The Royal National Orthopaedic Hospital is a world leader in researching and managing complex hip joint problems, including those involving significant bone loss in the pelvis (acetabular defects). The hospital is renowned for its expertise in using personalised implants to address these challenges. In this article, authors Anna Di Laura, Johann Henckel, Alister Hart, of the Royal National Orthopaedic Hospital NHS Trust, Stanmore, UK, discuss the current advances in custom implant designs.
Understanding bone loss around hip replacements
One major issue in hip replacements is acetabular osteolysis, a condition where bone loss around the implant weakens its attachment and leads to loosening. This complication significantly affects the long-term success of hip replacement surgery. A better understanding of the factors associated with the progression of osteolysis around implants has been limited by a lack of sensitivity of plain radiography[1].
There are three main sources/causes of significant bone loss around acetabular hip implants.
Metal-on-Polyethylene (MoP) bearings:
These materials were widely used in hip replacements but were found to wear over time, producing debris that triggers osteolysis.
Metal-on-Metal (MoM) bearings: Introduced as a solution to MoP wear, this generation of implants was expected to reduce bone loss. However, research has shown that metal debris from these implants can also cause osteolysis, sometimes more aggressively than in MoP implants. In some patients the osteolysis from MoM is severe within 10 years of the primary operation.
Peri-prosthetic infection: Infections can cause bone loss by releasing bacterial toxins and triggering inflammation. This inflammation activates cells that degrade bone. Bacteria can also form a biofilm on the implant, which allows the infection to persist and continue harming the surrounding bone. The infection must be treated before definitive reconstruction.
Challenges in detecting bone loss
Diagnosing bone loss early is critical, but standard X-rays are not very effective at detecting osteolysis[1]. This limitation means that the severity of the bone loss may be underreported [2, 3].
- MoP implant osteolysis: Easier to detect on X-rays since polyethylene is X-ray transparent, causing bone loss to show as a dark area.
- MoM implant osteolysis: More challenging to detect, as metal debris deposits appear as “bubble” or “cloud” signs rather than clear bone loss. Blood tests measuring cobalt and chromium ion levels can be useful, as elevated levels may indicate implant wear [4].
- Infection – Even in cases of significant bone loss, X-rays may be limited in detecting peri-prosthetic infections due to poor contrast around the implant and overlapping metal artefacts.