rad review of skeletal imaging and bone densitometry Imaging of central metatarsalgia RAD Magazine, 45, 528, 29-30 Dr Ayesha Niaz Consultant radiologist Dr Akash Ganguly Consultant radiologist Dr Hifz Aniq Consultant radiologist Royal Liverpool University Hospital A Metatarsalgia refers to a variety of disorders presenting with forefoot pain ranging from traumatic lesions (acute or chronic repetitive), inflammatory and infective disorders, non-neoplastic soft tissue lesions, and benign tumours to malignant lesions. Patients often present with symptoms of localised pain in the forefoot that worsens on weight bearing (walking or running), which can be sharp or dull and can be perceived as a lump felt inside or underneath the foot and described as walking on a marble or pebbles. Such patients are labelled as having metatarsalgia and further evaluated with ultrasound or MRI to establish a diagnosis. We describe imaging features of commonly occurring conditions that pro- duce central metatarsalgia. B Morton’s neuromas are masses composed of perineural fibrosis and nerve degeneration as a result of compression of the interdigital nerve against the deep transverse inter- metatarsal ligament. The most common clinical symptom is localised pain in the forefoot. Pain is worse on weight bear- ing, classically described as walking on a marble and can be associated with numbness and tingling.1 They appear as a peanut- or dumbbell-shaped nodule at the level of the metatarsal head, projecting plantar to the deep transverse intermetatarsal ligament on coronal images.2 Morton’s neuroma appears iso- to mildly hyperintense on C T1-weighted MR images and iso- to hypointense on T2- weighted MR images. Intense enhancement typically occurs on fat-suppressed T1-weighted images. Ultrasound has good specificity and sensitivity for Morton’s neuromas despite being user dependent. They are best assessed on ultrasound from the plantar aspect and the younger lesions show hypo- echogenicity while older lesions show hyperechogenicity2 (figure 1). Classic differentials of Morton’s neuroma include inflamed metatarsal bursa, lesser MTP joint instability, plantar plate tears, collateral ligament tears, and pericap- sular fibrosis which can mimic symptoms like those of D Morton’s neuroma.3 Two types of bursae are described: synovial lined bursae Figure 1 in recognised locations such as intermetatarsal bursae and Morton’s neuroma. Ultrasound shows hypoechoic adventitious bursae, which are not lined by mesothelial tis- lesion in the third intermetatarsal space on longitu- sue. Submetatarsal adventitious bursae can present with dinal (A) and transverse scan (B). MR scan forefoot pain, tenderness, pain on walking, or a lump underneath (different patient) short axis T1 and T2 fat satu- the metatarsal head. They develop at sites where subcuta- rated sequences (C, D) show hypointense Morton’s neous tissue is exposed to high pressure and friction. neuroma (arrows) arising from the third inter- Adventitious bursa shows high signal intensity on T2- metatarsal space. weighted MR images. On dynamic ultrasound, adventitious bursae appear as compressible hypoechogenicity in the plan- submetatarsal bursa. Peripheral enhancement may be seen tar fat pad.4 after administration of gadolinium.4 Intermetatarsal bursitis often presents with the same Intermetatarsal bursitis can occur in isolation, or it can symptoms and often mimics Morton’s neuroma. Small fluid accompany Morton’s neuroma, plantar plate tears, or both. collections in the first three intermetatarsal bursae with a Intermetatarsal bursae are located cranial to the deep trans- transverse diameter of 3mm or less may be physiologic.1 If verse intermetatarsal ligament, whereas a Morton’s neuroma bursae become inflamed, they can resemble a cystic struc- involving the digital neuromuscular bundle lies caudal (plan- ture. An intermetatarsal bursa has similar signal intensity tar) to the deep transverse intermetatarsal ligament. When on MR images and similar echogenicity on ultrasound to large, each extends beyond the deep transverse inter- rad review of skeletal imaging and bone densitometry metatarsal ligament, but the epicentre of the lesion often helps in differentiation.5 The plantar plate is a broad, thick, trapezoidal fibro- cartilaginous bandlike structure extending from the plantar surface of the metatarsal head to the base of the proximal phalanx. It is thickest medially and laterally on the base of the proximal phalanx and relatively attenuated centrally, with tear of the lateral aspect being more common.4 Plantar plate injury occurs commonly in women because of the increased weight-bearing load and hyperextension forces placed on the MTP joints. Plantar plate tears have been described in 40% in patients