Unusual accessory peroneal muscles, peroneus quartus, peroneus digiti quinti, and their association with peroneus brevis tendon tear

This cadaveric study was approved by Institutional Review Board (IRB), Faculty of Medicine, Chulalongkorn University (IRB No. 110/61; certificate of approval No. 007/2018). We retracted legs from 109 formaldehyde-embalmed cadavers from Thai donors (55 male and 54 female) while the specimens were prone. The average age of cadaver donors at death was 77.44 ± (standard deviation, SD) 12.91 years (range from 34 to 94 years), and were obtained from the Department of Anatomy, Faculty of Medicine, Chulalongkorn University. After removing the skin, the sural nerve and small saphenous vein were distracted anteriorly. The SPR was then identified and incised to expose the superior peroneal tunnel. The presence of accessory muscles, for example, PQ and PDQ, was identified. The origin and insertion of these muscles were described. The presence of any peroneal tendon tear (Grade III or IV as described by Sobel et al.) was noted [2, 19].

Accessory peroneal muscles were found in 48 of 109 cases (44%). Coexistence of the PQ and an unusual accessory peroneal muscle was present in one male cadaver. We found 13 (12%) PQ, 33 (30%) PDQ, and 2 (2%) unusual peroneal muscles.

The most frequent accessory peroneal muscle was the PDQ (Figure 1). The PDQ was found bilaterally in 12 (36%) cadavers (6 male and 6 female). All of these muscles originated from the PB tendon, but their insertions were varied as summarized in Table 1. On the dorsum of the foot, almost all of the PDQ were tendinous in structure, except for one that consisted of muscle fibers (Figure 1). Most of the PQ originated from the PB muscle belly and less frequently from the PB tendon. The PQ inserted on various sites including retrotrochlear eminence, peroneal tubercle, cuboid, and dorsolateral surface of the base of the 5th metatarsal as described in Table 1 and Figure 2. The PQ was found bilaterally in 2 (15%) cases (1 male and 1 female).

Figure 1

Figure 2

Unusual accessory peroneal muscles were observed in 2 specimens (1 male, 1 female). In one, an unusual accessory peroneus muscle originated from the PL muscle and inserted on the peroneal tubercle (Figure 3). The other had 2 heads of origin: one from the PL tendon and the other from the lower part of the PB muscle at the distal end of fibula. Both parts united and coursed over the PL tendon at the lateral malleolus before splitting into 2 tendons. One tendon inserted on the retrotrochlear eminence and the other on the talus and peroneal tubercle (Figure 4). The PQ coexisted and inserted at retrotrochlear eminence (Figure 4).

Figure 3

Figure 4

Evidence of a PB tendon tear was found in 14 of 109 cases (13%; Figure 5). However, there was no PL tendon tear in any case. The peroneal tubercle was found in 51 (47%) cases and retrotrochlear eminence was found in 87 (80%). All tendon tears were asymmetrical. No significant association was found between the presence of accessory peroneal muscles (either PQ or PDQ) and the prominence of the retrotrochlear eminence or peroneal tubercle (Tables 2 and 3). Moreover, there was no significant association between PB tendon tears and prominence of the retrotrochlear eminence or peroneal tubercle (Table 4). There was no coexistence of PQ and PB tendon tears in any case. PB tendon tears coexisted with a PDQ in 6 (43%) cases. However, there was no significant association between PB tendon tears and the presence of either a PQ or PDQ (Table 5).

Figure 5

In the present study, we found 44% prevalence of accessory peroneal muscles in cadavers of Thai individuals. These muscles were PDQ, PQ, and unusual accessory peroneal muscles. The prevalence of PDQ was 30%, which is similar to that found previously (30%–50%) [20, 21]. The PDQ was commonly found bilaterally and predominantly in men [22]. By contrast, the prevalence of bilateral PDQ in an Indian population was only 5% [20]. The PDQ separates from the PB tendon as a slender tendon [20, 21]. In the present study, we found one PDQ with muscle fibers. Most of the PDQ inserted on the different parts of the 5th toe including shaft of metatarsal and bases of proximal, middle, and distal phalanges. Moreover, the PDQ tendon merged with that of the extensor digitorum longus and inserted at the base of distal phalanx in 3 cases. Demir et al. described the insertion pattern of the PDQ as having 2 different types: a single tendon attached to the 5th metatarsal bone and 2 separated tendons attached to different parts of the 5th metatarsal bone [21]. Moreover, dual insertion of PDQ on the 4th and 5th metatarsals was reported in 3% of PDQ cases [20]. In the present study, all PDQ had a single tendon.

