Talus bipartitus (also known as talus partitus or frontal split) is a rare anatomic variant of the talus, first described by Strehle in 19281. Since then, to the best of our knowledge, only seven more cases have been described in the medical literature, and all of these cases occurred in adolescents who were between thirteen and eighteen years of age2-7. Three recent reports utilized a computed tomography (CT) scan for evaluation5-7, but none outlined the exact three-dimensional anatomy of the ossicle. A detailed follow-up has not been provided on any of the prior patients.
The term talus bipartitus describes a large, separate piece of bone that constitutes about one-third of the posterior aspect of the talar body and is separated from that structure by a frontal split. A talus bipartitus is therefore considerably larger than an os trigonum, which contains part of the lateral tubercle of the posterior process of the talus. The os trigonum is not uncommon, with a calculated prevalence of 7% to 13%8,9.
Over a period of ten years, we have observed four young adult patients with radiographic findings of talus bipartitus. The clinical and radiographic findings were different in all four cases, and treatment consequently had to be tailored individually.
This is the first case series of this rare anatomic variant, and we report for the first time on the three-dimensional appearance of this separate ossicle. An average follow-up of 2.5 years after treatment was obtained. The patients were informed that data concerning the cases would be submitted for publication, and they all consented.
Case 1. A thirty-one-year-old woman presented with a fourteen-month history of occasional pain in the right ankle. She had a history of repetitive inversion sprains of both ankles during competitive sports during childhood, and the last severe sprain occurred three months prior to presentation. Pain was marked during exercise and had been increasing steadily over the last year. On clinical examination, there was chronic ligamentous laxity in both ankles. Slight edema was noted over the posterolateral aspect of the right ankle, but no pain on palpation could be elicited. Right ankle motion was restricted by 15° as compared with the motion on the left side (10° vs. 15° of dorsiflexion and 40° vs. 50° of plantar flexion), while inversion motion at the right subtalar joint was restricted only by 10°.
Standard radiographs showed a bipartite talus with a separate bone on the posteromedial aspect, which appeared displaced (Fig. 1). CT and magnetic resonance imaging (MRI) revealed an accessory bone, approximately one-third the size of the talar dome, that extended into both the ankle and subtalar joints. No signs of osteonecrosis of the talar body or the additional ossicle were detected.
A posteromedial approach to the talus was carried out via a longitudinal incision made approximately 4 cm parallel and 1 cm medial to the Achilles tendon. The posterior tibial neurovascular bundle was retracted medially, together with the flexor hallucis longus tendon and muscle. Capsular release provided good surgical exposure of the posterior aspect of the ankle and subtalar joints. During surgery, the posterior ossicle was loosely connected to the main talar body with fibrous tissue. The articular cartilage on the ankle and subtalar joints appeared normal. The distal surface of the ossicle was covered completely by cortical bone and patches of fibrocartilage, as was the corresponding portion of the talus. Because of the size of the fragment, the presence of talar congruency, and the intact articular cartilage, the accessory bone fragment was fixed to the anterior part of the talar body with two 3.5-mm cortical screws after predrilling the corresponding facets to enhance osseous union. A split below-the-knee cast was applied postoperatively and was worn for one week. Range of motion exercises were begun at the second postoperative day. The patient was restricted to partial weight-bearing for eight weeks.
Six months after fixation of the fragment, CT scanning showed anatomic alignment. On final follow-up three years postoperatively, the patient was pain-free during activities of daily living (ADL) and had full ankle motion. Inversion at the subtalar joint was slightly restricted as compared with that on the contralateral side (35° vs. 45° of inversion, respectively, with an eversion of 15° for both ankles).
Case 2. A twenty-five-year-old man presented with a three-year history of intermittent pain in the right ankle, with no prior history of trauma. During routine medical examination for military service eight years previously, an osseous abnormality had been discovered at the talus, and he had been suspended temporarily from service.
On first presentation at our institution, he reported occasional mild pain in the right ankle with exercise. On clinical examination, there was no apparent deformity at the right ankle, but atrophy of the calf muscle was present. When walking barefoot, the patient displayed a slight right-sided limp. Standing on tiptoes caused right ankle pain, while there was only slight pain on palpation over the posterior aspect of the talus. Right ankle motion was limited to 10° of plantar flexion (compared with 40° on the left side) while dorsiflexion was 20° at both ankles. Eversion and inversion of the right foot were limited by 10° and 15°, respectively.
Radiographs showed a large fragment that constituted about one-third of the posteromedial aspect of the talar body (Fig. 2). CT demonstrated the irregular shape of the fragment, with narrowing of the joint space and the formation of subchondral cysts within the talus, both of which are signs of degenerative arthritis, on the posterior aspect of both the ankle and the subtalar joints. MRI did not demonstrate osteonecrosis of the talus but showed effusion at the ankle and subtalar joints.
