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A Closer Look At The Recent Literature On Peroneus Quartus
Although symptomatic peroneus quartus is rare, it can be a factor in various pathologies. Accordingly, these authors offer pertinent diagnostic tips as well as current insights on treatment.
Peroneus quartus, a common variant muscle of the foot and ankle with many variations of origin and insertions, can lead to many pathologies such as peroneal tendinopathy, stenosis, tenosynovitis, tendon attrition and tears. Magnetic resonance imaging (MRI) is a good imaging modality to verify the presence of peroneus quartus and the associated pathologies.
Although the presence of a symptomatic peroneus quartus muscle is quite rare, consider conservative measures first. Surgical intervention such as excising the peroneus quartus or an exostectomy of the peroneal tubercle can be a last resort.
Peroneus quartus is the most common variant muscle of the foot and ankle found in the lateral compartment of the leg. Researchers have estimated its prevalence in the general population ranging from 10 to 21.7 percent.1 Sobel and colleagues conducted a study and found the peroneus quartus muscle was present in 21.7 percent of their 124 legs from 65 fresh human cadavers.2 Bilgili and coworkers dissected 115 cadaver legs to investigate the prevalence of peroneus quartus and found the muscle was present in 5.2 percent of the legs.3 Zammit and Singh also found a similar prevalence of 6.5 percent after cadaver dissection and MRI reviews.4 Ultimately, there is a wide range in the prevalence rates and there is little research regarding the prevalence of whether the muscle is bilateral.
Understanding The Anatomy Of The Peroneus Quartus
There are many different variations in the morphology of the peroneus quartus with numerous different origins and insertions. In the years since Otto published his findings, numerous researchers have described variations in the anatomy of the muscle, most of which arise from its various insertion sites.5 Currently, though, peroneus quartus is an all encompassing term for all of the variations of the lateral compartment leg muscle.
Otto initially described the muscle as originating from the distal portion of the lower fibula and inserting into the lateral surface of the calcaneus, but research has found this to be just one variation.1 In addition to the distal fibula, authors have found the peroneus quartus also originates from the peroneus brevis. Bilgili and colleagues found that in five of their six cadavers with a peroneus quartus, the muscle originated from the peroneus brevis.3 Several case studies have found that the muscle originates in greater percentage from the peroneus brevis. Sobel and colleagues found that 63 percent of specimens with a peroneus quartus had an origin at the peroneus brevis in the lower one-third of the leg.2 They also found that the muscle could originate from the peroneus longus as well.
Most of the variation of the peroneus quartus muscle is due to its diverse insertion locations. The most frequent insertion in the literature was at the peroneal tubercle of the calcaneus. Sobel and coworkers found that of the 63 percent of specimens that originated from the peroneus brevis all inserted at the peroneal tubercle.2 At the insertion, the authors found that the peroneal tubercle was often hypertrophied and palpable. In one particular case by Zammit and Singh, the peroneal tubercle was raised approximately 4 mm.4 The muscle also inserted at either the peroneus brevis or the peroneus longus just distal to the fibular groove. Zammit and Singh also found that in one of their cadaver specimens, the muscle inserted into the cuboid.4
Overall, there is a lot of variation in the origin and insertion of the supernumerary muscle and now all these variations are considered variants of the peroneus quartus.
Assessing The Impact Of The ‘Crowding Effect’
The peroneus quartus is unique to humans and we believe it to be an evolutionary adaptation to bipedal gait. When the peroneus quartus muscle is present, it theoretically lifts the lateral edge of the foot and stabilizes pronation during gait. Although mostly asymptomatic, peroneus quartus has been implicated in lateral ankle pathology such as peroneal tendinopathies and recalcitrant lateral ankle pain.
