Isolated, periosteal stripping injuries of the Flexor Retinaculum: Case series of 3 patients with clinico-radiological review

Abstract

Isolated periosteal avulsion injuries of the flexor retinaculum on the medial side of the ankle are rare and may mimic osseous injuries or deltoid ligament tears. We describe a case series of 3 individuals with isolated, stripping injuries of the tibial attachment of the flexor retinaculum without underlying fracture or deltoid ligament disruption. A new classification system of flexor retinaculum periosteal stripping injuries of the ankle is proposed with clinico-radiological features.

1. Introduction

The ankle joint is a complex joint with movement occurring in the sagittal plane at the talocrural joint (dorsi-flexion and plantar flexion), along with inversion and eversion occurring in the coronal plane at the sub-talar joint.¹ Dynamic stability is provided by the medial (deltoid ligament complex), lateral ligament complex and the retinacular structures around the ankle.2, 3, 4 The retinacula of the ankle are regions of localized thickening of the investing deep fascia and important dynamic stabilizers around the ankle joint. These consist of the extensor retinaculum (superior and inferior), the peroneal retinaculum, and the flexor retinaculum.⁵ They function to retain and protect the tendons, nerves, and vessels as they pass from the back of the leg into the sole of the foot providing mechanical strength by preventing bowstringing of the contained tendons.⁶

The Flexor retinaculum present on the medial side of the ankle is roughly triangular shaped, thickened band of deep fascia, attached anteriorly to the posterior border and tip of the medial malleolus. Distally it is connected to the medial tubercle of the calcaneum, where it blends with the tibialis posterior tendon sheath and superficial deltoid ligament.⁷ In their cadaveric study, Numkarunarunrote et al. found the thickness of flexor retinaculum averaged 0.9 mm.⁶ The Flexor retinaculum forms the roof of the tarsal tunnel, sending septae to divide the medial space into 4 compartments carrying the tendons, nerves and vessels from medial to the lateral side (Fig. 1):

• 1.The tendon of Tibialis posterior (TP) tendon with its tendon sheath
• 2.The tendon of Flexor Digitorum Longus (FDL) tendon with its tendon sheath
• 3.1The posterior tibial artery and its accompanying veins
• 3.2The posterior tibial nerve with its terminal branches.
• 4The tendon of Flexor Hallucis Longus (FHL) tendon

Fig. 1.

Sagittal and axial schematic of the ankle showing the flexor retinaculum (Tib post, TP (tibialis posterior), Nav(navicular), Cun (cuneiform), ,Met (metatarsal), Med mall (medial malleolus), Cal(calcaneum), H(flexor hallucis longus), D (flexor digitorum longus).

Ankle injuries involve a spectrum of osseous and/or soft-tissue injuries either in combination or as isolated conditions.^8,9 There is a high incidence among physically active individuals including through participation in sport and dance. Soft tissue injuries of the ankle are common and may account for 16%–40% of all sports-related injuries.¹⁰ Ankle soft tissue injuries generally settle with good functional outcomes; however, they can also lead to complications with significant longer-term impairment.¹¹ Deltoid ligament tears represent the commonest medial ligament complex injuries and can occur in isolation or in combination with flexor retinaculum injuries.^12,13

Isolated injuries of the flexor retinaculum are rare. We describe a case series of 3 isolated, periosteal stripping flexor retinaculum injuries presenting as medial ankle pain without a clear history of a significant eversion or inversion injury mechanism. Clinical characteristics, anatomic correlation, complementary Magnetic Resonance Imaging (MRI) findings and management of this condition are described with a proposed classification of such injuries.

2. Case presentations

Three individuals (1 female and two male) presented with insidious onset of medial ankle pain (following sporting activity). Two were jumping athletes and one a runner. All were under the age of 30 years with no co-morbidities. There was no history of direct impaction, or a significant inversion or eversion trauma mechanism reported. Consequently, all patients sought clinical review due to niggling, medial ankle pain impacting their ability to train. All three individuals could fully-weight bear although initially with some discomfort. Clinical examination revealed focal tenderness over the medial malleolus with associated mild but visible soft tissue swelling in all 3 individuals. There was no medial or lateral ankle instability identified. They demonstrated full range of ankle movements with a normal neurovascular examination.

In view of the persisting medial ankle pain which was not responding to analgesics, training modifications and review of functional movement patterns and orthoses were undertaken. A MRI was performed to characterise any structural cause for the pain and guide subsequent graded return to training.

