by Brion Benninger, MD 1 , Jessica Kloenne 2
The Foot and Ankle Online Journal 4 (2): 2
Introduction: The purpose of this study was to assess radiograph incidence versus cadaver incidence rates of the os perineum (OP) within the fibularis (peroneus) longus tendon (FLT) and to assess why broad variants have occurred in previous radiograph and cadaver studies. The OP or sesamoid bone in the FLT has a history of controversy regarding the terminology and frequency. Recent histological studies have proven sesamoid terminology. Cadaver studies have revealed high incidence rates (IR), yet virtually all texts and atlases exclude it. Clinicians recognize it in routine foot radiographs. No studies have compared IRs between cadavers and injured patients of the general population.
Methods: A literature search of texts, atlases, journals and websites was conducted identifying incidence of OP within the FLT. Dissection of 82 embalmed cadavers (M 52, F 30) identified the IR of the OP. Oblique foot radiographs from 1,025 individuals were examined.
Results: A literature review revealed OP in 20% of atlases, 7.69% in texts, and previous cadaver study results are 46%, 90%, and 14.9%. This study’s cadaver results reported an IR of 88.46% with an average age of 78.1 (45 – 89yrs). Radiographic results revealed 15.12% incidence with an average age 41.97 (10 – 89yrs). The average IR from 10 to 70 years was 13.32%. From 70 onwards it increased to 32.98%. The p value per decade from radiographic analysis was 0.0005.
Conclusion: This study suggests there is a high IR of an OP in cadavers (88.46%). This may be a result of the average age of cadavers 78.1 and the technique used to locate the OP. Radiographic results were significantly lower and may be explained by an age factor. Radiographs reviewed were from an emergency room where the majority of patients receiving foot radiographs were between the ages of 19 – 45. The clinical importance has been understated regarding the area of the os peroneum, which can be mistaken for styloid and Jones’ fractures.
Key words: Os peroneum, incidence, foot injury, sesamoid bone, Jones fracture, styloid process.
Accepted: January, 2011
Published: February, 2011
The purpose of this study was to examine the important clinical relevance of the ‘os peroneum’ (OP) within the fibularis longus (FLT) by investigating the incidence of the OP between cadaveric specimens and radiological images.
The ‘os peroneum’ within the fibularis (peroneus) longus tendon has a history of controversy regarding its incidence in both individual and combined radiographic and cadaver studies. Radiographic and cadaveric studies have reported incidence rates of the OP within the FLT, however, a limited number of anatomical textbooks and atlases used by healthcare professionals and trainees mention or illustrate the OP.
Research to date reports a wide variation of incidence rates (IR), which are different in radiographic and cadaveric research. Previously Cilli, et al., in 2005 conducted a radiographic study on males only reporting 31.8% IR. A cadaveric study was conducted by Muehleman, et al., in 2009 which reported a 46% IR. Despite the various studies conducted thus far, no previous studies have conducted a comparison of incidence rates between radiographs and cadavers from separate populations. The objective of this research project was to assess radiograph incidence versus cadaver incidence rates of OP within the FLT and to assess why broad variants have occurred in previous radiograph and cadaver studies. (Fig. 1 and Fig. 2)
Figure 1 Os peroneum in the fibularis longus tendon. (Reproduced with kind permission from Lippincott Williams & Wilkins, Grant’s Anatomy, 12th edition.)
Figure 2 Location of os peroneum in the foot. (Reproduced with kind permission from Elsevier, Gray’s Anatomy, 40th edition.)
A literature search was conducted of anatomical texts and atlases, specialty texts, journals and websites regarding the presence or incidence of the OP within the FLT. Oblique foot radiographs from 1,025 individuals (range 10-89) were examined to identify OP within the FLT. (Fig. 3A and 3B) Dissection of 82 (156 sides) embalmed cadavers (52M, 30F) with a mean of 78.1 years (range 45- 89) was performed to identify the incidence rate of the OP within the FLT. A skin incision was made from the fifth toe to the styloid process distal to proximal along the lateral border of the foot. Then an oblique incision was made to the lateral malleolus to expose the FLT. (Fig. 4A)
Figure 3A and 3B Oblique radiographs of the OP in the FLT. (A and B) (Thanks to the OHSU Radiology Department for radiograph.)
At the styloid process a horizontal incision was made along the surface of the foot to the opposite side, then exposing the FLT within the tunnel it traverses. The FLT was freed from its attachment point distally and reflected back. Palpation of the FLT was performed to identify the OP. A longitudinal incision was performed 2cm proximal and distal to the OP and then opened to reveal the ‘sesamoid bone’s’ existence or not. (Fig. 4B) A second examiner palpated and analyzed the longitudinal incision and reported their findings. A paired t-test was conducted on the radiographic data.
Figure 4A and 4B OP within the FLT in a cadaver.
The literature search revealed the OP within the FLT was discussed in anatomy texts (7.69%), contemporary atlases (20%) and specific imaging texts (16.6%). This study’s radiographic evaluation of OP within the FLT from 1,025 individuals with a mean age of 41.97 years had an incidence rate of 15.12% overall.
