The Influence of Coating Length on Osseointegration of Uncemented Implants: A Computational Study
+ Puthumanapully, P K; + Browne, M
+ University of Southampton, Southampton, UK
[email protected]
Introduction
Uncemented implants can employ porous coated surfaces to
encourage bone ingrowth, thus securing and providing biological
fixation. The design of these implants has varied, in terms of stem shape,
size and extent of coating employed. The coating layer used has
normally extended through the length of the stem, with the rationale of
employing all or most of the implant surface to secure long term
secondary fixation through bone ingrowth. However, concerns of
proximal stress shielding have led to implant designs that employ lesser
coating lengths, sometimes limited to the metaphyseal regions of the
femur. The effect of reducing the coating length on the type of tissues
formed at the interface has not been investigated. The study aims to
study the influence of three different coating lengths on osseointegration
of a conventional uncemented implant.
coated implant and only 10% of bone tissue for the fully coated implant.
At stabilisation (figure 2), the majority of the tissue type surrounding the
three implants was a combination of mature bone and immature bone,
around 60% in all cases. The most bone formation in terms of
percentage area occupied was seen for the 1/3 coated implant, with 72%
of the tissue space as opposed to 68% and 63% for the 3/4 and fully
coated implants respectively. Fibrous tissue was found to occupy very
little of the tissue space, with the rest of the surrounding tissue
consisting mainly of the harder cartilage tissue.
Methods
Three different coating lengths on the AML (DePuy®) implant were
chosen; proximally 1/3 coated, 3/4 coated and the fully coated (figure 1).
To simulate tissue differentiation around the three porous coated
versions of the implant, finite element models were combined with an
underlying mechanoregulatory algorithm for each of the implants.
Figure 1: The AML, with the three different coating lengths employed in
the study
Figure 2: Tissues formed around the coatings at stabilization for a. 1/3
coated, b. 3/4 coated and c. fully coated
The algorithm and modeling technique used for simulating tissue
differentiation around the implant was from a previous study described
elsewhere [1] that quantified a mechanoregulatory hypothesis developed
by Carter et al for fracture healing [2]. To summarize, the model used a
granulation tissue layer around the implants that would differentiate to
various tissues depending on the mechanical stimuli locally. The
mechanical stimuli were described by an osteogenic index ‘I’, which
regulated tissue differentiation at the interface. High ‘I’ values would
encourage bone formation with lower values associated with fibrous
tissue formation. The loading and boundary conditions used were based
on the data reported by Bergmann et al [3], involving normal walking and
stair climbing loads. The simulations were run iteratively, updating
material properties of the tissues formed at the interface, proceeding
towards stabilization. Three specific tissue types were monitored; bone,
cartilage and fibrous tissue, with the amount of bone formed used to
characterize the effectiveness of a specific coating length.
Discussion
The study predicted bone ingrowth for all three coating lengths used,
consistent with clinical studies [4]. Although the 1/3 and 3/4 coated
implants show more percentage bone formation with regards to available
surface area for tissue formation, considering the larger coating length of
the fully coated implant, overall bone formation is more with a larger
absolute surface area covered and hence offering more stability. The
added advantage of early stabilization through bone formation shown by
the 1/3 coated implant is not justification of its preferential use over the
other two coating lengths. Based on the results obtained in this
computational study, where it has been shown that there is little
difference, if any, with regards to bone formation, it would appear that
the fully coated implant should still be considered the more viable option
as it also has added advantages of better initial stability, ease of
repeatable implantation and more absolute surface area for ingrowth.
Results
The change in tissue type was noticeable as the iterations progressed
for all three AML implants. As expected, at the start of the iterations, the
majority of tissue formed was fibrous (>90%) across all the models. This
was seen extensively in the medial regions of the implant, occupying the
proximal to mid-distal regions. As the iterations progressed, there was a
noticeable change in the stiffness of the tissues formed. Immature bone
formation was observed across all three coating lengths around iteration
10, but in varying quantities. The 1/3 coated AML showed the most
bone formation at this stage with over 30% of occupied space around the
implant consisting of immature bone, in similar proportions to fibrous
and cartilage tissue. There was negligible bone formation for the 3/4
Significance
The work aims to address the effect of reducing the coating length on
bone formation at the interface of a porous coated implant. If a specific
coating length can be associated with more osseointegration, it could
have an implication for the design of future implants with better stability
and longevity
References
[1]
Puthumanapully et al., JORS 29:787; [2] Carter et al., JORS 6:736; [3]
Bergmann et al., J.Biomech 34: 859; [4] Engh et al., CORR 298:89
Acknowledgements
Research partly funded by DePuy, a Johnson & Johnson company
Poster No. 0975 • ORS 2012 Annual Meeting