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Whilst attempting to adapt design to improve
function, the large range of sizes and expanding
ability for femoral – tibial size dissociation
inherently illustrates the difficulty in matching
a patient’s specific anatomy and as such the
potential risk in choosing the incorrect size for
the femur and/or tibia of a given patient.
To further improve upon sizing issues, gender
specific prosthetic sizes have been developed
to counter the significant difference in distal
femur proportions between men and women, as
dimensions propagation of femoral components
for TKA traditionally followed men’s distal
femoral anatomy dimensions. In addition,
sizing of modern femoral components was
traditionally based on an assumption that ML
dimension increases proportionally to AP
dimension [55]. Despite best attempts to
modify shape and size the problem has not
been completely addressed, and there is still a
tendency for ‘overhang’ and ‘underhang’. Ana
lysis of six contemporary femoral components
with multiple ML/AP shape offerings and an
increased number sizes (Persona™, NexGen®,
Sigma®, Genesis™II, Triathlon®,Vanguard®)
has demonstrated either persistent overhang or
underhang characteristics of each prosthesis,
despite some superiority of some prosthesis for
greater cross ethnicity fit [56]. However these
additional sizes for gender and enhanced ranges
of size have previously not appeared to
influence short-term outcomes [57].
Ongoing pain is also associated with soft-tissue
impingement and may occur in up to 25%, of
total knee arthroplasties. It is associated with a
femoral component with a shallow trochlear
groove or with a sharp transition to the
intercondylar region of the implant, and poor
patellofemoral tracking [58].
Tibial sizing and shape
Tibial sizing is an important issue as well. In
any case, tibial tray or polyethylene liner
overhang may lead to soft tissue impingement
and subsequent pain (fig. 4). Asymmetrical
Table 1
Femoral sizes
options
Tibial Size Options
Max. Compatability Range between
femur/tibia Size
Persona
12
9
6
Attune
10
10
4
Triathlon
8
8
Complete interchangeability
Journey
10
8
6
Genesis II
9
8
5
Vanguard
10
9
Complete interchangeability
Nexgen
11
10
5
Fig. 4: TKR revision for medial pain. Intra-operative
view of the polyethylene liner medial that is
overhanging in a rotating platform knee in external
rotation. The metal tray is well aligned with the
bone cut.