eMedicine - Ankle, Tibialis Posterior Tendon Injuries : Article by Sherif Wassef

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Ankle, Tibialis Posterior Tendon Injuries

Article Last Updated: Feb 5, 2008

Background

Tibialis posterior tendon dysfunction presents one of the most challenging problems that a foot and ankle specialist faces. This dysfunction often results in significant disability for the patient and in the progressive loss of function. The condition is recognized as a disabling cause of progressive flatfoot deformity.^{1, 2} Many cases of posterior tibial tendon dysfunction may go undiagnosed. The tibialis posterior is, by far, the most frequently ruptured tendon in the rear foot, but injuries to this structure are often overlooked.^{3, 4, 5, 6}

Pathophysiology

Tibialis posterior tendon dysfunction is encountered as a result of altered mechanics of the foot or as a response to systemic articular disease. Systemic risk factors are noted more frequently in dysfunction of the tibialis posterior tendon than in disorders of the Achilles tendon. These risk factors include hypertension; obesity; lupus; gout; rheumatoid arthritis; and, less commonly, Reiter syndrome. Patients with rheumatoid arthritis more frequently develop synovitis than tears. This synovitis causes fibrosis from recurrent inflammatory episodes and, eventually, tibialis posterior tendon dysfunction.^{7, 8, 9, 10}

Young athletes involved in tennis, soccer, ice hockey, basketball, and ballet dancing are vulnerable to traumatic injury to the tendon. Prior trauma and surgery are not strong predictors of tibialis posterior tendon disorder, nor is systemic steroid exposure. However, the direct injection of steroids into the tendon can cause tibialis posterior tendon tears. Unilateral involvement occurs in approximately 90% of cases, with a left-sided predominance.

Human leukocyte antigen Cw6 is a marker of a subtype of seronegative arthropathies, and a positive result with a Cw6 test is associated with tears of the tibialis posterior tendon.

Jahss has classified disorders of the tendons around the ankle and in the foot according to the following general categories: tenosynovitis, tears, tethering, dislocations, tumors and pseudotumors, ossification, congenital anomalies, contractures, and iatrogenic injuries.^{11, 12}

Clinically, the initial presenting stage of tibialis posterior tendon tenosynovitis is paratendinitis (or preferably peritendinosis) or synovitis (see Image 1). The next stage is tendinitis, which is correctly termed tendinosis. Tendinitis is the less-preferred nomenclature because the pathophysiology is degenerative dysfunction without a true inflammatory component. True tendinitis of the tibialis posterior tendon is unusual (see Image 2).

Many cases that are clinically believed to be tendinitis are in fact synovitis. In tendinosis, patients have degeneration in the tibialis posterior tendon. This is usually associated with paratendinosis/synovitis (see Images 3-4). Histologically, no inflammation is present, but there is evidence of intratendinous collagen degeneration, local necrosis, calcification, and hypocellularity similar to that seen in Achilles tendon degeneration.

By definition, a tethered tendon is one with a limited range of movement, owing to its abnormal fixation to an adjacent structure. Causes of such tethering include anatomic anomalies, such as a single tendon sheath surrounding 2 tendons that normally have separate sheaths or an accessory tendon that increases the volume of tissue within a single sheath; fractures with resulting deformities that lead to abnormal fixation or displacement of the tendon or its sheath; and fracture-dislocations of the ankle that may lead to tethering or even incarceration of 1 or more regional tendons.

The transition stage of tibialis posterior tendon disorder involves microscopic and eventually macroscopic tears of the tendon fibers. Partial tears can scar over and lead to tendon thickening; they can retract and lead to tendon thinning; or they can severely weaken the tendon and result in a gap.

Thin tendons are atrophic (see Images 5-6), and thick tendons are hypertrophic (see Image 7). Most patients have mixed regions of hypertrophic and atrophic tendons. This mixture occurs because of interstitial tendon tears with bulbous hypertrophic proximal tendon fibers and because of retraction of the distal atrophic fibers (see Image 8).

Partial or complete tears of the tendons of the foot and ankle may be related to laceration, especially in the sole of the foot; more commonly, they occur spontaneously. Spontaneous rupture of these tendons usually implies some type of intrinsic pathologic process because normal tendons rarely rupture in this fashion. In older persons or in younger persons (particularly athletes) with chronic inflammation, tendon degeneration may predispose them to spontaneous rupture. However, this is classically seen in persons aged 40-60 years (average age, 55 y), and two thirds of cases occur in women. A tibialis posterior tendon tear with a gap is unusual. The tibialis posterior tendon and the Achilles tendon are most frequently affected.

A similar continuum of tibialis posterior tendon disorders occurs in the Achilles tendon, and a similar concept of cumulative injury is useful in understanding tibialis posterior tendon disorders. However, in distinction to injuries of the Achilles tendon, complete tears with a gap that shows no evidence of fibrosis are fairly unusual manifestations of tibialis posterior tendon dysfunction, and ischemia appears to be a more important causal factor.

