Knee Replacement Considerations
Knee Pain
A primary goal of joint replacement is to reduce pain. Though most patients
experience some pain during recovery from surgery, they find that the
long-term relief is a welcome liberation from the pain caused by diseased
knee joints.
Zimmer’s knee systems feature a unique “deepened
trochlear groove” in the femoral component. This allows smooth movement of
the patella through full range of motion, which reduces clicking, stress and
wear.
| Natural trochlear groove… | …recreated in our knee implants… | …helps enable full range of motion. |
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| (Illustration by: Calhoon, Borman, Mueller) |
Alignment
Joint alignment follows the same principals as aligning your tires: if your joints are out of alignment, one side is going to wear down. This can leave patients with moderate to severe bowleggedness (varus) or knock-knees (valgus).
Zimmer has special instrumentation to help surgeons to replicate natural,
optimum alignment.
This alignment is crucial. If the knee joint is
not aligned correctly during total knee replacement, it could result in
several problems, including:
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Decreased range of motion
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Patellar wear
The patella acts as a delicate balancing point for the muscles that propel the knee in motion. If it’s out of balance, it won’t be able to leverage the same degree of movement, and the added stress can eventually wear it down.
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Instability
If the implant doesn’t fit correctly, or if the ligaments can’t stretch enough to support it, it will become unstable
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Limited bending and extending
The patient’s ability to flex and extend his/her leg will be compromised.
Trabecular Effect
| Trabeculae are small bundles of fibers that make up the framework of organs, including the spongy interior cancellous bone. Consider the analogy of balancing something on sticks: the sticks must be at a 90° angle to provide full support: |
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| Cutting on a posterior slope provides a 90° angle for full support. |
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A perpendicular cut, on the other hand, puts the tibial tray at an awkward angle to the trabaculae, weakening the support structure.
Bone Preservation
Bone is living tissue and like most living things, needs stimulation to stay
healthy.
A challenge in knee implant design is not to let the implant
do all the work. If the implant overly shields the bone from stress
(“stress-shielding”), bone can be resorbed or broken down by the body,
resulting in bone loss for the patient.
On the other hand, if the
bone is required to take on too much stress, abnormal growth can occur –
called “adaptive remodeling.”
Zimmer implants are
designed to provide a balance between stable fit and natural stress loading.
Stability
Ligaments are tough bands of tissue that connect the thigh bone and the shin
bone. They stretch as you flex, extend and rotate your knee.
The
cruciate ligaments are so called because they cross over each other, forming
a cross when viewed from the side.
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The anterior cruciate ligament (ACL) keeps the shin from sliding forward.
- The posterior cruciate ligament (PCL) behind the ACL keeps the thigh bone from sliding all the way back.
The medial collateral ligament on the inside of the knee and the lateral
collateral ligament on the outside of the knee prevent the thigh bone from
sliding sideways when you walk.
When putting in knee implants, 99% of
surgeons cut or “sacrifice” the ACL since it’s usually too damaged and tends
to get in the way during surgery. To compensate for the ACL, the knee
implant includes a tibial insert to provide stability.
The decision
whether or not to sacrifice the PCL is more subjective, depending upon
surgeon philosophy, functionality, etc. Some surgeons want to retain as much
of the natural anatomy as possible. Others feel that a partially damaged
ligament (as is usually the case) is unpredictable after surgery.
If
the PCL is sacrificed, it must be substituted.
Zimmer offers varying degrees of stability in its knee implant designs to compensate for deficient ligaments -- from the minimum thickness for patients with good quality ligaments to the posterior stabilized design (bottom left) to provide the most stability for patients whose PCL’s have been sacrificed.
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Range of Motion
A goal of total knee replacement is to help patients gain as much range of
motion (ROM) as possible. Zimmer implants’ “deepened trochlear groove”
feature helps maximize ROM.
Another unique feature is the
Natural-Knee’s asymmetrical tibial baseplate, one of the first designs to
truly replicate the natural anatomy.
Designing surgeon Dr. Aaron Hofmann's team measured 100 cadaveric tibias and then designed the tibial baseplate to precisely match the asymmetrical shape. This helps minimize the “pinching” often found in symmetrical baseplates and maximize the patient’s range of motion.
Lifetime of Implant
The longevity of a prosthetic knee (how long it will last) varies from patient to patient. It depends on many factors, such as a patient's physical condition and activity level, body weight and the surgical technique. A prosthetic joint is not as strong or durable as a natural, healthy joint, and there is no guarantee that a prosthetic joint will last the rest of a patient's life. All prosthetic knees may need to be revised (replaced) at some point.
