The recent increase in the popularity of minimally invasive surgery has
dramatically altered the technique that most surgeons use to perform
unicondylar knee arthroplasty. This change in technique brings new
difficulties to both the surgeon and the patient, such as the potential for
new complications related to the limited surgical exposure and the need for
surgeons to learn the new operative procedures. In this report, we describe
the cases of four patients who required arthroscopic removal of a loose cement
fragment after a minimally invasive unicondylar knee arthroplasty. We believe
that this complication was related to the decreased visualization in the
posterior compartment of the knee associated with the use of small incisions
and cementing of the all-polyethylene tibial component. Our patients were
informed that data concerning the cases would be submitted for
publication.
Case 1. A seventy-year-old man with a history of a left
unicondylar knee replacement presented with increasing pain in the right knee.
The findings on physical examination and radiographs were consistent with
severe osteoarthritis of the medial compartment of the knee. In April 2003,
the patient underwent a right unicondylar knee replacement through a minimally
invasive incision. Both the femoral and the tibial components (Preservation;
DePuy, Warsaw, Indiana) were cemented (Fig.
1). The thickness of the all-polyethylene tibial component was 9.5
mm.
In June 2003, the patient presented with acute pain after feeling a
"popping" in the right knee. Physical examination revealed a mild
effusion and some ecchymosis about the posterior aspect of the knee.
Radiographs revealed a fragment of cement that appeared to be lying free
within the joint space (Fig.
2). Previous radiographs from the six-week postoperative
examination showed that the cement fragment had been fixed to the posterior
aspect of the tibial component. The patient underwent arthroscopy of the right
knee two weeks later, at which time a large fragment of free cement was
removed from the posterior aspect of the knee through standard arthroscopic
portals. The patient initially had a minor effusion, but all symptoms and the
effusion had resolved by four months after the arthroscopic procedure.
Case 2. A seventy-nine-year-old woman presented with bilateral
knee pain that was localized to the medial aspect of the knees. The findings
of radiographic and physical examinations were consistent with bilateral
osteoarthritis of the medial compartment of the knee. In May 2002, the patient
underwent bilateral unicondylar knee replacement through minimally invasive
incisions. The femoral components (Preservation; DePuy) were cemented, as were
the 7-mm all-polyethylene tibial components.
At the one-year follow-up examination, the patient stated that she was
doing well but that a sharp pain had developed in the left knee one month
prior to that visit. She described pain in the central, posterior portion of
the knee. Physical examination revealed an effusion of the knee and focal
tenderness in the posterior aspect of the knee. Radiographs revealed a large,
loose fragment of cement in the medial compartment of the knee.
Arthroscopy of the left knee was performed, at which time a large fragment
of cement was removed from the posteromedial aspect of the knee
(Fig. 3). The patient tolerated
the procedure well. At the latest follow-up evaluation, the patient had no
pain and the effusion had resolved.
Case 3. A sixty-year-old woman presented with bilateral knee
pain. Radiographs revealed advanced osteoarthritis of the lateral compartment
in both knees as well as moderate valgus alignment. In January 2002, the
patient underwent bilateral, lateral unicondylar knee replacement through
minimally invasive incisions. Seven-millimeter all-polyethylene tibial
components (Preservation; DePuy) were cemented in place.
Two months after her annual visit, the patient presented with acute pain in
the posterolateral aspect of the left knee and a moderate effusion.
Radiographs revealed a fragment of cement lying adjacent to the tibial
component. Comparison of the new radiographs with the postoperative
radiographs confirmed that this was a new fragment.
The patient underwent arthroscopy of the left knee, at which time one large
fragment and two smaller fragments of cement were removed. At the subsequent
follow-up evaluation, the patient had complete resolution of all symptoms as
well as resolution of the effusion.
Case 4. A sixty-six-year-old woman was evaluated for chronic
pain in the medial aspect of the left knee. Radiographs revealed a varus
deformity and osteoarthritis of the medial joint space. The patient underwent
a unicondylar arthroplasty through a minimally invasive incision in January
2003. Both the 7-mm all-polyethylene tibial component and the femoral
component (Preservation; DePuy) were cemented.