Normally, a small amount of fluid is present around the tendon (see Image 9). If too much fluid is present, the patient may have pain and dysfunction. Also, fibrotic synovium contributes to the pathophysiologic process, causing a thickened tendon. Additionally, fibrosing tenosynovitis, related to paratendinitis and synovitis, causes thickening of the tendon. In fibrosing tenosynovitis, the synovium may appear black on MRIs. The dark, adherent synovium makes the tendon appear hypertrophic.

The typical location of tibialis posterior tendon disorders is perimalleolar, although they are centered somewhat distal to the malleolus. A second location at which disorders occur is distal. Distal locations are typical for injuries in young athletes and in patients with inflammatory arthropathies.

Dislocation of the tibialis posterior tendon is a rare injury that is often diagnosed late. The tibialis posterior tendon can also sublux outside its groove, often subtly.

Frequency

United States

The tibialis posterior is, by far, the most frequently ruptured tendon in the rear foot, but injuries to this structure are often overlooked.

Bilateral abnormalities of the tibialis posterior tendon are more common in cases associated with underlying disease, particularly rheumatoid arthritis (see Risk factors, under Pathophysiology, for other systemic risk factors). Unilateral involvement occurs in approximately 90% of cases, with a left-sided predominance.

Mortality/Morbidity

Race

No data suggest that any particular race is more vulnerable to tibialis posterior tendon dysfunction.

Sex

Tibialis posterior tendon dysfunction is a disorder that primarily occurs in women who are middle-aged or elderly. Classically, two thirds of spontaneous ruptures occur in women in their fifth or sixth decade of life.

Age

This disorder is bimodal, manifesting in young athletes and, to a greater extent, in middle-aged and elderly women. Classically, two thirds of spontaneous ruptures occur in women in their fifth or sixth decade of life.¹³

Anatomy

Numerous tendons extend from the lower portion of the leg across the ankle and into the foot. With the exception of the Achilles tendon, all of these tendons change from a vertical orientation in the lower leg to a horizontal orientation near the level of the ankle or in the foot (see Image 10). This modification in direction is accomplished by means of a pulley system consisting of either bone (eg, malleoli) or retinacula to promote their smooth, angular movement about the ankle, subtending a smooth curvy course, in contradistinction to disease process when this is lost (see Image 11).^{14, 15, 16, 17, 18}

The tibialis posterior muscle originates from the interosseous membrane and the adjacent posterior surface of the tibia in the proximal third of the leg. The tendon forms in the distal third of the leg and lies closely apposed to the tibia in the posteromedial aspect. Distally, the tibialis posterior tendon sits in a medial or posterior concavity on the medial edge of the posterior tibia. Just lateral to the tibialis posterior tendon lies the flexor digitorum tendon. The tibialis posterior tendon curves distally around the medial malleolus. At this level, the position of the tendon beneath the flexor retinaculum (laciniate) prevents the flexor tendons from bowstringing as they curve around the malleolus. The flexor retinaculum is the roof of the tarsal tunnel. The tarsal tunnel contains the 3 ankle flexor tendons, the adjacent posterior tibial artery and vein, and the tibial nerve (see Image 10).¹⁹

The tibialis posterior tendon next passes under the flexor retinaculum and over the deltoid ligament into the foot and, then, beneath the plantar calcaneonavicular ligament. The tendon contains a sesamoid fibrocartilage, as it runs under the plantar calcaneonavicular ligament. It is inserted into the tuberosity of the navicular bone and gives off fibrous expansions: one expansion passes backward to the sustentaculum tali of the calcaneus, and others pass forward and lateralward to the 3 cuneiforms, the cuboid, and the bases of the second, third, and fourth metatarsal bones (see Image 12).

Because an abnormal size may be the only indicator of tendon dysfunction, the relative sizes of the tibialis posterior tendon, the flexor digitorum tendon, and the flexor hallucis tendon should be examined. In a healthy person, the tibialis posterior tendon is roughly twice the size of the 2 adjacent tendons (see Images 13-15). Additionally, the tibialis posterior tendon should be slightly smaller than the tibialis anterior tendon, and the tibialis posterior tendon should be slightly smaller than the summated measurements of the peroneus brevis and peroneus longus tendons (see Images 13-14).

The proximal aspect of the tibialis posterior tendon is supplied by branches of the posterior tibial artery. The distal aspect of the tendon, at the enthesis, is supplied by the posterior tibial and dorsalis pedis arteries. The midtendon, similar to the Achilles tendon, is poorly supplied with blood. In addition, the mesotendon is absent distally because the synovial sheath ends at the mid portion of the talus. Because of the zone of hypovascularity and because of the absence of a mesotendon, the level of the medial malleolus in relation to the tubercle is the most common location for tibialis posterior tendon dysfunction.

