| BUECHEL-PAPPAS™
TOTAL KNEE REPLACEMENT SYSTEM |
Total
Knee Design Rationale
Articular Surface
The New Jersey Knee uses a common generating curve to form
the articular surface of the femur. A similar curve is used
to form the tibial and patellar articulating surfaces insuring
that at least near line contact will be maintained for all
motion phases. A slight incongruity was used with the first
generation design to accommodate the motion of the meniscal
bearings. This inconguity was retained in the second generation.
The third generation B-P knee, however, uses the identical
curve as shown in Figure below producing perfect contact,
thereby reducing contact stress by about 50%.
This
generating curve is swept around a series of parallel axes
to form the femoral articular shape to provide 162° of
flexion.
Segment
2 the "Principal Load Bearing Segment" is generated
by rotating the generating curve about an axis through the
centers of the two "A" radii of the figure above.
This produces two spherical regions in the Principal Load
Bearing Segment.
The tibial
bearings of the first and second generations have similar
complimentary spherical surfaces, and thus all articulation
in this region have near congruent area contact. This produces
near congruent tibiofemoral contact during peak load phases
of walking and congruent patellar articulation during all
motion phases except near full extension where patellofemoral
compressive loads are very small.
The tibiofemoral
and patellaformoral articulations of the third generation
B-P knee are, however, fully congruent in the high loading
phases reducing contact stresses by about 50% compared to
the earlier generations.
This
spherical surface allows varus-valgus motion and patellar
tilt without loss of congruent contact.
In the
first and second generation devices beyond about 30° of
flexion the tibiofemoral articulation of the first generation
is typified by near line contact. A reduced posterior radius
of curvature for the femoral condyles is needed to provide
anatomical motion without bearing dislocation and to provide
full flexion. This increases the contact stress in deep flexion.
Thus, although this design has near conguent contact at peak
load, loading at about 40° of flexion is still substantial.
Here, the first and second generations reduce to near line
contact with its associated higher stresses.
The third
generation B-P knee is, however, fully congruent until about
50° of flexion when full, rather than near, line contact
exists. Thus contact stress for all loading phases is substantially
lower.
This
compromise also occurs in the natural knee. The designers
of the Oxford Knee failed to make this compromise resulting
in excessive lateral meniscal bearing dislocation problems
when used as a lateral hemi-arthroplasty.
Femoral
Component
The current
Buechel-Pappas Femoral Component is an advanced third generation
NJ device. It differs from the first generation device in
four significant ways:
- The
primary load bearing segment arc is greater by 19° increasing
the degree of congruent contact during flexion.
- The
minor incongruity needed to accommodate the meniscal bearings
of the first and second generations is eliminated, reducing
contact stress by 50%.
- The
distal and proximal condylar thicknesses are the same so
that the prosthetic gaps can be precisely reproduced. The
LCS posterior condylar thickness is about 1.5mm smaller
than the distal thickness.
The Figure
below shows the differences in the Lateral Shape of the
First and Third Generation NJ Femoral Components
- The
fixation side of the sulcus is flat rather than curved as
in the LCS providing contact with bone, rather than clearance
in this region.
- The
medial anterior flange side wall angle is greater eliminating
the overhang often found in the first generation NJ Knee
in this region.
The Figure below shows the differences in the Frontal Shape
of the First and Third Generation NJ Knee
Tibial
Component
The B-P Tibial Platform is anatomically shaped. It contains
a Stop Pin on its superior surface which engages a slot or
hole in the inferior surface of the Bearing to limit bearing
rotation to prescribed limits.
The B-P
knee provides ±45° axial rotation which is in excess
of that needed for any normal activity as shown below.
The limit
on axial rotation is produced by a Stop Pin on the Tibial
Platform acting against the ends of a slot in the inferior
surface of the Bearing. These limits are not encountered during
any activity but are reached only in the event of subluxation
of the bearing from the Femoral Component.
Rotational
dislocation of the rotating platform in the New Jersey LCS
rotating platform knee is a significant complication (1.2%
in the two PMA clinical trials). By proper attention to the
maintenance of collateral ligament tension during implantation
the rate of such dislocation can be kept acceptably low. Nevertheless,
due to the absence of the cruciate ligaments, the principal
anterior-posterior (A-P) and medial-lateral (M-L) stabilizers
of the knee, the potential for such dislocation remains.
This instability
characteristic is illustrated above. Significantly, the combined
effects of an A-P shearing load, distraction of one of the
condylar compartments, and a lax collateral ligament associated
with the distracted compartment, the rotating bearing can
be forced to rotate to a dislocated position. Only ligament
tension sufficient to prevent the femoral condyle on the distracted
side from climbing over the lip of the bearing can prevent
such dislocation.
Both A-P
and M-L shift of the femur relative to the tibia as illustrated
in (e) and (f) must accompany such dislocation. This action
is called “spin-out”. The shearing force and the
effect of the vertical rotation axis of the bearing accentuate
it.
The spin-out
problem is solved by a rotational stop. The controlling concept,
in providing a successful anti spin-out stop, that will not
adversely effect function, is to provide enough rotatory motion
for all needed functions. The motion must be limited, however,
such that if distraction allows disengagement and partial
spin-out, reapplying compressive force to the distracted condyle
will produce self-reduction of the bearing.
(c) and
(d) shows a distracted femoral condyle on the bearing lip.
In this position the anterior lip of the bearing is anterior
to the center of the femoral condyle. If the bearing cannot
rotate further so as to sublux as in (e) and (f) then when
a compressive load is applied to the distracted condyle this
side of the bearing will be forced anteriorly until the bearing
has been returned into its normal position as shown in (a)
and (b).
The self-reducing
feature of the tibial stop can also be applied to the stop
used in the patellar component by reducing the available motion
in this device from 90°of the first generation design
to about 30°.
Patellar
Component
The
fundamental principles in the design of a successful patellofemoral
replacement are to follow the teachings of nature, observe
the laws of physics and utilize the principles of engineering.
The prosthetic femoral sulcus should be anatomical, and should
accommodate the natural lateral patellar tilt during flexion.
This is done with the New Jersey Knees. A natural femoral
component sulcus cross sectional shape with a normal sulcus
angle and a conforming patella are used. The spherical shape
of the femoral condyles together with the lateral and middle
patellar facets provide for patelar tilt without loss of congruency.
The patellar
articulating surface, shown below, is likewise anatomic containing
all but the odd facet.
A rotating
patellar bearing is used to accommodate the normal axial patellar
rotation.
Materials
The B-P Knee metallic components are made of TiN ceramic coated
titanium alloy, a combination that is superior to Co-Cr alloy.
Titanium is less expensive, stronger and more biocompatable.
The ceramic coating is harder and more wear resistant and
greatly reduces polyethylene wear.
The bearings
are made of wear resistant 1050 UHMWPe. This material is superior
in wear properties to some "enhanced" polyethylenes.
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