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Degeneration of the Accessory Navicular Synchondrosis Presenting as Rupture of the Posterior Tibial Tendon*

YEUNG-JEN CHEN, M.D., ROBERT WEN-WEI HSU, M.D. and SHIH-CHIEH LIANG, M.D., TAIPEI, TAIWAN, REPUBLIC OF CHINA

Investigation performed at the Department of Orthopaedic Surgery, Chang Gung Memorial Hospital, Taipei

	Abstract

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Degeneration of the accessory navicular synchondrosis may be associated with decreased function of the posterior tibial tendon in patients who are middle-aged or older. We investigated the role of ultrasonography in differentiating between degeneration of the accessory navicular synchondrosis with separation of the accessory navicular from the navicular, which has not been previously reported to our knowledge, and a rupture of the posterior tibial tendon. We studied fourteen patients (mean age, fifty-five years; range, forty-one to seventy-two years) who had an operatively confirmed injury of the accessory navicular synchondrosis. The mean duration of follow-up was thirty-nine months (range, twenty-seven to fifty-four months). Preoperative radiographs demonstrated a type-II accessory navicular (an accessory navicular with a synchondrosis) in all fourteen patients. Ultrasonography, which was performed for twelve patients, demonstrated a defect in the synchondrosis in eleven patients and a normal posterior tibial tendon in all twelve. The operative findings included incomplete separation of the synchondrosis in four of the fourteen patients, complete separation of the synchondrosis and the periosteum in eight, and avulsion of the accessory navicular in two. On the basis of our findings, we concluded that post-traumatic degeneration of an accessory navicular synchondrosis may present clinically as a variant type of avulsion or rupture of the posterior tibial tendon in this age-group. Ultrasonography is useful for distinguishing between complete or partial separation through the synchondrosis and rupture or attenuation of the posterior tibial tendon.

	Introduction

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Rupture of the posterior tibial tendon has received attention in many recent reports and is considered to be a common cause of acquired flatfoot^3,14-16. The posterior tibial tendon is subject to a variety of pathological conditions and injuries, such as avulsion, longitudinal tear, and rupture of the mid-substance^4,6-10.

An accessory navicular can be classified into three types on the basis of the size and location of the ossicle and the presence or absence of a synchondrosis^17,20,23. According to Geist's classification¹⁷, type I is a small accessory bone within the substance of the posterior tibial tendon, without attachment to the body of the navicular; type II, an accessory navicular with a synchondrosis that has the appearance of a cartilaginous unit containing islands of hyaline and fibrocartilage; and type III, a cornual navicular that probably represents an end stage of type II. A tear of the synchondrosis of a type-II accessory navicular can cause symptoms in young patients^5,13. After an injury of the synchondrosis has resulted in a chondro-osseous disruption, the combined forces of tension and shear from the posterior tibial tendon and the foot aggravate the injury and prevent it from healing^19,20. However, few authors have mentioned the prognosis of post-traumatic degeneration of the synchondrosis in patients who are middle-aged or older.

A type-II accessory navicular occasionally is noted in patients who have a rupture of the posterior tibial tendon^12,18. However, the possible involvement of the synchondrosis in these patients has not been addressed. In addition, the possibility of a separation through the accessory navicular synchondrosis presenting clinically as a rupture of the posterior tibial tendon rarely has been mentioned¹⁹.

The diagnosis of a rupture of the posterior tibial tendon depends mainly on the findings of clinical examination¹⁵. However, because of the non-specific findings associated with the various etiologies of the lesion, it is often difficult to make a correct diagnosis when there is clinical suspicion of a rupture of the posterior tibial tendon³. If a patient with the clinical symptoms has an accessory navicular and there is tenderness near the insertion of the posterior tibial tendon on the navicular, the differential diagnosis should include separation of the synchondrosis in addition to a tear or avulsion of the mid-substance of the tendon. Accurate preoperative identification of these different lesions is necessary because the treatment varies.

Ultrasonography is a highly effective method for evaluation of tendinous abnormalities, as evidence of a diffusely edematous tendon with surrounding fluid or frank rupture can easily be detected with high-resolution equipment^2,22. However, ultrasonography is not commonly used for the diagnosis of an injury of the synchondrosis between the accessory navicular and the navicular.