Tibialis posterior tendon disorders are predominantly ischemic, and similar to myocardial infarction, they are senescent diseases. Impingement also plays a role in tibialis posterior tendon dysfunction because the tibialis posterior tendon has a focal point of stress as it curves around the medial malleolus. This point of stress can be analogous to the pressure on the rotator cuff in the subacromial space. This combination of ischemia and mechanical compression causes most tibialis posterior tendon disorders.

The tibialis posterior muscle plantarflexes the ankle and inverts the foot. During normal gait, this muscle unit creates a rigid midfoot lever for forward propulsion by locking the calcaneus to the cuboid and the talus to the navicular. If tibialis posterior dysfunction is present, the lack of a rigid midfoot causes gastrocnemius and soleus flexion to occur at the midfoot instead of at the metatarsal heads. This problem eventually leads to midfoot collapse, forefoot abduction, and heel valgus. These deformities are exacerbated by the action of the peroneus brevis, the antagonist muscle to the tibialis posterior. Because the cross-sectional area of the peroneus brevis is half that of the tibialis posterior tendon, a significant degree and length of time of tibialis posterior dysfunction must be present before these abnormalities appear.

Clinical Details

The presenting signs and symptoms are pain, difficulty walking, and swelling along the medial malleolus and the arch of the foot. These signs and symptoms may occur gradually or suddenly as a result of trauma, and they may be difficult to attribute to a particular cause.

The clinical examination may reveal the anatomic locus of the symptoms, but the findings are often not precise in distinguishing other causes of similar symptoms, such as plantar fasciitis, tendinosis, and subtalar and talonavicular synovitis. These problems require different treatments; therefore, imaging studies play an important part in the diagnosis of posterior tibial tendon disorders. In addition, imaging studies are most useful to determine whether the abnormality is limited to the peritendinous area or the tendon itself.

Preferred Examination

Different imaging techniques can be used to assess tendon and tendon sheath abnormalities.^{14, 20, 21, 22, 23}

Tendon abnormalities can be evaluated with tenography. This is accomplished with a needle puncture of the tendon sheath.^{24, 25}

Sonography is becoming an increasingly important imaging modality for evaluating musculoskeletal disorders because of its availability, noninvasiveness, lack of ionizing radiation, multiplanar and real-time capabilities, and low cost. Higher-resolution transducers and the dynamic real-time capability of sonography make it attractive for evaluating muscles and tendons. Because of its superficial location, the posterior tibial tendon is particularly amenable to evaluation with sonography.

In the delineation of tendon calcification and retinacular avulsions of bone, CT is superior to MRI. However, in analysis of tendon dislocation both CT and MRI are of nearly equal value

With its superior soft-tissue contrast resolution and multiplanar capabilities, MRI is the imaging procedure of choice for evaluating the musculoskeletal system, particularly in detecting tenosynovitis and in assessing partial and complete ruptures of the tendons. Both MRI and sonography can be used to distinguish tendinosis from peritendinosis. This distinction is important because a more rigorous treatment is needed if the tendon is involved, because it might lead to partial and complete tear.

Imaging also provides insight into the pathophysiology of the disease process. Tendinosis and peritendinosis are often seen together (45% of cases); this observation is readily explained by a common causal mechanism of injury to the 2 sites. The finding of peritendinosis by itself, without tendinosis, is more common (20% of cases) than tendinosis alone without peritendinosis (7%), possibly because the tendon is stronger than the peritendinous tissue and therefore more resistant to injury.

Limitations of Techniques

An inherent drawback of both MRI and sonographic modalities is an inability to further categorize tendon abnormalities. Inhomogeneity of the tendon on MRI could be due to tendinitis, partial tears, degeneration, or other tendinopathies. All these entities fall into a spectrum of pathologic disorders, and determining when one ends and the second begins is difficult. One can speculate that inhomogeneity alone without enhancement is indicative of partial tear or chronic tendinopathy, but those disorders cannot be diagnosed on MRIs, and sonography does not help in resolving this problem.

CT is valuable only when an associated bony abnormality is present; however, tendinous or peritendinous abnormalities are least confidently detected by using imaging.

Enhancement of the tendon and the area around it on MRIs and increased flow on color-flow Doppler sonograms are the most useful features for diagnosing tendinosis and peritendinosis. Other useful, but less specific and less sensitive, criteria are as follows: for tendinosis, a change in signal intensity of the tendon on MRIs and inhomogeneity of the tendon on sonograms; for peritendinosis, increased soft tissue and fluid in the area around the tendon.

In the diagnosis of tendinosis, use of the combined criteria of flow and inhomogeneity of the tendon yields the best positive predictive value (90%) and the best negative predictive value (83%) for sonography, as compared with MRI. The addition of the abnormal size of the tendon as a criterion does not improve the sensitivity, specificity, or predictive values in the diagnosis of tendinosis.

Similarly, in the diagnosis of peritendinosis, the combined criteria of flow and increased soft tissue in the area around the tendon yield the best positive predictive value (89%) and the best negative predictive value (75%) for sonography.

