Radiographic technique

This view is taken either weightbearing or non-weightbearing with the X-ray beam angled 10° towards the heel (so that it is perpendicular to the metatarsals) and directed to the base of the third metatarsal.

Anatomical review areas

See Box 12.1.

This general purpose view shows the majority of the foot from the midtarsal area distally (Fig. 12.1). The neck of the talus, the distal edge of the calcaneum, the tarsus, metatarsals and phalanges are clearly seen. The calcaneocuboid and talonavicular joints which make up the midtarsal (Chopart) joint are visualised as are the tarsometatarsal (Lisfranc) joints. The sesamoid bones are seen through the head of the first metatarsal. Usually the bodies of the talus and the calcaneum are occluded by superimposition of the lower ends of the tibia and fibula.

Figure 12.1 Dorsiplantar view.

Radiographic technique

The oblique view is a non-weightbearing view which may be taken in several ways. In the most common method, the patient sits and inclines the plantar surface of the foot 45° to the cassette, with the X-ray beam directed to the base of the third metatarsal perpendicular to the dorsum of the foot.

Anatomical review areas

See Box 12.2.

This view gives a distorted image of the midtarsal area, but is particularly useful for the open view given of the articular facets in the area, particularly the talonavicular, navicular-cuneiform and calcaneocuboid joints (Fig. 12.2). It should be possible to see the outlines of the cuneiforms, but it can be difficult to see the facets of all the intercuneiform joints due to superimposition. The anterior aspect of the subtalar joint and the tarsometatarsal joints are also well seen. Good views of the distal calcaneus, metatarsals and phalanges are obtained.

Figure 12.2 Dorsiplantar oblique view.

Radiographic technique

This is taken with the X-ray beam perpendicular to the foot, centred on the base of the fifth metatarsal with the medial side of the foot against the cassette in the neutral position.

Anatomical review areas

See Box 12.3.

This view shows the tibial and fibular malleoli superimposed over the trochlear surface of the talus which should be domed and smooth in outline. The posterior lip of the tibia, also known as the posterior or third malleolus, is best seen on this projection. The facets of the subtalar joint and the calcaneus are well seen as are the calcaneocuboid and talonavicular joints. The sustentaculum tali and sinus tarsi are usually apparent. The midtarsal complex is partially obscured by the multiple superimpositions of the cuneiforms and metatarso-cuneiform articulations, although the first metatarso-cuneiform joint is usually well seen. Just proximal to this joint is the second and third metatarso-cuneiform joints which can be visualised with some superimpositions. The lesser metatarsals are partially superimposed over each other, although the first metatarsal, hallux, and first metatarsophalangeal joint should be easily distinguishable (Fig. 12.3).

Figure 12.3 Lateral weightbearing view.

Radiographic technique

This view is taken with the X-ray beam directed along the longitudinal axis of the foot towards the ankle, centred on a point midway between the malleoli.

Anatomical review areas

See Box 12.4.

The ankle mortise including the trochlear surface of the talus and the articulations with the tibial plafond and malleoli are well seen with this view (Fig. 12.4). It is useful if ankle injury is suspected and avulsion fractures, which occur with inversion and eversion injuries, can usually be seen on this view. Virtually no detail of the forefoot is seen due to marked superimposition.

Figure 12.4 Anteroposterior ankle view.

This view is sometimes taken as a non-weightbearing stress radiograph in which the foot is held in inversion or eversion and the amount of ligamentous damage is checked by assessing the degree of tilt of the talus within the joint (Fig. 12.5). This may be a painful procedure and the patient may require local anaesthesia.

Figure 12.5 Stress radiograph. Anteroposterior ankle view with inversion stress of the talus shows varus tilt of the talus and widening of the lateral ankle joint space secondary to lateral ligament injury.

