Intra-articular displacement of an avulsed medial (internal) epicondyle ossification centre in the paediatric elbow: a radiographic finding not to be missed

Abstract

We present the case of an 11-year-old girl who was presented to the Emergency Department with right elbow pain and swelling following a fall. Radiography demonstrated intra-articular displacement of an avulsed medial epicondyle ossification centre, which was not readily identified at presentation. She proceeded to an uncomplicated open reduction and internal fixation.

Background

Intra-articular displacement of an avulsed medial epicondyle ossification centre can be easily overlooked on radiography and has significant clinical implications if missed. Prompt and accurate diagnosis facilitates appropriate and timely management, obviating exposure to ionising radiation by way of Computed Tomography (CT), which should be limited in the radiosensitive paediatric population.

The radiographic findings in this case were not readily appreciated at the time of presentation. As such, we discuss the ossification centres of the paediatric elbow and an approach to radiographic interpretation to increase awareness of this important radiographic finding.

Case presentation

An 11-year-old girl was presented to the Emergency Department (ED) following a fall from a swing. She described attempting to stop herself from falling by grabbing onto the swing as she fell, resulting in a valgus hyperextension to her right elbow. There was no stated history of transient dislocation by either the patient or her caregivers. There was no medical history of note.

On examination, there was soft tissue swelling over the medial aspect of her right elbow. She was unable to fully flex or extend her right elbow with limitation to her range of movement, and she described a ‘pins and needles’ sensation in her right little finger. A supracondylar fracture was queried by the ED clinicians and radiographs of her right elbow were obtained.

Investigations

The anteroposterior (AP) projection (figure 1) demonstrated soft tissue swelling over the medial aspect of the right elbow centred over the expected position of the medial epicondyle ossification centre; however, the medial epicondyle ossification centre was projected over the trochlea ossification centre having been displaced anteriorly and inferiorly. There was also a concurrent fracture of the right radial neck. Radiocapitellar alignment was preserved.

Figure 1.

AP projection of the right elbow in an 11-year-old girl. There is soft tissue swelling over the medial aspect of the elbow, centred over the expected position of the medial epicondyle ossification centre, which itself is projected over the trochlea ossification centre. This anterior and inferior displacement may lead to misidentification of the medial epicondyle ossification centre as the trochlea ossification centre. There is also a concurrent radial neck fracture. AP, anteroposterior; R, rigth.

The contemporaneous lateral projection (figure 2) demonstrated elevation of the anterior fat pad consistent with an elbow joint effusion, with soft tissue swelling over the posterior aspect of the right distal humerus. Anterior humeral alignment was maintained, which, along with the normal radiocapitellar alignment on the AP projection, confirmed that the right elbow was normally aligned (congruent). The displaced medial epicondyle ossification centre was projected superior to the olecranon ossification centre and over the posterior aspect of the right elbow joint space. The findings of both radiographs confirmed that the avulsed medial epicondyle ossification centre had been displaced into the joint space.

Figure 2.

Contemporaneous lateral projection of the right elbow in the same 11-year-old girl. There is elevation of the anterior fat pad consistent with a joint effusion. There is soft tissue swelling over the posterior aspect of the right distal humerus. The medial epicondyle ossification centre is projected over the posterior aspect of the joint space and lies superior to the olecranon ossification centre. The findings in this image, alongside those from figure 1, confirm intra-articular displacement of an avulsed medial epicondyle ossification centre. The concurrent radial neck fracture is less well appreciated on this projection.

The orthopaedic team who reviewed the patient felt that the clinical examination was not consistent with a supracondylar fracture. A CT was performed, which confirmed the radiographic findings.

Treatment

The medial epicondyle avulsion fracture required open reduction and internal fixation with two Kirschner wires (figure 3) following removal of the ossification centre from the right elbow joint space.

Figure 3.

Postoperative (A) AP and (B) lateral radiographs of the right elbow in the same 11-year-old girl following open reduction and internal fixation. Images were obtained in cast. There are two Kirschner wires through the relocated medial epicondyle ossification centre. AP, anteroposterior.

Outcome and follow-up

No complications were reported in the immediate postoperative period or on the follow-up radiographs performed 1 month later (figure 4).

Figure 4.

Postoperative (A) AP and (B) lateral follow-up radiographs performed at 1 month later in the same patient as figures 1 and 2. Images were obtained out of cast. The Kirschner wires have been removed with no complication. The medial epicondyle ossification centre now lies in its expected anatomical position with a small amount of residual overlying soft tissue swelling. There is normal radiocapitellar and anterior humeral alignment. There is ongoing healing of the right radial neck fracture with periosteal reaction along the radial (A) and volar (B) cortices of the right proximal radius. AP, anteroposterior.

