The role of ultrasound as a problem-solving tool in the assessment of paediatric musculoskeletal injuries

Introduction

Musculoskeletal (MSK) ultrasound (US) is a non-invasive technique that is well-tolerated by children of all ages with no need for sedation or general anaesthesia (GA). Outcomes are best when the examination takes place in a quiet environment with a relaxed operator working together with the parent/carer to keep the child calm. Many departments have a child-friendly room with a television, colourful pictures, toys and other techniques available to distract the child.

MSK US has no role in acute trauma care. Apart from focussed assessment with sonography for trauma (FAST) to quickly evaluate the abdomen, computed tomography (CT) is the modality of choice in imaging acute head and body injury.^1,2 Acute MSK injuries are initially evaluated with plain radiographs, detailed assessment often occurring in the sub-acute setting. US, magnetic resonance imaging (MRI) or CT may be employed but high temporal and spatial resolution, the ability to perform dynamic imaging and examine the patient, the chance to talk to the carer and to observe the child, as well as the low cost and the lack of ionising radiation make US an important first line imaging tool in children.^3–5

The purpose of this article is to describe and illustrate commonly encountered injuries unique to the paediatric skeleton that can be evaluated with US. Some injury patterns in the older child that overlap with adult pathologies are also covered.

Technique

Ever-improving, high frequency (9–17 MHz) and small foot-plate (hockey stick) linear transducers are widely available and with spatial resolution superior to MRI allow exquisite delineation of the internal architecture of muscles, tendons, joints and other superficial soft tissues. ⁶ These high-resolution transducers depict superficial structures well, and are generally sufficient in infants and young children, whereas lower frequency and curvilinear transducers may be used for deeper structures and to gain an overview in the pelvis, buttock or thigh, in older or larger children. Small foot-plate transducers should be used to image small structures, such as finger joints in young children and to not frighten the child while imaging the neck or face, for instance.

When scanning superficial tiny structures in young children, it is important to minimise pressure with the US probe so as not to compress any fluid collections or vessels in the assessment of injury-related hypervascularity. ⁷

As anatomy varies significantly across the paediatric age group, the operator needs to be familiar with normal structures and commonly encountered variants. For instance, non-ossified cartilage around a joint that is mistaken for fluid may significantly alter diagnosis and treatment. It is useful to remember that in most cases a finding in one limb can be compared with the opposite normal side.

Normal paediatric skeletal anatomy

On US, the appearance of the skin and subcutaneous tissues is similar in children and adults. Reflective connective tissue septa separate hypoechoic fat lobules. This layer is separated from the underlying muscles by a thick hyperechoic band formed by apposition of superficial and deep fascial layers. Hypoechoic muscle fascicles are separated by reflective septa, which converge on the reflective internal aponeurosis producing a featherlike appearance in muscles.

Most skeletal muscles attach to bone via a tendon forming the muscle–tendon–bone complex. Injury will occur at different sites depending on the age and skeletal maturity of the patient and the type of sporting activity. The muscle–tendon–bone complex of paediatric joints varies from their adult counterparts as the tendon inserts into an incompletely ossified or purely cartilaginous apophysis, rather than fully mature bone. ⁸ Unlike in adults, the muscles, tendons and ligaments in the younger patient have comparatively greater tensile strength than the apophysis, explaining why the latter is the most common site for injury.^9–11 As the apophysis ossifies and fuses to the bone in older sporting adolescents, avulsion injuries become less common and the adult pattern of myotendinous muscle injury predominates. With increasing age, tendons undergo mucoid degeneration and weaken with rupture of the tendon itself becoming more common in middle and old age. This knowledge of changing biomechanics is important to bear in mind while scanning and trying to formulate a differential diagnosis.

