The present study was conducted on 36 embalmed cadavers bilaterally in the Department of Anatomy, Sri Aurobindo Medical College & P.G. Institute, Indore, India. No consideration was given to the age and sex of the specimens examined. The study was conducted over the period of two years (2012-2014). Lateral part of foot and sole was dissected to expose PL tendon and OP. Various dimensions of OP (max. length, max. breadth and max. thickness in accordance of its axis) and PL tendon (max. breadth and max. thickness, 1 cm proximal and distal to OP) were measured with the help of Scale and Digital Vernier caliper. The PL tendon approximately 10-12 cm long was removed which included OP. The specimens were submitted for radiological and Histological study.

A flattened oval enlargement (OP) was observed in all the PL tendons examined. The deep surface of OP was articular, concave, smooth and shiny [Table/Fig-1a]; sometimes divided into two parts [Table/Fig-1b]. Reciprocal convex articular facet was present on the Cuboid [Table/Fig-1a] at the proximal end of the peroneal sulcus (cuboid tunnel). In a few cases the articular surfaces extended on to the Calcaneum [Table/Fig-1b].

[Table/Fig-1]:

(a) Single large facet on OP & cuboid, (b) Showing double facets on OP (arrow heads) and reciprocal facets on Cuboid {Yellow arrows} & Calcaneum {white arrows}

On the right the breadth of proximal tendon (4.92 mm) was greater than the distal part (4.55 mm); whereas, the thickness in the distal part was more (2.44 mm) than the proximal part (2.23 mm). While on the left side the breadth & thickness of PL tendon was the same, both in the proximal and distal parts [Table/Fig-2].

The average length and breadth of OP was lesser on right side as compared to the left but the average thickness on the right was found to be slightly greater than the left side [Table/Fig-3].

The anterio-posterior diameter of articular surface on Cuboid was greater when there was single facet (average 11.43 mm) as compared to when double facets were present (average 9.59 mm) [Table/Fig-1a,b].

On the right double facets on Cuboid & Calcaneum were more (25.8%) as compared to left (16.2%). When double facets were present; the facet on Cuboid (average anterio-posterior diameter - 9.59 mm & transverse diameter – 8.82 mm) was larger than the facet on Calcaneum (average anterio-posterior diameter – 7.30 mm & transverse diameter – 7.57 mm) [Table/Fig-1b].

In this study, OP was present in all the PL tendons examined. When OP was examined radiologically it was found as non-calcified thickening in 65% & Calcified thickening in 35% [Table/Fig-4,5].

[Table/Fig-4]:

Radiograph showing ossific nodule in OP

Histologically, OP were either cancellous bone [Table/Fig-6a], fibrocartilage [Table/Fig-6b] or only thickened collagen fibres [Table/Fig-6c].

[Table/Fig-6]:

6]: Photomicrographs of OP showing (a) Cancellous bone (b) Fibrocartilage (c) Collagen fibres [H&E (10X)]

Le minor while investigating the comparative anatomy and significance of the OP has stated that during evolution of higher apes and man OP is associated with a substantial decline in the frequency. In man, the OP may be interpreted as a regressive form of that bone constantly found in the Cercopithecidae and in the Hylobatidae [3].

The function and origin, as well as incidence of OP is controversial, and Girnbaum et al., have stated that it ranges from 2.3% to 9% in radiological studies while in anatomical research it varies from 8.5 to 26% as reported in the literature [4]. Many support the idea that the OP is always present, but is completely ossified in only about 20% of the population [6]. Seon Jeong et al., while stating about Painful Os Peroneum Syndrome (POPS) presenting as lateral plantar foot pain have reported the presence of an OP between 4.7% and 30% of normal feet, and in some cases they were unilateral [7]. In the present study OP was found as a thickening on PL tendon where it was angulated and passed through peroneal sulcus of cuboid; and was present bilaterally in all the limbs examined.

Xiao-Tian Wang et al., stated that the prevalence of the OP is unknown. When ossified, it is visible in 20% of foot radiographs [5]. When the tendons were subjected to radiological investigations, we could observe calcification of these nodules in 35% of the OP only.

Muehleman et al., stated that 30% tendons displayed an OP both radiologically and histologically [8]. We've found that the OP was either cancellous bone or fibrocartilage or mere thickening of collagen fibres. Where OP was present as a hypertrophy of collagen bundles, the deep surface was covered by fibrocartilage or hyaline cartilage. In the present study, the average length of OP was 13.14 mm, average breadth of OP was 8.96 mm & average thickness of OP was 4.11 mm; but as the literature reviewed does not show similar parameters being studied by other workers; hence, no comparison can be drawn.

The OP is almost always single but may be multipartite in some individuals [6]. They are sometimes be mistaken for a fractured sesamoid. Fracture lines in the sesamoids usually run transversely, resembling bipartite sesamoids. A fracture line, however, will be irregular, whereas the margin of a bipartite sesamoid is smooth and regular [1].

The Painful Os Peroneum Syndrome (POPS), a term coined by Sobel et al., includes OP fracture or a diastasis of a multipartite OP. Clinical diagnosis of the POPS can be facilitated by the Single stance heel rise and Varus inversion stress test [9]. The presence of an OP can predispose to rupture of the PL tendon at the cuboid level, with or without, concomitant fracture [10,11].

Investigators studying cadaveric specimens find more sesamoids than when the data is obtained from just x-ray evaluation. This is an important fact for the Podiatric surgeon to keep in mind, particularly in cases where there may be symptoms of POPS and no radiographic evidence of OP [1].

The play of PL on the Cuboid can be easily appreciated when one finds presence of a synovial joint with the articular surfaces which may be single or double. On the right double facets (on Cuboid & Calcaneum) were more (25.8%) as compared to the left (16.2%). When double facets were present; the facet on Cuboid was larger than on Calcaneum [Table/Fig-1b]. We did not come across a reference to double articular facet of OP in the literature reviewed.

Every time when we see the nature's ingenuity in the structural and consequently in the functional adaptation one is awestruck by it and OP is one such example. It is present at the site of angulations of PL tendon with attendant change in direction and the exposure to various stresses and strains leading to the thickening and formation of OP. Although present in all the limbs studied; it was fibrous, cartilaginous or bony. Thus, it seems that Os Peroneum is a misnomer and should be called as: Peroneal sesamoid.