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STAND!! Introduction to FOOT Radiography

STAND UP FOR YOUR FEET!!                                                        Jan. 2021

Understanding foot radiography begins with understanding foot anatomy and function.  For reasons still unclear to anthropologists, at some point pre-apes began to switch to bipedal gait and posture. There was no going back from this ape/hominid divide—and this likely preceded, perhaps enabled, the further distinguishing evolutions of the larger brain and the making of tools. Upright plantigrade posture and gait make many demands, from centering the head over the feet, aligning the spine to the plumb line while giving it curves and flexibility to relate to gravity from many positions, developing large stabilizing muscles around the pelvis and thighs,  changing the shape of nearly every bone involved head to toe, and modifying the foot and ankle to accommodate these challenges.  The ape foot has an opposable hallux and is more V-shaped, like a hand.  Humans have a hallux which is more in line with the other toes and which becomes the primary weight-bearing and push-off platform during gait and balance. The longitudinal and transverse arches are strong springs which can tolerate weight and release repeatedly, like shock absorbers.

As with other introductory hand-outs on this site, this is a radiography starting point, a basic review of the standard views, assessing for technical adequacy/limitations, reviewing most common checkpoints, and underlining residents’ commonly over-looked areas or findings—ie, filling in the gaps and odd corners not always covered by the many excellent conventional teaching sites ‘out there’. By focusing only on the radiography it presents a smaller more digestible starting point for R1s and students; MR and CT are not covered here.

 Orthopaedists prefer weight-bearing (WB) images since that is the position of function, and since that results in more uniform positioning than non-weight-bearing (NWB), with a dangling foot, which is often plantar-flexed and/over over-arched on the laterals. NWB allows more toe flexion, further limiting exam.

       PRONATION / SUPINATION

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     INVERSION / EVERSION  and   DORSIFLEX / PLANTAR FLEX

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Describing the position of the foot is confusing. “anterior/posterior” are replaced with ‘dorsal/plantar’. ‘Proximal/distal’, ‘medial/lateral’, remain.  Pronation/supination rotate the initially-plantigrade plantar surface off the floor externally/internally along the heel-toe axis, inversion/eversion rotate the plantigrade foot inward/outward while it remains plantigrade (parallel to level ground). “Dorsi/plantar flexion” relate to the baseline plantigrade position, plantar foot on flat ground and foot at right angle to ankle.

HINDFOOT is talus and calcaneus; distal margin is (Chopart or) midtarsal/transtarsal jointà

MIDFOOT rest of tarsals—navicular, cuboid, 3 cuneiforms; distal margin is (Lisfranc or) tarsal-MT jointà

FOREFOOT MTs and phalanges (which can be numbered 1-5, medial-lateral, unlike the hand). The first toe also goes by great toe or hallux; has the 2d most consistent sesamoids in body, plantar 1st MT head.

3 STANDARD VIEWS:  Lateral, frontal, and oblique.

AP or FRONTAL (DP,dorsal-plantar):  starting point. Long axis talus runs down 1st MT diaphysis, long axis calcaneus nearly runs down 4th MT. Distal talus should be covered by navicular.

        Accesory ossicle navicular—Immediately medial, 1 mm typeI , 2-3mm II, are NORMAL variants; larger type III is ‘ANATOMIC variant, extends/enlarges medial margin, extends talar coverage, that may impinge overlying soft tx or footwear’  . May resemble fx.

        Lisfranc space    —Lf. ligament runs medial cuneiform to base of 2d MT; this space should be under 2mm (if no hallux valgus) . Widened space and/or radiodensity/fragment here implies destabilizing injury.  I prefer ‘tarsal-MT fx-dislocation’ but the Universe calls it (DWG) ‘Lisfranc fx-dislocation’.

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     1st MTP joint—articular surfaces should be perfectly parallel , ‘spooned’ parentheses, ‘C” shaped..  Early arthritis narrows the “C”, then progresses to osteophytes and increasing deformity/reactive change.. HALLUX VALGUS is valgus of the 1st proximal phalanx/varus 1st MT, which also widens Lisfranc space.  Hallux valgus most commonly results from ‘stupid shoes’, in particular high heels; other factors—genetics, ligamentous laxity, neurologic disease, pes planus—may also contribute. The toes in a bare footprint are normally wider than the hind foot; repeatedly forcing them into a narrow shoe box (‘pointed toe”), then elevating the hind foot on a high shoe heel, produces progressive permanent deformity (the ‘bunion’) and pain.