PD (Proton density) and PDFS (Proton density fat suppressed) axial, coronal, and sagittal MRI demonstrated stripping of periosteal attachment of the flexor retinaculum from the medial malleolus with mild soft tissue and osseous edema. The entire periosteum over the medial malleolus that connected the flexor retinaculum with the extensor retinaculum was elevated in all three cases (Fig. 2, Fig. 3). No discrete tear of the flexor retinaculum itself was noted. The medial tendons, in particular the tibialis posterior tendons were intact and in their normal position. The superior extensor retinaculum of the ankle was of normal thickness too. The superficial and deep component of the deltoid ligaments were intact. The lateral ligament complex was intact too. There was no osseous edema or fracture in relation to medial malleolus. Tibialis posterior, FHL, FDL The peroneal tendons and anterior tendon complex were intact with no tenosynovitis or tears identified. ATFL, AITFL, PITFL, PTFL and CFL were intact. Small tibiotalar joint effusion was present in all 3 cases which is a common finding in athletes. No osteochondral lesion in relation to talar done was noted. Tendo-Achilles, plantar fascia and sinus tarsi appeared normal. Normal marrow signal was noted in the rest of the bones. A clinico-radiological diagnosis of isolated, periosteal soft-tissue injury of flexor retinaculum was made. Clinico-radiological features are depicted in Table 1.

Fig. 2.

Coronal PD(a), PDFS(b), axial PDFS(c) and PD(d) showing stripping of the tibial attachment of the flexor retinaculum with soft tissue and osseous edema (arrow).

Fig. 3.

Coronal PDFS(a) and axial PDFS(c) showing stripping of the tibial attachment of the flexor retinaculum (arrow).

Table 1.

Patient characteristics, clinical features and Magnetic resonance imaging (MRI) findings of 3 patients with stripping injuries of the Flexor Retinaculum and medial ankle pain.

Patient/case number	Age/Sex	Vocation	Clinical symptom/s	Diagnostic Modality	Findings	Classification	Treatment
1	30/F	Athlete- Jumper	Medial Ankle pain, difficulty in walking,	MRI	Stripping of periosteal (tibial) attachment of flexor retinaculum	Type 2	Analgesics
							Rest
							Activity modification
							Gradual return to sports
2	29/M	Athlete- Jumper	Medial ankle pain	MRI	Stripping of periosteal (tibial) attachment of flexor retinaculum from the medial malleolus	Type 2	Analgesics
							Rest
							Activity modification
							Gradual return to sports
3	25/M	Runner	Medial ankle pain	MRI	Stripping of periosteal (tibial) attachment of flexor retinaculum from the medial malleolus	Type 2	Analgesics
							Rest
							Activity modification
							Gradual return to sports

Abbreviations: M: Male; F: female; MRI: Magnetic resonance imaging.

The injury was explained to the individuals and conservative management commenced. Management included training modifications whilst specifically optimising foot and ankle strength and proprioception within the wider kinetic chain function, in a sport specific manner. Physiotherapy alongside coaching technical input was sought with podiatry review. All three individuals have returned to training.

3. Discussion

The three distinct retinacula of the ankle: medial flexor retinaculum, extensor retinaculum (superior and inferior) and the peroneal retinaculum (superior and inferior) are important dynamic structures, protecting and preventing bowstringing of tendons during biomechanical performance of the joint.^1,6 Retinacular injuries are common with associated medial or lateral complex injuries of the ankle.⁵ The significance of injury to the medial flexor retinaculum is its anatomical continuum with the deltoid ligament. The anterior origin of the flexor retinaculum is continuous with the deltoid ligament (superficial component) with an aponeurotic expansion over the medial malleolus.⁶ Consequently, tears of the superficial deltoid ligament are often associated with medial flexor retinaculum injury with a quoted incidence of 37.0%.¹³

The injuries of the flexor retinaculum are centred over its tibial origin. These can represent a spectrum of injuries from subtle thickening of the attachment, partial stripping/elevation from the tibial origin or complete stripping/elevation of the periosteum connecting the flexor retinaculum and superior extensor retinaculum signifying a tibialis posterior fibro-osseous tunnel injury. (Fig. 4, Fig. 5). Such flexor retinaculum injuries can be associated with medial subluxation or frank dislocation of the tibialis posterior tendon and can easily be missed.¹⁴ (Table 3). There can be associated tendinopathy of tibialis posterior tendon too which one needs to look for (Table 2: Proposed Classification).

