Acetabular bone defects: classification and diagnosis

The Paprosky Classification is particularly useful in severe bone loss

Classification systems are important in that they should serve as a guidance to preoperative planning. However, surgeons should make the final reconstruction plan intraoperatively according to the defects after the original implant has been removed. The Paprosky Classification was constructed with clinical utility in mind. By determining the amount of support remaining, surgeons can develop a plan for treatment and grafting and fixation strategies can be made [13]. The Paprosky Classification is a particularly useful treatment guide in case of severe bone loss [13]. Further, among the currently available classifications, the Paprosky is the only one that has achieved moderate reliability and validity [9, 14].

Preoperative assessment

Like any complex surgical procedure, rTHA requires meticulous preoperative planning. Patient demographics and medical history such as age, gender, history of osteoporosis or other metabolic bone diseases, and lab test abnormalities (eg, calcium, vitamin D, and elevated thyroid hormone) can provide valuable information on a patient’s bone quality. Gait patterns, use of assistive devices (eg, crutches and walkers), and other movement patterns (eg, how the patient rises from a sitting position) can also reveal important information about a patient’s condition such as strength of the abductor muscles and hip range of motion. All this information can in turn affect surgical planning, for example, the amount of hip exposure needed [13].

A hierarchical approach to preoperative imaging

Radiological evaluation of acetabular bone loss and classification should start with plain x-rays [13, 15]. The following views of plain x-rays should be obtained for most patients:

• Plain AP pelvis view provides fundamental information about acetabular bone loss and implant migration. All patients being evaluated for a painful THA should have this.
• Standing AP pelvis view allows for understanding the dynamic pelvic position such as tilt and obliquity under load.
• AP and lateral view of the affected hip help visualize the femoral component in orthogonal views so as to assess fixation and osteolysis around the femur.
• Judet view helps visualize the anterior and posterior columns and the direction of cavitary defects or migration of the acetabular component.

Other views such as frog lateral view and cross-table orthogonal view have also been used. The frog lateral view helps assess the quality of femoral component fixation, overall femoral bone quality, and deformity in the sagittal plane. It does not provide additional information about acetabular bone loss [13]. The cross-table orthogonal view, on the other hand, may provide additional information on the extent of posterior column and ischial osteolysis [13].

Based on the information provided by the AP pelvis x-ray, one can identify whether advanced imaging is needed for further assessment [13].

Limitations of current classifications

As commented by Gozzard et al [16], “many of the current systems are hard to remember and difficult to apply to all revision cases”— although the classification is helpful clinically and has moderate reliability and validity (as compared to the AAOS system, where none had studied the validity of the AAOS system [9]), an accurate diagnosis according to the Paprosky Classification is still not easy. Yu et al [14] demonstrated that participants were able to achieve good interobserver reliability, but only after they had received a training session. Other weaknesses of the current classification systems include the reliance on x-rays instead of incorporating more advanced imaging techniques (such as CT or magnetic resonance imaging [MRI]) for preoperative diagnosis and a lack of clear guidelines to determine optimal reconstructions with current techniques and implants [11, 13].

When to engage computed tomography?

A CT scan is more helpful in acetabular bone loss with complex patterns. It should be conducted [13] when an evaluation of the above x-rays suggests the existence of:

• A pelvic discontinuity, ie, a pelvic fracture extending through the Köhler’s line is visible and with a concomitant rotation of the hemipelvis
• Significant osteolysis extending into the ischium
• Medial migration of the acetabular component

Computed tomography scans provide detailed cross-sectional images of the pelvis and therefore allow an assessment of all structures in axial, coronal, and sagittal views. These views can be very useful in assessing the extent and location of osteolysis. Information provided by CT scans helps determine if bone grafts or acetabular augments may be needed.

Although the utility of CT scans has traditionally been limited due to image distortion generated through metal artifacts of the existing implants, modern thin-cut technique and metal artifact reduction protocol have helped reduce such problems, making it the most reliable way for bone loss assessment [13].

In case of complex acetabular bone defects, 3D CT reconstruction and 3D modelling provide additional benefits in volumetric estimation of bone loss and size and shape of the bony defects [13]. Both techniques are helpful to surgeons in preoperative planning, potentially allowing accurate prediction of the final cup size and position or allowing surgeons to practice reaming and implantation on a 3D model. In case custom implants are needed, a preoperative CT reconstruction is required to design and fabricate a custom implant [13].

Magnetic resonance imaging

Magnetic resonance imaging is primarily used for visualizing soft tissue around the hip, such as the abductor muscles and capsule. It can be useful in assessing hip infection with soft-tissue extension, osteomyelitis, and identifying pseudotumors secondary to metal-on-metal articulation [13].

Conclusion

An accurate classification of acetabular defects is of vital importance in rTHA planning and execution. Preoperatively, it can assist the surgeon with planning to ensure that the potentially needed equipment and grafts are at hand in case a change of plan is necessary. However, one should be aware that the available classifications have moderate to poor inter- and intraobserver reliability. Additionally, the resulting bone defect after component removal can be more extensive than the original, preoperative assessment. For these reasons, rTHA workup should ideally include cross-sectional imaging to help surgeons visualize not just the defect but the remaining host bone location and quality.