Radiographic technique

The X-ray beam is angled towards the sesamoids with the foot flexed at the metatarsophalangeal joints. A special positioning platform may be used in some departments, but more commonly the patient lies in a prone position with the all of the toes dorsiflexed and resting on the cassette or in a supine position using a loop of bandage to retract the toes.

Anatomical review areas

Although not commonly standard, this view clearly demarcates the sesamoids which should be smooth in outline and positioned under the head of the first metatarsal on either side of the intersesamoidal ridge. The ends of the lesser metatarsals are also seen and may determine whether a long or plantarflexed metatarsal disturbs foot dynamics (Fig. 12.6).

Figure 12.6 Axial sesamoid view.

Dorsiplantar view

Commencing at the hindfoot, the longitudinal axis of the hindfoot is a line parallel to the distal portion of the lateral border of the calcaneus. In the normal foot, it is parallel to the long axis of the fourth metatarsal (Fig. 12.7). The talocalcaneal angle is measured between the longitudinal axis of the hindfoot and a line along the midline axis of the talus (Fig. 12.7). There is usually a 15–35° more lateral axis of the calcaneus compared with the talus. A smaller angle means that the calcaneus is directed closer to the midline and there is hindfoot varus or supination with increased superimposition of the talus on the calcaneus. A larger angle means that the calcaneus is directed further from the midline and there is hindfoot valgus or pronation.

Figure 12.7 Alignment dorsiplantar view. Line 1 – longitudinal axis of hindfoot. Line 2 – midline axis of talus. Angle A, talocalcaneal angle.

Forefoot varus and valgus is often discussed as a relationship between the talus and the first metatarsal, even though these bones are not truly adjacent. Simply put, the midline axis of the talus normally goes through the base of the first metatarsal (Fig. 12.7). If the axis of the talus lies lateral to the first metatarsal, the metatarsal is directed closer to the midline and there is forefoot varus or supination (also referred to as inversion). The bases of the metatarsals tend to converge more than normal and the proximal metatarsals are superimposed. If the axis of the talus lies medial to the first metatarsal, the metatarsal is directed further from the midline and there is forefoot valgus or pronation (also referred to as eversion). The metatarsals are less convergent than normal, more parallel with less overlap.

The longitudinal axis of the lesser tarsus is perpendicular to a line that transects the lesser tarsus which extends across the tarsus from halfway between the medial aspect of the talonavicular joint and the medial aspect of the first tarsometatarsal joint to halfway between the lateral aspect of the calcaneocuboid joint and the lateral aspect of the fifth tarsometatarsal joint (Fig. 12.8).

Figure 12.8 Alignment dorsiplantar view. Line 1 – bisection of lesser tarsus. Line 2 – longitudinal axis of lesser tarsus. Line 3 – longitudinal axis of hindfoot. Angle A – lesser tarsus abduction angle. Angle B – talonavicular angle.

Normally, 75% of the head of the talus articulates with the navicular. In a pronated foot the head of the talus moves out of alignment with the cupped surface of the navicular and there is progressive abduction of the lesser tarsus. The lesser tarsus abduction angle is the angle between the longitudinal axis of the lesser tarsus and the longitudinal axis of the hindfoot (Fig. 12.8). This angle increases with pronation and decreases with supination. The talonavicular angle is between the midline axis of the talus and the bisection of the lesser tarsus (normal 60–80°) (Fig. 12.8). This angle is greater than 80° in the supinated foot and less than 60° in the pronated foot.

The longitudinal axis of the metatarsus is a longitudinal bisection of the second metatarsal (Fig. 12.9). The metatarsus adduction angle is the angle between the longitudinal axis of the metatarsus and the longitudinal axis of the lesser tarsus (normal <15°) (Fig. 12.9). A higher figure indicates that metatarsus adductus or deviation of the forefoot to the midline is present. Severe metatarsus adductus is associated with hindfoot valgus. The metatarsus primus adductus angle, also known as the metatarsus primus varus or first intermetatarsal angle, by strict definition refers to a varus relationship of the first metatarsal with respect to the medial cuneiform. In everyday practice, metatarsus primus adductus is measured by considering the angle between longitudinal bisections of the first and second metatarsals (it is assumed that the second metatarsal axis gives a reasonable approximation of the medial cuneiform axis) (Fig. 12.9). An angle of more than 10° is considered abnormal and is associated with hallux valgus.

