abnormalities associated with this condition. Prospective studies are needed to understand the significance and importance of individual MRI findings in producing the symptoms of PAIS. q 2004 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.

Introduction Foot and ankle pain is common in ballet dancers. Injuries commonly sustained include ligamentous sprain, impingement syndromes and stress fractures. The classical positions in which ballet dancers stand, force the foot and ankle into extreme positions and predispose to these types of injuries.1 In addition, the tendons and ligaments are subject to repetitive stress during intense practise and demanding performances. Posterior ankle impingement syndrome (PAIS) refers to a group of pathological entities that result *Guarantor and correspondent: K.A.L. Peace, Department of Clinical Radiology, Chelsea and Westminster Hospital, 369 Fulham Road, London SW10 9NH, UK. Tel.: þ44-20-8746-8570; fax: þ 44-20-8746-8576. E-mail address: [email protected]

from repetitive plantar flexion of the foot when the foot is in the en-pointe or demipointe position.2 Entrapment of soft tissues, bony processes, or unfused ossicles occurs between the posterior tibial plafond and the superior surface of the calcaneum. Variations in normal osseous and soft-tissue anatomy that predispose to PAIS include, the presence of an os trigonum, prominent posterior talar process (Stieda process), prominent down-slope to the posterior tibia,2 prominent tuberosity arising from the superior calcaneum2 – 4 and the presence of the posterior intermalleolar ligament (PIML).5 Although common in ballet dancers, it is also encountered in other sporting activities that involve forced plantar flexion of the foot such as soccer, and athletics.1,3 Patients present with recurrent posterolateral ankle pain and swelling with limited plantar flexion.

0009-9260/$ - see front matter q 2004 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.crad.2004.02.010

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Rest from activity usually results in symptom relief, although return to dance often causes symptom recurrence. During clinical examination, forceful plantar flexion of the relaxed foot reproducing pain experienced when dancing is considered by many orthopaedic surgeons as the hallmark of PAIS.6 An injection of local anaesthetic into the posterolateral heel helps confirm the diagnosis if symptoms subsequently resolve. Imaging can also be used to help confirm the diagnosis. Plain radiographs of the plantar flexed foot can identify an os trigonum and demonstrate bony impingement at the posterior heel. Radionuclide bone scans may identify abnormal tracer uptake at the posterior talus.3,7 Magnetic resonance imaging (MRI) with its excellent soft-tissue resolution and multiplanar capability can demonstrate in detail the various osseous and soft-tissue components involved, which helps in deciding further management. If the predominant feature is posterior synovitis, the patient may benefit from ultrasound-guided steroid injections. If however, an os trigonum, Stieda process or superior calcaneal tuberosity are shown, the patient may require surgical decompression to treat their symptoms of PAIS. The value of MRI in PAIS has not, however, been widely reported. We describe the MRI features of PAIS in the largest series of patients, all of whom were ballet dancers.

Materials and methods Over a 26-month period, 23 professional ballet dancers (9 males, 14 females, mean age 18 years, range 14 – 35 years) presented with a clinical diagnosis of PAIS made by an experienced paediatric orthopaedic surgeon with an interest in foot and ankle injuries. The majority of these patients were training at the Royal Ballet School and a confirmatory injection of local anaesthetic into the posterolateral heel was not felt appropriate. All patients were assessed with MRI. Two patients underwent bilateral imaging. The MR studies were performed using a 1.0 T scanner (Magnetom, Siemens) with a dedicated extremity coil. Standard images obtained were sagittal and coronal T1weighted (repetition time (TR) 570, echo time (TE) 20, field of view (FOV) 160, matrix 256 £ 512) and short tau inversion recovery (STIR: TR 3675, TE 30, TI 130,FOV 150, matrix 182 £ 256), and axial high resolution T2-weighted (TR 3597, TE 96, matrix 196 £ 512, FOV 160) with the foot in the neutral position. Axial fat-saturated, T1-weighted (TR 490, TE 20, matrix 192 £ 512, FOV 160) images pre and