5th MT base—most common fx in adult foot is 5th MT styloid.  This is an avulsion fx, tends to be minimally displaced and heals well because of blood supply. Fx is transverse or oblique; do NOT mistake the normal peri-pubertal apophysis, a longitudinally oriented ossification center at the lateral base separated from the 5th MT by a fine longitudinal lucency, for fx!

AP is often a  limited exam 2d-4th toes, especially if NWB since toes tend to flex.  Document ‘Limited exam 2d through 5th toes” if so. Zoom and inspect all toes; stub or crush injuries common, as are fractures, and the hx. given is often just the nonspecific ‘Pain”. Impression can include “Coned and centered F/U of any area of interest can be obtained if clinically indicated”

Normal/Anatomic variants mistaken for fxs: At least 40 accessory ossicles and sesamoids commonly seen in the foot, several often mis-identified as abnormals. Type II or III accessory ossicle navicular with irregular apposition to medial navicular margin; os peroneum lateral/plantar to cuboid; 5th MT styloid base apophysis; os subfibularis at the distal fibular tip; bifid hallux sesamoids; os trigonum posterior to the posterior subtalar joint.  Even more confusing—if larger than average or predisposed by other anatomic considerations, most of these normal findings can be on a spectrum including symptoms and contributing to/causing pain.

Stress fxs: Follow the MT cortices carefully, very early stress/insufficiency may be seen as subtle focal periosteal reaction or focal cortical deformity. Most common in 2d 3rd or 4th MTs; often associated with modified weight-bearing (injury or surgery to contralateral extremity increasing burden on nl. side; ipsilateral injury/surgery modifiying  weight-bearing); new activity/sport/level of exercise, modified footwear; military training/marching with heavy packs; marked osteopenia.

LATERAL: (See also ANKLE, lateral view). WB or NWB? Markedly changes alignment (and leads to inaccurate ‘pes planus/cavus’ ). Proper positioning should give clear visualization of the tibio-talar joint which should be parallel curved lines; assess for ankle joint effusion. If suboptimal positioning, limited visualization tibio-talar or subtalar joints, if limited exam toes due to superimposition, say so.

 Assess anterior/posterior tibial lips for reactive change or deformity/fracture. Long axis talus should about parallel long axis MTs. ‘Relatively vertical talus’ if plantar-flexed, which can be congenital or acquired, often with neurologic disease or foot alignment anomalies.  Calcaneal axis, drawn along plantar cortex, should tilt cephalad slightly.  WB will have slightly more flattened arch and less calcaneal incline, although the talar-MT co-linear relationship should remain.  CAVEAT:  residents markedly over-call ‘pes planus’  and ‘pes cavus’, particularly in NWB views. Unless TALUS axis markedly abnormal, safer to describe as ‘relatively flattened longitudinal arch, WB view”; “relatively high-arched lateral, NWB view”.  Dorsal surface foot should be continuous and flow, not stepped-off (look for subluxation of midfoot relative to hind or fore foot; neuropathic disorganization or Lisfranc/other trauma may cause subluxations).

Check/mention dorsal 1st MT head for osteophytes which if more than 1-2 mm (not an official number) ‘may impinge overlying soft tx. or footwear’.  (Resection of such dorsal osteophytes is a ‘cheilectomy’).

Achilles tendon silhouette (yes DWG but common usage) should form elongated band with straight parallel margins (on WB), of homogeneous tissue density, clearly defined by pre-Achilles clear space (fat pad).  Bulges, widening, loss of definition, irregular contour should be described; “If the Achilles tendon is the area of clinical interest MR would give better visualization” (review old notes—very helpful if you can say ‘unchanged from…’ ‘History given of previous trauma/instrumentation …”).  NWB, with plantar flexion, crumples the tendon making assessment limited.  Note insertional enthesopathy, calcifications distal tendon (indicative of chronic/healed trauma, inflammation, surgery). ‘beaked’ elongated humped contour posterior superior margin calcaneus (Haglund deformity or ‘pump bump’),

The bony arch is stabilized by the ‘bow string’ of the plantar aponeurosis, a radiographically homogenous straight tissue density, fascia/connective tissue forming a band connecting the plantar aspect of the bony arch from the posterior plantar calcaneus (medial calcaneal tubercle) to the bases of the proximal phalanges.  It is difficult to assess radiographically.  Deep to it is the plantar fat pad, best seen posterior 2/3 plantar soft tissue; an 18-20 mm thick (NWB) ‘bubble wrap’ tissue cushion formed by criss-crossing strands of collagen/fascia or ‘honeycombing’, creating reticulated chambers filled with fat, cushioning every step.  Compromise by trauma, infection, diabetes, lead to constant pain. Look for the honeycombing. If penetrating trauma or diabetes or neuropathy, look for induration of the honeycomb/tissue loss/foreign matter.