Figure 12.9 Alignment dorsiplantar view. Line 1 – bisection of lesser tarsus. Line 2 – longitudinal axis of lesser tarsus. Line 3 – longitudinal axis of metatarsus. Line 4 – midline axis of first metatarsal. Angle A – metatarsus adduction angle. Angle B – metatarsus primus adductus angle. Angle C – hallux valgus angle.

In hallux valgus, the proximal phalanx of the great toe (hallux) is directed further from the midline with respect to the first metatarsal. If a longitudinal bisection of the first proximal phalanx is compared with a longitudinal bisection of the first metatarsal, the hallux valgus angle can be measured (Fig. 12.9). The hallux may normally deviate laterally by a small amount and an angle of less than 15° is generally considered acceptable. Hallux valgus is mild with an angle of 16–25°, moderate with an angle of 26–35° and severe with an angle of greater than 35°. The common term for hallux valgus is ‘bunion’ from which the term ‘bunionette’ (also known as tailor’s bunion) is derived. This describes a prominence lateral to the fifth metatarsal head secondary to a varus angulation of the fifth proximal phalanx with respect to the fifth metatarsal.

Deviation of the cartilaginous surface of the first metatarsal can be identified by construction of the proximal articular set angle (PASA) which may be relevant in planning hallux valgus surgery. This angle is between a line representing the limits of the articular cartilage on the first metatarsal head and a perpendicular to the longitudinal axis of the first metatarsal (normal PASA <10°). A deviation in the shaft of the first proximal phalanx is occasionally responsible for the valgus deformity seen in a hallux when the first metatarsophalangeal joint alignment is normal and can be identified by drawing the distal articular set angle (DASA). This angle is between a line representing the articular surface of the first proximal phalanx and a line perpendicular to the longitudinal axis of the phalangeal shaft (normal DASA = 0–6°).

The hallux interphalangeal angle is formed between the longitudinal axes of the proximal and distal phalanges of the hallux. A normal angle is less than 10°. A larger angle indicates an interphalangeal joint valgus deformity which is sometimes referred to as ‘terminal valgus’ or hallux interphalangeus.

Lateral view

The calcaneal inclination axis is a line along the inferior surface of the calcaneus connecting the most inferior point of the calcaneal tuberosity with the most distal inferior point of the calcaneus at the calcaneocuboid joint (Fig. 12.10). The calcaneal inclination angle is between the calcaneal inclination axis and the supporting surface and reflects the height of the foot framework (Fig. 12.10). Measurements of 10–20° would be considered low, 20–30° medium and >30° high. The measured angle varies according to whether the foot is abnormally pronated or supinated and clinical judgement is important.

Figure 12.10 Alignment lateral view. Line 1 – calcaneal inclination axis. Line 2 – midline axis of talus. Angle A – calcaneal inclination angle. Angle B – talar declination angle. Angle C – lateral talocalcaneal angle. Arrows D and E – the cyma line.

The talar declination angle is the angle between a bisection through the body and neck of the talus and the supporting surface (normal approximately 21°) (Fig. 12.10). A continuation of the midline talar axis should bisect the first metatarsal shaft. If the first metatarsal axis extends downwards compared with the talar axis, i.e. the first metatarsal is steeper in relation to the horizontal, the talar declination angle decreases and the line falls above the first metatarsal. This indicates that the longitudinal arch is high and there is a cavus foot (pes cavus). Conversely, if the talar axis extends downwards compared with the first metatarsal axis, the talar declination angle increases and the line falls below the first metatarsal. This is indicative of a low to flat longitudinal foot arch or a planus foot (pes planus).