Ossified bone and its interface to soft tissue or muscle is highly reflective on US, whereas unossified cartilage appears hypoechoic and may be mistaken for fluid by the inexperienced operator. Young children's joints contain large amounts of cartilage and the epiphysis in the very young child may be purely cartilaginous. It is important to recognise this potential pitfall, which can be avoided by changing the gain during scanning and by trying to compress potential fluid. Sonographic appearances of normal joints and entheses in healthy children and changes that occur with age are documented in recent articles.^12,13

Apophyseal avulsion injuries

Acute apophyseal injury often occurs during sporting activity when a sudden forceful muscular contraction applies violent traction tension to the tendon attachment at the apophysis. The apophysis can partially or completely detach from the underlying bone, occasionally with complete functional loss of the attaching muscle. ¹⁴ There are many sites in the pelvis where acute apophyseal injury occurs. In their study of 1000 pelvic X-rays from sporting adolescents, Rossi and Dragoni ¹⁵ found 203 avulsion injuries. The most common site of injury was the ischial tuberosity, insertion site of the hamstring muscles, with 54% of injuries, followed by the anterior inferior iliac spine (AIIS), insertion site of the rectus femoris muscle, with 22% and the anterior superior iliac spine (ASIS), insertion site of the sartorius muscle, with 19% of injuries encountered. Further avulsion sites in the pelvis include the greater trochanter where the gluteus medius and minimus muscles insert, the lesser trochanter as insertion site of the iliopsoas muscle and the origin of the adductor muscles from the pubic tubercle and inferior pubic ramus. A radiograph may be reported as normal if the diagnosis of avulsion injury is not considered as radiographic findings in acute injury can be very subtle.^14,16 Lazovic et al. ¹⁷ proved in a large series of patients with suspected apophyseal injuries that US was more sensitive than plain radiographs with the added advantage that dynamic examinations can identify unstable avulsion injuries. US can demonstrate haematoma at the injured site, irregularity within the bone, increased distance to the apophysis, dislocation of the apophysis and early neovascularisation increasing diagnostic accuracy.^14,16,17 This is especially helpful if injury occurs at unusual sites and in younger patients unable to communicate and localise their symptoms well, or in whom the apophysis has not ossified (Figure 1).^8,17 In our practice, it also has an important role in cases where presentation has been delayed or where symptoms persist and can be shown to be due to tendinopathy or failure of the avulsion fracture to unite (Figure 2). MRI may be used as an adjunct to US when deeper tendons or areas that are difficult to scan need to be evaluated or if US and radiography are negative, to identify injury based on bone marrow oedema. ⁸ OPEN IN VIEWER

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Figure 2.

Fifteen-year-old teenager with chronic avulsion of the AIIS. He presented with persistent left groin pain five months after rugby injury. (a) Longitudinal US image of the left AIIS shows elongation of the left AIIS apophysis with a widened growth plate (arrow) and tendinopathic changes in the rectus femoris (RF) tendon. (b) Longitudinal US image of the normal right AIIS for comparison. (c) AP frog-leg radiograph of the pelvis shows an enlarged, displaced left AIIS apophysis (arrow).

Chronic apophyseal avulsion injury or ‘traction apophysitis’ is an insidious, overuse injury, common in sporting adolescents who may struggle to recall a single traumatic episode or determine the exact onset of pain. ¹⁸

Injury results from episodic lower energy traction trauma on the apophyseal attachment of the tendon. These episodes are repetitive and cumulative, with no time for recovery before the insult occurs again. ¹⁹ Unlike acute injuries, chronic injuries are most common at the knee (Figure 3). ²⁰ Patients present with activity-related anterior knee pain and signs related to the patellar tendon insertions, at the lower pole of the patella in Sinding–Larsen–Johansson or the tibial tuberosity in Osgood–Schlatter's disease.^21,22 Radiographs show bony fragmentation at the site of tendon insertion and overlying soft tissue swelling. US is useful to confirm the diagnosis without the use of ionising radiation. US findings include soft tissue and cartilage swelling, thickening of the tendon insertion, fragmentation of the ossification centre and sometimes associated bursitis.^22–25 In the acute phase, hyperaemia may be seen locally and evaluated with colour and power Doppler. ²⁵ US is especially useful during exacerbation of symptoms to assess the tendon insertion. ¹⁴ OPEN IN VIEWER

