ANATOMY

A detailed understanding of the anatomy and anatomical variations of the peroneal tendons is essential to appreciate their role in lateral ankle stability. The peroneal tendons, composed of the peroneus longus (PL) and peroneus brevis (PB), are dynamic stabilisers of the ankle and act as evertors of the foot. Both originate from the fibula and descend laterally toward the foot, where they traverse the retromalleolar groove of the lateral malleolus. At this level, they share a common synovial sheath and are stabilised by the superior peroneal retinaculum (SPR), their primary stabiliser [1-3]. At the fibular insertion of the superior peroneal retinaculum (SPR), a fibrocartilaginous bridge is present. This structure serves to reinforce the SPR and enhances the concavity of the retromalleolar groove, contributing to the stability of the peroneal tendons.

Fig 1: (A and B) Illustration of the retromalleolar groove structures, showing the peroneus brevis (PB) tendon adjacent to the bone plane and the peroneus longus (PL) tendon positioned immediately posterior. The superior peroneal retinaculum (SPR) is depicted stabilizing the tendons, while the fibrocartilage (FC) at the fibular insertion enhances the groove's concavity.

Peroneus Brevis Tendon (PB):

Originates from the distal two-thirds of the fibula and inserts at the base of the fifth metatarsal. Its anteromedial position relative to the PL makes it susceptible to longitudinal tears and intra-sheath subluxation in cases of instability [1,2].

Peroneus Longus Tendon (PL):

Originates from the fibular head and proximal third of the fibula, passing beneath the cuboid tunnel before inserting at the base of the first metatarsal and medial cuneiform. The PL can exert pressure on the PB, especially in instability cases [2,3].

Key Anatomical Variants

1. Peroneus quartus muscle:Present in up to 20% of individuals, arises from the PB tendon and inserts into the calcaneus or nearby structures, increasing groove content and promoting tendon entrapment [3,5].
   

   Fig 2: The peroneus quartus is an accessory muscle or tendon present in up to 20% of individuals. It typically originates from the peroneus brevis, fibula, or distal fascia and inserts on the retrotrochlear eminence of the calcaneus, peroneal tubercle, or cuboid bone. Variations in its origin, insertion, and function may influence its role, which can include assisting ankle eversion or stabilizing the peroneal tendons. Its presence can contribute to crowding or tenosynovitis within the retromalleolar groove.
2. Retromalleolar groove morphology:Normally concave to stabilise the tendons, but in 11% of cases it is flat, and in 7%, convex, reducing natural containment and facilitating extra-sheath dislocation [6].
   

   Fig 3: Figure 3. Anatomical variations of the retromalleolar groove in the fibula and their impact on peroneal tendon stability: Concave (82%): The typical stabilizing shape, facilitating smooth tendon gliding. Flat (11%): A shallow or absent groove, increasing the risk of tendon subluxation or dislocation. Convex (7%): A raised bony surface, potentially causing friction or tendinopathy. These variations are crucial for imaging diagnosis and surgical planning in peroneal tendon pathologies.
3. Os peroneum:A sesamoid bone located within the PL tendon near the cuboid tunnel. Fractures or displacements of the os peroneum can lead to tendon rupture and persistent pain [5,6].
4. Low Muscle Extension of the Peroneal Tendons:This variant is observed in approximately 20-30% of individuals and occurs when the muscular plane extends to or projects distal to the SPR. This variant increases the risk of instability, compression, and tenosynovitis of the tendons due to reduced space. It is a critical factor in the surgical planning of peroneal tendon pathologies.
   

   Fig 4: This anatomical variant increases the risk of tendon instability, crowding, and tenosynovitis by reducing the space necessary for smooth tendon gliding. It is a critical consideration in the diagnosis and management of lateral ankle pain, as well as in surgical planning for peroneal tendon pathologies.