Other Problems to be Considered

Flat foot (pes planus)
Plantar fasciitis, tendinosis
Subtalar synovitis
Talonavicular synovitis

Findings

Routine radiographic findings associated with abnormalities of the tendons and tendon sheaths of the foot and ankle include the following: soft tissue swelling; a change in the contour, calcification, or ossification of a tendon; bone proliferation; fracture fragments; and sesamoid displacement. Soft-tissue swelling and fullness may accompany synovitis, but the finding is not specific (see Image 16).

Osseous proliferation or erosion is a recognized manifestation of inflammation of tendons and tendon sheaths that are close or directly on the surface of a bone. In the foot and ankle, this finding is most commonly observed in the posteromedial portion of the tibia in patients with rheumatoid arthritis or seronegative spondyloarthropathy who have involvement of the tibialis posterior tendon and sheath. Infections of tendons and tendon sheaths also can lead to infective or reactive periosteitis in the subjacent bone.

Radiographically, a dislocated tibialis posterior tendon can be diagnosed by noting the presence of a small avulsion fracture near the insertion of the flexor retinaculum on the medial malleolus.

Tenography is a procedure in which the tendon sheath is directly opacified with contrast medium. The peroneal tendon sheath is the first to be studied with tenography.

The frequency of tenography performed throughout the United States and worldwide is not known. However, about half of the cases of tenography are performed to evaluate the tibialis posterior tendon. Although MRI and CT may have replaced tenography in many institutions, tenography still has a role in the management of chronic foot and ankle pain; it is used in verifying that a patient's pain is coming from the tendon sheath, in surgical decision making, and in the injection of therapeutic steroids.

After the administration of a local anesthetic, a 25-gauge needle is inserted into the tendon. A 10-mL syringe filled with a mixture of contrast material diluted 2:1 with a local anesthetic is attached via flexible tubing. During the gentle injection, the needle is withdrawn until free filling of the tendon sheath is observed under fluoroscopy. The injection continues until the tendon sheath fails to fill at its distal end, until contrast material flows proximally into the fascial sheath around the muscle, or until the patient feels discomfort. Betamethasone has the lowest risk of a flare response after the injection.

Normally, the tendon sheath demonstrates a smooth contour. At the level of the tibial plafond, there is normally extrinisic compression on the tibialis posterior tendon sheath produced by the flexor retinaculum. This should not be confused with pathologic adhesion or stenosis (see Image 17).

Radiographically, according to the number of sacculations on the tendon sheath, mild tenosynovitis is manifest with 1-5 sacculations; moderate tenosynovitis, 6-10 sacculations (see Image 18); and severe tenosynovitis, more than 10 sacculations or an area of adhesion larger than 3 cm (see Image 19). However, this tenographic classification does not correlate well with the clinical classification of peritendinitis and chronic tenosynovitis. Moreover, a long segment of stenosis (longer than 3 cm) is considered as severe, representing stenosing tenosynovitis. Stenosis or nonfilling of segments of the sheath could occur from sheath fibrosis or from enlargement of the tendon occluding the sheath. Tibialis posterior tendon rupture can be seen as a filling defect suggestive of a mass effect (see Images 20-21).

For a discussion of the therapeutic effects and complications of tenography, see Intervention.

Findings

Transaxial CT images are the easiest to acquire, and they provide the most useful information, although reformatted transaxial images in the coronal and sagittal planes are occasionally required.

The CT features of a normal tendon include a smooth contour, a size similar to that on the opposite side, a well-defined margin, and attenuation values 75-115 HU (Hounsfield unit) higher than those of the respective muscles. Tenosynovitis is manifest as an enlarged tendon with an inhomogeneous appearance. The surrounding swollen, fluid-containing tendon sheath has a lower attenuation value than that of the tendon itself. Tendon displacement, tethering, or rupture may be evident, and the relationship of the tendon to the adjacent bone is identified readily. Tendon ruptures are associated with partial or complete discontinuity of the fibers and a decrease in the attenuation values (30-50 HU).

Degree of Confidence

Diagnostic difficulties are encountered with CT, owing to beam-hardening artifacts that cause inaccurate assessment of the attenuation values and to the presence of surrounding inflammation that obscures the contour of the tendon and the tendon sheath.

MRI is superior to CT in delineating small amounts of fluid around the tendon and in allowing differentiation of scar tissue from edema and fluid. CT is superior to MRI in demonstrating regions of tendon calcification and avulsion fractures related to the retinacula.

False Positives/Negatives

Rosenburg et al found that CT is sensitive in 90% of cases of tibialis posterior rupture and specific in 100%.²⁸ They defined 3 categories of injury: type 1 is a partially torn bullous or hypertrophied tendon with vertical splits and defects; type 2, partially torn and attenuated; and type 3, complete tendinous disruption with an intratendinous gap. CT has an accuracy of 91% in detecting tendon ruptures.