Discussion

The anatomy and physiology of paediatric bones are distinct from the mature adult skeleton, resulting in unique patterns of injury. Almost 20% of children presenting to the ED with an acute injury will have a fracture.¹ Furthermore, it is estimated that 70% of all paediatric fractures involve the upper extremity with 8%–10% involving either the humerus or the proximal radius and ulna.²

Elbow injuries can be overlooked due to a lack of understanding and unfamiliarity with the secondary ossification centres unique to the paediatric elbow. There are six paediatric elbow secondary ossification centres: four at the distal humerus (capitellum, internal (medial) epicondyle, trochlea and external (lateral) epicondyle); and the remaining two are formed by the radial head (proximal radius) and the olecranon (ulna).² These ossification centres are initially cartilaginous and are therefore not visualised on radiography but become visible as each centre ossifies. Each ossification centre can be identified by its expected anatomical position often having well-corticated margins; however, they may also be fragmented in appearance, notably the trochlea and the lateral epicondyle ossification centres. As the child ages, the physes (growth plates) of each ossification centres fuse (close) to form the adult elbow.

The chronological order of ossification of these centres and their appearance on radiography is predictable and consistent: knowledge of this is key to the comprehensive radiographic evaluation of the paediatric elbow. A mnemonic used to remember the order in which these centres ossify and are visualised on radiography is CRITOE (capitellum, radial head, internal (medial) epicondyle, trochlea, olecranon and external (lateral) epicondyle.^{3 4} There are various time frames presented in the literature by which the elbow ossification centres are visualised on radiography (table 1).^{2 5–7} By comparison, the age at which the secondary ossification centres fuse is independent of the age at which they ossify, typically doing so between the ages of 10 and 18 years.⁸

Table 1.

Broad age ranges by which the secondary ossification centres of the paediatric elbow ossify and are typically visualised on radiography. A suggested aide-mémoire is also presented.

Ossification centre	Age range (years)	Suggested aide-mémoire age (years)
Capitellum	1–2	1
Radial head	2–4	3
Internal (medial) epicondyle	4–6	5
Trochlea	8–11	7
Olecranon	9–11	9
External (lateral) epicondyle	10–11	11

The medial epicondyle ossification centre is an apophysis which is the site of a tendinous or ligamentous attachment that does not contribute to the longitudinal growth of the humerus.⁹ It is the last ossification centre to fuse with the distal humerus, typically between the ages of 13 and 17 years,⁸ although it has been reported to fuse even later, up to 20 years of age.¹⁰ The anterior aspect of the medial epicondyle receives the common tendon of the flexor pronator muscle group, in addition to serving as a point of attachment for the key stabiliser against valgus stress: the medial ulnar collateral ligament (UCL).¹¹

In children, tendons are stronger than the apophyses onto which they attach, such that a strong muscular contraction can result in an avulsion fracture rather than a sprain or tendon tear (as in adults).¹² Avulsion of the medial epicondyle ossification centre is the most common avulsion injury in the paediatric elbow¹³ comprising 11%–20% of all paediatric elbow fractures.⁵ These fractures typically occur in children between the ages of 9 and 14 years⁵ following a fall onto an outstretched hand (FOOSH) with the elbow in full extension resulting in a valgus stress (hyperextension) on the elbow.¹⁴ Rarely, these fractures can result from a direct blow to the elbow.

The radiographs in our case (presentation radiographs, figures 1 and 2; magnified, cropped and annotated radiographs, figures 5 and 6) demonstrate intra-articular displacement of an avulsed medial epicondyle ossification centre. The valgus hyperextension described in the case presentation can temporarily open the medial elbow joint space, which, along with the combination of traction from the UCL and the flexor pronator muscle group, pulls the medial epicondyle ossification centre into the joint space. As this force dissipates, the joint space closes and incarcerates the medial epicondyle ossification centre.¹¹

Figure 5.

Cropped and magnified images from figure 1. (A) The medial epicondyle ossification centre is projected over the trochlea ossification centre, which is outlined by the red dashed line in (B). The medial epicondyle ossification centre may be mistaken for the trochlea ossification centre if avulsed and displaced anteriorly and inferiorly.

Figure 6.

Cropped and magnified images from figure 2. (A) The medial epicondyle ossification centre is projected over the posterior aspect of the right elbow joint space, which is outlined by the red dashed line in (B).

The ulnar nerve runs in a groove in the posterior aspect of the medial epicondyle and may be subject to complications arising from medial epicondyle fractures given its location. Reported ulnar nerve dysesthesia, neuropraxia, dysfunction or paresis⁹ may be present in 10%–16% of patients with fractures¹⁵ but is typically transient and resolves following observation⁷; however, in those with an incarcerated intra-articular fracture fragment, this can rise to 50%.¹⁶ This correlates with the right little finger ‘pins and needles’ sensation experienced by our patient, and importantly, should alert ED physicians and orthopaedic surgeons to a possible medial epicondyle fracture in the context of FOOSH, prompting meticulous inspection of the radiographs obtained.

