Anna Di Laura, Johann Henckel, Harry Hothi, Alister Hart, of the Royal National Orthopaedic Hospital, discuss the challenge of revision surgery
Compared to primary THA, revision surgery is more challenging to plan and execute. It often involves the management of significant acetabular bone loss, a potential indication for the use of custom-made implants . Despite the improvements in implant design, technology and surgical techniques, the revision rate remains high .
Additive manufacturing, or 3D printing, has enabled the production of custom-made titanium implants with integral porosity providing a new tool to manage massive acetabular defects (Paprosky type 3) in revision hip surgery and help patients walk again.
The challenge in treating extensive bone osteolysis is the loss of the normal bony architecture affecting both primary stable fixation and the restoration of the hip centre.
The custom-made implant, by design fits only the one patient, one shape with one orientation and has a finite shelf-life as bone remodels over time.
3D Surgical planning has become a key tool in managing these patients and is dependent on the quality of the images acquired.
Accurate 3D planning and the adoption of custom 3D printed implants show promising results in complex hip revision surgery . The presented cohort of patients has a mean follow-up time of 46 months (32-62), with good clinical scores, the implants remain stable.
What are the main challenges yet to be overcome?
- Poor visualisation of the bone in diagnostic imaging due to the indwelling metal implants and consequent spray artefacts.
- Intra-operative bone loss whilst removing the failed metalwork.
- Non-standing patient orientation during cross-sectional imaging with relatively narrow field of view.
- Understanding the distribution of viable bone for screw placement.
Metal artefacts can obscure the true dimension and shape of the bones. We have studied the use of a two-stage procedure for the reconstruction of massive acetabular defects, with cross-sectional imaging taken both pre-operatively and following the first stage procedure. This imaging strategy had previously been showed effective as an additional benefit of two-stage surgery in planning reconstructive surgery in patients with periprosthetic infection.
Application of this technique was then expanded to cases where large amounts of metalwork obscured the bone leading to uncertainty in the evaluation of the bone stock and hence the implant design.
We observed that the virtual 3D-CT bony integrity was different between the two imaging timepoints. The changes were predominately associated with the shape and distribution of the acetabular defects. This is of clinical significance as it led to implant design modifications, change in the complexity of the surgical procedure and the patient specific guides needed. If not anticipated, these changes can lead to increased intra-operative uncertainty, difficulty in fitting the implant and suboptimal implant position .
The restoration of centre of rotation (CoR) of the hips and leg length (LL) are key factors in achieving good clinical outcome . Due to the current CT scanner designs, patients are imaged with a limited field of view and usually in the supine position. This limited field of view restricts imaging to the pelvis excluding the spine and legs and occasionally does not allow a full appreciation of LL discrepancies that do not involve the hip; including abnormal pelvic orientation, bone length inequality, fixed flexion and asymmetrical femoral and tibial torsion . Low-dose biplanar radiography, EOS imaging, offers upright full length imaging and through 3D reconstruction of the limbs, measurement of LL. The ideal strategy should involve the co-registration of both imaging modalities.
A good understanding of the bone quality is essential for both the design of the implant and to optimise its position it the best quality bone. An example of the latest image processing software solutions offers the ability to extract more accurate bone density information from the CT scan (Simpleware ScanIP, Synopsys Inc, Mountain View). The bone quality distribution can be displayed in the form of a heatmap on the surface of the virtual model to aid the engineer’s implant design and can also be printed in different colours representing the varying bone density to assist the surgeon in the orthopaedic theatre (Rize Inc, Concord, Boston, Massachusetts).
Revision arthroplasty in patients with massive acetabular defects is challenging, even for the most experienced orthopaedic surgeon. An accurate representation of the geometry of the patient’s bone as well as the individual study of the patient’s biomechanics in pre-operative planning are crucial to ensure long-term survivorship of the prosthesis. Optimisation of CT scanning protocols and strategies to minimise metal artefacts are still an unmet need.
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 Di Laura A, Henckel J, Dal Gal E, Monem M, Moralidou M, Hart AJ. Reconstruction of acetabular defects greater than Paprosky type 3B: the importance of functional imaging. BMC Musculoskeletal Disorders 2021;22(1):1-10.
Authors: Anna Di Laura, Johann Henckel, Harry Hothi, Alister Hart, of the Royal National Orthopaedic Hospital.
Royal National Orthopaedic Hospital are leaders in the treatment and research into the management complex acetabular defects associated with the revision hip surgery. We perform a large number of joint replacements each year for patients from across the UK and abroad and are recognised for our specialist expertise in custom-made prostheses.