Findings

MRI has been applied to the assessment of the tendons and other structures in the ankle and foot. The tibialis posterior tendon and the Achilles tendon have received the greatest attention.^{29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45}

Technical aspects

The technical aspects of performing MRI of the tibialis posterior tendon are controversial. MRIs can be obtained in the sagittal, coronal, or transaxial (plantar) plane or in a combination of these. The specific plane selected depends on the particular anatomic regions and structures to be evaluated and on the clinical questions involved. The axial plane is optimal; however, some institutions prefer oblique axial imaging perpendicular to the long axis of the tibialis posterior tendon. Sagittal imaging is the secondary plane, with the coronal plane used only as a supplement.

Two sets of axial images are ideal. One set of images should be morphology weighted to optimize the signal-to-noise ratio (see Images 22-23). Parameters for this imaging may include the following: sequence, fast spin echo; repetition time/echo time, 4000/35; echo train length, 4; field of view, 14; and matrix, 256 x 256. Another set of images should be T2-weighted by using fat-suppression and fast spin-echo protocols with a repetition time/echo time of 6000/75.

Sagittal images should be T1-weighted (see Images 24-25) and acquired with either T2-weighting with fat suppression or a short-tau inversion recovery (STIR) sequence.

The sagittal images depict the distal tibialis posterior tendon and its malleolar curve (see Image 11 and Images 26-28), and the axial images depict perimalleolar abnormalities (see Image 29). A dedicated extremity coil is necessary, and some institutions slightly plantarflex the foot to minimize the magic-angle artifact. T1- and STIR-weighted coronal imaging might be helpful (see Images 30-31).

Contrast material is useful only in some patients. Contrast material can be used when nonenhanced MRIs show subtle or no findings suggestive of abnormality but when the clinician suspects an abnormality of the tibialis posterior tendon. Moreover, contrast material can be used for the evaluation of suspected synovitis, infection, or inflammatory arthritis. Lastly, contrast material is helpful for the assessment of insertional tendinitis (see Image 32).

General findings

On MRIs, the tibialis posterior tendon is normally black without any internal signal intensity. The exception to this lack of signal intensity is the result of the magic-angle artifact (see Image 33) because the tibialis posterior tendon curves around the medial malleolus (see Image 11 and Images 26-28). In comparison with the Achilles tendon, the distal tibialis posterior tendon has no normal internal signal intensity. However, the signal intensity varies distally; the variations are related to volume averaging of the spring ligament (extremely distal), the tibial navicular, and the tibiotalar components of the deltoid ligament (slightly more proximal) (see Image 34).

On sagittal images, the tibialis posterior tendon should have a smooth curve around the medial malleolus to limit focal compression and impingement (see Image 11 and Image 26). A small amount of fluid in the synovial sheath of the tibialis posterior tendon is normal; this measures no more than 1-2 mm and is almost never circumferential. Because no normal sheath is present around the distal tibialis posterior tendon (see Image 27), fluid observed at the distal 1-2 cm is abnormal and related to the metaplastic synovium.

In common with the findings derived from ultrasonography and CT, the major MRI finding of tenosynovitis is abnormal accumulation of fluid within the tendon sheath. This fluid has low signal intensity on T1-weighted spin-echo images and high signal intensity on T2- and STIR-weighted images (see Image 35). Pannus and scar formation around a tendon are characterized by intermediate signal intensity on T1-weighted spin-echo images and intermediate-to-high signal intensity on T2-weighted spin-echo images (see Images 36-37).

Tendinitis is accompanied by focal areas of high signal intensity within the substance of the tendon on proton density– and T2-weighted spin-echo images (see Image 38). With chronic tendinitis, the tendon is enlarged and of low signal intensity in both T1-weighted and T2-weigted spin-echo images (see Image 2).

The MRI appearance of paratendonitis is similar to that seen in the Achilles tendon, with partially circumferential areas of high signal intensity located distally around the tibialis posterior tendon. This signal intensity is usually slightly less than that of fluid. Because normally no fluid is present distally around the tibialis posterior tendon on MRIs, the term synovitis should be used to describe this disorder only when it occurs more proximally (see Image 36). If apparent synovitis is seen distally, it is anatomically a paratendinitis, and images often reveal fluid with signal intensity slightly lower than what is typical for bland fluid (see Image 2 ). At this stage of the disorder, the tendon itself is normal and should not show intratendinous hypersensitivity. Tibialis posterior tendon disorders manifested by synovitis are often acutely symptomatic.

Although degeneration is histologically common, signal abnormalities caused by degeneration are infrequently seen on MRIs. In most patients, degeneration occurs with an apparently normal tibialis posterior tendon, as shown on MRIs.

In a transitional stage of tibialis posterior tendon disorder, microscopic and eventually macroscopic tears of the tendon fiber occur. Few partial tears are seen on MRIs, although most are seen on sonograms. On MRIs, subtle focal areas of high signal intensity may be visible in the tendon. At surgery, the disruption is often more extensive than it appears on MRIs. Therefore, what may appear as synovitis or tendinitis on images may in fact be a partial tear.