The current study was undertaken in order to identify the clinical, radiographic, ultrasonographic, and operative findings of degeneration of the accessory navicular synchondrosis and to determine the efficacy of ultrasonography for distinguishing between a rupture of the posterior tibial tendon and degeneration of the accessory navicular synchondrosis with separation of the accessory navicular from the navicular. Such separation has not been reported previously, to our knowledge.

	Materials and Methods

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The study included fourteen women who had a symptomatic accessory navicular associated with decreased function of the posterior tibial tendon between 1990 and 1993 (Table I). The mean age of the patients was fifty-five years (range, forty-one to seventy-two years). The diagnosis for all patients was made on the basis of pain in the medial aspect of the hindfoot and weakness of the posterior tibial muscle associated with radiographic or ultrasonographic evidence of degeneration of the synchondrosis with partial or complete separation of a type-II accessory navicular from the navicular. Eleven patients had an accessory navicular bilaterally, although none had symptoms bilaterally. The mean preoperative duration of the symptoms was twenty months (range, six to thirty-two months). Twelve patients could recall a mild twisting injury of the foot that had resulted in an insidious onset of pain and deformity. The remaining two patients had had an acute injury leading to severe pain and deformity. None of the patients had rheumatoid arthritis or seronegative disease.

Radiographs demonstrated a type-II accessory navicular in all fourteen patients. In Group A, the longitudinal arch was mildly decreased in four patients and severely decreased in two. The latter two patients, who had been clinically diagnosed as having a complete tear of the posterior tibial tendon, had evidence of a complete avulsion of the accessory navicular on the radiographs. In Group B, all patients had a normal arch.

Preoperatively, twelve of the fourteen patients had ultrasonographic evaluation after the radiographic examination (Table II). Ultrasonography was performed with use of a ten-megahertz probe (Aloka 1200, Tokyo, Japan) in the longitudinal and transverse planes to detect possible separation of the synchondrosis between the accessory navicular and the navicular and to assess the continuity of the posterior tibial tendon. The remaining two patients did not have ultrasonography because avulsion of the accessory navicular was confirmed with radiography alone.

Ten patients had excision of the accessory navicular with reattachment of the posterior tibial tendon. Four patients had an arthrodesis because of a fixed and irreducible flatfoot deformity.

	Results

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Ultrasonography
The osseous contours of the navicular and the accessory navicular were clearly visible on the ultrasonograms of the twelve patients for whom they were made, and the findings corresponded well with those on the plain radiographs. The continuity of the posterior tibial tendon was assessed by moving the transducer proximally along the tendon. Ultrasonographic images of a normal synchondrosis are echogenic and homogeneous, and the ultrasonograms of the unaffected synchondrosis in the patients who had an accessory navicular bilaterally demonstrated these findings (Fig. 1-A). The homogeneous zone representing a normal synchondrosis was demonstrated on sequential images, made with the transducer in different positions. In contrast, the ultrasonograms of the injured and degenerative areas demonstrated a diastasis (Fig. 1-B), with a wider gap seen in the patients who had complete separation of the synchondrosis and the periosteum (Fig. 2-A). In some patients, the space between the navicular and the accessory navicular is small, and marginal osteophytes may narrow the space evenly, preventing proper assessment of the synchondrosis. Thus, to prevent false-negative results, the transducer was placed in different positions during the examination. Eleven of the twelve patients had a diastasis and heterogeneous changes in the synchondrosis. The images of four patients showed a two to four-millimeter area representing fluid that had collected around the synchondrosis and the distal aspect of the posterior tibial tendon. The continuity of the posterior tibial tendon and its insertion into the accessory navicular were normal in all twelve patients (Fig. 2-A). One patient (Case 14) had a small amount of fluid around the synchondrosis, but ultrasonograms failed to demonstrate separation of the synchondrosis because of the remodeling process, which included marginal osteophyte formation on the accessory navicular. This patient was managed operatively because of persistent pain and decreased function of the posterior tibial tendon. Complete separation of the synchondrosis was found intraoperatively.

View larger version (143K): [in this window] [in a new window]   Fig. 1-A Ultrasonogram showing a normal synchondrosis (arrow), which is echogenic and homogeneous. NAVI. = navicular and A.N. = accessory navicular.

View larger version (76K): [in this window] [in a new window]   Fig. 1-B Ultrasonogram showing a synchondrosis (arrow) with degenerative and heterogeneous changes, including a diastasis. NAVI. = navicular and A.N. = accessory navicular.