Skeletal injuries

There is no doubt that plain X-ray remains the primary imaging modality for the diagnosis of fractures following trauma. The sensitivity of X-rays relies on the presence of ossified bone. In young children where cartilage predominates and the bone remains immature, some fractures are difficult to diagnose especially around the unossified epiphysis. In these cases, US is a useful complimentary modality especially where strong clinical suspicion remains (Figure 4).^8,26–29 OPEN IN VIEWER

US demonstrates a fracture as a step or break in the bone surface, shows epiphyseal or cartilage injury and highlights secondary signs such as sub-periosteal haematoma, joint effusion and changes in the surrounding soft tissues (Figure 5).^8,30,31 Radiologists and sonographers should be aware of these findings as sometimes fractures are identified unexpectedly, perhaps in a child with a swollen joint that was thought to represent infection (Figure 6). Using US, one can examine the entire circumference of the bone while radiographs usually rely on two projections. If strong suspicion remains after normal radiographs, a focussed US may reveal a subtle fracture (Figure 7). ³² OPEN IN VIEWER

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Figure 6.

Two-year-old boy with pathological fracture of the distal tibia and fibula due to osteomyelitis. The child presented with a swollen right ankle suspicious for septic arthritis. He had sensory and motor dysfunction in both legs after previous cord compression due to malignancy. (a) Longitudinal US image of the anterior ankle shows a large, complex collection with displacement of the distal tibial epiphysis from the metaphysis (arrows). (b) AP radiograph of the ankle confirms fractures (arrows) of the distal tibia and fibula which were thought to be related to osteomyelitis and reduced sensation in the legs.

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Figure 7.

Thirteen-year-old child with tibial and fibular fractures. The initial radiograph demonstrated a fracture of the proximal fibula only (not shown). US was requested to exclude quadriceps tendon injury as the child had extensive soft tissue swelling and was unable to straight leg raise. (a) Longitudinal scan of the proximal tibia showed a normal extensor mechanism, but US also reveals an occult fracture (arrows) of the postero-medial tibia. (b) Repeat X-ray demonstrates a subtle Salter-Harris type 2 fracture of the proximal tibia (arrows).

In recent years, numerous papers documented the use of US in diagnosing common paediatric fractures mostly in the forearm, wrist and elbow. ^33–38 A meta-analysis by Douma-den Hamer et al. ³⁹ including 16 studies with 1204 patients and 641 forearm fractures reports a sensitivity of 97% and a specificity of 95% and concludes that US should be considered as a reliable alternative to radiographs in the diagnosis of forearm fractures. Most US examinations were performed by emergency physicians as point of care examinations. Even operators with minimal training and examinations performed as FAST scans to detect joint effusions in the elbow reached high correlation with radiographs.^35,37,38 US can be used to measure angulation of the fracture site but displaced and severely angulated fractures are usually also assessed with radiography.^33,34,36,39 US has been highly successful in diagnosing many different paediatric fractures including long bone fractures, clavicle fractures, skull fractures and in bed-side assessment of poly-trauma patients in the emergency setting.^30,40–42 Waterbrook et al. ⁴⁰ remark that US may never be used exclusively without radiographic confirmation because of medico-legal and cultural reasons. However, this does not preclude successful use in the resource-poor setting, wilderness areas or war zones.^40,43 Radiation exposure could be avoided in cases with negative initial US where clinical follow up could be sufficient. ⁴⁰ Many authors report that the examination is well tolerated by children and US gel has an added cooling effect.^{30,33,37,39,40} The use of US is limited in follow up as most fractures are immobilised in a cast and cast removal could be painful and compromise fracture position. ³⁶

In our institution, radiography remains the primary imaging test for suspected fractures but US is frequently used as a problem-solving tool to answer specific clinical questions or to avoid MRI under GA (Figures 6 to 9). OPEN IN VIEWER

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Figure 9.

