Preparation and Approach
The type of anesthesia is determined by patient preference, in consultation
with the anesthesia staff.
The involved lower extremity is prepared and draped in the usual sterile
fashion. A tourniquet is inflated after the limb is exsanguinated. Landmarks,
including the patella, patellar tendon, and tibial tubercle, are outlined. The
surgical technique consists of a standard median parapatellar arthrotomy
performed through an incision approximately 10 cm in length placed slightly
medial to the midline (Fig.
1).
A medial arthrotomy is a familiar approach that allows simple conversion to
total knee replacement if unicompartmental knee arthroplasty is abandoned
intraoperatively, which can occur in up to 50% of candidates for lateral
unicompartmental knee arthroplasty (Fig.
2)10.
This technique provides excellent visualization of the lateral compartment as
well as the remaining compartments of the knee. An appreciation of knee
anatomy, an opportunity for minor débridement, and both patellar
subluxation and eversion are possible with this approach
(Fig. 3). A medial arthrotomy,
if reused for revision surgery at a later time, is also associated with fewer
complications11.
Moist towels are used to protect the subcutaneous tissue and surrounding soft
tissues throughout the procedure (Fig.
4).
Exposure
As the arthrotomy approaches the anterior horn of the medial meniscus, the
dissection is taken laterally and anterior to the coronary ligament to avoid
injury to the medial meniscus (Fig.
5). The patella is subluxated, or everted if necessary, and the
knee is flexed (Fig. 6).
Resection of the fat pad is performed only as necessary for improved
visualization for the lateral tibial resection. The medial and patellofemoral
compartments are thoroughly evaluated at this point to reaffirm the intention
to perform unicompartmental knee arthroplasty, or to abandon it and proceed to
total knee arthroplasty. Inspection and minor débridement of the medial
and patellofemoral compartments are facilitated with this approach.
Osteophytes are removed from the lateral compartment with the purpose of
defining the true condylar anatomy to allow proper prosthesis placement. The
improved visualization of the knee joint also allows better appreciation of
the condylar anatomy and the relationship to the other compartments, thereby
facilitating prosthesis
placement12
(Fig. 7).
Femoral Preparation
The distal end of the femur is first cleared of residual cartilage with
either a scalpel or an oscillating saw
(Figs. 8-A and 8-B). This
maneuver is performed to avoid underresection of bone, which hinders recession
of the leading edge of the femoral component
(Fig. 9-A). This step is
important in order to avoid impingement between the leading proximal edge of
the femoral component and the patella. Precise identification of the junction
of the cut surface of the distal femoral condyle and the overlying cartilage
is critical in determining the placement of the anterior extent of the femoral
component (Fig. 9-B). In medial
unicompartmental knee arthroplasty, the prosthesis can be placed so that it
abuts this junction. In lateral unicompartmental knee arthroplasty, however,
the anterior edge of the femoral prosthesis should be 1 to 2 mm below the
line. This difference is due to the laterally based tracking of the patella
with knee motion and the greater tendency for patella-prosthesis impingement
on the lateral side of the femur. For this reason, to err toward undersizing
the anteroposterior dimension of the femoral component is preferable in order
to avoid patellar impingement (Fig.
9-C). When downsizing the femoral component, however, one should
be sure that metal-to-plastic contact is maintained in full extension
(Fig. 10).
The femoral cutting guide is placed flush against the prepared distal
femoral surface. The guide is placed parallel to the long axis of the femoral
condyle. Removal of femoral osteophytes and complete visualization of the
anatomy assist in proper placement. As an additional guide to prosthesis
rotation, the planned posterior condylar resection should also be parallel to
the expected tibial plateau resection. A spacer block can be placed between
the femoral cutting jig and the tibia to assist in this process, if the tibial
cut is made first (Fig.
11).
The periphery of the lateral plateau extends beyond the lateral border of
the femoral condyle. Therefore, placement of the femoral and tibial prostheses
requires careful positioning to ensure congruency
(Fig. 12). The femoral
component is displaced laterally in the medial-lateral dimension in order to
maximize tibiofemoral component congruency in extension
(Fig. 13).
Tibial Preparation
An extramedullary tibial alignment guide is placed parallel to the tibial
mechanical axis. The amount of femoral resection dictates only a minimal
initial tibial resection in order to avoid the need for a thicker tibial
component (Fig. 14). Only a
small amount of posterior tibial slope (<5°) is created in order to
minimize posterior wear (Fig.
15). The guide is secured with one or two pins as close to the
tibial midline as possible to minimize the risk of a postoperative tibial
plateau fracture. A reciprocating saw is used to make a sagittal cut
immediately adjacent and lateral to the cruciate ligaments to facilitate
removal of the cut tibial plateau. A rectangular rasp is used as needed to
freshen the cut edges. The meniscus is removed carefully with electrocautery.
Flexion and extension gaps should be equal and are evaluated with spacer
blocks, and the tibial alignment is checked with an alignment rod. Sclerotic
bone can be drilled to enhance cement interdigitation.
Implantation
Preparation for prosthesis implantation begins with thorough pulsatile
lavage followed by drying of the osseous surfaces. A single batch of cement is
mixed at this time. The tibial prosthesis is cemented into place with use of a
cement gun and finger pressurization. Excess cement on the prepared tibial
surface is removed so that only a thin layer of well-pressurized cement
remains (Fig. 16-A). The
remainder of the cement is placed on the undersurface of the tibial
prosthesis, which is deliberately inserted by impacting the component from
posterior to anterior in order to displace excess cement anteriorly to
facilitate removal (Fig.
