Anterior hip ultrasound: A useful technique in developmental dysplasia of the hips

Abstract

Introduction: Infant developmental dysplasia of the hips arises when there is an interruption to normal joint development, usually present at birth but may be a later development. It has a wide spectrum of severity with potentially disastrous long-term complications if left untreated. Incidence is highly variable, often being attributed to differences of opinion in definition and diagnosis; however, there is consensus that ultrasound be used in screening and management. Topic Description: The anterior hip ultrasound technique is underutilised but is of value due to the additional confidence it provides regarding joint stability. Discussion: The ability to relate the acquired ultrasound image to hip anatomy and a technical appreciation of the technique are crucial to successful use. The method is described and its use within previous literature briefly outlined. Using pictorial ultrasound imaging examples to demonstrate the technique, we aim to highlight the anterior approach as a useful addition to ultrasound assessment of infant developmental dysplasia of the hips. Conclusion: Knowledge and understanding of different techniques is essential for practitioners involved in the diagnosis and management of this multifaceted and vigorously debated condition.

Introduction

If diagnosed early, developmental dysplasia of the hips (DDH) can often be treated easily, potentially avoiding significant morbidity in terms of corrective surgery, risk of arthritis and lifelong disability.^1,2 With highly variable incidence reports ranging from 0.06 to 76.1 per 1000 live births,³ diagnosis is essential, and ultrasound imaging (US) is well established in neonatal hip assessment.^3,4 Anterior approach US use in the diagnosis and management of DDH is described in the literature; however, it is less commonly utilised in practice when compared to the more widely used lateral approach techniques, such as the universally familiar Graf method.⁵ As an adjunct technique, anterior US can allow for a more complete examination. It is used in some international screening programmes; however, due to the imaging window used, it can be especially useful when treating DDH within an abduction harness, splint or cast to ensure adequate femoral head to acetabulum position. The sonographic projection directly relates with the direction of femoral head instability, thus enabling clear and logical appreciation of proximal, anteroposterior and lateral displacement of the femoral head. Locally, we adopted an anterior approach in 2004, through a requirement to adjust our imaging access due to a simple modification of Pavlik harness use. An anterior US approach assists us in the management of hips that are not stabilised in a Pavlik harness applied conventionally and has shown to be successful within this centre.⁶ In our practice it is now used routinely for monitoring the hip position once the hip is harnessed and in the first three weeks of Pavlik harness management while the hip stabilises. This is particularly useful in those more unstable hips which can dislocate if adducted during a standard lateral approach scanning plane. Our experience is that the additional qualitative information provided by anterior US may improve diagnostic confidence thus enhancing patient management.

Background

DDH is the all-encompassing preferred term used to describe a range of paediatric hip disorders that arise when there is an interruption to normal hip development during early embryonic, foetal and/or infant growth stages. The hip joint is still developing at birth, and normal development relies on the symbiotic relationship of femoral head located within the acetabulum. Ranging from mild laxity to frank dislocation, if undiagnosed and untreated, then long-term morbidity may result as the hip joint matures and establishes abnormally. As the infant becomes older, i.e., after three months, then adequate reduction becomes significantly more difficult and will likely require progressively more invasive surgical procedures to correct.

The Barlow and Ortolani clinical manoeuvres in assessment of neonatal hips are universally used.^4,7 However, clinical examination is poorly sensitive except in experienced hands, whereas US enables confident and reproducible appreciation of hip morphology, degree of dysplasia and dynamic assessment of femoral head movement within the acetabulum.^2,3 Radiographs are of limited use as the majority of the neonatal hip joint comprises cartilage rather than bone. Ultrasound has the major technical advantage in the ability to visualise and provide much more information of the immature cartilaginous skeleton. Whilst other imaging modalities including computed tomography (CT) and magnetic resonance imaging (MRI) may have a role in late diagnosis and planning surgical management of more complex cases, it is accepted that US evaluation for screening diagnosis and treatment management is essential.³

Ultrasound in infant hip assessment developed through the 1980s by numerous workers.^5,8–14 The main patient-related advantages of US are lack of potential harmful ionising radiation and a relatively non-threatening examination. Four main assessment methods emerged and are now the most universally known: Graf, Harcke, Terjesen and Suzuki.³ All four focus on slightly different quantitative and/or qualitative aspects of the hip joint to achieve an assessment of DDH, and the former three share a similar imaging approach from the lateral hip. The fourth, Suzuki technique, examines the hips in a different plane, viewing the hip from an anterior groin approach. Anecdotally, this fourth method is used less in routine practice and, when compared with the other methods, is sparsely referenced within literature.

