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Original Report |
1 Department of Radiology, Musculoskeletal Division, Box 3808, Duke University
Medical Center, Durham, NC 27710.
2 Department of Surgery, Division of Orthopaedic Surgery, Box 2923, Duke
University Medical Center, Durham, NC 27710.
Received April 29, 2004;
accepted after revision August 18, 2004.
Address correspondence to L. R. Toye
(leont{at}baylorhealth.edu).
Abstract
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CONCLUSION. MRI findings present in surgically proven spring ligament tears include an abnormal spring ligament caliber, signal intensity, waviness, a full-thickness gap, and posterior tibial tendonopathy. The finding unique to cases with surgically proven tears is a full-thickness gap in the ligament, seen in 79% of the cases in our series. When multiple abnormalities are seen in the spring ligament in conjunction with a full-thickness gap, the diagnosis of a tear can be made with confidence.
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Little literature exists regarding the MRI appearance of spring ligament tears. In a series of 13 patients, Yao et al. [2] described MRI to be 54–55% sensitive and 100% specific for the detection of spring ligament "insufficiency," with insufficiency defined as either laxity or rupture of the ligament. Increased signal heterogeneity within the medial portion of the spring ligament on axial short-TE or proton density spin-echo images was the most useful finding in that study.
We retrospectively reviewed the MRI examinations of 14 patients with surgically proven spring ligament tears in an attempt to further define the MRI appearance of spring ligament tears.
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All patients underwent a detailed history and physical examination by an experienced foot and ankle attending orthopedic surgeon. Group A and B patients underwent surgery by one of two experienced foot and ankle attending orthopedic surgeons.
All patients underwent preoperative MRI of the ankle and hindfoot using a 1.5-T MR scanner (Signa, GE Healthcare) and were examined in the axial, coronal, and sagittal planes with both T1- and T2-weighted sequences. The T1-weighted sequences were performed with a spin-echo technique using 506–650/9–14 (TR range/TE range), 256 x 192 matrix, 14- to 18-cm field of view, 2 excitations, and 4-mm slice thickness with 0.4-mm spacing. The T2-weighted images were obtained with a fast spin-echo fat-saturated sequence using 4,000–4,700/70–85 (TR range/effective TE range), 256 x 192 matrix, 13- to 16-cm field of view, 2 excitations, and 4-mm slice thickness with 0.4 mm spacing.
The MRI examinations were evaluated by at least one senior university-based attending musculoskeletal radiologist and by a musculoskeletal radiology fellow. All MR images were retrospectively compared with each other and correlated with operative reports. Imaging criteria used to evaluate the spring ligament included superomedial ligament size (normal; thick, > 5 mm; or thin, < 2 mm), morphology (contiguous or full-thickness gap, presence or absence of waviness), signal (dark or increased), and the presence or absence of posterior tibial tendonopathy. The normal range of spring ligament thickness was based on spring ligament imaging findings of Yao et al. [2] and cadaveric data of Taniguchi et al. [3].
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All four group B patients (abnormal MRI but surgically "intact") had an abnormal thickening of the spring ligament (Fig. 5A, 5B). None of these four patients had discontinuity of the spring ligament. The spring ligament in two patients had a wavy appearance. Abnormal increased signal on T2-weighted images was seen in the spring ligament in all four patients. The posterior tibial tendon was also found to have a grossly abnormal MRI appearance in all four patients.
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Of the 12 group C patients (control group), the spring ligament was normal in thickness in 10 patients (Fig. 1A, 1B). One patient had abnormal thickening of the ligament, and one patient had thinning. None of the 12 patients had discontinuity of the spring ligament. The spring ligament in two patients had a wavy appearance. None of the 12 patients had spring ligament signal abnormalities, and none of the 12 patients had an abnormal appearance of the posterior tibial tendon.
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The spring ligament (plantar calcaneonavicular ligament) serves as a slinglike structure, wrapping under the talar head from its calcaneal origin to its navicular insertion. When it fails, the talus rotates in a plantar fashion, and the calcaneus undergoes valgus angulation [1–4]. The result is the so-called pes planovalgus deformity or adult-acquired flatfoot deformity.
The spring ligament complex traditionally has been described as containing the superomedial and inferior ligaments [1, 2, 4, 7–9]. The larger superomedial ligament originates from the calcaneus (sustentaculum talus and anterior facet) and has a fanlike insertion on the navicular bone. The average normal superomedial ligament thickness was reported as 4.8 ± 1.4 (SD) mm in one recent anatomic study [3]. The much smaller inferior component is located plantar and lateral to the superomedial ligament, originating between the middle and anterior calcaneal facets and inserting in a fanlike fashion on the navicular bone. The inferior ligament's navicular insertion is lateral to the superomedial ligament, with fatty tissue interposed between the two structures [3, 4] Fatty tissue also spans the lateral border of the inferior ligament, adjacent to the bifurcate ligament [7].
