Non-constrained total knee prosthesis

Careful balancing of flexion extension gaps is important in both type of prosthesis but might be made more difficult in a PCL retaining knee where exposure is more limited. Newer semi-constrained knees that sacrifice the PCL avoid many problems of superfluous constraint while mimicking the function of the PCL. Excellent results from both types of prosthesis are possible. Gait analysis comparisons while level walking did not show any significant difference between retaining or removing PCL. However, patients without a PCL shift their weight farther forward with each step while climbing stairs. This is thought to be due in part from excess leaning forward while stair climbing with a shortened quadriceps mechanism lever arm and less roll back provided by the PCL. A recent report comparing functional outcome between matched pairs cruciate retention and posterior stabilized knees showed no statistically significant functional difference between the two groups in the clinical evaluations. (44) There is no definitive answer as to weather PCL retaining or sacrificing knees provide a superior construct but there is a trend toward PCL sacrificing knees.

Results with the semiconstrained GSB-prosthesis

[Semi-constrained prosthetic arthroplasty of the knee following tibial osteotomy]

Semi constrained knee replacement - Answers on …


She began whined quietly, in doubt and growing pain: the beautiful "bedroom slippers" had no heel
or toe padding, and the poor girl had no idea how she was supposed to walk on her toes with all her
weight balanced on paper-thin "soles" no more than an inch across. Ruta stepped in front of her
before releasing the strap above her breasts, and with beefy hands steadied her precarious
two-pointed stance.


"Yes, is hard to walk, but I help you, and you learn. Oh, one more thing..." Reaching down with
one hand she produced the spongy exercise gag and pressed it between Jasmine's sore lips, where it
was drawn in with a slurp. "Now, turn around for us." As Ruta's big hands steered and steadied her
the gagged doll-trainee shuffled about, until she leaned forward slightly with her big tits pressed
against the black mesh of the table for balance. Ruta left her for a moment and returned with a set of
nylon straps.


She had begun binding her charge's arms for most of their therapy sessions, except those parts when
the arms themselves were included in the stretches or exercises. It forced even more pronounced hip
rotation when the girl walked, and kept the dangling, nearly paralyzed arms from getting in the way
of the rest of the workout. Ruta varied the bindings at each session, to provide some variety for
them both and to test the girl's range of motion. One of her favorite ties bound Jasmine's elbows
together behind her head, with her hands projecting free to the sides where they would bounce up
and down with her stride. It was simple but dramatic and showed off the girl's helplessness and
flexibility. But it also restricted her head movements somewhat and Ruta knew that would not be
appropriate for today's session.


Instead she bound the unresisting arms behind the girl's back, with elbows tightly together and hands
twisted up between her shoulder blades. She turned the hands palm-outwards, and used three straps
to bind the elbows, wrists, and hands firmly together. Then a final long strap attached to the wrist
ties and passed over the biceps, crossing Jasmine's chest above the swelling breasts to fix her arms
securely behind her with her fingertips just below the base of her neck.


With her useless arms up and out of the way, Jasmine was turned slowly about again. Despite their
internal reinforcement her toes were beginning to cramp and burn, and she mewed through the gag
in appeal. Ruta's expression turned cross.


"No belly-aching yet. This big day, you just get started! You don't want Feng to think you
chicken." With a nudge she turned her charge toward the door and stepped behind her, steadying the
teetering doll with a single huge hand wrapped about both her bound wrists as they made their way
out of the room and down the hall.


By the time she'd tottered the twenty yards to the therapy room, her heavy jugs swinging chaotically
and her new golden rings forgotten, Jasmine's toes were in agony. She hung her head and focused
on her balance, and the pain, as she and the supporting Ruta wobbled with tiny steps across the
room, past other subjects who groaned under the demands of their own therapists. Their destination
was a mat in the far corner of the room, where a small female clad in burgundy awaited them. At
last they stopped a few feet in front of her, Jasmine tossing her head in complaint.


"See, you walk all this way," Ruta spoke gently in her ear, "even though bedroom slippers not
meant really for walking. On your knees now." Jasmine obeyed gratefully, dropping to her knees
and haunches while her huge mams bounced. With her toes relieved she looked up at the figure in
front of her. It was a familiar-looking Asian woman, slim and wiry with sharp, sloping features.

evidence of sufficient sound bone to seat the prosthesis


Ruta stood directly before the girl as the restraining bands were undone one by
one, and her feet took on more and more of her weight. The soreness in her feet
reappeared, but it was bearable, the shoes being well designed to support and
distribute the load, and the discomfort was insignificant next to her eagerness to
try her legs. She noted sadly that when her arms were released they hung limp
at her sides. She barely had the strength to swing them a few degrees.