with metatarsalgia, with 90% occurring at the second MTP joint. MRI is slightly more specific than ultrasound but both have similar sensitivities, with ultrasound more cost-efficient. The vascularity suggests inflammation, whereas fibrosis alters the appearance and thickness of the plate. The tears are effectively evaluated Figure 3 on ultrasound with a plantar approach, laterally at the base Freiberg’s disease. Plain x-ray of second toe shows of the proximal phalanx with dynamic assessment by apply- collapse and sclerosis of the third metatarsal head. ing a dorsiflexion stress. Abnormalities range from thicken- Long axis T2 fat-saturated sequence shows flatten- ing and hypoechogenicity to discontinuity and entheseal ing and sclerosis of third metatarsal head (arrow) irregularity at the base of the proximal phalanx4 (figure with marrow oedema of the distal shaft. 2A). MR findings include high T2 signal intensity around the within the intermetatarsal space and is generally associated plantar plate, persistent hyperextension of the proximal pha- with degenerative tears. When it is sufficiently thick, it can lanx with discontinuity of the plantar plate, MTP joint replace intermetatarsal fat, mimicking a Morton’s neuroma.4 synovitis, flexor sheath tenosynovitis, or combination of the Stress fractures are common in runners, gymnasts, preceding (figure 2B). Oedema can sometimes be seen at dancers and military recruits, secondary to abnormal stress the base of the proximal phalanx. Varus drift of the second on normal bone. Typically, stress fractures are imaged with metatarsus is seen in the setting of lateral plantar plate radiography and appear entirely normal initially, but with tear. MR arthrography further improves visualisation. time a fracture line or periosteal reaction can be identified. Pericapsular fibrosis is seen as low- to intermediate signal Inconclusive cases can be imaged with bone scintigraphy. intensity soft tissue thickening on T1- and T2-weighted MR Ultrasound can also be used to evaluate the cortical fracture images, eccentrically abutting the capsule and plantar plate lines and callus formation.4 Stress response manifests as an ill-defined area of low signal intensity within the bone marrow on T1-weighted images, with corresponding high signal intensity on T2- weighted and STIR images. Oedema may be seen in the adjacent soft tissues. These appearances can progress to a stress fracture, which is characterised by a band of low sig- nal intensity contiguous with the cortex on both T1- and T2- weighted images.4 Freiberg’s disease, or osteonecrosis of the metatarsal head, most commonly affects the second metatarsal and is characterised by pain, tenderness, swelling and limitation of motion in the affected MTP joint. Changes are seen on MRI before they are seen on radiography, with low signal intensity on T1-weighted images and corresponding high sig- A nal intensity on T2-weighted and STIR imaging. With time, the metatarsal head flattens and shows low signal intensity on T2WI, consistent with sclerosis4 (figure 3). The foot is the third most common site for a ganglion after the hand and wrist. Ganglions are seen on ultrasound as a uni- or multilocular anechoic lesion with posterior acoustic enhancement, and occasionally a stalk can be seen, indicating communication with the joint or tendon sheath. On MRI, a ganglion appears as a sharply delineated cyst with water-equivalent signals often septated and the walls can show moderate enhancement after administration of gadolinium.5 Epidermoid cysts or implantation dermoid on ultrasound appear as well-defined and hypoechoic with variable echogenic debris and posterior acoustic enhancement. On MRI, they appear as iso- or slightly hyperintense on T1- and B hyperintense on T2-weighted imaging, sometimes with areas of low signal intensity debris. There may be faint wall Figure 2 enhancement after contrast media enhancement.5 Plantar plate tear. Long axis second toe ultrasound Granulomas caused by a retained foreign body show a shows irregular hypoechoic plantar plate tear central echogenic foreign body on ultrasound with evidence (arrow). Sagittal T2-weighted sequence shows torn of an acoustic shadow. On MRI, the foreign body is seen as plantar plate with proximal retraction (white hypointense on T1- and T2-weighted imaging, with the arrow). surrounding reactive change of low T1-weighted and high rad review of skeletal imaging and bone densitometry T2-weighted signal with strong enhancement after admin- istration of gadolinium-based contrast material.5 Plantar fibromatosis (Ledderhose disease) is most com- monly seen in patients with diabetes and epilepsy. On ultra- sound, a plantar fibroma is a well-defined, hypoechoic lesion with vascularity on colour Doppler