Clinical History: A 32 year old male patient presented with pain in the tarsal region and burning in the dorsal foot, after a twisting injury. (1a) Sagittal T1, (1b) sagittal fat saturated fast spin echo (FSE) T2, and (1c) coronal fat saturated proton density (PD) weighted magnetic resonance (MR) images of the ankle are provided. What are the findings? What is your diagnosis?
Bipartite medial cuneiform, with degenerative changes in both segments.
Bipartite medial cuneiform is a rare segmentation anomaly of the midfoot, which can be mistaken for a fracture. Most commonly, the bipartite medial cuneiform is asymptomatic, but it can be a source of traumatic or non-traumatic foot pain, which may require surgical intervention. This developmental anomaly has a characteristic appearance on radiographs and magnetic resonance imaging (MRI), which helps to identify and distinguish this entity from a fracture.
The medial (or first) cuneiform is the largest of the three cuneiform bones. The name cuneiform derives from the Latin cuneus (wedge) and forma (likeness), based on its appearance. The medial cuneiform articulates with the navicular posteriorly, the middle (or second) cuneiform laterally, and the base of the first and second metatarsals anteriorly. In addition to the naviculocuneiform, first tarsal-metatarsal, and Lisfranc ligament attachments, portions of the tibialis posterior, tibialis anterior, and peroneus longus tendons also insert on the medial cuneiform.
The medial cuneiform develops by endochondral bone formation. Typically, the lateral cuneiform ossification center appears first at one year of age followed by the medial and middle cuneiforms at two and three-four years, respectively. The medial cuneiform ossification center typically appears between one and four years of age, but can occur earlier, and reaches adult morphology by approximately 6 years1,2,3. Single, bipartite, and multicentric ossification of the medial cuneiform can all be normal developmental findings and do not necessarily result in a permanent adult type bipartite form1.
Most commonly, medial cuneiform embryogenesis occurs by a single primordial mesenchymal structure2,3. Rarely, embryogenesis occurs by separate dorsal and plantar primordia. Bipartition may occur by initial division of the mesenchymal primordium or when a single cartilaginous anlage gives rise to two separate ossification centers2,4,5,6,7.
Bipartite medial cuneiform is a rare anatomic variant that occurs in approximately 0.3% of the population4,7,8,9. Higher frequencies have been reported, but do not correlate with clinical incidence, are felt to be less reliable and likely skewed by sample size 2 and/or sample population. Bipartite medial cuneiform demonstrates a male frequency, is commonly bilateral, and is probably familial2.
Numerous clinical, radiologic, anatomical, and archaeological reports of bipartite medial cuneiform are available. Recently, Jashashvili et al. reported the archaeological discovery of a bipartite medial cuneiform at the Early Pleistocene site of Dmanisi, the oldest known instance in the Hominin fossil record2.
Bipartition of the medial cuneiform can be partial or complete (Figures 4 & 5). In complete bipartition, the medial cuneiform is divided into dorsal (upper) and plantar (lower) segments. The two segments have been referred to as os cuneiform I dorsale and os cuneiform I plantare2,8. Typically, the plantar segment is larger than the dorsal segment. Partial bipartition can result in two partially fused segments with a cleft demarcating the dorsal and plantar segments or when there is division of the distal articular surface.
The dorsal and plantar surfaces of the bipartite medial cuneiform have been described to articulate with each other by a gliding or arthrodial joint, synchondrosis, syndesmosis, or some combination2,4,6,8,10,11. (Figures 6,7 & 8) Both the dorsal and plantar components articulate with the first metatarsal. Typically, both dorsal and plantar components articulate with the navicular and middle cuneiform; however reports of only one portion of the bipartite cuneiform articulating with the navicular or middle cuneiform have been described8.
When bipartition occurs, the overall shape of the medial cuneiform is conserved, but the size of the two combined bones is larger than that of a normal medial cuneiform. The base of the first metatarsal is also larger at the articulation with the bipartite cuneiform and demonstrates two articulating facets. The combined overall size of the bipartite form and size of the first metatarsal base help in differentiating this normal variant from a fracture.