View larger version (138K): [in this window] [in a new window]   Fig. 2-A Longitudinal ultrasonogram showing degeneration of the synchondrosis (white arrow) and an intact posterior tibial tendon (PTT. [black arrow]). AN. = accessory navicular and NAVI. = navicular.

Operative Findings
Three pathological conditions were identified in the current series. Twelve of the fourteen patients had an intact posterior tibial tendon; the pathological findings included a partial tear of the periosteum and partial separation of the synchondrosis in four of these patients and a complete tear of the periosteum and complete separation of the synchondrosis in eight. In the two remaining patients, the accessory navicular was completely avulsed. The operative findings confirmed that the posterior tibial tendon was intact in the four patients who were noted to have accumulation of fluid around the tendon on ultrasonography.

The severity of the clinical symptoms corresponded well with the operative findings. The four patients who had an incomplete tear of the periosteum and incomplete separation of the synchondrosis had pain localized to the prominence without deformity of the foot. The function of the posterior tibial tendon was found to be mildly affected when forceful inversion was tested. Eight patients (four in each group) had a complete tear of the periosteum and complete separation of the synchondrosis; these patients had marked tenderness over the synchondrosis, and the affected posterior tibial tendon was weaker than the tendon on the normal side. Of these eight patients, the four in Group B performed the single-heel-rise test with moderate pain and the four in Group A had severe pain during the test and mild collapse of the talonavicular joint seen on radiographs. The two patients, in Group A, who had complete avulsion of the accessory navicular were totally incapable of performing the single-heel-rise test and had a severe deformity of the foot.

Excision of the accessory navicular with suturing of the posterior tibial tendon to the navicular was performed in eight patients; excision of the accessory navicular with distal advancement of the posterior tibial tendon, in two; talonavicular and calcaneocuboid arthrodesis, in two; and triple arthrodesis, in two (Table I). In Group A, two patients (Cases 5 and 12), who had a completely avulsed accessory navicular and a severe flatfoot deformity with a valgus deformity of the hindfoot, were managed with a triple arthrodesis. Two other patients (Cases 2 and 4) in Group A, who had associated pain in the lateral aspect of the hindfoot because of the fixed deformity of the foot, had a talonavicular and calcaneocuboid arthrodesis. The remaining two patients (Cases 1 and 8; Figs. 2-A, 2-B, and 2-C) in Group A, who did not have associated pain in the lateral aspect of the hindfoot and had a flexible flatfoot deformity, had excision of the accessory navicular with distal advancement of the posterior tibial tendon. The foot then was immobilized for eight weeks in approximately 30 degrees of equinus and 10 degrees of inversion. All eight patients in Group B had excision of the accessory navicular with suturing of the posterior tibial tendon to the navicular. The foot then was immobilized in 10 degrees of equinus and 5 degrees of inversion with a below-the-knee cast for six weeks.

View larger version (127K): [in this window] [in a new window]   Fig. 2-B Anteroposterior radiograph of a forty-one-year-old woman (Case 1) who had a type-II17 accessory navicular (arrow).

View larger version (182K): [in this window] [in a new window]   Fig. 2-C Intraoperative photograph showing a complete tear of the synchondrosis (arrow) with a diastasis.

Functional Results
The functional result was graded as good if the patient was pain-free, had good strength of the posterior tibial tendon, and demonstrated good function on the single-heel-rise test; as fair if the patient had occasional pain and good function of the posterior tibial tendon; and as poor if the patient had persistent pain with weakness of the posterior tibial tendon. The duration of follow-up ranged from twenty-seven to fifty-four months (mean, thirty-nine months). In Group A, the four patients who had had an arthrodesis had a solid fusion, and three of the four had a good functional result. The fourth patient (Case 5), who had had a severe deformity before the triple arthrodesis, had a fair result because of occasional pain in the hindfoot. Of the other two patients in Group A, who had had advancement of the tendon, one (Case 1) had a good result and one (Case 8) had a fair result. The latter patient had mild pain in the hindfoot on weight-bearing, although the function of the posterior tibial tendon was good. In Group B, seven of the eight patients had a good result. The eighth patient (Case 3) had a fair result because of dull tenderness at the site of the insertion of the posterior tibial tendon on the navicular.