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post-gadolinium administration were performed in nine (36%) cases. Additional sagittal T1-weighted, STIR or gradient echo T2-weighted (TR 110, TE 10, flip angle 30, matrix 256 £ 512, FOV 160) sequences were performed during plantar flexion in 11 cases (44%). Standard imaging protocols were adjusted for each patient accordingly. Two observers independently and retrospectively reviewed the imaging studies, and any disagreement was resolved by means of a consensus opinion. The images were assessed for the presence of a number of osseous and soft-tissue features. The presence of bony lesions that predispose to PAIS including the presence of an os trigonum, Stieda process, prominent down-sloping posterior tibia and prominent posterosuperior calcaneal tuberosity were sought. A prominent down-sloping tibia is defined as greater than 5 mm protruberance of the posterior malleolus below a line drawn tangential to the anterior articular margin of the distal tibia, parallel to the epiphyseal plate (Fig. 1(a)). A prominent posterior –superior calcaneal tuberosity is defined as any bone protruding above the superior parallel pitch line8,9 (Fig. 1(b)). The parallel pitch lines are constructed by dropping a perpendicular line from the posterior lip of the talar articular facet on to the base line, which is a line that joins the medial and anterior calcaneal tuberosities. A parallel line (the upper parallel pitch line) is drawn from the posterior lip (Fig. 1(b)). In addition, abnormal bone marrow signal and its distribution were identified. The presence of the PIML was also noted, after assessment of the coronal MRI images. This is seen as a distinct, recognizable hypointense band traversing between the posterior talofibular and inferior tibiofibular ligaments on coronal T1 and T2-weighted images, coursing from the tip of the lateral malleolus superomedially to the inner surface of the medial malleolus.5 Soft-tissue abnormalities, including posterior soft-tissue oedema or enhancement indicating synovitis, posterior capsular thickening, high signal change and/or enhancement within the musculotendinous junction of the flexor hallucis longus (FHL) muscle belly and FHL tenosynovitis were identified. FHL tenosynovitis was diagnosed when fluid around the FHL tendon came to an abrupt halt at the level of the posterior talus10 or when there was synovial enhancement after contrast enhancement. Posterior synovitis was diagnosed if posterior soft-tissue enhancement was noted after intravenous gadolinium. In the absence of gadolinium administration, diagnosis was achieved by identifying surrounding soft-tissue oedema or by altering the window settings on the STIR images allowing

MRI features of posterior ankle impingement syndrome in ballet dancers: a review of 25 cases

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Table 1 The MRI features of posterior ankle impingement syndrome (PAIS) in the classical ballet dancers of the present series ðn ¼ 25Þ Anatomical variants at the posterior ankle Os trigonum Stieda process Posterior intermalleolar ligament Down-sloping posterior tibia Calcaneal tuberosity

28% ðn ¼ 7Þ 16% ðn ¼ 4Þ 48% ðn ¼ 12Þ 25% ðn ¼ 6Þ 64% ðn ¼ 16Þ

MR imaging features of PAIS Bone marrow oedema Os trigonum Posterior talus Talar dome Posterior calcaneum Posterior tibia Generalized patchy Posterior synovitis Posterior capsular thickening Tenosynovitis of FHL High signal at muscle/tendon junction FHL Impingement during plantar flexion Tibiotalar joint effusion

86% ðn ¼ 6Þa 40% ðn ¼ 10Þ 20% ðn ¼ 5Þ 24% ðn ¼ 6Þ 8% ðn ¼ 2Þ 40% ðn ¼ 10Þ 100% ðn ¼ 25Þ 52% ðn ¼ 13Þ 68% ðn ¼ 17Þ 36% ðn ¼ 9Þ 100% ðn ¼ 11Þb 44% ðn ¼ 11Þ

FHL, flexor hallucis longus. a Only 7/25 scans demonstrated an os trigonum. b Only 11/25 scans were performed in plantar flexion.

Figure 1 (a) Diagram demonstrating a prominent downsloping tibia. This is defined as greater than 5 mm protuberance of the posterior malleolus below a line drawn tangential to the anterior articular margin of the distal tibia, parallel to the epiphyseal plate. (b) Diagram demonstrating the parallel pitch lines and a prominent superior calcaneal tuberosity.

thickened oedematous synovium to be identified. Finally, the presence of soft-tissue entrapment within the posterior ankle recess was noted on the images performed in plantar flexion.

Results A summary of the MRI findings is listed in Table 1. Overall the most common bony anatomical variant noted was a prominent superior calcaneal tuberosity (n ¼ 16; 64%; Fig. 2(a)). Eleven images demonstrated an os trigonum or Stieda process (Fig. 2(b)). The PIML was seen in nearly half of the cases (n ¼ 12; 48%; Fig. 2(c)).