Clinical Relevance

These anatomical variants, alongside individual characteristics of the retromalleolar groove and SPR, play a crucial role in the pathophysiology of peroneal tendon instability. The low-lying PB muscle belly and the peroneus quartus increase the likelihood of friction, entrapment, and tendon displacement, favouring intra-sheath subluxation. Conversely, groove morphology alterations and SPR injuries are primary factors in extra-sheath dislocation [2,5,6].

CLASSIFICATION OF PERONEAL TENDON INSTABILITY

This study highlights the main patterns of peroneal tendon instability, emphasizing the role of dynamic ultrasound (US) in its assessment. Instability is categorized into two main subtypes: extra-sheath dislocation and intra-sheath subluxation, each with distinct clinical and pathophysiological features.

1. Extra-sheath dislocation: SPR rupture and anterior displacement

The most frequent and clinical form of instability, characterised by anterior displacement of the peroneal tendons over the lateral malleolus, outside the retromalleolar groove.

Eckert and Davis Classification (Oden):

Four categories based on the severity and extent of SPR involvement.

Fig 5: Schematic representation of the classification of Superior Peroneal Retinaculum (SPR) injuries. The diagram illustrates the relationship between the Peroneus Brevis (PB), Peroneus Longus (PL), and fibrocartilage (FC) structures.

• Type I:The SPR is elevated from the fibula, and the peroneal tendons are located between the bone and periosteum. The fibrocartilage remains intact.
  

  Fig 6: Figure 6. Illustration and ultrasound correlation of a type I SPR injury. 
Upper panel (A and B): Axial retromalleolar image of the fibula demonstrating a thickened and detached SPR along with the fibular periosteum (arrows), with the peroneal tendons remaining in their anatomical position. Lower panel (C and D): Following dynamic maneuvers (ankle dorsiflexion and eversion), anterior dislocation of the peroneus longus (PL) tendon is observed into a pocket formed by the detachment of the fibular periosteum (arrows).
  

  Fig 7: MRI correlation. Axial FSPD (A) and T2-weighted (B) images demonstrate a tear at the peroneal insertion of the SPR (short arrow), in continuity with detachment of the fibular periosteum (long arrows).
• Type II:The fibrocartilaginous bridge behind the lateral SPR insertion detaches along with the retinaculum, allowing the tendons to displace beneath it.
  

  Fig 8: Figure 8. Illustration and imaging correlation of a type II SPR injury. Upper panel (A and B): Axial retromalleolar ultrasound images of the fibula demonstrate a thickened and detached SPR along with the fibrocartilaginous bridge (arrows) and mild subluxation of the peroneal tendons. A pocket formed by periosteal lifting is indicated (blue arrow). Lower panel (C and D): MRI correlation clearly depicts the findings previously identified on ultrasound, including SPR detachment and periosteal lifting.
• Type III:Involves an avulsion of a small cortical bone fragment from the fibular insertion, with the tendons dislocating beneath the fibular fragment.
  

  Fig 9: Imaging correlation of an avulsion fragment in the posterolateral peroneal malleolus. (A) Anteroposterior (AP) X-ray mortise view showing an avulsion fragment at the posterolateral aspect of the fibula (arrows). (B) Axial ultrasound image of the retromalleolar groove demonstrating the avulsion fragment (long arrows). (C) Axial retromalleolar ultrasound image illustrating subluxation of the peroneus longus (PL) tendon toward the avulsion bed, located immediately deep to the avulsion fragment (long arrows). Irregularity of the avulsion bed (short arrows) is noted, which becomes the new sliding surface for the peroneal tendons, predisposing them to degenerative tearing.
  

  Fig 10: Dynamic evaluation of a type III SPR injury.
• Type IV:Avulsion or complete rupture of the SPR, with the tendons located externally and superficially to the retinaculum.
  

  Fig 11: Imaging correlation of a type IV SPR injury.
Upper panel (A and B): Illustration and axial ultrasound images of the retromalleolar region demonstrate a thickened SPR with a full-thickness defect proximal to its fibular insertion (blue arrow).
Lower panel (C and D): MRI correlation highlights the findings observed on ultrasound, including the SPR defect and its proximal detachment.
  