A case report from 1974 was the first study to hypothesize a “crowding effect” secondary to an anomalous peroneal tendon in the superior peroneal tunnel as a cause of posterolateral ankle pain.6 The superior peroneal tunnel is the continuation of the lateral compartment of the leg.6 The literature suggests the superior peroneal retinaculum forms the roof of the tunnel and the floor of the tunnel is formed by both the retromalleolar groove of the fibula and the distal aspect of the posterior intermuscular septum of the leg.7 Normally, the tunnel is occupied by the peroneus longus and brevis tendons, but studies have shown additional content can be present, including muscle fibers of a low-lying peroneus brevis, muscle fibers of peroneus quartus and the tendon of peroneus quartus.7,8 The literature suggests that crowding of the superior peroneal tunnel by the peroneus quartus may impede the normal functioning of the superior peroneal retinaculum by causing laxity and predisposing it to tearing.9,10
A stretched retinaculum would not provide the necessary support to the peroneus longus and brevis tendons as they pass through the peroneal tunnel. Greater laxity in the area of the posterolateral ankle could lead to increased friction and thus synovitis as well as tendon tears, subluxation and an overall greater propensity for inversion ankle sprains.
Sobel and colleagues also talk about the “crowding effect” within the fibular groove.11 The contributing factors within the peroneal tendon sheath and fibular groove may include tenosynovitis, peroneus longus or brevis hypertrophy secondary to attrition, an anomalously distal muscle insertion into the brevis tendon and the presence of the peroneus quartus muscle.
These authors found that peroneus brevis splits are common dynamic lesions that result from a combination of superior peroneal retinacular laxity, either traumatic or degenerative; a shallow fibular groove or anomalous muscles or tendons leading to incompetence of the superior peroneal retinaculum; peroneus longus compression; and a sharp posterior edge of the fibula. This mechanical etiology explains why the lesion can occur in young athletic patients with subluxating peroneal tendons and/or lateral ankle instability, as well as the elderly with no identifiable risk factors.12
What The Research Reveals About The Link Between Hypertrophic Peroneal Tubercles And Peroneus Quartus
Samaras and Kingsford have linked the presence of peroneus quartus to hypertrophy of peroneal tubercle, which can be associated with peroneal tendon pathology including tendinosis, stenosis tenosynovitis, tendon attrition and tears.13 Multiple studies have noted that the presence of hypertrophied peroneal tubercle has a certain influence on lateral ankle pathology. The peroneal tubercle is located on the anterior third of the lateral border of the calcaneus. It functions as an insertion point of the inferior peroneal retinaculum and also separates the common peroneal tendon sheath into separate sheaths for the peroneus longus and brevis tendons. Finally, the peroneal tubercule assists in plantarflexion of the first ray, acting as a pulley for the peroneus longus tendon.14-16
A hypertrophied peroneal tubercle has been present in both planus type and cavus type feet.17,18 The most common insertion site of the peroneus quartus muscle is the peroneal tubercle.2 In referring to hypertrophy, Rosenberg and colleagues studied computed tomography (CT) scans in 25 normal and 30 abnormal ankles after trauma, finding some variations of the peroneal tubercle predispose the peroneal tendons to abnormal mechanical stress.19 Burman suggests enlarged peroneal tubercles may have a congenital origin.20 Interestingly, in one report, peroneus quartus was present in all individuals with radiographic signs of hypertrophy of the peroneal tubercle.2
Additionally, Sobel and coworkers also noticed an association between the presence of peroneus quartus and hypertrophy of the peroneal tubercle, which can be a non-traumatic contributor to peroneus brevis tears or attrition.11 In the authors’ study, of the 27 ankles with a peroneus quartus muscle, there were 15 male and 12 female cadavers with 14 right feet and 13 left feet involved. The origins, insertions and size of the peroneus quartus varied. Radiographs of the calcaneus revealed significant peroneal tubercle hypertrophy in those specimens with peroneus quartus muscle insertion at the peroneal tubercle. Additionally, in five of the 27 ankles with the peroneus quartus muscle, longitudinal attrition of the peroneus brevis was present in the fibular groove.
When it comes to clinical assessment and imaging studies of the lateral ankle, pay close attention to the size and potential hypertrophy of the peroneal tubercle as well as any signs of the peroneus quartus.
What You Should Know About MRI And Peroneus Quartus
Magnetic resonance imaging can be a good choice for verifying the presence of peroneus quartus and quantifying associated pathologies. Furthermore, MRI has proven to be 83 percent sensitive and 75 percent specific in predicting tendon tears. The authors used MRI to determine the presence of a peroneus quartus muscle to be an individual structure that coursed parallel to the peroneus brevis muscle and the peroneus longus tendon, and was separated by an adipose weighted fascial plane.21,22 In the photo at left, the peroneus quartus is visible just posterior to the peroneus longus and brevis at the level of its insertion.