The most common MRI feature of PAIS was posterior tibiotalar joint synovitis seen in 100% (n ¼ 25; Figs. 2(b), 3(a) and (b)). Gadolinium was administered in nine cases (36%) and enhancement of the synovium was noted in all cases. Posterior capsular thickening was identified in 13 cases (52%; Fig. 3(c)). Eleven examinations (44%) were performed in both the neutral and plantar flexed positions and all cases demonstrated entrapment of soft-tissues posteriorly between the posterior tibia and calcaneum (Fig. 4(a) and (b)). Associated tenosynovitis of the FHL was identified in 17 cases (68%; Fig. 5(a)). An additional observation (n ¼ 9; 36%) was the presence of STIR/T2 high signal, noted at the musculotendinous junction of FHL (Fig. 5(b)). Bone marrow oedema within one or more of the tarsal bones was seen in all cases. This commonly occurred within the posterior talus (n ¼ 10; 40%; Fig. 6(a)). Of the seven patients with an os trigonum, six demonstrated bone marrow oedema within the ossicle (Fig. 3(a)) and five patients had posterior talar oedema. Of the 11 patients imaged in plantar flexion, five of these patients demonstrated an os trigonum. Bone marrow oedema was also seen in the posterior calcaneum (n ¼ 6; 24%) but infrequently in the posterior tibia (n ¼ 2; 8%). Generalized patchy oedema was seen throughout the hindfoot in 10 cases (40%; Fig. 6(b)).

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Figure 2 Anatomical variants that predispose to posterior ankle impingement syndrome. (a) A 26-year-old female ballet dancer. Sagittal T1-weighted turbo spin-echo image during plantar flexion showing a prominent tuberosity arising from the superior surface of the calcaneum (straight arrow). Note also the presence of an os trigonum (curved arrow). (b) A 17 year-old female ballet dancer. Sagittal T2-weighted gradient-echo image during plantar flexion showing a Stieda process (short arrow) with associated inflammation and compression of the soft tissues at the posterior ankle (long arrow). (c) A 15 year-old male ballet dancer. Coronal STIR image showing the anatomical location of the posterior intermalleolar ligament (arrows).


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Figure 3 A 16-year-old female ballet dancer showing MRI features of PAIS. (a) Sagittal STIR image during plantar flexion showing marrow oedema within the os trigonum (curved arrow) and increased signal within thickened soft tissues indicating posterior synovitis. (b) Sagittal STIR image during plantar flexion demonstrating posterior synovitis. Note thickened oedematous synovium surrounding a fluid collection (black arrow). Note also the associated feature of bone marrow oedema within the posterior talus (white arrow). (c) Sagittal T1-weighted turbo spin-echo showing impingement of an os trigonum during plantar flexion (open arrow). Note thickening of the adjacent posterior capsule (closed arrow).

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Figure 4 A 17-year-old male ballet dancer demonstrating posterior soft-tissue impingement during plantar flexion. (a) Sagittal gradient-echo T2-weighted image in the neutral position depicting fluid in the posterior synovial recess (arrow). (b) Sagittal gradient-echo T2weighted image in plantar flexion showing forward subluxation of the talus with respect to the tibia causing the posterior tibia and superior calcaneum to come into close apposition with consequent posterior displacement and entrapment of the synovial fluid between these osseous structures (arrow).

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Figure 5 A 19-year-old female ballet dancer showing flexor hallucis longus pathology in PAIS. (a) Axial T1weighted fat-saturated image post-intravenous gadolinium showing enhancing inflammed soft tissue (arrow) surrounding the flexor hallucis longus tendon (arrowhead) in keeping with tenosynovitis. (b) Axial T1 fatsaturated image post-intravenous gadolinium depicting enhancement in and around the muscle belly of flexor hallucis longus at the level of the musculotendinous junction (arrow).