In a recent anatomic study, a "third ligament" of the spring ligament complex was described as being interposed in the fatty tissue between the superomedial and inferior components [3]. In prior studies, this "third ligament" was likely regarded as part of the inferior ligament. The larger superomedial component is thought to play the most significant role in the spring ligament's contribution to ankle stability and is the component most often torn [1, 4, 8, 10].
A spring ligament tear is typically a chronic degenerative process that occurs in conjunction with posterior tibial tendon insufficiency [1, 2, 8, 10]. A few case reports of acute isolated spring tears have been published [5, 6], but these cases are rare. The typical patient with posterior tibial tendonopathy and a torn spring ligament is a middle-aged woman [10]. This same demographic was observed in our group A population, with 13 (93%) of the 14 patients being women, ranging in age from 36 to 67 years (average, 48.6 years). Twelve of these 14 patients had surgically proven posterior tibial tendonopathy.
On MRI, the normal spring ligament is a continuous band of low T2 signal. A contiguous ligament was seen in all our control group patients. Signal heterogeneity of the inferior portion of the spring ligament at its calcaneal sustentacular attachment is routinely seen and should not be misinterpreted as a ligament tear. This heterogeneity is likely due to interposed fat, which has been described at cadaveric dissections.
In 1993, Rule et al. [7] nicely described the normal spring ligament MRI anatomy using tailored oblique sagittal and axial imaging. The main advantage of these MRI techniques over routine ankle imaging appears to be better visualization of the inferior portion of the ligament. The thicker superomedial portion of the ligament is usually seen on routine axial and coronal images. Because the superomedial portion is the greater contributor of stability and is the usual location of spring ligament tears, attention should be focused on this component during MRI evaluation. Furthermore, the inferior portion of the spring ligament is not visualized on routine surgical evaluation.
MRI findings seen in all of our surgically proven spring ligament tears (group A) included an abnormal caliber of the spring ligament, increased signal in the spring ligament on T2-weighted images, and an abnormal appearance of the posterior tibial tendon. Less commonly seen findings were a full-thickness gap and a wavy appearance of the ligament, seen in 79% and 64% of the cases, respectively.
Our group B patients, those with a spring ligament described as "intact," warrant separate discussion. On retrospective review of the MR images, these patients had findings quite similar to those with surgically proven tears. In all four cases (100%), MRI showed abnormal thickening of the ligament, increased spring ligament signal on T2-weighted images, and an abnormal appearing posterior tibial tendon. Half had a wavy appearance of the ligament.
The similarities of the MRI appearances for groups A and B are thought to be due to the limitations of surgical evaluation of the spring ligament. The surgical approach to the spring ligament involves a medial incision of the mid-foot and retraction of the overlying posterior tibial tendon. This allows visualization of the superomedial spring ligament. A rent in this portion of the spring ligament will be visible with abduction stress. A complete tear will allow a probe to pass directly into the talonavicular joint (Fig. 3A). Beyond the superomedial portion, direct inspection of the spring ligament is not possible. Furthermore, violating a visibly intact ligament for either pathologic analysis or determination of tissue thickness is not part of surgical assessment. Thus, it is conceivable that our group B patients did in fact have spring ligament abnormalities but that these abnormalities were not surgically visible by current standard surgical methods.
Limitations of this study include its retrospective and nonblinded nature. The anatomic proximity of the spring ligament to the posterior tibial tendon also makes it difficult to assess for spring ligament tears independent of posterior tibial tendonopathy; however, this scenario appears uncommon.
In conclusion, the MRI findings present in a surgically proven spring ligament tear include abnormal spring ligament caliber, increased signal intensity in the ligament, waviness, a full-thickness spring ligament gap, and posterior tibial tendonopathy. Several of these findings were seen in our surgically proven "intact" ligaments, which may in fact be histologically abnormal, but not surgically visible. The finding that was unique to cases with surgically visible tears was a full-thickness gap in the ligament, seen 79% of the time.
When multiple abnormalities are seen in the spring ligament in conjunction with a full-thickness gap, the diagnosis of a surgically visible tear can be made with confidence. As surgical techniques for spring ligament repair evolve, it will become imperative to accurately evaluate the spring ligament with preoperative MRI.
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