Another method for assessing cavus or planus is measuring the angle of the longitudinal arch between the calcaneal inclination axis and a line which extends along the undersurface of the fifth metatarsal (normal 150–170°). Cavus is present with an angle of less than 150°. In planus the angle is increased approaching 180°. The lateral talocalcaneal angle is between the calcaneal inclination axis and the midline axis of the talus (normal 35–50°) (Fig. 12.10). Boehler’s angle is measured between a line drawn along the superior margin of the subtalar joint and a line drawn along the posterior superior border of the calcaneus (normal 20–40°).

Calcaneus and equines are used to express a relationship between the tibial axis and the calcaneus. Normally the undersurface of the calcaneus is a bit higher anteriorly than posteriorly. If the anterior calcaneus extends lower than the posterior, or if they are at the same level, an equinus calcaneus is present. If the anterior calcaneus extends more upwards than normal, above the rest of the upper surface of the bone, a calcaneus calcaneus is present.

The sinus tarsi is seen as a radiolucent area that lies above the sustentaculum tali and between the anterior and posterior subtalar facets. The subtalar facets and the sinus tarsi become obscured in a pronated foot (where the great toe moves down and the little toe assumes the higher position) and more visible in a supinated one (where the great toe moves up and the little toe assumes the lower position on the lateral view).

The talonavicular and calcaneocuboid joints together produce a superimposition on a lateral X-ray known as the cyma line (Fig. 12.10). These curved joints together form a reverse ‘lazy S’ as an intact curve in a normal foot. In a pronated foot the ‘S’ becomes broken as the talonavicular joint moves anterior and plantar to the calcaneocuboid joint and the reverse occurs in a supinated foot.

In a severely pronated foot the talus and navicular plantarflex and exert a downward force on the posterior aspect of the medial cuneiform, resulting in a naviculo-cuneiform fault. In such cases the intermediate cuneiform, not normally visible, may be seen protruding above the medial cuneiform. Less commonly, a calcaneocuboid fault may be seen in a high-arched foot. In this situation the cuboid may become partially displaced under the anterior plantar process of the calcaneum. In a supinated foot the metatarsals become more clearly outlined. In a pronated foot the superimposition of the metatarsals becomes more complete, and it may only be possible to clearly distinguish the first metatarsal.

The hallux should normally lie in line with the metatarsal, being neither dorsiflexed nor plantarflexed. The lesser metatarsophalangeal joints are occasionally visible and it may be possible to detect dorsal subluxation or dislocation.

Os trigonum

This ossicle is a secondary epiphysis, found on the posterior aspect of the trochlear surface of the talus, which fails to fuse to the talus in about 8–15% of the population. The os trigonum can give rise to posterior ankle pain when it is compressed between the posterior tibia and posterior calcaneus during forced plantar flexion, as occurs in certain sports such as football and tennis, and with other activities such as ballet or yoga. This is referred to as the os trigonum syndrome or posterior impingement.

Os tibiale externum

The accessory navicular bone may cause pain for several reasons. Painful degenerative changes may occur between the accessory ossicle and navicular, a painful bursa can develop in the soft tissues superficial to the ossicle and there is a higher incidence of tibialis posterior tendon tears or dysfunction. The incidence of the bone in radiographic studies is 2–20%.

Os vesalianum

This ossicle is a secondary epiphysis at the fifth metatarsal base and must be differentiated from an avulsion fracture. Generally, a fracture will have a longitudinal orientation and an irregular outline, compared with the transverse orientation and smooth contour of the ossicle.

Os peroneum

This lies within the peroneus longus tendon in the peroneal groove of the cuboid. It is the commonest accessory bone present in up to 26% of the population. It can be injured with repeated ankle sprains or following a forced dorsiflexion injury.