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Discussion PAIS is a painful disorder that limits full range of movement. Symptoms are thought to be secondary to repeated compression of osseous and soft-tissue structures between the superior calcaneum and the posterior plafond of the tibia, likened to a nut in a nutcracker.11 This results in bone contusions and a local synovitis involving the posterior recess of both the tibiotalar and subtalar joints.7 The use of MRI in the diagnosis of PAIS has only been reported twice in the literature. Wakeley et al.3 reported MRI findings in three patients with os trigonum syndrome. Bone marrow oedema within the os was noted in two patients and fluid around the os was demonstrated in the third patient. In addition, dynamic images demonstrated mobility of the os and inflammatory change within the surrounding soft tissues. Bureau et al.2 reported the MRI findings in seven patients who presented with posterior ankle pain, three of whom were classical ballet dancers. Bone marrow oedema within the os trigonum or lateral talar tubercle was seen in all cases. Posterior synovitis was seen in the tibiotalar joint in two cases and subtalar joint in four cases. In addition, associated FHL tenosynovitis was noted in three patients. Bureau et al. discussed the variations in the normal anatomy of the posterior ankle as aetiological factors of posterior impingement but only the presence of an os trigonum was noted in the study cohort. In the present study we divided the MRI findings in PAIS into the presence of osseous and soft-tissue structures that predispose to the syndrome and the signs of osseous and soft-tissue inflammation that result.

Involvement of the bony structures in PAIS

Figure 6 A 17-year-old male ballet dancer showing bone marrow oedema patterns in PAIS. (a) Sagittal STIR MR image showing the presence of posterior talar oedema in keeping with posterior ankle impingement (arrow). (b) Sagittal STIR MR image showing patchy bone marrow oedema within the tarsal bones. This is a common finding in ballet dancers feet and represents a generalized stress response impingement.

Understanding the anatomy of the posterior aspect of the ankle is fundamental to understanding the aetiology and MR features of PAIS. The posterior process of the talus comprises lateral and medial tubercles between which the FHL tendon runs in its fibro-osseous tunnel.12 A secondary ossification centre posterior to the lateral process often appears between the ages of 8 and 13 years.3 This usually fuses to the posterior talus within 1 year. Failure of fusion results in an os trigonum and this occurs in approximately 7% of the population.3,7,11,13 Seven out of 23 (30%) of our patients had an os trigonum. It is possible that ballet dancing in the developing skeleton predisposes to failure of fusion of this ossification centre. Other osseous structures involved in PAIS including

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a prominent Stieda process, and down-sloping of the posterior tibial articular were commonly identified.7 In this study the most common anatomical variant at the posterior ankle was the prominent tuberosity arising form the superior calcaneum (64%).4,7,11 The incidence in the general population is unknown, however in one study evaluating the Haglund syndrome, the incidence in the control group was 32% ðn ¼ 78Þ:8 In the present study the figure appears very high and this may be hypothesized as relating to the unusual stresses imposed on the developing calcaneum by the Achilles tendon, in ballet dancers. These variants in the normal osseous anatomy of the posterior ankle reduce the interval between the tibia and calcaneum allowing compression of the soft tissues during plantar flexion.

Bone marrow oedema pattern in PAIS Marrow oedema within the tarsal bones or distal tibia occurred in 100% of our cases, and is therefore a very common finding in this population. Marrow oedema was seen in the os trigonum and at the synchondrosis with the posterior talar tubercle in six of the seven patients suggesting that marrow oedema in an os trigonum is a reliable sign of PAIS. Other common sites include the posterior talus (40%) and posterior calcaneum (24%). It is of interest that oedema was not frequently seen in the posterior tibia which entraps the posterior soft tissues, talus and calcaneum. Traumatic forces within the tibia can obviously be distributed throughout a larger bony mass, which might explain this finding. In 40% of cases we noted diffuse patchy, marrow oedema throughout the hindfoot, not previously reported. The median age of our group is 18 years (range 14 – 35 years) and although bone-marrow oedema is seen as a normal variant in the adolescent population, patchy bone marrow oedema is not frequently seen in the adult nonathletic population. Although it may represent a stress response to repeated episodes of trauma its frequency suggests that it might represent a normal variation in ballet dancers related to the intensity of their training. Evaluation of non-symptomatic ballet dancer would be interesting to investigate the significance of this finding.