The last band to go was the one above her boobs. Ruta put her big hands up
under her subject's arms as the developer released it.

"Step forward, I have you."

Tentatively, the girl slid her left foot forward a few inches. She felt tension
building at her back, as though she was stuck to the table. She pushed the right
foot forward with greater force, and there was an audible peeling sound as the
table released her. She stumbled forward into Ruta's arms, her knees shaking
violently and toes groping for the floor.

The huge woman laughed and hugged her much smaller charge, the girl's huge
round tits pressing against her therapists' midsection. "Is OK, OK! Take your
time, get feet under you!"

The girl calmed herself and with an effort pressed her toes to the floor, then
straightened her knees again. "See? Like riding bike, they say." Ruta stepped
back slowly, steadying the wobbly girl at arm's length. "Come to me."

The girl put one foot forward, then the other, taking halting steps a few inches
long. Balancing was tricky, especially with the ponderous new counterweights
that projected in front of her. She had to thrust her shoulders well back, so that
her hands dangled behind her wide hips, to balance the load of the volleyball-
sized orbs. With each step Ruta retreated a like distance, steadying the girl but
allowing her to carry her own weight.

When they had traveled a few feet, the doctor spoke. "Let her turn around and
see where she's been all this time."

Like a lead dancer Ruta pivoted the pair clockwise, the girl shuffling her feet as
they slowly spun. When she had turned completely around Ruta looked her up
and down to be sure she was steady, then braced her with one hand and
stepped to the side.

The "table" was like nothing the girl had imagined. There was no flat surface at
all - it was really a conglomeration of sculpted shapes, each one designed to
support a particular body part. The material that had supported her from behind
was a black mesh, through which she could glimpse the complex steel armature
that had carried her weight while allowing her limbs to be flexed or rotated
individually. The whole contraption was mounted inside a pair of sturdy steel
posts bolted to the floor, which allowed the "table" to spin between them. It was
an impressive piece of equipment - but sinister. As it stood there empty, the
open bands seemed to beckon to her, calling her back into their grasp. She
shuddered and took an unsteady step back.

"OK, is OK, I have you." said Ruta, stepping again between her charge and the
threatening device. "Come this way, we walk."

Slowly she turned them again and led her charge toward the door. When her big
butt bumped against it, she stopped and moved behind the girl, again placing
one hand under each arm. "Do you want go outside?"

The girl was suddenly afraid, feeling for a moment as though this little white room
was all she knew, and the world outside the door was a mysterious and
frightening place. But she nodded her helmeted head slowly, pressing her chin
against the padded collar. Ruta reached out and touched a hand-sized metal
plate next to the door, which swung open silently.

For a moment the girl remembered her dream, but there were no curtains or
plush carpet - only a bare, white-tiled hallway. Ruta gave her a nudge and
together they shuffled haltingly out of the room. They turned and began to make
their way slowly down the corridor, which was perhaps 60 feet long and broken
by several closed and windowless doors to either side. At the end of the corridor
was a large double door, which did have small windows at eye level.

"Very good!" the developer called from behind them. "When you get back from
therapy there'll be more exciting things waiting - the orders for your custom mods
have come in! We'll get started as soon as Ruta is done with you."

Ruta whispered to her: "Don't worry that now. Focus your steps, you do very
good."

Restricted by the collar from looking down at her feet, the girl had no choice.

Class 2 Device Recall Proven Cemented Semi Constrained Total Knee: Date Initiated by Firm: ..
use of a constrained condylar knee prosthesis.

18/04/1989 · Semi-stable total knee prosthesis ..

TKA designs that remove the PCL may have less of a flexion arc and produce a quadriceps force which is weaker.
Theoretically, in semiconstrained components where the sagittal radius of the femoral component is constant though out, knee flexion would be expected to be limited to 120º with less roll back in flexion than a PCL retention device. Newer PCL sacrificing devices provide AP stability with an intercondylar spine or post built into the polyethylene and articulates with the transverse cam situated between the femoral condyles. This device mimics the kinematics of the PCL and augments flexion. The disadvantage of this device is that if the flexion gap is not balanced and is too large, a “cam jump” can occur, effectively dislocating the tibial post and tibia anterior to the femur. This must be reduced expeditiously and often requires a general anesthetic to do so. Cam jump can occur in the hyper-flexed knee even with careful balancing and therefore patients with expected knee motion > 130 º flexion should consider a PCL retaining implant.