	Discussion

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Rupture of the posterior tibial tendon with loss of function was first reported, to our knowledge, by Key, in 1953. Kettelkamp and Alexander, in 1969, reported on four patients who had a spontaneous rupture of the posterior tibial tendon with flatfoot deformity; two of these patients had a rupture of the mid-portion of the tendon, one had an avulsion rupture at the site of the insertion, and one had attenuation. Since then, various patterns of rupture of the posterior tibial tendon have been reported^3,4,6,16. The wide range of operative findings, including avulsion, attenuation, and complete rupture, suggests that the clinical symptoms may result from several different types of pathological conditions^{1,3,6,7,15,16}. However, loss of function of the posterior tibial tendon caused by avulsion of an accessory navicular from the navicular through the synchondrosis has rarely been described.

The accessory navicular is a congenital anomaly in which a tuberosity develops from a second center of ossification^11,21,23. The synchondrosis is a fibrocartilaginous plate between a type-II accessory navicular and the main navicular^5,13,17. A separation of the synchondrosis is considered to be one of the main causes of symptoms in a patient who has an accessory navicular^5,13. Sella et al. suggested that the injury of the synchondrosis might be the result of a simultaneous force of compression and tension, caused partially by contracture of the posterior tibial tendon^19,20.

Most of our patients recalled a mild twisting injury of the foot. They had had immediate pain but had ignored it. The pain had became insidiously worse until the time that they had visited our clinic. These patients described recurrent, occasional pain in the medial aspect of the midfoot after the injury, especially after prolonged exercise. On examination, the function of the posterior tibial tendon was found to be decreased because of the pain near the synchondrosis. Some patients were able to perform the single-heel-rise test with moderate-to-severe pain, whereas others were unable to perform the test.

The clinical presentation of degeneration of the synchondrosis in middle-aged patients rarely has been mentioned in the literature. The proposed pathomechanism of the injury may explain the variety of operative findings. Unless an injury of the synchondrosis is initially treated with immobilization, biomechanical forces will prevent it from healing and will cause chronic repetitive injury^5,19,20. Therefore, the synchondrosis is probably in a continuous process of destruction, repair, and remodeling. Finally, by middle age, the painful non-united synchondrosis is completely destroyed^5,13,20. The interface between the navicular and the accessory navicular is gradually worn down, and the subchondral bone is exposed¹³. The bond between the accessory navicular and the navicular becomes loosened by the plantar slip of the posterior tibial tendon and the periosteum between the accessory navicular and the main navicular¹⁹. In this situation, an acute twisting injury may decrease the function of the posterior tibial tendon by additional tearing of the residual fibers (Fig. 2-C). If the injuring force is so severe that the residual connecting tendon is completely torn, complete avulsion of the posterior tibial tendon may occur.

Decreased function of the posterior tibial tendon rarely is seen in young patients who have a symptomatic accessory navicular. The clinical symptoms of separation of the accessory navicular synchondrosis varied in our patients, who were middle-aged or older, and included local tenderness near the accessory navicular with or without tenderness along the distal part of the posterior tibial tendon. Functional testing of the posterior tibial tendon revealed various degrees of disability ranging from pain with activity to total loss of function.

Radiographic examination provided valuable information on patients who had a collapsed arch and a complete avulsion rupture of the posterior tibial tendon with obvious separation of the accessory navicular. However, incomplete or complete separation of the synchondrosis without displacement could not be reliably detected with radiography (Fig. 2-B).

Ultrasonography is accurate for the diagnosis of soft-tissue lesions^2,22. To our knowledge, there has been no previous report of the ultrasonographic findings associated with a tear of the synchondrosis. In the current study, the normal synchondrosis was echogenic and homogeneous on ultrasonography, and degeneration and diastasis were readily recognizable. Thus, ultrasonography can demonstrate the status of an injury of the synchondrosis, thereby allowing a determination of whether a tear of the synchondrosis is the cause of pain in the medial aspect of the midfoot in patients who have an accessory navicular. An additional advantage of ultrasonography is its efficacy for the differential diagnosis of peritendinous and intratendinous lesions of the posterior tibial tendon². An accessory navicular occasionally is found in patients who have decreased function of the posterior tibial tendon^12,18. However, the etiological role of pathological changes in the accessory navicular synchondrosis in these patients has long been ignored. Ultrasonography allows the location of the lesion to be identified. In our small series, this was useful for demonstrating the integrity of the posterior tibial tendon (Fig. 2-A).