Tendon ruptures may be acute or chronic and partial or complete. Recent tendon tears frequently reveal regions of increased signal intensity on T2-weighted spin-echo images and on certain gradient-echo images, owing to the presence of edema and hemorrhage. Remote tendon tears generally do not have these high-signal-intensity characteristics, owing to the presence of scar tissue.

Patterns of tendon tears

With regard to the extent of tendon tears, 3 MRI patterns have been described: type 1, type 2, and type 3.

Type 1 tears are partial tendon ruptures with tendon hypertrophy. The involved tendon appears hypertrophied or bulbous, and it reveals heterogeneous signal intensity. Focal areas of increased signal intensity are noted within its substance. The MRI pattern corresponds to a surgically evident, partially torn tendon with vertical splits and defects. The presence of an interstitial tear with a longitudinal split of the tibialis posterior tendon is also common (see Image 7 and Images 39-43). This is the only type of tibialis posterior tendon disorder that appears with high signal intensity on T2-weighted MRIs, and it is almost invariably associated with synovitis.

Type 2 tears are partial tendon ruptures with tendon attenuation. The involved tendon is stretched and attenuated in size; the MRI findings correspond to those found at surgery (see Image 8).

Type 3 tears are complete tendon ruptures with tendon retraction. The involved tendon is discontinuous; in some cases, a gap is evident that is filled with fluid, fat, or scar tissue, depending on the age of the tear. The size of the gap is variable both on MRI and at surgery, and this gap reflects the degree of tendon retraction. A tibialis posterior tendon tear with a gap is unusual. Usually, what is seen is severe thinning of the tibialis posterior tendon with thin residual threads that appear as a dysfunctional tendon during clinical examination.

Additionally, involvement of the spring ligament may be seen with severe tibialis posterior tendon tears.

Tendon subluxation or dislocation is easily detected with MRI, as with CT, owing to an abnormal relationship of the tendon with the adjacent tissues. The tendon itself may be enlarged or partially torn, and associated soft-tissue and bony injury may be evident (see Images 44-45).

Exceptions to general findings

With minor exceptions, the normal tendons in the ankle and foot are homogeneous and of low signal intensity with all MRI sequences. They generally are equal in size on the 2 sides of the body, and they have a smooth contour. However, some exceptions to these general rules include the following: magic-angle effect, tenosynovial fluid, bulbous tendon insertion sites, and tendon striations.

As indicated earlier, increased signal intensity may be seen in normal tendons oriented obliquely with respect to the main magnetic field; this effect is greatest when this orientation is at 55° to that of the magnetic field (see Image 13 and Image 33). This effect is greater when the MRI involves a spin-echo technique with short echo times or a gradient-echo technique.

The tibialis posterior tendon approximates this orientation at its site of attachment to the navicular bone, resulting in a normal appearance of increased signal intensity or heterogeneous signal intensity in this area. This alteration in signal intensity may be accentuated by volume averaging of different signal intensities derived from the joint capsule and fat in this region. Furthermore, repeating the MRI examination with a foot in plantar flexion diminishes or eliminates this magic-angle phenomenon.

The differentiation of thickened tendons from one surrounded by a fluid-filled synovial sheath is difficult on T1-weighted spin-echo MRIs. Moreover, the presence of small or even moderate amounts of fluid within a tendon sheath, by itself, is not diagnostic of an abnormality, because such fluid is seen in asymptomatic persons. Tenosynovial fluid is more common in flexor tendons, as compared with extensor tendons, and this may be particularly prominent around the flexor hallucis longus tendon.

Insertion sites of tendons may appear bulbous (see Image 26 and Image 28). This appearance is perhaps related to volume averaging of their signal intensity with that of adjacent cortical bone. This appearance can simulate that of a tendon disruption, particularly one of the tibialis posterior tendons.

When 3-dimensional gradient-recalled-echo MRIs are obtained, longitudinal lines of intermediate signal intensity may be noted in the distal portion of the tibialis posterior tendon. These lines probably represent branches of the tendon, although their appearance may simulate that of a tendon tear.

Secondary signs of tibialis posterior tendon dysfunction

The abnormal mechanics of the tibialis posterior tendon can result in anatomic changes that appear on MRIs. Although most MRI examinations are not performed while the tendons are bearing weight, MRI is a tomographic technique, and subtle mechanical disturbances may be apparent. These secondary signs can increase the diagnostic confidence in describing subtle tibialis posterior tendon disorders. Most of these signs are not pathognomonic of tibialis posterior tendon dysfunction, because they can be seen with other causes of pes planus and foot disorders. In addition, reducible and nonreducible deformities are distinguished clinically.

On MRI, the only distinction is that nonreducible deformities tend to be more severe, with secondary osteoarthritic changes. Excessive plantar flexion of the talus results in a mechanical disturbance called talonavicular fault. On the sagittal MRI on which the base of the first metatarsal is visible, a long axis is drawn on the talus and extended into the navicular. The failure of this line to divide the navicular into equal superoinferior parts, with the line positioned inferiorly, is a manifestation of the talonavicular fault and hence a dysfunctional tibialis posterior tendon (see Image 46).