A constrained prosthetic knee having a ..

Semi-stable total knee prosthesis:

Both PCL retaining and PCL sacrificing (AKA posterior/semi constrained prosthesis and PCL-substituting) implants are used. The eccentric femoral origin of the PCL produces a tensile force in the ligament as the knee flexes that is converted into a translatory force, shifting tibia forward or forcing femur to “roll back.” Retention of the PCL theoretically increases flexion with normal rollback but requires that the slope of replaced femoral condyles closely match that of the native condyles and that the knee be well balanced. The initial unconstrained implant designs that sacrificed both the PCL and ACL did not allow for rollback with a maximum flexion of 95º. These designs also had significant flexion instability. Preserving the PCL mitigates against flexion instability and cause rollback even without the ACL. However, this ACL deficient rollback is a combination roll and slide that can cause increased poly wear. This rollback also requires polyethylene to be relatively flat with less conformity on the femoral condyles. This effectively concentrates contact forces and can lead to rapid poly wear. Newer PCL retaining implants have a more congruent polyethylene that allows for less rollback but also spreads out contact forces leading to less poly wear. However, overly constrained implants can cause aseptic loosening because of excess stress transferred to the implant/cement/bone interfaces. TKA designs can reduce loosening by lessening stresses generated at the fixation interfaces but at the expense of accelerated polyethylene wear from increase peak pressures at the joint surface. The newest designs have developed an excellent balance between excessive stress transfer to the fixation site from over conformity and excessive poly wear from high contact pressures. The polyethylene inserts are “dished” in appearance in both the coronal and sagittal planes. Improvements in surgical instrumentation and techniques (ligamentous balancing, etc) have resulted in dramatic reduction in implant loosening as these problems were better addressed. The current goals of TKA design focus on minimizing polyethylene wear while protecting the integrity of the fixation interface.

The present invention provides an improved constrained knee prosthesis having a ..

with a semi-restraining total knee prosthesis constructed ..

Types of Implants
Kinematically, the native knee joint is more complicated than a simple “hinge” joint. Ideally, a TKA device should impart excellent range of motion, multiplanar stability, and maximal articular surface contact area. It should attempt to recreate the complicated sagittal and rotational kinematic motions that occur during flexion and extension of the normal knee. Solidly hinged and overly conforming TKA system increases the stability of a TKA by increasing its constraint. However, the torsional, coronal, and sagittal stresses that are normally shared by the surrounding soft tissue structures are more fully transferred through to the bone-cement or bone-implant fixation interface, which can result in premature failure from aseptic loosening. (42-43) The failure of early TKA designs was due predominantly to premature aseptic loosening associated with malalignment, ligamentous instability, and especially the use of excessively constrained simple hinge-like implant devices. However, in the normal knee, there is a complicated pattern of motion that occurs between the femoral and tibial articular surfaces during flexion and extension which is not on a fixed transverse axis of rotation like a simple hinge, but rather about a constantly changing center of rotation (polycentric rotation). Motion is achieved by a complex coupled mechanism in which the femoral condyles simultaneously glide and roll back on the tibial plateaus about multiple simultaneous axes. This femoral rollback and is a predictable and controlled posterior rollback of the femoral condyles on the tibial plateau during flexion of the intact knee. (Figure 36-37) Roll back is due impart to the stabilizing nature of the relatively immobile medial meniscus, PCL, ACL and various other ligamentous structures on top of a very low friction articular surface. Roll back increases posterior clearance by putting the tibia further out in front of the femur, permitting greater flexion with out impingement and increases the moment arm of quadriceps by 20 to 30%. Interestingly, the rollback on the lateral side is more than the medial side averaging (14.1-19.2mm laterally while the medial condyle translates posteriorly less than 5 mm). Therefore, there is external rotation of the femur on the tibia during flexion and internal rotation during extension. (ie, screw home mechanism) adding another axis of rotation to the knee. Rollback also increases the length of quadriceps moment-arm. If femoral roll back is not designed in prosthetic knee, effective strength of quadriceps is reduced by about 30%.