Incidence by ten-year increments revealed 12.16% for 10-19 years, 11.31% for 20-29 years, 13.87% for 30-39 years, 16.17% for 40-49 years, 15.15% for 50-59 years, 10% for 60-69 years, 41.38% for 70-79 years, and 19.44% for 80-89 years. The number of radiographs analyzed per ten-year increment was from approximately 100 individuals. (Graphs 1 and 2) The p value for the radiographic images was 0.0005. In this study, the incidence rate of the cadaver dissections was 88.46% with a mean age of 78.1.
Graph 1 Radiographic incidence of the OP within the FLT per ten-year increment.
Graph 2 Identification of the OP within the FLT in educational texts.
The OP is a sesamoid bone that is located within the FLT.  The shape of the OP can be round, oval, triangular, irregular and can also be found as bipartite or multipartite. [6,7]
The etiology of the OP is unknown; however, it has been thought that it arises from both mechanical and genetic factors. [7,11] A literature search of contemporary anatomical texts, atlases and specialty radiographic texts revealed incidence rates of 7.69%, 20% and 16.6%, respectively. This lack of recognition of the OP in commonly used texts and atlases contradicts radiographic and cadaver evidence from our study.
Our study’s incidence rate of identifying the OP within the FLT from radiographs (15.12%) was consistent with other radiographic studies. (Table 1). Radiographic studies report incidence rates of 31.8%, 4.7%, 14%, 14% and 9%. [3,4,1,11,13] The reason the incidence of the OP within the FLT in the images of our study was less than cadaveric results may be due to the average age of the individuals from the radiographs (41.97 years).
Table 1 Review of OP within the FLT in radiographic and cadaveric studies.
The radiographs reviewed were from an emergency room where on average the majority of patients receiving foot radiographs are between the ages of 19-61.  The incidence rate might have been higher if the average age of the individuals was higher. Another study had a mean age of 51 years.  Two other studies only provided the range of their subjects and none were greater than 72 years. [3,4] Two other radiographic studies did not provide any information on the mean age or range. [1,13] To collect comprehensive research on the radiographic incidence of the OP within the FLT, further data is required in the age range of 60 and up.
The IR of the OP within the FLT in the cadavers (88.46%) was consistent with one cadaveric study with an incidence rate of 90%.8 This consistency may be related to similar methods of identification of the OP within the FLT. Recent cadaveric studies have reported incidence rates of 46% and 14.9%. [7,10]
One study assessed radiographic imaging from the 33 cadavers dissected, but did not look at separate populations for radiographic and cadaveric data.  Furthermore, that study did not report the incidence rate of the OP within the FLT when solely palpating the FLT on cadavers; their incidence rate was reported after radiographic and histologic investigation. A combined radiographic (500 individuals) and cadaveric (20 cadavers) study showed 12.3% incidence rate; this study did not separate incidence rates for radiographic and cadaveric results. 
In our study, the average age of the cadavers (78.1 years) is much older than the average age of the individuals in the radiographs, which may contribute to the high incidence rate of the OP within the FLT in the cadavers. Other studies that researched incidence rate of the OP within the FLT in cadavers had mean ages that were consistent with our study (81.0, 75.2 and 77.7 years). [7,8,10] However, the age range (33-97 years) was only given for one of these studies. 
The method of identification may also contribute to the high incidence rate of the OP within the FLT because our study palpated and dissected open the OP, but did not use histology or radiology to confirm presence of OP from cadavers.
A conflict between cadaveric and radiographic results is the fact that cadaver incidence can be based on a partially ossified OP. Therefore a partially ossified OP could be recorded as positive. In contrast, an incomplete ossification may not be obvious or present on typical radiographs. A partially ossified OP can be cartilaginous and at times cartilage cannot be recognized on ordinary (not over or under penetrated images) radiographic images. 
A possible factor affecting the radiographic results of our study is that we used only those patients who presented to the emergency room with foot pain. It is not common for people over age 60 to present to the emergency room with sprained ankles or fractured 5th metatarsals. The over 60 age group population present acutely with hip fractures or with gout of the great toe. There may have been different results if we had randomly chosen from the general population for OP in the FLT.
The clinical importance has been understated regarding the area of the os peroneum, which can be mistaken for styloid and Jones’ fractures. The radiograph IR was always over 10% regardless the age group while the cadaveric incidence rate was 88.46%. This suggests that teaching the OP in the FLT is clinically relevant because lower limb injuries are common.
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Address correspondence to: Oregon Health & Science University
611 SW Campus Drive, Portland, OR 97239.
1 Department of Surgery, Department of Integrative Biosciences, Department of Orthopaedic Surgery & Rehabilitation, Department of Oral Maxillofacial Surgery, Oregon Health & Science University, Portland, OR.
2 Department of Integrative Biosciences, Oregon Health & Science University, Portland, OR.
© The Foot and Ankle Online Journal, 2011