Eight-year-old child with patellar sleeve fracture after fall from scooter. (a) Lateral radiograph of the knee demonstrates a patella sleeve fracture with overlying soft tissue swelling. (b) Longitudinal US image of the knee demonstrates the patellar sleeve fracture. Minimal displacement of the avulsed fragment allowed conservative management.

US is especially useful in the early evaluation of pre-verbal infants or toddlers presenting with a painful, immobile or swollen limb, a limp or refusal to weight bear. ⁴⁴ Occasionally in these children, US will reveal injuries that raise concern for non-accidental injury (NAI) (Figures 4 and 10). Radiologists and sonographers should be aware of the implications of unexpected injuries and fractures which should prompt discussions with local paediatricians to instigate child protection proceedings. ⁴⁵ US is not a screening tool to investigate children in whom NAI is suspected but may be used as a complimentary tool to investigate areas of concern after radiography.^27,46–48 US can demonstrate acute rib fractures which are difficult to see on radiographs and in young infants are highly suspicious of child abuse. ⁴⁷ In adults, US has been used successfully to diagnose rib fractures after chest trauma.^49–51 OPEN IN VIEWER

Superficial injuries

Injury to the superficial tissue layers is common and usually related to extrinsic trauma. It includes laceration, penetrating trauma, for instance due to foreign bodies (FB), but also blunt trauma causing soft tissue haematomas and fat layer injury. Children are very active, running about, frequently tripping and falling, often playing contact sports or are involved in other activities that may result in trivial trauma. They frequently present in the sub-acute stage with persistent pain and swelling or weeks to months later with a palpable lump and no recollection of a traumatic episode.

Penetrating injuries and lacerations leave an open skin wound and disruption of the underlying soft tissues. However, they may also involve important superficial structures such as nerves or tendons, especially at the wrist and ankle (Figures 11 and 12). US can detect retained FB as small as 2.5 mm with 92% accuracy.^52,53 Radiographs are useful to assess radiopaque FB such as metal and glass but cannot show their relationship to soft tissue structures, which is easily achieved with US. ⁵³ All FB are echogenic on US.^54–56 Wood and plastic is better identified with US than radiography as they demonstrate posterior acoustic shadowing.^54,55 Metal and glass produce reverberation artefact and round metal objects comet tail artefact. ⁵³ After approximately 24 hours, a hypoechoic halo develops around the FB representing inflammatory granulation tissue and haematoma which aids diagnosis.^53,54 A puncture wound may be visible on the skin but fragments may migrate, so a wider search area is necessary. Once located, the site of the FB should be marked on the skin surface and the depth and distance from the puncture wound measured to aid removal. The surgical extraction of FB can be guided with US. ⁵⁷ In the acute stage, with a laceration, or during attempts at removal of the FB, introduced subcutaneous air may decrease accuracy.^55,56 OPEN IN VIEWER

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Figure 12.

Foreign bodies identified with US. (a) US image of young child's buttock shows a 1 cm linear echogenic splinter (callipers) with surrounding hypoechoic collection. The child presented with a lump but no history of trauma and on further enquiry remembered pain after sitting on a wooden bench approximately eight months earlier. (b) Longitudinal US image of the volar aspect of the wrist in a 15-year-old child demonstrates a splinter (callipers). The splinter was within 3 mm of the median nerve and extending into the muscle in the region of the myotendinous junction (arrow), necessitating surgical removal. The child had sustained a penetrating injury whilst painting a wooden fence.