The ratio of tendon to muscle of the peroneus quartus vastly varies and inconsistently ranges from 0 to 100 percent of the tendon and therefore one cannot use this ratio to distinguish the anatomic structures.21
One can diagnose tears of the peroneal tendons by an increased signal on T1 and T2 weighted pulse sequences, contour irregularities and/or changes in girth. Fluid was also present within the peroneal tendon sheaths and adjacent soft tissues in one study.8 Artifact signals can appear as tendon changes on T1 weighted MRI due to the magic angle phenomenon. This phenomenon can occur when fibers within a tissue run parallel, which occurs in muscles and tendons. The falsely elevated signal on MRI occurs when the tissue fibers are at 55 degrees to the magnet. One can cross check these abnormalities for consistency on T2 weighted images.8,10 Other sources of error could stem from false positives and negatives, which authors have noted to account for as high as 18 percent of diagnosed tendon tears.23
Pertinent Pearls On Treatment
The presence of a symptomatic peroneus quartus muscle is quite rare. However, the literature suggests that the symptomatic peroneus quartus muscle is overwhelmingly recalcitrant to conservative measures. According to the American Orthopaedic Foot and Ankle Society (AOFAS), it is common practice to have success with treating peroneal injuries with conservative measures as such injuries can heal with rest.
If there is significant pain, prescribe a controlled ankle motion (CAM) walker boot for several weeks with the patient non-weightbearing. If tenderness and instability are minimal, an ankle brace, supportive orthotics or taping for lateral instability could be successful options. Patients should limit activity while they are actively treating the lateral ankle pain as this usually takes several weeks to show signs of improvement. A gradual return to activity should then occur over several weeks. If no signs of improvement happen, the next step should be to order an MRI to rule out the presence of peroneus quartus and identify other peroneal tendon pathologies. When one has exhausted all conservative therapy, the literature strongly suggests surgical exploration with possible debridement/excision.13,24,25
Despite the small sample size and lack of randomized studies, surgical excision of peroneus quartus has shown some promising signs after failed conservative therapy. Multiple case reports focus on a 15-year-old football player with an acute inversion ankle sprain, a 20-year-old college athlete with a potential overuse injury, a middle-aged woman without history of ankle trauma and two middle-aged women with histories of ankle sprains.13,24,25 The common theme illustrated in case reports was that upon excision of the anomalous peroneus quartus, in addition to post-surgery recovery time and/or physical therapy, all patients became asymptomatic and were able to return to their normal activity level.
In the case of a hypertrophied peroneal tubercle, one should address the peroneal tubercle when performing repair or release of the peroneal tendons. Hypertrophied tubercles can lead to stenosis of one or both of the peroneal tendons as they pass anterior or posterior to the tubercle. Consider an exostectomy of the hypertrophied peroneal tubercle with applications of bone wax product to the resection site as an adjunctive procedure to the tendon repair.26,27
In Conclusion
The peroneus quartus muscle is not uncommon with a 21.7 percent incidence according to Sobel and colleagues, but it also is not classically remarkable or regularly identified in the lateral column of the foot and ankle on physical exam or MRI.2 In addition, it is not typically discussed as a potential cause of lateral ankle tendonitis, tendinosis or instability. Attention to the peroneus quartus should exist in the discussion and differential diagnosis of lateral ankle instability and/or lateral ankle tendinopathy.
Dr. Sun is a resident in foot and ankle surgery at St. Mary’s Medical Center in San Francisco.
Ms. Castellucci is a podiatric medical student at the California School of Podiatric Medicine at Samuel Merritt University.
Ms. Harrington is a podiatric medical student at the California School of Podiatric Medicine at Samuel Merritt University.
Ms. Benson is a podiatric medical student at the California School of Podiatric Medicine at Samuel Merritt University.
Mr. Katz is a podiatric medical student at the California School of Podiatric Medicine at Samuel Merritt University.
Dr. Miller is an Assistant Professor in the Department of Podiatric Surgery at the California School of Podiatric Medicine at Samuel Merritt University.
The authors would like to thank Jana M. Crain, MD, and Christine E. Sun, BS.
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