A PS DESIGN PERSPECTIVE
James R. MacKenzie, BSc
M.D, F.R.C.S.(C) Clinical Lecturer, Department of Surgery, Faculty of Medicine University of Calgary Calgary. Alberta WHY DO I USE
A PS TYPE OF KNEE REPLACEMENT?
KINEMATIC
CONFLICT
“Either the articular geometry must be
free to determine how the parts will move with respect to each other, or
anatomic structures (PCL) must be allowed to pull the femur across the
surface of the tibia.”
Kelly
Vince, ICL 1993
The list of potential reasons for using a PCL
substituting knee includes the following: 1. Technically Easier
2. Proven Clinical Results
3. Knee Deformities can be Corrected
4. Allows for Greater Conformity
5. Less Tibial Resection
LESS TIBIAL
RESECTION
Most
CR knees require the surgeon to remove additional tibial bone for a tight
PCL. With a PCL substituting knee there is no need to remove additional bone
to compensate for a tight PCL. Effectively preserves bone on the Tibial side
GREATER CONFORMITY SAGITTAL PLANE CONFORMITY
Greater conformity is achievable in the sagittal plane,
rollback being facilitated by the cam and spine mechanism. Most CR knees rely
on the PCL and flat polyethylene surfaces to produce rollback. These flat
surfaces have been shown in clinical studies to be inferior. RANGE OF
MOTION
Some clinical studies have shown a better range of
motion with PCL substituting knees. The cam and spine mechanism has been
shown to work extremely well in clinical studies. CORRECTION OF
DEFORMITIES
Most
surgeons recommend the use of a posterior substituting device for knees with
significant deformity. This includes surgeons who routinely use a CR knee.
PCL substituting knees are suitable even for patients with severe varus or
valgus, flexion contractures, and PCL deficiency. This wide range of
application allows the PCL substituter the opportunity to use the same
implant for most patients.
CLINICAL
RESULTS
Hirsch, Lotke, and Morrison: The Posterior Cruciate Ligament in Total
Knee Surgery. Clin Orthop 309: 64-68, 1994.
“The
posterior cruciate ligament-substituting device historically has demonstrated
excellent survivorship and appears to offer greater ROM.”
Stern and Insall: Posterior Stabilized Prosthesis. J. Bone and
Joint Surg., 74-A: 980-986, 1992.
“This report provides additional evidence that a knee
arthroplasty with cement, accompanied by sacrifice of both cruciate
ligaments, yields long-term results that are as good as or better than those
provided by other implants.”
EASE OF
PROCEDURE
Some proponents for cruciate retaining advocate an
algorithm for how to handle the PCL under differing conditions. The options
include PCL retention with a cruciate sparing device, PCL recession with a
cruciate sparing device, PCL supplementing insert, and a PCL substituting
device. On the other hand, the PCL substituting surgeon has already decided
preoperatively how to handle the PCL. Even most CR surgeons acknowledge that
to substitute for the PCL is easier than trying to get it balanced correctly. DISADVANTAGES
There
are some potential disadvantages. However, clinical studies have not shown
any inferior results with the PS knee compared to CR knees. One must balance
the flexion and extension gaps or the knee could dislocate. Some might view
this as an advantage as it forces the surgeon to insure that the knee is
relatively balanced. Somewhat more femoral bone is removed to make room for
the cam and spine mechanism. Potentially, a CR knee could have greater
flexion range than a PS knee. A PS knee which flexes greater than 125 degrees
may have the spine dislocate behind the cam. Some clinical studies have shown
better average range of motion with PS compared to CR.
One
particular PS knee design (IB2) was associated with a syndrome known as
patella clunk. It was not a frequent complication and most of the time was
not painful. In my experience, the few patients who have required treatment
for it have responded well to arthroscopic debridement of the scar tissue.
The modified new version of this design (LPS) in my experience, in over one
hundred knees I have no patients with patella clunk.
COMMON SENSE
CONFLICT
There
is no significant evidence to suggest that CR knees do better. PS knees are
technically easier and allow correction of greater deformities. PS knees may
give better ROM. Why doesn’t everyone use a PS knee?
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