Ultrasonography allowed an accurate diagnosis of the lesion in our patients, enabling us to distinguish it from various causes of impaired function of the posterior tibial tendon and to select a proper treatment. The treatment for a rupture of the posterior tibial tendon is a tendon transfer with the flexor digitorum longus or a calcaneal osteotomy, or both^6-8,15,16. However, the initial treatment of a symptomatic accessory navicular is non-operative¹⁵. If the symptoms persist and ultrasonography demonstrates changes in the synchondrosis, operative intervention may be performed.

We could not differentiate between complete and incomplete separation of the synchondrosis on the basis of quantification of the diastasis of the synchondrosis in our small series. The ability to differentiate between these lesions may improve as more experience is gained. There were no false-positive findings in our series; however, one patient had false-negative ultrasonographic findings, which may have been partially due to marginal osteophytes. A technetium bone scan may aid in the differentiation and localization of the pathological characteristics of an injury of the synchondrosis when there is a high likelihood of negative ultrasonographic findings²⁰.

The pathomechanism of the changes in the longitudinal arch after separation of the synchondrosis in our patients is not clear. In Group A, the two complete avulsions of the posterior tibial tendon with complete separation of the synchondrosis represent special cases of common avulsion rupture of the posterior tibial tendon from the insertion on the main navicular. The pathomechanism of the acquired flatfoot deformity in these two patients probably was similar to that of avulsion rupture of the posterior tibial tendon from the main navicular¹⁴. In the four patients in Group A who had complete separation of the synchondrosis, the preoperative radiographs revealed collapse of the arch of the midfoot with sagging of the talonavicular joint. Two (Cases 2 and 4) of these patients had asymptomatic flatfoot on the contralateral side, and two (Cases 1 and 8) had a normal arch on the contralateral side. The cause of the sagging of the talonavicular joint in these four patients is not clear because operative exploration was not performed; however, it may have been associated with a concurrent injury of the spring ligament.

Simple excision of the accessory navicular with plication of the posterior tibial tendon is performed in patients who have a structurally normal foot. If the symptoms in the region of the synchondrosis are associated with a structurally abnormal foot, and if the abnormality has been insidiously present for a long time and has caused collapse of the arch and valgus angulation of the hindfoot, then plication of the posterior tibial tendon alone may not correct the deformity¹⁵. If the deformity is flexible and reducible, distal advancement of the posterior tibial tendon may then be performed. The efficacy of distal advancement for the treatment of decreased function of the posterior tibial tendon due to avulsion from the insertion has long been controversial^3,14. Such treatment may fail if the patient has tendinosis of the posterior tibial tendon¹⁶. If there is avulsion, there is no tendinosis and repair of the avulsed tendon usually will provide a satisfactory result. However, one patient (Case 8), in Group A, who had an obvious sag deformity of the talonavicular joint, had only a fair result. When the associated deformity is severe and not reducible, realignment followed by arthrodesis should be considered^3,8,15-17. The decision to perform a triple arthrodesis or a talonavicular and calcaneocuboid arthrodesis is based on the extent of involvement of the joint^8,15.

In conclusion, when loss of function of the posterior tibial tendon is noted in a patient who has an accessory navicular, it is necessary to determine whether the lesion is in the tendon or the synchondrosis. Degeneration of the accessory navicular synchondrosis in patients who are middle-aged or older may represent a lesion that is different from avulsion of the posterior tibial tendon from its insertion on the navicular. For such patients, ultrasonography is effective for the differential diagnosis.

	Footnotes

 
*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study.

Department of Orthopaedic Surgery, Chang Gung Memorial Hospital, Chang Gung Medical College, 5, Fu-Hsing Street, Kweishan, Taoyuan, Taiwan, 333, Republic of China. Please address requests for reprints to Dr. Yeung-Jen Chen.

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This Article Abstract Full Text (PDF) Free Letters to the Editor: Submit a response Alert me when this article is cited Alert me when Letters to the Editor are posted Alert me if a correction is posted Services E-mail this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to My File Cabinet Download to citation manager Reprints and Permissions Citing Articles Citing Articles via Google Scholar Google Scholar Articles by CHEN, Y.-J. Articles by LIANG, S.-C. Search for Related Content PubMed PubMed Citation Articles by CHEN, Y.-J. Articles by LIANG, S.-C. Social Bookmarking             What's this?

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