Hallux sesamoids

The medial and lateral sesamoids should appear as two smooth ovoid structures approximately 0.5 cm proximal to the articular surface of the first metatarsal head. Sesamoids may be symptomatic for several reasons: fractures, osteonecrosis, inflammation (sesamoiditis), infection and involvement in degenerative or inflammatory joint disease (Fig. 12.11). In a case of developing hallux valgus, the sesamoids progressively move laterally, eventually ending up in the interspace between the first and second metatarsals. Sesamoids may be bipartite or multipartite and occur under any of the other metatarsophalangeal or interphalangeal joints.

Figure 12.11 Sesamoiditis. Coronal short tau inversion recovery (STIR) MR image. High signal within the lateral sesamoid (arrow) is consistent with inflammation or sesamoiditis. Compare with the normal marrow signal of the medial sesamoid (arrowhead).

Polydactyly is the presence of additional phalanges or complete digits (Fig. 12.12). Brachydactyly is due to partial failure of development of a metatarsal or phalangeal segment leading to shortening of the digit. Congenital aplasia is complete failure of development of a segment and is rare (Fig. 12.13).

Figure 12.12 Polydactyly. Dorsiplantar radiograph. Polydactyly with additional fifth digit bilaterally and additional first digit on the left.

Figure 12.13 Congenital aplasia. Dorsiplantar radiograph. Congenital aplasia with a fracture of the medial digit (arrow).

Tarsal coalitions may be fibrous (syndesmosis), cartilaginous (synchondrosis) or osseous (synostosis) and are thought to represent failure of proper segmentation of the tarsal bones. Coalitions occur in about 6% of the population, may be bilateral and result in restricted subtalar joint motion. The more common forms are calcaneo-navicular and talocalcaneal, which usually occurs at the middle facet of the subtalar joint. Calcaneo-navicular coalition is usually well seen on oblique views (Fig. 12.14A). Talocalcaneal coalition may be associated with a prominent talar beak arising from the dorsal aspect of the head or neck of the talus. Suspected tarsal coalitions may be confirmed with CT or MRI (Fig 12.14B). Limited subtalar joint motion results in increased stresses elsewhere in the tarsus and this may result in bone marrow oedema adjacent to the coalition on MR images.

Figure 12.14 Osseous calcaneonavicular coalition. A Lateral radiograph. B Sagittal T1-weighted MR image. There is union of the anteromedial corner of the calcaneus with the navicular (arrows) consistent with an osseous coalition.

Osteoporosis

The commonest cause of osteoporosis is primary osteoporosis which is most commonly seen in post-menopausal women. Up to 30% of postmenopausal women will develop osteoporosis, which is largely preventable with hormone replacement therapy. Secondary osteoporosis is seen with a large number of underlying diseases which cannot usually be accurately distinguished by looking at a radiograph. More common causes include renal disease, long-term steroid use and endocrine disorders such as hyperthyroidism and Cushing’s syndrome (increased levels of cortisol either secondary to hyperactivity of the adrenal glands or a pituitary adenoma).

Decreased bone density confined to a region of the appendicular skeleton is referred to as regional osteopenia. The most common cause is disuse osteoporosis which occurs following a period of disuse of a limb such as with immobilisation of fractures. The osteoporosis usually appears after 8 weeks of immobilisation and reverses when stress and function return. Another cause of localised osteopenia is complex regional pain syndrome (Sudeck’s atrophy), which is mediated via a neurovascular mechanism usually following trauma. There is pain and soft-tissue swelling which are out of proportion to the injury and radiographic evidence of irregular mottled osteoporosis distal to the injury site. Regional migratory osteoporosis is a disease of unknown aetiology that typically occurs in middle-aged men and is characterised by pain and localised osteoporosis which migrates from one joint to another.

