Notas / Notes - Active Congress.......

VIERNES / FRIDAY
196
knee designs have as a goal the restoration of knee motion
to the maximum achievable by each patient. Those patients
who can achieve motion greater than 135 degrees can benefit
from the “ high flexion” knees. The key to obtaining optimum
knee flexion is designing the articular surface geometry so
that femoral roll back is achieved with either a retained posterior
cruciate ligament or a ligament substituting cam mechanism.
Optimization of the articular surface position is more critical
for the retained cruciate design. Once the anterior/posterior
curvature of the articular surface and its location is optimized,
the knee joint’s wear rate may be minimized by choosing appropriate
medial/lateral radii of curvature. An additional design
requirement for joint surface curvature is to balance knee joint
laxity against knee joint stability. The choice of materials also
has a major impact on minimizing joint wear.
WEAR ANALYSIS OF RETRIEVED
UHMWPE TIBIAL COMPONENTS FROM
ROTATING PLATFORM TOTAL KNEE
REPLACEMENTS
Victor M. Goldberg, M.D; Ryan Garcia, M.D;
Matthew Kraay, M.D; Clare Rinmac, Ph.D.
Case Medical Center, Department of Orthopaedics
Cleveland, Ohio (USA)
Mobile-bearing total knee replacements (TKRs) were designed
to increase conformity, decrease contact stresses, and decrease
polyethylene damage. Our objective was to evaluate the
performance of retrieved mobile-bearing TKRs with respect
to wear damage of the polyethylene in a series of components
obtained at revision surgery. Tibial component polyethylene
superior and inferior surface damage and radiographic radiolucency
analysis was conducted on 40 retrieved mobile-bearing
TKRs. Higher levels of superior articulating surface damage
were found to be associated with higher levels of inferior surface
damage in this retrieval study. Greater levels of damage
were present on both surfaces in components with greater
radiographic radiolucency scores and mechanically loose component.
The mobile bearing TKR remains vulnerable to polyethylene
wear damage at the superior surface and introduced
an independent inferior surface also vulnerable to wear damage.
MOBILE BEARINGS:
ARE THEY WORTH IT?
Peter Bonutti, MD
Bonutti Clinic. Effingham, Illinois (USA)
Mobile Bearing is a technology which adds additional cost
to Knee Arthroplasty. Are there added clinical benefits to justify
this cost? Numerous factors have been suggested to add
clinical value in MB TKA. However, a critical review of clinical
data may not support this. Alleged clinical benefits for MB
TKA include: 1) “The only knee that bends and rotates” -
False. 2) Improved function - Not accurate. 3) Improved ROM
- False. 4) Improved long term survivorship - unproven. 5)
Reduced polyethylene wear - not proven in in vivo studies.
6) Improved patellofemoral tracking - inaccurate. 7) “Surgical
forgiveness” - actually more challenging. In addition, MB TKA
may have additional complications not found in fixed bearings
such as bearing subluxations, dislocations, and insert fractures.
Two recent clinical studies, Ball et al (2006) and Delport et
al (2006), have suggested possible functional benefit in
flexion and stair climbing, but these may be design or
technique specific.
MB TKA theoretical advantages possibly may be design or
technique specific, but the majority of clinical literature does
not prove benefit associated with MB TKA and there is published
technical complexity and risk. Based on current review
MB TKA is not worth the additional cost.
HIGH FLEXION, ROTATING PLATFORM
TOTAL KNEE ARTHROPLASTY:
SCIENTIFIC BASIS AND CLINICAL
EXPERIENCE
Richard “Dickey” Jones, M.D.
U.T. Shoutwestern Medical Center.
Dallas, Texas, (USA)
High flexion in TKA is any flexion beyond 125°. High flexion
lifestyle activities such as kneeling or praying are done with
internal rotation, external rotation, and in neutral. Therefore,
maximum flexion TKA requires a rotating platform. Superior
range of motion and fluoroscopic kinematics of femoral rollback
have been shown with the Sigma RP posterior stabilized
knee. This knee was modified to produce the Sigma RP-F,
a maximum flexion knee, by adding a third contact area at
the post-cam mechanism to significantly increase bearing
conformity and decrease poly stresses in the range of 125-
155. Clinical experience is reported with statistically significant
increase in postoperative range of motion in patients with the
Sigma RP-F. Patient selection criteria for the Sigma RP-F
include patients with high flexion lifestyle, younger, more active
patients demanding a better ROM, and those patients
with less than 100° preoperative ROM. This knee system
shows significant promise in providing maximum flexion and
system longevity is expected.