Ultrasound technique

The technique used is modified from the method initially described by Suzuki et al.¹³ in 1991. Modern small footprint transducers prohibit an exact replication of that described but the fundamental philosophy of the method is maintained. Our technique has further evolved to highlight the landmarks as described by van Douveren et al.¹⁵

In a supine position, each hip is scanned individually using a linear array transducer with a frequency of between 7 and 10 MHz. This is dependent upon the size of the infant but usually provides adequate resolution to achieve good visualisation of hip morphology with sufficient penetration to see the posterior aspect of the acetabulum, ischium and full cartilaginous femoral head. A convex curvilinear, lower frequency transducer can be used with some effect on larger infants, but there will inevitably be some lateral distortion and loss of resolution, especially of the deep joint anatomy (Figure 1).

Figure 1.

Comparison of same normal left hip using different imaging settings. Red dot – ossific nucleus of femoral capital epiphysis/blue dot – pubic reflection/white dot – ischial reflection/white-dotted line – acetabulum/white arrow – reflection of the ligamentum teres (not visualised on (b)) (a) linear 7–10 MHz transducer – good detail and resolution of hip morphology; (b) convex curvilinear 3 MHz transducer – loss of clarity of structures with poorer resolution.

The infant lies with hips in a mildly flexed abducted but neutral, relaxed position. Gentle holding of the child to stop wriggling is all that is normally required to keep them reasonably still. This approach also allows easy access when the infant is in a treatment harness (Figure 2). The transducer is placed on the pubis in the transverse plane and slid laterally to the side of interest keeping the reflection of the pubis just off centre and bringing the acetabulum and femoral head into view. The bony acetabulum is delineated by the pubic bone anteriorly and ischium posteriorly (Figure 3). Maintaining the transducer angle perpendicular to the infant’s trunk, the lateral transducer edge is minimally rotated distally towards the infant’s feet until the dense, calcified reflection of the femoral metaphysis elongates into view. The pubis and ischium are seen as bright echoes separated by the ‘dark gap’ of the triradiate cartilage, which is the as yet unfused, immature Y-shaped epiphyseal plate at the junction of the ischium, ilium and pubis. Occasionally, it may also be possible to see the ligamentum teres femoris (Figure 1(a)). The next step is to focus to the triradiate cartilage and use a moderate level of dynamic range (approximately 60–70 dB) to appreciate the differing grey-scale structures of the hip anatomy. Depth of field should be adjusted so that the bright echo of the pubic bone is at the anterior aspect of the image and the ischium at the posterior aspect with the structures of interest filling the field of view. The normal non-ossified femoral head cartilage is seen as a rounded, hypoechoic, homogeneous, mildly speckled structure snug within the acetabulum. A comparison view of both hips side by side can be useful and be achieved by using the dual screen functionality that is standard on modern equipment (Figure 4). The hyperechoic ossific nucleus of the femoral capital epiphysis becomes visible at approximately three months of age normally, so may or may not be visualised within the femoral head centrally (Figure 1). When in this plane, if it is difficult to locate the femoral head within the acetabulum, it may be dislocated. To confirm, the practitioner scans distally down the femur where the linear bony reflection of the femoral shaft is easily identified, following the shaft proximally to locate the femoral head. This is especially useful when trying to assess the dislocated hip but also useful when learning the technique and understanding the anatomical landmarks required to make a confident assessment.

Figure 2.

Anterior scan technique (Maclean et al. 2005 Reproduced with kind permission from Wolters Kluwer Health, Inc.).

Figure 3.

Right hip. P: reflection of pubis blue line; I: reflection of ischium white line; F: cartilage of femoral head with red outline; femoral metaphysis dotted white line/red dot: ossific nucleus of femoral capital epiphysis: (a) anterior US of normal hip; (b) anterior US same hip as 2a for illustration; (c) corresponding anatomy on CT image (for illustration and not same child).

Figure 4.

A comparison view of both normal hips using dual screen functionality P: reflection of pubis; I: reflection of ischium; F: cartilage of femoral head; T: triradiate cartilage.

Anterior hip ultrasound in the literature

Although appropriate to consider previous anterior techniques briefly and refer to other techniques, detailed descriptions are beyond the scope of this paper.