Fig. 4.

Schematic showing the different types of injuries of flexor retinaculum, arrow showing intact flexor retainculum (a), stripping of flexor retinaculum (b,c) and anterior dislocation of tibialis posterior (d). (arrow) TP( tibialis posterior), FD (flexor digitorum longus), FHL( flexor hallucis longus).

Fig. 5.

Axial PD(a) and PDFS(b) showing anteriorly dislocated tibialis posterior tendon (arrow).

Table 3.

Table showing findings of similar various flexor retinaculum injuries in literature.

	Injury	Other key findings	Reference
1	Acute medial flexor retinaculum avulsion with small osseous fragment	This was associated with subtalar fracture dislocation	Lewis SD, Chew FS. Ankle medial flexor retinaculum avulsion fracture. Radiol Case Rep. 2019 Jul 17; 14 (9):1144–1147. https://doi.org/10.1016/j.radcr.2019.06.023
			No.7 in Bibliography
2	Tibialis posterior dislocation	Type 1. Normal	Gkoudina A, Graikos G, Chatziargiriou M, Saloupis P. Posterior Tibialis Tendon Dislocation: Case Report and Review of Literature. Cureus. 2021 Nov 6; 13 (11): e19301. https://doi.org/10.7759/cureus.19301.
		Type 2 – ruptured flexor retinaculum and anteromedial dislocation of tibialis posterior	No.14 in Bibliography
		Type 3. Avulsion of flexor retinaculum with false pouch containing dislocated tibialis posterior.

Table 2.

Proposed Magnetic Resonance Imaging (MRI) classification of periosteal stripping injuries of the Flexor Retinaculum of the ankle.

Classification Type	Description/Site of injury
Type 0	Normal flexor retinaculum
Type 1	Thickening of flexor retinaculum at the medial malleolus corner.
Type 2	Stripping of periosteal attachment of the flexor retinaculum from the medial malleolus with mild soft tissue and osseous edema
Type 3	Stripping of periosteal attachment of the flexor retinaculum from the medial malleolus with mild soft tissue and osseous edema with displacement of tibialis posterior tendon
Type 4	Stripping of periosteal attachment of the flexor retinaculum from the medial malleolus with mild soft tissue and osseous edema with displacement of tibialis posterior tendon with tendinopathy

A possible mechanism of injury of the flexor retinaculum involves a combination of forced eccentric contraction of tibialis posterior during the take-off phase of a jump particularly if the foot has a slightly collapsed (pronated) medial arch.⁷ This results in stripping of the periosteum over the medial malleolus and can be associated with subluxation or dislocation of tibialis posterior tendon. Jumping events in athletics require athletes to combine speed, strength, and agility from a take-off point. There can be an inherent tendency of the foot and ankle to go into pronation on landing stressing the medial ligamentous structures, exacerbating the stretch on the injured structures. The mechanism of injury in all three cases in the present series, would be consistent with proposed mechanism. The case of tibialis posterior dislocation demonstrated in our paper is not a part of the case series. It demonstrates the possible spectrum of injury.

Dynamic ultrasound can be vital in evaluation of the flexor retinaculum and tibialis posterior tendon, in particular if there is subluxation or dislocation of the tibialis posterior and to assess for tendinopathy.

MRI has excellent sensitivity with high diagnostic accuracy in evaluating ankle ligament complex and its associated injuries.^{6,12,13,15,16} It provides additional information about integrity and position of the tibialis posterior tendon to guide management. Three cases in our series had stripping/elevation of the periosteum with soft tissue and osseous edema representing Type 2 in our proposed classification of isolated, periosteal medial retinaculum injuries. The thickness of Flexor retinaculum was 1mm. No focal tear of Flexor retinaculum was noted. There was no subluxation or dislocation of the tibialis posterior tendon. MRI complemented the clinico-radiological diagnosis. All three were managed conservatively with initial rest, modified training and analgesics and gradual return to activities.

4. Conclusion

Isolated, flexor retinaculum injuries are rare but however, should be considered in the differential diagnosis when evaluating patients with medial ankle pain. Inherent knowledge of anatomy, biomechanics relevant to individual sports, dance genres and trigger activities combined with clinico-radiological correlation is essential to guide appropriate management. The proposed classification might be useful in diagnosis and treatment of these retinacular injuries.

Funding

No funding to declare.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.