16-B). Knee flexion and varus stress aid in femoral component
placement, which is cemented with similar pressurization techniques with the
remainder of the cement batch. The trial insert is placed, and the cement is
allowed to harden with the knee in extension to pressurize the prosthesis-bone
interfaces. All excess cement is thoroughly removed.
Postoperative rehabilitation protocols are initiated on postoperative day 1
with continuous passive motion. Patients are allowed to bear weight as
tolerated with emphasis on achieving maximal extension and flexion. Patients
are typically discharged from the hospital after two to three days to home or
a rehabilitation facility as determined by the physical therapist and the
patient's social situation. Patients are seen postoperatively at four weeks,
one year, two years, five years, and then every two to three years for routine
follow-up evaluation.
CRITICAL CONCEPTSINDICATIONS:Noninflammatory unicompartmental arthritisA patient deemed inappropriate for a realignment osteotomyPreservation of the medial joint space radiographicallySymptoms isolated to the lateral sideClinically correctable valgus alignmentIntraoperative inspection confirming arthritis predominantly isolated to
the lateral compartmentCONTRAINDICATIONS:Eburnated bone (Outerbridge Grade IV) in the patellofemoral compartment
(Grades I, II, and III are
accepted)13Medial ligamentous laxityAn absent or deficient anterior cruciate ligamentA fixed flexion deformity of >10°A fixed axial deformity of >10° from the neutral mechanical axisPITFALLS:Overresection of the tibial plateau necessitates use of a thick insert.Failure to remove residual cartilage of the distal end of the femur leads
to insufficient distal femoral resection.Oversizing of the femoral component or inadequately recessing the leading
edge of the femoral prosthesis causes patellar impingement.When downsizing the femoral component, one should make sure there is
sufficient metal-to-plastic contact when the knee is in full extension, or use
the larger femoral prosthesis and/or recess the leading edge of the
component.Mismatch may occur during placement of the femoral and tibial components in
the medial-lateral plane.Improper rotation of the components may occur when they are placed without
complete appreciation of the anatomical landmarks.A lateral arthrotomy approach likely makes intraoperative abandonment to a
total knee replacement unfamiliar and difficult.Excessive posterior slope can encourage posterior wear patterns.AUTHOR UPDATE:The described surgical technique has not changed substantially since the
publication of the original paper. The frequency of lateral unicompartmental
knee arthroplasties has remained constant throughout the career of the senior
author.
CRITICAL CONCEPTS
INDICATIONS:
Noninflammatory unicompartmental arthritisA patient deemed inappropriate for a realignment osteotomyPreservation of the medial joint space radiographicallySymptoms isolated to the lateral sideClinically correctable valgus alignmentIntraoperative inspection confirming arthritis predominantly isolated to
the lateral compartment
Noninflammatory unicompartmental arthritis
A patient deemed inappropriate for a realignment osteotomy
Preservation of the medial joint space radiographically
Symptoms isolated to the lateral side
Clinically correctable valgus alignment
Intraoperative inspection confirming arthritis predominantly isolated to
the lateral compartment
CONTRAINDICATIONS:
Eburnated bone (Outerbridge Grade IV) in the patellofemoral compartment
(Grades I, II, and III are
accepted)13Medial ligamentous laxityAn absent or deficient anterior cruciate ligamentA fixed flexion deformity of >10°A fixed axial deformity of >10° from the neutral mechanical axis
Eburnated bone (Outerbridge Grade IV) in the patellofemoral compartment
(Grades I, II, and III are
accepted)13
Medial ligamentous laxity
An absent or deficient anterior cruciate ligament
A fixed flexion deformity of >10°
A fixed axial deformity of >10° from the neutral mechanical axis
PITFALLS:
Overresection of the tibial plateau necessitates use of a thick insert.Failure to remove residual cartilage of the distal end of the femur leads
to insufficient distal femoral resection.Oversizing of the femoral component or inadequately recessing the leading
edge of the femoral prosthesis causes patellar impingement.When downsizing the femoral component, one should make sure there is
sufficient metal-to-plastic contact when the knee is in full extension, or use
the larger femoral prosthesis and/or recess the leading edge of the
component.Mismatch may occur during placement of the femoral and tibial components in
the medial-lateral plane.Improper rotation of the components may occur when they are placed without
complete appreciation of the anatomical landmarks.A lateral arthrotomy approach likely makes intraoperative abandonment to a
total knee replacement unfamiliar and difficult.Excessive posterior slope can encourage posterior wear patterns.
Overresection of the tibial plateau necessitates use of a thick insert.
Failure to remove residual cartilage of the distal end of the femur leads
to insufficient distal femoral resection.
Oversizing of the femoral component or inadequately recessing the leading
edge of the femoral prosthesis causes patellar impingement.
When downsizing the femoral component, one should make sure there is
sufficient metal-to-plastic contact when the knee is in full extension, or use
the larger femoral prosthesis and/or recess the leading edge of the
component.
Mismatch may occur during placement of the femoral and tibial components in
the medial-lateral plane.
Improper rotation of the components may occur when they are placed without
complete appreciation of the anatomical landmarks.
A lateral arthrotomy approach likely makes intraoperative abandonment to a
total knee replacement unfamiliar and difficult.
Excessive posterior slope can encourage posterior wear patterns.
AUTHOR UPDATE:
The described surgical technique has not changed substantially since the
publication of the original paper. The frequency of lateral unicompartmental
knee arthroplasties has remained constant throughout the career of the senior
author.