Injury to the superficial and subcutaneous layers may result from compressive forces associated with a direct blow, often sustained during contact sports in older children. Depending on the force of contusion, as well as pre-existing soft tissue abnormalities and treatment such as anticoagulation or steroid therapy, abnormalities may range from simple haemorrhagic infiltration to haematomas, fat necrosis or degloving injury. On US haemorrhagic fat infiltration is shown as a diffuse area of increased echogenicity.^58–60 A haematoma is a subcutaneous collection of blood, the US appearance of which varies over time. Initially the haematoma may appear highly echogenic to semisolid as fibrin and erythrocytes form multiple acoustic interfaces. With time the clot liquefies and the haematoma becomes anechoic. Sometimes a network of fibrin strands can be detected on US. Fluid levels may also be seen representing separation between anechoic (serum) and echogenic cellular blood components. An organising haematoma may have a well-defined capsule with surrounding vascularity seen on US. ⁶¹ Over several months the haematoma resolves and a fibrous scar remains (Figure 13).^58,62 OPEN IN VIEWER

Trauma to the subcutaneous fat may cause initial swelling, oedema and haemorrhage that progresses over time to atrophy and fat necrosis, often with a persistent palpable lump (Figure 14).^63,64 Fat necrosis appears as a poorly defined hyperechoic focus containing hypoechoic infarcted areas of fat or an isoechoic area with a hypoechoic halo. ⁶⁵ Closed degloving injuries occur when the fat plane is sheared from the muscular fascial plane, with disruption of perforating vessels and lymphatic channels causing a haematoma. This is called a Morel-Lavallée injury, and has characteristic appearances (Figure 14). ⁶⁶ US demonstrates an elongated fluid collection overlying the linear appearing fascia. OPEN IN VIEWER

Although bruising and haematomas are commonly seen, true vascular injuries are rare but may occasionally be encountered following blunt trauma (Figure 15). OPEN IN VIEWER

Patients may present with a lump, causing anxiety, when the traumatic event has been forgotten, especially when the trauma was minor.^61,63,65 In these circumstances, it is important to be able to confidently diagnose benign trauma-related injuries, in order to allay fears and prevent unnecessary additional complex imaging investigation. If there is initial doubt about the diagnosis, a repeat US after 2–6 weeks is usually helpful as these injuries evolve and resolve over time. ⁶²

Muscle and tendon injuries

Muscular injury may result from compressive extrinsic forces such as a direct blow, sustained in contact sports, but occurs most commonly as an intrinsic distraction injury caused by internal forces, such as a sudden strong eccentric forced muscle contraction, often occurring during rapid acceleration. Extrinsic forces cause compression injuries where tissue is compressed between the force applied and an underlying hard bony surface whereas intrinsic injury usually leads to muscle fibre disruption in the region of the myotendinous junction. The vastus muscles and especially the vastus intermedius is most commonly affected by extrinsic injuries (Figure 16(d)). Muscles, which span two joints, act in an eccentric fashion (i.e. lengthening as they contract) and which contain a high percentage of type II fast-twitch fibres are most at risk from intrinsic injury e.g. rectus femoris, biceps femoris and medial gastrocnemius muscles.^67–69 These muscles usually limit the range of motion of the joint they cross. Injury occurs during sporting activities that require high speed and rapid acceleration, such as football, rugby and sprinting – more common in older sporting adolescents. ⁶⁷ Isolated muscular injury is less common than apophyseal avulsion in young children, in whom the tendon-apophyseal insertion is relatively weaker than the myotendinous junction. ⁷⁰ OPEN IN VIEWER

Muscle strain injuries on US appear as avulsion and retraction of fibres from the tendon or aponeurosis to which they attach. Some muscles have complex internal tendon structures such as the rectus femoris muscle.^68,69 Therefore, the injury may also appear in the mid-portion of the muscle belly rather than in its distal portion. On US avulsion and retraction of fibroadipose muscle septa is seen. The space between the retracted septa and the aponeurosis fills with fluid and haematoma. Small tears may be difficult to identify but larger blood collections make the injury more conspicuous (Figure 16(a) and (b)).

