Restoration of normal knee joint function through surgical reconstruction
is dependent upon load-sharing between the implant and the surrounding
soft-tissue structures. Mobile-bearing knee designs offer the advantage
of maximally conforming geometry while diminishing constraint forces
to fixation interfaces through plateau mobility. The degree of mobility
afforded by these designs in the anterior-posterior, medial-lateral,
and rotational directions defines the required interaction between
the soft tissues and the design geometry to maintain a stable articulation.
This study characterizes nine contemporary mobile-bearing designs
in terms of the force generated during a prescribed displacement.
Among the designs evaluated, only the LCS Deep Dish Rotating Platform
is available for clinical use in the United States.
A dynamic testing system capable of applying biaxial loads (Instron
Testing Machine, model 1115; Instron, Canton, Massachusetts) was
utilized to assess the intrinsic performance characteristics of
nine non-hinged, mobile-bearing knee designs. Anterior, posterior,
medial, lateral, and rotational constraints were determined for
each total knee design under a compressive load consistent with
normal walking gait1,2. A compressive
load of four times body weight and 0° of flexion was chosen to represent
a position of gait where maximum shear forces act in the posterior
and lateral directions as well as in rotation1,2.
Anterior and medial shear forces are presented at the same gait
position for completeness. A body weight of 163 lb (74 kg), which
corresponds to the average for a sixty-year-old, 5-ft, 8-in (172.7-cm)
male subject, was used in this evaluation3.
Anterior-Posterior and Medial-Lateral Shear
Testing
Three tibial inserts were evaluated in each test direction for each
system. Under the prescribed in vivo compressive
load (652 lbf), shearing displacements were applied to the system
until the implant subluxated (Fig. 1). Anterior, posterior, medial,
and lateral subluxation was defined as the dislocation of the tibial
component relative to a stationary femoral component. The shear
forces that were determined provided a measure of the maximum ability
of the knee design to constrain displacement during gait.
Rotational Testing
Under the prescribed in vivo compressive load,
the system was rotated both internally and externally in the transverse
plane and the torque versus angular displacement was recorded (Fig. 1). Three tibial
inserts were evaluated for each system. These results provide a
measure of the ability of the knee design to constrain rotation
during gait.
Intrinsic Constraint
Stability of non-hinged total knee replacements is achieved through
geometric variation of the condylar surfaces. The intrinsic constraint
of an implant system is defined as the capacity of the implant to
limit rotational, anterior-posterior, and medial-lateral displacements
to within normal ranges. In the absence of gross material deformation,
intrinsic constraint due to geometric variation may be described
in terms of the shear forces and the torques that act orthogonal
to the physiologic compressive contact loads between the femoral
and tibial components.
The graphs presented for each design (Fig. 2 (LCS Deep Dish Rotating Platform, T.A.C.K.)(SAL, TRAC)(Genesis II, Interax I.S.A.)(MBK, Profix, Rotaglide))
consist of force-displacement plots measured in the anterior, posterior,
medial, and lateral directions as well as a rotation plot that represents
the amount of torque produced during angular displacement of the
tibial component. The plots show the average constraint forces for
the three tibial inserts evaluated for each knee design.
In general, the graphs provide a visual description of the mobility
and constraint offered by each total knee design. All plots begin
well before neutral and proceed in the direction of testing. For
example, the testing of posterior constraint starts with joint contact
substantially anterior of neutral and proceeds in the posterior
direction.
The maintenance of a relatively low shear force over a defined displacement
is indicative of low constraint motion such as sliding, with only
friction providing resistance. This frictional resistance is characteristic
of sliding between the tibial tray and insert as well as sliding
between the tibial insert and the femoral component when the condylar
geometry is flat. Conversely, rapidly increasing constraint is evidence
that the femoral component has engaged a sloped region of the insert
that is now immobilized by mechanical stops on the tibial tray.
One of the principal features of mobile-bearing knee designs is
promotion of load-sharing through displacements between the tibial
and femoral components. Simply put, these designs allow the torques
and shear forces of gait to be transferred by means of displacements
to the soft tissues in a fashion similar to what occurs in the normal
knee. Load-sharing has many potential advantages. It reduces loosening
stresses transferred to the implant-bone interface and promotes
soft-tissue strengthening. These tissues, unlike the inert prosthesis,
have the capacity to respond and remodel to the challenges of expanding
activities as the pain-free patient undergoes rehabilitation. Finally,
load-sharing contributes to the reduction of articular wear of these
devices by reducing joint loads. In general, soft-tissue involvement
should be encouraged in order to minimize the dependency on intrinsic
constraints afforded by condylar geometry.