Isolated medial cuneiform fractures are rare. Medial cuneiform fractures occur more commonly in conjunction with other fractures and typically, demonstrate bone marrow edema in the acute and subacute settings. The cleavage plane between the fractured medial cuneiform is typically irregular, whereas the bipartite medial cuneiform demonstrates well corticated, and frequently smooth margins. (Figure 9)
In 2008, Elias et al. described the characteristic “E-sign” on sagittal MR images related to the configuration of joint spaces between the base of the first metatarsal and bipartite medial cuneiform bones9. The “E-sign” can also be seen on lateral radiographs. (Figures 10 and 11) The bipartite medial cuneiform is well depicted on a 30 degree oblique radiograph of the foot11.
Although bipartite medial cuneiform is usually asymptomatic, symptomatic forms related to biomechanical factors and trauma have been described, including fracture through the synchondrosis10,11,12. Successful surgical treatments have been performed including excision of the synchondrosis and fusion of the two components with lag screws, and excision of a portion of the bipartite medial cuneiform. Both procedures have yielded satisfactory post-operative results with resolution of symptoms10,11.
Additional congenital anomalies have been reported in individuals with bipartite medial cuneiform, including os intermetatarseum, accessory navicular, spinal segmentation anomalies, lunotriquetral coalition, accessory temporal bone suture, and bipartite temporal bone8.
Bipartite medial cuneiform is a rare, congenital segmentation anomaly of the foot, which is most often incidental and asymptomatic. The bipartite medial cuneiform can be differentiated from a fracture by the characteristic appearance on MRI and radiographs, including the “E-sign”, well corticated margins, the size of the combined cuneiform segments, the size of the base of the first metatarsal, and edema pattern. Rarely, a bipartite medial cuneiform can be symptomatic, in which case surgery has proven successful in alleviating symptoms.
1 Freyschmidt J, Brossmann J, Wiens J, Sternberg A. Borderlands of Normal and Early Pathological Findings in Skeletal Radiography, Fifth revised edition. Thieme. New York, NY; 2003.
2 Jashashvili T, Ponce de Leon MS, Lordkipanidze D, Zollikofer CP. First evidence of bipartite medial cuneiform in the hominin fossil record: a case report from the Early Pleistocene site of Dmanisi. Journal of Anatomy 2010; 216(6):705-16.
3 Scheuer L, Black S. Developmental Juvenile Osteology. Academic Press. London; 2000.
4 Barlow TE. Os cuneiforme 1 bipartitum. Am J Phys Anthropol 1942; 29:95-111.
5 Cihak R. Ontogenesis of the skeleton and intrinsic muscles of the human hand and foot. Adv Anat Embryol Cell Biol 1972; 46:7-177.
6 O’Neal ML, Ganey TM, Ogden JA. Fracture of a bipartite medial cuneiform synchondrosis. Foot & Ankle International 1995; 16(1):37-40.
7 Pfitzner W. Beitrage zur Kenntniss des menschlichen Extremitatenskelets. [Contributions to the knowledge of the human limb skeleton.] Morphol Arbeiten 1896; 6,245-528.
8 Burnett SE, Case DT. Bipartite medial cuneiform: New frequencies from skeletal collections and meta analysis of previous cases. Journal of Comparative Human Biology 2011; 62,109-125.
9 Elias I, Dheer S, Zoga AC, Raikin SM, Morrison WB. Magnetic resonance imaging findings in bipartite medial cuneiform – a potential pitfall in diagnosis of midfoot injuries: a case series. Journal of Medical Case Reports 2008; 2:272.
10 Azurza K, Sakellariou A. “Osteosynthesis” of a symptomatic bipartite medial cuneiform. Foot & Ankle International 2001; 22(6):499-501.
11 Chiodo CP, Parentis MA, Myerson MS. Symptomatic bipartite medial cuneiform in an adult athlete: a case report. Foot & Ankle International 2002; 23(4):348-351.
12 Fulwadhva U, Parker RJ. Symptomatic bipartite medial cuneiform. Applied Radiology 2007; 3:42-44.