Because of gradually increasing pain and limitation of ankle motion over the next six months, surgery was scheduled. The radiographic findings of arthritis were confirmed during surgical exploration via a posterolateral approach to the posterior aspect of the talus. The flexor hallucis longus tendon was retracted medially to expose the posterior aspect of the ankle and subtalar joints. Because of the incongruity of the osseous fragment and the existing damage to the cartilage, the fragment was completely excised. Postoperatively, the patient was allowed to bear weight as tolerated and ankle motion exercises were begun immediately.
The patient continued to have pain over the lateral aspect of the hindfoot for about four months after surgery. However, on final follow-up, two years after resection of the ossicle, the symptoms were substantially reduced. The patient experienced only mild pain over the right ankle with exercise and was unable to play soccer. He worked full time as a mechanic and did not experience any functional restriction during ADLs. Plantar flexion of the ankle joint was restored to 35°, but subtalar motion was still restricted (Fig. 3).
Case 3. A twenty-seven-year-old woman was referred to our outpatient clinic after radiographs for the evaluation of an inversion ankle sprain appeared to show a talar nonunion. MRI revealed a bipartite talus with a frontal split in the posterior aspect of the talar dome, without accompanying joint effusion (Fig. 4, A). At the time of presentation, the patient had no pain during ADLs. Foot function during walking and running was not affected.
On clinical examination, there was slight edema but no pain on palpation over the posterolateral aspect of the ankle. Plantar flexion of the ankle was restricted by 10° as compared with the unaffected side. Because discomfort was minimal, the patient declined any further interventions and no special treatment was initiated. At the time of follow-up, three years after the first presentation, the symptoms were still minimal and the clinical findings were unchanged. A CT scan did not reveal any signs of arthritis at the ankle or subtalar joints (Fig. 4, B).
Case 4. A twenty-two-year-old female patient was referred for a suspected talar nonunion. She had experienced increasing pain in the left ankle over the previous two years but did not recall any trauma to her ankle. Plantar flexion of the ankle was restricted to 20° as compared with 40° on the unaffected side. In addition, flexion of the great toe was markedly restricted and elicited pain on the posteromedial aspect of the ankle. Radiographs revealed a large separate bone fragment posterior to the talar body (Fig. 5). CT and MRI showed the typical features of talus bipartitus. The fragment was incongruent with both the ankle and subtalar joints, and there were signs of degenerative arthritis. Impingement of the flexor hallucis longus tendon was seen on surgical exploration via a posterolateral approach similar to that described in Case 2 (Fig. 6). Consequently, the separate bone fragment was removed completely and was evaluated histologically (Fig. 7).
At the final follow-up, one year after surgery, the patient reported that she had no pain during the performance of ADLs and that she experienced mild pain over the posterior aspect of the ankle after dancing and when walking long distances over uneven ground. Plantar flexion of the ankle had increased to 30°. Flexion of the great toe was still restricted but was no longer painful. No progression of the preexisting radiographic signs of arthritis was noted at the time of final follow-up.
Talus bipartitus is a rare anatomic variation of the foot. Pfitzner, in his exhaustive review of abnormal bones in more than 1000 consecutive macerated cadaver feet and extensive review of the anatomic literature, did not describe a single case of this variant8. Tsuruta et al., when reviewing the radiographs of 3460 feet, did not find a single case of talus bipartitus9. They noted an os trigonum in 438 feet (12.7%) and a talus accessorius beside the trochlea tali in six feet (0.2%). Since the first description by Strehle in 1928, to the best of our knowledge only seven more cases have been added to the literature2-7.
Strehle1 described the case of a fourteen-year-old girl who had an eight-year history of exercise-induced ankle pain and mild swelling. In contrast to the case of our patient in Case 2, the patient had pain when walking on her heels and relief when walking tiptoe. On radiographs, a frontal split in the talus was apparent. Both the talar body and the additional bone seemed somewhat deformed and, if combined, would not have made up a normal talus, a finding that was similar to those in our patients in Case 2 and Case 4. Strehle suspected either a bipartite talus or a talar malformation with an additional os trigonum. No mention was made regarding the treatment.
Weinstein and Bonfiglio2 reported the case of a thirteen-year-old boy who had pain and swelling over the lateral malleolus over a period of five months. The fragment, which measured 2.0 × 2.5 cm, was excised. Histologic examination showed normal endochondral ossification in the accessory ossicle. The facet facing the anterior talar body displayed degenerative hyaline cartilage, signs of repair with fibrocartilage, and bone remodeling. The authors suspected a second ossification center at the talus. No follow-up was provided.
