Lower Limb X-Ray

Lower limb radiography covers the femur, knee, tibia and fibula, ankle, hindfoot, midfoot and forefoot (the hip is covered under pelvis and hip imaging).

At least two views at right angles are taken for each area.

• Femur: anteroposterior and lateral including the knee, with the hip imaged for femoral neck injury in high energy shaft fractures.
• Knee: anteroposterior and lateral, with a skyline view of the patellofemoral joint. In trauma, add a horizontal beam lateral view to show a lipohaemarthrosis.
• Tibia and fibula: anteroposterior and lateral including both joints.
• Ankle: anteroposterior, a mortise view with 15 to 20 degrees of internal rotation, and lateral.
• Foot: anteroposterior, oblique and lateral. Calcaneus: lateral and axial.

Weight bearing films (knee, ankle and foot) show the true joint space and alignment in arthritis and deformity. The dose is small (0.001 to 0.01 mSv for a distal limb view, about 0.01 to 0.05 mSv for the femur or knee).

Validated clinical rules reduce unnecessary radiography after ankle and foot injury.

• An ankle series is needed if there is pain in the malleolar zone with bony tenderness at the posterior edge or tip of either malleolus, or inability to bear weight for four steps both immediately and in the emergency department.
• A foot series is needed if there is pain in the midfoot zone with bony tenderness at the base of the fifth metatarsal or the navicular, or inability to bear weight.

Similar rules for the knee apply (age 55 or over, isolated patellar tenderness, fibular head tenderness, inability to flex to 90 degrees, and inability to bear weight). Rules of this kind have a sensitivity near 100% for fractures and reduce radiography by 30 to 40%.

Indications. Injury with bone tenderness or inability to bear weight, meeting clinical rules. Knee or ankle joint effusion or locked knee after trauma. Chronic knee, ankle or foot pain with deformity, swelling or stiffness (osteoarthritis, inflammatory arthritis, gout, infection). Bone pain, mass or limping in a child.

Diabetic foot with ulceration, swelling or suspected osteomyelitis or neuropathic joint. Suspected stress fracture. Suspected infection or tumour. Follow up of fractures, arthroplasty and fixation. Not for acute knee pain without trauma or mechanical features in a young adult, or for minor soft tissue injury without bony tenderness.

Contraindications and precautions. No absolute contraindication. Pregnancy: low dose, with shielding of the pelvis for knee and distal views. Suspected femoral shaft or knee dislocation: do not move the limb for positioning until neurovascular status is documented and the limb is splinted. Remove footwear and dressings that obscure detail.

Preparation and technique. Weight bearing films are taken standing where possible. For the ankle, the lateral view includes the base of the fifth metatarsal and the full calcaneus; for the foot, a weight bearing lateral film shows the arches. For the knee, the horizontal beam lateral allows fat and fluid to layer in the suprapatellar pouch.

Normal appearances. Knee: joint space of 3 to 6 mm with smooth tibial and femoral cortices, a patella lying centrally with a patellar height ratio (Insall-Salvati ratio of patellar tendon length to patellar length) of 0.8 to 1.2, and no joint effusion (the suprapatellar fat pad is clear).

Ankle: a uniform joint space, a medial clear space of 4 mm or less, a tibiofibular clear space of under 6 mm, and a talar dome parallel to the tibial plafond with no talar tilt. Foot: the hindfoot has a Bohler angle of 20 to 40 degrees (calcaneus lateral view) and a critical angle of Gissane of 120 to 145 degrees.

In the midfoot, the medial border of the second metatarsal is in line with the medial border of the middle cuneiform, and the medial border of the fourth metatarsal is in line with the cuboid. The first metatarsophalangeal joint is smooth, with the hallux valgus angle under 15 degrees and the intermetatarsal angle under 9 degrees.

Interpretation.