  Fig 12: Dynamic evaluation of a type IV SPR injury. Dynamic dislocation of the peroneus longus (PL) tendon is observed superficial to the SPR and fibular periosteum, caused by a full-thickness tear proximal to the fibular insertion.

Mechanism:

Typically secondary to acute rupture or chronic weakening of the SPR. This pattern is often associated with acute ankle trauma, such as forced dorsiflexion and eversion injuries, although it may also result from repetitive microtrauma.

Dynamic US Findings:

• Tendon displacement outside the groove during resisted dorsiflexion and eversion manoeuvres. If there is no significant pain or apprehension, the tendons can be gently pushed with the free hand's finger to assist in their evaluation.
• Visualisation of SPR discontinuity or thinning.
• Audible "snap" during dynamic manoeuvres.

MRI Correlation:

• Complete or partial rupture of the SPR.
• Possible soft tissue oedema and tendon involvement, including associated longitudinal tears.

2. Intra-sheath subluxation: Tendon slippage with an intact SPR

A more subtle pattern where the peroneal tendons abnormally slide over each other within the common synovial sheath, without exiting the retromalleolar groove. There are two types: Type A, which involves a positional change of the tendons but with intact tendons. Type B involves the subluxation of the peroneus longus through a split in the peroneus brevis.

Fig 13: Dynamic evaluation of intrasheath instability. (A)Type A intrasheath instability is characterized by dynamic positional reversal of the peroneal tendons. This condition is often associated with a clicking sensation, which may or may not be symptomatic. While not necessarily pathological, it is important to evaluate for tendinopathy, tenosynovitis, and other associated symptoms. (B) Type B intrasheath instability involves subluxation of the peroneus longus (PL) tendon through a longitudinal split in the peroneus brevis (PB) tendon. Dynamic maneuvers, such as dorsiflexion and eversion of the ankle, are useful for reproducing and assessing this condition.

Fig 14: Type A Intrasheath Instability. The dynamic evaluation demonstrates a positional change of the peroneal tendons, with the peroneus brevis (PB) moving into a more superficial position relative to the peroneus longus (PL). This finding is often observed during ankle circumduction maneuvers. Proper control of movement speed and direction with the free hand is crucial to ensure clear imaging, as rapid or sudden patient movements can obscure visibility.

Fig 15: Type B Intrasheath Instability. Dynamic evaluation during ankle dorsiflexion and eversion reveals the peroneus longus (PL) tendon interposed within a partial split of the peroneus brevis (PB) tendon. This observation aids in differentiating between partial and complete splits, as the formation of two hemitendons may be seen. Notably, in a resting position, the tear may not be visible due to the apposition of the hemitendons.

Mechanism:

Occurs in the presence of anatomical variants that create friction or crowding within the groove, such as a low-lying peroneus brevis muscle belly or the presence of an accessory peroneus quartus muscle. It is also associated with repetitive microtrauma in athletes.

Dynamic US Findings:

• Abnormal movement of the peroneal tendons relative to each other is observed in dynamic axial views.
• Reduced groove space due to anatomical variants.
• An intact superior peroneal retinaculum (SPR) with no evidence of discontinuity.

MRI Correlation:

• Tendinosis or longitudinal tears of the peroneus brevis due to increased friction.
• Synovial thickening or stenosis of the tendon sheath.

CLINICAL SIGNIFICANCE:

The differentiation between these two subtypes is essential for accurate diagnosis and treatment planning: Extra-sheath dislocation often necessitates surgical repair of the superior peroneal retinaculum (SPR), particularly in chronic or recurrent cases. Intra-sheath subluxation, on the other hand, can initially be managed conservatively. However, in severe cases with significant anatomical variants, targeted surgical intervention may be required.Dynamic ultrasound, using resisted dorsiflexion and eversion maneuvers, enables real-time visualization of the instability patterns and tendons, providing precise correlation with clinical and MRI findings.

Comparative Table

The following table highlights key differences between extra-sheath dislocation and intra-sheath subluxation.