Title: The Interprosthetic Gap as a Risk Factor for Interprosthetic Fractures of the Femur
Other Titles: De interprothetische afstand als risicofactor voor interprothetische breuken van het femur
Authors: Quirynen, Thomas
Issue Date: 3-Jun-2016
Abstract: An interprosthetic (IP) gap is created when an arthroplasty is performed on both joints of the same bone to replace deteriorated joints. Specifically in this
thesis, placement of a hip and knee prosthesis in the femur was examined. Such an ipsilateral placement of stemmed prostheses into the same bone creates a gap of varying size and location depending on the prosthesis stem lengths. The IP gap is measured in between the tips of the prosthesis stems. Nowadays, the occurrence of the IP gap and IP fracture is infrequent (around 1.25% of patients with an IP gap) and predominantly found in patients with advanced age. As life expectancy is increasing and prosthesis placement in younger patients is rapidly rising, an exponential increase in the occurrence of the IP gap in both young and old patients can be expected, as well as an increase in prosthesis revisions. These prosthesis revisions feature longer stems and lead to smaller gaps. Since the clinical hypothesis is that the creation of an IP gap acts as a stress riser and that small IP gaps further increase the fracture risk of the femur, the IP gap might become a serious clinical concern in the near future. Still, the influence of the IP gap on femoral fracture risk has not yet been studied in detail and even though IP fracture reconstruction is a challenging treatment, a treatment protocol has not yet been defined. This thesis thus aims to offer a deeper insight (1) in the properties of the IP gap (size & location), (2) in other clinical relevant parameters that could influence femoral fracture risk (cortical thickness, bone E-modulus and prosthesis shape such as neck and stem length), (3) in the fracture morphology and (4) in the options for fracture treatment.
Two different approaches were utilized to fulfil these research aims. Firstly, an experimental test set-up was build to investigate the effect of creation of the IP gap and the effect of several IP gap sizes on femoral fracture load. This was achieved by loading synthetic bone specimens with implanted prostheses, along the mechanical axis of the specimen until failure occurred. Subsequently, fracture morphology was recorded and the specimens were reconstructed following different treatment protocols. These reconstructed specimens were again loaded to failure to define the optimal fracture reconstruction. Secondly, finite element models were created to compare strains on the femoral cortex for models with different IP gaps. The simplified cylindrical and parametric model was loaded along the mechanical axis of the femur mimicking stance on one leg. The anatomically relevant model was based upon a CT scan of a synthetic femur combined with laser-scanned prostheses as used in the experimental tests. This model was loaded along the mechanical axis of the femur and by two even more physiologically relevant loads representing walking and stair climbing. Both models allowed for variation of the IP gap size, gap location and bone E-modulus. The simplified model was also used to investigate the effect of cortical thickness alteration. Likewise, the anatomical model was used to study the impact of the prosthesis shape.
Experimental tests and finite element analyses reached similar conclusions. An IP gap did not act as a stress riser, but models with such a gap had, on
the contrary, a higher fracture load and lower strains than a model with a hip prosthesis only. Based on fracture load and femoral strains, small IP gaps
and more distal gaps were proven to be favourable. Hence, this thesis rejected both clinical hypotheses. Furthermore, smaller cortical thickness and longer
prosthesis neck lengths had a noticeably increased femoral fracture risk. As for fracture reconstruction, fractures with a medial butterfly fragment were the
most common fracture morphology. An optimal reduction of the fragments was a prerequisite for construct stability. Such a reduction, held in place with a long locking plate and fixed to the bone with screws proved to be the most beneficial treatment, both mechanically and biologically. Results showed that addition of an anterior strut was only advisable when the plate construct failed to achieve sufficient initial stability.
In conclusion, this thesis investigates the effect of several clinically relevant parameters on the immediate post-op characteristics of the IP gap. Influence of IP gap size, IP gap location, bone mechanical properties and hip prosthesis neck and stem length on femoral fracture load and femoral strains, for an immediate post-surgery situation, were quantified. Interprosthetic fracture morphology and fracture reconstruction were studied as well. Addition of bone remodelling simulations to the finite element model could offer insights in the long-term behaviour of the IP gap size and location. This could be investigated in a future study.
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Biomechanics Section

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