• Knee: lipohaemarthrosis (a fat fluid level in the suprapatellar pouch on a horizontal beam lateral film) indicates an intra-articular fracture even when no fracture line is seen.
• Tibial plateau fractures (lateral commoner, graded by the pattern of split and depression) need CT. A Segond fracture (small avulsion of the lateral tibial plateau) indicates anterior cruciate ligament injury.
• Patellar fractures (transverse, stellate), with the bipartite patella (superolateral corner) as a mimic; patellar dislocation with a medial patellar avulsion; quadriceps and patellar tendon rupture show patella alta or baja, respectively.
• Osteochondritis dissecans (a subchondral lucent fragment at the lateral part of the medial femoral condyle), Osgood-Schlatter disease (fragmentation and soft tissue swelling at the tibial tuberosity in adolescents), and loose bodies.
• Knee osteoarthritis: medial joint space narrowing (weight bearing views), osteophytes, subchondral sclerosis and cysts.
• Knee osteoarthritis is graded 0 to 4 by the Kellgren-Lawrence scale (grade 2 definite osteophytes, grade 3 moderate narrowing, grade 4 severe narrowing with bone deformity).
• Inflammatory arthritis shows uniform joint space loss, marginal erosions and osteopenia; chondrocalcinosis points to calcium pyrophosphate deposition.
• Tibia and fibula: spiral fractures (twisting injuries), transverse or comminuted fractures (direct trauma), open fractures (commonest long bone), and tibial toddler fracture (spiral fracture without displacement in a child).
• Tibia and fibula fractures are described with fibular involvement, and rotational alignment is judged with both joints.
• Ankle fractures: classified by the level of the fibular fracture relative to the syndesmosis (Weber A distal to the syndesmosis, B at its level, and C proximal to it).
• Signs of ankle instability are a medial clear space over 4 mm, talar tilt, syndesmosis widening (tibiofibular clear space over 6 mm, overlap under 6 mm on anteroposterior view or under 1 mm on mortise), and a posterior malleolar fragment.
• A Maisonneuve injury (a proximal fibular fracture with a medial malleolar fracture or deltoid disruption and syndesmotic injury) is easily missed. Examine the full length of the fibula when the ankle shows a medial injury with no lateral fracture.
• A pilon fracture (distal tibial plafond) needs CT.
• Foot: calcaneal fractures (a reduced Bohler angle under 20 degrees, a flattened critical angle of Gissane, with associated spine fractures in falls) and talus fractures (risk of avascular necrosis at the talar body).
• Lisfranc injury: widening between the first and second metatarsal bases of over 2 mm, loss of alignment of the second metatarsal with the middle cuneiform, a "fleck" sign. It is often subtle and requires a weight bearing film or CT.
• Metatarsal fractures: fifth metatarsal tuberosity avulsion, Jones fracture at the metaphyseal diaphyseal junction with poor healing, and stress fractures in the shafts of the second and third metatarsals. Sesamoid fractures also occur.
• Stress fractures: early films are normal in the first 2 to 3 weeks; later, periosteal reaction, a sclerotic line or cortical thickening appears, and MRI shows oedema earlier.
• Diabetic foot: soft tissue gas, ulcers, and osteomyelitis (cortical loss, periosteal reaction, and lucency adjacent to an ulcer, 10 to 14 days after onset, with MRI for early disease).
• Neuropathic (Charcot) joint shows fragmentation, disorganisation, subluxation of the midfoot (rocker bottom foot) and densification.
• Paediatric conditions: rickets (cupping, fraying and widening of the metaphyses, a widened growth plate, bowed legs), Blount disease (tibia vara with a medial proximal tibial depression), slipped epiphysis in the knee (referred hip pain), clubfoot, flatfoot, and tarsal coalition (calcaneonavicular bar, talar beaking).
• Tumours: osteosarcoma (a lesion around the knee with a Codman triangle and sunburst periosteal reaction in adolescents) and Ewing sarcoma (a diaphyseal permeative lesion with layered periosteal reaction).
• Giant cell tumour (a lytic epiphyseal lesion in skeletally mature patients) and bone metastases also occur.