The prevalence of the PQ in this study was 12% and predominantly found in male cadavers in accordance with previous studies (5%–22%) [12, 14, 22, 23, 24]. The PQ muscle arose only from the PB muscle belly and its tendon, and not from the distal shaft of the fibula or posterior intermuscular septum as previously described [22, 23, 24]. The PQ had a single tendon and inserted at various sites including retrotrochlear eminence, peroneal tubercle, base of 5th metatarsal, and cuboid. However, Bilgili et al. reported the case of a PQ with 2 separated tendons that inserted at different points. In that case, the PQ tendon was bifurcated and coursed above and beneath the PB tendon to insert at the retrotrochlear eminence and cuboid bone [23]. The PQ is reported to coexist with hypertrophy of the peroneal tubercle or retrotrochlear eminence [15, 25]. In the present study, only 7 of 51 cases with a prominent peroneal tubercle and 12 of 87 cases with a prominent retrotrochlear eminence were found to have this coexistence (Table 2). Therefore, the association between the prominence of the retrotrochlear eminence or peroneal tubercle and the presence of the PQ was not significant. Moreover, the hypertrophy of a peroneal tubercle and a prominent retrotrochlear eminence was present in an asymptomatic ankle without the presence of a PQ in accordance with previous findings [14, 26].

Tubbs et al. reported an unusual accessory muscle called the peroneotalocalcaneus muscle. This muscle originated from the anterior intermuscular septum and PL muscle and inserted on the superior surface of the talus and calcaneus [17]. Moreover, Fabrizio reported an anomalous fibularis (peroneal) muscle. This muscle arose from the muscle belly of the PL muscle in the proximal half and gave rise to a long slender tendon, which coursed posteriorly to lateral malleolus, and inserted on the superficial aspect of inferior peroneal retinaculum [18]. In the present study, unusual accessory peroneal muscles were found in 1 instance in each sex (2%). One of these muscles arose from the PL and inserted at the peroneal tubercle between the inferior peroneal retinaculum septum. The other arose from the PL tendon and PB muscle and gave rise to 2 tendons inserted at different points. One tendon inserted on the talus and peroneal tubercle, while the other inserted on the retrotrochlear eminence.

A peroneal tendon tear or attrition was reported as a consequence of acute mechanical or the repetitive injury such as trauma, mechanical irritation, or attrition within the retromalleolar groove, subluxation of the SPR, incompetent SPR, and ankle instability [4, 9, 14]. Zammit and Singh stated that there were several anatomical factors associated with peroneal tendon tears, such as the prominence of the retrotrochlear eminence and the thinning or laxity of the SPR [14]. In the present study, only 13 of 87 specimens with a prominent retrotrochlear eminence coexisted with a PB tendon tear. Therefore, no significant association was found. Previous studies had reported that the presence of a PQ has the potential to create attrition or a tear of the peroneal tendon at the retromalleolar groove [2, 15, 16]. However, none of the cases of PB tendon tear in the present study coexisted with the presence of a PQ. Moreover, the presence of a PDQ in the present study was not significantly associated with a PB tendon tear (Table 5).

The prevalence of PQ, PDQ, and unusual accessory peroneal muscle in Thai specimens in the present study was in agreement with previous findings in other populations. Coexistence of a PQ and an unusual accessory peroneal muscle was found in one male specimen. There was no association between PQ or PDQ and the prominence of either the retrotrochlear eminence or peroneal tubercle. Moreover, the PB tendon tears were not significantly associated with the presence of an accessory peroneal muscle or the prominence of either the retrotrochlear eminence or peroneal tubercle.