In the mid-1980s, a Swedish group described an anterior US approach,¹² subjectively appreciating the hip morphology of the acetabulum and position of the femoral head but importantly also evaluating femoral head movement. Dahlstrom et al.¹² highlighted the extremely poor long-term outcomes of using clinical examination alone; over treating but with still high late DDH presentation rates. They also emphasised the greater accuracy for assessing hip instability and the improved access of the anterior approach to enable easy visualisation of the stressed hip using the Barlow clinical manoeuvre, thus understanding the dynamics of a dislocatable hip. The authors claimed that it was difficult to successfully perform dynamic manoeuvres from alternative scanning windows such as in Graf’s lateral technique and the medial approach of Novick et al.^5,9 In 1991, Japanese group Suzuki et al.¹⁶ further explored endeavouring to reflect an US appearance more similar to the established pelvic radiographs, examining both hips simultaneously together in one plane ‘from in front’. As with Dahlstrom et al.,¹² they also scanned the hips in abduction then adduction to assess for a dislocating hip but introduced a ‘standard plane’ to establish a potentially more objective reproducible image and outcome measurement. Their results drew attention to better positional identification and appreciation of a dislocated hip although acknowledging the limitations of the technique in appreciating acetabular dysplasia.

In subsequent years, there have been multiple uses of anterior approach ultrasound for DDH assessment. It is advocated in some screening programmes^17,18 but has been especially successful in managing DDH treatment in a harness or spica cast.^6,15,19–21 The Dutch group, van Douveren et al.’s¹⁵ description of the anterior US technique is especially helpful. They described the alignment of US landmarks of pubic ramus, femoral metaphysis and femoral head all depicted in one plane and directly relate this to the CT-modified Shenton’s line, as described by Smith et al.²² A line from the anterior aspect of the pubis reflection to the antero-medial edge of the femoral metaphysis reflection is drawn and, to confirm a normal positioned hip, should be parallel or very near parallel to the angle of the transducer lens (Figure 5).

Figure 5.

Anterior US illustrating the modified Shenton’s line^15,22: F: femoral head; P: pubic reflection; I: ischial reflection; femoral metaphysis: white arrow: (a) anterior US of normal right hip modified Shenton’s line – dotted yellow line drawn along anterior border of pubic bone to anteromedial femoral metaphysis traversing through the anterior cartilaginous femoral head; (b) anterior US of posteriorly dislocated right hip Modified Shenton’s line – dotted yellow line drawn along anterior border of pubic bone to anteromedial femoral metaphysis – steep angle and not traversing through the femoral head, which is posteriorly dislocated; (c) normal hip illustrating the modified Shenton’s line as in (a). The line should be very near parallel to the transducer; (d) dislocated hip illustrating the modified Shenton’s line as in (b). Note very steep angle created between dotted line and plane of transducer.

Discussion

Research often produces differing opinions over the gamut of issues surrounding DDH. Fuelled by difficulties of achieving an accurate diagnosis, literature reflects the many considerations for screening programmes and differing attitudes over choice and management of treatment preferences. The plethora of articles on the many aspects of DDH is contrasted by a stark lack of further novel research since the early original works developed through the 1980s and 1990s. In several decades, there has been little advancement in consensus regarding screening, diagnosis and management. Contemporary papers reflect this with a majority of references cited being more than 10 years old.^3,23

There are numerous recommendations for US assessment of DDH, but broadly, they fall into two categories: hip morphology and hip stability. Graf’s technique is generally favoured in Europe and emphasises hip morphology, classifying hips based on anatomical appearance and objective angles of inclination of the acetabular bony and cartilaginous roofs to demonstrate the degree of hip dysplasia or shallowness.^5,24 Harcke and co-workers^10,25,26 in the United States of America place greater importance on position and stability of the femoral head with a more dynamic, ‘multiplanar’ method to subjectively assess hip instability using ‘stress manoeuvres’ very similar to that used in a clinical examination. Scandinavia’s ‘Terjesen modified technique’ measures the percentage of acetabular bony roof coverage of the femoral head from a static image,^11,14 intimating objective standardisation. However, it is primarily a measurement of displacement and joint stability. Suzuki et al.¹⁶ examined both hips simultaneously from an anterior approach and introduced intersecting lines to assess femoral head position within the acetabulum. This, similar to Terjesen, introduces some objective reproducibility but again is a method which principally considers the degree of femoral head displacement. The question of which US method should be used is still debated globally, and choice seems often dependent upon which continent of the world you live and geographical predilection, with critics for and against the competing doctrines.^3,27 It seems logical therefore that all techniques play a part and should not be dismissed.