Critical values and findings needing immediate action.

• Knee dislocation (or a spontaneously reduced knee dislocation suggested by multiple ligament injury and avulsion fractures): urgent assessment of the popliteal artery (pulses, ankle brachial index, CT angiography), because arterial injury occurs in about 10 to 40%.
• Open fracture: antibiotics within 1 hour, tetanus prophylaxis, wound irrigation, splintage and orthopaedic and plastic surgery.
• Compartment syndrome after tibial fracture: emergency fasciotomy based on clinical signs.
• Displaced fractures with skin tenting, absent pulses or a cold foot: immediate reduction and vascular review.
• Lisfranc injury, talar neck fracture, calcaneal fracture with a displaced joint, tibial plateau fracture with depression, or pilon fracture: urgent orthopaedic review and CT.
• Suspected septic arthritis or osteomyelitis (fever, inability to bear weight, raised inflammatory markers): urgent aspiration and antibiotics.
• Pathological fracture or suspected sarcoma: orthopaedic oncology referral before any biopsy or fixation.
• Diabetic foot infection with gas, abscess or sepsis: urgent surgical review.

Limitations and artefacts.

• Plain films miss about 5 to 10% of fractures (undisplaced tibial plateau, occult hip, stress, osteochondral and Lisfranc injuries). A negative film with a lipohaemarthrosis, severe pain or persisting inability to bear weight needs CT or MRI.
• Plain films do not assess ligament, meniscal, cartilage and tendon injury, and effusions are only indirect signs.
• Non-weight-bearing films underestimate joint space narrowing and instability, and weight bearing views reveal the true alignment.
• Rotated or oblique positioning mimics fractures or hides syndesmotic widening.
• Normal variants (accessory ossicles such as os trigonum and os peroneum, bipartite patella, fabella, nutrient canals and growth plates) mimic fractures and avulsions.
• A second injury accompanies many fractures (calcaneal fracture with lumbar spine fracture, tibial fracture with ankle or knee injury, and Maisonneuve injury).
• Osteomyelitis changes lag 10 to 14 days after onset, and a normal film does not exclude early infection.
• Radiographic osteoarthritis grade correlates imperfectly with symptoms.
• Measurement errors in angles and alignments result from patient position and projection.

Special groups.

• Children: toddler fractures, buckle fractures, physeal injuries, avulsion fractures of the tibial spine, and limping child (differential of transient synovitis, septic arthritis, Perthes disease and slipped epiphysis); consider non-accidental injury with unexplained fractures in infants, especially metaphyseal corner fractures.
• Older adults: osteoporotic fractures of the tibial plateau, ankle and metatarsals; falls and fragility fractures need bone density assessment.
• Patients with diabetes or peripheral neuropathy: Charcot foot and osteomyelitis, which need MRI and early specialist care to avoid amputation.
• Athletes and military recruits: stress fractures of the tibia, metatarsals, navicular and fibula.
• Patients with sickle cell disease: bone infarction and osteomyelitis (including Salmonella) are hard to tell apart.
• Regions: injuries from road traffic and firearms, osteomyelitis in children, rickets, Buruli ulcer and mycetoma (Madura foot) with bone destruction, and podoconiosis related foot deformity are seen.

Follow up tests. MRI for suspected occult fracture, stress fracture, ligament, meniscal and cartilage injury, osteomyelitis and tumour, CT for intra-articular and complex fractures (tibial plateau, pilon, calcaneus, Lisfranc), weight bearing films and alignment views for deformity, ultrasound for effusions, tendon injury and joint aspiration, bone scintigraphy,

vascular studies (ankle brachial index, CT angiography) for suspected arterial injury, full blood count, C-reactive protein and erythrocyte sedimentation rate, blood cultures, uric acid, bone profile and vitamin D, and orthopaedic review.