A widely-accepted classification system of muscle injuries defines 4 grades of injury. ⁷¹ Grade 0 injury corresponds to normal US appearances despite local clinical symptoms. Grade 1 injury shows subtle US findings such as ill-defined hypo- and hyperechoic areas or a swollen aponeurosis. Partial and complete or full thickness muscle tears are classified as grade 2 and 3 injuries. US can establish whether there is a partial or full thickness tear of the muscle – and where retraction of muscle has occurred in full thickness tears – the gap between retracted muscle/tendon and the insertion site should be identified and measured.^68,69,71 This helps the clinician decide when sporting activities can be re-started or to offer surgical repair. The timing of US with respect to the injury is important as acute haemorrhage is echogenic and may mask the true extent of a tear.^68,71 Haematoma starts to organise after about 48 hours, becoming hypo/anechoic and the torn muscle shows hyperaemia, these features make the tear more conspicuous and easily visible on US, allowing more accurate assessment of extent. The ideal time to scan muscle tears in the thigh is approximately 48–72 hours after injury (Figure 16(c)). ⁷¹ During healing the haemorrhagic cavity shrinks, its walls thicken and eventually collapse. Muscular injuries heal with the development of a fibrous scar, which may persist indefinitely. On US, scars are ill-defined hyperechoic zones within the muscle that are bigger in higher grade tears. ⁷¹

Sometimes, as a complication of a muscle tear, myositis ossificans (MO) develops when the associated haematoma undergoes cystic transformation and progressively calcifies. MO is most commonly seen within the vastus intermedius muscle the brachialis muscle and the soleus muscle following severe contusion commonly in athletes involved in contact sports (Figure 16(d)).^71,72 MO is a benign self-limiting non-inflammatory condition. The US findings change over time. Initially on US, MO appears as a well-defined ovoid intramuscular hypoechoic mass. With maturation, a hyperechoic rim develops that can be seen on radiographs as a ossified peripheral egg-shell-like density.^72,73 This rim ossification and sometimes central areas of ossification is seen about two weeks earlier with US than radiographs and begins 3–4 weeks after injury. ⁷¹ The ossified rim and normal underlying bone can be confirmed with a radiograph or CT, and if shown, is very suggestive of MO rather than malignancy. ⁷⁴ In sarcomas, the necrotic centre typically calcifies first not the periphery. ⁷¹ Demonstration of normal muscle between the ossified mass and the underlying femur can help distinguish MO from parosteal sarcoma.^72–74

Where there is no recollection of trauma and the child presents with a palpable, painless intramuscular mass, anxiety will be high as clinicians want to avoid missing an intramuscular soft tissue sarcoma. ⁷⁵ This may have similar clinical presentation and can have overlapping US appearances, for instance when a sarcoma bleeds and gives rise to intra-tumoral haematoma. ⁷⁶ As opposed to soft tissue sarcomas, muscle tears that can present with a palpable swelling are found in typical locations at the myotendinous junction. ⁷⁵ Intramuscular haematomas evolve in a similar manner to those in the subcutaneous tissues and reassuring features include lack of internal vascularity, decreasing size over time and confinement to a single compartment. ⁷⁷ When the diagnosis of intramuscular haematoma is in doubt, a repeat US can be helpful to demonstrate evolution and eventual resorption. In our practice, we tend to repeat the US scan after two weeks and again after six weeks, especially if the child has been referred via oncology. We reserve MRI for cases where appearances follow an atypical course or some intralesional vascularity is seen.

Acute tendon and ligament injuries are uncommon in the paediatric population due to the relative strength of these structures in relation to their attachments, avulsion injuries being the norm. ⁷⁰ Chronic overuse tendinopathy can occasionally be seen in older sporting individuals.

Conclusion

Musculoskeletal injuries are common in the paediatric population and US is a child-friendly, relatively cheap, widely available, high-resolution modality that can be used for primary diagnosis, complementary imaging and problem solving. Varied structures may be involved and diagnoses made in the setting of MSK trauma. Therefore, practitioners should be aware of injury patterns in order to inform interpretation of findings. US assessment of MSK injuries allows us to ascertain the affected anatomical structures and the severity of the injury. The radiologist is well-placed to guide the referrer towards the most appropriate further management of the patient, be it reassurance, conservative management, more complex imaging investigation or surgical intervention.