Another morphologic abnormality, talonavicular unroofing, results from the unchecked pull of the peroneus brevis shifting the entire midfoot and forefoot laterally. This causes a navicular subluxing in relationship to the talus. Normally, the articular aspect of the talus, when evaluated on proximal axial images, is 85% covered by the navicular. Unopposed peroneal brevis pull causes the uncovering of the talus. In the uncovered talus, less than 85% of the articular surface is covered by the navicular (see Image 47).

A focal spur in the distal tibia is another secondary finding of a tibialis posterior tendon disorder. Because the tibialis posterior tendon normally sits in a slight concavity along the posterior medial aspect of the tibia, this spur is a sharpening of the medial or uppermost aspect of this concavity (see Image 6).

Additionally, a heel valgus as revealed on coronal images is an indirect sign of a tibialis posterior tendon tear (see Image 48). The long axes of the calcaneus and the tibia normally subtends an angle with 0-6° of valgus.

Marrow abnormalities

Bone marrow findings related to tibialis posterior tendon disorders include the accessory navicular, the cornuate navicular (see Image 49), and marrow edema. The first 2 entities lead to a more proximal insertion of the tibialis posterior tendon, reducing the curve around the malleolus. This straightening of the curve leads to focal attritional wear and tear of the tibialis posterior tendon (see Image 50 and Image 51).

Tibialis posterior tendon disorders can also cause focal areas of marrow edema. This marrow edema is typically seen underneath the course of the tibialis posterior tendon, typically in the tibia and less commonly in the talus and navicular. Usually, patients with marrow edema under the course of the tibialis posterior tendon are symptomatic (see Images 29 , 42 , 52, and 53).

The presence of marrow edema is somewhat more frequent in people with seronegative or seropositive arthropathies. However, most findings of marrow edema are seen in patients with routine degenerative tibialis posterior tendon disorders. Interestingly, marrow edema is frequently seen around the tibial spur, and it may be part of the evolution of this spur.

The development of a pseudoarthrosis between the accessory navicular and the native navicular is related to the tibialis posterior tendon. A chronic tibialis posterior tendon pull can lead to fracture of the normal synchondrosis. On MRIs, fluid is visible between the 2 bones, with kissing marrow edema on either side of the pseudoarthrosis (see Image 54).

On MRIs, the tibialis posterior tendon is seen subluxed anteriorly and medially, and it is seen as the most medial aspect of the tibia rather than behind it (see Image 44). Although a tibialis posterior tendon dislocation is uncommon, this is the second most common dislocation of the ankle tendons, after peroneal dislocations. Repetitive transient subluxation may also be part of the pathophysiology of more typical tibialis posterior tendon tears. The retromalleolar groove is usually shallow in patients with a tibialis posterior tendon dislocation, and the retinaculum may be visibly stripped off or torn. Infrequently, a related tear in the tendon is discovered.

Degree of Confidence

MRI is the current standard imaging technique for the diagnosis of foot and ankle problems. When inhomogeneity of the tendon is seen on MRIs, it could be due to tendinitis, a partial tear, degeneration, or another tendinopathy. All these entities fall into a spectrum of disorders, and determining when one ends and another begins is difficult. Hence, all these entities should be considered in the differential diagnosis.

While applying their classification, Rosenberg et al found MRI for diagnosing tendon ruptures to be sensitive in 95% of cases and specific in 100%. MRI has a 96% accuracy in detecting tendon rupture. The overall accuracy, which reflects a percentage of cases correctly diagnosed, as well as those correctly classified, was 59% for CT and 73% for MRI.

Findings

High-resolution sonography has gained acceptance for musculoskeletal abnormalities. It has the advantages of ready availability, noninvasiveness, and low cost.^{37, 38, 46, 47, 48, 49, 50, 51, 52}

On sonograms, the posterior tibial tendon normally shows homogeneous echogenic longitudinal fibers (see Image 9 and Images 55-56). No flow is seen in or around the tendon on color-flow Doppler sonograms. Minimal fluid is often seen adjacent to the tendon.

The findings in tendinosis are flow within the tendon on power Doppler sonography and inhomogeneity of the tendon. Flow in the tendon is seen in about 36% of the tendons (see Image 57). Inhomogeneity with mixed echogenicity and disruption of echogenic fibers is seen in 48% of the tendons. The tendon is also enlarged more prominently in the anteroposterior dimension than the transverse dimension (see Image 58). The findings in peritendinosis are increased flow in the peritendinous area on power Doppler sonograms in 45% of cases, and hypoechoic tissue is seen around the tendon in 36% of patients (see Image 59).

Other than size, the 2 MRI criteria used for the diagnosis of tendinosis are contrast enhancement and the abnormal signal intensity of the tendon. These criteria are compared with color Doppler findings of flow in the tendon and with the sonographic inhomogeneity of the tendon.