Schreiber et al.3 reported on a fifteen-year-old girl who had pain for four years. A technetium-99 bone scan showed increased uptake at the posterior part of the talus. Because symptoms were slight, no intervention was indicated at that time.
Blauth et al.4 described the case of an eighteen-year-old female patient who had a two-year history of ankle pain during walking and substantial reduction in ankle motion. The frontal split in the talus measured several millimeters on conventional tomography, and there was sclerosis at the bone margins facing the split. Because the posterior fragment extended far into the subtalar joint, the patient was treated conservatively with an orthopaedic shoe. Six years later, the pain had been reduced and the shoe was worn only intermittently.
In a book chapter on radiographic imaging of the foot and ankle, Hamel5 showed conventional tomographic and CT images of talus bipartitus in a thirteen-year-old boy who had had ankle pain for more than one year. The ossicle had been resected, but the patient continued to have pain and a varus ankle at the time of short-term follow-up. A single axial CT image shows the typical outline of the ossicle in the horizontal plane, as seen in our four patients.
Griffet et al.6 described talus bipartitus in a fifteen-year-old girl with daily ankle pain during walking, with resultant fragment excision. The space between the ossicle and the talar body was filled with fibrous tissue. A subtalar fusion with a bone block from the iliac crest was carried out because the articular cartilage showed arthritic changes. Short-term pain relief was reported, but pain recurred two years later along with 10° of restriction of ankle dorsiflexion.
Recently, Eichenbaum et al.7 reported on two patients with symptomatic talus bipartitus. In an eighteen-year-old man who had a history of ankle pain for more than four years, surgical exploration revealed a dysmorphic fragment with signs of arthritis at the subtalar joint. Because the subtalar joint was judged to be unstable after excision of the accessory fragment, a subtalar bone block arthrodesis was performed. A histologic section was provided that showed dense fibroelastic tissue and sclerotic bone on the margin of the ossicle, but the location of this section was not specified. The other patient, a sixteen-year-old girl with a fourteen-month history of ankle pain, was treated with fragment excision only and had improvement in pain and stability after six months.
In contrast to the previously reported cases in adolescents only, our four patients were between twenty-two and thirty-one years of age at the time of first presentation, so it is possible that talus bipartitus may remain clinically silent beyond adolescence. The data of all twelve cases reported as of this writing are summarized in Table I. Common to all reported cases was the gradual onset of pain over several months or years with no history of trauma other than ankle sprains. The posterior fragment of a bipartite talus is considerably larger than an os trigonum, which has a calculated prevalence of 7% to 13%8,9. The CT axial scans and MRI scans in our four patients revealed a varying oblique course of the cleft between the anterior talar body and the additional posterior ossicle. In the patients in Cases 1, 3, and 4, the cleft was running obliquely, resulting in an ossicle that was broader on the medial side than on the lateral side. In the patient in Case 2, the course of the cleft also started obliquely behind the lateral malleolus and continued to run in the frontal plane. Consequently, the resulting ossicle was broader on the lateral side because of the bigger size of the lateral tubercle of the posterior process as compared with the medial tubercle. These findings are consistent with the findings of the single axial CT images shown in two previous case reports5,7. Because all six cases were associated with an oblique course of the cleft, the term "frontal split" seems inaccurate and would best not be used as a synonym for this skeletal variant.
Talus bipartitus usually becomes symptomatic during adolescence and early adulthood. The clinical and radiographic symptoms may mimic those seen with a nonunion of a talar body fracture, but a history of appreciable trauma is missing. The junction between the ossicle and the talar body appears to resemble a symphysis or syndesmosis, so low-velocity injury may produce symptoms by loosening the fragments. Symptomatic loosening of an os trigonum has been described10,11, and a similar mechanism may have been present in some of the patients with talus bipartitus.
In contrast to the os trigonum, the posterior ossicle of the bipartite talus always articulates broadly with the subtalar joint and, to a lesser extent, with the ankle joint. It is covered by hyaline cartilage on the superior and inferior borders (Fig. 7). If the posterior ossicle is incongruent, this may lead to pain, restricted range of motion, impingement of the flexor hallucis longus tendon, and even arthritis of the ankle and subtalar joints. The severity of symptoms is related to the shape and displacement of the ossicle. Consequently, therapy has to be tailored to the individual patient. In the present case series, three different approaches were pursued. With minimal symptoms, the patient may be observed. If the ossicle is symptomatic but congruent to both the ankle and subtalar joints, refixation may be attempted. If signs of subtalar or ankle arthritis are seen, or if the ossicle is asymmetric and joint congruity cannot be achieved by fixation, resection is the treatment of choice. With progressive arthritis, arthrodesis may be contemplated6,7. Both the posteromedial and posterolateral approaches provide complete exposure of the posterior aspect of the ankle and subtalar joints, if surgical treatment is chosen. In our experience, a medial malleolar osteotomy is not necessary.