WB arch is flatter than the NWB arch.  Long axis talus continues to run through MT long axis, on both; if it is nose-diving or pointing morr plantar, ‘increased verticality talus”, “relatively vertical talus’  (most common in neurologic conditions, or ligamentous failures in RA, DM, etc). slight calcaneal incline should be preserved even on WB. Plantar calcaneus parallel to ground is ‘loss of calcaneal incline’; neurologic collapse/deformity may produce ‘calcaneal decline”.  If the arch is reversed—plantar alignment goes from concave arch to convexreversal, ‘rockerbottom lateral arch”.  

Normal os peroneum shows up as small rounded corticated lateral/plantar cuboid ossification (can be multiple)—need not mention.  Deeper/more plantar amorphous or streaky calcifications  must be described: plantar fascia streaks can be sequelae of fasciitis or steroid injections; while amorphous soft tissue calcifications, especially in the presences of soft tissue induration/swelling, raise the possibility of sarcoma.

Check base of 5th MT, some fxs. here best seen on lateral.

The calcaneus has 3 flowing cascades of trabeculae similar to the femoral neck, including a relatively stress-shielded lucent area of trabecular rarefaction, junction middle/anterior thirds similar to Ward triangle.

Follow the flowing trabeculae and cortices looking for compression, deformity, zigzags, discontinuity.  Can only see part of anterior and posterior subtalar joints; they undulate. Posterior talus may be elongated (Steida process) or have accessory ossicle/os trigonum—both normal variants until excessive in size, then “…which may impinge posterior to the posterior subtalar joint during range of motion”.

Dorsal distal third talus: small irregularities or densities may represent capsular avulsions, acute or chronic; is there overlying soft tx induration? Mature margins? Old images?

Check anterior process calcaneus for subtle fx. (or nl. variant)

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OBLIQUE: As with other bulky complex joints such as the knee, shoulder, hand, and spine, two views are not a complete exam (although often requested by non-Orthopaedists). We need this 3rd view to see more of the complex tarsal margins/joints and relationships, as well as better view of the lateral column of the foot.

As with other views, look at inter-tarsal spaces, cortical continuity and flow, trabeculae, tissue planes. Oblique is particularly good for looking into the ‘four corners’, the intersection of hind foot (calcaneus and talus) with midfoot (navicular and cuboid). Follow the MT cortices and bases (overlap at bases masks subtle fxs.). 

Check that 5th MT bases again, for fx. and for further distortion of the slight ‘S’ shaped step-off of the base of 5th MT/lateral distal cuboid (trans-MT subluxation, or Lisfranc fx-dislocation).   

Anterior process calcaneus fxs. may only show up here; ditto coalitions (bony, cartilaginous, or fibrous connections, limiting motion and often causing symptoms) which are a common cause of painful foot, and/or flattened arch. Since children are incompletely ossified, it may not show up til adolescence or adulthood. Calcaneal-navicular and talo-calcaneal coalitions are most common (~90%), and bilaterality is seen in about 50%.  Bony bands are best seen in the ‘four corners’ on the oblique; while cartilage or fibrous bands are radiolucent, the long-term modified weight-bearing/mobility of the bones often gives clues with chronically modified, elongated, or irregular margins  projecting into the ‘four corners’.

Check Lisfranc space (base of 1st-2d MTs) for fragments, deformity, widening.

 Zoom and study the toes.

IMPRESSION: As always, if no relevant findings, “Unremarkable’ or ‘No definite acute abnormailty’ are the preferred summaries; “Normal’ is a risky claim in such a complex structure so incompletely seen on radiography, and so prone to subtle abnormalities, acute or chronic. “Advise F/U coned to any area of interest if clinically indicated’, “Further F/U 10-14 days following onset/injury if clinically indicated (and if we know date of trauma—someone with trauma 4 weeks ago, or pain for 3 years, unlikely to benefit from ‘F/U 10-14 days’ and makes us look irrelevant—extract as much data as you can to supplement the usual non-informative indication of ‘Pain”)