The main radiographic finding in osteoporosis is decreased bone density with thinning of the cortex. There is accentuation of primary trabeculae with thinning of secondary trabeculae and an increased incidence of fractures with delayed healing. Bone density may be assessed on plain radiographs, but bone loss of up to 30% can occur before radiological changes are apparent. Some scientific methods that may be used for the evaluation of bone density by radiologists include:

Osteomalacia

In osteomalacia, there is a failure of the osteoid tissue to mineralise. The commonest causes of osteomalacia are renal osteodystrophy and disorders of vitamin D metabolism. A distinctive radiographic feature is Looser’s zones, which are areas of unmineralised osteoid that may develop insufficiency fractures, and tend to occur in the pelvis and scapula. The trabecular pattern is often coarse and indistinct. Osteomalacia in children is called rickets. Rickets causes characteristic enlarged and irregular epiphyseal plates with splayed metaphyses and bowing of the bones from bone softening (Fig. 12.15). As in adults, the commonest cause is renal disease, although other causes such as biliary disease and dietary insufficiencies are seen.

Figure 12.15 Rickets. AP radiograph. Rickets is characterised by decreased bone density with splayed metaphyses, enlarged irregular epiphyseal plates and bowing of the distal tibia and fibula.

Hyperparathyroidism

Hyperparathyroidism occurs from excess parathyroid hormone and is due to hyperplasia of the parathyroid glands (primary type), persistent stimulation due to low serum calcium levels (secondary type) or an adenoma (tertiary type). Parathyroid hormone causes increased bone resorption by osteoclasts which leads to osteoporosis and osteomalacia.

The classic radiographic sign of hyperparathyroidism is subperiosteal bone resorption which is most commonly seen on the radial aspect of the middle phalanges of the hand but can be seen in any long bone in the body. Other radiographic features include distal phalangeal tuft resorption, soft-tissue calcification, osteosclerosis typically involving the vertebral end plates giving rise to the ‘rugger jersey spine’ and brown tumours which are lytic expansile bone lesions.

Osteosclerosis

Many disorders have been reported to cause osteosclerosis and only the more common entities are covered.

Renal osteodystrophy is a constellation of musculoskeletal abnormalities that occur with chronic renal failure. Osteosclerosis is one of the more common manifestations occurring in up to 20% of patients. Other features are those of hyperparathyroidism, osteomalacia and soft-tissue calcification.

Myelofibrosis is caused by progressive fibrosis of the marrow in patients over 50 years of age. This leads to a generalised increase in bone density with anaemia, splenomegaly and extramedullary haematopoiesis.

Paget’s disease is a common disorder of unknown origin which can involve the entire skeleton or isolated bones, including the bones of the feet. The disease causes excessive bone resorption followed by haphazard new bone formation and remodelling. The bones are enlarged and appear dense with a coarse trabecular pattern and thickened cortex (Fig. 12.16). Although the bones are enlarged and appear dense on X-ray they are of poor quality and microfractures are common. Paget’s disease predisposes to an increased incidence of sarcoma, although less than 1% progress to malignancy.

Figure 12.16 Paget’s disease. Lateral radiograph. Paget’s disease of the calcaneum, appearing dense and enlarged with a coarse trabecular pattern and thickened cortex.

Metastatic bone disease with diffuse sclerotic deposits may rarely mimic osteosclerosis. Primary tumours are usually carcinoma of the breast and prostate. Radiolucent components of the metastasis and cortical destruction may be present.

Osteopetrosis (Albers–Schönberg disease or marble bone disease) is a rare hereditary disorder causing extremely dense bones throughout the skeleton. A characteristic finding is a ‘bone in bone’ appearance seen in the vertebral bodies, which contain a small replica of a vertebral body inside it. Densely sclerotic endplates give the appearance of ‘sandwich’ vertebrae. Although the bones are dense they are more susceptible to shear forces and may fracture more easily.

Increased fluorine ingestion (fluorosis) over many years may result in the laying down of new bone inside the medullary cavity of long bones, leading to a sclerotic appearance. Ligamentous calcification, particularly of the sacrotuberous ligament, may be present.