We have also observed a slight malposition of the talus as compared with the position of the talus in the unaffected, contralateral foot. This was probably due to the nonphysiological shape of the subtalar joint leading to increased talar inclination with subsequent pes cavus (Figs. 2, 3, 5, 6, and 7) or increased declination of the talus with subsequent pes planus (Fig. 1). No attempt was made to correct this malposition of the talus, while subtalar distraction bone-block fusion has been undertaken in two of the reported cases6,7. The relative paucity of symptoms in all of the surgically treated patients in the present series at a mean of 2.5 years of follow-up so far supports this less aggressive approach in treating this rare entity. Patients need to be counseled that, while full ankle motion will most likely be regained, subtalar motion will remain restricted as a result of the long-standing morphologic changes at the subtalar joint.
The origin of a bipartite talus is unclear. The talus ossifies from one center, which appears in the seventh month of gestation, but a two-part composition has been described in some cases12,13. It has been speculated that ossicles like talus bipartitus and os trigonum may be the consequence of a persisting secondary ossification center2,12. The presence of a strong cortical shell, covered in part by hyaline cartilage and dense fibrous tissue at the surface of the ossicle facing the anterior part of the talar body, seems to support this theory. The inferior part of this area in our patient in Case 4 showed signs of bone formation similar to the observation of Weinstein and Bonfiglio2. Some anatomists, in comparing different species, have held that the talus is phylogenetically derived from the os centrale I and os intermedium cruris in reptiles and that an os trigonum may represent failure of fusion of these two formerly independent bones, thus representing an atavistic trait14,15. The same may be true for a talus bipartitus. This view, however, was contradicted by other anatomists16. The three histologic specimens of a talus bipartitus that have been provided so far could not prove the existence of a secondary ossification center because they were retrieved in skeletally mature patients.
Weinstein and Bonfiglio synonymously termed the ossicle accessory talus2. However, the term talus accessorius has already been used by Pfitzner and others for an ossicle beneath the medial malleolus, lying beside the talar trochlea, most likely representing an osseous avulsion of the deltoid ligament8. Tsuruta et al. saw this variant in six of 3600 radiographs, resulting in a calculated prevalence of 0.2%9.The term talus accessorius should therefore not be used synonymously for talus bipartitus.
It can only be speculated why we found four cases of this unusual anatomic variant over a period of just ten years while previously only eight cases have been reported and while series with 1000 feet and more did not encounter a single case8,9. Possibly, the prevalence is higher than previously reported and some of the cases remain clinically silent, as in the case of our patient in Case 3, in whom the condition was found incidentally. In many of the reported cases, including three of the cases in the present series, the anatomic variant was initially mistaken for a fracture nonunion. However, the absence of a history of trauma and the typical shape of the ossicle exclude a posttraumatic genesis because talar body fractures are typically produced by high energy17. Furthermore, the histologic appearance and the irregular shape of the posterior ossicle and the posterior facet of the calcaneus are not compatible with a fracture nonunion.
Eichenbaum et al.7 raised the theory of a traumatic injury during osseous immaturity, as both of their patients with talus bipartitus had a history of tibial torsion along with possible attempted manipulation by the family or a therapist. As the authors themselves concede, this is purely hypothetical, as none of the patients in the other reported cases had a history of tibial torsion. Furthermore, fractures of the immature talus are even less frequent than the talar fractures that are seen in adults because even higher forces may be required to fracture the mostly cartilaginous talus in children18. On the basis of the current data on this rare skeletal variant, neither a traumatic nor a phylogenetic origin can be fully dismissed or proven, although the clinical findings associated with the reported cases support a phylogenetic origin.
In summary, talus bipartitus is a rare skeletal variant of unclear phylogenetic origin that may represent a secondary ossification center in the talus. It is probably underreported and has to be distinguished from an os trigonum or fracture nonunion of the posterior part of the talar trochlea or posterior process. If the fragment is asymmetric or unstable, it may affect the ankle and subtalar joints. Treatment is tailored to the individual symptoms, functional restriction, congruity of the fragment, and the presence of arthritic changes. The outcome in the reported cases is generally favorable.
Note: The authors thank Professor T. Sato (Okayama University Medical School, Japan) for his review of the Japanese literature, Professor J. Hamel (Orthopaedic Foot and Ankle Surgical Center, Munich, Germany) for providing details on his case, and Mr. T. Albrecht, DGPh (University Hospital Dresden, Germany) for the clinical photographs.