The anterior technique can add confidence when assessing the position of the femoral head, and the hip can be manipulated reasonably easily in real time to assess the degree of instability. In combination with a lateral coronal assessment, a fuller understanding of hip morphology and instability can be achieved by a single operator and with no requirement for any specialised accessory equipment. Figure 6 demonstrates the left hip US imaging of a four-week-old infant with severe bilateral DDH. The shallow bony roof is significantly dysplastic (Graf alpha angle of approx. 39°), and the hip is slow to normalise in Pavlik harness. The labrum is displaced, inverting into the joint and the cartilaginous roof is hypertrophic; hyperechoic thickening with structural alteration due to the instability and abnormal pressure on the joint anatomy. The position and dynamic movement of this postero-superiorly dislocated but relocate-able hip is assessed with confidence and can guide management decision for harness continuance or progress to alternative orthopaedic intervention. This is also true when making an assessment of hip position whilst in an abduction treatment harness. Due to the hip’s abducted position, it is especially difficult to access and scan with dexterity from the lateral aspect in the coronal plane, and subsequently, it is difficult to assess if the hip position is satisfactory. Figure 7 demonstrates the positional assessment that can be made to determine if a successful femoral head location has been achieved and assess for any ‘blocking’ obstacles to this relocation such as soft tissue within the joint. This information is crucial to harness treatment success or failure.

Figure 6.

Left DDH – anterior imaging with manipulation of the hip joint (Barlow manoeuvre) in abduction and adduction (neutral hip flexion). Blue dot: pubic reflection/white dot; IS: ischial reflection; FH: femoral head; T: triradiate cartilage; white arrow: soft tissues between femoral head and acetabulum; red arrow: inverted labrum and thick hypertrophied cartilage: (a) anterior view in abduction – postero-anteriorly, the femoral head is central to acetabulum although laterally displaced due to cartilage thickening and soft tissue interposition. Notably, the FH also appear misshapen (ovoid); (b) anterior view in adduction – clearly not in joint postero-anteriorly and dislocates significantly postero-laterally; (c) coronal lateral view – dislocated left hip with abnormal superior positioned femoral head.

Figure 7.

Repositioning – right DDH anterior imaging in Pavlik harness: blue dot: pubic reflection; white dot: ischial reflection; FH: femoral head; T: triradiate cartilage; white arrow: soft tissues between femoral head and acetabulum: (a) the femoral head is in a poor position within the Pavlik harness, lying posterior to ischium; (b) rescanned after repositioning of Pavlik harness and the FH is now more central to the acetabulum.

There are however important considerations that seem probable contributing factors to the underutilisation of this method. First, there is a degree of subjectivity in this technique if compared to Graf’s more quantitative, reproducible measurement. Observer variations in performing and interpreting an ultrasound examination are recognised even in Graf’s standardised technique.^3,28–30 It is unsurprising therefore that the more subjective a technique, then the more susceptible that technique is to potential observer differences in interpretation, which will in turn hinder confident user uptake. Second, the anatomical landmarks are arguably more subtle relative to other techniques, and like any new technique, anterior evaluation of a dislocated hip in inexperienced hands can be confusing, requiring a little perseverance. Previous authors have commented on this, highlighting that knowledge of the technique and understanding of the anatomy are crucial to interpretation and appropriate use.^15,31–33 The subjectivity of the anterior approach does pose specific issues around reproducibility and auditing, and we recognise the benefits of a more objective approach in routine assessments. At this centre, we use a Graf-based method for routine and screening examinations. Sonographers use the additional anterior technique in more complex cases in consultation with orthopaedic consultants.

Conclusion

Anterior approach imaging of the infant hip is well documented in literature but underutilised. It provides uncomplicated imaging access during dynamic manipulation of the hip joint that replicates clinical examination to assess for joint instability. Acetabular landmarks are less obvious especially in the dislocated hip, thus requiring good practitioner anatomical knowledge as well as familiarity of the technique. Poor familiarity compared to alternative techniques seems a likely reason for its underutilisation, but, once mastered, the additional information can enhance overall assessment. It is especially useful in monitoring hip position in a treatment harness or spica cast.