Abstract
Background/Aims
The development of extendable prostheses has permitted limb salvage surgery in paediatric patients with bone tumours in proximity to the physis. Prostheses are extended to offset limb length discrepancy as the child grows. Aseptic loosening (AL) is a recognised complication. The implant stem must fit the narrow paediatric medullary canal and remain fixed while withstanding growth and increasing physical demands. Novel designs incorporate a hydroxyapatite (HA) coated collar that manufacturers claim improves bony ongrowth and stability, providing even stress distribution in stem and shoulder regions and providing a bone-implant seal, resulting in decreased AL and prolonged survival. This study aims to assess whether there is a relationship between bony ongrowth onto a HA collar and AL. Hypothesis: Bone ongrowth onto the HA collar of extendable prostheses is associated with more stable fixation and less AL despite patient growth.
Methods
Retrospective review of 51 primary partial femoral extendable prostheses implanted over 12 years from 1994–2006 (followed up to death at a mean of 2.5±2.2 years or last clinical encounter at a mean of 8.6 years) and 24 subsequent revisions, to ascertain failure rate and mode, together with a cohort study reviewing bony ongrowth onto the HA coated collar in 10 loose and 13 well fixed partial femoral, humeral and tibial implants. Patient growth was measured as a change in bone:implant-width ratio.
Results
21 (41.2%) primary femoral implants failed at a mean 42.8 months, 5 through AL. 1 secondary implant was revised for AL. 2 implants displayed evidence of progressive AL but had not failed at last follow-up. 5 of 11 tibial component revisions in distal femoral replacement were due to AL. 1 major complication occurred after revision surgery for AL in a primary implant: deep infection requiring 2 stage revision. Bony collar ongrowth was significantly higher in all 4 quadrants (anterior, posterior, medial and lateral) in the well-fixed as opposed to loose group, demonstrating a strong negative relationship in each quadrant between bony ongrowth and AL (p0.001) in the presence of patient growth as shown by increased bone:implant width ratio. In both groups, collar ongrowth was greatest in the posterior quadrant. Summary and Conclusions: AL has been confirmed as a common cause of failure in massive extendable endoprostheses. Revision surgery is difficult and may cause serious complications. For the first time, a significant relationship between a well fixed implant stem and bony ongrowth onto a HA coated collar in the context of massive implants used in tumour surgery has been demonstrated. This newly-proven relationship may result in longer-term implant survival and thus a reduced need for revision surgery. It is hoped that this study will provide the basis for further study of this relationship.