The consistent order of ossification and visualisation on radiography highlights that the medial epicondyle ossification centre always appears before the trochlea ossification centre. Thus, on radiography, if the trochlea ossification centre is identified but the medial epicondyle ossification centre is not, it is imperative that the radiograph is scrutinised for an avulsed medial epicondyle ossification centre and, in particular, intra-articular displacement.¹⁷ An avulsed ossification centre may mimic another ossification centre if significantly displaced. An anteriorly and inferiorly displaced avulsed medial epicondyle ossification centre can be small and difficult to locate on radiography¹⁴; it may be projected over the trochlea ossification centre which may lead it to being mistaken for this ossification centre (figure 5).¹⁸

While elbow dislocations are relatively uncommon in the paediatric population, approximately half of all medial epicondyle ossification centre avulsion fractures are associated with elbow dislocations.⁹ This may not only make identification of the medial epicondyle ossification centre difficult but, in up to approximately 15% of patients,¹⁹ it can become incarcerated within the joint space following reduction.¹⁵ Moreover, it is vital that all other imaged bones are evaluated for acute bony injury and that ‘satisfaction of search’ is avoided; a concurrent radial neck fracture is visualised in figures 1 and 2, with evidence of healing on follow-up radiographs (figure 4).

Relative indications for surgical intervention include ‘concurrent elbow dislocation, displacement greater than a radiographic threshold (typically 5 mm), and fracture among upper-limb-dependent athletes’.^{20 21} Intra-articular incarceration of a fracture fragment is an absolute indication for surgery.^{7 11} Prolonged incarceration can result in chronic elbow instability.²⁰

The combination of radiography and ultrasonography can be effective in evaluating the paediatric elbow for fractures.^{22 23} Ultrasonography, which is inexpensive, readily accessible and does not employ ionising radiation,³ can be useful in the assessment of the paediatric elbow given that the ossification centres are partially ossified or unossified making them readily identifiable, in addition to allowing a dynamic examination of muscles, tendons and the elbow joint space.²⁴

Indirect signs of fracture on ultrasonography include lipohaemarthrosis and the ‘fat pad sign’, defined as the presence of fat superior to the distal line of the humerus on a longitudinal view.²³ A prospective study evaluating these signs, along with direct cortical disruption, identified 13 occult fractures in 34 patients who had been diagnosed as having no fracture on initial radiography. The ultrasonographic sensitivity of the fat pad sign and lipohaemarthrosis for fracture was 100% and 92%, respectively.²³ A larger prospective study involving 130 patients assessed whether indirect markers of elbow fracture on ultrasonography could be employed as a screening tool to diagnose elbow fractures.²⁵ It demonstrated a 98% sensitivity for elbow fracture when an elevated posterior fat pad sign or lipohaemarthrosis was present on ultrasonography compared with a 93% sensitivity on radiography. The absence of an elevated posterior fat pad and lipohaemarthrosis on ultrasound could reduce the use of radiography in children with acute elbow injuries.

Comparative radiographs of the unaffected elbow are of ‘questionable value’,²⁶ are seldom required and should not be obtained routinely. Furthermore, it has been shown that this practice does not increase diagnostic accuracy when evaluating the paediatric elbow in acute trauma.²⁷ As such, the increased exposure to ionising radiation in the paediatric population is not justified. In the first instance, discussion with an experienced clinical radiologist is always advocated. When this is not possible, it has been suggested that a bank of abnormal radiographs which demonstrate common paediatric elbow injuries may be helpful for less experienced physicians to improve the diagnostic accuracy of acute paediatric elbow injuries.²⁸

Intra-articular displacement of the medial epicondyle ossification centre is an important radiographic finding not to be overlooked. If the information discussed above is applied correctly, the diagnosis can be made on radiography alone obviating the need for imaging with CT and further exposure to ionising radiation; moreover, CT is not indicated in the evaluation of simple fractures.²⁰ This is particularly relevant for junior physicians who may have to interpret radiographs and manage these patients out-of-hours, where a (paediatric) radiologist opinion or report is not available. Our case underscores the need for a better appreciation of the normal anatomical development and growth patterns of the paediatric elbow, alongside employing knowledge of the predictable pattern of ossification and visualisation of the elbow ossification centres on radiography.

Learning points.

• There are six paediatric elbow secondary ossification centres; these ossify and are visualised on radiography in a predictable and consistent order. CRITOE is a mnemonic for this sequence: capitellum, radial head, internal (medial) epicondyle, trochlea, olecranon and external (lateral) epicondyle.

• An avulsion fracture of the medial epicondyle ossification centre can be overlooked, may be mistaken for the trochlea ossification centre and may be displaced into the elbow joint space.

• An elbow dislocation should prompt scrutiny of all the ossification centres to exclude avulsion fractures, intra-articular displacement and fractures of the other imaged bones.