HipRadiographic assessment of primary hip arthroplasty
Section snippets
Introduction: the need for radiographic review
Radiographic assessment of total hip arthroplasty (THA) is required to establish the technical success of the procedure, monitor progress, identify patients in need of revision surgery or specialist referral and potentially to predict outcome. Investigations such as computed tomography (CT), magnetic resonance imaging (MRI) and radio-isotope scanning (RIS) are increasingly undertaken but radiographic assessment remains the mainstay of arthroplasty review.1 THA failure occurs in a bimodal
Standards and timing of radiographs
Radiographs are taken immediately postoperatively and thereafter at the discretion of the unit, depending on resources for follow-up or research requirements. The absence of any radiographic changes at 1 year is a good prognostic indicator9 and many units discharge their patients at this stage (or sooner). However, changes may appear after this time and follow-up radiographs are indicated in younger patients. Radiographs are typically an anteroposterior (AP) of the pelvis, centred on the pubis
Principles of osteolysis
The principles of implant biomechanics and subsequent construct behaviour are well covered in basic texts and recent reviews16, 17 but a brief discussion of the relevant bone biology is warranted. Periprosthetic radiographic osteopaenia (reduced radiodensity than otherwise similar radiographs of the same patient) may only become apparent when 30–70% of bone mass is lost, hence quantitative assessment with plain radiographs is unreliable.18, 19, 20 Osteopaenia may be assessed quantitatively with
Pelvic landmarks
The pelvic landmarks with minimal variance in relation to the acetabular component are illustrated in Figure 1 and are used as reference points for measurement of acetabular position or migration.37 Although described for uncemented cups, the technique can be used for cemented cups. Migration on a vertical axis is best measured between the centre of the cup and perpendicular (Figure 1a) to the “inter-teardrop” line (Figure 1b), which touches the most inferior point of both the acetabular
Cemented acetabular components
On the pelvic AP view, the acetabulum is divided radially from the centre of rotation to the periphery of the cup into three zones39(Figure 2). Note that Figure 2 shows an uncemented cup for demonstration purposes. These zones are used to localize any radiographic findings such as radiolucency in bone or cement. In the immediate postoperative radiograph, the cement mantle around the cup should be of even thickness (2–5 mm) in the three zones. “Pooling” of cement at the inferomedial cup/bone
Radiographic changes with time
Abnormalities immediately apparent on routine postoperative radiographs are usually a result of technical error or difficulties during surgery.3, 40 However, as the bone–implant construct ages, changes may develop, irrespective of surgical technique or initial appearance, and it is difficult to predict when a construct will fail based on a single image, even in the presence of profound abnormalities. Therefore, progression of these features is the key to identifying potential problems. The
Conclusion
The behaviour and radiographic appearances of cemented and uncemented implants in vivo have been well-documented and these findings are consistent with biomechanical principles. It should be borne in mind that different designs (of which there are many) may manifest these signs to a variable extent and it is not always possible to extrapolate the behaviour of one implant to another. Nevertheless, radiographic assessment of THA is an important part of monitoring the status of the implanted
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