The importance of this study lies in the analysis of the mobility
offered by mobile-bearing knee systems and the extent to which a
design can exploit the benefits of soft-tissue load-sharing while
maintaining joint stability during gait.
In mobile-bearing knee systems, mobility can occur at the femoral
component-tibial insert articulation (as is found in fixed plateau
designs); at the tibial insert-tibial tray articulation; or, in
many cases, at both articulations. From a holistic approach, where
mobility occurs is irrelevant when addressing load-sharing. Although
it is important to issues concerning wear location and insert entrapment,
these topics are outside the scope of the current study.
There are substantial differences in the degrees of mobility offered
by the different mobile-bearing knee designs. A unique classification
system was implemented to group the designs according to the clinical
implication of their mobility in each of the directions tested.
The six directions of mobility were reduced to three: internal-external
rotation (R), medial-lateral displacement (ML), and anterior-posterior
displacement (AP). Directional constraint was grouped into one of three
categories based on known physiologic constraints and displacements:
unconstrained (
),
semiconstrained (
),
and constrained (
).
Unconstrained designs are characterized by very low constraint over
the entire range of normal displacements. Semiconstrained designs
have near-physiologic constraint that increases over the range of
normal displacements. Constrained designs are characterized by constraint
that exceeds physiologic levels and increases sharply over the range
of displacements. The values for these constraints and displacements
vary from patient to patient; however, normal values gleaned from
the literature are shown in Table I.
By applying these criteria to the constraint-versus-displacement
plots for each system, three distinct groups emerge, as shown in Table II.
Rotation in the transverse plane is a primary requirement of normal
gait. All of the mobile-bearing designs (Groups 1, 2, and 3) demonstrated
unconstrained rotational mobility (
) within a range of 15° of internal-external rotation.
This is the primary characteristic that defines mobile-bearing designs and
is an important feature in promoting longevity at the fixation interface.
Designs exhibiting unconstrained rotational constraint require soft-tissue
involvement, particularly balanced collateral ligaments, to achieve
knee stability.
No design that was evaluated demonstrated unconstrained medial-lateral
mobility. Group 3 presented with semiconstrained medial-lateral
mobility (
), whereas
Groups 1 and 2 had constrained medial-lateral mobility (
). Constrained or
semiconstrained medial-lateral mobility is a characteristic that
is common to all knee designs, fixed and mobile. This characteristic,
although not promoting soft-tissue load-sharing, does not adversely
affect clinical performance and may be advantageous in situations
of minor varus-valgus malalignment.
Major differences in the anteroposterior constraint were found among
the groups. Anteroposterior mobility was constrained (
) in Group 1 and unconstrained
(
) in Groups 2 and
3. Groups 2 and 3 require competent soft tissue—that is,
balanced collateral ligaments and/or posterior cruciate
ligament—to ensure joint stability. Although Group-1 designs
do not require substantial soft tissue for stability, the benefits
of load-sharing are not fully realized.
Mobile-bearing knee designs offer the orthopaedic surgeon a unique
option for returning the patient to normal, pain-free activity.
Because of the mobility that they provide, slight positional malalignment
of the components should not substantially affect the expected in
vivo service life of the device as long as that malalignment
corresponds with a region of mobility. In addition, this compliance to
position, within the mobility displacement envelope (defined by
the interaction between the soft-tissue structures and the device),
should allow these designs to function in patients with minor aberrant
gait patterns.
When mobile-bearing total knee systems are analyzed, it is important
to understand the actual mobility that is being offered by each
design. All of the designs tested permitted uninhibited physiologic
rotation of the tibial plateau, but the amount of displacement permitted
in the anterior-posterior and medial-lateral directions was highly
variable. Not all mobile-bearing knee systems are the same, and
to achieve clinical longevity of a total knee prosthesis it is important
to attain the correct balance between the intrinsic characteristics of
the device and the patient’s presenting pathology.
These ongoing laboratory evaluations assist in the understanding
of the anticipated performance of contemporary mobile-bearing implant
designs4,5. The results are intended
to aid surgeons in device selection when they are considering patient
factors. Furthermore, they provide manufacturers with design criteria
and assist regulatory agencies in determining the safety and efficacy
of specific knee designs.