For the diagnosis of peritendinosis, the criteria used for MRI are contrast enhancement of the peritendinous tissues and an increase in the amount of soft tissue and fluid in the peritendinous area. The corresponding criteria used for sonography are flow in the peritendinous area on color Doppler images and an increase in the amount of soft tissue and fluid in the peritendinous area.

Sonography is performed by using a small-parts 10-MHz transducer. The patient is placed in a prone oblique position with his or her ankle slightly elevated on a rolled towel so that the posterior tibial tendon and flexor digitorum longus tendon can be optimally evaluated.

The posterior tibial tendon is first identified just posterior to the medial malleolus. The tendon is followed along its entire length to the insertion into the navicular tuberosity. The anteroposterior diameter is measured on the longitudinal view of the posterior tibial tendon at approximately 1 cm distal to the tip of the medial malleolus. The transducer is then turned 90°, and transverse scans and measurements of the transverse diameter of the posterior tibial tendon are obtained.

The flexor digitorum longus tendon (which lies slightly posterior to the posterior tibial tendon) is then evaluated in a similar manner. Anteroposterior and transverse diameters of the posterior tibial tendon and the flexor digitorum longus tendon are measured 1 cm distal to the medial malleolus. Color and power Doppler sonography are then used to evaluate both tendons and the area around the tendon. The Doppler gain is set so that no flow is present in the cortical bone.

Degree of Confidence

Enhancement of the tendon and peritendinous area on MRIs and increased flow on color-flow Doppler sonograms are the most useful features for diagnosing tendinosis and peritendinosis.

Other useful, but less specific and sensitive, criteria are as follows: for tendinosis, a change in signal intensity of the tendon on MRI and inhomogeneity of the tendon on sonography; for peritendinosis, increased soft tissue and fluid in the peritendinous area.

In the diagnosis of tendinosis, use of the combined criteria of flow and inhomogeneity of the tendon yield the best positive predictive value (90%) and the best negative predictive value (83%) for sonography compared with MRI. The addition of the abnormal size of the tendon as a criterion does not improve the sensitivity, specificity, or predictive values in the diagnosis of tendinosis.

Similarly, in the diagnosis of peritendinosis, the combined criteria of flow and increased soft tissue in the area around the tendon yields the best positive predictive value (89%) and the best negative predictive value (75%) for sonography.

Findings

Nuclear medicine provides a number of sensitive techniques for the evaluation of foot pain. However, the techniques are not always specific. In subacute or chronic injuries in which prolonged pain is unexplained, the 3-phase bone scan may play a significant role.^{53, 54}

Bone scanning may be useful in differentiating soft-tissue pathology from bone pathology, and being a sensitive test, it may indicate the region that needs further specific radiologic examination. It may also indicate the clinical significance of a radiologic finding.

Careful attention to the technique enhances the efficiency of bone scintigraphy, and single-photon emission compute tomography (SPECT) allows better investigation of the hindfoot. With improved technique and instrumentation, the finding of a focal abnormality in the ankle or foot on bone scintigraphy is no longer sufficient. More precise information about perfusion, the blood pool, and the specific location of a lesion can be obtained with high-resolution and tomographic images.

Degree of Confidence

Nuclear medicine studies must be interpreted with knowledge of the patient's history and symptoms and with close correlation with the plain radiographic findings.

Findings

False Positives/Negatives

The therapeutic effect of tenography is likely a combination of local anesthesia, the anti-inflammatory effect of the steroids, and the actual mechanical dilatation of the tendon sheath due to fluid placed in the sheath. Patients with stenosing tenosynovitis may not receive a therapeutic effect from the tenography if the injected liquid does not extend through the stenotic area. Subsequently, patients with more advanced disease may need surgery. Almost half of the patients, regardless of the radiographic appearance of the tenosynovitis, have long-term complete or near-complete relief of symptoms after the therapeutic injection of steroids, contrast material, and local anesthesia during tenography.^{55, 56, 57, 58, 59}

Complications include the loss of skin pigmentation. This almost always occurs in dark skin. Transient bruising, infection, and partial paresthesia can occur. Tibialis posterior tendon rupture as a complication of steroid injection is reported in less than 1% of cases. Therefore, to decrease the likelihood of tendon rupture, patients should refrain from participating in sports for 6 weeks. The use of a removable walking brace or a cast for 6 weeks may be prudent after a tendon injection. Moreover, tenography, and particularly steroid injection, is not recommended in cases of a suspected tendon rupture.

The extent of the tendon tear, as well as its location, influences the type of surgical repair that may be attempted. A small focal area of tendon rupture may be treated with surgical resection and end-to-end tendon anastomosis, and a large area of tendon rupture may require side-to-side tendon anastomosis. A large tendon rupture with tendon retraction may preclude direct anastomosis of the torn ends of the tendon; in some cases, arthrodesis is required to prevent further foot deformities.