Ten weeks after the unicondylar knee arthroplasty, the patient presented
with acute pain in the posteromedial aspect of the knee. Physical examination
revealed that, although the knee range of motion was 0° to 120°, there
was mild effusion, posteromedial joint-line tenderness, and an antalgic gait.
Radiographs showed a fragment of cement lying posterior to the medial femoral
condyle. During arthroscopy, one large piece of cement and a second, smaller
piece were removed through a posteromedial portal. At the latest follow-up
evaluation, the patient had no symptoms except for tenderness at the medial
aspect of the portal site. The range of motion was still excellent, and the
effusion had resolved.
Unicondylar knee replacement is advocated as an alternative to total knee
replacement for selected
patients1-3.
The ten-year prosthetic survival rates after unicondylar knee arthroplasty
have been reported to range from 85% to 98% in studies of twenty-eight to 100
patients4-10.
Excellent clinical and radiographic results also have been
documented2,3,10.
Complications from this procedure have been noted as well, although previous
studies have typically focused on component
failure11,12.
Implant design, surgeon experience, and patient selection have traditionally
influenced
outcome3,13.
The popularity of the so-called minimally invasive approach has added a new
level of complexity to unicondylar knee replacement. The shortened incision
prevents a full examination of the posterior aspect of the knee and the
posterior portions of the prosthesis. The use of an all-polyethylene tibial
component further restricts visualization of the posterior compartment.
Because instruments cannot pass through this small area, it is difficult to
clean the posterior margins of the implant. As a result, there is an increased
chance that loose cement, bone, or soft tissue will remain in the posterior
area at the completion of the procedure.
In a six to ten-year follow-up study of patients with unicondylar knee
replacement, Berger et al. reported one case of revision surgery to remove
retained cement14.
We found no other reports of this complication in association with a
unicondylar knee arthroplasty. Previous studies have documented the use of
arthroscopy to treat complications of total knee
arthroplasty15.
Typically, the arthroscopic removal of foreign bodies has been associated with
good results. In our patients, arthroscopy proved to be an effective technique
that was associated with quick functional recovery and no major complications.
Standard portals were used, and the procedures were not technically difficult.
In one patient, a posteromedial portal was used successfully to remove a loose
fragment.
These cases highlight one complication of a minimally invasive technique.
We recommend ruling out evidence of polyethylene failure, loosening of any
component, infection, or contralateral compartment disease prior to
arthroscopy. We suspect that retained cement is occasionally an unrecognized
complication of both unicondylar and total knee arthroplasty.
To avoid this complication, we use a minimal amount of cement on the
components and the bone. We carefully inspect the components after they are
implanted, and we use specially designed nerve hooks to remove extraneous
cement from the posterior compartment. Occasionally, we also use a small
dental mirror to examine the posterior aspect of the implants. We remain
concerned, however, that the process of removing pieces of extruded cement
from behind the tibial component may loosen additional cement fragments.
Therefore, we now attempt to remove only very large fixed fragments or grossly
loose cement.
The size of the tibial component also may be a factor contributing to this
complication. The use of thinner polyethylene components typically results in
less bone removal and less space in which to operate, thus further inhibiting
visualization and the passage of instruments to the posterior aspect of the
knee; however, we continue to favor the use of a thinner (7-mm)
all-polyethylene insert with the Preservation system.
These four cases highlight the need for surgeons to be meticulous in the
preparation of cement and the placement of the implant and to recognize that
the use of an all-polyethylene tibial component prevents easy visualization of
the posterior aspect of the knee. Arthroscopy should be considered for
patients who present with acute mechanical symptoms, an effusion, and
radiographic findings that are consistent with a new loose body. Serial
radiographs are needed to identify such fragments. The arthroscopic removal of
a loose cement fragment was successful in this limited series of patients.