Biomedical Engineering Reference
In-Depth Information
This design also worked reasonably well but due to the failures discussed
earlier the Mark I design was changed in order to reduce blood supply disruption
and risk of bearing subluxation. The result was a design with a deeper sulcus and
shorter, dual fins. This dual fin arrangement also reduces the tendency of a fin to
transfer load distally, thus reducing stress protection. Further, finite element
analysis of the Mark I indicated that the tibial plate was too thin [81]. Its thickness
was, therefore, increased in the Mark II.
This new design is more difficult to implant since it requires additional
sculpture of the talus and another fin slot. The expected improved performance,
however, outweighed this difficulty in the design decision to adapt the new
configuration. The Mark II design is shown in Fig. 4.21. The differences between
the Mark I and Mark II are illustrated in Fig. 4.29.
MARK I
SINGLE FIN SHALLOW SULCUS
MARK II
DUAL FIN DEEP SULCUS
Fig. 4.29
Comparison of the Mark I and Mark II Devices
The Mark II was developed in 1989. Two additional sizes were added to
provide more anatomically conformance.. By this time the authors had also
developed a ceramic coating which apparently provided greatly improved wear
resistance, based on simulated wear tests [82], and enhanced biocompatibility
[83].
The new design was successful in greatly reducing talar subsidence as
illustrated by comparing the results given in Table 4.7 [25].
Based on the published literature, the B-P Mark II performed reasonably well in
both short and long term clinical use. Although the rate of talar subsidence is well
below that of the LCS Mark I Ankle it was felt, nevertheless, further improvement
was possible. The B-P Mark III is an attempt to address the problems associated
with the Mark II device.