Soft-tissue involvement in PAIS The soft tissues involved in PAIS include the joint synovium, posterior capsule and talofibular, posterior intermalleolar and tibiofibular ligaments.14 Posterior synovitis was the most common finding

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occurring in 100% cases. Posterior capsular thickening was frequently seen (52%) as irregular, low signal soft-tissue thickening best identified on the sagittal and axial images, which can be difficult to distinguish from the posterior ligaments. Its presence may suggest a chronic history with scar formation in the posterior recess of the tibiotalar joint. The PIML represents a further anatomical variant, not present in everyone that can contribute to PAIS. A previous study5 documented the presence of the PIML in a cadaveric population (56%), however, noted that it was only seen on 19% of MR images, performed on a group of elderly patients who presented with a variety of symptomatic foot problems (excluding trauma or ankle ligamentous injuries). The authors identified whether the ligament was present or not on the coronal images and stated that the normal PIML was thin and smaller than the adjacent posterior ligaments. They postulated the difference between the two populations might be attributed to the relatively poor spatial resolution of MRI and the need for inter-slice gap between images. Also, they stated that the normal PIML is only seen on coronal images due to the oblique course of the ligament. A further study15 looked at the PIML on MRI in three patients with PAIS and concluded that the ligament was considered “prominent” if it was the same size as the adjacent two ligaments and if it was easily visualized in all three imaging planes. They also postulated that the PIML might be a cause for PAIS in the absence of other structural abnormalities. Both studies noted that the PIML can be meniscus shaped in many patients and can demonstrate intraarticular extension during plantar flexion, which accounts for its role in PAIS. The presence of the PIML was common in our series, seen in 12 cases (48%), which is more frequent than reported by Rosenberg et al.5 (19% of MR images of normal feet). The PIML may become more prominent on MRI as a direct consequence of repetitive trauma producing scarring and inflammation, however, it would be interesting to compare the incidence of the PIML in three groups (symptomatic ballet dancer group, asymptomatic ballet dancer group and a group of matched controls) in a further study to help clarify the role of the PIML in PAIS. FHL tenosynovitis occurred in 68%. This is a common accompanying feature of PAIS and occurs due to the anatomical location of the FHL tendon within the groove at the posterior talus.10,12,16 Note was also made of oedema within the FHL at the level of the musculotendinous junction within the distal calf in nine cases (36%), a finding that has not been previously reported. Hamilton12 describes the


occasional abnormal distal insertion of the FHL muscle fibres into the tendon and states that, during dorsiflexion, this can lead to the muscle fibres being pulled into the sheath “like a cork in a bottle”. The oedema at the musculotendinous junction of the FHL may be secondary to impingement of the muscle belly within the tendon sheath during the excessive dorsiflexion, seen in the grand plie position. Entrapment of the soft tissues posteriorly between the posterior tibia and calcaneum was demonstrated in all cases performed in plantar flexion, performed to simulate en-pointe. This may explain the frequency of posterior synovitis in PAIS, which occurs as a result of repetitive compression and resulting inflammation of the soft tissues within the posterior osseous interval, however, it would be interesting to evaluate aymptomatic ballet dancers and normal controls during plantar flexion to define the normal range. In addition prominent anterior translation of the talus was noted in two cases. This may be a consequence of ligamentous laxity in extreme plantar flexion allowing the dorsum of the calcaneum and posterior tibia to come into even closer contact whilst en-pointe.6 However, it could also be secondary to repeated medial and lateral ankle ligament damage. In conclusion MRI has an important role in characterizing the features of PAIS. Anatomical variations around the posterior ankle that contribute towards the aetiology of the syndrome are clearly depicted. MRI is ideally suited to demonstrate the variety of resulting soft tissue and osseous pathologies that constitute PAIS. In addition, MR during plantar flexion clearly shows entrapment of soft tissues between the posterior tibia and calcaneum. Imaging features in the present series which occur with high frequency include the presence of a prominent posterosuperior calcaneal tuberosity, os trigonum, posterior intermalleolar ligament, bone marrow oedema, especially at the posterior talus or os trigonum, posterior synovitis, and FHL tenosynovitis. Demonstrating specific imaging features can aid in the clinical management of this syndrome in individual patients. However, the major limitation of our study is the lack of a control group. It is interesting to note that all patients with a diagnosis of PAIS had abnormalities on MRI, particularly posterior synovitis and bone marrow oedema. These may represent variations of normal in this particular group of patients related to intensity of

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training. Prospective studies are needed to understand the significance and importance of individual MRI findings in producing the symptoms of PAIS.

Acknowledgements The authors wish to thank the Medical Illustrations Department at Chelsea and Westminster Hospital.

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